A COMPARISON OF THE THERMODYNAMICS OF METAL ADSORPTION ONTO 2 COMMON BACTERIA

The cell walls of bacteria are known to adsorb a variety of metals, an d thus they may control metal mobilities in many low-temperature aqueo us systems. In order to quantify metal adsorption onto bacterial surfa ces, recent studies have applied equilibrium thermodynamics to the spe cific chemical and electrostatic interactions occurring at the solutio n-cell wall interface. However, to date, few studies have used this ap proach to compare the surface properties and metal affinities of diffe rent species of bacteria. In this study. we use acid-base titrations t o determine the concentrations and deprotonation constants of specific surface functional groups on Bacillus licheniformis. The cell wall di splays carboxyl, phosphate and hydroxyl surface functional groups, wit h pK(a) values and 1s errors of 5.2 +/- 0.3, 7.5 +/- 0.4 and 10.2 +/- 0.5, respectively. We perform metal-B. licheniformis adsorption experi ments using Cd, Pb, Cu and Al. The average log K values for the Cd-, P b-, Cu-and Al-carboxyl stability constants, with 1s errors, are 3.9 +/ - 0.5, 4.6 +/- 0.3, 4.9 +/- 0.4 and 5.8 +/- 0.3, respectively. Finally , we compare the surface characteristics and metal affinities of B. li cheniformis to those of Bacillus subtilis, as determined by Fein et al . [Fein, J.B., Daughney, C.J., Yee, N., Davis, T., 1997. A chemical eq uilibrium model of metal adsorption onto bacterial surfaces. Geochim. Cosmochim. Acta 61, 3319-3328]. Our investigations indicate that these two species of bacteria have different relative and absolute concentr ations of surface sites and slightly different deprotonation and metal adsorption stability constants. We relate these variations in surface properties to variations in metal affinity in order to predict metal mobilities in complex, natural systems. (C) 1998 Elsevier Science B.V.