Zn Surface Complexes on Birnessite. A Density Functional Theory Study

Birnessite (layer-type MnO₂), produced mainly by bacteria and fungi, is a ubiquitous environmental nanosized mineral participating in important geochemical processes, particularly trace metal scavenging. Among the trace metals of major interest, Zn appears to be influenced strongly in its biogeochemical cycling by sorption on birnessite minerals found in soils, aquifers, streams, and wetlands. Synchrotron based X-ray studies of sorbed Zn²⁺ revealed that Zn forms both tetrahedral (Zn^IV) and octahedral (Zn^VI) triple-corner-sharing surface complexes (TCS) with the three surface O at Mn(IV) vacancy sites in birnessite. The octahedral complex is expected on the basis of the local coordination environment of Zn in the mineral, chalcophanite (ZnMn₃O₇•3H₂O), but the reason for the occurrence of the four-coordinate Zn surface species remains unclear. We address this issue using spin-polarized density functional theory (DFT).

Structural parameters of the Zn^IV-TCS and Zn^VI-TCS species with single or double occupancy at a Mn vacancy (i.e., Zn^IV-TCS; Zn^VI-TCS; Zn^IV-TCS + Zn^IV-TCS; Zn^IV-TCS + Zn^VI-TCS; or Zn^IV-TCS + Zn^VI-TCS) obtained by DFT geometry optimization were in excellent agreement with available experimental data on Zn-birnessite. Their total energies, magnetic moments, and electron-overlap populations showed that tetrahedral Zn^IV-TCS is stable in birnessite without a need for Mn(III) substitution in the Mn octahedral sheet and that it is more effective at reducing vacancy O undersaturation than is octahedral Zn^VI-TCS. Our DFT study also revealed details of structural distortion of a Mn vacancy site: Mn nearest to Zn move toward a vacancy by 0.08 – 0.11 Å, while surface O bordering a vacancy move away from it by 0.16 – 0.21 Å, relative to their positions in an ideal vacancy-free MnO2. The structures of the Zn-TCS species in birnessite determined in the present study can serve as unique end-member species of Zn-TCS to facilitate the detailed interpretation of spectroscopic data on Zn sorbed by birnessite.