Luận văn thạc sĩ Kỹ thuật hóa học: Insight assessment of the synthesis, morphology, and photocatalytic properties of ZnO@CN interlayer composite for emerging antibiotic removals in water under visible light

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY ---o0o--- HOANG AN INSIGHT ASSESSMENT OF THE SYNTHESIS, MORPHOLOGY, AND PHOTOCATALYTIC PROPERTIES

VIETNAM NATIONAL UNIVERSITY HO CHI MINH CITY HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY

-o0o -

HOANG AN

INSIGHT ASSESSMENT OF THE SYNTHESIS, MORPHOLOGY, AND PHOTOCATALYTIC PROPERTIES OF ZnO@CN INTERLAYER COMPOSITE FOR EMERGING ANTIBIOTIC REMOVALS IN WATER UNDER VISIBLE LIGHT

NGHIÊN CỨU TỔNG HỢP, ĐẶC TRƯNG VÀ KHẢ NĂNG QUANG XÚC TÁC LOẠI BỎ DƯ LƯỢNG KHÁNG SINH

TRONG NƯỚC DƯỚI ÁNH SÁNG KHẢ KIẾN CỦA VẬT LIỆU COMPOSITE ZNO@CN ĐA LỚP

MASTER’S THESIS

HO CHI MINH CITY, June 2024 Major: Chemical engineering Major code: 8520301

i THIS THESIS IS COMPLETED AT

HO CHI MINH CITY UNIVERSITY OF TECHNOLOGY – VNU-HCM

Supervisor: Nguyen Thi Anh Nga, Ph.D

Assoc Prof Nguyen Huu Hieu, Ph.D

Examiner 1: Assoc Prof Tran Hoang Phuong, Ph.D

Examiner 2: Pham Trong Liem Chau, Ph.D

This master’s thesis is defended at HCM City University of Technology – HCM City on 25/6/2024

VNU-Master’s Thesis Committee: Assoc Prof Nguyen Thi Phuong Phong, Ph.D – Chairman of the thesis committee Assoc Prof Tran Hoang Phuong, Ph.D – Reviewer 1

Assoc Prof Nguyen Tuan Anh, Ph.D – Scientific secretary Approval of the Chairman of Master’s Thesis Committee and Dean of Faculty of Chemical Engineering after the thesis being corrected (If any)

CHAIRMAN OF THESIS COMMITTEE (Signature with full name)

HEAD OF FACULTY OF CHEMICAL ENGINEERING

(Signature with full name)

ii Ho Chi Minh City University of Technology-VNU HCMC

Faculty of Chemical Engineering

SOCIALIST REPUBLIC OF VIETNAM Independence – Freedom – Happiness

THESIS TASK ASSIGNMENT

1 Title:Title in Vietnamese: Nghiên cứu tổng hợp, đặc trưng và khả năng quang xúc tác loại bỏ dư

lượng kháng sinh trong nước dưới ánh sáng khả kiến của vật liệu composite ZnO@CN đa lớp

Title in English: Insight assessment of the synthesis, morphology, and photocatalytic

properties of ZnO@CN interlayer composite for emerging antibiotic removals in water under visible light

2 Assignment:2.1 Literature review

Graphitic carbon nitride (CN), zinc oxide (ZnO), zinc oxide-anchored graphitic carbon nitride(ZnO@CN), antibiotics, tetracycline (TC), ciprofloxacin (CP), cephalexin (CL), erythromycin A (ER), photocatalytic degradation performance, and mechanism study

1.2 Experimental

- Synthesis of ZnO via reduction method and synthesis of CN via calcination method

- Synthesis of ZnO@CN from ZnO with CN via sonication-assisted ex-situ method with

various ZnO dosages from 05% to 25% (ZnO@CN-05 – ZnO@CN-25)

- Characterization of CN, ZnO, and ZnO@CN

- Investigation of ZnO@CN performance of TC, CP, CL, and ER antibiotics photodegradation - Investigation of ZnO@CN photodegradation mechanism via radicals scavenging experiments and assessment of the reusability of ZnO@CN

3 Assignment date: 01/20244 Completion date: 6/20245 Supervisor:

Nguyen Thi Anh Nga, Ph.D.; Assoc Prof Nguyen Huu Hieu, Ph.D

SUPERVISOR

(signature with full name)

Ho Chi Minh City June 25th, 2024

HEAD OF DEPARTMENT

(signature with full name)

HEAD OF FACULTY OF CHEMICAL ENGINEERING

(signature with full name)

iii

ACKNOWLEDGEMENT

“Ego gigantus humerus sto” “To stand on the shoulder of giants”

Science is a lonely journey However, alone the path, there are family, friends, and colleagues to travel along, for help, for guidance, and for comfort Firstly, the author would like to express the utmost appreciation to the his mother and father for have always been there in every moment of life Every step that he has accompanied has led to this particular present, and with the unconditional love and dedication from all of the family members, it would be futile for the author to be who he is today The encouragement and guidance has always and always will be the beacon for the determination of the author to hold in the daily struggles

Secondly, the author would like to gratitude to the instructor, Nguyen Thi Anh Nga, Ph.D and Assoc Prof Nguyen Huu Hieu, Ph.D for their dedicated advice and guidance And to the end of the journey, they have shown great patience on the guidance to the right path of the research The invaluable guidance has effectively helped the author complete the thesis proposal with a high willingness and effort

Thirdly, the author is solely appreciative to all of the colleagues and seniors in Key Laboratory of Chemical Engineering and Petroleum Processing (Key CEPP Lab) for their precise supports and advices, not only for the thesis but also for all of the knowledge accumulated during the working period Finally, the author conveys an acknowledgement to all of the friends from across all platforms and the progenitors for having been a company in the process of finishing the project Without the immense spiritual and academically support, the pathway of the author could have been more difficult

Ho Chi Minh City, June 2024

Author

iv

ABSTRACT

In this thesis, zinc oxide-modified graphitic carbon nitride interlayer (ZnO@CN) was synthesized for the photocatalytic antibiotics removal in water under visible light In detail, graphitic carbon nitride (CN) was prepared by the calcination of melamine and activated using the mixture of K2Cr2O7 and concentrated H2SO4 Zinc oxide (ZnO) was synthesized from of Zn(CH3COO)2 using the mangosteen (Garcinia

mangostana) pericarp extract as a stabilizer and was calcined at high temperature

ZnO@CN was fabricated by combining ZnO and CN with different ratios (5, 10, 15, 20, and 25%) under sonication

