Reduction Removal of Cr(VI) Using Oxalic Acid With MOF-525(Fe) as Catalyst: Performance and Mechanism

Hexavalent chromium (Cr(VI)) has attracted great considerations due to their high toxicity, teratogenicity, and carcinogenicity. In this study, four Zr-MOFs were synthesized and used to comparative study their catalytic performances of oxalic acid (OA) for Cr(VI) removal. Results showed that the removal efficiency for Cr(VI) was reached 65.91%, 78.64%, 53.58%, and 96.32% by MOF-525, MOF-525(Co), MOF-525(Zn), and MOF-525(Fe) in the presence of OA when the initial Cr(VI) concentration was 100 mg·L⁻¹, respectively. The single-factor experiments for further improving the Cr(VI) removal by MOF-525(Fe)/OA demonstrated that the optimal conditions were OA dosage of 790 mg·L⁻¹, and MOF-525(Fe) dosage of 500 mg·L⁻¹ and pH of 2.0, and the MOF-525(Fe)/OA system could be used to treat Cr(VI)-containing wastewater at concentrations below 50 mg·L⁻¹. Ionic strength studies indicated that Na⁺, K⁺, Mg²⁺, Cl⁻, and SO₄²⁻ had little effect on the catalytic reduction of Cr(VI), while high concentrations of Ca²⁺ (0.10 and 0.25 mol·L⁻¹), NO₃⁻ (0.25 mol·L⁻¹), and PO₄³⁻ (0.25 mol·L⁻¹) had significant inhibition effects. The reusability experiments showed that the stability of MOF-525(Fe) was excellent, and it could be used for potential applications. Based on the results of Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and Electron Paramagnetic Resonance spectrometer (EPR), the possible reduction mechanism of Cr(VI) by MOF-525(Fe)/OA system was proposed as follows: The complexation of OA with Fe³⁺ was first complexed to produce Fe²⁺ and CO₂^•−, then Cr(VI) was combined with Zr–O and Fe–O clusters in MOF-525(Fe) to obtain activation energy, and the reduction ability was enhanced at the same time; finally, Cr(VI) obtained electrons from CO₂^•− and was sequentially reduced as Cr(V) and Cr(III).