Shujie Guo,Jiao Du,Fangzheng Yan,Zhi Wang*,Jixiao Wang

Chemical Engineering Research Center,School of Chemical Engineering and Technology,Tianjin University,Tianjin 300350,China

Tianjin Key Laboratory of Membrane Science and Desalination Technology,State Key Laboratory of Chemical Engineering,Collaborative Innovation Center of Chemical Science and Engineering,Tianjin University,Tianjin 300350,China

Haihe Laboratory of Sustainable Chemical Transformations,Tianjin 300350,China

Keywords:Streptomycin Polyamide Nanofiltration membrane Anti-adhesion Anti-bacterial

ABSTRACT Polyamide(PA)thin-film composite(TFC)nanofiltration(NF)membrane has extremely broad application prospects in separation of monovalent/divalent inorganic salts mixed solution.However,membrane fouling is the main obstacle to the application of PA,TFC and NF membrane.Streptomycin (SM) is a hydrophilic antibiotic containing a large number of hydroxyl and amino groups.In this work,the NF membrane was prepared via interfacial polymerization (IP) between trimesoyl chloride (TMC) in the organic phase and SM/piperazine (PIP) mixture in the aqueous phase.The NF membrane structure and performance were characterized in detail.The results showed that SM successfully participated in the IP.The negative charge and hydrophilicity of membrane surface were improved.The prepared membrane exhibited good anti-adhesion and anti-bacterial performance.Additionally,when the SM concentration was 2%,the prepared membrane exhibited the optimal permselectivity.The water permeance was 89.4 L·m-2·h-1·MPa-1.The rejection of NaCl and Na2SO4 were 17.17% and 97.84%,respectively.The NaCl/Na2SO4 separation factor of the SM2-PIP/TMC membrane in 1000 mg·L-1 NaCl and 1000 mg·L-1 Na2SO4 mixed solution was 40,which was 3.3 times that of PIP/TMC membrane.It indicated that SM2-PIP/TMC demonstrated excellent monovalent/divalent salts separation performance.This work provided an easy and effective approach to preparing anti-fouling NF membrane while possessing superior monovalent/divalent salts separation performance.

1.Introduction

With the continuous development of chemical,environmental and food industries,it is increasingly urgent to separate the mixed solution of monovalent and divalent inorganic salts.The separation of monovalent/divalent salts mixture solution has important requirements in water softening[1],reverse osmosis pretreatment[2,3],resource recovery [4],pollution monitoring [5] and other fields.The traditional separation methods of monovalent/bivalent salts mixture solution include evaporation,freezing concentration,solvent exchange,ion sieve adsorption and chemical precipitation[6].Compared with these methods,membrane separation technology has shown great application potential in separating monovalent/divalent salts mixed solution due to its advantages of low cost,small footprint and high energy saving [7,8].

Nanofiltration(NF)membrane is used for efficient separation of monovalent/divalent salts due to its moderate pore size and charged membrane surface [9].At present,polyamide (PA) thinfilm composite (TFC) membrane still is the state-of-the-art membrane for NF process because of its excellent permselectivity [10].PA TFC NF membrane is preparedviathe interfacial polymerization(IP) reaction between diamines and acyl chloride [11-13].That is,the amine monomer dissolved in the aqueous phase and the acyl chloride monomer dissolved in the organic phase form a PA layer at the interface of the two incompatible solutions through IP.However,PA TFC NF membrane application is limited due to the membrane fouling [14,15].

Membrane fouling refers to the adsorption and deposition of various types of foulants on the surface of the PA,TFC and NF membrane,which in turn causes degradation of membrane performance [16,17].Common foulants mainly include inorganic foulants (colloids and silica),organic foulants (biomolecules and natural organic matter) and biological foulants (various bacteria and fungi)[18].The addition of active chlorine to the feed solution is a common pretreatment method to alleviate membrane fouling.However,the active chlorine can damage the PA structure of the membrane,which can lead to membrane performance decrease[14,19].In addition,cleaning or replacing fouling membrane can also improve water production efficiency and quality [20].However,post-processing or pre-processing requires additional operational steps,materials or equipment.It not only wastes space and resource but also increases operational costs.So,development of PA TFC NF membrane with anti-fouling performance can reduce membrane fouling fundamentally [21].

