Reaction intermediates during the photocatalytic degradation of emerging contaminants under visible or solar light

7 Reaction intermediates during the photocatalytic degradation of emerging contaminants under visible or solar light Prasenjit Kar,1 Govindasamy Sathiyan,1 Raju Kumar Gupta1, 2 1

Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India; 2Center for Environmental Science and Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India

Chapter outline 1. Introduction 164 2. Study of intermediates during photocatalytic degradation of emerging pharmaceutical pollutants 166 2.1 Antipyrine 166 2.2 Triclosan 167 2.3 Sulfa pharmaceuticals 168 2.4 Atrazine 170 2.5 Acetaminophen 171 2.6 17a-Ethinyl estradiol 172 2.7 Diclofenac 175 2.8 Norfloxacin 176 2.9 Amoxicillin 177 2.10 Ampicillin 179 2.11 Naproxen 181 2.12 Ketoprofen 182 3. Conclusions 183 Acknowledgments 183 References 183

Visible Light Active Structured Photocatalysts for the Removal of Emerging Contaminants. https://doi.org/10.1016/B978-0-12-818334-2.00007-9 Copyright © 2020 Elsevier Inc. All rights reserved.

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1. Introduction Emerging contaminants are the new class of potential danger to aquatic life owing to their frequent detection in surface water even at very low level of concentration [1e4]. The emerging contaminants generally include pesticides, pharmaceuticals (antibiotic, antipyretic, beta-blocker etc.), personal care products (p-benzylphenol, romandolide, lilial, linalool, etc.), and endocrinedisrupting compounds [5e7]. Emerging contaminants are new class of molecules without regulatory status because the risk they pose to living organism in the aquatic environment is not yet fully understood [8e11]. For last few years, many researchers have reported the presence of such kind of new molecules, called “emerging pollutants,” in aquatic environments [12e14]. The emerging contaminants are generally originated because of inefficient treatment of pharmaceutical wastewaters and use of household wastes that result in toxic effects in aquatic organism and human life [11,15,16]. Pesticides and herbicides (basically comes from agriculture use and household productions) have also been frequently detected in the ground water [1,17,18]. Therefore, extensive research and development is necessary for complete removal of these compounds, thus providing safe water to public health and aqueous organism. Conventional wastewater treatment such as adsorption, chemical and biological treatment, reverse osmosis, and treatment of activated sludge is not sufficient enough for the removal of emerging pollutants completely [17,19e21]. Therefore, there is an increased interest in advanced oxidation processes (AOPs) as an alternative efficient technology that include “in situ” formation of strong oxidation radicals (usually holes, superoxides, hydroxyl radicals), which are sufficient enough for degrading and eliminating different contaminants (dye, metal, pharmaceutical compounds, etc.) in waters [22,23]. AOPs generally include Fenton oxidation, ozonation, and photocatalysis [24,25]. Photocatalysis is one of the green technologies that have attracted tremendous interest in the last few years because of low operating cost, nontoxicity, and efficient reduction of contaminants [26,27]. Furthermore, photocatalysis is a promising technology for water treatment processes as it requires relatively low water volumes with contaminants at low concentration, as in the case of emerging pollutants [28,29]. In this regard, photocatalysis is highly green appealing technology that requires harnessing of light energy through a semiconductor that is able to excite the electrons from valence band to the conduction band by irradiating with photons.

