Shujuan Xiao,Xiaowen Huo,Shuxin Fan,Kui Zhao,Shouwu Yu,,Xiaoyao Tan

1 College of Material Science and Engineering,North China University of Science and Technology,Tangshan 063210,China

2 School of Chemistry and Chemical Engineering,Tiangong University,Tianjin 300387,China

Keywords:Mixed matrix membranes Antifouling Metal–organic frameworks Membrane separation technology

ABSTRACT To enhance the performance of the polyphenylene sulfone(PPSU)membrane,a novel mixed matrix membrane with hydrophilicity and antifouling properties was prepared.Using PPSU as the basic membrane material,polyvinyl pyrrolidone(PVP)as the porogen,N-Methyl pyrrolidone(NMP)as the solvent,and MOF-CAU-1(Al4 (OH)2 (OCH3 )4 (H2 N-BDC)3 ·xH2O)as the filler,PPSU/CAU-1 mixed matrix membrane(MMM)was prepared by an immersion precipitation and phase transformation technique.By changing the amount of MOF-CAU-1,the properties and performance of the MMM membrane were investigated in terms of hydrophilicity,pore morphology,surface roughness,and dye removal.The results show that the highest pure water flux of the mixed reached 47.9 L·m?2·h?1,when the CAU-1 addition amount was 1.0 wt%,which was 23%higher than that of the pure PPSU membrane.Both the rejection rate and the antifouling performance of the MMM membrane also noticeably improved.

1.Introduction

Membrane separation is a mass transfer process of component separation,realized by the difference of permeability between the components in mixture[1–3].It has been widely used in chemical engineering[4,5],food processing[6,7],water treatment[8–10]and other fields.A high-efficiency membrane filtration process must have high throughput,high rejection and excellent pollution resistance.The performance of the separation membrane is mainly determined by the pore structure and the surface properties[11].

Metal organic framework(MOF)is a kind of organic–inorganic hybrid material with nano pore diameter,which is centered on metal ions and connected with organic ligands.It not only has the common properties of conventional porous materials such as high porosity,large specific surface area and low density,but also has its own advantages,among which adjustable structure and chemical properties are the main ones [12–14].This unique advantage comes from the fact that there are many possibilities in the combination of metal ions and organic ligands.As long as one of the ligands or one of the metal elements is changed,a new material can be formed,which has completely different properties from the previous materials [15–17].With the unique chemical properties,MOF has become an important additive for improving the performance of organic membranes[18].Compared with the conventional nanostructured porous materials(such as zeolite with rigid framework),MOF not only has a stronger affinity with the polymer-based membrane materials to avoid the formation of nonselective voids,but also selectively intercepts substances with different molecular weight by adjusting its size[19,20].

At present,the main methods of preparing MMM by combining MOF and organic basement membrane are blending[20–23],interfacial polymerization [24–26],layer by layer self-assembly [27–29]and in-situ growth[30].Meng et al.blended polyethyleneimine(PEI)aqueous solution with a BUT-8(A)MOF aqueous solution,and then spin-coated on hydrolyzed polyacrylonitrile (HPAN) substrate to form membrane.The obtained composite membrane of BUT-8(A)/PEI-HPAN-50 showed a high water permeability(396–683 L·m?2·h?1·MPa?1)and excellent dye retention[31].Yang[11]et al.directly mixed graphene oxide(GO)@HKUST-1(metal organic framework)with cellulose acetate(CA)to synthesize mixed matrix membrane by immersion precipitation induced phase transformation.Compared with only adding GO,the combination of HKUST-1 and GO has better hydrophilicity,larger pores and smoother membrane surface.Karim[32]et al.used the method of convection diffusion synthesis to form a uniform zeolites imidazolate framework(ZIF-8)layer on porous polyvinylidene fluoride(PVDF)ultrafiltration membrane.The water flux of the membrane nearly doubled that of the pure PVDF membrane.Wang[29]et al.also used ZIF-8 to grow the ZIF-8 layer in situ on a polysulfone(PSF)support membrane.A sandwich-like structure with improved hydrophilic properties was prepared by interfacial polymerization with PA layer.Recently,Wang[33]et al.constructed a continuous ZIF-8 crystal layer on the surface of flexible polysulfone (PS) membrane modified by poly dopamine(PDA).The ZIF-8/PDA/PS composite membranes showed a high water flux of 9.6 L·m?2·h?1and a low solute reverse flux of 3.8 g·m?2·h?1when using 1 mol·L?1MgCl2as draw solution and deionized water as feed solution under forward osmosis mode.It showed good forward osmosis(FO)performance and excellent antibacterial ability.

