Surface modification of polypiperazine-amide membrane by self-assembled method for dye wastewater treatment☆

Yong Zhou *,Zhenan Dai,Ding Zhai,Congjie Gao

1 The Development Center of Water Treatment Technology,State Oceanic Administration,Hangzhou 310012,China

2 Zhejiang Sci-Tech University,Hangzhou 310018,China

Keywords:Surface modification Polypiperazine-amide Self-assembled Nano filtration membrane

ABSTRACT Polypiperazine-amide membranes were modified with poly(ethyleneimine)(PEI)by self-assembled method,through which PEI molecules were fixed on the membrane surface by ionic interaction.In the experiments,the PEI concentration ranged from 50 to 2000 mg·L-1 while the depositing time was fixed at20 min.The results showed that low PEI concentration resulted in a slight increase of pure water flux,which was attributed to the enhanced membrane surface hydrophilicity.The PEI adsorption on membrane surface had less effect on the rejections to neutral PEG and sucrose,but improved the rejections to divalent cationic ions and methylene blue as the result of reversion of the membrane surface charge from negative to positive according to the XPS analysis and zeta potential measurements.The membrane modified at PEI=1500 mg·L-1 exhibited high rejection to methylene blue(MB)and is potential to be applied in the treatment of ef fluents containing positively charged dyes.

1.Introduction

Nano filtration(NF)is a pressure-driven membrane separation process employing a semi permeable membrane with the pore size between reverse osmosis and ultra filtration membranes.Both steric and charge effects play important roles on the separation characteristics of the NF membrane,which can be modulated by membrane materials,membrane formation methods,etc.Nowadays NF has been applied in many fields,such as water softening,COD removal from wastewater,concentration ofdyes and so on[1–6],due to its advantages ofimproved selectivity for mono and multivalent ions,low operating pressures and relatively low capital and operating costs.Most commercially available NF membranes are either neutral or negatively charged in an aqueous processing environment,and the performance of them is not good for multi-valent cations and positively charged dyes,while positively charged NF membranes may have excellent hydrophilicity and exhibit high rejections to multi-valent cations and positively charged dyes[7].The lack of positively charged NF membranes of the commercial environment stimulated some researchers to study and utilize positively charged NF membranes.Some novelmaterialand membrane formation method were obtained.The positive charged membranes may be obtained by two methods.One is preparation of membrane with the positive charged poly electrolyte；the other is to modify the membrane material to positive charge after membrane formation.

The positive charged poly electrolyte was one of materials to prepare positively charged NF membrane.Du[8,9]prepared a positively charged NF membrane using poly(N,N-dimethylaminoethyl methacrylate).The membrane was cross-linked by immersion in p-xylylene dichloride/heptane solution.The resulting NF membrane exhibited the best rejection of 90%for MgSO4and a membrane flux of 10–20 LMH at 0.8 MPa.Han and co-workers[10]investigated positively charged quaternized poly(phthalazinone ether sulfone ketone)(QAPPESK)membrane.The effect of different operation conditions was systematically studied based on the rejection and desalination of the dye.Flux of 14.5 L·m-2·h-1,dye rejection of 92.3%and salt rejection of less than 10%were observed in the long term operation,indicating that the QAPPESK membranes could be successfully applied in the treatment of sulfur black dye wastewater and the reuse of inorganic salts.Xu[11]prepared a new membrane using 2,6-dimethyl-1,4-phenylene oxide.It was dissolved in chlorobenzene and brominated,and then precipitated by adding methanol and dried to form the primary membrane polymer.This brominated primary polymer was dissolved in chloroform and cast onto a substrate membrane(prepared from similar materials)and then cross-linked in situ using a mixture of trimethylamine and ethylenediamine.The desired resulting membrane achieved a very favorable flux of 63 LMH at 0.25 MPa but exhibited a low rejection value of 73%for MgCl2.Integrally skinned asymmetric membranes offer a direct alternative and are fabricated using the more simplistic phase inversion technique[12].Tang et al.[13–15]prepared a membrane using the same materials as Xu[11].Huang et al.[16–18]prepared positively charged NF membranes using the ultra filtration(UF)membrane as a support layer,the quaternized chitosan as an active layer by the method of cross-linking.The obtained NF membrane showed high rejection of MgCl2and low rejection of Na2SO4,and the positively charged characteristic of the NF membranes was demonstrated through the measurements ofpositive streaming potential.Li et al.[19]prepared positively charged NF membrane using quaternized chitosan as active layer；the NF membrane revealed high PEG1000 rejection and low Na2SO4rejection.Chung et al.[20–23]crosslink polyimide substrate with hyperbranched polyethyleneimine and obtain stably attached on the membrane surface that brings in the very stable positively charged surface property.

