Conceptual framework and mathematical model for the transport of metal-chelant complexes during in situ soil remediation

Understanding the transport of metal-chelant complexes is a challenging but necessary task for assessing the in situ chelant applications for land remediation and the potential environmental risks. This study presented an integrated conceptual framework for delineating primary and secondary interactions between target metals, chelants and soil components. The mathematical transport model based on primary interactions reasonably simulated the breakthrough curves of multiple target metals (Cu, Zn, Pb, Cr, and Ni) and mineral cations (Fe, Al, Mg, Mn, and Ca) during EDTA flushing of a field-contaminated soil. The first-order extraction rates of target metals were on the order of 10-6s-1, except Zn (10-4s-1) due to exceptionally large extractable amount in the soil. These rates compared well with previously reported values for field-contaminated soil, but were much smaller than those for artificially contaminated soil. The first-order dissolution rates of mineral cations (10-6-10-5s-1) were similar to the reported values for crystalline minerals, except Ca (10-4s-1) because of substantial proton-induced dissolution of carbonates. Nevertheless, due to a wide spectrum of extraction and dissolution rates at different stages, the model provided a more conservative prediction (i.e., overestimation) of metal-chelant transport while underestimated the transport of free chelant. Further revision of the proposed model may improve its prediction accuracy but attention should be paid to the model complexity and the number of adjustable parameters.