Molybdenum (Mo) and vanadium (V) are important trace elements and potential environmental contaminants in many surface and subsurface environmental systems. Mo commonly occurs as the tetrahedral oxyanion molybdate (Mo(VI)O42-) and V can occur in several oxidation states including V(III), V(IV), V(V), however it is environmentally transportable in the more toxic, oxidised forms as vanadyl (V(IV)O2+) and vanadate (V(V)O43-). The widespread iron oxides have a controlling effect on the mobility of contaminated metals due to their high surface areas. This project applied multiple techniques including ATR-FTIR and XAS to holistically characterise the adsorption behaviour of Mo(VI) and V(V) to iron oxides under a range of environmental conditions (e.g. pH, adsorbate concentrations), and identify the changes of oxidation states and species, and determine bonding environment of Mo and V onto these mineral surfaces. The first study investigated Mo(VI) adsorption to hematite and indicated low pH (3-4) and high Mo(VI) concentrations (> 0.5 mM) contribute to the formation of polymolybdate and at low concentration and neutral to high pH an octahedral corner-sharing bidentate binuclear adsorption complex was observed. The second study of V(V) adsorption to hematite showed a gradual change of coordination environment from a mixture of octahedral decavanadate and tetrahedral polyvanadate at pH 3 to a tetrahedral monovanadate (via corner-sharing bidentate complex) dominating at pH 9. The third project investigated Mo(VI) adsorption to Fe(II)-bearing iron oxides (e.g. magnetite) and suggested three processes: 1. adsorption occurred at pH 5.5 via octahedral corner-sharing bidentate binuclear complex, 2. reduction of Mo(VI) to Mo(IV) to MoO2 occurred at pH>9, and 3. Mo dominantly adsorbed on magnetite via corner-sharing octahedra at pH 7 and 3.7% of Mo(VI) reduced to Mo(IV)O2. Collectively, these thesis results provide a detailed molecular scale understanding of the poly- or mono- Mo(VI) and V(V) adsorption and reduction mechanisms and give a comprehensive dataset for determining the environmental mobility of Mo and V in a variety of geochemical scenarios.
Date of Award | 1 Aug 2022 |
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Original language | English |
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Awarding Institution | - The University of Manchester
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Supervisor | Samuel Shaw (Supervisor) & Victoria Coker (Supervisor) |
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- FTIR
- Iron oxides
- X-ray absorption spectroscopy
- PHREEQC
- XANES
- Molybdenum
- Biogenic magnetite
- Magnetite
- Hematite
- Vanadium
- EXAFS
The Adsorption of Molybdate(Mo(VI)) and Vanadate(V(V)) to Iron Oxides
Zhang, J. (Author). 1 Aug 2022
Student thesis: Phd