INTRODUCTION
Research background
Antibiotics have revolutionized the treatment of infections, but the rising concern over antibiotic residues in the environment has garnered significant attention Studies indicate the presence of antibiotic resistance in various environmental matrices, including surface and groundwater, sediments, soils, and food These residues contribute to the development of antibiotic-resistant bacteria, posing risks to both ecosystems and human health, including allergies, mutations, and reproductive disorders The misuse of antibiotics has led to a surge in antibiotic-resistant bacteria and genes, threatening the effectiveness of life-saving medicines In 2021, the World Health Organization identified antimicrobial resistance (AMR) as a critical global health issue requiring urgent collective action.
Conventional wastewater treatment plants (WWTP) are unable to fully eliminate antibiotics and antibiotic-resistant bacteria, as they primarily utilize physical and biological methods (Baquero et al., 2008; Manoharan et al., 2022; K Wang et al., 2021) Therefore, it is essential to create supplementary treatment technologies to effectively remove these significant pollutants from wastewater.
Research significance
In recent years, photocatalytic materials have gained prominence as an effective and cost-efficient solution for antibiotic treatment and water disinfection Various photocatalysts, including TiO2, ZnO2, In2O3, and CdSe, have been extensively researched for their ability to treat organic pollutants in the environment.
Graphitic carbon nitride (g-C3N4) has gained significant attention in academia due to its properties as a metal-free, non-toxic, and photochemically stable semiconductor capable of driving photo-oxidation reactions under visible light However, its photocatalytic efficiency is hindered by a high recombination rate of photo-induced electrons and holes, low light absorption efficiency, and a limited specific surface area Therefore, it is essential to investigate the synthesis of g-C3N4-based photocatalysts to overcome these challenges and harness their potential for removing organic pollutants from aqueous environments.
CoMoO4 is a transition metal molybdate known for its narrow band gap energy (2.1 – 2.8 eV), excellent redox activity, and strong catalytic electrochemical properties, making it a promising visible-light-driven photocatalyst This photocatalyst has demonstrated effectiveness in removing organic pollutants and inactivating bacteria However, its application in organic compound photodegradation is hindered by the low potential energy of its conduction band and the rapid recombination of photo-induced electrons and holes.
g-C3N4 and CoMoO4, despite their individual photocatalytic limitations, can be effectively combined due to their compatible bandgap energies, resulting in a heterojunction that significantly enhances photocatalytic activity.
This study addresses the lack of research on developing a composite material aimed at treating tetracycline antibiotics and E coli antibiotic-resistant bacteria By focusing on creating a high-potential photocatalyst, the research aims to tackle the pressing problem of antibiotic and antibiotic-resistant bacteria residues in wastewater.
Research objectives
This research is implemented to achieve the following objectives:
(1) Synthesize g-C3N4/CoMoO4 heterojunction photocatalyst exhibiting high photocatalytic activity under visible light h
(2) Characterize the synthesized g-C3N4/CoMoO4 heterojunction photocatalyst
(3) Investigate the synthesized material’s photocatalytic removal efficiency with the Tetracycline antibiotic
(4) Investigate the synthesized material’s photocatalytic inactivation efficiency with E coli antibiotic-resistant bacteria.
Thesis structure
This thesis is structured into 5 chapters The main contents of each chapter are presented below:
Chapter 1 introduces the research context and describes the significance of the topic, as well as points out the research objectives
Chapter 2 highlights the issues of the antibiotic and antibiotic-resistant bacteria residue in wastewater, current technologies to address these issues, and introduces the photocatalytic oxidation process utilizing photocatalyst as an emerging strategy
Chapter 3 describes the materials and methods to prepare and characterize the target photocatalysts Experimental designs to investigate the photocatalytic activity of the synthesized photocatalyst in removing antibiotic and antibiotic-resistant bacteria are also elaborated
Chapter 4 presents and discusses the results obtained from all the characterization analyses and experiments conducted
Chapter 5 concludes the key results of this research and recommendations for further research to be conducted h
LITERATURE REVIEW
Issue of antibiotic and antibiotic-resistant bacteria residues in wastewater
2.1.1 Issue of antibiotic residues in wastewater
Antibiotics are crucial antimicrobial agents specifically designed to combat bacterial infections They function by either killing bacteria or inhibiting their growth, making them essential in the treatment of various bacterial diseases (Waksman, 1947).
The widespread use of antibiotics in human health, veterinary medicine, and agriculture has led to significant environmental contamination Studies indicate that 40–90% of prescribed antibiotics are excreted unchanged in feces and urine, which adversely affects soils, waterways, and plant life (Baquero et al., 2008).
Antibiotic residues in the environment can adversely affect ecosystems and human health, leading to issues such as allergies, mutations, and reproductive disorders through the consumption of contaminated food and water (M Wang et al., 2017).
The misuse of antibiotics contributes to the development of antibiotic-resistant bacteria and genes, which can transfer from the environment to humans Increased antibiotic usage is believed to undermine treatment effectiveness in human medicine, leading to prolonged illness, increased morbidity, and higher mortality rates (World Health Organization, 2018).
Improper disposal of unused medications into sewage systems poses a significant environmental concern, as wastewater treatment plants (WWTPs) struggle to completely eliminate antibiotics Common antibiotic classes detected in WWTPs include macrolides, sulfonamides, trimethoprim, quinolones, and tetracyclines Consequently, both the sludge produced by these plants and the treated effluent may contain residual antibiotics, with sludge potentially being used as fertilizer and effluent discharged into surface waters.
Vietnam faces a critical situation due to the reckless use of antibiotics in human healthcare and agriculture The lack of effective wastewater treatment systems exacerbates the issue, allowing antibiotic residues to contaminate aquatic environments extensively Furthermore, various activities contribute to significant antibiotic emissions into water sources, with aquaculture being a primary source (Binh et al., 2018).
Tetracycline is a widely utilized antibiotic known for its broad-spectrum efficacy against various pathogens, including Gram-positive and Gram-negative bacteria, fungi, rickettsia, and parasites As the second most produced and consumed antibiotic globally, it is favored for its affordability, low toxicity, and oral administration Tetracycline plays a significant role in human medicine, veterinary practices, and agricultural feed (Daghrir & Drogui, 2013).
Tetracycline is categorized into three types based on its manufacturing methods: natural, semi-synthetic, and synthetic Natural tetracyclines, such as tetracycline, oxytetracycline, and chlortetracycline, are derived from the fermentation of Streptomyces sp bacteria In contrast, semi-synthetic and synthetic tetracyclines are produced through chemical transformations of precursor compounds (Borghi & Palma, 2014).
Tetracycline exhibits three distinct pKa values at pH levels of 3.3, 7.7, and 9.7, corresponding to the protonation of specific functional groups within its structure At pH 3.3, tetracycline exists in a cationic form, transitioning to a zwitterionic state between pH 3.3 and 7.7, and becoming anionic when the pH exceeds 7.7 This shift results in an increase in the compound's negative charge, with 25% of tetracycline existing in anionic form at pH levels above 7.0 Notably, tetracycline is soluble in alcohols but not in organic solvents, as illustrated in the accompanying speciation diagram.
