Transformation of an Amine Moiety of Atenolol during Water Treatment

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Cite This: Environ. Sci. Technol. 2019, 53, 7653−7662

Transformation of an Amine Moiety of Atenolol during Water Treatment with Chlorine/UV: Reaction Kinetics, Products, and Mechanisms Jiwoon Ra,† Hoonsik Yoom,‡ Heejong Son,‡ Tae-Mun Hwang,§ and Yunho Lee*,†

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†

School of Earth Sciences and Environmental Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju 61005, Republic of Korea ‡ Busan Water Quality Institute, Gimhae, Gyeongsangnam 621-813, Republic of Korea § Water Resources and Environmental Research Division, Korea Institute of Construction Technology, 2311, Goyang, Gyeonggi 411-712, Republic of Korea S Supporting Information *

ABSTRACT: Transformation of atenolol (ATN), a micropollutant containing a secondary (2°) amine moiety, can be significantly enhanced in water treatment with sequential and combined use of chlorine and UV (chlorine/UV) through photolysis of the N−Cl bond. This study investigated the transformation kinetics, products, and mechanisms of the amine moiety of ATN in chlorine/UV (254 nm). The fluencebased, photolysis rate constant for N−Cl ATN was 2.0 × 10−3 cm2/mJ. Transformation products (TPs) with primary (1°) amines were mainly produced, but TPs with 2° and 3° amines were also formed, on the basis of liquid chromatography (LC)/quadrupole-time-of-flight/mass spectrometry and LC/ UV analyses. The amine-containing TPs could be further transformed in chlorine/UV (with residual chlorine in post UV) via formation and photolysis of new N−Cl bonds. Photolysis of N−Cl 1° amine TPs produced ammonia as a major product. These data could be explained by a reaction mechanism in which the N−Cl bond was cleaved by UV, forming aminyl radicals that were transformed via 1,2-hydrogen shift, β-scission, intramolecular addition, and 1,2-alkyl shift. Among these, the 1,2-alkyl shift is newly discovered in this study. Despite enhanced transformation, only partial mineralization of the ATN’s amine moiety was expected, even under chlorine/UV advanced oxidation process conditions. Overall, the kinetic and mechanistic information from this study can be useful for predicting the transformation of amine moieties by chlorine/UV water treatment.

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INTRODUCTION Chlorine has been widely used as a disinfectant for drinking water and swimming pool water.1 Chlorination, however, has drawbacks, such as formation of toxic disinfection byproducts (DBPs) and low inactivation efficacy against protozoan microorganisms.2,3 Ultraviolet (UV) light has been increasingly used as a secondary disinfectant in drinking water, due to its high germicidal effectiveness to chlorine-resistant microorganisms and the general absence of toxic byproduct formation.4,5 UV disinfection is typically followed by chlorination, as it is mandatory to maintain a residual disinfectant in drinking water distribution systems in many countries.6 Application of UV has also been increasing for swimming pools, as it can improve the water and air quality of pools by removing volatile chlorinated byproducts.7,8 Chlorination followed by UV is relevant for drinking water treatment as chlorine (referring here to free available chlorine, HOCl/OCl−) is used for a preoxidation process before coagulation and filtration processes.9,10 In this sequential use of chlorine and UV, chlorinated waters are subjected to UV © 2019 American Chemical Society

photolysis in the presence or absence of residual chlorine. Combined use of chlorine and UV has been proposed as an advanced oxidation process (commonly denoted as UV/ chlorine AOP)11,12 and intensively tested in bench-scale studies13−22 and also occasionally in pilot- and full-scale studies.23,24 The performance of UV/chlorine as an AOP is superior or comparable to that of UV/H2O2 at acidic and neutral pH but lower at basic pH, due to the pH-dependent hydroxyl radical (•OH) oxidation efficiency.13,25 Elucidating the major reactive species responsible for organic contaminant degradation in UV/chlorine has been the subject of investigations, since UV photolysis of chlorine produces a range of radical species, such as •OH, chlorine radicals (•Cl), dichlorine radicals (Cl2•−), and oxychlorine radicals (ClO•) as described in reactions R1−R5.25 It has been found that •OH Received: Revised: Accepted: Published: 7653

March 6, 2019 June 4, 2019 June 5, 2019 June 5, 2019 DOI: 10.1021/acs.est.9b01412 Environ. Sci. Technol. 2019, 53, 7653−7662

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Environmental Science & Technology

(iii) the mechanisms of UV-induced decomposition of organic chloramines.

was the main oxidant for recalcitrant contaminants with electron-deficient substituents, while •Cl, Cl2•−, and ClO• (termed as reactive chlorine species, RCS) become important for the degradation of contaminants with electron-rich moieties.15,18 HOCl(OCl−) + hv → •OH(•O−) + •Cl

(R1)

•

O− + H+ → •OH

(R2)

•

Cl + OH− F ClOH•− F •OH + Cl−

(R3)

•

Cl + Cl− F Cl 2•−

(R4)

•

OH + OCl− → ClO •+OH−

(R5)

