Environ. Sci. Technol. 1994, 28, 2402-2406
Complexation of Cadmium by a Rhamnolipid Biosurfactant Hua Pan, dill T. Champion, danick F. Artlola, Mark L. Brusseau, and Raina M. Miller'
Department of Soil and Water Science, University of Arizona, Tucson, Arizona 85721 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+ ! of Cd2+ (0.72 mM) was concentration. For example 92 % complexed by 7.3 mM rhamnolipid, and 97 5% 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+/rhamnolipidon 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 Kp = 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.
Introduction Contamination of the environment by heavy metals is of growing concern because of the health risks posed by human and animal exposure. A survey of 395 remedial action sites revealed that heavy metals (e.g., cadmium, copper, lead, zinc, etc.) were the most prevalent class of contaminants ( I ) . Traditionally, the remediation of heavy metal contaminated soils has involved the excavation and transport of contaminated soil to hazardous waste sites for landfilling. However, the rapidly increasing cost of excavation and landfilling is fostering interest in the development and use of alternative, cost-effective remediation approaches. Two widely used approaches for remediating contaminated subsurface environments are in situ soil washing, which is used primarily for surface soil contamination, and pump and treat technology, which is used at sites with deep soil or aquifer contamination. The time required for the removal of heavy metals by soil washing or pump and treat is often greater than is economically or politically acceptable due to sorption interactions of the contaminant with the soil. Strategies to enhance the mobilization of heavy metals must be developed to shorten the time required for soil washing or pump and treat remediation. One approach being investigated in our laboratory is the use of microbially-produced surfactants (biosurfactants) to complex and mobilize heavy metals in soils. It is wellestablished that whole cells and bacterial exopolymers can effectively recover metals from wastestreams such as sewage sludge, industrial effluents, and mine water ( 2 , 3 ) . However, the large size of whole bacterial cells and
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[email protected]. 2402
Environ. Sci. Technol., Vol. 28,No. 13, 1994
exopolymers (molecular weight approximately 106) makes it likely that metal-cell and metal-exopolymer complexes would be filtered out by the soil during washing or flushing. Thus, biosurfactants have a distinct advantage over whole cells and exopolymers due to their small size (generally molecular weight
