Synthesis of reduced graphene Oxide (rgo) for the removal of Tetracycline from aqueous Solutions

Effect of antibiotic residues ... Regulations on antibiotic content in water ... Tetracycline pollution in water ... Technologies of the treatment of antibiotics in water ... Filtration [r]

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HA THI MY TRINH

SYNTHESIS OF REDUCED GRAPHENE OXIDE (rGO) FOR THE REMOVAL OF

TETRACYCLINE FROM AQUEOUS SOLUTIONS

VIETNAM NATIONAL UNIVERSITY, HANOI VIETNAM JAPAN UNIVERSITY

HA THI MY TRINH

SYNTHESIS OF REDUCED GRAPHENE OXIDE (rGO) FOR THE REMOVAL OF

TETRACYCLINE FROM AQUEOUS SOLUTIONS

MAJOR: ENVIRONMENTAL ENGINEERING CODE: 8520320.01

RESEARCH SUPERVISORS: Dr TRAN DINH TRINH Dr NGUYEN THI AN HANG

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ACKNOWLEDGEMENTS

This work could not have been completed without the collaboration and help of many people whom I want to thank

First of all, I would also like to extend my deepest gratitude to Dr Tran Dinh Trinh, who inspired me to develop invaluable insight into a new field to me I’m also deeply indebted to Dr Nguyen Thi An Hang, who provided me with relentless support and constructive advices

Many thanks to Mrs Dao Thi Huong – the laboratory technician – as well as staffs of the Master’s Program in Environmental Engineering for all practical instructions and useful contributions Thanks also go to my classmates and teammates, who enthusiastically supported me during this study

Finally, I cannot begin to express my thanks to my family and friends for their patience and support until I finished this work

I’d like to acknowledge the assistance of the staffs from the Academic, Research and Development Promotion Department of VNU Vietnam Japan University as well as Japan International Cooperation Agency for guiding and supporting me to complete my thesis

Thank you all for everything

Ha Noi, August 7th, 2020

Student

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TABLE OFCONTENTS

Acknowledgements i

List of figures v

List of tables vi

List of abbreviations vii

Introduction

Chapter Literature review

1.1 Antibiotics pollution

1.1.1.Definition, classification and sources of antibiotics

Sources of antibiotics

Application of antibiotic in human and veterinary dedicines

1.1.2 Occurrence of antibiotics in water and environmental effects

Occurrence of antibiotics in water

Effect of antibiotic residues

1.1.3 Regulations on antibiotic content in water

1.1.4 Tetracycline pollution in water 10

1.2 Technologies of the treatment of antibiotics in water 11

1.2.1 Filtration and sorption processes 11

1.2.2 Photodegradation and oxidation 12

1.2.3 Biodegradation 12

1.2.4 Other techniques 13

1.2.5 Technologies applied for the treatment of TC in water 14

1.3 Synthesis and application of rGO in antibiotic adsorption 14

1.3.1 Synthesis of rGO 15

Chemical reduction 15

Thermal reduction 15

Solvothermal/hydrothermal reduction 16

Other methods 16

1.3.2 Application of rGO in antibiotic adsorption 17

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2.1 Materials 18

2.2 Methods 18

2.2.1 Literature review 18

2.2.2 Sample analysis in laboratory 18

Characterization of materials 18

a) Fourier-transform infrared spectroscopy 18

b) Energy-dispersive X-ray spectroscopy 20

c) Scanning Electron Microscopy 21

d) X-ray diffraction 22

e) Surface area and pore volume calculation 23

f) pH point of zero charge 25

Determination of Tetracycline concentration 25

2.2.3 Data calculation 26

a)Removal efficiency 26

b) Adsorption capacity 26

c) Kinetic parameters 27

d) Isotherm parameters 27

e) Thermodynamic study 28

2.2.4 Statistical analysis 28

2.3 Experiment setup 29

2.3.1 Material synthesis 29

2.3.2 Factors influencing the efficiency of TC adsorption 29

a)Contact time 29

b)pH 30

c) Dosage of rGO 30

d) Initial concentration of Tetracycline 30

2.3.3 Isotherm tests 30

2.3.4 Kinetics tests 30

2.3.5 Thermodynamic tests 31

Chapter Results and discussion 32

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3.1.1 Fourier-transform infrared spectroscopy 32

3.1.2 Energy-dispersive X-ray spectroscopy 33

3.1.3 X-ray diffraction 34

3.1.4 Surface area and pore volume 35

3.1.5 pH point of zero charge 37

3.2.Adsorption study 38

3.2.1 Factors influencing TC adsorption 38

a) Contact time 38

b) pH 39

c) Dosage 40

d) Initial concentration 41

e) Temperature 42

3.2.2 Adsorption isotherms 43

3.2.3 Adsorption kinetics 45

3.2.4 Adsorption thermodynamics 46

Conclusions and recommendations 48

Conclusions 48

Recommendations 48

References 50

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LIST OF FIGURES

Figure 1.1 Pathways of antibiotics into water

Figure 1.2 Global antibiotic consumption in livestock 2010

Figure 1.3 Legislation on antibiotics as growth promoters

Figure 2.1 FT-IR 4600 Jasco 20

Figure 2.2 Principle of EDX measurement 21

Figure 2.3 JSM-IT100/JED-2300 Analysis Station Plus, JEOL 21

Figure 2.4 MiniFlex 600 23

Figure 2.5 TriStar II Plus, Micromeritics 25

Figure 2.6 Calibration curve for Tetracycline 26

Figure 3.1 FT-IR result comparison of GO and rGO 32

Figure 3.2 SEM images of (a) graphite and (b) rGO 33

Figure 3.3 XRD results of GO and rGO 35

Figure 3.4 (a) N2 adsorption and desorption isotherms, (b) Types of physisorption isotherms, and (c) Types of hysteresis loops (IUPAC) 36

Figure 3.5 Pore size distribution of rGO obtained from DFT method 36

Figure 3.6 The plot of ΔpH versus pHi 38

Figure 3.7 Effect of contact time on TC adsorption by rGO 39

Figure 3.8 Effect of pH on TC adsorption by rGO 40

Figure 3.9 Effect of dosage effect on TC adsorption by rGO 41

Figure 3.10 Effect of initial concentration on TC adsorption by rGO 42

Figure 3.11 Effect of temperature on TC adsorption capacity of TC of rGO 43

Figure 3.12 Comparison of experimental data and modeled data on adsorption isotherms 43

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LIST OF TABLES

Table 1.1 Antibiotics classification according to chemical structure

Table 1.2 Chemical properties of tetracycline 10

Table 3.1 IR Spectrum by frequency range 33

Table 3.2 C/O ratio comparison of GO and rGO 34

Table 3.3 Properties of materials used for TC removal in previous studies 37

Table 3.4 Langmuir and Freundlich isotherm parameters on TC adsorption 44

Table 3.5 First- and Second-order kinetic parameters for TC adsorption 45

Table 3.6 Thermodynamic parameters of TC sorption process by rGO 47

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LIST OF ABBREVIATIONS

BJH Barrett-Joyner-Halenda method

DFT Density Functional Theory

EDA Electron donor-acceptor

EDX Energy-dispersive X-ray spectroscopy FT-IR Fourier-transform infrared spectroscopy

GO Graphene oxide

IUPAC International Union of Pure and Applied Chemistry

pHpzc Point of zero charge

rGO Reduced graphene oxide

SEM Scanning electron microscopy

TC Tetracycline

WWTPs Wastewater treatment plants

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INTRODUCTION

Since its discovery in 1928, antibiotics have played a very important role in human health protection and livestock industry It is estimated that millions of people have been saved from bacterial diseases (smallpox, cholera, typhoid fever, syphilis, etc.) thanks to antibiotics Antibiotics have revolutionized the treatment of bacterial diseases, which probably increase the average life span of Americans from 47 years in the early 20th century to 78.8 years (Chain et al 2016) Antibiotics are also widely known as growth stimulant in fisheries, livestock and cultivation

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Many studies have been carried out with the purpose of eliminating antibiotics from environments The preferred methods used today are: adsorption; degradation (photo-degradation, catalysis, bio-degradation); and oxidation processes (ozonation, UV raddiation, ) In particular, adsorption is a commonly applied method thanks to its advantages such as high efficiency and ease of operation Many materials have been used for adsorption process such as activated carbon (Rivera-Utrilla et al., 2013) graphene oxide (Gao et al., 2012), activated sludge (Prado et al 2009) However, studies showed that these materials are time consumed to reach high efficiency Therefore, reducing processing time is one of major approaches to widespread the application of this method

Graphene is a single layer of carbon with thickness as a carbon molecule, dense with carbon molecules containing sp2 in honeycomb lattice (Fitzer, et al 1995,

