Particulate and dissolved metal(loid) release from mine tailings is of concern in (semi-) arid environments where tailings can remain barren of vegetation for decades and, therefore, become highly susceptible to dispersion by wind and water. Erosive weathering of metalliferous tailings can lead to arsenic contamination of adjacent ecosystems and increased risk to public health. Management via phytostabilization with the establishment of a vegetative cap using organic amendments to enhance plant growth has been employed to reduce both physical erosion and leaching. However, prior research suggests that addition of organic matter into the oxic weathering zone of sulfide tailings has the potential to promote the mobilization of arsenate. Therefore, the objective of the current work was to assess the impacts of phytostabilization on the molecular-scale mechanisms controlling arsenic speciation and lability. These impacts, which remain poorly understood, limit our ability to mitigate environmental and human health risks. Here we report on subsurface biogeochemical transformations of arsenic and iron from a three-year phytostabilization field study conducted at a Superfund site in Arizona, USA. Legacy pyritic tailings at this site contain up to 3 g kg-1 arsenic originating from arsenopyrite that has undergone oxidation to form arsenate-ferrihydrite complexes in the top 1 m. Tailings were amended in the top 20 cm with 100, 150, or 200 g kg-1 (300-600 T ha-1) of composted organic matter and seeded with native halotolerant plant species. Treatments and an unamended control received irrigation of 360 ± 30 mm y-1 in addition to 250 ± 160 mm y-1 of precipitation. Cores to 1 m depth were collected annually for three years and sectioned into 20 cm increments for analysis by synchrotron iron and arsenic X-ray absorption spectroscopy (XAS) coupled with quantitative wet chemical and mass balance methods. Results revealed that > 80% of arsenic exists in ammonium oxalate-extractable and non-extractable phases, including dominantly ferrihydrite and jarosite. Arsenic release during arsenopyrite oxidation resulted in both downward translocation and As(V) attenuation by stable Fe(III)(oxyhydr)oxide and Fe(III) (hydroxy)sulfate minerals over time, highlighting the need for sampling at multiple depths and time points for accurate interpretation of arsenic speciation, lability, and translocation in weathering profiles. Less than 1% of total arsenic was highly-labile, i.e. water-extractable, from all treatments, depths, and years, and more than 99% of arsenate released by arsenopyrite weathering was attenuated by association with secondary minerals. Although downward translocation of both arsenic and iron was detected during phytostabilization by temporal enrichment analysis, a similar trend was measured for the uncomposted control, indicating that organic amendment associated with phytostabilization practices did not significantly increase arsenic mobilization over non-amended controls.

中文翻译：

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黄铁矿尾矿植物稳定过程中砷和铁的形态和动员

在（半）干旱环境中，尾矿中的颗粒和溶解金属（液体）释放是一个令人担忧的问题，在这些环境中，尾矿几十年来一直没有植被，因此非常容易受到风和水的影响。含金属尾矿的侵蚀性风化会导致邻近生态系统受到砷污染，并增加对公众健康的风险。通过植物稳定管理，使用有机添加物建立植物上限以促进植物生长，已被用于减少物理侵蚀和浸出。然而，先前的研究表明，在硫化物尾矿的氧化风化带中添加有机物有可能促进砷酸盐的迁移。所以，当前工作的目标是评估植物稳定性对控制砷形态和不稳定的分子尺度机制的影响。这些影响仍然知之甚少，限制了我们减轻环境和人类健康风险的能力。在这里，我们报告了在美国亚利桑那州 Superfund 站点进行的为期三年的植物稳定现场研究中砷和铁的地下生物地球化学转化。该地点的遗留黄铁矿尾矿含有高达 3 g kg-1 的砷，源自砷黄铁矿，在顶部 1 m 处经过氧化形成砷酸铁水合物复合物。用 100、150 或 200 g kg-1（300-600 T ha-1）的堆肥有机物质对顶部 20 cm 处的尾矿进行修正，并种植本地耐盐植物物种。除了 250 ± 160 mm y-1 的降水外，处理和未经修改的对照还接受了 360 ± 30 mm y-1 的灌溉。三年内每年收集 1 m 深的岩心，并将其切成 20 cm 的增量，以通过同步加速器铁和砷 X 射线吸收光谱 (XAS) 结合定量湿化学和质量平衡方法进行分析。结果表明，> 80% 的砷存在于草酸铵可萃取和不可萃取相中，主要包括水铁矿和黄钾铁矾。随着时间的推移，砷黄铁矿氧化过程中的砷释放导致稳定的 Fe(III)(羟基)氧化物和 Fe(III)（羟基）硫酸盐矿物向下易位和 As(V) 衰减，突出表明需要在多个深度和时间点采样准确解释砷形态、不稳定性、和风化剖面中的易位。在所有处理、深度和年份中，少于 1% 的总砷是高度不稳定的，即水可提取的，并且毒砂风化释放的砷酸盐中 99% 以上因与次生矿物的结合而减弱。尽管通过时间富集分析在植物稳定过程中检测到砷和铁的向下易位，但未堆肥对照也测量到类似的趋势，表明与植物稳定实践相关的有机改良剂没有显着增加砷动员。毒砂风化释放的砷酸盐99%以上因与次生矿物的缔合而衰减。尽管通过时间富集分析在植物稳定过程中检测到砷和铁的向下易位，但未堆肥对照也测量到类似的趋势，表明与植物稳定实践相关的有机改良剂没有显着增加砷动员。毒砂风化释放的砷酸盐99%以上因与次生矿物的缔合而衰减。尽管通过时间富集分析在植物稳定过程中检测到砷和铁的向下易位，但未堆肥对照也测量到类似的趋势，表明与植物稳定实践相关的有机改良剂没有显着增加砷动员。