This study investigated the solubility characteristics of several polyaluminium coagulants with different chemistries. Solubility in deionized water was studied at 20 and 5°C between pH 4 and 9 for seven coagulants: alum, a low and a high basicity polyaluminium chloride (PACl) containing no sulphate, a medium and a high basicity PACl with sulphate, aluminium chlorohydrate and a low basicity polyaluminium sulphate (PAS). Solubility was defined by measuring the concentration of aluminium passing through a 0.22 µm pore size membrane filter. Aluminium solubility diagrams were produced from the dissolved Al—pH data. Experimental solubility data were compared with published thermodynamic data to infer the Al-species present.
Solubility characteristics differed significantly between the different polyaluminium coagulants. At 20°C, the pH of minimum solubility was 6.0 for alum and PAS, between 6.2 and 6.4 for the PACls, and 6.7 for aluminium chlorohydrate. At 5°C, the pH of minimum solubility increased for all coagulants. Among the PACls, the pH of minimum solubility increased as the basicity increased. The solubility data indicated the presence of the Al7+13 polymer for the high basicity PACls and a mixture of polymers and monomers for the low and medium basicity PACls. There was no evidence of the Al7+13 polymer for alum or PAS. The presence of low concentrations of sulphate (2%) in the high basicity PACls did not affect solubility.