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MATERIALS AND METHODS Standards and Reagents. All chemicals and solvents (the highest purity available) were used as received from various commercial suppliers. Further details of chemical sources and stock solutions have been provided in SI-Text-1. Chlorination and UV Experiments. ATN and other amine compounds (1−100 μM) were prepared in phosphate(2−10 mM, pH 6−8) or borate-buffered solutions (5 mM, pH 9) and in a surface water matrix. These samples were treated with chlorine, UV, and chlorine/UV in laboratory, bench-scale experiments. UV irradiation was carried out in a quasicollimated beam system, equipped with a low-pressure Hg lamp, emitting UV light at 254 nm with UV intensity of 3−5 mW/cm2. Details of the chlorine and UV experiments have been provided in SI-Text-2. Analytical Methods. A liquid chromatography (Infinity 1260, Agilent) coupled to a quadrupole time-of-flight mass spectrometer (LC/Q-TOF/MS) with an electrospray ionization (ESI) source (6520, Agilent) was used for the identification of the TPs. A LC (Ultimate 3000, Dionex) with a UV detector (LC/UV) was used to quantify ATN and its TPs. The LC/UV was also used to quantify low molecular weight (LMW) carbonyl and amine products after precolumn derivatizations. Chlorinated samples were analyzed directly within a few hours to minimize further transformation; otherwise, the samples were quenched by thiosulfate and stored at 4 °C before the analyses. Further details of the analytical methods are provided in SI-Text-3.

Synthetic amine compounds are often present in water sources impaired by the discharge of municipal or industrial wastewaters.26−29 This is related to the fact that primary (1°), secondary (2°), tertiary (3°), and quaternary (4°) amine moieties are abundant in the structures of wastewater-relevant contaminants, such as pharmaceuticals and personal care products.30,31 The presence of amines in drinking water sources is a concern, as they are the precursors of toxic nitrogenous DBPs (N-DBPs) such as halonitriles, halonitroalkanes, or nitrosamines.32 The chlorination chemistry of amines shows that, apart from 4° amines, they all react rapidly with chlorine to produce organic chloramines with mono(R2N−Cl) or dichloro (RN−Cl2) bonds.33−35 Organic chloramines are relatively stable and can revert to the parent amines by chlorine transfer to reductants, such as sulfite or thiosulfate.35−37 Decomposition of organic chloramines sometimes yields toxic products, including halonitriles, halonitroalkanes, and aldehydes.38,39 Transformation of amine compounds can be significantly enhanced in sequential and combined chlorine and UV treatment (denoted as chlorine/UV hereafter), compared to chlorine or UV alone, due to their reactions with radicals (•OH and RCS) and the UV photolysis of the organic chloramines. UV photolysis of N−Cl bonds generates aminyl radicals (R2− N•) and Cl•.33,40 The latter pathway is more specific to the transformation of an amine moiety, compared to the less selective •OH-induced transformation of compounds. The role of organic chloramine photolysis during water treatment with chlorine/UV has been shown to be relevant for the enhanced elimination of microcystin-LR41 and for the formation of NDBPs such as cyanogen chlorine,42−44 chloropicrin,45 and nitrosamines.46 Nevertheless, organic chloramine photolysis has sometimes been neglected as an additional transformation pathway of amine-containing contaminants.15,47,48 Furthermore, information on the reaction pathways and mechanisms is currently too limited to be able to assess the fate of amine moieties in the chlorine/UV process. To fill this information gap, the transformation of a 2° amine moiety of atenolol (ATN) during water treatment with sequential and combined use of chlorine and UV (254 nm) was investigated in this study. ATN is a β-blocker pharmaceutical and wastewater-relevant contaminant.37 Systematic investigations were carried out on the following: (i) the kinetics and transformation products (TPs) of UV photolysis of organic chloramines of ATN, plus several 1° and 2° amine compounds, including the TPs from ATN or structural model compound of ATN, (ii) the transformation pathways of a 2° amine moiety of ATN in chlorine/UV, and

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RESULTS AND DISCUSSION Transformation of ATN in Dark Chlorination. N−Cl ATN was formed from the reaction of ATN with chlorine. The identity of N−Cl ATN could be confirmed by its mass spectrum (Figure S1). The reaction stoichiometry was close to 1:1 for the decrease of ATN and formation of N−Cl ATN per chlorine consumption (Figure S2). Thus, reaction R6 can describe the reaction of ATN with chlorine forming N−Cl ATN. ATN + HOCl → N−Cl ATN

(R6)

The formation of N−Cl ATN was rapid and completed in less than one min. Using the typical reactivity of HOCl toward deprotonated 2° amines (k = 107−108 M−1 s−1) and much less reactivity of OCl− and protonated amines,34,49 the apparent k for the reaction of chlorine with the 2° amine of ATN (pKa = 9.6) was calculated to be 2 × 103−2 × 107 M−1 s−1 in the pH range of 6−9. Thus, the N−Cl formation during typical water chlorination conditions (e.g., a few mg/L of chlorine) could actually have been completed within a few seconds. N−Cl ATN was stable in water (Figure S3). The decay of N−Cl ATN (10 μM) was less than 5% in 4 h in the pH range of 6−8. At pH 9, the decay of N−Cl ATN became faster, with a pseudo first-order rate constant of 7.6 × 10−5 s−1. N-Desisopropyl ATN and acetone were the major products from the N−Cl ATN decay, indicating hydrolysis of the N−Cl moiety. Similar hydrolysis of the N-halo moiety has been reported previously.33,37 N−Cl ATN was found to revert readily to ATN upon reaction with thiosulfate (Figure S4), which was consistent with a previous report.37 Overall, N−Cl ATN, a 7654

