A comprehensive update on antibiotics as an emerging water pollutant and their removal using nano-structured photocatalysts

Journal of Environmental Chemical Engineering xxx (xxxx) xxx

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A comprehensive update on antibiotics as an emerging water pollutant and their removal using nano-structured photocatalysts Namrata Roy a, Sruthi Ann Alex b, N Chandrasekaran c, Amitava Mukherjee c, *, Krishnan Kannabiran a, ** a b c

School of Biosciences and Technology, VIT, Vellore, India Centre for Nano Science and Technology, Anna University, Chennai, India Centre for Nanobiotechnology, VIT, Vellore, India

A R T I C L E I N F O

A B S T R A C T

Editor: Teik Thye Lim

Diverse antibiotic drugs have been used for the treatment of bacterial infections; however, their overuse, improper handling, and limitations in treatment facilities can result in their accumulation and biotransformation in the water systems. These bio-accumulated antibiotics can lead to more detrimental effects due to the emer­ gence of multi-resistant bacterial genes at the micro-level, which in turn enhance the chances of bacterial sur­ vival. The current review discusses the prevalence of this contaminant in India and different countries around the globe. Since prevention of antibiotic release is the most effective strategy for controlling water pollution, the importance of several governing bodies and their guidelines for antibiotic use and release have been discussed. As a solution to the increasing threat of antibiotics in treated wastewater and natural waters, various treatment strategies have been explored, of which photocatalytic degradation is one of the significant treatment options. In particular, the review focuses on the multiple approaches of photocatalytic degradation in a stand-alone system and in combination with other techniques and nanomaterials. Different photo-active materials, reactor systems, oxide molecular release, kinetic mechanisms, influence of different parameters and characterizations involved have been discussed in detail in order to understand the optimum conditions for photocatalytic degradation of antibiotics.

Keywords: Antibiotics Pollutant Semiconductor photocatalysts Photocatalysis mechanism Nanoparticles

1. Introduction Antibiotics have achieved significant recognition since the 20th century as their consumption has been related with development of resistance. As every boon has its own set of drawbacks, antibiotics are posing a serious threat to the environment despite their numerous ad­ vantages to humans [1]. The occurrence of antibiotics in the water system is because of their indirect permeation via anthropogenic stress caused by pharmaceutical production industries, personal care product manufacturers, clinical institutions, and agricultural lands [2]. It has been reported at higher concentrations than the guidelines assigned for disposal, thereby posing a potential threat in the future [3]. Antibiotics in the active form (small doses) together with their primary compounds can cause changes to the microbial colonies in the water bodies over time [4]. The study conducted in the waters of Beijing, China, showed that the receiving water bodies of wastewater treatment plants had

shown the presence of tetracycline, sulfonamides, and quinolones (in ng. L− 1), suggesting that receiving waters were contaminated many-fold higher. Similar studies in Spain showed the existence of antibiotics, ciprofloxacin and sulfamethoxazole, at higher concentrations in the downstream waters [5]. An initial study conducted on the effluent spots of Vietnam, such as pharmaceutical companies and hospitals, detected high concentration levels of antibiotics, which resemble that of the developed countries (in mg.L− 1), and similar high concentration of an­ tibiotics prevail in other parts of Asia [6]. From the studies conducted on regions of North America and those of Europe, ciprofloxacin was detected in wastewater treatment plant (WWTP) at higher concentra­ tions in the influent waters when compared to effluent samples [7]. Additionally, the release of agricultural runoffs due to animal feeding operations and excessive utilization of bactericides/bacteriostatic drugs aid antibiotic penetration into groundwater systems through surface waters [8]. Contaminant release can be aggravated due to the increasing

* Corresponding author at: Centre for Nanobiotechnology, VIT, Vellore, 632014, India. ** Corresponding author at: School of Biosciences and Technology, VIT, Vellore, 632014, India. E-mail addresses: [email protected], [email protected] (A. Mukherjee), [email protected] (K. Kannabiran). https://doi.org/10.1016/j.jece.2020.104796 Received 15 August 2020; Received in revised form 8 November 2020; Accepted 12 November 2020 Available online 25 November 2020 2213-3437/© 2020 Elsevier Ltd. All rights reserved.

Please cite this article as: Namrata Roy, Journal of Environmental Chemical Engineering, https://doi.org/10.1016/j.jece.2020.104796

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population, overuse of antibiotics, inappropriate prescription of medi­ cine, self-prescription, overuse on agricultural lands, and improper treatment of wastewater by pharmaceuticals [9]. Documented experi­ ments and reviews support the evidence of pharmaceuticals (antibiotics) in urban wastewater treatment plants (UWWTP), hospital sewage treatment (HST), sewage treatment plants (STP), municipal sewage treatment plants (MSTP), wastewater treatment plant (WWTP), and pharmaceutical treatment plants (PTP) [7]. The release of treatment plant effluents into water bodies can lead to the inoculation of residual pharmaceuticals into natural water systems, which can be substantiated by the experimental evidence listed in Table 1. Generally, similar antibiotics are used for treating animals as those used for humans like norfloxacin, tetracycline, sulfamethazine, chlor­ amphenicol, and β-Lactam antibiotics [20]. β-Lactam antibiotics that are widely used for animal feeding operations, such as amoxicillin, ampi­ cillin, and mezlocillin, are reported to be present in surface waters (concentration range of ng.L− 1) [18]. Though the harmful effects of antibiotic accumulation are prominent on the environment, human health can also be affected as the primary antibiotics are losing their impact and functionality due to microbial resistance including some ‘last resort’ antibiotic agents [21]. Microorganisms that are able to create a physical environment to overcome the effects of new and improved drugs are entitled as the ‘superbugs’ [22]. Antibiotic-resistant genes (ARGs) are naturally present at the source point or can develop over time due to continuous interaction with antibiotics [23]. To overcome the effects of antibiotics in the environment, various strategies are applied at an organizational and experimental scale. This review paper focuses on the organizational contributions to reduce the antibiotic contamination followed by the experimental studies on various forms of photocatalyst nanoparticles. Advanced

Oxidation Process (AOPs) are effective innovations for the treatment of wastewaters containing non-effectively removable organic contami­ nants such as antibiotics, and among these processes, photocatalysis specifically is the most reliable [24]. The photocatalytic reaction effi­ ciency depends on the threshold energy of photons [25]. During pho­ tocatalysis, superoxide [a major reactive oxygen species (ROS) is generated through a reductive pathway, which involves reaction of electrons (e− ) with molecular oxygen [26]. Catalysts like semi­ conductors, metal oxides, and nanoparticles have been studied to enhance the superoxide generation for pollutant degradation [27]. The photocatalytic reaction can serve as a sustainable eco-friendly remedi­ ation alternative to chemical-based remediation [28,29]. Chemical-based techniques, employing chlorine and hypochlorite for antibiotic degradation, can stimulate the production of halogenated species from some pollutants, which are possibly carcinogenic [30]. AOPs have been preferred over chemically-driven oxidation techniques [31]. They can oxidize and degrade antibiotic agents depending on the • concentration of the generated ROS, such as hydroxyl radicals ( OH) and 2 other non-radical species like singlet oxygen (1O ) [32,33]. The photo­ catalysis reaction of the suspended photocatalysts is influenced by the irradiation conditions provided. The electrons (e− ) are excited from the ground state to the higher energy level. This is analogous to e− excita­ tion from valence band to conduction band [34–36]. An ideal photo­ catalyst should be less toxic, highly stable, cost-efficient, easily producible, and highly photoactive. The review article summarizes various guidelines and concentration limits (regional standards) for the discharge of different types of anti­ biotics into aquatic bodies. The review mainly focuses on removal strategies for antibiotics that involve photocatalytic degradation alone or in combination with other techniques and residual content analysis.

