An easily recoverable thermo-sensitive polyelectrolyte as draw agent for forw ard osmosis process☆

Yanni Wang ,Hairong Yu ,Rui Xie ,*,Kuangmin Zhao ,Xiaojie Ju ,Wei Wang ,Zhuang Liu ,Liangyin Chu ,2,*

1 School of Chemical Engineering,Sichuan University,Chengdu 610065,China

2 State Key Laboratory of Polymer Materials Engineering,Sichuan University,Chengdu 610065,China

1.Introduction

With the rapid grow th of population and worldwide energy consumption,we are faced with unprecedented crises of energy and resources.Traditional desalination methods using reverse osmosis(RO)not only costs great amount of energy,but also has poor efficiency.As a promising alternative method,forward osmosis(FO)technology can produce clean water with higher efficiency and lower energy consumption.Moreover,FO technology can also be used to generate energy since there are natural salinity gradient in estuary,which can generate water flux through a proper semipermeable membrane[1,2].Thus,FO seems to be a potential solution to both freshwater shortage and energy crises.Compared to reverse osmosis(RO),FO process has several advantages,including no external hydraulic pressure needed[3],less energy input[4]and lower fouling tendency[5,6].How ever,the development of FO technology is limited by the exploration of desirable draw agents that can generate high osmotic pressure and can be recovered easily.High osmotic pressure can ensure high water fluxes during FO process,and it is important to find easy recovery methods for reducing total energy consumption.To date,there have been few draw agents that can meet these two requirements at the same time.Thus,development of a draw agent,which can generate high osmotic pressure and meanwhile be easily recovered,is still of great importance to improve the efficiency of FO processes and reduce the overall energy consumption.

So far,salts,sugars,hydroacid complexes and stimuli-responsive materials have been studied as draw agents in FO processes.Some salts can generate relatively high osmotic pressures.For example,KCl solution as a draw agent in FO processes can generate an osmotic pressure of 9.05MPa at the concentration of 2 mol·L-1and produce a water flux of 22.6 L·m-2·h-1(LMH)w hen DI water is used as the feed[7].How ever, finding the methods of recovering the diluted KCl solution still remains a challenge.Also,the same problems exist when MgCl2solution is used as the draw agent[8].Ammonia-carbon dioxide can provide high water fluxes[9,10];how ever,ammonia-carbon dioxide encounters problems including high reverse salt diffusion[10],insufficient removal of ammonia[1]and the alkalescence that may be harmful to the commonly used FO membranes based on cellulose triacetate(CTA).When sugars are applied in the FO processes as draw agents[11,12],the limitations are the complex recovering methods.Hydroacid complexes can generate considerably high water flux when they are used as draw solution in FO process.However,pressure-driven NF process is needed to concentrate the diluted draw solution[13,14].To solve the recovery problems,smart materials,such as thermo-sensitive and magnetic-responsive materials,are being explored as new kinds of draw agents.Super paramagnetic nanoparticles as the draw agents can generate high water flux and can be easily separated from water after the FO processes;but,the problems of the agglomeration of magnetic particles w hen a strong magnetic field is applied and the decreased water fluxes during a few runs of recycles remain unsolved[15].Thermo-sensitive hydrogels have been developed as novel draw agents in FO processes and they can be de watered by using solar energy[16-20].However,the generated water fluxes in FO processes are still relatively low.Because polyelectrolytes and thermo-sensitive polyelectrolytes can dissolve w ell in water and then ensure high osmotic pressure and high water fluxes created by the resultant aqueous solutions in FO processes,they are now a new kind of draw agents that have a promising application prospect[3,21].In previous work,the pH values of the thermo-sensitive polyelectrolyte synthesized by free-radical polymerization of sodium acrylate(SA)and N-isopropylacrylamide(NIPAM)in aqueous solution have no specific pattern and are not very controllable since the acrylic acid is neutralized before the polymerization reaction,and using hot ultra filtration to recover the diluted polyelectrolyte directly after FO process is not effective enough[21].Therefore, finding more rational methods for synthesizing the the rmosensitive polyelectrolyte and simpler ways for recovery are of great importance.

