A novel strategy for efficient removal of hazardous metal ions based on thermoresponsive phase separation of the PNIPAM/GO system

A temperature-responsive phase separation system based on poly(N-isopropyl acrylamide) (PNIPAM) and graphene oxide (GO) has been developed for efficient removal of metal ions from polluted water. PNIPAM undergoes a coil-to-globule conformational transition upon temperature increase to above its lower critical solution temperature (LCST), which can induce phase separation of the system into a condensed gel phase and a water phase. In this process all GO sheets can be transferred to the gel phase by having strong interactions with PNIPAM molecules via hydrophobic interaction, electrostatic interaction, and hydrogen bonding. Moreover, by having abundant carboxyl, hydroxyl, and epoxy groups to chelate with heavy metal ions, GO sheets can adsorb metal ions efficiently and enrich them into the gel phase. The gel phase can then be separated from the water phase in an extraction-like process, achieving easy removal of the pollutant metal ions from water. This method takes advantages of PNIPAM's temperature responsive phase transition and GO's high adsorption capacity, enabling high versatility, easy treatment and separation, and good reusability in treatment of metal ion pollutions, being highly potential for practical industry applications.