The material ZnO@CN was investigated on the antibiotics removal performance on tetracycline The material ZnO@CN with different ratios was applied in the same experimental setup for assessment Control test with ZnO and CN was also conducted in the same setup The application of ZnO@CN was extended to three organic compounds ciprofloxacin (CP), cephalexin (CL), and erythromycin A (ER) with the same conducted setup

Moreover, radical scavenging experiments were conducted to reveal the main photoactivity participants and elucidate the photodegradation mechanism The reagent vitamin C were used to capture the photogenerated hole, isopropanol to capture the hydroxyl radicals and ethylenediaminetetraacetate, to capture the superoxide radicals Besides, recyclability of ZnO@CN was conducted in the same manner for five consecutive cycles

The abstract of the thesis is illustrated in Figure 1

v

Figure 1: Graphical abstract of the thesis

vi

TÓM TẮT

Trong luận văn này, vật liệu graphitic carbon nitride (CN) đa lớp biến tính bằng kẽm oxide (ZnO@CN) được tổng hợp để ứng dụng vào quá trình quang phân hủy dư lượng kháng sinh trong nước dưới ánh sáng khả kiến Cụ thể, CN được tổng hợp bằng cách nung melamine và được hoạt hóa bằng hỗn hợp K2Cr2O7 và H2SO4 đặc Kẽm oxide (ZnO) được tổng hợp từ muối Zn(CH3COO)2 sử dụng dịch chiết vỏ măng cụt (Garcinia

mangostana) vói vai trò chất ổn định và được nung ở nhiệt độ cao Vật liệu ZnO@CN

được tổng hợp bằng phương pháp phối trộn huyền phù với sự hỗ trợ siêu âm với nhiều tỷ lệ giữa ZnO và CN khác nhau (5, 10, 15, 20 và 25%)

Vật liệu ZnO@CN được thử nghiệm và đánh giá khả năng quang phân hủy đối với kháng sinh tetracycline Đầu tiên, các mẫu vật liệu ZnO@CN với tỷ lệ ZnO và CN khác nhau được khảo sát và đối chiếu trong cũng một điều kiện Thí nghiệm đối chứng với từng vật liệu là ZnO và CN cũng được thực hiện Khả năng ứng dụng của ZnO@CN được đánh giá thông qua các ba loại chất hữu cơ khác nhau như là ciprofloxacin (CP), cephalexin (CL) và erythromycin A (ER)

Để đánh giá cơ chế xúc tác quang, thí nghiệm bắt gốc tự do được thực hiện Các tác nhân được dùng để bắt gốc tự do bao gồm vitamin C bắt các lỗ trống quang sinh, isopropanol bắt gốc tự do hydroxyl và ethylendiaminetetraacetate để bắt gốc tự do superoxide Bên cạnh đó, khả năng thu hồi tái sử dụng của ZnO@CN được đánh giá cũng điều kiện thí nghiệm qua 5 chu kì liên tục

Tóm tắt nội dung luận văn được thể hiện ở Hình 1

vii

Hình 1: Tóm tắt nội dung nghiên cứu của luận văn

viii

COMMITENT OF THE THESIS’AUTHOR

I hereby declare that the work is originally implemented by the author and was carried out under the instructions of Nguyen Thi Anh Nga, Ph.D and Assoc Prof Nguyen Huu Hieu, Ph.D in Ho Chi Minh City University of Technology – Vietnam National University Ho Chi Minh City

I confirm that this work is the result of my research and is solely my work All the contribution-related to this thesis have been fully acknowledged I affirm that any formulation, idea, research, reasoning, or analysis borrowed from a third party is correctly and accurately cited in both techniques and the author’s rights

The author takes full responsibility for the full work

Ho Chi Minh City, June 2024

Author

LIST OF FIGURES xii

LIST OF TABLES xiv

ABBREVIATIONS xv

INTRODUCTION 1

CHAPTER 1: LITERATURE REVIEW 2

1.1 Graphitic carbon nitride (CN) 2

Definition 2

1.1.1 Structure 2

1.1.2.1.2 Effect of precursor on the graphitic carbon nitride structure 6

Melamine 6

1.2.1 Cyanamide and dicyandiamide 7

1.2.2 Urea and thiourea 7

1.2.3 Combination 9

1.2.4.1.3 Zinc oxide (ZnO) 10

ZnO characteristic traits 10

1.3.1 ZnO advantages 10

1.3.2 ZnO disadvantages 11

1.3.3.1.4 Zinc oxide modification on graphitic carbon nitride (ZnO@CN) 12

Graphitic carbon nitride modification 12

1.4.1 Modified properties of ZnO@CN 15

1.4.2 Synergetic effects of ZnO and CN 17

1.4.3.1.5 Overall performance and mechanism ZnO@CN 21

1.6 Antibiotics pollution treatment methods 23

Current situation 23

1.6.1 Pollution treatment 24

1.6.2 Antibiotic photocatalytic degradation mechanism 261.6.3

x

1.7 Domestic and international scientific research 27

Domestic 27

1.7.1 International 28

1.7.2.1.8 Objectives, contents, methods, essentiality, novelty, and contribution 28

Objectives 28

1.8.1 Contents 28

1.8.2 Research methods 29

1.8.3 Essentiality 36

1.8.4 Novelty 36

1.8.5 Contribution 37

1.8.6.CHAPTER 2: EXPERIMENTAL 38

2.1 Chemicals, materials, facilities, equipment, and research location 38

Chemicals and materials 38

2.1.1 Facilities 39

2.1.2 Equipment 39

2.1.3 Research location 40

2.1.4.2.2 Experiments 40

Synthesis and characterization of ZnO, CN, and ZnO@CN 40

2.2.1 Investigation of tetracycline photodegradation performance of ZnO@CN 44

2.2.2 Investigation of degradation mechanism and ZnO@CN reusability 46

2.2.3.CHAPTER 3: RESULTS AND DISCUSSION 47

3.1 Characteristics of CN, ZnO, and the ZnO@CN 47

3.2 The antibiotics removal of ZnO@CN 59

Tetracycline removal of ZnO@CN 59

3.2.1 Tetracycline removal of ZnO, CN, and ZnO@CN-15 61

3.2.2 Antibiotics removal of ZnO@CN-15 62

3.2.3.3.3 The photocatalytic degradation mechanism and reusability assessment 65

Photocatalytic degradation mechanism 65

3.3.1 Recyclability assessment 69

3.3.2.CHAPTER 4: CONCLUSIONS AND RECOMMENDATION 71

4.1 Conclusions 71

xii

LIST OF FIGURES

Figure 1.1: (a) The triazine tectonic unit and (b) the heptazine tectonic unit 2

Figure 1.2: The synthesis pathways of (a) triazine and (b) heptazine tectonic unit and the constructive evolution of intermediate monomers 3

