Observation of surface precipitation of ferric molybdate on ferrihydrite: Implication for the mobility and fate of molybdate in natural and hydrometallurgical environments

Abstract

Adsorption of molybdate (Mo(VI)) on the surfaces of ferrihydrite is one of the most critical processes that control its mobility and fate in the environment. However, the sorption mechanism and the effect of pH on the speciation of Mo(VI) on ferrihydrite surfaces are not well understood. In this study, X-ray diffraction (XRD), Raman, Fourier transform infrared (FTIR), and Mo K-edge and L3-edge X-ray absorption spectroscopy (XAS) have been utilized to characterize the Mo(VI) species sorbed on ferrihydrite under various pH conditions. XRD, Raman, and FTIR results show that at acidic pH, surface precipitation of poorly crystalline ferric molybdate (PCFM) occurs under apparently undersaturated conditions (theoretical log IAP < log Ksp) and is enhanced by the aging process, whereas Mo(VI) is mainly present as surface adsorbed species at circum-neutral pH. The Mo K-edge and L3-edge X-ray absorption near edge structure (XANES) analyses show that a mixture of tetrahedrally and octahedrally coordinated Mo(VI) simultaneously exists at pH 3–7 and the octahedral Mo(VI) species decreases with increasing pH. The Mo-Fe interatomic distances (3.52–3.56 Å) derived from EXAFS fittings suggest the corner-sharing complexation of both MoO4 and MoO6 with FeO6 octahedra. As the pH decreases from 7 to 3, the coordination number of the Mo-Fe shell (CNMo-Fe) increases from 0.6(3) to 1.9(3), possibly due to the gradual transformation of surface adsorbed Mo(VI) to PCFM. These findings on the observation of Mo(VI) complexation, surface precipitation, and their marked pH dependence during the Mo(VI) adsorption on ferrihydrite have important implications for both understanding the mobility and fate of Mo(VI) in natural and hydrometallurgical industry impacted environments and developing optimal applications for the remediation of Mo contamination in aqueous environments.