Tailoring the Structure of Thin Film Nanocomposite Membranes to Achieve Seawater RO Membrane Performance
M A R Y L A U R A L I N D,†
D A N I
E L E U M I N E S U K,‡
T H E-V I N H N G U Y E N,§A N D
E R I C M.V.H O E K*
Department of Civil&Environmental Engineering and California NanoSystems Institute,University of California, Los Angeles(UCLA),Los Angeles,California,United States
Received May9,2010.Revid manuscript received September19,2010.Accepted September20,2010.
Herein we report on the formation and characterization of pure polyamide thinfilm composite(TFC)an
d zeolite-polyamide thinfilmnanocomposite(TFN)reverosmosis(RO)membranes. Fourdifferentphysical-chemicalpost-treatmentcombinations wereappliedaftertheinterfacialpolymerizationreactiontochange the molecular structure of polyamide and zeolite-polyamide thinfilms.Both TFC and TFN hand-cast membranes were more permeable,hydrophilic,and rough than a commercial awater RO membrane.Salt rejection by TFN membranes was consistently below that of hand-cast TFC membranes;however, two TFN membranes exhibited32g/L NaCl rejections above 99.4%,which was better than the commercial membrane under the test conditions employed.The nearly defect-free TFN
films that produced such high rejections were achieved only with wet curing,regardless of other post-treatments.Polyamide films formed in the prence of zeolite nanoparticles were less cross-linked than similarly cast pure polyamidefilms.At theverylownanoparticleloadingvaluated,differencesbetween pure polyamide and zeolite-polyamide membrane water and salt permeability correlated weakly with extent of cross-linking of the polyamidefilm,which suggests that defects and molecular-sieving largely govern transport through zeolite-polyamide
thinfilm nanocomposite membranes.
Introduction
Fresh water is esntial to human survival and is integral in the global economy for its us in agricultural irrigation, industrial process,oil and gas exploration,and electricity production(1).Continuous population growth and industrial development stress the limited supply of freshwater.This water stress cannot be eliminated by conrvation efforts alone.Hence,production of fresh water from alternative sources such as reclaimed wastewater,brackish groundwater, and ocean water must be considered.Commercially available rever osmosis(RO)membranes can produce high quality water from such alternative water sources,but improvements on existing RO membranes are needed to further reduce operating costs,energy demand,and chemical consumption. Improved RO membranes might exhibit higher water per-meability,solute lectivity,or fouling resistance.
Mixed matrix membranes s in which afiller material is embedded within a polymeric matrix s are already ud in a variety of industrial and environmental process including fuel cells,pervaporation,and gas parations(2-6).This concept has added a new degree of freedom in the develop-ment of membranes with novel paration , lection of the unique properties of thefiller material,which may include enhanced permeability,lectivity,stability, surface area,or catalytic activity.More recently,mixed matrix membranes are being explored to tailor the perfo
rmance and add new functionality to membranes for water purifica-tion applications.Thinfilm nanocomposite membranes for rever osmosis applications have been developed that incorporate pure metal,metal oxide,and zeolite molecular-sieve nanoparticles.The recent efforts are briefly reviewed here.
Kwak et al.deposited titanium dioxide nanoparticles onto hand-cast polyamide composite membranes and demon-strated differences in hydrophilicity andflux(7).Lee et al. incorporated silver nanoparticles into a thin polyamide layer during interfacial polymerization(8).While the membranes exhibited some antibacterial effects,they produced nano-filtration-like lectivity(96-97%rejection of2000ppm magnesium sulfate)without significant changes in water permeability.Lee et al.similarly incorporated30-nm titanium dioxide nanoparticles into polyamide thinfilms during the interfacial polymerization reaction(9).In brackish water chemistry,the waterflux of the titanium dioxide-polyamide membranes incread with nanoparticle loading up to5wt %with minimal changes in salt rejection;however,at particle loadings above5wt%salt rejection by the membranes decread dramatically suggesting theflux enhancement largely resulted from defects in the polyamide created by the solid nanoparticles(9).Singh et al.incorporated16-nm silica nanoparticles into polyamide thinfilm composite mem-branes(10).The membranes were tested using500ppm dioxane solutions
and the structure of the polymer was investigated with small angle neutron scattering.In later work from the same laboratory,Jadav et al.incorporated16-and 3-nm silica nanoparticles into the polyamide layer of thin film composite membranes(11).The resultant silica-poly-amide membranes exhibited brackish water desalination performance,but salt lectivity decread with nanoparticle loading indicating defect formation in the polyamide thin film as the principle mechanism offlux enhancement(11).
