The Construction And Application Of Forward Osmosis Water Flux Model Using Thermo-responsive Hydrogel As Draw Agent

Forward osmosis（FO）,an emerging green membrane separation technology with high water recovery rate and low pollution,has attracted more and more attention in the past decade.The development of novel draw agent was the main approach for the improvement of FO performance.A range of traditional draw agents,such as inorganic salts,macromolecular solutes and magnetic nanoparticles,have been shown to generate appreciable water fluxes.However,regeneration of draw agents that has absorbed water remained a major challenge for FO.As novel draw agents,smart hydrogels solved the above problem.Because they had the characteristics of absorbing water driven by their unique swelling properties and releasing water to restored their swelling properties by environmental stimulation（such as temperature,pressure and electricity,etc.）.In addition,hydrogels could effectively avoid reverse solute diffusion,resulting in clean product water.But now,the application of hydrogel-FO system was less researched;the systematic and in-depth relation between hydrogel and FO performance was still lack;and the driving mechanism of whole FO process was not yet clear.Therefore,it was necessary to explore the mechanism by means of mathematical methods.First,thermo-responsive hydrogels were synthesized by polymerization of N-isopropylacrylamide（NIPAM）with the addition of ionic polyglutamic acid（γ-PGA）and pore-forming polyethylene glycol（PEG）.Using its porous structure,hydrogels were prepared into sheets to investigate the effects of specific operating conditions（preparation thickness,initial water content,dewatering duration and regeneration times）on water flux in hydrogel-driven FO process.The optimal operating conditions were obtained as follows:preparation thickness of 500～800μm,initial water content of 5%,dewatering regeneration time of 5 min,and the water flux decreased less than 15%after three cycles.Secondly,the FO mass transfer model of salt solution and swelling pressure calculation model of hydrogel were used to develop the water flux FO model using thermo-responsive hydrogel,and then the validities of the redefined parameters and this model were verified.The influence of different operating conditions（concentration of feed solution,temperature,flow rate,hydrogel dosage and type）on water flux was analyzed.The determination coefficient(R_Jw²)of simulated and experimental water flux was above 0.86,which proved the accuracy of the water flux model.The swelling pressure was the real driving force of hydrogel in FO process,and the development of hydrogel with high swelling pressure was the key to increase water flux.Finally,the hydrogel-driven FO water flux model was used to guide the hydrogel development,and the hydrogel optimization was carried out in two aspects.The thermo-responsive PNIPAM/γ-PGA-Na/PEG hydrogel was prepared by introducingγ-PGA-Na monomer.It was found that the swelling pressure and swelling ratio could be improved with the addition ofγ-PGA-Na,and the internal concentration polarization（ICP）could be effectively alleviated.The water flux of PNIPAM/γ-PGA-Na/PEG could reached 1.79 LMH in the first 1 h.Polyurethane（PUF）foam was introduced into PNIPAM/γ-PGA-Na.It was found that PNIPAM/γ-PGA-Na/PUF hydrogel had higher hydrophilicity,more abundant pores and higher mechanical properties,which can increase the diffusion rate of water in hydrogel and effectively alleviate the ICP effect.In FO process test,the water flux of PNIPAM/γ-PGA-Na/PUF in the first 1 h was 2.09 LMH.