DOI: 10.1021/acs.est.9b01412 Environ. Sci. Technol. 2019, 53, 7653−7662

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scavengers. The quantum yield (Φ, mol/einstein) for the photolytic decomposition of N−Cl ATN was calculated to be 0.54 (see Table S1 for further details). Transformation Products of N−Cl ATN Photolysis. N−Cl ATN was prepared at 20 μM (or 80 μM) by reacting 40 μM of ATN with 20 μM of chlorine (or 100 μM of ATN with 80 μM of chlorine) at pH 7 for 10 min. The N−Cl ATN samples (containing 20 μM of unreacted ATN) were then treated with UV (0−1000 mJ/cm2) and analyzed using LC/QTOF/MS and LC/UV. Figure 1 shows selected chromato-

chlorinated product of ATN, was found to be persistent (t1/2 of >38 h at pH 6−8) in water chlorination. Transformation Kinetics of ATN in Chlorine/UV. The transformation kinetics of ATN during treatment at pH 8 with UV, chlorine, and chlorine/UV (with and without tertbutanol), respectively, are shown in Figure S5. Note that all chlorinated samples were quenched with thiosulfate (1 mM). The transformation of ATN was negligible ( SW > PB−BuOH matrices. The difference in the ATN transformation rate was most significant at pH 6; the kUV values were 8.2 × 10−3, 5.2 × 10−3, and 2.0 × 10−3 cm2/mJ for PB, SW, and PB−BuOH, respectively. With increasing pH from 6 to 9, the differences in the ATN transformation rates decreased. At pH 9, the kUV values were 2.6 × 10−3, 2.5 × 10−3, and 2.0 × 10−3 cm2/mJ for PB, SW, and PB−BuOH, respectively. Notably, the transformation rate of ATN in the PB−BuOH matrix was almost constant at kUV of ∼2 × 10−3 cm2/mJ, regardless of the pH. This indicated that the transformation of ATN in the PB−BuOH matrix was mainly driven by the photolysis of N−Cl ATN, with little contribution from the •OH (or RCS) reaction. In the PB and SW matrices, the •OH (or RCS) reaction also contributed to the transformation of ATN, with the contribution increasing with decreasing pH.13,25 Thus, the photolysis of N−Cl ATN can become the dominant transformation pathway for ATN at basic pH or in the presence of high concentrations of •OH

Figure 1. (a) LC/MS and (b) LC/UV (225 nm) chromatograms for N−Cl ATN treated by UV irradiation at 600 mJ/cm2. The initial concentration of N−Cl ATN was 20 μM for LC/MS ([ATN]0 = 40 μM and [Chlorine]0 = 20 μM) and 80 μM for LC/UV ([ATN]0 = 100 μM and [Chlorine]0 = 80 μM), which were prepared by chlorinating ATN at pH 7 for 10 min.

grams of (a) LC/MS and (b) LC/UV of the N−Cl ATN (20 μM) sample treated with UV at 600 mJ/cm2. An overlay of all LC/UV chromatograms before and after UV treatment of N− Cl ATN is also shown in Figure S7. The elution of N−Cl ATN was at 6.0 min in LC/MS and 9.5 min in LC/UV, and its peak gradually decreased with increasing UV fluence. With the decreasing N−Cl ATN peak, six other peaks evolved at retention times (RTs) of 1.1, 1.5, 2.1, 2.3, 3.1, and 4.6 min in LC/MS (Figure 1a). These LC/MS peaks could be matched to those from LC/UV at RTs of 3.0, 4.1, 5.8, 6.2, 7.5, and 8.1 min, respectively (Figure 1b), on the basis of their elution order in the same column used for the LC/MS and LC/UV analyses. These six peaks were named as TP-225, TP-239, TP265-I, TP-206, TP-194, and TP-265-II, on the basis of their protonated molecular masses (M + H+) (Table 1). The ATN peak changed little in the tested UV fluence range (Figure S7). To identify the structure of the TPs, their MS/MS (MS2) spectra (Figures S8−S14) and UV absorption spectra (Figure S15) were investigated. The MS2 spectrum of ATN showed 12 7655


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Table 1. Atenolol (ATN) and Its Transformation Products (TPs) from UV Photolysis of N−Cl ATNa and N−Cl DIP-ATNb Identified by LC/Q-TOF/MS

a TPs from N−Cl ATN. bTPs from N−Cl des-isopropyl ATN (DIP-ATN), cΔ[TP]/Δ[N−Cl ATN] (Figure S16) and Δ[TP]/Δ[N−Cl DIPATN] (Figure S22). dLow molecular weight (LMW) product that is coupled with each TP containing the phenylacetamide moiety.

fragment ions, which could be explained by the fragmentation patterns proposed in Figure S8. Among the ATN fragment ions, m/z 190.0859, 145.0646, and 133.0645 were formed from 2-(4-(2-hydroxypropoxy)phenyl)acetamide moieties. In addition, m/z 116.1070 and 98.0964 were formed from (2hydroxy-3-isopropylamino)propoxy moieties. These characteristic ATN MS2 spectra were used to interpret the MS2 spectra of the TPs (Table S2). The UV absorption spectrum of ATN showed peaks at 226 and 276 nm, which were assigned to phenylacetamide moieties. Changes in these characteristic absorption peaks indicated structural modification of the phenylacetamide moieties (Figure S15). Finally, a chlorine reactivity test was performed to check the presence of chlorinereactive amine moieties in TPs, in which the rapid

disappearance of the TP peak upon chlorine treatment was interpreted as the presence of the amine moiety. Details of the structural identification of the TPs have been provided in SIText-4, and the identified structures have been summarized in Table 1. TP-225 was confirmed to be N-des-isopropyl ATN (DIP-ATN), using a commercially available chemical standard. TP-239 was formed by replacement of the ATN isopropylamino by the methylamino moiety. TP-265-I was formed by a dehydrogenation in the isopropyl moiety of ATN. TP-206 was a product from removal of the isopropylamine (deamination) and a water (dehydrogenation) and TP-194, from removal of isopropylamine (deamination) and a carbon from ATN. Finally, TP-265-II was a 3° aromatic amine compound that could be formed via an intramolecular cyclization mechanism 7656


