A pH-responsive supramolecular draw solute that achieves high-performance in arsenic removal via forward osmosis

A pH-responsive, charge switchable piperazine derivative, 1,4-bis(3-propane- sulphonate sodium)-piperazinediethanesulfonic acid disodium-sulfate (4), has been designed via a stepwise synthesis and proposed as a draw solute to remove arsenics (As^III, As^V) from water through forward osmosis (FO). Having multiple sulfonic groups, 4 generates a high osmotic pressure and produces a water flux as high as 58.4 LMH at a dilute concentration (0.24 M), surpassing most of the existing draw solutes in water transfer rate under the similar experimental conditions. Compound 4 at 0.24 M yields a water flux of 52.9 LMH with a 100% As^V rejection, and 57.8 LMH with a 96.0% As^III rejection when 50 ppm As^V or As^III as the corresponding feed, manifesting the best performance in arsenic removal and concurrent water recovery efficiency. Remarkably, being a polymeric configuration in water, 4 has a negligible solute loss in the FO process. 4 can be readily regenerated for reuse in FO by precipitation from its solution through acidification. The abundance in ionic groups and the pH-responsive property coupled with a supramolecular configuration make 4 an ideal draw solute for FO wastewater treatment.