Preparation And Performance Of Carboxylate Polyelectrolyte And Electric-sensitive Hydrogels As Draw Agent For Forward Osmosis

Water shortages and pollution problems have become increasingly prominent with the population growth and rapid economic development.Since the 20th century,membrane filtration technology such as microfiltration,ultrafiltration,nanofiltration and reverse osmosis etc.has started to be applied to drinking water trwatment,groundwater recharge and industrial wastewater treatment for changing unsustainable patterns of water use.Forward osmosis(FO)is a membrane separation process which only depends on the osmotic pressure difference to enable spontaneous transport of water molecules from low osmotic pressure of feed solution to high osmotic pressure of draw solution.And it can draw drinking water from sewage or seawater.Compared with the reverse osmosis(RO)which can also be used for extracting drinking water,energy consumption is reduced because FO uses osmotic pressure difference as driving force without external pressure.Besides,it has high water recovery rate and low membrane fouling propensity.And its system has so low requirement on the water quality of influent that the sewage pretreatment process can be simplified.However,the wider application of FO is still restricted by the reverse osmosis of draw solution,complex regeneration and high regeneration consumption.Therefore,the synthesis and selection of appropriate draw solution have become a top topic in the current study of FO.A new polyelectrolyte is prepared for FO,macromolesize polyelectrolyte can reduce reverse osmosis,and using ultrafiltration process to regenerate diluted draw solution makes energy consumption decrease.Meanwhile,electric-sensitive hydrogels are synthesized as a new draw agent,which not only avoids reverse diffusion completely,but also achieves rapid regeneration under electric filed.The specific contents of this study are as follows.(1)Free radical polymerization was adopted to synthesize carboxylate polyelectrolyte using hydrophilic monomer of acrylic acid and sodium p-styrenesulfonate.Orthogonal experiment was carried out to obtain the optimum conditions for the reaction which were the initiator content accounting for the monomers(16%)and the monomer molar ratio(4)at 70 ℃for two hours.And it obtained the mass average molecular weight(MW)of 96308 g/mol,and there was a linear relationship between the osmotic pressure and concentration,but the viscosity did not have linear relationship with the concentration.At two membrane orientations,the water flux and reverse solute flux for carboxylate polyelectrolyte solutions were determined.The results showed that the water flux and reverse solute flux both tended to enhance with the increasing concentration of carboxylate polyelectrolyte solution.At the active layer facing the draw solution,0.18 g/mL carboxylate polyelectrolyte solution as draw solution produced the initial water flux of 11.77 LMH and the reverse solute flux of 1.26 gMH;and it produced the initial water flux of 6.68 LMH and reverse solute flux of 0.75 gMH at the active layer facing feed solution.Due to its large molecular structure,the reverse solute flux was far less than that of inorganic salt.Using polyethersulfone ultrafiltration membrane for regeneration could achieve the recovery efficiency of 94.78%.(2)Chemical cross-linked method integrated with repeated freezing-thawing was adopted to prepare PVA/PAA hydrogels using acrylic acid(AA)and poly(vinyl alcohol)(PVA)monomer,which were denoted as PVA3M0.25,PVA3M0.5,PVA3M1.0 and PVA3M1.5 hydrogel,respectively.The swelling ratio,morphology,mass retention rate and repeated swelling-deswelling behaviors were investigated.The results indicated that PVA/PAA hydrogels were sensitive to subtle electric stimuli,and PVA3M0.5 hydrogel released water more quickly when 5 V was applied.With the increasing content of cross-linker,the pore structure of hydrogel surface reduced.At the active layer facing the draw agent,PVA3M0.5 and PVA3M1.0 hydrogel were selected to adopt as draw agent,these hydrogels could yield the initial water flux of 1.04,0.72 LMH and 0.73,0.54 LMH,respectively when the deionized water and 2000 ppm NaCl solution were used as the feed solution,separately.(3)Aqueous solution polymerization was adopted to fabricate a series of electric-responsive AMPS/DMAEMA hydrogels with the different monomer ratios and contents of cross-linker by using 2-Acrylamido-2-methyl-1-propanesulfonic acid(AMPS)and 2-(Dimethylamino)ethyl methacrylate(DMAEMA)as the monomer,N,N’-methylenebisacrylamide(MB A Am)as the cross-linker and ammonium persulfate(APS)/sodium bisulfate(NaHSO3)as a redox initiator.These hydrogels were labelled as A0.5D0.5、A0.53D0.47、A0.55D0.45、A0.5M2、A0.53M2 and A0.55M2.Gravimetric method was used to examine the swelling ratio via a dialysis bag,so the swelling ratios of AO.55D0.45,A0.53D0.47 and A0.55M2 hydrogel were 20.07,10.78 and 16.91,respectively.Meanwhile,the infrared structure,morphology and different water states in hydrogels were investigated.The results indicated that the prepared hydrogels had obviously porous structure after freezing dry method,and the introduction of sulfonic groups enhanced the binding ability of the hydrogels and water molecules as the content of AMPS monomer increased.At the active layer facing the draw agent,the initial water fluxes were 2.09,1.63 and 1.11 LMH for them using 2000 ppm NaCl solution as the feed and 2 g xerogel particles as the draw agent.Three hydrogels were used to do dehydration test for regeneration under the electric stimuli.After the second regeneration,their water recovery rates were 59.06%,32.19%and 40.73%,and their initial water fluxes achieved 1.54,1.08 and 0.86 LMH in FO process,separately.