Current studies of the utilization of Co-doped carbon for heterogeneous PMS activation to degrade organic pollutants .... Figure 3.4 X-ray diffraction of jackfruit after treatment, and c
LITERATURE REVIEW
Jackfruit tree
Artocarpus heterophyllus Lam, a member of the Moraceae family, commonly known as jackfruit or Ceylon Jack tree, holds a prominent position among the diverse array of trees found in the home gardens of Vietnam With its botanical classification firmly established, this species has become an integral part of horticultural landscape [1] The jackfruit tree is a moderate-sized, perennial tree that commonly grows to a height of 8-25 meters with a stem diameter ranging from 30-80 centimeters [2] Young jackfruit trees typically feature a conical or pyramidal shape, while older ones develop a spreading and domed canopy The tree provides a dense shade due to its foliage It tends to have abundant lower branches Notably, when any part of the tree is wounded or injured, it secretes a sticky white latex [2]
The jackfruit tree is characterized by its monoecious nature, meaning that it possesses both male and female inflorescences on the same tree [3] Following a successful pollination, the fruit development process spans approximately three to seven months [1] The fruit itself can be divided into three primary components: the fruit axis, the persistent perianth, and the true fruit The fruit axis forms the core of the fruit and is not edible It is comprised of laticiferous cells, which contribute to a latex-rich composition and serve to hold the individual fruits together [2] The jackfruit has an outer skin that ranges in color from green to yellow-brown and grows in clusters The skin is composed of thick, rubbery, whitish-to-yellowish inner layer encased in hexagonal, sharply conical pieces called carpel apices The fruit has a fibrous core that holds it together and contributes to its weight Jackfruits are elongated, cylindrical fruits that are typically between 30 and 40 centimeters long [2] The capacity of the jackfruit tree to give more fruit than any other tree in the Moraceae family-between 70 and 200 kg of fruit per tree, depending on variety, cultural customs, and environmental factors-sets it apart from other trees [4]
1.1.1 Current situations of jackfruit in Vietnam
As of 2023, the cultivation area dedicated to jackfruit in Dong Thap province exceeds 3,000 hectares, with the largest concentration found in Thap Muoi, Thanh Binh, Cao Lanh, and Chau Thanh districts On average, jackfruit trees are harvested after a growth period of over 2 years Each jackfruit tree typically yields 2 to 3 fruits, with each fruit weighing more than 10 kilograms Across the province, it is customary to plant over 1,000 trees per hectare, and the harvest occurs after more than 2 years As a result, the average annual output ranges from 40 to 50 tons per hectare [5]
The structure of the jackfruit, as illustrated in Figure 1.2, consists of five main components: peel as thorn layer, mesocarp layer, tubular layer, seed, and core The peel refers to the outer covering of the fruit, while the core represents the central portion During the processing of jackfruit, the core is typically extracted and removed, as it is not commonly consumed This removal of the core serves the purpose of separating the edible parts of the fruit from the inedible portion Once the core is removed, it is often discarded or utilized as animal feed, particularly for animals like cows
Figure 1.2 The inside of the jackfruit with cross-sections in both vertical and horizontal directions [6].
Textile dye wastewater and remediation techniques
These days, there are major environmental contamination issues because of the growth of several enterprises The three primary categories of pollutants are chemical, physical, and biological, depending on the specific type of pollution [7] There are two primary categories of chemical pollutants: inorganic, which mostly consists of heavy metals, and organic, which includes toxins derived from carbon Physical pollutants include things that are radioactive, whereas biological pollutants include things like bacteria, viruses, and parasites Among the three groups of pollution mentioned above, chemical pollutants are mainly organic pollutants, which are the main causes of water and wastewater pollution Organic pollutants include insoluble pollutants such as oils and solution pollutants including dyes, antibiotics, and pesticides [8]
The process of turning natural or synthetic fibers into yarn and fabrics for industrial use is a drawn-out and intricate one The escalating amount of wastewater containing toxic dyes, produced by diverse sectors, is a major environmental and public health problem This presents a formidable obstacle to the current international water treatment facilities The textile industry, which is a recognized traditional and stakeholder business in the world economy, is confronted with significant environmental concerns The textile industries are home to a variety of chemical- based processes, ranging from early sizing to final washing, which raise significant environmental problems [9] The process of dyeing involves applying different colors and tints of a substance to a cloth to improve its look Any step of the textile production process, including the creation of fiber, yarn, fabric, and completed textile products like clothes and accessories, might involve dyeing Dyes are used to absorb and reflect light at wavelengths so that the human eye can see color A dyeing process is defined as the interaction of a dye with fiber and the migration of dye into the interior part of the fiber Not all the dye is attached to the fiber during the dyeing process; some dye is left in the dye bath and is released with the effluents Consequently, unless these effluents pose a risk to the environment, they must be treated [10] The consumer usually looks for some basic product attributes, such good fixation regarding light, sweat, and washing, both initially and after lengthy use, in addition to the design and appealing color The materials that provide color to the fiber required must have high affinity, consistent color, fade resistance, and be commercially viable in order to guarantee these qualities [11] In addition to weaving and the creation of synthetic materials, textile finishing procedures include washing, bleaching, coating, and dyeing as they relate to bulk textiles or clothing These are energy-intensive and consume a lot of water, which is mostly released as wastewater from Figure 1.3 [12]
Figure 1.3 Industrial textile waste dispersing both on land and in water [13]
Dyes have structures that are more complex than those of most organic substances Despite their complexity, dye structures have a few key traits in common In most dye compounds, a few aromatic rings, such naphthalene, benzene, are connected in a fully conjugated structure This suggests that there is a lengthy series of alternating single and double bonds between the carbon and other atoms across much of the formal textual structure This kind of arrangement is known as the chromophore, or color-donating unit [14] Based on their solubility, dyes are classed differently in specific situations As shown in Figure 1.4, soluble dyes include, for instance, acid, basic, direct, mordant, reactive, and insoluble dyes like azo, dispersion, sulfur, solvent, and vat [15] Reactive dye and chemical residues are present in printing and dyeing wastewater, which is processed before being released Water usage for textile processes varies depending on the techniques and chemicals employed; an approximate 50-240 liters of water are needed for every kilogram of finished material [16] High levels of organic matter, dyes and dye-related materials, hazardous chemicals, and inorganic substances such detergents, sodium hydroxide, hydrochloric acid, and sodium chloride are the constituents of wastewater Large concentrations of salts and high levels of biological and chemical oxygen demands (BOD/COD) give wastewaters their color These colors are undesirable from an ecological perspective [17, 18]
Figure 1.4 Synthetic dye classifications according to solubility [19]
One synthetic dye often used as a colorant for papers, wool, silk, and cotton is methylene blue (3,7-bis(dimethylamino) phenothiazine chloride tetra methylthionine chloride) [20] With a planar structure, MB is a heterocyclic aromatic chemical molecule It is chemically defined as C16H18N3ClS, illustrated in Figure 1.5, and has a molecular weight of 319.85 g/mol [21] The textile industry has extensively used
MB dye, a widespread blue, cationic, thiazine dye, as a fiber coloring agent [22] A significant amount of MB dye is consumed by the food, cosmetics, and pharmaceutical sectors for their manufacturing processes [23]
Figure 1.5 The model and the structure of methylene blue [24]
Nevertheless, there might be a lot of health problems associated with the release of wastewater from any of the companies that has been partly or untreated and laden with MB dye As an example, MB dye can cause cyanosis, tissue necrosis, the development of Heinz bodies, vomiting, jaundice, shock, and an increased heart rate in humans [25] Further, employing MB-loaded water incorrectly or untreated might cause skin necrosis, redness, and itching when the substance comes into contact with the skin [26] Additionally, the presence of MB has become a significant problem for plants, as seen by the growth suppression, pigment decrease, and reduction of protein content of the microalgae Spirulina platensis and Chlorella vulgaris [27] Therefore, efficient removal prior to industrial discharge is necessary due to the harmful impacts associated with wastewater laden with MB dye Also, the lack of clean water in society has lately been linked to the contamination of aquatic bodies caused by untreated MB dye effluents released by factories [28] A significant number of these compounds may be released into the environment as trash by manufacturing businesses including paints, textiles, cosmetics, medicines, and paints that use dyes comprising MB in their manufacturing processes The textile industry accounts for around 67% of the dyestuff market or consumption, and for every ton of produced fiber, over 120 cubic meters of industrial effluent are released into the environment [29] At 664 nm wavelength, MB typically has a high molar absorption coefficient
(about 8.4 × 10 4 L mol -1 cm -1 ), which can attenuate sunlight transmission and prevent light from reaching such streams and rivers [30] Hence, the entire aquatic environment may be impacted by the presence of MB, which can have a detrimental impact on the photosynthetic process, chemical oxygen demand (COD), biological oxygen demand (BOD), and oxygen requirement levels [31]
From Figure 1.4, the textile industry is a major source of dye release (54%), along with concurrent dyeing factories (21%), paper and pulp factories (10%), paint and tanneries (8%) and dye producing factories (7%) [15] These sectors also contribute to dye discharge into the environment In addition, the fast escalation of hazardous dye effluent produced by diverse sectors remains a grave threat to human health and the environment, presenting a formidable obstacle to the established conventional water treatment infrastructure Untreated wastewater containing dyes that are directly discharged into natural water bodies have an adverse effect on photosynthetic activity in aquatic ecosystems It has mutagenic or teratogenic effects on fish species and aquatic creatures because of the presence of aromatics and metals
Figure 1.6 Industries resulting in dyes by sector [15]
Because of this, dye removal from industrial effluent is a difficult task that also has an impact on the environment However, as seen in Figure 1.7, several cutting-edge techniques and technologies have recently been developed to both fulfill the needs for clean water and remove dye from wastewater For instance, advanced oxidation processes (AOPs), physical, chemical, biological methods are utilized to remove dyes [32-34]
Advanced oxidation processes (AOPs): Water treatment is the primary application for advanced oxidation processes (AOPs), which function under certain temperature, pressure, UV light, and oxidizing agent conditions [35] The efficacious treatment of polluted water is achieved by the in-situ production of hydroxyl radicals (OH • ), a potent oxidizing agent [36] For the treatment of water or wastewater, a number of process approaches, including ozonation, Fenton, photo-Fenton, photocatalysis, electrochemical oxidation, ultrasonic, plasma, and UV-based processes, have been studied as stand-alone or hybrid technologies [37] As a result of ozone photolysis in water, OH• radicals may be seen emerging (Eq 1.1) relates to the initiation phase, (Eq 1.2), and (Eq 1.3) to the propagation steps, and (Eq 1.4), and (Eq 1.5) to the termination steps [38]:
At certain wavelengths, hydrogen peroxide may absorb ultraviolet (UV) radiation Information on the production of OH• radicals can be found in the mechanism (Eq 1.6 –1.11) [38]:
OH • + HO → HO • + OH (Eq 1.9)
Physical processes: The implementation of physical techniques used for dye removal is based on the mass transfer mechanism, and literature reports have shown outstanding efficiency of elimination in the 85–99% range [39] Physical techniques often involve little complexity and offer several benefits, including low cost, easy unit operation, low design complexity, high efficiency, little chemical demand, and no inhibitory impact from the presence of harmful compounds
Chemical processes: With the exception of electrochemical technology, chemical treatments are often more costly than biological and physical ones Chemical dye removal techniques have a number of drawbacks that have rendered them unappealing for usage in the marketplace, including the need for specialized equipment, high electrical energy requirements for reactors, and significant chemical demand [40] Additional disposal issues arise from the chemical color removal process's production of hazardous secondary metabolites and byproducts [39]
Biological processes: Biological treatment is a less expensive, less harmful method that creates less sludge It produces full mineralization of the dyes and uses minimal chemical reagents It also offers energy-saving properties and is economically feasible to use in underdeveloped nations [41] Some of microorganisms have the capacity to change dye molecules into less hazardous forms, including yeast, fungi, algae, and bacteria [42]
Figure 1.7 Techniques for treating wastewater [19].
Peroxymonosulfate (PMS) activation results in oxidizing agents for the
The hydroxyl (OH • ) and sulfate (SO • ) radicals that are formed when AOPs degrade organic pollutants in wastewater have been shown to be highly effective reactive oxygen species (ROS) that can be produced [43] This has led to a significant amount of interest in AOPs due to their potential In addition, AOPs have a number of special benefits that set them apart from other technologies: 1) the potential to remove organic pollutants without generating secondary pollution; 2) the capacity to withstand challenging operating conditions; 3) low cost; and 4) the ability to remove highly toxic and persistent organic pollutants [44] While AOPs can produce ROS using a number of techniques such as photocatalysis, ultrasonication, microwave irradiation, and electrochemical processes, the most often used technique at the moment is the use of chemical oxidants [45] In order to produce ROS for the elimination of organic pollutants, hydrogen peroxide (H2O2), ozone (O3), and persulfate (S2O8 2−) have been employed extensively as chemical oxidants [46]
Dionysiou et al investigated the utilization of peroxymonosulfate (PMS), a novel chemical oxidant, to break down organic contaminants [47] As shown in Figure 1.8, PMS is marketed commercially as a solid powder under the brand name Oxone, having a chemical composition of 2KHSO5ãKHSO4ãK2SO4 The PMS anion has the structure HOOSO3 −, which is similar to HOOH but with one H atom substituted out for a SO3 − group
Figure 1.8 Molecular structure of PMS [48]
Although PMS has a high redox potential of 1.82 V and is generally a thermodynamically powerful oxidant, its direct interaction with the majority of organic contaminants is quite slow, necessitating activation [49] For PMS activation, a range of activation techniques have been used, such as heat, UV irradiation, alkali, ultrasound, transition metal catalysis, and electrochemical procedures [50] Due to their great effectiveness and simplicity of application, transition metal ions (such as
Co 2+ , Fe 2+ , Mn 2+ , Cr 3+ , and V 3+ ) are attracted a lot of attention recently as PMS activators It does not require any specialized equipment, which is quite helpful for large-scale applications [51].
