Defect-engineered MIL-100(Fe) via acid modulation for enhanced adsorption and selectivity in dye removal from wastewater

Defect engineering in metal-organic frameworks (MOFs), particularly linker defects via acid modulation, has emerged as a promising strategy to enhance their performance in wastewater treatment. In this work, the defective MIL-100(Fe) was synthesized via modulation using four acid modulators – acetic acid, citric acid, benzoic acid, and gallic acid (GA) – to induce linker vacancies while tailoring its micro/mesoporous characteristics to rationalize these impacts on the dye adsorption efficiency. The introduction of defect sites led to significant changes, including reduced crystallinity, lower micro/mesopore volume ratio, and increased specific surface area. Notably, GA induced defective MIL-100(Fe), MIL-GA, exhibited a significantly large surface area (1060.56 m2/g) with a low micro/mesopore volume ratio (0.734) and higher unsaturated Fe defect density based on its high Fe/C ratio. Enhanced mesoporosity in the defective frameworks facilitates improved molecular diffusion and accessibility, leading to significantly improved adsorption capacity and selectivity. MIL-GA achieved a maximum adsorption capacity of 621.74 mg/g for reactive orange 16 (RO16) and demonstrated enhanced selectivity in binary dye systems (RO16/rhodamine B ratio = 2.59 and RO16/methylene blue ratio = 1.7). The adsorption behaviour followed the Elovich and Redlich-Peterson models, suggesting chemisorption and hybrid mono−/multilayer adsorption mechanisms. The nanomaterial retained over 80% adsorption efficiency after four regeneration cycles using dilute acidic ethanol, highlighting its reusability. These findings underscore the potential of defect-engineered MOFs as high-performance, selective, and recyclable adsorbents for practical dye removal from industrial wastewater.