Removal of organic phosphonate HEDP by Eu-MOF/GO composite membrane

The efficient removal of organic phosphonate 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) from reverse osmosis brine concentrate was investigated by coupling adsorption and membrane separation. The prepared adsorbent of europium-metal organic framework (Eu-MOF) has an adsorption capacity of 37.31 mg-P/g for organic phosphine, which is the highest reported so far. Eu-MOF and graphene oxide (GO) were mixed and deposited on a supporter polyethersulfone by vacuum filtration to form Eu-MOF/GO composite membrane. The micro-space channels between the Eu-MOF straw-sheaf crystals and intrinsic nanosized pores in the Eu-MOFs served as separation membrane adsorbent with both high selectivity and high flux. The composite membrane could achieve a 100% rejection rate of HEDP from the simulated RO concentrate solution at pH 5–8, and the flux was more than six times that of a pure GO membrane. The increase in the interlayer spacing of GO nanosheets adjusted by Eu-MOF would enhance the permeation flux of the composite membrane. Most importantly, the Eu-MOF with the most probable pore size of 3.371 nm, embedded in the composite membrane, also increased permeation flux and at the same time enhanced the adsorption capacity of HEDP molecules. The separation mechanism of the composite membrane was mainly ligand exchange and hydrogen bonding. The trade-off effect between permeation flux and selectivity was overcome by the prepared composite membrane. This work provides a new strategy for designing composite membranes with adsorption function and such composite membrane can serve as a promising material for removing organic phosphonates from water.