The impact of redox conditions on OH• scavenging in surface water matrix and implications for testing of scavenging
Analytics
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Abstract
Advanced oxidation processes (AOPs) in water treatment utilizes chemical oxidation to react with and promote the degradation of trace contaminants present in wastewater. Hydroxyl radicals (OH•) generated in AOPs are highly reactive and nonselective, capable of targeting these trace contaminants. Background water constituents like bicarbonate, carbonate, and dissolved organic matter (DOC) are termed hydroxyl radical scavengers that will compete with these trace contaminants and decrease the effectiveness of AOPs. In AOP testing for scavenging, DOC is interpreted as a bulk parameter where an average reaction rate constant has been reported and used. This interpretation can provide inaccurate scavenging predictions as there is a wide range of variability in reaction rate constants between organic compounds and OH•. Therefore, measurement of scavenging is a preferred approach, however there is currently no guidance in literature on handling of such samples. Scavenging was quantified for water samples from two surface water sources (Concord and Mount Holly) over the course of two seasons at different hold times and storage conditions using a bench-scale collimated beam UV design with a low-pressure lamp. Samples were stored with headspace space and no headspace after collection and scavenging was analyzed at hold times of 0 HR, 24 HR, 48 HR, 7 HR, 7 days and 14 days. Results reveal that 52% of all measured scavenging significantly (α = 0.05) drifted compared to initial day scavenging indicating that testing should be within 24 HR of hold time. Furthermore, 69% of all headspace samples drifted while 63% of no headspace samples drifted. Concord samples revealed significant differences in storage conditions for all winter samples analyzed (α = 0.05), however no differences in the summer samples. Mount Holly samples revealed significant differences in both winter and summer seasons (α = 0.05). As a result, samples should be stored with no headspace for testing to preserve the sample as much as possible. Additionally, as reactions with OH• are oxidation reactions, the oxidation state of carbon was calculated for over 1,000 organic compounds and correlated with their respective reaction rate constants with OH• to study the relationship between the two variables. Results show that reaction rate constants with OH• between organic compounds with positive versus negative oxidation states of carbon is significantly different with a p-value of 0.0000204 (α = 0.05), thus drawing the conclusion that reaction rates are influenced by the average oxidation state of carbon. Additionally, negative versus zero oxidation states of carbon yielded a p-value of 0.000818 (α = 0.05). The results of the experiments with the surface water samples confirmed that the redox condition of the sample measured as oxidation-reduction potential (ORP) has influence on the reaction rate of dissolved organic matter with hydroxyl radicals, possibly by changing the redox state of the carbon on dissolved organic matter. However, the trends were localized for some sample sets and no universal trend was observed for all water samples. The results also highlight the variability of scavenging rates attributable to dissolved organic matter and emphasize the importance of experimental measurement of scavenging rather than calculation based on water quality parameters.