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Photodegradation of iodinated trihalomethanes in aqueous solution by UV 254 irradiation
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
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Photodegradation of iodinated trihalomethanes in aqueous solution by UV 254 irradiation Yongjun Xiao a,b, Rongli Fan b, Lifeng Zhang b, Junqi Yue b, Richard D. Webster c, Teik-Thye Lim a,d,* a
School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore b Water Research Analytical Laboratories, Water Quality Office, Public Utilities Board, 82 Toh Guan Road East, #04-03, Singapore 608576, Republic of Singapore c School of Physical and Mathematic Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Republic of Singapore d Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141, Republic of Singapore
article info
abstract
Article history:
Photodegradation of 6 iodinated trihalomethanes (ITHMs) under UV irradiation at 254 nm
Received 3 September 2013
was investigated in this study. ITHMs underwent a rapid photodegradation process
Received in revised form
through cleavage of carbon-halogen bond with first-order rate constants in the range of 0.1
19 November 2013
e0.6 min1. The effects of matrix species including nitrate, humic acid (HA), bicarbonate,
Accepted 24 November 2013
sulfate, and chloride were evaluated. The degradation rate increased slightly in the pres-
Available online 3 December 2013
ence of nitrate possibly due to generation of HO at a low quantum yield via direct photolysis of nitrate, while HA lowered the photodegradation rate of ITHMs due to its
Keywords:
competitive UV absorption. Moreover, bicarbonate, sulfate, and chloride had no significant
Direct photolysis
effect on photodegradation kinetics, as there is no UV absorption for these 3 species. In the
Iodinated disinfection by-product
study using surface water, treated water, and secondary effluent from a wastewater
Quantum yield
treatment plant, high turbidity and natural organic matters present in the water inhibited
Nitrate
the photodegradation of ITHMs. The degradation rates of 6 ITHMs in UV/H2O2 system were
Humic acid
rather comparable and significantly higher than those achieved in the UV system without
Water matrix
H2O2. To develop a quantitative structureereactivity relationship (QSAR) model, the loga-
QSAR model
rithm of measured first-order rate constants was correlated with a number of molecular descriptors. The best correlation was obtained with a combination of 3 molecular descriptors, namely the bond strength of carbon-halogen to be broken in the ratedetermining step, steric and electronic effects of all substituents to the carbon center. ª 2013 Elsevier Ltd. All rights reserved.
* Corresponding author. School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore. Tel.: þ65 6790 6933; fax: þ65 6791 0676. E-mail address: [email protected] (T.-T. Lim). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.11.039
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1.
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Introduction
It is well known that iodinated trihalomethanes (ITHMs) could be formed as disinfection by-products in water treatment processes when iodide is present in the source waters (i.e., from natural sources, seawater intrusion, or brine) (Bichsel and Von Gunten, 2000). Early concerns about ITHMs are attributed to their characteristic pharmaceutical or medicinal odors and taste (Hansson et al., 1987). For instance, it was reported that the odor threshold concentration of iodoform was 0.02 mg/L, which is in the significantly low level (Hansson et al., 1987; Cancho et al., 2000). This could explain the reported nuisance arising from low concentration of ITHMs that are able to cause strong offensive odors and taste in drinking water. Recently, ITHMs have been found to be more genotoxic and mutagenic than the regulated chlorinated and brominated THMs (Richardson et al., 2008). Therefore, significant research efforts have been directed toward increasing the understanding of ITHMs formation (Gallard et al., 2009; Pressman et al., 2010; Smith et al., 2010), analysis (Cancho et al., 1999; Cancho et al., 2000; Allard et al., 2012), occurrence (Cancho et al., 2000; Allard et al., 2012), and health effects (Richardson et al., 2008; Pressman et al., 2010). To date, only a few studies have been reported on the formation and fate of ITHMs during water and wastewater treatment processes (Cancho et al., 2000; Allard et al., 2012; Farre´ et al., 2012). In these studies, conventional water treatment process (sedimentation, sand filtration, and ozonation) and advanced technologies like reverse osmosis (RO) could not effectively remove ITHMs (Cancho et al., 2000; Farre´ et al., 2012). In contrast, UV-based advanced oxidation process (AOP) has been demonstrated as a promising technology to reduce concentration of ITHMs (Farre´ et al., 2012). Moreover, it was reported that ITHMs analog, brominated THMs (BTHMs) have been largely degraded by UV irradiation (direct photolysis) and UV-based AOP (Nicole et al., 1991; Kwok et al., 2004; Rudra et al., 2005; Jo et al., 2011). Therefore, it is believed that UV direct photolysis or UV-based AOP has the potential to be the most effective technology to degrade ITHMs. Quantitative structureeactivity relationship (QSAR) is often used to find the relationship between chemical structures and biological or chemical activity of the compounds studied. In QSAR application, it is assumed that the reactivity differences of similar compounds are due to the type, number, as well as the steric and electronic effects of their functional groups (Chen et al., 2010). To our knowledge, there is no experimental kinetic study on ITHMs photodegradation and only a few of QSAR-predicted degradation kinetics have been reported. Generally, the predicted degradation rate of iodinated disinfection by-products (DBPs) species were based on the experimental results of degradation of brominated/chlorinated DBP species and the QSAR models were established thereof (Zhang and Minear, 2002; Chen et al., 2010; Chen, 2011). However, there was no experimental result of ITHMs to validate the predicted reaction kinetics. In this context, it is necessary to perform laboratory experiments to investigate the degradation kinetics of ITHMs. In the present study, photolysis with germicidal UV 254 nm was chosen to remove ITHMs, as carbon-halogen bond could
be photo-cleaved under UV 254 irradiation (energy of CeI, CeBr, and CeCl bonds are 209, 280, and 397 kJ/mol, respectively, compared to molar photon energy of 472 kJ/mol for UV 254 nm) (Weast et al., 1986; Jones and Carpenter, 2005). An experimental kinetic study of photodegradation of ITHMs was firstly investigated in this study. The influence of common matrix species such as nitrate, humic acid, bicarbonate, sulfate, and chloride on the photodegradation kinetics was also investigated. Subsequently, UV photolysis of ITHMs in different types of real water was examined. Photodegradation of ITHMs in UV/H2O2 system was also evaluated. Finally, QSAR model was established based on the experimental results of photodegradation of ITHMs and BTHMs.
2.
Materials and methods
2.1.
Reagents and sample preparation
Samples for UV irradiation were prepared using deionized (DI) water (Millipore, USA). Dichloroiodomethane (CHCl2I, 95þ%), chlorobromoiodomethane (CHClBrI, 95þ%), dibromoiodomethane (CHBr2I, 90e95%), chlorodiiodomethane (CHClI2, 90e95%), and bromodiiodomethane (CHBrI2, 95þ%) were purchased from Cansyn Chemical Corp (Canada). Iodoform (CHI3, 99%) and bromoform (CHBr3, 97%) were purchased from SigmaeAldrich (Singapore). Atrazine was purchased from AccuStandard (USA). HPLC grade methanol was purchased from Merck (Singapore). H2O2 (35% w/w aqueous solution) was purchased from Alfa Aesar (Singapore). For quantitative chromatographic analysis, stock standard solutions were prepared in methanol by weighing approximately 10 mg of individual neat ITHMs into a 10-mL volumetric flask and diluting to volume. The secondary standard solutions were prepared by dilution of the primary standard to 100 mg/L and 10 mg/L, respectively.
2.2.
Chemical analysis
A reliable and sensitive method for determination of ITHMs at ng/L level in water sample was developed using automatic purge-and-trap (P&T) extraction (Tekmar, Atomx) coupled with GC (Agilent, 6890A)/MS (Agilent, 5973C). Water sample was heated at 65 C in P&T in order to improve the sensitivity of ITHMs. Trap #1, Tenax was used as the sorbent material. A DB-624 column (J&W) with helium as carrier gas was used. The oven temperature program was as follows: 45 C for 2 min, then increased at a rate of 10 C/min to 100 C, held at 100 C for 2 min, then ramped up to 200 C at 5 C/min, to 250 at 20 C/ min, and held at 250 C for 5 min. Selective ion monitoring (SIM) mode was operated in MS for quantitative analysis of ITHMs. More details about analytical method of ITHMs could be found in Supplementary material (Fig. S1, Table S1, and Table S2). Brominated/chlorinated THMs were analyzed according to EPA method 8260C. Atrazine was determined by high performance liquid chromatography (HPLC, Agilent-1200 series) coupled with tandem mass spectrometry (MS/MS, ABSCIEX-4000Q Trap). Total organic carbon (TOC) was analyzed by a TOC analyzer (Shimadzu, TOC-VCSH). UV absorption of ITHMs and THMs were measured by a UV/vis
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
Fig. 1 e Molar extinction coefficients (ε) and first-order rate constants (k) of 6 ITHMs and 4 THMs at UV 254 nm. Error bars denote standard deviations obtained from at least triplicate datasets.
spectrophotometer (Shimadzu, UV-2550). Iodide and bromide were detected by an ion chromatography (IC) coupled with ICP-MS (Agilent, G3151A). Chloride and formate were detected by IC coupled with thermal conductivity detector (Dionex, ICS3000). H2O2 concentration was measured by a WTW Photolab S12.
2.3.
UV Photoreactor and photodegradation experiment
Photodegradation experiments were conducted in a 740 mL cylindrical glass reactor filled with 740 mL of fresh ITHMs aqueous solution at room temperature. A low-pressure mercury vapor lamp (nominal power 5 W, produced by Philips) primarily emitting irradiation at 254 nm was placed coaxially with the reactor in a quartz sleeve. Photodegradation of 6 ITHMs in the multi-species system (each at initial concentration of 100 mg/L) in DI water under UV 254 irradiation was conducted in triplicate. For comparison, photodegradation of 6 ITHMs in the single-species system (each at initial concentration of 100 mg/L) was conducted once under the same conditions. Photodegradation of 6 ITHMs and 4 brominated/ chlorinated THMs (CHCl3, CHCl2Br, CHClBr2, and CHBr3) in the multi-species system under UV 254 irradiation was also performed once. Additionally, photodegradation of 6 ITHMs and an actinometer (i.e., atrazine or CHBr3) in multi-species system was conducted in duplicate to determine the photon flux emitted by the present UV lamp and quantum yields of 6 ITHMs. Aliquots were sampled at predetermined time intervals and analyzed immediately by purge-and-trap coupled with GC/MS.
