Free-Radical Chemistry of Disinfection Byproducts ... - ACS Publications

Nov 11, 2009 - UniVersity of California at IrVine, IrVine, California 92697; CiVil and ... Kaufman Hall, Old Dominion UniVersity, Norfolk, Virginia 23...
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J. Phys. Chem. A 2010, 114, 117–125

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Free-Radical Chemistry of Disinfection Byproducts. 3. Degradation Mechanisms of Chloronitromethane, Bromonitromethane, and Dichloronitromethane Bruce J. Mincher,*,† Stephen P. Mezyk,*,‡ William J. Cooper,§ S. Kirkham Cole,|,# Robert V. Fox,† and Piero R. Gardinali⊥ Aqueous Separations and Radiochemistry Group, Idaho National Laboratory, PO Box 1625, Idaho Falls, Idaho 83415; Department of Chemistry and Biochemistry, California State UniVersity at Long Beach, Long Beach, California 90840; Urban Water Research Center, Department of CiVil and EnVironmental Engineering, UniVersity of California at IrVine, IrVine, California 92697; CiVil and EnVironmental Engineering Department, Kaufman Hall, Old Dominion UniVersity, Norfolk, Virginia 23529; and Department of Chemistry and Biochemistry, Florida International UniVersity, Miami, Florida 33199 ReceiVed: July 30, 2009; ReVised Manuscript ReceiVed: October 14, 2009

Halonitromethanes (HNMs) are byproducts formed through ozonation and chlorine/ chloramine disinfection processes in drinking waters that contain dissolved organic matter and bromide ions. These species occur at low concentration but have been determined to have high cytotoxicity and mutagenicity and therefore may represent a human health hazard. In this study, we have investigated the chemistry involved in the mineralization of HNMs to nonhazardous inorganic products through the application of advanced oxidation and reduction processes. We have combined measured absolute reaction rate constants for the reactions of chloronitromethane, bromonitromethane, and dichloronitromethane with the hydroxyl radical and the hydrated electron with a kinetic computer model in an attempt to elucidate the reaction pathways of these HNMs. The results are compared to measurements of stable products resulting from steady-state 60Co γ-irradiations of the same compounds. The model predicted the decomposition of the parent compounds and ingrowth of chloride and bromide ions with excellent accuracy, but the prediction of the total nitrate ion concentration was slightly in error, reflecting the complexity of nitrogen oxide species reactions in irradiated solution. Introduction Drinking water is disinfected using chemicals such as chlorine, chloramines, and/or ozone prior to distribution to the public. However, in the presence of natural organic matter, these chemicals react to form halogenated disinfection byproducts (DBPs). Among the earliest DBPs to be identified and regulated were the trihalomethanes (THMs) and haloacetic acids (HAAs).1 On a weight basis, these are the most common DBPs found in surveys of United States drinking waters.2 However, there is now a growing recognition of many more DBPs, including the halonitromethanes3-5 (HNMs, X3-nHnCNO2, where X ) Cl, Br) in drinking water treatment. Trichloronitromethane (chloropicrin, Cl3CNO2) was recognized as a DBP and included in the first Drinking Water Priority List of 1988.1 Along with other DBPs, it was surveyed at 296 utilities by USEPA during 1997-1998.6 Additional HNMs were the object of another USEPA survey of drinking waters7 and it is now recognized that chloropicrin, monochloronitromethane, dichloronitromethane, and other bromochloro and bromo ana* Authors to whom correspondence should be addressed. Phone: (208) 533-7499 (B.J.M.), (562) 985-4649 (S.P.M.). Fax: (208) 533-4207 (B.J.M.), (562) 985-8557 (S.P.M.). E-mail: [email protected] (B.J.H.), E-mail: [email protected] (S.P.M.). † Idaho National Laboratory. ‡ California State University at Long Beach. § Director and Professor, Urban Water Research Center, University of California at Irvine. | Old Dominion University. ⊥ Florida International University. # Present address: NASA Langley Research Center, Ground Facilities & Testing Directorate, Subsonic/Transonic Testing Branch D501, East Reid Street, Building 1247T, Mail Stop 164E, Hampton, VA 23681-2199.

