key: cord-0736241-lau142ay
authors: Yang, Xiuru; Chen, Zhi; Zhao, Wan; Liu, Chunxi; Qian, Xiaoxiao; Zhang, Ming; Wei, Guoying; Khan, Eakalak; Hau Ng, Yun; Sik Ok, Yong
title: Recent Advances in Photodegradation of Antibiotic Residues in Water
date: 2020-08-31
journal: Chem Eng J
DOI: 10.1016/j.cej.2020.126806
sha: 081b14ac3a34c1d52aa6536b9bd6993dfe3b6ad8
doc_id: 736241
cord_uid: lau142ay

Antibiotics are widely present in the environment due to their extensive and long-term use in modern medicine. The presence and dispersal of these compounds in the environment lead to the dissemination of antibiotic residues, thereby seriously threatening human and ecosystem health. Thus, the effective management of antibiotic residues in water and the practical applications of the management methods are long-term matters of contention among academics. Particularly, photocatalysis has attracted extensive interest as it enables the treatment of antibiotic residues in an eco-friendly manner. Considerable progress has been achieved in the implementation of photocatalytic treatment of antibiotic residues in the past few years. Therefore, this review provides a comprehensive overview of the recent developments on this important topic. This review primarily focuses on the application of photocatalysis as a promising solution for the efficient decomposition of antibiotic residues in water. Particular emphasis was laid on improvement and modification strategies, such as augmented light harvesting, improved charge separation, and strengthened interface interaction, all of which enable the design of powerful photocatalysts to enhance the photocatalytic removal of antibiotics.

In the last few decades, antibiotics have been broadly utilized not only for human therapies, such as curing infectious diseases like COVID-19, transplants, chemotherapy, and surgical interventions, but also for therapeutic and non-therapeutic purposes in aquaculture and animal husbandry and for enhancing crop production [1, 2]. However, the use and overuse as well as the delayed metabolism of antibiotics have led to the inevitable discharge of their residues into aquatic environments, resulting in refractory pollution sources [3] . Conventional wastewater treatment plants (WWTPs) cannot efficiently remove antibiotic residues [4, 5] . This leads to their distribution into ecological systems [6, 7] and eventually into the human body through the food chain or drinking water [8] [9] [10] . Moreover, the long residence times of antibiotic residues in aquatic environments, even at low concentrations, may lead to the propagation of bacteria with antibiotic resistance and even multiple drug resistance, which may cause life-threatening infections [3, 11, 12] . Therefore, developing an effective approach to degrade or remove antibiotic residues from aquatic environments is crucial.

Different methods have been developed to treat antibiotic residues in water and wastewater before their final release into the environment. These current methods can be loosely classified into three types according to the treatment mechanisms or principles involved: physical removal, biological treatment, and chemical degradation. As shown in Table 1 , physical methods such as adsorption, sedimentation, flocculation, and filtration only separate the antibiotic residues from the water and generate problematic products such as brine and contaminated adsorbents.

Alternatively, biological approaches have recently emerged, and most antibiotic residues in the environment can be removed through this route [36] [37] [38] . However, the artificial introduction of active organisms into aquatic environments may disrupt the ecological balance of their biomes, which may cause irreversible ecosystem damage.

Moreover, biological processes suffer from some key disadvantages such as being timeconsuming and often unreliable. Therefore, the chemical degradation of antibiotics has gained strong interests.

Different chemical approaches such as ozonation, chlorination, and Fenton's oxidation have been developed for the treatment of antibiotic residues in water, as shown in Table   1 . Unfortunately, complete mineralization is difficult to achieve or may otherwise be prohibitively lengthy. In some cases, these methods may kill non-target organisms due to their low selectivity, which causes unintended damages [39, 40] . Additionally, this method incurs high capital and high operating costs. Combinations of physical and chemical degradation processes can significantly reduce the toxicity of treated effluents during the removal of antibiotic residues from water. Nevertheless, these methods are complex and costly [41] .

Alternatively, photocatalysis has broad application prospects for environmental remediation due to its unique advantages, including (1) easily-attainable reaction conditions (i.e., near ambient temperature and mostly ambient pressure), the use of oxygen in the air to produce a powerful oxidant, and the use of solar radiation as an energy source; (2) potentially complete decomposition of organic pollutants into innocuous inorganic molecules such as carbon dioxide and water; (3) strong redox ability, low cost, no adsorption saturation, and long durability. Therefore, photocatalysis has increasingly garnered worldwide interest and has been broadly implemented in novel energy extraction and environmental control strategies.