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Environmental Science & Technology Scheme 1. UV-Induced Transformation Pathways and Mechanisms for N−Cl ATNa

a

N−Cl ATN marked with a rectangle. Five pathways are proposed for the transformation of the ATN aminyl radical, on the basis of four different mechanisms: 1,2-H-shift (R8 and R11), β-scission (R15), intramolecular addition (R18), and 1,2-R-shift (R20). The parentheses indicate the molar yields of TPs and LMW products.

Figure 2. (a) Decrease of N−Cl ATN and evolution of transformation products (TPs) containing a phenylacetamide moiety and (b) evolution of low molecular weight (LMW) products and their associated TPs as a function of UV fluence during UV photolysis of N−Cl ATN. For (a), the “total” indicates the summed molar concentrations of N−Cl ATN and its six TPs. For (b), the associated formation of LMW products and TPs via different reaction mechanisms is depicted on the right. The experimental conditions were the same as those for Figure 1b.

(see Scheme 1 and the section below for the transformation mechanism). The evolution of the TPs as a function of UV fluence was also investigated. Figure 2 shows the results from UV photolysis of 80 μM of N−Cl ATN. The six TPs could be quantified on the basis of their relative peak areas compared with those of ATN in the LC/UV analysis. This semiquantification method was deemed to be acceptable as ATN

and its TPs (except TP-265-II) showed almost the same UV absorption spectra, due to the presence of a common phenylacetamide moiety (Figure S15). The suitability of this approach was also supported by almost the same peak area responses from ATN and DIP-ATN under LC/UV analysis. Using the data in Figure 2a, Δ[TPs] vs Δ[N−Cl ATN] were plotted, which showed good linear relationships for all TPs (Figure S16a). From the slopes of these linear plots, the molar 7657


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206, and TP-194 (Figure S22a) and 0.61 and 0.27 for ammonia and formaldehyde (Figure S22b), respectively. The sum of the molar yields of the three TPs was 0.73. The molar yield of ammonia (0.61) was close to the sum of molar yields of TP-206 and TP-194 (0.62 = 0.25 + 0.37), in which the latter two compounds lost an amino moiety from DIP-ATN. These data indicated an association of the formation of ammonia with both TP-206 and TP-194 (Table 1). Photodecomposition of N−Cl2 DIP-ATN. N−Cl2 DIPATN was prepared by reacting 20 μM of DIP-ATN with 50 μM of chlorine at pH 7, and then, the mixture was treated with UV in the presence of tert-butanol (10 mM). The latter was used to minimize degradation of N−Cl2 DIP-ATN by •OH (or RCS) and to allow its transformation exclusively by the direct photolysis. With increasing UV fluence, the N−Cl2 DIP-ATN peak decreased, and two new peaks evolved at RTs of 6.2 and 7.5 min, respectively (Figure S23). The kUV value of 3.9 × 10−3 cm2/mJ (Φ = 0.73, Table S1) was determined for the UV photolysis of N−Cl2 DIP-ATN, which was higher than kUV of N−Cl DIP-ATN by a factor of 2. The two product peaks were identified as TP-206 (RT 6.2 min) and TP-194 (RT 7.5 min), which were the same as the products formed from N−Cl DIPATN. TP-223 was not formed. Figure S24a shows the decrease of N−Cl2 DIP-ATN and evolution of the two products with increasing UV fluence. The molar yields were 0.51 for TP-194 and 0.10 for TP-206 (Figure S24b). Overall, the UV photolysis of N−Cl2 DIP-ATN produced products similar to N−Cl DIP-ATN but with different yields. Notably, the yield of TP-194 was higher and the yield of TP206 was lower for N−Cl2 DIP-ATN than for N−Cl DIP-ATN. Some level of N−Cl DIP-ATN was initially present as a minor product, despite the fact that a molar excess of chlorine over DIP-ATN was used. Interestingly, the peak area of N−Cl DIPATN changed little in the tested UV fluence range (0−500 mJ/cm2) (Figure S24). This indicated that N−Cl DIP-ATN was produced from the UV photolysis of N−Cl2 DIP-ATN, which counterbalanced the decrease of N−Cl DIP-ATN by UV photolysis. Transformation Pathways and Mechanisms of N−Cl ATN Photolysis. Scheme 1 shows the transformation pathways and mechanisms of the photolysis of N−Cl ATN, forming six TPs containing the phenylacetamide moiety and LMW products. As the first step, homolytic cleavage of the N− Cl bond of ATN by UV generated an aminyl radical and Cl• (R7a).51,52 Aminyl radicals can be protonated, forming aminyl radical cations. The pKa of the conjugated acids of the dimethyl- and diethyl-aminyl radicals have been reported to be 6.8 and 5.3, respectively.53 The aminyl radical of ATN (and its conjugated acid) is transformed following the five reaction pathways (Scheme 1, R8, R11, R15, R18, and R20) that are based on four reaction mechanisms (1,2-hydrogen (H) shift, βscission, intramolecular addition, and 1,2-alkyl (R) shift) described further in the following subsections. The Cl• (or • OH from •Cl) was expected to react with ATN and N−Cl ATN or be scavenged by tert-butanol; however, it was found that the rate and product formation pattern of N−Cl ATN photolysis were similar with or without tert-butanol (data not shown). This indicated that the Cl• (or •OH) from the UV photolysis of N−Cl ATN had an insignificant effect on the transformation of N−Cl ATN in the experimental conditions applied. 1,2-H Shift (Scheme 1, R8 and R11). Aminyl radicals are known to undergo 1,2-hydrogen(H) shifts, in which the

