Nickel desorption kinetics from hydrous ferric oxide in the presence of EDTA

Introducing 10 mu M EDTA to a system of nickel adsorbed to hydrous ferric oxide (HFO) at pH 8 promoted desorption of Ni as well as dissolution of HFO. By monitoring changes in the solution concentrations of Ni and Fe after the introduction of EDTA, it was possible to discern the important processes controlling Ni desorption. This study focused on the effect of the Ni concentration while all other variables were held constant. Under these conditions, Ni desorption and HFO dissolution occurred over two time domains. The first time domain was short (<0.25 h), where desorption and dissolution were fast parallel processes. In the second time domain (>0.25 h), Ni desorption was slower and the solution Fe concentration did not increase measurably over an 8h period. The rates of the observed processes were dependent on the Ni surface concentration, and were attributed to weakly bound surface species in the first time domain, and involved more strongly bound surface species in the second time domain. In the first time domain, increases in the surface concentration increased the Ni desorption rate and decreased the HFO dissolution rate for all surface concentrations studied. The Ni desorption rate in the second time domains also increased linearly with Ni surface concentration, but at surface concentrations >3.2mmol Ni per mol Fe the Ni desorption rate began to approach a maximum value. At total Ni concentrations greater than or equal to 40 mu M, decreases in the solution Fe concentration were observed in the second time domain, and was attributed to metal exchange. This process may become rate limiting at long equilibration times.