Study Of Preparation Of Solar Evaporation Materials And Coupling Forward Osmosis In Brackish Water Desalination

With the rapid increase of human society’s demand for fresh water resources,relatively clean natural water bodies represented by seawater and brackish water have become one of the most promising sources of fresh water.Compared with the traditional high energy-heating treatment technology,membrane separation treatment is a new kind of cleaner technology.In recent decades,Reverse osmosis(RO)technology gradually occupies the saline desalination market in our country by relying on relatively low energy consumption and high quality water production.However,the high pressure driven RO technology is still an energy intensive process.Forward osmosis(FO)is a membrane separation process that uses the osmotic pressure difference on both sides of the semipermeable membrane as the driving force to promote the spontaneous diffusion of water molecules from the low concentration side to the high concentration side.As an emerging technology with ultra-low energy consumption and low membrane pollution trend,FO has a huge application prospect in the field of separation and desalination of brine.However,with the transmembrane transport of water molecules,the osmotic pressure difference on both sides of the FO membrane decreases,and the driving force decreases accordingly,resulting in a rapid attenuation of the final water production flux.Therefore,how to reconcentrate the Draw solution(DS)to maintain continuous and stable transmembrane water flux has always been a research hotspot in the field of FO.Currently,FO-coupled Nanofiltration,RO and Membrane distillation are the mainstream research directions of reconcentrated DS.However,the pressure energy dissipation of Nanofiltration and RO and the heat energy dissipation of Membrane distillation are contrary to the concept of osmotic pressure driven FO process,prompting scholars to devote themselves to the research of new and more cost-effective technologies.Solar evaporation(SE)is a process that uses renewable energy sources to desalinate saline water.It uses photothermal conversion materials to efficiently convert captured solar energy into heat energy,which can realize local heating at the gas-liquid interface and cause water evaporation.FO and solar evaporation hybrid(FO-SE)technology proposed in this study enables rapid evaporation of water in DS through photothermal evaporation,so as to achieve multiple objectives of high efficiency and low consumption separation and reuse of diluted DS and collection of desalinated water,so as to ensure sustainable operation of FO technology in the treatment of brackish water.There have been no reports of this.The main conclusions of this study are as follows:(1)The first kind of photothermal evaporation material was the polyethylene glycol(PEG)modified Enteromorpha cornerstone graphene aerogel,denoting PEGA.The absorption rate of solar spectrum was more than 70%,and the maximum evaporation rate was 2.55 kg/(m2·h)and the photothermal conversion rate was as high as 105.7%under the optimal preparation conditions.At the same time,PEGA material has excellent salt-pollution resistance,which meets the demand of concentrated DS initially;(2)The second material is the melamine sponge(MS)composite with polypyrrole(PPy),denoted as PPy@MS,The material is prepared by an extremely simple soaking polymerization process,which has the advantage of large-scale and lowcost production.A maximum evaporation rate of 2.80 kg/(m2·h)can be achieved with very low PPy loading concentration.In addition,MS is naturally super hydrophilic,which ensures adequate water supply and makes the salt concentration of Feed solution always lower than that of precipitation,thus alleviating salt pollution.The evaporation rates of PPy@MS at 1 M,2 M and 3 M NaCl solution are 3.502,2.738 and 2.437 kg/(m2·h),respectively,which meet the needs of hybrid process;(3)When PEGA is used in the hybrid process of evaporation and concentration DS,the cylindrical block cannot cover the evaporation surface,reducing the effective area.Meanwhile,the extremely low density of PEGA makes it difficult for dozens of PEGA pieces on the evaporation surface to fit the liquid surface at the same time,and the overall water supply situation is extremely unbalanced.Finally,the graphene lamellar structure of PEGA is extremely easy to break chips and cannot withstand the impact of FO water flow.In conclusion,the effect of PEGA on hybrid process is not good;(4)PPy@MS has the advantages of excellent evaporation performance,super hydrophilic,pollution resistance,customizable shape,high mechanical strength,non-toxic substances,simple preparation and low cost,making it more suitable for FO-SE hybrid process.In the coupling experiment,the maximum evaporation flux is 17.4 L/(m2·h),calculated by the evaporation area of about 54 cm2 and the membrane area of 5 cm2.With the operation of FO-SE,the attenuation of the water flux was significantly inhibited,and even a continuous rise in the water flux was observed with a lower concentration of NaCl solution as DS.The water flux of FO-SE hybrid process corresponding to high concentration DS is higher than that of FO-alone process.In addition,evaporation does not increase the reverse salt flux.In terms of film contamination,evaporation process makes DS driving force greater than that of simple FO group,and the increase of water flux leads to slightly aggravated film contamination on the FS side,but the polluted layer is loose and easy to clean.In conclusion,the feasibility of applying SE to DS concentration is systematically explored in this paper,and the influence of SE process parameters on FO process and the feasibility of FO-SE hybrid technology are expounded.It provides a theoretical reference for the application of forward osmosis in the field of brackish water desalination.