Surface Sorption Site and Complexation Structure of Ca2+ at the Goethite–Water Interface: A Molecular Dynamics Simulation and Quantitative XANES Analysis
The molecular-level surface complexation structure of Ca2+ at the goethite/water interface remains unclear. We investigated the sorption of Ca2+ on the surfaces of goethite using classical molecular dynamics (MD) simulations and Ca K-edge X-ray absorption near edge structure (XANES) spectroscopy. The XANES results showed that Ca2+ was sequestered by goethite dominantly via sorption at pH ≤ 9, whereas the Ca(OH)2 surface precipitate formed at pH 10 for an initial Ca2+ concentration of 2 mM. The MD simulations showed that Ca2+ dominantly absorbed on the (100) and (110) surfaces of goethite via bidentate binuclear complexation by forming ≡(Fe-OH)2Ca2+·5H2O species, whereas little Ca2+ adsorbed on the (021) surface. The theoretical Ca K-edge XANES spectrum calculations gave a mean Ca-O interatomic distance of 2.34 Å (2.23–2.55 Å) and a Ca-Fe interatomic distance of 3.80 Å (3.79–3.81 Å) at goethite/water interface. Our results may shed some light on the geochemical cycling of calcium and other related cations and anions.
KeywordsCa sorption Molecular dynamics XANES analysis Local structure
We gratefully thank the financial support from the National Key R&D Pro-gram of China (2017YFD0801505), National Natural Science Foundation of China (41530643, 41877393, and 41673130), Hundred-Talent Program (Y8SBR121SS) and the Strategic Priority Research Program of the Chinese Academy of Sciences (No. QYZDJ-SSW-DQC038).