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Impact of UV Disinfection Combined with Chlorination/ Chloramination on the Formation of Halonitromethanes and Haloacetonitriles in Drinking Water Amisha D. Shah,† Aaron D. Dotson,‡ Karl G. Linden,‡ and William A. Mitch†,* †
Department of Chemical and Environmental Engineering, Yale University, Mason Lab 313b, 9 Hillhouse Ave., New Haven, Connecticut 06520, United States ‡ Department of Civil Architectural and Environmental Engineering, University of Colorado at Boulder, Boulder, Colorado 80309, United States
bS Supporting Information ABSTRACT: The application of UV disinfection in water treatment is increasing due to both its effectiveness against protozoan pathogens, and the perception that its lack of chemical inputs would minimize disinfection byproduct formation. However, previous research has indicated that treatment of nitrate-containing drinking waters with polychromatic medium pressure (MP), but not monochromatic (254 nm) low pressure (LP), UV lamps followed by chlorination could promote chloropicrin formation. To better understand this phenomenon, conditions promoting the formation of the full suite of chlorinated halonitromethanes and haloacetonitriles were studied. MP UV/postchlorination of authentic filter effluent waters increased chloropicrin formation up to an order of magnitude above the 0.19 μg/L median level in the U.S. EPA’s Information Collection Rule database, even at disinfection-level fluences ( dichloronitromethane > chloronitromethane (Figure 2A). This trend was observed in similar chlorination studies of nitromethane13 or monomethylamine;14 the rate of chlorine addition to nitromethane increases with chlorine substitution as electronwithdrawing chlorine substituents promote deprotonation to nitronate anions (e.g., CH2NO2) susceptible to chlorine addition. At low fluences, dichloronitromethane and chloropicrin formation were about 100% higher during postchlorination than postchloramination but were comparable at the highest fluence. During postchlorination, chloropicrin concentrations increased to 30 nM (4.9 μg/L) with 280 mJ/cm2, a range relevant to disinfection, and to 55 nM (9 μg/L) with 1500 mJ/cm2, a range relevant to advanced oxidation treatment. Nitrite formed linearly with fluence, reaching 70 μM at 1500 mJ/cm2, a ∼10% yield from nitrate (SI Figure SI-2). Nitrite formation likely contributed to oxidant scavenging; chloramine residuals decreased with increasing fluence, while no residual was measurable following postchlorination for any fluence (see SI for modeling of these reactions). For 010 mg/L-N nitrate at 280 mJ/cm2 MP UV, most of the increase in chloropicrin occurred at low nitrate concentrations (Figure 2B). After postchlorination, chloropicrin concentrations increased to 37 nM (6.1 μg/L) at 3 mg/L-N nitrate, leveling off at 55 nM (9 μg/L) at 10 mg/L-N nitrate (Figure 2B). Similar trends were observed for postchloramination, but chloropicrin concentrations were lower. Halonitromethane formation was rapid; with 10 mg/L-N nitrate at 280 mJ/cm2 MP UV they formed within 1 day (SI Figure SI-4). Chloropicrin concentrations remained constant during postchloramination, but declined over 72 h during postchlorination. In our confirmatory experiments in authentic waters, MP UV fluences significantly enhanced halonitromethane formation at relatively low nitrate concentrations. For Utility A water, chloropicrin was the only halonitromethane observed. Treatment with 01500 mJ/cm2 MP UV and postchlorination for 24 h formed 1.82.4 nM (0.30.4 μg/L) chloropicrin (Figure 2C). With 10 mg/L-N nitrate, Utility A water formed 8.2 nM (1.3 μg/L) chloropicrin at 186 mJ/cm2 and up to 17 nM (2.8 μg/L) at 1300 mJ/cm2 MP UV. The 1.3 μg/L chloropicrin observed at 186 mJ/cm2 far exceeds the 0.19 μg/L median level in the U.S. EPA’s Information Collection Rule (ICR) database. At 280 mJ/cm2 fluence, chloropicrin increased to 11 nM (1.8 μg/L) at 1.0 mg/L-N nitrate, leveling off at 15 nM (2.5 μg/L) at 10 mg/L-N nitrate (Figure 2B). For Cincinnati water, with 1.1 mg/L-N nitrate, dichloronitromethane was the only halonitromethane observed. Dichloronitromethane formed at 7.5 nM (1.0 μg/L) at 280 mJ/cm2 and up to 16 nM (2.1 μg/L) at 1500 mJ/cm2 during postchlorination (Figure 2C). With the native 1.1 mg/L-N nitrate at 280 mJ/cm2, dichloronitromethane formation was nearly maximized, as additional nitrate to 10 mg/L-N only increased dichloronitromethane to 9 nM (1.1 μg/L) (Figure 2B). From this utility, Reckhow et al.7 had observed up to 1.8 μg/L chloropicrin
