Abstract
The effect of nine Tween surfactants (polyoxyethylene sorbitan fatty acid esters) on methanogenesis and the microbial reductive dechlorination of hexachlorobenzene (HCB) was examined using three anaerobic, HCB‐dechlorinating mixed cultures, one glucose fed and two lactate fed, that were derived from contaminated estuarine sediments. The two lactate‐fed cultures differed significantly in their metabolic activity, especially with respect to the rate and extent of HCB reductive dechlorination. The objective of the study was to screen and select biologically compatible surfactants for use in subsequent research on surfactant‐enhanced bioavailability of sorbed‐phase chlorinated organic contaminants. Addition of the Tween surfactants resulted in lower rates of methanogenesis and HCB dechlorination compared with the control (i.e., surfactant‐free culture) for both the glucose‐fed culture and the lactate‐fed culture, which exhibited a high metabolic activity. Tween−61 and −65 exhibited the lowest inhibition of HCB dechlorination at initial surfactant concentrations of 200 and 1,000 mg/L. On the other hand, the Tween surfactants, especially Tween−81 and −85, enhanced the rates of both methanogenesis and HCB dechlorination when the low metabolic activity lactatefed culture was used. The observed rate enhancements may be attributed to bacterial cell membrane modification by the surfactants. Attempts to correlate methanogenesis rates to surfactant properties (such as the hydrophile‐lipophile balance number, fatty acid carbon chain length, and the number of ethylene oxide groups) were not successful. In general, increased surfactant concentrations resulted in lower rate and extent of HCB dechlorination. Although HCB dechlorination was completely inhibited at a surfactant concentration of 1,000 mg/L, the extent of methanogenesis was not affected. Results from this study demonstrate that despite compositional similarities among the Tween surfactants, their effects on biological activities, such as methanogenesis and reductive dechlorination, can differ significantly.