2.4.
3.
Results and discussion
3.1.
UV absorbance
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The efficiency of the photodegradation process is affected by the micropollutant light absorption and quantum yield at the wavelength in question (Prados-Joya et al., 2011). To check the applicability of photodegradation of ITHMs under UV 254 irradiation, molar extinction coefficients (εl) of ITHMs and THMs were measured. The molar extinction coefficients of the non-chlorinated ITHMs including CHI3, CHBrI2 and CHBr2I are very close, approximately 1100 M1 cm1, and are higher than those of the chlorinated ITHMs (Fig. 1). In general, the molar extinction coefficients of ITHMs and THMs followed the order: CHI3 z CHBrI2 z CHBr2I > CHClI2 > CHBr3 z CHClBrI > CHClBr2 z CHCl2I > CHCl2Br > CHCl3. The result suggests that the photodegradation of ITHMs under UV 254 irradiation is viable due to their evident UV absorbance.
3.2. Photodegradation of ITHMs in deionized water: kinetic analysis The photodegradation of 6 ITHMs and 3 BTHMs followed the first-order kinetics (Fig. 2). There is insignificant difference in the degradation rate constants of 6 ITHMs obtained in the multi-species system and single-species system (Table 1). No change of concentration of 6 ITHMs in control solution (stored at 4 C in a refrigerator) was observed within 6 days. Additionally, experimental results showed that pH (5 and 10) and the initial concentration of ITHMs (10e400 mg/L) had no significant effect on the photodegradation kinetics of ITHMs (Figs. S2eS3). Thus, pH adjustment was not considered and the initial concentration of 100 mg/L for each ITHM was used in the subsequent investigations. The photodegradation rate constants of the non-chlorinated ITHMs (CHBr2I, CHBrI2, and CHI3) are almost identical, which are higher than those of the chlorinated ITHMs (CHCl2I, CHClBrI, and CHClI2) and the 3
Statistical analyses
To address experimental uncertainty, the statistical significance of the difference between the obtained results was analyzed using One-Sample t-Test. The QSAR model was established by simple and multiple linear regressions analysis in Microsoft Excel 2007.
Fig. 2 e Linear plots for UV 254 direct photolysis of 6 ITHMs; inset: linear plots for UV 254 direct photolysis of 4 THMs.
97.68 97.94 98.86 99.03 99.20 99.32 4.65 (0.45) 12.65 (0.97) 16.89 (1.08) 25.80 (2.66) 23.57 (1.81) 23.18 (1.66) 12.80 (1.37) 5.01 1.02 e
84.26 (1.54) 87.72 (1.03) 91.47 (1.07) 91.85 (1.36) 92.35 (0.85) 93.47 (0.73) (1.51) (2.56) (2.48) (4.59) (3.26) (3.10) (3.54) 16.77 40.59 50.92 68.14 64.40 63.70 40.95 17.96 3.88 e
Based on the BeereLambert Law and the definition of quantum yield, the overall photodegradation rate of micropollutants could be described as follows (Fang et al., 2013). dC ¼ Fl Io 1 10εl Cb dt
(0.13) (0.31) (0.35) (0.93) (0.63) (0.57) (0.43)
3.3.
c
Atrazine as actinometer. Standard deviations were obtained from at least duplicate datasets. There was no observable loss of CHCl3 in this study within 60 min of experimental run. b
a
(0.04) (0.04) (0.03) (0.05) (0.03) (0.03) (0.05) 0.43 0.47 0.45 0.39 0.37 0.35 0.45 0.33 0.42 e 0.997 0.993 0.989 0.992 0.990 0.988 0.995 0.996 0.991 e 3.41 (0.34)b 9.68 (0.79) 13.23 (0.93) 21.37 (2.53) 19.23 (1.69) 18.86 (1.54) 9.80 (1.12) 3.67 0.73 e 0.093 (0.009) 0.26 (0.02) 0.36 (0.02) 0.58 (0.07) 0.52 (0.04) 0.51 (0.04) 0.27 (0.03) 0.10 0.02 ec 0.10 0.27 0.36 0.58 0.53 0.52 e e e e CHCl2I CHClBrI CHClI2 CHBr2I CHBrI2 CHI3 CHBr3 CHClBr2 CHCl2Br CHCl3
(Multispecies) (Singlespecies)
(Multispecies)
R2 kf$104 (cm2 mJ1) k (min1) k (min1)
BTHMs (Table 1). The degradation rate constant of CHCl2I is the lowest among 6 ITHMs. This explains why CHCl2I could be commonly detected in tap water among 6 ITHMs (Brass et al., 1977; Cancho et al., 2000). It was also observed that the degradation rate constants of CHClBrI and CHCl2I are comparable to those of CHBr3 and CHClBr2, respectively. Hence, it can be said that ITHMs still become relatively recalcitrant to UV 254 direct irradiation when Cl atom is also present in the molecule. In general, the degradation rates of ITHMs and THMs followed the order: CHBr2I z CHBrI2 z CHI3 > CHClI2 > CHClBrI z CHBr3 > CHCl2I z CHClBr2 > CHCl2Br > CHCl3, which agrees with the order of their molar extinction coefficients (Fig. 1).
1.32 3.80 5.15 8.19 7.40 7.27 3.84 1.41 0.28 e
6 mg/L H2O2 (multi-species) (Multispecies)
(Multispecies)
(Multispecies)
540 mJ cm2 140 mJ cm2
Va
40 mJ cm
2
% ITHMs and THMs removal (R254)
6 mg/L H2O2 (single-species)
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Compound
Table 1 e Summary of time-based first-order rate constant (k), fluence-based first-order rate constant (kf), regression coefficient (R2), quantum yield (V), and removal percentages (R254) of 6 ITHMs and 4 THMs at different treatment conditions in DI water.
278
Determination of quantum yield (V) of ITHMs
(1)
where C is the concentration of the specific micropollutant (mol L1); dC/dt is the photodegradation rate of the specific micropollutant (mol L1 s1); Vl is the quantum yield at the wavelength in question (mol E1); Io is the UV irradiation intensity of the lamp used (E L1 s1); εl is the molar extinction coefficient at the wavelength in question (M1 cm1); and b is the optical path length (cm). When the UV absorbance of micropollutant (A ¼ 3 lCb) is very low (i.e., A value is less than 0.02), 10εl Cb in Equation (1) through Taylor expansion could be simplified as 1e2.3033lCb. Additionally, as the photodegradation follows the first-order kinetics (Equation (2)), the determination of quantum yield in diluted solution under monochromatic wavelength irradiation can be written as Equation (3) (Prados-Joya et al., 2011; Fang et al., 2013). dC ¼ kl C dt Fl ¼
kl 2:303$qp $εl
(2)
(3)
where kl is the photodegradation first-order rate constant (s1); qp is the photon flux emitted by the lamp (E s1 cm2). To derive the efficiency of the photodegradation process, the quantum yield (Vl) of ITHMs were determined from simultaneous photodegradation of the target ITHMs and an actinometer with a known quantum yield at 254 nm (Javier Benitez et al., 2004; Liu et al., 2009; Prados-Joya et al., 2011). Atrazine, a common actinometer with a known quantum yield (0.033 mol E1) was used to determine the photon flux (qp) of the present UV lamp (Bolton and Stefan, 2002). The firstorder rate constant and molar extinction coefficient of atrazine in this study was measured as 0.15 min1 and 3424 M1 cm1, respectively. According to Equation (3), the photon flux (qp) was determined as 9.61 109 E s1 cm2, which corresponds to UV intensity of 4.54 mW cm2 for the present lamp. Due to its similar physicochemical properties to ITHMs, CHBr3 with a known quantum yield (0.43 mol E1) was also selected as another actinometer to validate the accuracy
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of photon flux (qp) measurement (Nicole et al., 1991). The photon flux (qp) was calculated as 1.01 108 E s1 cm2, which is almost the same as that obtained by the atrazine actinometry. This good agreement led us to choose atrazine as the actinometer to determine the photon flux and quantum yield in this study. The derived quantum yields of 6 ITHMs in the photoreactor ranged from 0.32 to 0.51 (Table 1), which indicates a high efficiency of quantum process in the ITHMs photodegradation under UV 254 irradiation. Since fluence-based first-order rate constant (kf) is independent of the fluctuation in photon flux (qp) of the UV lamp, it allows for direct comparison with the photodegradation rate constants obtained with different photoreactors (Canonica et al., 2008; Prados-Joya et al., 2011). For comparison purpose, kf values are calculated through dividing the time-based first-order rate constant (kl) by the photon flux and photon energy emitted by the UV lamp according to Equation (4). The obtained kf values for ITHMs were summarized in Table 1. kf ¼
kl qp $hc=l
(4)
where kf is the fluence-based first-order rate constant (cm2 mJ1); kl is the time-based first-order rate constant (s1); qp is the photon flux emitted by the lamp (E s1 cm2); hc/l is the energy of photon at a specific wavelength (kJ mol1). To check the applicability of UV 254 irradiation in ITHMs photodegradation in real water treatment process, the removal efficiency of ITHMs were determined under a specific UV 254 dose. The irradiation doses of 40 and 140 mJ cm2 are chosen because they define typical range used in disinfection application (Baeza and Knappe, 2011). As the UV intensity of the present lamp is 4.54 mW cm2, the time required to reach the irradiation doses of 40 and 140 mJ cm2 are 8.8 and 30.8 s, respectively. The removal percentage of ITHMs (R254) can be calculated according to Equation (5). R254 ¼ 1 ekl t %
279
also identified as the end-product, which was also reported in the previous study regarding direct photolysis of CHBr3 (Kwok et al., 2004). HCOOH increased slightly during the photodegradation process and reached 40 mg/L at the end (Fig. 3), which accounts for less than 5% of TOC concentration initially spiked in the reaction solution. It suggests that HCOOH is the minor end-product in the direct photolysis of ITHMs. Moreover, formaldehyde was not detected during the photodegradation process. According to previous studies, UV 240 nm could cleave the primary CeBr bond of CHBr3, but the photon energy was insufficient to induce secondary dissociation of the CHBr2 radical formed (McGivern et al., 2000; Kwok et al., 2004). In our study, the energy of UV 254 photon (471 kJ/mol) is higher than those of C-X bonds (209, 280, and 397 kJ/mol for CeI, CeBr, and CeCl, respectively) (Weast et al., 1986). Homolytic photocleavage of CeI bond upon UV 254 absorption could be the first step of ITHMs photolysis. As described in the previous section, the degradation rate constants of ITHMs are proportional to their corresponding molar extinction coefficients (Fig. 1). It suggests that the first-step reaction (cleavage of CeI bond) is the rate-determining step for photodegradation of ITHMs. Isobromoform (BrCHBreBr) has been previously identified as the intermediate in the second step of CHBr3 photodegradation, and subsequently a water-catalyzed mechanism proceeds to transform isobromoform into its following intermediates which will react with water molecule through OH insertion/HBr elimination resulting in complete conversion to HBr, CO (major product) and HCOOH (minor product) (Kwok et al., 2004). Thus, in our study, photocleavage of CeI bond by UV 254 followed by a series of water-catalyzed reactions might be the possible photodegradation pathway for ITHMs in water.
