Preparation and properties of PPSU/GO mixed matrix membrane☆

Shujuan Xiao ,Shouwu Yu ,Li Yan ,Ying Liu ,Xiaoyao Tan *

1 School of Material Science and Engineering,Tianjin Polytechnic University,Tianjin 300387,China

2 Qing gong College,North China University of Science and Technology,Tangshan 063009,China

3 College of Materials Science and Engineering,North China University of Science and Technology,Tangshan 063009,China

4 College of Foreign Languages,North China University of Science and Technology,Tangshan 063009,China

1.Introduction

Membrane separation technology is widely used in seawater desalination and industrial wastewater purification[1],organic polymer is an important membrane material.Polyphenylene sulfone(PPSU)with the structure shown in Fig.1 is a novel material of great interest in the field of ultra filtration(UF)membranes[2,3].Due to the formation of conjugated structure connected by two benzene rings,the rigidity of material can be maintained.Moreover,the biphenyl structure gives good liquidity[4–6],and the existing ether bond makes the molecule tougher.PPSU also has the advantages of good chemical corrosion resistance and high mechanical strength.However,the hydrophobic of PPSU membrane due to the low surface energy causes poor anti-fouling ability,which restricts its wide applications.Due to membrane fouling,the permeate flux and membrane selectivity are decreased,leading to increasing operating costs[7,8].Membrane fouling can be divided into reversible fouling and irreversible fouling.Reversible fouling is induced by the weak interaction of foulants on the membrane surface,and thus can be eliminated by simple washing.Irreversible fouling is caused by the strongly plugged membrane pores and cannot be removed by simple cleaning,which shortens the service life and reduces the separation performance of the membrane.Hence,the PPSU membrane has to be modified to improve the hydrophilicity and anti-fouling ability for practical applications.In recent years,the preparation of mixed matrix membrane with an inorganic nano additive into the organic membrane material has become a research hotspot[9].The commonly used inorganic components include montmorillonite(MMT)[10],ZnO[11],Al2O3[12],SiO2[13],carbon nanotubes(CNTs)[14],HNTs-Dextran[15]etc.

Graphene oxide(GO)is a graphene derivatives and has good hydrophilicity due to the contained hydroxyl and carboxyl groups[16,17].Besides,GO also has a high specific surface area,strong mechanical properties and good resistance to pollution.As such more and more researchers blend it into organic membrane for preparation of mixed matrix membrane[18–21].For example,Zinadiniet al.reported the PES/GO mixed matrix membrane,the results showed when the content of GO was 0.5 wt%,the membrane had the optimized mean pore radius and maximum water flux.The prepared GO nanocomposite membrane showed noteworthy reusability during filtration[22].Luet al.prepared GO-PA/PSF mixed matrix reverse RO membrane.It exhibited better separation performance and higher chlorine resistance than the common polyamide RO membrane.When the content of GO added in aqueous phase was 0.005 wt%,the flux of the membrane was maximized up to 63 L·m−2·h−1[23].

To date,few studies are reported about the PPSU/GO ultra filtration membranes that enhance membrane performance and resistances to fouling.In this work,PPSU/GO mixed matrix ultra filtration membranes with different GO contents are prepared by immersion precipitation phase inversion technique.SEM and TGA are used to characterize the microstructure and thermal properties of membrane.The membrane hydrophilicity,the permeation behavior as well as the anti-fouling properties of the membrane are investigated.

Fig.1.Structural of PPSU.

2.Materials and Methods

2.1.Synthesis of the PPSU/GO mixed matrix membrane

Polyphenylene sulfone(PPSU,Basf Company);bovine serum albumin(BAS,Mn=68,000,Beijing Solarbio Technology Co.Ltd.);graphene oxide(GO)prepared in laboratory;N,N-dimethylacetamide(DMAC,AR),and polyethylene glycol(PEG-1000),are bought in Sinopharm Chemical Reagent Co.,Ltd.Deionized water was used as the coagulant.

The mixed matrix membrane was prepared by the phase inversion technique(see Fig.2).To prepare the membrane casting solution,the concentration of 15%PPSU was firstly dissolved in DMAc solvent.6 wt%PEG-1000 was then added as the pore former into the solution,followed by stirring until it was completely dissolved.GO powder with the mass fraction of 0,0.5 wt%,1 wt%,1.5 wt%,and 2 wt%was respectively dispersed into the solution by intense magnetic stirring for 2 h,followed by curing for 24 h and then light static for 12–24 h at room temperature to avoid foaming.The casting solution was placed on the surface of a clean glass plate statically for 10 s,and then quickly put into the coagulating bath of distilled water.After immersion in distilled water for24 h to remove DMAC,the PPSU/GOmixed matrix membrane was finally obtained.

Fig.2.Illustration of PPSU/GO membrane preparation process.