Many anti-fouling membranes were fabricatedviabuilding anti-adhesion and anti-bacterial membrane surface [22,23].The NF membrane with anti-adhesion and anti-bacterial surface can be prepared by improving the membrane surface hydrophilicity,reducing the membrane surface roughness,regulating the membrane surface charge or introducing anti-bacterial materials on the membrane surface[14,24,25].At present,surface modification has been extensively applied in developing anti-fouling membranes [26,27].Wanget al.[26] modified the surface of the commercial NF membrane by grafting zwitterionic poly-(carboxy betaine methacrylate) (PCBMA).The grafted zwitterionic made the membrane surface charge electrically neutral and enhanced hydrophilicity,thus improving the membrane anti-adhesion performance.Leeet al.[27] modified the PA membrane with the chemical functionalization triclosan (TC),which was a nonionic organic biocide.TC was chemically immobilized on the TFC membrane through chemical functionalization of TC followed by strong covalent binding onto the membrane surface to enhance membrane anti-bacterial performance.Membrane surface modification can significantly improve the PA TFC membrane anti-fouling performance,but the amount of modification steps increases the preparation process complexity.Moreover,membrane surface modification generally increases the resistance of water molecules transport across the membrane,which may affect the PA TFC membrane permselectivity.

Introducing new hydrophilic and anti-bacterial monomers into nanofiltration membranes through the proven and simple IP process is expected to improve the membranes’ anti-fouling performance [28,29].The selection of monomers is critical to ensure that they contain at least two reactive groups and have high hydrophilicity and anti-bacterial performance.Jinet al.[30] used 2,2′-oxybis-ethylamine (2,2′-OEL) and PIP as aqueous comonomers to prepare NF membrane with TMC as organic monomerviaIP.2,2′-OEL possessed an ethyloxy group in the structure,which was more hydrophilic than hydrocarbon diamine.As a result,the membrane surface hydrophilicity improved,and the anti-fouling performance of the membrane was enhanced.Wenget al.[31] synthesized a novel zwitterionicN-aminoethyl piperazine propane sulfonate (AEPPS).The PA TFC NF membranes were preparedviathe IP reaction of AEPPS with TMC.The prepared NF membranes exhibited superior hydrophilicity and showed an excellent anti-adhesion performance.Moreover,the quaternary zwitterionic monomers used had excellent bactericidal performance,which in turn significantly improved the membrane’s anti-bacterial performance.However,most of the new monomers used in these studies were designed and synthesized by the researchers,and their preparation was complex and relatively expensive.To simplify the preparation process,we chose the cheap and commercialized streptomycin (SM) as the aqueous phase monomers.SM is a hydrophilic antibiotic containing a large number of hydroxyl and amino groups [32,33].SM can destroy the integrity of the bacterial cytomembrane and bind to the ribosomes in the bacteria,preventing the normal protein synthesis and ultimately killing the bacteria [34].

In this work,we used SM and PIP as aqueous co-monomers,and TMC was used as organic monomer.The NF membranes for separation monovalent/bivalent inorganic salts mixed solution were prepared on polysulfone(PSf)ultrafiltration membrane by IP.Regulate SM addition on the aqueous phase to optimize the pore structure and surface performance of NF membrane.Anti-adhesion performance and anti-bacterial performance of the prepared membrane were evaluated through anti-fouling experiments.The membrane prepared with the novel inexpensive monomer has significant potential for separating monovalent/divalent salts.

2.Experimental

2.1.Materials

TMC was purchased from J&K Scientific Reagent Co.ltd(China).NaOH and PIP was purchased from Aladdin Reagent Co.ltd(China).N-heptane was purchased from Jiangtian Chemical Technology Co.ltd (China).PSf ultrafiltration membrane as the support was acquired from Vontron Membrane Technology Co.ltd (China).PEG (200,400,600,800 Da) and SM (C21H42N7O18S1.5,728.69 g·mol-1)were purchased from Sciens Biochemical Technology Co.ltd (China).NaCl,MgCl2and MgSO4were purchased from Comere Chemical Reagents Co.,ltd(China).Na2SO4was purchased from Tianjin Best Chemical Co.ltd (China).Bovine serum albumin(BSA) as purchased from Aobox Biotechnology Co.ltd.(Beijing,China).

