Introduction
Research significance
Antibiotics play a crucial role in treating infectious diseases in humans and animals, with applications in preventive medicine and as growth promoters in agriculture (Binh et al., 2018) Their use has significantly reduced mortality and morbidity rates associated with common infections However, the extensive production and application of antibiotics have led to their widespread presence in the environment (Kümmerer, 2009b) They have been found in various aquatic environments, including lakes, rivers, and even treated drinking water Alarmingly, even at low concentrations, these antibiotics can contribute to the development of antibiotic resistance in the environment (Kümmerer, 2009a; Yu et al.).
2019) Therefore, antibiotics are considered as significant emerging environmental pollutants (Hu et al., 2019)
Water pollution from the improper disposal of antibiotics, particularly ciprofloxacin (CIP), has become a significant issue in recent years (Yu et al., 2019) CIP, commonly used to treat mild-to-moderate urinary and respiratory infections, poses risks to public health and aquatic ecosystems when it contaminates water sources Its presence in drinking water can lead to symptoms such as nervousness, nausea, and vomiting, while also contributing to the rise of antibiotic-resistant bacteria (Ahmadzadeh et al., 2017; Mandal et al., 2012) Therefore, effective degradation of CIP residues in water is essential Unfortunately, traditional wastewater treatment methods like activated sludge and trickling filters often fail to completely eliminate antibiotics due to their low biodegradability (Yu et al., 2019).
Advanced oxidation processes (AOP) utilizing photocatalysts have gained significant attention for their ability to generate reactive oxygen species, such as hydroxyl radicals and superoxide radicals, which can effectively decompose persistent organic pollutants into carbon dioxide and water A major challenge in this field is the efficient separation and recovery of photocatalysts post-use Recent advancements have highlighted the use of spinel ferrites, particularly magnesium ferrite nanoparticles (MgFe2O4 NP), which offer magnetic recovery capabilities, making them easier to retrieve MgFe2O4 NP is particularly appealing due to its narrow band gap of 2.0 eV, magnetic properties, and absence of toxic metals To further improve the photocatalytic efficiency of MgFe2O4, researchers are exploring the combination of this material with tungsten oxide (WO3) and reduced graphene oxide (rGO), leading to the development of the MgFe2O4/WO3/rGO nanocomposite.
Research novelty
Recent studies on nanocomposites for chemical industrial wastewater (CIP) treatment have highlighted limitations, including reliance on UV irradiation and the inability to recover materials using magnetic forces (Chen et al., 2019; Costa et al., 2021; Malakootian et al., 2019; Tamaddon et al., 2020) This research addresses these gaps by developing an advanced photocatalytic system at the lab scale, which effectively degrades CIP from aqueous solutions Key features of this system include the simultaneous generation of potent reactive oxygen species (HO• and O2 -•), a reduced recombination rate of photogenerated holes (h +) and electrons (e -) during advanced oxidation processes (AOP), efficient operation in the visible light spectrum, and the capability for post-use separation through external magnetic forces.
Research objectives
This research involves four main objectives, including:
This study explores the key factors affecting the removal of ciprofloxacin (CIP) using the MgFe2O4/WO3/rGO nanocomposite It examines how variables such as pH, catalyst dosage, and initial CIP concentration influence the efficiency of CIP removal The research aims to identify optimal conditions for maximizing removal efficiency.
• Study of photodegradation kinetics and mechanism by radical scavengers
• Investigation of stability and regeneration of MgFe2O4/WO3/rGO nanocomposite.
Thesis structure
This thesis is composed of 5 chapters The main contents of these chapters are presented below:
Chapter 1 introduces the research context and significance The novelty, objectives, and scope of the research are highlighted The first chapter closes with the thesis outline
Chapter 2 provides information on the sources, occurrence, negative impacts on the aquatic environment, and existing treatment methods of CIP Especially, AOP by photocatalysts in general and nanocomposites in particular are focused with their merits and demerits as well as possible pathways to overcome the bottlenecks
Chapter 3 demonstrates the materials, instruments and procedures utilized in this investigation The details of experiments are presented, including chemical preparation, experimental setup, analytical methodologies, and experimental instruments
Chapter 4 presents the main research results, including characteristics of the synthesized MgFe2O4/WO3/rGO nanocomposite, influential factors, treatment performance of CIP, photodegradation mechanism and kinetics
Chapter 5 summarizes the major research findings Additionally, it ends with recommendations for future research directions h
Literature review
Ciprofloxacin pollution in aquatic environment
Ciprofloxacin (CIP), a fluorinated quinolone resembling nalidixic acid, is known for its high bioavailability, effective tissue penetration, and minimal adverse effects It is primarily utilized in treating urinary tract infections and prostatitis, while also being effective against bacterial enteric infections, biliary tract infections, sexually transmitted diseases, and for preventing infections in neutropenic patients (Sharma et al., 2010).
DNA gyrase inhibition is the primary mechanism through which ciprofloxacin (CIP) exerts its antibacterial effects The presence of a fluorine atom at the C6 position and a piperazine group at the C7 position significantly boosts CIP's activity against both Gram-negative and Gram-positive bacteria.
Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus (Zhang et al.,
CIP has two pKa values of 5.90 and 8.89, with an isoelectric point of 7.4, where the molecules exist as zwitterions, possessing both anionic and cationic functional groups At pH levels above 8.89, CIP molecules lose a hydrogen ion, resulting in an anionic form characterized by the -COO- radical Conversely, at pH levels below 5.90, the secondary amine radical -NH gains an additional proton, transforming into the -NH2+ form.
, leading to the cationic form (Jalil et al., 2015)
Figure 2.2 Species of CIP at different pH values
2.1.2 Occurrence of CIP in aquatic environment
A study by Binh et al (2018) examined the presence of Ciprofloxacin (CIP) in Vietnam across five key sectors: aquaculture, husbandry, hospitals, pharmaceutical manufacturing, and household use In aquaculture, CIP is approved for limited use, particularly during the larvae stage, while it is banned in husbandry and allocated through a bidding system in hospitals by the Ministry of Agriculture and Rural Development (MARD) Despite the ban on fluoroquinolones, including CIP, since 2014, their stability in water and sediment has led to extensive use in aquaculture (Le & Munekage, 2004) In hospitals, CIP is one of seven quinolone antibiotics available through a pharmaceutical bidding system, and significant quantities of quinolone antibiotics, particularly CIP, were imported during 2014-2016, as reported by the Vietnam Customs Department.
Figure 2.3 The amount of antibiotics imported into Vietnam in the period of 2014-2016
Figure 2.4 Sources of CIP in aquatic medium
Ciprofloxacin (CIP) concentrations in aquatic environments in Vietnam have raised concerns, with sludge containing 2.42 mg/kg of CIP Kümmerer (2009b) demonstrated that CIP is not biodegradable, indicating persistent environmental toxicity In a study by Binh et al (2018), CIP was found in wastewater and surface water from 178 aquaculture sites, including shrimp farms and fishponds linked to livestock operations Hospital wastewater analysis revealed CIP in all 370 samples, with peak concentrations of 87.3 µg/L in influent and 53.3 µg/L in effluent, marking the highest recorded levels in Vietnam (Lien et al., 2016) Notably, CIP concentrations in contaminated rivers in Vietnam (98.6 ng/L) surpassed those found in the United States (30 ng/L) (Binh et al., 2018; Kolpin et al., 2002) Furthermore, CIP-resistant bacteria were identified at all sampling locations, with resistance rates between 0.1% and 15% (Takasu et al., 2009).
