Combined effects of pH and biosurfactant addition on solubilization and biodegradation of phenanthrene

Phenanthrene solubilization and biodegradation with a biosurfactant (rhamnolipid) solution were investigated as a function of pH. Batch phenanthrene solubilization experiments were performed in the pH range 4-8 and the highest solubilities with the biosurfactant were detected around a pH of 4.5-5.5. The apparent solubility at pH 5.5 was 3.8 times greater than at pH 7 in the presence of 240 ppm rhamnolipid, probably due to the rhamnolipid-an anionic surfactant-forming different pH-dependent structures. Biodegradation experiments using Pseudomonas putida CRE 7 were performed in the absence and the presence of the rhamnolipid solution. Without the biosurfactant, the specific growth rate (mu) at pH 6 was higher than at other pH values, and analysis for the total phenanthrene loss confirmed the trends in mu, with the greatest phenanthrene removal at pH 6. In presence of the rhamnolipid, the maximum mu value shifted to around pH 5, which showed maximum enhancement of solubility in the abiotic experiment. Although there was an increase in the observed specific growth rate with the biosurfactant, this increase was not as great as the increase in solubilization. For example, the 1.44 times increase in the mu value at pH 5 was lower than the 3.8 times enhancement in the solubility at the same pH. Thus, as observed by others, not all of the solubilized phenanthrene was bioavailable to the microorganisms. Interestingly, the results of a size distribution experiment showed that a large portion of the phenanthrene-rhamnolipid aggregates existed at a molecular weight of >300,000. Furthermore, this fraction appeared to be the most available for biodegradation, although not all the phenanthrene was bioavailable.