Iron adsorption by Bacillus subtilis bacterial cell walls

Adsorption of Fe to bacterial surfaces is known to occur and is a possible precursor to biomineralization of Fe-oxides. However, the extent of adsorption and the resulting effect on saturation conditions in the presence of bacteria have not been adequately explored. We conduct four types of experiments: (1) kinetic precipitation-type experiments with and without bacteria at starting Fe concentrations of 2.5 and 5.0 ppm at a constant of pH of 3; (2) batch adsorption experiments as a function of bacteria concentration and at a total Fe concentration of 2.5 ppm; (3) batch adsorption/desorption experiments at low pH (<4) with 1 ppm Fe and 2 g/L bacteria; (4) abiotic precipitation experiments at low pH (<4) and 1 ppm total Fe. sults of the experiments conducted in oversaturated conditions demonstrate that bacteria significantly enhance the rate and extent of Fe removal from solution relative to the bacteria-free controls. We use the results of the separate batch adsorption experiments to determine stability constants for Fe-bacteria surface complexes assuming zero precipitation at these low Fe, low pH conditions. The observed increase in Fe removed in the presence of bacteria in ‘oversaturated’ conditions can be accounted for by adsorption only, using a surface model based on the reaction:Fe3++R>L1−↔R>L1Fe2+ log K=6.1±0.4where R>L1− represents the deprotonated form of the bacterial surface functional group with a pKa value of 3.3. The stability of the Fe–bacteria sorption reaction is orders of magnitude stronger than that observed for the other metal–bacteria systems, emphasizing the importance of Fe(III) adsorption in bacteria-bearing systems. Some Fe removal by the bacteria was irreversible on the time scale of our experiments, indicating that, although adsorption has the potential to account for the observed Fe removal, the process is likely more complicated than simple metal adsorption onto bacterial cell wall functional groups.