Metal ion leaching from contaminated soils: Model development

A mathematical model is developed for metal leaching from contaminated soils subjected to acid extraction in a batch reactor. The model considers transport by pore diffusion and film transfer, leaching of metal bound to reversible and irreversible phases, and metal complexation by ions in solution. As currently developed, the model is semiempirical, with the primary objective of exploring the relative significance of chemical kinetics and diffusional transport under acidic leaching conditions. Simulation results and sensitivity analyses show that leaching kinetics vary according to the metal binding mechanism and location within a soil particle. The effects of pH, pore diffusion, film transfer, chemical reaction rate, particle size, and initial metal distribution are examined using parameter estimates derived from experimental data for a hazardous-waste-site soil contaminated with lead. Depending on leaching conditions, diffusion, reaction, or both may control metal leaching for time-scales of interest in soil washing. Both the distribution of contaminant metal between the reversibly and irreversibly sorbed fractions and the initial spatial distribution within the soil particle are important in determining leaching behavior.