Coupled and Selective Ti--Zr Release in Fe--Al Borosilicate Glass--Zirconolite Wasteforms
Abstract
The purpose of zirconolite-bearing glass--ceramic wasteforms is the immobilization of chemically complicated radioactive wastes using a combination of the glass host and crystalline phase able to contain refractory and actinide surrogates. The stability of these materials in water is defined by both the performance of the glassy matrix and crystalline zirconolite component in water attack. The presented work compares the aqueous durability of CaZrTi$_2$O$_7$ zirconolite ceramics and a composite made of Fe--Al borosilicate glass and CaZrTi$_2$O$_7$, with the weight ratio of zirconolite to glass being equal to 40 wt\%. Both CaZrTi$_2$O$_7$ ceramics and Fe--Al-BG--CaZrTi$_2$O$_7$ composite had almost the same surface areas of 30.00, 30.05, and 30.00 cm$^{-1}$, which allowed us to compare Ti and Zr leaching rates due to differences in behavior of materials rather than their shapes. Ti and Zr release-rate patterns at 1, 3, 5, 8, 15, and 30 days have been analyzed considering the early-to-terminal attenuation, logarithmic rate reduction, Ti/Zr congruence, terminal paired-element severity, composite-to-ceramic amplification, and time-integrated element burden. In the case of CaZrTi$_2$O$_7$ ceramics, the initial rates of Ti and Zr leaching, $6.3\times10^{-5}$ and $6.5\times10^{-5}$ g m$^{-2}$ d$^{-1}$, were reduced to $7.9\times10^{-7}$ and $7.9\times10^{-7}$ g m$^{-2}$ d$^{-1}$ after 30 days, with attenuation factors of 79.7 and 82.3. The CaZrTi$_2$O$_7$ composite started from $1.7\times10^{-3}$ and $1.3\times10^{-3}$ g m$^{-2}$ d$^{-1}$ and, after 30 days, became $4.4\times10^{-5}$ and $3.6\times10^{-5}$ g m$^{-2}$ d$^{-1}$, with attenuation factors of 38.6 and 36.1. While the ceramic demonstrated coupled Ti and Zr release with a mean ratio of 1.047, the composite reached a peak ratio of 3.053 at 8 days and decreased to 1.222 at 30 days. Terminal severity for the composite material was $3.98\times10^{-5}$ g m$^{-2}$ d$^{-1}$, which is 50 times higher than that for the ceramic. From these data, it can be stated that CaZrTi$_2$O$_7$ shows better capability for maintaining refractory element content in the system, whereas the composite behavior is dictated by the early availability of glass--ceramic interfaces and the subsequent partial shift back to coupled Ti--Zr release. Hence, the main goal for Fe--Al-BG--CaZrTi$_2$O$_7$ will be boundary-availability control.