Temperature dependence of Cd and Pb binding onto bacterial cells

Abstract

We measured aqueous Cd and Pb adsorption onto bacterial surfaces at 5 °C, 50 °C, and 80 °C. We performed the adsorption experiments using one Gram-positive species, Bacillus subtilis, and one Gram-negative species, Pseudomonas mendocina. We use a surface complexation approach to determine the thermodynamic stability constants for the important metal–bacterial surface complexes for each temperature and each bacterial species. Our results indicate that temperature affects metal adsorption onto the surfaces of Gram-positive and Gram-negative species differently. B. subtilis exhibits little difference in its metal-binding capacity over the temperature range of this study. At elevated temperatures, Pb(OH) + is the dominant form of Pb that binds to the surface while Pb 2+ adsorption predominates at lower temperatures. However, the overall extent of Pb adsorption does not vary significantly with temperature. Conversely, the extent of both Cd and Pb adsorption onto P. mendocina is higher at 5 °C relative to the 50 °C and 80 °C data, but only under low pH conditions. The temperature dependence of metal adsorption onto bacteria depends upon both the metal and the bacterial species involved in the reaction. For both bacterial species studied here under circumneutral conditions, there is no significant temperature dependence to the extent of Cd adsorption or to the speciation of the important Cd–bacterial surface complexes. Therefore, a single set of averaged stability constant values can be used to account for Cd adsorption onto the cells as a function of temperature. Conversely, Pb more readily hydrolyzes in solution with increasing temperature than does Cd. Therefore, although the extent of total Pb adsorption does not change with temperature above pH 5, the speciation of the important Pb–bacterial surface complexes does vary with temperature. For metals such as Pb, the effect of temperature on the identity and stability constants of the bacterial surface complexes must be determined in order to accurately model metal speciation and transport as a function of temperature in bacteria-bearing geologic systems.