Prsenjit Kr,Koml Shukl,Prtyush Jin,Govindsmy Sthiyn,Rju Kumr Gupt,b,＊

a Department of Chemical Engineering,Indian Institute of Technology Kanpur,Kanpur,208016,Uttar Pradesh,India

b Center for Environmental Science and Engineering,Indian Institute of Technology Kanpur,Kanpur,208016,Uttar Pradesh,India

Keywords:Emerging pharmaceutical pollutants Photocatalysis Nanomaterials Degradation mechanism

ABSTRACT The presence of emerging pharmaceutical pollutants at low concentration levels in the surface and ground water has caused a potential threat to the marine and human lives.The emerging pharmaceutical pollutants generally include analgesics and anti-inflammatories,lipid-lowering drugs,antiepileptics,antibiotics,andβ-blockers compounds.In recent years,various processes have been developed and advanced oxidation process is the most effective for decontamination of emerging pharmaceutical pollutants till date.Semiconductor based photocatalysis technology has recently received a great interest for the removal of new emerging pharmaceutical pollutants.This review article highlights the removal of emerging pharmaceutical pollutants especially through photocatalysis as well as recent progress using different nanostructures.Additional focus has been given over fundamental key dynamics processes of nanomaterials and degradation pathways of emerging pharmaceutical pollutants.Finally,this review concludes with the perspectives and outlook over future developments in photocatalysis technology for the degradation of emerging pharmaceutical pollutants leading to a solution for realworld in near future.

1.Introduction

To meet clean water demand and sustain aquatic life,removal of emerging pharmaceutical pollutants in wastewater is one of the serious concerns worldwide[1-3].Nowadays,detection of pharmaceutical pollutants in water is considered as one of the social concerns in aquatic environment due to unscientific disposal of used medicines,pharmaceutical side products,and wastewater from pharmaceutical industry and hospitals[4-6].Thus,in 21stcentury,threat to the integrity of our water resources from emerging pharmaceutical pollutants is demonstrated as one of the severe environmental problem worldwide.

In the environment,the commonly occurring pharmaceuticals include antibiotics,painkillers,lipid regulators,β-blockers and neuroactive compounds[7-9].Till date,there are different types of pharmaceutical compounds have been listed that exhibit potential risk to the aquatic life and human beings as they are gradually discharged into the environment depending on their mode of use in variety of applications(e.g.discharge of wastes from industry,medical and household products)[10-12].Detection of various pharmaceutical pollutants in the inlet and outlet water streams of various wastewater treatment plants confirm inefficacy of such treatment towards removal of many pharmaceuticals[13].Environmental risk assessment studies of these pharmaceutical chemical evaluate their acute(lethal)effects and,in some cases,study their long-term effects(reproduction).Most of the chemicals are generally found in aquatic system at very low concertation(ng/L)range.However,they exhibit several effects in human health like metabolic change,endocrine disruption as well as ecotoxicological effect in aquatic environment[14-16].Till date,wide variation of emerging pharmaceuticals chemicals were analyzed in aquatic environment at varying levels ranging from ng/L to mg/L[17,18].Many literature are reported over the presence of a variety number of potentially toxic pharmaceutical pollutants in the environment depending on their occurrence,fate and ecotoxicological effect[18-20].

Fig.1.Sources of different emerging pollutants in water.

Different techniques(e.g.sludge treatment,biological filtration,primary settling etc.)have been utilized as a conventional water treatment technology for potential removal of pharmaceutical pollutants[13,21].Tremendous efforts have been made to develop technologies like coagulation,biological,membrane filtration(activated carbon adsorption)for removal of pharmaceutical pollutants in waste water[22-25].Efficient removal of endocrine disrupting compounds(EDCs)using granular activated carbon columns was observed by Noutsopoulos et al.through adsorption mechanism[26].In another study,knappe et al.found that carbonaceous based adsorbents(activated carbon and carbonaceous resin)are more efficient for the removal of emerging pollutant(~200-900 ng/L)from lake water compared to zeolites[27].The problem with reported water treatment technology are high cost and low processing efficiency[13,28].Additionally,these developed methods have some limitations for practical purpose e.g.high processing time,toxic sludge generation,toxic byproduct formation etc.[29-31].However,the development of new technologies is essential for efficient removal of pharmaceutical pollutants for wastewater treatment.As recently suggested,advanced oxidation process(AOP)is considered as an effective water treatment technologies which involves formation of reactive oxygen species for the degradation of pollutants into nontoxic products,and may therefore be used for the removal of pharmaceutical pollutants[32,33].In this context,use of solar photocatalysis is highly beneficial as it significantly reduces the operating cost and makes AOPs highly beneficial for water treatment purpose[32,34-36].Heterogeneous photocatalysis proceeds via utilization of a semiconductor,where excitation of electron from valence band to the conduction band takes place under irradiation of the light[37,38].Simultaneously,photogenerated electron and hole react with the dissolved oxygen in water to generate different reactive oxygen species,which can degrade the pollutants[39-41].Application of a semiconductor for photocatalysis depends on band gap energy and facilitation of electron-hole pair separation upon absorption of photon[42].There should be a suitable overlap between photon energy and band gap of the semiconductor for the absorption of photon by semiconductor.Further,band gap of the semiconductor should be sufficiently enough so that generated electron-hole pairs migrate to the surface of semiconductors for redox reactions before recombination of electron-hole pair[43].A wide range of semiconductor photocatalysts e.g.ZnO,g-C3N4,TiO2,Fe2O3,Fe3O4,SnO2,Mn2O3have been developed for the wastewater treatment applications[44-51].Out of reported photocatalysts,TiO2enjoys efficient photocatalytic degradation of pharmaceutical pollutants due to various advantages e.g.low processing cost,non-toxicity and high environmental stability[43,52-54].TiO2-based solar photocatalysis has exhibited a significant potential for degradation of emerging pollutants e.g.trimethoprim,ofloxacin,enrofloxacin,clarithromycin,acetaminophen,diclofenac,caffeine,atenolol,thiabendazole and carbamazepine[55-57].TiO2immobilized titanium meshes were used by Rimoldi et al.to degrade wide variety of pollutants,as a single pollutant and in mixture under UV and simulated solar irradiation[58].Maldonado et al.developed solar photocatalytic device via decoration of TiO2on glass spheres and observed efficient degradation of a mixture of emerging contaminants[59].Other than TiO2,ZnO nanoparticles have been found to be an effective photocatalyst for photodegradation of pharmaceutical pollutant as reported by karaca et al.[60].Development of heterostructure(e.g.SnO2-BiOI)has been reported for excellent degradation of pharmaceutical because of improved charge separation and wide light harvesting ability[61].Therefore,utilization of nanomaterials and their suitable modification(tuning of charge separation,migration of charge to semiconductor surface and stability etc.)make them superior photocatalysts for the degradation of emerging pharmaceutical pollutants.In general,photocatalytic degradation of pharmaceutical pollutants by photocatalyst can be expressed as follows:

Fig.2.Proposed pathways of antipyrine degradation.(The asterisks indicate newly reported products)[82].

