Defect-engineered Fe-O covalency in MOF for enhanced antimony removal: Unifying Sb(III) and Sb(V) remediation

The effective removal of highly toxic antimony species (Sb(III)/Sb(V)) from textile wastewater remains a significant challenge. In the present study, a defect-engineering strategy assisted by acetic acid was proposed, through which defect-engineered MOF materials enriched with exposed Fe-O active sites (D-MIL-88B(Fe)) were successfully synthesized. The simultaneous removal efficiency of both antimony valences from real textile wastewater was markedly enhanced. Characterization results demonstrated that the defect-engineering process optimized surface roughness, pore structure, and specific surface area, while significantly increasing the abundance of Fe-O sites. Batch experiments confirmed that, under conditions of pH = 5 and 80 mM acetic acid as a modulator, removal rates of 93.15 % for Sb(V) and 68.75 % for Sb(III) were achieved, with a notably higher efficiency observed for Sb(V). Kinetic and isotherm analyses fitted the pseudo-second-order and the Freundlich models, respectively, indicating a chemisorption process dominated by coordination and a multilayer adsorption mechanism. Maximum adsorption capacities of 66.49 mg/g and 103.01 mg/g were recorded. Based on XPS, FTIR, and first-principles (DFT) calculations, adsorption and oxidation mechanisms were elucidated: coordination complexation between Fe-O sites and Sb(OH)₆^-/Sb(OH)₃ was identified as the primary adsorption pathway. Simultaneously, Fe-O sites were found to facilitate the oxidation of Sb(III) to Sb(V), and PDOS analysis of DFT calculations revealed significant electron transfer from Sb(III) to Fe(III) (adsorption energy: Sb(III) -2.39 eV > Sb(V) -1.2 eV). For practical application, a D-MIL-88B(Fe)@PVDF blend membrane was successfully fabricated, exhibiting excellent hydrophilicity, surface roughness, and structural stability, thereby ensuring high Sb retention during dynamic filtration (>68.8 % total Sb removal after five cycles). This study provides a theoretical and material basis for the development of high-efficiency antimony adsorbents and the advancement of textile wastewater treatment technologies.