Fig.1.Schematic illustration of design strategies for MOF membranes.

Polyphenylene sulfone resin(PPSU)is widely used as membrane material for water treatment due to its good chemical resistance,hydrolysis stability and excellent mechanical strength[34].By sulfonation of PPSU,the hydrophilicity can be significantly improved,but the mechanical strength of the membrane is also damaged[34].At present,it has been reported that SnO2is used as a membrane modification additive to improve the hydrophilicity and stability of the membrane[35].Due to the poor compatibility between rigid SnO2and membrane matrix,in order to avoid non selective defects,only a small amount of modified particles can be added to the membrane,which greatly limits the membrane flux and retention performance.MOF materials with large specific surface area and high porosity can be used as excellent modified particles,and the presence of organic ligands in MOF materials can be better compatible with organic membrane matrix.Dai et al.[36]prepared PPSU/ZIF-8 @ RMS MMM by growing MOF on the surface of macroporous resin microspheres(RMS)to reduce non selective defects.However,due to the hydrophobicity of ZIF-8,the addition of MOF cannot make the membrane obtain better anti-pollution performance.CAU-1 (Al4(OH)2(OCH3)4(H2N-BDC)3·xH2O) has a cubic structure consisting of deformed octahedral and tetrahedron cages.The effective diameters of the cages are 1 nm and 0.45 nm,respectively.Therefore,water molecules can pass freely,but the pollutants with large molecular weight may be intercepted and retained[37,38].Since CAU-1 has good water stability,high surface area and porosity,it can be added into MMM as nanoparticles[39,40].Till now,none of the researchers have reported the usage of PPSU/CAU-1 NPs polymer matrix for any other applications in membrane technology.This research gap had inspired us to fabricate the highly stable,novel PPSU/CAU-1 MMM for dye removal application.Moreover,the presence of amino group in CAU-1 may increase the hydrophilicity,leading to the improvement of the antipollution performance of the MMM(In this paper,BSA solution was used as a pollutant).

In this work,the hydrophilic MOF(CAU-1)containing amino group was prepared and mixed with PPSU.The morphology,hydrophilicity and water permeation performance of the membrane were extensively investigated.

2.Experimental

2.1.Materials

Polyvinylpyrrolidone and salts (NaCl,KCl,KH2PO4and NaH2PO4)were purchased from Tianjin Yongda Chemical Co.,Ltd.,and were used without further purification.The salts were used to prepare phosphate buffer solution(PBS).2-Aminoterephthalic acid(99%)was purchased from Aladdin.NMP,aluminum chloride hexahydrate(AlCl3·6H2O,crystal,≥97.0%)and anhydrous ethanol(99.5%)were purchased from Tianjin Guangfu Fine Chemical Research Institute.PPSU polymer particle with molecular weight of 650,000 was purchased from Solvay advanced polymers in the United States.BSA was purchased from Biopped.Methyl violet was purchased from Tianjin Institute of chemical reagents.Deionized water was used throughout the work.

2.2.Synthesis of CAU-1

CAU-1 powder was synthesized by solvothermal reaction,as described in literature[38].AlCl3·6H2O(2.967 g,12.3 mmol)and NH2-BDC(0.746 g,4.1 mmol)were mixed in methanol(30 ml).The suspension was transferred to a partial steel autoclave with Teflon liner.After heated at 125°C for 12 h,the yellow products were collected by centrifugation.The product was then washed with methanol to remove the unreacted NH2-BDC.In order to remove chloride ions from the framework,the product was stirred in deionized water for further purification.The product was recovered by centrifugation,and finally dried in vacuum for 24 h.

Table 1 Compositions of the casting solutions for MMM

2.3.Preparation of PPSU/CAU-1 mixed matrix membrane

Fig.1 schematically illustrate the preparation process of the PPSU/CAU-1 MMM(The dosage was based on PPSU/1.0 wt%).Firstly,a certain amount of CAU-1 (0.043 g) nanoparticles were uniformly dispersed under ultrasound in 30.84 g NMP(see Table 1 for the composition of the casting solutions for each membrane),PPSU(8.59 g)particles together with PVP(3.436 g)were added to the mixture and stirred for 12 h.The mixture was sonicated for 1 h and allowed to stand overnight to remove air bubbles.MMM with a thickness of about 200 μm were prepared by pouring the casting solution onto a smooth glass plate evenly.After a few seconds,the dope solution was quickly immersed into a coagulation bath containing deionized water to form MMM.The membranes were finally dried and reserved for characterization.

