Submitted to: Journal of Environmental Science and Technology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 8/12/2002
Publication Date: 9/25/2002
Citation: Interpretive Summary: Pesticides with low water solubility are often bio-degraded slowly or not at all. This is because biodegradation usually occurs in the water phase of soil. Surfactants, such as detergents, are sometimes used to increase the solubility of pesticides in water and enhance their degradation rate and hence, their removal from the environment. Synthetic chemical detergents such as Triton X-100 or lauryl sulfate can markedly increase the movement of pesticides to the water fraction of soil but are often toxic to the pesticide-degrading microbial communities. Rhamnolipids are natural surfactants produced by bacteria that can aid in the solubilization and degradation of organic chemicals with low-water solubility. This study compared the ability of rhamnolipids to solubilize 3 different pesticides in soil with that of Triton X-100. It was observed that only high concentrations of rhamnolipid or Triton X-100 significantly enhanced the solubility of the pesticides. At low doses rhamnolipids bound to soil particles and reduced the water solubility of the pesticides because they tended to stick to the rhamnolipid attached to soil. These results indicate that rhamnolipids may actually inhibit biodegradation of pesticides in soil if they are applied at low concentrations. A mathematical model was developed to describe the behavior of pesticides in soil in the presence of rhamnolipids or Triton X-100. This model will aid scientists and engineers using surfactants to enhance biodegradation of organic chemicals in soil.
Technical Abstract: The effectiveness of a rhamnolipid mixture (Rh-mix) in desorbing trifluralin, atrazine, and coumaphos from clay soil [Hagerstown B (HTB)], and silty soils [Beltsville A (BVA) and Hagerstown A (HTA)] was tested against that of Triton X-100 (TX-100). Sorption of both surfactants by the soils was significant and adequately described by the Langmuir isotherm equation. Values of maximum sorption capacity and Langmuir constant for HT soil were more than twice those for the high organic matter content soils HTA and BVA. The results suggest that besides soil organic matter (SOM), clay surfaces play an important role in the partition of surfactants onto soils. Pesticide desorption was only enhanced at surfactant dosages high enough to approach soil saturation and still promote, at equilibrium, an aqueous micellar phase where pesticides could partition. At dosages below soil saturation, surfactants sorbed onto soil, increasing its hydrophobicity and enhancing the sorption of the analytes by a factor of compared to conditions of surfactant absence. Similar values of water-soil partition coefficients for aged and fresh added pesticides to soils indicate that the aging process used did not significantly alter the capability of both surfactants to desorb them. A model able to estimate equilibrium distributions of organic compounds in soil-aqueous-micellar systems was tested against experimental results. Koc (fit) values obtained by the model for the three pesticides suggest that soil sorbed Rh-mix is, on a carbon normalized basis, a much better sorbent of hydrophobic organic compounds (HOC) than TX-100 and soil organic matter (SOM).