Leaching Behaviour of Zirconolite in 0.001M Citric Acid at 90°C Under Various Flow Regimes

Abstract

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.

SHS/QP Synthesis and Characterization of Ce‐Bearing Zirconolite‐Rich Waste Forms Using MoO3 as the Oxidant

“…. The LR Ca and LR Zr values decrease congruently as the leaching duration is prolonged, which remain almost unchanged after 28 d. The elemental leaching becomes steadily as the soaking time is prolonged . The LR Ca values are 3–4 orders of magnitude higher than the LR Zr values.…”

“…Thus, zirconolite holds the capability to accommodate many metallic cations over a wide range of valence and ionic radius (such as rare‐earths, actinides, alkaline‐earth ions, and transition‐metal ions) . From the view of waste loading, aqueous durability, chemical flexibility, and radiation stability, zirconolite is a promising ceramic phase for the immobilization of separated plutonium and minor actinides …”

Two‐step synthesis of zirconolite‐rich ceramic waste matrice and its physicochemical properties

“…Compared with Synroc waste forms prepared by hot pressing (HP) and hot isostatic pressing (HIP), the leaching rates of Ca, Ti, and Zr are comparable or even lower in this experiment. And the leaching rates are significantly lower than typical borosilicate glass (> 1 g·m −2 ·d −1 , 90°C) . According to the LR i values, the elemental leaching becomes almost constant as the soaking time is higher than 7 days .…”

“…2,3,39 According to the LR i values, the elemental leaching becomes almost constant as the soaking time is higher than 7 days. 39 This steady stage is related with the production of a protective layer at the sample surface. A near equilibrium state is achieved at this stage because of the precipitation and dissolution of a protective layer.…”

Self-propagating high-temperature synthesis of ZrO2 incorporated Gd2Ti2O7 pyrochlore

“…LRvalue shows the highest one (2.54 g•m 2 •d 1 ) and abruptly declines to 1.29×10 1 g•m2 •d 1 after 7 The elemental leaching becomes steady with the extension of soaking time[37], and the 42 dayCu LR value remains below 1.27×10 2 g•m 2 •d 1 .The 1 day Zr LR value is 3.54×10 5 g•m 2 •d 1 , which subsequently decreases to 5.29×10 6 g•m 2 •d 1 after 14 days. The 42 Zr LR value (8.44×10 7 g•m 2 •d 1 ) is 4-5 orders of magnitude lower than that of Cu and Gd elements.…”