(5)
As seen from Table 1, R254 of 6 ITHMs at UV doses of 40 and 140 mJ cm2 are less than 10 and 26%, respectively. It suggests that a higher irradiation dose than the usual dose for water disinfection is required to significantly photodegrade ITHMs.
3.4.
End-product identification
A previous study has shown that the lifetimes of CHBr3 intermediate photoproducts in water were in picoseconds and special equipment (like Picosecond Time-Resolved Resonance Raman (ps-TR3) Spectroscopy) was required to identify them (Kwok et al., 2004). In our study, through identification of the end-products, we could identify the possible photodegradation pathway of ITHMs. The photodegradation endproducts in this study were chloride, bromide and iodide (Fig. 3). There was a good mass balance between total amount of produced halides and the introduced halogens in ITHMs (i.e., 92e109% recovery). No formation of chlorate, bromate, and iodate was observed. Additionally, TOC decreased concomitantly during the photodegradation process (Fig. 3), indicating that the ITHMs could be mineralized via UV 254 photolysis. However, it is worth to mention that the HCOOH is
Fig. 3 e Time course of halide formation during direct photolysis of 6 ITHMs (multi-species system, each at initial concentration of 200 mg/L); inset: Time course of TOC reduction, CHI3 reduction and HCOOH formation during direct photolysis of CHI3 (initial concentration around 10 mg/L).
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3.5. Photodegradation of ITHMs in the presence of matrix species Further investigation was performed to evaluate the effects of matrix species including nitrate, humic acid (HA), bicarbonate, sulfate, and chloride on ITHMs photodegradation. In the following discussion, the degradation rates of each ITHM under different conditions are compared to those observed under the reference condition (DI water, initial concentration of 100 mg/L, and pH around 5). When the p value is less than
Fig. 4 e (A). Effect of nitrate on photodegradation of 6 ITHMs; (B). Effect of humic acid (HA) on photodegradation of 6 ITHMs; (C). Effect of bicarbonate, sulfate, and chloride on photodegradation of 6 ITHMs. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained in DI water. Error bars denote standard deviations obtained from at least duplicate datasets.
0.01 in One-Sample t-Test, it suggests that the degradation rate constant of the ITHM species under the specific condition is significantly different from those obtained under the reference condition. The degradation rate constant of CHCl2I in the presence of 5 mg/L nitrate is comparable to that obtained in DI water (Fig. 4A). However, when the nitrate concentration was increased from 20 to 200 mg/L, the degradation rate of CHCl2I increased correspondingly. The degradation rate constant of CHCl2I reaches 0.31 min1 in the presence of 200 mg/L nitrate, which is 3 times higher than that obtained in DI water. However, nitrate had much less influence on the photodegradation of the other 5 ITHMs which are highly susceptible to UV 254 photolysis compared to CHCl2I (Table 1). The photodegradation rate constants of the 5 ITHMs increased by less than one-fold as compared to those observed in DI water even the nitrate concentration was increased to 200 mg/L. It is reported that some chemicals such as salicylic acid and benzoic acid are employed as probe for trapping HO to form the stable and quantifiable products (Jankowski et al., 2000; Yang et al., 2004; Wu et al., 2007). On the basis of this methodology, some studies demonstrated that direct photolysis of
Fig. 5 e (A). Absorbance in DI water of nitrate, humic acid (HA), bicarbonate, sulfate, and chloride at the concentrations used in this study in the range of 220e300 nm; (B). Absorbance in DI water of nitrate, HA, bicarbonate, sulfate, and chloride at the concentrations used in this study in the range of 250e260 nm.
281
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nitrate under UV irradiation could generate HO (Yang et al., 2004; Boucheloukh et al., 2012; Ji et al., 2012). Indeed, the formation of HO in the UV direct photolysis of nitrate follows a complex pathway. The primary reactions in nitrate photolysis are the generation of the radical anion Oˉ and nitrite ion, which continues to produce the radical anion O under UV irradiation. Subsequently, the radical anion O is rapidly reacting with water molecule to form the highly reactive HO. Generally, the reaction pathway for generation of HO via nitrate photolysis can be simplified as follows (Keen et al., 2012).
NO 3 þ hv/NO2 þ O
NO 3 þ hv/NO2 þ
(6)
1 O2 2
(7)
NO 2 þ hv/NO þ O
(8)
O þ H2 O/HO þ HO
(9)
However, due to weak UV 254 absorption of nitrate (Fig. 5), HO was generated at a low quantum yield (Mack and Bolton, 1999; Keen et al., 2012). This can explain why the photodegradation rate of the 5 ITHMs (CHClBrI, CHBr2I, CHClI2, CHBrI2, and CHI3) only increased slightly in the presence of nitrate in this study. Additionally, it is believed that the main reason for the more significant effect of nitrate-induced indirect photolysis of CHCl2I is that the efficiency of its direct photolysis is low (i.e., its k value is 3e6 times lower than those of other 5 species). In other words, the effect of indirect photolysis due to HO attack in the presence of nitrate for other 5 species is possibly masked by their efficient direct photolysis. It has been reported that natural organic matter (NOM) present in water has two opposite effects on photodegradation of micropollutants (De la Cruz et al., 2012). On the one hand, it can inhibit direct photolysis due to inner filtering of UV light by NOM. On the other hand, with exposure to UV irradiation, a photosensitizer present in NOM can be excited to its reactive triplet state (3NOM*) or singlet state (1NOM*), which can react with the micropollutant (De la Cruz et al., 2012; Marin et al., 2012). Moreover, 3NOM* also can react with oxygen present in water to generate singlet molecular oxygen, 1O2 that will subsequently attack the micropollutant (De la Cruz et al., 2012). In the latter case, the formation of reactive oxygen species (ROS, e.g. 3NOM*, 1NOM*, and 1O2) increases the photodegradation rate of micropollutant. In this study, humic acid (HA) was selected as a matrix species to investigate the effect of NOM on ITHMs photodegradation, as HA is considered as the main component of NOM (Prados-Joya et al., 2011).
HA consists of a great number of aromatic rings and oxygenrelated functional groups, which could strongly absorb UV light (Steelink, 2002). Experimental results showed that the photodegradation rates of 6 ITHMs decreased with increasing HA from 1 to 20 mg/L (Fig. 4B). The presence of HA significantly reduced the photodegradation rates of 6 ITHMs to the same extent. This result suggests that filtering and screening of UV light by HA reduces the fraction of UV light absorbed by ITHMs, which results in the decease of the photodegradation rates of ITHMs. In other words, indirect photolysis in the presence of a photosensitizer such as HA could be less significant than the competitive UV absorption effect for ITHMs photodecay. In contrast, when bicarbonate, sulfate, or chloride was spiked into the reaction solution, the phenomenon of filtering and screening of UV 254 light as well as the ROS attacking of ITHMs was not observed, because these 3 anions at the concentration of 100 mg/L have no UV 254 absorption (Fig. 5). As such, bicarbonate, sulfate, and chloride had no statistically significant effect on the photodegradation kinetics of ITHMs (Fig. 4C).
3.6. water
Photodegradation of ITHMs in different types of
The viability of UV 254 irradiation to photodegrade ITHMs in different types of water including DI water, surface water, treated water, and secondary effluent (collected from a wastewater treatment plant, Singapore) was also evaluated. The characteristics of the water samples are presented in Table 2. The degradation rates of 6 ITHMs in the surface water and secondary effluent dramatically decreased, while their degradation rates in the treated water were comparable to those in DI water (Fig. 6). According to the preceding discussion, the effect of pH, sulfate, bicarbonate, chloride and nitrate with concentration less than 5 mg/L on ITHMs photodegradation could be excluded in the study using real water sample as the matrix. The reduction of photodegradation rates in surface water could be mainly attributed to its high turbidity which shielded target ITHMs from UV 254 irradiation. Although nitrate concentration in the secondary effluent was very high (45.2 mg/L), the increase of photodegradation rate of ITHMs was not evident, as high concentration of HO scavengers such as bicarbonate and NOM were also present in the secondary effluent (Table 2). In contrast, high concentration of NOM present in the secondary effluent led to decrease of photodegradation rates of ITHMs due to competitive absorption of UV 254 irradiation by NOM. It suggested that direct photolysis was still the predominant mechanism for ITHMs
Table 2 e Characteristics of different types of water used in this study. Water
pH
Turbidity (NTU)
TOC (as C, mg/L)
Total alkalinity (as CaCO3)
Nitrate (mg/L)
Sulfate (mg/L)
Chloride (mg/L)
DI water Surface water Treated water Secondary effluent
6.00 6.40 8.00 6.70
<0.1a 100 0.22 5.3
0.080 2.40 0.79 9.70
<5 8 13 36
<0.05 3.38 3.20 45.2
<0.045 4.14 21.6 45.1
<5 9 10 86
a
It represents that the result is below detection limit.