Choi et al., 2010; Ray, 2015) Graphene Oxide (GO) and Reduced Graphene Oxide (rGO) is oxidized graphene and has the presence of epoxy and hydroxyl functional groups; There were differences in functional groups or C:O ratio (Haubner et al., 2010) Although the presence of oxygen-containing groups made GO able to be hydrophilic which is suitable for water treatment, these functional groups weakened the π-electron activity linked to high fraction of sp3 C atoms which is important

interaction for adsorption process (Ai et al., 2019) On the other hand, rGO with the large specific surface area (Nidheesh, 2017), significantly fewer functional groups than GO, and its powder form made it more efficient and economic

The main objectives of this research are to:

1) Develope rGO as adsorbent for the removal of TC from aqueous solutions 2) Investigate the effect of pH, adsorbent dose, initial TC concentration, contact time,

and temperature on the adsorption of TC by rGO in the batch experiments 3) Estimate the TC adsorption capacity and removal efficiency of the as-synthesized

rGO and compare to other adsorbents

4) Elucidate the TC adsorption mechanisms by rGO The thesis consists of main chapters as follows:

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ecological and environmental impacts, treatment technologies, along with the synthesis and application of rGO in antibiotic adsorption

b) Chapter 2: Materials and methods: Describes materials, chemicals and methods utilized for the synthesis, characterization and application of rGO in the adsorption tests

c) Chapter 3: Results and discussion: Refers the main research results, including characterization of rGO, factors influencing the adsorption of TC, adsorption isotherms, and adsorption kinetics

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CHAPTER LITERATURE REVIEW

1.1 Antibiotics pollution

1.1.1 Definition, classification and sources of antibiotics

According to IUPAC, antibiotic is a substance produced by, and obtained from, certain living cells (especially bacteria, yeasts and moulds), or an equivalent synthetic substance, which is biostatic or biocidal at low concentrations to some other form of life, especially pathogenic or noxious organisms (Duffus, 2007)

Antibiotics can be classified based on their chemical structure, action mechanism, action spectrum, and the route of administration (Gothwal and Shashidhar, 2015) For further study of kinetics and mechanism, classification according to chemical structure is preferred in Table 1.1

Table 1.1 Antibiotics classification according to chemical structure (Kebede et al., 2014)

Group Internal group Representative with practical importance

Carbohydrate antibiotics

1.Aminoglycosideantibiotics Other (N- and C-)

glycosides Streptomycin, Neomycin Macro cyclic lactone (lactam) antibiotics 1.Macrolide antibiotics 2.Polyeneantibiotics Macrolactam antibiotics

Erythromycin Amphotericin Quinone and

similar antibiotics Oligomycin

5 Sources of antibiotics

Antibiotics mostly demonstrated degradation performance <30% over days, except of tetracycline is <44% over days (Wilkinson et al 2019) Tetracycline is relatively inert within human body (Hirsch et al., 1999) Then, remaining antibiotic in environment would become long-term contaminants and accumulate in biomass Antibiotic residues enter the environment primarily through human and animal waste and from manufacturing (Gelband and Miller-petrie, 2016) Antibiotics used in human and veterinary care are excreted which sometimes are used as agricultural fertilizer, most of times are discharged to environment Up to 90% of antibiotic dose can be excreted in animal urine and up to 75% in their feces Combined with expired industrial drugs, all of un-used antibiotic go into WWTPs and landfill Several studies reported that conventional treatment techniques cannot remove these persistent compounds completely, therefore significant amounts enter the aqueous environment, and end up to be influent of water treatment plants with inefficient treatment techniques

1.Furan derivatives 2.Pyran derivatives Alicyclic

antibiotics

1.Cycloalkane derivatives 2.Small terpenes 3.Oligoterpene antibiotics

Streptovitacins

Aromatic antibiotics

1.Benzene compounds 2.Condensedaromatic comp

3.Non-benzene aromatic comp

Chloramphenicol Grisefulvin Novobiocin

Aliphatic antibiotics

1.Alkane derivatives 2.Aliphatic carbocyclic acid

derivatives

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Figure 1.1 Pathways of antibiotics into water (modified from Frade et al 2014) Application of antibiotic in human and veterinary dedicines

Ever since antibiotics has been discovered, their market has been massively grown They have been extensively and effectively used in human and veterinary medicines and their benefits have also been recognized in agriculture, aquaculture, bee-keeping, and livestock as growth promoters (Gothwal and Shashidhar, 2015)

But over-use of these chemical compounds for both human and veteran care were attending concern due to its consequence Antibiotic use in humans was increasing worldwide for first-line and some last-resort antibiotics Only about 20% was used in hospitals, and non-prescription use of antibiotics is approximated 90% outside USA and Europe High-income countries tend to had higher in consumption in per capita, but low and middle income countries had the greater increase in antibiotic use (Gelband and Miller-petrie, 2016)

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Figure 1.1 Global antibiotic consumption in livestock 2010 (Van Boeckel et al., 2015)

Figure 1.3 Legislation on antibiotics as growth promoters illustrated according to (Organization for Economic Co-operation and Development, 2015)

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1.1.2 Occurrence of antibiotics in water and environmental effects Occurrence of antibiotics in water

Aus der Beek et al (2016) collected data of more than 1000 publications about pharmaceutical concentrations and found out that pharmaceuticals in the environment is truly topic of global concern when it detected in 71 countries all around the world According to Bu et al (2013), sulfonamides, fluoroquinolones, macrolides, tetracyclines and other antibiotics were detected in surface water, rivers, sea waters and sediments with concentrations mostly at the level of µg/L up to dozens of µL They implied that the concentration of these contaminants is related with high population density cities Some extremely high concentrations were found in certain areas indicating local point sources or inefficiency of WWTPs to remove these contaminants in sewage (Bu et al., 2013) (David, 2017) reported presence of 18 pharmaceuticals in surface waters in lower Great Lakes with highest concentrations were noted at 0.79µg/L (ibuprofen), 0.55µg/L (naproxen), and 0.65µg/L (carbamazepine) This review also said that in Grand River watershed, southern Ontario, 14 in 28 surveyed pharmaceuticals were detected with highest concentrations belonged to monensin and sufamethazine which are used for livestock In Tinkers Creek, 12 antibiotics were detected at 18 upstream and downstream from WWTP discharges to the mainstream In summarizing, pharmaceutical compounds are most detected near the discharge of WWTPs or agricultural production, in less diluted water bodies Wilkinson et al (2019)were detected 31 active pharmaceutical ingredients including antibiotics in tap, surface, wastewater treatment plant (WWTP) influent and WWTP effluent water collected globally

The statistics show that the more regions surveyed, such as Europe, USA, China, the more antibiotic contaminations are detected, also meaning that the less surveyed areas, like low- and middle-income countries are less contamination, but it is data limitations

Effect of antibiotic residues

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There are reports about near-extinct vultures due to feeding by dead drug treated livestock (Oaks et al., 2004) In aquatic, diclofenac is suspected of causing damage to viscera of the rainbow trout (Triebskorn et al., 2007) 17α-ethynylestradiol at 5-6ng/L is the reason for collapse population of fathead minnow (Pimephales promelas) in a male fish feminization experiment in Canada (Kidd et al., 2007)

The widespread of antimicrobials use has created selection pressure, promote the process of formation and spread of antimicrobial resistant pathogens worldwide Resistant microbes and resistance genes can come forth and back between human, animals, food, water and the environment antibiotic resistance genes formed in animals due to growth stimulation can be transferred directly to humans via food route or indirectly through the environment It is estimated that about 33,000 death each year from drug-resistant bacteria in the EU-EEA (European Centre for Disease Prevention and Control, 2019) Resistant strain, H58, originated in Asia and Africa was increased from 7% to 97% prevalence rate in years The resistance rates and trends are becoming global concerning 77% of E faecium healthcare-associated infections in the United States were resistant to vancomycin (Lahsoune et al., 2007) Among of high-income countries, United States was reported having higher rates of resistance to many Gram- positive bacteria, while resistance rates of Gram-negative bacteria were high in Southern and Eastern Europe In Asia, median resistance of K pneumoniae to ampicillin was 94%, and to cephalosporins, 84%, these contaminants in Africa was 100 and 50%, respectively Multi-resistance appeared in 30% of strains in Asia and 75% of strains in Africa (Gelband and Miller-petrie, 2016)

Antibiotic-resistant infections also contribute to the financial burden on healthcare systems Europe cost an estimated €1.5 billion annually, including healthcare expenditures and productivity losses (America and America, 2009); as United States is as much as $20 billion, and productivity losses total another $35 billion (Lahsoune et al., 2007)

1.1.3 Regulations on antibiotic content in water

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most antibiotics for animal husbandry The reason is the large part of antibiotics are overlapping between use of disease prevention and growth promoters Due to limited quantitative data, toxicity assessments for antibiotics cannot lead to a standard of legal concentration in the environment, just Predicted No Effect Environmental Concentration (PNEC) (Table S1) values have been suggested as maximum levels in an environmental matrix

