Sorption hysteresis of pyrene on zeolite

Systematic studies were performed to investigate the conditions and the extent of irreversible sorption and sorption hysteresis of a hydrophobic compound on a mineral surface. Adsorption and desorption isotherms of pyrene on zeolite were established. Various solvents were used, such as surfactant solution, 2-propanol, 2-propanol/water (70/30) or a microemulsion. Different adsorption and desorption isotherms were found for pyrene physisorbed on the mineral surface. Dependence of sorption on the adsorbent particle density was observed. Kinetic experiments were carried out in order to determine desorption rates. The curves obtained by fluorescence spectroscopy were fitted to the Henry model (monoexponential fit) which can be obtained from the Langmuir kinetics and to modified models (biexponential fit). The latter fitted the curves perfectly. The dynamics of the equilibrated system were studied by radio-tracer experiments. After the preceding desorption process a portion of pyrene containing solution was replaced by an equal amount of radio-labelled pyrene solution of the same concentration. No exchange could be detected on the time scale of the desorption experiments. The results can be explained in terms of metastable equilibrium. They suggest energy distribution of adsorption sites and high activation energies for at least one of the processes (adsorption or desorption). (C) 1999 Elsevier Science B.V. All rights reserved.