The activated methods of PMS
1.4.1 Homogenous transition metals for activation of PMS
In order to generate free radicals that can effectively degrade organic molecules, transition metals have been used extensively in oxidant activation H2O2 and Fe 2+ , traditional reagents utilized in the Fenton process as (Eq 1.12), have been effectively applied to the degradation of a variety of contaminants [52]
Fe + H O → OH • + Fe + OH (Eq 1.12) Anipsitakis et al developed a highly effective technique based on the Fenton reagent, in which PMS was activated by cobalt ions to form sulfate radical for the purpose of decomposing the organic pollutants [47] Co 2+ /PMS has the potential to be an additional technique for the decomposition of organic substances, as demonstrated by Anipsitakis et al Superior performance at neutral pH is a useful advantage of the
Co 2+ /PMS combination over the Fenton reagent In addition, PMS was initially reported to be broken down by cobalt ions, depicted in (Eq 1.13) by Ball and Edwards in 1956 [53]
Co + HSO → SO • + Co + OH (Eq 1.13)
Figure 1.9 PMS activation by transition metals as homogeneous catalyst [50]
After the activation of PMS by employing homogeneous catalyst method, PMS gives rise to a hydroxyl and sulfate radicals, which can degrade organic pollutants in wastewater, depicted in Figure 1.9 Dionysiou et al demonstrated the ways some transition metals activate PMS, leading 2,4-Dichlorophenol (2,4-DCP) to degrade Studies demonstrated the following sequence for PMS activation by transition metals for 2,4-DCP degradation: Ni 2+ < Fe 3+ < Mn 2+ < V 3+ < Ce 3+ < Fe 2+ < Ru 3+ < Co 2+ [51]
As with the Fenton process, the ratio of PMS to catalyst dosages must be adjusted to attain the ideal conditions for maximum sulfate radical production This demonstrated which higher PMS and transition metal dosages can operate as limiting factors for the effectiveness of sulfate radical-based processes [50] With a molar ratio of 10,000 for PMS, the oxidant concentration was often more than the catalyst dose [54] The scavenging action of excessive PMS produced • with less reactivity (E 0 = 1.1 V) in contrast with • (E 0 = 2.5 V), which is the reason why PMS concentrations beyond the optimal range reduced the efficiency of treatment [55, 56] Free radicals cannot be generated mainly by PMS utilization, as organic pollutants can not decompose at any PMS concentration in the absence of a homogenous catalyst [57] Cobalt (II) is the most effective homogeneous catalytic activator for PMS [58] Iron was the most observed material for this application despite being partially ineffective since it is low-cost, ecologically benign, and generally harmless when compared to other possibilities [59] Although homogeneous metal catalysts have achieved excellent results, there are an array of disadvantages to this technique, the primary among which is the challenge of recovering metal ions and the resulting increased level of these ions in the treated water [56]
1.4.2 Heterogeneous transition metals for PMS activation
The accessible catalyst in the aqueous solution immediately interacts with PMS in homogeneous systems, significantly reducing the effect of mass transfer constraint The application of homogeneous catalysts is prone to numerous restrictions Usually, it is extremely challenging to recover homogeneous catalysts, and more processes must be employed to separate them, which are frequently not technically or financially possible [60] Furthermore, particularly when treating wastewater with high strength, the stoichiometric quantities of catalysts required for the activations associated with elevated PMS concentrations might be quite high Their residues in effluents can thus be seen as a secondary issue The rate at which sulfate, and hydroxyl radicals are generated is impacted by both sub- and over-stoichiometric quantities of the catalysts Finally, because transition metal species produce hydrated species in acidic pH and hydroxide precipitates in alkaline pH, they are extremely sensitive to pH and decrease catalyst availability
Regarding cobalt ion, while modest concentrations of cobalt are utilized to trigger PMS, the primary concern is to the possible health and environmental consequences of cobalt in aquatic environments Humans who are exposed to cobalt may have severe health impacts include liver, thyroid, and asthma symptoms, as well as allergies [61] In live cells, cobalt may potentially alter genetic information [62] A tolerance limit of 0.05 mg/L for Co(II) has been set for drinking water Therefore, further operations are needed to get this concentration [63] The expenses of the treatment procedure are in fact increased by the homogenous post-treatment of residual cobalt [64]
Because it is in the solid phase and can be easily separated from the liquid phase for the heterogeneous activation, this makes the catalyst less hazardous to the environment Different heterogeneous catalysts, including carbon catalysts, have been employed in the case of PMS activation These catalysts are based on different transition metals and their combinations Figure 1.10 shows a schematic representation of the mechanism of heterogeneous PMS activation
Figure 1.10 The heterogeneous activation of PMS [50]
The development of cobalt-doped materials as heterogeneous catalysts for activating PMS for the removal of organic pollutants has drawn a lot of attention recently Recyclability is another benefit of these insoluble cobalt doped materials, which is shared by other heterogeneous cobalt-based catalysts A variety of solid materials may be utilized as substrates for cobalt doping It has been shown that certain solid substrates, such as FePO4 and metal organic frameworks (MOFs), can create an optimal environment that promotes the activity of cobalt sites [48] Other solid materials which may be employed as substrates consist of carbon-based materials and metal oxides and hydroxides This indicates that a stimulating or synergistic influence from the substrate, which could substantially decrease the dose of the harmful Co 2+ ion, can lead the doped cobalt species to operate as more advantageous active sites for PMS activation [48] There are five different forms of cobalt oxide that have been identified: CoO, CoO2, CoO(OH), Co2O3, and Co3O4 CoO, Co2O3, and Co3O4 were investigated as potential PMS activators to break down different types of contaminants [62] These oxides can be utilized individually as nanoparticles or in combination with a variety of supports, including zeolites and carbon-based compounds In the event of PMS breakdown, Co3O4 nanoparticles work very well because of the existence of Co(II) and Co(III) in the molecule structure [65] Consequently, the production and use of Co3O4 nanoparticles as diverse catalysts have been contemplated Co3O4 nanoparticles have considerable catalytic activity, nevertheless because of their propensity to aggregate during catalytic reactions, their catalytic effectiveness is decreased [66] To increase catalytic activity, cobalt oxides and metallic cobalt can be immobilized on a variety of substrates (Figure 1.11) Cobalt species immobilization may restrict nanoparticles from being released into the environment and minimize the number of cobalt ions that leaking [67] Additionally, assisted catalysts improve the catalyst's stability and, as a result, its functionality It should be noted that supported catalysts could make the process of separating catalyst from solution faster
Figure 1.11 Various cobalt species immobilization with different substrates [50]
1.4.3 PMS activation by UV irradiation
UV light has been effectively employed to break down PMS for the production of sulfate and hydroxyl radicals by activating a variety of oxidants, including H2O2, percarbonate, ozone, and chlorine [68] Scission is directed towards the peroxide bond in PMS, which uses energy for PMS activation [69] This method makes use of a variety of energy sources, including heat, ultraviolet light, and ultrasound Applying
UV light to break chemical bonds and activate peroxide is a safe method [70] UV irradiation has an energy that ranges from 300 kJ/Einstein in UV-A radiation to 1200 kJ/Einstein in vacuum UV irradiation [71] In the range of 248 to 351 nm, the quantum yields of sulfate radicals declined as the UV wavelength increased Around 1.4 was the maximum quantum yield at 248 and 253.7 nm in wavelength [72] Generally, persulfate radiation is detected at a wavelength of 254 nm [50] By contrast, at a wavelength of 248 nm, the quantum yield for peroxymonosulfate was 0.12 Another research found that when the sink of sulfate radical into hydroxyl radicals was taken into consideration, the quantum yield for UV-254/PMS was 0.52 [72] It has been reported that the scission of the peroxo band in the PMS structure may be accomplished with sufficient energy from this particular wavelength of electromagnetic radiation, and that the PMS absorbs energy in the UV range By using sulfate and hydroxyl radicals, the UV/PMS process can either directly or indirectly decompose organic contaminants by photolysis [73] Two pathways can be implicated in the UV-induced activation of PMS First, as shown in the equation below, is the fission of an O-O bond by UV light [49]
The second is that UV irradiation from water molecules can generate electrons, and these electrons can then activate PMS by electron conduction, as shown by the following equation [49]
Some organic pollutants can be efficiently degraded by a single ultraviolet light source, although UV light has a limited ability to remove some organic contaminants For instance, direct photolysis can affect nitrosodimethylamine This indicates that
UV photolysis can quickly decompose it [74] It is not possible to directly photolyze the amine group [75] Even though these compounds with chromophores are UV active, they can still deteriorate if they include an amine group [76] When PMS and
UV were combined, the removal efficacy of these compounds was improved, suggesting that the energy input is mostly responsible for the organic contaminant degradation caused by a single UV source
1.4.4 PMS activation by ultrasound and conduction electron
PMS, an oxidant, can be used in the photocatalysis process as an electron acceptor Based on (Eq 1.18), both sulfate and hydroxyl radicals may be generated in a similar way [77] When the resulting hole (h + ) combines with PMS, the peroxymonosulfate radical (SO • ) generates (Eq 1.19) [78] By using (Eq 1.20) as a basis for its reaction, peroxymonosulfate radical can simultaneously make sulfate radical [79]
Semiconductor + hv → e + h (Eq 1.17) HSO + e → OH + SO • or HO • + SO (Eq 1.18)
Conduction electrons break down particulate matter and generate free radicals, which accelerate the breakdown of organic contaminants [77] PMS has recently been involved in the nonmetal photocatalysis process as well Visible light irradiation activated the graphene-like carbon nitride [80], polyimides [81], carbon nitride materials [82], and orthorhombic a-sulfur [83] to activate PMS via conduction electron.
Solid supporter types for the synthesis of cobalt-doped material
Titania, alumina, manganese oxide, ferric oxide, and numerous multicomponent oxides/hydroxides are the most suitable metal oxides/hydroxides for application as cobalt dopant supporters because of their stability, large surface area, and simple production [48] Frequently, doping cobalt into these solid substrates is accomplished by substituting Co 2+ ions for the cations in the supporter lattice The principal synthesis techniques include ion exchange–calcination, coprecipitation, hydrothermal, sol–gel, and solution combustion The distribution of the cobalt dopant is significantly influenced by the radius ratio of the host cation to the guest Co 2+ ion, it should be emphasized It is advantageous for the Co 2+ sites to increase their utilization when the guest Co 2+ species is preferentially situated near the substrate's surface when Co 2+ is smaller than the host cation [84] Cobalt doping usually leads to the production of a large number of oxygen vacancies, which can boost catalytic performance overall More significantly, a typical phenomenon that cannot be disregarded is that the cations of the substrate lattice can effectively contribute to PMS activation or increase the redox characteristics of Co 2+ /Co 3+ , leading to improved catalytic performance [48]
The distinctive properties of carbon-based materials, such as chemical inertness, large specific surface area, excellent electrical conductivity, strong adsorption properties, and high availability, lead to them being potential substrates for cobalt doping [85] Examples of these materials are carbon aerogels, N-doped porous carbon, and N-doped graphene [86] Typically, the pyrolysis process is used for the fabrication of cobalt-doped carbon compounds A cobalt salt and an organic ligand are used as the cobalt and carbon sources, respectively [87] The reason for the presence of Co(0) in cobalt-doped carbons, as opposed to cobalt-doped metal oxides/hydroxides, is that at high temperatures, carbon reduces the cobalt ion, which results in the overall presence of cobalt species [85] When using cobalt-doped carbon for PMS activation to decompose organic contaminants, it has usually multiple advantages over other cobalt-doped materials First off, the carbon substrates themselves have some catalytic activity for PMS activation, and it's possible that the cobalt species and carbon substrate cooperate in conjunction to promote PMS activation [88] As a result, cobalt doped carbons typically demonstrate stronger catalytic efficiency than conventional catalysts based on cobalt In addition, carbon substrates frequently show an outstanding adsorption affinity towards a range of organic contaminants [89] This allows for the efficient concentration of organic pollutants on the catalyst surface, ultimately resulting in an enhanced effectiveness of elimination More recent attempts have been conducted to add heteroatoms including N, S, B, Si, and I to the carbon substrate to increase the catalytic effectiveness of cobalt-doped carbon-based materials It has been determined that because of the increased electrical transfer characteristic and potential synergism between Co and heteroatoms, heteroatom-doped carbons tend to be superior substrates than pure carbon [90]
Other solid materials, including as metal-organic frameworks, metal phosphate, hydroxyapatite, metal sulfide, carbon nitride, and mesoporous silica, can also be utilized as efficient substrates for cobalt doping to activate PMS [91-93] Usually, a little quantity of cobalt salt and a significant portion of another metal salt or silicate are used as precursors for cation substitution, which explains how cobalt doping in these solid substrates is carried out [48] In addition, the standard hydrothermal- calcination process is used in the production of Co-doped g-C3N4, similarly to how it is for Co-doped carbon compounds [48] Specific techniques to produce this kind of cobalt-doped material have been discovered For instance, Pang et al developed Co- doped hydroxyapatite (CoHAP) through calcination and ion exchange, two steps that catalytically activate PMS for the decomposition of organic contaminants [94].
