Zinc orthophosphate

(endorsed 2005)

Zinc orthophosphate is used to inhibit corrosion of lead, copper and iron, and to prevent the release of asbestos or cement from water pipes.

General description

Zinc orthophosphate, $\text{Zn}_{3}\text{(PO}_{4}\text{)}_{2}$ solution is a clear odourless liquid that is soluble in water; it is available in various ratios of phosphate to zinc.

Appropriate materials for handling zinc orthophosphate include cast iron, steel, fibreglass-reinforced plastic, polyethylene and polyvinyl chloride; rubber-lined containers can also be used.

Chemistry

Zinc orthophosphate is manufactured using zinc salts (chloride or sulfate) and orthophosphate. Zinc orthophosphate limits the release of lead, copper and iron from metal surfaces by forming a microscopic protective film on these surfaces, and by electrochemical passivation. Water with a high pH (> 8.1) should not be treated with zinc orthophosphate because of zinc hydroxide precipitation.

Reactions between orthophosphate and lead in water result in the formation of several solids that are less soluble than basic lead carbonate over a wide range of pH values. The most likely solid phase formed is hydroxypyromorphite ($\text{Pb}_{5}\text{(PO}_{4}\text{)}_{3}\text{OH}$). Tertiary lead orthophosphate ($\text{Pb}_{3}\text{(PO}_{4}\text{)}_{2}$) is another solid formed. The formation of lead orthophosphate films depends on the concentration of dissolved inorganic carbon (DIC; e.g. carbonates), acidity, temperature and orthophosphate content. These phosphate films may not form as rapidly as the basic lead carbonate solids. Carbonate competes with orthophosphate for control of lead solubility. Hence, lead orthophosphate films can be formed in water with low levels of carbonate or DIC (these two characteristics are often found together), in which case the effectiveness of a phosphate control program may need to be evaluated over a longer time.

Typical use in Australian drinking water treatment

In drinking water treatment, zinc orthophosphate is used to inhibit corrosion. It is particularly effective at inhibiting lead corrosion, because it reduces lead solubility in waters with both low and high alkalinity. The chemical is used to treat waters that are soft and corrosive. Zinc orthophosphate suppresses corrosion of carbon steel, and the release of asbestos fibres from asbestos–cement (A–C) pipe. It also inhibits corrosion of cast iron, and mildly inhibits corrosion of copper.

A few milligrams per litre of orthophosphate are sufficient at pH values in the 7–9 range.

Contaminants

The purity of chemicals used in Australia for the treatment of drinking water varies, depending on the manufacturing process. The following chemical contaminants may be present in this product:

• chloride

• sulfate

Residual and by-product formation in drinking water

When employed in drinking water treatment, zinc orthophosphate should be used in such a way that any contaminant or by-product formed by the use of the chemical does not exceed guideline values in the Australian Drinking Water Guidelines.

Status

Zinc orthophosphate was endorsed by the NHMRC for use as a drinking water treatment chemical in 1987. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

Clesceri LS, Greenberg AE and Eaton AD (eds) (1998). Standard Methods for the Examination of Water and Wastewater, 20th edition. American Public Health Association, Washington, DC.

Gosselin RE, Smith RP, Hodge HC (1984). Clinical Toxicology of Commercial Products, 5th edition. Williams and Wilkins, Baltimore, II-121.

Lewis RJ (1993). Hawley’s Condensed Chemical Dictionary, 12th edition. Van Nostrand Reinhold, New York.

NRC (National Research Council) (1981). Drinking Water & Health, Volume 4. National Academy Press, Washington, DC.

Schock MR (1999). Internal Corrosion and Deposition Control. In: Water Quality and Treatment, A Handbook of Community Water Supplies, Letterman RD (ed), American Water Works Association, 5th edition. McGraw-Hill Professional, 17.1–17.109.

Shibata H, Morioka T (1982). Antibacterial action of condensed phosphates on the bacterium Streptococcus mutans and experimental caries in the hamster. Archives of Oral Biology 27(10): 809–16.