Large nickel isotope fractionation caused by surface complexation reactions with hexagonal birnessite

Type Article
Date 2020-03
Language English
Author(s) Sorensen Jeffry V.1, Gueguen Bleuenn2, 3, 4, Stewart Brandy D.1, Peña Jasquelin5, Rouxel OlivierORCID4, Toner Brandy M.1
Affiliation(s) 1 : University of Minnesota-Twin Cities, MN 55108, USA
2 : CNRS, Univ Brest, UMR 6538 Laboratoire Géosciences Océan, F-29280 Plouzané, France
3 : CNRS, Univ Brest, UMS 3113, F-29280 Plouzané, France
4 : IFREMER, Centre de Brest, Unité Géosciences Marines, F-29280 Plouzané, France
5 : Institute of Earth Surface Dynamics, University of Lausanne, CH-1015 Lausanne, Switzerland
Source Chemical Geology (0009-2541) (Elsevier BV), 2020-03 , Vol. 537 , P. 119481 (13p.)
DOI 10.1016/j.chemgeo.2020.119481
WOS© Times Cited 21
Keyword(s) Ni stable isotope fractionation, Ni EXAFS, Surface complexation, Birnessite, Tracer development

Manganese oxides are an important sink for Ni in the ocean. To explore the potential of Ni stable isotopes as a geochemical tracer, we conducted two types of sorption reactions between Ni and hexagonal birnessite in 0.05 M NaNO3 media: one where we varied pH from 5 to 8 (constant initial Ni concentration = 170 μmol/L), and a second where we varied the initial dissolved Ni concentration from 17 to 426 μmol/L (constant pH = 7.7). Isotopic measurements were made on both the solid phase and the supernatant solutions to determine the Ni isotope fractionation factors (∆60/58Nimin-aq = δ60/58Nimin − δ60/58Niaq) between the mineral and aqueous phases. Nickel extended X-ray absorption fine structure (EXAFS) spectroscopy showed Ni in two distinct bonding environments: one where Ni atoms incorporate into the MnO2 sheet and a second where Ni atoms associate with the mineral surface sharing oxygens with 3 Mn tetrahedra (TCS, triple corner sharing). As pH and net negative surface charge increase, the coordination of Ni shifts to higher proportions of incorporation. The number of structural vacancies in birnessite, which are locations for TCS coordination of Ni, are controlled by pH and increase with decreasing pH. These vacancies are preferentially occupied by lighter Ni isotopes leading to fractionation factors, ∆60/58Nimin-aq, ranging from −2.76‰ (lowest TCS) to −3.35‰ (maximum TCS). These Ni isotopic fractionation factors are among the largest observed in natural geological and biological materials to date. Our findings reveal a relationship between Ni coordination environment and pH that may ultimately be used as an isotopic geochemical tracer of past ocean conditions. However, the results are inconsistent with current isotopic fractionation factors for marine ferromanganese deposits relative to seawater and point to unaddressed processes that modify Ni isotopic fractionation for ferromanganese deposits. Further research is needed to develop Ni as an isotopic tracer.

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