In this study,a thermo-sensitive poly(N-isopropylacrylamide-coacrylic acid)(PNA)polyelectrolyte,which has a friendly p H value to the ester bond of CTA FO membranes and can generate relatively high osmotic pressure after adjusting its p H value to around 6.0,is prepared as a FO draw agent and two easy and simple methods to recover the PNA polyelectrolyte are proposed.PNA polyelectrolytes with different compositions are synthesized by thermally initiated free-radical polymerization of N-isopropylacrylamide(NIPAM)and acrylic acid(AA).After an overall consideration of the hydrophilic/hydrophobic properties of PNAand the complexity of the recovery process,onecomposition named PNA-10,w hose molar ratio of AA to NIPAM in the feed is 10:90,is chosen to be the draw agent in the subsequent experiments.After using NaOH to adjust the p H value of PNA-10 polyelectrolyte to around 6.0,consistent FO performance of CTA FO membranes is resulted,because the hydrolysis rate of the ester bond in CTA FO membranes at this p H value is relatively low[21].Furthermore,adjusting the p H value can also increase the solubility of PNA-10 polyelectrolyte in water and raise the ionization degree of its carboxyl groups so that high osmotic pressure and water flux is generated.To recover the draw agent,acetic acid is added to decrease the solubility and the lower critical solution temperature(LCST)of PNA-10 polyelectrolyte.After heating the resultant solution at 70°C for a certain period,the polyelectrolytes become so hydrophobic that they separate from water and aggregate together.Afterwards,they can be easily recovered by either centrifugation or simply standing at 70°C.The results in this study provide valuable guidance for designing efficient and easily recoverable draw agents for FO processes.

2.Materials and Methods

2.1.Materials

N-Isopropylacrylamide(NIPAM,TCI)is purified by recrystallization with a hexane/acetone(50/50,v/v)mixture solution.Acrylic acid(AA,purity≥99.5%)and sodium hydroxide(NaOH,purity≥96.0%)are supplied by Chengdu Kelong Chemical Reagents.2,2′-Azobisisobutyronitrile(AIBN,Chengdu Kelong Chemical Reagents)is recrystallized with ethanol and used as initiator.The CTA FO membranes are kindly provided by HTI.The FO membrane is cellulose triacetate with an embedded polyester screen mesh.The thickness of the membrane is less than 50 μm,and the sodium chloride rejection is around 95%[10].All other chemicals are of analytical grade and used as received.Deionized water(18.2 MΩ at 25°C)from a Milli-Q Plus water purification system(Millipore)is used throughout the experiments.

2.2.Preparation of polyelectrolytes

PNApoly electrolytes with different compositions are synthesized by thermally initiated free-radical polymerization of NIPAM and AA in 50 ml tetrahydrofuran(THF)using AIBN as initiator.The concentration of total monomers(NIPAM and AA)is 0.3 mol·L-1.The molar ratios of AA to NIPAM in the feeds for preparing three PNA polyelectrolytes are 5:95,10:90 and 15:85 respectively,and the resultant PNA polyelectrolytes are labeled as PNA-5,PNA-10 and PNA-15 according to the molar ratio of AA in the feed.The molar ratio of AIBN to the total monomers is 1%.The copolymerization is carried out under the stirring speed of 600 rpm and nitrogen atmosphere in a constant-temperature bath(70°C)for 24 h.After that,the obtained PNA polyelectrolytes are purified three times by reprecipitation with an excess of hydrousethyl ether from THF to thoroughly remove the unreacted monomers and impurities.Then,the PNA polyelectrolytes are dried under vacuum at 50°C for 24 h.The w eight average molecular w eights(MW)of the three PNA polyelectrolytes(PNA-5,PNA-10 and PNA-15)are determined by gel permeation chromatography(GPC,Waters-2410,Waters)after methanol esterifiable treatment,using THF as the mobile phase and polystyrene as the standard.Also,the measured MWvalues of PNA-5,PNA-10 and PNA-15 are 3175 g·mol-1,3526 g·mol-1and 3676 g·mol-1,respectively.