Figure 1.3: Five main precursors of CN synthesis and the purpose 5

Figure 1.4: The proposed heterojunction model of S-scheme and Z-scheme 14

Figure 1.5: Synergetic effect between CN and ZnO in ZnO@CN: 18

Figure 1.6: (a) S-scheme and Z-scheme models of both “ZnO on CN” 20

Figure 1.7: ZnO and CN bandgap with ROS potentials and forming mechanism 22

Figure 1.8: Molecular structure of (a) TC, (b) CP, (c) CL, and (d) ER 23

Figure 1.9: POC treatment mechanism of ZnO@CN 27

Figure 1.10: Principle of XRD 29

Figure 1.11: Principle of FTIR spectrometer 30

Figure 1.12: Principle of (a) SEM and (b) TEM and HR-TEM 31

Figure 1.13: Principle of EDS 32

Figure 1.14: Principle of XPS 33

Figure 1.15: Principle of UV-Vis spectroscopy 34

Figure 2.1: The Garcinia mangostana pericarps 39

Figure 2.2: Garcinia mangostana pericarp extract preparation 41

Figure 2.3: Synthesis scheme of ZnO 42

Figure 2.4: Fabrication scheme of CN 42

Figure 2.5: Synthesis scheme of ZnO@CN 43

Figure 2.6: TC photocatalytic degradation experimental scheme 45

Figure 3.1: (a) XRD patterns and (b) FTIR spectra of CN, ZnO, 48

Figure 3.2: SEM images of (a) ZnO, (b) CN, and ZnO@CN samples (c) ZnO@CN-5, (d) ZnO@CN-10, (e) ZnO@CN-15, (f) ZnO@CN-20, and (g) ZnO@CN-25 50

Figure 3.3: TEM images and HR-TEM images of CN and ZnO@CN-15 51

Figure 3.4: Elemental mappings of (a) CN, (b) ZnO@CN-5, (c) ZnO@CN-10, (d) ZnO@CN-15, (e) ZnO@CN-20, and (f) ZnO@CN-25 53

Figure 3.5: (a) XPS survey profile and HR-XPS spectra of CN and ZnO@CN-15 55

xiii

Figure 3.6: Electrochemical of ZnO, CN, and ZnO@CN-15 in dark and illuminated

conditions (a) CV curves and (b) area of the CV curves 57

Figure 3.7: (a) Nyquist’s plot, and (b) electric resistance and electric conductivity

of ZnO, CN, and ZnO@CN-15 in illuminated conditions 58

Figure 3.8: TC photocatalytic degradation of ZnO@CN-05 to ZnO@CN-25 60Figure 3.9: TC photocatalytic degradation of ZnO, CN, and ZnO@CN-15 61Figure 3.10: Comparative antibiotics photocatalytic degradation of ZnO@CN-15 62Figure 3.11: Reactive sites and degradation targets of the antibiotics strain of 64Figure 3.12: Radical scavenging experiments on TC photocatalytic degradation 65Figure 3.13: VB-XPS spectra of (a) ZnO and (b) ZnO@CN-15 66Figure 3.14: (a) UV-Vis spectra of ZnO and ZnO@CN-15 and (b) Tauc plot and

bandgap analysis of ZnO and ZnO@CN-15 67

Figure 3.15: ZnO and ZnO@CN-15 photocatalytic mechanism 68Figure 3.16: Reusability tetracycline photocatalytic degradation of ZnO@CN-15

assessment in five consecutive cycles 69

xiv

LIST OF TABLES

Table 1.1 Summary of the effects of the precursors on the synthesized CN 8Table 2.1: List of used chemicals 38 Table 2.2: List of used equipment 39

xv

ABBREVIATIONS

Abbreviations Full names

HR-TEM High resolution-transmission electron microscopy

1

INTRODUCTION

Currently, the concentrated acceleration in agricultural production has appeared as the concentrated spearhead for developing countries Accompanying the growth is an alarming increment of pollution, especially via uncontrolled utilization of antibiotics throughout the economic movement Particularly, livestock has been fed with excessive antibiotics, and antiviral means of farming also scrutinize antibiotics as a cheap tactic to exterminate pestilence Such usage undoubtedly results in antibiotic contamination in wastewater, natural streams, and even underground reservoirs Concurrently, there are several technologies for water remediation on different scales such as porous material adsorption, ozonation, coagulation and sedimentation, and bio-degradation These methods provide noticeable efficiency in the removal process However, the costs of facilitation as well as the advanced requirement of equipment are among their crucial disadvantages Therefore, the search for a more affordable, approachable, and efficacious methodology is in huge necessity as the environment is in peril

One spearhead in the environmental remedy technology is the photocatalytic process, which is deployed as a potential resolution thanks to energy conservativeness and environmental friendliness properties As a result, global researchers have extensively studied numerous nanotechnology materials Recently, graphitic carbon nitride has reconciled attention as a promising photocatalytic material, proposing utilizations in the water splitting process, organic pollutants deterioration, and photoproduction Zinc oxide has been utilized in industries for decades, yet only for commercial and cosmetic applications Nowadays, the attention for zinc oxide nanoparticles has been concentrated on the semiconductor field, regarding the cathode lamp, electrical semiconductor, and photocatalyst Hence, the combination of them into a composite material can not only overcome the inherent disadvantages of each precursor but also further expand the application range for wastewater treatment under visible light Therefore, the thesis is implemented with

the title “Insight assessment of the synthesis, morphology, and photocatalytic properties of ZnO@CN interlayer composite for emerging antibiotic removals in water under visible light”

2

LITERATURE REVIEW CHAPTER 1:

1.1 Graphitic carbon nitride (CN)

Definition 1.1.1.

Generally, graphitic carbon nitride (CN) is a polymeric species belonging to the family of carbon nitride compounds Based on the molecular formula, the definition of CN can be expeditiously extended to a compound that contains only C and N to a certain ratio and albeit undesirable, might still contain some H atom [1] Certainly, the polymeric structure of CN is complex and constituted of different monomers with different compositions and effects

Structure 1.1.2.

