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Removal of Dyes, Sugars, and Amino Acids from NaCl Solutions Using Multilayer Polyelectrolyte Nanofiltration Membranes Seong Uk Hong,† Matthew D. Miller,‡ and Merlin L. Bruening*,‡ Department of Chemical Engineering, Hanbat National UniVersity, Daejeon 305-719, South Korea, and Department of Chemistry, Michigan State UniVersity, East Lansing, Michigan 48824
Several recent studies demonstrated highly selective ion and neutral molecule transport through multilayer polyelectrolyte nanofiltration (NF) membranes. This work examines the potential of such membranes in the selective removal of dyes, sucrose, and amino acids from NaCl solutions. Remarkably, simple deposition of 4.5-bilayer poly(styrenesulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) films on porous alumina supports yields membranes that exhibit NaCl/sucrose selectivities of ∼130 and NaCl/dye selectivities >2200. These high selectivities stem from rejections of 99.4% for sucrose and >99.9% for reactive dyes, along with small (∼20%) rejections of NaCl. Moreover, the solution flux (>1.7 m3/m2‚day at 4.8 bar) through these membranes is comparable to or greater than that reported for commercial membranes. In contrast to results with sucrose and reactive dyes, the NaCl/glutamine selectivity of a [PSS/PAH]7 membrane is only 3.7 because of the relatively small size of glutamine. With increasing NaCl concentrations (up to 0.5 M) in feed solutions, solution fluxes decrease due to an increased osmotic pressure drop across the membrane, but this flux decrease is minimized by low rejections of NaCl. Both salt passage and rejections of organic molecules are not strong functions of NaCl concentration. Introduction Nanofiltration (NF) is a pressure-driven, membrane-based separation technique that is similar to reverse osmosis (RO), but NF membranes have relatively high permeabilities that allow low operating pressures.1,2 The molecular weight cutoff (MWCO) of NF membranes ranges from 200 to 1000 Da, which is between the MWCOs typical for RO and ultrafiltration (UF). In general, RO membranes reject nearly all solutes and desalinate water, while UF membranes reject proteins and allow permeation of salts and low molecular weight components. In contrast, NF membranes allow removal of small molecules without complete desalination and are thus becoming important tools for recovering small molecules or removing pollutants from effluent streams. One potential application of NF studied in this research is the purification of effluent streams from textile plants.3-7 In some dyeing processes almost 50% of the color of reactive dyes is discharged into the effluent due to the poor fixation characteristics of the dyes, and disposal of these streams in public waterways is not acceptable.5 NF may provide an economical method for both recovering the salt from dye solutions and decreasing the volume of contaminated water. This technique could also prove useful in the sugar industry, where ion-exchange resins are used to remove high molecular weight (5000-20 000 Da) colorants such as melanins, melanoidines, caramels, and polyphenols from cane sugar liquor.8 The colorants are first adsorbed by ion exchange and finally released from the exhausted resin into an alkaline NaCl solution. NF could be employed either to remove the colorants or to recycle the alkaline NaCl solution, whereas UF would not reject the colorants, and RO would not be capable of separating the NaCl from the colorants.9
NF membranes usually contain a selective skin layer on a porous support that provides the mechanical strength of the system.1,2 Composite membranes are especially attractive because only a small amount of a potentially expensive skin material is required. A number of recent studies showed that alternating adsorption of polycations and polyanions on highly permeable supports allows the formation of an ultrathin skin layer with tailorable properties.10-18 This layer-by-layer technique affords control over film thickness through variation of the number of adsorbed layers and allows the formation of defect-free skins with thicknesses less than 50 nm.16,19 In addition, a wide range of polyelectrolytes are capable of forming multilayer films, and careful selection of constituent polyelectrolytes allows tailoring of flux, selectivity, and potentially fouling rates.20-22 This work examines the potential of a polyelectrolyte NF membrane for isolation of dyes, sugars, and amino acids from NaCl solutions. Owing to the industrial importance of these separations, several researchers studied the performance of commercial membranes in this area.6-9,23-32 Although dye and sugar rejections are often high (>96%),7,8,24-29 in some cases sodium rejection is 60% or more, limiting the utility of such membrane separations.24,25 The polyelectrolyte membranes examined here show dye rejections >99.9%, which is considerably higher than rejections reported for any commercial membranes.6,7,25-32 Moreover, the polyelectrolyte films simultaneously allow ∼80% salt passage. Sucrose rejection is also >99%, and flux through the polyelectrolyte films is comparable to that through the best commercial NF systems.33 Hence these polyelectrolyte membranes have the potential to provide significant performance improvements relative to current separation techniques. Experimental Section
* To whom correspondence should be addressed. Tel.: (517) 3559715, ext 237. Fax: (517) 353-1793. E-mail:
[email protected]. † Hanbat National University. ‡ Michigan State University.