1.1.4 Tetracycline pollution in water

Tetracycline antibiotics are one of the primarily antibiotics groups used for veterinary purposes, for human therapy and in agriculture sector as feed additive (Daghrir and Drogui, 2013) According to (Xie et al 2010; Cheng et al 2005) show that tetracycline antibiotics are ranked second in the production and usage of antibiotics worldwide and are ranked first in China Tetracycline, one of three most commonly used in the tetracyclines, showed that they are high water solubility (0.041mg/L) and low volatility (low log KOW) Physic-chemical properties of

tetracycline are listed in Table 1.2 Therefore, these antibiotics likely stay persistence in the aquatic environment Removal efficiency of conventional wastewater treatment plant was range from 12% to 80% (Daghrir and Drogui, 2013)

Table 1.2 Chemical properties of tetracycline (Rivera-Utrilla et al., 2013)

Volume nm3

Cross area

nm2

Chemical Structure

Solubility g/L

Log KOW

pKa

0.403 3.969 22 -1.3

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tracked in municipal waste water treatment plants effluence samples, in the concentration range of 280 to 540 ng/l in Tehran, Iran (Javid et al., 2016)

Ecological risk and potential toxic effects of tetracycline antibiotics residues in the environment also paid attention As many antibiotics, their residues in environment promoted the formation and the development of antibiotic resistant microorganisms These antimicrobial agents may disturb the microflora of the human intestinal and increase the risk of certain infections (Heuer et al., 2009)

1.2 Technologies of the treatment of antibiotics in water

Techniques that based on physical reactions (sedimentation, scour/re-suspension, adsorption/desorption, and gas transfer), biological transformations (biodegradation and co-metabolism), and chemical reactions (hydrolysis, oxidation, photo-degradation) play a important role in water treatment plants But as Gothwal and Shashidhar (2015) reviewed, removal efficiencies of conventional sewage treatment are found to vary substantially due to not designed to deal with new pollutants like antibiotics

1.2.1 Filtration and sorption processes

Several studies reported that membrane processes like a reverse osmosis-ultrafiltration system can reach ≈ 87.5 % efficiency in remove oxytetracycline from pharmaceutical wastewater (Li et al 2004) and 50~80% tetracyclines in synthesis water by nanofiltration (Koyuncu et al., 2008) But these studies also warned that high concentrations of organic substances present in the environment would hinder treatment performance

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Sorption processes had high removal rate in synthesis water (up to 94%) while hardly exceeded 67% in real river water (Choi et al., 2008) Hence, this process was also interfered by organic matter in the same way as that in the filtration process, which has been aforementioned

1.2.2 Photodegradation and oxidation

Photolysis processes using light are simple, clean and less expensive Some antibiotics, which are sensitive with UV irradiations like tetracyclines, can be degraded at the rate of up to 90% Although some studies indicated the presence of intermediate compounds which can be more toxic than original one (Daghrir and Drogui, 2013) In the photodegradation process, parameters such as light source, pH, temperature, time, type of matrix, and the type/amount of impurities in the matrix (salts, organic compounds, soils, etc.) are important In addition, maintenance and electrical cost are usually considered as limiting factors

The process of oxidation involves the use of strong oxidizing agent such as hydroxyl radicals, ozone, potassium permanganate, and chlorine Advanced oxidation processes which enhance the formation of strong oxidant like free radicals are attracting attention from many projects Treatment efficiencies often reach 90% in an extremely short time with a dosage material in mg/L Some wise used oxidation techniques is ozonation, chlorination, Fenton system Most of the studies are lack of information on by-products which formed during process Belongs on the minority, Li et al (2008) indicated that the first by-product of oxytetracycline was more toxic than the parent compound after ozonation It is possible by-products can be ecotoxic, and operation and optimization of oxidation process should be monitor through toxicity tests and determining transformation products

1.2.3 Biodegradation

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Benzylpenicillin Those antibiotic also was only compound can be biodegradation up to 87% of ThCO2 degradation in combined test (Gartiser et al., 2007) Many

antimicrobials reported that can not be biodegradable or negligible biodegradable by bacteria On the other hand, degradation of antibiotics by fungus show a more delight result Studying in Trametes versicolor, Rodríguez-Rodríguez et al (2012) reported that Sulfapyridine was completely degraded, and sulfathiazole reach 88% after 96h incubation 72h hydraulic retention time successfully eliminated sulfapyridine, sulfamethazine and sulfathiazole in a mixture and their metabolites

1.2.4 Other techniques

Some common water treatment processes were evaluated for antibiotics removal by Adams et al (2002), included metal salt coagulation, excess lime/soda ash softening, powdered activated carbon sorption, ultraviolet photolysis, ion exchange, and reverse osmosis The results demonstrated that metal salt coagulation, excess lime/soda ash softening, ultraviolet photolysis, and ion exchange were not effective for antibiotic removal; powdered activated carbon sorption and reverse osmosis could be used to these compounds, but it only available at selected water treatment plants Conventional biological wastewater treatment processes were reported that it was effective for the removal of some antibiotics, but 10-1000 ng/L concentrations in secondary treated effluents were occurred Application of advanced treatment after conventional biological improved the removal of antibiotics, but operation and maintenance cost goes up (Le-Minh et al., 2010)

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1.2.5 Technologies applied for the treatment of TC in water

Many treatments of tetracycline antibiotics in waters were suggested, included photocatalysis, oxidation, Fenton, adsorption, filtration, gamma radiation (table S2) Adsorption, most wised used in conventional wastewater treatment plants, was reported in previous studies that it was not so effective for TC removal Choi et al (2008) studied about adsorption using granular activated carbon in combination with coagulation process for TC antibiotics removal The results showed that removal rate of tetracyclines could be up to 94 % from synthetic water, whereas did not exceed 67 % from river water Rivera-Utrilla et al (2013) reported about TC removal by activated carbons, which reached 375.4 mg/g adsorption capacity calculating by Langmuir model, whereas it took about 200h to achieve equilibrium Graphene oxide was reported that the maximum adsorption capacity calculated by Langmuir model could be reach 313 mg/g (Gao et al., 2012), but the materials could not be recoverable Therefore, the material that improved in adsorption capacity while reducing contact time and being able to recovery was necessary to widely apply adsorption technique in large-scale for TC removal

1.3 Synthesis and application of rGO in antibiotic adsorption

15 1.3.1 Synthesis of rGO

GO reduction methods included chemical reduction, thermal reduction, solvothermal/hydrothermal reduction, microwave and photo reduction, and photo catalyst reduction (Nidheesh, 2017) Confirmation of effective reduction process are characterized by Scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy (EDX), Fourier-transform infrared spectroscopy (FT-IR), and X-ray diffraction (XRD) technique

Chemical reduction

Chemical reduction is the most popular method to synthesize rGO GO dispersion was reduced with reduction agents, oxygen functional groups are eliminated and the π-electron conjugation within the aromatic ring structure was partially restored Investigated reduction agents were L-ascorbic acid, D-glucose, tea polyphenol (Xu et al., 2015), NaBH4, Hydrazine hydrate (Alibeyli et al 2017),

borohydrides (Chua and Pumera, 2013), oxalic acid (Song et al 2012), sodium hydrosulfite (Zhou et al., 2011) All of these studies reported that rGO have been synthesized with appropriate characteristics such as C/O ratio, oxygen functional groups, diffraction peaks Zhou et al (2011) studied about synthesis of PVA/graphite oxide (GO) nanocomposites films by using sodium hydrosulfite as reduction agent They included that a 40% increase in tensile strength and 70% improvement in elongation at break have been obtained with only the addition of 0.7 wt.% of reduced graphite oxide The highest conductivity achieved is 8.9× 10-3 S/m for the composites

containing wt.% rGO, and the conductivity achieved is comparable to those achieved with hydrazine Chemical reductions usually propose high reducing efficiency in a short time

Thermal reduction

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2016) Li et al (2009) developed N-doped-rGO sheets through thermal annealing of GO in ammonia They concluded that the high temperature is needed to achieve the better reduction of GO Schniepp et al (2006) also reported the similar result, which the C/O ratio would increase (from <7 to >13) when heating temperature increasing (<5000C to 7500C) Therefore, this method exhibited the high reduction performance

but it also consumed a lot of energy for heating Solvothermal/hydrothermal reduction

Solvothermal or hydrothermal reduction occurs at low temperature and high pressure inside the sealed container, basically reach supercritical condition of the solvent which turn into reduction agent Some solvents were studied is water-NH4OH

(Johra and Jung, 2015), N,N-dimethylformamide (Wang et al., 2009), N-methyl-2-pyrrolidinone (Dubin et al 2014) rGO, which reduced by solvothermal treatment (1800C at 12h) N,N-dimethylformamide as solvent, was reported that had higher C/O

ratio than chemical reduction by hydrazine reduction at normal pressure (Wang et al., 2009) On the other hand, reducing GO in N-methyl-2-pyrrolidinone results confirmed the formation of single sheets of the solvothermal rGO platelet The obtained C/O ratio was low, 5.15 at 2000C, and demands 10000C to achieve 6.36

(Dubin et al., 2014) Similar with thermal method, for water treatment, this method may costly due to required heating in long time

