Nonideal Transport of Solute and Colloidal Tracers Through Reactive Zeolite/Iron Pellets

We conducted solute and colloidal tracer tests in laboratory columns to examine the hydraulic properties of a foamed zeolite/iron pellet material that was developed for in situ remediation of contaminated groundwater. The packed pellets had large inter-pellet pore spaces (several mm in dimension) and small intra-pellet pores (< 1 mm), with an overall porosity of 0.8. The colloidal tracer (1-micron polystyrene microspheres) transported through the columns much faster than the conservative solute tracer (tritiated water), reflecting the inter-pellet preferential flow pathways in the packed material. Flow interruption experiments with tritium and bromide showed concentration rebound of both tracers after the interruption, indicating the existence of non-advective zones inside the pellets. As expected, standard equilibrium transport models could not adequately describe the solute breakthrough curves. Physical non-equilibrium and dual-permeability models also were unable to describe the data. A model incorporating both intra-aggregate diffusion and dual permeability is required to reflect the complex transport processes in the pellets.