Introduction
Research significance
Antibiotics play a crucial role in treating infectious diseases in humans and animals, serving various purposes such as preventive medicine and growth promotion 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 prevalence in the environment, with traces found in lakes, rivers, wastewater, groundwater, and even treated drinking water (Kümmerer, 2009b) Alarmingly, even low concentrations of antibiotics can foster antibiotic resistance in environmental settings (Kümmerer, 2009a; Yu et al.).
2019) Therefore, antibiotics are considered as significant emerging environmental pollutants (Hu et al., 2019)
Water pollution from antibiotics, particularly ciprofloxacin (CIP), has become a significant issue due to increased consumption and improper disposal (Yu et al., 2019) CIP, commonly used for treating urinary and respiratory infections, poses risks to public health and aquatic ecosystems when present in water sources, leading to symptoms like nervousness, nausea, and vomiting Moreover, CIP contributes to the emergence of antibiotic-resistant bacteria (Ahmadzadeh et al., 2017; Mandal et al., 2012) Therefore, effectively degrading CIP residues in water is essential, yet traditional methods like activated sludge and trickling filters often fail to eliminate these contaminants due to their low biodegradability (Yu et al., 2019).
Advanced oxidation processes (AOP) utilizing photocatalysts have garnered significant attention for their effectiveness in degrading persistent organic pollutants into carbon dioxide and water through the generation of reactive oxygen species like hydroxyl and superoxide radicals A major challenge in photocatalyst application is the difficulty in separating and recovering the catalyst post-use Recent developments in spinel ferrites, particularly magnesium ferrite nanoparticles (MgFe2O4 NP), offer a solution due to their magnetic properties that facilitate catalyst recovery MgFe2O4 NP is particularly appealing because of its narrow band gap of 2.0 eV, magnetic recovery capabilities, and absence of toxic metals To further enhance the photocatalytic efficiency of MgFe2O4, this research explores the combination of MgFe2O4 with tungsten trioxide (WO3) and reduced graphene oxide (rGO), both of which are attracting considerable research interest This results in the creation of a MgFe2O4/WO3/rGO nanocomposite aimed at improving photocatalytic performance.
Research novelty
Recent studies on nanocomposites for CIP treatment have highlighted limitations, such as 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) To address these gaps, the current research focuses on developing a lab-scale advanced photocatalytic system designed for efficient CIP degradation in aqueous solutions This innovative system features the simultaneous generation of effective reactive oxygen species (HO• and O2 -•), a reduced recombination rate of photogenerated holes (h +) and electrons (e -) during advanced oxidation processes (AOP), optimal performance under visible light, and the potential for magnetic separation post-use.
Research objectives
This research involves four main objectives, including:
This study investigates the factors influencing the removal of ciprofloxacin (CIP) using the MgFe2O4/WO3/rGO nanocomposite Key parameters examined include pH levels, catalyst dosage, and initial CIP concentration, with a focus on determining the optimal conditions for maximum CIP 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.
Literature review
Ciprofloxacin pollution in aquatic environment
Ciprofloxacin (CIP), a fluorinated quinolone similar to nalidixic acid, is known for its high bioavailability, effective tissue penetration, and minimal side effects This antibiotic is primarily utilized in treating urinary tract infections and prostatitis, but it also effectively addresses bacterial enteric infections, biliary tract infections, sexually transmitted diseases, and helps prevent infections in immunocompromised neutropenic patients (Sharma et al., 2010).
CIP's primary mechanism of action is the inhibition of DNA gyrase, as noted by Campoli-Richards et al (1988) The presence of a fluorine atom at the C6 position and a piperazine ring at the C7 position significantly boosts CIP's antibacterial efficacy against both Gram-negative and Gram-positive bacteria.
Pseudomonas aeruginosa, Escherichia coli, and Staphylococcus aureus (Zhang et al.,
CIP has two pKa values, 5.90 and 8.89, with an isoelectric point of 7.4, where CIP exists as a neutrally charged zwitterion due to its anionic and cationic functional groups When the pH exceeds 8.89, CIP molecules lose an H+ ion, resulting in the anionic form -COO- Conversely, at a pH below 5.90, the secondary amine -NH gains an additional proton, transforming into -NH2+.
