Complexation of cadmium by a rhamnolipid biosurfactant

The potential of microbially-produced surfactants (biosurfactants) to complex heavy metals was investigated in this study. Batch solution studies using a model metal, cadmium, and an anionic monorhamnolipid biosurfactant produced by Pseudomonas aeruginosa ATCC 9027 showed that complexation of the metal and biosurfactant was rapid and stable and achieved high reductions in the free Cd²⁺ concentration. For example 92% of Cd²⁺ (0.72 mM) was complexed by 7.3 mM rhamnolipid, and 97% of Cd²⁺ (0.36 mM) was complexed by 3.9 mM rhamnolipid. Under the experimental conditions used, the maximum complexation capacity of the rhamnolipid was 0.2 Cd²⁺/rhamnolipid on a molar basis. The calculated cadmium-rhamnolipid stability constant (log K = -2.47) was higher than those reported for cadmium-sediment and cadmium-humic acid systems. The calculated cadmium-rhamnolipid equilibrium partition coefficient (log K_p = 4.89) was similar to values reported for sorption of organic contaminants by dissolved organic matter. Quantitative separation and recovery of the rhamnolipid from the rhamnolipid-Cd²⁺ complexes were achieved by acid precipitation of the rhamnolipid followed by centrifugation.