Table 1 Concentration range of antibiotics in different regions of the world. S. No.

Country

1.

Lagos, Nigeria

2.

Morelos, Mexico

3.

Spain

4.

Korea

5. 6.

Hanoi, Vietnam Vietnam

7.

China

8.

Kajang, Malaysia

9.

Romania

10.

Northern Colorado, USA

11.

Netherlands

Antibiotic Chloramphenicol Erythromycin-A dehydrate Erythromycin Sulfadiazine Sulfamethoxazole Trimethoprim Sulfamethoxazole Trimethoprim Sulfa-pyridine Sulfamethazine Sulfamethoxypyridazine Sulfadiazine Oxytetracycline Florfenicol Chlortetracycline Enrofloxacin Cefixime Ceftazidime Sulfacetamide Sulfadiazine Sulfamonomethoxine Sulfadimethoxine Sulfamethoxypyridazine Sulfamerazine Sulfamethoxazole Sulfathiazole Triclosan Sulfamethoxazole Macrolide class of antibiotics Sulfonamide class of antibiotics Piperacillin Trimethoprim Benzylpenicillin Cloxacillin Cephalosporin class of antibiotics Polyether ionophore antibiotics Sulfonamides and trimethoprim

Concentration range

Source of detection

Reference

Unknown

[10]

Surface waters

[11]

Veterinary and agricultural wastewaters

[12]

Surface waters

[13]

Sewage waters of the pharmaceutical plant Treated hospital wastewater

[6] [14]

Treated pharmaceutical wastewater

[15]

Drinking water

[16]

Groundwater Sewage treatment plant

[17]

− 1

0.36 mg.L 0.48 mg.L− 1 0.04 mg.L− 1 1.00 mg.L− 1 1.50 mg.L− 1 0.40 mg.L− 1 1.22 μg.L-1 0.395 μg.L− 1 12 μg.L− 1 6192 ng.L− 1 3704 ng.L− 1 2312 ng.L− 1 390–1410 ng.L− 1 17–340 ng.L− 1 13–793 ng.L− 1 10–133 ng.L− 1 19.24–43.33 ng.L− 1 5.0 μg.L− 1 5 ng.L− 1 10 ng.L− 1 6 ng.L− 1 9 ng.L− 1 4 ng.L− 1 287 ng.L− 1 9 ng.L− 1 21 ng.L− 1 0.1145 ng.L− 1 0.2345 ng.L− 1 568 ng.L− 1 579 ng.L− 1 571 ng.L− 1 730 ng.L− 1 1.11–43.31 μg.L− 1 5.05 μg.L− 1 0.97 μg.L− 1 945–1077 μg.L− 1 1–275 ng.L− 1

2

Agricultural manure application grounds Lagoon and agricultural run-offs

[18]

Wastewater treatment plant

[19]

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Moreover, few aspects and advancements to consider in the future have been identified. For preventing antibiotic accumulation in aquatic bodies, the various preventive measures taken to monitor the antibiotic concentrations in released wastewater and different remediation tech­ niques have also been addressed. Overall, the present accessible data regarding these issues do not provide all the necessary information, and these points are also discussed in this review.

resistance of microbes [45]. 2. Impact of antibiotics on the environment The rampant utilization of antibiotics has made their occurrence omnipresent in the surroundings, and nearly the entire of the world has recognized their existence common in natural and artificial set-ups. Environmental systems have been detected for contamination of anti­ microbials in sludge, sediments, soil, surface and groundwater, and other natural water sources. The emerging antibiotics continue to persist in the ecosystem due to their lower degradation rates, improper disposal, misuse, bulk manufacturing, and wastage. The increasing levels of emerging pollutants have instigated new scopes in research areas to combat these issues [46]. These are present in the ecosystem due to lower degradation rates. Their release into the water sources are through human excretion that depends on the amount of intake and metabolism of the person [47–49]. These composite materials need to be removed timely to avoid their accumulation in the discharge. The increasing presence of new contaminants necessitates modification of old scientific techniques to combat such new chemical constituents with unknown side effects on living organisms and their surrounding envi­ ronment. The acceptable amount of a new contaminant, which is less harmful to the existing ecosystem when present along with other bio-chemicals and manufactured chemicals, remains unknown [22]. This category of pollutants not only sabotages the environmental bal­ ance, but also influences human life. When a drug designed for a particular ailment becomes ineffective, new drugs are formulated for curing the same ailment but could simultaneously result in other side effects or affect other organs [47]. When a chain of new drugs gets accumulated with old, unused drugs that are released into the envi­ ronment, emergence of persisters, AMR, and ARG can be expected [50]. The reported occurrence of persisters is equivalent to that of the emerging resistant strains. The detection of new resistance requires time, and in the meantime, the microorganism may undergo several other changes [51]. In sewage removal structures, WWTPs are the final regions where antibiotic agents are treated before going into the aquatic systems, where they can get blended horizontally as well as vertically and get carried downstream by shifts in weather conditions and diffusion pro­ cesses. Pure natural, safe drinking water is now uncommon as most nations are confronting concerns over water quality. Indeed, even the faucet water which was trusted to be secure as a drinking water source has not been exempted from antibiotic pollution [52]. A study con­ ducted in Madrid, Spain, confirmed the contamination of drinking tap water with macrolides, erythromycin, and clarithromycin among other drugs [53]. During infusion and transportation in WWTP, the drugs might be adsorbed to suspended particles or may aggregate over residue and re­ turn into the column (pipelines of WWTP) via resuspension. Antibiotics with adsorption affinity can remain in the column, and the rest will be moved to the water body sediments. The percolation of antibiotics further hampers the balance of the aquatic systems, causing a negative impact that is difficult to alter [54]. As an entire alternative removal strategy, the sewage water can be used on farming areas as the natural bio-composition of soil can provide antimicrobial effects. However, the presence of natural contaminants in these water sources has now been questioned on these methods of removal as anthropogenic exercises have led to antimicrobial resistance and made urban aquatic systems accessible to antimicrobial contami­ nation [55]. Though soil obstructs the development of contaminants in the auxiliary surface water, it is hard to treat the water once it gets contaminated. Some of the sources of groundwater contamination are natural infiltration and water outlet pipes. The repetitive release and accumulation of sludge and fertilizer have created some major hotspots that can lead to spread of antibiotics into the land. From the sludge soils, the contaminant translocation typically takes place to the plants through