Fig.3.Possible photocatalytic reaction mechanism of acetaminophen over Bi-TNR under visible light irradiation[93].

In this review,we have discussed over the development of semiconductor based photocatalysts for effective degradation of emerging pharmaceutical pollutants from water resources.Here,we have also discussed key fundamental processes of nanomaterials and associated degradation pathways of emerging pollutants.The review also focuses on the future challenges and scopes of semiconductor based photocatalysts for degrading the emerging pollutants from environment,with more focus on innovative photocatalysis technologies for practical application in near future.

2.Occurrence and fate of the emerging pharmaceutical pollutants

Fig.4.The proposed degradation pathways of acetaminophen in Bi-TNR[93].

Fig.5.SEM images of the as-prepared samples:(A)pure BiVO4,(B)pure AgI,and(C,D)AgI(20 wt %)/BiVO4;typical TEM images of pure BiVO4(E),AgI(20 wt%)/BiVO4(F),and HRTEM images of the fresh(G)and the used(H)AgI(20 wt %)/BiVO4[104].

The emerging pharmaceutical pollutants basically are the pharmaceutical compounds that exhibit potential danger to the living organism in ecological system due to their presence in water resources,which are identified from ng/L toμg/L range[9,62-64].The wide variation of pharmaceutical compounds in aquatic environment commonly include anti-inflammatory,antidepressants,analgesics,antipyretics,antibacterial,beta-blockers,steroids drugs.The detection of these emerging contaminants in aquatic environment can be correlated with in-efficient treatment of wastage from pharmaceutical industry as well as wastage from household products and veterinary wastes.Fig.1 shows different routes for the contamination of emerging pollutants.Rosal et al.reported presence of several pharmaceutical pollutants e.g.diclofenac,indomethacin,ketoprofen,mefenamic acid etc.in urban water in ng/L level using GC-MS technique[62].Another excellent study was carried out by Rodil et al.where they detected nineteen emerging compounds in wastewater,surface and drinking water in Galicia(Spain)at median values higher than 0.1μg/L[65].Mohapatra et al.observed presence of pharmaceutical pollutants in sea water(India)due to inefficient treatment of wastewater by the treatment plants for complete removal of pharmaceutical contaminants[66].The presence of pharmaceutical pollutants demonstrates significant toxicity even at the very low level of concentration[67-69].These pollutants so-called endocrine disruptors(EDCs)can alter hormonal activity in the early stage of human life.In addition,pharmaceutical pollutants used for medicinal purpose,reach to the environmental via industrial effluents and water.As a result,these chemicals are prone to damage the soil,agricultural land and ground water[69-71].Quinn et al.also observed acute and chronic toxicity of pharmaceuticals(ibuprofen,naproxen,bezafibrate,gemfibrozil and carbamazepine)and categorized them as toxic compounds even at the low concentration(320μL mg/L)[72].Another potential danger about emerging pollutant in the aquatic environment is slowly development of antibiotic resistant strains in natural bacterial populations.In a recent survey,increased resistance of two natural bacterial strains in water was reported due to presence of several antibiotics in the effluent of a wastewater treatment plant in Australia[73].Thus,presence of pharmaceutical pollutant in water has a detrimental effect on the naturally occurring bacteria.Additionally,emerging pharmaceutical pollutants in fresh and ground water environment exhibits an adverse effect depending upon its toxicity,dose and duration of exposure.To prevent undesirable toxic effects of such pollutants,photocatalytic degradation of such emerging pharmaceuticals is highly demanding.In the next section,we will discuss development of various semiconductor based photocatalysts towards degradation of the pharmaceutical pollutants under light illumination.

3.Recent development of semiconductor photocatalysts for degradation of emerging pharmaceutical pollutants

Till now different semiconductor based photocatalysts have been widely used for degradation of emerging pharmaceutical pollutants.In this section,we will discuss over degradation of emerging pollutants via semiconductor based photocatalysts and degradation pathways for these pollutants.

Fig.6.(A)Photocatalytic degradation of tetracycline over AgI/BiVO4 samples.(B)Effect of different quenchers on the photocatalytic oxidation of tetracycline by AgI(20 wt %)/BiVO4.(C)Cycling test for the photocatalytic degradation of tetracycline(20 mg/L)by AgI(20 wt %)/BiVO4[104].

3.1.Antipyrine

Antipyrine(AP)is getting great importance as an emerging pollutant due to its availability in groundwater and surface water[74-76].Therefore,an extensive semiconductor based photocatalysis research has been carried out in order to degrade this compound efficiently.TiO2has been widely used for the degradation of pharmaceutical pollutant,i.e.antipyrine[77,78].Spinning disc reactor(SDR)using TiO2nanoparticles was used by Duran et al.for the photocatalytic degradation of antipyrine under UV light[78].Complete degradation was observed at pH 4 after 120 min with intact efficiency of the photocatalyst even after 10 cycles.Glass sphere coated with TiO2particles via sol-gel dip-coating technique demonstrated excellent degradation ability of antipyrine under UV light illumination[59].Doping of semiconductor was also found as an efficient strategy for degradation of emerging pharmaceutical pollutants as it significantly reduces the band gap of the semiconductor,leading to improvement in solar light absorption ability.Zr doped TiO2as synthesized by Belver et al.reported almost complete(90%)antipyrine degradation within 6 h via excellent charge carrier separation in presence of Zr[79].Development of heterojunction between ZnO and TiO2over a delaminated clay exhibited an improvement in antipyrine degradation due to efficient separation of photocarriers[80].Similar kind of improved photocatalytic activity for antipyrine degradation was also reported by Wang et al.using CoFe2O4/TiO2heterojunction[81].Generation of?OH,h+,O2-?radicals were reported as the main active species during photocatalytic degradation of antipyrine[80].Many studies have proposed different kind of intermediates during photocatalytic degradation of antipyrine[78,79,82].Generally,?OH radical attacks at the C-C bonds in the pentacyclic ring of antipyrine and results in the formation of different kind of hydroxylated products as shown in Fig.2.

