RES³T - Rossendorf Expert System for Surface and Sorption Thermodynamics

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 (BSr_max) 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 (K^S_Sr) 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 BSr_max and K^S_Sr 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.