Furthermore,it is useful to modify the membrane material to positive charge after membrane formation.Su et al.[24]manufactured an asymmetric base membrane using the phase inversion technique from chloromethylated poly(phthalazinone ether sulfone ketone)dissolved in N-methyl-2-pyrrolidine,which was cast onto glass sheets at 60°C prior to immersion in water.The base membrane was then immersed into a solution containing trimethylamine to induct quaternary nitrogen groups into the membrane,and thereby applying a surface modi fication to the original asymmetric membrane and introducing a positively charged quaternized surface layer.

Our previous work[25]has shown that modification of polyamide RO membrane surface could decrease the fouling resistance to cationic surfactant agent through enhancement of membrane hydrophilicity and optimization of membrane chemical structure.Polypiperazineamide has the similar chemical structure as polyamide membrane,so modification of commercial NF membrane to reverse the surface negative charge into positive charge is a potential way to solve the problem of positively charged NF membrane material absence.The purpose of this paper is to deposit positively charged hydrophilic polymer on the surface of negatively polypiperazine-amide membrane by ionic interaction.Analysis of the hydrophilic of membrane surface,effect of modifying condition on NF performance to monoand divalent cations,as well as its antifouling performance to cationic dye were systematically studied.

2.Experimental

2.1.Materials

Thin- film composite polypiperazine-amide nano filtration membrane,kindly supplied by The Beidouxing membrane manufacture company,Hangzhou,China,was used as the base membrane,which was made from TMC and PIP through interfacial polymerization technology on the polysulfone porous membrane.Branched poly(ethyleneimine)(PEI,averaged Mw=70000,50%aqueous solution)was obtained from Sigma-aldrich.Methylene blue(99%)was from Acros Co.Sodium chloride,magnesium chloride(MgCl2·6H2O),sodium sulfate and glycol 400(PEG400)were purchased from Aladdin Reagent Co.,LLC,Shanghai,China.Deionized water with a conductivity of less than 5 μs·cm-1was used as the solvent for rinsing the membranes and preparation of feed aqueous solutions.All chemical agents were used without further purification.

2.2.Modification of NF membrane

Modification of NF membrane was adsorption of PEI on membrane surface,which is similar to formation of polyelectrolyte complex with positively charged polyelectrolyte and negatively charged polyelectrolyte,such as Chit in(CS)and poly acrylic acid(PAA)[26–28].The pH value played an important role in the modification process.In this study,the pH=6 to modify membrane was chosen because most of wastewater was near to pH=6 or could be adjusted to that pH value,the modified membrane can be stable at this situation.

Polyelectrolytes(PEI)were dissolved in aqueous medium at a concentration of 50,300,500,1000,1500,and 2000 mg·L-1,pH was adjusted to 6 with HCl.The base membrane sheet(20 cm2)was fixed in the testing cell(Fig.1 Equipment 1),which is mobile and suitable for both membrane modification and performance testing.The membrane samples were tested for pure water permeate and NF performance firstly.Then polyelectrolyte(PEI)solution was poured into the tank and circulated for 20 min to deposit an individual layer on the base membrane.The polyelectrolyte(PEI)solution was removed from modified membrane equipment and de-ionized water was re filled to rinse the membrane for 2 h.The membranes were kept wet in testing cell during the whole modifying process.

