Enhanced anaerobic transformations of carbon tetrachloride by soil organic matter

Abstract

AbstractAaerobic, reductive dehalogenation of carbon tetrachloride (CT) by Shewanella putrefaciens 200 is enhanced by the presence of a high‐organic‐carbon soil. In microbial incubations without soil, 29% of the initial 3 ppm (19.5 μM) CT was transformed after 33 h, whereas 64% was transformed after only 18 h when soil was present. In sterile, biomimetic systems using a chemical reductant, 20 mM dithiothreitol, similar results were observed, suggesting that abiotic electron‐transfer mediators in the soil were catalyzing the reaction. Destruction of 62% of the soil organic carbon by H2O2 resulted in a soil that was less effective in enhancing CT dechlorination. Following separation of the soil organic matter into three humic fractions, the humic acid (HA) fraction catalyzed the dechlorination reaction to a greater extent (270–442 μg of CT per gram of HA) than did the fulvic acid (FA) fraction (149–234 μg of CT per gram of FA), and both were more effective than the fraction containing humin and inorganic minerals (19–26 μg of CT per gram). The results are consistent with a mechanism involving humic functional groups that serve as electron‐transfer mediators able to enhance the reductive transformation of CT in the presence of a microbial or chemical reductant. Humic functional group analyses showed that the FA contained more total acidity and carboxylic acidity than did the HA; however, both fractions contained similar amounts of total carbonyl groups and quinone carbonyls. Abiotic, HA‐mediated CT transformation in the presence or absence of 20 mM dithiothreitol was found to be pH dependent. At pH 3.6, little CT transformation was observed regardless of whether dithiothreitol was present or not. At circumneutral pH, HA‐mediated CT transformation required the presence of dithiothreitol. At pH 8.7, HA‐mediated reductive CT transformation occurred both in the absence or presence of dithiothreitol although the transformation was greater in the presence of a reductant. Trichloromethane (chloroform [CF]) production at pH 8.7 was much lower than at circumneutral pH, and volatile organic compounds other than CF were not detected as products in any case.