2.2.Characterization of PPSU/GO membranes

The membranes were characterized by SEM,TGA,FTIR spectroscopy and contactangle measurements.SEM(HitachiS-4800,Tokyo,Japan)images at low and high magnifications were captured from the membrane cross section and top surface.TGA was performed under the protection of nitrogen gas,in temperature range of 20–800 °C with the heating rate of 10 °C·min−1.The functional groups of the membrane were tested by FTIR using VERTEX70.Contact angles of the membranes were measured using JC2000C1 static contact angle measuring instrument.The membranes were cut into 1 cm×1 cm pieces for measurement.Take four different positions to measure and average the results[24].

The membrane porosity was measured by the immersion method using distilled water as the medium[25].The porosity(ε)was calculated using Eq.(1).

wherem1is the wet mass of the membrane(g);m2is the dry mass of the membrane(g);ρ is the density of water(g·cm−3);Ais the effective membrane area(cm3)andLis the membrane thickness(cm).

The formula for calculating the average pore size(r)of the membrane is shown in Eq.(2).

where η is the water viscosity(Pa·s),Jis the quantity of permeate per unit time(m3·m−2·h−1),ΔPis the operating pressure(Pa),Lis the membrane thickness(cm)and ε is the porosity of the membranes.

2.3.PPSU/GO membranes separation performance tests

The membrane separation performance was measured with a selfmade ultra filtration device.The first water preloading 20 min under 0.01 MPa,and then transferred to the test pressure 0.1 MPa,measured in a given period of time through the membrane water permeability.Three samples of each membrane were tested and the average data were taken as the membrane performance.Water flux(J)was calculated by

whereQis the volume of the water permeate(m3);Ais the area of membrane(m2),andtis the permeation time(h).

Membrane rejection is an important index to evaluate the membrane separation performance.Bovine serum albumin(BSA)solution instead of pure water was used for the permeation measurement.The permeation was conducted for 30 min under a trans-membrane pressure drop of 0.1 MPa.The BSA concentrations in the feed and the retentate were measured by UV spectrophotometer(UV-8000 s)under 280 nm wavelength.The rejection(R)was calculated by,

whereC1andC2are the concentrations of BAS in the feed solution and the retentate(mg·L−1),respectively.

The anti-fouling property of the membranes can be demonstrated using the flux recovery ratio(FRR)as it determines the volume of permeate flux that can be recovered after repeated filtration cycles.The pure water flux was measured under 0.1 MPa,the pure water fluxJw,1was recorded.Then the pure water was replaced by BAS solution,and the fluxJpof BSA solution was recorded with the change of time.After the BAS will be three times,the pollutant of membrane was cleaned with pure water,determination of pure water flux ofJw,2,membrane flux recovery rate calculation(FRR),the total pollution parameterRt,pollution parameterRr,irreversible fouling parameterRirwere calculated by Eqs.((5)–(8)),

Fig.3.SEM images of graphene oxide.

3.Results and Discussion

3.1.Characterization of GO

GO was prepared by the improved Hummers method[26].The morphology of the prepared GO is shown in Fig.3.The surface of GO shows small folded sheet,edge thin flaky.The FT-IR of GO is shown in Fig.4,the carbon network surface of GO introduces a large number of functional groups such as--OH,--C=O and--COOH,and so on.All these results via SEM and FTIR collectively con firmed the formation of GO.

Fig.4.FTIR spectrum of graphene oxide.

3.2.Effect of PPSU and PEG-1000 on membrane separation performance

The concentration of PPSU is one of the most important factors to determine the membrane separation performance.Several casting solutions with the PPSU concentration of 10%,12%,15%,18%and 20%in DMAc were used to prepare PPSU membranes.

As shown in Fig.5,with increasing the PPSU concentration,the resultant membrane exhibited decreased water flux but gradually increased BAS retention rate.This is because as the casting solution viscosity rises,the internal molecular chain winding degree between polymer becomes more serious,leading to difficulty in movement.As a result,the membrane becomes more and more dense,re flected in the decrease of pure water flux and the increase of retention rate.Considering such a trade-off between the flux and the retention,the concentration of PPSU was selected as 15%for membrane preparation.

Fig.6 shows the effect of the PEG-1000 concentration on the membrane separation performance.As can be seen,the water flux of the membrane initially increases with increasing the PEG content,reaching the maximum 127.2 L·m−2·h−1at 6%,and then declines with further increasing the PEG-1000 content,while the retention rate remains 97%without obvious change as the PEG-1000 concentration increases.At lower concentrations,the increase in the PEG-1000 content is helpful for enlarging the polymer micelle aggregates size and the formation of micropores,leading to higher permeation flux.However,after the PEG-1000 concentration increases to a certain value,i.e.6%,further increase in the PEG-1000 content makes the casting solution viscous,leading to a decreased porosity and thickening of the cortex,and as a result,the water flux decreases.

