ADSORPTION OF VOCS FROM THE GAS-PHASE TO DIFFERENT MINERALS AND A MINERAL MIXTURE

Vapor adsorption is an important process for the environmental fate of many organic pollutants by influencing the transport and degradation processes in the environment. This work is part of an attempt to under stand vapor adsorption by systematically studying the influence of ads orbent, adsorbate, temperature, and relative humidity (RH) on the adso rption equilibrium. Here adsorption to three well-defined minerals (he matite, corundum, and lime)was studied by a chromatographic method. An exponential decrease of the adsorption coefficients of ail tested com pounds on all adsorbents was found between 30 and 90% RH. Below 30%, t he influence of relative humidity was even stronger. The same function al relationship had been found for quartz and clay minerals tested ear lier (1, 2). We therefore hypothesize that this relationship may be va lid for all minerals in general. Differences in the surface area-norma lized adsorption on different mineral surfaces were found, and they we re most distinct at low relative humidities. These differences became smaller with increasing relative humidity and disappeared when humidit y approached 100% RH. The extrapolated adsorption coefficients at 100% RH agree with those reported for a bulk water surface. At close to 10 0% RH, the adsorbed water film apparently was thick enough to prevent influence of the mineral surface on adsorption. The heats of adsorptio n did show a slight dependence on the mineral surface. As expected, th ey increased for adsorbents and compounds with increasing adsorption c oefficients. Finally, the adsorption of organic vapors to a mixture of two minerals, Ca-kaolinite and corundum, was found to equal the linea r sum of the contributions of the single minerals.