Article pubs.acs.org/est
Impact of UV/H2O2 Pre-Oxidation on the Formation of Haloacetamides and Other Nitrogenous Disinfection Byproducts during Chlorination Wenhai Chu,*,† Naiyun Gao,*,† Daqiang Yin,† Stuart W. Krasner,‡ and William A. Mitch§ †
State Key Laboratory of Pollution Control and Resources Reuse, College of Environmental Science and Engineering, Tongji University, Shanghai, 200092, China ‡ Metropolitan Water District of Southern California, 700 Moreno Avenue, La Verne, California 91750-3399, United States § Department of Civil and Environmental Engineering, Stanford University, Y2E2 145, 473 Via Ortega, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Haloacetamides (HAcAms), an emerging class of nitrogenbased disinfection byproducts (N-DBPs) of health concern in drinking water, have been found in drinking waters at μg/L levels. However, there is a limited understanding about the formation, speciation, and control of halogenated HAcAms. Higher ultraviolet (UV) doses and UV advanced oxidation (UV/H2O2) processes (AOPs) are under consideration for the treatment of trace organic pollutants. The objective of this study was to examine the potential of pretreatment with UV irradiation, H2O2 oxidation, and a UV/H2O2 AOP for minimizing the formation of HAcAms, as well as other emerging N-DBPs, during postchlorination. We investigated changes in HAcAm formation and speciation attributed to UV, H2O2 or UV/H2O2 followed by the application of free chlorine to quench any excess hydrogen peroxide and to provide residual disinfection. The results showed that lowpressure UV irradiation alone (19.5−585 mJ/cm2) and H2O2 preoxidation alone (2−20 mg/L) did not significantly change total HAcAm formation during subsequent chlorination. However, H2O2 preoxidation alone resulted in diiodoacetamide formation in two iodide-containing waters and increased bromine utilization. Alternatively, UV/H2O2 preoxidation using UV (585 mJ/cm2) and H2O2 (10 mg/L) doses typically employed for trace contaminant removal controlled the formation of HAcAms and several other N-DBPs in drinking water.
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the brominated and iodinated species.8,9 One study investigated the formation of nine chlorinated and brominated HAcAms from seven source waters in China using a high chlorine dose.10 It was found that all of the source waters formed mostly diHAcAms. However, HAcAm concentrations observed in this study, using untreated source waters and high chlorine doses, may not be representative of HAcAm formation in practice (rather these conditions were more appropriate for formation potential testing). It is important to investigate HAcAm formation and speciation, including Cl-, Br-, and I-species, using realistic disinfection conditions and authentic treated/ filtered waters. Five HAcAms, including chloro- (CAcAm), dichloro- (DCAcAm), trichloro- (TCAcAm), bromo(BAcAm), and dibromoacetamide (DBAcAm), were quantified as part of a U.S. nationwide DBP occurrence study, and the median total concentration of the five HAcAms in the finished
INTRODUCTION Haloacetamides (HAcAms), an emerging class of halogenated nitrogen-based disinfection byproducts (N-DBPs), have been reported to be very cytotoxic and genotoxic in mammalian cell assays (142× more cytotoxic and 12× more genotoxic than regulated haloacetic acids [HAAs]).1,2 The elevated toxicity was also found in a recent study based on metabolomics.3 During chlorination, HAcAms can form from the reaction of chlorine with dissolved organic nitrogen (DON) precursors; studies using 15N-labeled chloramines have indicated that haloacetamides can form during chloramination by either reaction of chloramines with DON precursors or incorporation of the chloramine nitrogen into organic precursors.4 Although specific reaction pathways and precursors are unclear, studies have indicated that HAcAms can form by hydrolysis of haloacetonitriles5 or other pathways independent of haloacetonitriles.4 Most of the chlorinated HAcAms (Cl-HAcAms) are less cytotoxic and genotoxic than their brominated and iodinated analogues.6 However, to-date, most studies on HAcAms in drinking water have focused on the chlorinated species,7 whereas only a small number of occurrence studies evaluated © 2014 American Chemical Society
Received: Revised: Accepted: Published: 12190
April 29, 2014 September 4, 2014 September 24, 2014 September 24, 2014 dx.doi.org/10.1021/es502115x | Environ. Sci. Technol. 2014, 48, 12190−12198
Environmental Science & Technology
Article
Table 1. Water Quality Characteristics of the Three Natural Waters Included in This Study no.
