Organic Acid Salts As Novel Draw Solutes For Forward Osmosis

Forward osmosis(FO)process,which utilizes the osmotic pressure difference between two sides of FO membrane to drive water molecules from the feed solution side(lower osmotic pressure)to draw solution side(higher osmotic pressure),has exhibited a great potential for various applications because of its unique characteristics of low energy consumption(without the draw solution recovery considered)and low fouling tendency.The FO membrane and draw solute are two key factors influencing FO performance.More and more researches have been focused on the development of high-performance FO membranes,while the study on the development of draw solution is relatively limited.The desirable draw solution should possess the following features:(1)high osmotic pressure,(2)low reverse draw solute flux,(3)easy regeneration,(4)nontoxicity and so forth.Because the organic acid salt can ionize easily in the aqueous solution and has suitable molecular weight,three organic acid salts as draw solutes are studied in this dissertation,i.e.,poly(4-styrenesulfonic acid-co-maleic acid)sodium(P(SSA-co-MA)-Na),sodium phytate(PA-Na),and carboxymethyl cyclodextrin sodium(CM-CD-Na).Firstly,we study the properties of the strong polyelectrolyte P(SSA-co-MA)-Na as a novel draw solute in FO process.The physicochemical properties and FO performance of P(SSA-co-MA)-Na draw solutes with different molar ratio of 4-styrene sulfonic acid and maleic acid are investigated systematically.The result finds that,with a higher molar ratio of maleic acid,P(SSA-co-MA)-Na draw solute can generate a higher osmotic pressure,relative viscosity and water flux,and a lower reverse draw solute flux.In PRO mode,a high water flux of about 15 LMH and a low reverse solute flux of about 0.04 gMH can be obtained with 0.25 g/mL P(SSA-co-MA)-Na solution as the draw solution and deionized water as the feed solution.Compared to most other polyelectrolytes,the P(SSA-co-MA)-Na draw solution produces the higher water flux and lower reverse solute flux.The P(SSA-co-MA)-Na draw solution is further applied in treating dye-containing wastewater and can be recovered efficiently by NF process,which demonstrates its great potential in FO process.Natural compounds PA-Na are further explored as novel draw solutes for FO applications due to its nontoxicity.The effects of pH and concentration of the PA-Na solution on the osmotic pressure,relative viscosity and FO performance are systematically studied.A high water flux of 30.35 LMH and a low reverse solute flux of 0.61 gMH can be achieved using the self-made membrane in PRO mode.The PA-Na draw solution exhibits a great potential for the brackish water desalination.What's more,the dilute PA-Na draw solution after FO process can be used in food or feeding production directly.Therefore,the energy consumption in the draw solution recovery can be saved,reducing the total cost.In addition,sugar,as an abundant natural compound in nature,cannot be used as draw solute due to the low osmotic pressure of the solution.In this work,a class of sugar derivatives(CM-CD-Na)are synthesized through a simple one-step nucleophilic substitution reaction and employed as novel draw solutes.Their chemical structures are confirmed by Fourier Transform Infrared Spectroscopy(FTIR)and Nuclear Magnetic Resonance Spectroscopy(NMR).Among the three carboxymethyl cyclodextrins(CM-?-CD-Na,CM-?-CD-Na,CM-?-CD-Na),CM-?-CD-Na has the best FO performance as the draw solute.The osmotic pressure,relative viscosity and FO performance of CM-?-CD-Na synthesized at different temperatures and concentrations are then studied systematically.Due to its non-toxic feature,CM-?-CD-Na draw solution exhibits great potential for the juice concentration via FO.The diluted CM-?-CD-Na draw solution can also be used for soil washing without recycling.