Figure 2.1 (a) Structure and (b) speciation diagram of tetracycline antibiotic
Tetracycline's high hydrophilicity and low volatility enable it to remain in the environment for extended periods, promoting gene mutation and lateral gene transfer among aquatic microbes This process leads to the emergence of antibiotic resistance genes (ARGs), which pose a greater threat than the chronic toxicity of anthelmintics due to their persistence and ability to easily transfer between bacteria.
2.1.3 Issue of antibiotic-resistant bacteria residues in wastewater
Antibiotic resistance occurs when bacteria are no longer affected by standard doses of antibiotics, allowing them to survive and multiply, which leads to increased harm The overuse of antibiotics contributes significantly to the spread of this resistance To assess bacterial susceptibility to antibacterial agents, the minimum inhibitory concentration (MIC) is measured, which indicates the lowest concentration needed to prevent bacterial growth (Guilfoile & Alcamo, 2007).
Multiple drug resistance refers to the ability of microorganisms to resist the effects of two or more drugs or drug classes Cross-resistance occurs when resistance to one antibiotic also leads to resistance against another antibiotic that the organism has not previously encountered Currently, over 70% of the bacteria responsible for hospital infections are resistant to commonly used antibiotics (Wang et al., 2020) Some pathogens have developed resistance to all approved antibiotics, necessitating treatment with experimental and potentially harmful alternatives (Bisht et al., 2009).
Antibiotic-resistant bacteria (ARBs) and antibiotic resistance genes (ARGs) are prevalent in various environments, including wastewater treatment facilities, hospital discharges, and pharmaceutical waste, where high bacterial densities coexist with low antibiotic levels This unique combination fosters the release of ARBs and ARGs into the ecosystem Key factors driving their spread include the selection pressure from even minimal antibiotic exposure and horizontal gene transfer among bacteria The global rise of antibiotic resistance poses a significant threat to public health, with projections estimating up to 10 million deaths by 2050 due to infections caused by these resistant strains.
Wastewater treatment plants receive sewage from various sources, including hospitals and households, representing important sources of antibiotics as well (Nagulapally et al., 2009; Zhang et al., 2009)
Current wastewater treatment plant (WWTP) technologies often fail to effectively eliminate all bacteria, creating ideal conditions for the proliferation of bacteria and antibiotic-resistant genes The biological treatment process fosters a suitable environment for bacterial growth and enhances horizontal gene transfer Continuous exposure to low, sub-inhibitory doses of antibiotics places selective pressure on susceptible bacteria, contributing to the emergence of antibiotic-resistant strains Consequently, WWTP effluents serve as a primary pathway for the dissemination of antibiotic resistance in aquatic environments.
The predominant bacterial species that are found in WWTPs include E coli, Coliforms,
Enterococci, Enterobacteria, Pseudomonads, and Acinetobacter, vancomycin-resistant Enterococcus spp, and methicillin-resistant Staphylococcus aureus (Noor et al., 2021)
2.1.4 Escherichia coli (E coli) antibiotic-resistant bacteria
E coli is a bacterium with a special position in the microbiological world due to its potential to cause serious infections in both humans and animals as well as its important function in the innate microbiota of the various hosts The possible transmission of virulent and resistant E coli between animals and humans through h various pathways, including direct contact, contact with animal excretions, or via the food chain is causing critical concern in both academia, government, and the public (Blount, 2015)
Technologies to remove antibiotics and inactivate antibiotic-resistant bacteria
This method focuses on the collection and elimination of organic pollutants from wastewater by using absorbent materials to extract harmful impurities, effectively purifying and disinfecting the effluent Extensive research has explored the effectiveness of different adsorbents, such as activated carbon, metals, and silica gel, in removing antibiotics and antibiotic-resistant bacteria (ARB) from wastewater (Lu et al., 2020).
Natural and modified adsorbents face challenges related to economic feasibility, applicability, removal efficiency, and regeneration Activated carbon, known for its excellent adsorption capability, is widely used to remove antibiotics and antibiotic-resistant bacteria (ARB) through physical adsorption This amorphous carbon material features a large surface area and high porosity A study by Zhang et al (2016) demonstrated that powdered activated carbon effectively eliminated 28 antibiotics across six groups, achieving a maximum removal efficiency of 91.9% in deionized water under optimal conditions (20 mg/L dose and 120 minutes contact time) However, various factors such as the type of activated carbon, initial concentration of target compounds, pH, temperature, and adsorbent concentration can influence antibiotic absorption efficiency, despite the effectiveness of activated carbon in treating high concentrations of antibiotic compounds (Le-Minh et al., 2010).
Hiller et al (2019) investigated the removal of antibiotic resistance genes (ARGs) from digested swine wastewater, achieving a 95% reduction in both antibiotics and ARGs While adsorption plays a crucial role in wastewater treatment by facilitating rapid mass transfer of pollutants, it does not achieve complete elimination or biodegradation of contaminants Furthermore, the disposal of contaminated adsorbents poses significant challenges that limit the effectiveness of this method in removing antibiotics and antibiotic-resistant bacteria (ARB) from wastewater.
Conventional wastewater treatment plants (WWTPs) effectively remove total organic carbon and nutrients such as nitrates and phosphates, but they fall short in addressing micropollutants like antibiotics and antibiotic-resistant genes (ARGs) Aerobic and anaerobic treatment methods not only excel in reducing antibiotic-resistant bacteria (ARBs) and ARGs but are also low-energy and environmentally friendly, significantly lowering chemical oxygen demand While conventional activated sludge is the most widely used biological treatment method, it is not specifically designed for antibiotic removal, achieving only up to 90% efficiency (Hazra & Durso, 2022) Moreover, the substantial byproduct of activated sludge can lead to the absorption of antibiotics into sludge flocs Research conducted by Zhu et al (2020) on ARG profiles in swine wastewater demonstrated that a combination of anaerobic digestion, primary sedimentation, and secondary sedimentation could achieve an overall reduction of 84% in ARGs.
Antibiotic residues in wastewater are adsorbed onto activated sludge, making sludge treatment effective for both biodegradation and adsorption of pollutants The binding of pharmaceuticals to sludge is influenced by electrostatic and hydrophobic interactions During biological processes, microorganisms produce extracellular polymers, such as polysaccharides and proteins, essential for antibiotic adsorption Additionally, the antimicrobial properties of these antibiotics can affect the overall efficacy of activated sludge treatment (Hazra & Durso).
Previous studies indicate that biological treatment units can serve as hotspots for the spread of antibiotic resistance genes (ARGs), attributed to the rich nutrient environment and high bacterial density in activated sludge, which promotes horizontal gene transfer among different bacterial species (Guo et al., 2017).
Chemicals play a crucial role in wastewater treatment by facilitating disinfection through chemical unit processes These processes, which trigger chemical reactions, work in conjunction with biological and physical methods to meet diverse water quality standards Among the latest chemical technologies for eliminating antibiotics and antibiotic-resistant bacteria (ARB) from wastewater are chlorination, ozonation, and photocatalysis.
