Molybdenum binding to soil constituents in acid soils: An XAS and modelling study

- Molybdenum(VI) is bound to ferrihydrite and Al hydroxide mostly as edge-sharing inner-sphere and outer-sphere complexes.
- On natural organic matter, molybdenum(VI) is bound at low pH as an octahedrally coordinated monomeric complex.
- Results for a spodic B horizon show an Al(OH)3-type phase to be an important sorbent for molybdenum(VI).
- The results were used to suggest new complexation constants for the CD-MUSIC and Stockholm Humic models.

Despite its importance as a trace element, the binding mechanisms of molybdenum in soils are not well known. In this study, we studied the binding of molybdenum onto selected soil samples, and we used X-ray absorption spectroscopy (XAS) to characterize the coordination of molybdenum on three important environmental sorbents: ferrihydrite (Fh), amorphous aluminium hydroxide (Al(OH)3) and fulvic acid. The X-ray near-edge structure (XANES) data showed that the added molybdenum(VI) was not reduced, although for the organic samples the coordination shifted from tetrahedral to octahedral. The EXAFS (extended X-ray absorption fine structure) analysis showed that molybdenum(VI) on Fh and Al(OH)3 was dominated by edge-sharing bidentate complexes with Mo ⋯ Fe and Mo ⋯ Al distances of 2.80 and 2.62 Å, respectively. For ferrihydrite, there was a minor contribution from a corner-sharing bidentate complex at 3.55 Å. Further, geochemical modelling suggested an additional role of an outer-sphere complex at high pH. A sample from a spodic Bs horizon had XANES and EXAFS features similar to those of Mo sorbed to Al(OH)3, highlighting the importance of Al(OH)3-type sorbents in this soil. However, in the studied organic samples molybdenum(VI) was present in a distorted octahedral configuration as an organic complex. The results were used to improve molybdenum binding reaction equilibrium constants in the CD-MUSIC model for ferrihydrite and in the Stockholm Humic Model. Collectively the results show that acid soils may contain sorbents able to bind molybdenum efficiently, and thus prevent its leaching to waters.