Multi-layer temperature-responsive hydrogel for forward-osmosis desalination with high permeable flux and fast water release

The development of forward-osmosis (FO) desalination is significantly restricted by the lack of suitable draw agents for both high permeable flux and fast release of water. In order to address this bottleneck, a multi-layer temperature-responsive hydrogel based on poly(N-isopropylacrylamide-co-sodium acrylate) (P(NIPAAm-coSA)) was developed. The drawing layer had a high concentration of polyelectrolyte for high osmotic pressure in the FO process. The releasing layer had no polyelectrolyte to preserve the lower critical solution temperature (LCST) of P-NIPAAm for fast water release when the temperature was above the LCST. The multi-layer hydrogel yielded a high capacity of water absorption, which was mainly determined by the composition of the drawing layer while the swelling kinetics was restricted by the releasing layer. Besides, the drawing layer also had a major impact on the permeable flux, enabling the multi-layer hydrogel to yield a comparable FO flux to that of the uniform P(NIPAAmco-SA) hydrogels. Compared to the uniform copolymer hydrogels which did not show the LCST phase transition for the fast release of water, the releasing layer in the multilayer samples preserved the LCST which facilitated the release of water as a result of inter-layer water transport from the drawing to the releasing layers. Such an effect allowed the further enhancement of water release by using the intermittent dewatering strategy. It was also found that reducing the gradient of SA concentration along the water transport path also improved the dewatering rate as a result of promoting this inter-layer water transport.