Jeong et al.demonstrated that incorporating zeolite molecular-sieve nanoparticles into polyamide thinfilms (during interfacial polymerization)could double the water flux without reducing obrved rejection of2g/L sodium chloride,magnesium sulfate,and polyethylene glycol solu-tions(12).A key feature of this work is that zeolite nanoparticle size was designed to be the same size as polyamidefilm thickness,thereby,creating a“percolation threshold”through the thinfilm with a single particle.It was hypothesized that zeolite molecular-sieves improved membrane permeability while maintaining good lectivity by acting as preferential flow paths for water transport.However,a ries of zeo-lite-polyamide coatingfilms prepared with impermeable
*Corresponding author tel:(310)206-3735;fax:(310)206-2222;
e-mail:emvhoek@ucla.edu.
†Current address:Arizona State University School of Engineering
of Transport,Matter and Energy,P.O.Box876106Tempe,AZ85287-
6106,United States.
‡Current address:Samsung Cheil Industries Inc.,E-Project Team,
332-2Gocheon-dong,Uiwang-si,Gyeonggi-do,Korea437-711.
§Current address:Vietnam National University-Hochiminh City,
Hochiminh City University of Technology,Faculty of Environment,
268Ly Thuong Kiet Street,District10,Hochiminh City,Vietnam.
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10.1021/es101569p XXXX American Chemical ,NO.xx,XXXX/ENVIRONMENTAL SCIENCE&TECHNOLOGY9A
zeolite nanoparticles(internal poresfilled with the polymer template)producedfluxes intermediate between the pure polyamide membrane and the TFNfilm prepared with pore opened zeolites at the same zeolite loading(12).This result offered indirect evidence that another mechanism,besides molecular-sieving,could be responsible for the enhanced membrane performance.Subquently,two additional mech-anisms for the obrvedflux enhancement were propod (13,14):(1)defect formation due to zeolite nanoparticle aggregation in the organic monomer solution and(2)heat relea from the zeolites by hydration during the interfacial polymerization reaction,which changed the cross-linked structure of the polyamidefilms.
To date,nanocomposite RO membranes have exhibited paration performance suitable for brackish water RO or nanofiltration applications.If defect formation is the primary mechanism responsible forflux enhancement,then it should be very difficult to produce nanocomposite RO membranes with awater RO membrane salt lectivity.However,if either molecular sieving or altered polyamidefilm structure is the primary mechanism responsible forflux enhancement, then awater RO lectivity should be attainable.
In this paper,methods for fabricating TFN membranes with water permeability and salt lectivity rivaling com-mercial awater RO membranes were explored.Specifically, four different post-treatme
nt regimes were applied to pure polyamide and zeolite-polyamide thinfilm composite membranes to change their molecular structure and mini-mize defect formation.Very low zeolite loadings were ud to minimize the effects of molecular sieving on the paration performance of the membrane.Hand-cast TFC and TFN membrane paration performance,interfacial properties, and polyamide thinfilm structure were characterized and compared to a commercially fabricated awater RO membrane.
Experimental Section
篮球训练营Nomenclature.The following nomenclature is ud in this manuscript.Polysulfone coated polyester membranes(PS, NanoH2O)were ud as the support on which the polyamide was deposited.During the synthesis of the thin polyamide film meta-phenylene diamine(MPD,98%Sigma-Aldrich)was dissolved in deionized water.The organic solvent Isopar-rafin-G(Isopar-G,Gallade Chemical,Santa Ana,CA)was ud to dissolve trimesoyl chloride(TMC,Sigma-Aldrich).Nano-particles of Linde type A(LTA,NanoH2O)zeolites,ap-proximately250nm in diameter,were ud.The chemical formula for fully hydrated LTA is Na12[(AlO2)12(SiO2)12]·27H2O.
Chemicals ud for post-treatment were sodium hy-pochlorite solution(NaOCl,available chlorine10-13%, Sigma-Aldrich),sodium metabisulfite(Na2S2O5,Fisher Chemi-cal),and sodium bicarbonate(Na
HCO3,Sigma-Aldrich). Membrane solute rejection was tested using sodium chloride (NaCl).Thinfilm composite(TFC)membranes are pure polyamide thinfilms without nanoparticles,whereas thin film nanocomposite(TFN)membranes are polymerized with nanoparticles prent in the Isopar-G-TMC monomer solu-tion.Membranes are designated in the following manner: TFC-1-A means a thinfilm composite membrane fabricated with curing conditions1and rinsing conditions A(found in Table1).A commercially available membrane,SWC3+ (Hydranautics)was ud for comparison with the hand-cast membranes.