Fig.5.Effect of PPSU concentration on performance of membrane.

Fig.6.Effect of PEG concentration on performance of membrane.

3.3.Effect of GO content on the morphology of membranes

The pure PPSU membrane and PPSU/GO membrane the surface and cross-section are compared in Fig.7.From the surface SEM of the membrane,pores are fewer in the pure PPSU membrane surface.More pores appeared in the surface of the mixed matrix membrane,and the membrane surface becomes more smooth with the addition of GO,which are beneficial to the improvement of membrane flux.

Seen from the section structure of composite membrane,all the membranes demonstrate an asymmetric structure with an upper microporous layer supported on a bottom macroporous layer.With the increase of GO content,the overall thickness of the mixed matrix membrane changes very little,but the thickness of the upper microporous layer will be slightly reduced,under the microporous layer of the finger structure is more obvious.When GO content is higher than 1.5 wt%,the pore size of the membrane becomes uneven.So after the phase transition,the space occupied by water molecules formed the transverse holes which affect the mechanical properties of the membrane.The original vertical distribution of the channel will also have some damage,affecting the separation performance of the membrane[27].Comprehensively analyzed,when the mass content of GO is 1.5 wt%,the pore size of the membrane is more uniform.

The pore size and porosity of the mixed matrix membrane are shown in Table 1.The pore size and porosity of the mixed matrix membrane are increased gradually with the increase of GO content,which is consistent with the SEM analysis.

Fig.7.Top surface and cross section SEM of PPSU/GO membranes.(a:0 wt%GO,b:0.5 wt%GO,c:1 wt%GO,d:1.5 wt%GO,e:2 wt%GO).

Table 1Pore structure parameters of PPSU/GO membranes

3.4.Effect of GO content on the thermal stability for membranes

As shown in Fig.8,mixed matrix membrane has a trace amount of mass loss before 400°C,which is due to the presence of incomplete dry water or organic solvent evaporation;when the temperature is about 450°C,the quality loss of all membranes becomes larger,the reason is that C=O open into C--O in GO,and on the other hand,polymer itself will decompose[28];when the temperature is about 600°C,mass loss rate slows.The thermal stability of mixed matrix membranes is improved after the addition GO.The reason is addition of GO reducing the proportion of the polymer,reducing the membrane and oxygen contact area;on the other hand,the addition of GO could inhibit the shift of molecular polymer chain,interaction between PPSU chains and GO nanoparticles made the fracture polymer chain need more energy,so to improve the thermal stability of the membrane[29].

Fig.8.The curve of TGA analysis for the PPSU/GO membranes.

3.5.FTIR analysis

Infrared pure PPSU membrane and PPSU/GO mixed matrix membranes at 1.5 wt%spectra are shown in Fig.9.The PPSU membrane on the 1155 cm−1is seen with symmetric stretching characteristic absorption peak of O=S=group in PPSU,and on the 1286 cm−1is characteristic peak of infrared anti symmetric contraction of O=S=O group.There are obvious characteristic peaks at 1486 cm−1and 1585 cm−1,for the C=C stretching vibration of the benzene ring.Compared with the infrared spectra of pure PPSU membrane and composite membrane,it is shown that the absorption peak of PPSU/GO in the 3420 cm−1are stronger and the peak width is larger,which indicates that the hydrophilicity of the membrane is improved by the addition of GO.

Fig.9.FTIR of PPSU and PPSU/GO membrane.

3.6.Effect of GO content on the contact angle

Fig.10 shows the relationship between the contact angle of the membrane and the GO content.The pure PPSU contact angle is 93.4°,hydrophobic.With the addition of GO,the contact angle of the PPSU/GO membranes decreases,because the hydrophilic groups with surface GO,the mixed matrix membrane enhanced affinity for water.When the GO content is greater than 1.5 wt%,the tendency of the contact angle increases to be gentle.It is considered that the GO content is too much to cause agglomeration phenomenon,weakening the tendency of the contact angle increases.

Fig.10.Effect of concentration of GO on contact angle.

3.7.Separation performance of membranes

Fig.11 shows the pure water flux and retention rate of bovine serum protein of the PPSU/GO membranes with different GO content.The pure PPSU membrane has the lowest pure water flux and the highest rejection rate,which are 132.2 L·m−2·h−1and 96.8%respectively,and the water flux increases gradually with the increase of GO content.Because the hydrophilicity of GO makes it easy to absorb water molecules on the surface of the membrane,and water layer on the surface of the membrane is formed which can make water molecules immersion priority through membrane materials,effectively reducing the mass transfer resistance of membrane,and increasing the pure water flux of the membrane.At the same time,when the GO content is 2 wt%,the rejection rate decreases by 5.6%,which is a comparatively large decline.The decrease of retention rate is related to the surface morphology and pore size of the membrane.After the addition of GO,the pore size of the membrane becomes larger,which should be the reason for the decline of the BSA retention rate of the mixed matrix membrane.It is generally believed if the flux increases but the retention rate is minor changes,which requires a large number of holes on membrane surface and more uniformpore size[30].So the conclusion is drawn that the separation performance of the membrane is better when the amount of GO is 1.5 wt%.

