Enhanced degradation of toxic azo dye, amaranth, in water using Oxone catalyzed by MIL-101-NH2 under visible light irradiation

The acidic azo dye, amaranth (AMR), is frequently applied on fabrics, and also extensively employed as food colorants. However, AMR is validated to be carcinogenic, and thus it should be eliminated from wastewaters preferably by advanced oxidation processes (AOPs). While Fe-based catalysts are attractive for sulfate-radical-based AOPs, few studies have been conducted to employ Fe-based catalysts for activating Oxone to degrade AMR. As Metal Organic Frameworks (MOFs) can be designed to contain Fe species and exhibit high surface areas/porosities, Fe-based MOF, MIL-101, should be promising for activating Oxone to degrade AMR. More importantly, MIL-101 can be functionalized with amino groups to form MIL-101-NH2 (MIL-N), which can exhibit visible-light-responsive photo-catalytic activities, enhancing Oxone activation and AMR degradation. The catalytic transformation of Fe species mediated by the amine functional group also leads to the formation of hydroxyl radicals to contribute to AMR degradation. MIL-N also exhibits a much lower Ea value (38.8 kJ/mol) than other reported catalysts for AMR degradation using Oxone, and MIL-N remains quite stable at a wide range of pH values. MIL-N-activated Oxone could be still able to eliminate AMR completely even in the presence of typical charged surfactants, and recyclable for multiple cycles of AMR degradation. These features validate that MIL-N can be an advantageous heterogeneous catalyst for Oxone activation to degrade AMR. The results obtained here can be quite useful for treating AMR wastewaters by sulfate-radical-based AOPs and the findings in this study offers important mechanistic insights into catalytic characteristics of MIL-N.