Experimental investigation and bottom-up life cycle assessment of forward osmosis desalination using thermo-responsive Janus microgels

Abstract

While thermo-responsive hydrogels like poly(N-isopropylacrylamide) [P(NIPAAm)] have been widely investigated as draw agents for forward osmosis (FO) desalination, the bottom-up assessment of their specific energy and life cycle environmental impact at the system level remains elusive, raising questions about their potential to tackle the water-energy-climate nexus. To address this critical issue, we developed an FO desalination system using novel thermo-responsive Janus microgels, with a spatially separated structure involving ionic poly(sodium acrylate) [PSA] for high osmotic pressure and thermo-responsive P(NIPAAm) for fast water release. It was experimentally validated that these Janus microgels achieved a high FO flux of over 1 liter per m² per hour (LMH) and released over 90% of absorbed water within 45 minutes upon heating above the lower critical solution temperature (LCST) of 35 °C, significantly outperforming their analogs. Using the experimentally proven parameters as input, a bottom-up life cycle assessment (LCA) model of the FO system with a thermal regeneration strategy (FO–TR) was established, which consumed only 0.30 kWh m³ of electricity and relied on low-grade waste heat for regeneration. Accordingly, the FO–TR system reduced the energy-related environmental impact to below 5%—far less than the environmental impacts caused by conventional desalination systems. On the contrary, membrane materials were found to be the main environmental stressors. In the synthesis of the thermo-responsive draw agent, the NIPAAm monomer was identified as the major environmental impact contributor. This study provides a comprehensive experimental and environmental evaluation to reveal the potential and challenges of thermo-responsive hydrogels for sustainable FO desalination.