A three-step model has been proposed for the adsorption of cationic dyestuffs (Basic Blue 69 and Basic Red 22) from aqueous solutions on to natural clay. The rate of adsorption is controlled initially by the boundary layer film and an external mass-transfer coefficient Ks has been determined for this process under the influence of a number of design variables. Two approaches, designated methods 1 and 2, are available for determining the external mass-transfer coefficient. Using log-log correlations, the external mass-transfer coefficient was found to vary linearly with the agitation speed, the clay particle size range and the mass of clay employed.
