Effect of the morphology of synthetic kaolinites on their sorption properties

Natural kaolinites often have a permanent charge due to mineralogical impurities preventing to link directly the morphology of the kaolinite particle to a selectivity coefficient between two cations for edge sites. In this study, kaolinites with no permanent charge were hydrothermally synthesized under different physicochemical conditions to obtain various morphologies (hexagon-shaped, more or less anisotropic). Na+ and H+ were chosen as the sorbed cations due to their ubiquitous presence in natural waters. For synthetic kaolinites for which no swelling layer was detected, an experimental sorption isotherm between Na+ and H+ was obtained. Data were interpreted using a surface complexation model, containing no electrostatic term, by considering the specific surfaces of lateral sites and sorption site density identified by crystallography for the different faces presented in the samples ((010), (110), (1-10)). Selectivity coefficients between Na+ and H+ for all lateral sites characterizing a given morphology were calculated and validated in the [4–10] pH range, corresponding to the pH range for which dissolution can be considered negligible. The results showed that the Na+/H+ selectivity coefficient depends strongly on the particle morphology and that the sorption properties of kaolinites cannot be obtained with good accuracy without a fine knowledge of the morphology of the particles