Use of Biosurfactants in the Removal of Heavy Metal Ions from Soils

Abstract

Heavy metal contamination of soils is one of the world’s major environmental problems, posing significant risks to human health as well as to the ecosystems. Conventional treatment technologies for heavy metal polluted soils such as excavation and transport of contaminated soil to hazardous waste sites for landfilling have several disadvantages. They cannot completely remove metals, they can only immobilize them in the contaminated soil. Novel technologies involving microorganisms and their products to remove heavy metals have been successfully applied to waste streams such as sewage sludge, industrial effluents, and mine water. Biosorption of metal-contaminated soils presents a more complex separation problem. Use of biosurfactants to improve the removal of heavy metal contaminants from aqueous media and soils has received increasing attention in recent years. Surfactin produced by Bacillus subtilis, rhamnolipids from Pseudomonas aeruginosa, sophoro- lipids from Torulopsis bombicola, Aescin from Aesculus hippocastanum, and saponin from quillaja bark have been employed to remove metals from con- taminated soils. The possible mechanisms for the removal of heavy metals by biosurfactants are ion exchange, precipitation–dissolution, and counter ion binding. Reports on the use of biosurfactants in metal removal are however scanty. Even though sorption isotherms have been widely used to measure the heavy metal accu- mulation in soils,the desorption of heavy metals and the possible hysteresis have been scarcely reported. This chapter highlights the use of biosurfactants of various origins in the removal of heavy metals from soils contaminated with metals.