Publication
Living bacteria offer an ecological and cost-effective alternative to the classical physical and chemical techniques for the treatment of environmental contaminations. Although their increasing use in toxic metal removal, systematic studies on the influence of physicochemistry of the medium on metal uptake and thus on their metal removal efficiency are very limited, even though it would deserve greater environmental interest. The overall goal of the present work is to improve the mechanistic understanding of the interactions between toxic metals and microorganisms. The emphasis is on the effect of different site-specific characteristics e.g. the presence and concentrations of different complexing agents, metal mixtures and pH- on the Cd and Pb uptake by a metal-resistant bacterium (Cupriavidus metallidurans CH34). Conversely, the influence of bacteria on metal speciation and mobility in the surrounding environment is also addressed. Different processes involved in the overall metal uptake, such as the diffusion of metal species, adsorption, internalization and efflux, were quantified and described with a consistent dynamic model for a range of metal concentrations representing slightly to heavily polluted environments. Obtained results demonstrated that at 10-8 and 10-6 M of initial Cd concentrations, the bulk Cd depletion determined both adsorbed and intracellular Cd amount. Cd efflux affected Cd internalization over the whole concentration range. The findings of the present study demonstrated the key role of metal depletion in the bulk medium and the efflux on the overall metal uptake by C. metallidurans. Therefore, also the need to account for these processes in order to improve the efficiency of metal removal from contaminated environments. The relationship between Cd and Pb speciation and the metal uptake was also explored. The presence of citric, nitrilotriacetic and humic acids decreased both intracellular and total cellular metal contents. Intracellular and total cellular metal contents were directly related to the experimentally determined free cadmium and lead ion concentrations rather than the total metal concentrations, emphasizing the important role of metal speciation in uptake. Furthermore, the influence of the major cations, such as Ca and Mg, trace metals, such as Zn, Cu, Co, Mn and pH on Cd and Pb uptake were studied. Increasing concentrations of Ca, Mg, Zn or protons decreased both intracellular and total cellular Cd contents. The intracellular and total cellular Pb contents decreased significantly in the presence of Ca, while Mg decreased only total cellular metal content. In order to enhance even more the environmental relevance of the studied conditions, both metal speciation and Cd and Pb uptake by bacteria were determined in six metal contaminated soil solutions. A Michaelis-Menten based equation taking into account both speciation and competition effects was used to predict metal uptake in the soil solutions. Predictions