Chromium occurrence in a nickel laterite profile and its implications to surrounding surface waters
Chromium occurrence in a nickel laterite profile and its implications to surrounding surface waters
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DOI：
10.1016/j.chemgeo.2020.119863
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发表时间：
2020-12
期刊：
影响因子：
3.9
通讯作者：
Ruth Esther G Delina;C. Arcilla;T. Otake;Jhonard John Garcia;M. Tan;A. Ito
中科院分区：
文献类型：
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作者：
Ruth Esther G Delina;C. Arcilla;T. Otake;Jhonard John Garcia;M. Tan;A. Ito
超镁铁岩是铬的重要地质来源，其六价形态是环境问题。在热带至亚热带地区（如新喀里多尼亚、巴西、菲律宾），超镁铁岩的长期和普遍风化作用产生镍红土矿，这些矿进一步富集铬。虽然在镍红土中发现了高水平的铬，但对这些矿床的铬可用性和出口及其对天然沃茨的贡献仍然知之甚少。因此，本研究通过连续提取和浸出实验，再加上物理，矿物学和地球化学表征的镍红土剖面中的铬的形态和通量。这些都与通过水化学调查研究的周围地表沃茨中Cr的行为和输出有关。这项工作表明，通过Rio Tuba镍红土存款的例子，在热带地区的超镁铁岩的普遍风化产生非常重要的水库铬（六）。尽管Cr主要固定在不溶性部分（>89%）中，并且最少固定在生物可利用和易于移动的相（<1%）中，但由于Ni红土的固有的高Cr含量（高达2.9wt%），后者仍然提供超过饮用水和淡水标准的水可提取的Cr（VI）含量。通过与富含硅酸盐的腐泥土单元相比，上覆的Fe-（oxyhydr）氧化物为主的红土层产生较低的Cr（VI）含量，由于其酸性和较高的有机质和Fe-（oxyhydr）氧化物，促进还原和再吸附的Cr（VI）。在水分入渗过程中，Cr（VI）以62 ~ 3446 t/km ~ 2/yr的速率沿剖面沿着向下迁移。虽然周围表面沃茨的碱性和氧化性条件有利于Cr（VI）的移动，但只有一小部分来自Ni红土的Cr（VI）到达这些水体。地表沃茨中的Cr（VI）浓度（≤213 μg/L）和通量（大多数≤50 kg/km 2/yr）明显低于镍红土，这反映了稀释、还原、吸附等过程的作用，这些过程减弱了该物种沿着水流路径的释放。
Ultramafic rocks are considered as the most important geogenic sources of Cr whose hexavalent species is of environmental concern. In tropical to subtropical areas (e.g. New Caledonia, Brazil, Philippines), prolonged and pervasive weathering of ultramafic rocks produces Ni laterite ores which are further enriched in Cr. While elevated levels of Cr have been identified in Ni laterites, Cr availability and export from these deposits and its contribution to natural waters remain poorly understood. Therefore, this study examined the speciation and flux of Cr in a Ni laterite profile through sequential extraction and leaching experiments coupled with physical, mineralogical, and geochemical characterization. These were correlated to the behavior and export of Cr in surrounding surface waters studied through hydrochemical investigation. This work demonstrates, through the example of the Rio Tuba Ni laterite deposit, that pervasive weathering of ultramafic rocks in tropical areas generate very important reservoirs of Cr(VI). Although Cr is primarily fixed in insoluble fractions (>89%) and least in bioavailable and easily mobilizable phases (<1%), due to the inherent elevated Cr content (up to 2.9 wt%) of the Ni laterites, the latter still provided water-extractable Cr(VI) contents surpassing drinking water and freshwater standards. By comparison with the silicate-rich saprolite unit, the overlying Fe-(oxyhydr)oxide dominated laterite layer yielded lower Cr(VI) contents due to its acidic nature and higher amounts of organic matter and Fe-(oxyhydr)oxides which promote reduction and readsorption of Cr(VI). During water infiltration, Cr(VI) migrates downward along the profile at a rate of 62 to 3446 t/km2/yr. While the alkaline and oxidizing conditions of the surrounding surface waters favor Cr(VI) mobilization, only a fraction of Cr(VI) from the Ni laterites reaches these water bodies. Cr(VI) concentrations (≤213 μg/L) and fluxes (mostly ≤50 kg/km2/yr) in the surface waters being significantly lower than that of the Ni laterites reflect the action of processes (e.g. dilution, reduction, adsorption) that attenuate the release of this species along water flow paths.