| Experiments were conducted to probe for the existence of positive sites on the cell wall of the Gram-positive bacterium Bacillus subtilis through a combination of electrophoretic mobility measurements and anion adsorption experiments with iodide. Iodide adsorption onto Bacillus subtilis was measured as a function of pH, ionic strength, solid:solute ratio, and time. The experimental data were interpreted using a surface complexation approach. The I− adsorption data were best fit with a single surface site reaction, with the iodide ion forming a surface complex with the positively-charged amino functional group located on the bacterial cell wall: R-NH3+ + I− ↔ R-NH3-I− (log K = 8.9 ± 0.2). Electrophoretic mobility measurements, conducted as a function of pH and electrolyte ionic strength, support the presence of positively-charged surface functionalities at low pH under experimental conditions. Amino-anion stability constants may be incorporated into surface complexation models in order to accurately predict the bioavailability and exposure risk of radioiodine in the environment.; Additional anion adsorption experiments in metal-anion-bacteria ternary systems were conducted to determine the effect of multi-valent cations on the electrokinetic properties of the cell wall and its ability to adsorb anions. Adsorption results demonstrated that both anions and (oxy)anions have a pH-dependent affinity for the cell wall of B. subtilis below pH 5. Increased adsorption of anions at higher pH values (>pH 4) was observed in the presence of 1 mM Ca. In contrast, no significant little adsorption was observed at similar pH values in the presence of both 0.1 mM and 1 mM La. The presence of positively-charged ions at the bacteria-solute interface can influence the mobility of anionic contaminants and further refines the surface complex approach. |
