Zinc surface complexes on birnessite: A density functional theory study

Biogeochemical cycling of zinc is strongly influenced by sorption on birnessite minerals (layer-type MnO2), which are found in diverse terrestrial and aquatic environments. Zinc has been observed to form both tetrahedral (ZnIV) and octahedral (ZnVI) triple-corner-sharing surface complexes (TCS) at Mn(IV) vacancy sites in hexagonal birnessite. The octahedral complex is expected to be similar to that of Zn in the Mn oxide mineral, chalcophanite (ZnMn3O7·3H2O), but the reason for the occurrence of the four-coordinate Zn surface species remains unclear. We address this issue computationally using spin-polarized density functional theory (DFT) to examine the ZnIV–TCS and ZnVI–TCS species. Structural parameters obtained by DFT geometry optimization were in excellent agreement with available experimental data on Zn-birnessites. Total energy, magnetic moments, and electron overlap populations obtained by DFT for isolated ZnIV–TCS revealed that this species is stable in birnessite without a need for Mn(III) substitution in the octahedral sheet and that it is more effective in reducing undersaturation of surface O at a Mn vacancy than is ZnVI–TCS. Comparison between geometry-optimized ZnMn3O7·3H2O (chalcophanite) and the hypothetical monohydrate mineral, ZnMn3O7·H2O, which contains only tetrahedral Zn, showed that the hydration state of Zn significantly affects birnessite structural stability. Finally, our study also revealed that, relative to their positions in an ideal vacancy-free MnO2, Mn nearest to Zn in a TCS surface complex move toward the vacancy by 0.08–0.11 Å, while surface O bordering the vacancy move away from it by 0.16–0.21 Å, in agreement with recent X-ray absorption spectroscopic analyses.