A novel ionic liquid-entrapped MIL-101(Cr) framework with enhanced removal efficiency towards phosphate from aqueous solution

Developing adsorbent materials with high adsorptive dephosphorization (ADP) is significant for treating phosphate from aqueous solutions and eutrophic water. Herein, the MIL-101(Cr) framework was entrapped ionic liquid (IL) of 1-butyl-3-methylimidazoliumbromide ionic liquid ([C₄mem]⁺[Br]⁻) using a ship-in-a-bottle approach to obtain novel adsorbents [C₄mem]⁺[Br]⁻@MIL-101(Cr) contained varied IL contents, namely C₄mem@MIL-101. The characterization results revealed that the formed [C₄mem]⁺[Br]⁻ molecules interacted with the MIL-101(Cr) frameworks, enhanced their stability, and offered additional adsorption sites. The batch adsorptions of phosphate showed that the optimized C₄mem@MIL-101 adsorbent loaded with ~ 7% IL-based N content had the highest phosphate absorbing capacity of ~ 200 mg/g, outperforming the pristine MIL-101(Cr) and other adsorbents. The ADP efficiency was facilitated in the acidic media, where the phosphate ions of H₂PO₄⁻ and HPO₄²⁻ captured onto the C₄mem@MIL-101 via several interactions, including electrostatic attraction, H-bonds, and chemical interactions. In the meantime, the coexisting anions diminished the phosphate adsorption because they competed with the pollutants at adsorption sites. Furthermore, phosphate treatment under the continuous fixed-bed conditions showed that 1 g of the polyvinyl alcohol (PVA)-mixed C₄mem@MIL-101 pellets purified 25 l of water containing phosphate with a 1 mg/l concentration. The results suggest that the novel [C₄mem]⁺[Br]⁻@MIL-101(Cr) structure had a high potential for treating phosphate in aqueous solutions.