sampling location
DOC (mg/L)
DON (mg/L)
DOC/DON
SUVA (L/mg·m)
bromide (μg/L)
iodide (μg/L)
1 2 3 4 5 6
MH raw water MH filtered water SY raw water SY filtered water ZQ raw water ZQ filtered water
3.7 2.3 6.1 2.7 5.9 2.5
0.31 0.24 0.37 0.29 0.62 0.49
11.9 9.6 16.5 9.3 9.5 5.1
4.5 3.4 2.4 1.6 2.6 1.7
137 130 145 139 24 21
14 11 17 15 10 K
5−10 K
1−5 K
0.5−1 K
0.05), which were significantly higher than that of ZQ filtered water (p = 1.5 × 10−4 < 0.05), due to the higher bromide levels in the former two waters (MH: 130 μg/L; SY: 139 μg/L) than in ZQ filtered water (21 μg/L). However, the BUF values of all three filtered waters were similar (p = 0.46 > 0.05), which indicated that bromide utilization was somewhat similar in each filtered water regardless of the initial bromide level. In regards to THM and HAA formation, previous research has indicated that bromine is more effectively incorporated into low-SUVA, low-AMW, and hydrophilic DOM fractions.25,26 However, Hua and Reckhow also found that less unknown total organic bromide (TOBr) was formed by the hydrophilic fraction than by hydrophobic and highAMW precursors.26 Brominated HAcAm formation occurred in both low and high-SUVA254 filtered waters. Symons and colleagues found that bromine incorporation into THMs was impacted by the bromide/DOC and chlorine/bromide ratios, not just the level of bromide.37 In this study, bromide/DOC was similar for MH and SY (56.5 and 51.5 on a weight basis)
and iodide initially present that is utilized in forming diHAcAms. Previous studies have calculated utilization factors for both bromide28−30 and iodide30−33 in regards to THMs, HAAs or total organic halogen (TOX) formation. To calculate the bromide utilization factor (BUF) for di-HAcAm formation, the total di-HAcAm concentration was normalized based on the number of bromines present in each species, and this was divided by the bromide molar concentration in water. A similar analysis for the iodine utilization factor (IUF) was completed as well. The following formulas were applied to calculate utilization (eqs 1 and 2, where the concentrations are molar): BUF(DHAcAms) =
[BCAcAm] + [BIAcAm] + 2[DBAcAm] [Br −]
(1) IUF(DHAcAms) =
[CIAcAm] + [BIAcAm] + 2[DIAcAm] [I−]
(2)
Bromide incorporation factor (BIF) and iodide incorporation factor (IIF) for di-HAcAms were also calculated by modifying the THM halogen incorporation factor formula first suggested by Gould et al.34 and developed by Jones et al.31 In this study, the formula was modified to include all six di-HAcAms species (DHAcAm6). BIF and IIF are used as an index to describe the proportion of the DBPs that were partially or totally brominated and iodinated. The following formulas were applied to calculate BIF and IIF (eqs 3 and 4, where the concentrations are molar): BIF(DHAcAms) =
[BCAcAm] + [BIAcAm] + 2[DBAcAm] × 100% [DHAcAm6]
(3) IIF(DHAcAms) =
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[CIAcAm] + [BIAcAm] + 2[DIAcAm] × 100% [DHAcAm6]
(4)
RESULTS AND DISCUSSION HAcAm Formation during Chlorination Without UV and H2O2 Pretreatment. As shown in Figure 1, besides three chlorinated HAcAms (CAcAm, DCAcAm, and TCAcAm), only a little BCAcAm was detected in ZQ filtered water, due to the low bromide level (Table 1). Alternatively, other brominated HAcAms were formed in the other two waters, which contained moderate bromide concentrations. However, ZQ filtered water exhibited the highest total HAcAm concentrations (36.8 nM 12193
dx.doi.org/10.1021/es502115x | Environ. Sci. Technol. 2014, 48, 12190−12198
Environmental Science & Technology
Article
Table 3. HAcAm Formation during Chlorination with or without H2O2 Pre-Oxidationa conditions
CEC
CEC
AECb
CEC
total HAcAms
CEC
DIAcAm
DHAcAm substitution
water
H2O2 oxidation
(μg/L)
(nM)
DCAcAm (μg/L)
(ng/L)
(ng/L)
BIF
IIF
BUF
IUF
MH
no yes no yes no yes
2.71 2.80 3.20 3.40 5.85 5.77
17.0 17.4 19.6 20.5 36.8 37.4
1.69 1.73 2.07 2.13 4.93 5.02