Current studies of the utilization of Co-doped carbon for heterogeneous PMS
Tian et al develop a method to synthesize biochar modified with amorphous Co3O4 nanoparticles and employed it to activate PMS for the degradation of sulfamethazine (SMT) [95] They fabricate hydrochar from corn straw through hydrothermal carbonization, which is then impregnated with CoCl2 solution to anchor cobalt nanoparticles onto the biochar surface before undergoing pyrolysis Under optimal conditions, they achieve complete degradation of SMT within 60 minutes using a 1.0 mmol.L -1 CoCl2 aqueous solution, a pyrolysis temperature of 800 o C, 400 mg.L -1 of cobalt-loaded biochar, 0.06 mM PMS, and a solution pH of 8 The findings indicate that the amorphous Co3O4 nanoparticles effectively activate PMS The biochar serves a dual function: it disperses Co3O4 nanoparticles and acts as an electron mediator between the Co3O4 and PMS/SMT molecules During repeat use, the predominant free radicals transition from OH • and SO • to solely SO • , and only minimal cobalt ion leaching is observed
Utilizing two distinct cobalt-based catalysts, Delgado et al activate PMS to degrade benzoic acid [96] The various catalysts are supported on biochar that is derived from woody pruning wastes by pyrolysis Wet impregnation of biochar with metallic (II) solutions yields metallic catalysts containing a combination of Fe, Co, and Cu In the analyzed conditions, the biochar catalysts based on Fe and Cu exhibit low stability and efficiency for the degradation of organic pollutants Conversely, the Co catalysts have strong activity in the aqueous solution towards the degradation of benzoic acid
At the 2h reaction period, the benzoic acid degradation reaches the maximum amount of mineralization (78.1%) A highly stable and reusable catalyst emerges when the
Co catalysts are synthesized in an alkaline environment Following the alkaline treatment, surface oxygen functional groups are generated, significantly reducing metal leaching in the catalyst and enabling more cobalt to be loaded onto its surface Following a study of the quenching experiment data, they conclude that the creation of SO • and OH • radicals, which ultimately prove to be responsible for the degradation of pollutants
Monolithic cobalt-doped carbon aerogel was created by Peidong Hu et al to effectively catalyze the activation of PMS in water In this work, graphene oxide sheets and resorcinol-formaldehyde resin were co-condensed in the presence of cobalt ions to create a macroscopic three-dimensional monolithic cobalt-doped carbon aerogel This was then thermally treated in air, lyophilized, and carbonized [86] Finally, cobalt species were preserved as both metallic cobalt and Co3O4, encased in layers of graphitized carbon Cobalt ions were used as a polymerization catalyst to bridge the ordered framework Following a heat treatment in the air, the material was obtained (CoCA-A) In the presence of PMS and CoCA-A, the removal efficiency of phenol is significantly increased, reaching 87% in 60 minutes In the presence of PMS and CoCA-A, the removal efficiency of phenol is significantly increased, reaching 87% in 60 minutes When the initial phenol concentration increases from 20 to 100 mg.L -1 , the final degradation efficiency in 60 minutes decreases from 87% to 21%
N-doped porous carbon with Cobalt(0/II) incorporation was created by NanZhou et al as a potent heterogeneous PMS catalyst for quinclorac breakdown [90] Thermal degradation of ethylene-diamine tetra-acetic acid and a Co salt was performed in one step to create a Cobalt(0/II)-incorporated N-doped porous carbon (Co/NAC) catalyst The graphene-like film-type N-doped carbon support allowed for the formation and well-dispersion of fine Co nanoparticles made comprised of metallic and oxidized Co species In order to produce sulfate and hydroxyl radicals during PMS activation, the
Co species was primarily responsible Through improving electronic transport, the N- doped porous carbon had a synergistic effect on the catalytic performance The outcome Co/NAC allowed for significantly increased quinclorac (QNC) degradation and proved to be a highly effective heterogeneous catalyst for PMS activation Using 0.08 g.L -1 Co/NAC and 20 mmol.L -1 PMS, 93% QNC (50 mg.L -1 ) elimination was typically accomplished
Carbon-supported Co (CS-Co) composite was created by Yang Liu et al using a carbonization procedure starting with a saturated resin [97] When PMS was activated, CS-Co showed good catalytic activities, producing sulfate and hydroxyl radical for the degradation of trimethoprim (TMP) A range of pHs (3.0-9.0), catalyst dosages (0.05-0.5 g.L -1 ), PMS doses (0.05-1.0 mM), TMP concentrations (5-20 mg.L -
1), and temperature ranges (15-30°C) were examined in relation to the performance of composites TMP deterioration increased considerably when CS-Co and PMS were present at the same time It is possible to get a removal efficiency of up to 96.5% in
60 minutes; in the system without Co, this is only 29.7%, suggesting that the cobalt element is a necessary component for catalytic oxidation
Yue Yang et al utilized a simple method to produce waste diaper carbon (Co-WDC) that has been implanted with cobalt for the decomposition of bisphenol A (BPA) [98] When used repeatedly to remove organic pollutants, Co-WDC demonstrated good catalytic performance and recyclability During the carbonization process, cobalt and nitrogen combined to create the Co-N structure in the waste diapers, which served as a catalytic active center and triggered the creation of 1 O2 by PMS Co-WDC was able to efficiently degrade BPA by co-degradation via the radical and nonradical routes when this 1 O2 generation was coupled with the radical pathway, which was augmented by cobalt compositing Co-WDC eliminated 0.02 g.L -1 of BPA in under 6 minutes
Utilizing renewable sodium alginate and melamine through sol-gel assembly and carbonization techniques, Xin Zhao et al synthesized nitrogen/cobalt-codoped multifunctional carbonaceous beads (MCB, SA/N-CoxOy-700) [99] The results demonstrated the outstanding performance of SA/N-CoxOy-700, of which 99.63% of
MB could be degraded in about 5 minutes The removal investigations in various decolorization systems shown that the combined action of embedded nitrogen, carbon skeleton, and cobalt oxides might be primarily responsible for the exceptional decolorization efficiency Sulfate radicals were shown to have a prominent function in quenching investigates and coexisting anions study, with little effect from the large number of anions that may have been present in the real wastewater SA/N-CoxOy-
X was able to eliminate medications and aromatic chemicals like phenols and tetracycline in addition to degrading certain dye types
Zhigao Zhu et al fabricated hierarchically structured ferro-cobalt alloyed crystals supported on nitrogen-doped activated porous carbon fibers (FeCo2@APCFs) by a combination of multicomponent electrospinning, activation, and carbonization Using a combination of multicomponent electrospinning, activation, and carbonization, Zhigao Zhu et al created hierarchically organized ferro-cobalt alloyed crystals supported on nitrogen-doped activated porous carbon fibers (FeCo2@APCFs) [100] Within 60 minutes, 27.7% of MB deteriorated in the PMS instance alone In addition, when the Co molar ratio range was adjusted from 0 to 1 with catalyst (0.1 g.L -1 ), methylene blue (20 mg.L -1 ), and PMS (0.2 g.L -1 ), the elimination efficiency of MB in 5 minnutes was drastically enhanced from 65.91 to 99.53%
In this study, the jackfruit core is utilized for fabricating carbon substrate via advantageous, environmentally friendly techniques such as pyrolysis and hydrothermal techniques without the addition of hazardous substances This method quickly pre-treats jackfruit core to get rid of undesirable elements including lignin and hemicellulose The carbon substrate is doped with cobalt oxide, which causes the heterogeneous activation of PMS As a result, PMS produces sulfate and hydroxyl radicals, which severely degrade organic materials into simple byproducts At last, because this system is heterogeneous, materials can be efficiently recovered after organic contaminants are degraded It has been shown that even after multiple recycling cycles, the removal efficiency remains high.
METHODOLOGY
Research objectives and contents
This study focuses on cobalt-doped carbon derived from jackfruit core, which is a naturally occurring byproduct that is produced during the separation of fruits from jackfruit The jackfruit core utilized in this investigation is sourced specifically from the Tien Giang province
The fabrication of cobalt-doped carbon from jackfruit has been accomplished, with the intended application being water treatment
Content 1: Investigating pyrolysis temperature and loading cobalt content on morphology, chemical structure, and crystallinity profile of the Co-doped carbon from jackfruit core;
Content 2: Evaluating MB degradation ability of the synthesized Co-doped carbon under various synthesis conditions;
Content 3: Investigating MB removal efficiency of the fabricated Co-doped carbon under different catalyst dosages, PMS concentrations, dye concentrations, anionic concentrations, temperatures, and pH values;
Content 4: Identifying main reactive oxidized species in heterogeneous PMS activation;
Content 5: Regeneration of the Co-doped carbon in treatment of MB- contaminated water;
Content 6: Evaluating the degradation ability of the Co-doped carbon for crystal violet, methylene orange, and congo red.
Materials and instruments
All essential reagents and starting materials in this study were commercially obtained These chemicals are listed in Table 2.1
Table 2.1 List of chemicals purchased and used in the study
Jackfruit core - Dong Thap province Cobalt (II) nitrate hexahydrate Co(NO3)2.6H2O China Peroxymonosulfate KHSO5.0.5KHSO4.0.5K2SO4 China
Disodium hydrogen phosphate dodecahydrate Na2HPO4.12H2O China
Sodium hydrogen carbonate NaHCO3 China
Tert-butyl alcohol (CH3)3COH China
Equipment: beakers, pipettes, volumetric flask, thermometers, glass rods, laboratory bottles, burettes, weighing papers, syringes, etc
Instrument: All instruments used in this study, listed in Table 2.2
Table 2.2 List of instruments used in this study
Autoclave thermos flask (500 mL) China
UV-Vis Lavionbon Model X7000 United Kingdom
Experimental techniques
2.3.1 Fabrication of aerogel derived from jackfruit core
Figure 2.1 Pre-treatment of jackfruit core procedure
The core of the jackfruit is divided into smaller spices that have a volume of 1-2 cm 3 These pieces are pretreated via hydrothermal technique by being placed in a Teflon- lined stainless-steel autoclave with RO water, making sure that the volume of the mixture made up 60% of the autoclave overall volume In a closed system, the autoclave is heated to 180 °C for 5 h while self-generating pressure is maintained Following hydrothermal treatment, the jackfruit core is submerged for two days in a 1:1, v/v solution of ethanol and water Every 6 hours, the solution is replaced to get rid of unwanted components The cleaned jackfruit core is then frozen for a night before freeze-dried for 48 h in vacuum condition
2.3.2 Fabrication of cobalt-doped carbon
Figure 2.2 Fabrication of cobalt-doped carbon procedure
Co(NO3)2.6H2O salt and urea are placed in a glass beaker containing ethanol and stirred well to completely dissolve the solid mixture The treated jackfruit core is soaked in above the solution for 2 h at room temperature After soaking, the mixture is transferred to hydrothermal device and the hydrothermal process is carried out for
10 h at 120°C The hydrothermal solid fraction is filtered and dried at 50°C for 12 h The solid part after drying is pyrolyzed in a furnace with a constant heating rate of 5°C.min -1 with various temperature In this study, the various of pyrolysis temperature, and Co 2+ content are investigated by characteristics determination of cobalt-doped carbon (Co-JA), which showed in Table 2.3:
Table 2.3 Experimental design of fabricating cobalt-doped carbon procedure
Pyrolysis temperature ( o C) Co 2+ content (mmol)
2.3.3 Degradation of dye via advanced oxidation processes a Preparation of dye solutions
MB solution with concentration of 1000 mg.L -1 was prepared by weighing 1.1865 g then placing it in a volumetric flask and diluting to 1 L of solution Depending on each experiment, MB solutions with other concentrations are prepared by diluting from 1000 mg/L MB solution Methylene orange (MO), crystal violet (CV), Congo red (CR) 1000 mg/L solution is also prepared by weighing 1 g of dye, then dissolving and diluting to 1 L of solution b Preparation of standard cure
Prepare 10 volumetric flasks of 10 mL each for each type of dye Dilute the stock solutions of MB and CV with a concentration of 1000 mg.L -1 to concentrations of 0.5, 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, and 5 mg.L -1 For MO, dilute the stock solutions to concentrations of 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, and 15 mg.L -1 from the stock concentration of 1000 mg/L For CR, dilute the stock solutions to concentrations of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 mg.L -1 Then, measure the absorbance using a UV-Vis Lavionbon Model X7000 with the corresponding wavelengths for each type of dye: MB (664 nm), CV (590 nm), MO (464 nm), and
CR (500 nm) c Degradation of MB experiments
In this study, the various of catalyst dosages, PMS concentration, MB concentration, temperature of MB solution, pH of MB solution, anions in MB solution are investigated by determination of removal efficiency, which showed in Table 2.4
Each experiment was repeated 3 times
Table 2.4 Experimental design for degradation of MB
MB concentration Temperature pH Anions
The effect of catalyst dosages on MB removal efficiency
200 mL of MB solution with concentration of 75 mg.L -1 is placed in a 600 mL beaker Then add cobalt-doped carbon with catalyst dosage from 0 to 0.8 g.L -1 and stir for 1 minute Then 4000 uL PMS to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured
The effect of PMS concentration on MB removal efficiency
200 mL of MB solution with concentration of 75 mg.L -1 is placed in a 600 mL beaker Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration from 0 to 800 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured
The effect of MB concentration on MB removal efficiency
200 mL of MB solution with concentration from 25 to 150 mg.L -1 is placed in a 600 mL beaker Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration of 400 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured
The effect of temperature on MB removal efficiency
200 mL of MB solution with a concentration of 75 mg.L -1 is placed in a 600 mL beaker, and the solution is heated using a thermostat tank to temperatures ranging from 25 to 70 o C Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration of 400 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured
The effect of pH values on MB removal efficiency
200 mL of MB solution with a concentration of 75 mg.L -1 is placed in a 600 mL beaker, and the solution is adjusted to a pH range from 3 to 11 using NaOH 1M solution and HNO3 1M solution Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration of 400 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured
The effect of anions on MB removal efficiency
For each different anion, the corresponding amount of inorganic salts NaCl, Na2CO3, NaHCO3, and Na2HPO4 are added sequentially to the 200 mL MB solution with concentration of 75 mg.L -1 and stirred to dissolve, to achieve the appropriate anion concentration from 1 to 100 mM Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration of 400 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first 10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of 664 nm Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured d Degradation of other dyes experiment
200 mL of each solution including CV, MO, CR with a concentration of 75 mg.L -1 is placed in different 600 mL beaker Then add cobalt-doped carbon with catalyst dosage of 0.2 g.L -1 and stir for 1 minute Then add PMS with concentration of 400 mg.L -1 to conduct the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments and the process use a Jartest device After adding PMS, in the first
