Date of this Version
Environ. Sci. Technol. 1986, 20, 620-627
The sorption of quinoline (pKa = 4.94) was investigated on low-organic-carbon subsurface materials that varied in pH. Sorption isotherms were measured from 10-7 to 10-4 M quinoline and were found to be nonlinear. The resulting Freundlich constants (KF), based on total aqueous quinoline concentration, were poorly correlated with subsoil properties, including organic carbon. Higher sorption in the acidic subsoils and favorable coefficents for regression of KF, normalized to cation-exchange capacity vs. the ionization fraction (Q), point to the importance of ion exchange of the protonated compound. When the subsoil pH is adjusted, it is shown that sorption parallels the ionization fraction and retention of the organic cation far exceeds that of the neutral species. Calculations of surface speciation and thermodynamic parameters of sorption (∆H°, ∆S°) point to ion exchange and/or surface protonation at pH levels exceeding pKa by greater than 2 log units. It is suggested that in subsurface materials of low carbon content, quinoline sorption is controlled by pH, the nature and capacity of the exchange complex, and groundwater ion composition.