Phase transformation of hydrous ferric arsenate in the presence of Fe(II) under anaerobic conditions: Implications for arsenic mobility and fate in natural and anthropogenic environments

Abstract

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.