yields of the six TPs from the UV photolysis of N−Cl ATN could be determined (Table 1). The sum of the molar yields of the six TPs was 0.85, indicating that most of the major TPs from the UV photolysis of N−Cl ATN were identified. Comparison of the structures of the six TPs with ATN indicated formation of LMW products (i.e., C1 −C 4 ) containing either an amine or carbonyl moiety from the cleavage of C−N or C−C ATN bonds. Figure 2b shows the evolution of isopropylamine, acetone, formaldehyde, and acetaldehyde during the UV photolysis of N−Cl ATN. The molar yields of these LMW products could be determined from the slopes of the linear plots of Δ[LMW product] vs Δ[N−Cl ATN] (Figure S16b). Notably, the molar yield of isopropylamine (0.45) was close to the sum of molar yields of TP-206 and TP-194 (0.44 = 0.20 + 0.24) in which the latter two compounds lost an isopropylamino moiety from ATN. The molar yields of acetone (0.25), formaldehyde (0.20), and acetaldehyde (0.08) were comparable to those of TP-225 (0.20), TP-194 (0.24), and TP-239 (0.10), respectively. These data indicated an association of the formation of isopropylamine with both TP-206 and TP-194, formaldehyde with TP194, and acetaldehyde with TP-239, respectively. Photodecomposition of N−Cl DIP-ATN. From the UV photolysis of N−Cl ATN, TPs containing a 1° amine moiety were formed as the major products (Table 1). These aminecontaining TPs can be further transformed during UV/chlorine AOP treatment via formation of a 1° N−Cl bond and its photolysis. To elucidate the full transformation pathways of the ATN amine moiety, DIP-ATN was selected as a representative 1° amine TP, and the UV photolysis of N−Cl DIP-ATN was investigated. N−Cl DIP-ATN was prepared by reacting 20 μM of DIPATN with 10 μM of chlorine at pH 7. From this reaction, N− Cl DIP-ATN was formed as the major product, but dichloro (N−Cl2) DIP-ATN was also formed at a relatively low yield (Figure S17). The photolysis of N−Cl2 DIP-ATN will be discussed in the next section. With increasing UV fluence, the N−Cl DIP-ATN peak decreased and three additional peaks evolved at RTs of 4.9, 6.2, and 7.5 min, respectively (Figure S17). The kUV value of 2.0 × 10−3 cm2/mJ (Φ = 0.49, Table S1) was determined for the UV photolysis of N−Cl DIP-ATN. The TP structures were identified by investigating their MS and MS2 spectra (Table S3, Figures S18−S20) and UV absorption spectra (Figure S15). The peaks at RTs of 6.2 and 7.5 min were identified as TP-206 (m/z 206.0813) and TP194 (m/z 194.0817), respectively. Note that these two are the identical TPs to those produced from the UV photolysis of N− Cl ATN (Table 1). The peak at 4.9 min showed m/z of 223.1088 and was denoted as TP-223. This product was derived through the loss of H2 from DIP-ATN and determined to be the product of an intramolecular cyclization of DIPATN, which was comparable to the formation of TP-265-II from ATN (Table 1). In order to determine product yields, 80 μM of N−Cl DIPATN (prepared by reacting 100 μM of DIP-ATN with 80 μM of chlorine) was irradiated with UV, and the products were quantified by LC/UV analysis. Note that the samples were analyzed after thiosulfate quenching, and thus, N−Cl DIPATN was determined in the form of DIP-ATN. Figure S21 shows the decrease of DIP-ATN and evolution of (a) TP-223, TP-206, and TP-194 and (b) LMW products, such as formaldehyde and ammonia, with increasing UV fluence. The molar yields were 0.11, 0.25, and 0.37 for TP-223, TP7658


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Environmental Science & Technology Scheme 2. UV-Induced Transformation Pathways and Mechanisms for N−Cl DIP-ATNa

a

N−Cl DIP-ATN is marked with a rectangle. Three pathways are proposed for the transformation of the DIP-ATN aminyl radical, on the basis of three different mechanisms: 1,2-H-shift (R23), β-scission (R27), and intramolecular addition (R30). The parentheses indicate the molar yields of TPs and LMW products.

