Experimental studies of bacteria-iodide adsorption interactions

Experiments were conducted to probe the reactivity of positively charged sites on the cell wall of the Gram-positive bacterium Bacillus subtilis through a combination of electrophoretic mobility measurements and anion adsorption experiments. Iodide adsorption onto the Gram-positive soil bacterium B. subtilis was measured as a function of pH, ionic strength, solid/solute ratio, and time. The experimental I- adsorption data were fitted using a linear programming model (LPM) and Langmuir-type sorption isotherm model. The LPM found a single site for iodide sorption onto the B. subtilis surface, with a concentration of 3.54+/-3.80 mumol/g bacteria. This site was interpreted to be a positively charged ligand located on the bacterial cell wall, where the -log(10) of the iodide dissociation constant K-s (pK(s)) was equal to 1.53+/-0.08. Electrophoretic mobility measurements, conducted as a function of pH and background electrolyte concentration, support the presence of positively charged surface reactive sites at low pH under the experimental conditions of this study. The presence of positively charged functional groups on the bacterial surface could potentially influence the mobility of anionic contaminants and further emphasizes the importance of bacteria as geochemically reactive solids. The use of an LPM in this study has further refined the current aspects of bacterial surface complexation modeling approaches. Anion stability constants may be incorporated in sorption models in order to accurately predict the transport, bioavailability, and exposure risk of radioiodine in pristine and contaminated environments. (C) 2004 Elsevier B.V. All rights reserved.