Grantee Research Project Results
Final Report: Mechanisms and Kinetics of Chloramine Loss and By-Product Formation in the Presence of Reactive Drinking Water Distribution System Constituents.
EPA Grant Number: R826832Title: Mechanisms and Kinetics of Chloramine Loss and By-Product Formation in the Presence of Reactive Drinking Water Distribution System Constituents.
Investigators: Valentine, Richard L.
Institution: University of Iowa
EPA Project Officer: Hahn, Intaek
Project Period: September 15, 1998 through September 14, 2001
Project Amount: $317,868
RFA: Drinking Water (1998) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
Objective:
The objectives of this research project were to enhance our understanding of the influence of selected reactive substances on the: (1) fate of monochloramine and the nature of inorganic reaction products; (2) kinetics of monochloramine loss; and (3) formation of selected organic disinfection byproducts (DBPs). Results also were used to extend existing mechanistic chloramine reaction models to include the effects of these reactive substances. In this context, the primary purpose of the reaction modeling activities was to provide support for elucidation of the proposed reaction pathways, mechanisms, and kinetics. The fate of chloramines in drinking water distribution systems and the nature of the reactions and byproducts, as well as factors that influence these, are largely unknown. It is hypothesized that several ubiquitous reactive distribution system constituents, especially natural organic matter (NOM), are important determinants of disinfectant stability and the occurrence of DBP.
The approach consisted of conducting batch laboratory experiments using collected and laboratory-prepared waters containing various amendments. NOM was obtained from several sources and included reverse osmosis (RO) concentrates and lyophilized isolates to allow experimentation with variable concentrations and characteristics. Studies focused on: (1) the mechanisms and kinetics of chloramine loss and formation of haloacetic acid (HAA) disinfection byproducts in the presence of NOM; (2) evaluating the influence of bromide on monochloramine stability and HAA formation in the presence of NOM; (3) verifying proposed reaction mechanisms and kinetics describing loss of monochloramine and formation of nitrate in the presence of nitrite; (4) examining relationships between NOM reactivity and NOM characteristics, including a comparison of free chlorine and monochloramine loss kinetics; and (5) investigating the formation of nitrosodimethylamine via a reaction involving monochloramine and a model organic nitrogen precursor.
Reaction kinetic models were extensively used to facilitate resolution of multiple reaction pathways into their components. For example, reactivity of monochloramine with NOM was resolved by subtracting the contribution to overall loss attributed to its autodecomposition as predicted from known reaction pathways using a reaction model incorporating monochloramine autodecomposition.
Summary/Accomplishments (Outputs/Outcomes):
Reactions of Monochloramine With NOM
When monochloramine reacts with NOM, the reaction is biphasic in nature, characterized by a fast and a slow second order reaction with monochloramine, and is attributed to two different types of reaction sites (slow and fast). The fast reaction is not dependent on pH or ammonia concentration. Typical site fractions were approximately 0.01 to 0.02 for the fast reactive sites and 0.4 to 0.7 for the slow reactive sites. The slow reaction is pH dependent and hypothesized to be primarily attributable to a reaction of hypochlorous acid (HOCl) existing in equilibrium with monochloramine. This assumption could account for both the pH and ammonia concentration dependency of the slow reaction. A smaller contribution by a direct reaction of monochloramine with NOM also is indicated, although this appeared to be mostly important with aldrich humic acid (AHA) and not naturally derived NOM.
Experimental data were used to develop a reaction model as an aid in elucidating the hypothesized reaction pathways. Figure 1 shows an example of the generally good agreement between predicted and measured monochloramine concentrations as a function of total dissolved carbon isolated from the Iowa River.
Figure 1. Monochloramine Decay in the Presence of Varying Concentrations of DOC (IRWC) at pH 9.0. [NH2Cl]o = 0.05 mM, [TOTCO3] = 4.0 mM, µ = 6.0 mM, Temperature=25°C, and Cl/N = 0.7. Lines represent model results.