2.3.NF performance and fouling tests

All tests for NF performance were conducted at 0–0.7 MPa using salt solution(pH 6.0)or other solution(pH 6.0)at25°Cin the testing equipment(Fig.1,Equipment 1).The permeate flux(J,L·m-2·h-1)was determined by directmeasurementof the permeate volume over a certain period,as:

where Q is the volume of permeate,s is the effective area of membrane sheet and t is the running time.

The pure water permeability(PWP)was obtained by getting the slope between the flux of pure water(Jw)and applied pressure(ΔP),which defined as:

The normalized pure water permeability(PWP/PWP0)was used in this paper to reduce the error which resulted from differences among base membrane samples.PWP0and PWP are the pure water permeability of unmodified membrane and modified membranes,respectively.

The solute rejection rate(R,%)was determined through the following equation:

where,CPand Cfare the solute concentrations in permeate and feed,respectively.The salt concentration was obtained through measurement ofthe electricalconductance using a conductance meter(Cany Precision Instruments Co.Ltd.DDSJ-308A,China).The concentrations of PEG and sucrose were analyzed using a TOC-620C analyzer(Toray Industries Inc,Japan).The concentration ofMB was analyzed using a 755B Spectrophotometer(Shanghai precision&scientific instrument Co.,Ltd,China)at a wavelength of 663 nm.

Fouling test was carried out by adding cationic dye-methylene blue(MB)in feed solution directly；the decline of total flux with the increase of running time was obtained.

2.4.Characterization of membrane surface

Surface chemical characterization was carried out by X-ray photoelectron spectra(XPS),which was a Perkin Elmer PHI 5000C ESCA System with Mg/Al Dual Anode Hel/Hell ultra violet source(400 W,15 kV,1253.6 eV).The spectra were taken with the electron emission angle at 54°to give a sampling depth 10 nm,by a concentric hemispherical energy electron analyzer operating in the constant pass energy mode at 29.35 eV,using a 720 μm diameter analysis area.Membranes were mounted on a sample holder without adhesive tape and kept overnight at high vacuum in the preparation chamber before they were transferred to the analysis chamber of the spectrometer for their analysis.Each spectral region was scanned several sweeps until a good signal-to-noise ratio was observed.For each membrane,XPS analysis was carried out three times using different samples.Membranes were irradiated separately and for a maximum of 20 min to minimize X-ray-induced sample damage.

Fig.1.Schematic diagram of the equipment for in-situ membrane modification and testing.

The zeta potential was determined with an electrokinetic analyzer(EKA,Anton Paar KG,Graz,Austria)based on the streaming potential method[29,30].Flat sheets of the membranes were used as samples.The streaming potential was measured by forcing the electrolyte solutions(KCl)through a thin slit channel(90 mm×10 mm×0.27 mm)formed by a PTFE spacer between two membranes facing each other,for continuously increasing values of pressure[up to(200–240) ×10-4MPa]at(25 ± 0.5)°C.

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Tantec Contact Angle Meter(US Patent 5,268,733)was used to measure the contact angles of membranes with water at room temperature.Volume of water drop is 0.5μl,and 8 points for each sample were tested to get the average value.

3.Results and Discussion

3.1.Zeta potential

Deshmukh et al.investigate the effects of actual membrane feed waters on the surface charge properties ofsome ROmembranes by streaming potential measurements.Results showed a more positive zeta potential in the presence of the Brazos River water,most likely due to the presence of divalent cations(Ca2+and Mg2+)[36–38].In this study,streaming potential measurements were used to determine the surface zeta potential of the modified membranes.Fig.2 presents the PEIconcentration dependence ofZeta potentialofmodified membranes in 1.0 mol·m-3KClatpH=6.0.The charge ofthe modified membranes increased from negative to positive with the increase of PEI concentration due to the adsorption of PEI on membrane surface.The results show that the amount of PEI adsorbed on membrane surface increases with the increasing PEI concentration and reaches to balance when the PEI concentration is higher than 1500 mg·L-1.

Fig.2.Effect of the PEI concentration on the Zeta potential of modified membrane.

