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

Abstract

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°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°C.