Strong enhancement effect of bisulfite on MIL-68(Fe)-catalyzed Fenton-like reaction for organic pollutants degradation

Highlights: • Bisulfite immensely enhanced the MIL-68(Fe)-mediated Fenton oxidation. • The MIL-68(Fe)/HSO₃⁻/H₂O₂ system exhibited a wider applicable pH range. • OH, SO₄⁻, HO₂, and ¹O₂ were main ROS in the MIL-68(Fe)/HSO₃⁻/H₂O₂ system. • The enhancing mechanisms of bisulfite were elucidated. • The MIL-68(Fe)/HSO₃⁻/H₂O₂ system was highly effective and stable. Metal organic frameworks (MOFs) have shown a great potential to act as heterogeneous Fenton-like catalysts. However, the lower catalytic efficiency and reaction rate are limiting their practical applications. Herein, a facile and efficient strategy was studied to boost the catalytic oxidation performance of MIL-68(Fe)-mediated Fenton-like process with the addition of a minor amount of bisulfite. The factors affecting the catalytic oxidation capacity of the MIL-68(Fe)/HSO₃⁻/H₂O₂ system were investigated using methyl orange (MO) as model pollutant. Under the optimal condition, the degradation rate of MO could be increased by 11.5 times compared to that in MIL-68(Fe)/H₂O₂ system, and the total organic carbon (TOC) removal in 120 min reached 46.8%. The MIL-68(Fe)/HSO₃⁻/H₂O₂ system also displayed a wide applicable pH range. Besides MO, other organic contaminants such as acid red 18 (AR18), rhodamine B (RhB), methylene blue (MB), xylenol orange (XO) and tetracycline hydrochloride (TC) could be effectively degraded in the MIL-68(Fe)/HSO₃⁻/H₂O₂ system as well. The quenching tests and electron paramagnetic resonance (EPR) spectrometry confirmed that OH, SO₄⁻, HO₂, and ¹O₂ were the main reactive species for MO degradation in the MIL-68(Fe)/HSO₃⁻/H₂O₂ system. The coumarin fluorometry further disclosed that much more hydroxyl radicals were generated in the MIL-68(Fe)/HSO₃⁻/H₂O₂ system than in MIL-68(Fe)/H₂O₂ system. Cyclic voltammetry analysis, on the other hand, indicated that the presence of HSO₃⁻ facilitated the Fe(III)/Fe(II) cycle in the MIL-68(Fe)/HSO₃⁻/H₂O₂ system. Based on these results, the enhancement mechanism of the Fenton-like MIL-68(Fe)/HSO₃⁻/H₂O₂ system was proposed. Finally, the cyclic experiments manifested the MIL-68(Fe)/HSO₃⁻/H₂O₂ system had good stability and reusability. This study suggests that the addition of bisulfite being an excellent approach to construct the Fe-MOFs mediated Fenton systems for eliminating organic contaminants in water with high efficiency.