Article pubs.acs.org/est
Ozone Promotes Chloropicrin Formation by Oxidizing Amines to Nitro Compounds Daniel L. McCurry,† Amanda N. Quay,† and William A. Mitch*,† †
Department of Civil and Environmental Engineering, Stanford University, Stanford, California 94305, United States S Supporting Information *
ABSTRACT: Chloropicrin formation has been associated with ozonation followed by chlorination, but the reaction pathway and precursors have been poorly characterized. Experiments with methylamine demonstrated that ozonation converts methylamine to nitromethane at ∼100% yield. Subsequent chlorination converts nitromethane to chloropicrin at ∼50% yield under the conditions evaluated. Similarly high yields from other primary amines were limited to those with functional groups on the β-carbon (e.g., the carboxylic acid in glycine) that facilitate carbon−carbon bond cleavage to release nitromethyl anion. Secondary amines featuring these reactive primary amines as functional groups (e.g., secondary N-methylamines) formed chloropicrin at high yields, likely by facile dealkylation to release the primary nitro compound. Chloropicrin yields from tertiary amines were low. Natural water experiments, including derivatization to transform primary and secondary amines to less reactive carbamate functional groups, indicated that primary and secondary amines were the dominant chloropicrin precursors during ozonation/chlorination. Ozonation followed by chlorination of the primary amine side chain of lysine demonstrated low yields (∼0.2%) of chloropicrin, but high yields (∼17%) of dichloronitrolysine, a halonitroalkane structural analogue to chloropicrin. However, chloropicrin yields increased and dichloronitrolysine yields decreased in the absence of hydroxyl radical scavengers, suggesting that future research should characterize the potential occurrence of such halonitroalkane analogues relative to natural radical scavenger (e.g., carbonate) concentrations.
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INTRODUCTION The decreasing availability of pristine waters is forcing certain utilities to consider the use of water supplies impaired by algal blooms or upstream wastewater discharges. These waters often feature elevated concentrations of organic nitrogen1 that may serve as precursors for nitrogenous disinfection byproducts (NDBPs).2 The formation of N-DBPs, including halonitromethanes (e.g., chloropicrin (trichloronitromethane)) and haloacetonitriles (e.g., dichloroacetonitrile (DCAN)), has raised concerns because many of them are far more genotoxic than the carbon-based disinfection byproducts that currently are regulated (e.g., trihalomethanes).3,4 Compared to the formation pathways for haloacetonitriles, far less is known about the formation pathways of halonitromethanes.2 Early research demonstrated that nitromethane forms chloropicrin at quantitative yield during chlorination.5 However, nitro-groups are absent from common biomolecules (e.g., cellulose, lignins, amino acids, nucleic acids, lipids) that are the ultimate source of natural organic matter (NOM). Previous research has focused on reactions involving nitrite and nitrate to explain the formation of nitrated organic intermediates for chloropicrin. Application of chlorine, ozone or chlorine dioxide to non-nitrogenous phenols in the presence of nitrite produces nitrophenols.6 However, significant © XXXX American Chemical Society
chloropicrin formation attributable to nitrite during chlorination of NOM6 or natural waters7 generally requires higher nitrite concentrations (e.g., 2 mg/L) than are typically observed in source waters. More recently, application of UV from a medium pressure mercury lamp at fluence relevant to drinking water disinfection to waters containing nitrate significantly increased chloropicrin formation after chlorination, even at 1 mg-N/L nitrate.8,9 Unlike the 254 nm emission from low pressure mercury lamps, the overlap between the medium pressure mercury lamp emission spectrum and the nitrate absorbance spectrum promotes nitrate photolysis to NO2•,9 and NO2• can nitrate phenols.10 Chloropicrin formation is particularly associated with ozonation followed by chlorination.6,7,11,12 Chlorination of NOM isolates produced low yields of chloropicrin (1 μg/mg DOC) considering the elevated exposures applied (e.g., 5 mg HOCl/mg DOC for 7 days).13,14 However, the ozonationchlorination combination has been noted to increase chloropicrin formation by 3-fold at typical ozone doses (∼1 Received: September 4, 2015 Revised: January 6, 2016 Accepted: January 11, 2016
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DOI: 10.1021/acs.est.5b04282 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Article
Environmental Science & Technology mg O3/mg DOC),12 and up to 10-fold at higher doses.7,15 Halonitromethane formation increased with pH and bromide concentration,7 and yields were highest from the organic nitrogen-rich, hydrophilic fractions featuring low aromaticity, as indicated by low specific UV absorbance at 254 nm (SUVA254).12 Addition of up to 2 mg/L nitrite only modestly promoted chloropicrin formation,7 suggesting that the enhancement by ozone does not involve nitrite oxidation. We hypothesized that the ozone effect was attributable to the efficient oxidation of organic amines to nitrated intermediates. Amines are common constituents of biomolecules, and therefore should be prevalent in NOM. Previous research has identified amides, aldehydes, hydroxylamines, and oximes as ozonation products of primary and secondary amines in water, and N-oxides as products of tertiary amines.16,17 While the formation of nitrated organic intermediates from ozonation of primary amines in water has been proposed,18−20 their formation was not demonstrated. Similarly, the promotion of chloropicrin formation associated with ozonation has been suggested to occur via conversion of amines to nitro compounds, but nitrated intermediates have not been demonstrated.20 Ozonation of the amino acid, serine, in water formed nitrate.18 Ozonation and chlorination of free amino acids generally produced chloropicrin at
