This study investigated cobalt-doped biochar CoC derived from durian shell – an abundant waste source in Southeast Asia – as a catalyst to activate peroxymonosulfate PMS for treating org
INTRODUCTION
Durian shells
Durian (Durio zibethinus), known as the “king of fruits”, is one of the most prolific fruits in Southeast Asia due to its unique flavor and aroma, as shown in Figure 1.1 According to the Food and Agriculture Organization, global durian production reaches more than 3 million tons annually, with the Southeast Asia region contributing to more than 97% of the total output [1] The top three countries in the world for durian production are Thailand, Malaysia, and Indonesia Currently, Indonesia has a larger durian production volume than Thailand and Malaysia, but it primarily focuses on its domestic market with about 1.71 million metric tons of durian produced in 2022 Thailand and Malaysia, on the other hand, are the major players in the worldwide durian export industry, with around 1.09 and 0.05 million tons in 2023, respectively, according to figures from the Ministry of Agriculture and Food Security
According to data provided by the Ministry of Agriculture and Rural Development in Vietnam, the cultivation of durian spans over 110,000 hectares, yielding approximately 850,000 tons annually The primary regions responsible for durian production in the country are the Central Highlands, which encompasses more than 52,000 hectares (approximately 47%); the Mekong Delta region, with 33,000 hectares (about 30%); and the Southeast region, occupying 21,000 hectares (about 19%) Statistical analysis further highlights the significant role of durian as Vietnam's leading fruit export In 2023, durian contributed to nearly 40% of Vietnam’s total export revenue from fruits and vegetables, amounting to a substantial income of $2.2 billion
Moreover, the growing trend of exporting frozen durian without shells, aimed at reducing shipping weight and facilitating handling and transportation, has inadvertently led to a notable increase in durian waste generation For every 10 tons of durian processed in this manner, an astonishing amount of over 7 tons of durian shells is generated [2] In common practice, durian residues are burned or sent to landfills, without taking care of the surrounding environment, nor consider any precautions to prohibit the percolation of contaminants into the underlying water channels [3] Hence, numerous researchers are interested in finding solutions to environmental issues and using this byproduct source
The durian shell maintains recalcitrant structures, accounting for about 60 – 70% of durian fruit [5] It can be categorized into the outer thorn layer (consisting mainly of the exocarp and mesocarp), and the white sac between durian flesh (endocarp) [6-8] (Figure 1.2)
Figure 1.2 The structure of durian shell [7]
The brown or green outer thorn layer that protects the fruit from falling impacts is known as the exocarp This thorn arises from the elongated pimples on the greenish bud developing from the durian flower after pollination and can be hardened and further lengthened as the fruit grows [8] The mesocarp of the durian is identified as the layer beneath the thorny exocarp, characterized by its thick and spongy texture Both the exocarp and mesocarp are composed of fiber bundles, forming intricate and complex structures On the other hand, the locule, also known as the endocarp, is the thin white layer beneath the mesocarp [9] Durian shells, particularly the thorny layers, are classified as lignocellulosic materials They exhibit a complex, uneven fiber bundle structure primarily composed of cellulose (40 – 60%), hemicellulose (15 – 22%), and lignin (9 – 11%) [10-12]
To address durian shell waste disposal issues, numerous methods have been studied to convert durian biomass waste into valuable products These byproducts hold a high variety of phytochemicals that can be extracted, mainly composed of esters and acid compounds, making them suitable for pharmaceutical applications [10] Furthermore, the ash remains after combustion of durian shell shows a high content of K2O (47.24%) and P2O5 (14.47%) which are major micro molecules that are suitable for agronomic applications in soil amendments and plant nutrient incorporation [13] In certain local cuisines, especially in Thailand and Malaysia, the white layer composed of the mesocarp, and endocarp has been utilized in the form of dried durian snacks, durian shell sweet soup, or locule water [14] Although durian shells possess considerable potential for upcycling into an array of applications, further comprehensive research is required to establish and strengthen their promising utilization in specific fields
Biochar is a black, carbon-rich, and porous solid material (similar to charcoal) that can be produced through the thermochemical conversion of biomass with the presence of little or no oxygen [15, 16] The unique physicochemical properties of biochar, such as high surface area, tunable porosity, and chemical functionality, have garnered significant interest in its utilization as a versatile material for numerous applications [17] Biochar has different pore sizes, which may be in micro– (< 2.00 nm), macro–pores (> 50.00 nm) and nano– (< 0.900 nm), respectively Biochar can be derived from a wide range of biomass feedstocks, including agricultural residues, forestry waste, and municipal solid waste [16] The production of biochar from renewable and abundant biomass sources offers an attractive approach to converting waste into valuable products and contributes to the development of a more sustainable circular economy [15, 16] The physicochemical characteristics of biochar, which are largely dependent on the feedstock and production conditions, play a crucial role in determining its suitability and performance in target applications, such as soil amendment, water treatment, energy storage, and catalysis [18]
The distinct characteristics of biochar make it a promising material for environmental remediation applications The large surface area, porous structure, surface charge, and mineral composition of biochar provide effective adsorption sites for the removal of various contaminants, including heavy metals, organic pollutants, and nutrients, from water and soil [19-22] Additionally, the presence of functional groups like hydroxyl, phenolic, and carboxylic groups on the biochar surface can enhance the adsorption and immobilization of these contaminants [22] Nowadays, serving biochar as catalyst support applied in degradation processes to remove organic pollutants from wastewater has been receiving a great deal of public attention The high surface area and porous structure of biochar provide an excellent platform for the immobilization of catalytic materials, such as metal nanoparticles or oxidizing agents [23] This catalytic system can facilitate the efficient degradation of a broad spectrum of organic pollutants, including dyes, pharmaceuticals, and industrial chemicals.
Dye-containing wastewater
The rapid growth of population and industrialization have intensified the problem of water pollution globally It is estimated that about 800 million people worldwide still lack access to clean drinking water of adequate quality for domestic purposes [24] To meet the challenge of industrialization, the use of synthetic dyes in the textile industry, dyeing and printing business, tannery and paint industry, paper and pulp industry, cosmetic and food industry, dye manufacturing industry, and pharmaceutical sector has grown tremendously (Figure 1.4) [25-27]
Figure 1.4 Various categories of dyes and their possible industrial applications
Among these industries, the textile industry is prone to water pollution due to the hefty consumption of water and the discharge of coloring materials in the effluent (Figure 1.5) Textile dyeing, printing, finishing, and washing, combined with the textile wet processing industry, uses 85% of the total water and 65% of the total chemicals used in the textile supply chain [28] It has been also reported that the textile and apparel industry exceeded the combined emissions from international aviation and maritime transport If the current trend continues, it could account for one-fourth of global carbon emissions by 2050 [26] The impact is particularly severe for developing countries, mostly Asians such as China, Bangladesh, India, Pakistan, and Vietnam, which are regarded as textile manufacturing hubs [29] Hence, the ecological-friendly treatment of dye-containing wastewater to minimize the detrimental effect on human health and the environment is the need of the hour
Figure 1.5 Industrial sources of dyes wastewater [28]