, 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 prevalence of ciprofloxacin (CIP) in Vietnam across five primary sectors: aquaculture, husbandry, hospitals, pharmaceutical manufacturing, and household use CIP is permitted for limited use in aquaculture, banned in husbandry, and distributed through a bidding system in hospitals by the Ministry of Agriculture and Rural Development (MARD) In aquaculture, CIP is commonly utilized during the larvae stage due to its stability in water and sediment, despite the ban on fluoroquinolones since 2014 In hospitals, CIP is one of seven quinolone antibiotics available through a pharmaceutical bidding system Additionally, significant quantities of quinolone antibiotics, with CIP being the most imported, were recorded by the Vietnam Customs Department from 2014 to 2016.
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 vary, with sludge containing 2.42 mg/kg Kümmerer (2009b) demonstrated that CIP is not biodegradable, indicating persistent environmental toxicity It has been detected in wastewater and surface water from 178 shrimp and fish farms linked to pig and duck farms (Binh et al., 2018) Hospital wastewater analysis revealed CIP in all 370 samples from hospitals in Ho Chi Minh City and Hanoi, with peak concentrations of 87.3 µg/L in influent and 53.3 µg/L in effluent, marking the highest level recorded in Vietnam (Lien et al., 2016) Additionally, CIP concentrations in contaminated rivers in Vietnam (98.6 ng/L) surpass those in the United States (30 ng/L) (Binh et al., 2018; Kolpin et al., 2002) Furthermore, CIP-resistant bacteria were found at all sites, with prevalence 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
CIP, even at low concentrations, poses a significant environmental risk, as highlighted by El-Shafey et al (2012) Conventional wastewater treatment methods, including activated sludge and trickling filters, have proven ineffective in removing CIP, leading to its release into the environment and subsequent contamination of surface water, soil, and groundwater The presence of CIP in drinking water has been associated with various health issues, including nervousness, nausea, headaches, and tremors Furthermore, exposure to high concentrations of CIP may result in severe health effects such as acute renal failure, thrombocytopenia, and elevated liver enzymes, as noted by Ahmadzadeh et al (2017).
The presence of CIP in water sources contributes to the development of antibiotic-resistant bacteria, posing a significant public health threat that demands urgent action from both government and society This bacterial resistance can be transmitted to humans via water or food, particularly when sewage sludge is utilized as fertilizer, surface water irrigates crops, or manure is applied to livestock.
In 2009, Kümmerer identified two main mechanisms for the transfer of resistance genes between organisms The first, known as vertical resistance transfer, occurs naturally through cell division within the same species The second mechanism, horizontal resistance transfer, is induced by the presence of antibiotics in aquatic environments, where exposure leads to the development of resistance genetic material that can be shared among different species through conjugation.
Wastewater serves as a major reservoir for antibiotic-resistant bacteria, with nutrient-rich environments like sewage facilitating horizontal gene transfer, which often involves the movement of plasmids and transposons that carry antibiotic resistance Consequently, wastewater treatment facilities, where both antibiotic-resistant and susceptible bacteria thrive, are identified as hotspots for the proliferation of antibiotic resistance Given the significant health risks posed by ciprofloxacin (CIP) and the swift emergence of CIP-resistant bacteria in wastewater treatment plants (WWTPs), it is crucial to implement effective CIP treatment strategies in wastewater, requiring coordinated efforts from government sectors and global communities.
Recent findings on antibacterial resistance highlight two key points: firstly, the use of a single antibacterial agent can lead to increased resistance not only to that specific drug but also to other drugs with different mechanisms of action, a phenomenon known as cross-resistance Secondly, antibacterial resistance does not always correlate with the number of drugs used or the environmental concentrations of these compounds (Weston, 1996).
Methods for treatment of CIP in aquatic medium
Conventional wastewater treatment methods, 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), highlighting the urgent need for advanced techniques to effectively remove CIP from water sources.
Various techniques have been developed for the removal of ciprofloxacin (CIP) from 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 potential for effective CIP removal Notably, photocatalytic degradation using photocatalysts is preferred over other methods due to its affordability, simplicity, and clean operational requirements.
(2) possible working under natural irradiation (sunlight), and (3) effective degradation of CIP
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 effectively decompose persistent organic pollutants into harmless substances like CO2 and H2O When exposed to light of an appropriate wavelength, the photocatalyst absorbs energy, exciting an electron from the low-energy valence band (VB) to the high-energy conduction band (CB), creating a hole in the VB This hole oxidizes water or hydroxide ions (OH-) to generate hydroxyl radicals (HO•) when 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 form superoxide radicals (O2 -•) if the CB's redox potential is between +0.5 and 1.5 V versus SHE Under optimal conditions, O2 -• can react with H+ ions to create HOO•, which quickly decomposes into additional HO• The high redox potential of HO• (𝐸 𝐻𝑂 /𝑂𝐻 − = 2.80 V vs Normal hydrogen electrode (NHE)) underscores its effectiveness in these 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, a study demonstrated that a 1.0 g/L ZnFe2O4@CMC solution effectively degraded 5 ppm CIP solution, achieving an impressive 87% efficiency in darkness after 30 minutes, and complete degradation under UV light after 100 minutes Additionally, Chen et al (2019) found that a concentration of 0.5 g/L biochar@ZnFe2O4/BiOBr could degrade 84% of a 15 ppm CIP solution when exposed to a 300W Xe lamp for 60 minutes.
