Cd and Pb removal from contaminated environment by metal resistant bacterium Cupriavidus metallidurans CH34: importance of the complexation and competition effects

The present study aims to improve the understanding of the role of complexation and competition effects on Cd and Pb accumulation by the metal resistant bacterium Cupriavidus metallidurans largely used in bioremediation. Adsorbed and intracellular metal content in bacteria were determined in model exposure medium within a concentration range spanning from 10(-9) to 5 x 10(-5) M of Cd or Pb and water extracts from soils. In parallel, the free metal ion concentrations ([M2+]) were measured by an ion exchange technique. Obtained results demonstrated that Cd and Pb accumulation by C. metallidurans was related to [M2+] in the solution. The adsorbed and intracellular M fractions were significantly reduced by nitrilotriacetic acid, Elliot or Pahokee Peat humic acids, as well as by a large excess of Ca, Mg and Zn. No effect on Cd and Pb bioaccumulation was observed in the presence of Mn, Cu or Co at a 10-fold excess for bacteria exposed to 10(-6) M of Cd or Pb. Adsorbed and intracellular metal determined when bacteria were exposed to water extracts of soil were in the same order as expected from the model experiments when complexation and competition effects are considered. The study emphasises the necessity of taking into account chemical site-specificity of soil solutions and water, including dissolved organic ligands, pH and the presence of other metals when developing metal removal technologies by living bacteria.