Chapter 7 Reaction intermediates during the photocatalytic

Different types of semiconductor photocatalysts have been developed by researchers for the degradation of emerging pharmaceutical pollutants [30e33]. TiO2 has been extensively used for mineralization of organic pollutants via generation of strong reactive oxygen species because of its low cost, high stability, easy availability, and enough generation of electronehole pairs under UV light illumination [31,34e37]. Silva et al. observed photocatalytic degradation of caffeine using TiO2 via photoexcitation generation of electron and hole under ultraviolet (UV) illumination [38]. Jing et al. studied photocatalytic degradation of acetaminophen using Zeolite Socony Mobil-5 supported TiO2 materials, synthesized using a solegel method [39]. They reported almost complete degradation of acetaminophen after 180 min irradiation of low-intensity light (0.97 mW/cm2). However, the use of TiO2 in practical application is very much limited because solar spectrum at Earth’s surface consists only of 3%e6% of UV radiation. This restricts application of TiO2 for practical photocatalytic applications. A tremendous effort over several years has been carried out toward improving visible light absorption ability, which has been considered to be a key factor for improving solar energyeharvesting ability [40,41]. Therefore, development of TiO2-based materials via narrow band gap coupling, metal/nonmetal doping, and dye sensitization allowing the maximum utilization of solar spectrum is the current topic of research as it results in reduction in operating cost through better utilization of the sunlight [42,43]. In this context, doping of TiO2 with several metals (Au, Ag, etc.) and nonmetal (C, S, P, etc.) has been considered successful approaches for better absorption of solar light [44,45]. Mesoporous graphitic carbon nitride coupled with titanium dioxide (mpg-C3N4/TiO2) nanocomposite was reported by Zhou et al. for the detoxification of sulfamethoxazole [46]. To overcome serious drawbacks of separation of the photocatalyst from the treated water, different continuous reactors were developed where photocatalyst were immobilized over a substrate. Malato et al. reported efficient photodegradation of emerging pollutants where TiO2 was deposited on glass spheres using solegel technique [47]. Study of fundamental processes associated with the degradation of emerging pollutants is very much important as it contains complex reaction pathways. Sometimes degradation of emerging pollutants into different intermediates results in a toxicological effect into the aquatic environment. Therefore, understanding the transformation of emerging pollutants into different intermediates during photocatalytic degradation is essential, as it will provide information about the toxicity of the intermediates as well as risk assessment

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during photocatalytic degradation of emerging pollutants. This chapter focuses on the recent progress of photocatalytic degradation of emerging pollutants under solar illumination utilizing different nanostructures. The photocatalytic pathways and degradation intermediates for each emerging pollutant during degradation have been discussed.

2. Study of intermediates during photocatalytic degradation of emerging pharmaceutical pollutants The understanding of intermediates path during photocatalytic degradation of emerging pollutants is crucial to investigate the overall mechanism during degradation. Here we focus the formation of different intermediates during photocatalytic degradation of different emerging pollutants.

2.1 Antipyrine In pharmaceutical industries, antipyrine (AP) is commonly used as an analgesic and antiinflammatory drug [8,12]. Therefore, presence of AP in surface and ground water has been considered as one of the emerging pollutants owing to endocrine-disrupting effects [48]. Recently, semiconductor-based photocatalysis has been widely used by many researchers for efficient removal of AP [49,50]. Zr-doped TiO2 was reported by Belver et al. for efficient degradation of AP under solar light irradiation [51]. Patterson et al. demonstrated UV-mediated photodegradation process of AP using a novel photocatalytic spinning disc reactor, where TiO2 was decorated on the surface of a glass disc via a solegel method [50]. Complete degradation of AP was observed in 120 min along with good recyclability (10 cycles) with almost no loss in the efficiency. They reported formation of main intermediates (benzenamine, anthranilic acid, and butanedioic acid) during photocatalytic degradation of AP along with some other minor intermediates having less concentration such as fumaric acid, 4-oxo-pentanoic acid, 1,4 benzenedicarboxylic acid, and N-phenyl propanamide (see Fig. 7.1). The formation of intermediates such as N-phenylpropanamide and benzenamine during photocatalytic degradation of AP was also identified by Tana et al. in UV/H2O2 treatment process [52]. They proposed that degradation of AP was proceeded via breaking of C]C bond in the pentacyclic ring by OH
. In the next step, breaking of CeC and NeN bonds resulted in generation of N-phenylpropanamide.

Chapter 7 Reaction intermediates during the photocatalytic

Figure 7.1 Antipyrine degradation pathway induced by UV/H2O2/TiO2 [50].

Subsequently, formation of benzenamine takes place via attack of hydroxyl radicals on the CeN bond. The formation of reactive oxygen species (hþ, OH
, and O2$ ) are predominantly responsible for the degradation of AP as confirmed by Belver et al. using TiO2eZnO/clay nanoarchitectures under solar irradiation [53].