Other methods

17 1.3.2 Application of rGO in antibiotic adsorption

Thanks to the outstanding properties like large surface area of 2D graphene sheets, porous structure, stability, conductivity, and flexibility, rGO and their modified material can be used as adsorbent (Ali et al 2019; Gao et al 2012; Huízar-Félix et al 2019; Maliyekkal et al 2013; Song et al 2016; Sun et al 2013); and photocatalytic/photochemical oxidation (Moussa et al., 2016; Sun et al., 2014)

Applications of rGO and their deviations in environment for now focus on hazardous pollutants such as heavy metals (Ali et al., 2019) rGO exhibits chlorpyrifos uptake capacity as ~1200mg/g, 10–20% higher than that of GO, as well as able to regeneration and reuse (Maliyekkal et al., 2013) (Huízar-Félix et al., 2019) reported about removal of TC using magnetic rGO material, which was expected to increase electrostatic interaction between rGO with TC and recoverability However, it was found that hybrid material exhibited a lower removal efficiency than precursor Keshvardoostchokami et al (2019) concluded that Ag+-reduced rGO could remove

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CHAPTER MATERIALS AND METHODS

2.1 Materials

Tetracycline powder (95.3% purity) were purchased from LKT Laboratories, Inc (Japan) Tetracycline stock solutions (100mg/L) were prepared by double distilled water and stored at 40C

Graphite fine powder extra pure were obtained for Merck (Germany)

The other chemicals used in the study included: concentrated sulfuric acid (H2SO4), sodium nitrate (NaNO3), potassium permanganate (KMnO4), hydrogen

peroxide (H2O2), hydrochloric acid (HCl), L-ascorbic acid (C6H8O6), nitric acid

(HNO3), sodium hydroxide (NaOH).The entire reagents used were of A R grade

2.2 Methods

2.2.1 Literature review

The references that used in this study were domestic and international journals, published studies Website of large organizations (IUPAC, AMR industry alliance, Sigma-aldrich) also reviewed

Information, methods, and explanations were cited and adopted in order to give an overview of the objectives And the data and mechanisms were compared and explained with obtained data

2.2.2 Sample analysis in laboratory Characterization of materials

a) Fourier-transform infrared spectroscopy

Fourier-transform infrared spectroscopy (FT-IR) based on the interaction between analyte and beam of multi-wavelength within infrared region (400-4000 cm -1) The result of that would lead to the analyte absorbing part of the energy and

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vibrations that change the angle of link and the length of links between atoms in the molecule

Optical density is calculated by Lambert-Beer equation:

𝐷 = = ε𝑙𝐶 (1)

Where D is optical density quang; Io and I respectively are light intensity

before and after going though analyte; ε is absorption coefficient; l is thickness of cuvet; C is analyte concentration (mol/l)

IR spectra usually is performed by transmittance against wavelength:

T(%) = (I0/I)x100 (2)

Infrared spectra curve represents the dependence of the intensity of the infrared absorption of the analyte on wavenumbers or wavelengths On the infrared spectrum, the horizontal axis represents the wavelength (μm) or the number of wavenumbers (cm-1), the vertical axis represents the absorption intensity (abs) or

transmittance (T%) Each maximum in the IR spectrum featured the presence of a functional group or oscillation of a bond Therefore, it can be based on these characteristic frequencies to determine the presence of links or functional groups in the target analyte Conventionally the IR region is subdivided into three regions, near IR, mid IR and far IR Most of the IR used originates from the mid IR region (400-4000 cm-1)

The sample was measured by transmittance technique, the analytical powder was mixed with the KBr substrate at the rate of 1-2% of the sample/KBr, measured in the area of 400-4000 cm-1

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Figure 2.1 FT-IR 4600 Jasco b) Energy-dispersive X-ray spectroscopy

Energy-dispersive X-ray spectroscopy (EDX) is mainly done in electron microscopes, in which, solid microstructure images are captured by using high-energy electron beams interacting with solid objects When a high-high-energy electron beam rays onto a solid sample, it will penetrate deep into the solid atom and ejected electrons out of atom The electron vacancies are filled by electrons from a higher state, and an x-ray is emitted to balance the energy difference between the two electrons' states This interaction results in the creation of X-rays with characteristic wavelengths proportional to the atomic number (Z) of the atom according to Mosley's law:

𝑓 = 𝑣 = (𝑍 − 1) = (2.48 × 10 )(𝑍 − 1) (3)

Where me is the mass of electrons; qe is the charge of the electron, h is Planck's

constant; ε0 is the permittivity of free space

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Figure 2.2 Principle of EDX measurement (Reinoud Lavrijsen, 2010)

EDX is measured by JSM-IT100/JED-2300 Analysis station, JEOL, at nano technology laboratory, VJU (Figure 2.2)

Figure 2.3 JSM-IT100/JED-2300 Analysis Station Plus, JEOL (source: www.jeol.co.jp)

c) Scanning Electron Microscopy

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Morphology includes the shape, the size of components that contribute object Appearance is the surface characteristic of object including its texture or hardness

SEM is a powerful tool for studying surface morphology by using electron beams to sweep across the surface of a sample In this work, SEM is used to assess the size and shape of particles in a sample SEM images of material were taken on JSM-IT100 InTouchScope at nano technology laboratory, VJU (Figure 2.3)

d) X-ray diffraction

The principle of the X-ray diffraction (XRD) technique is based on the diffraction phenomenon of X-rays on the crystal lattice When X-ray radiation interacts with some matter, elastic scattering effect would appear with electrons of atoms in crystal structure material, which will lead to X-ray diffraction phenomenon Theoretically, lattices are made up of atoms or ions that are uniformly distributed in space in a certain order When the X-ray beam reaches the crystal surface and goes deep inside the crystal lattice, the lattice acts as a special diffraction grating Atoms or ions excited by the X-ray beam and form centers that emit reflected rays

Based on the position and intensity of the diffraction peaks on the recorded diagram, crystal lattice parameters and the distance between reflective surfaces in the crystal were determined

23

Figure 2.4 MiniFlex 600 (source: www.rigaku.com) e) Surface area and pore volume calculation

Brunauer–Emmett–Teller (BET) method

Brunauer–Emmett–Teller (BET) method still is a common method to evaluating the surface area of porous and finely-divided materials Surface area of material is determined by physisorption of a gas on its surface An adsorption isotherm of amount of adsorbed gas (typically N2) against a range of increasing

pressures at a constant temperature (77K for N2 liquid) The amount of adsorbed gas

are recommended to be in cm3/g units Conversely, desorption isotherm is achieved

by removed gas as pressure is reduced The bet equation is defined as follows (Naderi, 2015):

/

( ) = + ( ) (4)

where V is the volume of the adsorbed gas at the relative pressure P/P at

standard temperature and pressure (STP) (cm3) STP is defined as 273 K and atm

Vm is the monolayer capacity of the adsorbed gas (cm3), P is the pressure, P is the

24

The BET specific surface area (ssa) can be calculated by known the average area of value am (molecular cross‐sectional area, equals 0.162 nm2 for an absorbed

nitrogen molecule) and implementing the following equation (Naderi, 2015):

𝑠𝑠𝑎 = 𝑉 ∙ 𝑁 ∙ 𝑎 /𝑚 𝑣 (5)

where NA is the Avogadro constant (~6.022×1023 mol-1), ms is weight of

adsorbent (g), and vm is the molar volume of N2 in gas and equals 22,400 (cm3)

Barrett-Joyner-Halenda method (BJH)

The classical Kelvin equation relates the vapor pressure of the curvature of liquid-vapor interface (P), to the radius of the droplet (rk), gas molar volume (Vm),

surface tension (γ), the gas constant (R), Temperature (T), and vapor pressure of the same liquid on a planar surface (P0)

𝑙𝑛 = (6)

This equation is the basis for calculating pore-size distributions, assuming cylindrical pores, from adsorption isotherms When taking into account multi-layer adsorption, rk meaning the radius of the condensation occurring, which is the function

of the pore radius and the statistical thickness of the adsorption film Density Functional Theory (DFT)

This method calculated the density distribution of adsorbent liquid in pore spaces at a given temperature and equilibrium pressure can be given for pore structures and molecular interactions In contrast to BJH method that only reliable with mesoporous material, the DFT technique yields pore-size distributions of micro- to meso-porous objects

25

Figure 2.5 TriStar II Plus, Micromeritics (source: www.micromeritics.com) f) pH point of zero charge

Preparing flasks containing 50ml of NaNO3 0.01M, the pH of each was

adjusted from to 12 (2,4,6,8,10,12) with HNO3 and NaOH Recording the pH after

reaching the constant value, 10 mg of material is added to each flask and shake for 12 hours Solutions are filtered to re-measure the pH value of the solution (pHf) The

difference between the initial pH (pHi) and the equilibrium pH (pHf) is pHf – pHi =