1.1. Antibiotic pollution in India Globally, India stands third in the pharmaceutical production and thirteenth in the consumption of pharmaceuticals [37]. Over-consumption and self-prescription cases, a large amount of dis­ charged pharmaceutical metabolites and unused active compounds are released indirectly into the water systems. Developing countries are generally predisposed to the presence of antibiotics and counter com­ pounds in the water system due to the large number of pharmaceutical activities, improper treatment strategies, insufficient knowledge, and lack of financial support. [38,39]. Due to such active conditions, there is a need for operational control measures. The released effluent needs to be checked if it is within the permissible limit (in mg/L) along with compulsory measurement of pH, BOD, COD, TSS, TDS, oil and grease, and bio-assay tests [40]. Additional parameters include nitrogen, chlo­ ride, metal, and active pharmaceutical ingredient-based compounds. The commonly used antibiotics in the domain of agriculture, hospitals (surgical incision infection), and domestic include chloramphenicol (3.20), tylosin (0.33), oxytetracycline (0.20), neomycin (0.01), strep­ tomycin (6.40), ciprofloxacin (0.02), ofloxacin (0.20), enrofloxacin (0.02), norfloxacin (0.20), pefloxacin (3.20), ampicillin (0.10), nalidixic acid (6.40), gentamicin (0.08), piperacillin (0.20), linezolid (2.68), tigecycline (0.40), amoxicillin (0.10), erythromycin (0.20), tetracycline (0.40), and clindamycin (0.04) [41] (these antibiotics can have release limit of effluent concentration at the outlet in μg.L− 1). The limited acceptable concentration of residual antibiotics in the animal-sourced foods are: benzylpenicillin (4 μg.kg− 1), oxytetracycline (100 μg.kg− 1), streptomycin (200 μg.kg− 1), oxacillin (40 μg.kg− 1), sulphonamides (100 μg.kg− 1), tylosin (50 μg.kg− 1), and erythromycin (40 μg.kg− 1) [42]. Studies in Indian territory for the detection of antibiotics in influent, treated effluent, hospital sewage, cultivation land, and water bodies near pharmaceutical production plants reported prevalence of active compounds at concentrations higher than the permissible limits. The antibiotics and antimicrobials (range in ng g− 1) in the surrounding urban and suburban regions of Hooghly river surface sediments (in Howrah, West Bengal) were measured. In the urban regions, triclosan (2–19 ng g− 1) were detected. In the suburban region of Howrah, these antibiotics were measured in the range: 6–376 ng g− 1, 1–340 ng g− 1, 2–87 ng g− 1, 2–62 ng g− 1, 2–84 ng g− 1, respectively, posing high ecological risk since most of the compounds detected were prohibited and showed accumulation in the fauna of the region [43]. Studies con­ ducted in the Hyderabad region, wherein pharmaceutical production and discharge of waste into the Musi river occurred on a regular basis, showed the presence of the fluoroquinolone, a class of antibiotics. The detected higher concentrations of ciprofloxacin (6.59–5528 μg.L− 1) followed by ofloxacin, enrofloxacin, norfloxacin, pefloxacin, dofloxacin, and lomefloxacin at 1.55–318.1, 2.57–123.4, 16.14–217.5, 0.74–44.34, 0.47–37.74, and 3.59–10.32 (range in μg.L− 1) respectively, strongly suggested that the presence of antibiotics in the Indian waters was greater than that present around the world [37,44]. Similarly, a study conducted in the Ganga river basin running through the outskirts of Patna, Varanasi, and Kanpur reported having the maximum number of pharmaceutical companies discharging effluent pollutants. The selected compounds were analyzed for the groundwater samples, triclosan (≤5.4 ng.L− 1) and sulfamethoxazole (≤4.13 ng.L− 1). The research concludes that the antibiotic concentrations in the groundwater were less than that of river water, stating that the discharge of unused or used pharma­ ceutical compounds into river bodies causes heavy pollution and 3

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uptake by root surfaces, root absorption, foliage absorption, soil and ingestion of plants by animals via grazing activities [56–58].

compounds [68,69]. Photocatalysis-mediated remediation is intrinsi­ cally unique as they have lower toxic effects and can mimic the natural process of contaminant degradation [69]. Additionally, the photo-active materials used are usually biodegradable or eco-friendly and have considerably lesser effects on the microflora and fauna [70]. Their prospective drawback is the expenditure for the photocatalytic set-up, need for selection of appropriate composite photocatalysts for new an­ tibiotics, regeneration of the used photocatalysts, analysis of toxic in­ termediates and/or end products, and the inability of photocatalysis to achieve complete mineralization of the pollutant [71,72]. The photo­ catalytic process solely or in combination with other methods have demonstrated that the final compounds produced were less harmful or within the acceptable concentration range [73–75]. For complete degradation of antibiotics, photocatalysis needs to be used in combi­ nation with other treatment processes, such as hydrodynamic cavitation, adsorption, sonocatalysis, microwave treatment, and ozonation [76–81]. Pang and co-workers have demonstrated previously a similar process for the efficient degradation of tetracycline hydrochloride using a combination of microwave treatment and photocatalytic oxidation [79]. After treatment of antibiotic contaminants, the recovery and regeneration of nanoparticles can be done by various methods such as microfiltration, membrane dialysis, and recovery methods. However, if attached to a binding material like the cross-linked biopolymer sodium alginate, the photocatalyst can be extracted by simple filtration methods. The methods used for extraction and regeneration of photo­ catalyst NPs can be expensive and/or time consuming [82]. Apart from photocatalysis, some of the other techniques used for antibiotic degra­ dation are mentioned in Table 2. These methods have drawbacks such as lower efficiency, longer duration, man power requirement, etc.

