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Jan 1, 2014 - Ardhendu Sekhar Giri and Animes Kumar Golder*. Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam 781 ...
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Fenton, Photo-Fenton, H2O2 Photolysis, and TiO2 Photocatalysis for Dipyrone Oxidation: Drug Removal, Mineralization, Biodegradability, and Degradation Mechanism Ardhendu Sekhar Giri and Animes Kumar Golder* Department of Chemical Engineering, Indian Institute of Technology, Guwahati, Assam 781 039, India S Supporting Information *

ABSTRACT: Dipyrone (DIPY), an analgesic drug, is very quickly hydrolyzed to 4-methylaminoantipyrine (4-MAA) in an acidic solution. Batch study was conducted to compare the performance of different oxidation processes such as Fenton (FP), photoFenton (PFP), UV/H2O2 photolysis (UVP), and UV/TiO2 photocatalysis (UVPC) for removal of 4-MAA from aqueous solution. The degradation efficiency was evaluated in terms of total organic carbon (TOC) reduction and enhancement of biodegradability. Maximum 4-MAA removals of 94.1, 96.4, 74.4, and 71.2% were achieved in FP, PFP, UCP, and UVPC, respectively, against mineralization of 49.3, 58.2, 47, and 24.6%. The proposed mechanisms suggest that the cleavage of three methyl moieties followed by pyrazolinone ring breakage led to formation of various intermediates with low errors (−0.88 to 0.11 g/mol). The intermediates primarily were hydroxylated and carboxylic derivatives. BOD5 to COD (BOD, biochemical oxygen demand; COD, chemical oxygen demand) ratio of ≥0.4 resulted from DIPY decomposition in all processes with highest improvement in PFP (BOD5/COD ≈ 1.5). The collapse of iron(III)-chelates under UV irradiation gave higher biodegradability.

1. INTRODUCTION Advanced oxidation processes (AOPs) are widely used for removal of pharmaceutical and personal care products from industrial and municipal wastewater.1 AOPs undergo conditions through different reacting systems such as homogeneous or heterogeneous phases, in light or dark, etc. However, they have common characteristics of formation of hydroxyl free radicals (OH•).2 It causes consecutive unselective degradation of organic materials. Complete mineralization and/or oxidization of contaminants occur even at very low concentration, and the byproducts formed may also be environmentally nonhazardous.3 The Fenton process (FP) is based on redox reaction between Fe2+ and H2O2 generating OH• radicals,4 whereas the photoFenton process (PFP) occurs additionally under UV−vis irradiation. Heterogeneous semiconductor photocatalysis using TiO2 causes oxidation of organic pollutants via hydroxyl radicals, OH•ad, and valence holes (h+) generated when the semiconductor is exposed to UV irradiation. TiO2 photocatalysis works at ambient conditions and may be induced by solar irradiation.5 UV photolysis of H2O2 (UVP) and TiO2 photocatalysis (UVPC) generally show a lesser extent of oxidation than FP and PFP. Both homogeneous and heterogeneous catalytic processes have shown promising results even at pilot-plant scale in treatment of nonbiodegradable and toxic compounds. Dipyrone (DIPY) is one of the most popular analgesics and antipyretic drug. It is also known as Metamizole and Novalgin. About one-third of it remains unchanged and can be found in urinary excretion. DIPY is rapidly hydrolyzed into its main metabolite, 4-methylaminoantipyrine (4-MAA). 4-MAA is absorbed and biotransformed by enzymatic reactions.6 The metabolites of DIPY can be subdivided into two groups: (i) decarboxylated metabolites and (ii) metabolites with a degraded pyrazolinone moiety. The metabolic route is shown in Figure 1. 4-MAA is metabolized to 4-aminoantipyrine (4-AA) in human © 2014 American Chemical Society

Figure 1. Chemical structure of dipyrone drug (sodium [(2,3-dihydro1,5-dimethyl-3-oxo-2-phenyl-1-pyrazol-4-yl) methylamino]methanesulfonate) and its metabolic pathways.

