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 (∆60/58Nimin-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.

中文翻译：

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与六方水钠锰矿的表面络合反应引起的大镍同位素分馏

摘要 锰氧化物是海洋中镍的重要汇。为了探索 Ni 稳定同位素作为地球化学示踪剂的潜力，我们在 0.05 M NaNO3 介质中进行了 Ni 和六方水钠锰矿之间的两种吸附反应：一种是我们将 pH 从 5 改变到 8（恒定的初始 Ni 浓度 = 170 μmol/L )，然后我们将初始溶解的 Ni 浓度从 17 更改为 426 μmol/L（恒定 pH = 7.7）。对固相和上清液进行同位素测量，以确定矿物相和水相之间的 Ni 同位素分馏因子 (Δ60/58Nimin-aq = δ60/58Nimin - δ60/58Niaq)。镍扩展 X 射线吸收精细结构 (EXAFS) 光谱显示 Ni 在两种不同的键合环境中：一种是 Ni 原子结合到 MnO2 片中，另一种是 Ni 原子与矿物表面结合，与 3 Mn 四面体（TCS，三角共享）共享氧。随着 pH 值和表面净负电荷的增加，Ni 的配位转移到更高的掺入比例。水钠锰矿中结构空位的数量是 Ni 的 TCS 配位位置，受 pH 控制，并随着 pH 的降低而增加。这些空位优先被较轻的 Ni 同位素占据，导致分馏因子 ∆60/58Nimin-aq，范围从 -2.76‰（最低 TCS）到 -3.35‰（最大 TCS）。这些 Ni 同位素分馏因子是迄今为止在自然地质和生物材料中观察到的最大的因子之一。我们的研究结果揭示了 Ni 配位环境与 pH 值之间的关系，该关系最终可能用作过去海洋条件的同位素地球化学示踪剂。然而，该结果与当前海洋锰铁矿床相对于海水的同位素分馏因子不一致，并指出改变锰铁矿床镍同位素分馏的未解决过程。需要进一步研究以开发镍作为同位素示踪剂。