Influence of nonionic surfactants on the bioavailability and microbial reductive dechlorination of hexachlorobenzene

Research was conducted to investigate the effect of nonionic surfactants on the microbial reductive dechlorination of hexachlorobenzene (HCB) and to evaluate the potential for surfactant-enhanced bioavailability of sorbed-phase HCB in contaminated sediments. The biological compatibility of sixteen surfactants for both methanogenesis and reductive dechlorination was evaluated using HCB-dechlorinating, mixed, enrichment cultures developed with a contaminated estuarine sediment sample. Polyoxyethylene sorbitan fatty acid esters (Tween surfactants) were the least inhibitory. Increasing surfactant concentration generally inhibited reductive dechlorination, but not necessarily methanogenesis. The anaerobic biodegradability of three similar surfactants (Tween 60, 61, and 65) was assessed and compared to their structural features. Subcultures were successfully developed in which these Tween surfactants were utilized as the sole carbon source for methanogenesis and served as a stable source of electrons to sustain the reductive dechlorination of HCB for over eighteen months. The equilibrium phase distribution of HCB and surfactant in a suspended-growth culture system was measured through batch sorption assays and depicted with a mathematical model accounting for the distribution of HCB to the micellar-, accumulated surfactant-, and biomass-phases. Assays were conducted to determine the effect of HCB availability on its dechlorination rate. Mathematical models utilizing Michaelis-Menten type kinetics, based on either the concepts of bioavailability-limitation or enzyme-inhibition, were developed and reasonably simulated HCB dechlorination in the presence of a biodegradable surfactant. The effect of surfactants on the concurrent desorption and biotransformation of sorbed-phase HCB was evaluated using a field-contaminated (over 40 years) sediment containing HCB and other polychlorinated organic compounds which are resistant to desorption. Anaerobic sediment slurry microcosms, fed with glucose or Tween surfactants, were established. Although the Tween surfactants served as electron donors and supported the microbial reductive dechlorination and desorption of HCB, they did not enhance the bioavailability of sediment-bound HCB compared to the glucose-fed system over 119 d. This outcome is attributed to both the high degree of HCB sequestration in the high organic content, historically-contaminated sediment as well as surfactant losses from the solution phase due to sorption/precipitation and biodegradation. These findings illustrate the inherent difficulties in remediating environmental systems containing long-term, strongly-sequestered and recalcitrant contaminants.