Table 2.1 Antibiotics and their respective highest concentration in Vietnam
Source of antibiotics (highest concentration in ng/L)
2.1.3 Negative impacts of CIP on aquatic medium
The presence of ciprofloxacin (CIP) in water, even at low concentrations, poses significant environmental risks, as conventional wastewater treatment methods like activated sludge and trickling filters fail to effectively eliminate it, leading to contamination of surface water, soil, and groundwater Exposure to CIP in drinking water has been associated with various health issues, including nervousness, nausea, vomiting, headaches, diarrhea, and tremors, while higher concentrations can result in severe conditions such as acute renal failure, thrombocytopenia, elevated liver enzymes, eosinophilia, and leucopenia.
The presence of CIP in water sources contributes to the rise of antibiotic-resistant bacteria, posing a significant public health threat This situation necessitates urgent actions from both government and society The transfer of bacterial resistance to humans can occur through contaminated water or food, particularly when sewage sludge is utilized as fertilizer, surface water irrigates crops, or manure fertilizes livestock.
In 2009, Kümmerer identified two primary mechanisms for the transfer of antibiotic resistance genes between organisms The first, known as vertical resistance transfer, occurs naturally through cell division among organisms of the same species The second mechanism, horizontal resistance transfer, is induced by the presence of antibiotics in aquatic environments, leading to the development of resistance genetic material that is shared among different species through conjugation.
Wastewater serves as a critical environmental reservoir for antibiotic-resistant bacteria, creating nutrient-rich conditions that facilitate horizontal gene transfer, particularly through plasmids and transposons This makes wastewater treatment facilities hotspots for the proliferation of antibiotic resistance, as they harbor both resistant and susceptible bacterial strains Given the significant health risks posed by ciprofloxacin-resistant bacteria in these treatment plants, effective strategies for treating ciprofloxacin in wastewater are urgently needed Collaborative action from government sectors and communities worldwide is essential to address this pressing issue.
Recent research highlights critical insights into antibacterial resistance: (1) Utilizing a single antibacterial agent can lead to resistance not only to that specific drug but also to various others, a phenomenon known as cross-resistance, and (2) The degree of antibacterial resistance does not necessarily correlate with the quantity of drugs used or the environmental concentrations of these compounds (Weston, 1996).
Methods for treatment of CIP in aquatic medium
Conventional wastewater treatment processes, such as activated sludge and trickling filters, have proven ineffective for the treatment of ciprofloxacin (CIP) (Ahmadzadeh et al., 2017) Furthermore, CIP does not biodegrade under aerobic conditions (Kümmerer, 2009b) Therefore, it is essential to develop advanced techniques for the effective removal of CIP from water sources.
Various methods have been developed for the removal of ciprofloxacin (CIP) in water, including adsorption, membrane bioreactors, ozonation, solid polymer electrolytes, ultrasound irradiation, and photocatalytic degradation (Nguyen et al., 2020) As shown in Table 2.2, all these methods demonstrate promising efficacy in CIP removal Notably, photocatalytic degradation using photocatalysts is preferred over other techniques due to its affordability, simplicity, and environmentally friendly apparatus.
(2) possible working under natural irradiation (sunlight), and (3) effective degradation of CIP h
Table 2.2 Methods for removal of CIP from aqueous solutions
Method Removal Efficiency (RE) Advantage Disadvantage Ref
Adsorption Maximum adsorption capacity by
MPC800 (up to 90.0 mg/g) at CCIP 5 ppm, dosage = 0.1 g/L and pH = 4
• High removal rate • Many influential factors (pH, organic matter, and mineral content in soil) and behavior of antibiotic (molecular structure, functional groups)
• Impossible degradation of pollutants into less toxic products
& Al- Tamimi, 2019; Van Tran et al.,
• Hospital WW, initial concentration 1.926-23.841 𝜇𝑔/𝐿, RE 76-93% (flat sheet);
• Insignificant effects of temperature and initial concentration variation
• Substantial effects of high concentrations of organic substances
(Alonso et al., 2018; Nguyen et al., 2017)
• Possible ecotoxic by- products (Li et al.,
(7.91 mg/L); RE up to 98.7% under the optimum pH 9 within 30 min and 3g of O3/h
CIP in tap water: RE up to 91.36 % within 20 min under 1.16A of electric current, 520 rpm of stirring rate and 40kHz of ultrasound irradiation
• Many influential factors (e.g., humic acids and ions) (Tasca et al., 2020)
RE nearly 100% during treatment of
5 ppm CIP solution by 0.5 g/L P- doped TiO2 with surface oxygen vacancies under visible light region
• Possible operation under sunlight and visible light irradiation
• Effective oxidation of organics into CO2 & H2O
• High cost of electricity if UV- lamp is used (Costa et al., 2021; Feng et al.,
2.2.2 Advanced Oxidation Processes (AOP) by photocatalyst
Advanced Oxidation Processes (AOP) utilizing photocatalysts are effective in breaking down persistent organic compounds into harmless substances like CO2 and H2O When light of the appropriate wavelength is absorbed by the photocatalyst in water, electrons are excited from the valence band (VB) to the conduction band (CB), generating holes in the VB These holes can oxidize water or hydroxide ions to produce hydroxyl radicals (HO•) if the VB's redox potential ranges from 1.0 to 3.5 V versus the standard hydrogen electrode (SHE) Simultaneously, photogenerated electrons in the CB can reduce dissolved oxygen to create superoxide radicals (O2 -•) when the CB's redox potential is between +0.5 to 1.5 V versus SHE Under optimal conditions, superoxide radicals can react with hydrogen ions to form hydroperoxyl radicals (HOO•), which quickly decompose into hydroxyl radicals (HO•) The high redox potential of hydroxyl radicals (𝐸 𝐻𝑂 /𝑂𝐻 − = 2.80 V vs Normal hydrogen electrode (NHE)) enhances their effectiveness in oxidative processes.
O2 -• (𝐸 𝑂 2 /𝑂 2 − = -0.33 V vs NHE), they are able to oxidize organic pollutants to form simpler molecules, such as CO2 and H2O (Suresh et al., 2021) The equations from these steps are:
Step 1: Generation of charge carriers by absorbing light
𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 + ℎ𝑣 → 𝑃ℎ𝑜𝑡𝑜𝑐𝑎𝑡𝑎𝑙𝑦𝑠𝑡 (𝑒 − + ℎ + ) Step 2: Oxidation of H2O or OH - at VB
𝑂𝐻 − + ℎ + → 𝐻𝑂 Step 3: Reduction of dissolved oxygen at CB
𝑂 2 + 𝑒 − → 𝑂 2 − Step 4: Neutralization of O2 ∙- and the decomposition of intermediate
Figure 2.5 Schematic diagram of AOP
Many studies have applied photocatalysts in degradation of CIP Malakootian et al
In 2019, research demonstrated that a 1.0 g/L ZnFe2O4@CMC effectively degraded a 5 ppm CIP solution, achieving 87% efficiency in darkness after 30 minutes and reaching 100% degradation under UV light within 100 minutes Similarly, Chen et al (2019) found that 0.5 g/L biochar@ZnFe2O4/BiOBr could degrade 84% of a 15 ppm CIP solution when exposed to 300W Xe lamp irradiation for 60 minutes.
In a study conducted by Tamaddon et al (2020), the nanocomposite CuFe2O4@MC demonstrated a remarkable removal efficiency of 81% for a 3ppm CIP solution after 90 minutes of UV-C irradiation, with an optimal concentration of 0.67g/L The research highlighted that the highest removal efficiency was consistently achieved at a pH of 7, which aligns with the typical pH levels found in wastewater Additionally, the studies investigated spinel ferrite nanocomposites, revealing their effectiveness in CIP degradation and the ability to be recovered using external magnetic forces.
Introduction of MgFe 2 O 4 nanoparticle
h disciplines of sciences, such as material science, medicine, agriculture, pharmacy (Mmelesi et al., 2020)
Spinel ferrites, a type of magnetic nanoparticle, have garnered significant attention due to their remarkable properties and diverse applications in sensors, biomedicine, catalysis, and energy storage devices Their unique characteristics, such as superparamagnetism, narrow band-gap responsiveness to visible light, and high resistance to heat and corrosion, make them a focal point of research Consequently, there has been a growing body of studies exploring spinel ferrite nanocomposites and their potential applications.