2.2.Preparation of NF membrane

This work prepared the NF membrane on the PSf ultrafiltration membrane by IP.Fig.1 shows the membrane preparation process.Firstly,50 ml aqueous phase solution(containing 1.1%NaOH,0.5%PIP and different concentrations of SM) was poured on the PSf membrane surface for 1 min.Subsequently,poured out the aqueous solution and removed the excess aqueous solution on the PSf membrane surface with a rubber roller.Then,50 ml organic phase solution (containing 0.2% TMCn-heptane solution) was poured on the PSf membrane surface.After 30 s of IP reaction,the organic phase solution was poured out,and the membrane surface was washed withn-heptane to remove residual unreacted TMC.Finally,put the membrane into the oven at 80°C for 5 min.The SM concentrations were 0%,1%,2%,3% and 4%,respectively.The corresponding NF membranes were denoted as PIP/TMC,SM1-PIP/TMC,SM2-PIP/TMC,SM3-PIP-TMC and SM4-PIP/TMC,respectively.

2.3.NF membrane characterization

Before characterization,the prepared membrane should be dried at 40°C for 24 h.Scanning electron microscopy(SEM,Nanosem 430,FEI,USA) and atomic force microscopy (AFM,Dimension Icon,Bruker,Germany) were used to analyze the membrane surface morphology and roughness.X-ray photoelectron spectroscopy(XPS,ESCALAB 250Xi,Thermo Fisher Scientific,USA) and attenuated total reflection-Fourier-transform infrared spectroscopy(ATR-FTIR,IRAffinity 1S,Shimadzu,Japan) were used to analyze the elemental composition and chemical structure of the membrane.

2.4.NF membrane hydrophilicity and zeta potential

Fig.1.Schematic diagram of the preparation process for NF membranes.

Membrane hydrophilicity was evaluated by solid-water interfacial free energy.The solid-water interfacial free energy of membranes was calculated to eliminate the effect of surface roughness to surface water contact angle.The water contact angle was measured using a contact angle goniometer (OCA15EC,Dataphysics,Germany).The solid-water interfacial free energy(-ΔGSW,mJ·m-2) was calculated by the Eq.(1) [35]:

where γw(72.8 mJ·m-2,25 °C) is the water surface tension,θ (°) is the water contact angle,Δ is the relative surface area.

The membrane surface zeta potential was detected by electrokinetic potential analyzer (Surpass,Anton Paar GmbH,Austria)with 1.0 mmol·L-1KCl solution at PH 7.

2.5.NF membrane permselectivity evaluation

In this work,cross-flow filtration equipment was used to test the NF membrane permselectivity.The NF membrane was placed in a membrane cell with an effective area of 28.26 cm2and tested at 0.6 MPa and 25 °C.The experiment was conducted at PH 7 and 1.0 L·min-1flow rate.The single salt solution used 2000 mg·L-1NaCl,Na2SO4,MgCl2and MgSO4,respectively.The mixed salts solution used 1000 mg·L-1NaCl and 1000 mg·L-1Na2SO4.Before the test,the NF membrane should be pressed for 30 min to stabilize the membrane performance.The water flux (J,L·m-2·h-1),permeance (A,L·m-2·h-1·MPa-1),salt rejection (R) and separation factor(NaCl/Na2SO4) (α) of NF membrane were calculated by the Eqs.(2)-(5),respectively.

whereS(m2) represents the membrane effective area,Δt(h) represents filtration time,V(L) represents the permeated water volume.

where ΔP-Δπ (MPa) represents the difference between operating pressure and osmotic pressure.

whereCp(mol·L-1)represents the concentration of permeated solution,Cf(mol·L-1)represents the concentration of feed solution.The concentrations of single salt were detected by a conductivity meter(DDSJ-308A,Cany Precision Instrument Co.,ltd.,China).High-performance ion chromatography (HPIC) (ICS-1100,Thermo Fisher Technologies,USA) was used to detect the concentrations of different ions in mixed salts solutions.

where α is the separation factor (NaCl/Na2SO4),RsandRcare the rejections of Na2SO4and NaCl,respectively.

