Precipitation characteristics of uranyl ions at different pHs depending on the presence of carbonate ions and hydrogen peroxide

This work studied the dissolution of uranium dioxide and precipitation characteristics of uranyl ions in alkaline and acidic solutions depending on the presence of carbonate ions and H₂O₂ in the solutions at different pHs controlled by adding HNO₃ or NaOH in the solution. The chemical structures of the precipitates generated in different conditions were evaluated and compared by using XRD, SEM, TG-DT, and IR analyses together. The sizes and forms of the precipitates in the solutions were evaluated, as well. The uranyl ions were precipitated in the various forms, depending on the solution pH and the presences of hydrogen peroxide and carbonate ions in the solution. In a 0.5 M Na₂CO₃ solution with H ₂O₂, where the uranyl ions formed mixed uranyl peroxy-carbonato complexes, the uranyl ions were precipitated as a uranium peroxide of UO₄(H₂O)₄ at pH 3-4, and precipitated as a clarkeite of Na₂U₂O_x(OH) _y(H₂O)_z above pH 13. In the same carbonate solution without H₂O₂, where the uranyl ions formed uranyl tris-carbonato complex, the uranyl ions were observed to be precipitated as a different form of clarkeite above pH 13. The precipitate of uranyl ions in a nitrate solution without carbonate ions and H₂O₂ at a high pH were studied together to compare the precipitate forms in the carbonate solutions.