The effect of polymer adsorption on the electrokinetic stability of dilute clay suspensions

Abstract

A detailed study has been made of the destabilization of dilute clay suspensions with a cationic polymer under controlled conditions of pH, ionic strength, initial clay concentration, and intensity and duration of agitation. A C 14-labeled cationic polymer was added to each concentration of kaolinite and montmorillonite clay suspended in a slightly buffered synthetic water (ph = 7; μ = 1.15 (10) −3), and the clay-polymers systems were mixed at a given intensity of agitation for a predetermined period of time, following which they were allowed to sediment. The degree of destabilization of the system was evaluated by residual turbidity measurements of the settled suspensions. Polymer residuals in the supernatant liquid were determined by liquid scintillation counting techniques and electrophoretic mobility measurements were made on the clay-polymer systems. The destabilization of dilute clay suspensions (i.e., <150 mg./l.) was found to depend upon the initial clay concentration and the polymer dosage. The kinetics of polymer adsorption are extremely rapid and consequently the rate-controlling step in the flocculation process is the frequency of interparticle collisions, which depends upon the initial clay concentration. A rather well-defined stoichiometric relationship between initial clay concentration and optimum polymer dosage, that is, the dosage which maximized the degree of suspension destabilization, was found. The polymer adsorption data fit the Langmuirian model very well and reveal an inverse relationship between the initial clay concentration and the limiting adsorption value as calculated from the linearized form of the Langmuir adsorption equation (i.e., mg. polymer/mg. clay). As the polymer dosage was increased, the polymer-clay system passed through a zone of optimum turbidity removal and into a repeptized region where the clay particle or particle aggregate surfaces became saturated with polymer molecules. This was substantiated by both polymer adsorption and electrophoretic mobility data. It is postulated that the cationic polymer acts both as a coagulant, in compressing the double layers of the clay particles, and as a flocculant, in bridging the particles, the latter process being controlled by the initial clay concentration of the suspension.