Flocculation and reflocculation: Interplay between the adsorption behavior of the components of a dual flocculant

Abstract

Many dual flocculants are used in various industries. Two-component flocculants often offer advantages over a single component flocculant, such as better control of flocculation kinetics and improved floc strength. Most dual component flocculants consist of two polyelectrolytes, two polymers, or a polyelectrolyte and a nanocolloid. Usually one of the components adsorbs on the surface of the particles to be flocculated and the second component bridges these polymer-coated particles. One example of a dual component polymeric flocculant is poly(ethylene oxide) (PEO) and carboxylated phenolic resin (CPR), usually referred to as a cofactor. We show that this dual flocculant can induce a richness of flocculation behavior depending on the concentration of the two components. Flocculation, deflocculation and reflocculation of cellulose particles were studied for various CPR–PEO ratios. It was found that reflocculation is a strong function of this ratio. For low ratios no reflocculation occurs after a few flocculation cycles, whereas for high ratios very limited flocculation and reflocculation occurs. For intermediate ratios extensive flocculation and reflocculation occurs, and the extent of reflocculation increases with the number of flocculation cycles. The results are interpreted in terms of the association-induced polymer adsorption theory, which allows for the cofactor CPR to play two roles: one the traditional role of one polymer (PEO) forming a bridge between particles coated by the other polymer (CPR), and a second role or forming a complex between CPR and PEO, which adsorbs on particles and causes flocculation. Increasing the concentration of CPR results in more CPR “patches” on the particles and thus in more bridging by PEO, which increases the flocculation, but too much cofactor also results in highly charged PEO/CPR complexes, which reduce flocculation. As such there is an optimum CPR–PEO ratio for the bridging association complex, which in general is different from the ratio of the added components, because free CPR can be present from a previous addition and CPR can also associate with previously adsorbed PEO and can adsorb on the surfaces of the flocculating particles.