Anaerobic biodegradability of Tween surfactants used as a carbon source for the microbial reductive dechlorination of hexachlorobenzene

Three structurally-related, nonionic, polysorbate surfactants (Tween 60, 61, and 65) were used as the sole carbon source to sustain the microbial, sequential reductive dechlorination of hexachlorobenzene (HCB) in a mixed, methanogenic culture derived from a contaminated estuarine sediment. The surfactants were partially degraded and fermented to methane with no measurable accumulation of volatile fatty acids, indicating that methanogenesis was rapid relative to the rates of hydrolysis and acidogenesis. Addition of the methanogenesis inhibitor 2-bromoethanesulfonic acid resulted in acetate accumulation without impact on the sequential dechlorination of HCB. An anaerobic biodegradability assay was performed and the following data were obtained for the Tween 60, 61, and 65, respectively: 53, 62, and 62% COD destruction; 35, 57, and 48% COD to methane conversion; and 38, 38, and 45% COD to acetate conversion. These data suggest that the hydrophobic moiety (stearate) of the surfactants was preferentially degraded, most likely through β-oxidation, to acetate and ultimately to methane and carbon dioxide. Between 38 and 47% of the initial surfactant COD remained after 46 d incubation, which most likely corresponds to the hydrophilic polyoxyethylene moiety. An anaerobic biodegradation pathway of the Tween surfactants is proposed.