Surface complexation of Cu on birnessite (δ-MnO_2): Controls on Cu in the deep ocean

摘要

Hexagonal birnessite (δ-MnO_2) is a close analogue to the dominant phase in hydrogenetic marine ferromanganese crusts and nodules. These deposits contain ~0.25wt.% Cu which is believed to be scavenged from the overlying water column where Cu concentrations are near 0.1μg/L. Here, we measured the sorption of Cu on δ-MnO_2 as a function of pH and surface loading. We characterized the nature of the Cu sorption complex at pH 4 and 8 using EXAFS spectroscopy and find that, at pH 4, Cu sorbs to birnessite by inner-sphere complexation on the {001} surface at sites above Mn vacancies to give a three to fourfold coordinated complex with 6 Mn neighbors at ~3.4?. At pH 8, however, we find that some Cu has become structurally incorporated into the MnO_2 layer by occupying the vacancy sites to give 6 Mn neighbors at ~2.91?. Density functional calculations on CuMn18O24(OH)30(H2O)3-4 and CuMn18O21(OH)33(H2O)3-1 clusters predict a threefold coordinated surface complex and show that the change from surface complexation to structural incorporation is a response to protonation of oxygens surrounding the vacancy site. Consequently, we propose that the transformation between sorption via surface complex and vacancy site occupancy should be reversible. By fitting the Cu sorption as a function of surface loading and pH to the formation of the observed and predicted surface complex, we developed a surface complexation model (in the basic Stern approximation) for the sorption of Cu onto birnessite. Using this model, we demonstrate that the concentration of inorganic Cu in the deep ocean should be several orders of magnitude lower than the observed total dissolved Cu. We propose that the observed total dissolved Cu concentration in the oceans reflects solubilization of Cu by microbially generated ligands.