2.6.NF membrane molecular weight cut off (MWCO)

When the rejection for neutral substance is equal to 90 %,the corresponding molecular weight of this neutral substance is the MWCO of NF membrane [36].For NF membrane,the rejection of neutral solutes is mainly based on the size sieving effect.The MWCO is related to the pore size of NF membrane[37].This work used a series of PEG aqueous solutions with an average molecular weight of 200,400,600 and 800 Da at a concentration of 2000 mg·L-1as the feed solutions to carry out filtration experiment.The NF membrane was placed in a membrane cell with an effective area of 28.26 cm2and tested at 0.6 MPa and 25 °C.The experiment was conducted at PH 7 and 1.0 L·min-1flow rate.PEG rejection was calculated by the Eq.(3).Ultraviolet-visible(Uv-vis) spectrophotometer (TU-1900,Beijing General Analyzer Co.,ltd.,China) was used to detect the concentration of PEG in solution.The principle of Uv-vis is to use the difference in light absorption capacity of different concentration solutions to determine the concentration of the solution [38].The optical densities of different PEG concentrations were measured on the Uv-vis,and the PEG standard curve was graphed against the optical densities with different PEG concentrations.The optical densities of the feed solution and permeated solution were measured,respectively.Furthermore,the corresponding PEG concentration was queried from the standard curve [39].The MWCO of NF membrane was obtained by fitting the rejection of different molecular weight PEG.The Stokes radiusr(nm) was acquired by the Eq.(6) [40].

2.7.Anti-adhesion performance evaluation

This work chose BSA as model protein pollutant to evaluate the prepared membrane anti-adhesion performance.Firstly,with 2000 mg·L-1Na2SO4as the feed solution,the membrane was pressed at 0.6 MPa for 30 min to obtain a stable initial flux (Jw)and Na2SO4rejection.Then,100 mg·L-1BSA solution was filled into the filtration cell to evaluate the decline of membrane water flux during fouling filtration.The membrane flux (J) and Na2SO4rejection were calculated by sampling every 5 min.The normalized flux(J/Jw) and Na2SO4rejection of the membrane with the test time were examined.After 360 min fouling filtration,the fouled membrane was rinsed with deionized water for 60 min.Finally,the water flux and Na2SO4rejection of the rinsed membrane were measured again using 2000 mg·L-1Na2SO4as the feed solution.The 360 min fouling and 60 min rinsing processes were viewed as one circle.The whole anti-adhesion tests were carried out for three circles.The water flux after the third fouling was denoted asJf,and the water flux after the third rinsed was denoted asJc.The flux recovery ratio(FRR)and total flux decline ratio(FDR)were calculated by the Eqs.(7)-(8).The experiment was conducted at PH 7 and 25 °C with a 1.0 L·min-1flow rate.

2.8.Anti-bacterial performance evaluation

This work used gram-negativeEscherichia coli(E.coli)and grampositiveBacillus subtilis(B.sub) as simulated bacteria to evaluate the NF membrane anti-bacterial performance.Before the test,100 μl bacterial suspension with a concentration of 106CFU·ml-1was added to the membrane surface.The membrane was placed in a constant temperature incubator at 37 °C for 2 h.Then,the membrane was put into a plastic tube containing 20 ml sanitary saline and shaken well to collect the bacterial suspension.The bacterial suspension was diluted several times.After that,100 μl bacterial suspension was coated on the culture medium and cultured in a constant temperature incubator at 37°C for 24 h.The number of colonies on PIP/TMC and SM-PIP/TMC was recorded asEandF,respectively.In addition,100 μl bacterial suspension at 106CFU·ml-1was directly coated on the culture medium as a blank control group.After 24 h of culture under the same conditions,count the colonies numberHon the medium.The anti-bacterial ratio (D) was calculated according to the Eq.(9).