A previous study showed that an average–sized textile industry utilizes about 200L of water per kg of fabric processed per day In the textile industry, the major source of waste comes from synthetic dyes that contaminate a large amount of water, if discharged without proper treatment Textile industries consume a large amount of water in two main processes, including dyeing of fabric and washing of fabric [27] The dyes used in the textile industry do not bind properly with the fabric and are ultimately discharged as effluent to waterbodies Wastewater generated from the textile industry consists of a mixture of metals, dyes, and various other pollutants The characteristics of textile effluent differ from industry to industry depending upon the type of equipment used, process, fabric produced, chemicals used, season, the trend of fashion, etc Wastewater from the textile industry has color, high pH, biochemical oxygen demand (BOD), suspended solids, chemical oxygen demand (COD), salts, and high temperature Table 1.1 shows the characteristics of the textile wastewater obtained by different researchers
Table 1.1 Characteristics of textile wastewater [30, 31]
Each year, an estimated 60,000 tons of industrial synthetic dyes are released into the environment globally [28] One of the most toxic dyes in textile wastewater is Rhodamine B (RhB) which has extensive value in the textile industry as a textile colorant because of its high stability and non–biodegradable [27] It is used to make ball pens, paints, leather, dye lasers, carbon sheets, stamp pad inks, crackers, and fireworks [28] RhB is a reddish violet powder with a chemical name, the molecular formula, and the molecular weight of N-[9-(ortho-carboxyphenyl)-6-(diethylamino)- 3H-xanthen-3-ylidene] diethyl ammonium chloride, C28H31N2O3Cl and 479 g/mol [32] As shown in Figure 1.6, the chemical structure of RhB features a central xanthene ring with two diethylamino groups and a carboxyphenyl substituent attached This arrangement of functional groups confers upon RhB its characteristic light absorption and fluorescence properties, making it a widely used dye in an array of applications The presence of the amino and carboxyl moieties also contributes to the solubility behavior of RhB As a member of the xanthene family, RhB shares structural similarities with other important fluorescent markers such as fluorescein [32] Despite its usefulness, when it is discharged into water, it is considered a dangerous source of contamination due to its intractable nature [33] Some studies suggest that RhB dye is considered carcinogenic and mutagenic origin in animals and humans It causes biological issues such as skin, respiratory inflammation, hemolysis, degenerative changes in the liver, and kidneys, etc [27, 34] In 2017, RhB was classified as a category three carcinogen by the World Health Organization Skjolding et al reported that the exposure with 12.5 mg/L of RhB for 96 h causes 20% lethality for the aquatic life but the possibility of the lethality increased to 100% with the exposure above 25 mg/L RhB concentration [33] Thus, keeping the hazardous nature and harmful effects in view, it was considered worthwhile to make systematic efforts to remove RhB from wastewaters
Figure 1.6 Chemical structure of RhB dye [32]
The annual global market growth rate has been reported to be 8.13% Hence, it is estimated that large amounts of effluents consisting of approximately 280,000 tons of refractory textile dyes will be produced by the textile industry [28] The choice of appropriate treatment method depends on the production process and the chemicals used in the textile plant The choice is also influenced by the composition of wastewater, discharge rule, location, business costs, operational costs, availability of land, selection, and availability of reuse/recycling of treated effluents, process, and expertise [35, 36] Thus, numerous treatment processes, such as physical, chemical, and biological, have been developed to treat wastewater before the wastewater is released into river bodies Physical treatment methods involve the removal of substances from wastewater by exploiting commonly occurring forces like electrical attractive, gravitational, and/or van der Waals forces or physical barriers [30] Physical techniques, including adsorption, membrane filtration, and reverse osmosis, have proven beneficial for the removal of a wide range of dyes [37] However, the use of these methods does not cause a change in the chemical structure of the substances present in water Hence, these approaches are limited by the fact that the dye molecules are not degraded but rather concentrated, necessitating proper disposal of the concentrated waste [37, 38] Biological approaches, such as anaerobic and aerobic treatments, microbial degradation, and the use of enzymes, can facilitate the complete mineralization of organic contaminants in a cost–effective manner [27] These biological methods are also capable of removing biological oxygen demand (BOD), chemical oxygen demand (COD), and suspended particles [39] Therefore, they are green techniques with cost – effective However, the drawback of these methods are sensitive to toxic organic dye Numerous chemicals are used during the execution of many processes to accelerate the process of disinfecting wastewater during purification These chemical processes involve various chemical reactions that are labeled as chemical unit processes These processes accompany several biological and physical processes [37] Conventional chemical methods (coagulation and flocculation), electrochemical techniques are some of the methods and techniques that are commonly used to treat textile waste These methods offer several advantages, such as ease of application, short reaction times, absence of sludge generation, and the ability to effectively remove both soluble and insoluble toxic dyes However, the management of secondary toxic pollutants generated during the dye removal process remains a challenge [40] A primary drawback of the physicochemical methods is their high cost, low efficiency, restricted adaptability, requirement for specialized equipment, and the need to manage the generated waste [37]
The limited success in achieving the desired degradation of organic molecules in wastewater has prompted substantial interest in advanced oxidation processes (AOPs) [14] These approaches have proved to be a potential alternative for emerging dye removal due to several advantages including the potential for organic pollutant removal without generating secondary pollution, the ability to withstand harsh operational conditions, and the effective elimination of highly toxic and persistent organic pollutants [41]
Advanced oxidation processes (AOPs) are based on the in–situ generation of strongly reactive free radicals that are capable of oxidizing complex organic molecules to partial or complete mineralization [42] These AOPs occur in a two-step process that includes the in–situ formation of the reactive radicals, followed by their reaction with organic pollutants [21] These technologies are based on the use of a broad range of photocatalysts, or the combination of oxidants such as hydrogen peroxide (H2O2), peroxymonosulfate, or persulfate with metal catalysts or ultraviolet (UV) radiation [24] The conventional AOPs exhibited good performance in the removal of emerging contaminants, and their mechanisms are mainly dependent on the hydroxyl radical ( • OH) • OH is a non–selective strong oxidant with a redox potential of 2.8 V, which can destroy the structure of organic compounds and even mineralize them to some extent One of the most widely investigated conventional AOPs is Fenton's reagent process, which utilizes iron species (primarily Fe 2+ ) as a catalyst and H2O2 as the oxidant [43, 44] However, Fenton's reagent technology presents some operational challenges that need to be addressed, such as the instability of H2O2, a restricted pH working range (pH 2 − 4), and the generation of sludge [43, 44]
To overcome these limitations, sulfate radical-based advanced oxidation processes (SR–AOPs) have emerged as a promising alternative SO4 •− possesses several advantages over • OH Firstly, SO4 •− has a high oxidation potential (2.5 – 3.1 V), comparable to or even higher than that of • OH [42-44] Secondly, SO4 •− exhibits more selective and efficient reactivity via electron transfer with organic compounds containing unsaturated bonds or aromatic π electrons, whereas • OH is a non–selective radical that may also react through hydrogen abstraction or electrophilic addition [43] Thirdly, SO4 •− can efficiently oxidize organic compounds over a wide pH range of 2 – 8, reaching a higher standard oxidation potential than • OH at neutral pH [43]
Lastly, the half-life of SO4 •− is estimated to be 30 – 40 μs, which allows for more stable mass transfer and better contact with target compounds compared to the 20 μs half-life of • OH [43]
One of the common oxidants used for generating SO4 •− is peroxymonosulfate (PMS, HSO5 −) PMS is a white solid powder that can be easily dissolved in water with a solubility higher than 250 g/L Its water solution is acidic It has an asymmetrical structure and the distance of the O–O bond is 1.453 Å with bond energy estimated to be in the range of 140 – 213.3 kJ/mol [26] Oxone, a commercial name of potassium peroxymonosulfate (KHSO5.0.5KHSO4.0.5K2SO4), is a versatile and environmentally friendly oxidant that has been widely utilized for bleaching, cleaning, and disinfection and more importantly as a favorable source of PMS [26] Although PMS is a strong oxidizer with a redox potential of 1.82 V, it reacted directly with the organic contaminants with a low reaction rate Hence, to generate the strong oxidizer, SO4 •− and • OH, appropriate activation is imperative for PMS [45]
