Molybdenum

(endorsed 1996)

Guideline

Based on health considerations, the concentration of molybdenum in drinking water should not exceed 0.05 mg/L.

General description

Molybdenum is present in ground and surface waters at very low concentrations, generally below 0.01 mg/L. Higher concentrations have been reported in the vicinity of molybdenum mining operations. Fly ash deposited onto soils from coal-fired power stations can be a significant source of molybdenum. Application of fertilisers may also increase the concentration of molybdenum in ground and surface water.

Molybdenum is used in the production of steel, electrical components such as spark plugs, and nonferrous metal alloys. Molybdenum compounds are used as lubricants in oils and greases, and in fertilisers to overcome molybdenum deficiency in soils.

Many foods contain significant amounts of molybdenum. Legumes, grains and liver have the highest concentrations and food is a significant source of intake.

Typical values in Australian drinking water

Australian drinking water supplies have not been routinely monitored for molybdenum.

Treatment of drinking water

There are no published methods for the removal molybdenum from drinking water.

Measurement

The concentration of molybdenum in drinking water can be determined by inductively coupled plasma emission spectroscopy or graphite furnace atomic absorption spectroscopy (APHA Method 3500-Mo Parts B or C 1992). Limits of determination are 0.04 mg/L and 0.005 mg/L respectively.

Health considerations

Molybdenum is an essential trace element for humans and other animals. The estimated requirement is between 0.15 mg/day and 0.5 mg/day for adults.

Approximately 30–70% of dietary molybdenum is absorbed in the gastrointestinal tract. Highest concentrations of molybdenum are found in the liver, kidney and bones. There does not appear to be any significant bioaccumulation of molybdenum in the body. Approximately 90% of ingested molybdenum is excreted in the urine.

Data are scarce on the long- and short-term toxicity of molybdenum in humans. One study of people consuming up to 0.2 mg/L of molybdenum in drinking water for 2 years reported no adverse effects. Another study has linked high intake of molybdenum in food with gout-like symptoms, joint pains of the legs and hands, and enlargement of the liver.

A number of long- and short-term animal studies have been undertaken, with considerable variability in the results depending on the chemical nature of the compound and the animal species. Effects included changes in skin and fur pigment, enlargement of joints, weight loss, diarrhoea and emaciation. Not all these effects were observed in each study and effects usually occurred only at high doses.

No relevant data are available on the carcinogenicity of molybdenum. Tests for mutagenicity with bacteria have been inconclusive.

Derivation of guideline

The guideline value for molybdenum in drinking water was determined as follows:

$$\text{ 0.05 mg/L } = \dfrac{\text{ 0.5 mg/day x 0.2 }}{\text{ 2 L/day }}$$

where:

• 0.5 mg/day is the upper range of the estimated adult requirement for molybdenum.

• 0.2 is the proportion of total daily intake attributable to the consumption of water.

• 2 L/day is the average amount of water consumed by an adult.

Studies with animals and humans, while unable to establish a no-effect level, reported no adverse effects due to molybdenum in drinking water at concentrations of 0.05 mg/L (Chappell et al. 1979).

Adverse human health effects have been reported with molybdenum intakes of 10 mg/day (Chappell et al. 1979). This is a hundred times higher than the guideline value, assuming that water consumption is 2 litres per day.

The World Health Organization guideline value of 0.07 mg/L was determined using a different approach which, upon review, was considered to be questionable. The difference between the two values is not significant.

References

APHA Method 3500-Mo Part B (1992). Molybdenum: Atomic Absorption Spectrometric method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.

APHA Method 3500-Mo Part C (1992). Molybdenum: Inductively Coupled Plasma method. Standard Methods for the Examination of Water and Wastewater, 18th edition. American Public Health Association, Washington.

Chappell WR, Meglen RR, Moure-Eraso R, Solomons CC, Tsangas TS, Walravens PA, Wilson PW (1979). Human health effects of molybdenum in drinking water. United States Environmental Protection Agency, EPA-600/1-79-006, Cincinnati, Ohio, 100 pages, NTIS PB-292755.