Thermo- and light responsive microgels for efficient brackish and seawater forward osmosis desalination

Abstract

Forward osmosis desalination presents a promising solution to address water shortages in areas near brackish or seawater sources. This study investigates the use of thermo-responsive poly(N-isopropyl acrylamide-rand‑sodium acrylate) microgels as innovative draw agents for forward osmosis desalination. The drawing ability and responsiveness of these microgels are significantly influenced by the charged comonomer content. Unlike bulk hydrogels, these microgels, owing to their core-shell morphology, maintain thermo-responsivity even at higher comonomer content. Incorporating graphene oxide as a light absorber allows for partial heating, required to reach the transition temperature, to be obtained using UV light radiation. In forward osmosis, microgels can be used either in a dried state or as a concentrated aqueous dispersion. A sample with 25 mol% charged units achieved a balance between water flux and responsiveness. It reached fluxes of 2.84, 4.79 and 4.39 L·m⁻²·h⁻¹ for the dried state, 40 wt% and 20 wt% dispersions, respectively, when tested with 5 g·L⁻¹ brackish water. Furthermore, a 40 wt% dispersion drew 35 g·L⁻¹ seawater at a flux of 1.36 L·m⁻²·h⁻¹. This sample, which contains graphene oxide, exhibited a volume phase transition temperature at 41 °C that can be achieved through UV light radiation or natural sunlight exposure. Water separation from the microgels was accomplished through filtration under UV-light radiation with a power of 4 kW·m⁻², at 2–4 bar pressure, with a microfiltration membrane and a flux of 36 L·m⁻²·h⁻¹. These findings highlight the potential of these thermo-responsive microgels for efficient forward osmosis desalination.