Sorption of selected cationic and neutral organic molecules on palygorskite and sepiolite

Palygorskite and sepiolite show a high sorption capacity for organic molecu les. Adsorption of 2 organic cations, methylene blue (MB) and crystal viole t (CV), by palygorskite and sepiolite were examined. The maximum sorption o f MB and CV far exceeded the cation exchange capacity (CEC) of these minera ls. This shows that, besides the contribution of free negative sorption sit es (P-), the sites satisfied with sorption of single cations (PXi(0)) and n eutral sorption sites (N) on clay surfaces may contribute to the sorption o f organic cations. The number of neutral sites was determined by examining the sorption of 2 neutral organic molecules, triton-X 100 (TX100) and 15 cr own ether 5 (15C5), and by application of the Langmuir isotherm. To determine the contribution of different sites, an adsorption model that applies the Gouy-Chapman equation and takes into account the formation of d ifferent clay-organic complexes in a closed system was employed. Applicatio n of this model to sorption data provided the calculation of binding coeffi cients for neutral sites, as well as the surface potential of the minerals at different sorbate concentrations. At sorption maxima, for both palygorskite and sepiolite, the contribution o f neutral sites for sorption of organic cations was the highest, followed b y the PXi(0) sites in case of CV sorption, while in sorption of MB the cont ribution of P- sites was the second highest. The Fourier transform infrared (FTIR) patterns of clay-organic cation complexes compared with pure clays confirm that the sorption of organic cations is by silanol groups located a t the edge of fibrous crystals, which account for neutral sorption sites.