Chlorination is an effective method for disinfecting drinking water by adding chlorine to eliminate parasites, bacteria, and viruses This process is commonly used in water and wastewater treatment facilities due to chlorine's low cost as an oxidant Studies indicate that chlorination can remove antibiotics at rates between 50% and 90% (Acero et al., 2010) Additionally, a log-3 inactivation of various bacteria strains in wastewater has been achieved with specific contact times and chlorine concentrations (Helbling & VanBriesen, 2007) However, chlorination has limitations, including the requirement for high concentrations of free chlorine, the need for pH adjustments, and the potential formation of harmful byproducts that may be more dangerous than the original contaminants (Acero et al., 2010).
Ozonation is a chemical water treatment method that infuses ozone (O3), a powerful oxidant, into water to effectively eliminate pollutants, including antibiotic residues and antibiotic-resistant bacteria (ARB) The process involves ozone's transformation into oxygen, where its highly reactive nature targets and binds with contaminants While some studies report successful degradation of antibiotics through ozonation, other research indicates that the method's mineralization and overall wastewater purification efficiency are limited Additionally, ozonation's effectiveness is influenced by pH variations and necessitates costly equipment and significant energy, making it less suitable for treating pharmaceutical-contaminated environments.
Photocatalysis is a key advanced oxidation process (AOP) that utilizes photocatalyst materials to produce powerful oxidation compounds when stimulated Due to their recyclable nature and low toxicity risks, photocatalysts have been extensively researched for their effectiveness in treating antibiotics and antibiotic-resistant bacteria (ARBs) in wastewater (Baaloudj et al., 2021; Chen et al., 2022; Hiller et al., 2019; Zhu et al., 2020).
The photocatalytic oxidation process is driven by the transfer of electrons from the valence band (VB) to the conduction band (CB) within a photocatalyst Research indicates that as the number of orbitals in the VB, or the highest occupied molecular orbital (HOMO), and the CB, or the lowest unoccupied molecular orbital (LUMO), increases, the energy required for this electron transfer decreases (J Zhang et al., 2018).
When light with energy equal to or greater than the bandgap of a semiconductor photocatalyst is irradiated, electrons on the surface of the photocatalyst are excited, moving from the valence band (VB) to the conduction band (CB) and creating holes These photo-generated electrons and holes, in an aqueous environment, facilitate the formation of reactive oxygen species such as superoxide anions (.O2-) and hydroxyl radicals (.OH-), which are essential for degrading organic pollutants into carbon dioxide, water, and less harmful byproducts.
According to Hofmann et al., photo-generated electrons and holes can recombine, releasing heat within 10 to 100 nanoseconds, which contributes to the photocatalyst's low quantum efficiency However, this recombination process can be significantly accelerated by incorporating suitable scavengers or creating trap sites on the surfaces through the introduction of flaws or surface adsorbents (Wenderich & Mul, 2016).
For effective photocatalytic reactions, it is essential to allow sufficient time for the separation of photo-generated charge carriers, namely holes and electrons, before their recombination These charge carriers migrate to the surface of the photocatalyst, where they can initiate redox reactions with adsorbed pollutants The VB hole (h+) exhibits significant oxidation power, with a redox potential ranging from +1.0 to +3.5 V, influenced by factors such as the type of photocatalyst and environmental conditions like pH The presence of holes is vital for the photocatalytic degradation of organic compounds on the catalyst surface Oxidation can occur directly through h+ interacting with the surface or indirectly via surface-bound hydroxyl radicals.
Figure 2.2 Schematic diagram of the photocatalytic oxidation process of organic pollutants in the aqueous environment
Surface hydroxyl radicals can effectively mineralize contaminants through oxidation To avoid excess charge accumulation in catalytic particles, it is crucial for photoexcited electrons to be efficiently removed Enhancing electron removal significantly boosts the photocatalytic oxidation process of these contaminants.
g-C 3 N 4 and CoMoO 4 photocatalyst
Graphitic carbon nitride (g-C3N4) is a non-metallic semiconductor with a layered structure similar to graphite It features a small bandgap energy of approximately 2.7 eV, enabling it to operate effectively under sunlight This material is not only stable but also capable of large-scale synthesis Due to its unique properties and potential applications, g-C3N4 is gaining significance in various fields, including photocatalysis, heterogeneous catalysis, and as substrates.
Recent advancements in nanostructures and nanocapillary g-C3N4 materials have opened up new applications due to their remarkable properties g-C3N4 is gaining recognition as a promising material because of its chemical and thermal stability, low density, non-corrosiveness, and impermeability to water Research conducted by Gillan highlighted g-C3N4's exceptional chemical stability, demonstrating its near insolubility in solvents such as water, ethanol, toluene, diethyl ether, and Tetrahydrofuran, likely attributed to the Van der Waals forces between its overlapping layers.
Graphitic carbon nitride (g-C3N4) features a graphite-like layered structure characterized by stable structural units, including triazine and tri-s-triazine Under normal conditions, g-C3N4 exhibits its most stable allotrope, consisting of layers that stack along the axis to form graphite faces made up of hexagonal triazine rings (C3N3) The bonds between these rings are reinforced by nitrogen atoms X-ray powder diffraction (XRD) studies confirm the layered structure of g-C3N4, with tri-s-triazine being recognized as the primary structural unit due to its enhanced stability.
Figure 2.3 Structure of graphitic carbon nitride
Graphitic carbon nitride (g-C3N4), with a low bandgap of approximately 2.7 eV, is capable of driving photo-oxidation reactions under visible light However, pure g-C3N4 faces challenges such as low redox potential and a high recombination rate of photo-induced electrons and holes, significantly hindering its photocatalytic efficiency Various strategies have been explored to enhance its performance, including modifications to the material's size and structure.
Recent studies have focused on enhancing photocatalytic performance through various methods, including nonmetal and metal doping, as well as coupling with other photocatalysts For instance, Liu et al (2017) achieved a remarkable increase in the degradation of Rhodamine B from 30% to 100% by synthesizing mesoporous g-C3N4 nanorods using the nano-confined thermal condensation method Similarly, Dai et al (2020) improved g-C3N4's effectiveness by doping it with Cu via a thermal polymerization route, resulting in a 90.5% degradation rate of the antibiotic norfloxacin These advancements highlight the potential of modifying g-C3N4 to enhance its photocatalytic capabilities.
& Ayyappan, 2020) synthesized hybridized g-C3N4/ZnBi2O4 for reduction of 4- nitrophenol and reached an optimal removal efficiency of 79.0%
The construction of heterostructure photocatalysts by coupling g-C3N4 with other semiconductors is an effective strategy to enhance photocatalytic efficiency for contaminant treatment by preventing electron and hole recombination.
CoMoO4 is recognized as a promising visible-light-driven photocatalyst due to its narrow band gap energy ranging from 2.1 to 2.8 eV, excellent redox activity, and robust electrochemical catalytic properties Research has extensively explored the structure of cobalt molybdate, which exists in three polymorphic forms Notably, the α-CoMoO4 modification crystallizes in the monoclinic space group C2/m (No 12), while the β-CoMoO4 phase is distinguished by the tetrahedral coordination of Mo 6+ atoms, setting it apart from the α-phase.