liver via demethylation and further acetylated to acetylaminoantipyrine (4-AAA) (Figure 1). 4-Formylaminoantipyrine (4-FAA) is generated by an uncharacterized oxidation of an n-methyl group of 4-MAA (Figure 1). Received: Revised: Accepted: Published: 1351

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ionization (ESI) method in positive ion mode over the mass range of 100−400 amu. The parent ion was fragmented on the basis of a suitable range of mass to charge (m/z) ratio. The most prominent daughter ions were selected for further fragmentation. Chemical oxygen demand was determined according to the HACH method. A closed reflux digester of HACH, Loveland, CO, USA (model DRB 200) was used for digestion. Total organic carbon (TOC) analyzer of O.I. Analytical, College Station, TX, USA (model 1030C Aurora) was employed for the measurement of TOC by nondispersive infrared method. A 5 day biochemical oxygen demand (BOD5) was found per the standard method of analysis.9 Dissolved oxygen (DO) was measured using a precision dissolved oxygen meter of Hanna Instruments, Smithfield, RI, USA (model HI 2400). Solution pH was measured using a precision pH meter of Eutech Instruments, Malaysia (model pH/ion 510). 2.3. Experimental Procedure. All experiments were conducted in batch mode with continuous stirring. A 1000 mL capacity cylindrical borosilicate vessel (i.d., 10.5 cm) was used as the reactor system. A drug solution of 400 mL was taken for the experimentation. The Fenton experiment was carried out with 50 mg/L DIPY (≈4-MAA) at room temperature (23−25 °C). The initial concentration of DIPY was chosen based on the maximum concentration reported in literature.10 pH of the solution was adjusted using 0.05 N H2SO4 prior to addition of ferrous catalyst. It was noted that the concentration of 4-MAA was almost invariant in 2 h at pH 3.5 and temperature 25 °C. Subsequently the oxidation reaction was performed after 2 h of an initial premixing period. Until and otherwise outlined, the efficiency of drug degradation and mineralization is reported with respect to the concentration of 4-MAA. A predetermined amount of Fe2+ was added first and mixed for about 5 min at 260 rpm on a magnetic stirrer of Tarsons, Kolkata, India (model Spinot 6020; stirring ba:, i.d., 0.8 mm; length, 40 mm). After that H2O2 was added to 4-MAA/Fe2+ solution. The agitation was maintained at the same speed. A sample volume of 10 mL was withdrawn at selected time intervals. A 1 mL aliquot of 0.1 N NaOH was immediately added into the sample to terminate the oxidation reaction. Addition of NaOH increased the solution pH at ∼12.3. Sludge was separated out by centrifuging (Remi Instruments Ltd., Mumbai, India; model R23 8/06) at 2000 rpm for 30 min. Clear liquid was pipetted out for pH, COD, TOC content, and drug concentration. Clear liquid sample was heated at 70 °C for the destruction of residual H2O2 (if any) prior to COD measurement. The supernatant was filtered using 0.45 μm cellulose filter (serial no. 08091ID0683) of Pall India Pvt. Ltd. (Bangalore, India), before LC-MS analysis. 4MAA sorbed in sludge, if any, was determined by washing it in distilled water followed by digestion (dissolution) in H2SO4 (1 M). In the case of PFP, the experiment was performed following the same procedure in the presence of UV light. An UV lamp of 9 W from Hong Kong Jie Meng International Lighting Ltd. Company, China (wavelength, 362 nm; intensity, 12 W/m2) was employed for this work. The UVP experiment was conducted with a fixed concentration of H2O2 under UV irradiation without Fe2+. The UVPC test was carried out using TiO2 photocatalyst. The above experimental procedure was adopted for the UVPC experiment with a fixed amount of TiO2 (1 g/L) only. All of the experiments were performed in duplicate, and the average values are reported.