Membrane Preparation.All steps for preparation of the interfacially polymerized polyamide thinfilm,other than solution preparation,were performed in a vertical sash fume hood.The methods prented below are a modification of a previously published summary of commercially relevant polyamide composite membrane post-treatments(15).TFC membranes were prepared as follows.First,the polysulfone support membrane was taped to an8×5×0.25in. borosilicate glass plate with Fisherbrand laboratory tape.Two parate monomer solutions were prepared:MPD in water and TMC in organic.The MPD in deionized water solution, 3.4%(wt/wt%),was prepared in a glass jar and the jar was wrapped in aluminum foil to prevent light-oxidation of the MPD.A0.15%(wt/wt%)solution of TMC in Isopar-G was prepared in a500-mL pyrex solution bottle.Both solutions were stirred with a magnetic stirrer at room temperature for a minimum of3h prior to u.
After stirring,the MPD solution was poured into a pyrex dish on an approximately20degree incline.The plate with the PS support taped to it was then placed into the MPD solution for2min,the support side was placed into the solution such that the backside of the plate not immerd in the solution.After2min the plate was lifted from the solution and the excess solution was allowed to drain from the surface.The plate was then placed on a rubber mat,with the glass down and the MPD-soaked support facing up.An air-knife(model110012SS-316,Exair,Cincinnati OH)with filtered,compresd air was ud to remove excess solution from the support membrane.The air-knife was operated with a pressure of20psi and held approximately0.5in.from the membrane surface;a complete pass of the8-in.length of the sample took a minimum of15s.Excess MPD solution was wiped from the taped edges and back of the glass plate.Then the plate and membrane were immerd vertically in the TMC solution for1min in a custom-fabricated250-mL container.After1min in the TMC solution,the plate and membrane were removed from the TMC container and held vertically for2min.
After vertical holding,the membranes were subjected to either wet or dry curing followed by a ries of post-treatment rins(Table1).Postcure exposure was to either of two different combinations of rins(200ppm NaOCl in deionized water,followed by1g/L Na2S2O5in deionized water,or2g/L NaHC
O3in deionized water).Hot water cures and rins were performed in a90°C deionized water bath in a pyrex dish on a hot plate.Dry oven cures were performed at90°C, with the membrane placed vertically on a rack in the center of the oven(737F Isotemp Oven,Fisher Scientific).For all rins,the membrane was removed from the glass support and the solutions were kept in1-L glass beakers.Chlorination of polyamide membranes is known to structurally degrade the membranes(16).Here the NaOCl is believed tofirst scavenge unreacted MPD that has not yet escaped the structure;then the Na2S2O5is believed to neutralize any of
TABLE1.Post-Treatment Conditions for Membrane Synthesis a
fabrication conditions cure temperature[C]cure time[s]cure medium rin A.1[s]rin A.2[s]rin A.3[s]rin B.1[s] 1-A90120water12030120-
1-B90120water---600 2-A90360oven12030120-
2-B90360oven---600
a Rin A.1)0.2g/L NaOCl.Rin A.2)1g/L Na
2
S2O5.Rin A.3)90°C deionized water.Rin B.1)2g/L NaHCO3.
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the unreacted NaOCl and prevent it from further attacking the polyamide.The NaHCO3rin is a basic solution(pH ∼11).
Zeolite-polyamide nanocomposite thinfilms were cast identically to TFC membranes,except0.2%(wt/wt%)of colloidal zeolite nanoparticles were disperd in the Isopar-G-TMC solution.The TMC solution including nanoparticles was ultrasonicated for40min at20°C immediately prior to u in the interfacial polymerization reaction.
Membrane Separation Performance.The synthesized thinfilms were evaluated for permeability of pure water and NaCl using a custom-fabricated6-cell rever osmosis testing system described by Jin et al.(17).The membranes were compacted for a minimum of12h at∼59bar(850psi),until the pure waterflow reached a steady state.After compaction, pure waterflow rate was measured at∼55bar(800psi). Next,the feedwater solution was changed to a32g/L NaCl solution.Permeateflow rate and conductivity of feed and permeate samples were measured after the system perfor-mance was stable for at least30min.Waterflux was determined from permeate waterflow rate as
where Q p is the permeate waterflow rate and A m is the effective membrane area(0.00194m2).Feed and permeate conductivities were ud to calculate the obrved salt rejection from
where K f and K p were the feed and permeate conductivity.