CoMoO4 has been synthesized through multiple methods in previous studies, including precipitation, sol-gel, solid-state reaction, hydrothermal processes, and complete evaporation (Adabavazeh et al., 2021; Gao et al., 2019; Rosić et al., 2018; Umapathy & Neeraja).
Figure 2.4 Crystal structure of cobalt molybdate
The CoMoO4 photocatalyst has demonstrated effectiveness in removing organic pollutants and inactivating bacteria (Adabavazeh et al., 2021; Feizpoor et al., 2019; Gao et al., 2019; Umapathy & Neeraja, 2016) However, its use for organic compound photodegradation is hindered by the low potential energy of the conduction band and rapid recombination of photo-induced electrons and holes To overcome these limitations, studies have been conducted, such as the work by Umapathy and Neeraja (2016), which improved the photocatalytic activity of CoMoO4 through the creation of CoMoO4/TiO2 nanocomposites, achieving a degradation efficiency of 97.5% for 4-chlorophenol, significantly higher than the 88.0% efficiency of pure CoMoO4.
Development of g-C 3 N 4 /CoMoO 4 heterostructure photocatalyst
The incorporation of g-C3N4 as a hybridization agent with CoMoO4 enhances the photocatalytic activity by creating a conjugated photocatalyst system g-C3N4 has a more negative conduction band (-1.24 eV) compared to CoMoO4 (0.67 eV), while CoMoO4 features a relatively positive valence band (2.63 eV) This arrangement theoretically promotes electron transitions within the coupled photocatalyst, extending electron-hole separation In the hybrid system, electrons from CoMoO4's conduction band can easily migrate to g-C3N4's valence band, effectively reducing the recombination of electrons and holes in both materials In contrast, traditional systems see rapid recombination of photogenerated electrons and holes, resulting in a significant decline in conversion efficiency.
Under sunlight, CoMoO4 and g-C3N4 can be excited to produce electron-hole pairs The electrons in the conduction band of CoMoO4 transfer and recombine with holes in the valence band of g-C3N4, leading to an accumulation of photogenerated electrons in g-C3N4's conduction band This process enhances the reduction of adsorbed O2, resulting in the formation of more •O2- Simultaneously, the holes remaining in the valence band of CoMoO4 oxidize adsorbed H2O, generating •OH Consequently, this interaction significantly boosts the photocatalytic activity of g-C3N4.
C3N4/CoMoO4 system would be significantly increased, leading to the decomposition of organic compounds by •O2- and •OH reactive species
Previous studies on the synthesis of g-C3N4 and CoMoO4-based heterojunction photocatalysts have highlighted their effectiveness in pollutant removal, as summarized in Table 2.1 For instance, Habibi-Yangjeh et al demonstrated that their synthesized visible-light-driven g-C3N4/Fe3O4/CoMoO4 photocatalysts achieved removal efficiencies of 62.9% for rhodamine B, 72.8% for methylene blue, and 71.5% for fuchsine Additionally, Zhang et al investigated the photocatalytic performance of g-C3N4 composites with varying CoMoO4 ratios, achieving a remarkable 94.0% degradation rate for methylene blue However, these studies have not systematically examined the influencing factors in the synthesis of g-C3N4/CoMoO4 photocatalysts.
To date, there has been no research into the development of this target composite for the treatment of tetracycline antibiotic and E coli antibiotic-resistant bacteria
Table 2.1 g-C3N4 and CoMoO4-based heterojunction photocatalyst
Photocatalyst Target pollutant Performance Ref g-C3N4/Fe3O4/CoMoO4
RbB: 72%, 60 min He et al
Ciprofloxacin antibiotic 93.4%, 60 min Hu et al
TiO2/CoMoO4/PANI Rhodamine B 99%, 90 min Feizpoor et al (2019)
CoMoO4-Fe-g-C3N4 Methylene blue 99.5%, 90 min
MATERIALS AND METHODOLOGY
Chemicals and apparatus
In this study, melamine sourced from Tianjin Damao Chemical Reagent Factory (China, 99.5%) served as the precursor for synthesizing g-C3N4 For the preparation of CoMoO4, cobalt nitrate from Guangdong Guanghua Sci-Tech Co., Ltd (China, 98.5%) and sodium molybdate from Dezhou Jinmao Chemical Co., Ltd (China, 99.0%) were utilized as precursors Additionally, absolute ethanol from Duc Giang Chemicals Group JSC Co (Vietnam, 99.7%) was employed to wash the synthesized materials.
In the reactive oxygen species trapping experiments, Tetracycline powder (AK Scientific Inc., United States, 95.3%) was utilized to prepare solutions, while p-benzoquinone (Xiya Reagent Co., Ltd, China, 99.0%) and isopropyl alcohol (Xilong Scientific Co., Ltd, China, 99.7%) served as scavengers To adjust the pH in the reactor, sodium hydroxide (Xilong Scientific Co., Ltd, China, 96.0%) and hydrochloric acid (Xilong Scientific Co., Ltd, China, 38.0%) were employed.
In microbiological experiments, Trypto-Soy Broth (Eiken Chemical Co., Ltd, Japan) was used as the cultivation medium of E Coli, and Tryptone Bile X-glucuronide
TBX agar from Merck Millipore Corp., Germany, was enhanced with 95.3% Tetracycline powder from AK Scientific Inc., United States, for the enumeration of bacterial colonies Additionally, Phosphate Buffer Saline (PBS) powder from Wako Pure Chemical Industries, Ltd., Japan, was utilized to prepare the buffer solution in the photocatalytic reactor, which contained E coli bacteria isolated from the To Lich River in Vietnam.
Photocatalyst preparation
Graphitic carbon nitride was synthesized through the calcination of melamine in a lidded crucible at a temperature of 550 °C for 4 hours After cooling to room temperature, the resulting yellow powder was finely ground using an agate mortar.
To prepare pristine CoMoO4, cobalt nitrate and sodium molybdate were dissolved in distilled water with magnetic stirring for 30 minutes, resulting in 1 M solutions of each precursor.
After that, the prepared cobalt nitrate solution was slowly added to sodium molybdate solution under a constant stirring condition for 1 hour
The mixture was placed in a stainless-steel autoclave and subjected to a hydrothermal process at 180°C for 6 hours Afterward, the product underwent centrifugation to eliminate the supernatant and was dried at 60°C for 24 hours, resulting in the formation of purple CoMoO4 powder.
The g-C3N4/CoMoO4 composite was synthesized using a hydrothermal-calcination method Initially, 1 g of g-C3N4 was mixed with distilled water and stirred magnetically for 30 minutes Concurrently, cobalt nitrate and sodium molybdate were combined in distilled water and stirred for 30 minutes to create the precursor mixtures.
The prepared g-C3N4 was mixed with CoMoO4 in a weight ratio of 6:4 and stirred for one hour The resulting mixtures were then placed in a 100 ml autoclave and subjected to hydrothermal reactions at 180℃ for either 3 or 6 hours After the reactions, the final samples underwent centrifugation and were washed twice with distilled water and ethanol.
