Effect of rhamnolipids on the dissolution, bioavailability and biodegradation of phenanthrene

Biodegradation rates for polycyclic aromatic hydrocarbons (PAH) in the environment are limited by their low solubility and sorption to solid surfaces. The purpose of this study was to quantify the effect of biosurfactants on the dissolution, bioavailability, and biodegradation of a slightly soluble PAH, phenanthrene, in a series of batch solution studies. A mathematical model that describes the combined effects of solubilization and biodegradation, including description of bioavailability within surfactant micelles, was used to analyze the experimental results. Two forms of the biosurfactant, a monorhamnolipid and a dirhamnolipid, were tested; it was found that both surfactants increased the solubility and enhanced the rate of phenanthrene biodegradation. Monorhamnolipid was more effective than dirhamnolipid for solubilization; however, overall rates of mineralization were essentially the same. This seems to result from variable bioavailability of substrate: phenanthrene within monorhamnolipid micelles was less bioavailable than phenanthrene within dirhamnolipid micelles. Therefore, the effect of a surfactant on biodegradation is a combination of the solubilizing power of the surfactant and the bioavailability of the substrate within the surfactant micelles. Model analysis of the solubilization data showed that the overall solubilization rate coefficient, K-L, increased with increasing biosurfactant concentration. Analysis of biodegradation data showed that enhanced biodegradation rates depend upon both K-L and alpha, the coefficient for substrate bioavailability from micelles. Model simulations using parameters developed from test data are discussed.