Formatted Title
Pathways and Kinetics of Dechlorination in Abiotic Natural Attenuation
Background/Objectives
Significant natural attenuation by abiotic processes (ANA) requires materials (mineral, organic matter, etc.) with sufficient reactivity toward contaminants (especially chlorinated solvents like TCE). Much of the recent work assessing the role of ANA on the fate and remediation of chlorinated solvents in groundwater has focused on the identification and characterization of the reactive materials. Some of this work has emphasized the relatively stable and abundant iron minerals such as magnetite. Our recent work has emphasized the relatively more reactive but less abundant authigenic materials we have called “reactive mineral intermediates” (RMI)s. However, a major part of characterizing both types of materials involves measuring their reactivity with one or more contaminants (or related model/probe compounds). In parallel with our recent work on RMIs, we have been working to clarify the pathways and kinetics of dechlorination by abiotic natural attenuation.
Approach/Activities
To achieve this goal, two approaches are being taken. To provide more and better experimental data on the reactivity of contaminants with putative reductants involved in ANA, we have been performing batch experiments with several contaminants other than TCE—mainly cis-1,2-dichloroethene (cDCE)—with a wide variety of minerals, over a range of solution chemical conditions that control the formation of RMIs. In general, the results show that significant reaction (based on appearance of chloride) is only observed with the more highly chlorinated contaminants and the most strongly reducing conditions. To put these results into a broader perspective, we have compiled a very large data set of previously published data on the rates of contaminant reduction by potential abiotic reductants and subjected these data to extensive analysis for consistencies, inconsistencies, trends, data gaps, etc. The results of this analysis highlight the binary character of the available data: with almost all of the reported rate data corresponding to a small number of relatively reactive contaminants and materials (e.g., TCE vs. FeS) and little if any data on the other combinations. Some of this can be explained with models to predict very slow rates from more readily measured faster rates, but some of the data gaps identified may be resolvable by direct experimentation. A related issue is that most data are from studies of parent compound disappearance only, so the fate of any intermediates and products is unclear. We have recently developed a uniquely comprehensive kinetic model for the pathways and kinetics involved in reduction of chlorinated ethenes, which should create opportunities for improved assessment of ANA of these contaminants.
Results/Lessons Learned
The role of reducing materials and the contaminants they reduce are intrinsically coupled, so consideration of both is required to advance assessments of ANA. This can be challenging, but it also provides opportunities when insight obtained with one contaminant or reduction material can be applied to other contaminants or reducing materials where data are lacking.