3.Results and Discussion

3.1.Chemical composition and structure of membrane

ATR-FTIR can be used to characterize various functional groups of the membrane and then analyze the chemical structure[41].As shown in Fig.2(a),the PA characteristic absorption peak could be found at 1542 cm-1(N-H in-plane bending),1610 cm-1(hydrogen bonded C=O stretching),and 1667 cm-1(C=O bond stretching)in the spectra [42].This result indicated the formation of PA on the support membrane.For the PIP/TMC membrane,the peak at 1250 cm-1represented the aryl-ether (Ar-O-Ar) bond of PSf.In the cases of SM-PIP/TMC membranes,the peak at 1250 cm-1was due to both the Ar-O-Ar bond of PSf and C-O-C bond of SM.Moreover,the peak intensity at 1250 cm-1enhanced with the SM concentration increasing.In Fig.2(b),the SM-PIP/TMC membranes exhibited new peaks at 3100 to 3500 cm-1,while no new characteristic peaks were detected on the PIP/TMC membrane.The broad peak at 3100 to 3500 cm-1corresponded to the -OH stretching vibration of hydrogen bonding in SM,indicating the successful incorporation of SM into the PA layer [43,44].XPS can be used to further verify that SM has been successfully introduced into membrane.The membrane surface element composition is shown in Table 1.Compared with PIP/TMC membrane,the content of N and O elements in SM-PIP/TMC membrane increased significantly.Moreover,as the SM concentration increasing (from 0% to 4%),the N/C and O/C ratios of prepared membranes increased from 0.17 and 0.21 to 0.21 and 0.32,respectively.This was because SM contained a large number of oxygen-rich groups and nitrogen-rich groups such as-OH and-NH2(see Fig.3).In addition,PIP does not have primary amino groups,but SM does.As shown in Fig.4,SM-PIP/TMC membranes showed a peak of primary amine while the PIP-TMC membrane did not.ATR-FTIR and XPS results demonstrated that the SM successfully participated in IP,and the SM-PIP/TMC membrane was prepared.

Table 1 Surface element composition of NF membranes prepared with different SM concentrations

Fig.2.ATR-FTIR spectra of NF membranes prepared with different SM concentrations.

Fig.3.(a) Chemical structural of SM,PIP and TMC;(b) PA structural of PIP/TMC membrane;(c) PA structural of SM-PIP/TMC membrane.

3.2.Surface properties of membrane

The surface morphology of the PIP/TMC membrane and the SMPIP/TMC membrane can be characterized by SEM.As shown in Fig.5,all membranes’surfaces showed ring-shaped nodules.These ring-shaped nodules were formed due to the reaction between PIP and TMC releasing a large amount of heat.The generated heat released the dissolved gas in the solution,which further aggravated the instability of the IP reaction area[45].With the SM concentration increase,the number of ring-shaped nodules decreased gradually,and the membrane surface became smoother.The reason may be as follows.When SM monomer was added to the aqueous phase solution,it would pull the PIP molecule through intermolecular interaction due to its hydrophilicity,which slowed down the diffusion rate of PIP into the organic phase.This made different diffusion coefficients between PIP and TMC monomers,reducing the amount of PIP molecules involved in IP,and the number of ring-shaped nodules decreased eventually.On the other hand,when the high molecular weight SM participated in the IP reaction,it led to a low reaction activity.Therefore,under the combining effect of the above two factors,the increase of SM concentration made the IP reaction gentler and the reaction area more stable,contributing to less ring-shaped nodules and smoother membrane surface [46].

The surface roughness of the membrane was characterized by AFM.The average roughness (Ra),root mean square roughness(Rms) and the average distance between ‘‘peak-valley” (Rp-v) were used to represent the roughness of membranes.As presented in Fig.6 and Table 2,when the SM concentration increased from 0%to 4%,Ra,RmsandRp-vdecreased from 19.2,14.44 and 177.71 nm to 10.37,7.99 and 92.3 nm,respectively.The characterization result showed that compared with the PIP/TMC membrane,the SM-PIP/TMC membrane surface became smoother,which was consistent with the SEM characterization result.In conclusion,SEM and AFM characterization results proved that SM successfully participated in the IP,and the SM-PIP/TMC membrane was prepared.

Table 2 Surface roughness of NF membranes prepared with different SM concentrations

Table 3 MWCO and Stokes radius of NF membranes prepared with different SM concentrations

-ΔGSWis an important index to evaluate the hydrophilicity of the membrane.It is generally believed that a hydrophilic membrane surface is beneficial for the permeability and anti-fouling performance of NF membrane.As seen in Fig.7,when the concentration of SM increased from 0% to 3%,the -ΔGSWgradually increased from 116.21 to 130.44 mJ·m-2,which indicated that the hydrophilicity improved of membrane surface.For a membrane with a hydrophilic surface,an increase in the -ΔGSWcould be induced by an increase in the surface roughness or surface hydrophilicity [47].However,combined with the AFM characterization,the surface roughness decreased with the SM concentration increasing from 0%to 3%.So,the increase of the-ΔGSWwas attributed to the enhancement of the surface hydrophilicity.When the concentration of SM was 4%,the -ΔGSWdecreased to 126.87 mJ·m-2.Combined with the AFM characterization,the SM4-PIP/TMC membrane had minor surface roughness.The smooth surface was not conducive to the spreading of water droplets.Thus,its -ΔGSWdecreased instead.

