Microcalorimetric and potentiometric titration studies on the adsorption of copper by P. putida and B. thuringiensis and their composites with minerals

In order to have a better understanding of the interactions of heavy metals with bacteria and minerals in soil and associated environments, isothermal titration calorimetry (ITC), potentiometric titration and equilibrium sorption experiments were conducted to investigate the adsorption behavior of Cu(II) by Bacillus thuringiensis, Pseudomonas putida and their composites with minerals. The interaction of montmorillonite with bacteria increased the reactive sites and resulted in greater adsorption for Cu(II) on their composites, while decreased adsorption sites and capacities for Cu(II) were observed on goethite-bacteria composites. A gram-positive bacterium B. thuringiensis played a more important role than a gram-negative bacterium P. putida in determining the properties of the bacteria-minerals interfaces. The enthalpy changes (ΔHads) from endothermic (6.14 kJ mol-1) to slightly exothermic (-0.78 kJ mol-1) suggested that Cu(II) is complexed with the anionic oxygen ligands on the surface of bacteria-mineral composites. Large entropies (32.96-58.89 J mol-1 K-1) of Cu(II) adsorption onto bacteria-mineral composites demonstrated the formation of inner-sphere complexes in the presence of bacteria. The thermodynamic data implied that Cu(II) mainly bound to the carboxyl and phosphoryl groups as inner-sphere complexes on bacteria and mineral-bacteria composites.