282
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
Fig. 6 e Effect of different types of water on photodegradation of 6 ITHMs. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained in DI water. Error bars denote standard deviations obtained from at least triplicate datasets.
photodegradation in the real water samples but the indirect photolysis due to generation of ROS was not evident. It can be said that UV photolysis is more suitable as a post-treatment after filtration to control ITHMs.
3.7. Comparison of photodegradation of ITHMs by UV alone and UV/H2O2 As mentioned previously, the usual UV dose of 140 mJ/cm2 for water disinfection is not adequate to remove ITHMs. In order to provide practical reference for water industry, preliminary investigation of ITHMs photodegradation by UV/H2O2 was also conducted. The concentration of H2O2 used in this study was 6 mg/L, which was within the range of its typical doses applied in pilot-scale studies (Paradis and Hofmann, 2006). Two sets of experiments on photodegradation of ITHMs in DI water by UV/ H2O2 were performed (Fig. 7), namely multi-species system containing 6 ITHMs (each at the initial concentration of 100 mg/ L) and single-species system (100 mg/L). With addition of H2O2, the overall micropollutant photodegradation involves both UV direct photolysis and HO indirect photolysis. Due to the relatively high concentration of H2O2 under constant UV irradiation, concentration of HO is assumed to be constant at the steady-state level with respect to micoropollutant concentration. Therefore, the pseudo-firstorder kinetics (Equation (10)) has been proposed in the previous studies (Sharpless and Linden, 2003; Jo et al., 2011; Shu et al., 2013). dC ¼ k0 C ¼ ðkd þ ki ÞC dt
Fig. 7 e Comparison of photodegradation of ITHMs in DI water by UV alone and UV/H2O2. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained by UV alone. Error bars denote standard deviations obtained from at least duplicate datasets.
The degradation rate constants of 6 ITHMs in the UV/H2O2 system were comparable and significantly increased as compared to those achieved with UV alone. However, due to competitive reaction of 6 ITHMs with HO in the multi-species system, the extent of increase in the degradation rate constants are less than those in the single-species system (Fig. 7). Compared to nitrate photolysis at UV 254, HO was generated at a higher quantum yield in the UV/H2O2 system. As such, indirect photolysis played an important role in the ITHMs photodegradation in the UV/H2O2 system. Additionally, according to equation (5), the removal percentages (R254) of 6 ITHMs were determined at a UV dose of 540 mJ/cm2 with 6 mg/ L H2O2, which is a typical UV/H2O2 treatment conditions (Kruithof et al., 2007; Baeza and Knappe, 2011). As shown in Table 1, the removal percentages (R254) of 6 ITHMs even in the multi-species system could reach more than 84% in the UV/ H2O2 system and increased obviously as compared to those under direct UV irradiation with the similar UV dose. Thus, UV-based AOP technology is promising to remove ITHMs effectively, due to a synergistic effect of indirect photolysis and direct photolysis.
3.8.
QSAR Modeling
An empirical equation is usually used to describe the QSAR model as follows. P¼
i¼n;j¼n X
aij Dij þ b
(11)
i¼1;j¼1
(10)
where k0 is the observed pseudo-first-order rate constant of overall photodegradation process (s1); kd is the pseudo-firstorder rate constant of direct photolysis (s1); and ki is the pseudo-first-order rate constant of indirect photolysis due to the reaction with HO (s1). In this study, the pseudo-first-order kinetics was also observed for ITHMs photodegradation in the UV/H2O2 system.
where P is the property of interest such as first-order rate constant, quantum yield etc.; aij and b are the constants obtained from simple and multiple linear regression; Dij are molecular descriptors characterizing the structure of the compounds studied (Dyekjær and Jo´nsdo´ttir, 2004; Ioele et al., 2009). To derive a QSAR model for photodegradation of ITHMs and BTHMs, the logarithm of measured first-order rate
283
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
constants (logkm) are correlated to a number of molecular descriptors by simple and multiple linear regressions analysis. 3 molecular descriptors are selected since they give an indication of the bond to be broken, steric and electronic effect of the substituents that determine the chemical reactivity of the compound studied. These 3 molecular descriptors include the bond strength (BS) of the carbon-halogen bond to be broken in the rate-determining step, the summation of the electronic effect constants (s), and the summation of the steric effect constants (Es) of all substituents to carbon center. Values of BS, s, and Es of 6 ITHMs and 3 BTHMs used to establish the QSAR model are presented in Table 3 (Weast et al., 1986; Hansch and Leo, 1995). The results of a number of correlations between logkm and individual BS, s, Es, as well as the combinations of these molecular descriptors are summarized in Table 4. The value of the regression coefficient, R2, is used to determine the degree of correlation between these descriptors and logkm. When 1 or 2 descriptors are correlated with logkm, regression coefficients are less than 0.84 in all cases. The best correlation is obtained with a combination of these 3 descriptors (BS, s, and Es), with R2 ¼ 0.95 (model 7 in Table 4). In addition, F test in multiple linear regressions analysis shows that all coefficients in QSAR model 7 are significant at the 95% confidence level. It suggests that these 3 molecular descriptors have a combined effect on the photodegradation of ITHMs and BTHMs. QSAR model 7 is in turn used to predict the photodegradation rate of ITHMs and BTHMs. The QSAR-predicted logkp, experimentally measured logkm, as well as the difference between logkm and logkp (D) are presented in Table 3. The correspondence between logkm and logkp is also displayed in Fig. 8. Generally, QSAR-predicted values agree with the experimental results, except for CHClI2 which shows 0.235-log relative standard deviation. Additionally, the predicted first-order rate constant of CHCl3 by QSAR model 7 is 0.0013 min1, which corresponds to half-time of 9 h. It indicates that CHCl3 is more stable than ITHMs under UV 254 irradiation, because the life-time of 6 ITHMs is in the range of 1e8 min. However, there was no
Table 4 e Correlations of the logarithm of the measured first-order rate constant (logkm) with a number of molecular descriptors. Item 1 2 3 4 5 6 7
Model logk logk logk logk logk logk logk
¼ ¼ ¼ ¼ ¼ ¼ ¼
0.0086BS þ 1.32 1.17Es 4.89 6.13s þ 7.02 0.0016BS 1.07Es 4.15 0.0020BS 5.50s þ 6.67 1.88Es 15.6s þ 25.6 0.0075BS þ 4.97Es 28.8s þ 55
R2 0.40 0.74 0.80 0.76 0.81 0.84 0.95
observable loss of CHCl3 in this study within 60 min of experimental run. The deviation suggests that further research is needed to explore more parameters to give a better description of ITHMs and THMs photodegradation process.
4.
Conclusion
ITHMs photodegradation under UV 254 irradiation followed the first-order kinetics with rate constants in the range of 0.1e0.6 min1. The quantum yields of 6 ITHMs were derived from 0.32 to 0.51, when atrazine was used as an actinometer. ITHMs could be mineralized and liberated halides. Experimental results also showed that nitrate had a slightly positive effect on the photodegradation kinetics possibly due to generation of HO at a low quantum yield via nitrate photolysis, while HA reduced the photodegradation rates due to its competitive UV absorption. In contrast, bicarbonate, sulfate, and chloride had no significant effect on ITHMs photodegradation, as these 3 species have no UV 254 absorption. Based on the above investigation of matrix species effect, it could be deduced that the decrease of photodegradation rate of 6 ITHMs in surface water and secondary effluent are mainly attributed to high turbidity and NOM, respectively. Additionally, it is determined that the removal efficiency of direct photolysis of 6 ITHMs at a common disinfection dose of
Table 3 e Values of molecular descriptors used in this study for establishing the QSAR models and experimentally measured logkm, predicted logkp by QSAR model 7, as well as the difference between logkm and logkp, D. Compound
BSa
Esb
sb
logkm
Logkp
D
CHCl2Br CHClBr2 CHBr3 CHCl2I CHClBrI CHBr2I CHClI2 CHBrI2 CHI3
280 280 280 209 209 209 209 209 209
3.08 3.28 3.48 3.32 3.52 3.72 3.76 3.96 4.20
1.36 1.31 1.26 1.32 1.27 1.22 1.23 1.18 1.14
1.699 0.998 0.569 1.027 0.585 0.237 0.444 0.284 0.292
1.576 1.130 0.684 1.084 0.638 0.192 0.679 0.233 0.274
0.123 0.132 0.115 0.057 0.053 0.045 0.235 0.051 0.019
a
Bond strength of C-X bonds to be broken in the rate-determining step were cited from literature (Weast et al., 1986). b Es and s values of halogen atoms were cited from literature (Hansch and Leo, 1995); the combined effects of multiple halides in a halo-organic are assumed to be the summative effects of individual halides in it (Zhang and Minear, 2002; Chen et al., 2010).
Fig. 8 e Correspondence between experimentally measured logkm and predicted logkp values by QSAR model 7 for 6 ITHMs and 3 BTHMs.