3.2.XPS

The composite membranes consisted ofa thin layerofpolypiperazineamide(about 0.1 μm)deposited on a polysulfone porous support.So the chemical information obtained through XPS analysis was derived only from the near surface of the top layer,and the polysulfone layer was not probed；thus,the main chemical elements studied by XPS were C,N,O(the polypiperazine-amide characteristic elements).As shown in Table 1,there was a distinguished excess of oxygen with respect to the corresponding theoretical ratio for polyamide(O/N=1),which indicated a high content of–COOH on the surface of these membranes which come from the hydrolysis of the acyl chloride.The chemical structure of the membrane skin layer was cross-linked poly(piperazineamide)with pendant group of–COOH and the NF membrane used was negatively charged.

Table 1 XPS result of TMC/PIP membrane modified with different PEI concentration

Furthermore,the O/N ratio of membrane without PEI was higher than thatofmembrane with PEIand the O/Nratio decreases with the increase ofPEIconcentration.Itfurther proved thatin the process ofmodification,the PEI adsorption on membrane surface increases with the increase of PEI concentration.

3.3.Effect of PEI concentration on pure water permeate

Fig.3.Effect of PEI concentration on normalized PWP.(PWP0:PWP of base membrane,PWP:PWP of modified membrane).

The base membrane was testfor PWP before modification and PWP/PWP0was obtained from the same membrane.Fig.3 shows thatthere is small increases of the normal pure water permeate with the increase of the PEIconcentration,and PWP/PWP0keepsstable afterthe PEIconcentration is more than 1500 mg·L-1,which attributed to the hydrophilicity of membrane surface.The reason is that the PEI adsorption on membrane surface increases and then reaches to balance with the PEI concentration increasing form the XPS and Zeta potential results.According to the solution–diffusion mechanism[31],two steps for the NF process were selective sorption of water molecules on the membrane surface firstly and selective diffusion of them through the active layer of the membrane.PEI is a hydrophilic polymer and can be used to modify the hydrophilicity of membrane surface in a low concentration which can lead to better adsorption of water on the surface of membrane[32,33].

Here the contacting angle on membrane surface modified by the PEI contacting angle was obtained because it is one of the most sensitive factors forobtaining information[34,35]on surface wetting and we correlate with wettability.Fig.4 shows that contact angle(water drop)of modified membrane decreases sharply with the increase of the PEI concentration.It suggests that the membrane hydrophilicity has been improved by modification with PEI.

Fig.4.Effect of PEI concentration on the surface contact angle of the modified membrane.

3.4.NF performance

Water flux and salt rejection are two important parameters for the performance of NF membrane.Fig.5 is the effect of PEI concentration on NF performance of modified membrane.It shows that the flux of modified membranes increases slowly with the increase of PEI concentration and keeps stable for allthe NaCl,MgSO4and MgCl2solutions,because the layer covering with PEI becomes more and more dense and then keeps stable until the adsorption reaches to balance,which is the same as the change of PWP.With the increase of PEI concentration,rejection to NaCl increases slowly from 65.0%to 67.5%,while rejection to MgCl2increases most quickly from 85.1%to 93.2%.

Fig.5.Effect of PEI concentration on NF performance of modified membrane.(Operating condition:90 mg·L-1 NaCl solution,75 mg·L-1 MgSO4 and 75 mg·L-1 MgCl2,0.8 MPa).

When an ion-charged membrane is used in NF process,the separation ofthe ionic compound was notonly decided by the chemicalpotential gradient but also affected by the electrostatic interaction between the ionic permeate molecules and the charged membrane:repulsion between the same ion-charged permeate and membrane,and attraction between the different ion-charged permeates and membranes.In this case,when the membrane becomes positively charged the order of rejection is R(MgCl2)＞R(MgSO4)＞R(NaCl)as both MgCl2and NaCl have a common counter ion and Mg2+should be more highly rejected than Na+for the reasons explained previously.The SO42-counter ion should experience higher transport across the membrane due to the increased electrostatic attraction when compared with the mono-valent ion Cl-.

To explain the transport of salts(NaCl,MgSO4and MgCl2)directly,the permeability ofNaCl,MgSO4and MgCl2was obtained through the solution–diffusion mechanism[27]as follow:

where Jsis solute flux,Bsis saltpermeability constant,andΔCsrepresents concentration difference of the two solutions across the membrane.The salt permeability constant is also normalized to get Fig.6 by Bs/Bs0,Bs0is salt permeability constant of unmodified membrane.