Fig.11.Effect of GO content on separation performance of membrane.

3.8.Anti-fouling properties of mixed matrix membranes

Fig.12.Time-dependent flux of PPSU and PPSU/GO the membranes.

Fig.12 is the time flux change curve of PPSU/GO mixed matrix membrane after BAS solution contamination.When the pure water is replaced by BAS solution,the flux of the composite membrane is decreased,because of the adsorption of BAS solution on the membrane surface,resulting in membrane pore blockage.When the membrane is washed with water,the water flux recovered and the water flux is basically stable.

In order to show the pollution resistance of the mixed matrix membrane with different GO contents,the flux recovery rate of the membrane is tested,as shown in Fig.13.The recovery rate of PPSU membrane in the BAS solution is only 50.1%,and the attenuation is more serious,but the flux recovery rate of mixed matrix membrane increases gradually.The flux recovery rate of PPSU/GO membrane reaches 86%when the content of GO reaches 1.5 wt%.When the content of GO is higher than 1.5%,the flux recovery rate decreased.Because of excessive addition of GO to the membrane,the membrane pore becomes uneven and increases roughness.BAS residues results in the obstruction of the pores,thus membrane flux recovery rate decreases.

Fig.13.The anti-fouling indexes of the PPSU and PPSU/GO membranes.

The pollution of membrane surface is divided into two parts:reversible and irreversible fouling.The fouling index of the membrane is expressed in Fig.13.The total fouling number of the pure PPSU membrane is 76.3%,and the irreversible fouling parameter is 50%,which shows that the PPSU membrane is the least resistant to contamination.When the added amount of GO is less than 1.5 wt%,the total number of membrane fouling and the irreversible fouling parameters decreased,which shows that the anti-fouling ability of the mixed matrix membrane is improved.When the amount of GO is 2 wt%,all the parameters of the membrane and the irreversible fouling parameters are increased.Too much GO into the membrane cannot be a good fusion of the polymer,so the membrane hole becomes rough,uneven,resulting in some of the pores easily blocked,and then the anti-fouling ability of the membrane decreased.

4.Conclusions

In this study,graphene oxide was used as a hydrophilic additive to blend with PPSU membrane.PPSU/GO mixed matrix membrane was prepared by phase inversion technique,and the following results were obtained.

GO has good compatibility with PPSU.The addition of GO improves the inner pore structure of the membrane.The hydrophobicity of the membrane can be improved and the thermal stability is improved.When the content of GO was 1.5 wt%,the pure water flux of the mixed matrix membrane reaches 231.7 L·m−2·h−1,which increase by 83%compared with the pure PPSU membrane,while the rejection rate of the BAS solution was not very large.At the same time,the maximum flux recovery rate of PPSU/GO mixed matrix membrane was 86%,and the total number of membrane fouling and irreversible fouling parameters was decreased.With the addition of GO,the equivalent of adding hydrophilic groups in the membrane,water molecules would be preferentially combined with polar groups on the surface of the membrane,which obstructed the pollutant and improved the anti fouling ability of membrane.

Appendix A

A.1.Materials

Graphite powder purchased from Sinopharm Chemical Reagent Co.,Ltd.(China);concentrated sulfuric acid(H2SO4)and potassium permanganate(KMnO4)and hydrogen peroxide(H2O2)purchased from Tianjin Yongda Reagent Co.,Ltd.(China).

A.2.The preparation of graphite oxide

(1)Pre oxidation of graphite Adding 12 ml concentrated sulfuric acid to the flask,and then added the 2.5 g P2O5and 2.5 g K2S2O8,the mixture was heated to 80°C,then adding 0.8 g graphite powder,stirring 4 h.The obtained black solid was dried at60°C to obtain the pre oxidized graphite.

(2)Re-oxidation Adding 120 ml concentrated sulfuric acid and pre oxidized graphite into the flask,then slowly added 12 g KMnO4,evenly mixing,and the reaction continued in the thermostat water bath at 35°C for 2 h.Then 10 ml H2O2were dropped,stirring for 10 min,adding 5%HCl,stirring again 10 min,then standing 12 h.The solution was separated by centrifugation,and the graphene oxide was dried in 60°C vacuum drying oven 24 h.

(3)Post treatment A certain amount of GO dispersed in 100 ml water,and the suspension of GO is obtained.The suspension was dispersed by ultrasonic treatment 2 h,and the stable GO solution was obtained.

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