Calculation of Membrane Transport Coefficients.Trans-port through RO membranes is generally considered to occur by a solution-diffusion type mechanism where water,J w,and salt,J s,flux were calculated by
Here,∆p is the(applied)trans-membrane hydraulic pressure,∆πis the trans-membrane osmotic pressure,and ∆c()c m-c p)is the(real)trans-membrane concentration gradient,where c m is the feed-side membrane surface salt concentration and c p is the permeate salt concentration.Also, A and B are the water and solute permeability coefficients. Trans-membrane osmotic pressure for NaCl was determined from
where R is the universal gas constant and T is the temperature. The osmotic coefficient has been assumed to equal1,this is reasonable becau at the solute concentrations ud in this study the error between the Gibbs(nonlinear)and van’t Hoff(linear)equations is only∼4.34%(18).Using this as a correction factor in our osmotic pressure calculations does not change our conclusions on structure-
performance relationships.The membrane surface salt concentration was estimated using
where c f is the feed concentration,X s()1-c p/c f)is the obrved salt rejection,and k s is the salt mass transfer coefficient.The channel average mass transfer coefficient in the laboratory scale crossflow membranefiltration system was estimated by
Here,Re is the Reynolds number,Sc is the Schmidt number, D is the solute diffusivity,and d h()2H c,where H c is the crossflow channel height)is the hydraulic diameter of the crossflow channel(19).Next,the real membrane salt rejection could be calculated directly from
Combining the measuredflux and rejection with the cal-culated mass transfer coefficient,the pure water permeability coefficient during salt water experiments was
which was obtained by combining eq3with eqs5-8.The salt permeability coefficient was calculated from
which was derived by substituting J s)J w c p and the parenthetical expression from eq5into eq4,and dividing both sides by c m.The water-salt lectivity of a membrane can be characterized by the ratio of water to solute perme-ability()A/B).
Membrane Surface Characterization.Root-mean squared (RMS)surface roughness and surface area difference(SAD) were quantified by atomic force microscopy,AFM,(Nano-scope IIIa;Digital Instruments,Santa Barbara,CA,USA).For AFM analysis membrane coupons were driedflat in a desiccator overnight prior to analysis.Attenuated total reflection Fourier transform infrared spectroscopy(ATR-FTIR)was performed(FT/IR670plus,Jasco,Easton,MD, USA)with a variable angle ATR attachment coupled to a germanium crystal operated at45degrees.Prior to ATR-FTIR measurement the samples were driedflat in a desiccator for a minimum of24h.
Membrane surface chemical composition was analyzed with X-ray photoelectron spectroscopy(XPS).XPS data were collected using a Surface Science Instruments M-Probe system that has been described previously(20).Ejected electrons were collected at an angle of35°from the surface normal,and the sample chamber was maintained at<5×10-9Torr.Survey scans from0to1000eV were performed to identify the elements prent on the surface.High-resolution spectra(with a step size of0.065eV)were collected for the C1s,N1s,and O1s regions.The XPS data were analyzed using the ESCA Data Analysis Application(V2.01.01; Service Physics,Bend,OR)and the ratio of carbon to nitrogen was ud to estimate the extent of cross-linking of the polyamide.Typically,the O/N ratio is ud to evaluate the extent of cross-linking in polyamide thinfilm
s(21).However, becau zeolite-polyamide thinfilms have oxygen prent in the zeolite as well as the polymer,the ratio of carbon to nitrogen is explored instead of the carbon to oxygen ratio (13).Thus,relative extent of cross-linking was estimated from
J w)Q p
A m
(1)
X s)1-K f
K p
(2)
J w)A(∆p-∆π)(3) J s)B∆c(4)
∆π)2RT(c
m -c
p
)(5)
c m c f )1-Xamerican family
surveillances
+X
s
exp(J w k s)(6)
k s)1.85(ReSc)1/3
D
d h
(7)
星期六英文R s()1-c p c m))1-1-X s
1-X s+X s exp(J w/k s)
(8)
A)
J w
∆p-2RTR
s
c m
(9)
B)J w
(1-R s)
R s
(10)
%crosslinked)1-((C/N)obrved-(C/N)fully-crosslinked
(C/N)fully-linear-(C/N)fully-crosslinked)(11)
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where(C/N)obrved is the C/N ratio measured by XPS.A fully cross-linked polyamide,(C/N)fully cross-linked,has a C/N ratio of 6and a theoretically fully linear polyamide,(C/N)fully linear, has a C/N ratio of7.5.
