Hydrous ferric arsenate (HFA) is an important secondary arsenic (As)-bearing precipitate in mining-impacted environments. However, the role of iron(II) (Fe(II)) ions input in the phase transformation of HFA is largely unknown. In this work, we investigated the phase transformation of HFA in the presence of Fe(II) ions at various pH values (2–8) and Fe(II)/As(V) molar ratios (1 and 4) at 22 °C under anoxic conditions. The concentrations of dissolved As and Fe were monitored kinetically up to 15 d. The results showed that the removal of dissolved As increased with increasing pH and Fe(II)/As molar ratio. X-ray diffraction (XRD), scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDX), and Fourier transform infrared spectroscopy (FTIR) were employed to characterize the solid samples. The results showed that the transformation of HFA into crystalline Fe(II)-As phases was highly controlled by pH, whereas Fe(II)/As molar ratio was less affected. No phase transformation was observed at strong acidic conditions. At pH 4, HFA can react with Fe(II) to form a semi-crystalline phase. At pH 6, crystalline symplesite and parasymplesite were formed and constituted the major As-bearing species in the host solids, regardless of Fe(II)/As molar ratio. In contrast, at higher pH and Fe(II) concentration, thermodynamic modeling and XRD results indicated that Fe(OH)₂ and green rust were formed and constituted an appreciable fraction in the precipitated solids in addition to the dominant crystalline parasymplesite. The present study may have important implications for understanding the geochemical cycle of arsenic in Fe(II) and HFA coexisting systems.

在采矿影响的环境中，含水砷酸铁（HFA）是重要的次生砷（As）沉淀物。然而，铁（II）（Fe（II））离子输入在HFA的相变中的作用在很大程度上是未知的。在这项工作中，我们研究了22°C下在各种pH值（2-8）和Fe（II）/ As（V）摩尔比（1和4）下存在Fe（II）离子时HFA的相变。在缺氧条件下。动态监测溶解的As和Fe的浓度，直至15 d。结果表明，随着pH值和Fe（II）/ As摩尔比的增加，溶解态As的去除率增加。X射线衍射（XRD），扫描电子显微镜-能量色散X射线光谱（SEM-EDX）和傅里叶变换红外光谱（FTIR）用于表征固体样品。结果表明，pH值可控制HFA向Fe（II）-As结晶相的转变，而Fe（II）/ As摩尔比的影响较小。在强酸性条件下未观察到相变。在pH 4时，HFA可以与Fe（II）反应形成半结晶相。在pH值为6时，无论Fe（II）/ As摩尔比如何，在主体固体中均会形成结晶对称和副对称，并构成主要的含As物质。相反，在较高的pH和Fe（II）浓度下，热力学模型和XRD结果表明Fe（OH）无论Fe（II）/ As摩尔比如何，均会形成结晶的对称和副对称，并构成主体固体中主要的含砷物质。相反，在较高的pH和Fe（II）浓度下，热力学模型和XRD结果表明Fe（OH）无论Fe（II）/ As摩尔比如何，均会形成结晶的对称和副对称，并构成主体固体中主要的含砷物质。相反，在较高的pH和Fe（II）浓度下，热力学模型和XRD结果表明Fe（OH）_{如图2所示}，除了占优势的结晶副象征物外，还形成了生铁锈，并在沉淀的固体中构成了相当大的比例。本研究可能对理解Fe（II）和HFA共存系统中砷的地球化学循环具有重要意义。