3.2.Acetaminophen

Direct disposal of acetaminophen without prior treatment from pharmaceutical industry to the water(surface,ground)causes significant toxicity to the human life and aquatic organism and hence,considered as an emerging pollutant[83-86].Use of TiO2as a semiconductor photocatalyst was extensively used for degradation of acetaminophen under solar light illumination[59,87,88].Wu et al.observed 95%degradation of acetaminophen by TiO2nanoparticles using halide lamp source(250 W)for a period of 100 min[89].Sol-gel mediated synthesis of potassium peroxodisulfate(K2S2O8)doped TiO2by Lin et al.was reported efficient visible light degradation of acetaminophen[87].Under optimal conditions(catalyst:1 g/L,pH:9,T=295 K),100%acetaminophen degradation was achieved within 9 h by using 0.1 mM acetaminophen.TiO2supported over Zeolite also reported good photodegradation of acetaminophen under UV illumination[88].96.6% of degradation was attributed due to promoted charge separation under optimum TiO2loading(40 wt% and concentration 1.0 g/L)over the surface of zeolite.Additionally,catalyst was reported to exhibit excellent recyclability with negligible change in the degradation efficiency at the end of three cycles.Another investigation was performed by Lu et al.where KAl(SO4)2and NaAlO2doped TiO2reported 95%degradation of acetaminophen using blue LED lamp[90].Optimum degradation efficiency of acetaminophen was observed with the catalyst loading of 1 g/L,as the excess photocatalyst concentration would hinder light penetration ability.Photocatalytic degradation of acetaminophen was widely dependent upon pH of the medium with optimum efficiency at pH 6.9[90].At the lower and higher pH values than the optimum value,a decrease in photocatalytic activity was observed.At lower pH,surface become positively charged,which reduces formation of OH?in the medium.At higher pH,the electrostatic repulsion between TiO2surface and acetaminophen(pKa=6.9)decreases surface adsorption of acetaminophen due to surface being negatively charged[91,92].Detailed photocatalytic mechanism was explained by Fan et al.where excellent charge carrier separation by Bi-modified titanate nanomaterials(Bi-TNR)results in formation of reactive species(?OH,h+)mainly responsible for the degradation of acetaminophen(Fig.3)[93].The intermediates generated through photocatalytic degradation of acetaminophen has been reported in a number of literatures[93-96].At first,attack in the para position by hydroxyl radical(?OH)results in formation of benzoquinone,which further oxidizes to benzaldehyde and finally to benzoic acid(Fig.4)[93].Further degradation of benzoic acid leads to the formation of either acids(acetic,formic,oxalic and malonic acids)or alcohols(1-penten-3-ol,2-methyl,4-heptanol and 2,3-butadiol).Finally,all the intermediate products underwent complete mineralization leading to the formation of H2O and CO2.

Fig.7.Proposed photocatalytic degradation pathways of tetracycline[105].

3.3.Tetracycline

Presence of tetracycline in aquatic environment is highly concerned because of its ecological impact(carcinogenicity,toxicity)on the environment[97].Photocatalyst such as TiO2was used for the degradation of tetracycline by Reyes et al.under UV light illumination[98].Significant mineralization of tetracycline was observed using TiO2nanoparticles(0.5 g/L)under UV light illumination for a period of 120 min.Palominos et al.evaluated TiO2and ZnO for photodegradation of tetracycline under simulated solar light[99].Modified TiO2was also used effectively for the degradation of tetracycline.Modification of TiO2through the addition of multi wall carbon nanotubes by Ahmadi et al.was found to be more promising towards degradation of tetracycline when compared to TiO2only[100].Complete degradation of tetracycline(10 mg/L)was observed within 100 min for the composite(0.2 g/L)where,wt% of MWCNT:TiO2=1.5:1 at pH 5 when used under constant UV illumination.Degradation efficiency was improved due to efficient charge carrier separation of the photocatalyst,which facilitates more generation of electrons on the conduction band of TiO2[101,102].In another work by Wang et al.,C-N-S tridoped TiO2was reported efficient degradation and mineralization of tetracycline under solar irradiation[103].Visible light driven heterostructure i.e.AgI/BiVO4has an excellent impact towards degradation of tetracycline as reported by Chen et al.[104].AgI nanoparticles with diameters of 10-150 nm were decorated on unique dumbbell-shaped morphology of BiVO4to form the heterojunction(Fig.5).HRTEM analysis further confirmed well crystallinity of the nanoparticle present in the heterojunction.Optimized photocatalytic activity of the photocatalyst was observed with 1:4 mass ratio of AgI:BiVO4(Fig.6A).Almost complete removal of tetracycline was observed for the heterojunction,which was reported to be more efficient compared to their counterpart with different ratios under identical conditions.Dramatic enhancement in photocatalytic activity of such kind of heterojunction was attributed due to efficient separation of electron-hole pairs.In addition,study with radical initiator and quencher demonstrated the role of hole and superoxide radical in the degradation processes(Fig.6B).Catalyst was also found to be well stable under repetitive irradiation over several cycles as shown in Fig.6C.Such kind of intact stability and photocatalytic activity of AgI:BiVO4under repetitive irradiation is very useful for long term practical applications.Photocatalytic degradation of tetracycline leads to the generation of several intermediates as reported by Zhou et al.[105].The fragment of m/z of 431,416 and 400 were identified via reaction of tetracycline with h+.Another intermediate with m/z of 461 was generated from tetracycline on reaction with?OH.Further degradation by?OH leads to the formation of several intermediates with m/z of 477,475 and 495.Further,ring opening(via oxidation)leads to the generation of CO2,H2O and inorganic ions(Fig.7).

3.4.7α-ethinylestradiol(EE2)

EE2,a hormonally active chemical,is regarded as an emerging pollutant as it exhibits carcinogenic effects on the endocrine systems of wildlife and humans[106-108].There are a number of papers based on TiO2photocatalyst for the degradation of EE2[109-111].Superior photocatalytic degradation of EE2 by ZnO compared to titania was reported by Frontistisis et al.under solar illumination[112].The mechanistic pathways for the degradation of EE2 by ZnO are reported as follows:

Fig.8.(A)Photocatalytic degradation of EE2 and(B)rate constants for the degradation of EE2 by different nanocomposite under visible light irradiation.(C)Proposed mechanism for photocatalytic degradation of EE2 with Ag-AgCl/ZnFe2O4[114].

Both experimental and theoretical EE2 degradation were carried out by Xekoukoulotakis et al.using TiO2nanoparticle and predicted artificial neural network in modeling water treatment process[113].For the visible light mediated photodegradation of EE2,Ag-AgCl/ZnFe2O4nanocomposite was synthesized by Li et al.where UV reduction method was employed for the decoration of Ag nanoparticle over AgCl[114].Complete degradation of EE2 was observed within 240 min by the composite having pseudo rate constant(0.0183 min-1)compared to bare ZnFe2O4(0.0036 min-1)(as shown in Fig.8A and B).Improved photocatalytic activity of nanocomposite was ascribed via efficient charge carrier separation and better light harvesting ability(Fig.8C).Such kind of improved charge separation for better degradation of EE2 was also reported by Zhang et al.employing Ag/ZnO hollow spheres as catalyst under UV irradiation[115].TiO2coated reduced graphene oxide(RGO)nanocomposite was found to be efficient for the degradation of EE2 due to improved EE2 adsorption and reduced charge carrier recombination by RGO[116].In a recent report,visible light assisted photocatalytic degradation of EE2 was performed using Ag/AgCl@chiral TiO2nanofiber[117].In another work,Wei et al.demonstrates that the Sr/Ag-TiO2nanoparticles have high photocatalytic stability and excellent reusability up to four cycles(94%)[118].Photocatalytic degradation pathways for EE2 have been reported by many research groups[110,118].Photogenerated electron and hole pair at the photocatalyst results in the formation of various reactive species like such as?OH,O2-?,and h+.Next,attack of these reactive species over the EE2 generates semiquinone(oxidized form of EE2),which further degrades into lower molecular weight acids and other ring-cleavage compounds(Fig.9)[113].