2.4.Characterization of the synthesized MOF and the prepared mixed matrix membranes

The scanning rate of XRD(D/max2500)is 0.02(°)·s?1,which was used to characterize the crystal structure of CAU-1 and MMM at room temperature.FTIR was used to characterize the molecular structure of MOF in the wavelength range of 4000–400 cm?1.The morphology of CAU-1 and membrane was obtained by 15 kV field emission scanning electron microscope(Japan,Hitachi s4800)and 200 kV transmission electron microscope (Japan,JEOL JEM 2800).Water contact angle meter(sindin,sdc-200)was used to measure water contact angle to characterize hydrophilicity.The surface roughness test was characterized by AFM(Bruker multimode8).

2.5.MMM performance evaluation

In this paper,the pure water flux,rejection and antipollution performance of BSA were tested with the self-made equipment shown in Fig.2.Put the membrane into the membrane tank,prepress at 0.4 MPa for 20 min,and then reduce the pressure to 0.3 MPa.Calculate the pure water flux(PWF)using the following formula:

J was defined as the permeation flux per unit area of membrane in unit time;V was the permeation amount (L);A was the membrane area,effective area is 19.63(m2);and t was the permeation time(h).

In the same way as the determination of pure water flux,the filtration experiment was carried out on the aqueous solution containing methyl violet,and the initial dye concentration was 200 mg·L?1.The formula for the rejection R(%)of the membrane was as followed:

Cpand Cfrepresent the concentration of methyl violet solution,respectively before and after the test(g·L?1).

In order to evaluate the antifouling performance of the membrane,BSA(pH=7.1)buffer solution was prepared for membrane fouling experiment.First,the membrane was subjected to pure water permeation for 60 min to obtain the pure water flux(Jw1).Then,BSA solution at a concentration of 1 g·L?1was used as the feed solution and filtered for 60 min to obtain the water flux of BSA solution(Jp).After this,the fouled membrane was cleaned with deionized water for 30 min before measuring the water flux of the cleaned membrane(Jw2).The membrane flux recovery ratio(FRR),total fouling ratio(Rt)reversible fouling ratio(Rr)and irreversible fouling ratio(Rir)were calculated by using the following equations:

Fig.2.Schematic diagram of filter equipment.

3.Results and Discussion

3.1.Characterization of CAU-1

The three-dimensional network structure of the synthesized CAU-1 was shown in Fig.3.From the XRD characterization results(Fig.4(a)),it can be clearly seen that the characteristic peaks with diffraction angle(2θ)of 6.9°(011),9.9°(002)and 13.8°(022)have appeared.In accordance with the previously reported literature,CAU-1 has been successfully prepared[38,41].In the infrared test(Fig.4(b)),the symmetric and asymmetric stretching between 1600 cm?1and 1400 cm?1are the characteristic peaks of the bridging carboxylate group.The C--O tensile vibration and C--H tensile vibration at 1071 cm?1indicate the presence of methoxy in the structure of CAU-1.N--H vibration is not observed at 3380 cm?1,because it is covered by a wide intensity band between 3200 and 3600 cm?1due to the presence of water molecules[34].At the peaks at 1313 cm?1,that are the characteristic bispectral bands of C-N(NH2)vibration.As shown in Fig.4(c),CAU-1 has a large specific surface area,and the adsorption desorption curve is a typical I-curve.The specific surface area of Langmuir is about 1619 m2·g?1,and the micropore volume is 0.55 cm3·g?1.The measurement results of transmission electron microscope CAU-1 are shown in Fig.4(d).The synthesized CAU-1 has a regular cubic structure with an average particle size of about 1 μm[42].

3.2.Characterization of asymmetric PPSU and PPSU/CAU-1 mixed matrix membranes

The presence of crystalline CAU-1 material in MMM was confirmed using X-ray diffraction(XRD).Fig.5 shows the XRD data for M2 and M5,compared to the XRD of pure CAU-1 crystals.Three distinct peaks at diffraction angles(2θ)of 6.9°,9.9°and 11.9°were clearly observed,the membrane M5 shows large peaks at these values,and which indicates that the crystallinity and structural features of CAU-1 were maintained in the membranes.Besides,the membrane M2 shows no sign of MOF crystallinity,and that should be due to the less loading of CAU-1.

Fig.3.The crystal structure of CAU-1.

Fig.4.Characterization results of CAU-1:(a)XRD patterns;(b)Infrared spectrum;(c)N2 adsorption and desorption isotherms at 77 K;(d)TEM image.

