LAUSR

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.