Sessile drop contact angles of deionized water were measured on air-dried samples of synthesized membranes in an environmental chamber mounted to the contact angle goniometer(DSA10,KRu¨SS).The equilibrium value was the steady-state average of left and right angles.The data reported are the averages of12measurements on at least3different membrane samples with the high and low values discarded before averaging and computing standard deviations.A modified form of the Young-Dupre equation was ud to determine the surface roughne
ss corrected solid-liquid interfacial free energy,which provides a robust measurement of“hydrophilicity”or“wettability”of a membrane.In our modification,we correct for the increa in surface area due to roughness(a.k.a.,the relative surface area)as suggested by Wenzel(22).The solid-liquid interfacial free energy was determined from
whereθis the average measured contact angle,γL()72.8 mJ/m2for water at25°C)is the liquid surface tension,and r is the actual surface area divided by the planar surface area ()1-SAD).A larger value of-∆G SL indicates a more water wettable surface.
Results
Separation Performance of TFC and TFN Membranes.All hand-cast membranes produced larger average pure water and salt waterfluxes than the commercial awater RO membrane(Table2).All four hand-cast TFC membranes (TFC-1-A,TFC-2-A,TFC-1-B,TFC-2-B)produced average obrved salt rejections greater than or equal to99.4%, whereas the commercial awater RO membrane produced 99.3%rejection.The two nanocomposite membranes post-treated with the chlorine-bisulfite-water rin quence (TFN-1-A,TFN-2-A)exhibited increadfluxes and main-tained the same salt rejection as hand-cast TFC membranes. Nanocomposite membranes post-treated with the
sodium bicarbonate solution exhibited significantly lower salt rejections s TFN-1-B(98.5%)and TFN-2-B(95.6%).Hand-cast TFC membranes expod to the sodium bicarbonate rin(TFC-1-B,TFC-2-B)did not exhibit lower salt rejection; hence,post-treatment with sodium bicarbonate may have weakened zeolite-polyamide bonds formed during inter-facial polymerization thereby creating molecular-to-nano-scale voids(defects)in zeolite-polyamide nanocomposite films.
Physical-Chemical Properties of TFC and TFN Mem-branes.Compared to the commercial awater membrane, all hand-cast membranes exhibited lower pure water contact angles and XPS measured C/N ratios,but higher AFM measured RMS roughness and SAD values(Table2).Hand-cast TFN membranes were universally rougher with more surface area(both RMS and SAD values were larger)than hand-cast TFC membranes.All hand-cast TFN membranes had higher C/N ratios than hand-cast TFC membranes and lowerC/NratiosthanthecommercialawaterROmembrane.
Infrared spectroscopy provides insight into chemical functionality and the type of chemical bonds prent in TFC and TFN membranes.In the region of the ATR-FTIR spectra from1900to800cm-1(Figure1b),the ri of the broad peak at1050-950cm-1is characteristic of the Si-O and Al-O functionality in LTA zeolites,and this confirms that zeolites are prent in nanocompositefilms(23-26).The peak at 1734cm-1corresponds to C d O carboxylic acid functionality and the peak at1544cm-
1corresponds to the C-N stretch of amide II(27).The peaks at1734cm-1are more inten in TFN membranes than in the TFC membranes;additionally the ratio of the amplitude of the carboxylic acid to amide II peak increas in the TFN compared to TFC membranes. Bad on the chemical structure of the polyamide formed from MPD and TMC there is more carboxylic acid func-tionality in the linear polymeric structures.A fully cross-linked MPD-TMC polymer has a molecular formula of C6H4ON where a fully linear MPD-TMC polymer has a molecular formula of C15H10O4N2(21).Therefore,the higher C/N ratio of the TFN membranes(indicating a more linear polymeric structure)is consistent with the apparent increa in carboxylic acid functionality en in the ATR-FTIR spectra in TFN compared to TFC membranes.