Spinel ferrite, characterized by the general formula AB2O4, consists of metallic cations A and B, with the essential presence of Fe3+ cations in its crystalline structure Within a unit cell, oxygen anions are arranged in a cubic closed packing, while cations A and B occupy the tetrahedral and octahedral sites, respectively Spinel ferrites can be classified into normal, inverse, and mixed varieties based on the arrangement of 2+ and 3+ cations within the lattice (Ren et al., 2016).
Table 2.3 The positions of cations, formula and example of normal, inverse and mixed spinel ferrites (Mmelesi et al., 2020; Ren et al., 2016)
Category Positions of cations Formula Example
• 2+ ions locate at all the tetrahedral sites
• 3+ ions locate at all the octahedral sites
• Half of the 3+ ions locate at all the tetrahedral sites
• Both 2+ and half of 3+ locate at octahedral sites
Mixed • 2+ and 3+ ions are distributed randomly on both sites [M1-xFex][MxFe2-x]O4 MnFe2O4 h
Figure 2.6 Diagram of spinel ferrite demonstrating tetrahedral (yellow), octahedral
(green) and oxygen atoms (red) units
Magnesium ferrite nanoparticles, a notable example of spinel ferrites, have garnered significant attention due to their narrow band-gap of 2.0 eV, impressive magnetic recovery properties, and the absence of toxic metals These MgFe2O4 nanoparticles can effectively absorb visible light, facilitating the excitation of electrons from the O-2p level in the valence band to the Fe-3d level in the conduction band Additionally, the conduction band edge (ECB) of MgFe2O4 is -1.40 eV, enabling the reduction of dissolved oxygen to generate reactive oxygen species (O2 ∙-) for advanced oxidation processes (AOP).
Recent studies have examined the photo-degradation capabilities of MgFe2O4 and its derivatives According to George et al (2021), 1 g/L of Cu–MgFe2O4 achieved a remarkable 97% removal of 25 ppm methylene blue solution after 180 minutes of UV-lamp irradiation Additionally, research by Van Tran et al (2021) demonstrated that nearly 100% degradation of a 10-ppm solution was achieved, highlighting the effectiveness of these materials in photodegradation processes.
The chemical synthesis of spinel ferrites is the predominant method utilized, encompassing various techniques such as hydrothermal, co-precipitation, sol-gel auto-combustion, and solid-state reactions Each of these synthesis methods has distinct descriptions, advantages, and disadvantages, which are detailed in Table 2.4.
The co-precipitation method stands out as a promising technique for synthesizing MgFe2O4 due to its simplicity, cost-effectiveness, and suitability for large-scale production Numerous studies have successfully utilized this method for MgFe2O4 synthesis, including research by Akbari et al (2017), Aliyan et al (2017), Chen et al (1999), George et al (2021), Hwa et al (2020), Naaz et al (2020), Tran et al (2020), and Van Tran et al (2021).
Table 2.4 Chemical synthesis methods of spinel ferrites
(Qin et al., 2021; Wang & Chen, 2018)
• Using water as carrier and requiring high temperature at high pressure in closed system
• High crystallinity without further high temperature annealing process
• More complicated system and instrument requirement
• Simple and cost- effective pathway
• After mixing two or more kinds of cations in solution, the targeted material can be obtained after precipitation and calcination
• Precursors containing • Uneasy control of synthesis h stirring and alkaline conditions
• The gel then is dried and calcined at 450-800 o C
• Grinding the iron and metal salts to powders at high temperatures
• Difficulty in monitoring of the operation process
• Difficulty in determination of the optimal condition
• Difficulty in separation of the pure product from the mixture of reactants h
Introduction of WO 3 nanoparticle
Tungsten trioxide (WO3) is a highly researched visible-light material known for its effectiveness in water purification Its diverse applications include gas sensors, photochromic devices, electrochemical systems, smart windows, paint manufacturing, and environmental photocatalysis, particularly in hydrogen production through water splitting (Vasudevan et al., 2018).
WO3 is an n-type metal oxide semiconductor with a temperature-dependent crystalline structure (Figure 2.7) Between 180 and 900 o C, WO3 exists in five crystalline phases: monoclinic II (ε-WO3, < -43 o C), triclinic (δ-WO3, -43-17 o C), monoclinic I (γ-WO3, 17–
330 o C), orthorhombic (β-WO3, 330–740 o C), and tetragonal (α-WO3, > 740 o C) (Peleyeju
Figure 2.7 Crystallite structure of WO3 at different temperature
WO3 is an effective visible-light-driven photocatalyst with a bandgap (Eg) ranging from 2.36 to 3.20 eV, corresponding to wavelengths between 387.5 and 525 nm, which varies based on its crystallite structure The electron configuration of W(VI) is [Xe] 4f14 5d0 6s0, with the conduction band (CB) derived from the 5d orbitals of tungsten This structure facilitates the conversion of water into hydroxyl radicals (HO•) Due to the strong oxidizing power of holes in the valence band (VB), WO3 is extensively utilized in photocatalytic research (Paula et al., 2019; Riboni et al., 2013).
Numerous studies have explored the photo-degradation potential of WO3 and its derivatives Liang et al (2019) demonstrated that WO3 achieved complete removal of 25 ppm methyl orange solution within 120 minutes when exposed to a 500 W Xe lamp Additionally, Mao et al (2018) found that a 0.54 ppm solution of Aflatoxin B1 was degraded by 92.4% using a WO3/rGO/g-C3N4 composite after 120 minutes under a 300 W Xe lamp.
Various synthesis methods for tungsten trioxide (WO3) include hydrothermal, calcination, and precipitation techniques Among these, calcination stands out as the most straightforward and time-efficient approach Notably, ammonium metatungstate hydrate, represented as (NH4)6H2W12O40·xH2O, can be effectively calcined to produce WO3 (Dozzi et al., 2016; Hunyadi et al., 2014; Vu et al., 2022).
Introduction of reduced graphene oxide (rGO)
Carbon-based materials are highly valued in various fields due to their accessibility, persistence, and multi-dimensional existence, particularly in photocatalysis (Suresh et al., 2021) Graphene, a notable nano-carbon material, exhibits exceptional properties, with graphene oxide (GO) being distinguished by its functional groups like epoxy, hydroxyl, and carboxyl GO, made from exfoliated multi-graphene sheets, has an extremely high theoretical surface area, enhancing its sorption capabilities (Nasrollahzadeh et al., 2020) Synthesized from graphite precursors using strong oxidants, GO offers high yield and cost-effectiveness (Nidheesh, 2017) Its multiple polar functional groups render it highly hydrophilic and electrically insulating, complicating recovery post-treatment In contrast, reduced graphene oxide (rGO) features a larger surface area with fewer surface groups, making it more suitable for photocatalytic applications rGO is characterized by high thermal stability, good conductivity, corrosion resistance, and recoverability, positioning it as a promising support material for photocatalysts (Nasrollahzadeh et al., 2020; Suresh et al., 2021).
Figure 2.8 Chemical structure of GO
Reduced graphene oxide (rGO) significantly improves photocatalytic performance through three key mechanisms Firstly, the electronic coupling between the π states of rGO and the conduction band of the photocatalyst facilitates the transfer of photogenerated electrons within the nanocomposite, effectively lowering the recombination rate of charge carriers Secondly, rGO minimizes ion leaching and enhances the interaction between the photocatalyst and pollutants, leading to more efficient photocatalytic processes.
2021) Thirdly, rGO involves high surface area, which increases the number of active sites for the nanocomposite and thereby improves the photocatalytic efficiency (Nasrollahzadeh et al., 2020)
Graphene oxide (GO) can be synthesized from graphite and then reduced to obtain reduced graphene oxide (rGO) Various synthesis methods for GO are detailed in Table 2.5, while Table 2.6 outlines different reduction techniques used to produce rGO.