corresponding C-centered radicals are formed.54 For the aminyl radical of ATN, the 3° and 2 °C-centered radicals could be formed via 1,2-H shifts following R8 and R11 (Scheme 1), respectively. It has been well established that Ccentered radicals with a neighboring amino group are converted into imines via formation of peroxyl radicals and following liberation of a hydroperoxyl radical (HO2•).55 Hydrolysis of the imines generates the corresponding carbonyls and 1° amines.55 Following this mechanism, the 3 °Ccentered radical of ATN was transformed into acetone and DIP-ATN following R9a and R10 (Scheme 1). The 2 °Ccentered radical of ATN was transformed following R12 and R13 (Scheme 1) into isopropylamine and 2-(4-(2-hydroxy-3oxopropoxy)phenyl)acetamide, from which the latter was further transformed into TP-206 via dehydration of an αhydroxy carbonyl moiety (Scheme 1, R14). Formation of TP265-I could be explained by C−C double bond formation instead of the imine from the peroxyl radical of the 3 °Ccentered radical of ATN (Scheme 1, R9b). Nevertheless, this was a minor pathway (molar yield of 0.01). β-Scission (Scheme 1, R15). Cleavage of C−C bonds at the β-position (β-scission) has been observed for aminyl radicals, especially for amino acids.56,57 For the aminyl radical of ATN, the scission was expected to occur mainly at the β-carbon with the hydroxyl (OH) group, considering the radical stabilization effect by the OH. As a result of the β-scission, Nisopropylmethanimine and the C-centered radical of 2-(4-(2hydroxyethoxy)phenyl)acetamide were formed (Scheme 1, R15). Hydrolysis of N-isopropylmethanimine generated isopropylamine and formaldehyde (Scheme 1, R16), and the C-centered radical of 2-(4-((2-hydroxyethoxy)phenyl)acetamide was converted into TP-194 via the corresponding peroxyl radical chemistry (Scheme 1, R17). Intramolecular Addition (Scheme 1, R18). Aminyl radical cations are known to react readily with aromatic or olefinic moiety via an addition mechanism generating amine compounds.52 These reactions of aminyl radical cations have been proposed as a novel synthetic route for various structured amine compounds.40,58 For ATN, the aminyl radical cation was expected to undergo intramolecular attack on the phenyl

moiety, forming TP-265-II following R18 and R19 (Scheme 1). TP-265-II was seen as a 3° aromatic amine compound, whose light absorption, mass fragmentation, and chlorine reactivity were distinct in comparison with those of the other TPs containing 2° or 1° amines. A similar intramolecular cyclization has been observed for the aminyl radical cations with a neighboring olefinic or aromatic moiety.59 1,2-R Shift (Scheme 1, R20). The formation of TP-239 and acetaldehyde from ATN could not be explained by the known reaction mechanisms of aminyl radicals as described above. To explain the unexpected transformation of N-isopropyl of ATN to N-methyl of TP-239 and formation of acetaldehyde, a mechanism based on a 1,2-alkyl (R) shift is proposed. As the result of a 1,2-methyl shift from the isopropyl to aminyl, which is analogous to the 1,2-H shift, a 3° amine intermediate with a C-centered radical at the N-ethyl moiety was formed (Scheme 1, R20). Subsequent transformation of the C-centered radical via the peroxyl radical (Scheme 1, R21) and imine hydrolysis (Scheme 1, R22) generated TP-239 and acetaldehyde. As this type of rearrangement based on a “1,2-R shift” is rare, additional confirmation experiments were conducted by treating N-isopropylmethylamine as a 2° amine model compound with chlorine/UV. As the products of UV photolysis of N−Cl isopropylmethylamine, dimethylamine and acetaldehyde were indeed formed with molar yields of 0.17 and 0.11, respectively (Figure S25a). This demonstrated the same type of carbon rearrangement occurring in both N− Cl ATN and N−Cl isopropylmethylamine via the 1,2-methyl shift (Scheme S1). It has also been found that UV photolysis of N−Cl isopropylamine, a 1° amine model compound, generated methylamine and acetaldehyde (Figure S25b). This indicated that the aminyl radicals derived from the 1° amine could also undergo transformations via the 1,2-R shift mechanism (Scheme S2). The effect of dissolved oxygen on the transformation pathway was tested by photolyzing N−Cl ATN in N2(g)-, Air(g)-, and O2(g)-purged solutions. The product formation pattern was almost the same for the Air- and O2-purged solutions while it was different for the N2-purged solution (Table S4). The results support that the aminyl radicals of 7659


