Heterocoagulation experiments of kaolinite with solvent-diluted-bitumen were carried out to investigate the effect of hydrolyzable metal cations and citric acid on the liberation of bitumen from kaolinite. The adsorption of Ca2+ and Mg2+ on kaolinite, and zeta potentials of kaolinite and bitumen droplets in solutions containing 10(-3) mol/L of Ca2+, Mg2+ and Fe3+ with or without citric acid were also measured. It was found that the heterocoagulation of bitumen with kaolinite was enhanced in the presence of the metal cations from pH 7 to pH 10.5, accompanied by a decrease in the magnitude of the zeta potentials and an increase in the adsorption of the metal cations on kaolinite and possibly on bitumen droplets. The addition of 5 x 10(-4) mol/L citric acid reduced the degree of coagulation from 90% to less than 40% in the presence of 10(-3) mol/L Ca2+ and Mg2+ cations at pH similar to 10, and at pH similar to 8 for Fe3+. It was found that hydrolyzable metal cations enhanced bitumen-kaolinite interactions through electrical double layer compression and specific adsorption of the metal hydrolysis species on the surface of kaolinite. The effect of metal cations was removed by citric acid through formation of metal-citrate complexes and/or the adsorption of citrate anions, which restored the zeta potentials of both kaolinite and bitumen. Therefore, electrostatic attraction or repulsion was responsible for the coagulation or dispersion of kaolinite particles from bitumen droplets in the tested system. (c) 2008 Elsevier Inc. All rights reserved.