Grantee Research Project Results
Final Report: Use of Differential Spectroscopy to Probe Reactions between Natural Organic Matter and Chlorinated Oxidants
EPA Grant Number: R826645Title: Use of Differential Spectroscopy to Probe Reactions between Natural Organic Matter and Chlorinated Oxidants
Investigators: Benjamin, Mark M. , Korshin, Gregory V.
Institution: University of Washington
EPA Project Officer: Aja, Hayley
Project Period: August 1, 1998 through July 31, 2001 (Extended to February 28, 2002)
Project Amount: $374,401
RFA: Exploratory Research - Environmental Chemistry (1998) RFA Text | Recipients Lists
Research Category: Sustainable and Healthy Communities , Land and Waste Management , Air , Safer Chemicals
Objective:
The objective of this research project was to gain a better understanding of the kinetics and the reaction pathways for the formation of halogenated disinfection byproducts (DBPs) that form when drinking water is chlorinated. The experiments involved chlorination of both natural organic matter (NOM) (the precursor for DBPs in drinking water systems) and pure, model compounds (resorcinol and hesperetin).
The approaches employed included the use of differential spectroscopy to monitor the progress of the chlorination reaction, analysis of specific, regulated DBPs, and mass spectroscopy to obtain information about the reaction intermediates. The kinetics of the reactions were studied both in conventional, batch experiments with durations from several minutes to several days, and using a stop flow kinetic analyzer, which allowed us to obtain information about spectral changes in the solutions undergoing chlorination at timeframes on the order of milliseconds to seconds.
Summary/Accomplishments (Outputs/Outcomes):
The differential spectra of chlorinated NOM (i.e., the change in the spectra caused by chlorination) can be divided into two significant components, with one being much larger than the other, but important information embedded in both. The primary component is uniformly negative (i.e., it reflects a decline in the absorbance of the NOM) and has a peak intensity near 270 nm. The shape of this band is largely independent of the chlorine dose, and its intensity increases steadily with increasing Cl dose, at least in the range of doses normally used for water disinfection. The second component is positive, with one peak (S1) near 283 nm and another (S2) in the 340-380 nm range. The intensity of this component increases with increasing Cl dose at low Cl/dissolved organic carbon (DOC) ratios, but at higher ratios, the intensity diminishes, and eventually the component disappears from the differential spectra. Comparison of these features with those for chlorination of resorcinol (the simplest model aromatic compound with metapositioned dihydroxy substitution) suggest that the secondary component of the differential spectra of NOM might be attributed to chlorinated intermediates formed prior to the release of products such as haloacetic acids (HAAs) and trihalomethanes (THMs).
Studies of chlorination of hesperetin, a more complex model compound, suggest that the response of this compound to chlorination is quite similar to that of NOM. Virtually all the classes of DBPs that have been identified from chlorination of NOM also are formed when hesperetin is chlorinated. Also, the dependence of DBP speciation on pH and chlorine dose is similar to that observed for aquatic NOM. Although differences exist between the behavior of hesperetin and NOM, the former compound nonetheless exhibits many important features characteristic of NOM when the two are subjected to chlorination. As such, hesperetin appears useful as a surrogate for NOM in studies intended to elucidate the chemistry of NOM halogenation, including the nature and abundance of halogen attack sites and the detailed pathways of their halogenation and breakdown. In all probability, other flavonoids could serve a similar purpose.
Hesperetin molecules have three aromatic rings, and chlorine attack on hesperetin appears to occur exclusively on the dihydroxy-substituted ring, which we designate as ring R1. No evidence was seen for the participation of the methoxy-substituted ring R2 in chlorination reactions for molar ratios up to 20 mole Cl per mole hesperetin. This result is significant, because ring R2 has similarities to the structural elements of lignin, whose importance in NOM chlorination reactions has frequently been suggested.
The incorporation of the first chlorine atom into hesperetin causes the keto-enol equilibrium of ring R1 to shift strongly toward the keto form. Subsequent reactions favor the incorporation of additional Cl atoms at the same reaction site, leading to formation of compounds with two or three Cl atoms bound to the same carbon atom, and the release of corresponding small, di- and trihalogenated DBPs even at relatively low Cl/Hsp ratios. A similar pattern of DBP speciation at low Cl/DOC ratios has been observed in experiments with NOM. The proposed pathway is distinct from the pathway through chlorophenolic intermediates, whose importance is frequently postulated in the current literature on NOM halogenation. While both pathways are expected to be active, the suggestion that the dienone pathway is dominant appears to be novel.
Though the incorporation of chlorine into NOM during chlorination of drinking water occurs over a wide range of time scales (from milliseconds to days), the reactions appear to involve similar functional groups and pathways throughout. The initial reactions appear to involve primarily a chlorine attack on dihydroxy-substituted aromatic rings, leading to incorporation of Cl into the ring and, subsequently, cleavage of the ring to release small, identifiable (and, in many cases, regulated) byproducts. The examination of the primary and secondary features of differential spectra of halogenated NOM, combined with detailed studies of the reactions of appropriate model compounds, can advance our understanding of the nature of chlorine incorporation into aquatic NOM, as well as factors governing the speciation of individual DBPs. Further investigation of the features of differential spectra of halogenated NOM can provide additional information about the nature of the intermediates and the chemical identities of the halogen attack sites in NOM.
While the detailed chemistry of the reactive sites in the NOM and the reaction pathways are of scientific importance, this research project also generated significant information of regulatory and public health importance. In particular, it reinforced the pre-existing data suggesting that differential spectroscopy could be a valuable tool for monitoring the formation of a wide variety of chlorinated DBPs, even at trace levels. As such, it offers a means of surveying DBP concentrations at far more locations, and far more frequently, than is currently the case, without increasing and perhaps even with a decrease in the overall costs.
Journal Articles on this Report : 2 Displayed | Download in RIS Format
Other project views: | All 11 publications | 2 publications in selected types | All 2 journal articles |
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Type | Citation | ||
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Korshin GV, Benjamin MM, Xiao HB. Interactions of chlorine with natural organic matter and formation of intermediates: evidence by differential spectroscopy. Acta Hydrochimica Et Hydrobiologica 2000;28(7):378-384. |
R826645 (Final) |
not available |
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Li CW, Benjamin MM, Korshin GV. Use of UV spectroscopy to characterize the reaction between NOM and free chlorine. Environmental Science & Technology 2000;34(12):2570-2575. |
R826645 (Final) |
not available |
Supplemental Keywords:
disinfection byproduct, DBP, natural organic matter, NOM, kinetic, drinking water, chlorine., RFA, Scientific Discipline, Air, Toxics, Water, National Recommended Water Quality, Environmental Chemistry, Chemistry, Microbiology, Ecological Risk Assessment, Drinking Water, Electron Microscopy, Engineering, Chemistry, & Physics, monitoring, public water systems, chlorinated oxidants, trihalomethane, chlorinated by products, chlorination, mass spectrometry studies, oxidation, haloacetonitriles, disinfection byproducts (DPBs), kinetics, spectroscopic studies, chlorinated DBPs, community water system, UV treatment, natural organic matter, drinking water distribution system, treatment, chemical kinetics, disinfection byproducts (DBP), haloacetic acids, water quality, stochiometry, DBP risk management, drinking water contaminants, drinking water treatment, water treatmentProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.