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Journal of Photochemistry & Photobiology A: Chemistry

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Photocatalytic degradation of acetaminophen over Ag, Au and Pt loadedTiO2 using solar light

Osama Nasra, Omima Mohameda, Al-Sayed Al-Shirbinib, Aboel-Magd Abdel-Wahaba,⁎

a Chemistry Department, Faculty of Science, Assiut University, Assiut, 71516, EgyptbNational Institute of Laser Enhanced Science (NILES), Cairo University, Giza, 12613, Egypt

A R T I C L E I N F O

Keywords:AcetaminophenSolar lightTiO2

Noble metalHydroxyl radical

A B S T R A C T

The sustainability and feasibility of using solar irradiation instead of UV light in photocatalysis is a promisingapproach for water remediation. In this study, photocatalytic degradation (PCD) of a widely used analgesic andantipyretic drug, acetaminophen (AP), with noble metal loaded TiO2 photocatalysts (Ag/TiO2, Au/TiO2 and Pt/TiO2) was investigated in aqueous suspension using solar light. The deposition of noble metals (Ag, Au and Pt)onto the TiO2 surface enhanced the PCD of AP under different operating conditions including pH, surfactants anddrug excipients. However, lower degradation rate constants of AP were obtained under simulated and directsolar light as compared to UV light. The degradation mechanism of AP under UV as well as simulated solar lightwas found to follow similar, though not identical, reaction pathways leading to hydroxylated intermediates (e.g.4-acetamidoresorcinol (4-AR), 4-acetamidocatechol (4-AC) and hydroquinone (HQ)) through competitiveroutes. The PCD of AP followed a pseudo first order kinetics according to Langmiur-Hinshelwood model. Noblemetal (Ag, Au and Pt) loaded TiO2 photocatalysts can be used effectively to degrade AP in water under both solarand UV light.

1. Introduction

Heterogeneous photocatalysis has received a great deal of attentiondue to its potential in energy production and environmental remedia-tion. In particular, TiO2-based photocatalytic materials can convertsolar energy into chemical energy via redox processes to yield usefulmaterials such as hydrogen and hydrocarbons, and to decompose pol-lutants in air and water [1]. Although TiO2 exhibits low toxicity, goodchemical stability and suitable band gap edges which can induce thedesired redox reactions, it only absorbs photons with light wavelengthsin the UV or near-UV region (which accounts for less than 5% of thetotal solar light spectrum) because of its large bandgap (3–3.2 eV) [2,3].However, noble metal (e.g. Au, Ag, Pd and Pt) deposits on the TiO2

surface can absorb visible light due to surface plasmon resonance (SPR),extending the photocatalytic performance to visible region, thus en-abling practical applications using solar energy [4].

Pharmaceuticals and personal care products have emerged as sig-nificant classes of organic contaminants which have been detected inwastewater, surface water, ground water and even drinking waterthroughout the world [5,6]. Their prevalence in aquatic environments,even at low concentrations, have attracted great attention because oftheir adverse health effects [7,8].

Acetaminophen (paracetamol, N-acetyl-p-aminophenol or AP) iswidely used over-the-counter analgesic and antipyretic drug. It is one ofthe top 200 prescription drug in the USA [9]. Although it is safer attherapeutic doses, AP may be fatal at higher doses via oxidativetransformation to N-acetyl-p-benzoquinone imine (NAPQI), which is atoxic compound leading to hepatic necrosis [10]. It is also associatedwith a risk of rare but serious skin reactions [11]. AP and its metabolitesare excreted from the body in 58–68% during therapeutic use and it hasbeen detected in different aquatic environments throughout the worldat concentration levels from ng/L to μg/L [12–16].

The photocatalytic degradation, kinetic optimization and degrada-tion pathways of AP using UV/TiO2 have been extensively studied bymany research groups [17–20]. However, few investigations related tothe photocatalytic degradation of AP as well as its degradation productsunder visible and/or solar light have been reported [21–23]. In addi-tion, the use of Au/TiO2 and Pt/TiO2 for AP removal has not beensubstantially examined [24,25].

In this work, Ag/TiO2, Au/TiO2 and Pt/TiO2 were prepared, char-acterized and employed to improve the activity of TiO2 for the de-composition of AP under solar light. Photocatalytic degradation of APunder direct solar radiation was examined to exploit the high intensityof solar radiation in Egypt. In addition, UV light (ACE lamp) was also

https://doi.org/10.1016/j.jphotochem.2019.01.032Received 9 October 2018; Received in revised form 24 January 2019; Accepted 29 January 2019

⁎ Corresponding author.E-mail addresses: wahab_47@hotmail.com, aawahab@aun.edu.eg (A.-M. Abdel-Wahab).

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Available online 30 January 20191010-6030/ © 2019 Published by Elsevier B.V.

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used for comparison in order to investigate the effect of photon energyon the photocatalytic degradation of AP. According to our knowledgefrom literature review there are no comparative studies so far on the useof these light sources for AP degradation. Effects of some operationalparameters (e.g. catalyst dose, light intensity, initial concentration ofAP, pH, surfactant and drug excipients) were also investigated. Moreinterestingly, the photocatalytic degradation products were identifiedusing GC and GC/MS techniques in order to rationalize a plausiblemechanism of the photocatalytic degradation of AP.

2. Experimental

2.1. Materials

TiO2 P25, AgNO3, HAuCl4.H2O and HPtCl6 were purchased fromDegussa, Euromedex, Electron Microscopy Sciences and BDH, respec-tively. Acetaminophen 98%, resorcinol 98% and 1,2,4-benzenetriol98% were purchased from Alfa Aesar. 4-Nitrocatechol 97% was ob-tained from Sigma Aldrich. N,O-bis(trimethylsilyl)trifluoroacetamidewith 1% trimethylchlorosilane (BSTFA+TMCS, 99:1) was purchasedfrom Supelco, USA. All other chemicals used in this study were ofanalytical or reagent grade.

