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Sources of Oxidant Inefficiency in Chemical Oxidation Processes
Citation:
Rusevova, K. AND Scott G. Huling. Sources of Oxidant Inefficiency in Chemical Oxidation Processes. American Geophysical Union Fall Meeting, Washington,DC, December 10 - 14, 2018.
Impact/Purpose:
Chemical oxidation treatment systems are widely used to support many EPA programs that require treatment of drinking water, wastewater, and ground water in the US. These treatment systems often rely on the role of radical intermediates to transform environmental contaminants. Unfortunately, these radical-mediated oxidative treatment systems can be highly inefficient. One source of treatment inefficiency occurs through the mechanism of radical scavenging. The objective of this study was to identify mechanisms of radical scavenging, and to quantify the rate of reaction. Through this research, the mechanism and significance of radical scavenging has been quantified. This information is critical in the identification and development of strategies to minimize radical scavenging and to improve treatment efficiency. Overall, this will reduce the cost of water treatment and is of significant interest to many environmental programs in which EPA has regulatory responsibilities including CERCLA, RCRA, UST, Drinking Water, and Clean Water.
Description:
Fenton and other oxidative systems frequently deployed for contaminant removal in water face two main sources of treatment inefficiency, nonproductive reactions and radical scavenging. Nonproductive reactions involve reactions that deplete the source of oxidant (e.g. hydrogen peroxide, H2O2) which in turn diminishes radical production (species responsible for contaminant removal). Radical scavenging involves the reaction of free radicals formed in the oxidative mechanism with non-target chemical species. Laboratory methods were developed to quantify the role of scavengers in H2O2-based oxidative treatment systems, and involved H2O2 catalysis by either iron or UV light. A non-volatile probe compound was used to limit volatile losses and to simplify the kinetic analysis. Degradation loss of the probe compound was used in conjunction with a new method of kinetic analysis to quantify the reaction rate constants of different radical species. Reaction rate constants were estimated for inorganic species commonly present in groundwater systems. The reaction rate constants for specific scavenging species, derived using both the Fe- and UV-catalyzed testing systems, were in agreement. Results indicated that reaction rate constants are species dependent, and can vary by orders of magnitude. Overall, the methods developed and the results derived from this study were used to identify and quantify the relative role of radical scavengers. Results indicate that these scavengers play a major role in limiting oxidative treatment efficiency. This information can be used to prioritize the elimination of specific scavenger species, and to develop methods to suppress radical scavenging. Through these steps, improvements in oxidative treatment efficiency can be achieved in either Fenton-like H2O2 activation (Fe2+/Fe3+/H2O2), or photolytic activation of H2O2 (H2O2/UV) treatment systems.