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fluence (0−1960 mJ/cm2). ATN almost fully disappeared at 1960 mJ/cm2. As the primary TPs, DIP-ATN, TP-239, and isopropylamine could be quantified. The concentration of these primary TPs reached the maximum at ∼500 mJ/cm2 and then decreased with increased UV fluence. The concentrations of TP-265-I and TP-265-II, as the other N-containing primary TPs, were below the method quantification limit. Methylamine and ammonia were formed as the secondary or (more than) tertiary degradation products. For instance, these compounds could be formed from the UV photolysis of N−Cl isopropylamine as the primary products (Figure S25b). The concentration of methylamine reached its maximum at ∼1500 mJ/cm2 , while the concentration of ammonia continued to increase up to 1960 mJ/cm2. It should be noted that some ammonia could be produced as (more than) tertiary products from the UV photolysis of 1° N−Cl TPs in addition to DIP-ATN and isopropylamine, which were not identified in this study. Nitrate was formed as the final amine oxidation product, and its concentration continued to increase up to 16% of the initial ATN concentration at 1960 mJ/cm2. Nitrate can be formed from the UV photolysis of chloramines that are produced from the reaction of ammonia with chlorine.62,63 Transformation of the amine moiety of ATN could be kinetically modeled on the basis of the results of this study (Scheme S4) and information from the literature (e.g., UV photolysis of chloramines,62,63 breakpoint chlorination64) (see SI-Text-5 and Table S5 for further details). The developed kinetic model was able to simulate the experimental results reasonably well (Figures S26−S28). Implications for the Fate and Control of Organic Amines in Water Treatment. A chlorine/UV process could significantly enhance the transformation of 2° amine to 1° amine moieties via the UV photolysis of N−Cl bonds. The transformation of 1° amine moieties to ammonia and then to nitrate is also accelerated. The UV photolysis rates of 2° N−Cl and 1° N−Cl and N−Cl2 varied within a factor of 5, and the photolysis rates of N−Cl2 were usually larger than those of N− Cl (Table S1). UV photolysis of 3° N−Cl is expected to be unimportant as 3° N−Cl is quickly transformed to 2° amines before its photolysis.39 The kinetic and mechanistic information obtained from this study for the UV photolysis of some selected 2° and 1° organic chloramines can be useful for a generalized prediction of the transformation of amine moieties in the chlorine/UV process, which is relevant for advanced treatments of impaired source waters by wastewater effluent or pool water. Only partial mineralization of the amine moieties is expected, with formation of lower grade amines and ammonia under typical chlorine/UV AOP conditions (see Figure S27). Transformation of amine moieties can be enhanced by •OH or RCS in addition to the pathway via N−Cl photolysis, but the radical pathway is usually less efficient due to its low selectivity. It remains unclear how such partial mineralization of the amine moieties affects their N-DBP formation potential in post chlorination, and this warrants further investigation.

ATN do not directly react with O2 due to its slow reaction60,61 and are rapidly converted into the C-centered radicals that subsequently react with O2 forming peroxyl radicals as summarized in Scheme 1. Overall, the following order was determined for the relative importance of each transformation pathway for the aminyl radical of ATN: 1,2-H shift (0.41) > β-scission (0.24) > intramolecular addition (0.1) ≈ 1,2-R shift (0.1), with the parentheses containing the molar TP yields generated by each mechanism. Transformation Pathways and Mechanisms of N−Cl DIP-ATN Photolysis. Scheme 2 shows the transformation pathways and mechanisms for the photolysis of N−Cl DIPATN. The aminyl radical of DIP-ATN is transformed following three mechanisms, 1,2-H shift, β-scission, and intramolecular addition. Overall, the transformation pathway of N−Cl DIP-ATN was similar to that of N−Cl ATN, except that the 1,2-alkyl shift mechanism was not observed for N−Cl DIP-ATN. The 1,2-H shift of the aminyl radical of DIP-ATN generated the 2 °C-centered radical (Scheme 2, R23), which was transformed into TP-206 and ammonia via imine formation (Scheme 2, R24), imine hydrolysis (Scheme 2, R25), and dehydration of an α-hydroxy carbonyl moiety (Scheme 2, R26). The β-scission of the aminyl radical generated N-methanimine and the C-centered radical of 2(4-(2-hydroxyethoxy)phenyl)acetamide (Scheme 2, R27). Hydrolysis of N-methanimine generated ammonia and formaldehyde (Scheme 2, R28) and the C-centered radical was converted into TP-194 (Scheme 2, R29), which was identical to the reaction described previously for ATN (R17 in Scheme 1). The intramolecular addition of the aminyl radical cation to the phenyl moiety of DIP-ATN produced TP-223 (Scheme 2, R30 and R31), which was a 2° aromatic amine compound. For N−Cl DIP-ATN, the relative importance of each transformation mechanism of the aminyl radical was: β-scission (0.37) > 1,2-H shift (0.25) > intramolecular addition (0.11), with the parentheses containing the molar TP yields. Transformation Pathways and Mechanisms of N−Cl2 DIP-ATN Photolysis. The transformation pathways of N−Cl2 DIP-ATN are shown in Scheme S3. The Cl-aminyl radical was generated from the UV photolysis of N−Cl2 DIP-ATN in the first step. Due to the presence of an electronegative Cl atom, the Cl-aminyl radical cation could be more acidic than the aminyl radical cation. This explained the observation that TP223 was not formed from N−Cl2 DIP-ATN, because only the aminyl radical cation, not the neutral aminyl radical, could undergo an addition reaction to the aromatic moiety (intramolecular addition in this case).52 The transformation pathways of the Cl-aminyl radical included the 1,2-H shift and β-scission mechanisms, which were comparable to those of the aminyl radical, except that the β-scission (0.51) became more dominant compared to the 1,2-H shift (0.10). This indicated that the electronegative Cl atom could facilitate C−C cleavage at the β position. Fate of the Amine Moiety of ATN during Chlorine/UV AOP condition. ATN (10 μM) was treated with a molar excess of chlorine (100 μM) followed by UV photolysis in order to investigate the full transformation pathway of the amine moiety of ATN in chlorine/UV (with residual chlorine in post UV). The experiment was conducted in the presence of tert-butanol (10 mM) to exclude N−Cl ATN degradation by • OH (or RCS). Figure S26 shows the evolution of ATN, its Ncontaining TPs, ammonia, and nitrate, as a function of UV

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b01412. Five texts, 5 tables, 28 figures, and 4 schemes for addressing materials, experimental procedures, addi7660