10 minutes, every minute 400 uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask Then, the absorbance is measured by using UV-Vis Lavionbon Model X7000 with the wavelength of each dye: CV (590 nm), MO (464 nm), and CR (500 nm) Next, 10 minutes later, every 2 minutes, 400uL of the solution is taken out with a micropipette, filtered by syringe filter, and titrated to 10mL with a volumetric flask, then the absorbance is measured e Identifying main reactive oxidized species by employing scavengers
To determine the reactive oxidized species, scavengers are sequentially added to the
MB solution before proceeding with decomposition using the heterogeneous action of PMS 200 mL of MB solution with a concentration of 75 mg.L -1 is placed in different 600 mL beakers with 1M TBA Then, cobalt-doped carbon is added with a catalyst dosage of 0.2 g.L -1 and stirred for 1 minute PMS with a concentration of 400 mg.L -1 is then added to initiate the decomposition reaction The stirring speed is fixed at 100 rpm in all experiments using a Jartest device After adding PMS, during the first 10 mins, 400 àL of the solution is taken out every minute with a micropipette, filtered using a syringe filter, and diluted to 10 mL with a volumetric flask The absorbance is then measured using a UV-Vis Lavionbon Model X7000 at a wavelength of 664 nm In the next 10 minutes, every 2 mins, 400 àL of the solution is taken out with a micropipette, filtered using a syringe filter, and diluted to 10 mL with a volumetric flask, and the absorbance is measured The experiment is repeated with the individual present of scavenger including EtOH 2M and FFA 1mM f Evaluation of reusability for MB degradation
Characterization
2.4.1 Determination of dye removal efficiency
Ultraviolet-visible spectroscopy (UV-Vis) is used to analyze the absorption intensity of light by a sample solution placed in a cuvette The operating principle of the UV- Vis spectrometer is based on directing a beam of light with a specific wavelength through the sample solution, which absorbs the incoming radiation and displays a value known as the absorption coefficient This is a simple and widely used method for sample analysis
Removal efficiency (H%): The removal efficiency is the ratio between the removed concentration of the solution and the initial concentration of the solution, calculated by (Eq 2.1)
Where: C0: the initial concentration (mg.L -1 )
Ct: the concentration at time t (mg.L -1 )
To study the kinetics of dye degradation, the pseudo-first-order kinetic equation has been used to describe the dye degradation The pseudo-first-order kinetic equation is represented by (Eq 2.2): ln C
Where: C0: the initial concentration (mg.L -1 )
Ct: the concentration at time t (mg.L -1 ) k: the rate constant (min -1 )
In this case, the apparent rate constant is considered a general rate constant that includes multiple intermediate steps such as adsorption, oxidation, and desorption
In the study of the effect of MB concentrations, after determining the dye removal efficiency over time for different initial concentrations, the amount of MB degraded per 1 g of catalyst material was calculated according to (Eq 2.3):
Where: A: amount of MB degraded by 1 g of catalyst material
C0: initial MB concentration (mg.L -1 ) mcatalyst: mass of catalyst material (g)
VMB: volume of MB solution (L)
In the study of the effect of reaction temperature, the Arrhenius equation is used to represent the relationship between reaction temperature and the reaction rate constant (k) The equation representing this relationship is shown in (Eq 2.4):
Where: kt: rate constant (min -1 )
A: the pre-exponential factor or Arrhenius factor or frequency factor
R: the universal gas constant (R = 8.314 J.mol -1 K -1 )
Ea: the activation energy (J.mol -1 )
(Eq 2.4) can be rewritten as follows: lnk = lnA − E
The Arrhenius plot of ln(k) versus 1/T allows for the determination of the activation energy (Ea) at 5 reaction temperatures in this study
Figure 2.3 UV-Vis instrument (Model: Lovibond XD7000)
2.4.2 Point of zero charge (pH pzc )
The point of zero charge (pHpzc) of an adsorbent is a crucial characteristic for determining the pH at which the adsorbent surface is electrically neutral When pH < pHpzc, the solution contains more H + ions than OH - ions, resulting in a positively charged (+) adsorbent surface, which will more readily adsorb anions Conversely, when pH > pHpzc, the adsorbent surface is negatively charged (-), making it easier to adsorb cations
Figure 2.4 pH meter (Model: Benchtop PH-B100B)
XRD is used to calculate the index of crystallinity of samples following each stage With a rapid shutter speed, a clear image based on a contemporary detector that can count up to one photon without noise, and an algorithm that can recover the whole image of the sample, this method has the outstanding benefit of being able to be measured in an ambient environment
Figure 2.5 XRD instrument (Model: Panalytical Aeris)
2.4.4 Fourier Transform Infrared Spectroscopy (FTIR)
The chemical structures were examined using Fourier-transform infrared spectroscopy (Model: Bruker Alpha II) in the 400–4000 cm -1 wavenumber range In FTIR spectroscopy, the transmittance versus wavelength is used to represent the data This technique involved exposing the sample to infrared light while using a reference solvent, where some of the radiation will pass through or be absorbed by the sample to send signals to a mathematician that would transform the data to the spectrum
Figure 2.6 FTIR instrument (Model: Bruker Alpha II)
To examine the nitrogen physisorption characteristics of the samples, we utilized a Micromeritics ASAP 2020 apparatus Before initiating the adsorption and desorption isotherm experiments at 77 K using high-purity nitrogen gas, the samples were subjected to a pretreatment process This pretreatment involved heating the samples under vacuum conditions at a temperature of 150 o C for a duration of 6 h This step was crucial to ensure that any adsorbed contaminants or moisture were removed, thereby preparing the samples for accurate measurement Following this pretreatment, the specific surface area of the samples was determined using the Brunauer-Emmett-Teller (BET) model This model was applied within a relative pressure range of 0.05 to 0.30 (p/p0), allowing for precise calculation of the surface area Additionally, to gain further insights into the porous nature of the samples, the Barrett-Joyner-Halenda (BJH) method was utilized during both the adsorption and desorption phases The BJH method provided detailed information regarding the total pore volume and average pore diameter, offering a comprehensive understanding of the pore size distribution and overall porous structure of the samples
Figure 2.7 Physisorption instrument (Model: Micromeritics ASAP 2020)
2.4.6 Field Emission Scanning Electron Microscopes (FE-SEM) and energy dispersive spectrometer (EDS)
The Field Emission Scanning Electron Microscopes (FE-SEM Tescan Mira 4) is utilized to analyse the morphologies of samples at an acceleration voltage of 10 kV
In accordance with the characteristics of the samples, liquid or solid form was characterized The operating principle can be described as follows First, an electron gun emits a beam of electrons, which are then accelerated and focused into a narrow beam by electromagnetic lenses This beam is then scanned across the surface of the sample using electrostatic scanning coils When the incident electrons scan a point on the sample surface, detectors capture the emitted radiation and convert it into a corresponding image signal on a display screen, where the brightness depends on the radiation intensity The electron beam scanning the sample surface is synchronized with the electron beam in the display tube The magnification of the microscope is determined by the ratio of the display screen size to the scan area size on the sample surface The resolution of the SEM is determined by the size of the focused electron beam scanning the sample surface Additionally, the resolution of the SEM also depends on the interaction between the material at the sample surface and the electron beam
EDS is an analytical technique used to determine the presence of chemical elements on the surface or within the structure of a material sample Additionally, EDS can identify the distribution of elements, different isotopes of elements, and the weight percentage of each element EDS analysis is often combined with SEM microscopy
In this setup, X-rays are generated when the electron beam emitted from the gun penetrates and interacts with the sample Typically, an accelerating voltage of around
20 kV is used to provide enough energy for the primary beam to excite the characteristic X-rays of all the elements of material
Figure 2.8 FE-SEM instrument (Model: Tescan Mira 4).
RESULTS AND DISCUSSION
Characterization
3.1.1 Investigating the effect of pyrolysis temperature on the Co-JA
Figure 3.1 SEM images of jackfruit after treatment a) JA, cobalt-doped treated jackfruit b) Co-JA, cobalt-doped carbon with various pyrolysis temperature c) Co-
JA-300, d) Co-JA-350, e) Co-JA-400, f) Co-JA-450, g) Co-JA-500
Figure 3.1 presents SEM images of jackfruit cores at various stages: after initial hydrothermal treatment and freeze-drying, after cobalt impregnation, and after cobalt-doped pyrolysis at different temperatures In Figure 3.1a, the porous structure resulting from the hydrothermal process is visible due to the degradation of the hydroxyl, carbonyl, carboxyl, and aromatic groups [101] The freeze-drying step plays a crucial role in expanding this porous structure, which in turn facilitates the subsequent impregnation of cobalt into the jackfruit core This expanded porous network is essential for the effective incorporation of cobalt Figure 3.1b depicts the morphology of the jackfruit core following impregnation with a cobalt salt solution The three-dimensional porous structure observed is a result of the secondary hydrothermal process applied during cobalt impregnation, which enhances the uniformity of the pores Nevertheless, the cobalt particles attached to the material are not distinctly discernible in this image
After the pyrolysis process at various temperatures, the fiber bundles have disappeared, replaced by carbon sheets formed due to the decomposition of cellulose and hemicellulose The porosity of the material varies with different temperatures At 300°C, as shown in Figure 3.1c, the carbon layers exhibit minimal porosity, likely because cellulose and hemicellulose have not fully decomposed, leaving fibers that obscure the pores [102] However, at 350°C, depicted in Figure 3.1d, cellulose has partially decomposed, creating pores while some fibers remain to support the pore structure, preventing collapse [103] This structure facilitates the even distribution of cobalt oxide on the carbon surfaces At higher temperatures, such as 400°C, 450°C, and 500°C, shown in Figures 3.1e, f, g, cellulose, hemicellulose, lignin have almost completely decomposed, resulting in carbon formation [102] However, the complete decomposition of fibers leads to the collapse of the porous structure, blocking the pores Additionally, during pyrolysis, cobalt particles crystallize together, forming larger crystalline clusters on the material's surface The specific crystal forms of cobalt present in the material will be further investigated using XRD analysis Additionally, the cobalt-doped carbon at 350°C will be analyzed using EDS to determine the distribution and composition of the elements within the material
Figure 3.2 EDS mapping of Co-JA-350 a) elements b) C, c) O, d) N, d) Co
Figure 3.2a shows the EDS mapping of Co-JA-350, highlighting the elements C, O,
N, and Co Figures 3.2b, c, d, e display the individual distributions of these elements, respectively From Figure 3.2a, it is evident that carbon is abundantly and evenly distributed, indicating successful carbonization through pyrolysis at 350°C, which facilitates cobalt doping Figure 3.2e demonstrates that cobalt oxides have been successfully doped onto the carbon substrate, with the oxides distributed almost uniformly across the carbon layers
Figure 3.3 Weight fraction cobalt-doped carbon with various pyrolysis temperature
Figure 3.3 illustrates the weight fractions of elements in cobalt-doped carbon at various pyrolysis temperatures The data shows that cobalt has been successfully doped into the carbon matrix, with weight fractions ranging from 8% to 13% Additionally, as the pyrolysis temperature increases, the weight fraction of carbon also rises from 50% to 61% This increase indicates the decomposition of remaining organic compounds into carbon during pyrolysis The predominant weight fraction of carbon in the substrate facilitates the effective doping and integration of cobalt The predominance of carbon in the substrate, as indicated by its significant weight fraction, provides a conducive environment for the effective doping and adhesion of cobalt Thus, the pyrolysis temperature plays a crucial role in optimizing the carbon content and enhancing the material's properties through successful cobalt doping
Figure 3.4 X-ray diffraction of jackfruit after treatment, and cobalt-doped carbon with various pyrolysis temperature
The XRD patterns from Figure 3.4 illustrate that the cellulose Iβ crystalline structure in the JA and Co-JA samples exhibits a characteristic (110) plane, which is prominently displayed at a 2θ angle of 16.46° [104] After pyrolysis, this peak disintegrates, demonstrating that the carbon structure within the cobalt-doped carbon has changed A peak for the formation of crystalline structures of the graphitic carbon is observed in all samples at a 2θ angle of approximately 25°, which is associated with the (002) plane, and another peak is observed at a 2θ angle of 40°, which is associated with the (100) plane of graphitic carbon [105] The XRD patterns disclose characteristic peaks at 2θ angles of 36.87°, and 44.83° These peaks can be attributed to the (311), and (400) planes of the Co3O4 phase, respectively [106] This indicates the presence of cobalt oxide phases in the samples after pyrolysis, further confirming the successful incorporation and transformation of cobalt within the carbon matrix The disappearance of the cellulose peak and the emergence of the cobalt oxide peaks highlight the structural changes undergone during pyrolysis for the fabrication of cobalt-doped carbon material
Fourier transform infrared (FTIR) spectroscopy
Figure 3.5 FTIR spectroscopy of of jackfruit after treatment, and cobalt-doped carbon with various pyrolysis temperature
The changes in functional groups of the catalyst material after pyrolysis at various temperatures were studied using FTIR spectroscopy The FTIR spectra of the material before pyrolysis and after pyrolysis at 300°C, 350°C, 400 o C, 450°C, and 550°C are shown in Figure 3.5 The broad peak at 3440 cm −1 is attributed to the O-H stretching vibration associated with proteins, cellulose, hemicellulose, lignin structures, or absorbed water in JA and Co-JA samples, but this band disappears after pyrolysis [107] Additionally, in JA, the bands at 2906 cm −1 and 1373 cm −1 are linked to the stretching and deformation vibrations of the C-H group in the glucose units of cellulose [108] The signal at 1061 cm −1 corresponds to the -C-O- group of secondary alcohols and ether functions in the cellulose chain for both JA and Co-JA, and this signal is eliminated during the pyrolysis process used to create cobalt-doped carbon [108] The adsorption peaks in the range of 400-700 cm -1 were associated with metal- oxygen (Co-O) such as CO3O4, which is showed in all spectra of cobalt-doped carbon [109]
The zero-point charge (pH zpc )
Figure 3.6 The zero-point charge (pHzpc) of Co-JA-350
The graph showing the zero-point charge (pHzpc) values in Figure 3.6 The experimental results indicate that the isoelectric point (pHzpc) of the Co-JA-350 material is 7.6 When the solution's pH is less than the pHzpc, the surface of the adsorbent becomes positively charged, which reduces the interaction with methylene blue (MB), a cationic dye Conversely, when the solution's pH is greater than the pHzpc, the adsorbent surface becomes negatively charged, increasing the adsorption capacity for MB This higher pH facilitates easier interaction and degradation of MB at the reactive sites on the catalyst material
Brunauer-Emmett-Teller (BET) surface area analysis
Figure 3.7 N2 adsorption-desorption isotherm of Co-JA-350
Table 3.1 Structure and morphology of Co-JA-350
Co-JA-350 BET Specific Surface Area (m 2 g -1 ) 54.6 Total Pore Volume (cm 3 g -1 ) 0.0832 Average Pore Diameter (nm) 6.1
