Incorporation of MIL-101 (Fe or Al) into chitosan hydrogel adsorbent for phosphate removal: Performance and mechanism

Metal-organic frameworks (MOFs) have become popular adsorbents for phosphate removal. However, in their special form (powder), the obvious disadvantage is poor recyclability and difficult operation. Therefore, this study incorporate MIL-101 (Al/Fe) into chitosan for phosphate removal. Batch experiments with different adsorption parameters including pH, reaction time, temperature, ionic strength are studied. The kinetics and thermodynamic study prove that the pseudo-second-order model and the Langmuir model could describe the phosphate adsorption on MOFs-chitosan adsorbents accurately, indicating that chemical adsorption and monolayer adsorption in the adsorption process. The maximum adsorption capacities for phosphate reached 49.8 ​mg/g (MIL-101(Al)-CS). A thermodynamics calculation results (△G ​< ​0, △H ​> ​0, and △S ​> ​0), indicating that phosphate adsorption onto MOFs-chitosan adsorbents is endothermic, spontaneous, and disordered. The main mechanisms of phosphate removal consists of electrostatic attraction and ligand exchange. MOFs-chitosan adsorbents has excellent selectivity to phosphate when adsorption process with foreign anion. In addition, adsorption capacity of MOFs-chitosan adsorbents can maintain 92% of the initial adsorption capacity after ten adsorption-desorption cycles. These findings demonstrate that MOFs-chitosan adsorbents is a promising adsorbent for phosphate removal. The current study provided a strategy for the design of adsorbents from the viewpoint of chemical structures.