Kinetic approach to heavy metal mobilization assessment in sediments: choose of kinetic equations and models to achieve maximum information

Studies of trace metal mobilization in sediments are generally performed using sequential extraction schemes at equilibrium. In the present work, a kinetic fractionation of trace metals in sediments has been developed to assess that information. The extraction rate data have been obtained using a single extraction scheme with EDTA and following a protocol previously optimized. Two kinetic equations and two kinetic models were used to fit the experimental data. The two constants equation fits well the extraction rate data used in this work but does not present any physico-chemical meaning. The diffusion model and the two first-order reactions model allow determining which parameter (the reaction between the metal M and the EDTA or the diffusion of the complex M/EDTA) is rate limiting in the trace metal extraction by EDTA. It appears that the two first-order reactions model is more efficient than the diffusion model to fit the present extraction rate data so it can be deduced that the diffusion of the complex M/EDTA is not the limiting step of the trace metal extraction by EDTA in estuarine sediments. In a second part, relationships between the fraction of metals determined with the two first-order reactions model and the sediments composition were established.