The metabolites of DIPY enter into the environment from several anthropogenic sources such as pharmaceutical industry and infirmary effluent, water disinfection, and waste incineration facilities. These compounds are not completely eliminated by biological treatment, and thus their presence has been referenced in sludge treatment plant effluents and surface water at high concentrations. Proper removal of DIPY and its metabolites present in water and wastewater has an important role in the prevention of diseases both in humans and animals. AOPs can be employed for the detoxification of such compounds until the biodegradability is improved to a level amicable for subsequent biological treatment.7 However, there is a lacuna of investigation for the selection of suitable oxidation process(es) to achieve biodegradability to such extent. Therefore, in this study, four AOPs, i.e., FP, PFP, UVP, and UVPC, are compared based on their performance for the: (i) oxidation and mineralization of 4-MAA (primary metabolite of DIPY), (ii) formation of degradation products, and (iii) variation of biodegradability (BOD5/COD where BOD is the biochemical oxygen demand and COD is the chemical oxygen demand). A unified mechanism of 4-MAA oxidation is presented. It would help to elucidate the effect of unreacted drug and degradation products on the variation of biodegradability of different oxidation processes.

2. MATERIAL AND METHODS 2.1. Reagents. Dipyrone (purity > 99%) was obtained from Sigma Aldrich Chemical Ltd. (Hong Kong, China). The chemical structure of DIPY is illustrated in Figure 1. DIPY is hydrolyzed rapidly at low concentration. Almost complete hydrolysis of DIPY to 4-MAA in a solution of 0.01 mM occurs in 30 min at pH 2.5 and temperature 21 °C.8 Methanol (98% (v/ v) purity) and acetonitrile (99% (v/v) purity) of high performance liquid chromatographic (HPLC) grade were procured from Merck Specialties Pvt Ltd. (India). Ferrous ammonium sulfate heptahydrate (99% purity), sulfuric acid (98% purity), titanium dioxide (TiO2; purity, 99% (w/w); crystal type, rutile; specific surface area, 391 m2/g), H2O2 (50% (v/v) purity), silver sulfate (Ag2SO4), and K2Cr2O7 (purity > 98%) were obtained from Merck. Milli-Q water (model Elix 3, Millipore, Billerica, MA, USA) was used to prepare all reagent and drug solutions. 2.2. Analytical Techniques. High performance liquid chromatography was used for the determination of 4-MAA concentration. A 20 μL sample was injected directly into a C18 column of 4.6 mm inside diameter and 25 mm length. HPLC instrument of Shimadzu, Japan (model LC-20AD) equipped with an UV−visible detector was employed for the chromatographic measurement. Methanol and water at the flow rate of 0.5 mL/min (80:20 (v/v)) was used as the mobile phase. The scanning was performed at a fixed wavelength of 254 nm. Liquid chromatography−time-of-flight mass spectrometry (LC-TOFMS; Waters Q-Tof Premier & Aquity UPLC) system was employed for the identification of drug fragments. The chromatographic separation was performed on a YMC (Wilmington, NC, USA) hydrosphere C18 reverse phase column (4.6 mm × 150 mm, 5 μm particle size) following a guard column (4 mm × 10 mm, 5 μm particle size) using a mobile phase flow rate of 0.8 mL/min at 25 °C. The mobile phase was consisting of H2O and acetonitrile with 0.1% (v/v) formic acid. A linear gradient of 95 to 50% H2O was applied over 10 min. A 10 μL aliquot of both sample and calibration solution was injected from an autoinjector. The samples were analyzed by electrospray 1352

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3. RESULTS AND DISCUSSION 3.1. 4-MAA Removal and Mineralization. Concentrations of Fe2+ and H2O2 and pH were varied from 1 to 3 mM, from 5 to 25 mM, and from 2 to 4, respectively. The results are shown in Figures S1−S3 of the Supporting Information. Drug removal efficiency increased with rise of one of these parameters, i.e., pH and Fe2+ and H2O2 doses. It reached a maximum and then fell. So, the catalytic effect of Fe2+ was reduced at lower as well as at higher concentration of a reactant. The best performance of Fenton oxidation in terms of 4-MAA and TOC removal is illustrated in Figure 2. PFP was carried out with the same

