Ferric vs. ferrous arsenate amorphous precursors: properties and controls on scorodite mineralization

• Published in: The American mineralogist
• Main Authors: Chang, Pei, Zhu, Xiangyu, Cai, Hongming, Zhang, Jianchao, Li, Siliang, Lu, Xiancai, Wang, Rucheng, Teng, Hui Henry
• Format: Journal Article
• Language: English
• Published: 01.08.2024
• ISSN: 0003-004X; 1945-3027
• DOI: 10.2138/am-2024-9326

Abstract Amorphous iron-arsenate precipitates are significant As sinks in natural and industrial settings, and often serve as precursors to the crystallization of ferric arsenate minerals such as scorodite (FeAsO4·2H2O) and kankite (FeAsO4·3.5H2O). These amorphous phases have varied structures and compositions depending on geochemical conditions under which they form, but have not been well characterized except the frequently encountered amorphous ferric arsenate (represented as AFeIII in present study). Here, we intend to characterize another phase, amorphous ferrous arsenate (AFeII), which is formed in partially oxidized Fe(II)-As(V) systems, and carry out a comparative study on the properties and related scorodite mineralization processes relative to those of AFeIII. We synthesized both AFeII and AFeIII, determined their compositions and structures, and finally examined the amorphous-crystalline phase transition processes leading to scorodite mineralization. Computed chemical formulae showed AFeIII and AFeII can be represented by Fe(III)0.99AsO4·4.1H2O and Fe(III)0.48Fe(II)0.43(HAsO4)0.7(AsO4)0.3·4.0H2O, respectively, with AFeII containing 47.3% Fe(II) deduced from the pre-edge data of Fe K-edge X-ray absorption spectroscopy. Structurally, the Fe-O bond at 2.02 ± 0.01 Å in AFeII was significantly longer than that of 1.99 ± 0.01 Å in AFeIII and scorodite. In addition, AFeII appeared to have a higher stability (slower kinetics of scorodite crystallization) under either atmospheric or aqueous conditions. Together with solution chemistry measurements, these findings suggest AFeII controls scorodite formation by limiting the occurrence of aqueous Fe3+ and retard amorphous-scorodite phase transition compared to the case of AFeIII. These findings improve our understanding of iron arsenate system and may find industrial applications to arsenic sequestration via scorodite mineralization.