REMOVAL OF CADMIUM, LEAD, AND ZINC FRON SOIL BY A RHAMNOLIPID BIOSURFACTANT

Complexation of cadmium, lead, and zinc (singly and in a mixture) by a monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027 was studied in batch solution and soil experiments. Conditional stability constants (log K-L) for metal-rhamnolipid complexation in a buffered medium (0.1 M Pipes, pH 6.8) were determined in duplicate using an ion-exchange technique and averaged 6.5 (Cd2+), 6.6 (Pb2+), and 5.4 (Zn2+); these values are similar or slightly higher than literature values for Cd2+ and Pb2+ complexation with fulvic acid and activated sludge solids. To determine the ability of rhamnolipid to desorb soil-bound metals, rhamnolipid solutions (12.5, 25, 50, and 80 mM) were added to soil containing sorbed Cd2+ (1.46 mmol kg(-1)), Pb2+ (1.96 mmol kg(-1)), or a mixture of Pb2+-Cd2+-Zn2+ (3.4 mmol kg(-1)). At 12.5 end 25 mM rhamnolipid, rhamnolipid sorption to soil exceeded 78%, and less than 11% of soil-bound Cd2+ and Zn2+ was desorbed. However, ion exchange of bound metals with Kf present in the rhamnolipid matrix could account for the removal of between 16 and 48% of the sorbed Cd2+ and Zn2+. At 50 and 80 mM rhamnolipid, rhamnolipid sorption to soil decreased to between 20 and 77%, and the removal of Cd2+ and Zn2+ could exceed the removal by ion exchange by as much as 3-fold. The behavior of Pb2+ was quite different. Less than 2% of soil-bound Pb2+ was desorbed due to ion exchange, although up to 43% was desorbed by 80 mM rhamnolipid.