2.2. Instrumentation

X-ray diffraction (XRD) patterns were recorded on a PW2103 Philipsdiffractometer. N2 sorption isotherms were obtained using an auto-mated gas sorption apparatus (Nova 3200, Quantachrome).Transmission electron microscopy (TEM) images were acquired using aFEI Tecnai G2 spirit microscope equipped with a Veleta camera oper-ating at 120 kV. UV–vis spectra were recorded on a PerkinElmerLAMBDA 750 Spectrophotometer. FTIR spectra were collected on aThermo-Nicolet-6700 FTIR spectrometer. 1H NMR and 13C NMR spectrawere recorded on Varian EM390 (90MHz) and Bruker (100MHz)spectrometers, respectively. Gas chromatography analyses wereachieved using a Thermo Scientific Trace Ultra GC equipped with TG-5MS capillary column (30m×0.25mm i.d., 0.25 μm film thickness)and coupled to a FID detector. GC/MS analyses were performed usingAgilent 7890B/5977 A GC/MS equipped with HP-5MS capillary column(30m x 0.25mm i.d., 0.25 μm film thickness).

2.2.1. Light sources2.2.1.1. Solar simulator. The photocatalytic experiments wereconducted in a Pyrex beaker (covered from the top with quartz plateto minimize water evaporation) irradiated from above, at a distance of15 cm, by using a solar simulator (Oriel, New port) equipped with a1000W xenon lamp and AM 1.5 G filter. The system was covered withaluminum foil to minimize light loss.

2.2.1.2. Direct sunlight. Photocatalytic experiments under directsunlight were conducted out in a Pyrex beaker covered from the topby quartz cover to minimize water evaporation. The intensities of bothnatural and simulated sunlight were measured at the reaction site usingan auto digital Newport solar meter (model 91150 V).

2.2.1.3. ACE lamp. A Vycor glass cell (29.5×2.5 cm), equipped with areflux condenser, and externally irradiated with a 450W mediumpressure mercury lamp (ACE glass Inc., USA, maximum emission at296.7–578 nm (4.18–2.15 eV)) immersed in Pyrex well. The distancebetween the sample cell and the lamp is 5 cm. The system was coveredwith aluminum foil to decrease light loss, and the apparatus was set upin a metallic cabinet [26]. The intensity of UV light at the reaction sitewas measured using Sper Scientific UV-A/B light meter, model1219B70.

2.3. Preparation of Ag/TiO2, Au/TiO2 and Pt/TiO2 photocatalysts

In a typical procedure, to a 50mL of AgNO3, HAuCl4 or HPtCl6 indeionized water, 1 g of TiO2 was added and sonicated for 15min. Themixture was stirred for 45min under N2 gas purging before irradiationwith a 450W ACE lamp for 1 h. The obtained powder was collected bycentrifugation, washed four times with deionized water and finallydried at 80 °C for 8 h [27].

2.4. Photocatalytic degradation of AP

2.4.1. General procedureTo a 50mL solution of AP in deionized water, the photocatalyst was

added and the mixture was sonicated for 15min to get homogeneoussuspension. The magnetically stirred suspension was irradiated usingthe light source. Aliquots (5 mL) were withdrawn from the suspensionat different intervals and then filtered through 0.2 μm membrane filter(CHMLAB group, Spain) to remove essentially all the catalyst. The de-gradation of AP was monitored by measuring its absorbance at 243 nm.The chemical oxygen demand (COD) was measured by the closed refluxcolorimetric method (5220D) [28].

2.4.2. Identification of photocatalytic degradation products2.4.2.1. Sample preparation and derivatization. Aliquot (1 mL) waswithdrawn from the reaction mixture, filtered through 0.2 μmmembrane filter to remove essentially all the catalyst, evaporatedunder vacuum at 40 °C to remove water and finally the residue wasderivatized as described in previous report [26].

2.4.2.2. GC and GC/MS Analyses. GC and GC/MS analyses of theaforementioned derivatized photolysates were carried out using thesame method described elsewhere [26]. The injector and MS detectorwere maintained at 250 and 200 °C, respectively. The temperatureprogramming was: 80 °C for 1min, 7 °C/min up to 150 °C, hold time for5min, 7 °C/min to 250 °C, hold time for 5min. The separated productswere identified using authentic samples or/and Wiley and NIST massspectral libraries.

2.4.3. Preparation of authentic samples2.4.3.1. 4-Acetamidocatechol (4-AC). 4-AC was synthesized via one potprocedure where 4-nitrocatechol was reduced by sodium hydrosulfite(Na2S2O4) followed by acetylation [29]. The obtained white solid(410mg, 76.2% yield) was stored under vacuum to avoid darkeningin air. mp 186–188 °C; FTIR (KBr): 3327 cm−1(sharp, NeH), 3214-3184 cm−1 (OeH), 1624 cm−1 (C=O); 1H NMR (90MHz, DMSO-d6):δ (ppm) 9.6 (s, 1H, NH), 8.9 (s, 1H, OH), 8.45 (s, 1H, OH), 7.1 (d, 1H,aromatic), 6.65 (dd, 1H, aromatic), 6.5 (d, 1H, aromatic), 2 (s, 3H,CH3). 13C NMR (100MHz, DMSO-d6): δ (ppm) 167.8, 145.3, 141.6,132, 115.7, 110.8, 108.4, 24.23.

