Strontium Adsorption on Manganese Oxide (δ-MnO2) at Elevated Temperatures: Experiment and Modeling

Strontium adsorption was studied by acid-base potentiometric titration at various pH, ionic strength, the sorbate/sorbent ratio, and temperatures (at 25, 50, and 75 degrees C). The experimental data were interpreted using two models of surface complexation with two different electrostatic models of the interface: the constant capacitance (CCM) and triple-layer (TLM) models. Although both models are able to take into account acid-base reactions and surface complexation of Sr on birnessite, we believe that TLM is more suitable for the description of the H+->MnOH-Sr2+ heterogeneous system. At a low ionic strength and negatively charged surface, Sr2+ ions compete with electrolyte ions and form both inner- and outer-sphere complexes. Although the application of CCM in describing Sr adsorption may be mathematically satisfactory, it has little physical sense. We suggest a model that involves both inner-sphere (>MnOHSr2+, >MnOSr+, >MnOSrOH0) and outer-sphere ([>MnO-Sr2+](+)) surface complexes. The corresponding constants of formation of these surface complexes were calculated at 25, 50, and 75 degrees C.