Indeed, CN is the direct product of the condensation polymerization to create a vast 2-D structure Overall, the polymer consists of multiple conjugated systems consisting of alternative C and N and interconnect by the = N̈ – The effect of conjugation CN possesses is somewhat similar to graphene, hence the name, graphitic carbon nitride The structure of CN can also embark on different monomers and different pathways for polymerization, which results in the triazine and the heptazine tectonic unit For the triazine tectonic unit, the structure is constructed by the repetitive decolonization of a hexacyclic structure which connects each other through the =N̈– group, shown in Figure 1.1(a) Otherwise, the heptazine tectonic unit is constructed by the distinctive tri-functional heterocyclic structure that is linked by the tertiary amine, as illustrated in Figure 1.1(b)

Figure 1.1:(a) The triazine tectonic unit and (b) the heptazine tectonic unit

3 Both structures are sufficient with the bonding angles and compositions, therein, they can exist separately, each with slightly different properties However, based on the stability of the structure, heptazine-based is more likely to be formed due to lower energy [2] It can be elucidated that the pathway revolving around intermediates is more susceptible as it provides steps for the forming process, which decreases the system energy instead of requirements of esoteric synthesized conditions [3] Moreover, the flexibility of heptazine-based CN is also higher to some extends, as the monomer is larger than triazine As a result, heptazine-based CN can have another form, which is the buckled structure, most likely to be bent by foreign species interacting with the tectonic plate [4] To further elucidate the properties of CN, it is crucial to analyze the monomer of the polymer structure from the bottom-up order As illustrated in Figure 1.2, triazine and heptazine tectonic units are constituted by dissimilar monomers For each pathway, the complex structure always starts from the same initial structure the melamine

Figure 1.2: The synthesis pathways of (a) triazine and (b) heptazine tectonic unit and

the constructive evolution of intermediate monomers The structure of melamine can be divided into a heterocyclic ring and the three substituted amines on the ring The aromatic ring consists of an alternatively positioned C and N on the benzene-like ring As N is a semi-saturated element, the pair of lone e–and the alternative system of π-bond constitute an aromatic system that expands through the three – N̈ H2 Retaining the heterocyclic ring structure, melamine also

4 resides on one molecular plane, which means every higher oligomer that was synthesized from melamine will retain the same 2-D configuration [5] As the three –N̈ H2 is expanded by the conjugated effect, melamine can participate in more polymerization by expanding the inter-conjugated structure with other molecules

The triazine tectonic unit is the simpler structure of the CN polymer, thus the formation pathway is also simpler, revolving only the melam and melamine For the thermal-induced melam molecule, two melamine interlink with each other through a –N̈H–, resulting in an intermolecular conjugated system through the lone pair of the connected N atom As mentioned, the conjugated system of melamine can be expanded to other melamine which generates the interlinked melam, thus the same procedure can be carried out another time to connect with other melam or melamine Therefore, in the synthesis, the same polymerization can be carried out simultaneously, resulting in the triazine tectonic unit

For the heptazine tectonic unit, a higher degree monomer is required For a different pathway, the heptazine structure revolves around the melon molecule [6] In general, melon is a linearly oligomerized molecule from melem, a tri-functional molecule derived from melamine Similarly to the formation of triazine units where melam is simultaneously connected, the condensation of melem can take part linearly, resulting in a linear polymer On the other hand, melons can perform copolymerization with other melons to create a 2-D structure [7]; however, the 2-D forming step is rather difficult, which drastically requires the input monomer to be coplanar with the existing structure [8] The melem molecule is the result of the oligomerization of melamine that is conducted directly on the heterocyclic structure Moreover, the pathway of forming melam and melem is interchangeable, which means from melamine, melem can be directly synthesized, or it can be transformed to melam before further deamination to form melem [9], [10] After the formation of the tri-functional monomer, the oligomer can further participate in another polymerization reaction with itself, resulting in a long linear chain of melon [11]

It is noticeable that the structure of CN tectonic units is versatile and compatible, thus various organic nitride compounds can be utilized to synthesize CN It is crucial to

5 understand that melamine is both a monomeric structure and a precursor for the synthesis of CN In various research about CN, there are five main precursors, including melamine, cyanamide, dicyandiamide, urea, and thiourea, which have been extensively utilized to synthesize CN, of which the results and effects vary Figure 1.3 summarizes the structure, property, and main purpose of the five popular precursors For each precursor used, the advantages and disadvantages will be discussed to explain the choice for each specific application Moreover, to diminish the drawbacks of each precursor, the combination of the initial compounds can be utilized, which integrates the advantages of each compound and opens up the application to harvest higher efficiency

Figure 1.3: Five main precursors of CN synthesis and the purpose

in the structure construction

6

1.2 Effect of precursor on the graphitic carbon nitride structure

Melamine 1.2.1.

As already discussed, CN can be directly synthesized from melamine, of which the reaction was carried out by high temperature Multiple studies have provided the thermal gravitational analysis of melamine that agrees with the temperature of around 400°C can lead to the rapid loss of its mass, signifies the condensation of melamine to form CN, and the generation of NH3 as a side product As described, the formation of CN from melamine can embark on two possible pathways: either it can form melam and construct the triazine structure; or it can form melem and melon, which results in the heptazine structure However, in comparison, the formation of heptazine is more favourable than triazine by evaluating the Gibbs free energy of formation from melamine, whose value has a lower threshold value, which means the heptazine structure is most likely to form the melamine-based CN [3] One disadvantage that needs to be considered for the usage of melamine in the synthesis is that melamine can be sensitive to high temperatures [12] A large amount of heat provided in a short period can promote the pyrolysis of melamine, which forms unwanted products and hinders the formation of CN Moreover, the dissociation of melamine under sublimated conditions can promote side reactions, thus decreasing the total yield [13] Therefore, the universal condition for the synthesis using the calcination of melamine must be set to maximize the selectivity and conversion Based on previous literature, melamine can be calcinated at a maximum temperature of 600°C for 2 – 4 h to ensure a slow heat treatment to prevent the burning of melamine and CN [14] Otherwise, lower temperatures cannot promote the condensation polymerization of melamine Thanks to the similarity in structure, melamine is scrutinized vastly by researchers for the synthesis of CN However, the search for alternative methods also resulted in a fruitful conclusion that other precursors with identical elemental composition can be used as well, of which

different properties can be endowed on CN

Urea and thiourea 1.2.3.

