On the Mechanism of Clay Particle Linking by Anionic Polyelectrolyte

Abstract

In our previous paper, the flocculating effect of anionic polyelectrolyte on clay particles suspended in water was studied in relation to the mechanism of the formation of polyanion linkage among the clay particles. A new theory on the bridging mechanism has been proposed. The purpose of this paper is to provide satisfactory evidences for the theory.Aqueous suspension of clay (a mixture of quartz and pyrophillite) used in this work was treated with sodium polyacrylate, a typical polyanion, and one of the following reagents: sodium hydroxide, aluminium chloride, hydrochloric acid, or positively charged colloid of ferric hydroxide. For each suspension the pH value, the turbidity, and the amount of waterstable floc formed by the polymer linkage were determined.The experimental results on clay suspension with aluminium chloride showed that, in the range of pH 5.6-7, clay flocculation occurred through the interparticle bridging by polymer chains. The maximum amount of water-stable floc was obtained at pH of about 6.2 Above pH 7, the clay particles became deflocculated owing to the disappearance of the polymer linkage and the electrostatic repulsion among the particles. At the acid pH, below 5.5, polyanions could not participate in the bridging, because they were combined with free aluminium, resulting into a firmly coiled state inactive to the linkage. However, in the clay suspension treated with hydrochloric acid, it was found that the increase of acidity increased the formation of polymer linkage, unless the polymer was curled by the free aluminium produced by the dissociation of the clay lattice.In the case of the interparticle bridging, the polymer chain could be expected to be anchored by positively charged sites on the clay surface through chemical bond, maybe a co-ordinated sort. From the result on the clay suspension treated with colloidal ferric hydroxide, such positively charged sites were estimated to be the colloids of ferric-and/or aluminium-hydroxide fixed on the clay surface, as well as the lattice aluminium on the brocken edges of the clay.Further experiments were carried out concerning the order of addition of sodium polyacrylate and another reagent, aluminium chloride or colloidal ferric hydroxide. These results might also be interpreted by the above mentioned mechanism.