Ternary Complexation on Bacterial Surfaces: Implications for Subsurface Anion Transport

The physical, chemical, and biological controls on contaminant mobilities in aquatic ecosystems must be determined to establish the threat that contamination poses to the environment. Quantitative models of contaminant mobilities are required as a prerequisite to guide remediation efforts and to prioritize the potential hazard to the ecosystem of each contaminated site. It is well established that mineral surface adsorption is an important control on contaminant mobilities, and many studies have utilized thermodynamics to quantify metal/organic adsorption in order to yield predictive models of contaminant transport. However, these models of contaminant transport may not be representative of the reactions which control contaminant mobilities as most mineral surfaces are coated with organic acids, bacteria, and extracellular polymers. Numerous laboratory studies have demonstrated that bacterial cell walls have a high affinity for binding metal cations, and field studies indicate that a significant proportion of bacteria cells and associated extracellular matrices are coated with small scale hydrous metal oxides. The small size of bacteria, and in many cases the nanoscale of their associated mineral phases, suggests these bacteria-mineral composites may represent a large proportion of surface area exposed to fluid flow. Therefore, due to the affinity of bacterial cell walls for cations and biominerals, bacteria may also have a significant impact on anionic contaminant mobility in many natural systems. The extent of metal-bacteria adsorption reactions varies drastically as a function of pH and solution chemistry. Current adsorption models have focused on the interactions of positively charged metal cations with bacterial surfaces, however in many oxidizing environments metals such as Cr exist as anions or anionic complexes. We have studied the ability of non-metabolizing cells of the bacterial species Bacillus subtilis and Shewanella putrifaciens to adsorb aqueous Cr(VI) and I^- in the presence of background electrolyte and aqueous Al (III), Cd(II), Ca(II) or cells coated with Al (oxy)hydroxide phases. We use a unique blend of XRD, electrophoretic mobility, SEM, and aqueous geochemistry measurements to quantify the mechanisms of Cr(VI) and I^- removal from solution. Our results indicate the removal of both anions is highly dependent on solution pH with significant removal at low pH and diminishing removal at higher pH values, without the presence of cations or precipitates. However, in the presence of aqueous Cd(II) and Ca(II) which adsorbs strongly from pH 3.5-8, the removal of Cr(VI) and I^- increases appreciably. Furthermore the loading of the cell surface with small amorphous mineral phases increases adsorption. Aging of the mineral-bacteria composites appears to decrease removal efficiency due to coarsening of the mineral phases and a decrease in charge density. Considering that many geologic environments include both cationic and anionic metal contaminants, our results suggest that mass transport of Cr(VI), I^-, and other anions may be affected by ternary complexation or other cation mediated surface reactions in bacteria-bearing systems.