Stimulating in-soil rhamnolipid production in a bioslurry reactor by limiting nitrogen

A soil with aged contamination from lubricating oil (LO) and polychlorinated biphenyls (PCBs) was treated in a bioslurry reactor to investigate in-soil biosurfactant production by Pseudomonas aeruginosa, the most abundant indigenous, culturable, hydrocarbon-degrading microorganism. After 2 days of growth on LO, a depletion of nitrogen stimulated the production and accumulation of rhamnolipids to levels roughly 20 times the critical micelle concentration. Surface tensions and concentrations of monorhamnolipid and dirhamnolipid, PCBs, and total petroleum hydrocarbons (TPH) were measured in a slurry filtrate. Soil-bound PCBs and TPH were also quantified. Rhamnolipid production was observed within 1 to 2 days after nitrogen depletion in each of the 10 batches tested. By day 6, total rhamnolipid concentrations increased from below detection to average values over 1,000 mg/L, which caused over 98% of soil-bound PCBs and over 99% of TPH to be emulsified and recovered in the filtrate. After 70 days, rhamnolipid concentrations were only reduced by 15%, because of nitrogen-limited rates of rhamnolipid biodegradation. The results show that in-soil biosurfactant production can be stimulated in a controlled way with nutrient limitation and can be used to achieve soil washing.