Materials. Poly(styrenesulfonate) (PSS, MW ) 70 000), poly(allylamine hydrochloride) (PAH, MW ) 70 000), sucrose (ACS reagent), reactive blue 4 (35%), reactive black 5 (55%),
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reactive orange 16 (50%), and L-glutamine (98%) were used as received from Aldrich. MnCl2 (Alfa Aesar), NaBr (Jade Scientific), and NaCl (Spectrum, ACS reagent) were also used as received. The dyes contain additives such as buffers and salts, but these additional materials, which are sometimes proprietary, were not available from the manufacturer. The porous alumina supports (0.02-µm Whatman Anodisk filters) were UV/O3 cleaned with the feed side up (Boekel UV-Clean Model 135500) for 15 min before film deposition. Deionized water (Milli-Q, 18.2 MΩ cm) was used for membrane rinsing and preparation of the polyelectrolyte solutions. The pH of polyelectrolyte solutions was adjusted with dilute NaOH or HCl. Film Deposition. A UV/O3-cleaned porous alumina support was first placed in an O-ring holder so that only the feed side of the support would be exposed to the polyelectrolyte solutions. Polyelectrolyte deposition started with immersion of the alumina support in an aqueous solution containing 0.02 M PSS in 0.5 M MnCl2 (pH 2.1) for 2 min (concentrations of polyelectrolytes are always given with respect to the repeating unit). The alumina support was then rinsed with deionized water for 1 min before immersion in 0.02 M PAH in 0.5 M NaBr (pH 2.3) for 5 min and another 1-min water rinse. Additional bilayers were deposited similarly until the target number of bilayers was produced. MnCl2 and NaBr were used as supporting electrolytes to increase layer thickness and to compare these systems with previous membranes.16,17 To enhance the surface charge, [PSS/ PAH]4PSS films were terminated with a layer of PSS that was deposited from 2.5 M MnCl2. Transport Studies. Nanofiltration experiments were performed using a home-built cross-flow apparatus that was described previously.16 The system was pressurized with Ar at 4.8 bar, and a centrifugal pump circulated the analyte solution through the system and across the membrane. The concentrations of sucrose and glutamine in feed solutions were 0.001 M, while those of reactive dyes were nominally 1 g/L. (Because the dyes contain a large amount of additives, the actual dye concentration may be as low as 0.35 g/L.) The pH values of NF solutions containing sucrose, glutamine, reactive blue 4, reactive orange 16, and reactive black 5 were 6.3, 6.5, 4.3, 5.8, and 6.3, respectively. Because PSS is a strong polyelectrolyte and PAH should be highly protonated below pH 7, we do not expect to see a strong change in NF performance due to small variations in pH, but there could certainly be a minor effect. To minimize concentration polarization, the flow rate across the membrane was kept at 18 mL/min, which is ∼100 times the permeate flow rate. The Reynolds number for flow across the membrane was about 50, and experiments published elsewhere showed no significant change in NF results with 2- to 4-fold changes in the flow rate across the membrane.16,34 The membrane area exposed to the feed solution was 1.5 cm2. After an 18-h equilibration time, four permeate samples were collected for 30 min using a graduated cylinder. Glutamine was analyzed by liquid chromatography (Dionex, DX-600, AminoPac PA-10 column, water (76%) and 250 mM NaOH (24%) as eluent) coupled with integrated amperometric detection (Dionex, ED50). For sucrose analysis, a CarboPac PA-10 column with 100 mM NaOH as the eluent was used. In determination of NaCl concentrations, ion chromatography with an Ionpac AS14A column was used for glutamine/NaCl solutions and dye/salt feed solutions, while conductivity measurements (Orion 115 conductivity meter) were employed for dye/NaCl permeates and sucrose/NaCl solutions. In two cases, we analyzed both permeate and feed Cl- concentrations in dye/NaCl separations using ion chromatography, and the Cl- rejection was similar to that