2.4.3.2. 4-Nitroresorcinol (4-NR). 4-NR was prepared as reported [30].The obtained crude was purified by flash column chromatography onsilica gel (EtOAc/hexane=1:4). The product, yellow solid (1.5 g,21.5% yield), has mp 121–122 °C; IR (KBr): 3358-3290 cm−1 (OeH),3066 cm−1 (aromatic C–H), 1532-1398 cm−1 (NO2); 1H NMR (90MHz,DMSO-d6): δ 10.3–10.8 (s, 2H, 2OH), 7.6–7.8 (d, 1H, aromatic),6.15–6.25 (m, 2H, aromatic).

2.4.3.3. 4-Acetamidoresorcinol (4-AR). 4-nitroresorcinol was used toprepare 4-AR using the same procedure applied for 4- AC [29]. Theobtained white solid (392mg, 72.8% yield) was stored under vacuum toavoid darkening in air. mp 188–191 °C; IR (KBr) 3368 cm−1 (NeH),3214-2569 cm−1 (broad, OeH), 1637 cm−1 (C]O); 1H NMR (90MHz,DMSO-d6): δ (ppm) 9.6 (s, 1H, NH), 9.2 (s, 2H, 2OH), 7.2–7.3 (d, 1H,aromatic), 6.4 (d, 1H, aromatic), 6.2–6.3 (dd, 1H, aromatic), 2.1 (s, 3H,CH3). 13C NMR (100MHz, DMSO-d6): δ (ppm) 169.3, 155.7, 150.3,

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124.6, 118.6, 106.5, 104, 23.6.

3. Results and discussion

3.1. Characterization of M/TiO2 photocatalysts

3.1.1. XRD analysisThe powder XRD patterns of the bare and metal loaded TiO2 cata-

lysts are shown in Fig. 1. All catalysts show diffraction peaks corre-sponding to anatase (2θ=25.58, 38.08,48.08 and 54.58°) (JCPDS, no.21-1272) and rutile (2θ=27.51, 36.51, 41.10, 54.11 and 56.51°)(JCPDS, no. 21-1276) phases of TiO2 P25. No diffraction peaks of Ag,Au or Pt were detected which might be attributed to the low loadingamount (1 wt%) which is below the detection limit of the instrument,and the extremely small size of the deposited metal nanoparticles [31].The average crystal size of the TiO2 nanoparticles was calculated usingDebye–Scherrer's equation with the full width at half-maximum(FWHM) of the (101), (004) and (200) peaks and found to be in therange of 22–25 nm for all the prepared catalysts.

3.1.2. TEM and EDX analysesAs shown in Fig. 2, HRTEM observation reveals small Pt and Au

nanoparticles (3–5 nm) deposited on the surface of TiO2 particles(30 nm) with a high dispersion, whereas Ag aggregates are observed inthe case of Ag/TiO2 sample. Furthermore, EDX analysis (Fig. 2) revealsthe presence of Ag, Au, Pt, Ti and O peaks which confirms the successfulloading of Ag, Au and Pt nanoparticles onto the TiO2 surface.

3.2. Photocatalytic degradation of acetaminophen

3.2.1. Effect of TiO2 doseFig. 3 indicates that the rate of PCD of AP under simulated solar

light increases as the dose of TiO2 is increased from 0.2 g/L to 1 g/L dueto the increased surface area and hence greater number of active sites.However, further increase of TiO2 concentration results in lower pho-tocatalytic degradation which can be attributed to light scatteringcaused by the turbidity of the solution [32].

3.2.2. Effect of the initial concentration of APFig. 4 shows that the removal of AP decreases as the initial con-

centration of AP is increased from 10 to 50mg/L. The lower degrada-tion efficiency at higher initial concentration can be attributed to thefact that the higher AP concentrations could absorb more photons,which reduces the available photons to activate the TiO2 and this re-sults in reduced degradation efficiency [33].

3.2.3. Kinetics of the photocatalytic degradation of APThe kinetics of AP degradation under simulated solar light was

found to follow pseudo first order kinetic (Fig. S4) and can be welldescribed by Langmuir-Hinshelwood (L-H) model (Fig. 5) [34,35]. Thetrue degradation rate constant, k1, and L-H adsorption rate constant, K,for AP during the PCD over TiO2 using simulated solar light were foundto be 0.385mg L−1 min−1 and 0.0970mg−1L, respectively.

3.2.4. Effect of oxygen purgingThe effect of air, oxygen and nitrogen bubbling on the degradation

of AP under simulated solar light is illustrated in Fig. 6, where the rateof AP degradation in oxygen bubbling solution is 3.5 times higher thanthat in nitrogen bubbling solution. This implies that oxygen plays acritical role in solar TiO2 photocatalysis, enhancing the degradationefficiency even under lower concentration (20% oxygen in air, i.e. staticoxygen).

3.2.5. Effect of simulated solar light intensityFig. 7 reveals that the degradation of AP increases as the light in-

tensity is increased from 25.0 to 100.0 mW/cm2 with degradation rateproportional to the square root of light intensity. With increasing lightintensity, the number of photogenerated charge carriers and free radi-cals increases, which in turn promotes the photocatalytic process.However, at high light intensity the rate of electron-hole formationbecomes greater than the photocatalytic rate, which favors the electron-hole recombination [34].

3.2.6. Effect of noble metal (Ag, Au and Pt) loading on the TiO2 surfaceFig. 8 shows that the PCD of AP over 1 wt% M/TiO2 photocatalysts

is higher than that of the bare TiO2. The degradation rate constantswere 0.0128, 0.0189, 0.0160 and 0.0203min−1 for TiO2, Ag/TiO2, Au/TiO2 and Pt/TiO2, respectively. The improved photocatalytic activity ofmetal loaded TiO2 photocatalysts under simulated solar light can beascribed firstly to the improved visible light absorption of M/TiO2

triggered by the surface plasmon resonance (SPR) of noble metal na-noparticles, and secondly to the ability of noble metals deposits (Ag, Auand Pt) to scavenge the photogenerated electrons from TiO2 whichboosts the e-/h+ pair separation and promotes the interfacial chargetransfers [27,36]. In addition, noble metal deposits can act as activesites for the formation of reactive species (e.g. •OH, O2

•−, •OOH andH2O2) leading to higher degradation efficiency [37,38]. The highestphotocatalytic activity of Pt/TiO2 is probably due to that Pt is a bettertrapping site for photoelectron than Ag and Au [39].

