Sodium aluminate

(endorsed 2005)

Sodium aluminate is used as a primary coagulant in drinking water treatment, especially in water with low alkalinity; it can also be used in combination with alum to control alkalinity and pH.

General description

Sodium aluminate, $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$, is a white powder that is hygroscopic, soluble in water and strongly alkaline. The aqueous solution is a clear, colourless to pale amber liquid, with a pH of 14.

Sodium aluminate can be supplied as a powder or as a solution. The solid product contains 70–90% $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$, whereas the liquid form contains 29–35% $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$. The liquid solution has a specific gravity of 1.4–1.6, with an excess alkali (as sodium hydroxide, NaOH) of 8–13% and $\text{Al}_{2}\text{O}_{3}$ equivalent of 18–21%.

Appropriate handling materials for sodium aluminate include iron, fibreglass-reinforced plastic, polyethylene, rubber, steel, stainless steel and concrete.

Chemistry

Sodium aluminate is produced by combining aluminium oxide with excess caustic soda.

The aluminium ion neutralises the negative charges on turbidity particles and also forms insoluble metal hydroxides that agglomerate the neutralised particles:

$$\text{Na}_{2}\text{Al}_{2}\text{O}_{4}+\text{4H}_{2}\text{O}\rightarrow\text{2Na}^{+}+\text{2Al}^{+3}+\text{8OH}^{-} \newline \text{Al}_{2}\text{O}_{3}+\text{3H}_{2}\text{O}\leftrightarrow\text{2Al(OH)}_{3}\text{(s)}$$

1 mg/L of $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$ (88%) increases the alkalinity of the water by 0.54 mg/L and reduces carbon dioxide, $\text{CO}_{2}$, by 0.47 mg/L.

Typical use in Australian drinking water treatment

In drinking water treatment, sodium aluminate is used as a primary coagulant, especially in water with low alkalinity. It can also be used in combination with alum to control alkalinity and pH. An advantage of sodium aluminate is that the chemical provides both aluminium and alkali. However, its use as a coagulant in water treatment is limited by cost and by its chemical properties, which make it more difficult to handle than alum or other metal salts.

Because sodium aluminate contains a high percentage of aluminium, a concentration of 1 mg/L of $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$ is equivalent to 3.5 mg/L of alum (on a dry weight basis).

Typical concentrations used are 2–60 mg/L (as $\text{Na}_{2}\text{Al}_{2}\text{O}_{4}$). The appropriate level should be determined by jar testing.

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:

• arsenic

• cadmium

• chromium

• iron

• lead

• mercury

• selenium

• silver

Residual and by-product formation in drinking water

When employed in drinking water treatment, sodium aluminate 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.

Aluminium residuals remaining after filtration can cause floc to form in the distribution system, which can lead to customer complaints.

Status

Sodium aluminate was endorsed by the NHMRC for use as a drinking water treatment chemical in 1983. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

NSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B405-00. AWWA CD-ROM (April 2003). Available at <www.awwa.org>

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.