Environ. Sci. Technol. 2007, 41, 7039-7046
Enhanced Nitrogenous Disinfection ByProduct Formation near the Breakpoint: Implications for Nitrification Control I. MARIE SCHREIBER AND WILLIAM A. MITCH* Department of Chemical Engineering, Yale University, Mason Lab 313b, 9 Hillhouse Avenue, New Haven, Connecticut 06520
Increasing the chlorine to ammonia molar ratio and breakpoint chlorination are two control strategies practiced by drinking water treatment utilities experiencing nitrification during chloramination. The first strategy will increase dichloramine formation, which increases nitrosamine formation. Moreover, our results indicate that dichloramine is also an important factor for nitrile formation. Near the breakpoint, nitrosamine formation is over an order of magnitude higher than that observed during chloramination. We propose that there are two nitrosamine formation pathways active in the breakpoint chlorination region: (i) a relatively slow reaction of dichloramine with amine precursors in the presence of dissolved oxygen and (ii) a fast reaction involving reactive breakpoint chlorination intermediates. Lastly, in the presence of nitrite, if breakpoint chlorination is conducted to achieve a significant free chlorine residual, nitrosamines and nitramines will form through a reaction with nitrite and hypochlorite. However, nitrosamine formation will be much lower than when breakpoint chlorination is conducted with no significant free chlorine residual.
Introduction Unintentional nitrification in distribution systems occurs in approximately two-thirds of all utilities practicing chloramination in the United States (1). Increased levels of nitrification have been linked to distribution systems featuring long detention times and to storage tanks exhibiting poor turnover rates (2). Excess ammonia present during chloramination mediates the growth of ammonia-oxidizing bacteria (AOB) that release nitrite (i.e., nitrification) and form unwanted biofilms in distribution systems. Two major strategies are used to control nitrification (2). First, utilities can increase the chlorine to ammonia molar ratio (Cl2:NH3) in treatment plant effluents to reduce available ammonia in distribution systems. Second, utilities can practice breakpoint chlorination at locations in distribution systems featuring nitrifying biofilms (e.g., in storage tanks) to leave a free chlorine residual that hinders biofilm growth, with chloramines often reapplied downstream. However, the former strategy will increase the formation of dichloramine (NHCl2), whereas the latter will form a series of potentially reactive, but poorly characterized, breakpoint chlorination * Corresponding author phone: (203) 432-4386; fax (203) 4324387; e-mail:
[email protected]. 10.1021/es070500t CCC: $37.00 Published on Web 09/12/2007
2007 American Chemical Society
intermediates (3). The impact of both strategies on the formation of two toxic (4) nitrogenous disinfection byproduct (N-DBP) families, nitriles (e.g., haloacetonitriles, cyanogen chloride) and nitrosamines (e.g., N-nitrosodimethylamine (NDMA)), has not been examined. Maximum cyanogen chloride formation during chloramination was demonstrated to occur at a Cl2:NH3 molar ratio of ∼1.5 when chlorinating river water (5), wastewater (6), and deionized water containing model precursors (7, 8) in the presence of ammonia. Previous research (5) attributed the enhanced nitrile formation to increased nitrile stability because cyanogen chloride is stable in the presence of monochloramine (NH2Cl) (9). However, two factors prompted us to re-examine nitrile formation over a range of Cl2:NH3 molar ratios. First, at a Cl2:NH3 molar ratio of 1.5, NH2Cl and NHCl2 are present in approximately equivalent molar concentrations at pH 7 (3). Second, a Cl2:NH3 molar ratio of 1.5 corresponds to the onset of breakpoint chlorination. Previously, we examined NDMA formation during chloramination of a model precursor, dimethylamine (DMA), over a range of Cl2:NH3 molar ratios (10) and noted two important regimes. First, maximum nitrosamine formation was observed at the breakpoint (Cl2:NH3 ∼ 1.7). Second, at Cl2:NH3 < 1, when chloramines were formed in situ and the order of reagent (i.e., DMA, hypochlorite, and ammonia) addition was varied, NDMA formation was greatest when chlorine was added after ammonia. This enhanced formation was attributed to partial NHCl2 formation. Although NH2Cl formation is anticipated at Cl2:NH3 < 1, the reaction between chlorine and ammonia is so rapid (3) that NHCl2 formation is possible on the time scale of reagent mixing. This phenomenon is enhanced when chlorine is added after ammonia because the Cl2:NH3 molar ratio may exceed 1 at the point of chlorine addition prior to complete mixing, promoting localized NHCl2 formation. Although previous research indicated that NDMA formation during chloramination occurs by a reaction between NH2Cl and DMA via an unsymmetrical dimethylhydrazine (UDMH) intermediate (11, 12), we recently demonstrated that this pathway could not explain NDMA formation, because UDMH was not a key intermediate (13). We modeled nitrosamine formation (Scheme 1) (13) as a nucleophilic substitution reaction between NHCl2 and unprotonated secondary amines (e.g., DMA) to form chlorinated unsymmetrical dialkylhydrazine intermediates (e.g., chlorinated UDMH). The intermediate is then oxidized by dissolved oxygen to form the corresponding nitrosamine (e.g., NDMA) or by chloramines to form currently unidentified byproducts.
SCHEME 1
Wastewater and drinking water treatment plants typically disinfect at Cl2:NH3 molar ratios 1 on nitrosamine and nitrile formation from the model precursor DMA using NDMA as a model nitrosamine and dimethylcyanamide as a model nitrile. Dimethylcyanamide was previously shown to form under similar chloramination conditions as NDMA from DMA (11).
Materials and Methods Materials. Acros dimethylcyanamide (>97%), tert-butyl alcohol (99.5%), and uric acid (99%), Calbiochem trolox (>97%), and Sigma-Aldrich humic acid reagents were used without further purification. Some solutions were purged with ultrahigh-purity N2 (99.999%). Cambridge Isotope Laboratories deuterated d6-NDMA (98%) was used as an internal standard for NDMA analysis. Ambersorb 572 resin beads (Sigma-Aldrich) were used for NDMA extractions. Dimethylnitramine was prepared by oxidation of NDMA with trifluoroperacetic acid (17); the purity was determined to be >94% with 1H NMR. Chlorinated DMA was prepared by 7040
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addition of sodium hypochlorite to DMA in equimolar concentrations. Cyanogen chloride was prepared by addition of sodium hypochlorite to sodium cyanide in a 1:20 molar ratio. Other reagents were described previously (18). Chloramine Formation and Analysis. Stock hypochlorite (OCl-) solutions were prepared daily by adding sodium hypochlorite to deionized water. Concentrations were quantified at λmax ) 292 as described previously (10). When chloramines are formed at Cl2:NH3 molar ratios
