The Adsorption of Molybdate(Mo(VI)) and Vanadate(V(V)) to Iron Oxides

  • Jing Zhang

Student thesis: Phd

Abstract

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 Award1 Aug 2022
Original languageEnglish
Awarding Institution
  • The University of Manchester
SupervisorSamuel Shaw (Supervisor) & Victoria Coker (Supervisor)

Keywords

  • FTIR
  • Iron oxides
  • X-ray absorption spectroscopy
  • PHREEQC
  • XANES
  • Molybdenum
  • Biogenic magnetite
  • Magnetite
  • Hematite
  • Vanadium
  • EXAFS

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