2.2 Triclosan Triclosan (2,4,40-trichloro-20-hydroxydiphenyl ether), commonly used as an antimicrobial compound, has been detected in surface water, ground water with different levels (mg/Leng/L) [54,55].

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Triclosan degradation was studied by a number of research groups using various AOPs UV/H2O2, UV/TiO2, ZnO, visible light/TiO2, and solar light/TiO2 [56e59]. An interesting study was reported by García et al. for the photocatalytic degradation of triclosan, where TiO2 was immobilized (with loading of 0.335 g/L) on glass spheres (B ¼ 6 mm) [47]. More than 90% degradation of triclosan was observed, when 100 ppb of triclosan was flown in a cylindrical reactor packed with catalyst and irradiated by a 2.2 kW xenon arc lamp (l > 300 nm) solar simulator. Photocatalytic degradation of triclosan proceeded via formation of electronehole pair on the catalyst surface on irradiation of light. Thereafter, generation of reactive oxygen species (OH
, hþ, O2$ ) due to reaction of electron with dissolved oxygen in the water favors degradation of triclosan. Similar kind of mechanism was reported by Dai et al. where AgeTieSi ternary modified a-Bi2O3 nanoporous spheres were utilized for the degradation of triclosan under both UV and visible light [60]. Au-coated Cu2O nanowire arrays have been used by Yin et al. for triclosan degradation under visible light illumination [32]. The formation of chlorophenoxyphenol, phenoxyphenol, chlorophenol, catechol, phenol, benzoquinone, and lower volatile acids were identified as degradation intermediates for triclosan. Yin et al. proposed that the degradation pathway of triclosan proceeded via initial formation of dichlorophenoxyphenol (II). In the next step, dichlorophenoxyphenol (II) was dechlorinated into monochlorophenoxyphenol (III) and phenoxyphenol (IV). Next, the cleavage of ether linkage led to formation of chlorophenol (V), catechol (VI), and phenol (VII). Meanwhile, formation of phenol (VII) was reported via dichlorination of chlorophenol (V) [32]. These compounds (VI and VII) further reacted with reactive oxygen species and generated semiquinone (VIII) and benzoquinone (IX). Further oxidation of compounds (VIII and IX) led to the formation of low-molecular acids (X), such as muconic acid, acetic acid, and oxalic acid. According to the major identified intermediates and byproducts, the proposed photodegradation pathway of triclosan is shown in Fig. 7.2.

2.3 Sulfa pharmaceuticals Recently, presence of sulfa drugs used as antibiotics are frequently detected in environmental waste water and surface water and they exhibit potential carcinogenicity to the environment [61e64]. Photocatalytic degradation of sulfa pharmaceuticals based on proposed degradation pathways and intermediates have been studied by researchers [65e67].

Chapter 7 Reaction intermediates during the photocatalytic

0

5

10

15

20

25

OH

Triclosan(l)

5’ m/z

287

289

6’ 1’ O 1 2’

Cl 4’

291

3’

Cl

[l]

Dechlorination

HCl

Cl

Cl [ll]

Dechlorination

m/z

2H

221

HCl

OH O

[ lll ]

Cl

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3 4

O

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Monochlorophenoxyphenol(lll) 219

2

OH

m/z 255

Cl

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2H

Dichlorophenoxyphenol(ll) 253

6

m/z 185

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2H

2H

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128

OH

O

m/z 127

Ether Cleavage

HCl

OH

Chlorophenol(V)

[lV] Cl

129

m/z 110

111

Phenol(Vll)

m/z 94

95

96

HCl

Ether Cleavage OH

109

[V]

2H

2H

Catechol(Vl)

OH

OH

[Vl]

Quinoid Oxidation

[Vll] ROS

O

-H

O

O

O O

Benzoquinone(lX)

[Vlll]

m/z 107

0

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10

169

15

108

20

Retention Time (min)

[lX]

O

109

ROS

Ring Cleavage 25

COOH COOH

HO

O OH

O

O OH

[X]

Figure 7.2 Intermediates identified by LCeMS on a C18 column (verified by GC-MS) and the proposed reaction pathway. All products are listed in the order they appeared in the photocatalytic reaction and verified by their mass spectra and authentic standards. Small molecular acids were separated on a ODS-3 column [32].

