Structural incorporation and thermodynamic properties of Mo6+ iniron oxides and their role in Mo immobilization in soils

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In this project we propose to investigate the importance of iron oxides on the mobility and bioavailability of Mo6+ in soils. Molybdenum represents a critical prerequisite for the biosynthesis of several metalloenzymes and is therefore essential for all known life (Mendel & Bittner, 2006; McGrath et al., 2010, Duval et al., 2015). Its mobility and bioavailability in soils is low and directly coupled with the presence of iron oxides (Lang & Kaupenjohann, 1999). In order to evaluate the importance of iron oxides for Mo immobilization, previous studies focused predominantly on adsorption mechanisms (e.g. Goldberg et al., 1996; Brinza et al., 2008), whereas the possibility of structural incorporation by a heterovalent substitution of Fe3+ by Mo6+ was almost completely ignored (Richmond et al., 2004; Carroll & Richmond, 2008; Zemberyová et al., 2010).Iron oxides possess several structural mechanisms which can permit a heterovalent substitution (e.g. Balko & Clarkson, 2001; Khan et al., 2008; Bolanz et al., 2013; Ciobanu et al., 2013). For Mo6+, the most abundant Mo specie in oxic systems (Barron et al., 2009), however, almost no convincing data for structural incorporation into iron oxides exist. This tremendous gap in the knowledge is transferable to the thermodynamic properties of Mo-bearing iron oxides. Minerals, which structurally incorporate foreign ions like Mo6+, can exhibit fundamentally different thermodynamic properties and therefore stabilities (Heaney, 2000). Molybdenum could therefore alter its own bioavailability.Therefore, this project has four milestones including (i) synthesis and characterization of Mo6+-bearing iron oxides, (ii) identification of the mechanisms which allow iron oxides to structurally incorporate Mo6+, (iii) to provide calorimetric data (enthalpies of mixing, enthalpies of formation) of Mo-substituted iron oxides and to evaluate the effects of structurally incorporated Mo6+ on the thermodynamic stabilities of iron oxides, (iv) a transfer of the acquired incorporation mechanisms from synthetic Mo6+-bearing iron oxides, formed under laboratory conditions and their thermodynamic properties, to natural iron oxide in various soil types. In order to achieve these goals, a combination of bulk, micro- and nano-resolution techniques will be employed comprising powder X-ray diffraction, inductively- mass spectrometry, mass spectrometer-coupled gas emission system, transmission electron microscope and X-ray absorption spectroscopy techniques.It is expected that this study will significantly improve our understanding about the structural incorporation mechanisms of Mo6+ in iron oxides by heterovalent substitution reactions and establish a foundation for future research on Mo mobility and bioavailability in soils.