To prepare PNA polyelectrolytes in batch,150 mmol monomers(NIPAM and AA),in which the molar ratio of AA to NIPAM is 10:90,are added into 500 ml THF.The other synthesis conditions are the same as those described above.The w eight average molecular w eight(MW)of the obtained PNA-10 polyelectrolyte is 3530 g·mol-1after methanol esteri fiable treatment,determined by gel permeation chromatography(GPC,Waters-2410,Waters)using THF as the mobile phase and polystyrene as the standard.Next,the PNA-10 polyelectrolyte is dissolved in DI water,and then NaOH solution with the concentration of 0.5 mol·L-1is added until an alkaline sodium salt solution of PNA-10 is achieved.The resultant solution is then put into a dialysis bag(MWCO:1 kDa)and then the dialysis bag is put into DI water to remove small molecules(MW＜1000)and the excess NaOH until the p H value of the solution reaches around 6.0.The sodium salt of PNA-10 is subsequently dried under vacuum at 50°C,and is labeled as PNSA-10 and used as draw agent.

2.3.Characterization of polyelectrolytes

The compositions of PNA polyelectrolytes are determined by nuclear magnetic resonance spectrometry(1H NMR,Varian-400,Varian).The actual contents of the AA units in the PNA polyelectrolytes are calculated from the1H NMR results by the Eq.(1)[22]:

where,ωAAis the actual contents of the AA units in PNA polyelectrolyte,ωNIPAMis the actual contents of the NIPAM units in PNA polyelectrolyte,I1is the integral of characteristic peaks at around 3.84,and I2+3is the summation of the integral of characteristic peaks at around 1.97 and 1.45.

The thermo-responsive behaviors of PNA polyelectrolytes with different compositions and at different p H values are investigated by measuring the optical transmittance of the polyelectrolyte aqueous solutions with the polyelectrolyte concentration of 0.005 g·ml-1at 500 nm using a UV-Vis spectrophotometer(UV-1700,Shimadzu)equipped with a temperature controlled cell(TCC-240A,Shimadzu).NaOH solution with the concentration of 0.01 mol·L-1is used to adjust the p H values of the polyelectrolyte aqueous solutions to 5.0,6.0 and 7.0.

2.4.Measurement of osmotic pressure of PNSA-10/PNA-10 polyelectrolytes

A series of PNSA-10 and PNA-10 aqueous solutions with different concentrations are prepared.The osmotic pressures of PNSA-10 and PNA-10 aqueous solutions are measured using an osmometer(SMC 30C,Tianjin Tianhe Medical Instrument),and the p H values of the solutions are measured by a p H meter(Seven Multi,Mettler).

2.5.Characterization of relative viscosity and FO performance of PNSA-10 polyelectrolyte

Tw o PNSA-10 aqueous solutions with the concentrations of 0.20 g·ml-1and 0.38 g·ml-1are prepared,and their relative viscosities compared with DI water are calculated by the following equation:

where,ηris the relative viscosity of polyelectrolyte solution at 25 °C,η is the dynamic viscosity(Pa·s)of polyelectrolyte solution measured by a viscometer(Lovis2000M,Anton Paar)at 25 °C,and η0is the dynamic viscosity(Pa·s)of DI water at 25 °C.

Then,these two solutions are used as draw agents in FO processes,while DI water is used as the feed solution.The draw solution is put against the selective layer of the FO membrane.Since the purpose of this paper is to evaluate the possibility of using PNA polyelectrolyte as draw agent and recovering it easily,the mode(selective layer face to the draw solution)in this case can tell how much the FO water flux can reach with the deionized water as the feed solution.The original volume of each draw solution is 5 ml.The FO processes are carried out in a side-by-side diffusion cell(TK-6H1,Shanghai Kaikai Tech Trade).The FO membranes are immersed in DI water overnight before use.The water permeation flux during the FO experiment conducted at 25°C is calculated by the following equation:

w here,J is the water flux(LMH),Δm is the mass change(g)of the feed solution over a specific period of time t(h),ρ is the density(g·L-1)of water at 25 °C,and A is the effective area(m2)of the FO membrane.

