The Effect of Ionic Strength on the Adsorption of H+, Cd2+, Pb2+, and Cu2+byBacillus subtilisandBacillus licheniformis:A Surface Complexation Model

Abstract

To quantify metal adsorption onto bacterial surfaces, recent studies have applied surface complexation theory to model the specific chemical and electrostatic interactions occurring at the solution-cell wall interface. However, to date, the effect of ionic strength on these interactions has not been investigated. In this study, we perform acid–base titrations of suspensions containingBacillus subtilisorBacillus licheniformisin 0.01 or 0.1 M NaNO3, and we evaluate the constant capacitance and basic Stern double-layer models for their ability to describe ionic-strength-dependent behavior. The constant capacitance model provides the best description of the experimental data. The constant capacitance model parameters vary between independently grown bacterial cultures, possibly due to cell wall variation arising from genetic exchange during reproduction. We perform metal–B. subtilisand metal–B. licheniformisadsorption experiments using Cd, Pb, and Cu, and we solve for stability constants describing metal adsorption onto distinct functional groups on the bacterial cell walls. We find that these stability constants vary substantially but systematically between the two bacterial species at the two different ionic strengths.