Theme: 01. Remediation Technology Innovations

Topic Area: 01a. Abiotic and In Situ Biogeochemical Processes: Applications and Lessons Learned

Formatted Title:

Naturally-Occurring Abiotic Dechlorination in Clay

Background/Objectives:

The naturally-occurring abiotic dechlorination of chlorinated solvents in clay matrices that contain ferrous minerals may play an important role in long-term contaminant attenuation at many sites. While recent studies have identified and quantified these reactions in natural clays, verification of tools that can be used to estimate dechlorination rates in clays remains elusive. In addition, the extent to which processes identified at the bench scale actually occur in the field remains largely unconfirmed. Data are thus needed to provide improved support for abiotic dechlorination as a part of a viable natural attenuation remedy.

Approach/Activities:

Bench-scale batch experiments using clay:water slurries were performed to evaluate the abiotic reductive dechlorination of trichloroethene (TCE) under anaerobic conditions, where generation of reduced gases was used to quantify first order rate constants. Natural clays used in this study were obtained from multiple chlorinated solvent impacted sites, and were characterized with respect to mineralogy using both X-ray diffraction (XRD) and multiple acid-based extractions. Batch experiments under aerobic conditions were performed to measure hydroxyl radical generation (facilitated by the presence of ferrous minerals), which in turn facilitate the oxidative dechlorination of TCE. Bench-scale testing evaluating TCE diffusion and reaction in clays beds was used to provide further insight into these reactive processes. Processes were confirmed in situ.

Results/Lessons Learned:

Results to date show that the observed first order dechlorination rate constants measured in the natural clays under anaerobic conditions were positively correlated to the ferrous mineral contents of the clay, as determined via dilute hydrochloric acid extraction. A notable exception to this correlation was observed for clays that contained pyrite or biotite. While biotite was non-reactive with respect to TCE dechlorination, experiments with clay containing pyrite showed substantial abiotic reductive dechlorination despite having a relatively low ferrous mineral content. The low ferrous mineral content was due to the fact that pyrite is not readily extracted via the dilute acid extraction employed in this study. However, when the ferrous mineral content associated with the pyrite was estimated using XRD, the correlation between first order dechlorination rate constant and ferrous mineral content aligned with those observed for the other clays. Other field indicators, including reduced gases (e.g., acetylene), CSIA, reduction potentials, magnetics susceptibility, and total iron content were typically not useful for identifying and quantifying abiotic reductive dechlorination within the clays. Potential for oxidative abiotic dechlorination, as evidenced by the generation of hydroxyl radicals and peroxide, was verified via bench-scale batch testing and via in situ measurement of peroxide at the clay interface.

Preliminary results from bench-scale TCE diffusion/reaction experiments show that abiotic reductive dechlorination reactions are occurring within the anoxic clay, as evidenced by acetylene generation. Concurrently, oxidative abiotic dechlorination reactions are occurring near the aerobic clay interface, as evidenced by generation of short-chained organic acids from the oxidation of TCE. Overall, results suggest that these reactions are occurring in the field and are contributing to TCE attenuation.

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Duncan Shih