H2O2: photo-Fries rearrangement versus hydroxyl radical induced hydroxylation.

w a t e r r e s e a r c h x x x ( 2 0 1 5 ) 1 e1 1

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New insights into the primary phototransformation of acetaminophen by UV/ H2O2: photo-Fries rearrangement versus hydroxyl radical induced hydroxylation Shixiang Feng a, Xu Zhang a,*, Yanxiang Liu b,* a b

School of Resources and Environmental Science, Wuhan University, Wuhan, 430079, PR China School of Chemical and Environmental Engineering, Jianghan University, Wuhan, 430056, PR China

article info

abstract

Article history:

The phototransformation of acetaminophen (APAP) by UV/H2O2 in deionized water and

Received 22 February 2015

sewage treatment plant (STP) effluents was studied systematically by a combination of

Received in revised form

analysis of the reaction intermediates and kinetic study. 1-(2-amino-5-hydroxyphenyl)

30 April 2015

ethanone (P1) and the reported N-(3,4-dihydroxyphenyl)acetamide (P2) were identified as

Accepted 3 May 2015

the main transformation intermediates during the transformation of APAP by UV/H2O2.

Available online xxx

There was no influence of OH on the formation kinetics of P1, while its decay was promoted. The formation and decay kinetics of P2 were accelerated by increases in the con-

Keywords:

centration of OH. The second-order rate constants for the reaction of OH with APAP, P1,

Acetaminophen

and P2 were 3.9 109, 8.1 109, and 4.7 109 M1 s1, respectively. The kinetic study

Photo-Fries

indicated that the main transformation of APAP also included transformation to 1,4-

Hydroxylation

hydroquinone, although the accumulated concentration of 1,4-hydroquinone was quite

UV/H2O2

2 , NO low. The presence of anions (Cl, HCO 3 /CO3 2 /NO3 ), humic acid, commercial drug

Kinetics

components or adjuvants, and dissolved organic matters in STP effluents not only changed

Mechanism

the transformation kinetics of APAP, but also altered the distribution of the intermediates. The kinetics and pathway of APAP transformation in STP effluent were markedly different from those in deionized water. © 2015 Elsevier Ltd. All rights reserved.

1.

Introduction

Pharmaceutical and personal care products (PPCPs) are considered to be emerging pollutants. PPCPs are released into the environment through the discharge of manufactured wastewater, human excrement, and the improper disposal of commercial drugs. PPCPs cannot be completely removed by

conventional biological processes and are generally found at low concentrations (from ng/L to mg/L) in sewage treatment plant (STP) effluent (Boyd et al., 2003; Hedgespeth et al., 2012). Acetaminophen (APAP) is one of the most frequently detected PPCPs. APAP, also known as paracetamol, is the major ingredient in most commercial medicines for the common cold and is used as a pain reliever worldwide. Combinations of conventional wastewater treatments, including activated sludge

* Corresponding authors. Tel.: þ86 27 68772910. E-mail addresses: [email protected] (X. Zhang), [email protected] (Y. Liu). http://dx.doi.org/10.1016/j.watres.2015.05.008 0043-1354/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Feng, S., et al., New insights into the primary phototransformation of acetaminophen by UV/ H2O2: photo-Fries rearrangement versus hydroxyl radical induced hydroxylation, Water Research (2015), http://dx.doi.org/ 10.1016/j.watres.2015.05.008

