Gallium sorption on montmorillonite and illite colloids: Experimental study and modelling by ionic exchange and surface complexation

Abstract

The migration of metals as gallium (Ga) in the environment is highly influenced by their sorption on clay minerals, as montmorillonite and illite. Given the increased usage of gallium in the industry and the medicine, the Ga-associated waste may result in environmental problems. Ga sorption experiments were carried out on montmorillonite and illite colloids in a wide range of pH, ionic strength and Ga concentration. A Ga sorption model was developed combining ionic exchange and surface complexation on the edge sites (silanol and aluminol-like) of the clay sheets. The complexation constants were estimated as far as possible from the Ga hydrolysis constants applying the linear free energy relationship (LFER), which allowed to reduce the number of free parameters in the model. The Ga sorption behaviour was very similar on illite and montmorillonite: decreasing tendency with pH and dependency on ionic strength at very acidic conditions. The experimental data modelling suggests that the Ga sorption reactions avoid the Ga precipitation,which is predicted in absence of clay colloids between pH 3.5 and 5.5. Assuming this hypothesis, clay colloids would affect Ga aqueous speciation, preventing precipitation in favour of sorption. Ga sorption on montmorillonite and illite can be explained on the basis of three main reactions: Ga3+ exchange at very acidic conditions (pH < 3.8); GaOH4- complexation on protonated weak sites in acidic-neutral conditions (between pH 5.2 and pH 7.9); and Ga(OH)3 complexation on strong sites at basic conditions (pH > 7.9).