A similar two-site model also described loss of free chlorine in the presence of NOM. The rate constants were several orders of magnitude larger than those for monochloramine. However, when the monochloramine reaction mechanism was reexpressed in terms of reaction, with the extremely low concentrations of HOCl in the presence of excess ammonia, the rate constants for both free chlorine and monochloramine loss were comparable. This supports the conjecture that free chlorine plays an important role in the reaction of monochloramine with NOM.
Reactions of Monochloramine With Nitrite and Bromide
Mechanisms and kinetics of the oxidation of nitrite and bromide are important because they can result in both chloramine loss and influence DBP formation, depending on the presence of appropriate organic material.
The stability of nitrite in the presence of monochloramine was successfully modeled by incorporating the mechanism proposed by Margerum, et al. (Environmental Science and Technology 1994;28(2):331-337), the auto decomposition model:
NH2Cl + H+ + NO2- NH3 + NO2Cl
HOCl + NO2- NO2Cl + OH-
NO2Cl + NO2- N2O4 + Cl-
N2O4 + OH- NO3- + NO2- + H+
Experimental results verify that the reaction can be slow so that both monochloramine and nitrite may coexist for significant time periods depending on water quality characteristics (see Figure 2).
Figure 2. Effect of Nitrite on Monochloramine Decay and Nitrate Formation. No NOM present. Solid and dashed lines represents model results.
Bromide is slowly oxidized by monochloramine to form bromamines. These may rapidly react via auto decomposition to produce nitrogen gas or react with NOM according to the net reactions:
Bromide catalyzed autodecomposition:
NOM oxidation: + products
Both pathways result in regeneration of bromide but involve reactions having two different net stoichiometries with respect to monochloramine loss. A reaction model was developed that incorporated the several elementary reactions that actually are involved in these reactions. A comparison of model and experimental results indicates that at typical NOM concentrations, most of the oxidized bromide and resultant brominated oxidants react with NOM. However, the presence of NOM can actually slow down the decay of monochloramine in solutions containing bromide compared to solutions without NOM, where bromide can only act to catalyze autodecomposition. This supports the conclusion that the influence of bromide is to enhance oxidative, as well as substitution reactions involving NOM, some of which result in the formation of DBPs.
Formation of HAAs by Reaction of Monochloramine With NOM
HAA formation occurs as a consequence of monochloramine reacting with NOM. The approach taken to modeling HAA formation was to assume that the amount formed directly was proportional to the amount of monochloramine that reacted with NOM. Therefore, the formation is described by a simple formation coefficient (qHAA). This value was determined by measuring HAA formed as a function of the amount of monochloramine reacting with NOM. This factor was incorporated into the overall monochloramine decay model via the following rate expression (see Figure 3).
The presence of bromide shifted the HAA distribution toward formation of brominated species and fewer chlorinated ones. Its presence also increased the total HAA concentration. The amount formed correlated with the increase in NOM oxidation caused by bromide. This supports the hypothesis that brominated oxidants, even if produced by relatively small bromide concentrations at slow rates, can have a significant impact on HAA formation by monochloramine.
Figure 3. Formation of DCAA by Reaction of Monochloramine With 3.3 mg/L DOC. [NH2Cl]o = 0.05 mM. pH 8.33, line equals model.
Correlations Between NOM Characteristics and Reactivity
The slow reactive site fraction correlates quite well to SUVA280 for a variety of NOM sources, with the exclusion of AHA from the analysis. AHA has an extremely high specific UV absorbance uncharacteristic of aquatic NOM. A similar result was found for the reaction rate constant characterizing the slow reaction of free chlorine with NOM. The fast reaction rate did not correlate with SUVA. We were unable to determine this because spectral changes were so small.
N-Nitrosodimethylamine (NDMA) Formation Studies
NDMA is a drinking water contaminant of emerging concern at the nanogram-per-liter level. It generally is believed that it is formed via a reaction of nitrite and an organic nitrogen-containing precursor (such as dimethylamine). Our studies indicate that it also is formed by a direct reaction between monochloramine and dimethylamine. This mechanism involves the intermediate formation of unsymmetrical dimethylhydrazine (UDMH), which is oxidized to NDMA among other products. Therefore, NDMA should be considered a "new" DBP. Possibly other nitroso compounds may be formed by a similar mechanism depending on the nature of the organic precursors.
