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 understand vapor adsorption by systematically studying the influence of adsorbent, adsorbate, temperature, and relative humidity (RH) on the adsorption equilibrium. Here adsorption to three well-defined minerals (hematite, corundum, and lime)was studied by a chromatographic method. An exponential decrease of the adsorption coefficients of ail tested compounds on all adsorbents was found between 30 and 90% RH. Below 30%, the influence of relative humidity was even stronger. The same functional relationship had been found for quartz and clay minerals tested earlier (1, 2). We therefore hypothesize that this relationship may be valid for all minerals in general. Differences in the surface area-normalized adsorption on different mineral surfaces were found, and they were most distinct at low relative humidities. These differences became smaller with increasing relative humidity and disappeared when humidity approached 100% RH. The extrapolated adsorption coefficients at 100% RH agree with those reported for a bulk water surface. At close to 100% RH, the adsorbed water film apparently was thick enough to prevent influence of the mineral surface on adsorption. The heats of adsorption did show a slight dependence on the mineral surface. As expected, they increased for adsorbents and compounds with increasing adsorption coefficients. Finally, the adsorption of organic vapors to a mixture of two minerals, Ca-kaolinite and corundum, was found to equal the linear sum of the contributions of the single minerals.