Forward osmosis for heavy metal removal: Multi-charged metallic complexes as draw solutes

Forward osmosis (FO) proves advantageous in various water treatment. However, its potential application largely relies on the availability of suitable draw solutes. Here we report a series of multi-charged betaine metallic complexes for the removal of Cu2+, Zn2+ and Ni2+ ions from electroplating wastewater through FO. All betaine metallic complexes surpass the currently reported synthetic draw solutes and conventional NaCl, NH4HCO3 by increasing water fluxes up to 350% along with insignificant solute losses. Meanwhile, the betaine metallic complexes can generate a water permeation rate at least 300% higher than most commonly used NaCl draw solute in treating electroplating wastewater containing Cu2+, Zn2+ and Ni2+ ions. The impressive FO performance arises from the fact that the betaine metallic draw solutes contain abundant –COO−, –NMe3+ ionic moieties and have a special polymeric configuration. Remarkably, all betaine metallic complexes are readily regenerated by solvent extraction with minimal energy input. The novel betaine metallic draw solutes not only tackle the low water recovery and solute loss problems confronted in FO processes, but also can be recycled by a more economic and eco-friendly approach than the current draw solutes, demonstrating the suitability of the multi-charged metallic complexes presented here for FO water treatment.