Critical aspects of the stability and catalytic activity of MIL-100(Fe) in different advanced oxidation processes

The Fe-based MOF MIL-100(Fe) has been synthetized and fully characterized by XRD, N2-adsorption-desorption, ATR-FTIR, elemental analysis, WDXRF, XPS, DR-UV-vis, TGA-DTA, and pHPZC. MIL-100(Fe) presented rather good crystallinity and porosity (SBET = 1468 m2 g−1, total pore volume 0.716 cm3 g−1), though some unreacted linker (trimesic acid, TMA) was detected trapped in the structure. Purified MIL-100(Fe), free of unreacted trapped TMA, could be further obtained by some polishing treatments involving ozone, radiation and hydrogen peroxide at different conditions. The stability of the synthesized MIL-100(Fe) was studied in long-term experiments (15 days) at different pH (4–10) in non-buffered and phosphate-buffered ultrapure water, and in secondary effluent samples taken from a municipal wastewater treatment plant. Also, MOF stability was studied under different oxidizing environments through five-cycle experiments reusing the solid. Oxidizing conditions were produced by simulated solar radiation (550 W m−2, 300–800 nm), ozone (doses 0.25–2.0 g O3/g MOF) and/or hydrogen peroxide (doses 0.25–2.5 g H2O2/g MOF). The material showed low stability in the presence of phosphates and, also, at high oxidant doses plus radiation with moderate to high MOF degradation during the treatments. Purified MIL-100(Fe) samples were used as catalysts for the removal of four contaminants of emerging concern (metoprolol, clofibric acid, N,N-diethyl-m-toluamide and ibuprofen) by means of heterogeneous photocatalysis, Fenton-like, photo-Fenton-like, catalytic ozonation and photocatalytic ozonation processes. High catalytic activity in photo-Fenton-like and moderate activity in photocatalytic ozonation processes were attained but still the stability of the materials was questionable because of TMA and iron leaching into the reaction medium. Therefore, although TMA and the organic contaminants could be effectively removed by the advanced oxidation processes studied, bare MIL-100(Fe) is not recommended for water treatment.