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The Elements in the Second Rank

8 Molybdenum

8.1 Identity

Table 8.1
CAS No., EINECS No and molecular weight for Molybdenum

8.2 Physico-chemical properties

Molybdenum is a heavy metal belonging to group VIA in the periodic table. It occurs in valences from -2 to +6 [11]. The high oxidation numbers are most common and stable [12]. Metallic molybdenum has a silver-white metal glance. Molybdenum is oxidized at high temperature. It is not soluble in water, but has high solubility in concentrated sulphuric acid and nitric acid.

Table 8.2
Physico-chemical data for metallic molybdenum. Data from [1,3, 8]

8.3 Uses and consumption

8.3.1 Uses

Molybdenum is used in alloys of steel and for specialty steel/stainless steel [4, 24, 30]. It is also used widely in:

	catalysts for the chemical industry, especially the petrochemical industry [27, 31]
	color pigments (lead molybdenum) [27, 32]
	plastic [4]

It has been used mainly in metal alloys, e.g. special alloys to airplanes, weapons, [31], electronic components [22], sports equipment (petanque-balls, skies and hockey skates [19, 20], but is also used in alloys for electrodes [31].

Molybdenum trioxide is used as a flame retardant in e.g. polyester products [26].

The layered structure of molybdenum sulphide is utilised in lubricants for the automobile industry due to its high heat resistance [24, 27, 31]. Molybdenum has also a good conductivity and has therefore been applied in heating elements in high temperature furnaces [18].

Molybdenum is sold as dietary supplement and vitamins to animals and humans [21, 23, 32] and as plant nutrient for soils being deficient with respect to molybdenum [31].

8.3.2 Consumption

About 250 kg of molybdenum was discharged from industrial sources in Denmark in 1998 [6].

In the USA, 15,000 tons of molybdenum was used in 2000 [27]. From the per capita consumption in USA it is estimated that the Danish consumption is 275 tons per year.

8.4 Emissions to and occurrence in the environment

Anthropogenic discharges of molybdenum come from many industrial uses, combustion of fossil fuels and incineration of waste.

The content of molybdenum in fossil fuels is not known, but the concentration of molybdenum in air is approximately a factor of 10 higher in the urban environment than in countryside areas [15]. The concentration of molybdenum in fly ash, cinders and sewage sludge is 26-49, 2.5-14 and 1.4-17.4 mg/kg dw respectively [14, 16, 17]. This is supported by the high concentration of molybdenum observed in waste gas and in leachate from land fills where waste products from waste gas cleaning have been disposed (Table 8.4).

Molybdenum occurs naturally in aquatic and terrestrial environments at low concentrations. In fresh water the concentration of molybdenum is between 0.03 and 10 µg/L and is typically found as MoO₄^2- in oxidised environments. Table 8.3 shows the typical concentration of molybdenum in the environment.

Table 8.3
Typical background concentration of molybdenum in the environment. Data from [9, 16]

A limited investigation of the levels of molybdenum (and the other elements covered by this study) in the major emissions and waste streams in society was conducted in the autumn of 2001, see Table 8.4.

Molybdenum was found in relatively high concentrations in leachate and waste water, see Table 8.4. Sewage sludge and sediment from road runoff retention basins do also contain high molybdenum concentration compared to the other metals included in this study.

Table 8.4
Levels of molybdenum in selected emissions and waste products from measurements conducted as part of this study in the autumn of 2001.

8.5 Danger classification

MoO₃ is, except from molybdenum orange (lead chromate molybdate sulphate, CI pigment red 104) and a molybdenum-doped vanadium compound [7], the only inorganic molybdenum compound, which is found on the Danish list of dangerous compounds. Molybdenum trioxide is classified as harmful with risk phrases R48/20/22 (harmful: danger of serious damage to health by prolonged exposure through inhalation and if swallowed) and irritant for eyes and respiratory system.

Table 8.5
Classification of and risk phrases for molybdenum and molybdenum compounds

8.6 Toxicology

Molybdenum forms part of many enzymes and is regarded as an essential metal. Hence, both deficiency effects and toxic effects can be observed depending on molybdenum concentration. Long term exposure of low molybdenum concentrations does not result in chronic adverse effects. It seems, however, that there is a relation between low cobber metabolism and assimilation of molybdenum, and the importance of excretion from the kidneys could indicate that sensible individuals are to be found among groups with these characteristics [8]. Typical effects from sub-chronic and chronic oral exposure of experimental animals to high molybdenum concentrations are destabilization of the gastrointestinal tract, growth retardation, anaemia, bone deformity, sterility, and kidney and liver abnormalities.

8.7 Environmental properties

Molybdenum is an essential micro-nutrient, which forms part of several enzymes in bacteria, fungi, plants and mammals [9]. Among the molybdenum-containing enzymes are oxidases, dehydrogenases and nitrogenases [13].

8.7.1 Environmental chemistry

Some molybdenum compounds, e.g. MoO₃, can evaporate, while molybdenum in the terrestrial environment is relatively immobile. The sorption potential in soil is high (0.15-2.02 mg/g) and depends on the content of clay and organic matter. Molybdenum is found as MoO₄^2- in the aquatic environment. Transport of molybdenum in soil and groundwater reservoirs occurs slowly, depending on pH and content of sulphate and phosphate.

Amendment of sewage sludge on farm land results in molybdenum supply to the soil. After intensive sludge amendment the concentration of molybdenum can be as high as three times the background concentration [2].

MoO₄^2- as found in the aquatic environment can be reduced and precipitate when the environment turns anoxic. Under these conditions, molybdenum forms compounds of low solubility such as ion monosulphides. Molybdenum can also adsorb to ion oxides or organic matter and thereby be removed from the water phase [2].

8.7.2 Environmental toxicology

Results from environmental toxicity tests with molybdenum and molybdenum compounds are shown in Table 8.6. The acute toxicity measured as LC₅₀ on Daphnia magna is low: 2.848 mg Na₂MoO₄/L [10]. The toxicity of an unspecified Mo-compound and MoO₃ to algae and crustaceans are higher, but does not enough to result in a classification as dangerous for aquatic organisms. Ruminants have been observed to develop anaemia, growth retardation and diarrhea when grazing on pastureland with 20-100 ppm molybdenum [11, 15].

Table 8.6
Test results for environmental toxicity. Data from [5, 10]

8.7.3 Bioaccumulation

Molybdenum is an essential micro-nutrient and it is assimilated naturally in all parts of the food web. No bioaccumulation of the metal has been observed in lipid rich biota. It is not expected that molybdenum biomagnifies in the food web.

8.8 Conclusions

Molybdenum is used in industry in e.g. alloys and lubricants. The consumption has increased partly because of its use as a substitute for cobber due to its low toxicity. Molybdenum occurs naturally in all environments and is an essential micro nutrient. Amendment of sewage sludge on farmland and resulting increments in molybdenum concentration is not expected to give adverse effects if current sludge amendment procedures are followed. Molybdenum and inorganic molybdenum compounds are not to be classified as dangerous for aquatic organisms.

8.9 References

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