3.5.Carbamazepine

Recently,several studies have been carried out on occurrence of carbamazepine in surface water at high concentration due to lack of proper treatment[119-121].Most of the investigations have been carried out for the degradation of carbamazepine using TiO2because of its economic viability and nontoxicity[122,123].Surface modified TiO2was found to be efficient photocatalyst for removal of carbamazepine.Pt modified TiO2was found to be efficient for the degradation of carbamazepine under visible light illumination compared to bare one as reported by Choi et al.[124].Here,tiny size Pt nanoparticles act as a sink in order to enhance charge-separation which facilitates generation of more reactive oxygen species[125].Other than TiO2,photodegradation of carbamazepine was studied by BiPO4with tuned morphology and crystallinity as reported by Xu et al.[126].Change in morphology(disordered to order)was observed with the change in reaction time(1 h-72 h).High crystallinity of BiPO4was prepared at 180°C for 72 h,while further increment in temperature leads to the formation of distorted crystal.Ordered morphology of the BiPO4photocatalyst(180°C,72 h and hydrothermal)was found to be more efficient for the degradation of carbamazepine compared to unordered one under UV light illumination.This kind of morphology tuned photocatalytic activity was arisen due to improved e--h+separation at the photocatalyst[127].Another morphologically tuned photocatalytic activity was observed for carbon loaded TiO2sample,where excellent photocatalytic activity for rice grain shaped C-TiO2was attributed due to high surface area and surface active(001)plane(Fig.10)[128].Recently,Bi(0)doped BiOCl was utilized by Dandapat et al.for the degradation of carbamazepine under solar illumination[129].GQDs loaded BiVO4heterostructure was fabricated by Tang et al.for improved removal of carbamazepine under solar light.Efficient photocatalytic activity for 1 wt% GQDs/BiVO4was attributed to better charge carrier separation at the interface[130].For practical application purpose,N-doped TiO2film was utilized as a membrane in a continuous flow reactor and significant photodegradation of carbamazepine was observed under solar illumination[131].Photocatalytic degradation of carbamazepine proceed via two pathways(hydroxylation and bond cleavage)as reported by Tang et al.[130].In both the pathways,several intermediates were formed,where molecular weight of 252 and 193 were formed via hydrogen abstraction,hydroxylation of heterocycle ring and cleavage of the amide group,respectively.Further ring opening of the fragments under attack by?OH lead to the formation of smaller molecular weight intermediates and mineralized to CO2and H2O at the end[132-134].

Fig.9.The proposed reaction pathways for the photocatalytic degradation of EE2 in UPW[113].

Fig.10.Degradation of carbamazepine by different morphologies of C-TiO2 NCs[128].

Recently,sulfamethoxazole has been detected at a very low concentration levels as a micropollutants in water and considered as an emerging pollutant because of its little information about toxicity in the aquatic environment[135-137].TiO2was chosen as an effective and economically sustainable photocatalyst towards degradation of sulfamethoxazole[138].Carbonaro et al.designed a benchtop continuous-flow reactor for degradation of sulfamethoxazole using TiO2photocatalyst[139].54% degradation of sulfamethoxazole was observed with a pseudo-first-order rate constant(0.58±0.06 h-1).There was another report,where TiO2nanoparticles at different annealed temperature were used for the degradation of sulfamethoxazole under UV illumination[140].Efficient photocatalytic activity for the TiO2nanoparticles upon calcination at 400°C was observed than those synthesized at 500°C.Improved photocatalytic activity at 400°C was attributed to the compact structure,adsorbent losing and decreased rutile/anatase ratio.Role of metal nanoparticle(Au,Pt)over photodegradation of sulfamethoxazole was investigated by Baba et al.where metal nanoparticles were sensitized over TiO2[44].Presence of both the metallic nanoparticles resulted in improved photocatalytic activity under UV illumination(Fig.11I).Excellent photodegradation ability was attributed to charge carrier separation via electron trapping by plasmonic nanoparticles(Fig.11II).Single wall carbon nanotubes(SWCNTs)loaded TiO2nanoparticles were found to be efficient for degradation of sulfamethoxazole by Murgolo et al.under both UV and solar light[141].Recently,multi-walled carbon nanotubes(MWCNTs)decorated co-catalyst WO3was found to act as an efficient heterojunction for sulfamethoxazole degradation(81.9%in 3 h)under visible light[142].Such kind of boost in photocatalytic activity in heterojunction was ascribed due to better solar absorption,narrower band gap and enhanced charge carrier separation ability[143,144].In another interesting work,a membrane decorated with novel heterojunction photocatalyst i.e.mpg-C3N4/TiO2was developed for degradation of sulfamethoxazole under solar light[145].Highest removal efficiency was observed by the membrane(with 1%mpg-C3N4/TiO2loading)over 30 h of consecutive irradiation because of extended surface area and enriched electron transfer rate.Such kind of membrane based photocatalyst can be used for industrial application due to good stability of catalyst before and after irradiation.Formation of different intermediates from sulfamethoxazole during photocatalysis was analyzed by Zhou et al.[142].First,hydroxylation of sulfamethoxazole(via attack by?OH)leads to the formation of mono-or di-hydroxyl derivatives.Further,cleavage of the S-N bond along with bond cleavage of the benzene ring and isoxazole ring generates different low molecular weights products(Fig.12).Finally,oxidation of all the intermediates took place to form mineral acids,CO2and H2O.

Fig.11.(I)Photodegradation of sulfamethoxazole over TiO2 and nanocomposite Pt and Au-TiO2 coatings as a function of UV light(365 nm,2 mW/cm2)irradiation times(a).Example of the measured absorption spectra of the SMX solution for different irradiation times under UV-A light and over Au-TiO2 coating(b).(II)(a)Pt and Au to investigate the benefit of a noble metal layer on the charge recombination reduction and(b)TiO2-Au to investigate the benefit of a plasmonic layer and extend the photocatalytic properties to the visible range[44].

Fig.12.Proposed SMX photocatalytic degradation pathways by WO3-CNT composites[142].

Fig.13.(A)(a)Pyrex glass plate coated with TiO2,(b)fiberglass sheet coated with TiO2,(c)fiberglass sheet coated with dye-sensitized TiO2.(B)Effect of photocatalyst on degradation rate of DCF.Experimental conditions:Q=1.15×10-5 m3/s,VL=4×10-4 m3,I=45 W/m2,[DCF]o=25 ppm,T=298 K,O2 saturated[151].

