Zinc surface complexes on birnessite: A density functional theory study

Abstract

Biogeochemical cycling of zinc is strongly influenced by sorption on birnessite minerals (layertype MnO 2 ), which are found in diverse terrestrial and aquatic environments. Zinc has been observed to form both tetrahedral (Zn IV ) and octahedral (Zn VI ) 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 (ZnMn 3 O 7 ·3H 2 O), 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 Zn IV -TCS and Zn VI -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 Zn IV -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 Zn VI -TCS. Comparison between geometry-optimized ZnMn 3 O 7 ·3H 2 O (chalcophanite) and the hypothetical monohydrate mineral, ZnMn 3 O 7 ·H 2 O, 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

Surface complexation of Pb(II) by hexagonal birnessite nanoparticles

“…Overall, our geometry-optimized fully-relaxed magnetic quenselite structures compared favorably with experimental results, although they have larger a and c lattice parameters than observed experimentally, a typical trend in DFT/GGA studies of layer type Mn oxides, such as LiMnO 2 and ZnMn 3 O 7 ·3H 2 O (Mishra and Ceder, 1999;Kwon et al, 2009). Full relaxation led to interatomic distances too large on average by 2 to 3 %, whereas constrained geometry optimization of the AFM structure, with lattice parameters and angles fixed at their experimental values while the internal coordinates of all ions were relaxed, showed excellent agreement with experiment, with interatomic distances that differed from experiment on average by << 1 % (fifth and sixth columns in Table 1).…”

“…Our geometry optimizations were performed with the CASTEP code (Clark et al, 2005), which implements DFT in a plane-wave basis set to represent wavefunctions and uses ultrasoft pseudopotentials (Vanderbilt, 1990) Perdew et al (1996)] with spin polarization and periodic boundary conditions, as discussed in detail by Kwon et al (2009), which should be consulted for additional information about our DFT methodology as applied to hexagonal birnessite.…”

“…Our principal objective is to examine whether Pb-DES is the only stable surface complex that can occur on the lateral edges of these nanoparticles. First-principles techniques such as DFT can be applied to reduce ambiguities and guide the interpretation of EXAFS spectra by providing complementary electronic structure and bonding information about metals adsorbed on mineral surfaces (Zhang et al, 2006;Sherman et al, 2008;Hattori et al, 2009;Mason et al, 2009;Kwon et al, 2009). In particular, spinpolarized DFT (i.e., electron densities are calculated separately for spin-up and spin-down electrons) accurately predicts crystal structures and electronic properties of Mn oxides (Singh, 1997;Mishra and Ceder, 1999;Pask et al, 2001), including birnessite (Kwon et al, 2009).…”

Zn sorption modifies dynamically the layer and interlayer structure of vernadite

“…1) showed that Zn coordination varies with surface loading: Zn is dominantly tetrahedral at low Zn/Mn atomic ratios ( IV Zn/ VI Zn % 2 for Zn/Mn % 0.008), and octahedral at higher Zn loading (Drits et al, 2002;Manceau et al, 2002a;Lanson et al, 2002b). Recent quantum mechanical calculations suggested that VI Zn is stabilized by H-bonding between neighboring H 2 O molecules located near the interlayer mid-plane and belonging to the coordination spheres of two Zn 2+ cations each adsorbed on one side of the interlayer space (Kwon et al, 2009b). If this is the case, the IV Zn/ VI Zn ratio should vary not only with the surface coverage, but also with the layer stacking order because disruption of the layer periodicity should hinder the formation of H-bonds.…”

Interaction of Natural Organic Matter with Layered Minerals: Recent Developments in Computational Methods at the Nanoscale

“…Only one paper on classical MD simulations of birnessite has appeared, but the reported interlayer structure agreed with diffraction experiments [132]. Quantum methods have been used to investigate the effect of vacancy disordering on structural and energetic properties [133,134] as well as cation adsorption [133,[135][136][137].…”