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and membrane bioreactors, with advanced technologies, such as UV photolysis or UV-based advanced oxidation processes (UV/H2O2, UV/O3), have the potential to remove PPCPs from wastewater (Homem and Santos, 2011). The phototransformation of APAP by UV photolysis and UV/H2O2 has been widely studied and some conclusions have been reached. APAP was shown to be rapidly removed by a UV/H2O2 process, with hydroxyl radical (OH) induced hydroxylation/oxygenation identified as the main degradation pathway (Andreozzi et al., 2003; Vogna et al., 2002). A kinetic assessment of the degradation of APAP has been conducted and the second-order rate constant of APAP with OH was found to be 2.2 109 M1 s1 (Andreozzi et al., 2003). The primary transformation intermediate, with a high accumulated concentration is N-(3,4-dihydroxyphenyl)acetamide (P2). N-(2,4-dihydroxyphenyl)acetamide (P3), 1,4-hydroquinone (P4), and 1,4-benzoquinone (P5) have been identified as transformation products with relatively low accumulated concentrations (Andreozzi et al., 2003; Vogna et al., 2002). The phototransformation of APAP by direct UV photolysis was systematically studied about 10 years after the phototransformation of APAP by UV/H2O2 (De Laurentiis et al., 2014; Kawabata et al., 2012; Martignac et al., 2013; Peuravuori, 2012; Pozdnyakov et al., 2014; Zhang, 2011). The transformation properties of APAP under UV direct photolysis are markedly different from its properties in the transformation by UV/ H2O2. Our previous study indicated that the UV transformation of APAP was dependent on wavelength (Pozdnyakov et al., 2014). APAP undergoes two photon photoionizations under UV irradiation at 266 nm. The quantum yield for photolysis of APAP at 266 nm (0.013) is approximately 10 times that at 254 nm (0.0014). P2 was identified as the major transformation intermediate under irradiation at 266 nm. Photo-Fries rearrangement was found to be the predominant transformation pathway for direct UV photolysis of APAP at 254 nm (Martignac et al., 2013; Peuravuori, 2012; Pozdnyakov et al., 2014; Zhang, 2011), leading to the generation of P1. The toxicity of P1 is much higher than that of APAP (Kawabata et al., 2012). However, P1 was not detected, or at least not reported, during the degradation of APAP by UV photolysis at 254 nm in the presence of H2O2 (Andreozzi et al., 2003; Vogna et al., 2002). The competition of H2O2 for irradiation photons can be ignored because the molar extinction coefficient of H2O2 is much smaller than that of APAP at 254 nm. Thus, it was expected that the formation of P1 through photo-Fries rearrangement could not be altered by H2O2. On the other side, P1 could possess high reactivity with OH, which made the observation of P1 conditional on its reaction kinetics. Although there have been previous reports of the degradation kinetics of APAP (Andreozzi et al., 2003; De Laurentiis et al., 2014), there have been no kinetic studies of the formation and degradation of phototransformation intermediates. Moreover, as OH is a nonselective oxidizer, coexisting inorganic ions, natural dissolved organic matter, and commercial drug constituents and adjuvants could compete for OH with APAP. Therefore, it is necessary to investigate the effects of this coexisting matter on the phototransformation of APAP by UV/H2O2. The first objective of this study was to determine the effect of H2O2 concentration on the phototransformation of APAP,

and assess the formation and degradation kinetics of the main intermediates in deionized water and STP effluents. The second objective was to determine the influences of the aquatic environment on the phototransformation of APAP and generation of transformation intermediates. Specifically, the phototransformation of 20 mM APAP in the presence of 50e800 mM H2O2 was investigated in deionized water and STP effluents. The effects of various anions and dissolved humic acids (HA, 0e4.8 mgC/L) on the transformation of APAP and generation of intermediates were investigated in deionized water. The phototransformation of APAP released from commercial drugs by UV/H2O2 was investigated to study the effects of co-constituents and adjuvants on the transformation of APAP. A kinetic study of the phototransformation of APAP, including the formation and decay of the main intermediates, was also performed.

2.

Materials and methods

2.1.

Chemicals

The chemical compound APAP (97%þ) was purchased from Alfa-Aesar (Morecambe, UK) and used as received. The photoFries rearrangement product (P1), and the hydroxylation products (N-(3,4-dihydroxyphenyl) acetamide (P2) and N-(2,4dihydroxyphenyl)acetamide (P3)) were synthesized according to previously reported methods (Evagelos et al., 2004; Perzyna et al., 2004; Whiteley, 2002). Proton nuclear magnetic resonance (1H NMR: 300 MHz, DMSO) analysis demonstrated that the synthesized compounds had purities higher than 95%. As standards, 1,4-hydroquinone (P4, 99.5%), 1,4-benzoquinone (P5, 98%) and 4-aminophenol (P6) were purchased from SigmaeAldrich (St. Louis, MO). 1-(5-Amino-2-hydroxyphenyl) ethanone (P7, 95%þ), the isomer of P1, was obtained from Bide Pharmatech Ltd. (Shanghai, China). N-acetyl-p-benzoquinone imine (NAPQI, P8) was obtained from SigmaeAldrich (St. Louis, MO). The chemical structures of APAP and its related primary phototransformation products are shown in Scheme 1. Crude humic acid (HA) and amantadine hydrochloride (99%) were purchased from Aladdin Industrial Corporation (Shanghai, China). The purification of HA was performed according to the recommended method (Stevenson, 1994) and the final concentration of HA in its stock solution used in this work was 30 mgC/L. Deionized water (18.2 MUcm) was used as the solvent unless otherwise specified. The STP effluent was collected from the outlet of an STP in Wuhan, China. The collected solutions were filtrated with a 0.45 mM mixed cellulose ester membrane and immediately used as solvent. The details of the effluent are provided in Table S1 (Supporting Information). Anions (Cl, 2 HCO 3 /CO3 , NO2 /NO3 ) were introduced in their form of sodium salts.