3.6.VII.Diclofenac

Recently,diclofenac is found to be one of the most frequently detected emerging pollutant in wastewater and urban water ranging from 0.1 to 1.5μg/L[146,147].Photocatalysis has been considered as a key process in order to successfully detoxify diclofenac from waste water[148].Calza et al.reported photocatalytic cleaning of diclofenac by TiO2nanoparticles under solar light illumination[149].Here,TiO2(200 mg/L)mediated complete degradation of diclofenac was observed within 1 h,where 15 mg/L was used as initial concentration of diclofenac.Detailed kinetic investigation of diclofenac was reported by Rizzo et al.using TiO2nanoparticles under UV light illumination[150].A pseudo-first-order kinetic was found with low concentration of diclofenac,while it follows pseudo-second order kinetics with high concentration.Another efficient dye-sensitization strategy was adopted by Ray et al.to improve visible light activity for diclofenac[151].Change in color upon dye sensitization for TiO2immobilized glass plate is shown in Fig.13A.Excellent visible light photocatalytic activity(see Fig.13B)is attributed to enhanced light harvesting ability upon dye sensitization along with easy electron injection from LUMO of dye to CB of TiO2.N,S,and C-doped ZnO particles were also reported for efficient photodegradation of diclofenac under UV light because dopants can retard the recombination process and thereby,fascinating charge separation[152].Very recently,visible light driven photocatalyst CQDs/BiOCOOH(BiOCOOH modified by CQDs)was designed by Chen et al.for the photocatalytic degradation of diclofenac[153].2.0 wt% CQDs/BiOCOOH composites exhibited almost complete degradation of pollutant within 1 h,which was significantly higher than its counterparts.Such kind of efficient interface(as observed in CQDs and BiOCOOH)was well documented in reported literature that can alter overall photocatalytic efficacy[154-156].The main reactive oxygen species(h+and O2-?)was confirmed for the photodegradation of diclofenac.

The photocatalytic intermediates of diclofenac were reported by Mugunthan et al.using TiO2-SnO2mixed oxide under ultraviolet irradiation[157].Photodegradation of diclofenac was proceeded mainly via three pathways:(i)Decarboxylation,(ii)hydroxylation and(iii)dichlorination(Fig.14).At first,diclofenac hydroxylation results formation of a new compound with m/z 312.This undergoes decarboxylation followed by dichlorination to form compound with m/z 197.Next,removal of-CHO from m/z 197 to form new compound with m/z 169.In another path,hydroxylation was followed by chlorination to form fragments like m/z 250,127 and 140.Fragment(m/z 242)was formed via hydroxylation of diclofenac followed by loss of HCl.Similarly,m/z 151 and m/z 163 was obtained via cleavage of the C-N bond after hydroxylation of diclofenac.

3.7.Ibuprofen

Occurrence of ibuprofen as an emerging pharmaceutical pollutant in surface and waste water ranging from ng L-1toμg L-1level is due to its widespread use in the pharmaceutical industry[158,159].Photocatalytic degradation of ibuprofen is reported adequately in literature using TiO2based photocatalyst[160-162].Photocatalytic degradation of Ibuprofen(600 mg/L)was studied by Jallouli et al.using TiO2(1 g/L)under UV light illumination[163].Photogenerated electron-hole pair upon excitation of TiO2nanoparticles was main driving force for the degradation of ibuprofen via formation of reactive oxygen species(ROS)[164,165].Braz et al.reported total removal of ibuprofen(5 mg/L)in less than 60 min of UV-irradiation using 20 mg/L of TiO2at pH 7.8[166].Visible light mediated photodegradation of ibuprofen was reported by Trari et al.using α-(Cu,Fe)2O3catalyst prepared by a hydrothermal method[167].Maximum photocatalytic activity was observed using photocatalyst and ibuprofen concentration of 0.25 g/L and 200 mg/L,respectively at pH~11 via efficient generation of reactive oxygen species(?OH,h+)as confirmed by other reports[168,169].Graphene quantum dots(GQDs)decorated AgVO3nanoribbons(with length of 1.2-4μm and width of 100-300 nm),i.e.GQD/AgVO3heterojunctions,were developed by Tang et al.for better visible light activity towards degradation of ibuprofen[170].3 wt%-GQD/AgVO3demonstrated complete degradation of ibuprofen within 180 min due to improved light harvesting ability and photogenerated charge carrier separation[171,172].The developed photocatalyst was reported to have good recyclability even after four consecutive cycles.Recently,visible light active g-C3N4/Bi2WO6heterojunction was developed by Wang et al.and heterojunction demonstrated 96.1%degradation efficiency of ibuprofen within 1 h compared to its counterpart[173].Improved photocatalytic activity was explained from strong interfacial interaction through the interface which favors the photoinduced charge separation.Similar kind of improved charge carrier separated mechanism was investigated by Lim et al.using visible light active Ag-AgBr/TiO2composite(see Fig.15)[174].The reactions during photocatalytic degradation of ibuprofen by Ag-AgBr/TiO2composite are as follows:

Fig.14.The probable degradation mechanism of diclofenac[157].

The intermediates during photocatalytic degradation of ibuprofen were proceeded through different hydroxylation and decarboxylation processes that lead to the formation of fragments with different m/z(Fig.16)[175,176].Final products after the degradation of all the fragmented species were formed as CO2and H2O.

3.8.Ketoprofen

Fig.15.Proposed photocatalytic mechanisms for ibuprofen degradation over the Ag-AgBr/TiO2 under visible-light LED irradiation.The standard potentials of several redox couples associated with the photocatalytic reactions are shown on a scale on the right scale[174].

Fig.17.Possible transformation pathways of naproxen in the aqueous SDAg-CQDs/UCN solution under visible light irradiation[195].

Now a days,presence of ketoprofen in waste water treatment plant are frequently detected and considered as an emerging pollutant in the environment[177,178].Photocatalytic degradation of ketoprofen has been widely investigated by Martínez et al.using heterogeneous photocatalyst MWCNT-TiO2[179].Enhancement in photocatalytic activity via heterojunction was attributed to reduced recombination between photogenerated e-/h+pairs[180].Photodegradation of ketoprofen followed pseudo first order kinetics and photogenerated electron and hole of the photocatalyst are responsible for the photocatalysis via ROS generation[37,181,182].Recent study was reported on Bi2S3/TiO2-montmorillonite(Bi2S3/TiO2-Mt)photocatalysts for the photodegradation of ketoprofen in a photochemical reactor by Djouadi et al.under solar irradiation[183].Complete degradation of ketoprofen was observed within 120 min using optimum loading of Bi2S3on TiO2(Bi2S3/TiO2(25/75).Electron and hole generated from the photocatalyst produced free radicals(O2-?and OH?),which eventually degraded the ketoprofen.Photocatalytic degradation pathways of ketoprofen were reported by a number of literature[183-186].Formation of different fragmented intermediates(m/z:210,242,228,226 and 316)were detected due to formation of reactive speciesby the photocatalyst under illumination.

