Impact of soil microstructure on the molecular transport dynamics of 1,2-dichloroethane

DRIFT spectroscopic experiments are described here that build on our previous time-resolved in situ experiments using dihaloethanes as molecular probes. The earlier studies produced the first molecular level evidence for differences between clay minerals, humic substances, and lysimeter soils as sorbents for these chemicals. In the current study, spectroscopic experiments with several established field soils are combined with porosity characterizations derived from both N-2 and CO2 isotherms. Density functional theory is used to generate porosity distributions from both types of isotherms which, in turn, are used here to define the microstructural character of the soil. Results from the combined techniques strongly suggest that sorption/desorption behavior of volatile organic chemicals (VOCs) can be understood in the context of soil particle architecture. The porosimetry provides evidence that the microstructural array in particles of three of the soils investigated is determined primarily by the platelet character of quasicrystalline clay minerals while that of another one of the soils, an Oxisol, appears to be dominated by its high iron oxide content. With all the soils, curve-fitting of the time-resolved DRIFT spectra indicates the presence of both a sorbed liquid and a persistently sorbed species that accumulate at different rates: band areas for the persistent species continue to increase during desorption. For the soils whose microstructure is dominated by clay minerals, rates of accumulation for both species increase and desorption of the sorbed liquid is facilitated after treatment with hypochlorite to partially remove organic matter. The treatment has little effect, however, on the sorption behavior observed with the Oxisol. The changes in sorption/desorption behavior correlate well with the differences in porosity distributions determined using N-2. In contrast, CO2 determined microporosity (< 2 nm), while related to the organic matter content of the untreated soils, was not a good predictor of any of the observed effects. These results are entirely consistent with those obtained using lysimeter soils as sorbents and support our earlier hypothesis that the persistent species band can be attributed to vapor phase chemical entrapped within the pore network of the sorbent particles. Soil organic matter again appears to impact the retention process primarily by impeding the flow of both liquid and vapor phase chemical into and out of that pore network rather than by associating with the chemical itself. The implications of these results for other volatile organic pollutants and the potential role of surface tension forces within mesopores (2 50 nm) in promoting the formation of persistently sorbed chemical are also explored. (c) 2004 Elsevier B.V. All rights reserved.