Characterization of the dissociation kinetics of Cd and Ni in soils based on diffusive gradients in thin films technique

A new theoretical method was established for the combinatorial calculation of the dissociation rate constant (K-1) of the metal-organic complexes (MLs), the concentration of free ionic soil metals (CM), the labile concentration of soil metal-organic complexes (CML) based on diffusive gradients in thin-films (DGT) technique with a range of diffusive layer thicknesses (0.053-0.173 mm) in soils. The fitting results agreed well with the determined values. The values of K-1, CML and CM were calculated without other morphological analysis software and the fitting results agreed well with the determined values with some advantages such as the use of fewer hypothetical parameters, ease of calculation, the full embodiment of the contribution of MLs to the labile content. According to the results of model fitting, cation exchange capacity and soil organic matter were found to be the key environmental factors for K-1 values of Cd and Ni, respectively. The labile contents of Cd and Ni in soil were closely related with pH, soil organic matter and the total contents of heavy metals.