Figure 3.7 and Table 3.1 provide detailed information about the specific surface area, average volume, and average pore diameter of the material being studied The data indicate that the material demonstrates characteristics of a type IV isotherm, which is typical for mesoporous materials These mesopores have diameters ranging from 2 to
50 nm, with an average diameter of approximately 6.1 nm The catalyst boasts a specific surface area of 54.6 m 2 g -1 This high specific surface area significantly provides numerous active sites, which are crucial for catalytic activity The Co-JA-
350 catalyst has a pore volume of 0.0832 cm 3 g -1 This relatively large pore volume is advantageous as it allows MP and PMS molecules to penetrate deeply into the catalyst
Figure 3.8 The MB removal efficiency of cobalt-doped carbon with various pyrolysis temperature
The influence of pyrolysis temperature on the efficiency of methylene blue (MB) removal via the heterogeneous activation of PMS by transition metals is presented in
Figure 3.8 The results indicate that within the pyrolysis temperature range of 300-
500°C, materials with higher pyrolysis temperatures exhibit better performance The highest removal efficiency after 20 minutes, 92.1%, was achieved with the material pyrolyzed at 500°C This can be explained that lignocellulosic materials consist of three main components: hemicellulose, cellulose, and lignin, each of which decomposes at different temperatures, depending on the specific biomass studied Hemicellulose, which decomposes at the lowest temperature range (220-315°C), breaks down to create pores in the material structure [102] These pores facilitate the penetration of PMS and MB molecules into the catalyst material Consequently, PMS is heterogeneously activated by cobalt oxides present in the material, releasing strong oxidizing radicals like hydroxyl and sulfate to degrade MB The material pyrolyzed at 350°C, with a removal efficiency of 87.2%, performs better than the material pyrolyzed at 300°C, which has a removal efficiency of 75% This is because hemicellulose is almost completely decomposed at 350°C, resulting in a more porous structure Although the pyrolysis temperature is lower compared to Co-JA-400, Co- JA-450, and Co-JA-500, the MB degradation efficiency of Co-JA-350 is only slightly lower Therefore, a pyrolysis temperature of 350°C is suitable for further studies
Table 3.2 Reaction rate constant of cobalt-doped carbon with various pyrolysis temperature
The reaction rate constant (k) for the material's decomposition of MB at a concentration of 75 mg.L -1 is presented in Table 3.2 The pseudo-first-order kinetic equation for the 5 pyrolysis temperatures has R 2 coefficient greater than 0.95 The reaction rate constant (k) corresponding to the pyrolysis temperature at 350 o C with the removal efficiency of 87.2% is 0.0491 min -1
3.1.2 Investigating the effect of loading cobalt content on the Co-JA
Figure 3.9 The SEM images of Co-JA before pyrolysis a) 0 mmol, c) 1 mmol, e) 2 mmol, g) 3 mmol, Co-JA after pyrolysis b) 0 mmol, d) 1 mmol, f) 2 mmol, h) 3 mmol
SEM images of materials with varying cobalt loadings, both before and after pyrolysis, are displayed in Figure 3.9 These SEM images show the porous structure of the materials Notably, after pyrolysis, the pore diameters gradually reduce, which is attributed to the degradation of cellulose fibers Despite the increase in cobalt concentration from 0 to 3 mmol.g -1 , there is no significant difference observed in the surface morphology of the materials With the SEM images, can not demonstrate the identification or distribution of elements, including cobalt, on the material's surface
To accurately determine the presence and distribution of cobalt at different concentrations within the cobalt-doped carbon material, EDS analysis is necessary This method provide detailed information about the elemental composition and confirm the successful doping of cobalt across varying concentrations
Figure 3.10 EDS mapping of Co-JA-350 with various loading cobalt content a) 0 mmol (C,N,O), b) 1 mmol (Co), c) 2 mmol (Co), d) 3 mmol (Co)
The EDS mapping of Co-JA-350 with various cobalt loadings is illustrated in Figure
3.10 In Figure 3.10a, the EDS map shows a strong and uniform distribution of carbon across the material's surface when the cobalt concentration is 0 mmol.g -1 , resulting in no detectable cobalt signal on the surface In contrast, Figures 3.10b, c, d show EDS mapping for materials with cobalt concentrations of 1, 2, and 3 mmol.g -
1, respectively In Figure 3.10b, the cobalt signal is present but its distribution is uneven and sparse, indicating incomplete or non-uniform doping at a concentration of 1 mmol.g -1 Figure 3.10c illustrates a more uniform distribution of cobalt across the surface at a concentration of 2 mmol.g -1 , suggesting a more effective and consistent doping process This even distribution is crucial for the catalytic properties of the material as it ensures that cobalt is available throughout the surface to interact with the PMS In Figure 3.10d, with a cobalt concentration of 3 mmol.g -1 , there is a dense distribution of cobalt This could imply potential aggregation of cobalt particles, which affect the overall performance and accessibility of the catalyst sites
Figure 3.11 Weight fraction cobalt-doped carbon with various loading cobalt content
Investigating the effect of catalyst dosages on MB removal efficiency
Figure 3.15 The MB removal efficiency of Co-JA-350 with various catalyst dosages
Figure 3.15 illustrates the MB removal efficiency with increasing catalytic dosages from 0 to 0.8 g.L -1 At a catalyst concentration of 0 g.L -1 , only PMS is involved in the
MB treatment, resulting in a low removal efficiency of 18.5% This low efficiency can be attributed to the absence of a catalyst, which means that PMS is not activated to generate free radicals, such as hydroxyl and sulfate radicals [119] These radicals are crucial for the effective degradation of MB, and without them, the removal efficiency significantly decreases However, it is important to note that PMS can spontaneously decompose in water to form singlet oxygen ( 1 O2) [120] This singlet oxygen acts as an active oxidant even in the absence of a catalyst, which plays a significant role in inactivated PMS systems Thus, while the presence of a catalyst greatly enhances the efficiency by generating more free radicals, the inherent decomposition of PMS still provides some degree of oxidation through the formation of singlet oxygen In addition, free radicals such as hydroxyl and sulfate have a greater oxidation capacity than singlet oxygen [115] Therefore, the MB removal efficiency in the presence of a catalyst is notably high, ranging from 87.2% to 95.8% Specifically, with catalyst dosages between 0.6 and 0.8 g.L -1 , the MB removal efficiency surpasses 80% within the first 5 min of the reaction This rapid efficiency highlights the effectiveness of the free radicals generated through PMS activation The presence of a catalyst facilitates both free radical and non-radical pathways, further accelerating the degradation of MB Even at a lower catalyst dosage of 0.2 g.L -1 , the MB removal efficiency still reaches 87.2% after 20 minutes, indicating that even a small amount of catalyst can significantly enhance the MB removal process by activating PMS and generating powerful oxidizing radicals
The degradation of MB begins with the breaking of the sulfur-chlorine (S − Cl) bond The next step involves the cleavage of the nitrogen-carbon (N − C) bond that connects the methyl group to the nitrogen atom [121] This results in the transformation of the carbon-sulfur-carbon (C − S = C) and carbon-nitrogen-carbon (C = N − C) bonds into carbon-sulfoxide-carbon (C − S(= O) − C) and carbon-nitrogen-hydrogen- carbon (C − NH − C) bonds, respectively [121] With the breakdown of the (C = N −C) functional groups, the dimethylamino (−N − (CH ) ) groups are also targeted and degraded by continuous attacks from hydroxyl radicals (OH • ) This attack further leads to the disintegration of aromatic rings in two sides [122] The destruction of both the (C − S = C) and (C = N − C) bonds results in the opening of the central atomic ring, which contains hydrocarbons, sulfur, and nitrogen [121] This ring opening occurs under the aggressive influence of reactive oxidative species The dominant process in this sequence is the addition reaction involving unsaturated olefins, alkynes, or compounds with carbon-carbon (C = C) double bonds Sulfate radicals (SO • ) are particularly effective in attacking these unsaturated bonds, causing them to break [123] As the degradation process continues, various intermediate products containing charges are formed, such as carboxylate ions (RCOO ) and ammonium ions (R − NH ) These intermediates undergo further reactions and are eventually mineralized into simpler substances, including carbon dioxide (CO2), water (H2O), and inorganic anions such as sulfate (SO ), ammonium (NH ), and nitrate (NO ) [122] This comprehensive breakdown process effectively converts the complex organic structure of methylene blue into simple inorganic compounds, completing its degradation
Table 3.4 Reaction rate constant of Co-JA-350 with various catalyst dosages
Table 3.4 presents the reaction rate constant (k) for the MB removal reaction using cobalt-doped carbon at various catalyst dosages The rate constant (k) increases significantly, from 0.027 to 0.1487 min -1 , as the catalyst dosage rises across five different concentrations, ranging from 0.1 to 0.8 g.L -1 This enhancement in the rate constant can be attributed to the increase in the dosage of cobalt-doped carbon, which in turn raises the number of surface-active sites available for the reaction More surface-active sites lead to a higher rate of PMS activation, which boosts the generation of reactive species responsible for the degradation of MB [124] Consequently, the higher the catalyst dosage, the more efficient the MB degradation process becomes, demonstrating the critical role of catalyst concentration in enhancing the removal efficiency of pollutants.
Investigating the effect of PMS concentrations on MB removal efficiency
Figure 3.16 The MB removal efficiency of Co-JA-350 with various PMS concentrations
Figure 3.16 depicts the MB removal efficiency with varying dosages of PMS, ranging from 0 to 800 mg.L -1 At a PMS dosage of 0 mg.L -1 , the MB removal efficiency is notably low, at just 6.4% This low efficiency indicates that without PMS, the formation of reactive oxidized species necessary for MB decomposition is impossible In this case, only adsorption occurs, which has an insignificant impact on
MB removal efficiency However, when PMS is introduced at a concentration of 100 mg.L -1 , the MB removal efficiency increases substantially to 60% As the PMS dosage is further increased to 400 mg.L -1 , the removal efficiency climbs to 87.2% This significant improvement demonstrates the essential role of PMS in the degradation process of MB molecules The presence of PMS allows it to be activated by cobalt-doped carbon, generating reactive oxidized species such as hydroxyl and sulfate radicals, which are crucial for the effective breakdown of MB Interestingly, when the PMS dosage is raised to 600 and 800 mg.L -1 , the improvement in MB removal efficiency becomes marginal, reaching at around 94.1% This can be explained by the overabundance of PMS, which leads to an excess of HSO Excessive HSO can act as a radical scavenger, reducing the availability of more reactive radicals like SO • , and instead leading to the formation of the less reactive
SO • This mechanism follows the below equation [124]:
HSO + SO • → SO • + SO + H (Eq 3.7)
This scavenging effect hinders further increases in removal efficiency, despite the higher PMS dosages Therefore, while increasing PMS concentration initially boosts
MB removal efficiency by enhancing the production of reactive species, there is a threshold beyond which additional PMS does not translate into significant gains This phenomenon highlights the importance of optimizing PMS dosage to balance the generation of effective oxidizing radicals and avoid the adverse effects of radical scavenging
Table 3.5 Reaction rate constant of Co-JA-350 with various PMS concentrations
Table 3.5 illustrates the rate constants for the MB removal reactions using cobalt- doped carbon material at various PMS dosages The data shows that, in general, as the concentration of PMS increases, the reaction rate constant also rises This trend is due to the synergistic effect between the cobalt-doped carbon catalyst and PMS, which leads to the generation of potent oxidizing radicals These radicals, including hydroxyl and sulfate radicals, play a crucial role in the effective decomposition of
MB molecules The increasing PMS dosage enhances the production of these reactive species, thereby accelerating the degradation process and improving the overall removal efficiency of MB.
Investigating the effect of MB concentrations on MB removal efficiency
Figure 3.17 The MB removal efficiency of Co-JA-350 with various MB concentration
The experiment studying the effect of MB concentration on the efficiency of MB degradation was conducted over a concentration range of 25 – 150 mg.L -1 The treatment efficiency for various initial concentrations of MB is shown in Figure 3.17
It is evident that as the initial dye concentration increases from 25 to 150 mg.L -1 , the dye degradation efficiency decreases Specifically, when the initial dye concentrations were 25 and 50 mg.L -1 , the treatment efficiencies were 97.3% and 93.6%, respectively However, as the dye concentration increased to 75, 100, and 150 mg.L -1 , the treatment efficiencies dropped to 87.2%, 78.9%, and 60.7%, respectively This decline in treatment efficiency with increasing initial dye concentration can be explained by the fact that the number of oxidizing agents produced from the reaction between PMS and the catalytic site on the material remains constant Therefore, as the dye concentration in the solution increases, a higher number of oxidizing agents is required It can be concluded that the treatment efficiency decreases when the dye concentration increases because the oxidizing agents produced from the reaction are insufficient to degrade the MB molecules Additionally, a higher MB concentration may reduce the chances of contact between the cobalt-doped carbon and PMS, which is detrimental to the generation of reactive oxidized species [124] This reduced interaction further limits the formation of the necessary reactive species, thereby lowering the overall efficiency of MB degradation as the initial concentration of the dye rises Thus, the balance between the number of oxidizing agents and the dye concentration is crucial for maintaining high degradation efficiency
Figure 3.18 Weight of treated MB for 1 g of Co-JA-350 with various MB concentration
After determining the dye removal efficiency over time for various initial concentrations, the amount of MB degraded per gram of catalyst was calculated and is shown in Figure 3.18 It is evident that the amount of MB degraded increases progressively with higher MB concentrations in the solution After 20 mins of treatment, the amounts of MB removed at concentrations of 25, 50, 75, 100, and 150 mg/L were 121.3, 232.5, 326.3, 390.1, and 450.7 mg.g -1 , respectively The data indicate that while the MB removal efficiency percentage may decrease at higher concentrations, the quantity of MB degraded still increases significantly This underscores the catalyst's robust performance across varying concentrations, demonstrating its potential for effective use in diverse scenarios with different levels of MB contamination
Table 3.6 Reaction rate constant of Co-JA-350 with various MB concentrations
The data presented in Table 3.6 shows that as the concentration of MB in the solution increases, there is a corresponding decline in the reaction rate constant (k) This trend can be attributed to the fact that higher concentrations of MB molecules tend to adsorb onto the surface of the catalytic material This adsorption creates a barrier that impedes the contact between PMS molecules and the active sites on the catalyst Consequently, the decreased interaction between PMS and the catalyst's active sites results in a lower rate of reaction This phenomenon suggests that at elevated MB concentrations, the efficiency of the catalyst in generating reactive oxidizing species is compromised, leading to a slower degradation process.
