Competitive adsorption of alkylbenzene and water vapors on predominantly mineral surfaces

Competitive adsorption of ethylbenzene (EB) and water on bentonite and of p-xylene (pXYL) and water on kaolin and silica gel was studied using a technique that allowed the amount of adsorbed water and alkylbenzene to be measured independently. EB adsorption on bentonite was not affected by water at relative humidities (RH) near 0.23 but was reduced significantly at RH's near 0.50. pXYL adsorption on kaolin and silica gel decreased with increasing RH, especially above a RH of about 0.2. Increasing RH not only decreased the amount of alkylbenzene adsorption but also resulted in a change from Type-II adsorption isotherms to ones that were essentially linear. Linear isotherms for the adsorption of hydrophobic organic compounds on hydrated soil have generally been attributed to partitioning into organic carbon (OC). However, since the clays and oxide used here had very low to trace amounts of OC, it is suggested that processes involving only mineral surfaces can give rise to linear isotherms. Based on solubility considerations alone, partitioning of EB and pXYL into adsorbed water films was not considered to be an important adsorption mechanism in this study. The effect of cation hydration on the amount of water adsorbed from a mixture of water and pXYL vapors was evaluated by comparing adsorption on Li- and Na-saturated kaolin.