Figure 1. Structures of reactive dyes used in this study. Table 1. Molecular Weights (MW) and Stokes’ Radii (rs) of the Organic Molecules Employed in Transport Studies solute L-glutamine
sucrose reactive orange 16 reactive blue 4 reactive black 5
MW (g/mol)
rs (nm)
146 342 618 637 992
0.28 0.47
determined when conductivity was used to analyze the permeate. Analysis of the permeate from NF of both reactive blue 4 and reactive black 5 solutions showed Cl- to be the predominant anion (∼98%), so permeate conductivity measurements allowed effective calculation of rejections even though the dyes contained added NaCl and buffer. Dye rejections were determined by UV absorption using a Perkin-Elmer, Lambda 40 spectrophotometer. Results and Discussion To investigate the ability of polyelectrolyte NF membranes to reject organic molecules while passing NaCl, we performed NF with NaCl solutions containing a sugar (sucrose), an amino acid (glutamine), and reactive dyes (reactive orange 16, reactive blue 4, and reactive black 5). As shown in Table 1, these molecules present a wide range of molecular weights and sizes,35,36 and the results below demonstrate that solute rejection is a strong function of molecular weight, as would be expected. Dye/NaCl Separations. Reactive dyes, such as those shown in Figure 1, are soluble anionic molecules that contain one or more reactive groups capable of forming a covalent bond with the hydroxyl groups of a fiber. Unfortunately, these molecules have low fixation rates and thus have higher effluent concentrations than other types of dyes. In fact, almost 50% of their color
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sufficient that its concentration varied only slightly during the experiment.
(
)
(1)
CA,perm CB,feed 100 - RA ) CA,feed CB,perm 100 - RB
(2)
R) 1R)
Figure 2. Field-emission scanning electron micrograph of a cross section of a porous alumina support coated with a [PSS/PAH]4PSS film. The film was sputter-coated with 5 nm of Au prior to imaging.
remains in the effluent,5 so dye baths produce large amounts of wastewater that should not be discharged directly into the environment.29 Dye effluents also frequently contain high concentrations of NaCl, which is present to aid the dye fixation. When using NF to remove the dye from these solutions, therefore, a low NaCl rejection is required to both minimize the osmotic pressure that must be overcome in the separation and possibly recover the NaCl. We previously showed that four to five layer pairs of PSS/ PAH are required for formation of defect-free membranes on porous alumina supports with 0.02-µm-diameter surface pores.16,19 Moreover, the use of [PSS/PAH]4PSS films affords fluxes that are comparable to or higher than those of commercial NF membranes,16,33 so in this study [PSS/PAH]4PSS films were used to remove reactive dyes from NaCl solutions. (The top PSS layer was deposited from a solution containing 2.5 M MnCl2 to increase surface charge.) Films with fewer layers are defective in ion separations, while membranes with more layers exhibit lower flux.19 Figure 2 shows an electron micrograph of a typical membrane and demonstrates that the polyelectrolyte film covers pores without filling them. The thickness of [PSS/PAH]4PSS films as determined previously by ellipsometry on Au surfaces is ∼20 nm,16 which is consistent with the micrograph. Table 2 contains percent rejection values, selectivities, and solution fluxes from NF experiments with [PSS/PAH]4PSS polyelectrolyte membranes and a feed solution containing 1 g/L reactive dye and 0.01 M NaCl. Percent rejection, R, is defined by eq 1, where Cperm and Cfeed are the solute concentrations in the permeate and feed, respectively. The selectivity, R, for solute A over solute B is defined by eq 2, which can conveniently be expressed in terms of rejections as shown. Percent rejection and selectivity were determined after 18 h of filtration to achieve steady-state permeate concentrations, and the feed volume was
