Removal of Cadmium, Lead, and Zinc from 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 and 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 K⁺ 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.