The High pKa-Guided Defect Engineering: Improving Fluoride Removal in Actual Scenarios by Benzimidazole Modulated Metal-Organic Frameworks

Abstract

We firstly present a novel strategy for enhancing fluoride removal from contaminated water using defect-engineered UiO-66 (Zr-MOFs), emphasizing the crucial role of pKa in modulator selection. By utilizing modulators with varying pKa values-specifically benzimidazole (BI), benzoic acid (BA), and acetic acid (AA)-we synthesized defect-rich Act-UiO-66-M(X). The higher pKa of BI facilitated greater defect formation, resulting in significantly improved adsorption capacity and faster diffusion rates. Act-UiO-66-BI(8), modulated with BI, showed a higher intensity peak at g = 2.003 in ESR analysis, indicating more oxygen vacancies. Its fluoride adsorption capacity reached 93.59 mg F/g, nearly six times higher than commercial ZrO₂, with rapid kinetics—evidenced by a kinetic rate constant (k_int) of 2.64 mg/g·min^0.5 and equilibrium achieved within 10 minutes. The kinetic performance was enhanced by 270% compared to raw Act-UiO-66. Furthermore, Act-UiO-66-BI(8) demonstrated high selectivity and stability in high-salinity environments, with a K_d coefficient consistently exceeding 17,900 mL/g. The study highlights that selecting modulators based on pKa enhances defect formation, improving active site exposure and pore diffusion, as confirmed by DFT calculations and XPS analysis. The ability of Act-UiO-66-BI(8) to treat up to 1160 kg of wastewater per kg of adsorbent highlights its potential for large-scale water purification, showcasing a promising approach for developing high-performance MOF materials.