Alum

About This Product

Alum, in water treatment usage, means aluminium sulfate, formula Al2(SO4)3 with CAS number 10043-01-3, supplied as solid lump, granules or kibbled material and as a liquid solution around 8 percent Al2O3. Its function is coagulation, and it works in two ways at once. Fine suspended particles in raw water carry a negative surface charge that keeps them mutually repelled and permanently in suspension. Trivalent aluminium ions neutralise that charge, allowing the particles to approach and aggregate. At the same time, the dissolved aluminium hydrolyses and precipitates as aluminium hydroxide, a voluminous gelatinous floc that physically entraps particles as it settles, an effect known as sweep flocculation. The trade-off is pH. Hydrolysis consumes alkalinity and drives pH down, and aluminium hydroxide is only minimally soluble across a narrow band around pH 6 to 7. Outside that band residual dissolved aluminium rises, so alum dosing usually requires alkalinity adjustment, which is the principal reason polyaluminium chloride has displaced it in many plants. It is used in drinking water and wastewater clarification, paper sizing, textile mordanting, tanning and as a fire retardant. Bio Green Chem supplies alum to specification on Al2O3 content, free acid, iron, insolubles, arsenic and heavy metals.