Finally, the samples were dried, and finally, the dried products were heated in a Muffle furnace at different calcination temperatures of 300°C, 400°C, and 500 °C for 4 hours to get the target composites
This study examines how different preparation conditions, such as hydrothermal time, calcination temperature, and the mixing ratio of two components, influence the photocatalytic activities of the final material The variations in these conditions across samples are detailed in Table 3.1.
Table 3.1 Optimization of preparation conditions of g-C3N4/CoMoO4 composite
Sample name g-C 3 N 4 :CoMoO 4 mass ratio
Photocatalyst characterization
Scanning Electron Microscopy (SEM) is a powerful imaging technique that generates detailed two-dimensional images by scanning a sample with a high-energy electron beam in a raster pattern This method provides valuable insights into the microstructure of coated surfaces, the distribution and homogeneity of photocatalysts, and the morphology of particles within coatings In SEM, a primary electron beam is created in a high vacuum and directed across the specimen's surface The interaction between the electrons and the sample's atoms produces various signals that reveal critical information about the surface and elemental composition of the material (Pinto et al., 2018; Perret et al., 2005).
Before conducting a Scanning Electron Microscope (SEM) investigation, samples are placed on a stub and sputter-coated with gold to create a protective layer, ensuring optimal results under high voltage The SEM is managed by a connected computer, which establishes a vacuum to connect the sample on an exchange rod to the electron column Once the stub is inserted into the sample holder within the electron column, it is crucial to maintain the sample's alignment and close the column After adjusting the working distance and magnification using the controller, SEM images are captured for subsequent analysis (Perret et al., 2005).
This research examines the surface morphology of synthesized materials using a Hitachi TM 4000 Plus scanning electron microscope (SEM) at the VNU Key Laboratory of Advanced Materials for Green Growth, University of Science, Vietnam.
3.3.2 Energy-dispersive X-ray analysis (EDX)
The elemental composition of the material is analyzed through energy-dispersive X-ray (EDX) analysis using a MisF+ instrument from Oxford Instruments plc., UK, which is coupled with a scanning electron microscope (SEM) at the VNU Key Laboratory of Advanced Materials for Green Growth, University of Science, Vietnam.
During analysis, a high-energy electron beam bombards the specimen, leading to the emission of X-rays from the surface atoms This interaction produces two types of X-rays: Bremsstrahlung X-rays, or continuous X-rays, and characteristic X-rays.
Bremsstrahlung X-rays are produced by the interaction of an incident electron with the specimen's atomic nuclei This takes the form of a background spectrum over which the particular X-ray spectra are overlaid Characteristic X-rays, on the other hand, are produced when high-energy electrons hit lower-energy state vacancies, resulting in the transition of higher-energy electrons to lower-energy state vacancies The energy difference between the higher and lower energy states correlates to the energy of emitted X-rays and is affected by the specimen's properties The X-ray lines in the X- ray spectrum represent the distinctive X-rays The shell holding the inner void is indicated by capital Roman letters such as K, L, or M, whilst the Greek letters and numbers indicate the group to which the line belongs in decreasing order from to and the line's intensity in decreasing order from 1 to 2, respectively
An X-ray detector captures both Bremsstrahlung and typical X-ray emissions, which are shown as a spectrum of X-ray energy versus intensity The energy of the distinctive X-rays allows qualitative analysis to show the components that contribute to the specimen, whilst the intensity of the corresponding X-rays allows quantitative analysis to reveal the concentration of the elements present (Bell & Garratt-Reed,
3.3.3 Brunauer-Emmett-Teller analysis (BET)
BET analysis was performed using a BET Nova Touch LX4 (Quantachrome Corp., USA) to investigate the pore structure of the material This analysis was conducted at the VNU Key Laboratory of Advanced Materials for Green Growth at the University of Science, Vietnam.
BET analysis measures the specific surface area of materials based on the physical adsorption of gas molecules on solid surfaces Adsorption refers to the adhesion of gas atoms or molecules to a surface, and the amount of gas adsorbed is influenced by factors such as the exposed surface area, temperature, gas pressure, and the strength of the gas-solid interaction.
Nitrogen is widely used in BET surface area analysis due to its high purity and strong interaction with various materials To achieve measurable adsorption levels, the surface is cooled with liquid nitrogen, and the sample compartment is filled with known amounts of nitrogen gas under partial vacuum conditions, creating relative pressures below atmospheric levels Once saturation pressure is reached, no further adsorption occurs, regardless of increased pressure High-precision pressure transducers monitor pressure changes during the adsorption process After the adsorption layers form, the sample is removed from the nitrogen environment and heated to release and quantify the adsorbed nitrogen The resulting data is illustrated in a BET isotherm, which plots the amount of gas adsorbed against relative pressure.
3.3.4 X-ray powder diffraction analysis (XRD)
An X-ray diffractometer is used to perform structural characterization on a wide range of specimens from powder to thin films When using XRD, the constructive interference of monochromatic X-rays and a crystalline sample allows for the analytical investigation of the crystal structure (Bunaciu et al., 2015) In fact, the detailed mechanism of an XRD follows the Bragg equation, which is shown in Equation 1:
In X-ray diffraction, the integer n, characteristic wavelength λ, interplanar spacing d, and angle θ are key components in Bragg's law, which describes the conditions for constructive interference and the resulting diffracted beam By analyzing the direction of this diffracted beam, researchers can determine the lattice spacing (d-spacing) of a specimen, allowing for the identification of various structural features such as strain, structural orientation, crystallite size, and chemical composition.
This study utilized an XRD MiniFlex 600 (Rigaku Corp., Japan) to analyze the crystal structures of materials at the VNU Key Laboratory of Advanced Materials for Green Growth, University of Science, Vietnam The analysis was conducted with a scanning step of 0.02 degrees over a 2θ range of 0 to 60 degrees.
3.3.5 Fourier transform infrared spectroscopy (FTIR)
Fourier transform infrared spectroscopy (FTIR) is a powerful analytical technique that captures the infrared spectrum of a sample's absorption, emission, and photoconductivity across solid, liquid, and gas phases This method reveals the molecular vibration spectrum, making FTIR analysis essential for identifying organic, inorganic, and polymeric materials by utilizing infrared light to scan samples effectively.
FTIR analysis offers numerous advantages, including a high signal-to-noise ratio, exceptional wavenumber accuracy, and increased sensitivity Its rapid scanning capabilities, with resolutions ranging from 0.1 to 0.005 cm -1, allow for detailed examination of peak positions, intensities, widths, and shapes, which all provide valuable information These benefits have led to FTIR spectrometers increasingly replacing traditional dispersive IR spectrometers (Moore, 2016; Titus et al., 2019).
Experimental setup
3.4.1 Photocatalytic removal of tetracycline antibiotic
The photocatalytic activity of the synthesized materials was evaluated by removing tetracycline antibiotic under visible light from a 20 W LED lamp During the experiment, a magnetic stirrer facilitated mixing, while a fan circulated air to maintain the system at room temperature.
In a standard experimental setup, specific amounts of photocatalyst were mixed with 200 ml of tetracycline solution, followed by a 60-minute stirring period in the dark to achieve adsorption-desorption equilibrium Every 30 minutes, a 5 ml sample from the reaction mixture was centrifuged at 5000 RPM for 10 minutes, and the tetracycline concentration in the supernatant was determined by measuring absorbance at 357 nm using a UV-visible spectrophotometer Each experiment was conducted in triplicate to ensure reliability of results.
