Molybdenum (Mo) is an essential trace element in the environment. However, at high levels Mo can become toxic to plants and animals, including humans. Mo commonly exists as the tetrahedral oxyanion molybdate (Mo(VI)O42-) in oxic environments in soils associated with mining, steel waste and other extractive industries. Iron oxide minerals (such as hematite (Fe2O3) and magnetite (Fe3O4)) are ubiquitous in subsurface soils and sediments and can strongly impact the mobility of contaminant metals; in particular through absorption to these high surface area minerals. Previous studies on the absorption characteristics of Mo onto iron minerals have focused primarily on goethite and ferrihydrite, and to some extent hematite, with little information on the associations of Mo with magnetite. The aim of this project is to characterise the adsorption behavior of Mo(VI)O42- to hematite and magnetite under varying environmental conditions including pH (4-14), adsorbent concentration and ionic strength. X-ray Absorption Near Edge Structure (XANES) (Fig. 1) indicates the oxidation states of Mo associated with hematite or magnetite is different depending on pH and suggests that Mo(VI) is partially or fully reduced to Mo(IV) when adsorbed to magnetite. Further work using Extended X-ray Absorption Fine Structure (EXAFS) analysis of Mo sorbed to hematite and magnetite provides a detailed molecular scale understanding of the Mo adsorption mechanisms. This is associated with solution analysis and geochemical modelling (PHREEQC) to provide a comprehensive dataset for determining the environmental mobility of Mo in a variety of geochemical systems.

EXAFS analysis for the interaction of molybdate (Mo(VI)O42-) with hematite and magnetite

2019 Spring National Meeting

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