Figure 1.7 Molecular structure of Oxone [46]
Numerous activation strategies have been applied to activate PMS to generate active radicals, including the use of exterior energy activation like heat, ultrasonic, and ultraviolet), transition metals like Fe, Co, Mn-based catalyst activation, or chemical activation via acidic, alkaline, and phenols, as shown in Figure 1.8 [42-44] However, using these strategies to activate PMS is an effective process albeit with some limitations in energy–consuming, high cost, and the risk of metal leaching
To overcome these limitations, heterogeneous catalytic activation of PMS has emerged as a highly promising alternative The main advantages of heterogeneous catalytic activation include the reusability of the catalyst, ease of spent catalyst separation from the reaction media, the ability to operate under diverse conditions, the prevention of uncontrolled catalyst release and subsequent environmental contamination, and the potential for effective activation without the requirement of energy inputs These features make heterogeneous catalytic activation of PMS an attractive option for advanced oxidation processes in environmental remediation applications [47, 48]
Figure 1.8 Main methods for PMS activation [49]
Transition metals, mainly Fe, Co, Mn, Cu, Ce, Ni, Zn, oxides, and bimetallic oxides have been reported as efficient catalysts for PMS activation Loading these transition metal catalysts on the carriers is a feasible way to overcome the drawbacks of agglomeration and poor stability of catalysts In recent research, biochar derived from pyrolyzing biomass under an oxygen-deficient atmosphere has been widely used as support for transition metal catalysts to synthesize composite catalysts with high activity in SR-AOPs Biochar with a high specific area can not only ensure the uniform distribution of transition metal-based materials and thus greatly reduce their agglomeration but also can provide electrons and accelerate the transfer of electrons in the catalytic reactions Besides, the transform of metal ions with different valence will be increased, which can indirectly increase the production rate of free radicals
It should be noted that in this transition metal-loaded biochar materials-based SR- AOPs, both free radical and non-free radical ways are involved in the degradation of pollutants In the free radical pathway, PMS reacts with transition metals attached to the surface of biochar to produce SO4 •−, and other free radicals are also generated in the reaction system In the non-free radical pathway, functional groups on the surface of biochar can directly transfer electrons from contaminants to PMS [50] Under many situations, the free radical pathway plays a vital role while the non-free radical pathway plays a secondary role in such a system The multifunctional micromotors based on carbon materials can propel automatically in the wastewater and provide efficient dynamic oxidation platforms for AOPs, improving the efficiency of water treatment, which is another aspect of transition metal-modified biochar materials in wastewater treatment
Among transition metals, Co 2+ /PMS has been demonstrated to outperform which is even superior to traditional Fenton reactions at neutral pH and with lower dosages of reagents [51] However, the biggest disadvantage of this system is the discharge of cobalt ions containing water, which is a potential threat to human beings and increases the operation cost due to the loss of cobalt In the form of Co3O4, the leakage of cobalt ions is much suppressed due to the bound of CoO in the net of
Related research
Recent advancements in catalytic materials have focused on cobalt oxide doped onto biochar supports, a subject gaining substantial attention In 2018, Chen et al demonstrated the synthesis of Co3O4 using rice straw-derived biochar, wherein acidification and urea-assisted hydrothermal treatment at 180 °C for 10 h, followed by calcination at 350 °C for 2 h, increased the specific surface area from 37.0 to 62.7 m 2 /g, enhancing its efficacy in degrading antibiotics like ofloxacin (OFX), achieving up to 90% degradation within 10 min [54] In 2020, another research by Tian et al decorated Co3O4 nanoparticles on biochar derived from corn straw, employing hydrothermal carbonization to produce hydrochar before impregnation with CoCl2 and pyrolysis at 800 °C This method not only facilitated the dispersion of Co3O4 but also acted as an electron shuttle between Co3O4 and pollutants like sulfamethazine (SMT), resulting in complete removal under optimized conditions within 60 min [55]
Hengduo Xu furthered these findings by synthesizing Co3O4-decorated biochar from wheat straw, employing Co(NO3)2 and biochar via precipitation under magnetic stirring, followed by calcination at 450 °C for 4 h This approach significantly enhanced the catalytic activity for pollutant degradation, exemplified by the superior performance of the Co3O4-BC/PMS system over Co3O4 alone [56] Liang et al explored a similar strategy using rice husk-derived biochar, achieving a high specific surface area of 77.04 m 2 /g through a two-step calcination process, thereby improving the degradation efficiency of 2,4-dichlorophenoxyacetic acid (2,4-D) [57] These studies collectively highlight the efficacy of cobalt-doped biochar catalysts in environmental applications, emphasizing their role in enhancing pollutant removal efficiency through advanced catalytic mechanisms involving SO4 •− and • OH radicals
Additionally, the preparation process of biochar plays a pivotal role in determining its physicochemical properties and structural characteristics, crucial for its effectiveness as a catalyst or adsorbent in environmental applications [58] Methods such as hydrothermal carbonization and pyrolysis yield biochars with distinct features Hydrothermal carbonization typically enhances surface area and introduces oxygen-containing functional groups like carboxyl and hydroxyl, facilitating interactions with organic pollutants through hydrogen bonding and n–π interactions [59] In contrast, pyrolysis produces biochar with higher porosity and aromatic structures that engage in π-π interactions with aromatic contaminants [60] Combining these methods can synergistically enhance the capability of biochar For instance, Tomul et al demonstrated that pretreatment hydrothermal treatment followed by pyrolysis improved naproxen adsorption through enhanced development of the internal pore network, creating more active adsorption sites [61] Therefore, this versatility in preparation methods can harness the potential of cobalt-doped biochar derived from durian shells for environmental remediation strategies By employing techniques such as hydrothermal pretreatment and pyrolysis, the durian shell biochar can be tailored to exhibit enhanced porosity and metal oxide attachment capabilities
To date, research on biochar loaded with cobalt oxide for activating PMS in the degradation of organic contaminants has primarily focused on agricultural residues such as rice straw or corn straw However, investigations into the potential of durian shells, an abundant waste source in Vietnam, are relatively scarce Durian shells present a promising yet underutilized resource that can be transformed into high-performance materials specifically designed for environmental applications This innovative approach not only tackles waste management challenges but also provides a sustainable solution to improve environmental remediation strategies.
EXPERIMENT
Research objectives and contents
Successful fabrication of cobalt-doped durian shell biochar for degrading RhB dye via PMS activation
− Fabrication and characterization of the Co-doped biochar from durian shell by hydrothermal and pyrolysis;
− Investigation of RhB removal efficiency of the fabricated Co-doped biochar under different catalyst dosages, PMS concentrations, dye concentrations, anionic concentrations, temperatures, and pH values;
− Evaluating recyclability of the Co-doped biochar for eliminating RhB from aqueous solution
− Evaluating the dye degradation ability of the Co-doped biochar for methylene orange, and congo red;
Chemicals, experimental equipment, and instruments
The durian shell was collected from a local fruit market in Ho Chi Minh City, Vietnam All chemicals used in this study were of analytical grade and were employed without undergoing additional purification Additionally, all reagents were prepared using reverse osmosis (RO) water These chemicals are listed in Table 2.1
Table 2.1 List of chemicals purchased and used in the research
Sodium sulfate Na2SO4 Xilong
Sodium carbonate Na2CO3 Xilong
Sodium hydrogen carbonate NaHCO3 Xilong
Methylene orange C14H14O3N3SNa Xilong Congo red C32H22N6Na2O6S2 Himedia
Tert-butyl alcohol, 99.5% (CH3)3COH Xilong
In this study, the equipment used included beakers, pipettes, thermometers, alcohol burners, test tubes, glass rods, laboratory bottles, burettes, falcon tubes, weighing papers, droppers, syringes, dialysis membranes, etc Additionally, the instruments were used like analytical balance, water distiller, centrifuge system, ultrasonic sonicate bath, overhead stirrer, fume hood, freezer, drying oven, etc.
