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

Abstract

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. To address this bottleneck, a multi-layer temperature-responsive hydrogel based on poly(N-isopropylacrylamide-co‑sodium acrylate) (P(NIPAAm-co-SA)) was developed. The drawing layer had a high concentration of SA for high osmotic pressure in the FO process. The releasing layer had no SA to preserve the lower critical solution temperature (LCST) of P-NIPAAm for fast water release. The multi-layer hydrogel yielded a high capacity of water absorption and high permeable flux due to the ionic drawing layer, which were comparable to that of the uniform P(NIPAAm-co-SA) hydrogels. Compared to the uniform hydrogels without LCST phase transition for the fast release of water, the releasing layer in our design 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 and reduction of energy consumption. It was also found that reducing the gradient of SA concentration along the water transport path also improved the dewatering ratio as a result of promoting this inter-layer water transport.