The effect of chloride on the adsorption of Hg onto three bacterial species

Bulk adsorption and X-ray absorption spectroscopy experiments were conducted in order to investigate the ability of three bacterial species to adsorb Hg in the absence and presence of chloride from pH2 to 10. Adsorption experiments were performed using non-metabolizing cells of Bacillus subtilis, Shewanella oneidensis MR-1, and Geobacter sulfurreducens suspended in a 0.1M NaClO₄ electrolyte to buffer ionic strength. After equilibration, the aqueous phases were sampled and analyzed using inductively coupled plasma-optical emission spectroscopy (ICP-OES) for remaining Hg concentrations. The biomass from some experiments was analyzed using Hg X-ray absorption spectroscopy (XAS) to determine the binding environment of the Hg.In both chloride-free and chloride-bearing systems, the three bacterial species studied exhibited similar adsorption behaviors. Chloride causes a dramatic shift in the adsorption behavior of each of the bacterial species. In the absence of chloride, each of the species exhibits maximum adsorption between pH4 and 6, with decreasing but still significant adsorption with increasing pH from 6 to approximately 10. The extent of Hg adsorption in the chloride-free systems is extensive under all of the experimental conditions, and the concentration of adsorbed Hg exceeds the concentration of any individual binding site type on the cell envelope, indicating that binding onto multiple types of sites occurs even at the lowest pH conditions studied. Because binding onto an individual site type does not occur exclusively under any of the experimental conditions, individual stability constants for Hg-bacterial surface complexes cannot be determined in the chloride-free system. In the presence of chloride, all of the bacterial species exhibit minimal Hg adsorption below pH4, increasing adsorption between pH4 and 8, and slightly decreasing extents of adsorption with increasing pH above 8. The low extent of adsorption at low pH suggests that neutrally-charged HgCl₂⁰ adsorbs only weakly. The increase in Hg adsorption above pH4 is likely due to adsorption of Hg²⁺ and HgCl(OH)⁰, and is limited by site availability and transformation to Hg(OH)₂⁰ as pH increases. We use the adsorption data to determine stability constants of the Hg-, HgCl(OH)-, and Hg(OH)₂-bacterial cell envelope complexes, and the values enable estimations to be made for Hg adsorption behavior in bacteria-bearing geologic systems.