Cperm × 100% Cfeed
The data in Table 2 indicate that the [PSS/PAH]4PSS films allow passage of 60-85% of the NaCl while strongly retaining the dye molecules. Although the molecular weights of the dyes ranged from 618 to 992 Da, the rejection of all of these molecules was very high, presumably because all dyes were larger than the film pores. Dye/NaCl selectivities exhibited large uncertainties that resulted from the very low concentrations of dye in the permeate. Still, the selectivities are extremely high (in the thousands), reflecting both the high passage of NaCl and the high rejection of the dyes. We are not sure why the rejection of NaCl in the presence of reactive blue 4 was significantly higher than with the other dyes, but it could be due to the slightly lower pH (4.3) of these solutions. These dyes often contain proprietary additives to control pH and enhance the dyeing process. Owing to the industrial importance of dye/salt separations, several researchers studied the performance of commercial membranes in this area. Using DS5 DK nanofiltration membranes, Koyuncu achieved 98.7% rejection of reactive black 5 (1 g/L) from 0.017 M NaCl solutions with a flux of 0.98 m3/ m2‚day at 8 bar.26 Similarly, in the separation of reactive orange 16 (1 g/L) from 0.017 M NaCl solutions, Koyuncu reported 99.6% dye rejection with a flux of 0.84 m3/m2‚day at 8 bar, but no salt rejection was given.26 Even with a lower applied pressure, the polyelectrolyte membranes described here exhibited >3fold lower dye passage (higher dye rejections) along with 2-fold higher fluxes (Table 2). Tang and Chen also investigated dye removal from NaCl solutions using a flat sheet polysulfonebased thin film composite (TFC-SR2) nanofilter from Fluid Systems.28 For the separation of reactive black 5 (0.45 g/L) from 0.34 M NaCl solutions, they reported 98.1% dye rejection and 11% NaCl rejection with a flux of 1.34 m3/m2‚day at 5 bar. Although the NaCl rejection and flux are comparable to the results in Table 2, percent passage (100 - percent rejection) of the dye is still more than an order of magnitude lower with the polyelectrolyte membranes. Other studies of commercial membranes in dye/salt separations showed even lower dye rejections and fluxes along with relatively high salt rejections.25,29-32 The remarkable dye/salt selectivities, high fluxes, and low salt rejections of multilayer polyelectrolyte NF membranes make them very attractive for dye removal and salt recovery. We should note, however, that in the utilization of polyelectrolyte NF membranes for practical applications, several issues such as development of inexpensive supports and examination of membrane stability and fouling are still being addressed. One recent study showed that it is possible to prepare high-flux
Table 2. Rejections, Solution Fluxes, and Selectivities from NF Experiments with [PSS/PAH]4PSS-Coated Alumina Membranes and Solutions Containing Reactive Dyes (1 g/L) or Sucrose (0.001 M), and 0.01 M NaCla organic molecules
flux (m3/m2‚day)
NaCl rejection (%)
organic molecule rejection (%)
NaCl/organic molecule selectivity
sucrose reactive orange 16 reactive blue 4 reactive black 5
2.2 ( 0.2 1.7 ( 0.1 2.0 ( 0.2 2.4 ( 0.2
29 ( 3 19 ( 3 40 ( 4 15 ( 3
99.4 ( 0.1 99.96 ( 0.02 99.98 ( 0.01 99.92 ( 0.07
130 ( 30 2700 ( 1200 2900 ( 1100 2200 ( 1500
a
NF was performed at 4.8 bar.