2.6.Recovery of polyelectrolytes

In the experiments,three draw solutions with different concentrations are first used to imitate the diluted draw solutions instead of using the diluted draw solution after FO process directly,in order to find the relationship between the concentration of the diluted draw solution and its recovery ratio.This relationship can be used to help to determine the end point of FO process so that the w hole process can be optimized.Three PNSA-10 solutions with concentrations of 0.01 g·ml-1,0.05 g·ml-1and 0.10 g·ml-1are respectively prepared to imitate the diluted draw solutions after 12 to 24 h FO process.As shown in Fig.1,acetic acid is added into the PNSA-10 solutions until the pH values of them decrease to that lower than the original pH values before the polyelectrolytes being neutralized by NaOH,which are respectively 3.3,3.0 and 2.9 at concentrations of 0.01 g·ml-1,0.05 g·ml-1and 0.10 g·ml-1.Then,the solutions are put into a water bath w hose temperature is constantly at 70°C.To recover the polyelectrolytes,two simple and easy methods are designed as follow s.(1) “Heating and Centrifugalizing”method.The heated solutions are centrifugalized at a rotational speed of 10000 r·min-1for 3 min.Then,the solutions are put into the water bath to reach 70°C again.10 min later,the solutions are centrifugalized at a rotational speed of 10000 r·min-1for 2 min.After that,the supernatant liquid is decanted from the centrifugal tubes,and the polyelectrolytes in the bottom are dried under vacuum at 50°C until the mass does not change anymore.(2)“Heating and Standing”method.The solution with the concentration of 0.05 g·ml-1is kept at 70 °C for 15 h,during which the polyelectrolyte aggregates and deposits gradually.Then,the supernatant liquid is decanted and the polyelectrolyte is dried under vacuum at 50°C until the mass does not change.

Fig.1.Schematic diagram of the FO process using the thermo-sensitive polyelectrolyte solution as draw solution and the recovery process by two simple and easy methods.

The recovery ratio of PNA-10 polyelectrolyte is calculated by the following equation:

w here,R is the recovery ratio of PNA-10 polyelectrolyte,mRis the mass of the PNA-10 polyelectrolyte recovered(g),V is the volume of the diluted PNSA-10 solution(ml)and CPis the concentration of the diluted PNSA-10 solution(g·L-1).

2.7.FO performance of recovered PNSA-10 polyelectrolytes

After the first FO test,the diluted PNSA-10 polyelectrolyte solution with the original concentration of 0.20 g·ml-1is acidated and then recovered by the method of“Heating and Standing”.The recovery ratio is calculated by Eq.(4).The recovered polyelectrolyte is then used to prepare new draw solution with the concentration of 0.20 g·ml-1for the second FO process.Before that,NaOH solution is used to re-adjust the p H value to around 6.0.The osmotic pressure of the new draw solution is tested and the second FO process is conducted for 12 h.The average water flux is calculated by Eq.(3).

3.Results and Discussion

3.1.Compositional characterization of PNA polyelectrolytes

The1H NMR results demonstrate that the PNA polyelectrolytes have been successfully synthesized(Fig.2).Since the synthesis reaction is a free radical polymerization,the monomer should become free radical first.Also,the number of the monomer's free radical is vital to the composition of the final polymer.In this study,the actual contents of the AA units in the PNA polyelectrolytes calculated by1H NMR results are close to the contents of the AA units in monomers(Table 1).These results indicate that the monomer NIPAM and AA have similar reactivity in the solvent of THF[23].

Fig.2.1H NMR spectra of PNA-5(a),PNA-10(b)and PNA-15(c)polyelectrolytes in DMSO-d6.

Table 1 AA contents in the feed and in the PNA polyelectrolytes

3.2.Thermo-sensitive behaviors of PNA/PNSA polyelectrolytes

Due to the addition of hydrophilic monomers,the lower critical solution temperatures(LCSTs)of PNA/PNSA polyelectrolytes are different from that of poly(N-isopropylacrylamide)(PNIPAM)polymer.The increased donators of hydrogen bond brought by hydrophilic AA/SA units make it need higher temperature to break the hydrogen bond between water molecules and donators[24].The more the hydrophilic groups introduced by AAunits,the higher the ionization degree of carboxyl groups.Thus,the increased AAunits in PNA/PNSAlead to higher LCST and higher solubility[25].That is,the thermo-sensitive behaviors of PNA polyelectrolytes reflect their hydrophilic/hydrophobic balance properties.

