Two-stage multi-fraction first-order kinetic modeling for soil Cd extraction by EDTA

A two stage multi-fraction 1^st-order kinetic model was established herein, which incorporates Cd species distribution in the contaminated site, chelate dosage and washing time, and two distinct extraction mechanisms are also emphasized there. The model was found to successfully simulate the experimental data of Cd extraction by EDTA; with the obtained parameters, we also got a similarly good agreement in other two Cd-contaminated soils. All normalized root-mean-square error, the index of agreement and modeling efficiency values showed that this model can be used to predict Cd kinetic extraction process in different types of soils with an excellent validity. Both simulated and experimental results indicate that a greater EDTA dosage reasonably leads to a higher Cd extraction efficiency and a faster extraction by the direct EDTA-complex. Different Cd species also show different extraction behavior. Part of Cd species associated with Fe/Mn hydro(oxides) (FeMnOx) become destabilized by slow EDTA-promoted dissolution but not yet detached, leading to an apparently high removal efficiency of Cd in FeMnOx fraction dependent on EDTA dosage. While the removal of exchangeable Cd and carbonates (EXCH+CARB) seemed unchanged with the EDTA dosage, due to the transformation of the undetached Cd in FeMnOx fractions. However, an extreme dosage (i.e. molar ratio of EDTA to metal equal to 20 herein) may accelerate the detachment of these destabilized Cd species, resulting in a substantially high extraction efficiency of EXCH+CARB fraction.