Ind. Eng. Chem. Res., Vol. 45, No. 18, 2006 6287 Table 3. Effect of NaCl Concentration on Rejections, Solution Fluxes, and Selectivities from NF Experiments with [PSS/PAH]4PSS-coated Alumina Membranes and Solutions Containing 1 g/L Reactive Blue 4 and NaCla
a
NaCl concn (M)
flux (m3/m2‚day)
NaCl rejection (%)
dye rejection (%)
NaCl/dye selectivity
0.01 0.05 0.1 0.5
2.00 ( 0.20 1.50 ( 0.10 0.98 ( 0.07 0.63 ( 0.08
40 ( 4 23 ( 1 19 ( 2 20 ( 2
99.98 ( 0.01 99.96 ( 0.01 99.95 ( 0.01 99.86 ( 0.06
2900 ( 1100 2000 ( 300 1700 ( 300 600 ( 200
NF was performed at a pressure of 4.8 bar.
Table 4. Effect of NaCl Concentration on Rejections, Solution Fluxes, and Selectivities from NF Experiments with [PSS/PAH]7-coated Alumina Membranes and Solutions Containing Glutamine and NaCla
a
NaCl concn (M)
flux (m3/m2‚day)
NaCl rejection (%)
glutamine rejection (%)
NaCl/glutamine selectivity
0.002 0.01 0.05 0.1
1.35 ( 0.03 1.22 ( 0.03 1.21 ( 0.04 1.02 ( 0.03
59 ( 1 30 ( 3 27 ( 5 25 ( 1
77 ( 1 81 ( 4 74 ( 2 76 ( 3
1.8 ( 0.2 3.7 ( 0.6 2.8 ( 0.2 3.1 ( 0.4
NF was performed at a pressure of 4.8 bar.
polyelectrolyte membranes on ultrafiltration supports, but optimization of the porosity and morphology of polymeric supports may be necessary to achieve the flux available with porous alumina substrates.37 Sucrose/NaCl Separations. We previously reported the separation of sucrose from NaCl using PSS/poly(diallyldimethylammonium chloride) (PDADMAC) membranes.18 In that case, the best membranes showed 94% rejection of sucrose and 20% rejection of NaCl. Wang et al. also studied the removal of sucrose (0.2 g/L) from 0.01 M NaCl solutions. They reported ∼92% rejection of sucrose and 20% rejection of NaCl with a flux of 0.6 m3/m2‚day at 4 bar.23 As shown in Table 2, [PSS/ PAH]4PSS membranes exhibit a 99.4% rejection of sucrose along with a 29% rejection of NaCl to give a sucrose/NaCl selectivity of 130. Again, flux through these membranes is very high, making them attractive candidates for practical separations. The high rejection of sucrose relative to our previous PSS/ PDADMAC system is expected since PSS/PAH films swell less in water than do PSS/PDADMAC films.38,39 Additionally, the high rejection of sucrose, which is a neutral molecule, strongly suggests that the rejection of the dye molecules is based on size rather than charge. Effect of NaCl Concentration on Reactive Blue 4/NaCl Separations. Table 3 shows how solution flux, reactive blue 4 rejection, and NaCl rejection vary with the concentration of NaCl in solution. The dye rejection decreased slightly at higher ionic strengths, whereas NaCl rejection decreased from 40% to 20% with increasing NaCl concentration. Solution flux declined with increasing NaCl concentration, as would be expected because of an increasing osmotic pressure drop across the membrane. On going from 0.01 M NaCl to 0.5 M NaCl, the osmotic pressure drop across the membrane increased from 0.1 to 2.3 bar (taking into account the sucrose > glutamine. High rejections of the organic molecules are maintained at high salt concentrations, but flux decreases due to the osmotic pressure drop across the membrane. Still, at a concentration of 0.5 M NaCl, flux across the membranes is 0.6 m3/m2‚day at an applied pressure of 4.8 bar because of low NaCl rejection. The remarkable flux, organic molecule/NaCl selectivity, and high NaCl recoveries possible with these polyelectrolyte NF membranes make them attractive for recovery of small molecules from salt solutions. Acknowledgment We thank the Department of Energy Office of Basic Energy Sciences and Hanbat National University (S.U.H.) for financial support.
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ReceiVed for reView February 25, 2006 ReVised manuscript receiVed May 23, 2006 Accepted June 19, 2006 IE060239+