might cause catalyst agglomeration and hindrance to light penetration.11 pH determines the surface charge of the TiO2 photocatalyst (pHzpc ≈ 6.8). Lower pH favors 4-MAA adsorption. However, too strong 4-MAA adsorption probably decreased the absorption efficiency of light quanta at lower pH. The best performance of TiO2 photocatalysis with these experimental conditions are in Figure 2. Temperature, solution volume, reaction time, and extent of mixing were the same for all of the processes in order to compare their efficiencies for degradation of 4-MAA. The dynamics of 4MAA and TOC removal are shown in Figure 2a,b. The order of percentage removal of 4-MAA at any time of the reactor was found to be PFP > FP ≫ UVP > UVPC. Percentage removal of drug achieved to 96.4, 94.1, 74.4, and 71.1%, respectively, in 45 min. Production of OH• radicals according to eqs 1−5 causes degradation of 4-MAA as a function of time based on the oxidation process. Fe 2 + + H 2O2 = Fe3 +aq + OH− + OH• (k = 76 M−1 s−1)

(1)

Fe(OH)2 + + hv → Fe 2 + + OH• (k = 3.1 × 10−3 M−1 s−1)

H 2O2 + hν → 2OH•

(2)

(k = 3.3 × 107 M−1 s−1) (3)

+

TiO2 (h

vb)

+ H 2Oad = TiO2 + OH

• ad

TiO2 (h+ vb) + OH−ad = TiO2 + OH•ad

+H

+

(4) (5)

4-MAA removal showed two distinct rate periods (Figure 2a), i.e., initial faster drug removal followed by virtually constant rate even though there was a notable amount of unreacted 4-MAA present in solution in the case of UVP and UVPC. The transition for these two processes lasted for a long period (2.5−15 min) compared to the Fenton reaction with and without UV light (2.5−10 min). It implies that OH• generation is predominant at 2.5 min). In the case of FP and PFP, drug removal of 73.5 and 83.2% at 2.5 min of oxidation rose to 94.1 and 96.4%, respectively, in 45 min. OH• + H+ + e− → H 2O

(6)

H 2O2 + H+ → H3O2+

(7)

Application of UV light during Fenton reaction generates excess hydroxyl radicals, and ferric ions are reduced into ferrous leading to further generation of OH• (eqs 1 and 2). Increase in gross OH• concentration gave a bit higher degradation of 4-MAA in PFP. It is evident from Figure 2a that UVP and UVPC are less effective compared to the other two processes. UVP showed higher removal efficiency than UVPC. The first process exhibits a rate constant of nearly 25 times higher than the UV/TiO2 process.12 The drug removal achieved was 56.4 and 44.5% within the first 2.5 min in UVP and UVPC, respectively. It increased to 74.4 and 71.1% in 45 min. OH• radicals are generated by photolysis of the peroxide bond (eq 3). It formed on the surface of TiO2 (eqs 4 and 5), and adsorbed 4-MAA on the surface is oxidized. Hence, the reaction occurs at the diffusioncontrolled regime in UVPC.13

Figure 2. (a) Removal of 4-MAA in different AOPs with reaction time. (b) Removal of TOC. Initial 4-MAA concentration, 50 mg/L; TOC = 23.4 mg/L; temperature, 25 °C. FP: Fe2+, 2.25 mM; H2O2, 22.5 mM; pH, 3.5. PFP: Fe2+, 2.25 mM; H2O2, 22.5 mM; pH, 3.5; UV light, 9 W. UVP: H2O2, 22.5 mM; pH, 3.5; UV light, 9 W. UVPC: TiO2, 1.0 g/L; pH, 2.5; UV light, 9 W.

operating conditions. UVP was conducted with the same amount of H2O2 added in the Fenton reaction. In the case of the heterogeneous catalytic reaction, catalyst dose and pH were varied from 0.5 to 2.5 g/L and from 2 to 4.5, respectively. Their effects on 4-MAA removal are in Figures S4 and S5 of the Supporting Information. Increase in the TiO2 dose from 1 g/L had a negative effect on drug removal. Higher concentration 1353

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Figure 3. Degradation pathways of 4-MAA and identification of intermediates were obtained in different oxidation processes at 10 min of oxidation (photoreaction with an UV lamp of 9 W; FP, PFP, and UVP conducted at pH 3.5 and UVPC at pH 2.5): (a) Cleavage of 4-MAA; (b) degradation of D2 (m/z = 279.11) continued from panel a; (c) D5 (m/z = 176.07) cleavage continued from panel b. The exact mass to charge ratio is in parentheses.

