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Biopolymer-based nanosystem for ferric ion removal from water
Separation and Purification Technology 112 (2013) 26–33
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Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Biopolymer-based nanosystem for ferric ion removal from water }thi b, Nóra Harmati b, Zsuzsanna Csikós a, John F. Hartmann c, Magdolna Bodnár a, István Hajdu a, Eszter Ro d d Csaba Balogh , Béla Kelemen , János Tamás b, János Borbély a,e,⇑ a
BBS Nanotechnology Ltd., H-4225 Debrecen, P.O.B. 12, Hungary Department of Water and Environmental Management, University of Debrecen, H-4010 Debrecen, Hungary c ElizaNor Polymer LLC, Princeton Junction, 08550 NJ, USA d Environmental Inspectorate, H-4025 Debrecen, Hungary e Department of Radiology, Medical and Health Science Center, University of Debrecen, H-4032 Debrecen, Hungary b
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 24 March 2013 Accepted 25 March 2013 Available online 2 April 2013 Keywords: Poly-gamma-glutamic acid Ferric ion Removal Ultrafiltration Micro-irrigation
a b s t r a c t The removal of ferric ions from aqueous solutions by a nanoparticle-enhanced ultrafiltration technique was investigated. Biodegradable poly-gamma-glutamic acid (c-PGA), a linear biopolymer, and its cross-linked nanoparticles were used to complex the metal ions by forming nanosized spherical particles with more or less deformability. These polymer–metal ion particles were then removed by membrane separation. Two ultrafiltration techniques were studied with the aim of developing a nanoparticleenhanced separation process for the efficient removal of ferric ions from model solutions. The influence of parameters such as the feed c-PGA and Fe3+ concentrations, their proportions and the extent of crosslinking of c-PGA were studied. It was observed that c-PGA could bind and remove ferric ions to produce water with low Fe3+ concentration. These studies show that optimal nano-membrane separation technology can be performed, depending on the environment and quality of treatable water by the combination of the filtration parameters and type of membrane techniques shown here. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Presence of iron in drinking water and water supplies can cause problems. Even at relatively low concentrations, it can be occur as aesthetic and functional problems. Iron can give water a metallic taste, reddish color, turbidity and odor [1,2]. The presence of iron can make the water unusable for food and beverage [3]. In addition, it can precipitate and make clogs for softeners or irrigation systems. Nowadays in agriculture, the water-saving techniques have growing interest, and more attention is given to micro-irrigation systems. Micro-irrigation is a term which describes a family of irrigation systems. It provides irrigation water slowly, directly to the plants. The water discharge patterns are different depending upon specific applications. There is a common property that the average discharge rate per emitter is usually only between 1 L/h and 4 L/h, which is given off dripped to the plants’ root zone through the water distributor system using 2.5 bar pressure as maximum [4– 6]. Micro-irrigation is getting more attention and playing an important role in modern agriculture, due to its many advantages, ⇑ Corresponding author. Address: H-4032 Debrecen, Kiserdo 4, Hungary. Tel./fax: +36 52 541 742. E-mail address: [email protected] (J. Borbély). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.03.043
such as high water application efficiency, ability to irrigate plants independently of the soil type or broken ground, which minimizes soil erosion or energy cost. Micro-irrigation components include flow control equipment, pipes, tubes, emitters, and several accessories. Clogging of these micro-systems is recognized as one of the most important concerns for micro-irrigation. Clogging can be categorized into three major groups, namely, physical [7], biological [8], and chemical clogging [9]. High concentration of metal ions, such as ferric and manganese ions present in irrigation water, as well as calcium and magnesium ions can cause clogging by their precipitation from the water. High ferric ion concentration (c = 2 mg/L) of irrigation water does not disease for the plants, but filter and manifold systems of micro-irrigation equipment can be obstructive because of the precipitation of iron-hydroxide. Therefore, for the micro-irrigation, the water must be treated and remove the ferric ions in order to produce a water with low ferric ion concentration (c = 0.1 mg/L) [10]. Several methods are known for removal of ferric ions from aqueous media, however nowadays there is an increasing interest to find environmentally friendly methods for desalination of water. For removal of metal ions, several natural polymers including chitosan [11,12], cellulose [13,14], alginic acid [15,16] or other natural biopolymers [17,18] have been investigated. The most valuable properties of these biopolymers are their biocompatibility, biodegradability, and flocculating activity for metal ions [19,20].
