Phosphate adsorption in flocculation processes of aluminium sulphate and poly-aluminium-silicate-sulphate

The adsorption of mono-phosphate ions from aqueous solutions by flocs formed upon hydrolysis of alum (Al2(SO4)3) and a basic alum (poly-aluminium-silicate-sulphate [PASS]) was investigated as a function of pH (5–7), phosphate concentration (0–0.53 mM or 0–50 ppm) and coagulant concentration. The PO4 adsorption processes were examined—first, on pre-formed flocs of alum or PASS and, second, upon in-situ floc formation, e.g. hydrolysis and flocculation in the presence of PO4 ions. The PO4 adsorption data, together with zeta-potential results, show that the PO4-floc interactions on pre-formed flocs involve mainly specific interactions accompanied by H+ or OH− exchange; with in-situ hydrolysis, complexation and charge neutralization effects play a significant part. On pre-formed flocs, PO4 binding does not distinguish between alum and PASS. Upon in-situ hydrolysis and coagulation, PASS floc binds less PO4 than alum floc, due to the higher cationic charge of the latter; however, PASS yields a floc which is larger and more resistant to shear stress than the floc obtained with alum, particularly at low pH.