Table 2.5 Different method for synthesis of GO
(Alam et al., 2017; Botas et al., 2013; Habte & Ayele, 2019; Poh et al., 2012; Zaaba et al., 2017)
Intensive chemical requirement for washing step h
Oxidizes graphite by KMnO4, NaNO3 in concentrated H2SO4
Generation of toxic gases (e.g., NO2,
Based on Hummer’s method, modification: no usage of NaNO3, increase KMnO4 in
Higher oxidation efficiency than Hummer’s method
Intensive consumption of concentrated acid
Table 2.6 Different methods for synthesis of rGO
(Alam et al., 2017; Bosch-Navarro et al., 2012; Chua & Pumera, 2016; Jakhar et al.,
2020; Mei et al., 2015; Vu et al., 2022; Yang et al., 2015)
Most popular method, using L-ascorbic acid, NaBH4, N2H4
High reduction efficiency in a short time
Utilization of toxic chemicals (e.g., NaBH4 & N2H4)
Reduce GO in high temperature & vacuum, or inert, or reducing atmosphere
Reduce GO in high temperature and high pressure in seal container
High oxidation efficiency, simple operation and mild synthesis condition
Energy consumption for long heating time
Utilizing microwave radiation to increase the instantaneous internal temperature to reduce GO
Short time and improved efficiency
Overview of Z-scheme photocatalytic system
Conventional photocatalysts face a significant challenge due to the rapid recombination of photogenerated electrons and holes, occurring within just 10 to 100 nanoseconds To address this issue, researchers have employed various strategies, including the use of photogenerated electron traps like reduced graphene oxide (rGO) A prominent approach that has emerged is the development of Z-scheme photocatalytic systems, which have evolved through three distinct generations, as illustrated in Figure 2.9 and detailed in Table 2.7.
Figure 2.9 Historical progression of Z-scheme photocatalytic system
Table 2.7 Comparison of three generations of Z-scheme photocatalytic system
Liquid-phase Z- • Combination of two photocatalysts with a h electron acceptor/donor pair under light irradiation
The use of commonly utilized electron acceptor/donor pairs, such as Fe2+/Fe3+, can interfere with the reactants involved in reduction and oxidation reactions Additionally, the application of these processes is restricted to the liquid phase, which limits their practical use in various applications.
All-solid-state Z- scheme photocatalytic system
• Combination of two photocatalysts with a noble-metal nanoparticles as electron mediator
• Less interference with reactants than 1 st generation Z-scheme system
• Disadvantages: the limitation in practical applications due to the rarity and high cost of noble metals
• Combination of two photocatalysts, which directly contact at interface without addition of electron mediator
• Inheritance of all advantages and overcoming disadvantages related to electron mediator in comparison with previous generations
• Simultaneously effective formation of both HO• and O2 -• and efficient reduction in recombination of charge carriers
• Enormous study for various photocatalytic applications
The direct Z-scheme system utilizes two semiconductor photocatalysts, where one features a low valence band (VB) position (EVB > 2.80 V) for hydroxyl radical (HO•) generation, and the other has a high conduction band (CB) position (ECB < -0.33 V) for superoxide radical (O2 -•) formation Under appropriate light irradiation, electrons in the VB of photocatalyst A are excited to its CB, then transferred to the VB of photocatalyst B before moving to its CB This innovative direct Z-scheme approach significantly reduces the recombination of charge carriers in both photocatalysts, enhancing their photocatalytic efficiency (Vu et al., 2022).
Figure 2.10 Mechanism of a direct Z-scheme photocatalytic system
MgFe 2 O 4 /WO 3 /rGO nanoparticle as a direct Z-scheme photocatalytic system 28 2.8 Conclusion of literature review
The ECB of MgFe2O4 is -1.40 V, enabling the reduction of dissolved oxygen to form superoxide radicals (O2 -•) for advanced oxidation processes (AOP) To improve the photocatalytic activity of MgFe2O4, it is essential to address two key challenges: first, the valence band (VB) potential of MgFe2O4 is lower than 2.72 V; second, there is a rapid recombination of photogenerated electrons and holes occurring within a mere 10 to 100 nanoseconds (Zhang et al., 2018b).
To overcome the first challange, WO3 nanoparticles should be added The reasons are
(1) the band-gap energy (Eg) of WO3 is 2.84 eV, suggesting that WO3 can absorb the visible light; and (2) EVB of WO3 is 3.35 V, which is higher than 2.72V, implying that
WO3 can oxidize H2O and OH - to form hydroxyl radicals (Ghattavi & Nezamzadeh- Ejhieh, 2021) The coupling of MgFe2O4 with WO3 is able create a direct Z-scheme h
The presence of MgFe2O4 in the system effectively prevents the recombination of electrons and holes in semiconductors Electrons in the conduction band of MgFe2O4 facilitate the reduction of dissolved oxygen, generating O2 -• radicals Meanwhile, holes in the valence band of WO3 oxidize water, producing HO• radicals These HO• and O2 -• radicals play a crucial role in the degradation of organic compounds, ultimately converting them into CO2 and H2O.
The incorporation of reduced graphene oxide (rGO) addresses the challenge of charge carrier recombination by acting as a photogenerated electron trap, thereby enhancing the efficiency of photocatalysts Additionally, rGO's high surface area promotes greater interaction between the photocatalyst and pollutants, leading to an increased number of active sites within the nanocomposite.
The MgFe2O4/WO3/rGO nanocomposite is anticipated to establish a Z-scheme that effectively generates reactive oxygen species (HO• and O2 -•), minimizes the recombination of photogenerated holes (h+) and electrons (e-), operates efficiently under visible light, and allows for easy separation post-use through external magnetic forces.
Water pollution from antibiotics, particularly ciprofloxacin (CIP), has become a significant issue due to increased consumption and improper disposal CIP is commonly used to treat mild-to-moderate urinary and respiratory infections, but its presence in drinking water can lead to adverse health effects such as nervousness, nausea, and vomiting Furthermore, CIP contamination contributes to the emergence of antibiotic-resistant bacteria, with wastewater treatment facilities serving as critical hotspots for the transfer of antibiotic resistance Therefore, effectively degrading CIP residues in water resources is essential for protecting public health and aquatic ecosystems.
Traditional technologies like activated sludge and trickling filters struggle to completely remove antibiotics due to their low biodegradability To address this issue, various effective antibiotic treatment methods have emerged, such as adsorption, membrane bioreactors, ozonation, solid polymer electrolytes, ultrasound irradiation, and advanced oxidation processes (AOP) like photocatalytic degradation Notably, AOP utilizing photocatalysts has gained significant attention for its capacity to generate reactive oxygen species, including hydroxyl radicals (HO•) and superoxide radicals (O2 -•), which can effectively decompose stubborn organic compounds into carbon dioxide (CO2).
Recent advancements in photocatalysis have highlighted the potential of spinel ferrites, particularly MgFe2O4, due to their narrow band-gap of 2.0 eV, magnetic recovery properties, and absence of toxic metals To improve the photocatalytic efficiency of MgFe2O4, researchers have developed a nanocomposite by incorporating WO3 and reduced graphene oxide (rGO) WO3 serves as a visible-light-driven photocatalyst that facilitates the generation of hydroxyl radicals (HO•), a process that MgFe2O4 alone cannot achieve Additionally, rGO acts as a photogenerated electron trap, effectively minimizing the recombination rate of charge carriers produced by the combination of MgFe2O4 and WO3.
The MgFe2O4/WO3/rGO nanocomposite is anticipated to create a Z-scheme that effectively generates reactive oxygen species (HO• and O2 -•), minimizes the recombination of photogenerated holes (h +) and electrons (e -) during advanced oxidation processes (AOPs), operates efficiently under visible light, and allows for convenient separation through external magnetic forces.
