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Nd-bearing zirconolite was leached at 90°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°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.
Geopolymers should be serious waste form candidates for intermediate level waste (ILW), insofar as they are more durable than Portland cement and can pass the PCT-B test for high-level waste. Thus an alkaline ILW could be considered to be satisfactorily immobilised in a geopolymer formulation. However a simulated Hanford tank waste was found to fail the PCT-B criterion even for a waste loading as low as 5 wt%, very probably due to the formation of a soluble sodium phosphate compound(s). This suggests that it could be worth developing a “mixed” GP waste form in which the amorphous material can immobilise cations and a zeolitic component to immobilise anions. The PCT -B test is demonstrably subject to significant saturation effects, especially for relatively soluble waste forms.
Nd-bearing zirconolite was leached at 90°C for 6 months in 0.001M citric acid, and also in deionised water, to determine the effect of organic-bearing solutions on durability. The pH of the citric acid solution was adjusted to 5 using KOH, approximating that of the water in the parallel tests, to avoid the influence of pH on chemical durability of the zirconolite.
Releases were incongruent in the tests carried out in water. Release rates of Ti, Zr and Nd were comparatively very low (commonly too low to be measured) over the first 80 days of leaching. Rates for Ca and Al were 2 to 4 orders of magnitude higher than Ti, Zr and Nd over this same period. At about 80 days, there was an anomalous decrease in pH from 6 to 4 which enhanced release rates of Ti and Nd in particular. There was development of titania crystals, and the suggestion of hydrolysed titania, on the surface after 6 months. Thermodynamic equilibrium between the leachates and hydrolysed species on the surface of the zirconolite may be the key to apparent cessation of alteration, at least during thefirst 80 days of leaching.
By contrast, zirconolite leached in 0.001M citric acid maintained release rates of Ti, Zr and Nd 2 to 4 orders of magnitude greater than those in water for the first 80 days, values sustained, within an order of magnitude, for the remainder of the leach tests. Releases were congruent. The surface of the zirconolite showed no signs of secondary phase development. This suggests complexation by citrate ions prevented control by hydrolysed species on zirconolite solubility.
Polished tiles (7×7×2 mm3) of Nd-bearing zirconolite were fabricated and then some were irradiated on both large faces with 3 MeV or 2 MeV Au2+ ions (total fluence of ≥ 1 × 1015 ions/cm2) in order to render the zirconolite amorphous and so simulate displacement damage caused by alpha decay. Both the irradiated and non-irradiated tiles were then subjected to static dissolution tests in 0.01M nitric solution (pH2) at 90 C, for periods of 0–1, 1–7, 7–14 and 14–28 days. It was found that radiation damage did not affect the dissolution rate of zirconolite as indicated by the elemental leach rates of Nd, Ti, Ca and Al. The results of solution analyses are consistent with those obtained from X-ray Photoelectron Spectroscopy (XPS) in that the Ca, Nd, Ti and Al concentrations in the top surface layer (< 5 nm) all decreased with respect to that of Zr after dissolution testing, and the leached surface composition of the non-irradiated zirconolite is very similar to that of the two irradiated specimens. The implications of these results are discussed in the context of previous work.
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