In this study,the LCSTs of PNA-5,PNA-10 and PNA-15 polyelectrolytes with p H values ranging from 3.7 to 7.0 are tested(Fig.3).For each PNA polyelectrolyte,the LCST increases with increasing the p H value because of the rising ionization degree of carboxyl groups that makes the PNA chain more hydrophilic and harder to accomplish the“coil-to-globule”change.For all the three PNA polyelectrolytes,the LCSTs are similar w hen the p H value is around 3.7.How ever,as the p H value in creasing from 5.0 to 7.0,the more AA content in the PNA polyelectrolytes,the more significant the increase rate of the LCST.The LCST of PNA-15 at p H=7.0 is even higher than 70°C.For each polyelectrolyte at a certain p H value,the higher LCST the polyelectrolyte has,the more hydrophilic the polyelectrolyte is.Therefore,higher LCST value of the polyelectrolyte can insure higher solubility and higher osmotic pressure of the polyelectrolyte in water.How ever,the most suitable p H values for FO membranes based on CTA materials are limited from 4.0 to 6.0[21].Furthermore,more AA content in the polyelectrolyte means more acetic acid is needed while recovering the PNA polyelectrolyte from diluted PNSA polyelectrolyte solution in the subsequent recovery operation,and too much sodium acetate remained in the system may cause reverse salt diffusion.Therefore,by considering all above-mentioned factors,PNSA-10(p H=6.0),which has moderate hydrophilic/hydrophobic balance property as well as moderate LCSTs at p Hfrom 4.0 to 6.0,is chosen to be the draw agent in the subsequent FO process experiments.

3.3.Osmotic pressure of PNSA-10/PNA-10 polyelectrolyte

Asavery important factor of being a good draw solution,the osmotic pressure of PNSA-10 polyelectrolyte is investigated systematically.Six different concentrations of PNSA-10 polyelectrolyte are selected as 0.05 g·ml-1,0.10 g·ml-1,0.20 g·ml-1,0.30 g·ml-1,0.34 g·ml-1and 0.38 g·ml-1in this study,among which the 0.38 g·ml-1concentration is up to the solubility of PNSA-10(0.40 g·ml-1).The p H values of the six solutions are all 6.0±0.3,which is suitable for the FO membrane.The solubility of polyelectrolyte before adjusting p H is low and 0.10 g·ml-1is close to its maximum concentration in water.After adjusting the p H value of PNA-10 polyelectrolyte to around 6.0,the solubility and the osmotic pressure of polyelectrolyte increases rapidly(Fig.4a).Also,after adjusting the p H value,the osmotic pressure of PNSA-10 polyelectrolyte increases with the increase of polyelectrolyte concentration(Fig.4b).When the concentration of PNSA-10 polyelectrolyte solution is 0.38 g·ml-1,the osmotic pressure can reach up to 72.

3.4.Relative viscosity and FO performance of PNSA-10 polyelectrolyte

Fig.3.Temperature-dependent phase transition behaviors of PNA-5(a),PNA-10(b)and PNA-15(c)polyelectrolyte solutions at different p H values as well as the corresponding p H-dependent LCST values(d).