2.2.

Phototransformation of APAP by UV/H2O2

The phototransformation of APAP was performed in a homemade reactor (Fig. S1, Supporting Information). The irradiation source was a TUV PLS 9-W lamp (Philips, Eindhoven, Netherlands). The photon flux at 254 nm determined


3


Scheme 1 e Chemical structure of APAP and related transformation products.

using a ferrioxalate actinometer was 7.47 106 mol s1 (Montalti et al., 2006). The temperature of reaction solution was maintained at 25 C by using a thermostat circulating water bath. For the direct UV photolysis of acetaminophen, reaction solutions (250 mL) containing 20 mM APAP were placed in the reactor and stirred with a magnetic stirrer. The lamp was then turned on, and 1-mL aliquots were drawn at various time intervals. For the UV/H2O2 oxidation of acetaminophen, a given volume of H2O2, acetaminophen and/or 2 other components (HA, Cl, HCO 3 /CO3 , NO2 /NO3 , or amantadine hydrochloride) aqueous solution were added to a 250-mL flask at the settled concentrations (50e800 mM for H2O2 and 20 mM for acetaminophen) and diluted with either deionized water or filtrated STP effluent. Then the solution was replaced to the reactor and the following procedure was the same as described for direct UV photolysis. Commercial drugs containing APAP were dissolved in deionized water and diluted to 20 mM (APAP). The solutions were then irradiated as described above to investigate the effects of pharmaceutical constituents on the phototransformation of APAP.

2.3. Competition method for measuring the second-order rate constants for the reaction of OH with APAP, P1, and P2 The rate constants for the reactions of OH with APAP and its main intermediates (P1 and P2) were measured using the competition reaction kinetic method. Benzoic acid (BA) was selected as the reference compound. [OH]ss is the quasisteady state concentration of OH during the reactions.

d½P1 ¼ k OH=P1 ½OHss ½P1 dt

(3)

d½P2 ¼ k OH=P2 ½OHss ½P2 dt

(4)

The rate constants for the reaction of OH with APAP (k OH/ P1 (k OH/P1), and P2 (k OH/P2) were determined by Eqs. (1)e(4) using the known reaction constant between OH and BA (k OH/BA, 4.3 109 M1 s1) (Anastasio and Newberg, 2007).

APAP),

2.4.

Sample analysis

The phototransformation of APAP and the generation of its transformation products were monitored by highperformance liquid chromatography (HPLC). The concentrations were calibrated using a working curve of the corresponding standard compounds. HPLC chromatograms were recorded on a Waters 484 UV detector (Waters, Milford, MA) at 250 nm. Separation was performed with a Zorbax SB-C18 column (5 mm, 4.6 250 mm; Agilent, Atlanta, GA) using a mobile phase of methanol/1% NH4Cl water (30/70, v/v) with a flow rate 1.0 mL min1. Total organic carbon (TOC) reduction was monitored by using a TOC analyzer (liquiTOC II, Elementar) to reveal the mineralization of APAP during the phototransformation process.

3.

Results and discussion Effects of H2O2 dosage on phototransformation of

d½BA ¼ k OH=BA ½OHss ½BA dt

(1)

3.1. APAP

d½APAP ¼ k OH=APAP ½OHss ½APAP dt

(2)

The effects of H2O2 concentration on the phototransformation of APAP and the formation of intermediates in deionized


4


aqueous solutions are shown in Fig. 1. As shown in Fig. 1a, APAP underwent rapid transformation under UV irradiation, and the presence of H2O2 accelerated the phototransformation process. Control experiments showed that the direct oxidation of APAP by H2O2 was negligible after 5 h without UV irradiation. TOC reduced dramatically with the degradation of APAP. 55% of APAP was mineralized in 60 min that indicating TOC removal was much slower than the disappearance of APAP because of the formation of intermediates (Figure S2, Supporting Information). P1 and P2

Fig. 1 e Effects of H2O2 concentration on the transformation of acetaminophen (a), formation and decay of P1 (b) and P2 (c) by UV/H2O2, curves represent the fitting of experimental data for APAP, P1 and P2 to Eq. (8), Eq. (11) and Eq. (12), respectively. [APAP]0 ¼ 20 mM.