ΔpH The pH value where the curve of ΔpH crosses pHi axis is pHzpc

Determination of Tetracycline concentration

26

Figure 2.6 Calibration curve for Tetracycline

The results show that the dependence of Tetracycline concentration on the absorbance in the solution follows the linear formula: y = 0.029x + 0.001, with regression coefficient R2 = 0.9995

2.2.3 Data calculation a) Removal efficiency

The absorption efficiency is obtained using the following equation(Jannat Abadi et al., 2019):

𝐻% = × 100 (7)

where: H% is the adsorption capacity; Co is the initial TC concentration in

solution (mg/L); Ce is equilibrium TC concentration in solution (mg/L);

b) Adsorption capacity

The adsorption capacity is determined by the following formula(Jannat Abadi et al., 2019):

𝑞 =( )× (8)

y = 0.029x + 0.001 R² = 0.9995

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35

0 10 12

A

bs

27

where: qe is the adsorption capacity (mg/g); Co is the initial TC concentration

in solution (mg/L); Ce is equilibrium TC concentration in solution (mg/L); V is

volume of solution (L); and m is amount of rGO (g) c) Kinetic parameters

Adsorption kinetics has been evaluated through pseudo-first-order model (Ocampo-Pérez et al., 2012):

ln(𝑞 − 𝑞 ) = ln(𝑞 ) − 𝑘 𝑡 (9)

And pseudo-second-order model (Hamoudi et al., 2018)

= + (10)

which qe is equilibrium adsorption capacity (mg/g); qt is adsorption capacity

at time t (mg/g); k1 is the rate constant for pseudo-first-order (min-1); k2 is the rate

constant for pseudo-second-order (g·mg-1·min-1)

d) Isotherm parameters

Adsorption isotherm of Tetracycline is evaluated by Langmuir model and Freundlich model

The Langmuir isotherm is given by:

𝑞 = (11)

Linearized form is given by the following equation:

= + ∙ (12)

The essential features of Langmuir isotherm can be expressed in terms of a dimensionless constant separation factor (RL) that can be defined by the following

relationship

28

where C0 is the initial concentration (mg/l) and b is the Langmuir constant

(L/mg) For favorable adsorption process the value of RL should be in the range

between zero and one The values of RL were determined at T= (298, 308, and 318)

K (Anirudhan and Radhakrishnan, 2008) Freundlich isotherm is given by:

𝑞 = 𝐾 𝐶 ⁄ (14)

Linearized form is given by the following equation:

𝑙𝑛𝑞 = 𝑙𝑛𝐾 + 𝑙𝑛𝐶 (15)

Where qe is equilibrium adsorption capacity (mg/g); Ce is equilibrium

concentration (mg/L); qm is ultimate adsorption capacity (mg/g); KL is the Langmuir

constant (L/mg); and KF (mg.g-1)(L.mg-1)1/n and n are Freundlich empirical constants

(Gao et al., 2012)

e) Thermodynamic study

The experiment was carried out at different temperature conditions and parameters that assumed constant included enthalpy (ΔH), entropy (ΔS), and Gibbs free energy (ΔG) are calculated by following equation:

∆𝐺 = ∆𝐻 − 𝑇∆𝑆 (16)

∆𝐺 = −𝑅𝑇𝑙𝑛𝐾 (17)

Where R is the natural gas constant; T is temperature (K); KC is the constant

at equilibrium KC would be obtained from constants of isotherm equations (Húmpola

et al., 2013)

2.2.4 Statistical analysis

29 2.3 Experiment setup

2.3.1 Material synthesis

Graphene oxide is synthesis by adopted modified Hummer’s methods Firstly, 23ml of sulfuric acid is cooled down to less than 50C, then gram of graphite powder

and 0.5 gram of sodium nitrate are added, the mixture is stirred in below 500C After

one hour, 0.3 gram of Potassium manganate (VII) is added to pre-oxidize, then 2.7gram Potassium manganate (VII) is add slowly while stirring, the temperature is controlled at 10~200C for two hours to get dark green suspension Increasing the

temperature to 30~350C for two hours to obtain the brown suspension, then 23 ml of

distilled water is added slowly drop by drop, and temperature is controlled at 90~950C

for 30 minutes Add 120 ml of distilled water, the reaction system is cooled down to ∼500C Then add 10 ml of 30% hydrogen peroxide solution (H2O2) and continue

stirring until the solution changes from brown to light yellow

The mixture is washed with 5% HCl, then the mixture is washed with distilled water until pH~6 The final mixture was ultrasonicated within hours and then dried at 500C overnight The ultrasonication separate the GO layers being filled with acid

molecules before, in addition to removing unwanted mechanical deposits from graphite

Reduced GO is synthesis by mild reduction method, using L-ascorbic acid gram of L-ascorbic acid is dissolved in 100 ml of distilled water After adding 0.1gram dried GO, mixture is ultrasonicated within 45 minutes The mixture is then heated at 90~950C for hour The black precipitate is filtered by vacuum pump and

further washed by 1M HCl and distilled water to neutral pH Finally, filter and dry in a vacuum oven at 500C for hours to obtain rGO

2.3.2 Factors influencing the efficiency of TC adsorption a) Contact time

30

condition until reaching equilibrium stage 5ml solution were collected each one hour and filtered, the filtrate was collected for measuring remaining concentration by UV– Vis spectroscopy at 357nm wavelength Experiment was replicate times

b) pH

To investigate the effect of pH, experiments were carried out in the pH range of 2–10 in ambient temperature Weighing 0.01g of rGO into 100mL of Tetracycline solution at a concentration of mg/L, shake at 120 rpm with time required to reach equilibrium Filtering the solution and determine the remaining Tetracycline concentration Experiment was replicate times

c) Dosage of rGO

Operating experiments as the above sections with the time to reach the adsorption equilibrium, the optimal pH determined above and adjust the amount of rGO to 0.02; 0.03; 0.04; 0.05 and 0.06g Experiment was replicate times

d) Initial concentration of Tetracycline

The experiments were conducted similarly to the section on the influence of time, pH, mass of adsorbents; however, the concentration of Tetracycline solution varies from 10 to 80 mg/L and the adsorption process is carried out to the time when adsorption equilibrium is reached, the optimum pH and the optimal amount of adsorbent are determined in the previous research sections Experiment was replicate times

2.3.3 Isotherm tests

Isotherm tests were adopted from (Lu et al., 2018) Sorption isotherms experiments were carried out by adding 0.04g of rGO into 100 ml of TC solutions with varying concentrations (10–80 mg/L) All the solutions were agitated at the desired temperature (298K, 308K or 318K) for 6h

2.3.4 Kinetics tests

31

periodically extracted to determine the amount of TC adsorbed as a function of time (Ocampo-Pérez et al., 2012)

2.3.5 Thermodynamic tests

32

CHAPTER RESULTS AND DISCUSSION

3.1 Material characterization

3.1.1 Fourier-transform infrared spectroscopy

The FT-IR spectrum of GO (Figure 3.1) shows similarities with previous studies, consisted an intense peak at 3446 cm-1 and other lower intensity peaks at

1728, 1627, 1397, and 1070 cm-1 After reduction, rGO spectrum witnessed the

diminished of the peak at 1728 cm-1 as well as the peak of 1397 and 1070 cm-1 which

indicated the decomposition of aldehyde groups, tertiary C-OH groups, and C-O stretching vibrations of alkoxy groups The appearance of intense peak at 1591 cm-1

confirmed the restoration of the sp2 carbon networks (Johra and Jung, 2015)

Compounds with corresponding to the wavelength ranges are listed in the table 3.1

33

Table 3.1 IR Spectrum by frequency range (www.sigmaaldrich.com) Absorption region

cm-1 Group Compound Class

Obtained result cm-1

3700-3100 O-H stretching Alcohol/water 3446.36

1740-1720 C = O stretching Aldehyde 1728.89

1680-1600 C=C stretching Alkene 1627.07

1600-1400 C=C stretching Aromatic 1591.72

1440-1395 O-H deformation Carboxylic acid 1397.27 1085-1050 C-O stretching Primary alcohol 1070.61

3.1.2 Energy-dispersive X-ray spectroscopy

Having the same fine powdered forms, graphite structure consisted multilayer overlap and no porosity, as rGO show typical wrinkled morphology (Figure 3.2) Powder particle size lies below 50 µm

Figure 3.2 SEM images of (a) graphite and (b) rGO

Table 3.2 lists the C/O ratio in GO and rGO summarized in this study compared to previous studies C/O ratio in this study is comparable with those in studies conducted by Alibeyli et al (2017) and Xu et al (2015), who used NaBH4

34

reductions generally showed a higher efficiency than microbial reduction (Chen et al., 2017)

Table 3.2 C/O ratio comparison of GO and rGO

Material C/O atomic ratio Reduction agent Reference

Graphite 23.75

L-ascorbic acid This study

GO 1.07

rGO 7.10

Graphite 17.87

L-ascorbic acid (Xu et al., 2015)