Relative viscosities of PNSA-10 polyelectrolyte solutions with the concentration of 0.20 and 0.38 g·ml-1are 8.3 and 59.8 respectively.To test the FO performance of PNSA-10 polyelectrolyte,FO process experiments are conducted for 12 h using these two PNSA-10 polyelectrolyte solutions as the draw solutions and DI water as the feed solution.The average water fluxes for every 2 h are calculated(Fig.5).At the first 2 h,the water fluxes are relatively high.For the next 2 h,the water fluxes decrease dramatically,which may be caused from the external concentration polarization(ECP).The draw solution close to the membrane is diluted by the water permeated through the membrane and the osmotic pressure there decreases rapidly.After the first 2 h,the water fluxes of the PNSA-10 polyelectrolyte solutions decrease slightly.The average water fluxes for 12 h of the PNSA-10 polyelectrolyte solutions with the concentrations of 0.20 g·ml-1and 0.38 g·ml-1are 2.09 LMH and 2.95 LMH respectively.Although the osmotic pressure of the polyelectrolyte solution with the concentration of 0.38 g·ml-1is much higher than the one with the concentration of 0.20 g·ml-1,their water fluxes are close.This phenomenon may be caused by the higher viscosity of the polyelectrolyte solution with the concentration of 0.20 g·ml-1and more serious ECP.Since the apparatus we use for conducting FO processes is quite simple and the mass transfer is not efficient enough,the water fluxes might be higher than the current ones if a better FO apparatus is used to enhance the mass transfer.As for the reverse salt flux of the draw solutions,previously published work has shown that the reverse salt flux of PNA-Na solution(C=0.72 g·ml-1,Mw=1800)is about 1.2 LMH,which is much smaller than that of seawater(about 37 LMH)due to the larger size and negatively charged property of PNA-Na[3].In this paper,the molecular w eight of our polyelectrolyte(PNA-10,Mw=3530)is much lager than 1800,and our PNSA polyelectrolyte is also negatively charged,which can be repelled from the negatively charged FO membrane.Thus,w e can speculate that the reverse salt flux of our tw o draw PNSA-10 solutions could be very low.

Fig.4.Osmotic pressures of the PNA-10 polyelectrolyte solutions be fore and after adjusting the pH value(a)as well as the effect of polyelectrolyte concentration on the osmotic pressure of PNSA-10 polyelectrolyte solution after adjusting p H value.

Fig.5.Water fluxesinduced by PNSA-10 polyelectrolytesolutions with the concentrations of 0.20 g·ml-1 and 0.38 g·ml-1 during 12 h FO processes.

3.5.Recovery of polyelectrolyte

Due to the thermo-sensitive phase transition behaviors of PNA-10 polyelectrolytes,two simple and easy methods based on heating and centrifugal or gravitational sedimentation are applied to recover the draw agents(Fig.1).Before heating,the PNSA-10 polyelectrolyte solution is acidified by acetic acid,which decreases the solubility and the LCST of the polyelectrolyte at the same time.After re-adjusting the p H value and heating,the polyelectrolyte chains aggregate together and separate from water because of the hydrophobic association.Thus,the draw agents can be easily recovered by centrifugation and standing at 70°C constantly using the centrifugal force and gravity respectively.To ascertain the single-run recovery ratio of PNA-10 polyelectrolyte,the method of“heating and centrifugalizing”is used to recover the PNA-10 polyelectrolyte in three PNSA-10 polyelectrolyte solutions with the concentrations of 0.01 g·ml-1,0.05 g·ml-1and 0.10 g·ml-1.The single-run recovery ratio increases from 72%to as high as 89%while the polyelectrolyte concentration increases from 0.01 g·ml-1to 0.05 g·ml-1,and then stays at around 89%w hen the polyelectrolyte concentration reaches to 0.10 g·ml-1(Fig.6).Thus,w e speculate that the diluted draw solution with concentration higher than 0.10 g·ml-1may also get a recovery rate around 89%.Therefore,a proper end of the FO process is vital to the recovery rate of the draw agent.

Fig.6.Recovery ratio of PNA-10 polyelectrolyte in PNSA-10 polyelectrolyte solution using the method of“heating and centrifugalizing”at 70 °C.