Fig.4.XPS N 1 s spectra of the NF membranes prepared with different SM concentrations: (a) 0%;(b) 1%;(c) 2%;(d) 3%;(e) 4%.

Fig.5.SEM surface morphology of NF membranes prepared with different SM concentrations: (a) 0%;(b) 1%;(c) 2%;(d) 3%;(e) 4%.

Fig.6.AFM 3D morphology of NF membranes prepared with different SM concentrations: (a) 0%;(b) 1%;(c) 2 %;(d) 3%;(e) 4 %.

Fig.7.Water contact angle and solid-water interfacial free energy of NF membranes prepared with different SM concentrations.

Fig.8.Zeta potential of NF membranes prepared with different SM concentrations.

Fig.8 shows the zeta potential of the PIP/TMC membrane and the SM-PIP/TMC membrane under neutral conditions (PH=7).All membranes showed negatively charged surface,which was mainly due to the hydrolysis of unreacted-COCl groups to-COOH groups on the TMC [48].With the increase of SM concentration(from 0% to 4%),the zeta potential of the membrane surface decreased from -24.40 to -46.75 mV,exhibiting that the negative charge of the membrane surface was enhanced.When only PIP molecules participated in the reaction,the diffusion rate of PIP molecules in the organic phase was fast,resulting in most of the TMC molecules participating in the IP.Only a few residual -COCl groups were hydrolyzed to -COOH groups.Thus,the membrane surface showed a low negative charge.As mentioned in Section 3.2,the amount of SM and PIP molecules involved in the IP decreased with the addition of SM to the aqueous phase.Therefore,more -COCl groups remained on the membrane surface and then hydrolyzed to -COOH groups,which ultimately enhanced the negative charge of the SM-PIP/TMC membrane.

Fig.9.The rejection of different molecular weights PEG for NF membranes prepared with different SM concentrations.

Fig.10.(a)Water permeance and the rejection of NaCl and Na2SO4 of NF membranes and(b)separation factors of NF membranes prepared with different SM concentrations(Test conditions: 2000 mg·L-1 NaCl,2000 mg·L-1 Na2SO4,0.6 MPa).

Fig.11.The rejection for different salts of SM2-PIP/TMC membrane and PIP/TMC membrane(Test conditions:2000 mg·L-1 single component salt solution,0.6 MPa).

3.3.MWCO evaluation

Fig.9 presents the rejection of the NF membrane to PEG of different molecular weight.It can be seen that the MWCO of the prepared membranes further increased(from 471.47 to 514.72)as SM concentration increasing (from 0% to 4%).The Stokes radius of the membrane also increased continuously,from 0.510 to 0.542 nm(see Table 3).The reason was as follows.The SM molecules were too large,which led to a slow IP reaction,thus forming a loose separation layer [49].

3.4.Membrane permselectivity

Fig.12.The mass transfer mechanism of different inorganic salts through a negatively charged NF membrane.

The salt rejection of the NF membrane was tested using 2000 mg·L-1NaCl solution and 2000 mg·L-1Na2SO4solution,respectively.Here,we evaluated the effect of SM concentration on the prepared membrane permeability by testing 2000 mg·L-1Na2SO4solution permeability.The result is shown in Fig.10.As the SM concentration increased from 0% to 2%,the water permeance of NF membrane increased from 65.0 to 89.4 L·m-2·h-1-·MPa-1,attributed to the enhanced hydrophilicity and enlarged pore size (Fig.7 and Table 3).According to the size sieving effect,the NaCl rejection decreased from 27.4%to 17.17%,primarily associated with the enlarged pore size.Therefore,the enhanced hydrophilicity and the enlarged pore size of the SM-PIP/TMC membrane made the water molecules and NaCl can transport across membrane easily.Thus,compared with the PIP/TMC membrane,the SM-PIP/TMC membrane had greater water permeance and lower NaCl rejection.According to the Donnan effect and electrostatic repulsion,the negative charge on the surface of the NF membrane was enhanced with the increase of SM concentration,increasing the electrostatic repulsion of[50].So,the Na2SO4rejection increased from 97.01% to 97.84% over the increase of SM concentration from 0%to 2%.Moreover,when the SM concentration increased to 4%,the water permeance of the membrane increased to 93.1 L·m-2·h-1·MPa-1,while the rejection of NaCl and Na2SO4decreased to 16.55% and 96.82%,respectively.It was likely that the structure of the SM4-PIP/TMC membrane was excessively loose,which reduced the mass transfer resistance of the membrane and finally led to the water permeance increase and the inorganic salts rejection decrease.Therefore,the separation factors (NaCl/Na2SO4) of NF membranes increased firstly and then decreased.At SM concentration of 2%,the NF membrane had an ideal separation factor of 49.Besides,when the SM concentration was 2%,the SM2-PIP/TMC membrane exhibited the optimal permselectivity.The water permeance was 89.4 L·m-2·h-1·MPa-1.The rejection of NaCl and Na2SO4was 17.17%and 97.84%,respectively.

