Nature of High- and Low-Affinity Metal Surface Sites on Birnessite Nanosheets

Birnessite
nanosheets (δ-MnO2) are key reactive
nanoparticles that regulate metal cycling in terrestrial and marine
settings, yet there is no molecular explanation for the sorption selectivity
of metals which controls their enrichment. This fundamental question
was addressed by optimizing the structure of Ni, Cu, Zn, and Pb surface
complexes on δ-MnO2 and by calculating the Gibbs
free-energy change (ΔG) of the sorption reactions
with density functional theory. The sorption selectivity follows the
order Pb > Cu > Ni > Zn in good agreement with experimental
data.
Cu, Ni, and Zn bind preferentially to layer edges at low surface coverage
forming double-edge-sharing (DES) complexes, whereas Pb binds extensively
with high selectivity over the three transition metals to both layer
edges (DES bonding) and to basal planes forming triple-corner-sharing
(TCS) complexes. Pb has a similar affinity for the DES and TCS sites
at pH 5 and a higher affinity for the TCS sites at circumneutral pH.
The Pb DES and TCS complexes are both dehydrated at the δ-MnO2–water interface and feature a trigonal pyramidal geometry
with three surface O atoms. The high stability of the two new Pb complexes
arises from the hybridization between the Pb 6s/6p and O 2p states,
forming a strong covalent Pb–O/OH bond at the δ-MnO2 surface. The quantum chemical results provide a mechanistic
and energetics insight into the metal uptake on δ-MnO2 that extends what extended X-ray absorption fine structure (EXAFS)
spectroscopy alone can provide.