Conclusions:
The loss of monochloramine in waters containing NOM is attributed to both its autodecomposition and to its reaction with NOM. This reaction primarily results in the oxidation of NOM to unknown products. Formation of some DBPs such as HAAs, although a comparatively minor reaction product, is nonetheless significant. NOM reactivity is characterized by a biphasic reaction that is consistent with the existence of two types of reaction sites. Reaction with the slow sites accounts for the majority of monochloramine loss. This dominant NOM reaction pathway likely is to involve reaction of NOM with HOCl existing in extremely small concentrations in the presence of excess ammonia. HAA formation can be modeled using a simple proportionation reaction approach. Furthermore, reactivity of the "slow" reactive NOM sites correlates with SUVA280. The presence of bromide in chloraminated water enhances the oxidation of NOM and the formation of HAAs, and greatly increases their speciation to brominated species. Nitrite oxidation is consistent with proposed mechanisms, showing that both chloramine and nitrite can coexist for relatively long time periods. NDMA is formed by a reaction of monochloramine, with dimethylamine, indicating that NDMA and probably nitroso compounds generally are a new class of DBPs. Lastly, a number of reaction kinetic models were developed that should be of use in elucidation of additional reactions, and to provide a rational basis for improved water quality models.
Journal Articles on this Report : 8 Displayed | Download in RIS Format
Other project views: | All 15 publications | 8 publications in selected types | All 8 journal articles |
---|
Type | Citation | ||
---|---|---|---|
|
Choi J, Duirk SE, Valentine RL. Mechanistic studies of N-nitrosodimethylamine (NDMA) formation in chlorinated drinking water. Journal of Environmental Monitoring 2002;4(2):249-252. |
R826832 (Final) |
Exit Exit |
|
Choi J, Valentine RL. Formation of N-nitrosodimethylamine (NDMA) from reaction of monochloramine: a new disinfection by-product. Water Research 2002;36(4):817-824. |
R826832 (Final) |
Exit Exit Exit |
|
Choi J, Valentine RL. A kinetic model of N-nitrosodimethylamine (NDMA) formation during water chlorination/chloramination. Water Science & Technology 2002;46(3):65-71. |
R826832 (Final) |
Exit |
|
Duirk SE, Gombert B, Choi J, Valentine RL. Monochloramine loss in the presence of humic acid. Journal of Environmental Monitoring 2002;4(1):85-89. |
R826832 (Final) |
Exit |
|
Duirk SE, Gombert B, Croue J-P, Valentine RL. Modeling monochloramine loss in the presence of natural organic matter. Water Research 2005;39(14):3418-3431. |
R826832 (Final) |
Exit Exit Exit |
|
Duirk SE, Valentine RL. Modeling dichloroacetic acid formation from the reaction of monochloramine with natural organic matter. Water Research 2006;40(14):2667-2674. |
R826832 (Final) |
Exit Exit |
|
Duirk SE. Erratum to "Modeling monochloramine loss in the presence of natural organic matter":[Water Research 39(14), 3418-3431]. Water Research 2006;40(4):851-852. |
R826832 (Final) |
Exit Exit |
|
Vikesland PJ, Ozekin K, Valentine RL. Monochloramine decay in model and distribution system waters. Water Research 2001;35(7):1766-1776. |
R826832 (2000) R826832 (Final) |
Exit Exit Exit |
Supplemental Keywords:
drinking water, disinfection, disinfection byproducts, chloramination, haloacetic acids, nitrosoamines., RFA, Scientific Discipline, Water, Chemical Engineering, Environmental Chemistry, Chemistry, Drinking Water, microbial contamination, public water systems, oxidation, disinfection byproducts (DPBs), community water system, kinetics of Chloramine loss, treatment, bromate formation, brominated DPBs, manganese, drinking water distribution system, iron, microbial risk management, chloramines, emerging pathogens, DBP risk management, water quality, drinking water contaminantsRelevant Websites:
The University of Iowa, College of Engineering, Civil and Environmental Engineering: Richard L. Valentine Exit
Progress 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.