Adsorption of paraquat on minerals. A thermodynamic study

The adsorption of the paraquat molecule on silica, kaolinite, illite and montmorillonite was studied with regard to both the determination of adsorption isotherms and the microcalorimetric measurement of adsorption enthalpies. Surface concentration at saturation can be related to the neutralization of the surface charge. Part of the observed variation in adsorption enthalpies from one mineral to another can be attributed to the degree of fitting of the adsorptive molecule to the spatial repartition of surface charges. A particularly interesting system is that involving montmorillonite, where there is evidence of the possibility of neutralization, by the same paraquat molecule, of two sites relating to different platelets.