were the main phototransformation products with high accumulated concentrations (Figure S3, Supporting Information). Gas chromatographyemass spectrometry (GCeMS) analysis confirmed P3, P4, and P6 as transformation products (Zhang, 2011). The maximal accumulated concentrations of these compounds were much lower than those of P1 or P2. Unlike the transformation of APAP by hypochlorite (Bedner and Maccrehan, 2006), N-acetyl-p-benzoquinone imine (NAPQI) (P8) was not detected during the transformation of APAP by UV/H2O2. NAPQI is a toxic byproduct produced during the xenobiotic metabolism of APAP mediated by cytochrome P-450 (Fischer et al., 1985), which causes the pathological effect of an APAP overdose. Therefore, UV/H2O2 has the potential to remove APAP from water without the generation of NAPQI. P1 was not detected, or was not reported, in the transformation of APAP by UV/H2O2 in previous studies (Andreozzi et al., 2003; Vogna et al., 2002). P1 was generated from the photo-Fries rearrangement of APAP and similar reactions have been observed for acetanilide derivatives, phenylurea herbicides, and aromatic esters (Amine-Khodja et al., 2004; pez et al., 2005; Shizuka, 1969a,b). The concentraCanle Lo tion of P1 changed during the reaction, as shown in Fig. 1b. As proposed in Scheme 2, CeN bond cleavage could result from either the first excited state of APAP (path a) or from the radical ion of APAP (path b). In path a, APAP jumps to its first excited state (S1) followed by the generation of an acetyl radical and an anilinyl radical, and rearrangement of the generated radicals leads to the photo-Fries product P1. No rearrangement product at the meta position (P7) was detected in this study. The results were consistent with the phototransformation of other p-substituted acetanilides (Shizuka, 1969b). The escape of the acetyl radical from the radical cage led to the formation of 4-aminophenol (P6). Photoionization of APAP led to the formation of a radical cation, which further generated an anilinyl radical and an acyl cation through cleavage of the CeN bond and subsequently resulted in the generation of P1. Hydroxyl radicals can attack the aromatic ring to form the hydroxylation products P2, P3, and P4 through the ipso-substitution reaction (Vogna et al., 2002). In this study, P2 was found to be one of the main transformation products in the presence of low H2O2 concentrations, and it was the predominant product with highest accumulated concentration in the presence of high H2O2 concentrations (Fig. 1c). The concentration of transformed APAP was 1.0 mM after irradiation for 45 s in the presence of 50 mM H2O2. The concentrations of accumulated P1 and P2 were 0.30 and 0.43 mM, which were 0.30 and 0.43 times that of the transformed APAP, respectively. When the concentration of H2O2 was increased to 100 mM, the concentration of transformed APAP was 2.0 mM after 45 s of irradiation. The concentrations of accumulated P1 and P2 were 0.34 and 0.91 mM, which were 0.17 and 0.46 times that of the removed APAP, respectively. These results indicate that H2O2 promoted accumulation of the hydroxylation product P2 and inhibited accumulation of the photoFries product P1. The concentrations of accumulated intermediates were lower in the presence of higher concentrations of H2O2, indicating that H2O2 also favored further degradation of the intermediates.


5


Scheme 2 e Proposed mechanism for the photo-Fries rearrangement of APAP in aqueous solution.

3.2. Kinetic assessment of APAP and its main intermediates A kinetic assessment of APAP phototransformation, and the formation and degradation of P1 and P2 was performed to determine the mechanism of APAP transformation during the UV/H2O2 process (Scheme 3). The transformation of APAP in UV/H2O2 can be described by Eq. (5).

d½APAP ¼ kd;APAP ½APAP þ k OH=APAP ½OHss ½APAP dt X þ kminor ½M½APAP X ¼ kd;APAP þ k OH=APAP ½OHss þ kminor ½M ½APAP

¼ kobs ½APAP (5) 1

1

1

s ) are the direct UV Here, kd (s ) and kminor (M photolysis reaction constant and the second-order rate

Scheme 3 e Mechanism for the transformation of APAP and its intermediates in the UV/H2O2 process.


6


constant of the other minor transformation process of APAP with other reactants (M), respectively, while kobs (s1) represents the observed pseudo-first-order reaction constant. The photo-Fries rearrangement of APAP was considered to be the only process for generation of P1. Direct UV photolysis and the reaction with OH are its decay processes. Therefore, Eq. (6) can be used to describe the kinetics of P1, where kd,P1 (s1) is the pseudo-first-order reaction constant for the direct photolysis of P1. d½P1 ¼ kformation;P1 ½APAP kd;P1 ½P1 k OH=P1 ½OHss ½P1 dt

(6)

Analogously, taking the reaction of APAP with OH as the formation path for P2 and direct UV photolysis (kd,P2, s1) and the reaction with OH as the decay processes of P2, Eq. (7) can be used to describe the kinetics of P2. d½P2 ¼ k OH=APAP ½OHss ½APAP kd;P2 ½P2 k OH=P2 ½OHss ½P2 dt