Tamaddon et al (2020) found that 0.67g/L of CuFe2O4@MC achieved an impressive 81% removal efficiency of a 3ppm CIP solution after 90 minutes of UV-C irradiation The optimal removal efficiency was consistently observed at a pH of 7, which aligns with typical wastewater conditions Additionally, the studies highlighted the effectiveness of spinel ferrite nanocomposites in degrading CIP and their recoverability through external magnetic forces.
Introduction of MgFe 2 O 4 nanoparticle
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 diverse applications in sensors, biomedicine, catalysis, and energy storage devices Their remarkable properties, including superparamagnetism, narrow band-gap responsiveness to visible light, and high heat and corrosion resistance, contribute to their growing popularity in research Consequently, there has been an increasing focus on the study of spinel ferrites 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 Fe 3+ cation in its crystalline structure In each unit cell, oxygen anions are arranged in a cubic close-packed configuration, while the A and B cations occupy tetrahedral and octahedral sites, respectively Depending on the positions of 2+ and 3+ cations within the lattice, spinel ferrites can be classified into normal, inverse, and mixed types (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
Figure 2.6 Diagram of spinel ferrite demonstrating tetrahedral (yellow), octahedral
(green) and oxygen atoms (red) units
Magnesium ferrite nanoparticles, a prominent type of spinel ferrites, have garnered significant attention due to their narrow band-gap of 2.0 eV, excellent magnetic recovery properties, and the absence of toxic metals These 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 superoxide radicals (O2 ∙-) for advanced oxidation processes (AOP).
Research has shown that MgFe2O4 and its derivatives possess significant photo-degradation capabilities For instance, George et al (2021) demonstrated that 1 g/L of Cu–MgFe2O4 could eliminate 97% of a 25 ppm methylene blue solution after 180 minutes of UV-lamp irradiation Additionally, a study by Van Tran et al (2021) found that nearly 100% degradation was achieved with a 10-ppm concentration.
The chemical synthesis of spinel ferrites is the most prevalent method, encompassing techniques such as hydrothermal, co-precipitation, sol-gel auto-combustion, and solid-state reactions Each synthesis method has its unique characteristics, along with specific advantages and disadvantages, which are detailed in Table 2.4.
The co-precipitation method stands out as a highly effective technique for synthesizing MgFe2O4, offering advantages such as 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 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.
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, photochromism, electrochemistry, smart windows, paint industries, and environmental photocatalysis, as well as 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, depending on its crystallite structure The electron configuration of tungsten (W) in the VI oxidation state is [Xe] 4f14 5d0 6s0, with the conduction band (CB) derived from the 5d orbitals of the W atom This structure facilitates the generation of hydroxyl radicals (HO•) from water, and 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 capabilities of WO3 and its derivatives For instance, Liang et al (2019) demonstrated that WO3 achieved a complete removal efficiency of 100% for a 25 ppm methyl orange solution after 120 minutes of exposure to a 500 W Xe lamp Similarly, Mao et al (2018) found that a 0.54 ppm Aflatoxin B1 solution was degraded by 92.4% using WO3/rGO/g-C3N4 after 120 minutes under a 300 W Xe lamp.
Various techniques for synthesizing WO3 include hydrothermal methods, calcination, and precipitation, with calcination being the most straightforward and time-efficient approach 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, particularly graphene and its derivative graphene oxide (GO), are highly valued in various fields due to their accessibility and unique properties GO, characterized by functional groups such as epoxy, hydroxyl, and carboxyl, boasts a high theoretical surface area that enhances sorption techniques Synthesized from graphite using strong oxidants, GO is cost-effective and yields significant results However, its numerous polar functional groups make it hydrophilic and electrically insulating In contrast, reduced graphene oxide (rGO) has fewer surface groups, offering a larger surface area and properties such as high thermal stability, good conductivity, and corrosion resistance, making it an ideal support material for photocatalysts.