Materials and methods
Chemicals and Apparatus
Chemicals used in the study includes:
• Magnesium chloride hexahydrate (MgCl2.6H2O) (Xilong, China)
• Ferric chloride hexahydrate (FeCl3.6H2O) (Xilong, China)
• Sodium hydroxide (NaOH) (Xilong, China)
• Ammonium metatungstate hydrate ((NH4)6H2W12O40ãxH2O) (Xilong, China)
• Sulfuric acid (H2SO4) (Xilong, China)
• Nitric acid (HNO3) (Xilong, China)
• Hydrochloric acid (HCl) (Xilong, China)
• Sodium nitrate (NaNO3) (Xilong, China)
• Absolute ethanol (C2H5OH) (Xilong, China)
• Potassium permanganate (KMnO4) (Xilong, China)
All chemicals used were Analytical Reagent grade
Apparatus used in the study includes:
The study utilized various advanced analytical instruments, including the X-ray powder diffraction (XRD MiniFlex 600) from Rigaku Corp., Fourier transformation infrared spectroscopy (FTIR 4600) by Jasco Corp., and scanning electron microscopy (SEM TM 4000 Plus) from Hitachi Corp Additionally, energy dispersive X-ray (EDX MisF+) from Oxford Instruments plc., UV–vis diffuse reflectance spectroscopy (UV-DRS UH 5300) by Hitachi Corp., photoluminescence (FluoroMax-4) from Horiba, and double beam UV-Vis spectroscopy (UH5300) from Hitachi Corp were employed to conduct comprehensive material characterization.
• Sealed Teflon lined stainless-steel autoclave
Material synthesis
The synthesis of MgFe2O4 was based on the method established by Van Tran et al (2021), involving the dissolution of 5.406 g of FeCl3.6H2O and 2.033 g of MgCl2.6H2O in 100 ml of distilled water The solution was mixed using magnetic stirring, and NaOH 5M was added dropwise until the pH reached 9-10 The mixture was then heated to 90°C and maintained for 2 hours, followed by natural cooling to room temperature After filtration and thorough washing with distilled water and ethanol until achieving a pH of 7, the precipitate was dried overnight, ground, and calcined at 900°C for 3 hours, resulting in the formation of the brown product, MgFe2O4.
The synthesis method of WO3 in this study was adapted from the research of Vu et al
(2022) Ammonium metatungstate hydrate was calcined at 450 o C for 4h to obtain WO3
The synthesis method of GO in this study followed the modified Hummer’s method h
To synthesize graphene oxide (GO), KMnO4 was initially added for pre-oxidation, with 2.7 grams introduced slowly while stirring at a temperature between 10 and 20°C for two hours to achieve a dark green suspension The temperature was then increased to 30-35°C for another two hours, resulting in a brown suspension Following this, 23 ml of distilled water was added dropwise while maintaining the temperature at 90-95°C for 30 minutes An additional 120 ml of distilled water was introduced, and the reaction mixture was cooled to 50°C Subsequently, 10 ml of a 30% H2O2 solution was added, stirring until the color shifted from brown to light yellow The mixture was then washed with 5% HCl and distilled water until a pH of approximately 6 was achieved, and finally, it was dried overnight to yield GO.
3.2.4 Synthesis of MgFe 2 O 4 /WO 3 /rGO nanocomposite
This study presents a modified synthesis method for the MgFe2O4/WO3/rGO nanocomposite, adapting techniques from the Fe3O4/WO3/rGO synthesis (Guo et al., 2021) Key modifications include the synthesis processes for MgFe2O4 and WO3, simultaneous coupling of MgFe2O4 and WO3 with GO layers, and the hydrothermal reduction of GO to rGO using distilled water as a solvent instead of glycol Additionally, the hydrothermal temperature was adjusted from 200 °C to 180 °C to align with available laboratory equipment The hydrothermal reduction of GO to rGO using water at 180 °C has also been documented by Bosch-Navarro et al (2012).
In this study, 0.012 grams of graphene oxide (GO) were fully dispersed in 50 ml of distilled water using an ultrasonic sonicator Following this, 0.3 grams of MgFe2O4 and 0.1 grams of WO3 were added at room temperature The resulting mixture was stirred and ultrasonicated for two hours before being transferred to a Teflon-sealed autoclave, where it reacted at 180 °C for six hours Finally, the MgFe2O4/WO3/rGO nanocomposite was collected, washed with distilled water and ethanol, and dried at 50 °C for six hours.
Material characterization method
X-ray Diffraction (XRD) is used to determine the crystal structure, phase composition, and size of crystalline substances The XRD method involves illuminating a sample with an X-ray beam and then analyzing the scattered beam The scattering angle is dependent h on the X-ray wavelength, crystal orientation, and atomic plane spacing, and can therefore be used to characterize structure of the material (Brundle et al., 1992)
Figure 3.1 Operation of XRD method
The distance between crystal planes is denoted as d, and an X-ray beam directed at the crystal forms an angle θ with these planes When X-rays penetrate the crystal lattice, the reflection from two consecutive planes creates a distinct optical effect.
The XRD instrument operates in accordance with Bragg's law of reflection:
𝜆: wavelength of the X-ray beam d: distance between parallel crystal plane
𝜃: angle made by the incident X-ray beam and scatter plane
The XRD analysis of the fabricated nanocomposite was conducted using the XRD MiniFlex 600 from Rigaku Corp., Japan, which employs CuKα radiation (λ=0.154 nm) The scanning was performed with a step size of 0.02° across a 2θ range of 10° to 80° at the VNU Key Laboratory for Advanced Materials for Green Growth (KLAMAG).
Figure 3.2 XRD MiniFlex 600, Rigaku Corp
Fourier-transform infrared spectroscopy (FT-IR) is a powerful technique for determining molecular structures across gas, liquid, and solid states by analyzing characteristic infrared radiation absorption The molecular vibrational spectrum, derived from the infrared spectrum, occurs when sample molecules selectively absorb specific wavelengths of infrared radiation, leading to changes in their dipole moments and transitions from ground to excited vibrational energy states The frequency of the absorption peaks is influenced by the vibrational energy gaps, while the number of vibrational freedoms correlates with the number of absorption peaks Additionally, the intensity of these peaks reflects the change in dipole moment and the likelihood of energy level transitions, enabling the structural analysis of molecules through their infrared spectra (Harris, 2015).
In this research, the sample was measured by FTIR 4600, Jasco Corp., Japan at the VNU KLAMAG h
Figure 3.3 FT-IR 4600, Jasco Corp
Figure 3.4 Schematic diagram of a FT-IR instrument
The scanning electron microscope (SEM) is crucial for evaluating solid objects across various fields It utilizes a focused beam of high-energy electrons to produce diverse signals from the surface of solid samples SEM provides images with a greater depth of focus compared to traditional light microscopy.
In electron microscopy, various types of electrons interact with the sample, producing images with unique characteristics Secondary and backscattered electrons are key contributors, as they are reflected from the sample upon impact, revealing important structural details (Pawliszyn, 2012; Kohli & Mittal, 2011).
In this research, SEM images of the sample was taken by SEM TM 4000 Plus, Hitachi High-Technologies Corp., Japan at the VNU KLAMAG
Figure 3.5 SEM TM 4000 Plus, Hitachi Corp
The Energy-dispersive X-ray (EDX) technique is utilized in electron microscopes to analyze the microstructure of solids through high-energy electron beam interactions When the electron beam strikes a solid, it penetrates the atom's inner electron layers, leading to the generation of X-rays These X-rays have a characteristic wavelength that is directly proportional to the atomic number (Z) of the atom, allowing for detailed elemental analysis.
The unique frequency of X-rays emitted by each substance allows for the identification of chemical elements in a solid sample By analyzing the X-ray spectrum, one can determine both the elements present and their relative quantities (Reichelt, 2007).