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140 mJ cm2 is less than 26%, while more than 84% of 6 ITHMs could be degraded at a UV dose of 540 mJ cm2 with 6 mg/L H2O2. However, more research is still needed to investigate the applicability of UV-based AOP for ITHMs removal in real practice. Finally, a good QSAR model with R2 ¼ 0.95 was established based on the correlation between logkm and the combination of 3 molecular descriptors, namely bond strength (BS) of the carbon-halogen bond to be broken, the electronic effect (s) and the steric effect (Es) of all substituents to carbon center. It suggests that these 3 parameters have a significant combined effect on the overall photodegradation of ITHMs and BTHMs.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.11.039
references
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Photodegradation of iodinated trihalomethanes in aqueous solution by UV 254 irradiation Yongjun Xiao a,b, Rongli Fan b, Lifeng Zhang b, Junqi Yue b, Richard D. Webster c, Teik-Thye Lim a,d,* a
School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore b Water Research Analytical Laboratories, Water Quality Office, Public Utilities Board, 82 Toh Guan Road East, #04-03, Singapore 608576, Republic of Singapore c School of Physical and Mathematic Sciences, Nanyang Technological University, 21 Nanyang Link, Singapore 637371, Republic of Singapore d Nanyang Environment and Water Research Institute (NEWRI), Nanyang Technological University, 1 Cleantech Loop, CleanTech One, Singapore 637141, Republic of Singapore
article info
abstract
Article history:
Photodegradation of 6 iodinated trihalomethanes (ITHMs) under UV irradiation at 254 nm
Received 3 September 2013
was investigated in this study. ITHMs underwent a rapid photodegradation process
Received in revised form
through cleavage of carbon-halogen bond with first-order rate constants in the range of 0.1
19 November 2013
e0.6 min1. The effects of matrix species including nitrate, humic acid (HA), bicarbonate,
Accepted 24 November 2013
sulfate, and chloride were evaluated. The degradation rate increased slightly in the pres-
Available online 3 December 2013
ence of nitrate possibly due to generation of HO at a low quantum yield via direct photolysis of nitrate, while HA lowered the photodegradation rate of ITHMs due to its
Keywords:
competitive UV absorption. Moreover, bicarbonate, sulfate, and chloride had no significant
Direct photolysis
effect on photodegradation kinetics, as there is no UV absorption for these 3 species. In the
Iodinated disinfection by-product
study using surface water, treated water, and secondary effluent from a wastewater
Quantum yield
treatment plant, high turbidity and natural organic matters present in the water inhibited
Nitrate
the photodegradation of ITHMs. The degradation rates of 6 ITHMs in UV/H2O2 system were
Humic acid
rather comparable and significantly higher than those achieved in the UV system without
Water matrix
H2O2. To develop a quantitative structureereactivity relationship (QSAR) model, the loga-
QSAR model
rithm of measured first-order rate constants was correlated with a number of molecular descriptors. The best correlation was obtained with a combination of 3 molecular descriptors, namely the bond strength of carbon-halogen to be broken in the ratedetermining step, steric and electronic effects of all substituents to the carbon center. ª 2013 Elsevier Ltd. All rights reserved.
* Corresponding author. School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Republic of Singapore. Tel.: þ65 6790 6933; fax: þ65 6791 0676. E-mail address: [email protected] (T.-T. Lim). 0043-1354/$ e see front matter ª 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.watres.2013.11.039
276
1.
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
Introduction
It is well known that iodinated trihalomethanes (ITHMs) could be formed as disinfection by-products in water treatment processes when iodide is present in the source waters (i.e., from natural sources, seawater intrusion, or brine) (Bichsel and Von Gunten, 2000). Early concerns about ITHMs are attributed to their characteristic pharmaceutical or medicinal odors and taste (Hansson et al., 1987). For instance, it was reported that the odor threshold concentration of iodoform was 0.02 mg/L, which is in the significantly low level (Hansson et al., 1987; Cancho et al., 2000). This could explain the reported nuisance arising from low concentration of ITHMs that are able to cause strong offensive odors and taste in drinking water. Recently, ITHMs have been found to be more genotoxic and mutagenic than the regulated chlorinated and brominated THMs (Richardson et al., 2008). Therefore, significant research efforts have been directed toward increasing the understanding of ITHMs formation (Gallard et al., 2009; Pressman et al., 2010; Smith et al., 2010), analysis (Cancho et al., 1999; Cancho et al., 2000; Allard et al., 2012), occurrence (Cancho et al., 2000; Allard et al., 2012), and health effects (Richardson et al., 2008; Pressman et al., 2010). To date, only a few studies have been reported on the formation and fate of ITHMs during water and wastewater treatment processes (Cancho et al., 2000; Allard et al., 2012; Farre´ et al., 2012). In these studies, conventional water treatment process (sedimentation, sand filtration, and ozonation) and advanced technologies like reverse osmosis (RO) could not effectively remove ITHMs (Cancho et al., 2000; Farre´ et al., 2012). In contrast, UV-based advanced oxidation process (AOP) has been demonstrated as a promising technology to reduce concentration of ITHMs (Farre´ et al., 2012). Moreover, it was reported that ITHMs analog, brominated THMs (BTHMs) have been largely degraded by UV irradiation (direct photolysis) and UV-based AOP (Nicole et al., 1991; Kwok et al., 2004; Rudra et al., 2005; Jo et al., 2011). Therefore, it is believed that UV direct photolysis or UV-based AOP has the potential to be the most effective technology to degrade ITHMs. Quantitative structureeactivity relationship (QSAR) is often used to find the relationship between chemical structures and biological or chemical activity of the compounds studied. In QSAR application, it is assumed that the reactivity differences of similar compounds are due to the type, number, as well as the steric and electronic effects of their functional groups (Chen et al., 2010). To our knowledge, there is no experimental kinetic study on ITHMs photodegradation and only a few of QSAR-predicted degradation kinetics have been reported. Generally, the predicted degradation rate of iodinated disinfection by-products (DBPs) species were based on the experimental results of degradation of brominated/chlorinated DBP species and the QSAR models were established thereof (Zhang and Minear, 2002; Chen et al., 2010; Chen, 2011). However, there was no experimental result of ITHMs to validate the predicted reaction kinetics. In this context, it is necessary to perform laboratory experiments to investigate the degradation kinetics of ITHMs. In the present study, photolysis with germicidal UV 254 nm was chosen to remove ITHMs, as carbon-halogen bond could
be photo-cleaved under UV 254 irradiation (energy of CeI, CeBr, and CeCl bonds are 209, 280, and 397 kJ/mol, respectively, compared to molar photon energy of 472 kJ/mol for UV 254 nm) (Weast et al., 1986; Jones and Carpenter, 2005). An experimental kinetic study of photodegradation of ITHMs was firstly investigated in this study. The influence of common matrix species such as nitrate, humic acid, bicarbonate, sulfate, and chloride on the photodegradation kinetics was also investigated. Subsequently, UV photolysis of ITHMs in different types of real water was examined. Photodegradation of ITHMs in UV/H2O2 system was also evaluated. Finally, QSAR model was established based on the experimental results of photodegradation of ITHMs and BTHMs.
2.
Materials and methods
2.1.
Reagents and sample preparation
Samples for UV irradiation were prepared using deionized (DI) water (Millipore, USA). Dichloroiodomethane (CHCl2I, 95þ%), chlorobromoiodomethane (CHClBrI, 95þ%), dibromoiodomethane (CHBr2I, 90e95%), chlorodiiodomethane (CHClI2, 90e95%), and bromodiiodomethane (CHBrI2, 95þ%) were purchased from Cansyn Chemical Corp (Canada). Iodoform (CHI3, 99%) and bromoform (CHBr3, 97%) were purchased from SigmaeAldrich (Singapore). Atrazine was purchased from AccuStandard (USA). HPLC grade methanol was purchased from Merck (Singapore). H2O2 (35% w/w aqueous solution) was purchased from Alfa Aesar (Singapore). For quantitative chromatographic analysis, stock standard solutions were prepared in methanol by weighing approximately 10 mg of individual neat ITHMs into a 10-mL volumetric flask and diluting to volume. The secondary standard solutions were prepared by dilution of the primary standard to 100 mg/L and 10 mg/L, respectively.
2.2.
Chemical analysis
A reliable and sensitive method for determination of ITHMs at ng/L level in water sample was developed using automatic purge-and-trap (P&T) extraction (Tekmar, Atomx) coupled with GC (Agilent, 6890A)/MS (Agilent, 5973C). Water sample was heated at 65 C in P&T in order to improve the sensitivity of ITHMs. Trap #1, Tenax was used as the sorbent material. A DB-624 column (J&W) with helium as carrier gas was used. The oven temperature program was as follows: 45 C for 2 min, then increased at a rate of 10 C/min to 100 C, held at 100 C for 2 min, then ramped up to 200 C at 5 C/min, to 250 at 20 C/ min, and held at 250 C for 5 min. Selective ion monitoring (SIM) mode was operated in MS for quantitative analysis of ITHMs. More details about analytical method of ITHMs could be found in Supplementary material (Fig. S1, Table S1, and Table S2). Brominated/chlorinated THMs were analyzed according to EPA method 8260C. Atrazine was determined by high performance liquid chromatography (HPLC, Agilent-1200 series) coupled with tandem mass spectrometry (MS/MS, ABSCIEX-4000Q Trap). Total organic carbon (TOC) was analyzed by a TOC analyzer (Shimadzu, TOC-VCSH). UV absorption of ITHMs and THMs were measured by a UV/vis
w a t e r r e s e a r c h 4 9 ( 2 0 1 4 ) 2 7 5 e2 8 5
Fig. 1 e Molar extinction coefficients (ε) and first-order rate constants (k) of 6 ITHMs and 4 THMs at UV 254 nm. Error bars denote standard deviations obtained from at least triplicate datasets.
spectrophotometer (Shimadzu, UV-2550). Iodide and bromide were detected by an ion chromatography (IC) coupled with ICP-MS (Agilent, G3151A). Chloride and formate were detected by IC coupled with thermal conductivity detector (Dionex, ICS3000). H2O2 concentration was measured by a WTW Photolab S12.
2.3.
UV Photoreactor and photodegradation experiment
Photodegradation experiments were conducted in a 740 mL cylindrical glass reactor filled with 740 mL of fresh ITHMs aqueous solution at room temperature. A low-pressure mercury vapor lamp (nominal power 5 W, produced by Philips) primarily emitting irradiation at 254 nm was placed coaxially with the reactor in a quartz sleeve. Photodegradation of 6 ITHMs in the multi-species system (each at initial concentration of 100 mg/L) in DI water under UV 254 irradiation was conducted in triplicate. For comparison, photodegradation of 6 ITHMs in the single-species system (each at initial concentration of 100 mg/L) was conducted once under the same conditions. Photodegradation of 6 ITHMs and 4 brominated/ chlorinated THMs (CHCl3, CHCl2Br, CHClBr2, and CHBr3) in the multi-species system under UV 254 irradiation was also performed once. Additionally, photodegradation of 6 ITHMs and an actinometer (i.e., atrazine or CHBr3) in multi-species system was conducted in duplicate to determine the photon flux emitted by the present UV lamp and quantum yields of 6 ITHMs. Aliquots were sampled at predetermined time intervals and analyzed immediately by purge-and-trap coupled with GC/MS.
