ADSORPTION OF MONOVALENT ORGANIC CATIONS ON SEPIOLITE - EXPERIMENTAL RESULTS AND MODEL-CALCULATIONS

Adsorption of neutral organic molecules and the monovalent organic cat ions methylene blue (MB) and crystal violet (CV) to sepiolite was dete rmined experimentally and investigated by an adsorption model. The lar gest amounts of MB and CV adsorbed were about l-fold of the cation exc hange capacity (CEC) of sepiolite. Consequently, it was proposed that most of the above described adsorption was to neutral sites of the cla y. The adsorption model considered combines the Gouy-Chapman solution and specific binding in a closed system. The model was extended by all owing cation adsorption to neutral sites of the clay, in addition to a dsorption to negatively charged sites and adsorption to neutral comple xes formed from 1 cation adsorbed to a negative surface site. The amou nt of available neutral sites was determined from the adsorption of th e neutral molecule Triton-X 100 (TX100). The model could adequately si mulate the adsorption of the neutral molecules TX100 and crown ether 1 5-crown-5 (15C5) as well as the organic cations. Due to aggregation of MB molecules in solution, their adsorption was somewhat less than tha t of CV at the larger added concentrations. A consideration of the mol ecular dimensions of TX100, MB and CV suggested that their adsorption was mostly to external sites of the clay and that their entry to the s epiolite channels was largely excluded. This interpretation is support ed by infrared spectroscopy (IR) measurements, which show large pertur bations of the peak corresponding to vibrations of external Si-OH grou ps of the clay and confirm complete occupancy of external sites by MB and CV.