GO 2.65

rGO 2.89-5.15

GO 1.79 NaBH4/hydrazine

monohydrate (Alibeyli et al., 2017)

rGO 4.31-9.47

GO 2.23 Azotobacter

chroococcum (Chen et al., 2017)

rGO 4.18

3.1.3 X-ray diffraction

Figure 3.3 shows characteristic graphite peaks at 2θ = 26.6o and 54o 26.6o

peak shifting to 10.62o, characteristic peak of GO (Alibeyli et al., 2017), indicated

35

Figure 3.3 XRD results of GO and rGO

The broad peak for rGO implied that the rGO structure was arranged randomly, resulting the formation of single or only a few layers of rGO Less intense peak at 2θ =42.88o attributed by the turbostratic structure (locally parallel) of disordered carbon

materials (Hidayah et al., 2017) 3.1.4 Surface area and pore volume

The BET surface areas of rGO were investigated using nitrogen adsorption– desorption experiments (Figure 3.4), is consistent with previous studies (Alazmi et al., 2016; Moussa et al., 2016) The N2 adsorption-desorption isotherm is fitted with

36

desorption branch does not reach the adsorption branch due to the presence of micropores (Moussa et al., 2016)

(a) (b) (c)

Figure 3.4 (a) N2 adsorption and desorption isotherms, (b) Types of physisorption

isotherms, and (c) Types of hysteresis loops (IUPAC)

Figure 3.5 confirms pore-size distribution in rGO which half pore width is highest at ~3.2nm, most pores have radius below 20nm Clearly synthesized rGO is meso-porous material

Figure 3.5 Pore size distribution of rGO obtained from DFT method

Pore volume and surface area was obtained by BJH method, causing this method are most suitable for meso-porous material Results of rGO are 341.224 m2/g

0 0.1 0.2 0.3 0.4 0.5 0.6

0 10 20 30 40

dV

(lo

gr

)c

c/

g

37

for surface area and 0.4025 cm3/g for pore volume which comparable with other

studies in Table 3.3 Surface area is quite lower than theory (2600 m2/g) likely due

to the large amount polar groups (hydroxyl, carbonyl ) remaining after reduction process which caused tight hydrogen binding and strong sheet associations (Moussa et al., 2016)

Table 3.3 Properties of materials used for TC removal in previous studies Textural

property

SBET (m2/g) Vtotal

(cm3/g)

Average pore diameter (nm)

Reference

GO 81.2 - - Song et al., 2016)

rGO 384.5 - -

rGO 75.77 ± 0.35 - - (Moussa et al., 2016)

NAS* 139 1.85 1.97 (Ocampo-Pérez et al.,

2012)

rGO 657.46 - - (Alibeyli et al., 2017)

rGO-CPD*

364 1.17 - (Alazmi et al., 2016)

Fe‑doped zeolite

98.82 0.2716 - (Jannat Abadi et al.,

2019)

Pumice 6.6 0.012 7.6 (Lu et al., 2018)

*NAS: NaOH-activated sludge; rGO-CPD: critical point drying

3.1.5 pH point of zero charge

According to IUPAC, a surface charge is at its point of zero charge ( pHpzc)

38

is negative charge; and if pH<pHpzc, the particle surface will have anion exchange

capacity, or positive charge Figure 3.6 show the pHpzc of rGO is approximately 5.2

Figure 3.6 The plot of ΔpH versus pHi

3.2 Adsorption study

3.2.1 Factors influencing TC adsorption a) Contact time

Figure 3.7 presented dependent of TC adsorbed amount of rGO in contact time The adsorption capacity of TC treatment in a solution with an initial concentration of mg/L using rGO was determined at different reaction times It can be seen that the adsorption capacity of the material reaches 41.84 mg/g after hours After that, the TC concentration in the solution was negligible change, suggesting that the time to reach adsorption equilibrium was hours

-2 -1.5 -1 -0.5 0.5 1.5

0 10 12 14

de

lta

p

H

39

Figure 3.7 Effect of contact time on TC adsorption by rGO b) pH

Theoretically, if electrostatic interaction has a significant impact in adsorption process, then the difference in adsorption capacity between pHs would depends strongly on pHpzc of the two substances Due to the existence of three functional

groups, the TC molecular can be positive (pH < 3.32) charged, neutral (3.32 < pH < 7.78), or negative (pH > 7.78) charged (Zhang et al., 2015) Comparing with pHpzc of

rGO (5.2), pH range which possible exhibits electrostatic interactions is 3.32-7.78, then adsorption capacity in this range must higher than others

The result in Figure 3.8 shows that there is not much difference in adsorption capacity at the pH range of 2-8 (28.6-30.9mg/g), in accordance with (Song et al 2016), which meaning electrostatic interactions does not affect processing efficiency much (Sun et al 2013) concluded that pH not influence the adsorption of polycyclic aromatic hydrocarbons This is consistent with previous studies which insists that the interaction between TC and rGO most likely occurred van der Waals forces (π–π interactions and cation–π bonding) and electron donor-acceptor (EDA) due to their large aromatic ring structure (Ai et al., 2019; Koyuncu et al., 2008; Song et al., 2016)

0 10 20 30 40 50 60

0 10

ad

so

rb

ed

a

m

ou

n

t

(m

g/

g)

40

Figure 3.8 Effect of pH on TC adsorption by rGO c) Dosage

The results in Figure 3.9 indicate that the greater the amount of adsorbent, the lower the adsorption capacity It also showed that adsorption efficiency increased from 67.42% to 81.64% when weight of rGO increased from 0.01 (g) to 0.04 (g) When continuing to increase the dosage of rGO, the adsorption efficiency of TC almost unchange

0 10 15 20 25 30 35

0 10 11

ad

so

rb

ed

a

m

ou

n

t

(m

g/

g)

41

Figure 3.9 Effect of dosage effect on TC adsorption by rGO

The reason is when the amount of the adsorbent is small, the active sites on rGO is small, and not enough to adsorb the presenting TC On the other hand, when the amount of the adsorbent is too high will lead to the aggregation of material particles, as a result processing efficiency and TC uptake decrease (Padmavathy et al., 2016)

d) Initial concentration

The results showed that as the initial concentration of TC increased (from 10 to 80mg/L), the removal efficiency of rGO material decreased (from 88.5% to 28.9%) (Figure 3.10) 10 20 30 40 50 60 70 80 90

0.1 0.2 0.3 0.4 0.5 0.6

Ad so rb ed a m ou nt (m g/ g) Ef fic ie nc y (H % )

Dosage of rGO (g/L)

42

Figure 3.10 Effect of initial concentration on TC adsorption by rGO e) Temperature

The temperature survey revealed that the adsorption capacity is proportional to the temperature (Figure 3.11) Adsorption capacity observed after hours for initial TC concentration 80mg/L at 298K, 308K, 318K reaches to 60.76, 107.33 and 137.41 mg/g, respectively, which meaning chemisorption is involved And at 308K and 318K, adsorption capacity can also increase when initial TC concentration increasing At initial TC concentration 100mg/L, adsorption capacities are 123.71 and 159.48 mg/g at 308K and 318K respectively

0 10 20 30 40 50 60 70 10 20 30 40 50 60 70 80 90 100

10 20 30 40 50 60 70 80

Ad so rb ed a m ou nt (m g/ g) Ef fic ie nc y (H % ) Co (mg/L)

43

Figure 3.11 Effect of temperature on TC adsorption capacity of TC of rGO 3.2.2 Adsorption isotherms

Figure 3.12 Comparison of experimental data and modeled data on adsorption isotherms

0 20 40 60 80 100 120 140 160 180 200

0 10 20 30 40 50 60 70

qe

(m

g/

g)

Ce(mg/L)

298K 308K 318K

0 10 20 30 40 50 60 70

0 10 20 30 40 50 60

qe

(m

g/

g)

Ce (mg/L)

44

Isotherm models which were calculated by equations (11-14) which presented in Figure 3.12 The results showed that Langmuir and Freundlich isothermal adsorption models fitting well with TC adsorption process by rGO, which consistent with previous studies (Huízar-Félix et al., 2019; Song et al., 2016) as Langmuir model described slightly better This is shown by the regression coefficient (R2) of the

Langmuir and Freundlich model are 0.986 and 0.942 respectively The maximum adsorption load calculated by Langmuir model reaches 58 mg/g of material, higher than pumice (4.89mg/g) and bamboo charcoal (22.7mg/g) NAS owned greater qm

(672 mg/g) is likely by higher surface area (139m2/g) and large pore volume

(1.85cm3/g) in NAS (Ocampo-Pérez et al 2012)

Table 3.4 Langmuir and Freundlich isotherm parameters on TC adsorption

Adsorbe nt

Experiment conditions

Langmuir Freundlich

Referen ce qm

mg/g

KL

L/mg R

2 KF* n R2

rGO

298K, pH~6, C0*=5mg/L,

ma*=0.1g/L,

time=6h

58 0.52 0.99 23.25 3.95 0.94 This study

Bamboo charcoal

298K, pH=3.6, C0

=8.33mg/L, ma=0.181g/L,

time=90min

22.7 0.02 0.92 0.76 0.65 0.99

(Liao et al., 2013)

rGO

298K, pH=7, C0=30-8mg/L,

ma=0.6g/L,

time=90min

44.23 0.09 0.98 4.65 1.57 0.99

(Huízar-Félix et

al., 2019)