Fig.18.Photocatalytic activity of NC and NCTs,pH=5.0,solid to liquid(s/l)ratio=1.5 g L-1.(◆)NC with UV,(?)irradiation of TiO2,(＊)5 wt%TiO2 NCT with UV,(Δ)10 wt% TiO2 NCT with UV,(■)25 wt% TiO2 NCT with UV,(+)50 wt% TiO2 NCT with UV[201].

Table 1Calculated kapp,EEO andΦvalues for the photodegradation of MEF using various NCTs[201].

3.9.Naproxen

Naproxen commonly considered as an anti-inflammatory drug,has been detected in water as an emerging pollutant because it exhibits an adverse impact on the environment[187-189].Semiconductor photocatalysts such as TiO2has been actively investigated for the mineralization of naproxen in wastewater[190,191].ZnO-TiO2nanopowder has shown great potential for the degradation of naproxen under UV light[192].Ksibi et al.reported that photodegradation of naproxen preferably follow pseudo first order kinetics with rate constant 0.0095 min-1.Visible light active nanosized NiS and NiO photocatalysts were also reported to have good degradation efficiency(50%)as observed by Torki et al.[193].Cerium doped nickel hydroxide nanosheets were synthetized via hydrothermal exfoliation for degradation of naproxen under UV light[194].Excellent photocatalytic activity after Ce doping(5 wt %)was explained because of reduction in recombination of photoinduced charge carriers.Recently,novel ternary photocatalyst i.e.silver and carbon quantum dots,co-loaded with ultrathin g-C3N4(SDAg-CQDs/UCN)was synthesized by wang et al.to improve broad band absorbance over entire range of solar spectra[195].1.0 wt% of CQDs and 3.0 wt% of Ag loaded photocatalyst(1 g/L)exhibited optimum photocatalytic activity(complete degradation of naproxen(4 mg/L)within 25 min)under visible light.Enhanced photocatalytic activity by heterostructure was explained via efficient charge separation and improved solar light harvesting ability.Photodegradation of naproxen was proceeded via formation of reactive oxygen species(1O2,O2-?).Formation of different intermediates(as shown in Fig.17)during photocatalytic degradation of naproxen were analyzed by Wang et al.[195].In pathway I,?OH radical attacks at the naphthalene moiety of naproxen and leads to formation of hydroxylation products m/z 262.In pathway II,radical attack leads to the formation of m/z 202,200 and 184,respectively[196].In pathway III,h+attack at naproxen and results in formation of m/z 158 via decarboxylation.Finally,ring opening of all the products from pathways I-III causes formation of lower molecular weight products i.e.m/z 134,148,178,and 208,which were finally oxidized to CO2and H2O.

Fig.19.(A)Photocatalytic activities of P25 TiO2,NH2-MOFXII@30Sm2O3(a),NH2-MOFXII@30ZnO(b),NH2-MOFXII@(c),NH2-MOFXII@m2O3-ZnO(d),NH2-MOFXII@-ZnO(e),NH2-MOFXII@-ZnO(f),NH2-MOFXII@(g),and NH2-MOFXII@(h)NCPs for AMX degradation under visible-light irradiation and dark condition.(B)UV-visible absorption spectra of AMX after different irradiation times during the degradation process using NH2-MOFXII@-ZnO NCPs after 180 min[217].

3.10.Mefenamic acid

Non-steroidal anti-inflammatory drug mefenamic acid(MEF,dimethylphenylaminobenzoic acid)is considered as an emerging pollutant as its presence in water is proven to be a significant impact on aquatic organism[197-199].Photocatalytic degradation of mefenamic acid was reported by Khalaf et al.using TiO2nanoparticles supported on borosilicate tubes(cut-off 290 nm)[200].They observed potential improvement in photocatalytic activity with no significant loss in photocatalytic activity of the photocatalyst,which is useful for potentially large scale application.Similar kind of improvement for the photodegradation of MEF was observed by Rathod et al.where they immobilized TiO2nanoparticles(5,10,25 and 50 wt%)on nanocellulose[201].Efficient degradation of MEF(90% in 160 min)was observed for TiO2supported on nanocellulose(10 wt%TiO2)(Fig.18).Table 1 shows detailed kinetic parameters of MEF using various nanocomposites.The enhancement in photocatalytic activity was explained due to effective interaction between-OH group of nanocellulose and TiO2,which promotes significant charge carrier separation.In order to harvest the maximized solar light during photocatalysis,hybridized CuO-ZnO supported onto clinoptilolite nanoparticles(NCP)was reported by Shirzadi et al.for the photodegradation of mefenamic acid under Hg light illumination[202].Excellent photocatalytic activity for heterojunction in compared to counter part was reported due to efficient electron-hole separation via transfer of electron from CB of CuO to CB of ZnO,due to proper band alignments[203,204].

Fig.20.Schematic of possible mechanism for photocatalytic degradation of AMX for the separation of photogenerated electron-hole pairs and the production of?O2-and?OH by NH2-MOFXII@ NCPs under visible-light irradiation[217].

Fig.21.Photodegradation pathways of AMX in the SCMPR[218].

3.11.Amoxicillin

Amoxicillin(AMX)is also a penicillin-type antibiotic medicine and is extensively used for the treatment of various bacterial infections like dental infections,chest infections and other infections(ear,throat and sinus).The detection of amoxicillin in water is considered as an emerging pollutant because of its presence causes several health effects to the aquatic life[75,205].ZnO,TiO2were widely used for the photodegradation of amoxicillin under solar illumination[206-209].Activated carbon supported TiO2nanoparticles was effectively used for the degradation of amoxicillin(89%)within 120 min under UV illumination[210].Martínez et al.investigated the oxidative degradation of amoxicillin using ND-TiO2(synthesized by pristine nano-diamond insertion in TiO2)under UV/Vis illumination[211].The complete photocatalytic degradation of amoxicillin(rate constant 83.3×10-3min-1)was achieved within 60 min using TiO2concentration of 1 g/L.Formation of various reactive species(such as HO?,O2-?and/or,HOO?radicals)and photogenerated holes(h+)were mainly responsible for photocatalytic degradation process as confirmed via using different scavengers for holes and radicals,respectively[35,212,213].Surface plasmon resonance(SPR)enhanced photodegradation of amoxicillin was reported by Leong et al.using Ag/TiO2photocatalyst[214].3.0 wt% Ag/TiO2was reported to achieve the highest efficiency of 63.48%.The depletion of charge carrier recombination and enhanced solar harvesting ability were responsible to get maximized efficiency[215,216].Recently,novel nanocomposite NH2-MOF@yxSm2O3-ZnO(x and y represent weight percentages of Sm2O3/ZnO and Sm2O3-ZnO/MOF)was synthesized by Abazari et al.for AMX degradation under visible light[217].Optimal photocatalytic activity was observed by NH2-MOFXII@within 3 h as shown in Fig.19.Excellent degradation ability and good recyclability(up to 5 cycles)was ascribed via formation of efficient heterojunction as depicted in Fig.20.Here,favorable charge separation via trapping electron by Sm2O3facilitates more generation of ROS(?O2-and?OH),which eventually degrade AMX.Photocatalytic degradation of amoxicillin was proposed by Li et al.through Q-TOF LC/MS technique(Fig.21)[218].In pathway I,hydroxylation leads to the formation of different fragments such as m/z 382.1,219.2 and 165.1.In pathway II,hydroxyl radical attack was followed by cleavage of amine and carbonyl group and formed m/z 165.1,219.2.Further attack by hydroxyl radicals enable them to degrade these intermediates to CO2,water,and inorganic ions[219,220].

