Nickel (Ni) is an essential micronutrient for phytoplankton. Its importance to both the modern and ancient Earth system has encouraged development of Ni and its isotopes as biogeochemical tracers. To interpret these signatures however, understanding of how Ni and its isotopes are recorded in marine archives is required. Here we simulate different inorganic and organic Ni species in seawater and investigate their adsorption behaviours to variably crystalline phyllomanganates and organo-mineral phyllomanganate. We conduct pH adsorption edge experiments to determine the binding affinity of the different Ni species to the minerals and then perform desorption experiments to operationally define Ni bonding strength. We also use thermodynamic surface complexation modelling to constrain Ni adsorption mechanisms. From the adsorption edges and stability constants generated from our modelling, the binding affinity increases in the order of Ni-formate (aq) < NiCl (aq) < Ni (aq). From the desorption experiments, desorption at pH 8 is non quantitative and increases in the opposite order of Ni-formate (aq) > NiCl (aq) ∼ Ni (aq). For the organo-mineral however, Ni desorption at pH 8 is non quantitative and similar for all three experiments, and is significantly higher compared to the variably crystalline phyllomanganates. Although both our adsorption and desorption experiments were performed over 48 h, it is possible that desorption is somewhat slower than adsorption such that a longer desorption period may result in further Ni loss to solution and thus greater adsorption reversibility. Taken together however, the Ni-formate (aq) and Ni organo-birnessite desorption experiments suggest that Ni bonding strength is decreased by the presence of organic carbon, compared to NiCl (aq) and Ni (aq). Because bonding strength governs equilibrium stable isotope fractionation, we use our experimental findings to suggest how Ni speciation in seawater might influence Ni isotope behaviour during adsorption to phyllomanganate. We find that our suggestions are consistent with isotopic measurements from natural sediments. Although the balance of Ni adsorption versus incorporation during uptake to phyllomanganates may play a greater part in explaining the variation in the Ni isotope composition in Mn-rich sediments, Ni speciation and the presence of organics might increase the range of δNi values measured in natural settings.

中文翻译：

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Ni对水钠锰矿和有机水钠锰矿吸附的形态控制

镍（Ni）是浮游植物必需的微量营养素。它对现代和古代地球系统的重要性促进了镍及其同位素作为生物地球化学示踪剂的发展。然而，为了解释这些特征，需要了解镍及其同位素如何记录在海洋档案中。在这里，我们模拟了海水中不同的无机和有机镍物种，并研究了它们对不同结晶层状锰酸盐和有机矿物层状锰酸盐的吸附行为。我们进行 pH 值吸附边缘实验，以确定不同镍物质与矿物质的结合亲和力，然后进行解吸实验，以确定镍结合强度。我们还使用热力学表面络合模型来约束镍吸附机制。根据我们的建模生成的吸附边缘和稳定性常数，结合亲和力按 Ni-甲酸 (aq) < NiCl (aq) < Ni (aq) 的顺序增加。从解吸实验来看，pH 8 时的解吸是非定量的，并且以 Ni-formate (aq) > NiCl (aq) ∼ Ni (aq) 的相反顺序增加。然而，对于有机矿物，pH 8 下的 Ni 解吸是非定量的，并且对于所有三个实验而言都是相似的，并且与可变结晶叶锰酸盐相比显着更高。尽管我们的吸附和解吸实验均进行了 48 小时以上，但解吸可能比吸附稍慢，因此较长的解吸时间可能会导致 Ni 进一步损失到溶液中，从而导致更大的吸附可逆性。然而，综合起来，Ni-甲酸盐（aq）和Ni有机水钠锰矿解吸实验表明，与NiCl（aq）和Ni（aq）相比，Ni键合强度因有机碳的存在而降低。由于键合强度控制平衡稳定同位素分馏，因此我们利用实验结果来表明海水中镍的形态如何影响页锰酸盐吸附过程中镍同位素的行为。我们发现我们的建议与天然沉积物的同位素测量结果一致。虽然页锰酸盐吸收过程中镍吸附与掺入的平衡可能在解释富锰沉积物中镍同位素组成的变化方面发挥更大作用，但镍形态和有机物的存在可能会增加自然环境中测量的 δNi 值的范围。