3. Treatment initiatives at organizational levels Since the recognition of pharmaceutical and personal care products as pollutant, various precautionary measures, analysis procedures, and removal techniques have been devised to reduce their levels to accept­ able amounts. The WHO has formed a committee, Global Antimicrobial Resistance Surveillance System (GLASS), that governs such facilities [59]. The WHO-supported program promotes a systemic approach to the compilation, interpretation, and disclosure of antimicrobial resistance-related evidence at a global level. A proper guide to risk management can promote the state, national, and international inter­ vention. The United Nations Secretary-General has set up Interagency Coordination Group on Antimicrobial Resistance (IACG) to improve coordination between global associations and to guarantee any activity against risk to health protection worldwide [60]. United Nations’ World Water Development Report (WWDR) was composed by the World Water Assessment Program (WWAP) [61]. The 2017 report of WWAP ‘Waste­ water’ suggested the influence of untapped resources of effluent from many industrial sources, pharmaceutics, and municipal waste expelled into the waters. European Union Water Framework Directive (WFD; 2000/60/EC) proposed by the European Commission (EC) implements regulations contrary to organic and inorganic pollution in waters [17]. The Com­ mission lists two categories based on priority pollutant and dangerous substances in List 1 released by the WFD, wherein in 2014, the updated list added 12 new compounds, of which three constituted of antibiotics: diclofenac, 17-beta-estradiol, and 17-alpha-ethinylestradiol [62]. The Commission has adapted to Directive 2009/90/EC to provide strategies and guidelines for monitoring and reporting pollutants in water sources to administrate the report upgradation periodically [63]. The approach to Sustainable Development Goals (SDGs) led to formation of the AMR Industry Alliance to combat antibiotic discharge and its resultant anti­ microbial resistance and to enforce policies at national standards [64]. SDG requires the contribution from life science sectors such as bio­ technologists, diagnostics, generics, and pharmacologists through vigi­ lance over research, medicine accessibility, prescription use, and manufacture of new improved drugs. A non-profitable organization such as the Access to Medicine Foun­ dation surveys leading pharmaceutical industries and ranks them ac­ cording to the approach towards access to medicine [65]. Currently, the organization has three on-going research programs, of which two are related to antibiotics usage and the Access to Medicine Index. Another organization, Antimicrobial Resistance Benchmark, encourages the effort of pharmaceutical companies to deal with antimicrobial resis­ tance, focuses on good practices, and emphasizes the importance of other medicine-based companies to do the same. Apart from organization-level interventions, there is a vast range of experimental evidence that provides approaches for removal of antibiotics from the environment.

4.2. Synthesis of hybrid/photo-activated nanomaterial There are various ways by which the photocatalytic activity of an element can be manipulated to stimulate high-efficiency photo­ luminescence property in the presence of a light source. Homogeneous nanoparticles with consistent shape, particle size distribution and lower mass transfer resistance encourage higher reaction rates. On the other hand, heterogeneous nanoparticles are NPs bound to a substrate mate­ rial and can demonstrate unique functionality by exhibiting different surface quantum chemistry in the form of ligands, functional groups, and hydrogen and covalent bonds. Nanoparticles exhibiting heteroge­ neous catalyst functions are usually unstable in nature, except under optimized chemical conditions. Heterogeneous systems are more advanced when compared to homogeneous NPs. They are designed to reduce e− /h+ pair formation by increasing the reaction rate [86–89]. The additional properties of the conjugate particle can assist in reducing the bandgap effects and satisfy the thermodynamic prerequisites; thereby, improving the photoactivity of the material [90]. The hetero­ junction can be classified in two-types: type II- heterojunction and Z-scheme heterojunction. The latter mimics the natural process of photosynthesis and is the advanced form of the previous mentioned systems. It is designed to overcome the lack of lower band-gap energies, blocking of e− /h+ pair formation and band-edge position [91]. The mechanism of Z-scheme is the formation of e− /h+ pair upon photon excitation between the two catalysts through a transportation pathway termed as e− mediator. The e− s from conduction band of photosystem II (PS II or oxidation site) is shifted to the valence band of photosystem I (PS I or reduction site) to form e− /h+ pair. The vectorial (Z-form) e− transfer allows the simultaneous recruitment of e− and h+ maintaining strong oxidation-reduction potential [92,93]. Minimum alteration of free Gibbs energy is mandatory for proper functioning of Z-scheme catalyst [93]. Fig. 1 schematically explains the mechanism of single-component and heterogeneous catalyst. The synthesis of photo-active compounds can be green-based or solution-based, though green-based synthesis is cost-effective and less toxic. Chemical synthesis has the advantage of lasting stability,

4. Nano-structured photocatalysts to treat antibiotic wastewater 4.1. Photocatalysis-based remediation processes Traditional treatment strategies have failed to single-handedly reduce the issue of antibiotic contamination. Lately, various advanced processes have been explored for the removal of antibiotics from the aqueous environment [66]. Advanced oxidation processes (AOPs) have gained increasing interest because of their fast reaction rate and robust oxidation ability, which are vital for antibiotic degradation in aquatic bodies [67]. The usage of specific photocatalytic conditions can generate oxidation molecules that can help to break down particular types of organic contaminants (antibiotics) and yield biodegradable 4

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Table 2 Examples of other techniques used for degradation of antibiotics. Techniques

Materials used for degradation

Antibiotics

Mechanism

Reference

Sonocatalytic degradation Biocatalytic degradation Thermophilic composting Pulsed discharge plasma

ZnO nanostructure with nanocellulose Enzyme lacasse

Tetracycline

Chemical effect of acoustic cavitation generated by ultrasonic waves Oxidative coupling and de-aniline

[81]

–

Tylosin, florfenicol, sulfadimethoxine, sulfamethazine Enrofloxacin

Plausible microbial activity, chemisorption, surface-catalyst reaction and other abiotic reactions Synergistic effect of ultrasound or sonication or electric field and radical species

[84]

WO3

Sulfamethoxazole

[83]

[85]

Fig. 1. Schematic illustration of a) single component photocatalysts, b) type-II heterojunction photocatalysts, and c) Z-scheme Photocatalytic system. Reprinted with permission [93]. Copyright 2020 WILEY-VCH Verlag GmbH & Co. KGaA.

structural control, bulk synthesis, and effectiveness. Production of photocatalyst through green methods can utilize the microorganism precursors that are readily present from wastewater treatment plants. Microbes are capable of metal reduction to form corresponding nano­ particles. Usage of plant-source precursor possesses medicative proper­ ties leading to fewer hazards at the treatment point while providing indirect on-site enrichment [94,95]. Furthermore, recent advancements in green approach for remediation involve the use of metal oxides for treating contaminants. Combinations of metal oxide nanoparticles, such as titanium dioxide (TiO2), zinc oxide (ZnO), copper oxide (CuO), and nickel oxide (NiO), with photon energy have also been studied. Nano­ particle production can be done using a microbial source, wherein the live medium can act as a reducing agent, and the chemical substrate can act as an oxidizing agent for the drug and can prevent nanoparticle aggregation with the help of stabilizing agents like carboxymethyl cel­ lulose (CMC) [95]. The improvisation or optimization of the photo­ catalytic activity of the element can be done through different ways that are inclusive of doping, heterojunction compositions, introduction of defects and voids, facet framework, and preventing recombination of holes [96]. The use of clay-based support material as a composite or substrate provides for expanded areas of interaction, stable surface support, less interference, and can serve as excellent semiconductors due to the presence of negative charge on the layers [97]. The various ho­ mogeneous and heterogeneous nanomaterials synthesized using different techniques are listed in the following Table 3.

