Polyaluminium chloride

(endorsed 2005)

Polyaluminium chloride is used as a primary coagulant in the treatment of drinking water. It is effective over a range of pH values. It is particularly effective on some waters and usually requires a lower dose than alum.

General description

Polyaluminium chloride (PACl), $\text{Al}_{2}\text{(OH)}_{3}\text{Cl}_{3}$, is also known as aluminium hydroxy chloride or basic aluminium chloride. In solution, PACl is colourless to pale yellow, clear to slightly cloudy liquid. It is usually supplied with a minimum of 10% $\text{Al}_{2}\text{O}_{3}$ content, a pH of 2.2–2.8 and a basicity of about 50% (w/w). PACl solution has a specific gravity of 1.18–1.22 at 20°C and is completely soluble in water. Its use requires less alkalinity adjustment than most coagulants because of its basicity.

The formula $\text{Al}_{2}\text{(OH)}_{3}\text{Cl}_{3}$, is simply a representation of the proportions of aluminium, hydroxide and chloride in the solution. A generic formula for the PACl species may be given as $\text{Al}_{2}\text{(OH)}_{m}\text{Cl}_{(6-m)}$ where the value of m typically ranges from 2.5 to 3.5.

PACl can be stored in fibreglass or plastics (polyethylene, polypropylene or polyfluorene), but is corrosive to most materials, including stainless steel (although 316 stainless steel can be used).

Chemistry

PACl is manufactured by the reaction of hydrochloric acid with aluminium-containing raw materials such as aluminium metal, alumina trihydrate, aluminium chloride or aluminium sulfate.

PACl solution is a complex, dynamic mixture of positively charged polynuclear aluminium species, with no single species predominating. When applied to water, these species interact with and destablise negatively charged colloidal matter, such as inorganic particles and the high molecular weight organic compounds that largely constitute natural organic matter. The polynuclear species also hydrolyse to form dense flocs of aluminium hydroxides that further act to entrap particles and remove some organic.

An example of one of the many polynuclear species that may be present in PACl solution is the so called Al-13 ion that has the formula $\text{[AlO}_{4}\cdot\text{Al}_{12}\text{(OH)}_{24}\text{(H}_{2}\text{O)}_{12}\text{]}^{13+}$.

The hydrolysis of PACl produces less acid than the hydrolysis of aluminium sulfate owing to the high degree of hydroxylation of the aluminium. As a result, PACl generally requires less pH correction with alkali than if alum were the coagulant.

Typical use in Australian drinking water treatment

PACl is used as a primary coagulant to reduce turbidity, metals, colour and natural organic matter.

The amount of PACl added as a coagulant depends on the properties of the raw water, including factors such as turbidity, dissolved organic carbon, temperature and alkalinity.

Typical PACl doses (with 10% $\text{Al}_{2}\text{O}_{3}$ content) are 5–100 mg/L, although higher doses can be required if the water is particularly dirty. Doses should be determined by laboratory trials.

PACl is the next most commonly used aluminium salt after alum. Compared to alum, it produces a relatively robust floc, generally requires lower doses and is effective over a wider pH range.

Contaminants

PACl solution is usually low in trace metals, because it is made from clean raw materials. However, the following chemical contaminants may be present in this product:

• antimony

• arsenic

• barium

• beryllium

• cadmium

• chromium

• copper

• fluoride

• iron

• lead

• magnesium

• manganese

• mercury

• nickel

• phosphorus

• selenium

• silver

• thallium

• zinc

Residual and by-product formation in drinking water

When employed in drinking water treatment, PACl 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.

Most of the aluminium ions resulting from the use of PACl as a coagulant are removed by conventional water treatment processes. Residual chloride will be present, but at low levels that do not adversely affect drinking water quality.

Status

Polyaluminum chloride was endorsed by the NHMRC for use as a drinking water treatment chemical in 1979. The revision undertaken in 2003 did not change the status of this chemical for the treatment of drinking water.

References

ANSI (American National Standards Institute)/AWWA (American Water and Wastewater Association) Standard no B408-98. 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.