Filtration of colloidal dispersions flocculated by anionic and cationic polyelectrolytes

Abstract

The state of flocculation produced by polyelectrolyte additions can be followed accurately by measurements of the improvement in the rate of subsequent filtration, which becomes a measure of size of flocs and the porosity of macroflocs forming the filter cake. This rate increases rapidly, as the eighth power of the concentration of polyelectrolyte, and reaches a sharp maximum whose value P_M is a reproducible characteristic for each polymer-substrate system. Flocculation is dominated by the extent of adsorption of the polyelectrolyte which leads to a bridging mechanism between particles, and not by electrostatic interaction as required by the DLVO theory. The adsorption theory of Smellie and La Mer, modified by Healy and La Mer to take into account molecular parameters, gives an accurate description of the re-filtration rate in terms of Langmuir's θ, the fraction of the surface covered, and (1–θ) the fraction uncovered.

The steady-state kinetics of flocculation, of deflocculation, and of repair of flocs, yields the Smoluchowski equation for rate of coagulation, but is modified by the factor [θ(1–θ)] to become the rate of primary flocculation. This factor is a quantitative measure of sensitization. The radius of the floc R depends on [θ²(1–θ²)] and the rate of filtration upon [θ⁴(1–θ)⁴], which is the theoretical basis for the experimental eighth power law. New data emphasizing cationics is presented and contrasted with that for anionics in respect to zeta potential, molecular weight, length of chain, and specific radicals affecting adsorption.