Costa et al. reported 82% degradation of sulfamethoxazole as well as 23% reduction in total organic carbon (TOC) through using 0.5 g TiO2/L under UV illumination [65]. Detailed photodegradation kinetics and pathways of different sulfa drugs (sulfanilamide, sulfacetamide, sulfathiazole, sulfamethoxazole, and sulfadiazine) were investigated by Baran et al. using TiO2, Fe salts, and TiO2/ FeCl3 catalysts [68]. The disappearance of sulphonamides

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and the formation of organic intermediates were analyzed by HPLC/MS using TiO2 [66]. The degradation kinetics of sulfa pharmaceuticals was studied in TiO2 aqueous suspension by An et al. [69]. They proposed that addition of hydroxyl radical generates different monohydroxylated intermediates such as monohydroxylated, dihydroxylated intermediates with m/z ¼ 302, 284, and 266 for sulfachlorpyridazine, sulfisoxazole, and sulfapyridine, respectively (Fig. 7.3). Sulfachlorpyridazine and sulfisoxazole were also reported for the formation of another daughter hydroxylated intermediates with m/z ¼ 318 and 300. In another secondary pathway, breaking of SeN bond was reported via attacking by hþ led to the formation of primary amines along with 4-aminobenzenesulfonic acid.

2.4 Atrazine Atrazine (2-chloro-4-(ethylamino)-6-(isopropylamino)-s-triazine) is recently considered as an emerging pollutant because of detection in surface water, ground water even at trace level [70e72]. Photocatalysis technology is considered one of the most green approach for the degradation of atrazine. Immobilized TiO2 films in a stirred tank reactor were effectively used for the photocatalytic degradation of atrazine [73]. In another report, different metallic nanoparticles (Au, Cu, and Ni)-modified TiO2 were used for the photocatalytic degradation and mineralization of the herbicide atrazine [74]. SPR-induced improvement in photocatalytic activity was reported owing to sufficient increment in the absorption of light. Additionally, the presence of metallic nanoparticles increased charge carrier separation of the electrone hole pair, thereby reducing their recombination. Zhao et al. investigated the intermediates during photocatalytic degradation

Figure 7.3 The proposed pathways of photocatalytic degradation of sulfa pharmaceuticals [69].

Chapter 7 Reaction intermediates during the photocatalytic

of atrazine using H3PW12O40/AgeTiO2 as photocatalyst [75]. Improved degradation efficiency of composite catalyst toward atrazine was reported because of generation of OH
, hþ, and O2$ generated by the reaction of photogenerated electron and hole with dissolved oxygen. Photocatalytic intermediates of atrazine were studied by Chen et al. through coupling of TiO2 with multiwalled carbon nanotubes [76]. Formation of different intermediates was analyzed via HPLC as shown in Fig. 7.4.

2.5 Acetaminophen Acetaminophen is regarded one of the emerging pollutants, which causes serious environmental impact because of high water solubility and persistence [77e79]. Photocatalytic decomposition of acetaminophen has been studied extensively using different photocatalysts under solar light irradiation [39,80,81]. Enhancement in visible light photocatalytic activity was reported by Lin et al. using Cu-doped TiO2 for the degradation of acetaminophen [82]. The photogenerated electrons and holes eventually interact with the adsorbed O2 and H2O. This interaction results in formation of reactive oxygen species (hydroxyl radicals [OH
] along with other oxidants, e.g., superoxide radical anion O2$ ), which eventually degrade the acetaminophen to end products. In another report, K2S2O8-doped TiO2 as synthesized by Lu et al. group exhibited excellent photocatalytic activity for the degradation of acetaminophen [83]. Nan et al. investigated reaction intermediates during photocatalytic degradation of acetaminophen using b-Bi2O3 nanospheres under visible light irradiation [84]. Photocatalytic degradation of

Figure 7.4 HPLC spectra for time 0e3 min of MAPC experiment, showing decrease of atrazine and the increasing formation of intermediates (catalyst: TiO2/CNTs with annealing at 573K for 2 h) [76].

