Tributyl phosphate degradation and phosphorus immobilization by MnO₂: Reaction condition optimization and mechanism...

The treatment of tributyl phosphate (TBP) extractant waste from specific industry, eg., nuclear industry, is a great challenge due to its stability and high environmental risk of phosphorus-containing species releasing. Inspired by chemical looping combustion (CLC) technology, a MnO₂-assisted thermal oxidation strategy is proposed for TBP degradation and simultaneously P immobilization. Under recommended reaction conditions of 220 °C, 10 g MnO₂ mL^-1 TBP and 3 h reaction duration, a high P immobilization efficiency of 93.99% is achieved. Material characterization results indicate that P is mainly immobilized in the form of Mn₂P₂O₇, which greatly reduces the environmental risk of P-containing species. TBP degradation intermediates are further identified by thermogravimetric-gas chromatography-mass spectrometry (TG-GC-MS), liquid chromatography-mass spectrometry (LC-MS) and gas chromatography-mass spectrometry (GC-MS), which facilitates understanding of reaction mechanisms as well as proposing possible pathways of TBP degradation. It is suggested that MnO₂ provides essential oxygen as oxygen carrier for flameless combustion. Meantime, MnO₂ reduction leads to the generation of Mn(III) species. The existence of oxygen vacancy in MnO₂ also facilitates •O₂^- radical generation. Under flameless combustion and attacks of Mn(III) and •O₂^-, TBP is firstly degraded into intermediates and finally mineralized into CO₂ and H₂O, while P is mainly immobilized as pyrophosphate.