Sorption of nonionic, hydrophobic organic chemicals to mineral surfaces

Equilibrium sorption coefficients (K-d) Of a series of polycyclic aromatic hydrocarbons (PAHs) and chlorobenzenes were measured on the pure mineral oxides alpha-Al2O3 and alpha-Fe2O3 using column chromatography. Sorbent surface area did not account for differences in the measured distribution coefficients (K-d) for a given compound on alpha-Fe2O3 and alpha-Al2O3. This observation indicates that the type of mineral surface is important in the sorption of nonionic, hydrophobic organic chemicals (HOCs). On both minerals, sorption was reversible as sorption isotherms of the PAHs were linear to at least 50% of solute aqueous solubility and the measured enthalpy of sorption similar to the compounds negative enthalpy of dissolution. Solution pH and the sign and magnitude of sorbent surface charge did not affect sorption, nor did a factor 50 change in ionic strength. The relationship between K-d and the aqueous activity coefficient, gamma(sat)(w), was log K-d = 0.70 log gamma(sat)(w) + -10.68R(2) = 0.94 on alpha-Al2O3 and log K-d = 0.98 log gamma(sat)(w) + -11.39R(2) = 0.92 on alpha-Fe2O3 for the PAHs, chlorobenzenes, and biphenyl. Our observations support the hypothesis that nonionic, HOCs interact weakly and nonspecifically with mineral surfaces. For the sorption of PAHs to mineral surfaces containing organic matter, the mineral contribution to K-d should become significant at values below log f(oc) = -4.43 to -4.00 for alpha-Al2O3 and log f(oc) = -4.33 to -3.46 on alpha-Fe2O3.