The effects of some electrolytes on flocculation with a cationic polyacrylamide

The effects of sodium chloride, sodium sulfate, and alum (aluminum sulfate) on the performance of a cationic polyacrylamide flocculant in a papermaking suspension consisting of bleached (hardwood: softwood, 50∶50) kraft wood-pulp fibres and anatase (TiO₂) were investigated. Sodium chloride and sodium sulfate, 1×10^?5 to 1×10^?2 M, in the presence of polymer, caused negligible changes in the electrophoretic mobility of the TiO₂ and in the first-pass retention of TiO₂ (heteroflocculation of TiO₂ and fibres). Alum at concentrations from 1×10^?5 to 1×10^?4 M at pH 4.0 and 4.5 increased retention with polymer; higher alum concentrations resulted in lower retentions. At pH 4.0 the electrophoretic mobility of the TiO₂ was positive over the entire range of alum concentrations investigated (1×10^?5?3.2×10^?3 M) whereas at pH 4.5 the mobility was negative at 1×10^?5 M alum and charge reversal was observed at about 4×10^?5 M alum. The intrinsic viscosity of the cationic polyacrylamide was decreased by the addition of alum, sodium chloride or sodium sulfate. The effect of alum on the polymer conformation appeared to be that of the non-specific interaction of sulfate ions with a cationic polyelectrolyte. Retention results are discussed in terms of the colloidal stability of TiO₂, the adsorption of polyacrylamide on TiO₂ and the conformation of adsorbed polymer.