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Trace metals and metalloids and Ga/Al ratios in grey shale weathering profiles along a climate gradient and in batch reactors
Geoderma ( IF 6.1 ) Pub Date : 2021-09-08 , DOI: 10.1016/j.geoderma.2021.115431
Justin B. Richardson 1, 2 , Ivan C. Mischenko 1 , Trevor J. Mackowiak 1, 3 , Nicolas Perdrial 3
Affiliation  

Shale is an important lithology globally due to its wide spatial abundance and potentially high trace metal and metalloid (TMM) geochemistry, which can be potentially inherited by its overlying soils. Unlike other soils, shale-derived soils inherit organic matter and oxides (Fe and Mn) which promote accumulation and retention of both geogenic and exogeneous TMMs. Here, we explore TMMs in seven grey shales weathering profiles along a north–south transect spanning the western flank of the Appalachian Mountains from New York to Tennessee. Overall, total TMM concentrations in the grey shales and their soils were below concentrations known to be toxic and below concentrations observed in black shales. Tau values show that shale-derived soils are net accumulators of many TMMs (As, Cd, Co, Cu, Ni, Pb, Sb, Sn and Zn) but others (Cr, W, and V) showed a net depletion. Many of the TMMs had high proportions (5–50%) that were sequestered in reducible phases (amorphous and crystalline Fe oxides) but few TMMs were associated with oxidizable phases (organic matter, reduced minerals). Sulfides and oxides (Mn nor Fe) were not detected by X-ray diffraction (<2% g/g). TMM accumulation and release during weathering was not extensively related to climate or soil development/age as we hypothesized, potentially due to localized effects of vegetation, geomorphology, pollution, and physicochemical parameters of the shale. Our laboratory batch reactor experiments indicated that some TMMs had highest release rates under oxic, acidic conditions with organic acids present (Cr, Ga, Sn, Sb, and W) implying aluminosilicate dissolution control or under slightly acidic, reduced conditions (As, Cd, Co, Cu, Ni, Pb, V, and Zn), suggesting association with geogenic oxides. Total Ga/Al ratios in rock to soil profiles were not varying significantly across the climate-soil development gradient, despite batch reactor acidic conditions generating low Ga/Al ratios. This implies weathering of shales is dominated by processes that do not fractionate Ga/Al ratio (organic or inorganic colloid production) as our laboratory results suggests oxic, acidic aluminosilicate weathering should generate a high Ga/Al solid phase.



中文翻译:

沿气候梯度和间歇反应器中灰色页岩风化剖面中的痕量金属和准金属以及 Ga/Al 比率

由于其广泛的空间丰度和潜在的高痕量金属和准金属 (TMM) 地球化学,页岩是全球重要的岩性,其上覆土壤可能会继承这些。与其他土壤不同,页岩衍生土壤继承了有机质和氧化物(Fe 和 Mn),它们促进了地质和外源 TMM 的积累和保留。在这里,我们沿着横跨阿巴拉契亚山脉西侧从纽约到田纳西州的南北横断面探索了七个灰色页岩风化剖面中的 TMM。总体而言,灰色页岩及其土壤中的 TMM 总浓度低于已知有毒的浓度,也低于黑色页岩中观察到的浓度。Tau 值表明,页岩衍生土壤是许多 TMM(As、Cd、Co、Cu、Ni、Pb、Sb、Sn 和 Zn)的净蓄积物,但其他(Cr、W、和 V) 显示净消耗。许多 TMM 具有高比例(5-50%),它们被隔离在可还原相(无定形和结晶 Fe 氧化物)中,但很少有 TMM 与可氧化相(有机物、还原矿物)相关联。X 射线衍射未检测到硫化物和氧化物(Mn 或 Fe)(<2% g/g)。正如我们假设的那样,风化过程中 TMM 的积累和释放与气候或土壤发育/年龄没有广泛的关系,这可能是由于植被、地貌、污染和页岩的物理化学参数的局部影响。我们的实验室间歇反应器实验表明,一些 TMM 在含氧、酸性条件下具有最高的释放率,其中存在有机酸(Cr、Ga、Sn、Sb 和 W),这意味着铝硅酸盐溶解控制或在弱酸性、还原条件(As、Cd、公司,Cu、Ni、Pb、V 和 Zn),表明与地源氧化物有关。尽管间歇反应器酸性条件产生低 Ga/Al 比,但岩石与土壤剖面中的总 Ga/Al 比在气候 - 土壤发育梯度上没有显着变化。这意味着页岩风化主要由不分馏 Ga/Al 比率(有机或无机胶体生产)的过程主导,因为我们的实验室结果表明,含氧、酸性铝硅酸盐风化应产生高 Ga/Al 固相。

更新日期:2021-09-08
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