logues are also formed in water treated with chlorine and chloramines, especially when pretreatment includes ozonation.8 In a recent survey of 12 treatment plants in the United States, the total HNM concentration varied from below detection limit to a maximum of 10 µg L-1.2 While the overall concentrations of halonitromethanes are lower than those of THMs and HAAs, recent evidence indicates that they have higher cytotoxicity and genotoxicity than the more common DBPs. For example, chloronitromethane, dichloronitromethane, and chloropicrin are more mutagenic than the corresponding chloromethanes.9 All nine HNM congeners were found to be mutagenic in Salmonella.10 Further, Plewa et al. reported that, while the maximum dichloronitromethane concentration in USEPA-surveyed drinking water occurred at only 1/27th the concentration of chloroacetic acid, its cytotoxicity was over 30 times greater.8 As potent genotoxins and cytotoxins, the presence of HNMs in drinking water poses a hazard to public health. Research into means of treating drinking water to remove them is therefore warranted. The quantitative removal of trace levels of organic chemicals from waters is difficult. One potential approach is the use of advanced oxidation or reduction processes (AO/RPs) that generate radicals, in situ, that directly react with and destroy contaminants. The AO/RP radicals include the oxidizing hydroxyl radical (•OH), and the reducing hydrated electron (eaq-) and hydrogen atom (H•). While many different AO/RP systems exist, the generation of all three of these radicals by electron beam irradiation of water is thought to be capable of the ultimate treatment of DBPs through mineralization to nonhazardous constituents.

10.1021/jp907305g  2010 American Chemical Society Published on Web 11/11/2009

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However, these technologies are expensive to implement, and therefore a full understanding of the chemistry involved is essential for their optimal use. Computer models that incorporate all the radical kinetics and reaction mechanisms offer the best understanding of the AO/RP treatment chemistry.11 Therefore, the objective of this study was to create a general kinetic model for the simplest partially halogenated HNMs through examining the kinetics and mechanisms of the reactions of the radiolytically produced hydrated electron and hydroxyl radical with the demonstrably mutagenic chloronitromethane (CNM) and dichloronitromethane (DCNM) and the bromonitromethane (BNM) analogue. Advantage was taken of the mechanistic insights gained during our previous studies of the radiolytic degradation of chloropicrin.12 Our previously measured rate constants for the reactions of the HNMs with eaq- and •OH radical,13 and the results of current byproduct analyses of 60Co steady-state irradiation experiments, were combined here into a general kinetic computer model to better understand the utility of electron beam irradiation for the treatment of these DBPs in drinking water. Experimental Section The HNMs used were supplied by Helix Biotech and were of the highest purity available. Solutions were prepared with water filtered by a Millipore Milli-Q system (>18.2 MΩ) that was constantly illuminated by a Xe arc lamp. The TOC concentration of this water was 96% through oxidation). At such low HNM concentrations, further hydroxyl radical loss though its reactions with other real-world water constituents could also be significant, but the fraction that actually reacts with CNM can be again derived through a consideration of the relative rates of the reactions involved. With sufficient radical reactions, the complete destruction of this HNM is predicted to occur. Moreover, these AO/RP radicals will also react with the species produced in the HNM destruction, thus also removing any harmful product chemicals. Our model predictions showed that, with the exception of the stable halide and nitrate ions, no other species/intermediate reached a concentration greater than 0.1% of the original HNM concentration at any time. Dichloronitromethane. The radiolysis of the dihalogenated nitromethane dichloronitromethane (DCNM) was also evaluated with our model in this study. The rate constants for the reaction of DCNM with the hydrated electron and the hydroxyl radical are again shown in Table 1.13,17 The measured steady state γ-radiolysis product yields are shown in Figure 3. The mass balance for chloride production is very good with 104% recovery for chloride and a slightly higher value of 61% for nitrate at the maximum absorbed dose of 8.5 kGy. The electron and hydroxyl radical reactions of DCNM are analogous to those for the monohalogenated species and are shown in eqs 15 and 16:

eaq- + CHCl2NO2 f Cl- + •CHClNO2

(15)

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OH + CHCl2NO2 f •CCl2NO2 + H2O

Mincher et al.

