Soil heterogeneity effects on acid flushing of lead-contaminated soil

Abstract

A compact model for evaluation of acid flushing of heavy-metal-contaminated soil in a small-scale on-site treatment plant is proposed. The model assumes that the soil was re-packed in a container after excavation resulting in a soil structure with heterogeneous and random physical and chemical properties. To evaluate the effects of heterogeneity on the efficiency of contaminant removal by acid flushing, a numerical analysis of lead transport in the heterogeneous soil medium was performed. The model examines cation exchange and surface complexation reactions involving three cations (Ca, Pb, and H) and one anion (Cl) in both dissolved and exchangeable forms, two Pb surface complexes (SOPbCl and SOPbOH), and one Cl surface complex (SOH 2 Cl). The transport of these species during flushing with acid in a synthetically generated two-dimensional heterogeneous soil was simulated in the model. Results indicated that the flushing fluid preferentially followed pathways with large permeability. The heterogeneous cation exchange capacity (CEC) distribution and surface complexation sites had a significant effect on the transport of dissolved species. Because the CEC was set to a relatively low value, Pb was adsorbed mainly as surface complexes (SOPbCl and SOPbOH). Simulation results suggest that blocks of low hydraulic conductivity located in the upper part of the model domain greatly impede solute transport. Ponding conditions did not significantly affect the efficiency of decontamination. The model and its results are useful in the design of smallscale treatment plants for acid flushing.

Modeling Solute Transport in Volcanic Ash Soils with Cation Exchange and Anion Retardation

“…The procedure for simulating the reactions and transport at an arbitrary numerical time step for cations was as follows [14]: (1) calculation of the temporary dissolved ion concentrations using the CIP method [22] for the convective phase and the finite difference method was used for the non-convective phase, along with the dissolved ion concentrations from the previous time step;…”

Comparison and modeling of leachate transportation dominated by the field permeability with an anisotropic characteristic based on a large-scale field trial study

“…As compared to the physicochemical characteristics of IBA, which can be quantitatively determined in the laboratory, field conditions exhibit significant heterogeneities that are site-specific which would cause IBA performances out of expectation. For instance, the heterogeneous geology may result in metal ponding and fingered flows, while biological activities would induce mixed results on metal mobility (Scheibe et al, 2006;Nakagawa et al, 2008;Mastrocicco et al, 2011;Atchley et al, 2013;Fan et al, 2019).…”