EFFECTS OF TEMPERATURE AND RELATIVE-HUMIDITY ON THE SORPTION OF ORGANIC VAPORS ON QUARTZ SAND

The sorption of 17 volatile and semivolatile organic compounds on quartz sand was studied at different relative humidities and temperatures. A chromatographic method was used which allowed measurements at very low concentrations, thus enhancing the environmental relevance of the results. The heat of sorption was determined for each substance and compared with the corresponding heat of condensation. The result suggest higher binding forces for polar than for nonpolar compounds, probably due to hydrogen bonds. These increased binding forces influence not only the temperature dependence but also the absolute extent of sorption. The measurements at different humidities suggest the existence of two processes. At humidities below the one corresponding to a monolayer of adsorbed water, the sorption coefficients were high and decreased rapidly as the exposed mineral surface was reduced by hydration. This is attributed to a strong competition between water and organic molecules for adsorption sites on the mineral surface. Above the water monolayer, coverage sorption occurred on the adsorbed water film and showed an exponential decrease with increasing relative humidity. Only the decrease in the sorption of methanol did not follow an exponential function. This was attributed to dissolution of methanol in the adsorbed water film as an additional sorption mechanism.