3.2.7. Effect of initial pHThe influence of initial solution pH on the PCD of AP was examined

in a pH range of 2.5 ‒ 10. As shown in Fig. 9, the efficiency of APdegradation is strongly affected by the initial pH of the solution. BareTiO2 P25 shows the highest photocatalytic activity at weakly alkalinemedium (pH 8) due to greater concentration of %OH radical [17].However, the reaction rate constant is significantly decreased when thesolution becomes more alkaline (pH 10), mainly due to the electrostaticrepulsion between the negatively charged TiO2 surface (pHpzc= 6.3)and the anionic form of AP molecules (pKa= 9.5), which result in pooradsorption of AP and decreased rate of AP degradation [17,40]. For Ag/TiO2, the highest AP removal is observed at pH 6.3, while Au/TiO2 andPt/TiO2 show the highest degradation efficiency for AP at pH 4.3 whichcan be attributed to the enhanced formation of the superoxide anion(O2

•−) and subsequent formation of %OH (Eqs. 1–4) [41].

+ →− +O H HO2

•2• (1)

+ → + →+ −HO H e H O2

•2 2 (2)

+ → +− −H O e OH OH2 2

• (3)

+ →hvH O 2 OH2 2• (4)

However, TiO2 particles tend to agglomerate under strong acidiccondition (e.g. pH 2.5), therefore the surface area and photon

Fig. 1. XRD patterns of (a) TiO2 P25, (b) 1%Ag/TiO2, (c) 1%Au/TiO2 and (d)1%Pt/TiO2 photocatalysts.

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absorption would be reduced, which consequently decreases the rate ofAP degradation [42]. Therefore, the pH range of 4.2–8 is the optimalrange for photocatalytic degradation of AP over the employed photo-catalysts under simulated solar light.

3.2.8. Effect of surfactantsSurfactants, from detergents and other products, are often present in

wastewater and enter the environment with pharmaceuticals throughwastewater discharge. To investigate the effect of surfactants on pho-tocatalytic degradation of AP, two surfactants of different categories

Fig. 2. TEM images (left) and EDX analysis (right) of metal loaded TiO2 photocatalysts.

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were employed; CTAB and SDS as cationic and anionic surfactants,respectively. Fig. 10 shows that, with all utilized photocatalysts, therate of photocatalytic degradation of AP is reduced in the presence ofCTAB or SDS surfactants at their critical micelles concentrations (CMC).This can be attributed to the fact that the surfactants can compete withthe AP for active sites on the photocatalyst, and thus retard the de-gradation process. Inhibition of photocatalytic degradation of phenol inthe presence of cationic and anionic surfactants has been observed byseveral research groups [43,44]. More importantly, the metal loadedTiO2 photocatalysts showed higher degradation of AP compared to bareTiO2 in the presence of surfactants (Fig. 10). These results further

demonstrate the role of metal deposition in improving the photo-catalytic efficiency of TiO2 even in the presence of other competitors,i.e. surfactants.

3.2.9. Photocatalytic degradation of Panadol drug under simulated solarlight

The effect of Panadol drug (commercial AP prescription) excipientson the photocatalytic degradation of AP over bare TiO2 P25 and 1% Pt/TiO2 photocatalysts under simulated solar light is illustrated in Fig. 11.After 180min simulated solar irradiation, 83% of Panadol was de-composed with 1%Pt/TiO2 compared to 99% of pure AP, while 69% ofPanadol was decomposed with bare TiO2 compared to 94% of pure AP.The reduced degradation efficiency of Panadol drug can be attributed

Fig. 3. Effect of TiO2 dose on the rate constant of the PCD of AP.([AP]o= 20mg/L, pH=6.3 (ambient), light intensity= 50.0mW/cm2).

Fig. 4. Effect of initial concentration of AP on the PCD of AP under simulatedsolar light. ([TiO2]=0.4 g/L, pH=6.3 (ambient), light intensity= 50.0mW/cm2).

Fig. 5. Langmuir-Hinshelwood plot for photocatalytic degradation of AP undersimulated solar light.

Fig. 6. Effect of gas purging on the photocatalytic degradation of AP undersimulated solar light. ([AP]o= 20mg/L, [TiO2]= 0.4 g/L, pH=6.3 (ambient),light intensity= 50.0 mW/cm2).

Fig. 7. Apparent degradation rate constant vs. different light intensity.([AP]o= 20mg/L, [TiO2]=0.4 g/L, pH=6.3 (ambient)).

Fig. 8. Photocatalytic degradation of AP under simulated solar light over bareTiO2 and metal loaded TiO2 photoctalysts. ([AP]o= 20mg/L, [catalyst]= 0.4g/L, pH=5.6–6.3 (ambient), light intensity= 50.0mW/cm2).

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firstly to the light scattering due to the turbidity of the solution causedby the insoluble excipients (e.g. starch, talc, povidone, etc.) and sec-ondly to the competition of the excipients with the AP molecules for theactive sites and reactive species on the photocatalyst surface. The su-periority of Pt/TiO2 photocatalyst in Panadol degradation proves therole of Pt deposits in enhancing the photocatalytic activity of TiO2.

