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

Abstract

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 (∆60/58Niminaq = 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. Highlights ►Manganese oxides are important nickel sink in oceans. ► Nickel exists in two distinct chemical bonding environments with manganese oxide birnessite. ► Both nickel surface bonding and nickel birnessite mineral incorporation forms exist. ► Nickel isotopic fractionation between aqueous phase and solid birnessite occurs. ► Future potential for nickel as geochemical tracer of past ocean conditions.

Towards balancing the oceanic Ni budget

“…5A). Sorensen et al (2020) suggest three possible explanations for this observation. The first involves differences in the mechanism of incorporation into the solid structure.…”

“…In particular, phyllomanganates like birnessite, which has a layered structure of edge-sharing MnO octahedra, are ubiquitous in the natural environment and are the main Mn-bearing and trace metal-sorbing phases in oxic marine sediments (e.g., Peacock and Sherman, 2007a;Little et al, 2014). Experimental estimates of the magnitude of Ni isotope fractionation on sorption to birnessite suggest a large light isotope effect, with a recent study reporting Δ 60 Ni MnO2-aqueous = -2.8 to -3.4‰ (Sorensen et al, 2020).…”

Bioactive Trace Metals and Their Isotopes as Paleoproductivity Proxies: An Assessment Using GEOTRACES‐Era Data

“…22; Gall et al, 2013;Gueguen et al, 2016), which are, on average, slightly isotopically heavier (at δ 60 Ni of +1.6‰) than seawater (δ 60 Ni at about +1.3‰). However, experiments suggest that sorption of Ni to birnessite (the primary Ni-hosting phase in Fe-Mn sediments) should be associated with a large negative isotope effect (of about 3 to 4 ‰; Wasylenki et al, 2014;Sorensen et al, 2020). It remains unclear why the full isotope effect is not expressed in Fe-Mn crusts.…”

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

“…61,88 The TCS-INC chemical reaction has attracted large scientific attention in the last decade. [88][89][90][91] Ni is totally incorporated into the octahedral sheets of phyllomanganates in ferromanganese precipitates and nodules from soils and marine environments. 6,9,10,31,92 The TCS coordination is considered to be the high energy surface site for Ni, and the main mechanism for Ni enrichment up to percent levels by mass in natural layer-type Mn(IV) oxide minerals is thought to follow the TCS-INC pathway.…”