Figure 2.8 Chemical structure of GO
Reduced graphene oxide (rGO) significantly enhances photocatalytic performance through three key mechanisms Firstly, its electronic coupling with the conduction band of the photocatalyst facilitates efficient transport of photogenerated electrons, thereby decreasing the recombination rate of charge carriers Secondly, rGO minimizes ion leaching, promoting stronger interactions between the photocatalyst and pollutants.
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, followed by a reduction process to obtain reduced graphene oxide (rGO) Various synthesis methods for GO are detailed in Table 2.5, while Table 2.6 outlines the 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
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 strategies such as utilizing photogenerated electron traps like reduced graphene oxide (rGO) and developing Z-scheme photocatalytic systems The evolution of Z-scheme systems has progressed through three 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 electron acceptor/donor pair under light irradiation
The disadvantages of this system include interference from commonly used electron acceptor/donor pairs like Fe²⁺/Fe³⁺, which can disrupt the reduction and oxidation reactions Additionally, its application is limited to the liquid phase, restricting its practical uses.
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 has a low valence band (VB) position (EVB > 2.80 V) for hydroxyl radical (HO•) formation, and the other has a high conduction band (CB) position (ECB < -0.33 V) for superoxide radical (O2 -•) generation Under appropriate light irradiation, electrons in the VB of photocatalyst A are excited to its CB, and 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 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 electrochemical behavior of MgFe2O4, with an ECB of -1.40 V, allows for the reduction of dissolved oxygen to generate O2 -• for advanced oxidation processes (AOP) However, to improve the photocatalytic efficiency of MgFe2O4, two significant challenges must be addressed: (1) the valence band potential of MgFe2O4 is lower than 2.72 V, and (2) the rapid recombination of photogenerated electrons and holes occurs within an extremely short timeframe of 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
The conduction band of MgFe2O4 facilitates the reduction of dissolved oxygen, generating O2 -• radicals, while the valence band of WO3 enables the oxidation of water, producing HO• radicals These radicals, HO• and O2 -•, play a crucial role in degrading organic compounds into carbon dioxide (CO2) and water (H2O), effectively preventing the recombination of electrons and holes in the semiconductors.
The incorporation of reduced graphene oxide (rGO) addresses the challenge of charge carrier recombination by acting as a photogenerated electron trap, thereby enhancing efficiency Additionally, rGO's high surface area promotes better interaction between the photocatalyst and pollutants, increasing the availability of active sites within the nanocomposite.
The MgFe2O4/WO3/rGO nanocomposite is designed 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 easy separation using external magnetic forces.
Water pollution from the increased consumption and improper disposal of antibiotics, particularly ciprofloxacin (CIP), has become a significant concern in recent years CIP, commonly used to treat mild-to-moderate urinary and respiratory infections, poses risks to public health and aquatic ecosystems when it contaminates water resources Its presence in drinking water can lead to symptoms such as nervousness, nausea, and vomiting, while also contributing to the emergence of antibiotic-resistant bacteria Wastewater treatment facilities serve as hotspots for the transfer of antibiotic resistance among organisms, making the degradation of CIP residues in water essential for safeguarding health and the environment.
Traditional technologies like activated sludge and trickling filters struggle to completely remove antibiotics due to their low biodegradability To address this issue, various advanced methods have been developed for effective antibiotic treatment, including adsorption, membrane bioreactors, ozonation, solid polymer electrolytes, ultrasound irradiation, and advanced oxidation processes (AOP) such as photocatalytic degradation Among these techniques, AOP utilizing photocatalysts has garnered significant attention for its capability to generate reactive oxygen species, such as hydroxyl radicals (HO•) and superoxide radicals (O2 -•), which effectively break down persistent 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 capabilities, and absence of toxic metals To improve the photocatalytic efficiency of MgFe2O4, researchers are integrating it with WO3 and reduced graphene oxide (rGO) to create the MgFe2O4/WO3/rGO nanocomposite WO3 acts as a visible-light-driven photocatalyst that generates hydroxyl radicals (HO•), a process that MgFe2O4 alone cannot achieve Furthermore, the incorporation of rGO serves as a trap for photogenerated electrons, effectively minimizing the recombination rate of charge carriers produced by both MgFe2O4 and WO3, thereby enhancing overall photocatalytic performance.
