Membrane-free osmotic desalination at near-room temperatures enabled by thermally responsive polyionic liquid hydrogels

Abstract

A novel thermally responsive chloride-anion rich polyionic liquid (TPIL) hydrogel was synthesized and its potential in membrane-free osmotic desalination for saline water and seawater was explored. The tunable lower critical solution temperature (LCST) originates from the subtle balance between hydrophilicity and hydrophobicity within the sole ionic liquid monomers, imparting the hydrogel with temperature-controlled reversible volume change in an aqueous salt solution. At temperatures below LCST, the TPIL hydrogel swells and significantly prevents salt ion infusion due to the high density of charges and chloride counterions within the TPIL hydrogel network. The swelled TPIL hydrogel shrinks and releases salt-depleted water at temperatures above LCST. Salt rejection for the membrane-free osmotic desalination of >80% in one cycle achieved from artificial seawater (0.6 M NaCl) is the highest observed thus far. In addition, water recovery can be realized solely by moderate heating of the TPIL hydrogel to only 40–50 °C, avoiding water vaporization or high-grade electricity consumption in alternative desalination technologies. This facile and economical water recovery technique enables desalination that utilizes low-grade thermal energy and reduces the carbon footprint. Utilization of solar energy is also demonstrated in this paper by incorporating photo-thermal additives of Ti₃C₂T_x MXene into the hydrogel network.