Phosphinic acid-functionalized MOF, MIL-101-PO₂H₂: Remarkable adsorbent to selectively remove cationic dyes from water

Phosphinic acid functionalized MOF (MIL-101-PO₂H₂, abbreviated as M101-PO₂H₂) was prepared from MIL-101-NH₂ through a post-synthetic modification. Phosphinic acid-loaded MOF, M101-PO₂H₂, together with MIL-101, MIL-101-NH₂, and activated carbon, was applied to the adsorptive elimination of organic dyes from water. M101-PO₂H₂, because of the loaded phosphinic acid and high porosity, showed a remarkable performance in the removal of cationic dyes like methylene blue (MB) and azure B (AzB), both in high and low (C_i: 1 ppm) concentrations. However, M101-PO₂H₂ was very poor in the adsorption of anionic dyes like methyl orange (MO) and acid red 1. Moreover, the developed adsorbent showed highly selective adsorption of a cationic dye (AzB) from an equi-concentration mixture of cationic and anionic (AzB and MO) dyes. In order to understand the mechanism of AzB adsorption, AzB and MO adsorptions under wide pH conditions, analysis of surface charges of the MOFs (also, at pH from 1 to 11), and calculations were carried out. The very favorable adsorption of AzB over M101-PO₂H₂ was mainly due to electrostatic and hydrogen bonding (AzB: H-donor) interactions. The more favorable AzB adsorption over M101-PO₂H₂, compared with the MB adsorption, could also be interpreted with hydrogen bonding. The developed M101-PO₂H₂ was reusable for at least ten cycles after conventional solvent washing. Based on the research, porous MOFs with phosphinic acid can be suggested as an effective adsorbent to eliminate cationic species, especially with H-donors like -OH or -NH₂ from water.