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acetaminophen led to the formation of organic acids as main products along with a small amount of hydroxy-acetic acid, formic acid, and succinic acid. Photocatalytic degradation pathways of acetaminophen proceeded mainly through hydroxylation of acetaminophen [39,85]. The pathway of acetaminophen degradation has been proposed in a recent work, where intermediates were analyzed through HPLC chromatograms [86]. Firstly, hydroxyl radicals attack at the para position with respect to OH functional group to produce hydroquinone and amide. The amide is further degraded into ammonium and ethylamine before converting into nitrates. On the other hand, hydroquinone is converted into benzaldehyde and then to benzoic acid. Intermediates further degrade into either acids or alcohols and finally converted into CO2 and H2O via complete mineralization. Similar kind of degradation was reported by another research group (Fan et al.), where photodegradation products of acetaminophen were analyzed using HPLCeMS as shown in Fig. 7.5 [87].

2.6 17a-Ethinyl estradiol Recently, inadequate treatment of wastewater treatment plants leads to the detection of 17a-ethinyl estradiol (EE2) in aquatic environment at ppb level and is considered one of the emerging pollutants [88,89]. Heterogeneous photocatalysis is considered one of the effective technologies to save ecosystem, human health, and drinking water from EE2 [90e92]. Photocatalytic degradation of EE2 was carried out using ZnO as a photocatalyst by Frontistis et al. under solar radiation [93]. Ag-modified TiO2 nanotube arrays (Ag/TiO2 NAs) were also employed as an efficient photocatalyst for the degradation of 17a-ethinylestradiol (EE2) [94]. In another report, photocatalytic degradation of EE2 was carried out by Ag/AgCl @ chiral TiO2 nanofiber under visible light illumination [95]. The separation of electron and hole pair by photocatalyst under solar illumination leads to the generation of various reactive species (RS) such as OH
radicals, O2$ radicals, and hþ, which subsequently degrade EE2. In another study, Li et al. reported excellent photocatalytic degradation of EE2 in the presence of multifunctional magnetic nanocomposite (AgeAgCl/ZnFe2O4) catalyst [96]. A complete degradation was observed within 240 min under visible light illumination. Based on the experimental results (LCeMS study) and several other reported literature Li. et al. proposed that EE2 was first oxidized into semiquinone as shown in Fig. 7.6. Further attack by superoxide radical on A (at the position 10 of aromatic ring) leads to the generation of several fragmentated compounds with m/z 315,

Chapter 7 Reaction intermediates during the photocatalytic

Figure 7.5 The proposed degradation pathway of acetaminophen in Bi-TNR [87].

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Figure 7.6 The proposed pathway for the photocatalytic degradation of EE2 over AgeAgCl/ZnFe2O4 [96].

Chapter 7 Reaction intermediates during the photocatalytic

269.2, 217, 255.2, and 205. Finally, formation of small acids, CO2, and H2O occurred via further oxidation of fragmented products. Another report by Pan et al. proposed EE2 photocatalytic degradation through two different mechanism (via
OH radical attack or direct hole oxidation) [97]. LCeMS study confirmed formation of three different compounds with m/z 313.1799, 313.1793, and 313.1804 with different retention time (17.65, 21.75, and 23.37 min). Finally, breaking of CeC, C]C, CeO, and OeH bonds in EE2 molecule takes place by strong RS (hþ, OH
, O2$ ) to generate other intermediates, which finally form CO2 and inorganic ions.

2.7 Diclofenac Recently, the presence of diclofenac in aquatic environment (ng/Lemg/L) has shown several adverse effects in the ecosystem and is considered one of the emerging pollutants [3,98]. Photocatalysis technology is reported one of the most efficient technology for the removal of diclofenac from industrial waste effluents [99,100]. TiO2, SnO2, C3N4, etc. semiconductors have been used for the efficient degradation of diclofenac [31,101,102]. Ag-modified C3N4 was found to be efficient for the degradation of diclofenac due to formation of efficient interface and charge carrier separation [103]. Improvement in lightharvesting ability and photo-induced charge carrier separation led to accelerated degradation of diclofenac as reported by Cui et al. using AgeAg2O/r-TiO2 photocatalyst [104]. Liu et al. reported visible lighteinduced photocatalytic degradation of diclofenac using carbon quantum dotsemodified porous g-C3N4 [102]. They observed mechanism for efficient degradation of the diclofenac due to prominent electron and hole separation under visible light illumination with good reusability for several consecutive cycles. Efficient photodegradation of diclofenac was mainly observed because of the formation of different reactive oxygen species (hþ, OH
, O2$ ) in aqueous solution [105,106]. Cheng et al. studied photodegraded intermediate pathways of diclofenac using N, SeTiO2/TiO2 NTs catalyst through identification of intermediates via LCeMS [107]. The probable degradation of diclofenac was reported through decarboxylation, hydroxylation, and CeN bond cleavage reaction (see Fig. 7.7). In the first path, hydroxylation of diclofenac resulted in the formation of hydroxy diclofenac with m/z 311. In the next step, repeated hydroxylation led to the formation of poly hydroxy diclofenac, which further mineralized into CO2, H2O, etc. In path II, CeN bond of diclofenac was reported to