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 easy 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 a range of advanced analytical instruments, including the X-ray powder diffraction (XRD MiniFlex 600 from Rigaku Corp., Japan) and Fourier transformation infrared spectroscopy (FTIR 4600 from Jasco Corp., Japan), to characterize materials Additionally, scanning electron microscopy (SEM TM 4000 Plus from Hitachi Corp., Japan) and energy dispersive X-ray (EDX MisF+ from Oxford Instruments plc., UK) were employed for detailed structural analysis The optical properties were examined using UV–vis diffuse reflectance spectroscopy (UV-DRS UH 5300 from Hitachi Corp., Japan) and photoluminescence (FluoroMax-4 from Horiba, Japan), along with UV-Vis spectroscopy via a Double Beam Spectrophotometer (UH5300 from Hitachi Corp., Japan).
• Sealed Teflon lined stainless-steel autoclave
Material synthesis
The synthesis of MgFe2O4 in this study followed the method of 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 a pH of 9-10 was achieved The mixture's temperature was then increased to 90 °C and maintained for 2 hours, followed by natural cooling to room temperature The mixture was filtered and washed several times with distilled water and ethanol until a pH of 7 was reached The collected 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
To synthesize graphene oxide (GO), 2.7 grams of KMnO4 was gradually added to a mixture while stirring, maintaining 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 two hours, resulting in a brown suspension Following this, 23 ml of distilled water was added dropwise while keeping the temperature at 90-95 °C for 30 minutes, and then 120 ml of distilled water was introduced before cooling the reaction to 50 °C A 10 ml portion of 30% H2O2 solution was added, stirring until the color changed from brown to light yellow The final mixture was washed with 5% HCl and distilled water until a pH of approximately 6 was reached, and then 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, building on the previous work of Guo et al (2021) on Fe3O4/WO3/rGO Key modifications include the synthesis processes for MgFe2O4 and WO3, simultaneous coupling of these materials to graphene oxide (GO) layers, and hydrothermal reduction of GO to reduced graphene oxide (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 use of water as a solvent for the hydrothermal reduction 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 path 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 placed in 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 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 when an X-ray beam strikes the crystal at an angle θ, it interacts with the crystal lattice This interaction results in X-rays being reflected from two consecutive planes, creating 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 of 0.02° over 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 used to analyze the structures of molecules in gas, liquid, and solid states by measuring their characteristic infrared radiation absorption The molecular vibrational spectrum, resulting from this absorption, occurs when sample molecules selectively absorb infrared radiation at specific wavelengths, leading to a change in their dipole moment and a transition from ground to excited vibrational energy levels The frequency of the absorption peaks is determined by the vibrational energy gap, 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 determination of molecular structures through infrared spectrum analysis (Harris, 2015).
In this research, the sample was measured by FTIR 4600, Jasco Corp., Japan at the VNU KLAMAG
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 analyzing solid objects across various fields, utilizing a focused beam of high-energy electrons to produce diverse signals from the surface of samples SEM images offer superior depth of focus compared to traditional light microscopy, enhancing the clarity and detail of the observations.
Different types of electrons interact with a sample in unique ways, producing images with distinct characteristics Secondary and backscattered electrons are reflected from the sample after they strike it, influencing the resulting imagery.
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, resulting in the emission of X-rays that have a characteristic wavelength corresponding to the atomic number (Z) of the element.
The unique frequency of X-rays emitted by atoms in a solid allows for the identification of chemical elements and their relative quantities in the sample, as demonstrated by 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-
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 1/2 for direct band gaps and 2 for indirect band gaps The band gap energy of a material is determined by the coordinate value at the point where the linear-fit line intersects the horizontal axis of the Tauc graph (Makuła et al., 2018).
This research determined the absorption spectrum of MgFe2O4, WO3, and MgFe2O4/WO3/rGO samples in the wavelength range of 200 to 800 nm using UV-Vis DRS UH 5300 from Hitachi Corp., Japan, at VNU KLAMAG.
Figure 3.8 UV-Vis DRS UH 5300, Hitachi Corp
Photoluminescence (PL) is a key analytical technique used to assess 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 comparative analysis of the sample's fluorescence intensity 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 their photocatalytic activity The pH value determines the isoelectric point and surface charge of the photocatalyst, with the point of zero charge (PZC) indicating when the surface charge is neutral At this isoelectric point, electrostatic forces are absent, resulting in minimal interaction between catalyst particles and pollutants When the solution's pH is below the PZC, the catalyst exhibits a positive surface charge, while a pH above the PZC leads to a negative surface charge, which hinders interactions with negatively charged compounds in water.