Investigating the effect of temperature on MB removal efficiency
Figure 3.19 The MB removal efficiency of Co-JA-350 with various temperature
Temperature generally plays a crucial role in aqueous solutions by affecting both the activation of PMS and the surface charge of the catalyst To examine how temperature impacts MB degradation in the cobalt-doped carbon/PMS system, a series of experiments were conducted at temperatures ranging from 25 to 70 o C Figure 3.19 demonstrates that as the reaction temperature was incrementally increased from 25 to
50 o C, the degradation efficiency of MB significantly improved from 87.7 to 91.5%
At 70 o C, the MB removal efficiency reached its peak at 95.7% This trend indicates that higher temperatures enhance the degradation of MB This improvement can be explained that elevated temperatures facilitate the decomposition of PMS, leading to the generation of more reactive oxidizing species such as hydroxyl and sulfate radicals These species are highly effective in breaking down MB molecules In addition, higher temperatures increase the kinetic energy of the molecules in the solution, resulting in more frequent and energetic collisions between MB molecules and reactive oxidizing species [124] This heightened collision frequency accelerates the overall reaction rate, leading to more efficient degradation of MB
Table 3.7 Reaction rate constant of Co-JA-350 with various temperature
The results from Table 3.7 indicate that the reaction rate constant (k) increases from 0.0381 to 0.0525 min⁻ạ as the reaction temperature is raised from 25 to 70 o C The value of k at 70 o C is 1.4 times higher than the reaction rate constant at 25 o C Higher temperatures not only facilitate the production of reactive species but also enhance the interaction between these species and MB molecules, thereby significantly improving the overall degradation performance In addition, the degradation of MB by PMS is an endothermic reaction process [120] Hence, the reaction temperature positively influences the degradation of MB through free radicals generated by the heterogeneous activation of PMS
As the rate constant (k) showed a significant increase with rising temperature, the relationship between k and temperature was analyzed using the Arrhenius equation The plot of ln(k) versus 1/T is presented in Figure 3.20, demonstrating a high regression coefficient (R² = 0.9309) This strong correlation confirms the reliability of the data Using the Arrhenius equation, the activation energy (Ea) for the Cobalt- doped carbon/PMS system was calculated to be 5.99 kJ mol⁻ạ This value indicates that the variation in reaction temperature positively influences the degradation of MB
Figure 3.20 Arrhenius plot of MB decomposition with various temperature.
Investigating the effect of pH values on MB removal efficiency
Figure 3.21 The MB removal efficiency of Co-JA-350 with various pH values
In an aqueous solution, pH generally affects the activation of PMS and the surface charge of the catalyst In order to investigate the impact of starting pH on MB degradation as a result of heterogeneous PMS activation, a range of pH values, from
3 to 12, were used in the investigations According to Figure 3.21, the degradation efficiency is 93.3% at pH 3, and it drop significantly to 92.6% and 88.9% at pH 5 and
7, respectively But in comparison with pH levels between 3 and 7, MB elimination is considerably lower at pH 9 (65.1%) and 12 (29.3%) These results indicate that the catalytic degradation of MB is more efficient under weakly acidic or nearly neutral conditions This inhibition can be explained by the interaction of hydroxide ions (OH⁻) with sulfate radicals (SO • ) In detail, SO • oxidizes OH⁻ to produce hydroxyl radicals (OH • ), as described by below mechanism at (Eq 1) [125] Hydroxyl radicals (OH • ), although still reactive, have a lower oxidation potential compared to sulfate radicals (SO • ) [126] The shift in dominant reactive species from SO • to OH • under basic conditions is a key factor in the observed decrease in MB degradation performance
OH + SO • → OH • + SO (Eq 3.8)
Table 3.8 Reaction rate constant of Co-JA-350 with various pH values pH K
Table 3.8 illustrates the reaction rate constant for the removal of MB across different pH values ranging from 3 to 12 It is evident that under alkaline conditions, the rate constants are 0.0119 and 0.0103 min⁻ạ at pH levels of 9 and 11, respectively In comparison, the rate constants at pH 3, 5, and 7 are significantly higher, indicating that the heterogeneous activation of PMS to produce free radicals occurs more efficient in acidic to neutral conditions.
Investigating the effect of anions on MB removal efficiency
Figure 3.22 The MB removal efficiency of Co-JA-350 in the presence of [Cl - ]
The performance of catalyst/PMS systems can be complexly influenced by a variety of inorganic anions witnessed in real aquatic environments These consist of capturing reactive oxidation species or conflicting with the heterogeneous activation of PMS As a result, evaluating the effects of these interferences is essential Because
Cl - , CO3 2-, HCO3 -, and HPO4 2- are prominent in the majority of wastewater their presence and concentrations are investigated In Figure 3.22, the effectiveness of removing MB is depicted under different concentrations of chloride ions (Cl - ) During a 20-minute period, the addition of 1 mM, 10 mM, and 100 mM Cl - to the degradation system resulted in a noticeable suppression effect, leading to degradation efficiencies of MB decreasing to 80.3%, 75.7%, and 72.1%, respectively The reason for this is that chloride ions (Cl - ) can undergo reactions with sulfate or hydroxyl radicals, producing chloride radicals such as Cl • and Cl • , as indicated in (Eq 3.9-3.12) [127, 128] These chloride radicals have a lower redox potential for mineralizing organic pollutants Consequently, the degradation of MB through the heterogeneous activation of PMS process is significantly impeded in the presence of Cl -
Cl + SO • → Cl • + SO (Eq 3.9)
Cl + OH • → HOCl • (Eq 3.10) HOCl • + H → Cl • + H O (Eq 3.11)
Figure 3.23 The MB removal efficiency of Co-JA-350 in the presence of [CO3 2-]
The effectiveness of eliminating MB in the presence of different carbonate anion (CO3 2-) concentrations ranging from 0 to 100 mM is demonstrated in Figure 3.23 The results illustrate that, within 20 mins, the degradation efficiency of MB drop considerably from 87.2% to 82.1% following the addition of 1 mM of CO3 2- When the concentration of CO3 2- rise from 10 to 100 mM, it causes a progressive decrease in MB degradation from 77.5% to 72.6% (Eq 3.13-3.14) illustrate how CO3 2- interacts with sulfate or hydroxyl radicals to produce carbonate radicals (CO • ) that have a lower redox potential, which explains this tendency [129, 130]
CO + SO • → CO • + SO (Eq 3.13)
CO + OH • → CO • + OH (Eq 3.14)
Figure 3.24 The MB removal efficiency of Co-JA-350 in the presence of [HCO3 -]
The effectiveness of MB removal in the presence of bicarbonate ions (HCO3 -) at concentration ranging from 0 to 100 mM is displayed in Figure 3.24 The results evidently show that the presence of HCO3 - inhibits the process of MB the decomposition significantly The degradation efficiency of MB diminishes significantly from 87.2% to 80.1% even with 1 mM HCO3 - added As HCO3 - concentrations rise, correspondingly increases the degree of MB degradation reduction Particularly, the degradation efficiency of MB decreases significantly to 45.9% in the presence of 100 mM HCO3 - (Eq 3.15-3.16) demonstrate the severe competition between HCO3 - and SO • and OH • for forming less active carbonate or bicarbonate radicals, which confirms the phenomena [129, 130] Therefore, MB degradation is substantially hindered in the presence of HCO3 -
HCO + SO • → HCO • + SO (Eq 3.15) HCO + OH • → CO • + H O (Eq 3.16)
Figure 3.25 The MB removal efficiency of Co-JA-350 in the presence of [HPO4 2-]
The efficiency of MB removal in the presence of HPO4 2- anion at concentrations that vary from 0 to 100 mM is illustrated in Figure 3.25 The MB degradation efficiencies are 87.2%, 82.5%, and 43.3% at concentrations of 0 mM, 1 mM, and 10 mM HPO4 2- in the reaction solution, respectively The results indicate that these HPO4 2- anion have a detrimental effect on MB decomposition and that the impact is more pronounced at greater HPO4 2- concentrations Nevertheless, as the concentration of HPO4 2- rises a lot more its limiting effect is seen to diminish The MB degradation effectiveness improves from 43.3% to 50.1% when the HPO4 2- concentration is increased from 10 mM to 100 mM The reduction in degradation efficiency reported at lower concentrations of HPO4 2- is related to its tendency to react with free radicals, producing hydrogen phosphate radicals (Eq 3.1-3.18) that are less reactive [129,
131] The decline in suppressive impact is ascribed to the reaction pH system being modified to weak acidity by the excessive contribution of HPO4 2-, which promotes the catalytic the decomposition of MB
HPO + SO • → HPO • + SO (Eq 3.17) HPO + OH • → HPO • + OH (Eq 3.18)
Identification of main reactive oxidized species
Figure 3.26 Influences of various radical scavengers in the removal efficiency of
To determine which reactive oxidizing species are primarily responsible for MB degradation in the heterogeneous activation of PMS, classical radical quenching studies were carried out Singlet oxygen ( 1 O2), hydroxyl radicals (OH • ), and sulfate radicals (SO • ) are among the reactive oxidizing species that can be produced when PMS is activated Tert-butanol (TBA), ethanol (EtOH), and furfuryl alcohol (FFA) are frequently utilized as scavengers for OH • , SO • , and 1 O2, in accordance with recognized protocols [132] Figure 3.26 illustrates that the addition of TBA to the cobalt-doped carbon system decreased the degradation efficiency from 87.2% to
65.5%, suggesting that ãOH had a minor role in MB degradation The MB degradation was dramatically suppressed by the addition of EtOH, a scavenger for both SO • and
OH • , with the removal efficiency falling to 38.1% after 20 minutes These results indicate that SO • is present and actively contributes to the decomposition of MB Nonetheless, it is reasonable to suspect the involvement of other reactive species, such as 1 O2, since the degradation is not completely suppressed when both SO • and
OH • are scavenged by EtOH FFA is added as a quenching agent to confirm this, causing the degrading efficiency to drop from 87.2% to 27.8% According to this, 1 O2 takes part in the MB degrading process as well FFA has the ability to quench not just
1O2, but also SO • and OH • [129, 132].
Investigating the regeneration of the cobalt-doped carbon
Figure 3.27 The MB removal efficiency of Co-JA-350 after regeneration
The effectiveness of MB removal by the Co-JA-350 material during multiple reuse cycles is demonstrated in Figure 3.27 Initially, the material demonstrates a high removal efficiency from 84.1% to 80.3% After 3 reuse cycles, the efficiency declines to below 80%, specifically reaching 76.9% This demonstrates that the material can maintain a relatively high level of MB removal even after being reused 3 times As the number of reuse cycles increases, there is a more noticeable decline in efficiency
By the 7 reuse cycle, the efficiency drops to 59.5% This reduction signifies that the material ability to remove MB decreases to 0.68 times its original efficiency Remarkably, even after 9 reuse cycles, the Co-JA-350 material manages to retain an
MB removal efficiency of over 50% This persistence indicates that although the material's efficiency decreases with repeated use, it still maintains more than half of its initial removal capability after extensive reuse
Weight fraction and EDS mapping
Figure 3.28 a) Weight fraction, b) EDS mapping of elements in Co-JA-350 after 9 th regeneration cycle
Figure 3.28 illustrates a reduction in the weight fraction of cobalt in the catalyst material after it has undergone nine cycles of reuse, specifically noting a decrease of 6.7% This indicates that the cobalt content diminishes slightly with repeated usage
In addition to cobalt, the weight fractions of other key elements within the catalyst are detailed: carbon constitutes 58.8% of the material, oxygen makes up 21.5%, and nitrogen accounts for 13.3% From the EDS mapping, it can be observed that the elements C, O, N, and Co are still evenly distributed.
Investigating the the ability of Co-JA-350 to decompose dyes
Figure 3.29 The dyes removal efficiency of Co-JA-350
The broad applicability of the catalyst was assessed by testing its effectiveness in degrading several other dye pollutants with varying chemical structures, specifically methyl orange (MO), congo red (CR), and crystal violet (CV) These experiments are performed under the same conditions as those used for methylene blue (MB) degradation, maintaining an initial dye concentration of 75 mg/L for each pollutant The results, as depicted in Figure 3.29, showed that MO was degraded with remarkable efficiency, achieving degradation rates exceeding 90.3% within just 20 minutes This high efficiency underscores the catalyst's strong performance with MO
In contrast, the degradation efficiency for CV was lower, falling below 80%, with a specific value of 75.3% This indicates that while the catalyst is effective, its performance varies depending on the dye's chemical structure The catalyst demonstrated impressive results with CR, achieving a degradation efficiency of 80.6% This further illustrates the versatility of catalysts in handling different types of dye pollutants The Co-JA-350 catalyst proved to be highly effective in degrading a range of dyes through the heterogeneous activation of PMS Its ability to maintain high degradation efficiencies across various dyes highlights its potential for widespread application in the treatment of wastewater containing diverse organic pollutants.
