Submitted to: Environmental Science and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: September 1, 2000
Publication Date: N/A
Interpretive Summary: Organic chemicals such as pesticides are often degraded slowly in soils because they are relatively insoluble in water. Surfactants and detergents are sometimes used to help get more of the pesticide dissolved into the water phase so microbes can degrade them. However, synthetic detergents and surfactants are often toxic to microbes and can be pollutants themselves so otheir use is limited. Rhamnolipids are natural surfactants produced by a number of bacterial species. Because they are natural products they may be more benign to bio-degrading bacteria and be more bio-degradable themselves and, therefore, may be better candidates for dissolving pesticides for biodegradation. In this study, we compared the pesticide dissolving ability of a rhamnolipid mixture produced by Pseudomonas aeruginosa with that of the commonly used synthetic surfactant Triton X-100. The ability of these materials to help solubilize atrazine, trifluralin, and coumaphos was compared. The rhamnolipid mixture was less effective than triton X-100 in solubilizing trifluralin or coumaphos but was as effective in solubilizing atrazine. A simple model was developed to describe each surfactant's ability to dissolve each of the three pesticides. This research will be valuable to scientists and engineers who try to develop methods for the use of surfactants in the clean-up of contaminated soils.
The ability of a rhamnolipid mixture produced by P. aeruginosa UG2 to solubilize the pesticides atrazine, trifluralin, and coumaphos was compared with that of the surfactant Triton X-100. The values of maximum micellar solubilization capacities (Ksupra [mmol pest/mol surf]) for trifluralin and coumaphos in Triton X-100, were double those for the rhamnolipid mixture, whereas, atrazine Ksupra value for the rhamnolipid biosurfactant was in th same range as that for the synthetic surfactant. In spite of having the second largest Kow value of the three pesticides, coumaphos had the lowest affinity for both surfactant micellar phases. Comparison of values of aqueous-micelle solubilization rate coefficients (kOWM ) obtained for trifluralin showed that the pesticide is solubilized at the same rate in both surfactant micellar phases. A much lower value of micellar-aqueous transfer rate coefficient (kOMW ) for trifluralin in the rhamnolipid mixture, suggests that the pesticide is bound more tightly to the biosurfactant micellar core, and diffuses out to the aqueous phase at a lower speed than that observed for the synthetic surfactant. Future research can benefit from this work by studying the effect that micellar solubilization can have on the bioavailability of organic pollutants for microbial uptake.