The trend of mineralization was similar to that of drug removal. The highest TOC removal took place in PFP. It was very close in the cases of FP and UVP (Figure 2b). The initial rapid mineralization was up to 2.5 and 5 min for FP and PFP. However, it was not so distinct for the other two processes. TOC removal of 28.4 and 42.78% was obtained in 2.5 and 5 min with

FP in addition to 53.6 and 56.0% being obtained in 2.5 and 5 min for PFP. The first stage of TOC removal was very fast due to mineralization of three-methyl moieties. The slow second stage is related to the opening and mineralization of pyrazolinone ring.14 Usually, large molecular weight intermediates were either mineralized or broken to lower molecular weight products like 1354

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oxalic and acetic acids.15 Kavitha and Palanivelu indicated that carboxylic acids are eliminated in PFP very quickly during the first stage of oxidation.16 Bauer et al. reported continuous formation as well as degradation of lightweight carboxylic acids during UVPC.17 There was a difference of about 20% in drug removal between FP and UVP. However TOC reduction was almost the same. It is necessary to mention that H2O2 (22.5 mM) alone gave around 12.8% 4-MAA decomposition. However, only UV irradiation did not show notable effect on drug removal. There is a significant role of iron-chelation on drug mineralization. Iron(III)-chelate complexes are more stable in PF because of lower possibility of further degradation.18 PFP would have higher ability to degrade such complexes under UV luminesce. Knight et al. suggested that Fe(III)-humate complexes are easily reduced to Fe(II) by H2O2 in PFP.19 In addition, humic acids themselves are photodegraded through formation Fe(III) complexes.20 Klamerth et al. pointed out that PFP could breakdown metal complexes of molecular acids and iron through separation between heavy metal and its complexing agent.21 TOC removed in PFP mostly corresponds to oxidation of hydrophobic components which predominate in natural organic compounds present in raw water.22 It can be explained by greater aromaticity of the hydrophobic fraction giving higher reactivity toward oxidizing agents. The refractory hydrophilic fraction is consisting of short-chain aliphatic amines, alcohols, aldehydes, esters, ketones, aliphatic amides ( 0.4.34 The initial BOD5 and COD were 3.75 and 41.3 mg/L for an aqueous solution with 50 mg/L DIPY. It implies the nonbiodegradable nature of DIPY (BOD5/COD ≈ 0.1). From Figure 2a it is evident that the extent of drug decomposition was around 5% higher in PFP than FP. Nevertheless, PFP resulted in notably higher biodegradability (Figure 4). A BOD5/COD = 1.51 was achieved in PFP in

the Supporting Information). Indeed, it can be seen from Supporting Information Table S1 that compounds formed in UVPC were consisting of lower DBE. The highest DBE of 9 was calculated for both D6 and D7. There are also possibilities for cleavage of D2 following path 3 and path 4 as shown in Figure 3b. Path 3 shows the route of formation of D11 and D12 corresponding to m/z of 186.11 and 171.21, respectively. D11 and D12 both were formed from the same intermediate (m/z = 164.11) via oxidation of D4 originated by hydrolysis of D3 compound (path 1) as presented before. This compound seems to be more stable in FP. Further attack of OH• at the α-carbon of D4 caused formation of D11 in the case of PFP. The carbonyl group is very susceptible to being oxidized at lower pH ( 8). It might have formed during pH adjustment at >12 for the termination of oxidation reaction. Further oxidation of D23 compound yielded the corresponding quinone−imine inter-

Figure 4. Variation of BOD5/COD in different oxidation processes and its initial value (initial 4-MAA, 50 mg/L; oxidation time, 10 min; temperature, 25 °C): with FP, 2.25 mM Fe2+, 22.5 mM H2O2, and pH 3.5; with PFP, 2.25 mM Fe2+, 22.5 mM H2O2, pH 3.5, and 9 W UV light; with UVP, 22.5 mM H2O2, pH 3.5, and 9 W UV light; with UVPC, 1.0 g/ L TiO2, pH 2.5, and 9 W UV light.