Figure 3.6 Fundamental principle of EDX instrument
In this research, elemental composition and electron mapping of samples were determined by EDX MisF+, Oxford Instruments plc., UK at the VNU KLAMAG
Figure 3.7 EDX MisF+, Oxford Instruments plc
3.3.5 UV–Vis Diffuse Reflectance Spectroscopy
UV–Vis Diffuse Reflectance Spectroscopy (UV-Vis DRS) is a crucial technique for calculating the band gap energy Eg of materials
This technique is based on the transition of electrons from low-energy orbitals to high- h
When a photon of sufficient energy excites semiconductor materials, the electron will jump from the VB to the CB
The energy of the material's band gap can be estimated using the Tauc plot Tauc graphs are frequently constructed utilizing Kubelka and Munk's equations (Eq.3.3) (Vu et al.,
In which: α: energy-dependent absorption coefficient, ℎ: Planck constant, 𝜈: photon’s frequency, and n: nature of electron transition
The Tauc graph illustrates the relationship between (𝛼ℎ𝑣) 𝑛 1 and ℎ𝑣, where n is either 1/2 for direct band gaps or 2 for indirect band gaps The band gap energy of a material is determined by the intersection point of the linear fit line, which passes through the inflection point of the Tauc graph, with the horizontal axis (Makuła et al., 2018).
This research involved determining the absorption spectrum of MgFe2O4, WO3, and MgFe2O4/WO3/rGO samples within the wavelength range of 200 to 800 nm The measurements were conducted using a UV-Vis DRS UH 5300 spectrophotometer from Hitachi Corp., Japan, at the VNU KLAMAG facility.
Figure 3.8 UV-Vis DRS UH 5300, Hitachi Corp h
Photoluminescence (PL) is an analytical technique used to measure the fluorescence intensity emitted by a chemical substance when it is excited by ultraviolet, visible, or other forms of electronic radiation This method allows for a comparison of the fluorescence intensity of the sample against that of a standard under identical conditions.
In this research, the recombination rate of charge carriers by PL was conducted by FluoroMax-4, Horiba at the VNU KLAMAG
3.3.7 pH point of zero charge
In heterogeneous photocatalysis, pH significantly influences the charge, size, and dispersibility of catalyst particles, thereby affecting photocatalytic activity The isoelectric point and surface charge of the photocatalyst are determined by pH values, with the point of zero charge (PZC) marking where the surface charge is neutral At this isoelectric point, electrostatic interactions between catalyst particles and pollutants are minimal When the pH is below the PZC, the catalyst surface carries a positive charge, while a pH above the PZC results in a negatively charged surface, leading to poor interaction with negatively charged compounds in water.
Prepare five conical flasks, each containing 50 ml of 0.01M NaCl solution adjusted to pH levels of 3, 5, 7, 9, and 11 To each flask, add 0.2g of MgFe2O4/WO3/rGO Place the flasks on a shaker for about 12 hours, then measure the pH values again (Tran, 2020).
To determine the point of zero charge (pHpzc) of MgFe2O4/WO3/rGO in a NaCl solution, the initial pH (pH_initial) and the pH after the addition (pH_after) are measured By plotting the change in pH (ΔpH) against the initial pH values, the pHpzc can be identified as the point where the ΔpH curve intersects the horizontal axis of initial pH.
O 4 /WO 3 /rGO
Factors influencing the CIP removal efficiency
0.5 g/L of MgFe2O4/WO3/rGO nanocomposite were added to 100 ml of 5 ppm CIP solution at various pH values of 3, 5, 7, 9 and 11 The solution was kept in the dark for h
After achieving adsorption-desorption equilibrium in 30 minutes, the solution was irradiated with a 20W white lamp for 180 minutes At 30, 90, 150, and 210 minutes, 5 ml samples were extracted, centrifuged to eliminate the nanocomposite, and the light absorbance of CIP was measured.
𝜆 𝑚𝑎𝑥 by UV-Vis spectroscopy (Double Beam Spectrophotometer UH5300, Hitachi Corp., Japan) b) Catalyst dosage
Different dosages of MgFe2O4/WO3/rGO nanocomposite, specifically 0.25, 0.50, 0.75, and 1.0 g/L, were introduced into 100 ml of a 5 ppm CIP solution at the optimal pH The mixture was allowed to sit in the dark for 30 minutes to achieve adsorption-desorption equilibrium before being irradiated with a 20W white lamp for 180 minutes Subsequently, 5 ml of the solution was extracted at 30-minute intervals for analysis.
The nanocomposite was centrifuged at intervals of 90, 150, and 210 minutes to effectively separate it, and the absorbance of ciprofloxacin (CIP) was measured at its maximum wavelength (𝜆 𝑚𝑎𝑥) using a UV-Vis spectrophotometer (Double Beam Spectrophotometer UH5300, Hitachi Corp., Japan) The initial concentration of CIP was also determined during this process.
The optimal dosage of MgFe2O4/WO3/rGO nanocomposite was added to 100 ml of ciprofloxacin (CIP) solutions with varying initial concentrations of 2.5, 5, 7.5, and 10 ppm at the optimal pH The solutions were kept in the dark for 30 minutes to achieve adsorption-desorption equilibrium before being irradiated with a 20W white lamp for 180 minutes Samples of 5 ml were extracted at intervals of 30, 90, 150, and 210 minutes, centrifuged to separate the nanocomposite, and then analyzed for light absorbance of CIP at the maximum wavelength using a UV-Vis spectrophotometer (Double Beam Spectrophotometer UH5300, Hitachi Corp., Japan).
Kinetic study
The kinetics of ciprofloxacin (CIP) removal using MgFe2O4/WO3/rGO nanocomposite was investigated through three models, focusing on optimal dosage, pH, and varying initial CIP concentrations (2.5, 5, 7.5, and 10 ppm) The study highlights the effectiveness of the nanocomposite in enhancing CIP removal rates under these specific conditions.
Co: initial concentration of CIP (ppm)
The concentration of CIP at time t (in minutes) is measured in parts per million (ppm) The rate constant for the Pseudo-zero-order model is expressed in ppm per minute (ppm.min -1), while the Pseudo-first-order model's rate constant is represented in per minute (min -1) Additionally, the rate constant for the Pseudo-second-order model is given in ppm per minute (ppm -1 min -1).
Radical scavengers
Previous research indicates that hydroxyl radicals (HO•) and superoxide anions (O2 -•) play crucial roles in photodegradation processes (Rimoldi et al., 2019; Vu et al., 2022) To determine if these species are dominant in the photodegradation of CIP, t-butanol and p-benzoquinone are typically employed as scavengers for HO• and O2 -•, respectively (Schneider et al., 2020).
In optimal conditions for the removal of ciprofloxacin (CIP), 1.5 ml of t-BuOH, a hydroxyl radical scavenger, was introduced to the reaction solution at time zero The solution was maintained in the dark for 30 minutes to allow for adsorption-desorption equilibrium before being irradiated with a 20W white lamp for 180 minutes At intervals of 30, 90, 150, and 210 minutes, 5 ml of the solution was extracted, centrifuged to eliminate particulates, and the light absorbance of CIP was measured at its maximum wavelength using UV-Vis spectroscopy.
Stability and recyclability of MgFe 2 O 4 /WO 3 /rGO nanocomposite
The recyclability of a catalyst is crucial for assessing its stability In this study, the MgFe2O4/WO3/rGO nanocomposite was magnetically recovered after each CIP treatment cycle, thoroughly washed with deionized water three times, and dried at 50°C for six hours before being reused in subsequent removal cycles under optimal conditions A total of three cycles were performed to evaluate the stability of the MgFe2O4/WO3/rGO nanocomposite.
