Hydrothermal phase transformation of hazardous beryllium-containing iron slag for selective beryllium extraction and toxicity reduction.

Selective separation of beryllium (Be) from hazardous Be-containing iron slag (BIS) generated in Be smelting is critical for resource recovery and waste detoxification. Trapped in ferric oxyhydroxide precipitates via multiple binding pathways, Be cannot be selectively separated by conventional acid leaching. Herein, a Na2S2O8-assisted hydrothermal treatment coupled with Na2CO3 washing was developed for reduction of Be-related leaching risk in the residue. Characterizations verified that Be was tightly bound to amorphous FeOOH in BIS. Surface adsorption alone could not fully account for Be retention, whereas coprecipitation and encapsulation within FeOOH were the dominant mechanisms responsible for Be immobilization. During hydrothermal reaction, thermal decomposition of Na2S2O8 acidifies the system to dissolve FeOOH and release Be. Dissolved SO42-, Na+ and Fe3+ then precipitate in situ as NaFe3(SO4)2(OH)6, leaving Be predominantly retained in the liquid phase. Under optimized conditions (210 °C, 0.8 mol/L Na2S2O8, liquid-to-solid ratio of 8 mL/g, 180 min), the Be extraction efficiency reaches 99.7% with only 2.6% Fe co-extracted. After washing the hydrothermal residue with 0.005 mol/L Na2CO3, the Be concentration in toxicity leachate of the residue drops to 0.20 μg/L, far below the national hazardous waste identification standard limit. This integrated strategy provides a novel route for selective Be recovery and risk reduction of hazardous Be-containing iron slag.