The conversion of organic sulfur and strontium into inorganic compounds during the molten salt oxidation of mixed resin

Molten salt oxidation (MSO) has the ability to capture sulfur and metal ions from radioactive mixed resin (MR). Nevertheless, the chemical conversion of sulfur (S) and strontium (Sr) in molten carbonate remains to be further investigated. Herein, MR doped with Sr2+ (Sr-MR) was used to replace spent mix resin containing radioactive 90Sr. The oxidation behaviors of MR and Sr-MR in ternary carbonate (Li2CO3-Na2CO3-K2CO3) were comparatively studied. Sr2+ could react with sulfur-containing species to form corresponding compounds in addition to the adsorption of SO2 by molten carbonate. The content of SO2 drops to 0 ppm at 800 °C, which indicates that the complete decomposition of sulfur-containing structures in Sr-MR and the efficient adsorption of SO2 by molten carbonate. Besides, Sr2+ could significantly promote the removal of organic sulfur, and the existence forms of strontium in molten salt are strontium sulfide (SrS, 350 °C), strontium oxide (SrO, 450 °C) and strontium carbonate (SrCO3, ≥ 550 °C). The presence of quaternary ammonium group (-N+(CH3)3) in Sr-MR indirectly stimulated the formation of sulfates from sulfur-containing compounds. The residual amounts of S and C in Sr-MR decreased to 1.44 % and 6.89 % at 650 °C, which were lower than that of pure MR under the same conditions. The proposed MSO system had excellent retention effects of Sr (98.51 %) and S (47.66 %) at 800 °C for 2.0 h, which provides a new perspective on the disposal of organic wastes containing sulfur radioactive metal ions. [Display omitted] •Sr2+ in Sr-MR could significantly promote the removal of organic sulfur.•The -N+(CH3)3 in Sr-MR could stimulate the spontaneous formation of sulfates from sulfur-containing compounds.•The existence forms of Sr in molten salt were SrS (350 °C), SrO (450 °C) and SrCO3 (≥ 550 °C).•98.51 % of Sr (800 °C) and 98.56 % of S (650 °C) in Sr-MR were efficiently remained in Li2CO3-Na2CO3-K2CO3 melt.