Fig.6 presents the SEM images of the cross sectional structure and top surface of the PPSU-based membrane incorporated with different CAU-1 loadings.The cross section of all prepared membranes shows a typical asymmetric structure.For pure membrane(Fig.6(a)),the top layer is a dense pore structure,and the sublayer is similar to finger structure[43].From Fig.6(b)to(f),with the increase of the amount of CAU-1,the finger structure in the MMM decreases,and the large spherical structure appears at the bottom.In addition,the macroporous structure is more obvious when the amount of CAU-1 is higher.This phenomenon is related to the increase of viscosity of polymer solution.Many studies have also shown that the increase of viscosity will slow down the exchange rate of solvent and non-solvent,which leads to the transition of phase transformation path from instantaneous phase separation to delayed phase separation.The change of phase separation process leads to the appearance of large spherical membrane pores[44,45].By observing the surface of the membrane,CAU-1 is randomly distributed on the surface,and the load is in regular cube shape,which indicates that MOF has been successfully added to the membrane.When the amount of CAU-1 in the membrane reaches 2.5%,the partial agglomeration will occur in the membrane matrix(Fig.6(f)).

AFM was used to characterize the roughness of the membrane surface.The size of the test sample is 4 μm×4 μm.AFM image and roughness parameters are shown in Fig.7 and Table 2.The Ra(average surface roughness)of PPSU pure membrane is 2.20,M2 and M5 are 7.11 and 21.9,respectively.A higher Ravalue indicates a higher surface roughness.Compared with the pure membrane,the nodular structure appears on the surface of the MMM [46],the increase of pore size and the large amount of MOF(Fig.6(f))will lead to agglomeration and increase of roughness[20].

3.3.Membrane hydrophilicity

Fig.5.XRD data for the membranes alongside the data for pure CAU-1 crystals.

Fig.6.SEM images of cross section and top surface of PSSU membranes embedded with different CAU-1 loadings(a)pure PPSU,(b)0.5 wt%,(c)1.0 wt%,(d)1.5 wt%,(e)2.0 wt%and(f)2.5 wt%.

Fig.6 (continued).

The hydrophilicity of the membrane is one of the main properties of the membrane,which directly affects the membrane flux and antifouling ability.In this study,the static water contact angle of the membrane was tested,and the surface hydrophilicity of the membrane was evaluated by the water contact angle.The results are shown in Fig.8.With the increase of CAU-1,the surface water contact angle of the membrane decreased.The initial water contact angle changed from 73.94° to 63.42°.The results showed that the hydrophilicity of the MMM was improved by adding MOF particles to the casting solution with the addition of MOF,the content of MOF in the membrane increases correspondingly,which will inevitably cause more MOF particles to appear on the membrane surface.Because of the hydrophilic functional group contained in CAU-1,the membrane surface with more MOF content has higher hydrophilicity,as shown in Fig.6.However,the size of the water contact angle not only depends on the surface hydrophilicity,but also is affected by the surface roughness,so there is not enough evidence to prove the hydrophilicity of the membrane surface.As shown in Fig.7,AFM test results show that MMM has larger surface roughness than pure membrane,and the roughness increases with the increase of CAU-1.If only according to the change rule of roughness,the antipollution ability of the membrane should be gradually reduced and easy to be polluted.However,the test results show that(Fig.9(b))FRR fluctuates up and down.The results further confirmed the improvement of the hydrophilicity of the membrane,and the hydrophilicity of the membrane played an important role in antipollution.

3.4.Membrane penetration and retention

In order to study the effect of CAU-1 addition in PPSU membrane,pure water flux and methyl violet retention are tested with pure water and 200 mg·L?1methyl violet solution respectively(Fig.10).It is found that the pure water flux increased first,then decreased,and finally increased abruptly with the increase of the amount of CAU-1.The addition of MOF breaks the traditional“trade off”effect.Because of the addition of hydrophilic CAU-1,the fluidity of casting solution increases and the hydrophilicity enhances the phase separation process.At the same time,as shown by the scanning electron microscope in Fig.6,the pores in the bottom layer tend to be widened,thus effectively improving the water flux of the membrane [47].In addition,the improvement of hydrophilicity on the surface of the MMM is conducive to the transport of water molecules.The increase of the surface roughness of the membrane leads to an increase in the area of water contact,which is also conducive to the improvement of water flux[48,49].The highest water flux of PPSU/1.0 wt% CAU-1 mixed membrane is 47.9 L·m?2·h?1,which is about 23%higher than that of pure membrane.When the CAU-1 content is added to 2 wt%,the effect of the increase in the viscosity of the casting membrane fluid is greater than the hydrophilicity,which slows the exchange rate of the solvent and non-solvent,promotes the formation of a dense layer,and leads to a decrease in water flux.At the same time,the retention of methyl violet reached 97%.However,when the dosage is 2.5 wt%,the water flux suddenly increases.The reason is that a large number of MOF agglomerates in the membrane matrix will result defects.As shown in Fig.6(f)by SEM,the liquid has no selective transmission,and the retention rate is reduced to 80%.