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Figure 7.7 Diagram process for PEC degradation of diclofenac solution by N, SeTiO2/TiO2 NTs photoelectrode [107].

form another intermediate with m/z ¼ 161, 2, 6-dichloro aniline. Formation of this intermediate (m/z ¼ 161) was also accelerated by side chain oxidation of polyhydroxy diclofenac with hþ. Subsequently, the intermediates were further attacked by radicals and converted to smaller fragments and finally to the CO2, H2O, etc. In path III, decarboxylation of diclofenac molecule led to the formation of 2,6-dichloro-N-o-tolylbenzamide with m/z ¼ 251. After that, formation of (2-(2, 6-dichlorophenylamino)phenyl) methanol was due to further attack by hydroxyl radicals. Dechlorination and oxidation further resulted in the formation of 2-(phenylamino) benzaldehyde with m/z ¼ 197. Complete mineralization of all the products resulted in the formation of CO2, H2O, and other ions.

2.8 Norfloxacin Norfloxacin (NFN) is considered as an emerging pollutant in fluoroquinolones drug family because of frequent detection in wastewater with high concentration (100 mg/L) [108,109]. Therefore, NFN is proven to execute adequate toxicity on human beings and environment [110,111]. Different semiconductorbased photocatalysts such as ZnO, TiO2, etc. are widely used for the degradation of NFN [112,113]. Reduced TiO2 was utilized effectively for the degradation of NFN under visible light illumination [114]. In situ construction of BiVO4/BiOCl heterojunction has proven efficient photocatalyst for the degradation of NFN due to prolonged charge carrier separation

Chapter 7 Reaction intermediates during the photocatalytic

of electronehole pair at the interface [30]. Generation of RS (hydroxyl radicals [OH
] and holes [hþ]) was mainly a driving force for the photocatalytic oxidation processes [115,116]. Another interesting study by Guo et al. reported magnetically recoverable photocatalyst, i.e., (BiOBr coupled with Fe3O4) exhibited superior photocatalytic activity toward degradation of NFN [117]. The photodegradation route of NFN was investigated by kumar et al. using Ag@BiPO4/BiOBr/BiFeO3 nanoassembly under visible light illumination [118]. Peak corresponding to m/z ¼ 319.3 was attributed to molecular ion peak (IM-1) for NFN as evident from LCeMS study. The three main pathways (defluorination, ring conversion, and decarboxylation) were reported for the degradation of NFN. In path I, hydroxyl group substitution by fluoride led to the formation of IM-1.2 (m/z ¼ 383.3) via addition to the quinolone ring. Further degradation and ring opening of IM-1.2 resulted in formation of low molecular weight products. In path II, attacking of radicals caused piperazine ring transformation or conversion, which formed fragments with m/z ¼ 336.3 and m/z ¼ 350.7. In another path (Path III), decarboxylation (loss of CO2) of parent ion led to amide formation with m/z values 276, 265.9, and 294, etc. At the end of all the stages, formation of different simpler molecules was detected. Finally, all the intermediate formed CO2, H2O, and other ions. Another report by Yang et al. also proposed formation of small intermediates on degradation of NFN due to loss of ammonia, carbon dioxide, and water molecules [114]. Formation of variety of intermediates was accelerated via reaction with different reactive oxygen species (hþ, OH
, O2 $ ) as shown in Fig. 7.8.