2.4.
3.
Results and discussion
3.1.
UV absorbance
277
The efficiency of the photodegradation process is affected by the micropollutant light absorption and quantum yield at the wavelength in question (Prados-Joya et al., 2011). To check the applicability of photodegradation of ITHMs under UV 254 irradiation, molar extinction coefficients (εl) of ITHMs and THMs were measured. The molar extinction coefficients of the non-chlorinated ITHMs including CHI3, CHBrI2 and CHBr2I are very close, approximately 1100 M1 cm1, and are higher than those of the chlorinated ITHMs (Fig. 1). In general, the molar extinction coefficients of ITHMs and THMs followed the order: CHI3 z CHBrI2 z CHBr2I > CHClI2 > CHBr3 z CHClBrI > CHClBr2 z CHCl2I > CHCl2Br > CHCl3. The result suggests that the photodegradation of ITHMs under UV 254 irradiation is viable due to their evident UV absorbance.
3.2. Photodegradation of ITHMs in deionized water: kinetic analysis The photodegradation of 6 ITHMs and 3 BTHMs followed the first-order kinetics (Fig. 2). There is insignificant difference in the degradation rate constants of 6 ITHMs obtained in the multi-species system and single-species system (Table 1). No change of concentration of 6 ITHMs in control solution (stored at 4 C in a refrigerator) was observed within 6 days. Additionally, experimental results showed that pH (5 and 10) and the initial concentration of ITHMs (10e400 mg/L) had no significant effect on the photodegradation kinetics of ITHMs (Figs. S2eS3). Thus, pH adjustment was not considered and the initial concentration of 100 mg/L for each ITHM was used in the subsequent investigations. The photodegradation rate constants of the non-chlorinated ITHMs (CHBr2I, CHBrI2, and CHI3) are almost identical, which are higher than those of the chlorinated ITHMs (CHCl2I, CHClBrI, and CHClI2) and the 3
Statistical analyses
To address experimental uncertainty, the statistical significance of the difference between the obtained results was analyzed using One-Sample t-Test. The QSAR model was established by simple and multiple linear regressions analysis in Microsoft Excel 2007.
Fig. 2 e Linear plots for UV 254 direct photolysis of 6 ITHMs; inset: linear plots for UV 254 direct photolysis of 4 THMs.
97.68 97.94 98.86 99.03 99.20 99.32 4.65 (0.45) 12.65 (0.97) 16.89 (1.08) 25.80 (2.66) 23.57 (1.81) 23.18 (1.66) 12.80 (1.37) 5.01 1.02 e
84.26 (1.54) 87.72 (1.03) 91.47 (1.07) 91.85 (1.36) 92.35 (0.85) 93.47 (0.73) (1.51) (2.56) (2.48) (4.59) (3.26) (3.10) (3.54) 16.77 40.59 50.92 68.14 64.40 63.70 40.95 17.96 3.88 e
Based on the BeereLambert Law and the definition of quantum yield, the overall photodegradation rate of micropollutants could be described as follows (Fang et al., 2013). dC ¼ Fl Io 1 10εl Cb dt
(0.13) (0.31) (0.35) (0.93) (0.63) (0.57) (0.43)
3.3.
c
Atrazine as actinometer. Standard deviations were obtained from at least duplicate datasets. There was no observable loss of CHCl3 in this study within 60 min of experimental run. b
a
(0.04) (0.04) (0.03) (0.05) (0.03) (0.03) (0.05) 0.43 0.47 0.45 0.39 0.37 0.35 0.45 0.33 0.42 e 0.997 0.993 0.989 0.992 0.990 0.988 0.995 0.996 0.991 e 3.41 (0.34)b 9.68 (0.79) 13.23 (0.93) 21.37 (2.53) 19.23 (1.69) 18.86 (1.54) 9.80 (1.12) 3.67 0.73 e 0.093 (0.009) 0.26 (0.02) 0.36 (0.02) 0.58 (0.07) 0.52 (0.04) 0.51 (0.04) 0.27 (0.03) 0.10 0.02 ec 0.10 0.27 0.36 0.58 0.53 0.52 e e e e CHCl2I CHClBrI CHClI2 CHBr2I CHBrI2 CHI3 CHBr3 CHClBr2 CHCl2Br CHCl3
(Multispecies) (Singlespecies)
(Multispecies)
R2 kf$104 (cm2 mJ1) k (min1) k (min1)
BTHMs (Table 1). The degradation rate constant of CHCl2I is the lowest among 6 ITHMs. This explains why CHCl2I could be commonly detected in tap water among 6 ITHMs (Brass et al., 1977; Cancho et al., 2000). It was also observed that the degradation rate constants of CHClBrI and CHCl2I are comparable to those of CHBr3 and CHClBr2, respectively. Hence, it can be said that ITHMs still become relatively recalcitrant to UV 254 direct irradiation when Cl atom is also present in the molecule. In general, the degradation rates of ITHMs and THMs followed the order: CHBr2I z CHBrI2 z CHI3 > CHClI2 > CHClBrI z CHBr3 > CHCl2I z CHClBr2 > CHCl2Br > CHCl3, which agrees with the order of their molar extinction coefficients (Fig. 1).
1.32 3.80 5.15 8.19 7.40 7.27 3.84 1.41 0.28 e
6 mg/L H2O2 (multi-species) (Multispecies)
(Multispecies)
(Multispecies)
540 mJ cm2 140 mJ cm2
Va
40 mJ cm
2
% ITHMs and THMs removal (R254)
6 mg/L H2O2 (single-species)
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Compound
Table 1 e Summary of time-based first-order rate constant (k), fluence-based first-order rate constant (kf), regression coefficient (R2), quantum yield (V), and removal percentages (R254) of 6 ITHMs and 4 THMs at different treatment conditions in DI water.
278
Determination of quantum yield (V) of ITHMs
(1)
where C is the concentration of the specific micropollutant (mol L1); dC/dt is the photodegradation rate of the specific micropollutant (mol L1 s1); Vl is the quantum yield at the wavelength in question (mol E1); Io is the UV irradiation intensity of the lamp used (E L1 s1); εl is the molar extinction coefficient at the wavelength in question (M1 cm1); and b is the optical path length (cm). When the UV absorbance of micropollutant (A ¼ 3 lCb) is very low (i.e., A value is less than 0.02), 10εl Cb in Equation (1) through Taylor expansion could be simplified as 1e2.3033lCb. Additionally, as the photodegradation follows the first-order kinetics (Equation (2)), the determination of quantum yield in diluted solution under monochromatic wavelength irradiation can be written as Equation (3) (Prados-Joya et al., 2011; Fang et al., 2013). dC ¼ kl C dt Fl ¼
kl 2:303$qp $εl
(2)
(3)
where kl is the photodegradation first-order rate constant (s1); qp is the photon flux emitted by the lamp (E s1 cm2). To derive the efficiency of the photodegradation process, the quantum yield (Vl) of ITHMs were determined from simultaneous photodegradation of the target ITHMs and an actinometer with a known quantum yield at 254 nm (Javier Benitez et al., 2004; Liu et al., 2009; Prados-Joya et al., 2011). Atrazine, a common actinometer with a known quantum yield (0.033 mol E1) was used to determine the photon flux (qp) of the present UV lamp (Bolton and Stefan, 2002). The firstorder rate constant and molar extinction coefficient of atrazine in this study was measured as 0.15 min1 and 3424 M1 cm1, respectively. According to Equation (3), the photon flux (qp) was determined as 9.61 109 E s1 cm2, which corresponds to UV intensity of 4.54 mW cm2 for the present lamp. Due to its similar physicochemical properties to ITHMs, CHBr3 with a known quantum yield (0.43 mol E1) was also selected as another actinometer to validate the accuracy
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of photon flux (qp) measurement (Nicole et al., 1991). The photon flux (qp) was calculated as 1.01 108 E s1 cm2, which is almost the same as that obtained by the atrazine actinometry. This good agreement led us to choose atrazine as the actinometer to determine the photon flux and quantum yield in this study. The derived quantum yields of 6 ITHMs in the photoreactor ranged from 0.32 to 0.51 (Table 1), which indicates a high efficiency of quantum process in the ITHMs photodegradation under UV 254 irradiation. Since fluence-based first-order rate constant (kf) is independent of the fluctuation in photon flux (qp) of the UV lamp, it allows for direct comparison with the photodegradation rate constants obtained with different photoreactors (Canonica et al., 2008; Prados-Joya et al., 2011). For comparison purpose, kf values are calculated through dividing the time-based first-order rate constant (kl) by the photon flux and photon energy emitted by the UV lamp according to Equation (4). The obtained kf values for ITHMs were summarized in Table 1. kf ¼
kl qp $hc=l
(4)
where kf is the fluence-based first-order rate constant (cm2 mJ1); kl is the time-based first-order rate constant (s1); qp is the photon flux emitted by the lamp (E s1 cm2); hc/l is the energy of photon at a specific wavelength (kJ mol1). To check the applicability of UV 254 irradiation in ITHMs photodegradation in real water treatment process, the removal efficiency of ITHMs were determined under a specific UV 254 dose. The irradiation doses of 40 and 140 mJ cm2 are chosen because they define typical range used in disinfection application (Baeza and Knappe, 2011). As the UV intensity of the present lamp is 4.54 mW cm2, the time required to reach the irradiation doses of 40 and 140 mJ cm2 are 8.8 and 30.8 s, respectively. The removal percentage of ITHMs (R254) can be calculated according to Equation (5). R254 ¼ 1 ekl t %
279
also identified as the end-product, which was also reported in the previous study regarding direct photolysis of CHBr3 (Kwok et al., 2004). HCOOH increased slightly during the photodegradation process and reached 40 mg/L at the end (Fig. 3), which accounts for less than 5% of TOC concentration initially spiked in the reaction solution. It suggests that HCOOH is the minor end-product in the direct photolysis of ITHMs. Moreover, formaldehyde was not detected during the photodegradation process. According to previous studies, UV 240 nm could cleave the primary CeBr bond of CHBr3, but the photon energy was insufficient to induce secondary dissociation of the CHBr2 radical formed (McGivern et al., 2000; Kwok et al., 2004). In our study, the energy of UV 254 photon (471 kJ/mol) is higher than those of C-X bonds (209, 280, and 397 kJ/mol for CeI, CeBr, and CeCl, respectively) (Weast et al., 1986). Homolytic photocleavage of CeI bond upon UV 254 absorption could be the first step of ITHMs photolysis. As described in the previous section, the degradation rate constants of ITHMs are proportional to their corresponding molar extinction coefficients (Fig. 1). It suggests that the first-step reaction (cleavage of CeI bond) is the rate-determining step for photodegradation of ITHMs. Isobromoform (BrCHBreBr) has been previously identified as the intermediate in the second step of CHBr3 photodegradation, and subsequently a water-catalyzed mechanism proceeds to transform isobromoform into its following intermediates which will react with water molecule through OH insertion/HBr elimination resulting in complete conversion to HBr, CO (major product) and HCOOH (minor product) (Kwok et al., 2004). Thus, in our study, photocleavage of CeI bond by UV 254 followed by a series of water-catalyzed reactions might be the possible photodegradation pathway for ITHMs in water.
