A kinetic investigation of the flocculation of alumina with polyacrylic acid.

Using a model colloidal system of alumina and polyacrylic acid (PAA), the kinetics of flocculation was investigated at low polymer concentrations and short durations (on the order of seconds). The polymer-induced flocculation processes obeyed Von Smoluchowski's bimolecular rate equation. Increases in the concentration of the polymer resulted in higher rate constants for the flocculation process. At a fixed concentration (say 50 ppb, parts per billion), the rate constant values showed a maximum value for 250,000 g mol(-1) polyacrylic acid. At this polymer concentration, calculations of the surface coverage of alumina by PAA molecules of different molecular weights show that for all the cases the coverage is nearly the same, approximately 1x10(-3), but the flocculation response and the rates are significantly different. This trend in flocculation characteristics is attributed to the critical polymer number density requirement for effective flocculation (at least partial charge neutralization and initiation of flocculation). The mechanism governing the flocculation at ultralow concentrations (50 ppb) is the synergistic effect of partial patch neutralization and bridging.