(7)

than in its absence. P1 could not be detected in the samples withdrawn after a long period of irradiation, which is probably why P1 was not detected, or at least not reported, in previous studies (Andreozzi et al., 2003; Vogna et al., 2002). Under UV irradiation, the OH concentration increased with increases in H2O2 concentration. Thus, it is expected that the kformation,P2 increased with increases in H2O2 concentration because P2 is formed through the reaction of APAP with OH. The values of kdecay,P2 were 1.3 103 s1 and 6.0 103 s1 in the presence of 50 mM and 200 mM H2O2, respectively. The observed kinetic constant for the direct UV photolysis of 10 mM of P2 was 0.4 103 s1, which is negligible to the overall kdecay,P2 in the presence of 200 mM H2O2. The kdecay,P1 in the presence of 200 mM H2O2 was approximately 40 times that of the kinetic constant for the direct UV photolysis of P1. Thus, the ratio between kdecay,P1 and kdecay,P2 in the presence of 200 mM H2O2 was approximately equivalent to the ratio between k OH/P1 [OH]ss and k OH/P2 [OH]ss. The second-order rate constant for the reaction of OH with P1 (k OH/P1) was 1.8 times (11.0/6.0) that for the reaction of OH with P2 (k OH/P2). As shown in Figures S5eS7 (Supporting Information), taking the reaction constant for the reaction of OH with benzoic acid (4.3 109 Me1 s1) as the reference (Anastasio and Newberg, 2007), the calculated second-order rate constants for the reaction of OH with APAP, P1, and P2 were 3.9 109, 8.1 109, and 4.7 109 Me1 s1, respectively. Thus, the ratio of k OH/P1 to k OH/P2 obtained from competition experiments was 1.7, which was close to the value obtained from the kinetic fit (1.8). It was apparent that the sum of kformation,P1 and kformation,P2 was much slower than the reaction constant for the overall transformation of APAP (kobs), indicating the existence of other major transformation pathways, although no other intermediates were detected with high accumulated concentrations besides P1 and P2. Vogna et al. reported that the formation ratio of acetamide/P2 was ca. 1:1 during the early stage of phototransformation of APAP by UV/H2O2 based on 1H NMR analysis, and concluded that hydroxylation of OH at the ortho and para positions proceeded to comparable degrees (Vogna et al., 2002). The attack of OH at the para positions led to the formation of P4 (1,4-hydroquinone). Nevertheless, the accumulated concentration of P4 was found to be much lower than P2 in both the report of Vogna et al. (Vogna et al., 2002) and this study, which was probably because of the fast decay or transformation of P4 to 1,4-benzoquinone (P5) or other hydroxylation products during the reactions. It is reasonable to assume that the formation constant of P4 (kformation,P4) is equivalent to that of P2 (kformation,P2), and it could be concluded that the sum of kformation,P1, kformation,P2, and kformation,P4 is 90% ± 5% the overall kinetic constant (kobs) for the phototransformation of APAP. The minor transformation process (kminor) made little contribution to the overall transformation of APAP. Thus, the predominant transformation path for APAP by UV/H2O2 included the formation of P1, P2, and P4 based on the results of analysis of intermediates and kinetic analysis.

By combining Eqs. (5)e(7) and the quasi-steady-state hypothesis for OH, the concentrations of APAP, P1, and P2 can be expressed as: ½APAP ¼ ½APAP0 ekobs t

(8)

kformation;P1 ½APAP0 ½P1 ¼ kd;P1 þ k OH=P1 ½OHss kobs ekobs t eðkd;P1 þk OH=P1 ½ OHss Þt

(9)

k OH=APAP ½OHss ½APAP0 ðkd;P2 þ k OH=P2 ½OHss Þ kobs ekobs t eðkd;P2 þk OH=P2 ½ OHss Þt

(10)

½P2 ¼

Taking kd,P1 þ k OH/P1 [OH]ss as kdecay,P1, Eq. (9) can be rewritten as Eq. (11)

½P1 ¼

kformation;P1 ½APAP0 kobs t e ekdecay;P1 t kdecay;P1 kobs

(11)

Analogously, by taking k OH/APAP [OH]ss as kformation,P2, and kd,P2 þ k OH/P2 [OH]ss as kdecay,P2, Eq. (10) can be rewritten as Eq. (12)

½P2 ¼

kformation;P2 ½APAP0 kobs t e ekdecay;P2 t kdecay;P2 kobs

(12)

