Sorption and speciation of molybdate in soils: Implications for molybdenum mobility and availability

Highlights: • Mo sorption and desorption isotherms were determined for different soils. • The main Mo species are sorbed Mo(VI), followed by Fe/Al- or Ca-bound Mo(VI). • Acidic soils exhibit reversible sorption/desorption of Mo due to Al-bound Mo(VI). • Sorption irreversibility is enhanced by Ca-bound Mo(VI) in soils at higher pHs. Molybdenum (Mo) is an emerging contaminant in the environment. To assess the mobility and availability of Mo in soils, this study investigated the effect of soil properties on the sorption and desorption of Mo in soils. The Mo K-edge X-ray absorption near edge structure (XANES) of the soils after Mo sorption showed that sorbed molybdate was the predominant species, with Fe/Al-molybdate and Ca-molybdate being the minor components in soils with low and high pH levels, respectively. Although acidic soils exhibited higher Mo sorptivity, they exhibited partial reversibility of Mo sorption, which may be attributed to the high solubility of Al-molybdate. Accordingly, the mobility of Mo may be relatively high in soils with a low pH, high exchangeable Al content, and high Fe-hydroxide crystallinity, such as Ultisols and Oxisols. At higher pHs, the sorption irreversibility of molybdate were enhanced due to the formation of Ca-molybdate precipitate. The results of this study indicated that sorption/desorption irreversibility and related mechanisms should be considered when evaluating the mobility and availability of Mo in soils.