Chapter 14 - Surface and Solution Speciation of AgI in a Heterogeneous Ferrihydrite-Solution System with Thiosulfate

Abstract

The presence of thiosulfate modifies the adsorptive characteristics of AgI on ferrihydrite through the formation of a ternary surface complex that adsorbs in a ligand-like manner, directly overmapping the adsorption edge profile of thiosulfate. In this study, experimental data were modeled using the Triple-Layer Surface Complexation Model implemented in the HYDRAQL code. Modeling included the evaluation of several alternative reaction stoichiometries for both inner-sphere and outer-sphere surface complexes, taken singly and two at a time. Modeling results suggest that AgS2O3- is bound as an inner-sphere ternary complex at low pH, overlapping the monodentate AgI outer-sphere surface complex that increases in a metal-like manner with increasing pH. The study proposes a mechanism for the formation of the ternary inner-sphere surface complex based on a condensation reaction leading to the transition from an outer-sphere to an inner-sphere ternary surface species. The proposed mechanism is initiated by the approach of the AgS2O3- species toward the protolyzed surface moiety and the formation of hydrogen bonds, which is enhanced by charge stabilization of the S2O3 molecule by Ag. Coordination of an O atom and subsequent bond formation facilitates H2O as a leaving group completing the dehydration reaction.