pH-Responsive Polyoxometalates that Achieve Efficient Wastewater Reclamation and Source Recovery via Forward Osmosis

Forward osmosis (FO) has been increasingly used for water treatment. However, the lack of suitable draw solutes impedes its further development. Herein, we design pH-responsive polyoxometalates, that is, (NH4)6Mo7O24 and Na6Mo7O24, as draw solutes for simultaneous water reclamation and resource recovery from wastewater via FO. Both polyoxometalates have a cage-like configuration and release multiple ionic species in water. These characteristics allow them to generate high osmotic pressures to drive the FO separation efficiently with negligible reverse solute diffusion. (NH4)6Mo7O24 and Na6Mo7O24 at a dilute concentration (0.4 M) produce water fluxes of 16.4 LMH and 14.2 LMH, respectively, against DI water, outperforming the frequently used commercial NaCl and NH4HCO3 draw solutes, and other synthetic materials. With an average water flux of 10.0 LMH, (NH4)6Mo7O24 reclaims water from the simulated glutathione-containing wastewater more efficiently than Na6Mo7O24 (9.1 LMH), NaCl (3.3 LMH), and NH4HCO3 (5.6 LMH). The final glutathione treated with (NH4)6Mo7O24 and Na6Mo7O24 remains intact but that treated with NaCl and NH4HCO3 is either denatured or contaminated owing to their severe leakage in FO. Remarkably, both polyoxometalates are readily recycled by pH regulation and reused for FO. Polyoxometalate is thus proven to be an appropriate candidate for FO separation in wastewater reclamation and resource recovery.