METAL-ION LEACHING FROM CONTAMINATED SOILS - MODEL CALIBRATION AND APPLICATION

A previously developed model that describes leaching of heavy metals f rom contaminated soils is applied to four hazardous-waste-site soils c ontaminated with Pb. Processes included in the model are intraparticle diffusion, rate expressions for irreversibly and reversibly sorbed fr actions, and metal complexation by ions in solution. The model is cali brated using laboratory experimental data in the pH 1-3 range, liquid- to-solid mass ratios from 5 to 20, and leaching times of 24 h. Paramet ers for the model are estimated through a combination of independent e xperiments, literature correlations, and mathematical optimization. Eq uilibrium data were used to estimate site density and an adsorption eq uilibrium constant. Two kinetic rate coefficients, a particle tortuosi ty factor, and a distribution coefficient (alpha(a)) that defined the amount of Pb in two contaminant fractions were adjusted to match kinet ic leaching data. Using one set of parameter estimates for each soil, the model successfully simulated experimental data collected under dif ferent leaching conditions. The fraction of Pb associated with easily leachable, irreversibly sorbed fraction (1 - alpha(a)) provides some i nsight to the geochemical distribution of Pb in the soils tested. The model is used to explore effects of process variables such as liquid-t o-solid ratio and sequential washes. The model should be useful for si mulating ex-situ soil washing processes and may, with further developm ent, have applications for in-situ flushing processes.