Surface complexation of Cu on birnessite (δ-MnO2): Controls on Cu in the deep ocean

Abstract

Hexagonal birnessite (d-MnO 2 ) is a close analogue to the dominant phase in hydrogenetic marine ferromanganese crusts and nodules. These deposits contain $0.25 wt.% Cu which is believed to be scavenged from the overlying water column where Cu concentrations are near 0.1 lg/L. Here, we measured the sorption of Cu on d-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 {0 0 1} surface at sites above Mn vacancies to give a three to fourfold coordinated complex with 6 Mn neighbors at $3.4 Å . 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 Å . Density functional calculations on CuMn 18 O 24 ðOHÞ 30 ðH 2 OÞ 3 À4 and CuMn 18 O 21 ðOHÞ 33 ðH 2 OÞ 3 À1 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.

Copper sorption by the edge surfaces of synthetic birnessite nanoparticles

“…The adsorption of divalent metals at vacancy sites involves the coordination of the metal to the three surface oxygen atoms surrounding the vacancy site, thus forming a triple-corner-sharing (TCS) complex (Manceau et al, 2002(Manceau et al, , 2007b. In birnessite suspensions equilibrated with Cu at low pH values (pH ≈ 4.0), Cu-TCS complexes were inferred from the Cu-Mn interatomic distance of 3.39-3.43 Å obtained from fitting extended X-ray absorption fine structure (EXAFS) spectra (Manceau et al, 2002;Sherman and Peacock, 2010;Arai, 2011). The occurrence of up to 20% Cu-incorporation (Cu-INC) at high pH values (pH ≈ 8.0) was proposed by Sherman and Peacock (2010) based on the appearance of a peak near 2.9 Å in the Fourier-transformed EXAFS spectrum for δ-MnO 2 sample with a loading of 0.068 mol Cu mol −1 Mn.…”

“…In birnessite suspensions equilibrated with Cu at low pH values (pH ≈ 4.0), Cu-TCS complexes were inferred from the Cu-Mn interatomic distance of 3.39-3.43 Å obtained from fitting extended X-ray absorption fine structure (EXAFS) spectra (Manceau et al, 2002;Sherman and Peacock, 2010;Arai, 2011). The occurrence of up to 20% Cu-incorporation (Cu-INC) at high pH values (pH ≈ 8.0) was proposed by Sherman and Peacock (2010) based on the appearance of a peak near 2.9 Å in the Fourier-transformed EXAFS spectrum for δ-MnO 2 sample with a loading of 0.068 mol Cu mol −1 Mn. However, in a recent density functional theory (DFT) study, Kwon et al (2013) demonstrated that the incorporation of Cu into a vacancy site strongly inhibits the JahnTeller distortion of Cu, destabilizing the Cu-INC species relative to the Cu-TCS species at any pH value.…”

“…Its high specific surface area and low vacancy content thus make it an optimal choice for studying edge adsorption experimentally. The study of Cu adsorption at birnessite particle edges is particularly relevant in light of the disagreement between experimental and computational findings regarding the incorporation of Cu into MnO 2 (Sherman and Peacock, 2010;Kwon et al, 2013). Moreover, the inability to explain the observed maximum surface excess of Cu in terms of the nominal vacancy content of the birnessite used (Sherman and Peacock, 2010) implies additional sorption mechanisms.…”

“…The study of Cu adsorption at birnessite particle edges is particularly relevant in light of the disagreement between experimental and computational findings regarding the incorporation of Cu into MnO 2 (Sherman and Peacock, 2010;Kwon et al, 2013). Moreover, the inability to explain the observed maximum surface excess of Cu in terms of the nominal vacancy content of the birnessite used (Sherman and Peacock, 2010) implies additional sorption mechanisms. Finally, Cu speciation on birnessite is not well-documented; only a few samples have been characterized to date, three of which were prepared at pH 4 and one at pH 8 (Manceau et al, 2002;Sherman and Peacock, 2010;Arai, 2011).…”

Palaeoceanography: motivations and challenges for the future

“…A missing link in contemporary palaeoceanography may be the modelling of molecular and particle-scale interactions of isotopic tracers [97]. There are fundamental gaps in our theoretical and modelling understanding of the underlying thermodynamics that control the broad biogeochemical processes used in the palaeoceanographic community.…”