Geological Survey of Denmark and Greenland (GEUS), Copenhagen, Denmark.
Department of Earth Sciences - Geochemistry, Faculty of Geosciences, Utrecht University, Utrecht, The Netherlands.
Environ Sci Process Impacts. 2019 Sep 18;21(9):1459-1476. doi: 10.1039/c9em00267g.
We investigated the impact of aging-induced structural modifications of carbonate green rust (GR), a mixed valent Fe(ii,iii) (hydr)oxide with a high oxyanion sorption affinity, on the partitioning and binding mode of arsenic (As). Suspensions of carbonate GR were produced in the presence of As(v) or As(iii) (i.e. co-precipitated with As(iii) or As(v)) and aged in anoxic and oxic conditions for up to a year. We tracked aqueous As over time and characterized the solid phase by X-ray absorption spectroscopy (XAS). In experiments with initial As(v) (4500 μg L-1, As/Fe = 2 mol%), the fresh GR suspension sorbed >99% of the initial As, resulting in approximately 14 ± 8 μg L-1 residual dissolved As. Anoxic aging of the As(v)-laden GR for a month increased aqueous As to >60 μg L-1, which was coupled to an increase in GR structural order revealed by Fe K-edge XAS. Further anoxic aging up to a year transformed As(v)-laden GR into magnetite and decreased significantly the aqueous As to <2 μg L-1. The As binding mode was also modified during GR transformation to magnetite from sorption to GR particle edges to As substitution for tetrahedral Fe in the magnetite structure. These GR structural modifications altered the ratio of As partitioning to the solid (μg As/mg Fe) and liquid (μg As per L) phase from 2.0 to 0.4 to 14 L mg-1 for the fresh, month, and year aged suspensions, respectively. Similar trends in GR transformation and As partitioning during anoxic aging were observed for As(iii)-laden suspensions, but occurred on more rapid timescales: As(iii)-laden GR transformed to magnetite after a day of anoxic aging. In oxic aging experiments, rapid GR oxidation by dissolved oxygen to Fe(iii) precipitates required only an hour for both As(v) and As(iii) experiments, with lepidocrocite favored in As(v) experiments and hydrous ferric oxide favored in As(iii) experiments. Aqueous As during GR oxidation decreased to <10 μg L-1 for both As(v) and As(iii) series. Knowledge of this interdependence between GR aging products and oxyanion fate improves biogeochemical models of contaminant and nutrient dynamics during Fe cycling and can be used to design more effective arsenic remediation strategies that rely on arsenic sorption to GR.
我们研究了碳酸亚铁（GR）老化引起的结构变化对砷（As）分配和结合模式的影响。碳酸亚铁是一种具有高含氧阴离子吸附亲和力的混合价态 Fe(ii,iii)（水合）氧化物，我们在存在砷（V）或砷（III）的情况下制备了碳酸亚铁悬浮液（即与砷（III）共沉淀或与砷（V）共沉淀），并在缺氧和有氧条件下老化长达一年。我们随时间跟踪水中的 As，并通过 X 射线吸收光谱（XAS）对固相进行了表征。在含有初始砷（V）（4500μg L-1，As/Fe=2mol%）的实验中，新鲜的 GR 悬浮液吸附了初始 As 的>99%，导致残余溶解的 As 约为 14±8μg L-1。GR 负载砷（V）的缺氧老化一个月后，水中的 As 增加到>60μg L-1，这与 Fe K 边 XAS 显示的 GR 结构有序性增加有关。进一步的缺氧老化长达一年将 GR 负载的砷（V）转化为磁铁矿，并将水中的 As 显著降低到<2μg L-1。GR 结构的转变也改变了砷的结合模式，从 GR 颗粒边缘的吸附到磁铁矿结构中四面体 Fe 的取代。这些 GR 结构的改变改变了新鲜、老化一个月和老化一年的悬浮液中砷分配到固相（μg As/mg Fe）和液相（μg As/L）的比例，分别为 2.0、0.4 和 14L mg-1。在缺氧老化过程中，GR 转变和 As 分配也存在类似的趋势，但在更短的时间尺度上发生：GR 负载的砷（III）在缺氧老化一天后转化为磁铁矿。在有氧老化实验中，溶解氧对 GR 的快速氧化使 Fe(iii)沉淀只需要一小时，无论是砷（V）还是砷（III）实验，都有利于针铁矿的形成，而有利于水合氧化铁的形成砷（III）实验。GR 氧化过程中水中的 As 减少到<10μg L-1，砷（V）和砷（III）系列均如此。这种 GR 老化产物与含氧阴离子命运之间的相互依存关系的知识提高了铁循环过程中污染物和养分动态的生物地球化学模型的准确性，并可用于设计更有效的砷修复策略，该策略依赖于 GR 对砷的吸附。
