Modeling the transport of metals with rate-limited EDTA-promoted extraction and dissolution during EDTA-flushing of copper-contaminated soils.

EDTA-flushing of artificially contaminated soils enhanced Cu extraction but also induced mineral dissolution simultaneously. The mobilization and transport of these metal-EDTA complexes was investigated with column experiments. A quantitative transport model was proposed for simulating the experimental breakthrough curves of Cu, Fe, Al, and Ca. The rate-limited EDTA-promoted extraction and dissolution could be described by respective second-order kinetic terms, which were necessary for explaining the time-dependent depletion of extractable metals (sorbed and indigenous) in soils with continuous EDTA-flushing. Simultaneous simulation of extraction of sorbed Cu and dissolution of soil Fe, Al, and Ca is more conceptually accurate than individual modeling of each metal because the latter approach tends to overestimate the concentration of free EDTA during transport and thus underestimate the rate coefficients of EDTA-promoted dissolution. The fitted rate coefficients of Cu were about an order of magnitude larger than those of Fe and Al; these values probably reflect Cu extraction from weakly sorbed fractions and Fe and Al dissolution from amorphous oxides. The apparent retardation of Fe, Al, and Ca transport had to be taken into account by empirical determination, which was attributed to the metal lability in soils and thermodynamics of surface complexation.