Targeted Selenite Adsorption Using Defective Fe-BTC: Effective in Acidic and Alkaline Conditions

Amorphous Fe-BTC, characterized by entirely defective metal nodes, has been employed for the effective adsorption of toxic selenite anions from aqueous solutions. Remarkably, Fe-BTC maintains high adsorption efficiency across a broad pH range (2-12), achieving a maximum adsorption capacity of 491 mg g^-1, ranking among the highest recorded for adsorbents, including MOFs. The adsorption process involves distinct chemical interactions depending on pH: weak and variable interactions under acidic conditions (pH 2) and strong, diverse coordination modes under alkaline conditions (pH 11). Notably, the strong coordination ability of selenite ensures high selectivity over selenate and competing anions such as Cl^-, NO₂ ^-, NO₃ ^-, CO₃ ^2-, SO₄ ^2-, and PO₄ ^3-. The abundance of metal defects endows Fe-BTC with superior adsorption capacity compared to crystalline Fe-MOF, MIL-100(Fe). This study provides a comparative analysis of selenite adsorption on Fe-BTC under acidic and alkaline conditions, emphasizing pH-dependent adsorption mechanisms and their implications for designing effective adsorbents for toxic species removal.