(5)
As seen from Table 1, R254 of 6 ITHMs at UV doses of 40 and 140 mJ cm2 are less than 10 and 26%, respectively. It suggests that a higher irradiation dose than the usual dose for water disinfection is required to significantly photodegrade ITHMs.
3.4.
End-product identification
A previous study has shown that the lifetimes of CHBr3 intermediate photoproducts in water were in picoseconds and special equipment (like Picosecond Time-Resolved Resonance Raman (ps-TR3) Spectroscopy) was required to identify them (Kwok et al., 2004). In our study, through identification of the end-products, we could identify the possible photodegradation pathway of ITHMs. The photodegradation endproducts in this study were chloride, bromide and iodide (Fig. 3). There was a good mass balance between total amount of produced halides and the introduced halogens in ITHMs (i.e., 92e109% recovery). No formation of chlorate, bromate, and iodate was observed. Additionally, TOC decreased concomitantly during the photodegradation process (Fig. 3), indicating that the ITHMs could be mineralized via UV 254 photolysis. However, it is worth to mention that the HCOOH is
Fig. 3 e Time course of halide formation during direct photolysis of 6 ITHMs (multi-species system, each at initial concentration of 200 mg/L); inset: Time course of TOC reduction, CHI3 reduction and HCOOH formation during direct photolysis of CHI3 (initial concentration around 10 mg/L).
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3.5. Photodegradation of ITHMs in the presence of matrix species Further investigation was performed to evaluate the effects of matrix species including nitrate, humic acid (HA), bicarbonate, sulfate, and chloride on ITHMs photodegradation. In the following discussion, the degradation rates of each ITHM under different conditions are compared to those observed under the reference condition (DI water, initial concentration of 100 mg/L, and pH around 5). When the p value is less than
Fig. 4 e (A). Effect of nitrate on photodegradation of 6 ITHMs; (B). Effect of humic acid (HA) on photodegradation of 6 ITHMs; (C). Effect of bicarbonate, sulfate, and chloride on photodegradation of 6 ITHMs. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained in DI water. Error bars denote standard deviations obtained from at least duplicate datasets.
0.01 in One-Sample t-Test, it suggests that the degradation rate constant of the ITHM species under the specific condition is significantly different from those obtained under the reference condition. The degradation rate constant of CHCl2I in the presence of 5 mg/L nitrate is comparable to that obtained in DI water (Fig. 4A). However, when the nitrate concentration was increased from 20 to 200 mg/L, the degradation rate of CHCl2I increased correspondingly. The degradation rate constant of CHCl2I reaches 0.31 min1 in the presence of 200 mg/L nitrate, which is 3 times higher than that obtained in DI water. However, nitrate had much less influence on the photodegradation of the other 5 ITHMs which are highly susceptible to UV 254 photolysis compared to CHCl2I (Table 1). The photodegradation rate constants of the 5 ITHMs increased by less than one-fold as compared to those observed in DI water even the nitrate concentration was increased to 200 mg/L. It is reported that some chemicals such as salicylic acid and benzoic acid are employed as probe for trapping HO to form the stable and quantifiable products (Jankowski et al., 2000; Yang et al., 2004; Wu et al., 2007). On the basis of this methodology, some studies demonstrated that direct photolysis of
Fig. 5 e (A). Absorbance in DI water of nitrate, humic acid (HA), bicarbonate, sulfate, and chloride at the concentrations used in this study in the range of 220e300 nm; (B). Absorbance in DI water of nitrate, HA, bicarbonate, sulfate, and chloride at the concentrations used in this study in the range of 250e260 nm.
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nitrate under UV irradiation could generate HO (Yang et al., 2004; Boucheloukh et al., 2012; Ji et al., 2012). Indeed, the formation of HO in the UV direct photolysis of nitrate follows a complex pathway. The primary reactions in nitrate photolysis are the generation of the radical anion Oˉ and nitrite ion, which continues to produce the radical anion O under UV irradiation. Subsequently, the radical anion O is rapidly reacting with water molecule to form the highly reactive HO. Generally, the reaction pathway for generation of HO via nitrate photolysis can be simplified as follows (Keen et al., 2012).
NO 3 þ hv/NO2 þ O
NO 3 þ hv/NO2 þ
(6)
1 O2 2
(7)
NO 2 þ hv/NO þ O
(8)
O þ H2 O/HO þ HO
(9)
However, due to weak UV 254 absorption of nitrate (Fig. 5), HO was generated at a low quantum yield (Mack and Bolton, 1999; Keen et al., 2012). This can explain why the photodegradation rate of the 5 ITHMs (CHClBrI, CHBr2I, CHClI2, CHBrI2, and CHI3) only increased slightly in the presence of nitrate in this study. Additionally, it is believed that the main reason for the more significant effect of nitrate-induced indirect photolysis of CHCl2I is that the efficiency of its direct photolysis is low (i.e., its k value is 3e6 times lower than those of other 5 species). In other words, the effect of indirect photolysis due to HO attack in the presence of nitrate for other 5 species is possibly masked by their efficient direct photolysis. It has been reported that natural organic matter (NOM) present in water has two opposite effects on photodegradation of micropollutants (De la Cruz et al., 2012). On the one hand, it can inhibit direct photolysis due to inner filtering of UV light by NOM. On the other hand, with exposure to UV irradiation, a photosensitizer present in NOM can be excited to its reactive triplet state (3NOM*) or singlet state (1NOM*), which can react with the micropollutant (De la Cruz et al., 2012; Marin et al., 2012). Moreover, 3NOM* also can react with oxygen present in water to generate singlet molecular oxygen, 1O2 that will subsequently attack the micropollutant (De la Cruz et al., 2012). In the latter case, the formation of reactive oxygen species (ROS, e.g. 3NOM*, 1NOM*, and 1O2) increases the photodegradation rate of micropollutant. In this study, humic acid (HA) was selected as a matrix species to investigate the effect of NOM on ITHMs photodegradation, as HA is considered as the main component of NOM (Prados-Joya et al., 2011).
HA consists of a great number of aromatic rings and oxygenrelated functional groups, which could strongly absorb UV light (Steelink, 2002). Experimental results showed that the photodegradation rates of 6 ITHMs decreased with increasing HA from 1 to 20 mg/L (Fig. 4B). The presence of HA significantly reduced the photodegradation rates of 6 ITHMs to the same extent. This result suggests that filtering and screening of UV light by HA reduces the fraction of UV light absorbed by ITHMs, which results in the decease of the photodegradation rates of ITHMs. In other words, indirect photolysis in the presence of a photosensitizer such as HA could be less significant than the competitive UV absorption effect for ITHMs photodecay. In contrast, when bicarbonate, sulfate, or chloride was spiked into the reaction solution, the phenomenon of filtering and screening of UV 254 light as well as the ROS attacking of ITHMs was not observed, because these 3 anions at the concentration of 100 mg/L have no UV 254 absorption (Fig. 5). As such, bicarbonate, sulfate, and chloride had no statistically significant effect on the photodegradation kinetics of ITHMs (Fig. 4C).
3.6. water
Photodegradation of ITHMs in different types of
The viability of UV 254 irradiation to photodegrade ITHMs in different types of water including DI water, surface water, treated water, and secondary effluent (collected from a wastewater treatment plant, Singapore) was also evaluated. The characteristics of the water samples are presented in Table 2. The degradation rates of 6 ITHMs in the surface water and secondary effluent dramatically decreased, while their degradation rates in the treated water were comparable to those in DI water (Fig. 6). According to the preceding discussion, the effect of pH, sulfate, bicarbonate, chloride and nitrate with concentration less than 5 mg/L on ITHMs photodegradation could be excluded in the study using real water sample as the matrix. The reduction of photodegradation rates in surface water could be mainly attributed to its high turbidity which shielded target ITHMs from UV 254 irradiation. Although nitrate concentration in the secondary effluent was very high (45.2 mg/L), the increase of photodegradation rate of ITHMs was not evident, as high concentration of HO scavengers such as bicarbonate and NOM were also present in the secondary effluent (Table 2). In contrast, high concentration of NOM present in the secondary effluent led to decrease of photodegradation rates of ITHMs due to competitive absorption of UV 254 irradiation by NOM. It suggested that direct photolysis was still the predominant mechanism for ITHMs
Table 2 e Characteristics of different types of water used in this study. Water
pH
Turbidity (NTU)
TOC (as C, mg/L)
Total alkalinity (as CaCO3)
Nitrate (mg/L)
Sulfate (mg/L)
Chloride (mg/L)
DI water Surface water Treated water Secondary effluent
6.00 6.40 8.00 6.70
<0.1a 100 0.22 5.3
0.080 2.40 0.79 9.70
<5 8 13 36
<0.05 3.38 3.20 45.2
<0.045 4.14 21.6 45.1
<5 9 10 86
a
It represents that the result is below detection limit.