Material preparation
The Figure 2.1 shows the pretreatment of the material Fresh durian shells were recovered within one day of deshelling and the white endocarp layer was separated The endocarp layer was then thoroughly washed to eliminate any remaining durian aril and subsequently cut into small pieces measuring approximately 1 – 2 cm 3 These endocarp pieces were subjected to hydrothermal pretreatment by placing them in a Teflon-lined stainless-steel autoclave along with
RO water The volume of the mixture was adjusted to constitute 60% of the total autoclave volume The autoclave was heated to 180 °C and maintained at this temperature for 5 h under self-generated pressure within a closed system After the completion of the hydrothermal treatment, the durian shells were immersed in a solution consisting of ethanol and water in a 1:1 ratio (v/v) for 2 days During this time, the solution was refreshed every 6 h to ensure the removal of soluble impurities Subsequently, the treated shells, named as DA, were frozen overnight, and subjected to freeze-drying under vacuum conditions for 48 h
Figure 2.1 Durian shell pretreatment procedure
For the preparation of the cobalt-doped biochar catalyst, a combination of hydrothermal and pyrolysis methods was used, as shown in Figure 2.2 The procedure began with immersing 1 g of DA into a solution containing Co(NO3)2.6H2O, urea, and 50 ml ethanol This mixture was allowed to react for 2 h at a temperature of 30 °C, followed by hydrothermal treatment at 120 °C for 10 h The resulting product, referred to as CoDA, was then dried at 50 °C for 12 h to ensure complete removal of the liquid remains Next, the dried CoDA product was placed in an MPCVD – 700 furnace, and a temperature ramp was applied from 300 to 500 °C at a rate of 5 °C/min The material was maintained at the final temperature for 2 h The as-obtained black catalyst was finely ground using a porcelain mortar and pestle and subsequently stored in an oven, dried at 50°C overnight, for further experimentation
Figure 2.2 Cobalt-doped biochar catalyst preparation procedure
The factors investigated in the CoC synthesis process include the pyrolysis time and the initial cobalt concentration, as presented in Table 2.2
Table 2.2 Factors investigated in the CoC synthesis process
Pyrolysis temperature (°C) Initial cobalt concentration (mmol/g)
Characterization
The surface morphology of the products was characterized using field emission scanning electron microscopy (FE–SEM) on a TESCAN Mira4 instrument (Figure 2.3) The FE–SEM was operated at an accelerating voltage of 20 kV and was equipped with an X-ray energy-dispersive spectroscopy (EDS) detector This allowed for the acquisition of mapping images to analyze the distribution of carbon, nitrogen, oxygen, and cobalt atoms within the samples The samples were sputter-coated with a thin Pt layer before SEM image processing in the 30s via Cressington 108 Auto Sputter Coater
Figure 2.3 Field emission scanning electron microscopy (Model: Mira4, TESCAN)
X-ray diffraction (XRD) is a powerful analytical technique that utilizes high- energy X-ray beams to probe the internal structure of solid materials When an X-ray beam interacts with the densely packed electrons within a material, it results in the scattering of the X–rays The characteristics of this scattered radiation are directly related to the chemical composition and crystallographic structure of the sample This makes XRD an invaluable method for determining the phase composition and crystal structure of materials, including those used in catalysts The XRD measurements discussed in this work were conducted using a state-of-the-art Aeris diffractometer manufactured by Malvern Panalytical, as shown in Figure 2.4 The instrument was operated at 40 kV and 40 mA, with measurements performed at room temperature
The data were collected over a 2θ range of 2 – 30°, using a scan rate of 0.6°/min and a step size of 0.01°
Figure 2.4 X-ray diffractometers (Model: Aeris, Malvern Panalytical)
The chemical properties of the samples were investigated using Fourier Transform Infrared Spectroscopy (FT–IR) When exposed to infrared radiation, the material absorbs energy at specific frequencies corresponding to the vibrational modes of its molecular structure This absorption profile can be used to identify functional groups and structural characteristics of the materials The FT–IR analysis was performed over the wavenumber range of 500 to 4000 cm -1 using an ALPHA II spectrometer from Bruker
Figure 2.5 Fourier Transform Infrared Spectroscopy (Model: Alpha II, Bruker)
Nitrogen physisorption measurements were also conducted on the samples using a Micromeritics ASAP 2020 apparatus to investigate the surface behavior of materials, as shown in Figure 2.6 Prior to the isotherm adsorption experiments at 77
K using high-purity nitrogen gas, the samples were pretreated by heating under vacuum at 150 °C for 6 h The specific surface area was calculated using the Brunauer-Emmett-Teller (BET) model in the relative pressure range of 0.05 – 0.30 (p/po), while the pore size distribution was evaluated using the density functional theory (DFT) method
Figure 2.6 Physisorption measurement (Model: ASAP 2020, Micromeritics)
The pH drift method is employed to analyze the pH at zero charge point (pHPZC) [62] NaOH and HNO3 with a concentration of 0.1 M for each are added to
50 mL of 0.01 M NaCl solution to adjust to the desired pH values ranging from 2 to
11 After that, 0.1 g of the catalyst is introduced into the adjusted pH solution and shaken by a shaker The pH of the solution is measured again after 24 h The pHPZC was defined as the pH at which the final pH value was equal to the original pH, indicating the point where the net surface charge of the catalyst was zero, as shown in Figure 2.7
Figure 2.7 pH meter (Model: PH-B100B, Benchtop)
Stock suspensions of dyes (RhB, MO, CR) and PMS were prepared by dispersing the pre-weighed amounts of respective chemicals in RO water using volumetric flasks UV-Vis spectrophotometer was used to determine dye concentration, as shown in Figure 2.8
Figure 2.8 UV-Vis spectrophotometer (Model: X7000, Lavionbon) a Investigation of operating parameters on the RhB degradation
Pyrolysis temperatures ranging from 300 – 500 °C and initial cobalt concentrations of 1 – 3 mmol/g were employed as operating parameters for activating PMS to degrade RhB A catalyst consisting of 0.3 g/L of CoC was introduced into a beaker containing 200 mL RhB solution with concentrations of 25 or 75 mg/L, and the mixture was mechanically stirred at 100 rpm The reaction was initiated by adding
150 mg/L of PMS to the solution, ensuring continuous stirring to promote thorough contact between the RhB solution and the CoC/PMS system At specific time intervals, 400 àL solution samples were extracted using a micropipette, and the UV/Vis absorbance of the RhB solution was measured The experiment was repeated three times for each material The RhB concentration was determined using a UV- Vis spectrophotometer (UV-Vis Lavionbon Model X7000) at a wavelength of 554 nm, and an external standard curve was established by correlating the concentrations of standard solutions with their corresponding absorbance values To determine the degradation efficiency of RhB (H, %) was calculated using the following equation:
C o ) × 100% b Investigation of difference systems on the RhB degradation
0.3 g/L catalysts like CoC or biochar were introduced into a beaker containing
200 ml RhB solution with an initial concentration of 50 mg/L, and the mixture was mechanically stirred at 100 rpm The reaction was initiated by adding 150 mg/L of PMS to the solution, ensuring continuous stirring to promote thorough contact between the RhB solution and the CoC/PMS system At specific time intervals, 400 àL solution samples were extracted using a micropipette, and the UV/Vis absorbance of the RhB solution was measured The experiment was repeated three times for each material The RhB concentration was determined using a UV-Vis spectrophotometer (UV-Vis Lavionbon Model X7000) at a wavelength of 554 nm, and an external standard curve was established by correlating the concentrations of standard solutions with their corresponding absorbance values
The degradation efficiency of RhB (H, %) was calculated using the following equation:
The kinetics of the RhB decomposition by the CoC/PMS system was expressed via pseudo-first-order and pseudo-second-order models as follow: ln (C t
Where C o and C t are the concentration (mg/L) of RhB at the initial time and a defined time t (min), respectively K app (min −1 ) and K′ app (L.mg -1 min -1 ) are the apparent rate constant of pseudo-first-order and pseudo-second-order models, respectively c Investigation of reaction parameters on the RhB degradation