As mentioned, the elemental compositions of CN including C, N, and probably H based on the completion of the polymerization process A different scenario is brought up when synthesizing CN from urea and TU The two molecules share relatively the same structure with one C connected with two – N̈ H2 Nonetheless, dissimilarity

8 between these precursors is that O and S are present in urea and TU, respectively This briefly indicated that the synthesis utilizing these two precursors could provide foreign elements into the mainframe of the material [19] By introducing S and O into the CN structure, the material can improve the photocatalytic properties by shifting the charge density and promoting the separation process

However, it is noteworthy that the condensation of these two precursors can produce multiple gaseous by-products such as CO2, H2S, SO2, and NH3 [20] In comparison, the amount of accumulated gas can rupture the newly-formed infrastructure of CN, thus decreasing the crystallinity [21] However, the collapsing structure by the plume of by-product could also lead to advantages The material synthesized from urea and thiourea has been considered a better adsorbent thanks to the leveling porosity of the material This attribute comes from the releasing gas that reformed the surface of the material, providing the partially-liquid material with a system of pores of different sizes In perspective, a rapid exhume of by-product could hinder the conversion from urea and thiourea to CN; whilst slower generation of the expelled gas, granted by low heat profile, could boost the exfoliation rapidly by the lifting effects of the said vapor cause the material [22] As a result, the choice of precursors can both directly benefit the properties and hinder the applications of the material Therefore, another strategy, which is the combination of reactants, is employed to overcome the drawback that each sole precursor possesses The different advantages of each precursor are presented in Table 1.1

Table 1.1 Summary of the effects of the precursors on the synthesized CNNo Precursors Attribute Advantages Disadvantages

1 Melamine Versatile Highly reactive Heat-sensitive

crystallinity

Higher dimension synthesized

ineffectiveness 3 Dicyandiamide

9 Along with the condensation pathway, some precursors like melamine or dicyandiamide have better crystallinity, suitable for composite fabrication, whilst urea-based and thiourea-based CN provide the resulting structure with more pores, which amplifies the adsorption process of the material In reality, to further expand the properties of CN, the precursors are usually combined

Combination 1.2.4.

A wise strategy that many pieces of literature adopt is the combination of urea/thiourea with melamine/cyanamide/dicyandiamide, respectively There are two main reasons for this strategy: First is the defect forming by foreign elements in the already built structure As mentioned, urea and thiourea are excellent sources of O and S for doping during the condensation reaction [12] With the combination of these elements, the urea or thiourea can be integrated into the CN structures already formed by the precursor However, in combination with many precursors, the inter-condensation of each compound can be selected to be performed, resulting in multiple heterocyclic intermediates which pose on contribution to the synthesis pathway [23]

The other reason for the combined precursors is to exploit the dissimilarity in CN isomeric structure Although the elemental composition is identical, different precursors could deviate the condensation degree, pore size, and crystal structure Various independent studies have fabricated CN from different sources and conducted the bandgap determination experiment As the results suggest, the precursor combination provides a homojunction, which relatively decreases the bandgap and increases the photocatalytic activity Moreover, the usage of thiourea and urea could decrease the bandgap in comparison to other precursors, suggesting the role of O and S in defect induction

Finally, the combination of precursors also serves a constructive purpose As mentioned, being the basic structure of both triazine and heptazine units, melamine is a versatile precursor that could be combined with many additives to form a more esoteric structure

As concluded, the combination of various precursors can bring out the best of each property Regarding the simplicity, cost-effectiveness, and conventional conversion,

10 melamine has been chosen as the precursor for the synthesis of CN in this thesis Surely, a robust product can be yield from the precursor choice However, there are still some drawbacks that limit the application potential of CN in reality Therefore, modifications of CN are required to overcome the disadvantages

1.3 Zinc oxide (ZnO)

ZnO characteristic traits 1.3.1.

ZnO is a widely used semiconductor that has been in research for substituted materials as a photocatalyst Possessing a bandgap value of around 3.34 eV, ZnO is a suitable choice for operations under UV irradiation [24] ZnO usually possesses the crystal configuration of wurtzite, which results in a wide capability to be morphed into different structures and lattices [25] On the nanoscale, ZnO can be fabricated in multiple intricate morphologies, for instance, nanoparticles provide a large specific area, promoting the absorption process A higher dimension configuration can be achieved, famously the ZnO nanorods, which is a suitable binder with a high surface area [26] The 2-D and 3-D structures of ZnO photocatalyst possess exceptional properties for various purposes, however, the synthesis method is proven to be difficult because of the usage of multiple surfactants and distinctive synthesis conditions [27]

ZnO advantages 1.3.2.

By the light of science, ZnO has been vastly utilized as a photocatalyst thanks to its benefits which lie in the possession of properties such as low toxicity and low cost [28] In comparison, Zn is an abundant element, therefore the cost of production of ZnO can be lower Moreover, the toxicity of ZnO is obstructed to some extent due to the bulk size in the exposure and diffusion [29] One of the huge benefits ZnO photocatalyst provides is great energy absorption [30] As a result, sometimes ZnO is utilized to be a UV absorber in sunscreen and cosmetics to limit the harmful effects on fabrics and garments

Corresponding to the huge bandgap, ZnO is capable of producing many reactive oxygen species (ROS), of which the redox potentials allocate between the CB and VB values of ZnO Previous literature concluded that the CB and VB values of ZnO are around -0.5 and 2.89 eV, respectively [31] Therefore, multiple ROS can be produced

11 by ZnO photocatalysts under the irradiation of UV light [32] The material can be applied in the aspects of photodegradation and photoproduction thanks to the ROS ZnO can produce

ZnO disadvantages 1.3.3.

Despite the universal application, ZnO still faces some drastic disadvantages that can hinder further deployment in the photocatalysis process The major drawback is the large bandgap, thus the charge separation process of ZnO requires a lot of energy to operate [33] As a result, the photocatalytic activity of the material is drastically retarded when the quantum is not enough to promote ROS production [34] Alongside this, the spectrum required for the energy provision is also distinctive compared to CN The adsorption region of ZnO is still mostly in the UV region, which limits its absorption of visible light [35] Without bandgap modification, the application of ZnO in visible irradiation is limited

Another disadvantage of ZnO in photocatalytic is the agglomeration phenomenon Based on the thermodynamic law, ZnO tends to agglomerate into larger particles, which in turn, decreases the photocatalytic performance of the material [36] As reported, the pretreated ZnO has surface charges of around 26 mV and the isoelectric point of ZnO lies around pH 9.3 [37], [38] However, it is crucial to notice that since ZnO is an oxide, the compound is sensitive to acidic environments as it can be dissociated into the corresponding soluble salt Therefore, a high pH is prioritized However, surface modification can also be utilized to prevent the phenomenon Per se, ZnO is a great material when it comes to photocatalyst, inexpensive, and non-toxic; however, ZnO is accompanied by certain disadvantages, of which sole performance is greatly hindering

A need of material improvement arises when it comes to the movement from theoretical to experimental application Truly, both CN and ZnO possess difficulties that partially hindering the embarkation to a fully-realized photocatalyst Catching up with the idea of modification, ZnO is considered to be paired with CN The inexpensiveness of both materials is scrutinized, after which great photochemical application is considered as a supporting basis for the combination

12

1.4 Zinc oxide modification on graphitic carbon nitride (ZnO@CN)

Graphitic carbon nitride modification 1.4.1.