3.2.10. Effect of light sourceFig. 12 shows the photocatalytic degradation of AP, normalized to

light intensity, under different light sources, namely UV, simulated, and

direct solar light. As clearly seen, AP degradation over the 1% Ag/TiO2

photocatalyst under UV light is much higher than those with simulatedand direct solar light. The normalized rate constants are 0.0034, 0.0003and 0.0001min−1/mW/cm2 with UV, simulated and direct solar light,respectively.

3.2.11. Determination of mineralization using COD analysisIn photocatalytic degradation processes, the accumulation of de-

gradation products is of great concern as they may pose health risks tothe ecosystem [45]. Therefore, it is important to identify the degrada-tion products, as will be discussed later (vide infra), and to determinethe amount of dissolved organic carbon during the photocatalytic de-gradation process. COD was measured to evaluate the AP mineraliza-tion, i.e. complete oxidation to CO2 and H2O. Fig. 13 demonstrates thatthe 1%M/TiO2 photocatalysts show higher mineralization than bareTiO2. After 180min of simulated solar light irradiation, the observedmineralization of AP over bare TiO2, Ag/TiO2, Au/TiO2 and Pt/TiO2

was 63.4, 73.7, 68.8 and 78.0%, respectively. On the other hand, after30min UV irradiation, the observed mineralization of AP was found tobe 81.0, 90.9, 85.7 and 89.4% with TiO2, Ag/TiO2, Au/TiO2 and Pt/TiO2, respectively, which are higher than those obtained in our pre-vious work (66% after 60min UV irradiation over TiO2/Fe2O3) andstudies by other research groups [24,25,27]. For instance, Yavas et al.reported 20 and 40% mineralization of AP with Au/TiO2 and Pt/TiO2,respectively, upon 60min UVC (254 nm) light irradiation [24,25]. Thehigher mineralization efficiency of the 1%M/TiO2 photocatalysts is dueto the enhanced interfacial charge transfer process, favorable

Fig. 9. Effect of initial pH on the simulated solar induced photocatalytic de-gradation rate constant of AP. ([AP]o= 20mg/L, [catalyst]= 0.4 g/L, lightintensity= 50.0mW/cm2).

Fig. 10. Effect of CTAB and SDS surfactants on the photocatalytic degradationof AP under simulated solar light. ([AP]o= 20mg/L, [catalyst]= 0.4 g/L,[CTAB]= 1mM, [SDS]= 6mM, light intensity= 50.0mW/cm2).

Fig. 11. Effect of Panadol drug excipients on simulated solar light inducedphotocatalytic degradation of AP over TiO2 and Pt/TiO2. ([AP]o= 20mg/L,[catalyst]= 0.4 g/L, pH=5.6–6.3 (ambient), light intensity= 50.0mW/cm2).

Fig. 12. Effect of light source on the photocatalytic degradation of AP over1%Ag/TiO2 under direct sunlight. ([AP]o= 20mg/L, [catalyst]= 0.4 g/L,pH=5.6 (ambient), simulated solar light intensity= 50.0 mW/cm2, directsunlight intensity= 82.0–89.0mW/cm2, ACE lamp intensity= 37.4mW/cm2).

Fig. 13. Photocatalytic mineralization of AP under (a) simulated solar light([AP]o= 20mg/L, [catalyst]= 0.4 g/L, pH=5.6–6.3 (ambient), lightintensity= 50.0mW/cm2, CODo= 40.35mg/L) (b) UV light ([AP]o= 50mg/L, [catalyst]= 0.1 g/L, CODo=92.6mg/L, light intensity= 37.4 mW/cm2).

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adsorption of degradation products onto the metal nanoparticles sur-face and ability of the metal nanoparticles to drive catalytic oxidation/reduction reactions (e.g. aerobic oxidation and catalytic hydrogenation)for some degradation products [46–48].

3.2.12. Identification and mechanism of degradation productsIdentification of the intermediate products generated during the

degradation process is of great concern as some intermediates may bepersistent or toxic themselves [45]. In addition, it is beneficial for theelucidation of reaction mechanism. GC and/or GC/MS analyses wereused to identify the photocatalytic degradation products. Due to the lowvolatility and thermal instability of the degradation products (phenoliccompounds and acids), prior to analysis, the mixture was treated withBSTFA+TMCS and converted into the more volatile trimethylsilylderivatives so that can be analyzed by GC and/or GC /MS techniques.

A total of twenty-one degradation products were recognized duringthe PCD of AP over bare and metal loaded TiO2 photocatalysts irra-diated with simulated solar light (Table S1, supporting information),while a total of nineteen products were identified with ACE lamp(Table S2, supporting information). Suggested degradation pathwaysleading to the formed products are sketched in Fig. 16. The resultsprovided in Tables S1 & S2, indicate that the photocatalytic degradationprocess could be elucidated to be dominated via the most powerfuloxidizing •OH radicals. This is obvious from the predominance of thehydroxyl derivatives of AP (4-AC and 4-AR) as well as hydroquinone inthe early stages of photocatalytic decomposition of AP as illustrated in

Figs. 14 and 15. Hydroquinone formation could be explained throughdirect attack of the non-selective %OH radical on C-4 of AP followed byremoval of acetamide molecule (route a, Fig. 16). Similarly, the samemechanism can be proposed for the formation of 1,2,4-benzenetriolfrom either 4-AC or 4-AR [28]. Alternatively, h+ induced one electronoxidation of AP and/or hydrogen abstraction by the %OH radical (routeb, Fig. 16) can be considered as competitive routes to the %OH ringattack (route a) leading to the formation of the compounds no. 5, 6 and7 (Table S1) with simulated solar irradiation. These compounds werenot detected in the PCD of AP with UV light in the present study, whichcan be ascribed to their rapid degradation by the higher energy of UVlight compared with simulated solar light. Also, these compounds werenot detected in our previous work [26] or in any reported related stu-dies found in the literature [17–20,24,25]. Moreover, compounds 5, 7have not appeared in Table S1 and S2, respectively, in case of bareTiO2. It is worthwhile to note that Yavas et al. could only detect hy-droquinone and benzoquinone on their work concerning the PCD of APover Au/TiO2, Pd/TiO2, Au-Pd/TiO2 and Pt/TiO2 photocatalysts usingUV light [24,25]. In addition, Moctezuma et al. [18] observed theformation of 4-aminophenol via deacetylation of AP, which is incon-sistent with our findings.