The curves in Fig. 1 represent the fit of the experimental data for APAP, P1, and P2 to Eqs. (8), (11) and (12), respectively. The fitting results are listed in Table 1. As shown in Table 1, the kobs for transformation of APAP increased linearly with increasing H2O2 concentration. In the presence of 200 mM H2O2, the kobs was 8.7 times higher than the kobs in the absence of H2O2. It was notable that the addition of H2O2 did not change the formation kinetic constant of P1 (kformation,P1). The molar extinction coefficients of H2O2 and APAP at 254 nm were 21.0 and 8300 M1 cm1, respectively. Thus, the presence of 200 mM H2O2 would not influence the absorption of photons by 20 mM APAP, and thereafter barely affected the photo-Fries rearrangement. The presence of H2O2 accelerated the overall decay kinetics of P1 (kdecay,P1) because of the reaction of P1 with OH. Therefore, the accumulated concentration of P1 was much lower in the presence of H2O2

3.3. Effects of dissolved HA on the phototransformation of APAP As shown in Fig. 2, the presence of HA clearly inhibited the transformation of APAP and the retardation effect increased


7


Table 1 e Kinetic constants for the transformation of APAP, and the formation and decay of P1 and P2. Parameters Compound H2O2 (mM)

HA (mgC/L)f

Cl (mM)f

a b c d e f

APAP

P1

P2

Concentration

kobsa (103 s1)

kformation,P1b (103 s1)

kdecay,P1b (103 s1)

kformation,P2c (103 s1)

kdecay,P2c (103 s1)

0 50 75 100 200 0.6 1.8 3.0 4.8 0.5 1 2 5

0.6 1.6 2.4 3.0 5.2 2.8 2.4 1.8 1.5 2.7 2.0 1.9 2.1

0.46 0.44 0.44 0.47 0.44 0.37 0.31 0.25 0.19 0.58 0.52 0.56 0.56

0.28 2.8 3.6 5.1 11.0 5.0 4.4 3.3 3.3 6.3 4.1 3.9 3.9

ed 0.52 0.79 1.2 2.1 1.1 0.92 0.57 0.49 1.3 0.70 0.70 0.75

0.4e 1.3 1.9 3.1 6.0 2.3 1.8 1.6 1.4 3.1 1.8 1.8 1.8

Results obtained from the fit to Eq. (8). Results obtained from the fit to Eq. (11). Results obtained from the fit to Eq. (12). The accumulated concentration of P2 was too low for fitting. The result was obtained from the direct photolysis of a 10 mM P2 solution (Figure S4, Supporting Information). The experiments were carried out in the presence of 100 mM H2O2.

as the HA concentration increased from 0.6 to 4.8 mgC/L. The kobs decreased to 2.8 103 and 1.5 10e3 s1 in the presence of 0.6 and 4.8 mgC/L HA, respectively. The second-order rate constant (1.08 1010 Me1 s1) for the reaction of APAP with triplet states of chromophoric dissolved organic matter (CDOM, using anthraquinone-2-sulphonate as a proxy) has been reported (De Laurentiis et al., 2014). The constant was much larger than that for the reaction of APAP with OH. Nevertheless, HA retarded the transformation kinetics of APAP by UV/H2O2 in this study, indicating that the reaction of APAP with the triplet states of HA was negligible. HA also retarded the formation of P1 and P2, and no other new intermediates were generated during the phototransformation of APAP. After 18 min (1080 s) of irradiation, the accumulated P1 concentrations were 0.01 and 0.32 mM in the absence and presence of 4.8 mgC/L HA, respectively. The accumulated P2 concentrations were 0.48 and 2.3 mM in the absence and presence of 4.8 mgC/L HA, respectively. The kinetic study showed that the addition of HA decreased kformation,P1 and kformation,P2. As OH showed no influence on the formation kinetics of P1, the inhibitory effect of HA on the formation of P1 was probably due to competition for irradiation photons. 3.0 mgC/L HA has comparable absorbance at 254 nm to 20 mM APAP (Figure S6, Supporting Information). As The decrease in irradiation photons could subsequently retard the photo-Fries rearrangement of APAP. Quenching the singlet states of APAP by HA may also contribute to the decrease of kformation,P1. The addition of HA linearly decreased the kformation,P2, which was mainly due to competition for OH with APAP. The presence of HA markedly inhibited kdecay,P1 and kdecay,P2. In the presence of 0.6 mgC/L HA, kdecay,P1 and kdecay,P2 were reduced by 2% and 26%, respectively. A further increase of HA to 4.8 mgC/L HA reduced kdecay,P1 and kdecay,P2 by 35% and 55%, respectively. These results indicated that the decay of P2 was very sensitive to the

presence of HA. The potential environmental risk during the transformation of APAP by UV/H2O2 in the presence of HA could increase to some extent because the presence of HA favored accumulation of P1 and P2 over long periods, and these intermediates have higher toxicity than APAP.

