Institute of Mineralogy & Geochemistry, Karlsruhe Institute of Technology, 76131 Karlsruhe, Germany.
Water Res. 2010 Nov;44(19):5545-55. doi: 10.1016/j.watres.2010.06.002. Epub 2010 Jun 9.
High concentrations of As in groundwater frequently occur throughout the world. The dissolved concentration, however, is not necessarily determined by the amount of As in the ambient sediment but rather by the partitioning of As between different minerals and the type of fixation. Sequential extractions are commonly applied to determine associations and binding forms of As in sediments. Due to the operational nature of the extracted fractions, however, the results do not provide insight into how and where precisely As is bound within mineral grains and no information about elemental associations or involved mineral phases can be gained. Furthermore, little is known about possible geochemical alterations that actually occur within a single grain during sequential extraction. Therefore, micro-synchrotron X-ray fluorescence analysis was applied to study the micro-scale distribution of As and other elements in single sediment grains. Arsenic was found to be mainly enriched in Fe oxy-hydroxide coatings along with other heavy metals resulting in high correlations. Phosphate leached 34-66% of As from the studied grains. The release of As in this leaching step was accompanied by the disappearance of correlations between As and Fe as well as by a higher Fe/As ratio compared to untreated samples. During the Fe-leaching step the coatings were largely dissolved leading to much lower concentrations of As and Fe. The correlation between As and Fe was preserved only in association with K, indicating the presence of both elements in silicate structures. Several distinctive features were observed such as the release of Fe, Mn and Cr during phosphate leaching as well as the lowering of mean K concentrations due to the Fe-leaching which indicates that not only target mineral phases were dissolved in these extraction steps. The importance of re-precipitation processes during sequential extraction was indicated by a consistently observed increase of the Fe/As ratio from the untreated to the Fe-leached samples.
高浓度的砷在地下水中经常在世界各地出现。然而，溶解浓度并不一定取决于环境沉积物中的砷含量，而是由砷在不同矿物之间的分配和固定类型决定的。连续提取通常用于确定沉积物中砷的结合和结合形式。然而，由于提取部分的操作性质，结果并不能深入了解砷在矿物颗粒中的确切结合方式和位置，也无法获得有关元素结合或涉及的矿物相的信息。此外，对于在连续提取过程中实际发生的可能的地球化学变化，人们知之甚少。因此，应用微同步加速器 X 射线荧光分析来研究单个沉积物颗粒中砷和其他元素的微观分布。结果发现，砷主要富集在铁氢氧化物涂层中，同时还与其他重金属一起富集，导致相关性很高。磷酸盐从研究的颗粒中浸出了 34-66%的砷。在这个浸出步骤中释放的砷伴随着砷与铁之间的相关性消失，以及与未经处理的样品相比，铁/砷的比值更高。在铁浸出步骤中，涂层被大量溶解，导致砷和铁的浓度降低。只有在与 K 相关联的情况下，砷和铁之间的相关性才得以保留，表明这两种元素都存在于硅酸盐结构中。观察到了几个明显的特征，例如在磷酸盐浸出过程中释放的铁、锰和铬，以及由于铁浸出导致平均 K 浓度降低，这表明不仅目标矿物相在这些提取步骤中溶解。连续提取过程中再沉淀过程的重要性通过从未处理到铁浸出样品中观察到的铁/砷比的持续增加得到了表明。