The PIP/TMC membrane and optimal membrane SM2-PIP/TMC were selected to test the separation performance in NaCl,Na2SO4,MgCl2and MgSO4salt solutions(2000 mg·L-1).The result is shown in Fig.11.For the SM2-PIP/TMC membrane,the salt rejection order was Na2SO4(97.84%) ＞MgSO4(94.78%) ＞NaCl (17.17%) ＞MgCl2(12.58%).Similarly,for the PIP/TMC membrane,the salt rejection order also was Na2SO4(97.01%) ＞ MgSO4(93.56%) ＞ NaCl(27.4%) ＞MgCl2(24.98%).These were consistent with the separation performance of negatively charged NF membranes reported in the literature [51].The mass transfer mechanism of different inorganic salts through the NF membrane is shown in Fig.12.Compared with Cl-,thehad higher negative charge density and stronger repulsion between the negatively charged membrane surface.Furthermore,the size ofwas larger than Cl-.Hence,the membranes could maintain the divalentwith negative charge and large size.The membranes exhibited high rejection of Na2SO4and MgSO4.The Mg2+showed higher positive charge density and stronger attraction between the negatively charged membrane surface than Na+,suggesting that Mg2+was more readily accessible to membrane pore and transported across the membrane.As a result,the rejection of Na2SO4is higher than that of MgSO4.Likewise,MgCl2was more accessible to transport across membranes than NaCl,resulting the rejection of NaCl was higher than that of MgCl2.For the negatively charged NF membrane,Na2SO4rejection was the highest and MgCl2rejection was the lowest.However,it could be found that SM2-PIP/TMC membrane had higher divalent salts rejection and lower monovalent salts rejection than the PIP/TMC membrane.These were mainly due to the stronger negatively charged surface and looser membrane structure of the SM2-PIP/TMC membrane.Therefore,SM2-PIP/TMC membrane was expected to be more preferable for monovalent/divalent salts separation.

Fig.13.The separation performance of the PIP/TMC membrane and the SM2-PIP/TMC membrane for mixed solution:(a)water permeance and NaCl and Na2SO4 rejection,(b)separation factors (NaCl/Na2SO4) (Test conditions: mixed solution of 1000 mg·L-1 NaCl and 1000 mg·L-1 Na2SO4,0.6 MPa).

Fig.14.Three times anti-adhesion tests of PIP/TMC membrane and the SM2-PIP/TMC membrane: (a) water flux change;(b) Na2SO4 rejection change (Test conditions:2000 mg·L-1 Na2SO4,100 mg·L-1 BSA,0.6 MPa).

Fig.15.Before and after fouling of (a,c) PIP/TMC membrane and (b,d) SM2-PIP/TMC membrane.

The separation performance of the PIP/TMC membrane and the SM2-PIP/TMC membrane for mixed solution is showed in Fig.13.Compared with the PIP/TMC membrane,the water permeance and Na2SO4rejection of the SM2-PIP/TMC membrane increased from 69.4 L·m-2·h-1·MPa-1and 94.2% to 92.3 L·m-2·h-1·MPa-1and 97.92%,respectively.While the rejection NaCl decreased from 28.37%to 16.3%.The trends of water permeance and salts rejection were the same as those of single salt feed solution,which were mainly ascribed to the improved hydrophilicity,the enlarged pore size and the enhanced negative charge of SM2-PIP/TMC membrane.Besides,the separation factors (NaCl/Na2SO4) of the PIP/TMC and the SM2-PIP/TMC membrane were 12 and 40,respectively.The SM-PIP/TMC membrane separation factor was 3.3 times more than the PIP-TMC membrane,which indicated SM2-PIP/TMC membrane exhibited an excellent separation performance for monovalent/divalent inorganic salts.