Pumice

293K, C0=20mg/L,

pH=6, ma=5g/L,

time=24h

4.89 0.258 0.98 0.97 0.47 0.94

(Lu et al., 2018) NAS* 298K, pH~7,

C0=700mg/L,

45 ma=1g/L,

time~200h

et al., 2012)

*C0: initial TC concentration in solution; ma: dosage of adsorbent; NAS: NaOH-activated

sludge; KF: Freundlich constant, unit (mg/g)(L/mg)1/n

All the RL values were found to be less than one and greater than zero

indicating the favorable adsorption of TC onto rGO 3.2.3 Adsorption kinetics

The first- and second-order kinetic model were exploited to evaluate reaction constant rate of adsorption process (equation 9,10) As shown in Table 3.5, the result was fitted better with second-order model than first-order, provided constant rate k2=0.018 g.mg-1.h-1 Table 3.5 compares constant rate of difference material for TC

adsorption It indicated that reaction rate of rGO is higher than graphite, its precursor, and rough material like pumice and biochar but lower than GO due to number of remaining functional groups on the surface The rate is higher than same material studied by (Huízar-Félix et al., 2019) properly causing the difference remaining functional groups in surface

Table 3.5 First- and Second-order kinetic parameters for TC adsorption (T=298K) First order kinetic

parameters

Second order kinetic parameters

Reference k1

(h-1)

qe

(mg/g) R

2

k2

(g/mg.h -1)

qe

(mg/g) R

2

rGO 0.365 29.16 0.91 0.018 49.32 0.99 This study

Graphite 0.014 31.5 0.99 - - - (Vedenyapina

et al., 2014) Biochar 0.081 3.266 0.877 0.069 7.001 0.99 (Wang et al.,

2018)

GO - - - 0.065 35 0.99 (Gao et al.,

46

rGO - - - 0.15

×102 44.23 0.99

(Hzar-Félix et al., 2019) Pumice 0.05

×10-3 2.9 0.99

0.049

×10-3 2.99 0.98

(Lu et al., 2018)

*C0: initial TC concentration in solution; ma: dosage of adsorbent; NAS: NaOH-activated

sludge; PACs: petroleum coke derived activated carbons

3.2.4 Adsorption thermodynamics

In order to assess energy exchange phenomenon of the TC sorption process, Gibbs free energy (ΔG), enthalpy (ΔH), and entropy (ΔS) were estimated using equation (16,17)

Based on Figure 3.13, one could obtain ΔG, ΔH, and ΔS, and the values ò these parameters were listed in Table 3.6 ΔG is negative at all temperature (7.79, -9.31, and -9.54 kJ/mol) which confirmed the spontaneous nature of TC onto rGO (Suresh et al., 2011) That study also reported that physisorption generally is occurred at ΔG ranges from -20 to kJ/mol, and -80 to -400 kJ/mol for chemisorption As such, the sorption nature in this work was physisorption The positive value of ΔH (17.97 kJ/mol) confirmed that the adsorption reaction is endothermic, the reason why sorption capacity increased proportional with temperature The positive value of ΔS (0.087 kJ.mol-1.K-1) reflected the affinity of rGO for TC and the more disorder at the

y = -0.0872x + 17.974 R² = 0.8474

-12 -10 -8 -6 -4 -2

295 300 305 310 315 320

Δ G ( J/ m ol ) T(K)

47

solid/solution during adsorption (Lu et al., 2018) For physical adsorption ΔH should be in range to 10kJ/mol, and for chemical adsorption it ranges between 30 and 70 kJ/mol (Suresh et al 2011) The calculated ΔH (17.97 kJ/mol) in this study indicated that TC adsorption on rGO can be attributed to a physic-chemisorption processes

Table 3.6 Thermodynamic parameters of TC sorption process by rGO Temperature (K) ΔG(kJ/mol) ΔH (kJ/mol) ΔS(kJ·mol-1·K-1)

298 -7.79

17.97 0.087

308 -9.31

48

CONCLUSIONS AND RECOMMENDATIONS

Conclusions

rGO was successfully synthesized from graphite precursor The X-Ray diffraction analysis revealed that characteristic peaks of graphite (2θ = 26.6o) was

shifted to those of rGO (2θ = 24.18o) The EDX spectra also revealed a decrease in

proportion of oxygen element in rGO compared to GO Finally, FTIR analyses also indicated the diminution of carboxyl and epoxy functional groups Moreover, B.E.T results also demonstrated rGO a micro-meso porous material, with 341.2 m2/g for

surface area and 0.4025 cm3/g for pore volume

The TC adsorption efficiency on rGO was depended on the dosage of adsorbent, initial TC concentration, and temperature, whereas it did not much reply on pH of the solution The TC absorption process was better described by Langmuir isotherm model (r2=0.986) compared to that of Freundlich (r2=0.942) The maximum TC adsorption capacity of rGO was 58 mg/g at pH=6, calculated by Langmuir isotherm model The adsorption kinetic of TC fitted well the pseudo second order kinetic (R2=0.99) The thermodynamic parameters confirmed that the TC adsorption

process of TC onto rGO was endothermic physic-chemisorption spontaneous process The combination with the results on the effect of process parameters, it can be said that TC adsorption mechanisms was dominated by both physisorption (Van der Waals forces, π–π interaction) and chemisorption (Electron donor-acceptor) via interactions of aromatic ring sheets (Ai et al., 2019; Gao et al., 2012)

Recommendations

All adsorption tests in this study were conducted in the batch mode with the synthetic wastewater Therefore, the further study on TC adsorption in the column mode with the real wastewater is necessary

49

50

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58

APPENDIX

Table S1 Antibiotic Resistance Alliance Science-Based PNEC Targets for Risk Assessments (AMR Industry Alliance, 2020)

Active Pharmaceutical Ingredient

PNECENV*

(µg/L)

PNECMIC**

(µg/L)

Lowest Value (µg/L)

