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Environmental fate & pathways

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The speciation of molybdenum in aqueous media as a function of pH and molybdenum concentration has been thoroughly investigated and reported in the open literature. Under physiological conditions (pH > 6.5) the sole molybdenum species is the molybdate anion, MoO42- (Cruywagen, 2000; Cruywagen et al, 2002). Molybdenum compounds, e.g., molybdenum trioxide and polymolybdates, transform rapidly to the MoO42- ion under environmental relevant test conditions (Greenwood and Earnshaw, 1987). For the Mo-substance in this dossier, the UV-spectra of aqueous solutions of this substance indeed demonstrated that the only dissolved molybdenum species, originating directly from the Mo-substance in this dossier, is molybdate.


The derivation of environmental fate data like adsorption/desorption coefficients and bioconcentration/bioaccumulation factors is based on measured Mo-levels, and relates to the properties of the molybdate anion. The latter form is the only form under which molybdenum originating from disodium molybdate will occur. The molybdenum-based environmental fate data in this section of the dossier are considered relevant for the behaviour of molybdenum (as molybdate) that is released into the environment from the molybdenum substance that is assessed in this report.

A pdf reader-friendly version of the Environmental Fate Properties of Molybdenum information is attached to the technical dossier in IUCLID Section 13.

Additional information

Background information on the fate of molybdenum in the environment


Under oxic conditions, dissolved Mo is dominated by the relatively unreactive tetrahedral Mo(VI)O42- ion. The hexavalent oxyanion, molybdate, is also the dominant species of Mo in seawater (Yamazaki and Gohda, 1990). Protonated forms (HMoO4- and H2MoO4) are found at pH < 5 (Jarrell et al, 1980; Oyerinde et al., 2008; Tkac and Paulenova, 2008; Kendall et al., 2017) but their concentration is likely to be very low because at pH <5 adsorption to minerals and organic matter is greatest (see below). Polynuclear species having linked MoO6 octahedra can occur at pH < 6 and high Mo concentrations (Aveston et al., 1964; Cruywagen, 1999). Under specific conditions several metal molybdates can be relatively stable both in solution and in the solid phase. For example, MgMoO4 could be a major Mo soluble species in seawater (Smedley and Kinniburgh, 2017).

Only in exceptional circumstances, as with acidic drainage from near Mo-rich mines and ore bodies, are the more exotic polynuclear species likely to be found.



Molybdenum in the sediment compartment


Part of the molybdate in solution in the water column will migrate into the sediment layer. Oxic and anoxic layered micro-environments will coexist in many sediments, with the oxic fraction typically extending to depths of 2 to 5 mm. Speciation and fate processes under oxic and anoxic conditions are therefore both relevant with regard to the fate of molybdenum in sediments. Firstly, the substances that bind molybdenum will differ under oxic or anoxic conditions (e.g, iron oxide or sulfide). Secondly, various other physicochemical properties determine the speciation of molybdenum (molybdates, thiomolybdates,…) which behave differently in the sediment compartment (differences in binding affinity). The fate of molybdenum in the sediment compartment will be determined by its binding: a fraction that is strongly bound will be fixed and immobilized and will not be released to the porewater compartment.



Xu et al (2013) reviewed existing data on the interaction between molybdenum and clay, and concluded that adsorption of molybdate on clay minerals is a maximum near pH 3 and decreases with increasing pH to zero near pH 7. Under anoxic conditions, in the deeper layers of lakes, interaction of molybdate with sulfide and iron causes molybdenum to move from the water column towards the sediment compartment. Conversion of molybdate to thiomolybdate occurs in the presence of HS- and H+ (Harmer and Sykes, 1980; Luther, 1986; Helz et al, 1996; Erickson and Helz, 2000). This reaction is favoured by acid conditions, but HS-, which is the most effective sulfide species for supplying S2- to the reaction, dominates the sulfide speciation only at high pH (Harmer and Sykes, 1980). The optimal pH for transformation of molybdate to thiomolybdate-species is around pH 7.3. Hence, at this environmentally relevant pH and in sulfur-rich environments where molybdate is converted to thiomolybdate, molybdenum adsorption will occur to sediment particles and to sediment fractions (e.g. goethite, pyrite) which under anoxic conditions precipitate or co-precipitate with iron sulfides.

With regard to interaction between molybdate and organics in sediments there is growing evidence that Mo adsorption to organic matter can be a driver of Mo-biochemistry (e.g. Algeo and Lyons, 2006; McManus et al., 2006; Wagner et al., 2017, Wichard et al., 2009; King et al., 2014, 2016; Marks et al., 2015; Siebert et al., 2015).



Molybdenum in the soil compartment


In line with the findings for the sediment compartment, the clay fraction in soil (Fe- and Mn-(oxyhydr)oxides) are important for Mo adsorption processes in the terrestrial compartment (Bibab and Borggaard, 1994; Goldberg et al, 1996, 2002). However, there is growing evidence that Mo adsorption to organic matter is a driver of Mo bio geochemistry in soils (Wichard et al., 2009; King et al., 2014, 2016; Marks et al., 2015; Siebert et al., 2015).

Typically, a soluble metal species added to a soil sample undergoes an initial, fast reaction, sorption onto solid phases (Ma et al, 2006a,b; Wendling et al, 2009). This is followed by slower reactions that remove the metal from the labile pool into a pool with lower desorption – the aging process as for soil-added Mo (Brennan, 2002,2006; Lang and Kaupenjohann, 2003).

Clay content and incubation time are the most important factors affecting the labile pool of MoO42- in soils with time after addition. Soluble molybdate was removed more rapidly into non-labile pools in neutral to alkaline clay soils than in acidic sandy soils. Labile molybdate concentrations in molybdenum contaminated soils were less than 10% of the total Mo concentration.