The recovery ratio of PNA-10 polyelectrolyte in PNSA-10 polyelectrolyte solution with the concentration of 0.05 g·ml-1is tested using the other method of“heating and standing”(Fig.7).At room temperature,the polyelectrolyte solution is transparent(Fig.7a).After adding acetic acid and heating to 70°C,the PNSA-10 polyelectrolyte solution becomes opaque due to the hydrophobic aggregation and separation from water of the polyelectrolyte(Fig.7b).After standing still at 70°C for 15 h,most of the PNA-10 polyelectrolytedeposit down to the bottom of the cuvette(Fig.7c).When the supernatant liquid is decanted and put into another cuvette at the temperature of 70°C,the supernatant liquid appears to be transparent(Fig.7d).This phenomenon indicates again that most of the PNA-10 polyelectrolyte have deposited down to the bottom during the still standing.The recovery ratio of PNA-10 polyelectrolyte,calculated from the mass loss of the PNA-10 polyelectrolyte in the first cuvette,also reaches up to 89%,just the same as that in the centrifugal recovery method.That is,both methods proposed in this study can successfully recover about 90%of the PNA-10 polyelectrolyte in PNSA-10 polyelectrolyte solution.The recovery results in Fig.6 also indicate that the recovery ratio of PNA-10 polyelectrolyte is related to the concentration of the diluted draw solution that needed to be recovered;thus,the operation time of FO process before recovery of draw agents should be chosen wisely based on this consideration.The“heating and centrifugalizing”and“heating and standing”recovery methods proposed in this study are actually pre-recovery methods for the draw agents.That is,for water recovery,the supernatant liquid should be de watered by hot ultra filtration[20]or nano filtration later to recover the draw agents completely.The acetate acid in the supernatant liquid can be separated by some simple methods,such as ion exchange via resins or membranes[26],or adsorption via activated carbons,and so on.It is worth to adjust the p H using acetic acid in the recovery of draw agents,because such a p H adjustment can significantly increase the efficiency of the whole recovery process.

3.6.Osmotic pressure and FO water flux of PNSA-10 polyelectrolyte after recovery

The diluted PNSA-10 polyelectrolyte solution with the original concentration of 0.20 g·ml-1is recovered after first FO test.The recovery ratio of the polyelectrolyte is around 90%.Then,the polyelectrolyte is reused to prepare new draw solution with the concentration of 0.20 g·ml-1in the second FO test.The osmotic pressure of the draw solution in the second FO process is even slightly higher than that in the first FO process(Fig.8).This may be caused by the residue of acetate ion,which decreases the average molecular w eight of the drawsolution.The average water flux of the second FO process is 2.16 LMH while the average water flux of the first FO process is2.09 LMH.This phenomenon may continue after several cycles of regeneration.In summary,the recovered polyelectrolytes can be reused with almost the same FO performance as fresh ones.

Fig.8.Osmotic pressure of PNSA-10 polyelectrolyte solution with the concentration of 0.20 g·ml-1 and the average water flux during 12 h of two FO tests.

4.Conclusions

A thermo-sensitive PNA polyelectrolyte has been successfully developed as an efficient draw agent in the FO process,and two easy and simple methods to effectively recover the PNA polyelectrolyte in the draw solutions have been demonstrated.After adjusting the p H value of the polyelectrolyte solution to around 6.0,which is a friendly p H value to the ester bond of CTA FO membranes,the good solubility of the polyelectrolyte in water and the increased ionization degree of carboxyl groups of the polyelectrolyte result in relatively high osmotic pressure and then relatively high water fluxes compared with previously-reported results[20].When the PNSA-10 polyelectrolyte solutions with concentrations of 0.20 g·ml-1and 0.38 g·ml-1are used as draw solutions in FO processes,the average water fluxes during 12 h are respectively 2.09 LMH and 2.95 LMH.After FO processes,the draw agents-thermo-sensitive polyelectrolytes-can be easily and efficiently recovered by either the“heating and centrifugalizing”method or the “heating and standing”method.When the polyelectrolyte concentration in the draw solution is 0.05 g·ml-1or higher,about 90%of the PNA-10 polyelectrolytes in PNSA-10 polyelectrolyte solution can be successfully recovered in a single run.The recovered polyelectrolytes can be reused with almost the same FO performance as fresh ones.In spite of the relatively lower water flux in the FO process comparing with that induced by other draw solutions like salt solutions and relatively high price of synthetic materials,the advantage of being easily recoverable can also make the thermo-sensitive PNA polyelectrolyte a highly potential draw agent in the future FO processes.

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