Amikacin N/A 16.00 16.00

Amoxicillin 0.57 0.25 0.25

Amphotericin B N/A 0.02 0.02

Ampicillin 0.60 0.25 0.25

Anidulafungin N/A 0.02 0.02

Avilamycin 125.00 8.00 8.00

Azithromycin 0.03 0.25 0.03

Aztreonam N/A 0.50 0.50

Bacitracin 114.59 8.00 8.00

Bedaquiline 0.08 N/A 0.08

Capreomycin N/A 2.00 2.00

Cefaclor N/A 0.50 0.50

Cefadroxil 0.14 2.00 0.14

Cefalonium 21.10 N/A 21.10

59

Cefalothin N/A 2.00 2.00

Cefazolin N/A 1.00 1.00

Cefdinir N/A 0.25 0.25

Cefepime N/A 0.50 0.50

Cefixime 0.60 0.06 0.06

Cefoperazone N/A 0.50 0.50

Cefotaxime 0.12 0.13 0.12

Cefoxitin N/A 8.00 8.00

Cefpirome N/A 0.06 0.06

Cefpodoxime proxetil 1.76 0.25 0.25

Cefquinome 1.60 N/A 1.60

Ceftaroline 0.12 0.06 0.06

Ceftazidime 1.30 0.50 0.50

Ceftibuten N/A 0.25 0.25

Ceftiofur N/A 0.06 0.06

Ceftobiprole 0.23 0.25 0.23

Ceftolozane 1.90 N/A 1.90

Ceftriaxone 29.40 0.03 0.03

Cefuroxime 1.70 0.50 0.50

Cephalexin 0.21 4.00 0.21

60

Chloramphenicol N/A 8.00 8.00

Chlortetracycline 5.00 N/A 5.00

Ciprofloxacin 0.45 0.06 0.06

Clarithromycin 0.26 0.25 0.25

Clinafloxacin N/A 0.50 0.50

Clindamycin 0.10 1.00 0.10

Cloxacillin 20.00 0.13 0.13

Colistin (Polymyxin E) 9.00 2.00 2.00

Daptomycin 510.00 1.00 1.00

Delamanid 0.03 N/A 0.03

Doripenem 0.46 0.13 0.13

Doxycycline 25.10 2.00 2.00

Enramycin 4.80 N/A 4.80

Enrofloxacin 1.91 0.06 0.06

Ertapenem 14.00 0.13 0.13

Erythromycin 0.50 1.00 0.50

Ethambutol N/A 2.00 2.00

Faropenem N/A 0.02 0.02

Fidaxomicin 891.00 0.02 0.02

Florfenicol 38.00 2.00 2.00

61

Fluconazole N/A 0.25 0.25

Flumequine N/A 0.25 0.25

Fosfomycin N/A 2.00 2.00

Fusidic acid N/A 0.50 0.50

Framycetine Testing

on-going 0.06 0.06

Gatifloxacin N/A 0.13 0.13

Gemifloxacin N/A 0.06 0.06

Gentamicin 0.15 1.00 0.15

Imipenem 0.41 0.13 0.13

Isoniazid N/A 0.13 0.13

Kanamycin 1.05 2.00 1.05

Levofloxacin 0.52 0.25 0.25

Lincomycin 0.81 2.00 0.81

Linezolid 3.50 8.00 3.50

Loracarbef N/A 2.00 2.00

Mecillinam N/A 1.00 1.00

Meropenem 1.50 0.06 0.06

Metronidazole N/A 0.13 0.13

Minocycline Testing

62

Moxifloxacin N/A 0.13 0.13

Mupirocin N/A 0.25 0.25

Nalidixic acid N/A 16.00 16.00

Narasin N/A 0.50 0.50

Natamycin Testing

on-going N/A N/A

Neomycin 0.03 2.00 0.03

Netilmicin N/A 0.50 0.50

Nitrofurantoin N/A 64.00 64.00

Norfloxacin 120.00 0.50 0.50

Ofloxacin 10.00 0.50 0.50

Oxacillin N/A 1.00 1.00

Oxytetracycline 47.00 0.50 0.50

Pefloxacin N/A 8.00 8.00

Penicillin G Procaine 16.00 0.25 0.25

Phenoxymethylpenicillin N/A 0.06 0.06

Piperacillin 4.30 0.50 0.50

Polymixin B 0.06 N/A 0.06

Pristinamycin Testing

on-going N/A N/A

63

Retapamulin N/A 0.06 0.06

Rifampicin Testing

on-going 0.06 0.06

Rifamycin N/A N/A N/A

Rifaximin N/A N/A N/A

Roxithromycin 6.80 1.00 1.00

Secnidazole N/A 1.00 1.00

Sparfloxacin N/A 0.06 0.06

Spectinomycin N/A 32.00 32.00

Spiramycin 1.09 0.50 0.50

Streptomycin N/A 16.00 16.00

Sulbactam N/A 16.00 16.00

Sulfadiazine 11.21 13.00 11.21

Sulfamethoxazole 0.60 16.00 0.60

Tedizolid 3.20 N/A 3.20

Teicoplanin 12.90 0.50 0.50

Telithromycin Testing

on-going 0.06 0.06

Tetracycline 3.20 1.00 1.00

Thiamphenicol N/A 1.00 1.00

64

Ticarcillin N/A 8.00 8.00

Tigecycline Testing

on-going 1.00 1.00

Tildipirosin 0.42 N/A 0.42

Tilmicosin 0.80 1.00 0.80

Tobramycin 4.30 1.00 1.00

Trimethoprim 312.45 0.50 0.50

Trovafloxacin N/A 0.03 0.03

Tulathromycin Testing

on-going N/A N/A

Tylosin 0.98 4.00 0.98

Vancomycin N/A 8.00 8.00

Viomycin N/A 2.00 2.00

Virginiamycin N/A 2.00 2.00

*PNEC‐Environment (PNECENV) values are based on eco-toxicology data generated by

Alliance member companies and relevant peer reviewed literature These values are intended to be protective of ecological species and incorporate assessment factors consistent with standard environmental risk methodologies

**PNEC-Minimum Inhibitory Concentration (PNECMIC) values are based on the published

65

Table S2 Removal of tetracycline antibiotics using different treatment processes

Antibiotics Matrix Treatment Operating conditions Results and comments Tetracycline Distilled water Semiconductor

photocatalysis

MP UV (125 W); pH = 6.0; [TC] = 10–50 mg/L; TiO2 (100 %

anatase or anatase/rutile = 4/1; and 0.4 g/L of catalyst

More than 98 % of tetracycline was oxidized within about h; 100 % of total organic carbon removal using TiO2 (anatase/rutile)

Deionised water Semiconductor photocatalysis

UV (254 nm, 365 nm); solarium device (300–400 nm); TiO2

catalyst; 05–1.0 g/L catalyst; [tetracycline] = 40 mg/L; treatment time = 120

100 % degradation and 90 % total organic carbon removal (UV 254 nm; 0.5 g/L TiO2; after 120 min)

100 % degradation and 70 % total organic carbon removal (Solarium device; 0.5 g/L TiO2; after 120 min)

50 % degradation and 10 % total organic carbon removal (UV 365 nm; 0.5 g/L TiO2; after 120 min)

Ultra-pure water Direct photolysis UV at 365 nm; pH=6; [tetracycline] = 10–40 mg/L

73 % tetracycline degradation; 15 % total organic carbon removal Distilled water Electrochemical

oxidation

pH = 3.9–10.0, current density = 15.9–63.5 mA/cm2, treatment

time = 60 min, [Na2SO4] = 0.05–

0.20 mol/L, Ti/ RuO2–IrO2:

anode, stainless steel: cathode, [OH] = 0–4.20 mmol/ L, [tetracycline] = 50–200 mg/L, volume = 200 mL

More than 90 % of tetracycline degradation at pH = 3.9, current density = 47.6 mA/cm2, [Na

2SO4] =

0.1 mol/L and [tetracycline] = 100 mg/L

Distilled water Photoelectrocatalytic process

TiO2 photoanode, UV light (254

nm, 2.5 mW/cm2), 0.5 V, pH =

5.5, [NaSO4] = 0.02 moL/L,

[tetracycline] = 10 mg/L, treatment time = h

More than 80 % of tetracycline degradation

Spicked STP effluent Surface Deionised water

Photo-Fenton Black light (15 W); solar irradiation; 1–10 mM H2O2; 0.20

mM ferrioxalate or Fe(NO3)3;

[tetracycline] = 24 mg/L

100 % tetracycline degradation under solar irradiation

Distilled Water Semiconductor photocatalysis

Xe lamp (300–800 nm); TiO2

and ZnO as catalyst; 0.5–1.5 g/L TiO2; 0.2–1.5 g/L ZnO; pH = 3–

10 (TiO2); pH = 6–11 (ZnO);

[tetracycline] = 20 mg/L

66

Chlortetracycline Oxytetracycline Tetracycline

Distilled water Adsorption with aluminium oxide

C = 20–110 lg/L, 0.8–3.5 g/L Al2O3

Rapid adsorption of tetracycline (43 %), chlortetracycline (57 %) and oxytetracycline (44 %)

Chlortetracycline Oxytetracycline Tetracycline Aqueous solution Photoelectrocatalytic process

[tetracycline] = 10 mg/L; [chlortetracycline] = 10 mg/L; [oxytetracycline] = 10 mg/L; medium pressure mercury lamp (15 W, kC 365 nm, 21.2 lW/ cm2, [Na

2SO4] = 0.1 moL/L,

potential applied = 0.6–3.0 V, treatment time = 0–180 min, pH = 3–12

About 95 % of tetracycline antibiotics degradation Chlortetracycline Doxycycline Oxytetracycline Tetracycline Aqueous solution Wastewater animal husbandry Electrochemical oxidation

Current intensity = 1.5 A, Ti/IrO2 (or Ti/PbO2) : anode, Ti:

cathode, electrode gap = 10 mm, [NaCl] (or [Na2SO4]) = 1,000

mg, treatment time = h, [Tetracycline antibiotics] = 100 mg/L, [oxytetracycline] = 100 mg/L

[Tetracycline antibiotics] final = 0.6 mg/L, [oxytetracycline] = 0.7 mg/L after h of treatment time

Chlortetracycline Doxycycline Oxytetracycline Tetracycline

Distilled water spicked with calcium chloride, humic acid and NaCl

Nanofiltration NF 200 membranes (14.6 cm2

area), pH = 7, T = 20 C

Degradation of tetracycline antibiotics between 50 and 80 % after 90

Chlortetracycline Doxycycline Oxytetracycline Tetracycline

Distilled water Oxidation/reduction pH = 7.0 ± 0.1, mM phosphate buffer, T = 22.0 ± 1.0 C, Xe arc lamp (172 nm), [TOC] = 13 lg/L, electron pulse radiolysis (472 nm, G = 5.2 10-4 m2/J

The efficiencies for OH reaction = 32–60 %

The efficiencies for ereaction = 15– 29 %, for chlortetracycline = 97 %

Chlortetracycline Oxytetracycline Tetracycline Ultrapure water Surface water Ground water Wastewater

Gamma radiation Temperature = 25 C ± 1.0 C, pH = 2–10, radiation dose = 1.66– 3.83 Gy/min, [HCO3-] = 0.0–7.2

meq/L, [SO42-] = 0.0–41.0 mg/L,

[NO3-] = 0.0–4.4 mg/L,

[Tetracycline antibiotics] = 20– 100 mg/L

67

Chlortetracycline Doxycyline Oxytetracycline Tetracycline Demeclocycline Minocycline

Spiked synthetic and river water

Coagulation adsorption with activated carbon

Contact time min, 10 lg/L for adsorption process, 100 lg/L for coagulation, coagulation PACI (5–60 mg/L), granular activated carbon filtration: calgon F400 and coconut-based carbon