Stability Constants for the Complexation of Various Metals with a Rhamnolipid Biosurfactant

Abstract

The presence of toxic metals in natural environments presents a potential health hazard for humans. Metal contaminants in these environments are usually tightly bound to colloidal particles and organic matter. This represents a major constraint to their removal using currently available in situ remediation technologies. One technique that has shown potential for facilitated metal removal from soil is treatment with an anionic microbial surfactant, rhamnolipid. Successful application of rhamnolipid in metal removal requires knowledge of the rhamnolipid-metal complexation reaction. Therefore, our objective was to evaluate the biosurfactant complexation affinity for the most common natural soil and water cations and for various metal contaminants. The conditional stability constant (log K) for each of these metals was determined using an ion-exchange resin technique. Results show the measured stability constants follow the order (from strongest to weakest): Al3+ > Cu2+ > Pb2+ > Cd2+ > Zn2+ > Fe3+ > Hg2+ > Ca2+ > Co2+ > Ni2+ > Mn2+ > Mg2+ > K+. These data indicate that rhamnolipid will preferentially complex metal contaminants such as lead, cadmium, and mercury in the presence of common soil or water cations. The measured rhamnolipid-metal stability constants were found in most cases to be similar or higher than conditional stability constants reported in the literature for metal complexation with acetic acid, oxalic acid, citric acid, and fulvic acids. These results help delineate the conditions under which rhamnolipid may be successfully applied as a remediation agent in the removal of metal contaminants from soil, as well as surface waters, ground water, and wastestreams.

Bioremediation of multi-metal contaminated soil using biosurfactant — a novel approach

“…The results above indicates that di-rhamnolipid selectively favours mobilization of metals in the order of Cd=Cr>Pb=Cu>Ni. Apart from differential sorption of metals to soil the extraction and mobilization of metals depends on various factor like stability constant of the metals of concern with the di-rhamnolipid [12].…”

Structure and Characterization of Flavolipids, a Novel Class of Biosurfactants Produced by Flavobacterium sp. Strain MTN11

“…The ability of an organic ligand to bind a metal is often expressed as a conditional stability constant, log K. An ion-exchange technique was used to determine the conditional stability constant for flavolipid and cadmium, as previously described (41). In brief, 0.1 g of sodium-saturated SP Sephadex C-25 (Pharmacia Biotech AB, Uppsala, Sweden) was placed into a 7-ml polypropylene scintillation vial.…”

“…The measured conditional stability constant for flavolipid and cadmium was 3.61. This can be compared to the reported stability constants for the following organic ligands and cadmium: rhamnolipid, 6.89; acetic acid, 1.2 to 3.2; oxalic acid, 4.1; and citric acid, 4.5 (30,31,41). Flavolipid very likely has a much higher stability constant for Fe 3ϩ due to its structural similarity to arthrobactin and aerobactin, which were selected to complex Fe 3ϩ .…”

Isolation and Antimicrobial Activity of Rhamnolipid (Biosurfactant) From Oil-Contaminated Soil Sample Using Humic-Acid Salts-Vitamin Agar

“…They can be efficiently used in handling industrial emulsions, biodegradation and detoxification of industrial effluents, control of oil spills, and in bioremediation of contaminated soils [13][14][15].…”