| Forward osmosis(FO)is an emerging membrane separation process that relies solely on osmotic pressure,which means that water molecules spontaneously penetrate through the semipermeable membrane from the feed solution side(the high water chemistry potential)to draw solution side(the low water chemistry potential).The technology has been successfully applied in the fields of seawater power generation,sewage treatment and emergency rescue.Compared with pressure-driven membrane filtration technologies,FO technology has the advantages of no external pressure drive,high water recovery rate and small membrane contamination tendency.However,its application and development are still restricted by reverse solute penetration,complex regeneration methods and high energy consumption of draw solution regeneration.For instance,the reverse penetration phenomenon exists using NaCl as draw solution in an industrialized application,which leads to the contamination of the feed solution and needs extra supplement of the draw solution.Simultaneously,the regeneration method is reverse osmosis,which needs complex equipment and high energy consumption.Therefore,developing an ideal draw solution which can meet the needs of FO technology,such as high water flux,low reverse penetration and easy regeneration method,has become a major research direction.In this paper,based on the characteristics and requirements of FO technology,a hydrophilic ferric-lactate complex(LAFe)draw solute with higher osmotic pressure and lower reverse solute penetration was synthesized.But this kind draw solution still has the problems of reverse solute penetration and high regeneration energy consumption.Therefore,a porous monolith with three-dimensional interpenetrating network structure was explored as the draw agent in FO technology.The porous monolith can effectively improve the regeneration efficiency and decrease the energy consumption,and the regeneration process can be accomplished by simply compression.In order to reduce the raw materials cost and simplify the preparation process,a self-standing hydrophilic compressible functional aerogel based on the porous monolith draw agent was used as draw agent,which shows an improved specific surface area,water flux and regeneration pressure.Moreover,a hydrophilic modified short chain multi-walled carbon nanotube(MWCNT)was introduced to enhance the compression and the FO performances.Specific contents and conclusions are below.(1)The LAFe complex was synthesized by a mild one-pot method using sodium lactate as a ligand and ferric sulfate as a coordination center.The osmotic pressure of 0.75 mol L-1 LAFe aqueous solution was as high as 3180 mOsm kg-1,which was 1.90 times than that of NaCl at the same concentration because it can disassociate a large number of ions.And it has a larger molecular size(bigger than NaCl),which could effectively reduce reverse solute penetration.In FO process,the water flux and reverse solute flux of 1 mol L-1 LAFe(18.78 ± 1.56 L m-2 h-1(LMH),1.00±0.09 g m-2 h-1)were 1.80 times and 1/4 than that of 1 mol L-1 NaCl,respectively.In the seawater desalination test,1 mol L-1 LAFe complex exhibited higher water flux(7.54 LMH),which is higher than NaCl.Compared with the reverse osmosis which was used to recovery NaCl,the LAFe complex was recovered by a lower energy consumption nanofiltration(NF)method and the recovery rate was as high as 96%.(2)Based on the study of LAFe complex draw solution,a compressible porous monolith material with three-dimensional interpenetrating network structure was used as a draw agent,to avoid reverse solute penetration and simplify the regeneration process.A hydrophilic compressible HA-GO/PVA porous monolith was prepared by chemical crosslinking and lyophilization method,with hydrophilic polyvinyl alcohol(PVA)sponge as a substrate,sodium hyaluronate(HA)and graphene oxide(GO)as raw materials.ATR-FTIR,XRD,XPS and SEM results confirmed that HA and GO were reacted with the crosslinking agent to form an amide bond and stably loaded on the PVA sponge substrate.Then the HA-GO/PVA-2 presented higher hydrophilia,larger specific surface area(78.97 m2 g-1)and better compression(degree of strain up to 80%).The drawing capacity of the porous HA-GO/PVA monolith was studied in FO.And the results showed that the pure water flux of the dry HA-GO/PVA-2 draw agent reached 13.9±1.09 LMH,which was 1.17 times than that of 1 mol L-1 NaCl,and completely avoided reverse solute permeation.In the continuous 300 cycles of FO drawing water test,the pure water flux of HA-GO/PVA-2 draw agent stabilized in the range of 5.6-7.8 LMH.In the seawater desalination test,the initial water flux of dry HA-GO/PVA-2 reached 7.2±0.45 LMH.The regeneration process of the HA-GO/PVA-2 was accomplished in a very short period by simple mechanical compression(～15 kPa),which significantly increased the regeneration efficiency than that of the hydrogel draw agent.Compared with the LAFe complex and stimulate-sensitized hydrogel,the HA-GO/PVA-2 porous monolith had a promoted regeneration efficiency.(3)Based on the study of HA-GO/PVA-2 porous monolith draw agent,a substrate-free aerogel with larger specific surface area and more hydrophilic performance was used as a draw agent,to simplify the preparation process,reduce the preparation cost and increase the water flux.The sodium alginate-graphene oxide(SA-GO)aerogel materials were prepared through cross-linking,gelatinization and lyophilization with the SA and GO as raw materials.ATR-FTIR,XRD,XPS and SEM results confirmed that SA and GO were cross-linked through ester groups to form a three-dimensional interpenetrating network structure.The research showed that the appropriate amount of GO was introduced to retardate the agglomeration of SA molecule,which leads to the thinnest layer and regular structure.The results showed that the SA-GO-1(SA:GO=80:1,mass ratio)presented higher specific surface area(311 m2 g-1),higher water flux(15.25±0.65 LMH),higher desalination flux of the seawater(7.49±0.61 LMH)and smaller regeneration pressure(1 kPa)than the HA-GO/PVA-2 monolith under the same condition.It corresponded with the results of the physicochemical properties.In addition,in the continuous 100 cycles of FO drawing water test,the pure water flux stabilized in the range of 5-6.5 LMH,which suggested that the material exhibited better regeneration.(4)In order to furtherly improve the compression resistance of the SA-GO aerogel draw agents,a hydrophilic short-chain carboxyl MWCNT modified SA-GO(SA-GO-CNT)was synthesized.ATR-FTIR,XRD,XPS and SEM results confirmed that MWCNT and GO were cross-linked through ester groups.The research showed that the appropriate amount of MWCNT was introduced to form a regular micro structure.The SA-GO-CNT-A3(SA:GO:MWCNT=160:1:1,mass ratio)was found to have a regular microstructure,larger specific surface area(383.96 m2 g-1)and higher compression resistance(degree of strain up to 80%)than the SA-GO-1 aerogel.The FO performance study showed that the initial pure water flux of dry SA-GO-CNT-A3 reached 14.01±0.65 LMH.In the continuous 100 cycles of FO draw water test,the pure water flux stabilized in the range of 6.3-7.4 LMH,which was better than that of the SA-GO-1 aerogel.In addition,in the FO drawing water test,the water flux of dry SA-GO-CNT-A3 draw agent in simulated seawater and biochemical wastewater reached 6.41±0.29 and 11.92±0.54 LMH,respectively,with the lower conductivity(<52 μS cm-1)and TDS(<30 ppm).The SA-GO-CNT-A3 aerogel has a similar regeneration pressure(1.5 kPa)with the SA-GO-1 aerogel,but it presented higher regeneration efficiency(65%)and better compressibility(degree of strain up to 80%). |
PDF Full Text Request