The concentration of tetracycline in samples was calculated using the following calibration equation [2]:
0.0341 [ 2 ] where 𝐶 (mg/L) is the concentration of tetracycline and 𝐴𝑏𝑠 (a.u.) is the absorbance measured at 357 nm by the UV-visible spectrophotometer
The tetracycline removal efficiency was calculated using the equation [3]:
𝐶 0 × 100% [ 3 ] where 𝐶 0 (mg/L) is the concentration of tetracycline solution at the initial time t = 0 (min), and 𝐶 (mg/L) is the concentration of tetracycline in the supernatant collected at each time interval
Figure 3.8 shows the calibration curve for Tetracycline concentration vs absorbance: h
Figure 3.8 Standard calibration curve of Tetracycline
This research explores how photocatalyst dosage, initial pollutant concentration, and pH levels affect the removal efficiency of tetracycline The study varied photocatalyst dosage between 1 to 5 grams in a small tetracycline solution, identifying the optimal amount for further testing The removal efficiency was then assessed across different pH values (3, 5, 7, 9, and 11) using the optimal photocatalyst dosage Additionally, experiments were conducted with initial tetracycline concentrations of 5, 10, and 15 mg/L to establish the best operating conditions for effective pollutant removal.
3.4.2 Photocatalytic inactivation of E coli antibiotic-resistant bacteria
All the antimicrobial experiments were carried out using autoclave sterilized glassware at 121 o C for 30 minutes
Firstly, E coli sample isolated from To Lich river (Vietnam) was cultured in the autoclaved Trypto-Soy Broth solution supplemented with tetracycline antibiotic (4
The culture solution was incubated at 40℃ with shaking at 120 rpm for 16 hours in a thermal water bath, until clear turbidity was observed.
After the incubation period, the culture was centrifuged at 3,500 rpm for 15 minutes, discarding the supernatant Subsequently, 5 mL of Phosphate Buffer Saline (PBS) was added, mixed thoroughly, and centrifuged again at 3,500 rpm for 15 minutes, with the supernatant discarded once more This washing step was repeated three times to ensure the complete removal of Trypto-Soy Broth from the sample Finally, the cultured E coli bacteria were preserved in a PBS solution.
Photocatalytic inactivation studies of E coli were performed under both dark conditions and light irradiation using a 20 W LED lamp A serial dilution of bacterial cultures was prepared with PBS to achieve a concentration of 1.4 x 10^8 CFU/L, followed by counting the E coli colonies For the experiments, a calculated amount of catalyst was added to 200 mL of the E coli PBS solution and placed on a shaker for 60 minutes to establish adsorption/desorption equilibrium between the photocatalyst and the bacterial cells.
To assess the number of viable cells through CFU count at 30-minute intervals, 1 mL of the catalyst-suspended bacterial sample was applied to nutrient agar plates enriched with tetracycline (4 µg/mL), mixed thoroughly, and incubated at 37°C for 16-18 hours A control experiment was performed without the photocatalyst The optimal amount of photocatalyst required for effective bacterial inactivation was established, and the log inactivation of bacteria was calculated by counting the surviving colonies in relation to the initial count, as outlined in equation [4].
𝑁 0 [ 4 ] where 𝑁 0 (CFU) is the number of E coli bacteria colonies at the initial time t = 0
(min), and 𝑁 (CFU) is the number of E coli bacteria colonies in the supernatant collected at each time interval h
3.4.3 Determination of photocatalyst's pH point of zero charge
The pH point of zero charge value (pHpzc) of the optimal g-C3N4/CoMoO4 composite was determined using the drift method (Noh & Schwarz, 1989)
A 0.01 M NaCl solution was prepared, and its initial pH was measured Subsequently, several flasks containing 50 ml of this NaCl solution were adjusted to pH values of 2, 4, 6, 8, 10, and 12 using sodium hydroxide and hydrochloric acid.
After the stable pH value was obtained in each solution, 150 mg of the optimal g-
The C3N4/CoMoO4 composite was introduced into each flask and shaken at 200 rpm for 24 hours under airtight conditions Following this period, the solution was filtered, and the final pH of each sample was recorded.
The pHpzc value of the material was determined by plotting the final pH against the initial pH This value is identified at the intersection of the plot with the line where the initial pH equals the final pH.
3.4.4 Reactive oxygen species trapping experiments
In the experiments, 0.2 mM 1,4-benzoquinone (BQ) and 5 mM isopropanol (IPA) were used as scavengers to identify the primary reactive oxygen species responsible for the photocatalytic activity of the synthesized material, targeting superoxide radicals (•O2 –) and hydroxyl radicals (•OH), respectively.
A blank experiment was performed without any scavenger, utilizing 200 ml of a tetracycline solution at a concentration of 5 mg/L The photocatalyst dosage was maintained at 4 g/L, and the experiment was conducted at a pH of 7.
Statistical analysis
In this research, all experiments are conducted 3 times The average figure of all measurements and calculation values are reported as final data The average of a set of
N numbers (X1, X2, X3,…, Xx) is denoted by 𝑋̅ and calculated by equation [5]:
In this thesis, error bars in all figures are depicted by the standard deviation derived from three observations of each measurement or calculation The standard deviation, represented by S, quantifies the variability of a set of numbers X1, X2, X3, , XN, as defined in equation [6].
RESULTS AND DISCUSSION
Optimization of photocatalyst synthesis conditions
The efficiency of tetracycline removal in aqueous solutions was influenced by variations in hydrothermal time, calcination temperature, and the g-C3N4 to CoMoO4 ratio in the composite The optimal synthesis conditions were identified as 6 hours of hydrothermal treatment followed by calcination at 500°C, using a 6:4 mass ratio of g-C3N4 to CoMoO4 This optimally synthesized composite will be referred to as g-C3N4/CoMoO4 in subsequent figures.
Figure 4.1 Change in tetracycline removal efficiency of synthesized materials at different preparation conditions h
Characterization of synthesized materials
The SEM images reveal that g-C3N4 is present in thin sheet-like layers, while CoMoO4 exhibits a rod-like structure Notably, the morphology of the g-C3N4:CoMoO4 composite shows CoMoO4 flakes uniformly dispersed on the g-C3N4 surface The average particle size of the prepared composite is estimated to be between 2-10 μm.
Figure 4.3 SEM image of the synthesized a) g-C3N4, b) CoMoO4 and c) g-C3N4/CoMoO4 composite
4.2.2 Energy-dispersive X-ray analysis (EDX)
The EDX spectra and elemental composition analysis of the synthesized materials indicate successful formation of the target chemical compounds without any detected impurities Additionally, EDX mapping images validate the creation of the g-C3N4/CoMoO4 composite, showing the distribution of Co, Mo, and O atoms within the flake-like structure of CoMoO4, while C and N atoms are dispersed throughout the enclosed g-C3N4 sheets.
