X-ray absorption spectroscopic investigation of molybdenum multinuclear sorption mechanism at the goethite-water interface

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Abstract

Understanding in situ metalloid surface speciation on mineral surfaces is critical to predicting the bioavailability in surface and subsurface environments. In this study, Mo K-edge X-ray absorption spectroscopy (XAS) was used to elucidate Mo(VI) surface speciation at the goethite-water interface. Effects of pH and loading levels were investigated. X-ray absorption near edge structure (XANES) analysis indicated that the Mo(VI) coordination environment changes from tetrahedral to octahedral with decreasing pH. At near neutral pH, Mo(VI) predominantly remains as tetrahedral molecules, forming inner-sphere surface species via corner- and edge-sharing attachment with iron octahedral structures (interatomic distance of Mo-Fe (RMo-Fe) at ∼2.8 and ∼3.4 Å, respectively). In contrast, a mixture of surface species comprising tetrahedrally and octahedrally coordinated Mo(VI) exists at pH ∼3-4. While the same Mo(VI) tetrahedral surface species are present at acidic pH, there was an additional MoO6 polymer attachment on iron octahedral structures, resulting in a RMo-Fe at 3.53 Å. The coordination number (CN) of a Mo-Mo backscatterer gradually increased with increasing loading level, suggesting the formation of surface polymerization. Overall, there seems to be a transition from Mo(VI) tetrahedral to octahedral coordination environment with decreasing pH. The XAS findings further support a Mo(VI) inner-sphere adsorption mechanism that was previously suggested in the pressure-jump relaxation study by Zhang and Sparks (Soil Sci. Soc. Am. J. 1989, 53 (4), 1028-1034). pH-Dependent multinuclear Mo(VI) surface speciation may be important in predicting Mo(VI) transport process in the soil-water environment.

Original languageEnglish (US)
Pages (from-to)8491-8496
Number of pages6
JournalEnvironmental Science and Technology
Volume44
Issue number22
DOIs
StatePublished - Nov 15 2010
Externally publishedYes

ASJC Scopus subject areas

  • Chemistry(all)
  • Environmental Chemistry

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