Formation of carboxylic acids could be explained as reported via

Fig. 14. Evolution of primary degradation products formed during photo-catlytic degradation of AP under simulated solar light in the presence of (a) bareTiO2, (b) 1%Ag/ TiO2, (c) 1%Au/ TiO2 and (d) 1%Pt/ TiO2 photocatlysts.([AP]o= 20mg/L, [catalyst]= 0.4 g/L, pH=5.6–6.3 (ambient), lightintensity= 50.0mW/cm2). Fig. 15. Evolution of primary degradation products formed during photo-

catlytic degradation of AP using UV light in the presence of (a) bare TiO2, (b)1%Ag/ TiO2, (c) 1%Au/ TiO2 and (d) Pt/ TiO2 photocatlysts. ([AP]o= 50mg/L, [catalyst]= 0.1 g/L, pH=5.6–6.3 O2 flow=100mL/min, lightintensity= 37.4mW/cm2).

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successive •OH oxidation of the hydroxyl aromatic derivatives followedby ring cleavage. For example, hydroquinone undergoes oxidation tobenzoquinone (not detected) which upon ring cleavage and furtheroxidation produce the obtained maleic, fumaric, succinic, malic, tar-taric, malonic, glycolic and oxalic acids as reported [29]. Likewise, 4-AC and 4-AR are logical precursors for 3-acetamido-4-hydroxyhex-2-enedioic acid, 3-acetamido-2,4-dihydroxypent-2-enedioic acid and 2-acetamido-3-hydroxysuccinic acid which are observed only during thePCD of AP with UV irradiation [29]. This can be attributed to the highconcentration of 4-AC and 4-AR (precursors for nitrogenous aliphaticcompounds) compared with that of HQ (main precursor for non-ni-trogenous aliphatic compounds) during the course of the reaction underUV light, which is opposite to what is observed with simulated solarlight (Figs. 14 and 15). Similarly, we could explain the appearance ofglycerol, dihydroxyacetone, adipic and glutaric acids only with simu-lated solar irradiation. Virtually, the observed variation between thedegradation products in Tables S1 and S2 can be ascribed to the factthat the formation of these compounds is dependent on energy andintensity of light, irradiation time as well as type of photocatalyst.However, conclusive explanation for certain compounds appeared onfurther degradation needs further extensive studies.

Oxamic acid, interestingly, could be produced through oxidation ofacetamide by •OH due to the electron attracting carbonyl group whichmakes the methyl hydrogen atoms susceptible to %OH oxidation formingoxamic acid. The latter is also a reasonable precursor for the producedoxalic acid [19].

4. Conclusions

Three nanosized noble metal loaded TiO2 photocatalysts (Ag/TiO2,Au/TiO2 and Pt/TiO2) were fabricated, characterized and employed forphotocatalytic degradation of AP under solar light and compared withUV light. The metal loaded TiO2 photocatalysts showed enhanced solarphotocatalytic degradation of AP in a wide pH range of 4.2–8.0 as wellas in the presence of other interferences such as surfactants, and drugexcipients and binding materials. The highest photocatalytic activityobserved with Pt/TiO2, which was ascribed to the fact that Pt is a bettertrapping site for photoelectron than Ag and Au. A total of twenty-onedegradation products (two are newly reported) were identified duringthe degradation of AP with simulated solar light, whereas a total ofnineteen products were identified when illuminated with UV light. Themain degradation products were 4-acetamidocatechol, 4-acetamidor-esorcinol and hydroquinone. The mechanism of photodegradation ofAP was proposed to be dominated by •OH radical attack onto the

aromatic ring in case of UV as well as solar light. Also, h+ inducedoxidation of AP and/or hydrogen abstraction by the %OH radicalpathway was proposed to account for formation of dimeric compoundsin case of simulated solar irradiation. The results demonstrated that thedegradation products are dependent upon light energy, irradiation timeas well as the type of photocatalyst. Photocatalysis with solar lightcould be implemented as a feasible and environmentally benign methodfor effective degradation of AP using noble metal loaded TiO2 photo-catalysts.

Declarations of interest

None

Appendix A. Supplementary data

Supplementary material related to this article can be found, in theonline version, at doi:https://doi.org/10.1016/j.jphotochem.2019.01.032.

References

[1] K. Nakata, A. Fujishima, TiO2 photocatalysis: design and applications, J.Photochem. Photobiol. C Photochem. Rev. 13 (2012) 169–189.

[2] S.G. Kumar, L.G. Devi, Review on modified TiO2 photocatalysis under UV/visiblelight: selected results and related mechanisms on interfacial charge carrier transferdynamics, J. Phys. Chem. A 115 (2011) 13211–13241.

[3] L. Wang, J. Fan, Z. Cao, Y. Zheng, Z. Yao, G. Shao, J. Hu, Fabrication of pre-dominantly Mn4+‐doped TiO2 nanoparticles under equilibrium conditions andtheir application as visible‐light photocatalyts, Chem. Asian J. 9 (2014) 1904–1912.

[4] L.G. Devi, R. Kavitha, A review on plasmonic metal–TiO2 composite for generation,trapping, storing and dynamic vectorial transfer of photogenerated electrons acrossthe Schottky junction in a photocatalytic system, Appl. Surf. Sci. 360 (2016)601–622.