4.3. Prerequisite of photocatalyst selection Oxide molecules can indirectly degrade the co-contaminants of metals and other pollutants [118]. General metal oxides used in research studies consist of TiO2 nanoparticles in anatase and rutile form, show­ casing sharp crystallinity and thermodynamic stability. Stand-alone or doped iron nanoparticles use ferrous ions for the photocatalytic reaction [119]. ZnO nanoparticles on calcination at different temperatures pro­ duce variant structures, like nano-floral formation that produce extended interaction surface area, which can be coupled with various metallic and non-metallic substrates. Graphene oxide (GO) displays multiple layers and hybridization levels depending on the modifications made to Hummer’s protocol; it is used as a support material that pro­ vides cost-effective absorptive and oxidative removal of contaminants [120,121]. Nonetheless, doping should be improvised for the productive functioning of the catalyst [69]. Doping involves introducing impurities onto the surface of the catalyst, which can indirectly produce a balance between the acceptor and donor. An electron–hole (e− /h+) pair gets generated due to photon activity by coordinating the balance between the conduction band and valence band. The photo-current density by induction that is determined based on the e− /h+ pair is represented in Eq. (1) [122]. hγ + semiconductor → hole + + e− + energy

(1)

The catalyst can function only in conditions where the photon 5

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Table 3 List of various homogeneous and heterogeneous nanomaterials synthesized using various techniques, band gap energy, and their degradation rate/percentage. S. No.

Catalyst

1.

TiO2 Anatase

2.

Cadmium oxide

3. 4.

α-Fe2O3 (A) α-Fe2O3 (N)

5.

Homogeneous materials

Rose-ZnO (R-ZnO)

6.

Hydrangea- ZnO (H-ZnO)

7. 8. 9.

Ag2ZnI4/AgI ZnO/Ag/Ag2WO4 Ni, Co doping ZnO

10.

NiO/graphene

11. 12.

CNTs@CoFe2O4 Co1-xZnxFe2O4 Carbon sphere template for TiO2 hollow structure BiVO4/WO3 ZnO-SnO2 RGO-Cd0.6Zn0.4S-Pt Co/BiOBr MgTiO3 nanofibers MgTiO3 nanosphere Ag2C2O4/TiO2 TiO2/g-C3N4 heterojunction catalyst MnFe2O4 ZnSe/PANI

13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23.

Heterogeneous materials

Method of preparation

Bandgap (eV)

Degradation rate/removal percentage

Reactive magnetron sputtering SILAR (Successive Ionic Layer Adsorption and Reaction) method Sol–gel technique Sol–gel technique SDS (Sodium dodecyl sulfate) assisted hydrothermal technique SDS (Sodium dodecyl sulfate) assisted hydrothermal technique Hydrothermal process Ultrasonic irradiation method Co-precipitation method NiO- thermal decomposition, NiO/graphenepre-graphenization route Chemical vapor deposition technique Microwave combustion method

3.23 eV

2.4 × 10−

2.6–2.8 eV

Unknown

[99]

2.63 eV 2.6 eV

98 % 86 %

[100] [100]

unknown

99.84 %

[101]

unknown

99.04 %

[101]

3.28 eV 3.2 eV 3.28 eV

89.3 % 121 × 10− 42 %

[102] [103] [104]

3

4

min−

min−

1

1

Reference [98]

3.3 eV

90.3 %

[105]

Unknown 2.43 eV

96 % 99.9 %

[106] [107]

Hydrothermal and Ultrasonication technique

3–3.2 eV

Unknown

[108]

Hydrothermal technique Polymer salt method DMSO-assisted Solvothermal method Solvothermal method Electrospinning technique Sol–gel technique Electrospinning-combined SILAR technique

2.37eV Unknown <2.59eV 2.58 eV 3.7eV 3.6eV 2.91eV

92.01 % 50 % – 99.5 % Unknown Unknown 99.9 %

[109] [110] [111] [112] [113] [113] [114]

Electrospinning technique

2.48eV

96 %

[115]

Auto combustion technique Co-precipitation method

Unknown Unknown

Unknown 50 %

[116] [117]

intensity is equal to or greater than the energy of the bandgap as shown in Fig. 2 [123]. The catalyst should also possess the property of ther­ modynamically stability and represent a bandgap that is higher than 1.23 eV. The bandgap should possibly occur in the photon range of visible light spectra (2.0–2.4 eV) so that the utmost utilization of the photon activity can be achieved with a minimal bandgap of 0.8 eV to prevent further energy losses. Knowledge of electronic configuration is the most crucial parameter for selection. Information of quantum electronic properties helps in understanding the valence to conduction band excitation of atoms with energy generation. Secondly, the absorbed photon of the exciton can induce ionization and lead to the energy transfer from valence to con­ duction band. The binding energy is affected by functional masses and dielectric constants [125]. Other parameters like stability, robustness, and durability of the photocatalyst can also play an influential role.

4.4. Mechanism of photocatalysis The development of a significant new activity in the field of water and wastewater treatment, especially photocatalysis. Preferably, a photocatalyst for the treatment of water ought to be chemically and biologically inactive, photo-catalytically dynamic, simple to produce and utilize, and activated under the influence of sunlight [126]. The basic electronic structure of nearly all semiconductor materials contains a highly populated band brimming with electrons called the valence band, and a least unoccupied band called the conductance band [127]. In order to have lower bandgap energy for activation, the surface of the catalyst is doped to form a heterogeneous catalyst. It is advisable to use a doping material with similar or smaller bandgap range of the catalyst so as to ensure that the reaction continues by preventing an energy drop from conduction to valence band [128]. Such a catalyst produces a redox reaction that generates hydrogen (H2) via photocatalysis reaction [129].

Fig. 2. Band positions and potential applications of some typical photocatalysts (at pH = 7 in aqueous solutions). Reproduced from [124] with permission from The Royal Society of Chemistry. 6

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The photocatalysis mechanism (Fig. 3) requires the smooth transition of 3 imperative steps to yield H2.

4.5. Radical nature of photocatalysis As mentioned previously, photo-excitation leads to the formation of the e− /h+ pairing which is unfavorable for photocatalytic reaction for treatment of organic contaminants. e-/h+ pairing which is unfavorable for photocatalytic reaction for treatment of organic contaminants [135]. • • The scavenging of electrons to form O−2 (superoxide) and OOH (hydroperoxyl) the protonated form followed by H2O2 (hydrogen peroxide).

a) excitation of e− from the valence to conduction band upon absorp­ tion of light energy b) formation of a combination of e− /h+ pairs on the exterior of the catalyst c) e− /h+ pair interacts with the water molecules leading to reduction and oxidation reactions

ϕ∝

κMϕ κMϕ + κEP

(3)

H2 + O2 + e− → OH + OH− •

The final step leads to the simultaneous degradation of organic compounds, i.e. antibiotics in this case. Considerably, the cycle of electron transport (or similarly for holes) is potential, if the electron acceptors are adsorbed on the molecule surface. The transfer rate relies on the general positions of the conduction and valence band, also upon the redox capability of the adsorbed species [130]. The effectiveness of a photocatalytic cycle is estimated by quantum yield, which is the proportion of the excitation occurrences per uptake of photon. Nevertheless, there is loss of photon in the case of heteroge­ neous photocatalyst via scattering after interaction limiting the accu­ racy. To overcome this drawback the total absorbed photon is taken into consideration. The complete quantum yield is stated by the Eq. (2) [131].