3.12.Norfloxacin

Norfloxacin is an antibiotic from the fluro-quinolones groups and is widely used for the treatment of urinary tract infections.Now a days,presence of norfloxacin in wastewater(specially hospital)contains high concentration(~100μg/L)and is considered one of the emerging pollutants in aquatic environment[221].TiO2/Ti film was reported by Sayed et al.towards degradation of norfloxacin in various natural water matrices[222].Reduced-TiO2was reported as another alternative visible active photocatalyst for the degradation of norfloxacin[223].They observed that h+is the main active species towards degradation of norfloxacin,while OH?plays minor role as confirmed by EPR(Fig.22 and Table 2).Another visible-light active carbon doped TiO2was reported by Chen et al.to degrade norfloxacin effectively[224].Development of heterojunction was another approach for enhanced photodegradation efficiency via maximizing solar harvesting ability.Another excellent work was demonstrated by Zhou et al.where plasmonic triangular silver nanoplates(T-Ag)exhibited remarkable visible light activity for the degradation norfloxacin,when decorated over ZnO nanoflower[225].Recently,a p-n heterojunction(BiOCl-BiVO4)demonstrated good visible light active photocatalyst for almost complete removal of norfloxacin within 1 h[226].This phenomenon was explained via formation of efficient interface that facilitates separation of photoinduced carriers.Very recently,a novel Ag deposited BiPO4/BiOBr/BiFeO3ternary heterojunction was designed to harvest maximum solar spectrum[227].Superior photocatalytic activity under entire solar range was observed by the ternary nano-heterostructure because of improved charge collection.Fabrication of Ag2O/TiO2

Fig.23.Proposed degradation pathways of norfloxacin on TiO2-Xunder visible light[223].heterojunction on Zeolite composite reported an unprecedented photocatalytic performance towards degradation of norfloxacin because of higher photoinduced charge separation efficiency[228].Photodegradation of norfloxacin was reported by kumar et al.,where?OH,h+,attack on norfloxacin leads to different fragmentated intermediates,which further undergo degradation to form simpler aliphatic compounds and inorganic minerals[227].HPLC-TOF-HRMS technique was utilized by Yang et al.to detect different intermediates upon degradation of norfloxacin due to loss of ammonia,CO2,and water molecules(Fig.23)[223].

3.13.Triclosan

Triclosan commonly used as an antibacterial and antifungal agent,is now considered as an emerging pollutant as it exhibits endocrine disrupting and potentially carcinogenic effects[229-231].Semiconductors based photocatalysts have been widely used to degrade and mineralize this compound in presence of solar light[232-235].TiO2mediated degradation of triclosan was reported by Rafqah et al.under UV illumination[236].Complete removal of triclosan(4.5 mg/L)was observed in 1 h and 3 h for P-25(mixture of anatase and rutile)and anatase TiO2(1 g/L),respectively.Another investigation by García et al.where immobilized TiO2with loading of 0.335 g/L(0.5 mg/g glass sphere)on glass spheres(?=6 mm)demonstrated good photocatalytic activity towards degradation of triclosan[59].Triclosan was removed more than 90%in 52.65 min,when triclosan(100 ppb)was flown in a cylindrical reactor filled with TiO2under xenon arc lamp(λ>300 nm)irradiation.Similar immobilization of TiO2over volcanic porous stones as substrate successfully degraded 50%of triclosan(12 mg/L)in a continuous reactor operation[237].Tailored photocatalytic activity for triclosan was reported for immobilized TiO2on single wall carbon nanotubes(SWCNTs)upon irradiation by solar light[141].Other than TiO2,unique morphology of ZnO(tetrapod shape)was found to be promising for complete degradation of triclosan compared to commercial ZnO due to high surface to volume ratio[238].Rinc'on et al.reported improved photocatalytic degradation activity for triclosan using ZnO modified GO[239].Almost complete degradation of triclosan was observed by ZnO/GO(0.5%)under visible light with high pseudo first-order rate constant.Efficient photocatalytic activity was reported due to improved photogenerated electron-hole pairs separation.Other than ZnO and TiO2,meta-stableβ-Bi2O3was reported to exhibit good visible light photocatalytic activity towards degradation of triclosan[240].Another visible light driven degradation of triclosan was reported by Dai et al.using Ag-Ti-Si ternary modifiedα-Bi2O3nanoporous spheres(TMBS)[241].Excellent visible photocatalytic activity was reported due to generation of reactive species(?OH,1O2and O2-?)as detected via ESR spin-trap technique.Photocatalytic degradation intermediates of triclosan was analyzed by Constantin et al.where?OH attack on triclosan led to the formation of 4-chlorocatechol and ortoquinone[242].In another pathway,direct interaction of photogenerated electrons with triclosan generates 2,2′-oxibis(5-chrolophenol)and 2-chloro-5-(4-chlorophenoxy)benzene-1,4-diol.Further,degradation of all the compounds result in the formation of carboxylic acids.Similar kind of photodegradation intermediates of triclosan was reported by Yin et al.through photogenerated electron,hole and superoxide radicals as shown in Fig.24[243].

3.14.Atenolol

Atenolol commonly known as a beta-blockers,has been frequently detected in surface water(μg-ng/L)and exhibits acute toxicity to the human beings[244].TiO2as a photocatalyst,was most widely reported for the degradation of atenolol[245,246].For practical application,mesoporous TiO2was reported advantageous over commercial P25 because of easy recyclability and separation from water[247].In order to harvest visible light,supported TiO2was reported as an efficient photocatalyst for degradation of atenolol.Graphene oxide decorated TiO2was found to be an excellent visible light active material for atenolol degradation compared to bare TiO2[248].Ozone mediated efficient atenolol degradation was reported by Liao et al.using Ag modified TiO2micro-tube[249].Here,ozone facilitated generation of?OH radical via Ag nanoparticle,which acted as an electron extractor.Other than TiO2,ZnO nanowire were also found to be beneficial for complete atenolol degradation under UV excitation[250].Stability and recyclability of the photocatalyst are very important for practical wastewater treatment applications[251,252].Recently,BiOCl nanosheets synthesized by Mao et al.demonstrated 100% atenolol degradation within 60 min under solar illumination with good reusability(>10 cycles)[253].Atenolol degradation preferably followed pseudo first order kinetics and?OH was primary responsible for the degradation process[254].Identification of different intermediates generated during photocatalysis of atenolol was carried out by Ji et al.via HPLC-MS/MS technique(Fig.25)[255].In all the pathways,?OH attack at atenolol followed by ring opening leads to generation of several low molecular weight carboxylic acids at different retention time as shown in Table 3.