3.5.Anti-adhesion performance evaluation

In this work,BSA was used as protein pollutant to evaluate the anti-adhesion performance of PIP/TMC membrane and SM2-PIP/TMC membrane.The variations ofJ/Jwand Na2SO4rejection with fouling time were evaluated for PIP/TMC and SM2/PIP/TMC.The result is displayed in Fig.14.The water flux of SM-PIP/TMC membrane decreased slower than that of PIP/TMC membrane during the fouling testing,suggesting that the SM-PIP/TMC membrane possessed a stronger capability to prevent fouling.After three times continuous fouling testing,the FDR of the PIP/TMC membrane and SM2-PIP/TMC were 41.2% and 32.0%,respectively.The water fluxes of both SM2-PIP/TMC membrane and PIP/TMC membrane were reduced,which should be the result of BSA molecules attached to the membrane surface.The SM2/PIP-TMC membrane was more hydrophilic,so the number of BSA molecules attached to its surface was small (see Fig.15).As a result,the FDR of SM2-PIP/TMC membrane was lower than the FDR of PIP/TMC membrane.The Na2SO4rejection increased for both membranes at the beginning of the fouling.This was due to the deposition of BSA on the membrane surface,which increased the mass transfer resistance of PA layer.After deionized water rinsing,the FRR of PIP/TMC membrane and SM2-PIP/TMC membrane were 70.2% and 83.1%,respectively.The result showed that SM2-PIP/TMC membrane exhibited superior anti-adhesive performance than PIP/TMC membrane,which could be attributed to the enhanced hydrophilicity of SM2-PIP/TMC membrane (see Fig.7) [52].The high hydrophilicity helped the formation of a dense hydration layer on the membrane surface,preventing the pollutants from the membrane surface.At the same time,the surface of SM2-PIP/TMC membrane was smoother (combined with AFM characterization).The smooth membrane surface was also not beneficial for the deposition of pollutants,thus improving the anti-adhesion performance of NF membrane.

Fig.16.Anti-bacterial performance: (a,d) blank group,(b,e) PIP/TMC membrane and (c,f) SM2-PIP/TMC membrane.

3.6.Anti-bacterial performance evaluation

The anti-bacterial performance of PIP/TMC and SM2-PIP/TMC membranes were evaluated usingE.CoilandB.subas model bacterial pollutants.The growth of colonies on the culture medium is shown in Fig.16.Compared with PIP/TMC membrane,SM2-PIP/TMC membrane showed excellent anti-bacterial performance with nearly 100 % killing ratio to gram-negative and grampositive bacteria.The excellent anti-bacterial performance of SM2-PIP/TMC membrane was due to the monomer SM used,which was a hydrophilic antibiotic.SM destroyed the integrity of the bacterial cytomembrane and bound to the ribosomes in the bacteria,preventing the normal protein synthesis and ultimately killing the bacteria.

4.Conclusions

In this work,the NF membrane was preparedviaIP between TMC in the organic phase and SM/PIP mixture in the aqueous phase.Compared with the PIP/TMC membrane,the SM-PIP/TMC membranes had looser structure,smoother surface,enhanced hydrophilicity and more negative charge,thus exhibited excellent permselectivity and anti-fouling performance.When BSA was used as protein pollutant and after three times continuous antiadhesion testing,the FRR of SM2-PIP/TMC membrane was 83.1%,higher 18.7% than that of PIP/TMC membrane.The SM2-PIP/TMC membrane had nearly 100% killing ratio toE.CoilandB.subdue to anti-bacterial performance of SM.Furthermore,the SM2-PIP/TMC membrane exhibited high water permeance(89.4 L·m-2·h-1-·MPa-1)and high Na2SO4rejection(97.84%)at 2000 mg·L-1Na2SO4feed.In the mixed solution of 1000 mg·L-1NaCl and 1000 mg·L-1Na2SO4,the separation factors (NaCl/Na2SO4) of the PIP/TMC membrane and the SM2-PIP/TMC membrane were 12 and 40,respectively.This suggested that the SM2-PIP/TMC membrane showed an excellent separation performance for monovalent/divalent inorganic salts.This work offered an efficient method for preparing a high-performance NF membrane by using SM as a reaction co-monomer.

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 the Joint Funds of the National Natural Science Foundation of China (U2006220).