Enhanced photocatalysis degradation of organophosphorus flame retardant using MIL-101(Fe)/persulfate: Effect of irradiation wavelength and real water matrixes

Metal-organic-frameworks (MOFs) are novel classes of porous materials with diverse potentiality and easily tailored structures. Great challenges for the applications of MOF-based heterogeneous photocatalysis still exist, such as its stability and usability for real water bodies. In this study, MIL-101(Fe) was prepared and a MOF/photo/persulfate system was used to degrade tris(2-chloroethyl) phosphate (TCEP). Synthesized MIL-101(Fe) is high-purity regular crystal with a band gap energy at 2.41 eV, indicating that short wavelength visible light (400-520 nm) and ultraviolet light (200-400 nm) can be utilized by this MOF. Mechanism of photocatalysis reaction involved activation of MIL-101(Fe) for the transformation of Fe(III) to Fe(II), which induced further transformation of S2O82- to center dot SO4-, and then TCEP was degraded by radical oxidation. The degradation pattern demonstrated a S-shape curve with two stages, induction period and radical oxidation. The former would be a process involving reactants adsorption and photocatalyst activation, while the later mainly included center dot SO4- and/or center dot OH reactions. Reaction rates of these two stages highly depended on activation wavelength and pH condition. Irradiation at 420 nm and acid condition were conductive to TCEP degradation, while high temperature and S2O82- addition accelerated the reaction. Five degradation products were elucidated, and the dominant degradation pathways included cleavage, hydroxylation, carbonylation and carboxylation. Furthermore, MIL101(Fe)/420 nm/persulfate system demonstrated a more robust performance than homogeneous ultraviolet/persulfate system for TCEP removal in real water matrixes, implying that it will be a potential technology for elimination of organic pollutants in water.