2.9 Amoxicillin Recently, amoxicillin (AMX) was reported as an emerging pollutant owing to its presence in the environment in ng/L level [119,120]. Photocatalysis is considered one of the efficient approaches for the degradation of AMX [121,122]. TiO2, ZnO, and TiO2/activated carbon have been reported as photocatalysts for the removal of AMX [123e125]. Improvement in AMX degradation (97.5% degradation within 5 h) under visible light was reported by Leong et al. using palladium nanoparticles (Pd NPs) on titania (TiO2) nano photocatalyst [126]. Recently, Yang et al. found MIL-68(In)-NH2/graphene oxide (GrO) composite efficient for the degradation of AMX under visible light illumination [127]. Improved charge carrier separation under solar illumination as well as generation of OH
, hþ, and O2$ were found

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Figure 7.8 Proposed degradation pathway of norfloxacin on TiO2X under visible light [114].

Chapter 7 Reaction intermediates during the photocatalytic

to be the main reasons for the degradation of AMX [128,129]. TiO2/natural zeolite was used for the degradation of AMX under UV illumination. Improvement of AMX degradation was observed for TiO2/natural zeolite (88%) in comparison to bare one (79%) [130]. However, intermediates for photocatalytic degradation were confirmed by LCeMS study. A diasteroisomeric compound with m/z 367 (amoxicillin penicilloic acids (APcA)) was observed via initial ring opening of b-lactum ring of amoxicillin. Further decarboxylation of these compounds led to the formation of a stereoisomeric mixture of AMX penilloic acids with m/z 323.

2.10 Ampicillin Recently, detection of ampicillin in aquatic environment is considered as an emerging pollutant because of their ecological toxicity even at low concentration [131,132]. Semiconductorbased photocatalysis has gained significant interest for the degradation of ampicillin compared to other reported processes [122,133]. ZnO/polyaniline nanocomposite was reported by Nosrati et al. for efficient photodegradation and removal of ampicillin in aqueous solution [134]. Construction of heterojunction, i.e., Bi2O3/BiOCl was reported for efficient photodegradation of ampicillin under solar illumination, when supported over graphene sand composite as well as on chitosan [135]. Improved photocatalytic activity of the heterojunction was due to efficient separation of the photogenerated electron holeepair as well as broad solar light absorption. Recently, Sharma et al. reported photocatalytic degradation of ampicillin using La/Cu/Zr trimetallic nanoparticles under solar light illumination [136]. The basic mechanism for degradation of ampicillin was found to be efficient separation of electronehole pairs under solar illumination. As a result, generation of hydroxyl and super oxide radicals occurred via reaction with dissolved oxygen, which subsequently caused degradation of ampicillin. Photocatalytic degradation pathways and characterization of intermediates were analyzed by LCeMS method. Ampicilloic acid (m/z 368) and diketopiperazines were identified as parent ions during photodegradation of ampicillin. The other fragmented compounds with different m/z (333.3, 209.2, 192.0, 190.0, 159.1, and 114.0) were also observed. Final mineralization of all the compounds into simpler inorganic ions and compounds took place via photocatalysis. Similar kind of different fragmentated ions were observed by Zhao et al. from ampicillin degradation using magnetically separable nanocomposite (Fe3O4@TiO2@Ag NCs), as shown in Fig. 7.9 [137].

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Figure 7.9 The molecular species corresponding to the different m/z ions during photocatalytic degradation of ampicillin by Fe3O4@TiO2@Ag NCs [137].