(16)

These formed carbon-centered radicals would also add oxygen to form the corresponding peroxyl radicals, in analogy with the reactions in eq 7. The peroxyl radicals would add to form the corresponding tetroxides, which, for the H-containing species would then decay by the Russell and Bennett mechanisms:20

ClH(NO2)C-O4-C(NO2)HCl f NO2C(O)Cl + NO2C(OH)HCl + O2 (17) ClH(NO2)C-O4-C(NO2)HCl f 2[NO2C(O)Cl] + H2O2 (18) The aldehydes and alcohols again hydrolyze as previously discussed. However, for the carbon-centered radical product of eq 16, the tetroxide product cannot decay by these mechanisms due to lack of hydrogen atoms on the R-carbon.17 The only remaining decay pathway is oxygen elimination, as shown in eq 19:

Cl2(NO2)C-O4-C(NO2)Cl2 f 2[Cl2(NO2)CO•] + O2 (19) The resulting alkoxy radical would undergo intramolecular rearrangement as we previously showed for chloropicrin.12 Ultimately, the products of these reactions will hydrolyze, to quantitatively produce chloride, carbon dioxide, and nitrite (nitrate). The hydrogen atom reaction for DCNM was also assumed to occur by reductive dissociation, with a model optimized reaction rate constant of 5.0 × 108 M-1 s-1:

H• + CHCl2NO2 f H+ + Cl- + •CHClNO2

(20)

This rate constant is again consistent with those predicted for CNM and BNM. The kinetic computer model for DCNM was constructed by combining the standard water radiolysis reaction set plus the same inorganic nitrogen-containing set used for the monohalide HNMs of this study with these DCNM reactions.

Figure 3. Measured results (individual data points) for the decrease in concentration of dichloronitromethane (0) and increase in concentration of chloride (O) and nitrate (4) anions versus absorbed dose in steady-state γ-irradiation of DCNM. Error bars are standard deviations as based on the results of triplicate determinations. Solid lines are the kinetic model results.

