NONLINEAR SORPTION AND NONEQUILIBRIUM SOLUTE TRANSPORT IN AGGREGATED POROUS-MEDIA - EXPERIMENTS, PROCESS IDENTIFICATION AND MODELING

The combined effects of nonlinear sorption, nonequilibrium mass transf er and the distribution of sorption sites on transport of organic cont aminants has been examined in porous media containing aggregates of cl ay minerals and organic matter as sorbents. The major goal was to deve lop 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 e xperiments were performed covering a wide range of time scales and oth er experimental conditions. Using a multiple reactive tracer approach, independent information was obtained on the hydrodynamic properties o f the columns, on the relative importance of the two different sorbent s present, and on the accessibility and the distribution of these sorb ents at the pore scale. The breakthrough curves (BTCs) of the nonlinea rly sorbing tracer generally exhibited sharp fronts and excessive tail ing, 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 incorporate d nonlinear sorption and retarded intra-aggregate diffusion, successfu lly described the results of our entire set of miscible displacement d ata using a single set of parameter values. Our study demonstrates tha t although nonlinear sorption and nonequilibrium mass transfer may hav e very similar effects on solute BTCs, these processes can be distingu ished from experimental data if experiments with different solutes, di fferent flow rates and different input concentrations are evaluated si multaneously. It is shown that a very small volume fraction of immobil e regions (< 0.1% of total porosity), which is insignificant for the t ransport of conservative solutes, may strongly affect the transport of sorbing solutes if sorption sites are concentrated within these regio ns. In soils and aquifers, clay minerals and other reactive surfaces a re often present in aggregates. Thus, the transport of solutes that st rongly interact with such sites generally is very susceptible to rate- limited mass transfer processes while the transport of conservative tr acers is poorly affected. (C) 1998 Elsevier Science B.V.