[5] A.J. Ebele, M.A.-E. Abdallah, S. Harrad, Pharmaceuticals and personal care pro-ducts (PPCPs) in the freshwater aquatic environment, Emerg. Contam. (2017).

[6] F.A. Weber, A. Bergmann, S. Hickmann, I. Ebert, A. Hein, A. Küster,Pharmaceuticals in the environment—global occurrences and perspectives,Environ. Toxicol. Chem. 35 (2016) 823–835.

[7] K. Fent, A.A. Weston, D. Caminada, Ecotoxicology of human pharmaceuticals,Aquat. Toxicol. 76 (2006) 122–159.

[8] W.C. Li, Occurrence, sources, and fate of pharmaceuticals in aquatic environmentand soil, Environ. Pollut. 187 (2014) 193–201.

[9] S. Wu, L. Zhang, J. Chen, Paracetamol in the environment and its degradation bymicroorganisms, Appl. Microbiol. Biotechnol. 96 (2012) 875–884.

[10] D. Nematollahi, H. Shayani-Jam, M. Alimoradi, S. Niroomand, Electrochemicaloxidation of acetaminophen in aqueous solutions: kinetic evaluation of hydrolysis,hydroxylation and dimerization processes, Electrochim. Acta 54 (2009) 7407–7415.

[11] V.H.-T. Thi, B.-K. Lee, Effective photocatalytic degradation of paracetamol using La-doped ZnO photocatalyst under visible light irradiation, Mater. Res. Bull. (2017).

[12] N. Muir, J. Nichols, M. Stillings, J. Sykes, Comparative bioavailability of aspirin and

Fig. 16. Proposed pathways for photocatalytic degradation of AP under both simulated solar and UV light [26].

O. Nasr et al. Journal of Photochemistry & Photobiology A: Chemistry 374 (2019) 185–193

192

paracetamol following single dose administration of soluble and plain tablets, Curr.Med. Res. Opin. 13 (1997) 491–500.

[13] D.J. Fairbairn, M.E. Karpuzcu, W.A. Arnold, B.L. Barber, E.F. Kaufenberg,W.C. Koskinen, P.J. Novak, P.J. Rice, D.L. Swackhamer, Sources and transport ofcontaminants of emerging concern: a two-year study of occurrence and spatio-temporal variation in a mixed land use watershed, Sci. Total Environ. 551 (2016)605–613.

[14] P.D. Scott, M. Bartkow, S.J. Blockwell, H.M. Coleman, S.J. Khan, R. Lim,J.A. McDonald, H. Nice, D. Nugegoda, V. Pettigrove, A national survey of traceorganic contaminants in Australian rivers, J. Environ. Qual. 43 (2014) 1702–1712.

[15] M. Gómez, M.M. Bueno, S. Lacorte, A. Fernández-Alba, A. Agüera, Pilot surveymonitoring pharmaceuticals and related compounds in a sewage treatment plantlocated on the Mediterranean coast, Chemosphere 66 (2007) 993–1002.

[16] M.S. Fram, K. Belitz, Occurrence and concentrations of pharmaceutical compoundsin groundwater used for public drinking-water supply in California, Sci. TotalEnviron. 409 (2011) 3409–3417.

[17] L. Yang, E.Y. Liya, M.B. Ray, Degradation of paracetamol in aqueous solutions byTiO2 photocatalysis, Water Res. 42 (2008) 3480–3488.

[18] E. Moctezuma, E. Leyva, C.A. Aguilar, R.A. Luna, C. Montalvo, Photocatalytic de-gradation of paracetamol: intermediates and total reaction mechanism, J. Hazard.Mater. 243 (2012) 130–138.

[19] L. Yang, L.E. Yu, M.B. Ray, Photocatalytic oxidation of paracetamol: dominant re-actants, intermediates, and reaction mechanisms, Environ. Sci. Technol. 43 (2008)460–465.

[20] C.J. Lin, W.-T. Yang, C.-Y. Chou, S.Y.H. Liou, Hollow mesoporous TiO2 micro-spheres for enhanced photocatalytic degradation of acetaminophen in water,Chemosphere 152 (2016) 490–495.

[21] C.-J. Lin, W.-T. Yang, Ordered mesostructured Cu-doped TiO2 spheres as activevisible-light-driven photocatalysts for degradation of paracetamol, Chem. Eng. J.237 (2014) 131–137.

[22] C.J. Lin, Y.H. Liou, Y. Zhang, C.L. Chen, C.-L. Dong, S.-Y. Chen, G.D. Stucky,Mesoporous Fe-doped TiO2 sub-microspheres with enhanced photocatalytic activityunder visible light illumination, Appl. Catal. B 127 (2012) 175–181.

[23] N. Jallouli, K. Elghniji, H. Trabelsi, M. Ksibi, Photocatalytic degradation of para-cetamol on TiO2 nanoparticles and TiO2/cellulosic fiber under UV and sunlightirradiation, Arab. J. Chem. 10 (2017) S3640–S3645.

[24] A. Ziylan-Yavaş, N.H. Ince, Enhanced photo-degradation of paracetamol on n-pla-tinum-loaded TiO2: the effect of ultrasound and OH/hole scavengers, Chemosphere162 (2016) 324–332.

[25] A. Ziylan-Yavas, Y. Mizukoshi, Y. Maeda, N.H. Ince, Supporting of pristine TiO2

with noble metals to enhance the oxidation and mineralization of paracetamol bysonolysis and sonophotolysis, Appl. Catal. B 172 (2015) 7–17.

[26] A.-M. Abdel-Wahab, A.-S. Al-Shirbini, O. Mohamed, O. Nasr, Photocatalytic de-gradation of paracetamol over magnetic flower-like TiO2/Fe2O3 core-shell nanos-tructures, J. Photochem. Photobiol. A: Chem. 347 (2017) 186–198.

