Significantly enhanced water flux in forward osmosis desalination with polymer-graphene composite hydrogels as a draw agent†
We report here a new strategy to dramatically increase water flux in a forward osmosis (FO) process using reduced graphene oxide (rGO)-composite hydrogels as draw agents as well as increasing the rate of the subsequent regain of pure water from the hydrogel particles. The composite hydrogels were prepared by incorporating 0.3 wt%–3 wt% rGO into two different hydrogels: poly(sodium acrylate) (PSA) and poly(sodium acrylate)-poly(N-isopropylacrylamide) (PSA-NIPAM). The amount of incorporated rGO sheets had a significant effect on the swelling pressure of the composite hydrogels and the composite hydrogels with contained small amounts of rGO sheets (0.3–1.2 wt%) showed significantly enhanced swelling ratios while those with more rGO (e.g., 3 wt%) exhibited decreased swelling ratios. Consequently, significant enhancements in water flux in the FO process were achieved for composite hydrogels with small amounts of rGO. When compared with the pure hydrogels, the composite hydrogels PSA-1.2 wt% rGO and PSA-NIPAM-1.2 wt% rGO showed increased water fluxes of some 310% and 227%, respectively, when 2000 ppm of a NaCl aqueous solution was used as the feed. When deionized water was used, even higher water fluxes were attained, i.e., 8.2 L m−2 h−1 for PSA-1.2 wt% rGO and 6.8 L m−2 h−1 for PSA-NIPAM-1.2 wt% rGO. The swelling process of the particles was investigated using optical microscopy where it was found that the addition of small amounts of rGO greatly increased the softness of the composite hydrogels and improved the inter-particle and particle-membrane contact, leading to dramatically improved water fluxes. In addition, the light-absorbing property of rGO produced much better outcomes in terms of dewatering of the composite hydrogels in the second stage of the FO process, in which the pure water from the hydrogels is harvested, with dewatering stimulated by heating induced from absorbed solar energy. The water recovery rate for composites with 1.2 wt% rGO was found to be twice as fast as that for pure hydrogels.