Sorption of Ni by birnessite: Equilibrium controls on Ni in seawater

Synthetic hexagonal birnessite (Hx-birnessite) is a close analogue to natural poorly crystalline phyllomanganate phases found in soils and marine ferromanganese deposits. These phases are often highly enriched in trace metals such as Ni and Co. We measured the sorption of Ni(II) onto synthetic hexagonal birnessite (Hx-birnessite) from pH 1 to 7. EXAFS spectra show that, at pH 3.7, Ni is adsorbed to the Hx-birnessite surface above vacancy sites on {001} as a tridentate corner-sharing complex. We developed a surface complexation model for Ni adsorption based on the equilibria3(≡Mn2O− 2/3) + Ni+ 2 = (≡Mn2O)3Ni03(≡Mn2O− 2/3) + Ni+ 2 + H2O = (≡Mn2O)3Ni(OH)− + H+ this surface complexation model, we predict the concentration of Ni in seawater in equilibrium with Ni-bearing birnessite found in hydrogenetic FeMn crusts and nodules. Our predicted results are in good agreement with observed Ni concentrations in seawater and suggest that the concentration of dissolved Ni in seawater is buffered by sorption to birnessite or a related MnO2 phase. However, in addition to the surface complex, Ni also sorbs by structural incorporation into the vacancy site. In our synthetic samples at pH 7, EXAFS shows ∼ 10% of Ni is structurally incorporated into Hx-birnessite. In natural birnessites found in marine ferromanganese crusts and nodules, EXAFS shows that all of the sorbed Ni is structurally incorporated. Structural incorporation suggests that Ni sorption may be irreversible.