Fig.24.Intermediates identified by LC-MS 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[243].

3.15.Oseltamivir phosphate

Fig.25.Possible photocatalytic degradation pathways of atenolol in illuminated aqueous TiO2 suspensions[255].

Table 3Identification of carboxylic acids arising from photocatalytic degradation of atenolol[255].

Oseltamivir phosphate(OP),an antiviral drug,known by its trade name Tamiflu,is used for medication during pandemic influenza outbreak[256].This drug has a potential of reaching at a significantly high concentration level in water bodies under an influenza epidemic and may cause development of antiviral drug resistant viruses[104,257].Wu et al.performed photocatalytic degradation of OP by using TiO2anchored on carbon sphere(CS)under the visible light illumination[258].OP(5 ppm)was degraded~40% by TiO2@CS(400 mg/L)under 400 min.Improved visible region absorption ability was attributed to existence of oxygen vacancy within the TiO2.Excitation of electron of TiO2to the oxygen vacancy under visible light further facilitated the charge separation due to formation of efficient interface upon contact with CS.Interfacial contact and amount of TiO2loaded on CS was reported to be key parameters in order to get better photocatalytic activity of TiO2@CS.Wang et al.has shown that TiO2(P25)can sufficiently degrade OP(24μM)completely in 90 min under UV light irradiation[259].Rate kinetics for the photocatalytic degradation was best fitted with pseudo-first-order kinetics(k=0.040 min-1).Variation of the catalyst loading has shown an increase in the degradation of OP with increase in the catalyst loading up to an optimized amount.Increased contact between catalyst active sites and pollutants had increased the degradation,however,after an optimized loading further increase led to light scattering.Performance of photocatalytic efficiency of different semiconductors towards degradation of different emerging pharmaceutical pollutants is summarized in Table 4.Further investigation revealed that h+and?OH were key active species during photocatalytic degradation of OP.Possible degradation of OP was reported by an attack of?OH radical(Fig.26)to produce different fragmented derivatives having m/z=329,345,361 and 377[259].In another pathway,?OH attack to the hydroxylated OP derivatives results fragments of m/z=261,277.Attack of?OH over OP may also result in substitution of ethoxycarbonyl group by-OH derivative,further leading to cleavage of ethylpropoxyl and amino groups and producing keto derivatives.

4.Limitations and future scope

This review discusses use of different semiconductor based photocatalyst for degradation of emerging pollutants along with key role of reactive species and photodegradation pathways of the pharmaceutical compounds.Despite so far development in this direction,there are still some fundamental and crucial issues that need more attention for further research.Semiconductor like TiO2,ZnO are widely used for the photodegradation of pharmaceutical pollutants under UV illumination.Photocatalytic pharmaceutical degradation of most of the photocatalyst reported to date remain unsatisfactory.Therefore,various strategies have been carried out in this direction and further improvement is necessary to get overwhelming photocatalytic activity(Fig.27).Although most of the semiconductors suffer due to only UV excitation,several efforts e.g.low band gap semiconductor coupling,doping,dye sensitization,polymer sensitization,immobilization of metal nanoparticles have been carried out for better solar energy harvesting in this direction.Utilization of NIR and IR light is still an open area for the degradation of emerging pharmaceutical pollutants as~52% of solar spectra is still unutilized.Utilization of low band gap coupling semiconductor for NIR photocatalytic application remains unexplored in this area.Alternation of band gap engineering with suitable different band gap coupling and its charge carrier dynamics at the interface of the heterostructure has proven to be an effective strategy by many researchers for better solar energy harvesting application.Although,more studies are required to understand the link between the charge transfer dynamics with photocatalytic ability of the photocatalyst.In thisdirection,theorical calculation of different charge state can be helpful for better understanding of heterostructure formation.It has been reported by many researchers that surface morphology and crystallinity play an important role for the photocatalytic degradation of emerging pollutants.More focus on design of novel and more effective photocatalysts is necessary and correlation needs to be established with morphology/crystallinity of samples along with different parameters e.g.light trapping,charge separation and pollutant adsorption ability.In most of the cases,photocatalytic activity of the photocatalyst were performed for a single pharmaceutical pollutant.It is necessary to explore the role of photocatalyst in degradation of different pharmaceutical pollutants in order to establish the presence of any connection within the properties of catalysts surface and substrate molecules.Examination of reaction kinetics and the role of several parameters e.g.pH,temperature etc.are studied by many research groups.However,applicability of such photocatalyst in complex water system,like in the presence of other dissolved metal ions,salts,dyes require detailed investigation.At present,most of the researchers have often focused on the laboratory scale without considering relevant and necessary aspects for real-world applications.Therefore,effect of photocatalyst for real water system(e.g.presence of metal,NOM,dissolved solids etc.)still needs to be explored towards degradation of emerging pharmaceutical pollutants.Special attention should be given to assess the applicability and efficiency of the prototype reactor in practical purpose before further development and commercialization studies.

Table 4Photocatalytic efficiency of different semiconductor towards degradation of emerging pharmaceutical pollutants.

5.Conclusions

Fig.26.Proposed pathways for photocatalytic degradation of OP by P25[259].

This review article has highlighted the development of nanomaterials for degradation of emerging pharmaceutical pollutants under light(UV/Vis/Solar)illumination.The unique properties of nanomaterials(hierarchical structure,surface porosity,charge carrier transport and separation)and their convergence explains great opportunities to revolutionize wastewater treatment from emerging pharmaceutical contaminants.The catalytic pathway and efficacy of the developed photocatalyst are critically reviewed.Unique role of surface morphology that influence catalyst loading and light trapping ability has been discussed.Tailoring of semiconductor band energy for effective charge carrier separation and transport is also elaborated in this review.Further research is needed in future for practical wastewater treatment application as most of the works discussed here were performed at lab scale under several limitations.We have also focused on the fragmentation pathway of the emerging pollutants during photocatalysis.Based on the investigation in this field,we hope our article will be helpful to motivate the scientific community towards developing novel photocatalysts that can solve various limitations and utilize them for continuous treatment of wastewater containing emerging pharmaceutical pollutants.

Fig.27.Future possible areas for the degradation of emerging pollutants using photocatalyst.

Declaration of competing interest

None.

Acknowledgements

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 thanks IIT Kanpur for Institute Postdoctoral Fellowship.