In a typical experiment, a catalyst consisting of a mount of CoC was introduced into a beaker containing 200 mL of RhB solution The mixture was mechanically stirred at 100 rpm, and the reaction was initiated by adding PMS to the solution Continuous stirring was maintained to ensure thorough contact between the RhB solution and the CoC/PMS system At specific time intervals, 400 àL solution samples were extracted using a micropipette, and the UV/Vis absorbance of the RhB solution was measured The experiment was repeated three times for each material The RhB concentration was determined using a UV-Vis spectrophotometer (UV-Vis Lavionbon Model X7000) at a wavelength of 554 nm, and an external standard curve was established by correlating the concentrations of standard solutions with their corresponding absorbance values While the initial solution was adjusted to the initial pH (from 3 to 11) by 0.1 mol/L NaOH or HNO3, a temperature experiment was conducted in a water bath Table 2.3 shows the investigated reaction parameters in this study, including catalyst dosage, PMS concentration, RhB concentration, pH, and reaction temperature
Table 2.3 Experimental design for degradation of RhB
In the investigation of the effect of anions on RhB degradation, varying concentrations of Cl - (0 – 10 mM), CO3 2- (0 – 10 mM), HCO3 - (0 – 10 mM), and SO4 2-
(0 – 10 mM) ions were added to the reaction mixture A catalyst consisting of 0.3 g/L of CoC was introduced into a beaker containing 200 ml RhB solution with an initial concentration of 50 mg/L, and the mixture was mechanically stirred at 100 rpm The reaction was initiated by adding 150 mg/L of PMS to the solution, ensuring continuous stirring to promote thorough contact between the RhB solution and the CoC/PMS system At specific time intervals, 400 àL solution samples were extracted using a micropipette, and the UV/Vis absorbance of the RhB solution was measured The experiment was repeated three times for each material
The degradation efficiency of RhB (H, %) was calculated using the following equation:
The kinetics of the RhB decomposition by the CoC/PMS system was expressed via a pseudo-first-order model as follow: ln (C t
Where C o and C t are the concentration (mg/L) of RhB at the initial time and a defined time t (min), respectively K app (min −1 ) is the apparent rate constant of pseudo-first-order
The relationship between reaction temperature and the reaction rate constant can be described by the Arrhenius equation: k t = A (− RT E a ) lnk t = lnA − E a
RT Where T is the reaction temperature (K); A is the pre-exponential factor, also known as Arrhenius factor or frequency factor; R is the universal gas constant (R 8.314 J.mol -1 K -1 ) and Ea is the activation energy of the reaction (J.mol -1 ) The Arrhenius plot of ln(k) versus 1/T allows for the determination of the activation energy (Ea) at 3 reaction temperatures in this study d Investigation of reactive oxygen species on the RhB degradation
To investigate the quenching of reactive radicals, varying concentrations of tert-butyl alcohol (0 - 500 mM), ethanol (0 - 500 mM), and furfuryl alcohol (0 – 25 mM) were added to the reaction mixtures before conducting the experiments e Reusability and applicability of the catalyst in wastewater treatment
RESULTS AND DISCUSSION
Influence of operating conditions on RhB removal by Co-doped biochar
Figure 3.1 SEM images of white durian shell, DA, and CoDA at difference magnifications (a–c) x1000 and (d–f) x5000
The surface morphology of the durian shell samples was analyzed using SEM at multiple magnifications, as shown in Figure 3.1 At 1000x magnification (Figure 3.1a-c), the SEM micrographs revealed an improvement in the porosity of the durian shell after the hydrothermal pretreatment and free-drying process This increased porosity could have facilitated the improved dispersion of cobalt on the surface of the material Furthermore, when the magnification was increased to 5000x (Figure 3.1d- f), the surface of the cobalt-doped durian shell (CoDA) exhibited a rough morphology that was covered by a uniform layer of predicted cobalt salt This surface morphology was noticeably different from the previous durian shell samples before cobalt doping, indicating the successful incorporation of the cobalt species onto the surface
Figure 3.2 shows the effect of pyrolysis temperature on the morphology of the catalyst The pyrolysis from 350 to 500°C led to the development of honeycomb-like porous carbon structures with defined pores, while the CoC-300 material exhibited a less cracked surface compared to others, despite undergoing the grinding step It is observed that as the pyrolysis temperature increases, there is an increase in the porosity of the material N2 adsorption-desorption isotherms as shown in Appendix 3 revealed that the CoC-350 catalyst displayed a type IV isotherm, indicative of a mesoporous structure, and had a significantly larger BET surface area of 33.23 m²/g compared to 4.01 m²/g for the CoDA sample [63] The average pore diameter of CoC-
350 (9.94 nm) was generally smaller than twice that of CoDA, which was attributed to the dispersion of cobalt oxide species within the porous biochar matrix, preventing particle agglomeration and potentially contributing to enhanced catalytic activity
Figure 3.2 Effect of pyrolysis temperature on morphology of catalyst
EDS mapping was conducted on both CoDA and CoC-n catalysts to assess the elemental distribution in Table 3.1 with measurements carried out at three distinct locations for each sample Carbonization of the CoDA led to a notable increase in carbon content and a decrease in oxygen content when increasing the pyrolysis temperature Additionally, the Co content observed in the resultant CoC-n catalysts ranged from 8.56 to 15.13 wt.% The element mapping indicated that the CoDA catalyst exhibited a more uniform distribution of Co elements compared to others (Appendix 1)
Table 3.1 Element composition of materials with different pyrolysis temperatures via EDS analysis
CoDA 48.37 ± 1.98 20.97 ± 2.74 22.54 ± 1.41 8.12 ± 1.23 CoC-300 48.08 ± 0.84 17.57 ± 0.80 19.38 ± 0.57 14.97 ± 0.27 CoC-350 57.48 ± 2.03 14.85 ± 0.80 19.11 ± 1.67 8.56 ± 0.69 CoC-400 63.08 ± 1.60 9.99 ± 0.06 19.25 ± 0.71 7.31 ± 1.22 CoC-450 50.82 ± 0.48 11.74 ± 0.72 22.30 ± 0.25 15.13 ± 0.22 CoC-500 63.08 ± 0.67 11.31 ± 0.72 16.85 ± 0.48 8.76 ± 1.04
The XRD patterns of the synthesized materials are shown in Figure 3.3 The presence of the (101) and (200) reflections around 16° and 25° in the DA and CoDA samples indicate the partial charring of the original cellulosic structure, suggesting the preservation of graphite oxide and graphite characteristics [64] After pyrolysis, disordered graphitic (002) and (100) planes were found between 20-30° and 43°, respectively due to the formation of the graphite structure of samples [64] Additionally, characteristic peaks at 21.87°, 36.08°, 42.59°, and 51.95° corresponded to the (111), (022), (131), and (040) planes, respectively, consistent with the cubic spinel-phase structure of Co3O4 [65] However, increasing the pyrolysis temperature to 500°C resulted in the formation of CoO, with peaks at 42.62° and 49.63° corresponding to the (111) and (020) planes, respectively [66] This phenomenon was consistent with previous studies showing that Co3O4 typically forms at 300 – 450 °C [67, 68] Notably, Co3O4 is more effective than crystalline CoO in activating PMS [55] The absence of the characteristic peak of cobalt oxide in the CoDA sample could be due to the existence of another form of cobalt
Figure 3.3 XRD patterns of DA, CoDA, and CoC-n
The results of the FT–IR analysis conducted on DA, CoDA, and CoC samples are illustrated in Figure 3.4 A broad peak observed at 3442 cm -1 was attributed to the stretching vibration of surface-adsorbed molecular water and hydrogen-bonded O–H groups [67] Additionally, a sharp peak centered around 3363 cm -1 of the CoDA sample, could be related to the modification of Co ions [69] The peak at 1611 cm -1 corresponded to C=C or C=O stretching vibrations [70, 71] As the pyrolysis temperature ramp from 300°C to 500°C, the peak intensities of C–O and C–H functional groups increased, while the proportion of C=O groups decreased This suggests that the oxygen-containing functional groups of the CoC materials were degraded at higher temperatures [72] Peaks at 1050 cm -1 and 873 cm -1 were attributed to C–O and C–H of aromatic compounds, respectively [73] The typical bands originated from the stretching vibrations of the metal-oxygen bonds (567 and 662 cm −1 ) were occurred in CoC samples, indicating the presence of Co3O4 [74]
