PHOSPHATE ADSORPTION IN FLOCCULATION PROCESSES OF ALUMINUM SULFATE AND POLY-ALUMINUM-SILICATE-SULFATE

The adsorption of mono-phosphate ions from aqueous solutions by flocs formed upon hydrolysis of alum (Al-2(SO4)(3)) and a basic alum (poly-a luminium-silicate-sulphate [PASS]) was investigated as a function of p H (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 Aocs of alum or PASS and, second, upon in-situ flee format ion, 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 hydrolysi s. complexation and charge neutralization effects play a significant p art. On pre-formed flocs, PO4 binding does not distinguish between alu m and PASS. Upon in-situ hydrolysis and coagulation, PASS flee binds l ess PO4 than alum floc, due to the higher cationic charge of the latte r; however, PASS yields a flee which is larger and more resistant to s hear stress than the flee obtained with alum, particularly at low pH. (C) 1997 Elsevier Science Ltd.