Insight into the effect of SO42− on the precipitation and solubility of ferric arsenate in acidic solutions: Implication for arsenic mobility and fate

The effect of sulfate (SO42−) on the precipitation of As(V) and Fe(III) and the stability of the generated solid phase as well as its underlying mechanisms, which strongly influences the transportation and fate of As in acidic waters (such as acid mine drainage), is still not well known. This work systematically investigated the precipitation, decomposition, and settling properties of the ferric arsenate phase in the presence of different amount of SO42− at pH 1.2 and 1.8. The results showed that on the one side, the presence of SO42− delayed the removal of As and Fe from the aqueous phase and enhanced the solubility of solid ferric arsenate. The geochemical modelling results showed that the coexisting SO42− induced the formation of aqueous SO42−-Fe(III) complexes, inducing the decomposition of produced ferric arsenate solid phase. On the other side, the addition of SO42− promoted the growth rate of generated ferric arsenate and accelerated the settling of solids. The particle size distribution and transmission electron micorscopy analyses elucidated that the presence of SO42− significantly enlarged the ferric arsenate aggregate size. The infrared spectroscopy and S K-edge X-ray absorption near edge structure results proved that SO42− is incorporated into the structure of ferric arsenate by forming ferric arsenate sulfate solid solution, with each SO42− tetrahedron coordinating with approximately two FeO6 octahedra. The S K-edge extended X-ray absorption fine structrure result gave an averaged S-Fe interatomic distance of 3.21 Å. These results demonstrated that the presence of SO42− enlarged the aggregate size of ferric arsenate dominantly via the S-O-Fe bridging, and then accelerated the settling of SO4-ferric arsenate. The contrasting role of SO42− in influencing As obtained in this study is significant for understanding the mobilization and fate of As and the formation mechanisms of ferric arsenate minerals in acidic systems.