Prepare five conical flasks, each containing 50 ml of 0.01M NaCl solution at varying pH levels of 3, 5, 7, 9, and 11 Add 0.2g of MgFe2O4/WO3/rGO to each flask and place them on a shaker for about 12 hours After this period, remeasure the pH values to assess any changes (Tran, 2020).
The initial pH (pH_initial) and final pH (pH_after) are measured before and after the addition of MgFe2O4/WO3/rGO to a NaCl solution By plotting the change in pH (ΔpH) against the initial pH values, the point of zero charge (pHpzc) can be determined The pHpzc is identified as the point where the ΔpH curve intersects the horizontal axis of the 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
After reaching adsorption-desorption equilibrium in 30 minutes, the solution was irradiated with a 20W white lamp for 180 minutes At intervals of 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 concentrations of MgFe2O4/WO3/rGO nanocomposite (0.25, 0.50, 0.75, and 1.0 g/L) were introduced into 100 ml of a 5 ppm ciprofloxacin (CIP) solution at the optimal pH The mixture was allowed to equilibrate in the dark for 30 minutes before being exposed to a 20W white lamp for 180 minutes Samples of 5 ml were taken at 30-minute intervals for analysis.
CIP absorbance was measured at its maximum wavelength (𝜆 𝑚𝑎𝑥) using a UV-Vis spectrophotometer (Double Beam Spectrophotometer UH5300, Hitachi Corp., Japan) after centrifugation at 90, 150, and 210 minutes to remove the nanocomposite The initial concentration of CIP was also determined during the analysis.
The optimal dosage of MgFe2O4/WO3/rGO nanocomposite was added to 100 ml of CIP solutions at varying initial concentrations of 2.5, 5, 7.5, and 10 ppm, maintaining 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 At intervals of 30, 90, 150, and 210 minutes, 5 ml samples were extracted, centrifuged to separate the nanocomposite, and the light absorbance of CIP was measured at its maximum wavelength using a UV-Vis spectrophotometer (Double Beam Spectrophotometer UH5300, Hitachi Corp., Japan).
Kinetic study
The kinetics of CIP removal using the MgFe2O4/WO3/rGO nanocomposite were investigated through three models, focusing on optimal conditions such as the dosage of the nanocomposite, pH levels, and varying initial concentrations of CIP solutions (2.5, 5, 7.5, and 10 ppm) The analysis involved the natural logarithm of the initial concentration (ln(C₀)), providing insights into the efficiency of the nanocomposite in degrading CIP under specified 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 rate constant for the Pseudo-first-order model is indicated in per minute (min⁻¹) Additionally, the rate constant for the Pseudo-second-order model is represented 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 predominant during the photodegradation of CIP, t-butanol and p-benzoquinone are typically employed as scavengers for HO• and O2 -•, respectively (Schneider et al., 2020).
In the optimal conditions for CIP removal, 1.5 ml of t-BuOH, a hydroxyl radical scavenger, was introduced to the reaction solution at time zero The solution was then 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 30, 90, 150, and 210 minutes, centrifuged to eliminate solid materials, 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 a crucial factor in 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 6 hours before being utilized 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 the synthesized materials, confirming the successful formation of the crystalline structures of MgFe2O4, WO3, rGO, and MgFe2O4/WO3/rGO, free from impurities The MgFe2O4 compound exhibits characteristic peaks at 2θ = 30.1°, 35.4°, 43.1°, 53.5°, 57.0°, and 62.6°, which correspond 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 identifies 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 previously reported data and the standard peaks for monoclinic WO3, confirming the absence of contaminants as noted in earlier studies (Ali et al., 2018; Van Tran et al., 2021).
The XRD analysis confirmed the formation of rGO, indicated by characteristic peaks at 2θ° without any impurities, aligning with previous studies (Bosch-Navarro et al., 2012; Vu et al., 2022) In the MgFe2O4/WO3/rGO nanocomposite, all characteristic peaks of MgFe2O4 and WO3 were observed, while the rGO peaks were largely obscured due to its lower proportion compared to the other components.
Figure 4.1 XRD spectra of MgFe2O4, WO3, rGO and MgFe2O4/WO3/rGO
FT-IR spectroscopy is an effective tool for confirming the formation of characteristic bonds in nanomaterials, as illustrated by the FT-IR spectra of MgFe2O4, WO3, rGO, and the MgFe2O4/WO3/rGO nanocomposite shown in Figure 4.2 Notably, the rGO and MgFe2O4/WO3/rGO spectra exhibit an adsorption band between 3600 – 3000 cm -1, indicating O-H vibrations from water molecules (Vu et al., 2022) For MgFe2O4, the observed peaks at 430 cm -1 and 577 cm -1 correspond to the stretching vibrations of metal-oxygen bonds at octahedral and tetrahedral sites, respectively These findings align with previous studies (Kurian & Mathew, 2017; Van Tran et al.).
