4.1. CHEMICAL IDENTITY

Molybdenum (Mo) is a naturally occurring metallic trace element found in natural minerals, but not as the free metal. Biologically, it is an important micronutrient in plants and animals, including humans. It is used widely in industry for metallurgical applications (EPA 1979).

Molybdenum metal is a dark-gray or black powder with a metallic luster (NLM 2020a). It is a transition element in Group 6 of the Periodic Table. It has oxidation states from −2 to +6. Commonly encountered compounds are those of molybdenum in oxidation state +6 (Mo(VI), MoO₃, molybdates) and +4 (Mo(IV), MoS₂). It does not occur naturally in the pure metallic form; it more commonly occurs in the mineral, molybdenite (Sebenik et al. 2012). Other naturally occurring molybdenum-containing minerals are powellite, wulfenite, ferrimolybdite, and ilsemannite; however, molybdenite is the primary commercial source of molybdenum. Molybdenum (VI) anions include molybdate (MoO₄⁻²) with molybdenum at the center of a tetrahedron of four oxygen atoms, and polyberic anions (‘isopolymolybdates’) of which the most common are heptamolybdate (Mo₇O₂₄⁶⁻) and octamolybdate (Mo₈O₂₆⁴⁻) (EPA 1979). These anions occur in sodium and ammonium salts, often hydrated, which are the common sources of molybdenum in commerce and industrial applications.

There are 33 known isotopes of molybdenum. Seven isotopes occur naturally: mass numbers 92, 94, 95, 96, 97, 98, and 100. ⁹⁸Mo is the most abundant isomer, comprising approximately 24.3% (Rumble et al. 2018). Radioisotopes of masses 83–91, 93, 99, and 101–115 have been reported. The only one of major worldwide importance is Mo-99 (⁹⁹Mo), a 100% beta-emitting isotope with a 65.976-hour radioactive half-life that is used to produce technetium-99m (Tc-99^m or ^99mTc) for medical scans (Doll et al. 2014; Parma 2009; Richards 1989).

Under physiological conditions (pH >6.5), the molybdate anion, [MoO₄]²⁻, is the sole molybdenum species in aqueous media (Cruywagen 2000; Cruywagen et al. 2002). Molybdenum compounds (e.g., molybdenum trioxide and polymolybdates) transform rapidly to the [MoO₄]²⁻ ion under environmentally relevant test conditions (Deltombe et al. 1974; Greenwood and Earnshaw 1997). Protonated forms, such as [HMoO₄]⁻ and H₂MoO₄, are found at pH <5 (Smedley and Kinniburgh 2017). Molybdenum tends to be more mobile under alkaline conditions, but adsorption increases with decreasing pH (Goldberg et al. 2002).

Information regarding the chemical identity of molybdenum and molybdenum compounds is provided in Table 4-1.

Table 4-1

Chemical Identity of Molybdenum and Compounds.

4.2. PHYSICAL AND CHEMICAL PROPERTIES

Metallic molybdenum, in the form of fine molybdenum powder, is considered nonflammable.

Information regarding the physical and chemical properties of molybdenum and molybdenum compounds is provided in Table 4-2. Much of the information presented was obtained from the chapter, Molybdenum and Molybdenum Compounds, in the Ullmann’s Encyclopedia of Industrial Chemistry (Sebenik et al. 2012), or handbooks such as the CRC Handbook of Chemistry and Physics or the European Chemicals Agency (ECHA) registration dossiers.

Table 4-2

Physical and Chemical Properties of Molybdenum and Compounds.