comparison to 0.62 in FP. It implies that the first process generated more (gross) biodegradable intermediates. UVPC exhibited about 20% higher BOD5/COD improvement than UVP (Figure 4). It was about 11% higher than the corresponding drug decomposition. Comparatively lower BOD5/COD in UVP and UVPC than FP and PFP was largely due to unreacted 4-MAA and intermediate products. Hyvonen et al. reported that a higher extent of conjugation due to formation of chelates of three different heavy metals, i.e., Cd(II), Hg(II), and Pb(II), and (dicarboxyethoxy)ethyl]aspartic acid (BCA6) improves biodegradability.35 Kummerer et al. reported that newly formed 1356

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(3) Malato, S.; Blanco, J.; Vidal, A.; Richter, C. Photocatalysis with solar energy at a pilot-plant scale: An overview. Appl. Catal. B: Environ. 2001, 37, 1−15. (4) Pignatello, J. J.; Oliveros, E.; MacKay, A. Advanced oxidation processes for organic contaminant destruction based on the Fenton reaction and related chemistry. Crit. Rev. Environ. Sci. Technol. 2006, 36, 1−84. (5) Malato, S.; Fernandez-Ibanez, P.; Maldonado, M. I.; Blanco, J.; Gernjak, W. Decontamination and disinfection of water by solar photocatalysis: recent overview and trends. Catal. Today 2009, 147, 1− 59. (6) Oliveira, R.; Almeida, M.; Santos, L.; Madeira, L. Experimental design of 2,4-dichlorophenol oxidation by Fenton’s reaction. Ind. Eng. Chem. Res. 2006, 45, 1266−1276. (7) Morais, J. L.; Zamora, P. P. Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates. J. Hazard. Mater. 2005, 123, 181−186. (8) Ergun, H.; Fratarelli, D. A. C.; Aranda, J. V. Characterization of the role of physicochemical factors on the hydrolysis of dipyrone. J. Pharm. Biomed. Anal. 2004, 35, 479−487. (9) APHA. Standard Methods for the Examination of Water and Wastewater, 20th ed.; American Public Health Association: Washington, DC, USA, 1998. (10) Huber, M. M.; Gobel, A.; Joss, A.; Hermann, N.; Loffler, D.; Mcardell, C. S.; Ried, A.; Siegrist, H.; Ternes, T. A.; Von Gunten, U. Oxidation of pharmaceuticals during ozonation of municipal wastewater effluents: A pilot study. Environ. Sci. Technol. 2005, 39, 4290−4299. (11) Sohrabi, M. R.; Ghavami, M. Taguchi experimental design used for Nano photo catalytic degradation of the pharmaceutical agent Aspirin. J. Chem. Pharm. Res. 2008, 153 (3), 1235−1239. (12) Jayson, G. G.; Parsons, B. J. Oxidation of ferrous ions by perhydroxyl radicals. Trans. Faraday Soc. 1972, 68, 236−242. (13) Petrovic, M. M.; Bianca, F. S.; Aleksandra, J. R. Oxidative transformation of fluoroquinolone antibacterial agents and structurally related amines by manganese oxide. Chemosphere 2011, 85, 1331−1339. (14) Gomez, M. J.; Martınez-Bueno, M. J.; Lacorte, S.; FernandezAlba, A. R.; Aguera, A. Pilot survey monitoring pharmaceuticals and related compounds in a sewage treatment plant located on the Mediterranean coast. Chemosphere 2007, 66, 993−1002. (15) Leonidas, A.; Perez-Estrada, S. M.; Ana, A. R. Degradation of dipyrone and its main intermediates by solar AOPs Identification of intermediate products and toxicity assessment. Catal. Today 2007, 129, 207−214. (16) Kavitha, V.; Palanivelu, K. The role of ferrous ion in Fenton and photo-Fenton processes for the degradation of phenol. Chemosphere 2004, 55, 1235−1243. (17) Bauer, R.; Waldner, G.; Fallmann, H.; Hager, S.; Klare, M.; Krutzler, T.; Malato, S.; Maletzky, P. The photo-fenton reaction and the TiO2/UV process for wastewater treatmentNovel developments. Catal. Today 1999, 53, 131−144. (18) Trovo, A. G.; Raquel, F. P.; Nogueira, A. A.; Carla, S. A. Photodegradation of sulfamethoxazole in various aqueous media: Persistence, toxicity and photoproducts assessment. Chemosphere 2009, 77, 1292−1298. (19) Knight, R. J.; Sylva, R. N. Spectrophotometric investigation of iron (III) hydrolysis in light and heavy water at 25°C. J. Inorg. Nucl. Chem. 1975, 37, 779−783. (20) Fukushima, M.; Tatsumi, K.; Nagao, S. Degradation Characteristics of Humic Acid during Photo-Fenton Processes. Environ. Sci. Technol. 2001, 35, 3683−3690. (21) Klamerth, N.; Malato, S.; Aguera, A.; Fernandez-Alba, A. PhotoFenton and modified photo-Fenton at neutral pH for the treatment of emerging contaminants in wastewater treatment plant effluents: A comparison. Water Res. 2013, 47, 833−840. (22) Buchanan, W.; Roddick, F.; Porter, N.; Drikas, M. Fractionation of UV and VUV pretreated natural organic matter from drinking water. Environ. Sci. Technol. 2005, 39, 4647−4654.

