Transformation of Ni-containing birnessite to tectomanganate: Influence and fate of weakly bound Ni(II) species
Résumé
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 Â 3 tunnel structure. Contrastingly and despite similar high Mn(III) contents in Ni-rich precursors, hydrolysis of interlayer Ni 2+ 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 MnO 2 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.
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