Dissolution of Ce, Zr and La-containing magnetites and nickel ferrite in citric acid-EDTA-gallic acid formulation

Abstract

It has been shown by us earlier that gallic acid (GA) can be used as a reductant in dilute chemical decontaminant formulations containing Ethylene Diamine Tetra Acetic acid (EDTA) as chelant. The results on the dissolution of magnetite in such a formulation were promising. Moreover, the superior radiation stability of gallic acid vis-à-vis other reductants such as ascorbic acid (AA) or oxalic acid (OA) is another plus point for this formulation. Besides having an inherent stability against radiation degradation, it is able to protect even EDTA against radiation-induced decomposition to a great extent unlike the case of ascorbic acid. In an extension of that work, dissolution experiments have now been carried out on nickel ferrite and magnetites containing fission product nuclides such as La, Ce and Zr. This is to simulate the presence of fission product oxides in magnetite resulting from a possible phase of operation with leaky fuel. The rate constants have been determined using both the inverse cubic rate law and also a general kinetic law model. Both models predicted the same trend in the behaviour of dissolution. They showed linear behaviour in their rate plots up to 80–90% of dissolution. Typically using the ICR model in the case of nickel ferrite, although there is an initial induction period, the rate constants for the dissolution were determined from the ICR model to be 0.016 and 0.0036 min −1 at 353 and 333 K, respectively. The presence of Ce in particular either alone or in combination with Zr/La at a level of about 0.5 at.% equivalent each in magnetite is seen to increase the surface area of the oxide. The rate constants (typical ICR model) for the dissolution at 353 K in a 11:44:4 mM citric acid-EDTA-gallic acid (CEG) formulation taken with magnetite and Ce, Zr and La-containing magnetites equivalent to yielding 22 mM Fe upon complete dissolution are as follows: 0.051 min −1 (magnetite), 0.071 min −1 (Ce 2O 3 containing magnetite), 0.063 min −1 (Ce 2O 3, ZrO 2 containing magnetite), 0.1 min −1 (Ce 2O 3, ZrO 2, La 2O 3 containing magnetite). The presence of Ce, Zr and La at ≈0.5 at.% level each has not resulted in any turbidity in solution at the end of magnetite dissolution suggesting a chemical dissolution of the oxides of rare earth elements and Zr in the formulation. Simple magnetite (worth 0.4 mM Fe upon complete dissolution) taken in a 1.4:1.4:1.7 CEG formulation yielded a rate constant of 0.076 min −1. It is concluded that the presence of rare earths in magnetite at low amounts has in fact improved the dissolution characteristics of magnetite in CEG formulation. A rate constant value of 0.1 min −1 was obtained for Ce,Zr,La-containing magnetite in 11:44:4 mM CEG as against a reported value of 0.031 min −1 in 11:44:4 mM CEA fomulation. Thus, CEG formulation appears to be better than CEA formulation for the dissolution of magnetite. Apart from better kinetics of dissolution, the major advantage of CEG formulation over CEA formulation seems to be its stability towards radiation-induced decomposition, a consideration important when chemical decontamination is planned under a short shutdown period (a few days) of the reactor.