Specifically, the first band at 567 cm −1 was associated with the OB3 vibration in the spinel lattice, and the second band at 662 cm −1 was attributed to the ABO3 vibration (A denotes the Co 2+ in a tetrahedral hole and B denotes Co 3+ in an octahedral hole) [75, 76]
Figure 3.4 FT–IR spectra of DA, CoDA, and CoC-n
Figure 3.5 shows the catalytic properties of CoC-n catalysts synthesized at different pyrolysis temperatures When tested with a 25 mg/L RhB solution, all catalysts demonstrated remarkable efficacy in RhB treatment CoC-300 and CoC-500 achieved over 90% RhB removal within 30 min However, as the concentration of RhB increased, the degradation efficiency of the catalysts decreased Nevertheless, the catalyst still exhibited effective performance, with removal efficiencies ranging from 58.98% to 74.30% Notably, among the tested catalysts, CoC-350 displayed the highest removal efficiency, achieving 92.72% and 74.3% within 30 min for 25 ppm and 75 mg/L RhB solutions, respectively The synthetic effect of the graphitic carbon matrix with dispersed Co3O4 on the CoC-350 surface enabled PMS activation for RhB degradation with the highest efficiency, hence a pyrolysis temperature of 350°C was selected for subsequent experiments
Figure 3.5 Influence of pyrolysis temperature of catalyst on RhB removal efficiency at (a) 25 mg/L and (b) 75 mg/L
([RhB] = 25 and 75 mg/L, [PMS] = 150 mg/L, [catalyst] = 0.3 g/L, T = 30 °C, pH
The morphology of the materials synthesized with varying initial cobalt content (1, 2, and 3 mmol/g, denoted as CoC-1, CoC-2, and CoC-3, respectively) was investigated using SEM imaging (Figure 3.7)
Figure 3.6 Effect of initial cobalt concentration on morphology of catalyst
The results indicate that increasing the initial cobalt concentration did not lead to significant differences in the material morphology The unpyrolyzed samples (CoDA-1, CoDA-2, CoDA-3) maintained the heterogenous surface structure of the precursor materials, while the pyrolyzed biochar samples retained their characteristic honeycomb-like porous architecture Furthermore, EDS mapping (Table 3.2 and Appendix 2) confirmed the presence of C, O, N, and Co in all samples, with the Co content scaling proportionally with the initial cobalt concentration
Table 3.2 Element composition of composites with difference initial cobalt concentration via EDS analysis
CoDA-1 50.18 ± 1.51 19.57 ± 0.70 24.53 ± 0.91 6.00 ± 0.34 CoDA-2 48.37 ± 1.98 20.97 ± 2.74 22.54 ± 1.41 8.12 ± 1.23 CoDA-3 41.26 ± 1.56 19.65 ± 0.80 29.23 ± 1.43 9.85 ± 0.92 CoC-1 60.97 ± 2.01 7.81 ± 1.66 23.02 ± 0.75 8.20 ± 1.42 CoC-2 57.48 ± 2.03 14.85 ± 0.80 19.11 ± 1.67 8.56 ± 0.69 CoC-3 52.18 ± 6.14 9.55 ± 0.57 22.78 ± 1.00 15.49 ± 4.63
XRD and FT–IR analysis were conducted to further characterize the samples with varying initial cobalt concentrations, as shown in Figure 3.7 According to the XRD patterns depicted in Figure 3.7a, the XRD patterns of the CoC-1, CoC-2, and CoC-3 samples showed diffraction peaks at 2-theta of 25° corresponding to the (002) planes of graphite structure [64] Additionally, characteristic peaks were observed at 2-theta values of 21.87°, 36.08°, 42.59°, and 51.95°, matching the crystal planes of
Co3O4 with (111), (022), (131), and (040) planes, respectively [65] As the initial cobalt concentration was increased from 1 to 3 mmol/g, the intensity of the 25° peak decreased, likely due to the increasing loading of the metal ions [69] The results of the FT–IR analysis conducted on the CoC and CoDA samples with different initial cobalt content are illustrated in Figure 3.7b A distinct peak centered around 3363 cm -1 was observed in the CoDA samples, which may be linked to the modification of the cobalt ions [69] Following the pyrolysis process, the decline in the intensity of this peak could be attributed to the transformation of the cobalt ions into an oxide form The characteristic bands originating from the stretching vibrations of the metal- oxygen bonds at around 567 cm -1 were present in the CoC samples, further confirming the existence of Co3O4 phase [74]
Figure 3.7 (a) XRD patterns and (b) FT–IR spectra of the catalyst with different initial cobalt content
As depicted in Figure 3.8, as the initial Co solution content increased from 1 mmol/g to 2 mmol/g, the RhB removal efficiency improved significantly However, a further increase in the initial cobalt concentration to 3 mmol/g led to a decrease in RhB removal efficiency This trend could be due to increased self-quenching between the excessive free radicals generated or reactions between the free radicals and HSO5 -
[55] Therefore, the 2 mmol/g initial Co content solution was selected for subsequent experiments
Figure 3.8 Effect of initial cobalt concentration on RhB degradation
([RhB] = 75 mg/L, [PMS] = 150 mg/L, [catalyst] = 0.3 g/L, T = 30 °C, pH 6.5)
Based on the results presented, the CoC catalyst with a pyrolysis temperature of 350 °C and an initial cobalt concentration of 2 mmol/g was selected for further investigations This will be referred to as the CoC going forward.
Influence of different reaction systems on RhB removal
To evaluate the catalytic performance of CoC for PMS activation, the removal efficiencies of RhB and rate constants in the different systems were compared and the results are indicated in Figure 3.9 and Table 3.3 Over 38% of RhB was removed in the Biochar/PMS system within 30 min, which may be due to its large specific surface area It was also noted that only 28% of RhB was degraded by PMS alone although it was a strong oxidizing agent with a redox potential of 1.82 V Additionally, the individual CoC system exhibited relatively insignificant RhB removal, less than 10% In contrast, the CoC/PMS system achieved over 85% RhB removal within 12 min, suggesting a synergistic effect of CoC and PMS in degradation The oxidative degradation of dyes in different systems was explored using kinetic models such as pseudo-first-order and pseudo-second-order, as shown in Table 3.3 Based on the obtained R 2 values, the pseudo-first-order model was best fitted to the oxidative degradation of RhB dye Under the pseudo-first-order model, a rapid increase in apparent rate constants (Kapp) was observed for the CoC/PMS system, reaching 0.1638 min −1 This value was 3 and 11 times higher than those for the PMS and Biochar/PMS systems, respectively This significant enhancement can be attributed to the increased generation of reactive oxygen species (ROSs) upon the introduction of PMS, which dramatically accelerated the dye degradation processes Moreover, these findings are in agreement with previous studies on the activated persulfate system for RhB dye degradation [77, 78]
Figure 3.9 Effect of different systems on RhB degradation
([RhB] = 50 mg/L, [PMS] = 150 mg/L, [catalyst] = 0.3 g/L, T = 30 °C, pH 6.5)
Table 3.3 Effect of the different systems on RhB degradation
Pseudo 1 st Order Pseudo 2 nd Order
K app (min -1 ) R 2 K’ app (L.mg -1 min -1 ) R’ 2
Influence of reaction parameters on RhB removal by Co-doped biochar
Figure 3.10 Effect of catalyst dosage on RhB degradation
([RhB] = 50 mg/L, [PMS] = 150 mg/L, T = 30 °C, pH 6.5)
The influence of catalyst dosage on the degradation of RhB by the CoC/PMS system was investigated across a concentration range of 0.1 – 0.5 g/L, as depicted in Figure 3.10 and Table 3.4 Upon increasing the catalyst dosage from 0.1 to 0.3 g/L, a substantial enhancement in RhB removal efficiency was observed, rising from 70.96% to 86.41% after 30 min Accordingly, the apparent rate constant experienced a four-fold increment as the catalyst dosage was increased to 0.3 g/L This performance improvement can be attributed to the increased number of active sites, which promotes a higher number of ROSs and results in enhanced RhB degradation Correspondingly, the Kapp value was just increased 1.18 times at the 0.5 g/L catalyst dosage compared to the value at the 0.3 g/L dosage This phenomenon could be attributed to the excessive catalyst dosage, which hindered the availability of PMS for activation, thereby limiting a significant alteration in the rate of ROS generation Additionally, an excess of cobalt ions may have led to the quenching of the radicals produced during the process [55] Therefore, 0.3 g/L catalyst dosage was selected for further investigation
Table 3.4 Effect of catalyst dosage on the rate constant
Pseudo 1 st Order Pseudo 2 nd Order
K app (min -1 ) R 2 K’ app (L.mg -1 min -1 ) R’ 2
3.3.2 Effect of the PMS concentration
