Influence of ionic strength on strontium sorption to bacteria, Fe(III) oxide, and composite bacteria-Fe(III) oxide surfaces

A series of experiments were conducted to investigate the influence of ionic strength on Sr2+ sorption by the bacteria Shewanella alga, hydrous ferric oxide (HFO) and bacteria-HFO composite solids. Sorption of Sr2+ to S. alga exhibited a strong dependence on ionic strength with the maximum concentration of solid phase Sr (BSrmax) decreasing from 79 μmol g−1 under dilute aqueous conditions to 4 μmol g−1 at high ionic strength (0.1M NaNO3). Corresponding apparent surface complex formation (KSSr) values for S. alga increased from 10−0.51 to 10−0.26 with increasing ionic strength, implying that only high affinity sites remain to bind Sr2+ under conditions of increased ionic strength. In contrast, Sr2+ sorption to HFO surfaces was independent of ionic strength with BSrmax and KSSr remaining relatively constant (approximately 1 μmol g−1 and 10−2.1, respectively) under increasing ionic strength conditions. The ionic strength dependent sorptive behaviour exhibited by S. alga is consistent with electrostatic outer-sphere complexation reactions occurring in the diffuse layer, whereas inner-sphere complexation reactions account for the Sr2+ sorption behaviour of HFO. The bacteria-HFO composite solid exhibited moderate ionic strength dependence with maximum binding capacities decreasing from 34 μmol g−1 (dilute conditions) to 24 μmol g−1 (0.1 M NaNO3). These results suggest that Fe3+ sorption and precipitation at the bacterial surface alters the electrochemical surface properties of the composite solid, buffering the effects of increased ionic strength on subsequent Sr2+ sorption.