A CHEMICAL-EQUILIBRIUM MODEL FOR METAL ADSORPTION ONTO BACTERIAL SURFACES

This study quantifies metal adsorption onto cell wall surfaces of Baci llus subtilis by applying equilibrium thermodynamics to the specific c hemical reactions that occur at the water-bacteria interface. We use a cid/base titrations to determine deprotonation constants for the impor tant surface functional groups, and we perform metal-bacteria adsorpti on experiments, using Cd, Cu, Pb, and AI, to yield site-specific stabi lity constants for the important metal-bacteria surface complexes. The acid/base properties of the cell wall of B. subtilis can best be char acterized by invoking three distinct types of surface organic acid fun ctional groups, with pK(a) values of 4.82 +/- 0.14, 6.9 +/- 0.5, and 9 .4 +/- 0.6. These functional groups likely correspond to carboxyl, pho sphate, and hydroxyl sites, respectively, that are displayed on the ce ll wall surface. The results of the metal adsorption experiments indic ate that both the carboxyl sites and the phosphate sites contribute to metal uptake. The values of the log stability constants for metal-car boxyl surface complexes range from 3.4 for Cd, 4.2 for Pb, 4.3 for Cu, to 5.0 for Al. These results suggest that the stabilities of the meta l-surface complexes are high enough for metal-bacterial interactions t o affect metal mobilities in many aqueous systems, and this approach e nables quantitative assessment of the effects of bacteria on metal mob ilities. Copyright (C) 1997 Elsevier Science Ltd.