Influence of Ferrous Iron and pH on Carbon Tetrachloride Degradation by Methanosarcina thermophila

Abstract

The influence of environmental conditions on the biological transformation of a contaminant must be well understood to optimize remediation processes. One factor that impacts the biological transformation of carbon tetrachloride (CT) is elemental iron (Fe 0). Previous research has shown that Fe 0 increases the methanogenic CT degradation rate by providing H 2 for cell growth and dechlorination. As Fe 0 oxidizes it also increases the pH and Fe 2+ levels, which may also impact the biological transformation of CT. Experiments were performed with Methanosarcina thermophila to investigate the influence of these factors on CT degradation. The transformation of CT and CF was greatly influenced by pH, with the rate of CT and CF degradation increasing with increasing pH. After 6 h, >90% of the CT had been degraded in the treatments containing cells at a pH of 8.5, whereas only about 51% of the CT had been degraded in similar treatments at a pH of 5.5. Fe 2+ did not significantly influence the degradation of CT; however, 60% less CF was formed in systems containing cells+Fe 2+ than in systems containing cells only. In addition, Fe 2+ promoted rapid transformation of CF when added to treatments containing cells. The product distribution after 9 days in all systems containing cells was very similar, with 98.04±5.46% (two-sided 95% confidence interval) of the originally fed CT present as soluble products. These results show that pH and Fe 2+ influence the degradation of CT and CF, although transiently. Because the residence time of contaminants in Fe 0 barriers varies with the thickness of the barrier, it is likely that this influence will be important for some flow-through systems. This implies that a combined Fe 0/organism remediation system may have previously unrealized advantages (due to pH and Fe 2+ changes).