Adsorption of Monovalent Organic Cations on Sepiolite: Experimental Results and Model Calculations

Giora Rytwo1, 2, Shlomo Nir2, Leon Margulies2, Blanca Casal3, Jesus Merino3, Eduardo Ruiz-Hitzky3 and Jose Maria Serratosa3
1 Tel Hai Academic College, Upper Galilee, 12210, Israel
2 The Seagram Center for Soil and Water Sciences, The Faculty of Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel
3 Instituto de Ciencia de Materiales de Madrid, CSIC, Cantoblanco, 28049 Madrid, Spain

Abstract: Adsorption of neutral organic molecules and the monovalent organic cations methylene blue (MB) and crystal violet (CV) to sepiolite was determined experimentally and investigated by an adsorption model. The largest amounts of MB and CV adsorbed were about 4-fold of the cation exchange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the clay. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by allowing cation adsorption to neutral sites of the clay, in addition to adsorption to negatively charged sites and adsorption to neutral complexes formed from 1 cation adsorbed to a negative surface site. The amount of available neutral sites was determined from the adsorption of the neutral molecule Triton-X 100 (TX100). The model could adequately simulate the adsorption of the neutral molecules TX100 and crown ether 15-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than that of CV at the larger added concentrations. A consideration of the molecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the sepiolite channels was largely excluded. This interpretation is supported by infrared spectroscopy (IR) measurements, which show large perturbations of the peak corresponding to vibrations of external Si-OH groups of the clay and confirm complete occupancy of external sites by MB and CV.

Key Words: Adsorption Model • Crystal Violet • Methylene Blue • Sepiolite

Clays and Clay Minerals; June 1998 v. 46; no. 3; p. 340-348; DOI: 10.1346/CCMN.1998.0460313
© 1998, The Clay Minerals Society
Clay Minerals Society (www.clays.org)