B₂O₃-assisted low-temperature crystallization of pollucite structures and their potential applications in Cs⁺ immobilization

Pollucite structures present remarkable chemical stability and good corrosion resistance against the leaching medium, which renders them promising for the permanent disposal of radioactive ¹³⁷Cs. However, it is still challenging to prepare pollucite with dense structures and high immobilization performance at low temperature so far. Additionally, the fundamental understanding of the mechanism underlying the leaching behaviors of Cs⁺ from pollucite remains largely unexplored. In this paper, we realize the low-temperature production of pollucite by tailoring the nature of geopolymer precursors and employing B₂O₃ additives. It has been shown that the introduction of B₂O₃ not only significantly reduces the crystallization temperature of pollucite, but also improves the immobilization performance of Cs through encapsulation effect. Differential scanning calorimetry and X-ray diffraction confirm the formation of crystalline pollucite from amorphous geopolymer at ∼700 °C in the presence of 7.5 wt% B₂O₃. By further increasing the treatment temperature to 1000 °C, pollucite grains can be well-encapsulated by glass phase, exhibiting leaching rates ∼2 order lower than their unencapsulated counterparts. Moreover, distinct leaching mechanisms that govern the leaching behaviors of Cs⁺ from specimens being treated at different temperatures are unveiled. We anticipate that the proposed scheme facilitates the low-temperature production of high-quality pollucite structures with promising applications in the immobilization of hazardous wastes.