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

Birnessite nanosheets (?-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 structure Ni, Cu, Zn, Pb surface complexes on ?-MnO2 calculating Gibbs free-energy change (?G) reactions with density functional theory. The follows order > Cu Ni Zn good agreement experimental data. bind preferentially to layer edges at low coverage forming double-edge-sharing (DES) complexes, whereas binds extensively high over three transition both (DES bonding) basal planes triple-corner-sharing (TCS) complexes. has a similar affinity DES TCS sites pH 5 higher circumneutral pH. dehydrated ?-MnO2–water interface feature trigonal pyramidal geometry O atoms. stability two new arises from hybridization between 6s/6p 2p states, strong covalent Pb–O/OH bond surface. quantum chemical results provide mechanistic energetics insight into uptake extends what extended X-ray absorption fine (EXAFS) spectroscopy alone can provide.