TABLE2.Obrved Membrane Physical-Chemical Properties
membrane type post-treatment
conditions a
pure water
flux(µm/s)
salt water
flux(µm/s)
salt rejection
(-)
灿烂千阳pdfcontact angle
who care
(deg)
RMS roughness
(nm)
roughness
area ratio(-)
C/N ratio
(-)
hand-cast TFC1-A28.4(1.811.7(1.299.5(0.063(9130(17 2.3(1.6 6.0 hand-cast TFC1-B24.4(1.710.7(1.799.4(0.158(11134(17 2.0(1.2 6.0 hand-cast TFC2-A23.4(0.28.8(0.199.7(0.166(4158(38 2.9(1.9 6.4 hand-cast TFC2-B54.8(3.616.9(0.799.4(0.266(11116(14 1.9(1.2 6.0 hand-cast TFN1-A31.8(1.911.7(0.999.4(0.248(7164(43 2.3(1.3 6.8 hand-cast TFN1-B24.0(1.210.4(1.198.6(0.860(13133(30 1.8(1.2 6.8 hand-cast TFN2-A27.2(5.710.8(1.999.5(0.051(5184(154 1.9(1.5 6.8 hand-cast TFN2-B29.8(7.711.6(2.695.7(3.072(12331(55 2.3(1.2 6.5 commercial SWRO proprietary23.1(1.29.2(0.699.3(0.288(599(22 1.7(1.17.2 a Fabrication conditions from Table1:1)wet cure;2)dry cure;A)NaOCl,Na
2
S2O5,water;B)NaHCO3.
-G
SL )γ
L(1+cosθr)(12)
FIGURE1.ATR-FTIR spectra from1900to800cm-1for(a)TFC
and(b)TFN membranes post-treated by wet(1)and dry(2)
curing followed by NaOCl-Na2S2O5-water(A)and NaHCO3(B)
rins.
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In the region of the spectra from 4000to 2500cm -1(Figure 2),TFN membranes exhibit slight broadening and develop-ment of a shoulder (around 3400cm -1)in the broad peak around 3274cm -1(compare Figure 2to 1).In the TFC membranes,this peak is centered around ∼3274,but in the TFN membranes,this peak downshifts ∼14to ∼3260cm -1.In the pure LTA spectrum (Figure 2),the peak at 3440cm -1corresponds to the inner hydroxyl stretches of the zeolite (28).In the TFN spectra the slight broadening and downshift of the peaks around 3274to 3260cm -1and the ri of this shoulder at 3400cm -,a downshifting of the peak at 3440cm -1in the pure zeolite)indicate some interaction between the inner hydroxyl group of the zeolite with acid chloride,carboxylic,amine,or amide groups in the polyamide (28).If the principle interface interaction between the zeolites and the polyamide consists of hydrogen bonding between reciprocal acid -ba functional groups between the surfaces,the high pH (∼11)of the bicarbonate rin could create defects at the zeolite -polyamide interface by interfering with hydrogen bonding.
Discussion
It is difficult to relate membrane structure to obrved paration performance data becau the latte
r is a function of membrane properties and operating conditions.In ad-dition,measured physical -chemical properties such as contact angle can be thrown off by surface roughness.Therefore,experimentally measured data (Table 2)were transformed into water permeability (Figure 3a),salt perme-ability (Figure 3b),roughness corrected solid -liquid inter-facial free energy (Figure 4a),and fractional extent of cross-linking (Figure 4b).
For all of the post-treatment combinations,hand-cast TFC and TFN membrane water permeability coefficients were higher than tho of the commercial awater RO membrane.All of the hand-cast TFC membranes and the TFN membranes of post-treatment condition A had similar salt permeability coefficients compared to the commercially available mem-brane (mirroring the salt rejection data);while TFN mem-branes of post-treatment condition B had significantly incread salt permeability coefficients compared to the TFC membranes and the commercial membrane (also mirroring salt rejection data).
The commercial awater RO membrane had a smaller solid -liquid interfacial free energy than all hand-cast ,it was less wetting or hydrophilic.For post-
treatment with conditions 1-A and 2-A (hypochlorite,bisulfite)TFN membranes had slightly larger -∆G
SL than the corre-sponding TFC membranes.However,there was no significant difference in -∆G SL between TFC and TFN membranes post-treated with bicarbonate rin (TFC/TFN-1-B and 2-B).Again,this suggested a complex interaction between the polyamide thin film,the zeolites,and the post-treatment conditions in changing the chemical surface functionality of the membranes.All TFN membranes were less cross-linked than the corresponding TFC membranes.Note the extent of cross-linking reported for the commercial membrane assumed a pure MPD -TMC film,which may not be the ca for this proprietary material.More linear polyamides (less cross-linked)have a higher ratio of C d O to amide II functionality,but a lower C/N ratio.On this point the FTIR (Figure 2)and XPS data (Figure 4b)were consistent;both indicated that TFN membranes were less cross-linked than TFC mem-branes.However,there was no correlation between water or salt permeability and extent of cross-linking (correlation coefficient 0.41and -0.01,respectively).Membrane lectiv-ity (A /B )was moderately inverly correlated with RMS surface roughness (correlation coefficient -0.87),which for TFN membranes is largely a function of nanoparticle content;hence,molecular-sieving may have played a major role
in
FIGURE 2.ATR-FTIR spectra from 4000to 2600cm
-
1for (a)TFC and (b)TFN membranes post-treated by wet (1)and dry (2)curing followed by NaOCl -Na 2S 2O 5-water (A)and NaHCO 3(B)rins.