Chapter 7 Reaction intermediates during the photocatalytic

2.11 Naproxen Naproxen with concentration of ng/L up to mg/L in wastewater could cause severe toxic effects in the aquatic ecosystem [138e140]. Use of semiconductors such as TiO2, ZnO, C3N4, etc. has significantly improved photocatalytic degradation of naproxen under solar illumination [141e144]. Efficient visible light photocatalytic activity was reported by Ye et al. using SiO2@Au@TiO2 coreeshell nanostructure having tunable Au content (0.1 wt%) [145]. Studies with different radical initiator and quencher have confirmed that formation of reactive oxygen species (1O2, O2$ ) played a vital role for the photodegradation of naproxen [146,147]. A novel ternary photocatalyst was reported by Wang et al., where single atomedispersed silver and carbon quantum dots, co-loaded with ultrathin g-C3N4 (SDAg-CQDs/UCN), showed efficient photocatalytic degradation of naproxen [146]. 1.0 wt% of CQDs and 3.0 wt% of Ag decorated g-C3N4 exhibited complete degradation of naproxen under visible light irradiation. Efficient charge carrier separation via plasmonic Ag NPs, up-converted fluorescent properties of CQDs, and narrowed energy gap was responsible for improvement in photocatalytic activity. Formation of reactive oxygen species was responsible for photocatalytic degradation of naproxen as evident from ESR and RS scavenging experiments. The formation of reactive intermediates and photocatalytic degradation products of naproxen was identified by liquid chromatography with mass spectrometry (HPLCeMS/MS) and gas chromatograph mass spectrometer (GCeMS) by Wang et al. [146]. They reported photocatalytic degradation of naproxen was preferably following two primary pathways, including hydroxylation and decarboxylation (see Fig. 7.10). In pathway I, firstly electrophilic addition reaction occurred via reaction of _OH with naphthalene ring, resulting in the formation of hydroxylation products m/z 262 [148]. In pathway II, O2$ attacked at the most positive point charge C(22) atom, thereby cleavage of carboxyl resulted in the formation of carboncentered radicals T1. Subsequently, fragmented intermediates with m/z 184, 200, and 218 formed via further oxidation by O2$ and 1O2 [114]. Another fragment having m/z 202 formed when T1 reacted with _OH [116]. In pathway III, hþ attacked at the C(1) and resulted in the formation of m/z 158 via decarboxylation reaction. Finally, all the fragmented intermediates underwent ring opening mechanism to give corresponding products with m/z 134, 148, 178, and 208. Finally, all the intermediates were converted to CO2 and H2O.

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Chapter 7 Reaction intermediates during the photocatalytic

Figure 7.10 Possible transformation pathways of naproxen in the aqueous SDAg-CQDs/UCN solution under visible light irradiation [146].

2.12 Ketoprofen Ketoprofen (2-(3-benzoylphenyl) propionic acid) in waste and surface water causes undesired effects on living organism present in the environment [149,150]. Recent development of photocatalysis technology has been proven most efficient methods to degrade ketoprofen in wastewater resources [27,151,152]. Multiwalled carbon nanotubes decorated with anatase TiO2 was found to be efficient for the degradation of ketoprofen [153]. Photocatalytic degradation of ketoprofen was also reported by Djouadi et al. using heterogeneous photocatalyst (Bi2S3/TiO2eMontmorillonite nanocomposites) under simulated solar irradiation [154]. Excellent photocatalytic degradation of ketoprofen was observed under UVeVis irradiation using Bi2S3/TiO2(25/75)-Mt. Intermediates during photocatalytic degradation were analyzed by HPLCeMS technique. Intermediates with m/z 210 (3-ethylbenzophenone), 242 (3-(1droperoxyethyl) benzophenone), 228 (1-[3-(hydroxy-phenylmethyl)-phenyl]-ethanol), 226 (3-(1-hydroxyethyl)benzophenone), and 316 ((3-(1-hydroxy-1-phenylpropan-2-yl)phenyl)(phenyl) methanone) were identified. Formation of intermediates was explained via generation of reactive oxygen species (HO
, O2$ ).

Chapter 7 Reaction intermediates during the photocatalytic

3. Conclusions The photodegradation study of different emerging pollutants has been investigated using different nanostructured materials under solar irradiation. Formation of different intermediates was identified using HPLCeMS and other spectroscopy tools during photocatalytic degradation process. This chapter highlights fundamental mechanism as well as associated degradation pathways during photocatalytic degradation of the emerging pollutants.

Acknowledgments RKG acknowledges financial assistance from Department of Science and Technology (DST), India, through the INSPIRE Faculty Award (Project No. IFA-13 ENG-57) and Grant No. DST/TM/WTI/2K16/23(G). PK acknowledges financial assistance from the Indian Institute of Technology Kanpur, India, for the Institute Postdoctoral Fellowship Award.

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