Grantee Research Project Results

1999 Progress Report: Kinetic-Based Models for Bromate Formation in Natural Waters

EPA Grant Number: R826835
Title: 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, 1998 through August 31, 1999
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 by-products (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 project objectives are to:

• Task 1 - Develop a comprehensive database of bromine and bromate formation during ozonation of Colorado River water.

• Task 2 - Determine hydroxyl radical concentrations present during ozonation.

• Task 3 - Determine rates of reaction between HOBr and OBr- and NOM.

• Task 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 was initiated and work is progressing. This section provides an overview of progress to date for each of the four tasks identified above. The schedule of work resulted in initiation of Task 3 related activities first. Work on Task 1 and Task 2 was initiated about three-quarters of the way through the first year of this project.

Task 1. Experimental and analytical methods were developed and subjected to quality control and quality assurance checks. Bromide and bromate were measured by ion chromatography in ultrapure and Colorado River (high ionic strength) waters. Bromine was measured by direct spectroscopic analysis at 330 nm (OBr-) and 261 nm (HOBr) in the absence of NOM, and by DPD colorimetric analysis in the presence of NOM. 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/mgDOC), 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. 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.

Task 2. Methods for assessing hydroxyl radical (HO) concentrations during ozonation were developed. Parachlorobenzoic acid (PCBA) was added to all 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.

Task 3. Several strategies were attempted to evaluate the direct reaction rate between NOM and bromine. The first set of approaches involved conducting reactions within a spectrophotometer quartz cell wherein liquid bromine was added to a sample containing a pure model compound. Experiments between pH 5 and 11 were unsuccessful at providing precise kinetics of bromine disappearance in the presence of an excess amount of model compound. This was attempted to facilitate first-order kinetics with respect to bromine. However, it was unsuccessful because the model organic compounds and NOM overlapped with the UV/Vis spectra for the bromine. After many attempts to reconfigure the experimental system, efforts were shifted to work with a dynamic tubular reactor. Using a syringe pump, two reactants (bromine and organics) are rapidly dispensed from separate syringes, and allowed to react in a section of Teflon tubing before being quenched. Using back-titration, bromine concentrations were calculated. Varying the length of Teflon tube or the speed of the pumps facilitated study of kinetics on the order of 1 to 100 seconds. Using the dynamic tubular reactor system, rates of reaction between NOM and bromine are currently being measured. Preliminary results suggest that the reaction rate is on the order of 1 s-1 for a soil humic acid and slower for aquatic fulvic acids. These rates of reaction suggest that bromine reactions with NOM are important during ozonation and bromate formation.

Task 4. Limited modeling work has been initiated and will be in a future phase of the project.

Future Activities:

Continued research on the project will focus on extending the oxidant (O3, HO) and by-product (BrO3-) work started under Tasks 1 and 2. Bromine reaction rates with the nanofiltration isolate from the Colorado River water will be defined. Mechanistic and predictive modeling also will be undertaken.

Journal Articles:

No journal articles submitted with this report: View all 9 publications for this project

Supplemental Keywords:

drinking water, oxidation, pathogens, human health, carcinogen., RFA, Scientific Discipline, Water, Health Risk Assessment, Environmental Chemistry, Analytical Chemistry, Drinking Water, microbial contamination, carcinogenicity, ozonation, removal of natural organic matter, database development, treatment, water quality, natural organic matter, natural waters, microbial risk management, analytical methods, brominated DPBs, kinetics, monitoring, drinking water system, drinking water contaminants, drinking water treatment, water treatment, bromate formation, chemical byproducts, hydroxyl radicals, predicting chemical concentrations, disinfection byproducts (DPBs), public water systems

Relevant Websites:

http://www.eas.asu.edu/~civil/

Progress and Final Reports:

Original Abstract

Final Report