FIGURE 3.Calculated values of (a)water and (b)salt permeability coefficients for the commercial awater RO membrane and hand-cast TFC and TFN membranes post-treated by wet (1)and dry (2)curing followed by NaOCl -Na 2S 2O 5-water (A)and NaHCO 3(B)rins.
FIGURE 4.Calculated values of (a)roughness corrected solid -liquid interfacial free energy and (b)the fractional extent of cross-linking in the polyamide films for the commercial awater RO membrane and hand-cast TFC and TFN mem-branes post-treated by wet (1)and dry (2)curing followed by NaOCl -Na 2S 2O 5-water (A)and NaHCO 3(B)rins.
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E
membrane paration performance despite the very low zeolite loadings.
The continued evidence supporting the role of molecular-sieving is important becau it implies(although does not prove)that thinfilm nanocomposite membranes made with ,impermeable)nanoparticles might only exhibit different paration performance through defect formation.This is a hypothesis that should be tested in the future.More importantly,herein TFN membranes exhibiting commercially relevant awater RO paration performance were demonstrated for thefirst time.The ability to fabricate nanocomposite membranes with very high salt lectivity (>99.4%rejection of32g/L NaCl)opens up new possibilities for tailoring the performance of awater RO membrane materials and process.
Acknowledgments
besides的用法Financial support for this rearch was provided in part by the U.S,Environmental Protection Agency(Award87888.01) and the UCLA California NanoSystems Institute.We are grateful to the Molecular Materials Rearch Center of the Beckman Institute at the California Institute of Technology for providing access to the XPS instrument.We are very grateful to Prof.Bruce Dunn in the UCLA Department of Material Science&Engineering for his assistance in providing access to the ATR-FTIR instrument.
Literature Cited
(1)Perlman,H.U.S.Department of the Interior/U.S.Geological
Survey Water Q&A:Water u.ga.v/edu/ qausage.html#HDR1(accesd April21,2010).
(2)Tin,P.S.;Chung,T.S.;Jiang,L.Y.;Kulprathipanja,S.Carbon-
zeolite composite membranes for gas paration.Carbon2005, 43,2025–2027.
(3)Zimmerman,C.M.;Singh,A.;Koros,W.J.Tailoring mixed matrix
composite membranes for gas parations.J.Membr.Sci.1997, 137(1-2),145–154.
(4)Okumus,E.;Gurkan,T.;Yilmaz,L.Development of a mixed-
初中化学实验基本操作matrix membrane for pervaporation.Sep.Sci.Technol.1994, 29,2451–2473.
(5)Vu,D.Q.;Koros,W.J.;Miller,S.J.Mixed matrix membranes
using carbon molecular sieves-I.Preparation and experimental results.J.Membr.Sci.2003,211(2),311–334.
(6)Chang,H.Y.;Lin,C.W.Proton conducting membranes bad
on PEG/SiO2nanocomposites for direct methanol fuel cells.J.
Membr.Sci.2003,218(1-2),295–306.
(7)Kwak,S.Y.;Kim,S.H.;Kim,S.S.Hybrid organic/inorganic
rever osmosis(RO)membrane for bactericidal anti-fouling.
1.Preparation and characterization of TiO2nanoparticle lf-
asmbled aromatic polyamide thin-film-composite(TFC) membrane.Environ.Sci.Technol.2001,35,2388–2394. (8)Lee,S.Y.;Kim,H.J.;Patel,R.;Im,S.J.;Kim,J.H.;Min,B.R.
Silver nanoparticles immobilized on thinfilm composite polyamide membrane:Characterization,nanofiltration,anti-fouling properties.Polym.Adv.Technol.2007,18,562–568.
(9)Lee,H.S.;Im,S.J.;Kim,J.H.;Kim,H.J.;Kim,J.P.;Min,B.R.
Polyamide thin-film nanofiltration membranes containing TiO2 nanoparticles.Desalination2008,219(1-3),48–56.
(10)Singh,P.S.;Aswal,V.K.Characterization of physical structure
of silica nanoparticles encapsulated in polymeric structure
of polyamidefilms.J.Colloid Interface Sci.2008,326(1),176–185.