3.4. Effects of chloride ions on the phototransformation of APAP The chloride ion (Cle) is the most common co-existing inorganic ion and is considered to be a scavenger of OH (Eq. (13)) (Jayson et al., 1973). The presence of Cle had different effects on degradation of the targets by UV/H2O2 because the reaction of the targets with the generated Cl and Cl2 could compromise the loss of OH (Yang et al., 2014). In this study, the addition of 0.5 mM Cle neither significantly affected the transformation of APAP nor influenced the formation and decay of P1 and P2 (Fig. 3).

OH þ Cl

4:3109 M1 s1

!

6:1109 s1

ClOH

(13)

The addition of 1 mM Cle to deionized water reduced kobs by 33% for the transformation of APAP by UV/H2O2. The kobs exhibited no further obvious changes with addition of more Cle to the reaction solution. The addition of high concentrations of Cle had different effects on the formation of P1 and P2, with a slight increase in the formation of P1 and a clear decrease in the formation of P2. It is expected that some OH could be captured by the addition of Cle to UV/H2O2, which would subsequently decrease the formation of P2. The beneficial effect of Cl on the formation of P1 was probably because the reaction of APAP with Cl , Cl2, or ClO could initiate photoFries rearrangement. Conversely, the addition of Cle decreased the decay of P1 and P2 due to its competition for OH.


8


3.5. Phototransformation of APAP released from commercial drugs Various co-constituents and adjuvants have been used to improve the performance or stability of APAP in commercial drugs (Table S2, Supporting Information). The phototransformation of APAP released from commercial drugs for the common cold was conducted to study the effects of their constituents and adjuvants on the transformation of APAP by UV/H2O2. As shown in Fig. 4, compared with transformation in deionized water, the presence of coexisting compounds released from different commercial drugs exhibited various

Fig. 2 e Effects of HA on the phototransformation of acetaminophen by UV/H2O2 (a), and its influence on the formation and decay of P1 (b) and P2 (c), [APAP]0 ¼ 20 mM, [H2O2]0 ¼ 100 mM.

effects on the transformation of APAP, formation and decay of P1 and P2 by UV/H2O2. The kobs for the transformation of APAP decreased by less than 10% in the presence of coexisting compounds. The presence of the coexisting compounds released from commercial drug 1 slightly increased kformation,P1 and kdecay,P1, but slightly decreased kformation,P2 and kdecay,P2. The presence of the coexisting compounds released from commercial drug 2 barely altered kformation,P1, but decreased kformation,P2 by ca. 30%. On the other hand, kdecay,P1 and kdecay,P2 were reduced by 19% and 32%, respectively, in the

Fig. 3 e Effects of Cl¡ on the phototransformation of acetaminophen by UV/H2O2 (a), and its influence on the formation and decay of P1 (b) and P2 (c), [APAP]0 ¼ 20 mM, [H2O2]0 ¼ 100 mM.


9


presence of coexisting compounds released from drug 2. The main constituent other than APAP in drug 2 was amantadine hydrochloride (Table S2, Supporting Information), with an APAP/amantadine hydrochloride molar ratio of 3.1. The rate constant for the reaction of amantadine with OH is 5.9 109 M1 s1 (Skolimowski et al., 2003), which is comparable to k OH/APAP, k OH/P1 and k OH/P2. Therefore, OH involved reaction constants including kformation,P2, kdecay,P1 and kdecay,P2 decreased because the coexisting amantadine hydrochloride could compete for OH. The influence of 6.4 mM amantadine hydrochloride on the phototransformation of 20 mM APAP, formation and decay of P1 and P2 were approximately the

same as the combined influence of coexisting constituents from commercial drug 2. The results indicated that other minor compositions in drug 2 exhibited slightly influence on the transformation of APAP. These results indicate that coexisting constituents or adjuvants from commercial drugs could interfere with the transformation kinetics and distribution of transformation intermediates.

3.6.