In 2021, a significant peak for WO3 was observed between 650-950 cm-1, indicating the stretching vibration of O-W-O bonds (Vu et al., 2022) Additionally, rGO exhibited characteristic peaks at 1720 cm-1, associated with carbonyl functional groups, and at 1615 cm-1.
The FT-IR spectrum of the MgFe2O4/WO3/rGO nanocomposite exhibited distinct characteristic peaks corresponding to the 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, confirming the absence of impurities (Guo et al., 2009; Wang et al., 2017).
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 in Figures 4.3 and 4.4 reveal that MgFe2O4 exhibits a cubic structure, while WO3 has a spherical shape Additionally, Figure 4.5 demonstrates that the majority of MgFe2O4 and WO3 nanoparticles are evenly distributed across the rGO sheet.
Figure 4.3 SEM image of MgFe2O4
Figure 4.4 SEM image of WO3
Figure 4.5 SEM image of MgFe2O4/WO3/rGO
Figures 4.6 and 4.7 illustrate the EDX spectrum, elemental composition, and mapping of the MgFe2O4/WO3/rGO nanocomposite These findings support the SEM results, indicating that although some regions exhibit agglomeration of WO3, the majority of MgFe2O4 and WO3 nanoparticles are uniformly 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
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 Diffuse Reflectance Spectroscopy (DRS) was utilized to analyze the absorption spectra of MgFe2O4, WO3, and MgFe2O4/WO3/rGO photocatalysts within the wavelength range of 200 to 800 nm The energy bandgap (Eg) was calculated using the Tauc plot derived from the spectral data, enabling the evaluation of the photocatalysts' effectiveness under visible light conditions.
The 20W white light has a wavelength of 450-460 nm, and as shown in Figure 4.8, MgFe2O4, WO3, and MgFe2O4/WO3/rGO demonstrate significant absorption at this wavelength, indicating their compatibility with this light source.
Figure 4.8 UV-Vis DRS absorption spectra of MgFe2O4, WO3 and
The Tauc plot equation for WO3, an indirect band gap semiconductor, is represented by Eq 4.1, with the value of n set to 2 (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 predominant presence of MgFe2O4 also categorizes the composite as a direct band gap semiconductor Consequently, the Tauc plot equations for both MgFe2O4 and the MgFe2O4/WO3/rGO composite can be represented as specified in the study by Van Tran et al (2021).
Figure 4.9 Tauc plot of MgFe2O4, WO3 and MgFe2O4/WO3/rGO
Table 4.2 Eg of MgFe2O4, WO3 and MgFe2O4/WO3/rGO
The band gap (Eg) values for MgFe2O4, WO3, and MgFe2O4/WO3/rGO were found to be 1.94 eV (639 nm), 2.41 eV (515 nm), and 1.87 eV (663 nm), respectively These findings align with previous studies by Cadan et al (2021) and Garcia-Muñoz et al (2020) Consequently, all materials are capable of facilitating 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 analysis reveals that the MgFe2O4/WO3/rGO nanocomposite shows a reduced emission peak intensity compared to pure MgFe2O4, indicating a lower recombination rate of charge carriers This observation underscores the crucial role of reduced graphene oxide (rGO) in functioning 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 nanocomposite surface exhibits a positive charge, while higher pH values result in a negative charge This finding is crucial for understanding the interactions between the MgFe2O4/WO3/rGO nanocomposite and the CIP molecule across varying solution pH levels.
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 treatment results indicate that the Ct/Co ratios for CIP photodegradation using MgFe2O4, WO3, and the MgFe2O4/WO3/rGO nanocomposite are 0.52, 0.64, and 0.17, respectively, reflecting CIP removal efficiencies of 48%, 36%, and 83% Notably, the MgFe2O4/WO3/rGO nanocomposite exhibited a significantly higher CIP removal efficiency compared to both pristine MgFe2O4 and WO3 nanoparticles, as illustrated in Figure 4.12.
Figure 4.12 Comparison of CIP removal between MgFe2O4, WO3, and
The photodegradation efficiency of CIP was found to be significantly enhanced when using the MgFe2O4/WO3/rGO nanocomposite, achieving an impressive 83% efficiency compared to 48% for MgFe2O4 and 36% for WO3 alone The corresponding Ct/Co values were 0.52 for MgFe2O4, 0.64 for WO3, and 0.17 for the MgFe2O4/WO3/rGO composite, highlighting its superior capacity for photodegradation.
