Interfacial reactions of Sn(II) with anatase (TiO2): EXAFS and surface complexation modeling

Sn(II) is a contaminant of general environmental interest, but little is known about its interaction and retention by mineral surfaces. We have selected the redox-inactive mineral anatase (TiO2) to study the surface complexation mechanism by combining EXAFSderived surface structures with surface complexation modeling to model the macroscopic sorption behavior. Three adjustable parameters required for diffuse double layer model were determined in this study: the surface site density (16 sites/nm2), and two log K+ values (4.76 ± 0.36 and −9.90 ± 0.42) of the protolysis reactions. EXAFS spectra show Sn(II)–Ti distances at 3.26 and 3.60 Å indicating the formation of monodentate and bidentate inner-sphere sorption complexes. Four surface complexes, Anat_sOSn+, Anat_sSnOH,
Anat_sOH(Anat_sO)Sn+, and (Anat_sO)2Sn, with log K values of 3.66, 3.68, 4.63, and 4.41, respectively,
were required to model the sorption data.