Dissolution of nickel ferrite and rare earths containing magnetites in citric acid - EDTA - gallic acid (CEG) formulation

It has been shown by us earlier that gallic acid can be used as a reductant in dilute chemical decontaminant formulations containing EDTA as chelant. The results on the dissolution of magnetite in such a formulation were quite promising. Moreover, the superior radiation stability of gallic acid vis-a-vis other reductants such as ascorbic acid or oxalic acid 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 rare earths like 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 the inverse cubic rate law. In the case of nickel ferrite, although there is an initial induction period, the rate constants for the dissolution were determined to be at 1.6 x 10-2 and 3.6 x 10-3 min-1 at 353 K and 333 K respectively. Presence of Ce in particular either alone or in combination with Zr/La at a level of 1 at.% equivalent each in magnetite is seen to increase the surface area of the oxide. The rate constants for the dissolution at 353 K in a 11:44:4 mM CEG formulation taken with magnetite and rare earth containing magnetites equivalent to yielding 22 mM Fe upon complete dissolution are as follows: 5.09 x 10-2 min-1 (magnetite), 7.06 x 10-2 min-1 (Ce2O3 containing magnetite), 6.33 x 10-2 min-1 (Ce2O3, ZrO2 containing magnetite), 1 x 10-1 min-1 (Ce2O3, ZrO2, La2O3 containing magnetite). The presence of Ce, Zr and La at 1 at.% level each has not resulted in any turbidity in solution at the end of magnetite dissolution suggesting chemical dissolution of these rare earth oxides 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 7.55 x 10-2 min-1. It is concluded that the presence of rare earths in magnetite at low amounts has not affected the dissolution characteristics of magnetite in CEG formulation. This conclusion corroborates a similar observed behaviour in citric acid-EDTA-ascorbic acid (CEA) formulation. The main 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. (authors)