Sorption of Strontium by Bacteria, Fe(III) Oxide, and Bacteria−Fe(III) Oxide Composites

This study provides the first quantitative comparison of the sorptive capacities of a bacteria−Fe oxide composite to its individual components. These results have enormous significance for understanding the fate and transport of inorganic contaminants in natural aqueous environments where heterogeneous bacteria−oxide composite solids are commonly found. We quantify Sr2+ sorption to the bacteria Shewanella alga, Shewanella putrefaciens, amorphous hydrous ferric oxide (HFO), and S. alga coated with HFO over a range of total Sr2+ concentrations (0.005−10 mM) and pH (2.5−11 at 0.5 pH increments), under well controlled laboratory conditions. Significant Sr2+ sorption occurred at significantly lower pH values to the bacteria and S. alga−HFO composite (5.5−5.9) compared to HFO (7.6). Geochemical modeling using a generalized Langmuir equation showed that the bacteria sorb significantly greater quantities of Sr2+ (maximum sorptive capacity: BSrmax = 0.079 and 0.075 mmol·g-1 for S. alga and S. putrefaciens, respectively) than the HFO (0.001 mmol·g-1). The observed BSrmax for the S. alga−HFO composite (0.034 mmol·g-1) was less than the combined sorptive properties of its components (BSrmax = 0.041 mmol·g-1), likely reflecting HFO masking of bacterial surface binding sites.