Grantee Research Project Results
2000 Progress Report: Kinetic-Based Models for Bromate Formation in Natural Waters
EPA Grant Number: R826835Title: Kinetic-Based Models for Bromate Formation in Natural Waters
Investigators: Westerhoff, Paul
Institution: Arizona State University
EPA Project Officer: Hahn, Intaek
Project Period: September 1, 1998 through August 31, 2000
Project Period Covered by this Report: September 1, 1999 through August 31, 2000
Project Amount: $99,500
RFA: Drinking Water (1998) RFA Text | Recipients Lists
Research Category: Drinking Water , Water
Objective:
Ozone (O3) is an effective disinfectant, but it can form carcinogenic byproducts (e.g., bromate). There is a need to develop tools to understand and predict bromate (BrO3-) formation while still achieving high levels of microbial disinfection. The central hypothesis is that a kinetic-based understanding of natural organic matter (NOM) reactions with hydroxyl (HO) radicals and aqueous bromine (HOBr/OBr-) over a range of temperatures is necessary to develop mechanistic-based models for bromate formation in bulk waters. The tasks for this project include: (1) develop a comprehensive database of bromine and bromate formation during ozonation of Colorado River water; (2) determine hydroxyl radical concentrations present during ozonation; (3) determine rates of reaction between HOBr and OBr- and NOM; and (4) calibrate and verify a BrO3- formation mechanistic-based model that includes NOM, and simulate BrO3- control measures necessary to meet proposed and future maximum contaminant levels (MCLs).Progress Summary:
The study is progressing well. A no-cost project extension was granted in the spring of 2000 so that a companion ozonation study involving bromate formation in the Colorado River could be undertaken and leveraged with this EPA project. Experimental work is approximately 50 percent complete, and model formulations have been developed. This section provides an overview of progress to date for each of the four tasks identified above.
1. Ozonation experiments were conducted in batch reactors. Kinetic experiments were performed in ultrapure water containing bromide, and in Colorado River water spiked with bromide to various levels. Mass balances on bromide in the form of bromide, bromine, and bromate were satisfied in ultrapure water after proper sample preparation (pH adjustment and stripping of residual ozone) prior to DPD analysis. A matrix of experiments with Colorado River water has been completed and includes kinetic measurement of ozone, bromide, bromine, and bromate in over 16 separate experimental conditions: ozone dose (0.5, 1.0, 1.5 mgO3/mg dissolved organic carbon [DOC]), temperature (10 and 22?C), and bromide concentrations (60, 300, 500 µg/L).
NOM has been isolated from Colorado River water by nanofiltration (300 daltons) and lyophilized (CAP-NF). The isolated organic material has been analyzed for structural properties by C13-NMR, UV/Vis absorbance, and fluorescence intensity; it contains approximately 40 percent carbon by weight, and approximately 2.5 grams were isolated. The isolated NOM will be used in mechanistic ozonation studies and studies on the reaction of NOM with bromine (Task 3). Comparative studies of bromate formation and ozone demand in the isolated and bulk Colorado River water have been completed.
2. Parachlorobenzoic acid (PCBA) was added during ozonation experiments as a HO probe, and measured by HPLC. Kinetic data on the disappearance of PCBA during ozonation was then used to compute HO concentrations. HO concentrations were lower in Colorado River water compared against ultrapure water, and varied as a function of pH and ozone dose. HO radicals play a key role in oxidizing bromide to bromate during ozonation. These measurements will provide data to calibrate numerical mechanistic models during later stages of this research.
3. After testing several experimental methods for determination of reaction rates between NOM and halogens (e.g., aqueous bromine), a colorimetric approach (ABTS) was adopted. A large experimental matrix of reaction rate constants have been determined for the reaction of HOBr and OBr- with un-ozonated and ozonated NOM. Although not directly related to bromate formation, companion experiments using HOCl and OCl- were simultaneously undertaken to better understand the reaction mechanisms and provide insight into organo-halogen formation. As an example of representative results, the second order reaction between HOBr and CAP-NF for HOBr was 31 M-1s-1, and only 1.4 M-1s-1 for HOCl. Hence, HOBr reacts faster with NOM than HOCl, but the rate of reaction is quite low given the rates of reaction between important oxidants (ozone and HO radicals) and bromine species during the formation of bromate. Therefore, the presence of NOM is likely to exert a larger effect on the amount of oxidant present than affect the intermediate bromine species during bromate formation.
4. A literature review of model formations has been completed. Based upon that work, it was decided that an oxidant-driven mechanistic model would be developed that accounted for the major pathways of bromate formation (molecular ozone, HO radical, or combination of the two pathways) and key water chemistry reactions for pH, alkalinity, and ammonia. Theoretical relationships for the relative importance of the pathways will be used rather than strictly first or second order rate equations. The current experimental work would suggest this approach to be more robust in predicting bromate formation than current models.
The most significant quality assurance/quality control (QA/QC) issue on this project centers around analysis on bromate, and serves as an example of QA/QC measures undertaken on other parameters. The current MCL for bromate is 10 µg/L, based upon a practical quantification level (PQL). Figure 1 is a standard curve from bromate in nanopure water, and is typical of the results run with each set of bromate samples. The high coefficient of determination signifies reliable calibration over the range of 1 to 25 mg/L. In addition, method detection limits (MDLs) for bromate in nanopure and natural water matrices have been determined. Samples of nanopure and Colorado River water were spiked with 5 µg/L bromate and analyzed nine times, then the MDL was calculated as 3.14 times the standard deviation of the nine samples. The MDL in nanopure water was 1.1 mg/L and 2.2 mg/L in Colorado River water. Due to the high alkalinity and other matrix effects in Colorado River water, this MDL is considered acceptable. In addition to calculating an MDL, duplicate samples of ozonated Colorado River water were sent to an EPA-certified laboratory for bromate analysis. Figure 2 presents results from the cross-laboratory QA/QC study. A slope of near one through the data and a high coefficient of determination indicate excellent cross-laboratory verification of our analysis over a range of bromate concentrations from 1.9 to 93 µg/L.
Future Activities:
Continued research on the project will focus on extending the oxidant (O3, HO) and byproduct (BrO3-) work started under Tasks 1 and 2. Specific experiments testing the effect of pH and ammonia on bromate formation in Colorado River water will be undertaken. Model calibration and predictions will be conducted.Journal Articles:
No journal articles submitted with this report: View all 9 publications for this projectSupplemental Keywords:
drinking water, oxidation, pathogens, human health, carcinogen., RFA, Scientific Discipline, Water, Environmental Chemistry, Health Risk Assessment, Analytical Chemistry, Drinking Water, microbial contamination, natural waters, public water systems, monitoring, predicting chemical concentrations, chemical byproducts, disinfection byproducts (DPBs), kinetics, database development, natural organic matter, analytical methods, bromate formation, brominated DPBs, carcinogenicity, treatment, microbial risk management, hydroxyl radicals, DBP risk management, water quality, drinking water contaminants, drinking water treatment, water treatment, drinking water system, ozonationRelevant Websites:
http://www.public.asu.edu/~westerho/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.