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 st udies using a model metal, cadmium, and an anionic monorhamnolipid bio surfactant produced by Pseudomonas aeruginosa ATCC 9027 showed that co mplexation of the metal and biosurfactant was rapid and stable and ach ieved high reductions in the free Cd2+ concentration. For example 99% 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-rha mnolipid stability constant (log K = -2.47) was higher than those repo rted for cadmium-sediment and cadmium-humic acid systems. The calculat ed cadmium-rhamnolipid equilibrium partition coefficient (log K(p) = 4 .89) was similar to values reported for sorption of organic contaminan ts by dissolved organic matter. Quantitative separation and recovery o f the rhamnolipid from the rhamnolipid-Cd2+ complexes were achieved by acid precipitation of the rhamnolipid followed by centrifugation.