Photocatalysis is an advanced oxidation process that has previously been applied in the treatment of antibiotic residues. As shown in Fig. 1 , less than 40 studies had been published on the photocatalytic treatment of antibiotic residues, prior to 2014. However, a sharply increasing number of publications on this subject have been released thereafter. Substantial progress has been made in recent years; however, photocatalysis still suffers from some key imperfections, including insufficient visible light utilization, rapid annihilation of photogenerated carriers, and incomplete mineralization, all of which strongly restrict its commercial application. A thorough overview concerning the fundamentals, improvement, and modification of strategies and challenges of the photocatalytic treatment of antibiotic residues was yet to be compiled, and therefore, these topics have been comprehensively addressed herein. This article comprehensively reviews the latest advances in the photocatalytic treatment of antibiotic residues in water. Specifically, the fundamentals of photocatalysis will be introduced. Different strategies will then be summarized and classified, followed by a discussion on the improvement and modification strategies to enhance the photocatalytic degradation of antibiotics. Finally, this review will conclude with a summary of major challenges and key perspectives.

When a semiconductor is exposed to irradiation with energy beyond its optical band gap, electrons are excited and shifted from its valence band (VB) towards the conduction band (CB), producing an equal number of positively charged holes in the VB. When the potential of VB vs NHE is more positive than H 2 O/ or , and the potential of CB OH ( + 2.72 V vs NHE)

OH -/OH ( + 1.89 V vs NHE)

vs NHE is more negative than −0.33 V vs NHE), the semiconductor will be O 2 /O -2 ( able to generate and . Thereafter, the photoinduced electrons and holes OH O -2 separate and migrate to the surface of the semiconductor, and redox reactions will occur at the reactive site on the semiconductor surface ( Fig. 2) 

The photocatalytic treatment of antibiotic residues in aquatic environments has recently become the focus of much attention, and diverse strategies have been proposed to improve the photocatalytic efficiency (Scheme 1). Examples of such reported strategies are summarized in Table 2 . Tetracycline 20 mg/L 80% optimum within 120 min [177] Porous materials Ultra-thin Bi 2 MoO 6 nanosheets 0.5 g/L-1.5 g/L

Ofloxacin 10 mg/L Approximately 71% optimum within 90 min [178] Tailoring morphology Rod-like SrV 2 O 6 2-9 g/L 500-W tungsten lamp (λ>400 nm)

Metronidazole 20 mg/L 98% optimum within 60 min [179] 33 

Defects play two distinct roles in the photocatalytic process: (1) they act as trapping whereas insufficient vacancies would fail to achieve the desired photocatalytic performance. Vacancies may be categorized into three types: 1) anion vacancies (such as sulfur [191] [192] [193] [194] [195] , halogen [196] [197] [198] [199] , nitrogen [200] [201] [202] and oxygen vacancies (OV) [203] [204] [205] [206] [207] [218] reported that anion vacancy could be produced in a TiO 2 lattice when Ti 3+ was present. Excess energy levels below the CB would be induced in the presence of these anion vacancies [219] [220] [221] [222] . Overlapping of these levels with CB occurs by increasing the concentration of these vacancies, resulting in a decreased bandgap of TiO 2 . Therefore, the visible-light-driven transition can take place from intrinsic VB to Ti 3+ states. This behavior can extend the optical response to an onset wavelength of longer than 400 nm [223] .

The introduction of heteroatoms or ions into the photocatalyst, in the form of dopant, is an effective light response management method. Non-metal dopants (e.g., C [224, 225] , B [226] [227] [228] [229] , P [57, 230], S [231, 232] , N [233] [234] [235] [236] , and halogen [237, 238] [241], Co 2+ [244, 245] and Er 3+ [246] ) were most widely studied in semiconductor photocatalysts, especially metal oxides. Generally, a method can either introduce an impurity energy level lower than the CB minimum or higher than the VB maximum to narrow down the bandgap, enhancing light absorption.

Recently, N-doped TiO 2 photocatalysts with tunable doping contents were prepared by calcining sol-gel TiO 2 powder in the presence of NH 3 flow at 450-800 °C [247] .

