Betaine-Based Polynuclear Metallic Complexes for Heavy Metal Removal Through Forward Osmosis

The availability of suitable draw solutes is essential to fully exert the advantages of forward osmosis (FO) in water pollution control. Herein betaine-based polynuclear complexes, Bet-Mn-TFSI (M = Cu, Zn, Ni, n = 3 - 5), are synthesized via one-pot ligation of betaine and MCl2 and used in FO for Cu2+, Zn2+ and Ni2+ removal from electroplating wastewater. Factors affecting FO performance are systematically studied by varying the chemical composition, the structure and charge state of Bet-Mn-TFSI. Compared to conventional NaCl, NH4HCO3 and other recently reported draw solutes, all Bet-Mn-TFSI have negligible solute losses and increase water fluxes by up to 350%, tackling the low water recovery and solute loss problems confronted with the current draw solutes. Bet-Mn-TFSI induce a water flow rate 300% (M = Cu), 451% (M = Zn) and 539% (M = Ni) higher than that of NaCl in purifying electroplating wastewater containing 2000 ppm metal ions. Having an expanded configuration and abundant –COO-, –NMe3+ and (CF3SO2)2N- ionic moieties are essential for Bet-Mn-TFSI to achieve satisfactory FO performance. Particularly, Bet-Mn-TFSI are readily regenerated by solvent extraction without energy input. These findings demonstrate the suitability of Bet-Mn-TFSI for FO water pollution control.