Phototransformation of APAP in STP effluent

The phototransformation of APAP in STP effluent was conducted to assess the performance of UV/H2O2 in dealing with real waters polluted with trace APAP. As shown in Fig. 5, the transformation of APAP in STP effluent was much smaller than the transformation in deionized water. In the presence of 100 mM H2O2, the kobs (0.0008 s1) for the transformation of APAP in STP effluent decreased by 74% compared to the kobs in deionized water. The kobs obtained in STP effluent in the presence of 800 mM H2O2 was equivalent to the kobs obtained in deionized water in the presence of 100 mM H2O2 (Figure S8, Supporting Information). Moreover, accumulated concentrations of P1 and P2 in STP effluent were much lower than the quantified concentration in deionized water. STP effluent contains high concentrations of organic matter and anions (Table S1, Supporting Information). As shown in Figure S6, STP effluent showed strong absorbance near 254 nm. Thus, photoFries rearrangement could be inhibited due to the competition for irradiation photons by the components of the effluent. Moreover, some components in the STP effluent may also quench the singlet states of APAP, followed by a decrease in photo-Fries rearrangement. The Cle, CO3 2 , HCO 3 , and other ions, as well as organic matter, in the STP effluent could compete for OH which would result in the inhibition of APAP transformation. In order to identify what constituents in STP effluent largely affected the transformation kinetics of APAP, phototransformation of APAP were investigated in the presence of different ions. The kobs (s1) for the transformation of APAP in 2 the presence of various anions, HCO 3 /CO3 , NO2 /NO3 was 0.0018, 0.0024 and 0.0027, respectively. Combined with the kobs for the transformation of APAP in deionized water and STP effluent, it can be concluded that anions and dissolved organic matters in STP effluent made a comparable contribution to the decrease of APAP's transformation. Compared with the other major anions in STP effluent, HCO 3 exhibited dominant influence on the transformation kinetics of APAP. As shown in Fig. 5, the presence of NO 2 /NO3 only slightly changed the transformation of APAP and P2, while the formation and decay pattern of P1 was highly changed. Other 2 showed different efanions including Cl and HCO 3 /CO3 fects on the formation and decay of P1 and/or P2. The results indicated the influence of anions on the formation and decay of intermediates was more complex. Reaction of anions and dissolved organic matters with OH led to the generation of e other inorganic radicals, including COe 3 , Cl2 , and triplet states, which may have high reactivity with APAP and intermediates. New pathways may be introduced for the transformation of APAP and intermediates in STP effluent. Further studies are underway to identify the mechanism of

Fig. 4 e Phototransformation of acetaminophen released from commercial drugs or in 6.4 mM amantadine hydrochloride aqueous solution (a), formation and decay of P1 (b) and P2 (c), [APAP]0 ¼ 20 mM, [H2O2]0 ¼ 100 mM.


10


transformation of APAP, P1 and P2 in STP effluents by UV/ H2O2.

4.

Conclusions

This study provided an in-depth understanding of the phototransformation of APAP by UV/H2O2. Based on the identified transformation products with high accumulated

concentration, P1 and P2, the predominant phototransformation pathways of APAP include photo-Fries rearrangement and hydroxylation at the ortho position to the phenoxyl group in APAP. The results of kinetic analysis indicated that the main transformation path also included the formation of 1,4-hydroquinone, although it was not detected at a high accumulated concentration because of the high decay or transformation rate. The results indicate that a combination of product and kinetics analysis could provide a more precise mechanism for the transformation of APAP by UV/H2O2. APAP, P1, and P2 all have high reaction affinity with OH. The formation constant of P1 was unaffected by the H O 2 2 concentration, while its decay increased with increasing concentration of H2O2. Therefore, the stationary concentration of accumulated P1 was much smaller in the presence of a large H2O2 concentration than that in the absence of H2O2. The formation kinetic constants for P2 and 1,4-hydroquinone were comparable and increased with increases in H2O2. Dissolved HA, anions, and coexisting commercial drug constituents and adjuvants not only influenced the transformation kinetics of APAP, but also affected the formation and decay of the main intermediates in deionized water. Anions and dissolved organic matters made comparable affection on the transformation kinetics of APAP in STP effluent and the transformation patterns of APAP in STP effluent were very different from those in deionized water.

Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant Nos. 21207104, 21477091) and the Postdoctoral Science Foundation of China (No. 2013T60743). We thank the reviewers for their suggestions that are helpful in improving the quality of this work, and professional editors from “textcheck” for improving the English quality in this document. We wish to dedicate this paper to Professor Nansheng Deng on the occasion of his 70th birthday.

Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2015.05.008.

references

Fig. 5 e Transformation of acetaminophen in STP effluents or deionized water solution with different anions (a), formation and decay of P1 (b) and P2 (c), simulated anions including 100 mg/L Cl¡, 0.05 mg/L NO 2 , 16.3 mg/L NO3 , 2 220.2 mg/L HCO3 and 0.16 mg/L CO3 , [APAP]0 ¼ 20 mM, [H2O2]0 ¼ 100 mM.

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