These predictions are contrasted with the experimental data in Figure 3. It can be seen that the agreement with the measured data is again very good for loss of DCNM and also for the ingrowth of chloride and nitrate ion. Conclusions A kinetic model was constructed by combining measured absolute reaction rate constants for the reactions of chloronitromethane, bromonitromethane, and dichloronitromethane with the hydroxyl radical and the hydrated electron with the measurements of stable products resulting from steady-state 60 Co γ-irradiations. The model predicted the decomposition of the parent compounds and ingrowth of chloride and bromide ions with excellent accuracy, but the prediction of the total nitrate ion concentration was slightly in error, reflecting the complexity of nitrogen oxide species reactions in irradiated solution. Sensitivity analyses of these general kinetic models developed in this study showed that the most important unknown reactions were the H• atom reaction with the primary HNMs, and the chemistry of the nitrogencontaining species that resulted in the production of volatile species. Future work will concentrate on developing a better understanding of the complicated chemistry of nitrogen oxides in aqueous solution. Acknowledgment. Some of this work was performed at the Radiation Laboratory, University of Notre Dame, which is supported by the Office of Basic Energy Sciences, U.S. Department of Energy. Partial financial support was also provided by McKim & Creed, PA. B.J.M. was supported by the US Department of Energy, Office of Nuclear Energy, Science and Technology under DOE Idaho Operations Office contract DE-AC07-99ID13727. This is contribution 44 from the University of California, Irvine, Urban Water Research Center. References and Notes (1) Pontius, F. W. Water Quality and Treatment; McGraw-Hill, Inc.: New York, 1990; Chapter 1, pp 1-61. (2) Krasner, S. W.; Weinberg, H. S.; Richardson, S. D.; Pastor, S. J.; Chinn, R.; Sclimenti, M. J.; Onstad, G. D.; Thruston, A. D., Jr. EnViron. Sci. Technol. 2006, 40, 7175–7185. (3) Garcia-Quispes, W. A.; Carmona, E. R.; Creus, A.; Marcos, R. Chemosphere 2009, 75, 906–909. (4) Chu, W.; Gao, N.; Deng, Y. Xiandai Huagong 2009, 29 (2), 86– 89. (5) Liviac, D.; Creus, A.; Marcos, R. EnViron. Res. 2009, 109 (3), 232– 238. (6) National Primary Drinking Water Regulations: Monitoring Requirements for Public Drinking Water Supplies; Final Rule. Fed. Regist. 1996, 61 (94), 24354. (7) Weinberg, H. S.; Krasner, S. W.; Richardson, S. D.; Thruston A. D., Jr. “The Occurrence of Disinfection By-Products (DBPs) of Health Concern in Drinking Water: Results of a Nation-wide DBP Occurrence Study”, EPA 600/R02/068; U.S. Environmental Protection Agency, National Exposure Research Laboratory: Athens, GA, 2002. (8) Plewa, M. J.; Wagner, E. D.; Jazwierska, P.; Richardson, S. D.; Chen, P. H.; McKague, A. B. EnViron. Sci. Technol. 2004, 38, 62–68. (9) Schneider, M.; Quistad, G. B.; Casida, J. E. Mutat. Res. 1999, 439, 233–238. (10) Kundu, B.; Richardson, S. D.; Swartz, P. D.; Matthews, P. P.; Richard, A. M.; DeMarini, D. M. Mutat. Res. 1995, 562, 39–65. (11) Crittenden, J. C.; Hu, S.; Hand, D. W.; Green, S. A. Water Res. 1999, 33 (10), 2315–2328. (12) Cole, S. K.; Cooper, W. J.; Fox, R. V.; Gardinali, P. R.; Mezyk, S. P.; Mincher, B. J.; O’Shea, K. E. EnViron. Sci. Technol. 2007, 41, 863– 869. (13) Mezyk, S. P.; Helgeson, T.; Cole, S. K.; Cooper, W. J.; Fox, R. V.; Gardinali, P. R.; Mincher, B. J. J. Phys. Chem. A 2006, 110, 2176–2180. (14) Klassen, N. V.; Shortt, K. R.; Seuntjens, J.; Ross, C. K. Phys. Med. Biol. 1999, 44, 1609–1624.

Degradation Mechanisms of Halonitromethanes (15) Buxton, G. V.; Greenstock, C. L.; Helman, W. P.; Ross, A. B. J. Phys. Chem. Ref. Data 1988, 17, 513–886. (16) Carver, M. B.; Hanley, D. V.; Chapin, K. R. “MAKSIMACHEMIST, A program for mass action kinetic simulated manipulation and integration using stiff techniques”; Chalk River Nuclear Laboratories Report; Atomic Energy of Canada, Ltd., 1979; pp 1-28. (17) Cole, S. K.; Cooper, W. J.; Mincher, B. J.; Fox, R. V.; Gardinalli, P. R.; O’Shea, K. E.; Mezyk, S. P. In Photocatalytic and adVanced oxidation processes for treatment of air, water, soil and surfaces; Ollis, D. F., AlEkabi, H., Eds.; Redox Technologies, Inc.: Sicklerville, NJ, 2005; pp 311317.

J. Phys. Chem. A, Vol. 114, No. 1, 2010 125 (18) Hart, E. J.; Anbar, M. The hydrated electron; Wiley-Interscience: New York, 1970. (19) Neta, P.; Huie, R. E.; Ross, A. B. J. Phys. Chem. Ref. Data 1990, 19, 413–513. (20) Von Sonntag, C.; Schuchmann, H.-P. Peroxyl radicals in aqueous solutions. In Peroxyl Radicals; Alfassi, Z. B., Ed.; John Wiley & Sons: New York, 1997; pp 173-234.

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