Physico-chemical factors affecting rhamnolipid (biosurfactant) application for removal of metal contaminants from soil

Contamination of soil and groundwater environments by toxic metals and organic compounds is of major concern because of the potential health hazard posed for humans. Remediation of such sites may require the addition of chemical agents that help in the mobilization of contaminants which are likely to be bound to solid surfaces. Surfactants are one of the agents proposed for addition to enhance the removal of soil-bound contaminants. In this dissertation, a series of laboratory experiments were conducted to investigate constraints to the potential application of a microbially produced surfactant (biosurfactant) for removal of metal contaminants from soil. The first part of the dissertation describes the measurement of stability constants and 13 metals including ten of the metals most frequently found in contaminated sites as well as three of the most common metal cations found in soil, Ca²⁺, Mg ²⁺, and K⁺. The second part of the dissertation describes a series of experiments designed to determine the interaction of the biosurfactant with soil matrix components including clays, metal oxides, and organic matter. The biosurfactant used in this research was monorhamnolipid produced by Pseudomonas aeruginosa ATCC 9027. A mixture of mono- and dirhamnolipid produced by P. aeruginosa UG2 was also used in some cases. Results showed that selectivity of the monorhamnolipid followed the order: Al³⁺ > Cu²⁺ > Pb²⁺ > Cd²⁺ > Zn²⁺ > Fe³⁺ > Hg ²⁺ > Ca²⁺ > Co²⁺ > Ni²⁺ > Mn²⁺ > Mg²⁺ > K⁺. These results suggest that monorhamnolipid binds common metal contaminants in preference over common soil cations such as Ca²⁺, Mg²⁺, and K⁺. Rhamnolipid was shown to bind to some soil constituents very strongly including hematite, illite, kaolinite, and montmorillonite. These results indicate that the effectiveness of rhamnolipid in soils with high amounts of iron oxide or clay may be limited due to extensive sorption. Finally, it was found that monorhamnolipid sorbed more strongly than a rhamnolipid mixture containing both monorhamnolipid and dirhamnolipid. This suggests that the use of a biosurfactant mixture may improve the effectiveness of rhamnolipid in the removal of organic and metal contaminants from soil