+

•

(4)

+

H2O + h → OH +H −

+

(5)

•

OH + h → OH

(6)

R + h + → R+

The pairing of O2 (oxygen) with the free e forms the O2 (superox­ • ide) radical. The newly formed O2 reacts with H+ (hydron) to form hydroperoxyl radical with the subsequent formation of H2O2 (hydrogen • peroxide). The OH plays a major role as the primary oxidant that is generated from the reduction of H2O2 (Eq. 3) and subsequent oxidation of water (Eq. 4 and Eq. 5). The direct oxidation of organic pollutants is possible via the h+ post absorption over the photocatalyst surface (Eq. 6) [29,69,136]. The photocatalytic reduction via ROS is explained in Fig. 4. Organic pollutants (antibiotics) are easily oxidizable compounds in a photocatalytic reaction. ROS, especially O2, has the limitation of −

(2)

where κMϕ denotes e− / h+ transfer rate and κEP denotes recombining rate. The understanding of the Φ parameter is crucial as it determines the difference of capabilities of various photocatalysts for a given re­ action and estimates the energy and cost efficiency of a photocatalyst [132]. The recombination of charge is essential or quantum yield would count as unity. The increase of photocatalytic activity via decreasing the rapid recombination of the e-/h+ pair is optimized by the modified heterogeneous photocatalysts [133]. To demonstrate the relation between photocatalytic reaction and degradation of organic compounds [134], a photoelectrochemical pho­ toanode doped with fluorine (F− ) was designed. By facilitating the reduction in bandgap energy of bismuth vanadate (BiVO4), the quantum efficiency of degradation of tetracycline improved through photo­ catalysis. The core–shell was doped with F− to reduce the oxidation reaction and produce high energy levels when they absorb the scarcely available UV light (4 %). The F-doped BiVO4 modified with NiFe-layered double hydroxide (F-BiVO4@NiFe-LDH) oxidizes and degrades the tetracycline hydrochloride while simultaneously helping in more energy production.

•

Fig. 4. ROS generated in the photocatalytic reduction and oxidation steps of oxygen and water. Reprinted (adapted) with permission from [137]. Copyright (2017) American Chemical Society.

Fig. 3. Schematic illustration of degradation of pollutants by the formation of photo-induced charge carriers (e− /h+) in the semiconductor surface [29]. 7

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oxidizing alkyl halides. The subsequent reaction of reduction and oxidation takes place simultaneously in the photocatalysis reaction. The production of ROS from oxygen (O2) and water molecules (H2O) post • • photocatalyst absorption can be sequentially placed as O−2 , H2O2, OH • • 2 − and OH, H2O2, O2 , 1O , respectively [137].

in Table 4. • • The formation of hydroxyl radical ( OH), superoxide radicals ( O−2 ), 1 − − singlet oxygen ( O2 ), and free e s is influenced by the pH. Fig. 5 in­ dicates the implication of antibiotic ciprofloxacin (CIP) on interaction with TiO2 photocatalyst at different pH from acidic to alkaline with miscellaneous by-products. • It is justifiable that OH radicals are likely to be added at basic pH to the piperazine ring of CIP since it has reducing properties (e− donor); h+ is the prevailing oxidizing species. Responses initiated by hydroxyl ions appear to dominate, this may result from the higher concentration of OH- at basic pH [142]. From the previously performed studies, it can be concluded that the sequential optimization of the parameters involved in the photocatalytic degradation of contaminants firstly involves the determination of the catalyst and the contaminant concentration. An optimum concentration of the catalyst would prevent saturation of the reaction system and would also facilitate the proper absorption of photons from the illumi­ nating light source [139,140]. Further, to improve the performance of the reaction system, the pH and temperature needs to be adjusted [36]. Considering the requirement of ROS for the degradation of the con­ taminants, an alkaline pH with abundant OH- ions is mostly preferable. Additionally, time-dependent irradiation with UVC light enhances the reaction rate, promoting faster elimination of the desired contaminant [141].

5. Influence of different parameters on the fate of antibiotics Photo-active materials are photochromic materials that can be manipulated to reach the optimized condition along with the substrate for accelerated activation, which under the influence of ultraviolet or visible light can oxidize and degrade organic compounds. Some of the parameters that can influence the antibiotic degradation are mentioned Table 4 Parameters for efficient antibiotic degradation. Parameter

Effects over degradation efficiency

Reference

pH

• Hydroxyl radicals are pivotal part of photocatalytic reaction • Point of zero charge (pHzpc) is the limiting pH defined by net surface charge to be 0. The pHzpc is negative with increase in synthesis pH. • pHzpc UVB > UVA) • High intensity of light facilitates the rapid recombination of e-/h + pair and lower intensity reduces the transfer of e-, thereby reducing the reaction rate. • Up to irradiation of ⁓25 mW/cm2, the degradation is subjective to the square root of intensity and depends on the gradual increment at lower intensity.

[138]

Contaminant concentration

Catalyst loading

Dopant concentration

Temperature

Light conditions

6. Characterization of the photocatalysts The photocatalyst synthesis can be performed by a vast range of procedures to produce a specific combination of structure, surface morphology, particle charge, conjugate hybrid materials, surface charge changes on interaction with the contaminant, and so on. The need for characterization is to understand the changes that take place on the photocatalyst, action of photocatalyst on the contaminant, liberation of charged molecules, oxidation–reduction potential of the photocatalyst, morphological changes, by-products produced, changes on the active faces of the catalyst, and to optimize the photocatalyst surface to improve the efficiency of the process. [143,144]. A representation of variable techniques for characterization along with the parameters detected is shown in Fig. 6. UV–vis spectrometry is the elementary stage of analysis used for conformation of the material. The contaminant analysis provides degradation percentage and the retention time required to attain removal and optimize the conditions accordingly. The formula: A0–A/ A0*100, (wherein A0 stands for the initial contaminant concentration and A stands for the final concentration after treatment) can be used for the quantitative study of the absorption or transmission capacity of the contaminant [145]. Fourier Transform Infrared (FTIR) analysis provides information of the changes in functional groups of contaminants and provides information about their rotational and vibrational energies. Absorption is studied in the mid-infrared region (4000–400 cm− 1). FTIR has different set-ups that can be changed according to the sample type, namely transmission (TIR), diffuse reflectance (DRIFTS), attenuated total reflection (ATR), and reflection absorption (RAIRS) [146]. X-Ray Diffraction (XRD) analyzes the crystalline structure of a material that can reflect the X-ray beam, which can be used to construct a pattern that yields diffraction peaks, representing the different facets over the crys­ tal. For instance, during the formation of TiO2 via magnetron sputtering process, the XRD peak pattern can confirm the pure composition of anatase form [98]. Apart from the characterization of the state of photocatalysts (preand post-treatment) by analytical methods, the toxic activity of photo­ catalyst over biota can also be studied. This test can be used to analyze the influence of residual antibiotic toxicity using algal constituents due to their high sensitivity compared to bacterial components. Terzic et al. showed that the possible biotransformation of degraded components from parent compounds (erythromycin) resulted in lesser toxicological

[139]

[140]

[140]

[36]

[141]

8

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Fig. 5. Radical attacking model of CIP on the surface of heterogeneous TiO2 [142].