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(16) Kong, X.; Wu, Z.; Ren, Z.; Guo, K.; Hou, S.; Hua, Z.; Li, X.; Fang, J. Degradation of lipid regulators by the UV/chlorine process: Radical mechanisms, chlorine oxide radical (ClO•)-mediated transformation pathways and toxicity changes. Water Res. 2018, 137, 242− 250. (17) Jin, J.; El-Din, M. G.; Bolton, J. R. Assessment of the UV/ chlorine process as an advanced oxidation process. Water Res. 2011, 45 (4), 1890−1896. (18) Sun, P.; Lee, W. N.; Zhang, R.; Huang, C. H. Degradation of DEET and Caffeine under UV/Chlorine and Simulated Sunlight/ Chlorine Conditions. Environ. Sci. Technol. 2016, 50 (24), 13265− 13273. (19) Wang, D.; Bolton, J. R.; Andrews, S. A.; Hofmann, R. Formation of disinfection by-products in the ultraviolet/chlorine advanced oxidation process. Sci. Total Environ. 2015, 518−519, 49− 57. (20) Wu, Z.; Fang, J.; Xiang, Y.; Shang, C.; Li, X.; Meng, F.; Yang, X. Roles of reactive chlorine species in trimethoprim degradation in the UV/chlorine process: Kinetics and transformation pathways. Water Res. 2016, 104, 272−282. (21) Wu, Z.; Guo, K.; Fang, J.; Yang, X.; Xiao, H.; Hou, S.; Kong, X.; Shang, C.; Yang, X.; Meng, F.; Chen, L. Factors affecting the roles of reactive species in the degradation of micropollutants by the UV/ chlorine process. Water Res. 2017, 126, 351−360. (22) Ye, B.; Li, Y.; Chen, Z.; Wu, Q.-Y.; Wang, W.-L.; Wang, T.; Hu, H.-Y. Degradation of polyvinyl alcohol (PVA) by UV/chlorine oxidation: Radical roles, influencing factors, and degradation pathway. Water Res. 2017, 124, 381−387. (23) Wang, D.; Bolton, J. R.; Andrews, S. A.; Hofmann, R. UV/ chlorine control of drinking water taste and odour at pilot and fullscale. Chemosphere 2015, 136, 239−244. (24) Scheideler, J.; Aflaki, R.; Hammond, S.; Robinson, K. Full scale UV Advanced Oxidation Process with Sodium Hypochlorite for Potable Reuse Treatment−an economic attractive option. Proceedings of the Water Environment Federation 2016, 2016 (8), 4786−4791. (25) Remucal, C. K.; Manley, D. Emerging investigators series: the efficacy of chlorine photolysis as an advanced oxidation process for drinking water treatment. Environmental Science: Water Research & Technology 2016, 2 (4), 565−579. (26) Kosaka, K.; Asami, M.; Ohkubo, K.; Iwamoto, T.; Tanaka, Y.; Koshino, H.; Echigo, S.; Akiba, M. Identification of a New NNitrosodimethylamine Precursor in Sewage Containing Industrial Effluents. Environ. Sci. Technol. 2014, 48 (19), 11243−11250. (27) Mitch, W. A.; Sedlak, D. L. Characterization and fate of Nnitrosodimethylamine precursors in municipal wastewater treatment plants. Environ. Sci. Technol. 2004, 38 (5), 1445−1454. (28) Zeng, T.; Glover, C. M.; Marti, E. J.; Woods-Chabane, G. C.; Karanfil, T.; Mitch, W. A.; Dickenson, E. R. V. Relative Importance of Different Water Categories as Sources of N-Nitrosamine Precursors. Environ. Sci. Technol. 2016, 50 (24), 13239−13248. (29) Merel, S.; Lege, S.; Yanez Heras, J. E.; Zwiener, C. Assessment of N-Oxide Formation during Wastewater Ozonation. Environ. Sci. Technol. 2017, 51 (1), 410−417. (30) Manallack, D. The acid−base profile of a contemporary set of drugs: implications for drug discovery. SAR and QSAR in Environmental Research 2009, 20 (7−8), 611−655. (31) Tolls, J.; Berger, H.; Klenk, A.; Meyberg, M.; Beiersdorf, A.; Müller, R.; Rettinger, K.; Steber, J. Environmental safety aspects of personal care productsa European perspective. Environ. Toxicol. Chem. 2009, 28 (12), 2485−2489. (32) Shah, A. D.; Mitch, W. A. Halonitroalkanes, Halonitriles, Haloamides, and N-Nitrosamines: A Critical Review of Nitrogenous Disinfection Byproduct Formation Pathways. Environ. Sci. Technol. 2012, 46 (1), 119−131. (33) Armesto, X.; García, M.; Santaballa, J.; Canle, M. Aqueous chemistry of N-halo-compounds. Chem. Soc. Rev. 1998, 27 (6), 453− 460.

tional data and discussions, and kinetic modeling (PDF)

AUTHOR INFORMATION

Corresponding Author

*Phone: 82-62-715-2468; fax: 82-62-715-2434; e-mail: [email protected]. ORCID

Yunho Lee: 0000-0001-5923-4897 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the National Research Foundation of Korea funded by the Ministry of Science, ICT and Future Planning (NRF-2017R1A2B2002593 and NRF2017M3A7B4042273).

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REFERENCES

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