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. Applying techniques developed with clay minerals, the processes are followed in situ from both kinetic and mechanistic perspectives using controlled environment diffuse reflectance infrared spectroscopy in conjunction with chemicals that exhibit isomerization properties. The rapid accumulation and ease of desorption of the first sorbed species detected 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 second species, detected only after several hours of sorption, increases in intensity during the sorption phase of the experiment and continues to increase even after days of desorption. Both of these species appear to be in the vapor state, consistent with structural porosity as a primary factor in controlling the activity of sorbed chemical. Spectral 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 accessed more slowly but retain sorbed chemical much more strongly.