Chemical Durability and Dissolution Kinetics of Iodoapatite in Aqueous Solutions

Abstract

Understanding the long-term release of radionuclides from nuclear waste to the environment is critical for public acceptance and sustainability of nuclear energy. Iodoapatite, a synthetic material similar to mineral vanadinite proposed for radioactive iodine-129 immobilization, is employed in this study as a model system for iodine waste forms and ceramic waste forms in general to understand its long-term chemical durability. Semidynamic leaching experiments were performed in cap-sealed Teflon vessels to evaluate the chemical durability at temperatures from 20 to 90 °C and pH values from 4 to 9 using deionized water and pH buffer solutions. The leachates were analyzed using inductively coupled plasma-mass spectrometry. The leached surfaces were examined by X-ray diffraction, scanning electron microscopy, and Raman spectroscopy. Effects of test variables including surface-to-volume ratio, leachant replacement interval, and environmental variables including temperature and pH on the dissolution rate were systematically investigated. The activation energy of the dissolution was 16.9 ± 1.5 kJ/mol for the matrix elements and 34.4 ± 3.9 kJ/mol for the diffusive iodine release. The order of the pH effect on the dissolution rate as a power law exponent was 0.87 ± 0.08. The effect of the surface-to-volume ratio and replacement interval was approximated by a single exponential function rise to maximum. Fully parametrized models were then combined to predict iodine release rate under various conditions. The result suggests that the long-term iodine release is controlled by iodide diffusion when the matrix dissolution rate is very low in near neutral pH solutions and by matrix dissolution when the dissolution rate is high at low pH. The present study demonstrates a mechanistic approach to parametrize models that can be used to evaluate the performance of nuclear waste forms under various disposal environments.

Effect of solution chemistry on the iodine release from iodoapatite in aqueous environments

“…Details of the synthesis and characterization of these samples were reported previously in separate publications. 25,29,37 The leaching method was adopted from ASTM C1308 standard test, as described in the previous study 29,37 . Four parallel experiments were conducted simultaneously for 14 days in four different leaching solutions: 0.1 mol/L NaCl, 0.1 mol/L Na2CO3, 0.1 mol/L Na3PO4, and 0.1 mol/L Na2SO4.…”

“…Interestingly, the trend of iodine released in Na2CO3 solution of pH 10.3 resembles that of leaching iodoapatite under pH 4. 37 Despite the rate difference, both surfaces leached by pH 4 and pH 10.3 formed secondary phases (chervetite and hydroxylvanadinite, respectively). Our previous study showed that the equivalent long-term rate of iodine release under pH 6 is 8.1 mmol/m 2 /d, over two magnitudes higher than that of the deionized water 0.036 mmol/m 2 /d.…”

“…Therefore, it can be problematic to use data from static leaching tests to predict waste form behavior in a repository environment. 37 To address the issues of solution feedback, a semi-dynamic leaching method was implemented by Zhang et al in 2018 to quantify the processes involved in the iodine release of an iodine-bearing apatite. 29 In their experiment, deionized water solutions, as the leachant, were replaced periodically to minimize the solution feedback.…”

“…This semi-dynamic approach was employed to produce essential datasets to parameterize a mechanistic model suitable for predicting the kinetics of iodine release under different conditions. 37 Since the aqueous systems in natural environment contain a variety of dissolved species, it is necessary to understand how these aqueous species affect the iodine release from iodine waste forms in an aqueous environment. For instance, the iodine release from apatite structured materials can be enhanced by rapid substitution of halogen element [38][39][40][41] or inhibited by precipitation of secondary phase.…”

“…For instance, the iodine release from apatite structured materials can be enhanced by rapid substitution of halogen element [38][39][40][41] or inhibited by precipitation of secondary phase. 33,35,37 In this study, we conducted semi-dynamic leach tests on single phase crystal ceramics of iodoapatite in 0.1 mol/L NaCl, Na2CO3, Na3PO4 and Na2SO4 solutions. The goal is to examine the impact of the solution chemistry on the kinetics of iodoapatite dissolution.…”