Figure 4.4 EDX spectrum of the synthesized a) g-C3N4, b) CoMoO4 and c) g-
Figure 4.5 EDX elementary mapping of the synthesized a) g-C3N4, b) CoMoO4 and c) g-C3N4/CoMoO4 composite
4.2.3 Brunauer-Emmett-Teller (BET) analysis
The surface area and total pore volume of the synthesized g-C3N4, CoMoO4, and g-
C3N4/CoMoO4 are summarized in Table 4.1:
Table 4.1 Surface area and total pore volume of the synthesized materials
It can be seen that coupling CoMoO4 with g-C3N4 significantly improved the composite’s porosity, exhibiting an increase in surface area and total pore volume h
This result partially explains the improved adsorption performance of the composite materials compared to the CoMoO4 pristine
4.2.4 X-ray powder diffraction analysis (XRD)
The XRD analysis was conducted to identify the crystalline structure of the g-
C3N4/CoMoO4 composite, compared with each of the pristine components
Figure 4.6 XRD patterns of the synthesized g-C3N4, CoMoO4, and g-C3N4/CoMoO4 composite
As shown in Figure 4.6, in these spectra, CoMoO4 pristine presents strong diffraction peaks at 2 = 17.7 o , 19.0 o , 25.3, 27.2 o , and 34.3 o , corresponding to the reflections of
(110), (100), (202), (112), and (221) planes respectively, which was well matched with the standard patterns of monoclinic CoMoO4 (JCPDS No 21-286)
The synthesized composite exhibited overlapping characteristic peaks, with pure g-C3N4 showing a typical peak at 27.5° corresponding to the (002) plane (JCPDS 87-1526) and CoMoO4 presenting a peak at 27.2°.
The shift in the peak position, peak broadening, and the change in peak intensity at 2
= 27.7 o in the composite sample provided evidence that the crystal structure of g-C3N4 was altered by growing CoMoO4 in-situ on that pristine h
4.2.5 Fourier transform infrared spectroscopy (FTIR)
Figure 4.7 presents the FTIR spectrum of synthesized g-C3N4/CoMoO4 composite, CoMoO4, and g-C3N4 pristine
The FTIR spectra of the synthesized g-C3N4, CoMoO4, and their composite reveal a broad stretching vibration in the wavenumber range of 3000-4000 cm⁻¹, indicating the presence of O-H bonding in the composite.
The absorption bands of g-C3N4, located between 1200 – 1700 cm -1, 1085-1150 cm -1, and 870-890 cm -1, correspond to the stretching vibrations of C-N, C-O, and C-H bonds, respectively In contrast, CoMoO4 exhibits characteristic bands at 869 cm -1 and 785 cm -1, attributed to the stretching vibrations of the Mo=O bond and the symmetric Mo-O-Mo bonds (Ma et al., 2020).
The detection of characteristic bonding vibrations in the composite, along with their slight shifts, indicates the successful synthesis of the targeted heterostructure photocatalyst and highlights the effective surface modification of its components.
4.2.6 UV–vis diffuse reflectance spectroscopy (UV-DRS)
Figure 4.8 UV-vis diffuse reflectance absorption spectra of the synthesized g-C3N4,
Figure 4.8 illustrates the absorption spectra of both the prepared composite and its individual components Notably, the composite photocatalyst exhibits a shift in its absorption edge towards a higher wavelength within the visible range when compared to pure g-C3N4 This shift signifies an improved capability for visible light absorption in the composite.
The optical bandgap of the composite, as displayed in Figure 4.9, was significantly decreased compared to its two pristine The bandgap energy of the synthesized g-
C3N4/ CoMoO4 composite was calculated to be 2.63 eV, using the Tauc equation, while pure g-C3N4’s bandgap is 2.81 and that of CoMoO4 is 2.98
The narrower bandgap of the composite leads to its stronger capability of visible light absorption, resulting in the improvement of photocatalytic activity (Pan et al., 2020) h
Figure 4.9 Tauc plot of the synthesized g-C3N4, CoMoO4, and g-C3N4/CoMoO4 composite
The PL spectra of the g-C3N4/CoMoO4 composite, shown in Figure 4.10, were recorded at an excitation wavelength of 602 nm The normalized PL spectrum indicates that the composite exhibits lower PL intensity compared to the individual components, g-C3N4 and CoMoO4 This reduction in PL intensity suggests enhanced charge separation within the composite, leading to a decreased recombination rate of photogenerated electrons and holes Consequently, this improved charge separation enhances the generation of reactive oxygen species, thereby increasing the photocatalytic activity of the composite.
Figure 4.10 PL spectra of the synthesized g-C3N4, CoMoO4, and g-C3N4/CoMoO4 composite
Removal efficiency of synthesized materials with tetracycline antibiotic
4.3.1 Enhancement of tetracycline removal efficiency of g-C 3 N 4 /CoMoO 4 composite
Figure 4.11 shows the removal efficiency of tetracycline by the synthesized materials The g-C3N4/CoMoO4 composite demonstrated higher tetracycline removal efficiency than the component material g-C3N4 and CoMoO4
In experiments conducted under dark conditions, the composite demonstrated a higher removal efficiency for tetracycline, achieving optimal results with 2.5 g of photocatalyst in 200 mL of antibiotic solution at a concentration of 10 mg/L.
After that, under the visible light condition, the total removal efficiency was 48.07%, 40.81%, and 81.9% for g-C3N4, CoMoO4, and the composite g-C3N4/CoMoO4 respectively. h
Figure 4.11 Tetracycline removal efficiency of the synthesized g-C3N4, CoMoO4, and g-C3N4/CoMoO4 composite
For the optimization of the antibiotic removal process, a series of experiments were conducted for different parameters including the dosage of composite g-
C3N4/CoMoO4, pH condition, and initial concentration of tetracycline
4.3.2 Effect of photocatalyst dosage on tetracycline removal efficiency
As shown in Figure 4.12, increasing the photocatalyst dosage from 1 g/L to 4 g/L enhanced the tetracycline removal efficiency of the composite photocatalyst from 58.34% to 92.56%
As the dosage of photocatalyst increases to 5 g/L, a decline in removal efficiency occurs due to excessive particle density, which obstructs the light source and hinders the photocatalytic process.
Figure 4.12 Effect of photocatalyst dosage on tetracycline removal efficiency
4.3.3 Effect of pH on tetracycline removal efficiency
The study investigated various pH conditions to identify the optimal environment for tetracycline removal from solutions Results indicated that both acidic and neutral pH levels enhance the treatment efficiency of the antibiotic Notably, at a pH of 7, the synthesized g-C3N4/CoMoO4 demonstrated a maximum removal efficiency of 92.88%.
The study determined that the composite's point of zero charge is 8.16, indicating that the material's surface is positively charged at pH levels above 8.16 and negatively charged below this threshold Understanding these pH conditions is crucial for optimizing the photocatalyst's performance.
