EVALUATION OF THE SORPTION MECHANISM OF CATECHOL AND ITS CHLORINATED DERIVATIVES IN A PALOUSE SILT LOAM SOIL (PHENOLICS, CALORIMETRY, THERMODYNAMICS

Catechol and its chlorinated derivatives are important in soil as intermediary metabolites in the degradation of many synthetic and naturally-occurring aromatics. This study examined the sorption mechanisms of catechol, 4-chlorocatechol, 4,5-dichlorocatechol, and tetrachlorocatechol in a Palouse silt loam soil. Sorption was evaluated by the batch equilibration method using ('14)C-labeled chemicals and varying solvent combinations and conditions. Data were fitted to the Freundlich isotherm equation. All the catechols were strongly sorbed with isotherm slopes of one or greater indicating strong intermolecular associations upon adsorption. Sorption decreased as the catechol molecule became more chlorinated, except tetrachlorocatechol which was one of the most sorbed. Increasing the ionic strength of the solution increased sorption of dichlorocatechol and tetrachlorocatechol. Increasing quantities of acetone or 2-propanol in the equilibration solution produced little change in the sorption of catechol and chlorocatechol, but a slight decrease in the sorption of dichlorocatechol and tetrachlorocatechol. These results indicate that solubility and hydrophobicity had a significant effect on the adsorption of dichlorocatechol and tetrachlorocatechol.;Heats of adsorption were calculated from Freundlich isotherm data taken at 1, 15, 22, and 30C and were compared with values obtained by direct measurement using microcalorimetry. These data show that catechols were sorbed to the most energetic sites first followed by less energetic sites, and that chlorinated catechols were sorbed by hydrogen bonds or van der Waals attractions while catechol itself was probably sorbed by stronger bonds. Intermolecular attractions between molecules on the soil surface were probably important for all of the catechols. Comparison of microcalorimetric and calculation methods showed that microcalorimetry gave more exothermic heats of adsorption for all catechols except tetrachlorocatechol, especially at low surface coverage. Microcalorimetric heats of adsorption became less exothermic with increasing chlorine substitution on the catechol molecule, corresponding to changes in its chemical and physical properties. Variability of both methods was high so that further research is necessary before one of the methods can be recommended.