Despite the portion of applications in various fields, CN still possesses some drawbacks For semiconductors, the fast recombination rate is one of the obstacles, of which CN is witnessed to occur rapidly [39] The repetitive structure of CN renders the e– density of the material isotropous across the sheet [40] Therefore, the photogenerated e–, without any interactions with alien species, can quickly recombine with the vacant h+ However, the rate of recombination can be adjusted accordingly to the degree of defect in the structure

1.4.1.1 Defect induction

The e– density is delocalized by the defects, which create an e– well that trap the newly-formed e– With the trapped e– separated from the h+, the recombination can be hindered, thus increasing the photocatalytic activity of CN [41] Subsequently, the degree of defect can be attributed to the precursor choice and the synthesis method There are multiple types of defects, including the vacancies of atoms, craters, crevices, or basal, which are all the gathering sites for photogenerated species during photocatalysis [42] The most common methodology for defect induction is an introduction of foreign elements into the molecular structure of CN, which can be done from the beginning stage of precursor mixing As aforementioned, CN synthesized by urea or thiourea can receive extra O or S into the tectonic plate of CN [43], [44] The discrete appearance of S and O, an e– rich element, can alter the e–density of the CN, causing a condensed state of e– at various sites in CN [45] The optical and electrical properties of the modified material, therefore, receive various upgrades, which aid in photocatalytic activities Extensively, defect induction can also be conducted by changing certain constituent monomers of the structure The modified CN materials by defect induction are reported to have greater photocatalytic and optical properties

Another major disadvantage of CN is that the light absorption region is relatively short and positioned near the UV-region with previous studies showing the wavelength absorption edge positions around 450 nm [46] With the bandgap of 2.7

13 eV, the energy level of CN is compatible with the near-UV region, which heavily discourages the application in visible light [47]

1.4.1.2 Heterojunction construction

Similar to the recombination problem, among the precursor or mixtures of precursors, the regions can deviate from each other Adapting from the literature, CN synthesized from urea, dicyandiamide, and melamine have a red shift in the absorption edge in the same order [48] Different precursors can result in different crystal structures, which passively modulate the bandgap Aforementioned, using a mixture of precursors can result in a homojunction CN, which also alters the bandgap of the CN and shifts the absorption range Another method to overcome the disadvantage of CN is the heterojunction construction with other substances Compared with the homojunction, the effects of heterojunction are superior in terms of introducing other materials into the same matrix to harvest further attributes

It is noticeable that the term “bandgap” does not apply to composite However, based on the operation scheme where a suitable irradiance can activate a composite material to participate in the photocatalytic activity, similarly to the bandgap, the term “bandgap” will be used ambiguously to depict the activated irradiance The first requirement for the heterojunction construction is that the different materials must come in contact, which creates an interface that differentiates the dipole moment [49] For CN, the reported bandgap value is around 2.7 eV, with the valence band (VB) allocated at around 1.3 eV [50] Based on the data, the usual model for the heterojunction of CN is a staggered type heterojunction, although some other types could be created when paired with the esoteric material Nevertheless, a normal CN synthesized from the usual precursors is deemed to be an n-type semiconductor [51] Under some specific synthesis procedure or modification, the material could be changed into p-type [52] Hence, the combination of CN with a staggered and compatible band material could lead to different scenarios of heterojunctions like type-II, S-scheme, Z-scheme, or p-n junction [53] The S-scheme and Z-scheme model for heterojunction, however, will be focused more in this review as various literatures regarding CN focus on the constructions of the two architectures

14

Figure 1.4: The proposed heterojunction model of (a) S-scheme and (b) Z-scheme

The more direct approach to construct the heterojunction of CN is S-scheme, where the requirement is a high interface for the charge movements, illustrated in Figure 1.4(a) Therefore, many pieces of literature propose the decoration of compounds on CN to increase the photocatalytic potential To ensure the highest junction efficiency, the synthesis must improve the surface interaction of CN and the decorated material, and usually, it is conducted through a 0-D – 2-D interface, however, higher dimensions are also under survey [54] The configuration can be more efficient as the CN sheet can perform the adsorption process as well, which amplifies the overall efficiency of the heterojunction photocatalyst Whether it is for the reduction of the energy gap for better performance in visible regions, or to achieve higher redox potential for the photoproduction fields, CN is utilized as both a beneficiary and a benefactor [55], [56] Previous results indicate that with the heterojunction construction, the performance of modified CN increases rapidly to a higher extent, which implies the application of the material in both photoproduction and pollution treatment

In comparison, the Z-scheme provides a more efficient pathway in charge separation As illustrated in Figure 1.4(b), the recombination of e– and h+ is performed directly between the conduction band (CB) and VB of the two materials, diminishing the chance for the same material recombination [57] In comparison, Z-scheme is based on a direct approach of the e– movement from CB to VB; whilst S-scheme shows an interaction at the interface of the material The movement of

reduction

reduction

15 charged species is different to aid in diverse purposes S-scheme construction requires charged-surface separation that is more difficult to render but the recombination is decreased Likewise, the construction of the Z-scheme can utilize the low VB value of CN, when combined with a high CB value, can result in a great redox material [58] Thus, Z-scheme CN can be applied to more aspects of photoproduction as more redox potential can be achieved compared to S-scheme However, the Z-scheme is harder to be constructed than the S-scheme as the e– and h+movement is more favorable because of the shorter energy gap In resolution, a third component is added to the Z-scheme heterojunction to act as the mediator The tertiary compound can alleviate the huge energy gap by acting as the recombination site for e– and h+, which shortens the energy gap both for the e– and the h+ [59] Furthermore, a complex heterojunction is also unfavorable as the synthesis method is complicated and the system can be unstable

Therefore, a less severe system is proposed to utilize the decoration of metal oxide onto CN There are several metal oxides, for instance, tin oxide, cupric oxide, rutile, and alumina, of which each compound possesses different crystal configurations, properties, and applications Considering the easy synthesis approach and inexpensive precursor, in this thesis, the zinc oxide (ZnO) decorated on CN will be extensively analyzed to explore the fundamental implications and vast applications For the specification, the ZnO decoration will alter the CN structure by creating defective sites and constructing a heterojunction system with CN

Modified properties of ZnO@CN 1.4.2.

