Application of rhamnolipid and surfactin for enhanced biodegradation of diesel-contaminated water and soil

Bioremediation of petroleum hydrocarbons has been proposed as an effective, economic, and environmentally friendly technology, although bioavailability of hydrophobic organic compounds (HOCs) to microorganisms could be a limiting factor during the biodegradation process. This study investigated the potential application of two biosurfactants, surfactin (SF) and rhamnolipid (RL), for enhanced biodegradation of diesel-contaminated water and soil with series of bench-scale experiments. In diesel/water batch experiments, addition of 40 mg/L of SF significantly enhanced biomass growth as well as diesel biodegradation percentage, compared to no SF addition. Addition of SF more than 40 mg/L, however, decreased both biomass growth and diesel biodegradation efficiency, with a worst diesel biodegradation percentage at 400 mg/L of SF addition. Similar trends were also observed for specific biomass growth rate and specific diesel degradation rate constant as SF addition increased from 0 to 400 mg/L. Addition of RL to diesel/water systems from 0 to 80 mg/L substantially increased biomass growth and diesel biodegradation percentage from 1000 to 2500 mg VSS/L and 40 to 100%, respectively. RL addition at a concentration of 160 mg/L provided similar results to those of 80 mg/L addition. In diesel-water systems with 50 mg/L of RL addition, an optimum pH condition for microbial growth and diesel biodegradation was found to be at a pH 7.2, while decreasing pH to 5.2 or increasing it to 8.4 reduced those parameters considerably. For the cases where 40 mg/L of SF was added, the enhancing ability shared a general trend with that observed for adding 50 mg/L of RL as the pH increased from 5.2 to 7.2. Finally, potential application of SF and RL in stimulating indigenous microorganisms for enhanced bioremediation of diesel-contaminated soil was also examined. The results confirmed their enhanced capability in both efficiency and rate of diesel biodegradation in diesel/soil systems.