The light absorption capacity of samples prepared at a temperature region of 450-600 °C increased gradually with temperature, and their colors varied from pale yellow to emerald green as a result of the production of N 2p (localized states) beyond the VB of TiO 2 and the generation of OV ( Fig. 4(a) ). When NH 3 treatment reached T > 600 °C, anatase TiO 2 was transformed into rutile, leading to a remarkable reduction in its 

The addition of photosensitizers is another simple method of enhancing the nanosheets to promote visible-light-induced photocatalysis [252] . The pristine (i.e., unmodified) LTO nanosheets had no optical response from 400 to 800 nm ( Fig. 5(a) ).

In contrast, all CQD/La 2 Ti 2 O 7 (C-LTO) composites exhibited distinct responses in the above range, and the response could be modulated accordingly by changing the amount of CQD.

Zhao et al. [253] successfully synthesized TiO 2 nanotube (CQD/TNTs) composites with an outstanding photocatalytic performance. These CQD co-catalysts had remarkable up-converting photoluminescence (PL) features. Longwave infrared rays (LWIR, >600 nm) absorption could be converted into optical light of shorter than 600 nm, enabling the production of IR-induced electrons and holes on TNTs ( Fig. 5(b) ).

Additionally, the introduction of CQDs can accelerate the harvesting of photoexcited electrons and prolong the lifetime of photoinduced carriers. 

The construction of a junction interface is a typical approach to facilitate photocatalysis by modulating the separation/migration of the photoinduced electrons and holes at the interface. Therefore, in-depth studies have been conducted to design photocatalyst junction interfaces to monitor carrier migration capabilities. According to their phase composition, junction interfaces can be divided into homogenous junction interfaces and heterojunction interfaces.

Homogenous junction interfaces are built by identical compounds in the absence of additional components and have been the focus of significant interest. The construction of homogenous junction interfaces is primarily based on the established phase junction or facet junction, creating an effective migration pathway for the photoinduced charges.

Most crystals (e.g., TiO 2 [262] [263] [264] [265] [266] [267] [268] [269] [270] , CdS [271] , Bi 2 O 3 [272] , MoS 2 [273] and ZnIn 2 S 4 [274] ) have many different natural or artificial phases. The construction of the phase junction is a significant approach to improve the photocatalytic performance through accelerating the migration of electrons and holes. Many studies have been conducted on the design of photocatalyst phase junctions to boost carrier migration characteristics and capabilities [275, 276] . Jia et al. [277] studied the theoretical basis of the phase changes and interfacial characteristics of Cu 2 ZnSnS 4 with a hetero-phase junction. A type-II band structure could be formed at the hetero-phase junction of (101), (110), and (100) facets, thereby facilitating the photoelectric capacity. However, type-I heterojunctions may be aligned at the phase junction of (112) and (001) 

Previous studies have reported that, unlike isolated surfaces, the coexistence of multiple crystal planes in single particles could be recognized as interactive surfaces with synergistic effects [279, 280] . This means that redox reactions occur locally at the separate planes, resulting in spatial charge separation in semiconductors. For example, multi-morphologic silver bromide (AgBr) crystals containing distinct exposed planes were prepared by synchronously injecting silver nitrate and potassium bromide precursors [74] . The shape and exposed planes of the target sample could be easily modulated by varying the density of Brions, which can significantly reduce the surface barrier of (100) and (111) planes and affect their growth rates. C-AgBr ((100) facet exposed cubes), T-AgBr ( (100) and (111) (111) facets. Therefore, electrons and holes could not merely be efficiently separated, but back-reaction was effectively prevented owing to the isolation of active redox sites.

The formation of homojunctions is considerably challenging owing to its complexity and high cost. Therefore, the creation of heterojunction interfaces has been proposed as an alternative to improve charge separation efficiency and has been widely investigated in the past decades [281] . Typically, heterojunctions are divided into five categories:

(1) Schottky heterojunctions, (2) type I heterojunctions, (3) type II heterojunctions, (4) p-n heterojunctions, and (5) Z-scheme heterojunctions.

At the semiconductor-metal interface, photoinduced electrons may typically flow from the former to the latter to match their Fermi energies and form a Schottky barrier. 94.2% TC could be degraded in 120 min ( Fig. 7(a) ). The improved photocatalytic properties resulted from the synergism of the photoinduced electrons on Bi 3 O 4 Cl and SPR from Ag NPs, which improved visible-light harvesting and facilitated the isolation of photoinduced electrons and holes ( Fig. 7(b) ).