Recent researches nowadays focus on composite nanomaterial to harvest the great attributes of both materials, thus the combination of CN and ZnO is carried out robustly with great application For the composite, many configurations have been fabricated, with each performing better than the previous The most abundant and most simplified morphology for ZnO-modified CN (ZnO@CN) is in the form of 0-D – 2-D, where the ZnO anchors itself onto the structure of CN However, higher dimension nanomaterials can be constructed when it comes to heterojunction to amplify the materials interface In general, the bonding of ZnO with the CN can be

16 ascribed to the newly-formed bonding between C and Zn, which indicates the sharing of electrons between the two species [60] Moreover, ZnO can adhere to the CN substrate through hydrogen bonding and electrostatic interaction, which strengthens the decoration of ZnO@CN [61] Concurrently, the synthesis of ZnO@CN is varied by different approaches and conditions, with some methods being far more complex but containing promising morphologies

1.4.2.1 Synthesis methodology

Up-to-date, there have been numerous studies about the composite material ZnO@CN, which utilized different precursors for both CN and ZnO as well as embarked on different synthesis procedures Some novel studies about ZnO@CN employ the usual precursor for ZnO synthesis is Zn(AcO)2 The choice probably comes from the fact that the form of Zn2+ is loosely attached and, therefore can easily be reduced to zero-oxidation number ZnO; moreover, AcO- contributes slightly to interference to the whole synthesis as a benign anion, thus no special treatments or eliminations are required For CN, as already mentioned, melamine is a top choice as it provides a versatile morphology and property compared to other sources of precursors Meanwhile, urea can also be utilized for the inexpensiveness and solubility in various matrices

From previous literature, ZnO@CN can be fabricated by many methods, of which calcination is the most popular The effect of heat can benefit both the synthesis of ZnO and CN As for ZnO, heat rapidly reduces Zn2+, with the exhaustion of gas in the case of Zn(NO3)2; while CN is polymerized by heat, as mentioned Therefore, researchers have deployed co-calcination to limit energy usage Another cost-effective method is the hydrothermal method, which utilizes lower heat and higher pressure However, this method requires an additive to be either a reductant or and medium Therefore, it is wise to choose a constructive medium such as PLA or zeolite P to encase the material in a built-in structure [62] For the mentioned method, the morphology usually cannot be controlled, which will return to the 0-D – 2-D configuration However, with sophisticated approaches like thermal atomic layer

17 deposition, the material not only be successfully synthesized but the morphologies are in higher dimensions

1.4.2.2 Morphologically study

Different morphologies are constructed for each nanomaterial with the aim of higher surface area for charge transference Furthermore, an additive sometimes can be introduced to increase the adhesion or amplify the properties of the material In ordinary cases, CN takes up a sheet-like structure as the nature of the material is a flat 2-D polymer ZnO, on the other hand, corrugates to nanoparticles or nanorods based on the temperature and duration of synthesis As the method is elevated, more unique forms of CN can be synthesized, for instance, nanorods can be synthesized by using SPME [63]; or nanoparticles can result if the bulk CN is treated with harsh, oxidative conditions, repetitively [64] Finally, with a suitable medium, the carrier can act as the support structure and electron carrier, thus the requirement of higher dimension morphologies is not sustainable Therefore, 0-D – 0-D heterojunction is enough

The construction of high-dimension heterojunction is essential to create a charge transfer interaction, of which the results usually show greater properties The reason behind the increment can be traced to the synergetic effects of both CN and ZnO, which amplify the properties of ZnO@CN

Synergetic effects of ZnO and CN 1.4.3.

As mentioned, the act of combining CN with ZnO is most likely to compose a heterojunction In the same conditions, it can be concluded that the VB and CB values of each constituent are in the order of VBZnO > VBCN > CBZnO > CBCN, thus showing the heterojunction is the staggered type, [65], [66] Multiple publications have fabricated both the S-scheme and Z-scheme for ZnO@CN and each material can be applied differently in different situations Whether it is Z-scheme or S-scheme depends on the charge transference on the interface between the two components [53] There is no doubt that the synergetic effects between CN and ZnO, illustrated in Figure 1.5, can brighten up various advantages to overpass the drawbacks that each material possesses

18

Figure 1.5: Synergetic effect between CN and ZnO in ZnO@CN:

(a) and (b) “ZnO on CN”; (c) and (d) “CN on ZnO”

1.4.3.1 Effects of ZnO on CN

To increase the application of CN nanosheets structure, defect induction could be carried out on the material to create the anisotropic coordination [67] In this particular scenario, the decoration of ZnO onto the CN substrate could also induce defects in CN as well The vast adherent of ZnO onto CN can increase the load of the sheet-like structure, which can be fragmented under enough reckoning force This results in a much coarser and more fragmented structure Besides the intermission of e– segue, ZnO nanoparticles can also create morphological sites that benefit the marginal of CN For instance, the anchoring site can become a defective crater, the folding of two fracture sheets can become crevices, and multiple folding of CN can create non-planar morphologies which also increase the surface area [68] Discussing the adsorption capability, the CN structure is amorphous and mesoporous, thus making the material a good adsorbent As mentioned, the decoration could create sites where the electronic structure is intermitted Close examination shows that the e– density of the material can be diminished or stuffed at those locations [69] As a result, the increment in the number of defective sites can create more active sites for CN in both reaction and adsorption

19 It should be noted that the photocatalytic activity of the material can be greatly hindered if a low amount of ROS is produced, resulting in a fast recombination rate However, with the defect induction, the transition is disrupted and the e– flow can focus on some locations In some cases, the e– flow can even focus on the decorated ZnO, which further promotes the production of ROS on the surface of ZnO [70] The proposed movement also fits the scheme of heterojunction where the photogenerated e– can move from CN to ZnO Furthermore, the crevice and crater can act as the e–trap, for the e– can be separated from the photogenerated h+ by focusing on the active site As a result, the decoration of ZnO can decrease the recombination rate significantly in comparison with CN alone Also, it is proven that with the increment in the amount of anchoring ZnO, the recombination rate drastically decreases, which signifies the greater diffusion of e– movement out of the generated site [71] Ultimately, the act of decorating ZnO on CN can boost the photocatalytic activity of CN to further frontier, surpassing what the pristine CN can do

1.4.3.2 Effect of CN on ZnO

On the other hand, CN also provides benefits for the performance of ZnO On the one hand, bare ZnO can only perform under UV irradiation as the bandgap of ZnO is higher than the 3 eV capstone Therefore, to open up the application, the bandgap needs to be reduced Energy gap modulation can be conducted by the construction of S-scheme and Z-scheme heterojunctions with CN and ZnO However, the results can vary between the two scenarios For instance, the construction of “ZnO on CN” S-scheme heterojunction shows that after the synthesis, the material has an energy gap value of 2.83 eV, lower than both bandgaps of the constituents (value of CN is 2.86 eV and value of ZnO is 3.19 eV) [72] On the contrary, the S-scheme of the “CN on ZnO” heterojunction shows the energy gap value positions in between the bandgaps of the constituents [73] It can be seen that, although embracing the same S-scheme, the overall bandgap position is in different regions The same conclusion can be drawn with the Z-scheme construction For instance, a proposed Z-scheme “ZnO on CN” photocatalyst modulated the energy gap to be 2.35 eV, lower than the bandgap of CN [50]; while the Z-scheme “CN on ZnO” composite shows the energy gap