As depicted in Figure 3.11, increasing the PMS concentration from 75 to 100 mg/L substantially improved its degradation efficiency from 67.06% to 97.92% This enhancement was attributed to the positive effect of PMS, which facilitates the generation of ROSs by attaching more HSO5 - ions to the active sites of the CoC catalyst At 500 mg/L PMS, RhB removal reached 98% within 6 min, representing the highest efficiency achieved - with the rate constant approximately 8.1 and 3.6 times greater than at 75 and 150 mg/L PMS, respectively (Table 3.5) However, to investigate the effect of other factors in further experiments, a PMS concentration of
Figure 3.11 Effect of PMS concentration on RhB degradation
Table 3.5 Effect of PMS concentration on the rate constant
Pseudo 1 st Order Pseudo 2 nd Order
K app (min -1 ) R 2 K’ app (L.mg -1 min -1 ) R’ 2
3.3.3 Effect of the RhB concentration
Figure 3.12 Effect of RhB concentration on RhB degradation
Table 3.6 Effect of RhB concentration on the rate constant
Pseudo 1 st Order Pseudo 2 nd Order
K app (min -1 ) R 2 K’ app (L.mg -1 min -1 ) R’ 2
As shown in Figure 3.12 and Table 3.6, the initial RhB concentration had a significant impact on its removal and the reaction rate Increasing the initial RhB concentration from 25 mg/L to 150 mg/L decreased the removal efficiency from 96.82% to 46.05% after 30 min This corresponded to a 5.7-fold reduction in the Kapp value The inverse relationship between initial RhB concentration and removal efficiency could be due to higher RhB concentrations limiting diffusion and contact with the catalyst This inhibited degradation and required longer times for the complete elimination of the RhB Additionally, the apparent rate constant of the reaction is then correlated to the initial dye concentration by a simple power law as follows:
Figure 3.13 shows the relationship between lnK and ln Co It is determined that the order (n) respective to dye concentration is 0.9629, thus confirming the first–order kinetics
Figure 3.13 Relationship between lnK and lnCo
Figure 3.14 Effect of pH on RhB degradation
([RhB] = 50 mg/L, [catalyst] = 0.3 g/L, [PMS] = 150 mg/L; T = 30 °C)
Table 3.7 Effect of pH on the rate constant pH
Pseudo 1 st Order Pseudo 2 nd Order
K app (min -1 ) R 2 K’ app (L.mg -1 min -1 ) R’ 2
The pH value of the solution plays an important role in the generation of ROSs upon PMS activation by the CoC catalyst As shown in Figure 3.14, pH 3 – 7 gave an increase in RhB degradation, which was inhibited in a strong alkali environment Considering the pKa values of PMS (pKa1 < 0, pKa2 = 9.4), the predominant species in the pH range of 3 to 7 is HSO5 – [45] Additionally, the pHPZC of the catalyst surface was determined to be 7.3 as shown in Figure 3.15 Hence, at a pH value lower than 7.3, the positive charge on the catalyst surface has a strong interaction with negatively charged HSO5 -, leading to higher RhB degradation efficiency At pH 3, the RhB degradation efficiency was slightly lower, reaching only 86% after 30 min of treatment This could be due to excessive H + ions, which may inhibit the generation of SO4 ●– and ● OH by forming inactive species like HSO4 ● and H2O, thereby, suppressing PMS activation At pH 9, the RhB degradation efficiency exhibited a lower performance during the initial 12 min of treatment This can be attributed to the electrostatic repulsion between the negatively charged catalyst surface (pH > pHPZC) and the anionic forms of both RhB and the PMS species (HSO5 - and SO4 2-) However, the RhB treatment had a high efficiency of 85% after 30 min This may be due to the reaction of SO4 ●- and HO - to form ● OH, thus the RhB removal efficiency increases under the effect of both radical reactive species Additionally, the existing forms of PMS are HSO5 - and SO5 2- at this pH, thus the reactions can occur to form single oxygen 1 O2, which then combined with SO4 ●– and ● OH and synergistically degrade RhB via radical and non-radical mechanisms At high pH 11, the surface of the CoC catalyst carries a negative charge, while PMS exists mainly in the form of
SO5 2-, thus increasing the repulsive force between the surface of the material and the oxidizing agent SO5 2– Furthermore, unstable HSO5 – decomposes into non-radical products like SO4 2– in a strongly alkaline environment [45] As shown in Table 3.7, the rate constant increases gradually from 3 – 7 with the highest rate constants of 0.1122 – 0.1761 min -1 observed in the pH range of 5 – 7, then gradually decreases and reaches the lowest value at pH 11 (0.0056 min -1 ) The reaction rate at pH 7.0 is
2, 1.5, and 31 times higher than those at pH 3, 9, and 11, respectively
Figure 3.15 Determination of isoelectric point (pHPZC) of catalyst
Figure 3.16 Effect of reaction temperature on CoC/PMS system
([RhB] = 50 mg/L, [catalyst] = 0.3 g/L, [PMS] = 150 mg/L; pH 6.5)
Table 3.8 Effect of temperature on RhB degradation kinetics
The temperature has a significant effect on the PMS activation ability of the catalyst during RhB treatment, showing a rapid removal of RhB at high temperatures (Figure 3.16) RhB treatment efficiency was 84% after 10 min at 30 °C, 87% after 10 min at 40 °C, 89.6% after 6 min at 40 °C, and 89.33% after 3 min at 60 °C, which increased slowly after that As the temperature increased from 30 to 60 °C, the rate constant increased from 0.1638 to 0.7303 min -1 (Table 3.8), which is because the higher reaction temperature can easily activate and break the O–O bond of HSO5 – and generates faster SO4 ●– radical [45] In addition, the higher reaction temperature favors the reactant molecules to overcome the activation energy barrier [79] Meanwhile, high temperature is an easier condition for the dye molecules to obtain the higher energy required to break the intermolecular forces, resulting in the degradation of the dye into smaller molecules [79] The activation energy for RhB removal in the CoC/PMS system was determined using the Arrhenius equation to be 46.52 kJ/mol
Furthermore, the effect of temperature on RhB degradation of PMS without a catalyst was investigated, as shown in Figure 3.17 and Table 3.8 It was found that increasing the temperature significantly enhanced the degradation efficiency of PMS for RhB, with a 2.3-fold increase at 60 °C compared to 30 °C The activation energy for RhB removal by PMS, calculated using the Arrhenius equation, was 43.13 kJ/mol, which is slightly lower than that of the CoC/PMS system This phenomenon might be attributed to the direct interaction between PMS and RhB, possibly following a simpler and more energetically favorable pathway In contrast, the presence of a CoC catalyst could introduce more complex reaction intermediates, which require higher activation energy but result in greater overall degradation efficiency, exceeding the non-catalyzed system by 1.3 to 2.6 times within the same time interval This phenomenon indicates that while catalysts increase the complexity and activation energy of the reaction, this material ultimately improves the overall degradation efficiency, demonstrating their effectiveness in facilitating more efficient degradation pathways
Figure 3.17 Effect of reaction temperature on RhB degradation of PMS
([RhB] = 50 mg/L, [PMS] = 150 mg/L; pH 6.5; 30 min)
Figure 3.18 Effect of (a) Cl - , (b) SO4 2-, (c) HCO3 -, and (d) CO3 2- on RhB removal ([RhB] = 50 mg/L, [catalyst] = 0.3 g/L, [PMS] = 150 mg/L; T = 30 °C)
Textile dyeing wastewater or other polluted water sources contain many anions that may have a great impact on its organic treatment efficiency Figure 3.18 depicts the effects of some anions, including Cl - , SO4 2-, HCO3 -, and CO3 2- at different concentrations from 0 – 10 mM on the RhB removal It was found that the RhB removal did not change significantly in the presence of SO4 2- ions with RhB removal efficiency is about 84% and the reason may be that an SO4 2- ion does not react with the SO4 •- The reduction in treatment efficiency may be due to anions creating an electrostatic force with RhB dye molecules, hindering their movement to reaction sites and leading to reduced efficacy
The presence of Cl - anions at a concentration of 1 mM resulted in a slight reduction in the removal efficiency, reaching 82% after 30 min However, when the
Cl - concentration was increased to 5 mM, the removal efficiency further dropped to 79% within the same time frame This decrease in efficiency can be attributed to the reaction between Cl - ion and SO4 ●-, leading to the generation of chlorine radicals, namely Cl ● and Cl2 ●-, which possess high redox potential (E°(Cl ● /Cl - ) = 2.4 V and E° (Cl2 ●-/Cl - ) = 2.1V) These chlorine radicals are weaker oxidizers compared to SO4 •- (E°(SO4 ●-/SO4 2-) = 2.5 – 3.1 V [80] Additionally, excessive Cl - concentrations may interfere with the RhB removal process
