Effect of solute size and mass transfer on transport of contaminants in porous media.

Abstract

Miscible displacement experiments were performed with solutes of different size and structure to examine their mass transfer, sorption, and transport in homogeneous and heterogeneous porous media. In homogeneous porous media, the contribution of axial diffusion becomes significant at pore-water velocities less than 0.1 cm/h, and the use of a tracer-derived dispersivity for solutes of different sizes would not be valid in this case. Comparison showed that dispersivities measured with a non-sorbing single-solute solution should be applicable to multi-component systems. Breakthrough curves exhibited both early breakthrough and tailing for solute transport in aggregated, stratified, and macroporous media. The extent of non-ideality was consistent with the impact of solute size on the relative degree of "non-equilibrium" experienced by solutes whose transport is constrained by diffusive mass transfer. Flow-interruption experiments with dual tracers of different size, performed for various interruption times, provided additional evidence regarding the effect of solute size on diffusive mass transfer. The relationship between sorbate structure and rate-limited sorption was examined using the QSAR (quantitative structure-activity relationship) approach for sorption of low-polarity compounds by two soils. The first-order valence molecular connectivity (¹xᵛ), accounting for the size and structure of the solutes, was found to be the best topological descriptor. This supports the contention that rate-limited sorption in these systems is analogous to the polymer diffusion model. Based on this model, the calculated diffusion-length ratios for two soils compare favorably to the values determined from the measured rate data. The synergistic effects of rate-limited sorption and mass transfer in heterogeneous porous media were examined. Independent predictions produced with the multiprocess non-equilibrium model (MPNE) provided very good descriptions of the experimental data for transport of several organic solutes with different solute structures in a saturated aggregated medium. The success of describing the mass transfer, rate-limited sorption, and transport of contaminants has important implications for understanding contaminant transport in the subsurface and for remediation practices of contaminated sites.