Results and discussion
Material characterization
Figure 4.1 displays the XRD patterns of synthesized materials, confirming the successful formation of a crystalline structure for MgFe2O4, WO3, rGO, and the composite MgFe2O4/WO3/rGO, all free from impurities Notably, MgFe2O4 exhibits characteristic peaks at 2θ = 30.1°, 35.4°, 43.1°, 53.5°, 57.0°, and 62.6°, corresponding to the (220) plane.
(311), (400), (422), (511) and (440) planes, respectively This result is in good agreement with standard characteristic peaks of cubic MgFe2O4 structure (JCPDS 71-
The study identified characteristic peaks of WO3 at specific 2θ values, including 23.1°, 23.6°, 24.3°, 26.7°, 34.12°, 35.5°, 40.7°, 47.4°, 49.1°, and 55.01°, corresponding to the (002), (020), (200), (120), (202), (122), (222), (040), (140), and (420) planes These findings align with previous research and the standard characteristic peaks of monoclinic WO3, confirming the absence of contaminants in the samples analyzed (Ali et al., 2018; Van Tran et al., 2021).
The formation of reduced graphene oxide (rGO) was confirmed by distinctive peaks at 2θ° without any impurity peaks, as noted in previous studies (Bosch-Navarro et al., 2012; Vu et al., 2022) The XRD spectrum of the MgFe2O4/WO3/rGO nanocomposite revealed all characteristic peaks of MgFe2O4 and WO3, with rGO peaks being less prominent due to its significantly lower proportion in the composite.
Figure 4.1 XRD spectra of MgFe2O4, WO3, rGO and MgFe2O4/WO3/rGO
FT-IR spectra serve as a valuable tool for confirming the successful formation of characteristic bonds in various nanomaterials, including MgFe2O4, WO3, rGO, and the MgFe2O4/WO3/rGO nanocomposite, as illustrated in Figure 4.2 Notably, the spectra of rGO and the MgFe2O4/WO3/rGO nanocomposite exhibit an adsorption band between 3600 and 3000 cm -1, attributed to the O-H vibrations of water molecules (Vu et al., 2022) In the case of MgFe2O4, distinct peaks at 430 cm -1 and 577 cm -1 indicate the stretching vibrations of metal-oxygen bonds at octahedral and tetrahedral sites, respectively These findings are consistent with previous studies (Kurian & Mathew, 2017; Van Tran et al.).
In 2021, research highlighted a significant peak for WO3 at 650-950 cm⁻¹, which is associated with the stretching vibration of O-W-O bonds (Vu et al., 2022) Additionally, for reduced graphene oxide (rGO), characteristic peaks were identified at 1720 cm⁻¹, indicating the presence of carbonyl functional groups, and at 1615 cm⁻¹.
The FT-IR spectrum of the MgFe2O4/WO3/rGO nanocomposite reveals distinct characteristic peaks corresponding to various components, including C=C vibrations of the graphene skeleton at 1600 cm^-1, C-O oscillations of hydroxyl groups at 1400 cm^-1, and C-O oscillations of epoxy groups at 1060 cm^-1, as noted by Guo et al (2009) and Wang et al (2017) Importantly, these peaks indicate the absence of impurities in the nanocomposite.
Figure 4.2 FT-IR spectra of MgFe2O4, WO3, rGO and MgFe2O4/WO3/rGO
The SEM images of MgFe2O4, WO3, and the MgFe2O4/WO3/rGO nanomaterial are presented in Figures 4.3, 4.4, and 4.5 The morphologies observed show that MgFe2O4 exhibits a cubic structure, while WO3 appears spherical Additionally, Figure 4.5 illustrates that the MgFe2O4 and WO3 nanoparticles are uniformly distributed across the rGO sheet.
Figure 4.3 SEM image of MgFe2O4
Figure 4.4 SEM image of WO3 h
Figure 4.5 SEM image of MgFe2O4/WO3/rGO
Figures 4.6 and 4.7 illustrate the EDX spectrum and elemental mapping of the MgFe2O4/WO3/rGO nanocomposite, confirming the elemental composition These findings support the SEM results, indicating that although some regions exhibit agglomeration of WO3, the majority of MgFe2O4 and WO3 nanoparticles are evenly distributed across the rGO sheet.
Table 4.1 reports the weight and atomic percentage of elements in the nanocomposite
It is evident that all elements were detected without any impurity h
Figure 4.6 EDX spectrum of MgFe2O4/WO3/rGO a)
Figure 4.7 Electron mapping of a) Mg, b) C, c) Fe, d) O, and e) W elements in the
Table 4.1 Elemental composition of MgFe2O4/WO3/rGO nanocomposite
4.1.5 UV–Vis Diffuse Reflectance Spectroscopy
UV-Vis DRS was utilized to analyze the absorption spectra of MgFe2O4, WO3, and MgFe2O4/WO3/rGO photocatalysts within the 200 to 800 nm range The energy bandgap (Eg) was determined using a Tauc plot from the collected data, enabling the evaluation of the photocatalysts' efficiency under visible light conditions.
The 20W white light emits wavelengths of 450-460 nm, which aligns with the strong absorption characteristics observed in MgFe2O4, WO3, and MgFe2O4/WO3/rGO This indicates that these materials are well-suited for effective performance with this specific lamp.
Figure 4.8 UV-Vis DRS absorption spectra of MgFe2O4, WO3 and
WO3, being an indirect band gap semiconductor, has a value of n equal to 2 in the Tauc plot equation Therefore, the Tauc plot for WO3 can be represented by the corresponding equation (Vu et al., 2022).
MgFe2O4 is identified as a direct band gap semiconductor, leading to a value of n equal to 1/2 in the relevant equation In the MgFe2O4/WO3/rGO nanocomposite, the predominance of MgFe2O4 suggests that the entire nanocomposite behaves as a direct band gap semiconductor Consequently, the Tauc plot equations for both MgFe2O4 and the MgFe2O4/WO3/rGO nanocomposite can be expressed as outlined in the study by Van Tran et al (2021).
Figure 4.9 Tauc plot of MgFe2O4, WO3 and MgFe2O4/WO3/rGO h
Table 4.2 Eg of MgFe2O4, WO3 and MgFe2O4/WO3/rGO
The bandgap (Eg) values of MgFe2O4, WO3, and MgFe2O4/WO3/rGO were measured at 1.94 eV (639 nm), 2.41 eV (515 nm), and 1.87 eV (663 nm), respectively, as shown in Table 4.2 These values align with previous studies (Cadan et al., 2021; Garcia-Muñoz et al., 2020) Consequently, all materials demonstrate the capability to facilitate electron transitions from the valence band (VB) to the conduction band (CB) when exposed to visible light, thereby generating reactive oxygen species (ROS) for the photodegradation of CIP.
The PL spectrum comparison between the MgFe2O4/WO3/rGO nanocomposite and MgFe2O4 reveals that the nanocomposite exhibits a lower emission peak intensity This indicates that the MgFe2O4/WO3/rGO nanocomposite has a reduced recombination rate of charge carriers compared to pristine MgFe2O4 nanoparticles The addition of reduced graphene oxide (rGO) plays a crucial role as a photogenerated electron trap, enhancing the performance of the nanocomposite.
Figure 4.10 Photoluminescene spectra of MgFe2O4/WO3/rGO nanocomposite and pristine MgFe2O4 nanoparticle
4.1.7 pH point of zero charge
At pH levels below 5.2, the surface of the MgFe2O4/WO3/rGO nanocomposite exhibits a positive charge, which is crucial for understanding its interaction with the CIP molecule across varying pH conditions.
