ROLE OF MICROSTRUCTURAL PROPERTIES IN THE TIME-DEPENDENT SORPTION DESORPTION BEHAVIOR OF 1,2-DICHLOROETHANE ON HUMIC SUBSTANCES/

This represents the second phase in our efforts to develop a molecular level understanding of sorption/desorption processes at soil surfaces contributing to prolonged retention of nonpolar organic chemicals. Ap plying techniques developed with clay minerals, the processes are foll owed in situ from both kinetic and mechanistic perspectives using cont rolled environment diffuse reflectance infrared spectroscopy in conjun ction with chemicals that exhibit isomerization properties. The rapid accumulation and ease of desorption of the first sorbed species detect ed as vapor phase chemical flows through humic sorbents is consistent with macroscopic partitioning behavior: its conformer populations are in accord with the nonpolar nature of humic substances. A band for a s econd species, detected only after several hours of sorption, increase s in intensity during the sorption phase of the experiment and continu es to increase even after days of desorption. Both of these species ap pear to be in the vapor state, consistent with structural porosity as a primary factor in controlling the activity of sorbed chemical. Spect ral evidence and CO2-determined microporosity support the existence of discrete regions in the macromolecular structures that are more polar , more dense, or more tightly coiled than others. These regions are ac cessed more slowly but retain sorbed chemical much more strongly.