Leaching behaviour of zirconolite in 0.001M citric acid at 90 deg. C under various flow regimes

Nd-bearing zirconolite was leached at 90 deg. C for 157 days in 0.001M citric acid under single-pass- flow-through conditions (modified MCC-4 protocol). Three different flow rates were used, ranging in an order of magnitude from 10 mL per day to 100 mL per day, to determine the effect of the rate of leachant replenishment on the durability of the zirconolite. Results of previous studies on the role of complexing agents on the leaching behaviour of single-phase zirconolite have been included in the discussion. The pH of the citric acid solution was adjusted to 5 using KOH, mimicking that of the water in the parallel tests, to avoid the influence of pH on chemical durability of the zirconolite. Simulated groundwater containing 0.001M citric acid at 90 deg. C led to congruency in elemental releases and a diminution of release rate with time of about an order of magnitude, reaching virtual constancy after about 50 to 60 days to a level of about 10-5 g m-2 day-1. The most significant finding was that the elemental release rates of Nd, Ti and Zr (and Ca and Al where detected) were similar for all flow rates. Clearly, varying flow rate by up to an order of magnitude had no effect on elemental releases i.e. there is no solubility limit control on releases at 0.001M citric acid concentration. An important finding of previous studies using identical leaching protocols with 0.001M citric acid, and inferred in our latest investigations reported here, was that there is no secondary layer development at the surface of the zirconolite to affect leach rates. In contrast, parallel tests carried out in deionised water instead of citric acid showed that hydroxides form in situ on the zirconolite surface, effectively forming hydrolysed zirconolite. This controls further dissolution of the zirconolite matrix due to the solubility limit being reached with respect to the hydrolysed phases rather than with zirconolite. Complexation by citrate ions prevents such control by hydrolysed species on zirconolite solubility. Even under the more aggressive conditions imposed in these studies (0.001M citric acid), and regardless of flow rate of the leachant, elemental releases from zirconolite are very low for a candidate wasteform and demonstrate its attributes as a ceramic-based wasteform for the containment of actinides. (author)