CONCLUSIONS AND RECOMMENDATIONS
Conclusions
The study successfully synthesizes cobalt-doped carbon material, with the carbon source derived from jackfruit core, using the hydrothermal method and pyrolysis at
350 o C with a heating rate of 50 o C.min -1 Additionally, the catalyst has a specific surface area, total pore volume, and average pore diameter of 54.6 m 2 g -1 , 0.0832 cm 3 g -1 , and 6.1 nm, respectively With a cobalt concentration of 2 mmol.g -1 , the catalyst demonstrated high efficiency in activating PMS to generate strong oxidizing radicals for dye degradation, achieving a removal efficiency of 87.2%
Additionally, SEM and EDS analyses show that cobalt is evenly distributed on the carbon substrate, with the weight fractions of the elements C, O, N, and Co being 52%, 16%, 15%, and 17%, respectively The chemical structure of catalyst is analyzed through FTIR and XRD, confirming the successful synthesis of cobalt- doped carbon with characteristic functional groups
The study also investigates the reaction conditions affecting the removal efficiency of methylene blue (MB), including catalyst dosage, PMS concentration, MB concentration, temperature, pH, and anions The catalyst performs well in solutions with temperatures ranging from 25 to 70°C In weakly acidic or neutral environments, the catalyst shows high MB removal efficiency, ranging from 88.9% to 93.3% when the pH is between 7 and 3 In the presence of anions such as Cl - , CO3 2-, HCO3 -, and HPO4 2- at a concentration of 1 mM each, the MB removal efficiency remain above 80% Additionally, the catalyst exhibits high reusability, with an MB removal efficiency of 51.2% after 9 regeneration cycles
Further investigation into the identification of oxidizing radicals reveal that PMS is successfully activated, generating strong oxidizing radicals such as sulfate and hydroxyl The catalyst also demonstrates high removal efficiency for other dyes, including MO (methyl orange), CR (congo red), and CV (crystal violet), with removal efficiencies of 90.3%, 80.6%, and 75.3%, respectively
In conclusion, this study demonstrates that cobalt-doped carbon exhibits significant potential as a material for dye treatment applications The material's high efficiency in generating strong oxidizing radicals for the degradation of various dyes, along with its favorable performance under different conditions and its reusability, underscores its promise for practical use in the removal of dyes from wastewater
Recommendations
Based on the achievements of this study, several future research directions can be proposed These include exploring the material's capability to remove other types of industrial dyes and various organic pollutants It would also be beneficial to investigate its effectiveness in real-world water environments, such as rivers, streams, ponds, lakes, and canals Additionally, testing its ability to treat actual household wastewater would provide practical insights
Further studies should focus on analyzing the reduction in Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC) levels to better understand the material's impact on water quality Utilizing advanced techniques such as Gas Chromatography/Mass Spectrometry (GC/MS) to identify the mineralization products formed during the treatment process would help propose a more accurate and detailed mechanism for pollutant degradation
Research should examine the potential of this catalytic material in other advanced oxidation processes This would help to expand the application scope of the synthesized catalyst, making it a more versatile solution for various environmental treatment challenges These directions will not only enhance the understanding of the material's capabilities but also contribute to the development of more effective and efficient water treatment technologies
[1] M S Baliga et al., "Phytochemistry, nutritional and pharmacological properties of Artocarpus heterophyllus Lam (jackfruit): A review," Food research international, vol 44, no 7, pp 1800-1811, 2011
[2] O Prakash et al., "Artocarpus heterophyllus (Jackfruit): an overview,"
[3] T Bose, "Jackfruit" in Fruits of India: tropical and subtropical, 1 st ed., vol 3,
B K N Mitra, Ed Calcutta: Naya Prokash, pp 488-498, 1985
[4] F Akter and M Haque, "Jackfruit waste: a promising source of food and feed,"
Ann Bangladesh Agric, vol 23, no 1, pp 91-102, 2019
[5] T Nguyen "Người trồng mít lãi từ 10 - 20 nghìn đồng/kg." Internet: https://baotintuc.vn/thi-truong-tien-te/nguoi-trong-mit-lai-tu-10-20-nghin- dongkg-20231020140621510.htm, Oct 20, 2023
[6] B S Lazarus et al., "Jackfruit: Composition, structure, and progressive collapsibility in the largest fruit on the Earth for impact resistance," Acta Biomaterialia, vol 166, pp 430-446, 2023
[7] E P Popek, "Environmental chemical pollutants" in Sampling and analysis of environmental chemical pollutants: a complete guide, 2 rd ed., vol 1, K Morrissey, Ed California: Elsevier, pp 13-23, 2017
[8] J Xiao et al., "Multifunctional graphene/poly (vinyl alcohol) aerogels: In situ hydrothermal preparation and applications in broad-spectrum adsorption for dyes and oils," Carbon, vol 123, pp 354-363, 2017
[9] S A S Chatha et al., "Enzyme-based solutions for textile processing and dye contaminant biodegradation—a review," Environmental Science and Pollution Research, vol 24, pp 14005-14018, 2017
[10] S Shang, "Process control in dyeing of textiles," in Process control in textile manufacturing, 1 st ed., vol 1, A Majumdar, Ed Cambridge: Woodhead Publishing, pp 300-338, 2013
[11] F D Chequer et al., "Textile dyes: dyeing process and environmental impact,"
Eco-friendly textile dyeing and finishing, vol 6, no 6, pp 151-176, 2013
[12] A Azanaw et al., "Textile effluent treatment methods and eco-friendly resolution of textile wastewater," Case Studies in Chemical and Environmental Engineering, vol 6, p 100230, 2022
[13] R P Singh et al., "Treatment and recycling of wastewater from textile industry," Advances in biological treatment of industrial waste water and their recycling for a sustainable future, pp 225-266, 2019
[14] A D Broadbent,"An introduction of dyes and dyeing" in Basic principles of textile coloration, 1 st ed., vol 1, Ed West Yorkshire: Society of Dyers and Colorists Bradford, pp 174-196, 2001
[15] S Samsami et al., "Recent advances in the treatment of dye-containing wastewater from textile industries: Overview and perspectives," Process safety and environmental protection, vol 143, pp 138-163, 2020
[16] I Holme, "Sir William Henry Perkin: a review of his life, work and legacy,"
Coloration Technology, vol 122, no 5, pp 235-251, 2006
[17] A Sule and M Bardhan, "Objective evaluation of feel and handle, appearance and tailorability of fabrics Part-II: The KES-FB system of Kawabata,"
[18] P Mallinson, "Application of surfactants in the textile industry," Special
Publications of the Royal Society of Chemistry, vol 230, pp 272-278, 1999
[19] H Solayman et al., "Performance evaluation of dye wastewater treatment technologies: A review," Journal of Environmental Chemical Engineering, vol 11, no 3, p 109610, 2023
[20] M Khodaie et al., "Removal of methylene blue from wastewater by adsorption onto ZnCl 2 activated corn husk carbon equilibrium studies," Journal of Chemistry, vol 2013, 2013
[21] R H Waghchaure et al., "Photocatalytic degradation of methylene blue, rhodamine B, methyl orange and Eriochrome black T dyes by modified ZnO nanocatalysts: A concise review," Inorganic Chemistry Communications, vol
[22] X.-J Liu et al., "Manganese-modified lignin biochar as adsorbent for removal of methylene blue," Journal of Materials Research and Technology, vol 12, pp 1434-1445, 2021
[23] S Dardouri and J Sghaier, "Adsorptive removal of methylene blue from aqueous solution using different agricultural wastes as adsorbents," Korean Journal of Chemical Engineering, vol 34, no 4, pp 1037-1043, 2017
[24] J Zhang et al., "Photocatalytic and degradation mechanisms of anatase TiO 2: a HRTEM study," Catalysis Science & Technology, vol 1, no 2, pp 273-278,
[25] R Ahmad and R Kumar, "Adsorption studies of hazardous malachite green onto treated ginger waste," Journal of environmental management, vol 91, no
[26] S Shakoor and A Nasar, "Adsorptive treatment of hazardous methylene blue dye from artificially contaminated water using cucumis sativus peel waste as a low-cost adsorbent," Groundwater for Sustainable Development, vol 5, pp 152-159, 2017
[27] A K Moorthy et al., "Acute toxicity of textile dye Methylene blue on growth and metabolism of selected freshwater microalgae," Environmental Toxicology and Pharmacology, vol 82, p 103552, 2021
[28] O S Bayomie et al., "Novel approach for effective removal of methylene blue dye from water using fava bean peel waste," Scientific reports, vol 10, no 1, p 7824, 2020
[29] P O Oladoye et al., "Methylene blue dye: Toxicity and potential elimination technology from wastewater," Results in Engineering, vol 16, p 100678,
[30] M M Hassan and C M Carr, "A critical review on recent advancements of the removal of reactive dyes from dyehouse effluent by ion-exchange adsorbents," Chemosphere, vol 209, pp 201-219, 2018
[31] M Imran et al., "Microbial biotechnology for decolorization of textile wastewaters," Reviews in Environmental Science and Bio/Technology, vol 14, pp 73-92, 2015
[32] D Ma et al., "Critical review of advanced oxidation processes in organic wastewater treatment," Chemosphere, vol 275, p 130104, 2021
[33] R Al-Tohamy et al., "A critical review on the treatment of dye-containing wastewater: Ecotoxicological and health concerns of textile dyes and possible remediation approaches for environmental safety," Ecotoxicology and Environmental Safety, vol 231, p 113160, 2022
[34] M Kurian, "Advanced oxidation processes and nanomaterials-a review,"
Cleaner Engineering and Technology, vol 2, p 100090, 2021
[35] N S Mishra et al., "A review on advanced oxidation processes for effective water treatment," Curr World Environ, vol 12, no 3, pp 270-490, 2017
[36] L Bilińska et al., "Textile wastewater treatment by AOPs for brine reuse,"
Process Safety and Environmental Protection, vol 109, pp 420-428, 2017
[37] M I Stefan, "Feton, photo Feton and Feton-like processes" in Advanced oxidation processes for water treatment: fundamentals and applications, 1 st ed., vol 1, Ed Lodon: IWA publishing, pp 297-323, 2017
[38] M A Oturan and J.-J Aaron, "Advanced oxidation processes in water/wastewater treatment: principles and applications A review," Critical reviews in environmental science and technology, vol 44, no 23, pp 2577-
[39] V Katheresan et al., "Efficiency of various recent wastewater dye removal methods: A review," Journal of environmental chemical engineering, vol 6, no 4, pp 4676-4697, 2018
[40] A B Dos Santos et al., "Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology," Bioresource technology, vol 98, no 12, pp 2369-2385, 2007
[41] A Pandey et al., "Bacterial decolorization and degradation of azo dyes,"
International biodeterioration & biodegradation, vol 59, no 2, pp 73-84,
[42] A F S Costa et al., "Color removal from industrial dyeing and laundry effluent by microbial consortium and coagulant agents," Process Safety and
[43] B C Hodges et al., "Challenges and prospects of advanced oxidation water treatment processes using catalytic nanomaterials," Nature nanotechnology, vol 13, no 8, pp 642-650, 2018
[44] C Comninellis et al., "Advanced oxidation processes for water treatment: advances and trends for R&D," Journal of Chemical Technology & Biotechnology: International Research in Process, Environmental & Clean Technology, vol 83, no 6, pp 769-776, 2008
[45] J Schneider et al., "Understanding TiO2 photocatalysis: mechanisms and materials," Chemical reviews, vol 114, no 19, pp 9919-9986, 2014
[46] A Buthiyappan et al., "Recent advances and prospects of catalytic advanced oxidation process in treating textile effluents," Reviews in Chemical Engineering, vol 32, no 1, pp 1-47, 2016
[47] G P Anipsitakis and D D Dionysiou, "Degradation of organic contaminants in water with sulfate radicals generated by the conjunction of peroxymonosulfate with cobalt," Environmental science & technology, vol
[48] Q Gao et al., "Utilizing cobalt-doped materials as heterogeneous catalysts to activate peroxymonosulfate for organic pollutant degradation: a critical review," Environmental Science: Water Research & Technology, vol 7, no 7, pp 1197-1211, 2021
[49] J Wang and S Wang, "Activation of persulfate (PS) and peroxymonosulfate
(PMS) and application for the degradation of emerging contaminants,"
Chemical Engineering Journal, vol 334, pp 1502-1517, 2018
[50] F Ghanbari and M Moradi, "Application of peroxymonosulfate and its activation methods for degradation of environmental organic pollutants,"
Chemical Engineering Journal, vol 310, pp 41-62, 2017
[51] G P Anipsitakis and D D Dionysiou, "Radical generation by the interaction of transition metals with common oxidants," Environmental science & technology, vol 38, no 13, pp 3705-3712, 2004
[52] J J Pignatello et al., "Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry," Critical reviews in environmental science and technology, vol 36, no 1, pp 1-84,
[53] D L Ball and J O Edwards, "The kinetics and mechanism of the decomposition of Caro's acid I," Journal of the American Chemical Society, vol 78, no 6, pp 1125-1129, 1956
[54] J Sun et al., "Oxone/Co2+ oxidation as an advanced oxidation process: comparison with traditional Fenton oxidation for treatment of landfill leachate," Water research, vol 43, no 17, pp 4363-4369, 2009
[55] S K Ling et al., "Oxidative degradation of dyes in water using Co2+/H2O2 and Co2+/peroxymonosulfate," Journal of Hazardous Materials, vol 178, no 1-3, pp 385-389, 2010
[56] S Guerra-Rodríguez et al., "Assessment of sulfate radical-based advanced oxidation processes for water and wastewater treatment: a review," Water, vol
[57] E R Bandala et al., "Degradation of atrazine using solar driven fenton-like advanced oxidation processes," Journal of Environmental Science and Health
[58] L Ismail et al., "Elimination of sulfaclozine from water with SO4− radicals:
Evaluation of different persulfate activation methods," Applied Catalysis B:
[59] A Rastogi et al., "Sulfate radical-based ferrous–peroxymonosulfate oxidative system for PCBs degradation in aqueous and sediment systems," Applied catalysis B: environmental, vol 85, no 3-4, pp 171-179, 2009
