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 significantly with increasing relative humidity levels in the vapor phase. It might be due to the competition of adsorption sites by water molecules on the soil surfaces. Generally, the surface of nature adsorbents, such as soil and clay mineral, is highly heterogeneous owing to their complex chemical composition and irregularities of physical structure. It is necessary to describe the adsorption phenomena of nature adsorbent microscopically. Based on gas-solid adsorption isotherm, the energy distribution function of sorbing sites on soil surface were constructed. The variation of adsorption energy distribution function reveals that the adsorption energy for water was higher than that for toluene and, therefore, the soil surface would prefer to adsorb water vapor rather than toluene vapor. In addition, the adsorption energy of water vapor shifts towards the lower side at higher relative humidity level. It appears that the water vapor is condensed onto the surface. It is found that the previously adsorbed water molecules modified the soil surface and changed the energy spectrum of the adsorption sites. The shift of the adsorption energy spectrum to lower adsorption energy is the reason of the suppression of the soil uptake of organic vapors by water vapor.