Saturday, 15 July 2006

Application of Cupriavidus Metallidurans CH34 and Escherichia coli to Bio-Remediate Zinc, Cadmium and Copper Contaminated Soils.

Jean M. F. MARTINS and Veronique Guiné. CNRS - LTHE, BP 53, Domaine Universitaire, Grenoble, 38041, France

The fate and impact of heavy metals produced by agricultural or industrial activities has become a crucial environmental problem. Bio-remediation processes of contaminated soils have made many progress. However the effect of geo-chemical factors on the behaviour of micro-organisms is not yet well understood. Metallic cations display high affinity for bacterial surfaces and the mobility of these contaminants in groundwater systems can be related to that of bacteria, depending on hydrogeologic and geochemical conditions, bacteria are either mobile or immobile (bacterial attachment or blocking). The aim of this work is to improve our understanding of the bio-physical-chemical factors controlling bacterial cell mobility/adhesion and their influence on heavy metal (Zn Cd or Cu) leaching, in order to remediate naturally or artificially contaminated soils. The bacterial strain used in this study are Cupriavidus metallidurans CH34 and Escherichia coli DH5alpha, both are rod-shaped, gram-negative and aerobic bacteria. Bio-facilitated transport and bio-leaching experiments were conducted in glass columns (2.6 cm diameter and 15 cm length), filled with about 150g of heavy metal contaminated soil. The physical factors controlling flow were kept constant in all experiments and pH was monitored. Experiments were conducted in duplicate under saturated conditions. Bacterial cells mobility was found to be controlled by bio-physical-chemical parameters (membrane properties, cell concentration, soil-solution chemistry). Bacteria were found to be excellent sorbents for Zn, Cd and Cu. The transport of these heavy metals in soil columns was strongly accelerated (up to 3.5 times for Cd and 2.5 times for Zn) in the presence of both bacteria. Moreover, biosorbed-metal transport process was 4 to 6 times higher than dissolved transport even when high amounts of heavy metal were retained by bacterial cells in sand. Bacterial cells strongly controlled the heavy metal mobility through saturated sand columns. From bio-remediation experiments, we observed both a transient and persistent effect of bacteria on metal leaching: a high amount of the metals were mobilised during the breakthrough of the cells and this metal leaching effect persisted very long after the bacteria inoculation, whatever the bacterial type. The metal removal was strongly increased (12 fold) in the polluted soils as compared to the bacteria-free leaching experiments. Consequently the time needed to detoxify these soils was similarly decreased. The increased metal leaching is assumed to be either an active transport for E. coli DH5alpha because of the bio-sorbed concentrations of heavy metals or an indirect in situ modification of the metal speciation and advective transport for C. metallidurans CH34. We evidenced the ability of bio-colloids to both enhance leaching and accelerate transport of heavy metals in contaminated soils.

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