Fig. 6. Variable characterization techniques and detection parameters.

9

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effects on Pseudokirchneriella subcapitata alga [147]. Studies were also conducted to test the residual activity over the bacterial strain Salmo­ nella typhimurium using the products obtained from the photocatalytic oxidation of ciprofloxacin, norfloxacin, and ofloxacin at concentrations similar to that of natural aquatic settings. The lesser antibacterial property of the oxidation products indicated that they have limited ac­ tivity post-treatment [148]. Similar studies were also conducted using antibiotic oxidation products of chloramphenicol following treatment using photocatalytic degradation combined with titanium dioxide. The toxicity evaluation was conducted over four bacterial cultures, namely V. fischeri, P.subcapitata, L. sativum, and D. magna, which showed toxicity of 10 % [70]. Octanol/water partition coefficient (log Kow) can help to determine the absorption of antibiotic agents into the biotic system through water transport and passive retention. The affinity of antibiotic agents toward sorption onto sludge is characterized by the consistent Kd,solid (L/kg), while the aggregation in sludge can likewise be surveyed by Kow (octanol/water segment coefficient). The productivity of WWTP is restricted by the adsorption and complexation of antibiotic flux in organic matter, which may limit their further bioavailability [149]. Water partition coefficient is an important factor as it influences adsorption, photodegradation, and moreover biodegradation of anti­ biotic agents [150–152]. The binding affinity of antibiotics can be described in 3 log-classes of the n-octanol/water dispersion coefficient (Kow): log Kow<2.5 is described as low sorption, log Kow in the range of 2.5 and 4 is moderate sorption, and log Kow>4 is high sorption [153].

time t and equilibrium, respectively. k1 and k2 stand for rate constant for pseudo-first and second-order (min− 1), β is desorption constant, Kp is intraparticle diffusion rate constant, and C denotes external layer effects. [MY]0, KLH, kc, and kobs are initial concentration of pollutant (mg L− 1), equilibrium constant of adsorption (L mg− 1), rate constant of surface reaction (mg L-1 min− 1), and observation rate constant, respectively. Langmuir–Hinshelwood (LH–) model is applied to most of the photo­ catalytic degradation studies. Mostly, kinetics of photocatalysis of organic substrates are analyzed under the criteria of L-H. The pivotal aspect of L–H is the use of experimental reading points obtained via dark experimental conditions. Because if there is a difference in the mea­ surements obtained between KLH (kinetic) and adsorption measurement the L–H mechanism is not applicable. The L–H model is fit for photo­ catalysis reactions conducted under UV [159,160]. The carrier trans­ formation should provide necessary oxidation and reduction potential in combination with the high crystallinity of the catalyst to prevent the loss of carriers within the material after reaction [119]. Alalm and others experimentally demonstrated the removal of amoxicillin, and ampicillin using activated carbon combined with the photocatalyst TiO2 using the Langmuir–Hinshelwood kinetics [161]. Similar studies using TiO2-­ doped carbon nanotubes and urea (Urea-CNT-TiO2) for triclosan removal was performed with 410-nm light using the pseudo-first-order kinetics, which showed 50–90 % removal [162]. Reduced graphene oxide as a support material for TiO2 (rGO-TiO2) was designed for the removal of sulfamethoxazole and was depicted using the Lang­ muir–Hinshelwood removal kinetics [163]. A recent analogous study was conducted using ZnO exhibiting photocatalytic reactive facets decorated with N,S–doped CQDs (ZnO/N,S CQDs) for testing the removal of various organic pollutants. Here, ciprofloxacin and cepha­ lexin were found to have degradation, 92.9 % and 86.7 %, respectively, and were observed to follow pseudo-first-order kinetics [164]. The re­ ported combinations of nanocomposites suggest that although photo­ catalysis using ultraviolet can produce antibiotic degradation, coupling with modified material can improve the removal efficiency.

6.1. Adsorption kinetics Adsorption is broadly applied separation technique, particularly in remediation of environmental pollution, because of its minimal cost and high effectiveness. Adsorption isotherm models can give mechanism data of the adsorption cycle, which is significant for the optimization of adsorption framework. The removal of antibiotics from the source can take place through their adsorption over a support material such as polymer, clay, carbon substrate, microporous substrate, and zeolite. They possess less chemical reactivity, good porosity, thermal resistance, and large surface range. Such a substrate component provides photo­ catalyst stability, reduces degradation effects over the photocatalyst, and increases the interaction of contaminants. The adsorption over the surface varies from substrate to substrate. Adsorption employing weak forces (Van der Waal’s forces) is termed as physisorption, whereas strong surface molecular binding is termed as chemisorption. Kinetic model designing helps to explain the mechanism and rate by which adsorption of the contaminant on the substrate occurs. The rate of adsorption is the most vital parameter, which is determined by adsor­ bent surface intricacy, adsorbate concentration, and retention time [154]. Depending on the various parameters of adsorbate–adsorbent interactions, four kinetic models are present: the pseudo-first-order re­ action, pseudo-second-order reaction, Elovich model, and intraparticle model. Table 5 summarizes the conditions and equation of the kinetics. Here, qt and qe stand for contaminant absorption over adsorbent at

7. Photocatalyst reactor systems Photocatalytic reactor is a significant unit in a photocatalytic wastewater treatment plant. The fundamental structure of the photo­ catalytic reactor is composed of a light source and a reactor design [165]. The plan and setup of the reactor are pivotal for the reaction performance. The degradation/removal of pollutants by the photo­ catalyst in the reactor includes the mass exchange of the related sub­ strates to the surface of catalyst, i.e., adsorption or desorption, and consequently, disintegration of the substrates using light. An ideal photocatalytic reactor ought to have a high-speed of mass transfer, good kinetics, and high surface area for surface reaction [166,167]. A number of reactor models have been designed due to various drawbacks faced during the optimization of experimental conditions. One of the difficulties is that the incredibly low pollutant fixation may bring about a moderate photodegradation rate as the contaminations are typically present in the water in miniscule level [168]. A high reaction

Table 5 Linear kinetic equations and criteria for application. S. No.

Kinetic model

Criteria

Sorption type

Equation

Reference

1. 2.