On the other hand, HCO3 - and CO3 2– anions strongly inhibit the decomposition of RhB, which is more severe with the increase of anion concentration HCO3 - and
CO3 2– can react with ROS to produce weaker radicals such as CO3 ●- (E°(CO3 ●-/CO3 2–
) = 1.78 V) and HCO3 ● (E°(HCO3 -/HCO3 ●) = 1.65 V) [81, 82] Furthermore, these ions can directly react with PMS to produce peroxymonocarbonate (HCO4 -, E°(HSO5 -
/SO4 2–) = 1.8 V), which is unstable and decomposes to hydrogen peroxide and HCO3
[83] On the other hand, CO3 2- and HCO3 - could increase the solution pH to alkaline conditions, thus causing a negative effect on RhB removal
2HCO3 - + SO4 ●- + ● OH → HCO3 ●- +CO3 ●- + SO3 2- +H2O 2CO3 2- + SO4 ●- + ● OH → 2CO3 ●- + SO4 2- + HO -
2HSO5 - + HCO3 - + CO3 2- → 2HCO4 - + CO4 2- + H + + SO4 2- HCO4 - + CO4 2- + 2H2O → 2H2O2 + HCO3 - + CO3 2-
3.3.7 Identification of active radical species
In the PMS activation AOPs, a variety of ROSs, usually including ● OH and
SO4 ●- are produced and participate in the degradation of pollutants, which is the free radical-based process [45] In addition, a previous study showed that carbon matrix composites could effectively degrade pollutants by singlet oxygen ( 1 O2) through a non-free radical mechanism without the participation of free radicals, which is a non- radical pathway [69] The relative contributions of different active radical species were distinguished through the chemical quenching studies by using excess ethanol (EtOH), tert-butanol (TBA), and furfuryl alcohol (FFA) as the radical scavengers Table 3.9 below shows the reaction rate constants of radicals with the corresponding scavenger EtOH can simultaneously quench both SO4 ●- and ● OH, while TBA has a much higher affinity to scavenge ● OH than to SO4 ●- With regard 1 O2, FFA is considered an effective quencher
Table 3.9 The reaction rate constants of radicals with the corresponding scavenger
Scavenger Targeted radicals Rate constant (M -1 s -1 )
As shown in Figure 3.19, RhB degradation efficiency significantly decreased from 86% to 46% after the addition of 500 mM EtOH Conversely, TBA showed less inhibition in RhB degradation efficiency It could be speculated that SO4 ●- was the main free radical in RhB removal, which was generated in the CoC/ PMS system The presence of FFA at concentrations of 10 and 20 mM strongly reduced RhB degradation to 34% and 15%, respectively while the FFA concentration of 25 mM almost completely inhibited the RhB degradation, proving that 1 O2 was formed in the CoC/PMS system and played an important role in the RhB degradation These findings indicated that ● OH, SO4 ●- and 1 O2 were responsible for RhB degradation, with SO4 ●- and 1 O2 playing the most significant roles
Figure 3.19 RhB removal in the presence of radical scavengers
([RhB] = 50 mg/L, [catalyst] = 0.3 g/L, [PMS] = 150 mg/L; pH 6.5; T = 30 °C)
Based on the analysis above and the previous research, a possible catalytic mechanism for the CoC/PMS system was proposed [17, 54, 85, 86] Initially, the metal ions in the catalyst act as Lewis acid sites, combining with water to form hydroxyl groups on the CoC surface The HSO5 − species then binds to these hydroxyl groups via hydrogen bonding The O–O bond in HSO5 − is subsequently cleaved as it accepts electrons discharged from active Co 2+ sites, generating SO4 ●− and oxidizing
Co 2+ to Co 3+ The standard electrode potential of Co 3+ /Co 2+ (1.81 V) is higher than that of HSO5 −/SO5 ●− (1.1 V), making the reduction of Co 3+ back to Co 2+ by HSO5 − thermodynamically favorable This regenerates the weaker oxidizing species SO5 ●−
Co 2+ + H2O CoOH + + H + CoOH + + HSO5 − → CoO + + H2O + SO4 ●−
Co 3+ + HSO5 − + OH − → Co 2+ + SO5 ●− + H2O
Additional reactive oxygen species, such as ● OH and 1 O2, can also be generated through reactions involving SO4 ●−, HSO5 −, and SO5 2−
HSO5 − → SO5 2− + H + HSO5 − + SO5 2− → HSO4 − + SO4 2− + 1 O2
The reversible Co 3+ /Co 2+ redox cycle, combined with the ability of the biochar support to facilitate electron transfer, was expected to contribute to the stability and high efficiency of the CoC catalyst in activating PMS Ultimately, these ROSs mediate the degradation of the model pollutant RhB to intermediates and final mineralization products of CO2, H2O, and SO4 2−
●OH, 1 O2, SO4 ●− + RhB → intermediates + SO4 2− → CO2 + H2O + SO4 2−
3.3.8 Reusability and applicability of the catalyst in wastewater treatment
The efficiency of RhB removal in the CoC/PMS system decreased from 86% to 56% over 8 cycles, as depicted in Figure 3.20 This decline can be attributed to two primary factors Firstly, RhB or its intermediates may adsorb onto the CoC surface, occupying active sites and impeding PMS reactivation Secondly, there was minor leaching of Co from the catalyst during the washing and drying processes after each cycle, leading to a gradual reduction in catalytic activity However, even after 8 cycles, the RhB removal efficiency remained higher than that of CoC alone or PMS alone, demonstrating the superior reusability of the CoC material SEM images in Figure 3.21 show no significant morphological changes after 8 cycles of use, except for clear cracks on the surface compared to the initial catalyst
Figure 3.20 Reusability study of the CoC/PMS system
([RhB] = 50 mg/L, [catalyst] = 0.3 g/L, [PMS] = 150 mg/L; pH 6.5; T = 30 °C)
Figure 3.21 The change of catalyst morphology after 8 cycles
The performance of the CoC catalyst was evaluated for the degradation of several organic dyes, including methyl orange, congo red, and RhB with the chemical structure shown in Figure 3.22
Figure 3.22 Chemical structure of Methyl orange, Congo red, and Rhodamine B
CONCLUSIONS AND RECOMMENDATIONS
The study successfully synthesized cobalt-doped biochar derived from durian shells (CoC) using hydrothermal and pyrolysis methods, confirming its composition and physicochemical properties through various analytical techniques The catalyst, treated at a pyrolysis temperature of 350 °C with an initial cobalt concentration of 2 mmol/g, demonstrated superior efficiency in activating peroxymonosulfate (PMS) for degrading Rhodamine B (RhB), achieving removal efficiencies of 93% and 74% at initial RhB concentrations of 25 mg/L and 75 mg/L, respectively Factors affecting reaction conditions such as catalyst dosage, PMS concentration, initial RhB concentration, pH levels, temperature, and the presence of specific anions were systematically investigated Higher removal rates were observed with increased catalyst and PMS dosages, while higher initial RhB concentrations resulted in lower removal efficiencies Optimal PMS activation occurred at pH 7, achieving maximum RhB degradation of 90% The presence of anions like Cl – , HCO3 –, CO3 2–, and SO4 2– influenced the efficiency of RhB removal, with RhB exhibiting the highest removal efficiency among tested dyes, followed by Methyl Orange and Congo Red RhB degradation followed a pseudo-first-order kinetic model with an activation energy of 46.52 kJ/mol, highlighting the synergistic effect of Co3O4 and the biochar matrix in PMS activation Quenching tests identified sulfate radicals (SO4 ●–), hydroxyl radicals ( ● OH), and singlet oxygen ( 1 O2) as crucial reactive species for effective RhB degradation Furthermore, the CoC catalyst demonstrated robust stability and recyclability, maintaining a removal efficiency above 50% over eight consecutive cycles The catalyst exhibited effective removal capabilities across several dye types, achieving efficiencies of 44% for RhB, 63% for Methyl Orange, and 86% for Congo Red
Moving forward, future research should focus on evaluating the performance of the synthesized catalyst under real environmental conditions using analytical methods such as total organic carbon (TOC), biochemical oxygen demand (BOD5), and chemical oxygen demand (COD to evaluate the effectiveness of water treatment processes Additionally, optimization of the catalyst for practical applications is essential to enhance its efficacy in environmental remediation processes These efforts will significantly contribute to advancing the practical applicability of the CoC catalyst for sustainable water treatment solutions
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APPENDIX 1 Elemental mapping of CoC by EDX
APPENDIX 2 Elemental mapping of a catalyst with different initial cobalt concentrations by EDX
APPENDIX 3 N2 adsorption-desorption isotherms of CoDA and CoC-350
APPENDIX 4 pH-dependent UV-visible spectra of RhB dye
APPENDIX 5 The calibration curve of RhB in aqueous solution
APPENDIX 6 The calibration curve of CR in aqueous solution
APPENDIX 7 The calibration curve of MO in aqueous solution.