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Fig. 6 e Effect of different types of water on photodegradation of 6 ITHMs. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained in DI water. Error bars denote standard deviations obtained from at least triplicate datasets.
photodegradation in the real water samples but the indirect photolysis due to generation of ROS was not evident. It can be said that UV photolysis is more suitable as a post-treatment after filtration to control ITHMs.
3.7. Comparison of photodegradation of ITHMs by UV alone and UV/H2O2 As mentioned previously, the usual UV dose of 140 mJ/cm2 for water disinfection is not adequate to remove ITHMs. In order to provide practical reference for water industry, preliminary investigation of ITHMs photodegradation by UV/H2O2 was also conducted. The concentration of H2O2 used in this study was 6 mg/L, which was within the range of its typical doses applied in pilot-scale studies (Paradis and Hofmann, 2006). Two sets of experiments on photodegradation of ITHMs in DI water by UV/ H2O2 were performed (Fig. 7), namely multi-species system containing 6 ITHMs (each at the initial concentration of 100 mg/ L) and single-species system (100 mg/L). With addition of H2O2, the overall micropollutant photodegradation involves both UV direct photolysis and HO indirect photolysis. Due to the relatively high concentration of H2O2 under constant UV irradiation, concentration of HO is assumed to be constant at the steady-state level with respect to micoropollutant concentration. Therefore, the pseudo-firstorder kinetics (Equation (10)) has been proposed in the previous studies (Sharpless and Linden, 2003; Jo et al., 2011; Shu et al., 2013). dC ¼ k0 C ¼ ðkd þ ki ÞC dt
Fig. 7 e Comparison of photodegradation of ITHMs in DI water by UV alone and UV/H2O2. Asterisk (*) represents p < 0.01, which implies that the k value is significantly different from the k values obtained by UV alone. Error bars denote standard deviations obtained from at least duplicate datasets.
The degradation rate constants of 6 ITHMs in the UV/H2O2 system were comparable and significantly increased as compared to those achieved with UV alone. However, due to competitive reaction of 6 ITHMs with HO in the multi-species system, the extent of increase in the degradation rate constants are less than those in the single-species system (Fig. 7). Compared to nitrate photolysis at UV 254, HO was generated at a higher quantum yield in the UV/H2O2 system. As such, indirect photolysis played an important role in the ITHMs photodegradation in the UV/H2O2 system. Additionally, according to equation (5), the removal percentages (R254) of 6 ITHMs were determined at a UV dose of 540 mJ/cm2 with 6 mg/ L H2O2, which is a typical UV/H2O2 treatment conditions (Kruithof et al., 2007; Baeza and Knappe, 2011). As shown in Table 1, the removal percentages (R254) of 6 ITHMs even in the multi-species system could reach more than 84% in the UV/ H2O2 system and increased obviously as compared to those under direct UV irradiation with the similar UV dose. Thus, UV-based AOP technology is promising to remove ITHMs effectively, due to a synergistic effect of indirect photolysis and direct photolysis.
3.8.
QSAR Modeling
An empirical equation is usually used to describe the QSAR model as follows. P¼
i¼n;j¼n X
aij Dij þ b
(11)
i¼1;j¼1
(10)
where k0 is the observed pseudo-first-order rate constant of overall photodegradation process (s1); kd is the pseudo-firstorder rate constant of direct photolysis (s1); and ki is the pseudo-first-order rate constant of indirect photolysis due to the reaction with HO (s1). In this study, the pseudo-first-order kinetics was also observed for ITHMs photodegradation in the UV/H2O2 system.
where P is the property of interest such as first-order rate constant, quantum yield etc.; aij and b are the constants obtained from simple and multiple linear regression; Dij are molecular descriptors characterizing the structure of the compounds studied (Dyekjær and Jo´nsdo´ttir, 2004; Ioele et al., 2009). To derive a QSAR model for photodegradation of ITHMs and BTHMs, the logarithm of measured first-order rate
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constants (logkm) are correlated to a number of molecular descriptors by simple and multiple linear regressions analysis. 3 molecular descriptors are selected since they give an indication of the bond to be broken, steric and electronic effect of the substituents that determine the chemical reactivity of the compound studied. These 3 molecular descriptors include the bond strength (BS) of the carbon-halogen bond to be broken in the rate-determining step, the summation of the electronic effect constants (s), and the summation of the steric effect constants (Es) of all substituents to carbon center. Values of BS, s, and Es of 6 ITHMs and 3 BTHMs used to establish the QSAR model are presented in Table 3 (Weast et al., 1986; Hansch and Leo, 1995). The results of a number of correlations between logkm and individual BS, s, Es, as well as the combinations of these molecular descriptors are summarized in Table 4. The value of the regression coefficient, R2, is used to determine the degree of correlation between these descriptors and logkm. When 1 or 2 descriptors are correlated with logkm, regression coefficients are less than 0.84 in all cases. The best correlation is obtained with a combination of these 3 descriptors (BS, s, and Es), with R2 ¼ 0.95 (model 7 in Table 4). In addition, F test in multiple linear regressions analysis shows that all coefficients in QSAR model 7 are significant at the 95% confidence level. It suggests that these 3 molecular descriptors have a combined effect on the photodegradation of ITHMs and BTHMs. QSAR model 7 is in turn used to predict the photodegradation rate of ITHMs and BTHMs. The QSAR-predicted logkp, experimentally measured logkm, as well as the difference between logkm and logkp (D) are presented in Table 3. The correspondence between logkm and logkp is also displayed in Fig. 8. Generally, QSAR-predicted values agree with the experimental results, except for CHClI2 which shows 0.235-log relative standard deviation. Additionally, the predicted first-order rate constant of CHCl3 by QSAR model 7 is 0.0013 min1, which corresponds to half-time of 9 h. It indicates that CHCl3 is more stable than ITHMs under UV 254 irradiation, because the life-time of 6 ITHMs is in the range of 1e8 min. However, there was no
Table 4 e Correlations of the logarithm of the measured first-order rate constant (logkm) with a number of molecular descriptors. Item 1 2 3 4 5 6 7
Model logk logk logk logk logk logk logk
¼ ¼ ¼ ¼ ¼ ¼ ¼
0.0086BS þ 1.32 1.17Es 4.89 6.13s þ 7.02 0.0016BS 1.07Es 4.15 0.0020BS 5.50s þ 6.67 1.88Es 15.6s þ 25.6 0.0075BS þ 4.97Es 28.8s þ 55
R2 0.40 0.74 0.80 0.76 0.81 0.84 0.95
observable loss of CHCl3 in this study within 60 min of experimental run. The deviation suggests that further research is needed to explore more parameters to give a better description of ITHMs and THMs photodegradation process.
4.
Conclusion
ITHMs photodegradation under UV 254 irradiation followed the first-order kinetics with rate constants in the range of 0.1e0.6 min1. The quantum yields of 6 ITHMs were derived from 0.32 to 0.51, when atrazine was used as an actinometer. ITHMs could be mineralized and liberated halides. Experimental results also showed that nitrate had a slightly positive effect on the photodegradation kinetics possibly due to generation of HO at a low quantum yield via nitrate photolysis, while HA reduced the photodegradation rates due to its competitive UV absorption. In contrast, bicarbonate, sulfate, and chloride had no significant effect on ITHMs photodegradation, as these 3 species have no UV 254 absorption. Based on the above investigation of matrix species effect, it could be deduced that the decrease of photodegradation rate of 6 ITHMs in surface water and secondary effluent are mainly attributed to high turbidity and NOM, respectively. Additionally, it is determined that the removal efficiency of direct photolysis of 6 ITHMs at a common disinfection dose of
Table 3 e Values of molecular descriptors used in this study for establishing the QSAR models and experimentally measured logkm, predicted logkp by QSAR model 7, as well as the difference between logkm and logkp, D. Compound
BSa
Esb
sb
logkm
Logkp
D
CHCl2Br CHClBr2 CHBr3 CHCl2I CHClBrI CHBr2I CHClI2 CHBrI2 CHI3
280 280 280 209 209 209 209 209 209
3.08 3.28 3.48 3.32 3.52 3.72 3.76 3.96 4.20
1.36 1.31 1.26 1.32 1.27 1.22 1.23 1.18 1.14
1.699 0.998 0.569 1.027 0.585 0.237 0.444 0.284 0.292
1.576 1.130 0.684 1.084 0.638 0.192 0.679 0.233 0.274
0.123 0.132 0.115 0.057 0.053 0.045 0.235 0.051 0.019
a
Bond strength of C-X bonds to be broken in the rate-determining step were cited from literature (Weast et al., 1986). b Es and s values of halogen atoms were cited from literature (Hansch and Leo, 1995); the combined effects of multiple halides in a halo-organic are assumed to be the summative effects of individual halides in it (Zhang and Minear, 2002; Chen et al., 2010).
Fig. 8 e Correspondence between experimentally measured logkm and predicted logkp values by QSAR model 7 for 6 ITHMs and 3 BTHMs.
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140 mJ cm2 is less than 26%, while more than 84% of 6 ITHMs could be degraded at a UV dose of 540 mJ cm2 with 6 mg/L H2O2. However, more research is still needed to investigate the applicability of UV-based AOP for ITHMs removal in real practice. Finally, a good QSAR model with R2 ¼ 0.95 was established based on the correlation between logkm and the combination of 3 molecular descriptors, namely bond strength (BS) of the carbon-halogen bond to be broken, the electronic effect (s) and the steric effect (Es) of all substituents to carbon center. It suggests that these 3 parameters have a significant combined effect on the overall photodegradation of ITHMs and BTHMs.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2013.11.039
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