The long-term stability of FeIII-AsV coprecipitates at pH 4 and 7: Mechanisms controlling the arsenic behavior

Highlights: • The stability of Fe^III-As^V coprecipitates at pH 4 is much higher than that at pH 7. • About 5% of the total As released for the pH 7 sample aged at 75 °C. • As speciation and specified Fe/As molar ratio control the fate of As. • Slight increase of the crystallinity of the solid phase trigger As release at pH 7. • Microcosmic distribution of As influences the transformation of Fe^III-As^V solids. -- Abstract: Hydrometallurgical Fe^III-As^V coprecipitates, which are amorphous in nature, could re-crystallize gradually and pose risks of contamination to the environment. However, the mechanisms controlling the As behavior when the Fe^III-As^V coprecipitates stored at different pHs is still not fully understood. This work systematically investigated the fate of As and the transformation process of the coprecipitates (Fe/As = 4) at different pHs and temperatures. XRD, EXAFS, HRTEM, and chemical extraction methods were employed to characterize the crystallinity degrees and the transformation products of the coprecipitates. The results showed that the coprecipitates are more stable at acidic pH than at neutral pH. For those samples aged at pH 4, the arsenic speciation includes poorly crystalline ferric arsenate (PCFA) and As adsorbed on 2-line ferrihydrite (Fh). Due to the presence of PCFA, the Fe/As molar ratio for the latter phase is much higher than the bulk Fe/As molar ratio (4 in this work) of the coprecipitates and controls As release. However, for those samples aged at pH 7, due to the fact that 2-line Fh is the major As-bearing phase, slight changes of the crystallinity degrees of 2-line Fh will trigger As release.