Zn2+, Co2+, and Ni2+ Adsorption at the Rutile-Water Interface to 250 degrees C

Potentiometric base titrations in stirred hydrogen electrode concentration cells (HECCs) were used to monitor Zn2+, Co2+, and Ni2+ adsorption on rutile surfaces from 25 to 250 ? in sodium triflate (NaTr) media. Adsorption, as monitored by both H+ release and adsorbed amounts, increased with an increasing temperature. These bulk adsorption data were augmented by room-temperature X-ray standing wave (XSW), extended X-ray absorption fine structure (EXAFS), and density functional theory (DFT) results, which found inner-sphere mono- and bidentate adsorption complexes, with the dominant monodentate complex being hydrolyzed. From these collective results, a basic Stern charge distribution multi-site (CD-MUSIC) surface complexation model (SCM) was formulated to rationalize the data. Mono- and bidentate adsorption constants increased systematically with the temperature, mirroring the data themselves. Moreover, the distribution between the mono- and bidentate adsorption complexes for Zn2+ at 25 ?, as determined by XSW experiments, was closely matched by SCM-predicted ratios. Finally, the monodentate adsorption constants up temperature were fit to estimate adsorption enthalpies, entropies, and heat capacities, and those estimates were compared to the corresponding first hydrolysis constants of the metal cations in aqueous solution. Enthalpies of the surface and solution hydrolysis reactions were endothermic and similar, but entropies were not. Adsorption entropies were positive, while the corresponding first hydrolysis entropies were much less positive or negative, indicating that adsorption is accompanied and driven by a net disruption of surface and cation hydration waters during adsorption. Correlations between the monodentate adsorption and first hydrolysis constants were highly linear and indicate that hydrolysis is enhanced at the surface by about 5 log K units.