Polyaluminium chloride (PAC) is a widely used inorganic polymer coagulant in water treatment. A common question from operators and engineers is whether PAC dissolves in water. The answer is yes. PAC has strong water solubility: when added to water it disperses quickly and forms an effective coagulating solution that supports both coagulation and flocculation. This article explains the solubility of PAC, what happens once it dissolves, and how the resulting solution is applied in practice.
Is PAC Water Soluble?
Yes, PAC is readily water soluble. Thanks to its unique chemical structure, which contains partially neutralized polynuclear aluminum species, PAC dissolves easily in water to form a polyaluminium chloride solution with coagulating properties. Both powder and liquid grades disperse rapidly under normal mixing conditions, and the solution can be dosed continuously into treatment streams with standard metering equipment.
Why PAC Dissolves Easily in Water
The molecular structure of PAC, with the general formula Al2Cln(OH)6-n and a typical molecular weight around 116.17, contains hydroxyl bridges that make the polymer hydrophilic. Because the hydrolysis steps that produce PAC are already partially completed during manufacturing, the solid simply rehydrates and ionizes when mixed with water. This pre-hydrolysis is also why PAC acts faster than conventional coagulants such as aluminum sulfate.
What Happens After PAC Dissolves
Once dissolved, PAC releases cationic aluminum hydrolysis products that neutralize the negative charge of suspended colloids, then bridge them into large flocs through adsorption. The resulting flocs settle or filter readily, making solid-liquid separation simple and efficient. In practice a dissolved PAC solution can effectively remove:
Suspended solids: fine particles and colloids that cause turbidity are aggregated and removed.
Organic matter: especially humic substances, which are coagulated and precipitated out of the water.
Heavy metal ions: lead, mercury and other metals are captured by the flocs and removed with the sludge, improving water safety.
Key Applications of PAC Solutions
PAC solutions are used in drinking water purification, industrial wastewater treatment, municipal sewage treatment, papermaking process water, textile and dyeing effluent, and swimming pool clarification. The solution form allows precise dosing and homogeneous distribution, which is particularly important in large continuous treatment plants where dose stability directly affects effluent quality.
Storage and Shelf Life
PAC is typically supplied as a yellow powder or as a liquid concentrate. Powder grades should be kept in a cool, dry, dark location in a tightly sealed container; under these conditions the shelf life is about 24 months. Liquid grades should be stored above their crystallization temperature and stirred before use to keep the solution homogeneous. Moisture pickup and contamination should be avoided, as they can reduce the effective alumina content and coagulation performance.
Frequently Asked Questions
Is PAC completely soluble in water?
Yes, PAC dissolves readily in water, and both powder and liquid grades form homogeneous coagulating solutions under normal mixing.
What is the CAS number and molecular formula of PAC?
PAC has the CAS number 1327-41-9 and a general molecular formula of Al2Cln(OH)6-n with a molecular weight around 116.17.
What can PAC remove from water?
PAC removes suspended solids, organic pollutants such as humic substances, and heavy metal ions such as lead and mercury, improving water clarity and safety.
How long can PAC powder be stored?
Stored in a cool, dry, dark, sealed container, PAC powder typically keeps for about 24 months.
What does PAC look like?
PAC is commonly supplied as a yellow powder, though liquid concentrate grades are also available.
Why is PAC preferred over other coagulants?
PAC offers fast dissolution, quick floc formation, good performance at low temperature and over a wide pH range, and often lower sludge production than conventional coagulants.