In other studies, bimetallic-supported semiconductor catalysts (Pt/Au/TiO 2 [283, 284] , Au/Ag/TiO 2 [285] , Pd-Cu/TiO 2 [286] and Ag-Cu/TiO 2 [287] ) exhibited super activity and high selectivity, which cannot be found in single metals owing to the synergistic effect of bimetals. For example, Xue et al.

[86] prepared plasmonic Au/Pt/g-C 3 N 4 via a simple calcination-photo-deposition process. The prepared heterostructure photocatalyst exhibited an optimized photodegradation performance for antibiotic tetracycline hydrochloride (TC-HCl) treatment, and the visible-light-driven degradation was 3.4 times higher than that of pristine g-C 3 N 4 (Fig. 7(c) ). This enhanced photodegradation performance was attributed to the synergism between the SPR absorption on Au and the electron-trap effect of Pt NPs, which facilitated the lightharvesting capability and isolation of photoinduced charges on g-C 3 N 4 , thereby jointly boosting the photocatalytic properties. Similarly, TiO 2 nanotube arrays (Au-Pt/TNTAs) with small quantities of Au-Pt were successfully prepared. Notably, these structures exhibited remarkably enhanced visible-light harvesting and carrier separation capacity.

These enhancements in photodegradation activity derived from the synergism from interfacial Schottky junctions and the synergistic effect between ternary Au, Pt, and TNTAs ( Fig. 7(d) ) [284] . [79], (c) photocatalytic kinetics of prepared g-C 3 N 4 , Pt/g-C 3 N 4 , Au/g-C 3 N 4 , and through a simple ionic-liquid-assisted solvothermal process (Fig. 8) . This 

Schottky and type II heterojunctions may successfully split electron-hole pairs.

However, it is still necessary to restrain the superfast electron-hole annihilation at the interface. Therefore, p-n heterojunctions have been implemented and widely used to 

The development of Z-scheme photocatalysis was inspired by photosynthetic systems in nature and has attracted tremendous interests since the first report of a conventional Z-scheme photosystem in 1979 ( Fig. 9(a) ) [290, 291] . Depending on the charge transport route involved, Z-scheme photosystems are classified as conventional, all- nanosheet/TiO 2 nanotube array) Z-scheme heterostructure ( Fig. 9(b) ) was successfully 51 fabricated in-situ by adding preformed g-C 3 N 4 nanosheets into the anodizing bath solution [296] . Enhanced photocatalytic performances and good stability were observed on nanosheet-supplemented photocatalysts, and g-C 3 N 4 (60)/TNTAs exhibited an optimum activity for the degradation of rhodamine B (RhB) and colorless TC-HCl. The optimized photodegradation performance was derived from an enhanced light harvesting capacity, a suppressed carrier recombination, and an extended carrier lifetime.

(a) (b) Fig. 9 (a) Roadmap of Z-scheme photocatalytic system evolution [293] ; (b) suggested photoinduced charge transport on g-C 3 N 4 (60)/TNTAs [296] 

In 

Although charges can be effectively separated, recombination in the process of electron transport remains problematic when the charges cannot be consumed promptly due to low activity or limited active sites on the semiconductor surface. Therefore, strategies have been proposed to promote intrinsic active sites on photocatalysts and the adsorption of pollutants.

Typically, the redox reactions that occur on the outer surface or interface of semiconductors are highly susceptible to the exposed facets. Based on crystal anisotropy data, it is widely known that different crystal facets have different atomic arrangements, which result in different physical and chemical properties, including anisotropic surface electronic structures, tunable surface energy, and diverse molecule absorption abilities and reactivities [279] . [298] . Particularly, h-WO 3 NSs with exposed (002) planes exhibited the highest visible-light photodegradation performance owing to their large BET surface and interior electron-hole separation on high-energy (002) reactive planes.

Porous materials have numerous notable advantages, including a high-density active core that facilitates photocatalytic reactions, high light absorption rates derived from the reflection and scattering of incident light within the pores, and high specific surface areas that improve the adsorption of contaminants and accelerate surface reactions. [305] [306] [307] , and g-C 3 N 4 [308] [309] [310] [311] [312] [313] [314] [315] [316] [317] [318] [319] [320] [321] [322] are mostly used in photocatalytic hydrogen evolution or photodegradation of organic dyes and are rarely applied for the degradation of antibiotics. Nonetheless, 2D porous materials could be promising candidates for the photocatalytic treatment of antibiotics.