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following MP UV treatment/postchlorination, although dichloronitromethane was not measured. Dichloronitromethane exerts higher cytotoxicity than chloropicrin in mammalian cell assays.11 For both waters, halonitromethane concentrations were about 2.5 times lower for postchloramination (Figure 2C). Reactive nitrogen species resulting from MP UV treatment of nitrate-containing waters are likely responsible for the enhanced halonitromethane formation. For both humic acid and Utility A waters, halonitromethane formation was only observed with nitrate present, reflecting the broad overlap between nitrate absorption and MP UV emission spectra (Figure 1); MP UV promoted halonitromethane formation after postchlorination/ chloramination, unlike LP UV treatment. MP UV treatment could promote three reactive nitrogen species. First, nitrate photolysis generates NO2• at λ < 250 nm and 280 < λ < 320 nm15 (eq 1); quantum yields for this reaction increase with decreasing wavelength from 0.009 at 305 nm to 0.21 at 205 nm.16 NO2• may directly nitrate phenolic constituents,8 or dimerize to nitrogen tetraoxide (N2O4; eq 3), a nitrating agent.10 Second, at λ < 250 nm, photoisomerization of nitrate generates peroxynitrite (ONOO; eq 415,16), the conjugate base of peroxynitrous acid (ONOOH; pKa 6.8), a nitrating agent;17 quantum yields for ONOO formation increase with decreasing wavelength from 0.02 at 270 nm to 0.28 at 205 nm.16 Reaction of ONOO with CO2 forms ONOOCO2, a nitrating agent.17 Third, nitrate photolysis at λ ∼ 305 nm can form nitrite (eq 5), although at low quantum yield (Φ = 0.00118). Upon postchlorination/chloramination, nitrite can react with free chlorine (reaction 6; 19) or monochloramine20 to form the nitrating species ClNO2,21 or N2O4 after reaction with NO2 (eq 7).19 NO3 þ hv f NO2 • þ O•
ð1Þ
O• þ Hþ f HO•
ð2Þ
2NO2 • f N2 O4
ð3Þ
NO3 þ hv f ONOO
ð4Þ
NO3 þ hv f NO2 þ O•
ð5Þ
NO2 þ HOCl f ClNO2 þ OH
ð6Þ
ClNO2 þ NO2 f N2 O4 þ Cl
ð7Þ
Interrelationships among these species render the identification of the most important nitrating species difficult. For example, ONOOH undergoes homolysis to NO2• (eq 8; ref 22), while ONOO decomposes at high pH to nitrite.16 However, the importance of nitrite formation and subsequent generation of ClNO2 upon postchlorination was evaluated by chlorinating (72 h with 7 mg/L-Cl2) a 4.0 mg/L-C humic acid solution with 10 mg/L-N nitrate after irradiation with 76 mJ/cm2 MP UV at pH 7.2. Directly after irradiation, 2 μM nitrite was measured, while 24 nM chloropicrin formed 72 h after postchlorination. When a solution containing 4.0 mg/L-C humic acid solution and 2 μM nitrite at pH 7.2 was treated only with 7 mg/LCl2 free chlorine for 72 h, ∼1 nM chloropicrin formed, indicating that the ClNO2 pathway was unimportant. The importance of ONOOH/ONOO was evaluated by adjusting 4.0 mg/L-C humic acid solutions to pH 5.69.6 with 5 mM phosphate or borate buffers and purging with either N2 or 3660
dx.doi.org/10.1021/es104240v |Environ. Sci. Technol. 2011, 45, 3657–3664
Environmental Science & Technology
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CO2. The solutions were transferred to 25 mL headspace-free vials and spiked with 72 μM ONOO. ONOOH rapidly isomerizes to NO3 (k = 0.9 s1) or undergoes homolysis to NO2• (eq 8) (k = 0.35 s1).18 To eliminate the influence of chlorine speciation during postchlorination, the pH was adjusted to 7.0 after 20 h, and then the samples were dosed with 7.0 mg/LCl2 free chlorine for 72 h. Chloropicrin formation increased by 900% from ∼4 nM to 30 nM when the pH increased from 6.0 to 9.0 in N2-purged samples while concentrations remained at
