Systematic structure control of ammonium iodide salts as feasible UCST-type forward osmosis draw solutes for the treatment of wastewater

Abstract

A variety of UCST-type thermoresponsive ammonium iodide salts is systematically designed and synthesised with desired aqueous solubility, phase-transition temperature, osmolality change, phase-transition concentration range, stability, and toxicity by controlling the ionic interactions, hydrophobicity, and symmetry of the salt structure. Suitable draw solutes can be selected based on the characteristics of the ammonium iodide salts as well as the feed solutions for feasible forward osmosis (FO)-based water purification. In this research, highly concentrated wastewater from a flue gas desulfuriser (FGD), with osmotic pressure three times higher than seawater, is targeted for purification by FO using the draw solutes. Two ammonium iodide salts (HM8I and HM10I) show a remarkable water flux from the wastewater samples and significantly lower salt leakage compared to conventional draw solutes. The osmolality of the phase-separated draw solution drops to less than one tenth of that of the initial feed solution, and reverse osmosis or nanofiltration can be applied to the solution with much lower external pressure for the final purification to produce fresh water. This systematic approach to the design and selection of suitable draw solutes can be an effective strategy for future practical FO-based wastewater treatment and seawater desalination.