20 position in between of the twos bandgap values [65] To draw the correlation, it can be seen that not whether Z-scheme or S-scheme can decrease the energy gap further but considering the relative ratio between CN and ZnO A summary is provided as illustrated in Figure 1.6

Figure 1.6: (a) S-scheme and Z-scheme models of both “ZnO on CN”

and “CN on ZnO” types of composite and (b) the measured energy gap

relative positions based on ZnO@CN types of composite For the “CN on ZnO” case, ZnO holds the dominance in the material, therefore the amount of CN is inferior and only acts as an e– mediator for ZnO Therefore, the energy gap can be decreased, but only in the threshold of both bandgaps Reversely, the opposite construction, the “ZnO on CN”, creates an abundance of CN, whence the constructed heterojunction can have a massive interface between two constituents Consequently, the e– diffusion through the materials is promoted and the charge transfer can be accelerated As previously mentioned, the CN structure consists of multiple tri-functional heterocyclic conjugated systems Therefore, the conjugated property can be

21 maintained throughout the structure, hence the ability to conjure e– movement [74] Therefore, for every photogenerated e– from ZnO, the e– can be dissipated quickly across the CN layer, probably to the active site of the material where the ROS can be produced, or to the adsorption site where the adsorbed molecule resides

Furthermore, the act of anchoring ZnO on the thin CN sheet can prevent the agglomeration of ZnO by the interaction between the surface and the nanomaterial The nanostructure of the material can be determined that ZnO NPs distributed evenly on the surface of CN, creating a uniform mesh of ZnO@CN [75] The reason is that the decorating is a bond between the substrate and the ZnO particles, probably from the functional group interactions [76] As a result, the material must be distributed evenly on the surface, thus agglomerating the NPs The same effect could also be witnessed in the opposite decorating model, CN on ZnO CN decorated on different morphologies of ZnO can also help segregate the material [77] As the nanostructure of CN tends to repel each other due to same-charge electrostatic interaction, the nanomaterial can be separated from each other

The modification of CN using the decoration of ZnO is indeed fruitful in property enhancement Harvesting the achieved advantages for the application in photocatalyst could benefit the utilization even more

1.5 Overall performance and mechanism ZnO@CN

As previously described, the bandgap of ZnO and CN is allocated in a staggered position, of which the Z-scheme is favored more in the construction of the composite material For a convenient view, the CB and VB values of ZnO and CN are illustrated in Figure 1.7

From the diagram, it can be seen that the bandgap of CN is 2.7 eV from 1.3 to -1.4 eV; while ZnO has a bandgap of 3.36 eV from 2.9 to -0.5 eV As reported, the main species that is relatively convenient for ZnO@CN to produce is O2•– as the redox potential lies in the bandgap of the material in various conditions [65] H2O2 is also a huge participant in photocatalytic reactions, which can also be produced directly from water or passively from O2•– [78]

22

Figure 1.7: ZnO and CN bandgap models with ROS potentials and forming mechanism

For the case of •OH, the radical forming step is a little bit trickier as the potential is outside of the VB value of CN Therefore, the Z-scheme proposal can be constructed to overcome this problem as the bandgap of ZnO covers the redox potential •OH/O2[79] Moreover, the passive pathways are available, ensuring the production of •OH from H2O2 or OH– ion [67] The generation of ROS under the influence of visible

light is summarized in Equations (1.1) – (1.5)

23 treatment is mainly focused as ZnO@CN can be applied with high performance and great adaptability

1.6 Antibiotics pollution treatment methods

Current situation 1.6.1.

For developing nations, the highest revenue of rapid development and concentrated industrial output acceleration have emerged as the highest priorities The uncontrolled usage of substances in various developing branches of the economy has been an outrageous catastrophe A worrying increase in pollution, particularly due to unchecked exhaustion of persistent organic compounds (POC) dumped into the environment, is occurring in tandem with the drastic increment Between the infamous POC such as benzene-based solvents, herbicides, organic dyes Antibiotics, since the initial discovery, have been used without less-to-none regulations To present the omnipotent coverage of antibiotics, in this thesis, four strains of the substances tetracycline (TC), ciprofloxacin (CP), cephalexin (CL), and erythromycin A (ER) are enlisted for investigation in Figure 1.8

Figure 1.8: Molecular structure of (a) TC, (b) CP, (c) CL, and (d) ER

24 It is to note that, antibiotics are not one strain of compounds but rather a complex system of multiple families Typical names could be spelt such as tetracycline (self-represent), flouroquinolone (representative: CP), β-lactam (representative: CL) macrolide (representative ER) as presented in this thesis More and more are being developed, and following up, more and more are being excessive exploited For instance, agriculture abuses antibiotics in the livestock specialty, where the farm animal has been fed with various substances Human over-consumptions the antibiotics just to treat simple diseases could also raise an alarm [80]

In the hydrosphere, the presence of persistent compounds could disrupt the metabolism of creatures, which causes harmful effects on the biosphere and reluctantly, human health is endangered by consuming the overdosed antibiotic polluted water The most severe impact has not yet fully arrived, as the mutated bacterial strains are developing antibiotic-resistant traits [81] More and more severe dosages of medicine are required to treat just conventional diseases; and in the future, fatality of the said bacteria is speculated to be higher

Wastewater treatment remains an enormous issue for the production line without access to contemporary methods and frequently causes unintentional negative effects on the environment Several technologies are currently present for water treatment on various scales, such as adsorption, ozonation, biodegradation, and photocatalyst

Pollution treatment 1.6.2.

1.6.2.1 Adsorption

The wastewater can be treated with high-quality adsorption techniques to remove dissolved antibiotic Because of the straight-forward design, minimal investment, and capacity to recycle, the adsorption process is frequently used to remediate the industrial wastewater of pollutants, both organic and inorganic Adsorbent characteristics and the chemical makeup of the solution represent two variables that affect adsorption efficiency Several disadvantages can outweigh the benefits of the method The drawbacks include a longer reaction time, challenges with desorption, and reusability following the adsorption process that somewhat hinder the antibiotics removal [82]