Figure 4.13 Effect of pH condition on tetracycline removal efficiency
Figure 4.14 pH point of zero charge of the synthesized g-C3N4/CoMoO4 composite h
Tetracycline exhibits three pKa values (3.3, 7.7, and 9.7), allowing it to exist in cationic, zwitterionic, and anionic forms under acidic, neutral, and alkaline conditions, respectively (McCormick et al., 1957) The adsorption of tetracycline on the surface of photocatalysts is most effective at acidic and neutral pH levels, specifically between 3.3 and 7.7 In contrast, the material demonstrates significantly reduced performance in alkaline conditions, such as at pH 9.
Therefore, this result aligned with the observation in Figure 4.13, and pH=7 was the optimized condition for the tetracycline removal process
4.3.4 Effect of initial pollutant concentration on tetracycline removal efficiency
Figure 4.15 Effect of tetracycline's initial concentration on the removal efficiency
The removal efficiency of the synthesized composite for tetracycline increases from 80.50% to 95.53% as the initial concentration of tetracycline decreases Higher initial concentrations lead to greater adsorption of pollutant molecules on the photocatalyst's surface, resulting in significant light absorption by the antibiotic molecules instead of the material particles This reduced light penetration to the photocatalyst's surface ultimately results in decreased pollutant removal efficiency.
To sum up, the highest removal efficiency of 95.93% was obtained with the g-
C3N4/CoMoO4 photocatalyst dose of 4 g/L for 5 mg/L tetracycline solution at pH=7.
Inactivation efficiency of synthesized materials with E coli bacteria
Figure 4.16 E coli inactivation efficiency with different dosages of photocatalyst
Figure 4.16 shows the inactivation efficiency of the inactivation of g-C3N4/CoMoO4 bateria in PBS solution containing E coli antibiotic-resistant bacteria at the concentration of 1.4 × 10 8 CFU/L
In the blank experiment, where no photocatalyst was introduced, there was minimal change in the reduction of bacteria concentration This finding confirms that the synthesized photocatalyst effectively contributes to the inactivation of E coli in the other experiments conducted.
When varying the photocatalyst dosage from 1 to 4 g/L, the synthesized 6:4wt g-
The C3N4/CoMoO4 composite demonstrates an impressive inactivation efficiency of log 2.3 against E coli at a concentration of 2 g/L This efficacy is attributed to both the adsorption phenomenon and the material's photocatalytic activity, with the latter playing a slightly more significant role in enhancing bacterial disinfection efficiency.
Proposed photocatalytic mechanism
This research highlights the enhanced performance of a g-C3N4/CoMoO4 composite in removing tetracycline antibiotics The incorporation of CoMoO4 effectively mitigates the limitations of g-C3N4's wide bandgap, resulting in a narrowed bandgap that allows for improved photon absorption from visible light Consequently, this facilitates the catalytic degradation of tetracycline, showcasing the composite's potential for effective environmental remediation.
Figure 4.17 demonstrates the impact of scavengers, specifically highlighting that 1,4-benzoquinone (BQ) targets hydroxyl radicals (•OH), while isopropanol (IPA) interacts with superoxide radicals (•O2–) to enhance removal efficiency The results of these scavenger experiments indicate that superoxide radicals (•O2–) are crucial in the oxidation of tetracycline in the solution.
Figure 4.17 Effect of different scavengers on the photocatalytic efficiency of the synthesized h
The synthesized material demonstrated effective removal of tetracycline through both adsorption and photocatalytic oxidation processes Figure 4.18 illustrates the photocatalytic mechanism involved in the degradation of this target pollutant.
Figure 4.18 The proposed photocatalytic mechanism for degradation of antibiotics and antibiotic-resistant bacteria
In the photocatalytic oxidation process, CoMoO4 and g-C3N4 are excited by visible light to generate electron-hole pairs The electrons from CoMoO4 are likely to transfer and recombine with holes in the valence band of g-C3N4 This interaction leads to an increased accumulation of photogenerated electrons in the conduction band of g-C3N4, which facilitates the reduction of adsorbed O2, resulting in the formation of more •O2–.
Meanwhile, the photo-generated holes left behind in the valence band of CoMoO4 can oxidize the adsorbed H2O to give •OH Therefore, the photocatalytic activity of the g-
The C3N4/CoMoO4 heterojunction exhibits a notable increase in photocatalytic activity, primarily driven by reactive species such as •O2– and •OH Among these, •O2– is identified as the critical contributor to the photocatalytic process This enhancement leads to improved photodegradation of organic pollutants, as illustrated in the subsequent equations.
CoMoO 4 /g-C3N4 + hv → CoMoO 4 /g-C3N4 (e − + h + ) (i)Photo-reduction:
H2O2 + e − → • OH + OH − (Chang et al.) Photo-oxidation:
•O2 −, • OH + organic pollutants → CO2 + H2O + byproducts
In the current case, during the illumination process, superoxide (•O2−) is the predominant active species formed by the synthesized heterojunction photocatalyst responsible for the removal of tetracycline h
CONCLUSION AND RECOMMENDATIONS
Conclusion
The g-C3N4/CoMoO4 heterostructure photocatalyst was effectively synthesized using a straightforward hydrothermal-calcination method, leading to a notable enhancement in the photocatalytic activity of g-C3N4 for the removal of tetracycline under visible light.
C3N4/CoMoO4 sample consisting of 60% mass CoMoO4, going through 6 hours hydrothermal at 180 o C, followed by calcination at 500 o C for 4 hours, displayed superior photocatalytic performance for tetracycline removal under visible light
The synthesized g-C3N4/CoMoO4 composite exhibited significantly enhanced physical and chemical properties due to the coupling of its two pristine components This improvement provides valuable insights into its effective performance in treating antibiotic and antibiotic-resistant bacteria in aqueous environments.
The study focused on the effective treatment of tetracycline antibiotics and antibiotic-resistant E coli bacteria using a g-C3N4/CoMoO4 composite photocatalyst Under optimal conditions, including a photocatalyst dosage of 4 g/L and a tetracycline concentration of 5 mg/L at pH 7, the composite achieved an impressive tetracycline removal efficiency of 95.93% Additionally, the highest inactivation efficiency for E coli was recorded at a photocatalyst dosage of 2 g/L, targeting an initial concentration of 1.4 × 10^8 CFU/mL.
Based on experiment results, the removal mechanism of synthesized composite with tetracycline was proposed, in which •O2− plays the most important role in the degradation process
This comprehensive research explores the synthesis and optimization of the g-C3N4/CoMoO4 heterostructure photocatalyst, highlighting how various preparation conditions influence its properties and efficiency The study demonstrates the photocatalyst's potential in effectively removing antibiotics and bacteria, making it a promising solution for wastewater purification.
Recommendations
This study highlights the significant potential of the g-C3N4/CoMoO4 composite photocatalyst, paving the way for various research avenues Future investigations should focus on optimizing the parameters and experimental conditions to enhance the effectiveness of this composite in photocatalytic applications.
E coli bacteria could be conducted For example, control experiments without irradiation time could be implemented to investigate the direct impact of photodegradation on the inactivation of E coli bacteria Besides, the material’s recycling ability should also be studied Furthermore, simultaneous treatment of both antibiotics and antibiotic-resistant bacteria is also recommended to investigate Moreover, the removal efficiency of this composite can also be studied by targeting other harmful organic pollutants such as dyes, antibiotics, and bacteria h
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