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
Figure 4.13 Comparison of CIP removal by MgFe2O4/WO3/rGO nanocomposite at various pH values
The MgFe2O4/WO3/rGO nanocomposite demonstrated varying CIP elimination efficiencies across different pH levels, with Ct/Co values recorded at 0.28, 0.22, 0.17, 0.36, and 0.54 for pH values of 3, 5, 7, 9, and 11, respectively This corresponds to CIP removal efficiencies of 72%, 78%, 83%, 64%, and 46% Notably, the highest CIP removal efficiency of 83% was achieved at a pH of 7.
The interaction between MgFe2O4/WO3/rGO nanocomposite and ciprofloxacin (CIP) is significantly influenced by pH levels, which affects the photodegradation efficiency The point of zero charge (pHpzc) of the nanocomposite is 5.2, meaning that at pH values below 5.2, the surface becomes positively charged, while it turns negatively charged at pH values above this threshold Additionally, CIP exhibits pKa values of 5.90 and 8.89, indicating its speciation varies with pH, further impacting its interaction with the nanocomposite.
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 CIP molecule contains functional groups such as -F, -COOH, =O, and benzene rings, which possess high electron affinity and are partially negatively charged This creates an electrostatic attraction between the negatively charged groups of the CIP molecule 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 a coordinate covalent bond with H + using its lone pair of electrons Additionally, the piperazine ring displays a partial positive charge due to its conjugation with the adjacent benzene ring This strong interaction between the negatively charged nanocomposite and the significantly positively charged -NH2 + enhances the overall stability and functionality of the system.
The MgFe2O4/WO3/rGO nanocomposite exhibits more active binding sites at a neutral pH of 7, leading to higher efficiency in CIP removal compared to acidic conditions at pH values of 3 and 5 This increased efficiency is attributed to the stronger interactions between the functional groups of the nanocomposite and the contaminants at pH 7.
At pH levels of 9 and 11, both the CIP species (A -) and the MgFe2O4/WO3/rGO nanocomposite carried a negative charge, resulting in reduced interaction between them and consequently low CIP removal efficiency Additionally, the dosage of the catalyst 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 demonstrated that the removal efficiencies of CIP using various dosages of MgFe2O4/WO3/rGO nanocomposite were significant, with Ct/Co values recorded at 0.37, 0.17, 0.09, and 0.15 for dosages of 0.25, 0.50, 0.75, and 1.0 g/L, respectively This translates to removal efficiencies of 63%, 83%, 91%, and 85%, indicating that the optimal dosage for maximum efficiency was found to be 0.75 g/L, achieving the highest removal rate of 91%.
The photodegradation efficiency of CIP improved with increasing doses of MgFe2O4/WO3/rGO nanocomposite, rising from 0.25 to 0.75 g/L due to enhanced active sites and the generation of reactive oxygen species (ROS) such as HO• and O2 -• radicals However, increasing the dosage further to 1.0 g/L led to greater turbidity in the solution, which obstructed and scattered incoming light, resulting in reduced electron transitions from the valence band to the conduction band This decrease in electron activity ultimately led to a lower generation of ROS and 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
After treatment, the CIP removal efficiencies at initial concentrations of 2.5, 5, 7.5, and 10 ppm were 79%, 91%, 84%, and 80%, respectively, with the highest efficiency observed at 5 ppm The Ct/Co values for CIP removal were recorded as 0.21, 0.09, 0.16, and 0.20, indicating the effectiveness of the treatment across varying concentrations.
Higher CIP removal efficiency was observed when the initial concentration of CIP was optimized, facilitating the effective binding of CIP molecules to the active 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, achieving a maximum CIP removal efficiency of 91%.
In this study, the optimal conditions for CIP degradation were evaluated against previous research, as shown in Table 4.3 Notably, the MgFe2O4/WO3/rGO nanocomposite demonstrated exceptional degradation efficiency, outperforming other photocatalysts, even under visible-light illumination instead of 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
Three pseudo-kinetic models were utilized to analyze the photodegradation of CIP solutions at varying concentrations (2.5, 5, 7.5, and 10 ppm) and a pH of 7, using a MgFe2O4/WO3/rGO nanocomposite at a dosage of 0.75 g/L The suitability of each model was assessed through R² values, with values closer to 1 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.