Abstract The geochemical behavior of nickel, an essential trace metal element, strongly depends on its interactions with Mn oxides. Interactions between the phyllomanganate birnessite and sorbed or structurally incorporated Ni have been extensively documented together with the fate of Ni along the transformation of these layered species to tunnel Mn oxides (tectomanganates). By contrast, interactions of phyllomanganates with weakly bound Ni species [hydrated Ni, Ni (hydr)oxides], that possibly prevail in natural Ni-rich (>10% NiO) manganates, have received little attention and the influence of these Ni species on the phyllomanganate-to-tectomanganate transformation remains essentially unknown. A set of phyllomanganate precursors with contrasting contents of Ni was thus prepared and subjected to a reflux treatment mimicking the natural phyllomanganate-to-tectomanganate conversion. Layered precursors and reflux products were characterized with a combination of diffractometric, spectroscopic, thermal, and chemical methods. Ni is essentially present as hydrated Ni(II) and Ni(II) (hydr)oxides in Ni-rich layered precursors whereas Ni(II) sorbed at particle edges prevail at low Ni content. No Ni sorbed at layer vacancy sites or structurally incorporated was detected in the initial vacancy-free layered precursors. Consistent with the high content (≈1/3) of Jahn-Teller distorted Mn(III) octahedra in layered precursors, which is favorable to their conversion to tectomanganates, Ni-free samples fully convert to an a-disordered todorokite, a common tectomanganate with a 3 × 3 tunnel structure. Contrastingly and despite similar high Mn(III) contents in Ni-rich precursors, hydrolysis of interlayer Ni2+ and polymerization of Ni(OH)2 in phyllomanganate interlayers is kinetically favored during reflux process. Asbolane, a phyllomanganate with an incomplete – island-like – octahedral layer of metal (hydr)oxides, is thus formed rather than todorokite. A nitric acid treatment, aiming at the dissolution of the island-like interlayer Ni(OH)2 layer, allows an easy and unambiguous differentiation between asbolane and todorokite, the latter being unaffected by the treatment. Both compounds exhibit indeed similar interplanar periodicities and can be confused when using X-ray diffraction, despite contrasting intensity ratios. Migration rate of Mn(III) out of the MnO2 layer relative to the metal hydrolysis and polymerization rate determines the formation of todorokite or asbolane. Here, Ni(OH)2 polymerization hampers the formation of tectomanganates and likely contributes to the prevalence of phyllomanganates over tectomanganates in natural Ni-rich environments. Most Ni is retained during the reflux process, part of Ni (≈20%) being likely structurally incorporated in the reaction products, thus enhancing the sequestration of Ni in Mn oxides.

中文翻译：

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含镍水钠锰矿向构造锰酸盐的转化：弱结合 Ni(II) 物种的影响和归宿

摘要 镍是一种必需的微量金属元素，其地球化学行为很大程度上取决于其与锰氧化物的相互作用。叶锰酸盐水钠锰矿与吸附或结构结合的 Ni 之间的相互作用以及随着这些层状物质向隧道锰氧化物（构造锰酸盐）的转变过程中的 Ni 命运，已被广泛记录。相比之下，叶锰酸盐与弱结合的 Ni 物质 [水合 Ni、Ni（氢）氧化物] 之间的相互作用，可能普遍存在于天然富 Ni（>10% NiO）锰酸盐中，但很少受到关注，这些 Ni 物质对叶锰酸盐到构造锰酸盐的转化基本上仍然未知。因此制备了一组具有不同 Ni 含量的叶锰酸盐前体，并进行回流处理，模拟天然叶锰酸盐到 tectomanganate 的转化。层状前体和回流产物的特征是结合衍射、光谱、热和化学方法。Ni 基本上以水合 Ni(II) 和 Ni(II)（氢）氧化物的形式存在于富含 Ni 的层状前驱体中，而在颗粒边缘吸附的 Ni（II）在低 Ni 含量下占优势。在初始无空位层状前驱体中未检测到在层空位位点吸附或结构上掺入的 Ni。与层状前驱体中 Jahn-Teller 畸变 Mn(III) 八面体的高含量 (≈1/3) 一致，这有利于它们转化为 tectomanganates，无 Ni 样品完全转化为无序的 todorokite，具有 3 × 3 隧道结构的常见构造锰酸盐。相比之下，尽管富镍前驱体中的 Mn(III) 含量相似，但在回流过程中，层间 Ni2+ 的水解和层间锰酸盐层中 Ni(OH)2 的聚合在动力学上是有利的。Asbolane 是一种叶状锰酸盐，具有不完整的岛状金属（氢）氧化物八面体层，因此形成而不是 todorokite。旨在溶解岛状中间层 Ni(OH)2 层的硝酸处理可以轻松明确区分 asbolane 和 todorokite，后者不受处理的影响。两种化合物确实表现出相似的面间周期性，并且在使用 X 射线衍射时可能会混淆，尽管强度比不同。相对于金属水解和聚合速率，Mn(III) 从 MnO2 层中的迁移速率决定了 todorokite 或 asbolane 的形成。在这里，Ni(OH)2 聚合阻碍了 tectomanganates 的形成，并可能导致在天然富 Ni 环境中，页锰酸盐超过 tectomanganates。大多数 Ni 在回流过程中被保留，部分 Ni (≈20%) 可能在结构上结合在反应产物中，从而增强了 Ni 在 Mn 氧化物中的螯合。