THE INFLUENCE OF RELATIVE-HUMIDITY ON THE ADSORPTION OF TOLUENE BY SOILS - INTERPRETATION WITH THE ADSORPTION ENERGY-DISTRIBUTION FUNCTIONS

It has been shown that the VOC adsorption capacity of soil decreases s ignificantly with increasing relative humidity levels in the vapor pha se. It might be due to the competition of adsorption sites by water mo lecules on the soil surfaces. Generally, the surface of nature adsorbe nts, such as soil and clay mineral, is highly heterogeneous owing to t heir complex chemical composition and irregularities of physical struc ture. It is necessary to describe the adsorption phenomena of nature a dsorbent microscopically. Based on gas-solid adsorption isotherm, the energy distribution function of sorbing sites on soil surface were con structed. The variation of adsorption energy distribution function rev eals that the adsorption energy for water was higher than that for tol uene and, therefore, the soil surface would prefer to adsorb water vap or rather than toluene vapor. In addition, the adsorption energy oi wa ter vapor shifts towards the lower side at higher relative humidity le vel. It appears that the water vapor is condensed onto the surface. II is found that the previously adsorbed water molecules modified the so il surface and changed the energy spectrum of the adsorption sires. Th e shift of the adsorption energy spectrum to lower adsorption energy i s the reason of the suppression of the soil uptake of organic vapors b y water vapor. (C) 1998 Elsevier Science Ltd. All rights reserved.