AbstractPartitioning of 15 volatile organic compounds (VOCs) between the vapor phase and dry and moist Al2O3samples was measured using a headspace technique. The best predictors of partition coefficients (K′p) from the vapor phase varied with the three Al2O3moisture contents (MCs) used; these variations may reflect alternate sorption mechanisms. For instance, sorption of chlorinated hydrocarbons at 1.9 MC was best explained by the hydrogen bond acceptor parameter β, suggesting hydrogen bonding between the —Cl group and surface groups or water films. However, sorption on humic acid‐coated Al2O3at approximately 45 MC was best explained by the log of the aqueous solubility and Henry's law constant. The importance of aqueous solubility and Henry's law constant suggested that dissolution of organic compounds in interstitial water is a primary sorption mechanism for VOCs at high Al2O3MCs. Sorption of the relatively insoluble hydrocarbons such asn‐alkanes occurred in excess of that explained by dissolution in the interstitial water and may reflect the importance of partitioning into the organic matter provided by the humic acid
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