1995 Volume 44 Issue 10 Pages 557-565
Chelating agents are being used to remove radioactive contaminants in the corrosion products of cooling systems of nuclear power plants. To elucidate the reaction mechanism the dissolution rate of sintered magnetite pellets in solutions with and without EDTA was measured as a function of potential (0.6- -1.2V vs. saturated Ag/AgCl electrode) and pH (1.1-4.6) at 80°C. It was found that two kinds of reaction can occur: (a) reductive dissolution and (b) chemical dissolution due to chelate formation. Without EDTA only reductive dissolution occurs in the potential range 0.6- -1.2V. At pH1.1 the dissolution rate shows a maximum at 0V, the dissolution peak shifts to more negative values, and the peak height decreases as pH is increased. In the presence of EDTA only reductive dissolution occurs at pH1.1, but at pH2.5 chemical dissolution proceeds as well as the reductive dissolution, showing a maximum dissolution rate at 0V. These results were explained by a model considering the effect of potential, pH, and EDTA on the two coupled processes: (1) the transfer of Fe ions from solid to aqueous phase to form aqua ions or EDTA chelates and (2) the transfer of lattice OH- or O2- ions by reaction with H+ ions to form H2O.