[60] K Pirkanniemi and M Sillanpọọ, "Heterogeneous water phase catalysis as an environmental application: a review," Chemosphere, vol 48, no 10, pp 1047-
[61] P R Shukla et al., "Activated carbon supported cobalt catalysts for advanced oxidation of organic contaminants in aqueous solution," Applied Catalysis B:
[62] P Hu and M Long, "Cobalt-catalyzed sulfate radical-based advanced oxidation: a review on heterogeneous catalysts and applications," Applied Catalysis B: Environmental, vol 181, pp 103-117, 2016
[63] D Manohar et al., "Adsorption performance of Al-pillared bentonite clay for the removal of cobalt (II) from aqueous phase," Applied Clay Science, vol 31, no 3-4, pp 194-206, 2006
[64] Z Huang et al., "Key role of activated carbon fibers in enhanced decomposition of pollutants using heterogeneous cobalt/peroxymonosulfate system," Journal of Chemical Technology & Biotechnology, vol 91, no 5, pp 1257-1265, 2016
[65] G Yi et al., "Facile chemical blowing synthesis of interconnected N-doped carbon nanosheets coupled with Co3O4 nanoparticles as superior peroxymonosulfate activators for p-nitrophenol destruction: Mechanisms and degradation pathways," Applied Surface Science, vol 593, p 153244, 2022
[66] J Deng et al., "Heterogeneous activation of peroxymonosulfate using ordered mesoporous Co3O4 for the degradation of chloramphenicol at neutral pH,"
Chemical Engineering Journal, vol 308, pp 505-515, 2017
[67] K.-Y A Lin and B.-J Chen, "Magnetic carbon-supported cobalt derived from a Prussian blue analogue as a heterogeneous catalyst to activate peroxymonosulfate for efficient degradation of caffeine in water," Journal of colloid and interface science, vol 486, pp 255-264, 2017
[68] M Moradi and F Ghanbari, "Application of response surface method for coagulation process in leachate treatment as pretreatment for Fenton process: Biodegradability improvement," Journal of Water Process Engineering, vol
[69] B.-T Zhang et al., "Sulfate radical and its application in decontamination technologies," Critical Reviews in Environmental Science and Technology, vol 45, no 16, pp 1756-1800, 2015
[70] T Olmez-Hanci et al., "Application of the UV-C photo-assisted peroxymonosulfate oxidation for the mineralization of dimethyl phthalate in aqueous solutions," Photochemical & Photobiological Sciences, vol 10, no
[71] F J Beltrán, "Ozone-UV radiation-hydrogen peroxide oxidation technologies," in Chemical degradation methods for wastes and pollutants:
[72] H Herrmann, "On the photolysis of simple anions and neutral molecules as sources of O−/OH, SO x− and Cl in aqueous solution," Physical Chemistry Chemical Physics, vol 9, no 30, pp 3935-3964, 2007
[73] N Getoff, "Purification of drinking water by irradiation A review," Journal of Chemical Sciences, vol 105, pp 373-391, 1993
[74] C Lee et al., "UV photolytic mechanism of N-nitrosodimethylamine in water: roles of dissolved oxygen and solution pH," Environmental science & technology, vol 39, no 24, pp 9702-9709, 2005
[75] P Devi et al., "In-situ chemical oxidation: principle and applications of peroxide and persulfate treatments in wastewater systems," Science of the Total Environment, vol 571, pp 643-657, 2016
[76] Y Xia et al., "Photoactivatable phospholipids bearing tetrafluorophenylazido chromophores exhibit unprecedented protonation-state-dependent 19F NMR signals," Organic Letters, vol 13, no 16, pp 4248-4251, 2011
[77] M Ahmadi et al., "Photocatalysis assisted by peroxymonosulfate and persulfate for benzotriazole degradation: effect of pH on sulfate and hydroxyl radicals," Water Science and Technology, vol 72, no 11, pp 2095-2102,
[78] Y Ji et al., "New insights into atrazine degradation by cobalt catalyzed peroxymonosulfate oxidation: kinetics, reaction products and transformation mechanisms," Journal of Hazardous Materials, vol 285, pp 491-500, 2015
[79] K Y A Lin and Z Y Zhang, "Metal-free activation of Oxone using one-step prepared sulfur-doped carbon nitride under visible light irradiation,"
Separation and Purification Technology, vol 173, pp 72-79, 2017
[80] K Y A Lin and Y C Chen, "Accelerated decomposition of Oxone using graphene-like carbon nitride with visible light irradiation for enhanced decolorization in water," Journal of the Taiwan Institute of Chemical Engineers, vol 60, pp 423-429, 2016
[81] Y Tao et al., "Polyimides as metal-free catalysts for organic dye degradation in the presence peroxymonosulfate under visible light irradiation," RSC advances, vol 5, no 119, pp 98231-98240, 2015
[82] Y Tao et al., "Metal-free activation of peroxymonosulfate by gC 3 N 4 under visible light irradiation for the degradation of organic dyes," Rsc Advances, vol 5, no 55, pp 44128-44136, 2015
[83] K Y A Lin and Z Y Zhang, "α-Sulfur as a metal-free catalyst to activate peroxymonosulfate under visible light irradiation for decolorization," RSC advances, vol 6, no 18, pp 15027-15034, 2016
[84] H Wang et al., "One-pot synthesis of a novel hierarchical Co (II)-doped TiO2 nanostructure: toward highly active and durable catalyst of peroxymonosulfate activation for degradation of antibiotics and other organic pollutants,"
Chemical Engineering Journal, vol 368, pp 377-389, 2019
[85] P Gouerec and M Savy, "Oxygen reduction electrocatalysis: ageing of pyrolyzed cobalt macrocycles dispersed on an active carbon," Electrochimica acta, vol 44, no 15, pp 2653-2661, 1999
[86] P Hu et al., "Monolithic cobalt-doped carbon aerogel for efficient catalytic activation of peroxymonosulfate in water," Journal of hazardous materials, vol 332, pp 195-204, 2017
[87] G R Bhadu et al., "Controlled assembly of cobalt embedded N-doped graphene nanosheets (Co@ NGr) by pyrolysis of a mixed ligand Co (ii) MOF as a sacrificial template for high-performance electrocatalysts," RSC advances, vol 11, no 34, pp 21179-21188, 2021
[88] Y Chen et al., "Insight into the degradation of tetracycline hydrochloride by non-radical-dominated peroxymonosulfate activation with hollow shell-core Co@ NC: Role of cobalt species," Separation and Purification Technology, vol 289, p 120662, 2022
[89] M M Sabzehmeidani et al., "Carbon based materials: A review of adsorbents for inorganic and organic compounds," Materials Advances, vol 2, no 2, pp 598-627, 2021
[90] N Zhou et al., "Cobalt (0/II) incorporated N-doped porous carbon as effective heterogeneous peroxymonosulfate catalyst for quinclorac degradation,"
Journal of colloid and interface science, vol 563, pp 197-206, 2020
[91] F Liu et al., "Heterogeneous activation of peroxymonosulfate by cobalt-doped
MIL-53 (Al) for efficient tetracycline degradation in water: Coexistence of radical and non-radical reactions," Journal of colloid and interface science, vol 581, pp 195-204, 2021
[92] J Zhu et al., "Durable activation of peroxymonosulfate mediated by Co-doped mesoporous FePO4 via charge redistribution for atrazine degradation,"
[93] P Chen et al., "Efficient Ofloxacin degradation with Co (Ⅱ)-doped MoS2 nano-flowers as PMS activator under visible-light irradiation," Chemical Engineering Journal, vol 401, p 125978, 2020
[94] Y Pang et al., "Facilely synthesized cobalt doped hydroxyapatite as hydroxyl promoted peroxymonosulfate activator for degradation of Rhodamine B,"
Journal of hazardous materials, vol 384, p 121447, 2020
[95] R Tian et al., "Amorphous Co3O4 nanoparticles-decorated biochar as an efficient activator of peroxymonosulfate for the removal of sulfamethazine in aqueous solution," Separation and Purification Technology, vol 250, p
[96] F Delgado et al., "Stable and efficient metal-biochar supported catalyst: degradation of model pollutants through sulfate radical-based advanced oxidation processes," Biochar, vol 2, no 3, pp 319-328, 2020
[97] Y Liu et al., "Highly efficient removal of trimethoprim based on peroxymonosulfate activation by carbonized resin with Co doping: Performance, mechanism and degradation pathway," Chemical Engineering Journal, vol 356, pp 717-726, 2019
[98] Y Yang et al., "Recycling of nitrogen-containing waste diapers for catalytic contaminant oxidation: occurrence of radical and non-radical pathways,"
[99] X Zhao et al., "Seaweed-derived multifunctional nitrogen/cobalt-codoped carbonaceous beads for relatively high-efficient peroxymonosulfate activation for organic pollutants degradation," Chemical Engineering Journal, vol 353, pp 746-759, 2018
[100] Z Zhu et al., "Magnetic Fe–Co crystal doped hierarchical porous carbon fibers for removal of organic pollutants," Journal of materials chemistry A, vol 5, no 34, pp 18071-18080, 2017
[101] K Lee et al., "Aerogel from fruit biowaste produces ultracapacitors with high energy density and stability," Journal of Energy Storage, vol 27, p 101152,
[102] A A Vaidya et al., "Green route to modification of wood waste, cellulose and hemicellulose using reactive extrusion," Carbohydrate polymers, vol 136, pp 1238-1250, 2016
[103] V C Tran et al., "Carbon aerogel from jackfruit waste as new material for electrodes capacitive deionization," Chemical Engineering Transactions, vol
[104] Y Liu et al., "Fabrication of functional biomass carbon aerogels derived from sisal fibers for application in selenium extraction," Food and bioproducts processing, vol 111, pp 93-103, 2018
[105] N K Kalagatur et al., "Application of activated carbon derived from seed shells of Jatropha curcas for decontamination of zearalenone mycotoxin,"
[106] N M Dang et al., "Cobalt oxide hollow nanoparticles as synthesized by templating a tri-block copolymer micelle with a core–shell–corona structure: a promising anode material for lithium ion batteries," New Journal of Chemistry, vol 39, no 6, pp 4726-4730, 2015
[107] B Yang et al., "Evaluation of activated carbon synthesized by one-stage and two-stage co-pyrolysis from sludge and coconut shell," Ecotoxicology and environmental safety, vol 170, pp 722-731, 2019
[108] B Abderrahim et al., "Kinetic thermal degradation of cellulose, polybutylene succinate and a green composite: comparative study," World J Environ Eng, vol 3, no 4, pp 95-110, 2015
[109] X Zhao et al., "Co-Mn layered double hydroxide as an effective heterogeneous catalyst for degradation of organic dyes by activation of peroxymonosulfate," Chemosphere, vol 204, pp 11-21, 2018
[110] C Trilokesh and K B Uppuluri, "Isolation and characterization of cellulose nanocrystals from jackfruit peel," Scientific Reports, vol 9, no 1, p 16709,
[111] E Apaydın Varol and ĩ Mutlu, "TGA-FTIR analysis of biomass samples based on the thermal decomposition behavior of hemicellulose, cellulose, and lignin," Energies, vol 16, no 9, p 3674, 2023
[112] N Akhtar et al., "Physico-chemical characteristics of leaf litter biomass to delineate the chemistries involved in biofuel production," Journal of the Taiwan Institute of Chemical Engineers, vol 62, pp 239-246, 2016
[113] S Ye et al., "Nitrogen-doped biochar fiber with graphitization from
Boehmeria nivea for promoted peroxymonosulfate activation and non-radical degradation pathways with enhancing electron transfer," Applied Catalysis B:
[114] Y Gao et al., "New insights into the generation of singlet oxygen in the metal- free peroxymonosulfate activation process: Important role of electron- deficient carbon atoms," Environmental science & technology, vol 54, no 2, pp 1232-1241, 2019
[115] W Zhao et al., "Cobalt-based catalysts for heterogeneous peroxymonosulfate
(PMS) activation in degradation of organic contaminants: Recent advances and perspectives," Journal of Alloys and Compounds, vol 958, no 43, p
[116] E.-T Yun et al., "Exploring the role of persulfate in the activation process: radical precursor versus electron acceptor," Environmental science & technology, vol 51, no 17, pp 10090-10099, 2017
[117] C Zhao et al., "Activation of peroxymonosulfate by biochar-based catalysts and applications in the degradation of organic contaminants: A review,"
[118] D T Oyekunle et al., "Review on carbonaceous materials as persulfate activators: structure–performance relationship, mechanism and future perspectives on water treatment," Journal of Materials Chemistry A, vol 9, no 13, pp 8012-8050, 2021
[119] Y Yang et al., "Oxidation of organic compounds in water by unactivated peroxymonosulfate," Environmental science & technology, vol 52, no 10, pp 5911-5919, 2018
[120] X Zuo et al., "Direct degradation of methylene blue by unactivated peroxymonosulfate: reaction parameters, kinetics, and mechanism,"
Environmental Science and Pollution Research, vol 29, no 50, pp 75597-
[121] S Wang and Y Zhang, "Degradation of methylene blue by an E-Fenton process coupled with peroxymonosulfate via free radical and non-radical oxidation pathways," New Journal of Chemistry, vol 47, no 7, pp 3616-3627,
[122] Y Liu et al., "Enhanced catalytic degradation of methylene blue by α-
Fe2O3/graphene oxide via heterogeneous photo-Fenton reactions," Applied Catalysis B: Environmental, vol 206, pp 642-652, 2017
[123] X Zhao et al., "One-step preparation of FexOy/N-GN/CNTs heterojunctions as a peroxymonosulfate activator for relatively highly-efficient methylene blue degradation," Chinese Journal of Catalysis, vol 39, no 11, pp 1842-
[124] G Zhu et al., "Co nanoparticle-embedded N, O-codoped porous carbon nanospheres as an efficient peroxymonosulfate activator: singlet oxygen dominated catalytic degradation of organic pollutants," Physical Chemistry Chemical Physics, vol 22, no 27, pp 15340-15353, 2020
[125] E Hayon et al., "Electronic spectra, photochemistry, and autoxidation mechanism of the sulfite-bisulfite-pyrosulfite systems SO2-, SO3-, SO4-, and SO5-radicals," Journal of the american chemical society, vol 94, no 1, pp
[126] Z Li et al., "A review of sulfate radical-based and singlet oxygen-based advanced oxidation technologies: recent advances and prospects," Catalysts, vol 12, no 10, p 1092, 2022
[127] X Y Yu et al., "Free radical reactions involving Cl•, Cl2-•, and SO4-• in the
248 nm photolysis of aqueous solutions containing S2O82-and Cl," The Journal of Physical Chemistry A, vol 108, no 2, pp 295-308, 2004
[128] C Liang et al., "Influences of carbonate and chloride ions on persulfate oxidation of trichloroethylene at 20 C," Science of the total environment, vol
[129] G V Buxton et al., "Critical Review of rate constants for reactions of hydrated electrons, hydrogen atoms and hydroxyl radicals (⋅ OH/⋅ O− in Aqueous Solution," Journal of physical and chemical reference data, vol 17, no 2, pp 513-886, 1988