Proton and metal adsorption onto bacterial consortia: Stability constants for metal-bacterial surface complexes

In this study, we conduct potentiometric titrations and metal adsorption experiments (Cd, Ca, Cu, Pb, Sr, and Zn) using bacterial consortia grown from three representative locations and sampled over the course of a year in order to determine whether bacterial diversity affects proton and metal uptake behaviors. We observe significant changes in bacterial diversity from one site to another, and from month to month during the study period. Despite these changes in diversity, all of the bacterial consortia studied exhibit similar proton and metal uptake, strongly suggesting universal adsorption behavior for the bacterial species present in these samples. We demonstrate that a single, metal-specific, averaged surface complexation model can be used to reasonably account for the acid/base and metal adsorption behaviors of each consortium. We use a four discrete site non-electrostatic model to describe the protonation of the consortia functional groups, with averaged pK_a values of 3.2 + 0.2 / - 0.4, 4.8 + 0.2 / - 0.3, 6.5 + 0.3 / - 0.8, and 9.2 + 0.1 / - 0.3, and site concentrations of (1.0 ± 0.28) × 10^{- 4}, (1.2 ± 0.23) × 10^{- 4}, (6.12 ± 1.1) × 10^{- 5}, and (9.7 ± 2.0) × 10^{- 5} moles of sites per gram wet mass of bacteria, respectively. The metal adsorption data are used to constrain site-specific bacterial surface complexation models, and we determine the stability constants for the important metal-bacterial surface complexes. These calculated stability constants correlate well to known stability constants for metal-acetate complexes, yielding predictive relationships that enable the estimation of the extent of adsorption of other metals onto bacterial consortia. This study demonstrates that a wide range of bacteria exhibit similar proton and metal adsorption behaviors, and that a single set of averaged acidity constants, site concentrations, and stability constants for metal-bacterial surface complexes can be used to model the adsorption behavior.