Effectiveness Research Of Using Electric-responsive Hydrogels As Draw Agents In Forward Osmosis

Forward osmosis (FO) uses the osmotic pressure difference between the draw solution and the feed solution to drive the water from the feed solution to the draw solution, which can directly draw pure water from polluted water. Compared to a reserve osmosis (RO) process, FO shows the advantages of lower energy consumption, higher water recovery rate, lighter membrane fouling and more environmentally friendly. However, reverse draw solute diffusion and the complicated regeneration process hinder the further development of FO process.Taking above into account, this study considered hyaluronic acid-polyvinyl alcohol (HA-PVA) hydrogels as draws agent of the FO process,which could avoid the reverse draw solute diffusion. The regeneration process could be simplified by applying an extra electric field. The effects of freeze-thaw cycle, voltage, operating time and concentrations of the feed solution on the FO process were examined, while regarding the as-prepared hydrogels as draw agents. The energy consumption was calculated as well.First of all, electric-responsive HA-PVA hydrogels were prepared by repeated freezing-thawing. Hydrogels with different freeze-thaw cycles were designated:HA-PVA-3, HA-PVA-5, HA-PVA-7and HA-PVA-9, respectively. The HA-PVA solution cannot form a gel within three repeated freeze-thaw cycles because of the low degree of cross-linking. When using HA-PVA-5, HA-PVA-7and HA-PVA-9hydrogels as draw agents and deionised water as the feed solution, the initial water fluxes were:1.2,0.91and0.9L·m-2-·h-1, respectively. When the voltage of the electric field was:0,3,6and9V, the total water fluxes produced by HA-PVA-5hydrogels in24h were:17.27,20.95,25.49and26.47L·m-2, respectively. The total water flux produced by continuous experiment and intermittent experiment at a voltage of6V in24h were50.84and62.39L·m-2, respectively. When using2000,5000and8000ppm sodium chloride solutions as feed solutions, the total water fluxes produced by HA-PVA-5hydrogels at a voltage of6V in24h were22.66,15.77and12.44L·m-2, respectively. When compared to other studies which use hydrogels as draw agents, the initial water fluxes and total fluxes are desirable.Secondly, the energy consumption was calculated when using HA-PVA-5hydrogels as draw agents, applying an extra electric field of6V. The total water fluxes of continuous operation mode and intermittent operation mode were50.84and62.39L-m"2, respectively. It turned out that intermittent operation mode would consume less energy (3.2×105J) to absorb the same quality of pure water (1L) when compared to continuous operation mode (3.6×105J). Intermittent operation mode is preferred for long-term running.Finally, the influence of addictives to the performance of HA-PVA-5hydrogels were investigated. However, the results have failed to meet the expectation. Research on addictives remains to be further discussed.