(11)Jadav,G.L.;Singh,P.S.Synthesis of novel silica-polyamide
nanocomposite membrane with enhanced properties.J.Membr.
Sci.2009,328(1-2),257–267.
(12)Jeong,B.H.;Hoek,E.M.V.;Yan,Y.S.;Subramani,A.;Huang,
上海英语口语学习X.F.;Hurwitz,G.;Ghosh,A.K.;Jawor,A.Interfacial polymer-ization of thinfilm nanocomposites:A new concept for rever osmosis membranes.J.Membr.Sci.2007,294(1-2),1–7. (13)Lind,M.L.;Ghosh,A.K.;Jawor,A.;Huang,X.F.;Hou,W.;Yang,
Y.;Hoek,E.M.V.Influence of zeolite crystal size on zeolite-polyamide thinfilm nanocomposite membranes.Langmuir 2009,25,10139–10145.
性格内向怎么办(14)Lind,M.L.;Jeong,B.H.;Subramani,A.;Huang,X.F.;Hoek,
E.M.V.Effect of mobile cation on zeolite-polyamide thinfilm
nanocomposite membranes.J.Mater.Res.2009,24,1624–1631.
(15)Hiroki,T.;Sugita,K.;Oto,K.Composite mipermeable
membrane,and production process thereof.U.S.Patent 7,279,097,2007.
(16)Kwon,Y.N.;Tang,C.Y.;Leckie,J.O.Change of chemical
composition and hydrogen bonding behavior due to chlorina-tion of crosslinked polyamide membranes.J.Appl.Polym.Sci.
2008,108,2061–2066.
(17)Jin,X.;Huang,X.F.;Hoek,E.M.V.Role of Specific Ion
Interactions in Seawater RO Membrane Fouling by Alginic Acid.
Environ.Sci.Technol.2009,43,3580–3587.
(18)Crittenden,J.;Trusll,R.R.;Hand, D.W.;Howe,K.J.;
Tchobanoglous,G.Water Treatment Principles and Design,2nd ed.;John Wiley&Sons,Inc.:Hoboken,NJ,2005;pp1447-1448.
(19)Hoek,E.M.V.;Kim,A.S.;Elimelech,M.Influence of crossflow
membranefilter geometry and shear rate on colloidal fouling in rever osmosis and nanofiltration parations.Environ.Eng.
Sci.2002,19,357–372.
(20)Bansal,A.;Li,X.L.;Yi,S.I.;Weinberg,W.H.;Lewis,N.S.
Spectroscopic studies of the modification of crystalline Si(111) surfaces with covalently-attached alkyl chains using a chlorina-tion/alkylation method.J.Phys.Chem.B2001,105,10266–10277.
(21)Tang,C.Y.Y.;Kwon,Y.N.;Leckie,J.O.Probing the nano-and
micro-scales of rever osmosis membranes-A comprehensive characterization of physiochemical properties of uncoated and coated membranes by XPS,TEM,ATR-FTIR,and streaming potential measurements.J.Membr.Sci.2007,287(1),146–156.
(22)Wenzel,R.N.Surface roughness and contact angle.J.Phys.
Colloid Chem.1949,53,1466–1467.
(23)Urban,M.W.Vibrational Spectroscopy of Molecules and
Macromolecules on Surfaces;John Wiley&Sons,Inc.:New York, 1993.
(24)Kyotani,T.;Shimotsuma,N.;Kakui,S.FTIR-ATR study of the
surface of a tubular zeolite NaA membrane ultrasonically reacted with water and acetic acid.Anal.Sci.2006,22,325–327. (25)Alfaro,S.;Rodriguez,C.;Valenzuela,M.A.;Bosch,P.Aging time
effect on the synthesis of small crystal LTA zeolites in the abnce of organic template.Mater.Lett.20
07,61,4655–4658. (26)Breck,D.W.Zeolite Molecular Sieves;John Wiley&Sons:New
York,1974;pp415.
(27)Singh,P.S.;Joshi,S.V.;Trivedi,J.J.;Devmurari,C.V.;Rao,A.P.;
Ghosh,P.K.Probing the structural variations of thinfilm composite RO membranes obtained by coating polyamide over polysulfone membranes of different pore dimensions.J.Membr.
Sci.2006,278(1-2),19–25.
(28)Clarizia,G.;Algieri,C.;Regina,A.;Drioli,E.Zeolite-bad
composite PEEK-WC membranes:Gas transport and surface properties.Microporous Mesoporous Mater.2008,115(1-2), 67–74.
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