The Relative Importance of Abiotic and Biotic Transformation of Carbon Tetrachloride in Anaerobic Soils and Sediments

Abstract

The relative contributions of abiotic and microbial processes and the role of dissolved species in the reductive dechlorination of carbon tetrachloride (CT) by natural soils and sediments were investigated. Microcosms were constructed using soils or sediments and site water from three locations, and then amended with electron acceptors and/or donors to stimulate the growth of iron- and sulfate-reducing bacteria and to promote the formation of minerals that can react with CT. Before spiking with CT, half the replicate microcosms were sterilized in order to measure the rates of abiotic CT transformation without any direct contribution from microbial dechlorination. Abiotic reaction rates were significantly greater than microbial rates for a range of initial CT concentrations, and for both iron- and sulfate-reducing conditions. In most cases, abiotic reaction rates were indistinguishable from total reaction rates (abiotic plus microbial), indicating a negligible microbial contribution to CT transformation. While in most microcosms the soil/sediment acted as the abiotic reductant, under certain conditions the supernatant was more reactive with CT than was the solid phase. For these conditions, we propose that the reactive species in the supernatant consisted of aqueous natural organic matter that underwent reduction or other transformation by S(-II) generated by sulfate-reducing bacteria.

Keywords:

• Abiotic transformation
• biodegradation
• carbon tetrachloride
• natural attenuation
• reductive dechlorination

Acknowledgements

This research was supported by the National Science Foundation under Grant No. 0093332. H. S. thanks the University of Oklahoma Graduate College for an Alumni Doctoral Fellowship. We thank Scott Christenson, Ernie Smith, and Jason Masoner from the U. S. Geological Survey in Oklahoma City, OK, for providing equipment and sampling assistance at the Norman Landfill, John Wilson and Kevin Smith from US EPA, Ada, OK, for providing the Altus AFB soil and ground water samples, and Tohren Kibbey, Xingdong Zhu, Yiran Dong, Xiaoming Liang, and Yao Tan for assisting in sampling at the Norman Landfill and Duck Pond. We also thank Lee Krumholz for helpful advice and three anonymous reviewers for critical review of the manuscript.

Notes

^a Uncertainties are one standard error obtained from least-squares regression.

^b Initial rates for the following sterilized, medium [CT]₀ conditions are also reported in Shao and Butler (2009): DP-IR, L-SR, and Altus AFB-SR. ^c L, DP, and Altus AFB refer to microcosms prepared from Norman Landfill, Duck Pond, and Altus Air Force Base soils or sediments, respectively. IR and SR refer to iron-reducing and sulfate-reducing conditions. ^d Low [CT]₀: 0.34–0.41 μM. ^e Medium [CT]₀: 1.5–1.9 μM. ^f High [CT]₀: 0.97–1.4 mM. ^g NM means not measured.