Pseudo-first order (Lagergren model) Pseudo-second order

Physisorption Chemisorption

ln (qe − qt) = ln qe – k1 t qt = t/(1/k2 q2e +t/qe)

[155] [156]

3.

Elovich model

Adsorbate on adsorbent The reaction rate is dependent on the available active sites on the adsorbent Adsorbate adsorption decreases as concentration of adsorbate molecules increases

Chemisorption

qt= 1/β ln (t+ 1/αβ- 1/β ln (αβ)

[155]

4.

Intraparticle model (diffusion controlledadsorption kinetics) Langmuir-Hinshelwood model

Adsorption of adsorbate (solute molecules) through mass transfer Dark conditions

Physicochemical adsorption –

qt =Kp√t+C

[157]

1/kobs = 1/kC KLH +[MY]0/kC

[158]

5.

10

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rate is needed to degrade the pollutants totally and deliver non-toxic products. Based on the setup, the reactors can basically be classified as suspended or fixed-bed reactors [169]. Membrane photocatalytic reactor (MPR) is a framework used to co­ ordinate photocatalysis and layer filtration methods to deliver a high caliber of permeate. The photocatalysis framework utilizes photo­ catalytic materials and photon excitation to create oxidation and reduction reactions to degrade harmful (organic) compounds [170]. On the other hand, film framework is utilized to isolate substrate particles present in the medium, and subsequently, treatment is done by AOP. It operates as a boundary for photocatalysts and eventually creates an opportunity for its reuse [171]. Pressure-driven filtration utilizing microfiltration, ultrafiltration, or nanofiltration films are the typical separation strategies utilized in MPR [172]. In membrane photocatalytic reactors, the submerged membrane photocatalytic reactor (SMPR) is the most widely used system (Fig. 7), wherein the photocatalyst is freely suspended within the structure of the reactor. The main parameter to be followed is the concentration of the photocatalyst. The slurry reactor has a huge surface with particles in suspension [173]. This can advance the rate of mass transfer between the reactants and photocatalyst and subsequently upgrade the photo­ catalytic proficiency [174]. In any case, the penetration of the light energy through the reactor will be diminished if the suspension is excessively concentrated. The excessive suspension can affect the cir­ culation of radiation, and consequently, the degradation/removal rate. Moreover, the suspended photocatalyst in the reactor is needed to be isolated before releasing the treated water to reduce the exposure of photocatalyst to the environment. The activation of photocatalysts in suspension by a given light source can create a lot of oxidizing radical species, which are utilized in degradation of the pollutants [175]. Besides the SMPR, the integrated membrane photoreactor (IMPR) is designed to solve the clogging issues associated with SMPR. The fabri­ cation of IMPR includes an in-built filtration set-up which separates the catalyst from the treated effluent before its discharge. Other photo­ reactors like the trough parabolic photoreactor (TPR) can be used, wherein collectors are placed for wastewater flow. The collectors are shaped parabolic and surface lined with reflectors. The reactor is designed in a way to concentrate the radiation at the focal line of the parabola [177]. The compound parabolic concentrator (CPC) is designed to overcome the limitations of TPR as it possesses the char­ acteristics of parabolic and static reactors [177,178].

an artificial source is steady and can be manipulated. The most wellknown light source of this category is the mercury light lamps. The mercury lights can be additionally classified into low-pressure light source (254 nm), fluorescent light source (~ 365 nm), medium-pressure light (UV–noticeable range), and high-pressure light source (UV) [179]. Light emitting diode (LED) has several benefits over traditional pressure mercury lamps that contribute to mercury pollution, short lifespan and cost inefficiency. LEDs provide extended lifespan, low-heating, strong radical generation, and water treatment abilities. The UVA LED lamps are studied for the activation of TiO2 particles with high photonic effi­ ciency (e− /h+) for the treatment of organic pollutants like antibiotics. The UVC lamps are extensively used in the AOPs such as the UV/H2O2 and Fenton process for treating water [141,180]. Thus, most photo­ catalytic experiments use artificial light to optimize light intensity ac­ cording to individual studies [181]. 8. Challenges and future aspects This review illustrates the scientific approaches for overcoming the emerging increase in antibiotics in water bodies and the formulation of various strategies to combat the problem, specifically focusing on ecofriendly photocatalytic strategies. Despite several governmental and organizational interventions, the lack of knowledge and proper under­ standing of antibiotics have further aggravated the problem. Therefore, there is a need for an appropriate scientific breakthrough. The presence of heavy metals along with antibiotics can further alter the extent of pollution and type of bioremediation needed. Based on the type of an­ tibiotics used, the concentration and type of co-exposed heavy metals can determine if the antibiotic resistance in microbes will be impacted positively or negatively. The photocatalysis is a well-studied and proven approach for the efficient antibiotic removal with lesser toxicity, eco-friendly nature, and has additional benefits of altering antibiotic-resistant genes; thereby, it can lower the overall side effects. Optimized nanomaterials show increased activity in ultraviolet and visible light sources, can imitate natural degradation processes, and their stability ensures reusability. Albeit the vast scope of advantages to the photocatalytic process, drawbacks and challenges are equivalent to the advantages in terms of scaling-up at an industrial-level. This level of up-gradation requires harvesting of solar energy as a supplying source of energy for the functioning of the photoreactor. Additionally, the need for substantial production of catalytic material is challenging irrespective of the financial investment. A green approach towards synthesis is also chal­ lenging due to the continuous changes in the batch produced due to the differences in biological make-up of the material used. Also, antibiotics tested at the laboratory scale for degradation are specific rather than using a multi-drug approach. Since scaling-up means the use of contaminated natural water, the source may contain antibiotics other than the specified contaminant that can cause hindrance to the treat­ ment process. Such an obstacle requires prior analysis of the treatment

7.1. Light source Photoreactors can have either natural visible light source or artificial light (in industries). The benefits of utilizing artificial light sources are consistency and reliability. The intensity of natural visible light relies upon the climate, and it can result in high heating temperatures, and are non-commercial for larg e-s cale application; however, the intensity of

Fig. 7. Scheme diagrams of (a) SMPR and (b) IMPR [176]. 11

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efficiency. Another setback is the difficulty in retraction of the photo­ catalyst material post treatment, which requires additional ultrafiltration attachment to the photobioreactor. There are several crucial aspects to be considered before choosing any scientific proposition. Photocatalytic removal when combined with other techniques can reduce the scale-up expenses. Natural adsorbents can reduce the toxic effects and accelerate the separation of nanomaterials by posing as a binding material. Optimization of the adsorbent material helps to reduce the retention time and energy consumption. Doping, heterojunction, and hybrid materials can increase the removal or degradation activity, and the use of biological sources combined with photocatalysis can reduce the toxic effects and benefit the water system. Even though the scientific community is striving for further advancements, which will reduce their antagonistic effects globally, the government norms and guidelines of different countries require strict supervision of the contaminant sources.

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