Adsorptive behavior of nickel with goethite in the presence of EDTA: Kinetics and reversibility

The influence of EDTA on the adsorption of nickel by goethite has been studied in batch experiments. The nickel adsorption reaches equilibrium in about 3 hours for free (hydrated) Ni and NI-EDTA, whereas up to 24 hours may be necessary to attain desorptive equilibrium for adsorbed Ni in the presence of EDTA. Nickel behavior is strongly influenced by the presence of EDTA. Nickel solid/solution partitioning depends on the solution pH, the EDTA/Ni ratio and the sequence of introduction of the adsorbates (Ni and EMA). Compared to the behavior of free Ni at the solid-solution interface, the occurrence of NI-EDTA complexes increases Ni adsorption at low pH values and decreases adsorption at high pH. The observed data are explained by the formation of a surface-ligand-metal ternary surface complex. Ni uptake depends on the sequence of introduction of the reactants in the goethite suspension (i.e., whether Ni is added with or prior to the EDTA). When EDTA is added to the suspension after Ni had time to equilibrate with the goethite surface, the behavior of Ni at the water-goethite interface is controlled by reaction kinetics. EDTA adsorbs faster than it can complex the dissolved Ni at low pH, and the adsorbed Ni is only slowly desorbed by the EDTA remaining dissolved at high pH.