The stability of membrane is critical in the practical application.Therefore,the stability of the PPSU/CAU-1 MMM for the water permeance was studied by continuous filter operation for 120 min.As shown in Fig.11(a),the permeability of the two groups of membranes decreased slightly in the first 20 min or so,and then stabilized with the increase of time.Compared with the pure membrane,the MMM showed better membrane flux stability during 40 min–100 min test.This can be attributed to the fact that CAU-1 is an ideal filler with good compatibility with the polymer matrix,thus strengthening the membrane structure and avoiding the flux drop due to long-term work.We also studied the effect of transmembrane pressure (TMP) on membrane performance,as shown in Fig.11(b).The permeability of the membrane increased with the increase of TMP,but the rejection was still above 90%.The results showed that the mixed membrane of CAU-1/PPSU could work under high pressure without affecting its separation performance.

Fig.7.Surface AFM images of the MOF mixed matrix PPSU membranes with different concentrations.(a)Unfilled PPSU,(b)1.0%and(c)2.5%.

Table 2 Surface roughness parameters of MOF embedded PPSU membranes resulted from analyzing three randomly chosen AFM images

Fig.8.Static water contact angle.

3.5.Membrane antifouling property

Fig.9 shows the pollution resistance test of PPSU/CAU-1 MMM.The higher FRR value means the better antifouling performance of the membrane.Fig.9(a)consists of three parts:pure water flux,dirt flux and water flux after cleaning dirt.In this study,a 1 g·L?1BSA(pH=7.1)buffer solution was used for contamination experiments.According to Fig.9(a),when pure water is replaced by BSA solution,the membrane flux decreases,indicating membrane fouling.After 30 min of circulating water cleaning,the membrane with different MOF contents had different FRR.Van der Waals,hydrophobicity,hydrogen bond and electrostatic action all have effects on protein polymerization and membrane fouling[50].Generally speaking,the membrane separation system is a water-phase system.The hydrophilic membrane surface is easy to form hydrogen bond with water,which makes the membrane surface form an ordered structure.When hydrophobic solutes want to approach the membrane surface,they must first break the ordered structure.This behavior is obviously not easy to carry out,so the membrane surface is not easy to be polluted.In contrast,hydrophobic membrane is more likely to be polluted because the surface with hydrophobic membrane has no hydrogen bond with water [51,52].Recent literature shows that the antifouling performance of the membrane mainly depends on its hydrophilicity and roughness [53–56].Adding CAU-1 to increase the hydrophilicity of the membrane facilitates the formation of a hydration layer on the membrane surface to reduce pollution,which is consistent with the change of water contact angle in Fig.8[57,58].However,excessive MOF can cause agglomeration and increase roughness(Fig.6),which makes BSA molecules more easily adsorbed on the membrane surface and reduces antifouling performance.As shown in Fig.9(b),FRR has a fluctuating trend,when the addition amount of CAU-1 is 0.5 wt%and 1.0 wt%respectively,FRR increases to 93%and 91%,and the irreversible pollution degree is lower than that of unmodified membrane.Combined with the performance analysis of water flux and methyl violet retention in Fig.10,M2 has a larger water flux and retention rate,compared with M1.Therefore,based on the analysis of multiple factors,the performance is optimal when the MOF addition amount is 1 wt%.

Fig.9.The permeate fluxes of PPSU and PPSU/CAU-1 membranes during BSA fouling tests.

Fig.10.Pure water flux and methyl violet rejection.

Fig.11.(a)Flux profile of PPSU and PPSU/1.0 wt%CAU-1 membranes.(b)Effect of TMP on the separation performance of the PPSU/1.0 wt%CAU-1 membrane.

4.Conclusions

PPSU/CAU-1 MMM is prepared by immersion precipitation phase transformation,compared with PPSU pure membrane,it has better hydrophilicity and anti-pollution ability.By contrast,PPSU/1 wt%CAU-1 membrane has the highest pure water flux of 47.9 L·m?2·h?1,and the retention rate of organic dye methyl violet is 93.8%.At the same time,the membrane flux recovery rate(FRR)increased to 90.7%.The high permeability and fouling resistance are attributed to the expansion of membrane pores and the excellent hydrophilicity of CAU-1 in the membrane.The MMM based on MOF has great potential in the water treatment of organic dyes.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by Key Research and Development of Tangshan(19140204F).