Kang et al. [311] prepared few-layer g-C 3 N 4 NSs with foamy pores by ultrafast liquid-N 2 -frozen exfoliation of its bulk counterpart within 10 s. The foamed porous super-thin g-C 3 N 4 NSs exhibited interesting advances such as well-crystallized characteristics, a narrowed energy band, an additional unveiled edge, a slightlyaugmented BET surface (135.6 m 2 /g) with an increased mesopore, and an enhanced charge transport capability. Unlike massive g-C 3 N 4 , the super-thin porous g-C 3 N 4 exhibited a remarkably promoted capability to generate reactive oxygen species (ROS) and a 4-fold improvement in performance for visible-light-excited RhB decomposition.

This primarily resulted from an increase in reactive sites and a shortened route for charge transport. These results suggest that this technology could be further developed to generate high-performance super-thin g-C 3 N 4 NSs for contaminant treatment.

Template-free 2D porous super-thin g-C 3 N 4 NSs with doped oxygen atoms were synthesized by She et al. [316] . The photodegradation performance (60.8%) of MO and the mean H 2 production speed (∼189.3 μmol·h -1 ) were nearly 71 and 5.2 times higher than those in the bulk phase, respectively. This enhanced photocatalysis derived from unique features such as an enriched adsorbable/reactive site, a reinforced oxidationreduction capability and enhanced charge migration. 

The hazards of antibiotics and their treatment methods were briefly discussed herein, followed by a discussion on the current state of photocatalytic degradation of antibiotics in water. Photocatalysis plays an important role in treating antibiotic residues in water due to its superb features. However, efforts have to be made to further improve the efficiency for the wide application of this technology. The strategies for the improvement of degradation efficiency are the following:

(1) increasing light-harvesting capacity via defect engineering

(2) enhancing charge separation via interface engineering

(3) accelerating surface reaction.

These strategies are promising based on previous studies. However, for further practical applications, many challenges remain to be addressed:

a) It is difficult to identify the exact occurrence, formation, concentration, and types of defects (especially for vacancies) using current characterization techniques. This produces barriers for understanding the relationship between the structure and performance, which limits the further development of these technologies.

b) The construction of traditional heterojunctions via interface engineering has been intensively studied. Nonetheless, only a few previous studies have focused on homojunctions and double heterojunctions, given that current preparation methods are complex, time-consuming, non-eco-friendly, and theoretically lacking.

c) The BET surface and the number of active sites of a catalyst are key factors for accelerating surface reactions. In addition to increasing the specific surface area by controlling catalyst morphology, the selective exposure of highly active high-specificsurface-area crystal faces has recently become a popular research topic. However, most of these materials are used for hydrogen production, with few instances of antibiotic treatment applications.

Despite these challenges, this review provides valuable information on improvement and modification strategies for the design of high-performance photocatalysts to treat antibiotic residues in water. Besides, it is difficult to treat the antibiotic residues in practice by a sole technology. Innovating a complex system comprising photocatalysis and photoelectrocatalysis or electrocatalysis could be a promising alternative for the efficient removal of antibiotic residues in water. This review provides new insights into the design of high-efficiency photocatalysts for the degradation of antibiotic residues, thereby furthering the development of photocatalysis for water treatment and other fields. [60] F. Cai, Y. Tang 

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Drastic Enhancement of Photocatalytic Activities over Phosphoric Acid Protonated Porous g-C 3 N 4 Nanosheets under Visible Light

Alkali-assisted fabrication of holey carbon nitride nanosheet with tunable conjugated system for efficient visible-lightdriven water splitting

Vopor-polymerization strategy to carbon-rich holey few-layer carbon nitride nanosheets with large domain size for superior photocatalytic hydrogen evolution

Porous defect-modified graphitic carbon nitride via a facile one-step approach with significantly enhanced photocatalytic hydrogen evolution under visible light irradiation

Oxygen-doped nanoporous carbon nitride via water-based homogeneous supramolecular assembly for photocatalytic hydrogen evolution

Facile one-step synthesis of hollow mesoporous g-C 3 N 4 spheres with ultrathin nanosheets for photoredox water splitting