Article pubs.acs.org/est
Formation Pathways and Trade-Offs between Haloacetamides and Haloacetaldehydes during Combined Chlorination and Chloramination of Lignin Phenols and Natural Waters Yi-Hsueh Chuang,† Daniel L. McCurry,† Hsin-hsin Tung,‡ and William A. Mitch*,† †
Department of Civil and Environmental Engineering, Stanford University, 473 Via Ortega, Stanford, California 94305, United States Graduate Institute of Environmental Engineering, National Taiwan University, No. 1, Sec. 4, Roosevelt Rd., Taipei 10617, Taiwan
‡
S Supporting Information *
ABSTRACT: In vitro bioassays have indicated that haloacetamides and haloacetaldehydes exhibit the highest cytotoxicity among DBP classes. Previous research has focused on their potential formation from the chlorination or chloramination of aliphatic compounds, particularly nonaromatic amino acids, and acetaldehyde. The present work found that acetaldehyde served as a relatively poor precursor for trichloroacetaldehyde and dichloroacetamide, generally the most prevalent of the haloacetaldehydes and haloacetamides, during chlorination or chlorination/chloramination. Using phenolic model compounds, particularly 4-hydroxybenzoic acid, as models for structures in humic substances, we found significantly higher formation of trichloroacetaldehyde and dichloroacetamide from prechlorination followed by chloramination. Evaluation of the stoichiometry of chlorine reactions with 4-hydroxybenzoic acid and several intermediates indicated that seven successive Cl[+1] transfers, faster with chlorination than chloramination, can form 2,3,5,5,6-pentachloro-6hydroxy-cyclohexa-2-ene-1,4-dione via chlorophenol and chlorobenzoquinone intermediates. Formation of 2,3,5,5,6-pentachloro6-hydroxy-cyclohexa-2-ene-1,4-dione may serve as a key branching point, with chloramines promoting the formation of dichloroacetamide and chlorination promoting the formation of trichloroacetaldehyde. The behavior of 4-hydroxybenzoic acid with respect to yields of dichloroacetamide and trichloroacetaldehyde during chlorination followed by chloramination was similar to the behavior observed for model humic acids and several surface waters, suggesting that phenolic structures in natural waters may serve as the predominant, and common pool of precursors for haloacetamides and haloacetaldehydes. Experiments with natural waters indicated that the branching point is reached over prechlorine exposures (100−500 mg-min/L) relevant to drinking water utilities using chlorine as a primary disinfectant and chloramines for maintenance of a distribution system residual.
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INTRODUCTION To reduce the concentration of regulated trihalomethane (THM) and haloacetic acid (HAA) disinfection byproducts (DBPs), utilities are increasingly switching from free chlorine disinfection to a combination of free chlorine for primary disinfection and chloramines for maintenance of a distribution system residual. Simultaneously, there is rising interest in an array of unregulated DBPs, including N-nitrosamines, haloacetonitriles, halonitromethanes, haloacetamides, and haloacetaldehydes. Research is needed to optimize chlorination/ chloramination combinations to minimize the overall exposure to DBPs, because some of these compounds (e.g., Nnitrosamines and haloacetamides), form at higher concentrations during chloramination.1,2 Although haloacetaldehydes and haloacetamides occur in the ng/L to low μg/L range in disinfected waters,3,4 their cytotoxicity exceeds that of haloacetonitriles, halonitromethanes, HAAs and THMs in in vitro mammalian cell toxicity assays.5,6 The formation pathways for haloacetaldehydes and haloacetamides have received less attention than those for the other © 2015 American Chemical Society
DBP classes. Previous research has focused on aliphatic amino acid precursors.7−11 During chlorination, the amino groups in amino acids can serve as the source of the haloacetamide nitrogens via hydrolysis of a dichloroacetonitrile (DCAN) intermediate. For example, chlorination or chloramination of free aspartic acid results in chlorination of the α-amino group8 (Scheme S1). Concerted decarboxylation followed by hydrochloric acid elimination from the dichlorinated intermediate yields a nitrile. Chlorination of the acidic α-carbons followed by hydrolysis yields DCAN. Hydrolysis of DCAN can produce dichloroacetamide.12 However, several factors suggest that this pathway is of low importance. The molar yields of dichloroacetamide from the chlorination of free amino acids are all below 0.025%,13 and the occurrence of free amino acids in raw waters is typically very low (i.e., sub μg/L levels).14 Received: Revised: Accepted: Published: 14432
September 29, 2015 November 12, 2015 November 16, 2015 November 16, 2015 DOI: 10.1021/acs.est.5b04783 Environ. Sci. Technol. 2015, 49, 14432−14440
Article
Environmental Science & Technology Table 1. Basic Water Quality Results
a
waters
source
pH
DOC (mg-C/L)
SUVA254 (L/mg-m)
NH4+ (mg-N/L)
NO3− (mg-N/L)
NO2− (mg-N/L)
Br− (μg/L)
water A water B water C
WTP influent WTP influent lake water
6.9 7.8 7.9
1.9 4.5 8.3
1.89 2.80 1.40