Figure 4.11 pHpzc of MgFe2O4/WO3/rGO nanocomposite
Study of photocatalytic removal of Ciprofloxacin by MgFe 2 O 4 -WO 3 -rGO
4.2.1 Comparative study on the removal of CIP by MgFe 2 O 4 , WO 3 and MgFe 2 O 4 /WO 3 /rGO
Ct/Co values obtained when 5 ppm CIP (pH = 7) solution was treated with 0.5 g/L MgFe2O4, WO3 and MgFe2O4/WO3/rGO are reported in Figure 4.12
The photodegradation treatment of Ciprofloxacin (CIP) using various catalysts revealed distinct removal efficiencies, with MgFe2O4, WO3, and MgFe2O4/WO3/rGO achieving 48%, 36%, and 83% CIP removal, respectively Notably, the MgFe2O4/WO3/rGO nanocomposite exhibited a significantly enhanced CIP removal efficiency compared to the individual MgFe2O4 and WO3 nanoparticles, as illustrated in Figure 4.12.
Figure 4.12 Comparison of CIP removal between MgFe2O4, WO3, and
The photodegradation of CIP was evaluated using MgFe2O4, WO3, and the MgFe2O4/WO3/rGO nanocomposite, yielding Ct/Co values of 0.52, 0.64, and 0.17, respectively This corresponds to photodegradation efficiencies of 48%, 36%, and 83% Notably, the MgFe2O4/WO3/rGO nanocomposite demonstrated a significantly higher capacity for CIP photodegradation compared to the pristine MgFe2O4 and WO3 nanoparticles.
4.2.2 Factors influencing the CIP removal efficiency a) pH of the solution
Ct/Co values attained when 5 ppm CIP solutions at pH = 3, 5, 7, 9, and 11 were treated with 0.5 g/L MgFe O /WO /rGO nanocomposite are shown in Figure 4.13 h
Figure 4.13 Comparison of CIP removal by MgFe2O4/WO3/rGO nanocomposite at various pH values
The study demonstrated that the MgFe2O4/WO3/rGO nanocomposite effectively eliminated CIP, achieving Ct/Co values of 0.28, 0.22, 0.17, 0.36, and 0.54 at pH levels of 3, 5, 7, 9, and 11, respectively This corresponded to CIP removal efficiencies of 72%, 78%, 83%, 64%, and 46% Notably, the highest removal efficiency of 83% was recorded at a pH of 7.
The interaction between the MgFe2O4/WO3/rGO nanocomposite and ciprofloxacin (CIP) is significantly influenced by the pH level, which affects the photodegradation efficiency The point of zero charge (pHpzc) of the nanocomposite is measured at 5.2; thus, at pH values below 5.2, the surface of the nanocomposite carries a positive charge, while it becomes negatively charged at pH levels above 5.2 Additionally, CIP exhibits two dissociation constants, pKa1 at 5.90 and pKa2 at 8.89, further impacting its speciation and interaction with the nanocomposite across varying pH conditions.
At pH < 5.90, CIP is in cationic form (A + ), at pH between 5.90 and 8.89, CIP is in neutral form (A) and at pH > 8.89, CIP is in anionic form (A - ) (Jalil et al., 2015)
At pH values of 3 and 5, both CIP species and the MgFe2O4/WO3/rGO nanocomposite exhibited a positive charge The presence of functional groups on the CIP molecule, such as -F, -COOH, =O, and benzene rings, contributed to a partial negative charge due to their high electron affinity This created an electrostatic attraction between the negatively charged functional groups and the positively charged nanocomposite.
At a pH of 7, the nanocomposite exhibits a negative charge, leading to the dominance of the positively charged CIP species A, characterized by a nitrogen atom (-NH2 +) that forms coordinate covalent bonds with H + using its lone pair of electrons Additionally, the piperazine ring displays a partial positive charge due to conjugation with the adjacent benzene ring This interaction results in a strong attraction between the negatively charged nanocomposite and the significantly positively charged -NH2 + group.
The MgFe2O4/WO3/rGO nanocomposite exhibits higher active binding sites, leading to enhanced efficiency in the removal of CIP at a pH of 7 compared to pH values of 3 and 5, due to the stronger interactions between its functional groups at neutral pH.
At pH levels of 9 and 11, both CIP species (A -) and the MgFe2O4/WO3/rGO nanocomposite exhibited negative charges, resulting in minimal interaction between them and consequently low CIP removal efficiency Additionally, the catalyst dosage plays a crucial role in this process.
Ct/Co values obtained when 5 ppm CIP solution at pH = 7 was treated with 0.25, 0.50, 0.75 and 1.0 g/L MgFe2O4/WO3/rGO nanocomposite are reported in Figure 4.15
The study revealed that the removal efficiencies of CIP using MgFe2O4/WO3/rGO nanocomposite at dosages of 0.25, 0.50, 0.75, and 1.0 g/L were 63%, 83%, 91%, and 85%, respectively, with the highest efficiency recorded at a dosage of 0.75 g/L The corresponding Ct/Co values for these dosages were 0.37, 0.17, 0.09, and 0.15.
The photodegradation efficiency of CIP improved with an increase in the dosage of MgFe2O4/WO3/rGO nanocomposite from 0.25 to 0.75 g/L, attributed to the enhanced active sites of the photocatalyst and the generation of reactive oxygen species (ROS) such as HO• and O2 -• radicals However, further increasing the dosage to 1.0 g/L resulted in greater solution turbidity, which obstructed and scattered incoming light, leading to reduced electron transitions from the valence band to the conduction band This decrease in electron activity resulted in lower ROS production and, consequently, diminished photodegradation efficiency (Zhang et al., 2018b).
Figure 4.15 Comparison of CIP removal at various dosage of MgFe2O4/WO3/rGO nanocomposite c) Initial concentration of CIP
Ct/Co values obtained when CIP solutions (2.5, 5, 7.5, and 10 ppm) at pH of 7 were treated with 0.75 g/L MgFe2O4/WO3/rGO nanocomposite are presented in Figure 4.16
The study revealed that the Ct/Co values for CIP removal at initial concentrations of 2.5, 5, 7.5, and 10 ppm were 0.21, 0.09, 0.16, and 0.20, indicating removal efficiencies of 79%, 91%, 84%, and 80%, respectively Notably, the highest CIP removal efficiency was achieved at an initial concentration of 5 ppm.
Higher CIP removal efficiency was observed when the initial CIP concentration was optimized, allowing for increased interaction between CIP molecules and the active binding sites of the MgFe2O4/WO3/rGO nanocomposite (Zhang et al., 2018b).
Figure 4.16 Comparison of CIP removal at various initial CIP concentration by
The optimal conditions for the photodegradation of CIP were identified as a 5 ppm CIP solution, an initial pH of 7, and a dosage of 0.75 g/L of the MgFe2O4/WO3/rGO nanocomposite Under these conditions, the maximum CIP removal efficiency reached 91%.
In this study, the optimal conditions for CIP degradation were evaluated against previous research findings (Table 4.3) Notably, the MgFe2O4/WO3/rGO nanocomposite demonstrated exceptional degradation efficiency, outperforming other photocatalysts, even under visible light rather than UV light.
Table 4.3 The comparison of CIP photodegradation between MgFe2O4/WO3/rGO nanocomposite and other photocatalysts
5 ppm CIP pH = 7 Dark: 30 min Irradiation: 100 min
3 ppm CIP pH = 7 Irradiation: 90 min
15 ppm CIP Dark: 30 min Irradiation: 60 min
5 ppm CIP pH = 7 Dark: 10 min
5ppm CIP pH = 7 Dark: 30 min Irradiation: 180 min
4.2.3 Kinetic study of CIP photodegradation
In a study on the photodegradation of ciprofloxacin (CIP) solutions at varying concentrations of 2.5, 5, 7.5, and 10 ppm, three pseudo-kinetic models were evaluated using a MgFe2O4/WO3/rGO nanocomposite at a pH of 7 and a concentration of 0.75 g/L The suitability of each model was assessed through R² values, with higher values indicating a better fit for the CIP removal process The findings are visually represented in Figures 4.17, 4.18, and 4.19, showcasing the plots for pseudo-zero-order, pseudo-first-order, and pseudo-second-order kinetic models.