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

Abstract

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.