Fig.6.Effect of PEI concentration on normalized permeability of NaCl,MgCl2 and MgSO4 through the membrane.

It can be seen from Fig.6 that,with PEI concentration increasing,the normalized permeability constant(Bs/Bs0)of MgCl2and MgSO4decreases more quickly than that of NaCl.The reason is that the charge of membrane surface changes from negative charge to positive charge after deposition PEI.

In this case,unmodified membrane is a negative charge,which leads to higher concentration of counter-ions(Na+,Mg2+)and lower concentration of co-ions(Cl-,)in the membrane than in the adjacent feed solution according to Donnan equilibrium[27].Thus,concentration difference in solute ions between the membrane and the adjacent feed solution acts as a driving force for a diffusive transport.The membrane excludes all co-ions and is permeable to counter-ions,as A in Fig.7.Rejection for MgCl2is lower than that for NaCl because Mg2+is a divalent cation,and more permeable than Na+through the unmodified membrane.

The ionic charge of modified membrane changes from negative to positive,and the strength of positive charge increases with the increase of PEI concentration.It appears higher concentration of counter-ions(Cl-,)and lower concentration of co-ions(Na+,Mg2+)in the PEI layer on the membrane than in the adjacentfeed solution,which result that the permeability of MgSO4decreases more quickly than that of NaCl with PEI concentration increasing.

The rejection values of PEG,sucrose,and MG were presented in Fig.8,from which one can see that the rejection of MB increases quickly butthose ofPEGand sucrose keep stable with PEIconcentration increasing because the MB is a positively charged dye(Fig.9)and PEG/sucrose are neutral.The charge of membrane surface changes from negative to positive(Zeta potential results),but the steric effect does not change with the increase of PEI concentration.

Fig.8.Effect of PEI concentration on PEG,sucrose and methyl blue rejection.

Fig.7.Schematic diagram of ion transport mechanism through unmodified and modified membrane.

Fig.9.Chemical structure of methylene blue.

3.5.MB removal by modified NF membrane

The best modification parameters for the membrane are obtained from above results:concentrations of PEI=1500 mg·L-1and contact time with membrane=20 min.The desired membrane was used to test MB removal.The first industrial application considered in this work was the removal of low concentration pollutants from a watercourse,in this case removal of coloration caused by low level dye contamination.

The retentions and fluxes to aqueous solutions containing different contents of dye MB at constant trans-membrane pressure are shown in Fig.10.It can be seen that both the MB retention and flux decrease slightly with the increase of MB concentration.It is reasonable like some salt solution[34]because permeability of MB increases with the increase of MB concentration and flux is banked on transmembrane pressure.

Fig.10.Effect of MB concentration on NF performance.(Transmembrane pressure:0.7 MPa).

The retention and flux of different trans-membrane pressures of the MB dye atconstant MB concentrations are shown in Fig.11.From Fig.11 we can find that both rejection for MB and flux increase with the increase of transmembrane pressure because the permeability of water increases with the increase of trans-membrane and the permeability of MB keeps stable at constant MB concentration.

4.Conclusions

Fig.11.Effect of trans-membrane pressure on NF performance.(MB concentration:1500 mg·L-1).

It is successful to modify polypiperazine-amide membranes with PEI through self-assembled method to obtain a novel positive charged NF membrane.The PEI layer covering on base membrane not only improved the hydrophilicity of membrane surface but also changed the charge of membrane surface from negative to positive.There is a small increase for the pure water flux at the low PEI concentrations,which is mainly attributed to the enhanced surface hydrophilicity.The PEI adsorption on membrane surface has no effect on rejections to neutral PEG and sucrose but improves rejections to divalent cationic ions and methylene blue because the surface charge of membranes changed from negative to positive according to the XPS and Zeta potential results.The membrane modified at PEI=1500 mg·L-1and contact time with membrane=20 min exhibits high rejection for methylene blue and is potential to be applied in the treatment of ef fluents containing positively charged dyes.