A kinetic investigation of cationic starch adsorption and flocculation in kaolin suspension

The flocculation kinetics of kaolin particles suspended in 0.01 mol/L NaCl solution and adsorption properties of cationic starch on kaolin surface have been investigated at pH 5.0. Adsorption kinetics of cationic starch followed pseudo-second-order model (R2 > 0.98). Flocculation kinetics showed that increase in flocculant dosage resulted in higher rate constants for the flocculation process and lower rate constant for the aggregate breakage. The rate of aggregation of particles and frequency of collisions of particles are very slow and the two steps determined the rate of flocculation process. The adsorption of cationic starch on the kaolin surface followed Langmuir isotherm (R2 > 0.99). Thermodynamic study indicated that the values of ΔGθ were strongly temperature-dependent and that cationic starch adsorption onto kaolin particles entropically dominated rather than enthalpically driven since |ΔHθ| < |−TΔSθ|. The negative values of ΔGθ and the positive values of ΔHθ suggested that the adsorption process is spontaneous and endothermic. The value of ΔHθ (37.262 kJ/mol) suggested that the electrostatic interaction is the dominant mechanism for the adsorption of cationic starch on kaolin. The fractional coverage decreased with increasing temperature, consistent with a reduced hydrodynamic diameter and a more contracted polymer conformation.