Rhamnolipid Adsorption in Soil: Factors, Unique Features, and Considerations for Use as Green Antizoosporic Agents

In aqueous solutions, rhamnolipids effectively kill the motile zoospores responsible for spreading many pathogens, including soy-infecting Phytophthora sojae. For use in soil, adsorption properties need to be considered. Having low critical micelle concentrations, rhamnolipids tend to form micelles/aggregates with unknown effects on soil adsorption. Effects of soil pH, rhamnolipid congener structure, and concentration were examined. Congeners were identified and each quantitated for adsorptive partitioning. The adsorption isotherm at pH 6.5 showed a multi-stage profile plateauing at 1700 μg/g of soil. Less hydrophilic congeners adsorbed preferentially: R-C10-C12 > R-C10-C12:1 > RR-C10-C12:1 > RR-C10-C12 > R-C10-C10 > RR-C10-C10 > R-C8-C10 > RR-C8-C10 (where R is rhamnose and C# is the carbon number of β-hydroxy fatty acid). Adsorptive selectivity among congeners was very clear in dilute solutions but diminished with increasing concentrations. Results were interpreted with aggregate formation in solutions and on the soil surface. The cost estimate made accordingly supported the economic feasibility of rhamnolipid antizoosporic uses in soil.