Mobilisation of arsenic, selenium and uranium from Carboniferous black shales in west Ireland
Mobilisation of arsenic, selenium and uranium from Carboniferous black shales in west Ireland
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DOI：
10.1016/j.apgeochem.2019.104401
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发表时间：
2019-10-01
影响因子：
3.4
通讯作者：
Feldmann, Jorg
中科院分区：
文献类型：
--
作者：
Armstrong, Joseph G. T.;Parnell, John;Feldmann, Jorg
关键元素在近地表环境中的固定和积累是理解元素在地壳中循环的重要因素，无论是为了勘探新的资源还是环境管理战略。与全球地壳平均值相比，碳质黑色页岩通常富含微量元素，其中许多元素对环境有潜在的影响，这取决于它们在地表的形态和流动性。可以通过研究与地表黑色页岩有关的次生矿化(风化)来研究这种微量元素的移动性。在这项研究中，爱尔兰西海岸的石炭系页岩被发现具有高于平均页岩浓度的As、Cd、Cu、Co、Mo、Ni、Se、Te和U，与横向相当的英国Bowland Shales类似。这些黄铁矿黑色页岩的地下水渗透和氧化风化作用产生了以针铁矿和黄钾铁矾矿为主的氧化物矿床，与母页岩中的浓度相比，它们显著富含As(44-468ppm)、Se(12-184ppm)、U(6-158ppm)和其他微量元素。主要元素的组成从3.5%到29.4%硫酸盐，0.6%到9.1%的磷和36.6%到47.2%的氧化铁。在这些在常压和温度条件下形成的样品中，观察到黄钾铁矾石中的磷酸盐被取代。形成这些次生矿床的主要和微量元素主要来自下伏的黑色页岩，通过地下水的活化。这一发现对于页岩气勘探和生产过程中对黑色页岩岩性的环境评估至关重要，在这种情况下，原地元素丰度的活动性可能表明在水力压裂过程中和之后产生的和地下水污染的一个重要来源。主要氧化相的比例对次生矿床中微量元素的富集量显示出明显的控制作用，其中Se和As浓度的增加与黄钾铁矾矿矿化中磷含量的增加相关。这对酸性矿山排水渗漏的修复具有重要意义，富磷黄钾铁矾相可用于更有效地去除痕量元素。
The fixation and accumulation of critical elements in the near surface environment is an important factor in understanding elemental cycling through the crust, both for exploration of new resources and environmental management strategies. Carbonaceous black shales are commonly rich in trace elements relative to global crustal averages, many of which have potential environmental impacts depending on their speciation and mobility at surface. This trace element mobility can be investigated by studying the secondary mineralisation (regolith) associated with black shales at surface. In this study, Carboniferous shales on the west coast of Ireland are found to have higher than average shale concentrations of As, Cd, Cu, Co, Mo, Ni, Se, Te and U, similar to the laterally equivalent Bowland Shales, UK. Groundwater penetration and oxidative weathering of these pyritic black shales produces oxide deposits, dominated by goethite and jarosite, which are significantly enriched in As (44- 468 ppm), Se (12-184 ppm), U (6-158 ppm) and other trace elements, compared to concentrations in the parent shales. Major elemental abundances vary in composition from 3.5 to 29.4% sulphate, 0.6-9.1% phosphate and 36.6-47.2% iron-oxide. Phosphate substitution within jarosite is observed in these samples, formed under ambient pressure and temperature conditions.The major and trace elements forming these secondary deposits are predominantly sourced from the underlying black shales through mobilisation by groundwater. This discovery is critical for the environmental assessment of black shale lithologies during shale gas exploration and production, where the demonstrated mobility of in situ elemental enrichments may indicate a significant source of produced- and groundwater contamination during and after hydraulic fracturing processes. The proportions of the major oxide phases exhibit a clear control on the trace elemental enrichments within the secondary deposits, where increasing Se and As concentrations correlate with increasing phosphate content of the jarosite mineralisation. This has implications for the remediation of acid mine drainage seeps, where phosphate-rich jarosite phases could be utilised for more efficient trace element removal.