intermediates act as chelating agents that significantly reduce their biodegradability.36 It can be seen from a to c of Figure 3 that D2, D3, D14, and D23 could form chelates with Fe3+. D1, D2, D6, and D25 traced in FP could exhibit more biodegradability because of extended conjugation. Therefore, the net effect of chelation and its stability under UV irradiance and formation of intermediate products and their different extent of conjugation resulted in higher biodegradability in PFP. The biodegradable nature of a number of intermediates as in the proposed structure is found out from the earlier reports. It is summarized in Table S1 of the Supporting Information.

4. CONCLUSIONS Dipyrone at lower concentration (0.149 mM) was almost completely hydrolyzed to 4-MAA within 2 h at pH 3.5 and temperature of 25 °C. Maximum 4-MAA removals of 94.1, 96.4, 74.4, and 71.2% were noted against mineralization efficiencies of 49.3, 58.2, 47, and 24.6% in Fenton, photo-Fenton, H2O2 photolysis, and TiO2 photocatalysis, respectively. PFP showed better mineralization efficiency due to additional amount of OH• formation and collapse of iron complexes under UV irradiation probably because of enhanced decarboxylation. However, in the case of TiO2 photocatalysis more hydroxylated products were identified that resulted from direct attack of OH• ad. A total of 29 intermediate products appeared in the mass spectra within the mass to charge ratio of 100−400 in four oxidation processes from the same parent molecule. Pyrazolinone ring was degraded preceded by cleavage of methyl moieties. The proposed mechanism implies that most of the intermediates were formed by pyrazolinone ring degradation. The order of biodegradability in different oxidation processes was found to be PFP≫FP > UVP > UVPC.

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ASSOCIATED CONTENT

S Supporting Information *

Table S1 and Figures S1−S9 containing additional information on intermediate products, experimental optimization of CIP decomposition, and mass spectra as outlined in the text. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +91-361-2582269. Fax: +91361-258-2292. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully thank the Indian Institute of Guwahati (India), for providing the research fellowship to Mr. Ardhendu Sekhar Giri and necessary research facilities to the Department of Chemical Engineering and Central Instruments Facility (CIF).

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REFERENCES

(1) Moldovan, Z. Occurrences of pharmaceutical and personal care products as micropollutants in rivers from Romania. Chemosphere 2006, 64, 1808−1817. (2) Torres, R.; Sarria, V.; Torres, W.; Peringer, P.; Pulgarin, C. Electrochemical treatment of industrial wastewater containing 5-amino6-methyl-2-benzimidazolone: toward and electrochemical-biological coupling. Water Res. 2003, 37, 7−13. 1357

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dx.doi.org/10.1021/ie402279q | Ind. Eng. Chem. Res. 2014, 53, 1351−1358