[27] A.A. Melvin, K. Illath, T. Das, T. Raja, S. Bhattacharyya, C.S. Gopinath, M–Au/TiO2

(M= Ag, Pd, and Pt) nanophotocatalyst for overall solar water splitting: role ofinterfaces, Nanoscale 7 (2015) 13477–13488.

[28] APHA, Standard Methods for the Examination of Waters and Waste Water, 21st ed,APHA, WWA and WEF, Washington DC, USA, 2005.

[29] D. Vogna, R. Marotta, A. Napolitano, M. d’Ischia, Advanced oxidation chemistry ofparacetamol. UV/H2O2-induced hydroxylation/degradation pathways and 15N-aided inventory of nitrogenous breakdown products, J. Org. Chem. 67 (2002)6143–6151.

[30] R. An, P. Wei, D. Zhang, N. Su, A highly selective 7-hydroxy-3-methyl-

benzoxazinone based fluorescent probe for instant detection of thiophenols in en-vironmental samples, Tetrahedron Lett. 57 (2016) 3039–3042.

[31] S. Yurdakal, B.S. Tek, Ç. Değirmenci, G. Palmisano, Selective photocatalytic oxi-dation of aromatic alcohols in solar-irradiated aqueous suspensions of Pt, Au, Pdand Ag loaded TiO2 catalysts, Catal. Today 281 (2017) 53–59.

[32] X. Luo, J. Wang, C. Wang, S. Zhu, Z. Li, X. Tang, M. Wu, Degradation and miner-alization of benzohydroxamic acid by synthesized mesoporous La/TiO2, Int. J.Environ. Res. Public Health 13 (2016) 997.

[33] S.S. Muniandy, N.H.M. Kaus, Z.-T. Jiang, M. Altarawneh, H.L. Lee, Green synthesisof mesoporous anatase TiO2 nanoparticles and their photocatalytic activities, RSCAdv. 7 (2017) 48083–48094.

[34] P. Chowdhury, J. Moreira, H. Gomaa, A.K. Ray, Visible-solar-light-driven photo-catalytic degradation of phenol with dye-sensitized TiO2: parametric and kineticstudy, Ind. Eng. Chem. Res. 51 (2012) 4523–4532.

[35] T.B. Nguyen, R.-a. Doong, Heterostructured ZnFe2O4/TiO2 nanocomposites with ahighly recyclable visible-light-response for bisphenol A degradation, RSC Adv. 7(2017) 50006–50016.

[36] L.G. Devi, R. Kavitha, A review on plasmonic metal–TiO2 composite for generation,trapping, storing and dynamic vectorial transfer of photogenerated electrons acrossthe Schottky junction in a photocatalytic system, Appl. Surf. Sci. 360 (2016)601–622.

[37] A. Zielińska-Jurek, Z. Wei, I. Wysocka, P. Szweda, E. Kowalska, The effect of na-noparticles size on photocatalytic and antimicrobial properties of Ag-Pt/TiO2

photocatalysts, Appl. Surf. Sci. 353 (2015) 317–325.[38] H. Yu, X. Wang, H. Sun, M. Huo, Photocatalytic degradation of malathion in aqu-

eous solution using an Au–Pd–TiO2 nanotube film, J. Hazard. Mater. 184 (2010)753–758.

[39] M.R. Khan, T.W. Chuan, A. Yousuf, M. Chowdhury, C.K. Cheng, Schottky barrierand surface plasmonic resonance phenomena towards the photocatalytic reaction:study of their mechanisms to enhance photocatalytic activity, Catal. Sci. Technol. 5(2015) 2522–2531.

[40] X. Zhang, F. Wu, X. Wu, P. Chen, N. Deng, Photodegradation of acetaminophen inTiO2 suspended solution, J. Hazard. Mater. 157 (2008) 300–307.

[41] H. Liao, T. Reitberger, Generation of free OHaq radicals by black light illuminationof Degussa (Evonik) P25 TiO2 aqueous suspensions, Catalysts 3 (2013) 418–443.

[42] M.A. Fox, M.T. Dulay, Heterogeneous photocatalysis, Chem. Rev. 93 (1993)341–357.

[43] N. Kashif, F. Ouyang, Parameters effect on heterogeneous photocatalysed de-gradation of phenol in aqueous dispersion of TiO2, J. Environ. Sci. 21 (2009)527–533.

[44] D. Fabbri, A.B. Prevot, E. Pramauro, Effect of surfactant microstructures on pho-tocatalytic degradation of phenol and chlorophenols, Appl. Catal. B 62 (2006)21–27.

[45] W. Jardim, S. Moraes, M. Takiyama, Photocatalytic degradation of aromaticchlorinated compounds using TiO2: toxicity of intermediates, Water Res. 31 (1997)1728–1732.

[46] A.E. de Los Monteros, G. Lafaye, A. Cervantes, G. Del Angel, J. Barbier Jr, G. Torres,Catalytic wet air oxidation of phenol over metal catalyst (Ru, Pt) supported onTiO2–CeO2 oxides, Catal. Today 258 (2015) 564–569.

[47] Y. Shen, Y. Li, H. Liu, Base-free aerobic oxidation of glycerol on TiO2-supportedbimetallic Au–Pt catalysts, J. Energy Chem. 24 (2015) 669–673.

[48] Z. He, X. Wang, Highly selective catalytic hydrodeoxygenation of guaiacol to cy-clohexane over Pt/TiO2 and NiMo/Al2O3 catalysts, Front. Chem. Sci. Eng. 8 (2014)369–377.

O. Nasr et al. Journal of Photochemistry & Photobiology A: Chemistry 374 (2019) 185–193

193