Biosurfactants metal complexation

Due to the anionic nature of rhamnolipids, they are able to remove metals from soil and ions such as cadmium, copper, lanthanum, lead and zinc due to their complexation ability [57-59], More information is required to establish the nature of the biosurfactant-metal complexes. Stability constants were established by an ion exchange resin technique [60], Cations of lowest to highest affinity for rhamnolipid were K+ < Mg + < Mn + < Ni " " < Co " < Ca2+ < Hg2+ < Fe + < Zn2+ < Cd2+ < Pb2+ < Cu2+ < M +. These affinities were approximately the same or higher than those with the organic acids, acetic, citric, fulvic and oxalic acids. This indicated the potential of the rhamnolipid for metal remediation. Molar ratios of the rhamnolipid to metal for selected metals were 2.31 for copper, 2.37 for lead, 1.91 for cadmium, 1.58 for zinc and 0.93 for nickel. Common soil cations, magnesium and potassium, had low molar ratios, 0.84 and 0.57, respectively. 

Rhamnolipids were applied to a soil in the presence of oil contamination [15] and from sediments to remove heavy metals [63]. Although 80-100% of cadmium and lead can be removed from artificially contaminated soil, in field samples the results were more in the range of 20-80% [64], Clay and iron oxide contents affected the efficiency of the biosurfactants but this has not been researched. Biosurfactant could be added as a soil washing process for excavated soil. Due to the foaming property of the biosurfactant, metal-biosurfactant complexes can be removed by addition of air to cause foaming and then the biosurfactant can be recycled through precipitation by reducing the pH to 2. 

CMC, which contribute to the changes in cell surface morphology. Simulation studies showed that rhamnohpids can be used for the bioremediation of marine oil spill e. Rhamnolipids have been reported as metal complexing biosurfactants and can reduce organism s toxicity to metals such as cadmium during biodegradation of naphthalene. Hence, rhamnolipids can be used to treat soils co-contaminated with metals and hydrocarbons. Rhamnohpids can be used in the formulation of soil washing ent for the removal of heavy metals and their activity is comparable to that of synthetic metal chelators. ... 

Although their role in nature is still not clear, there are extracellular microbial products, such as biosurfactants, bioemulsifiers and siderophores, that complex or chelate metals quite efficiently. Examples of these compounds are shown in Figure 10.1. 

Micro-foam, or colloidal gas aphrons have also been reportedly used for soil flushing in contaminated-site remediation [494—498], These also have been adapted from processes developed for enhanced oil recovery (see Section 11.2.2.2). A recent review of surfactant-enhanced soil remediation [530] lists various classes of biosurfactants, some of which have been used in enhanced oil recovery, and discusses their performance on removing different type of hydrocarbons, as well as the removal of metal contaminants such as copper and zinc. In the latter area, the application of heavy metal ion complexing surfactants to remediation of landfill and mine leachate, is showing promise [541]. 

Fig. 4. Potential mechanism for metal removal by the rhamnolipid biosurfactant. (1) Adsorption of the biosurfactant on the soil surface and interaction via electrostatic attraction and solubilization of soil fractions containing the metal. (2) Removal of the metal from the soil surface. To maintain electrostatic neutrality, two carboxylic groups are needed per divalent metal. (3) Incorporation of the metal-biosurfactant complex into micelles (adapted from Mulligan et al. [68]).

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