Nonlinear sorption and nonequilibrium solute transport in aggregated porous media: Experiments, process identification and modeling

The combined effects of nonlinear sorption, nonequilibrium mass transfer and the distribution of sorption sites on transport of organic contaminants has been examined in porous media containing aggregates of clay minerals and organic matter as sorbents. The major goal was to develop general concepts for describing, deterministically, the transport processes of solutes with different adsorption characteristics in such systems. Various sets of batch adsorption and miscible displacement experiments were performed covering a wide range of time scales and other experimental conditions. Using a multiple reactive tracer approach, independent information was obtained on the hydrodynamic properties of the columns, on the relative importance of the two different sorbents present, and on the accessibility and the distribution of these sorbents at the pore scale. The breakthrough curves (BTCs) of the nonlinearly sorbing tracer generally exhibited sharp fronts and excessive tailing, consistent with the Langmuir–Freundlich type adsorption at clays. The effect of nonequilibrium mass transfer was most evident from the tailing of the self-sharpened fronts of the BTCs and from the results of interrupted flow experiments. A two-region model, which incorporated nonlinear sorption and retarded intra-aggregate diffusion, successfully described the results of our entire set of miscible displacement data using a single set of parameter values. Our study demonstrates that although nonlinear sorption and nonequilibrium mass transfer may have very similar effects on solute BTCs, these processes can be distinguished from experimental data if experiments with different solutes, different flow rates and different input concentrations are evaluated simultaneously. It is shown that a very small volume fraction of immobile regions (<0.1% of total porosity), which is insignificant for the transport of conservative solutes, may strongly affect the transport of sorbing solutes if sorption sites are concentrated within these regions. In soils and aquifers, clay minerals and other reactive surfaces are often present in aggregates. Thus, the transport of solutes that strongly interact with such sites generally is very susceptible to rate-limited mass transfer processes while the transport of conservative tracers is poorly affected.