Performance Of Electro-responsive AMPS-based Hydrogels As Forward Osmosis Draw Agents For Desalination

Compared with reverse osmosis(RO),forward osmosis(FO)is a spontaneous membrane separation process with low energy consumption and less membrane fouling.It has great potentials in the applications of seawater desalination,industrial wastewater,concentration and dewatering of landfill leachate and sludge,farmland irrigation,food processing and power generation,which have aroused great concern of scientific workers.However,fabricating sorbents with high osmotic pressure,low reverse solute diffusion,easy regeneration and low energy consumption is a critical issue to promote the commercial application of FO technology.Smart hydrogel,a kind of three-dimensional network structure polymers,can absorb a large number of water molecules with water insolubility.Moreover,it can contract and swell reversibly with the stimulation of external environment(such as temperature,p H,light,magnetic field,pressure or electric field).Such characteristics can be employed to design as novel forward osmosis draw agents.Rapid regeneration of sorbents and reduce energy consumption can be achieved because the electric field stimulation mode is easy to be controlled and applied.Herein,electro-responsive hydrogels based on 2-acrylamido-2-methy-1-propanesulfonic acid(AMPS)were fabricated via free radical polymerization and their performance when used as positive osmotic sorbents were investigated.The main research content and results are as follows:(1)Poly(N,N-dimethylacrylamide-co-2-acrylamido-2-methyl-l-propanesulfonic acid)(P(DMAM-co-AMPS))is used as novel forward osmosis draw agents.The chemical structure,morphology,water-adsorbing capacity,mechanical properties and water state of hydrogels were characterized,and the response behaviors and mechanism of hydrogels under external electric field were explored.Besides,the water flux and regeneration properties of hydrogel draw agents in the process of forward osmosis were explored.The results show that the equilibrium swelling ratio of the prepared hydrogel is 532 g/g,and the swelling process accords with the Fickian diffusion type.The hydrogels showed obvious and reversible anti-deswelling via the stimulation of external electric field,and the dehydration rate of the hydrogels increased with the increase of applied voltage,which may be related to the electroosmosis process and electrochemical reaction in the system.Using 2 g/L Na Cl as feed solution,the average water flux produced by the hydrogel in 1.0 h was 1.81 LMH.When the stimulation voltage was 15 V,the recovery rate of reclaimed water was 67.7 %.(2)To improve the swelling of hydrogels,a series of poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide)(P(AMPS-co-AM))hydrogels with different monomer ratios were prepared using acrylamide(AM)and AMPS as monomers.The morphology,chemical structure,swelling and water state of the hydrogel were characterized by SEM,FTIR,DSC.In addition,the response behaviors and mechanism of hydrogel extractant under external electric field were studied,and the water flux and regeneration performance of hydrogel draw agents in the process of forward osmosis were explored.The results show that compared with P(DMAM-co-AMPS),P(AMPS-co-AM)possessed higher swelling ratio and the water flux of 2.76 LMH was shown in the FO test(the feed solution is 2 g/L Na Cl solution).Under 15 V electric field,the recovery rate of reclaimed water was about 71 % in 40 min.(3)Poly(2-acrylamido-2-methyl-1-propanesulfonic acid-co-acrylamide)/polyacrylic acid hydrogel P(AMPS-g-AM)/PAA with semi-interpenetrating network(Semi-IPN)structure was prepared by chemical cross-linking method.With the construction of semi-interpenetrating network,the hydrogel not only kept its original properties,but also showed good performance.Meanwhile,the hydrophilicity and electrical sensitivity of the hydrogels were further improved by the incorporation of polyelectrolyte polyacrylic acid(PAA),which aimed to improve the forward osmosis performance.The results show that the swelling ratio of P(AMPS-g-AM)/PAA was higher than that of pure P(AMPS-g-AM).Besides,under the same voltage stimulation,the dehydration rate of P(AMPS-g-AM)/PAA after water swelling via forward osmosis was faster,and the recovery rate of reclaimed water was 1.43 times higher than that of the former.