Experimental measurements of the reversibility of metal-bacteria adsorption reactions

This study tests the reversibility of metal-bacteria interactions by comparing estimated extents of desorption based on surface complexation modeling, to those we observed in the experimental adsorption/desorption systems. The experiments also determine if extended adsorption contact time affects desorption kinetics. The experiments involved Ca and Cd adsorption/desorption onto the surface of a gram positive bacterium: Bacillus subtilis. Three types of experiments were performed: (1) Ca and Cd desorption from the cell wall of B. subtilis after 1 h of adsorption contact; (2) Cd and Ca desorption from the cell wall of B. subtilis after > 15 h of adsorption contact; and (3) Ca and Cd desorption as a function of pH after 1 h of adsorption contact. Both the adsorption and desorption reactions are rapid, and the desorption kinetics are independent of adsorption contact time. Steady-state conditions are attained within 2 h for all adsorption reactions studied, and within 1 h for all desorption reactions studied. Furthermore, the extent of adsorption or desorption remains constant for at least 24 h (and up to 80 h for Cd). The observed extent of desorption in the experimental systems is in excellent agreement with the amount estimated from a surface complexation model based on independently conducted adsorption experiments. Therefore, this study indicates that the adsorption of Cd and Ca on B. subtilis is a rapid, fully reversible, and hence an equilibrium process. Therefore the use of surface complexation modeling of aqueous metal adsorption onto bacterial surfaces yields accurate estimates of the distribution of metals in bacteria-bearing solutions. (C) 2000 Elsevier Science B.V. All rights reserved.