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Flocculation is a process that enhance agglomeration or collection of smaller floc particles into larger, more easily sedimentable particles through gentle stirring by hydraulic or mechanical means. The addition of flocculating agents may promote the formation of flocs. Flocculating agents that are usually used for water treatment can be categorized into three major groups, namely, inorganic flocculants [21], organic synthetic polymer flocculants [22,23], hybrid flocculants [24,25] and natural biopolymer flocculants [26– 28]. Recently, biodegradable flocculants have been investigated to minimize the risk for the environment and humans. Flocculation of polyelectrolytes in the presence of bivalent ions is an important process, and is widely used in water treatment technologies. For separation of metal ions, natural c-PGA [29,30] and other natural polymers [15,31] have been investigated. Poly-c-glutamic acid is a water soluble, biodegradable, edible and nontoxic polyanion [32–34]. Therefore, c-PGA and its derivatives have been employed extensively in a variety of commercial applications such as cosmetics, food, medicine, and water treatment [35–38]. c-PGA has appropriate flocculating activity and binds metal ions with high affinity. Therefore, this biopolymer may be useful for water treatment. Optimal pH range, temperature, concentrations and other main factors were determined in different reaction conditions for improving the flocculation activity of c-PGA. In our previous work, formation of complexes of c-PGA with Pb2+ was studied [39]. The size and solubility of these nanoparticles in the dried and swollen states were discussed, and the factors determining the physico-chemical properties of nanoparticles were described. The removal of toxic lead ions from aqueous solution by a combined nano-membrane separation technique was also investigated [40]. It was found that c-PGA could bind and remove more than 99.8% of the lead ions from water through a convenient, low-pressure ultrafiltration technique, resulting in a permeate that satisfied the standard for drinking water recommended by the WHO. The present investigation focuses on the removal of Fe3+ from an aqueous environment by combined nano-membrane technology. Biodegradable linear c-PGA and its cross-linked nanoparticles were used to make complex and remove Fe3+, and different ultrafiltration techniques were compared with the aim of efficient Fe3+ removal with biodegradable systems. The permeate flux and efficiency of Fe3+ removal were studied as a function of the FeCl3 6H2O and c-PGA concentrations, their proportions, the crosslinking of c-PGA and type of membrane separation techniques. 2. Materials and methods 2.1. Reagents
c-PGA (Mw = 400 kDa) was purchased from Vedan Group, Taiwan. Iron (III) chloride hexahydrate was purchased from Sigma–Aldrich Co., Hungary. 2,20 -(Ethylenedioxy)bis(ethylamine) was used as cross-linking agent and 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide methiodide as coupling agent in the cross-linking process to produce c-PGA nanoparticles. All solutions were prepared with deionized water from a Milli-Q Gradient system (conductivity <0.05 lS/cm).
to pH 5.5 with 0.1 M hydrochloric acid solution. The diamine solution was added to the c-PGA solution and mixed for 30 min at room temperature. After the dropwise addition of carbodiimide solution, the reaction mixture was stirred at 4 °C for 4 h and at room temperature for 20 h. The solution containing cross-linked c-PGA nanoparticles was purified by dialysis for 7 days against distilled water and freeze-dried. 2.3. Binding measurements
c-PGA linear biopolymer and cross-linked c-PGA nanoparticle solutions were used for the removal of Fe3+ from aqueous solution. c-PGA solutions (c = 10 mg/mL, 2 mg/mL, 0.2 mg/mL), crosslinked c-PGA nanoparticle solutions (c = 10 mg/mL) and FeCl3 6H2O solutions (c = 1 mg/mL, 0.1 mg/mL, 0.01 mg/mL) were prepared to study the binding affinity. The c-PGA solution (V = 0, 2, 4, 8, 12 or 16 mL) was added dropwise to the FeCl3 6H2O solution (V = 100 mL, pH = 2.6). The reaction mixture was stirred for 30 min at room temperature. The pH of the reaction mixture was adjusted to 9.0 to facilitate the precipitation of Fe(OH)3 based on the residual free ferric ions. Reaction mixture was stirred, and after that the pH of the reaction mixture was adjusted to 7.0 for the membrane separation. These combined nano-membrane techniques could be effective for water treatment in one step membrane filtration at appropriate reaction conditions, but could require a two-step treatment including centrifugation and membrane separation at not designed reaction parameters. The concentration and quantity of biopolymer have to be calculated according to the ferric ion concentration. Table 1 summarizes the reaction conditions. 2.4. Characterization 2.4.1. Transmission electron microscopy (TEM) A JEOL2000 FX-II transmission electron microscope was used to characterize the size and morphology of the dried nanoparticles. For TEM observation, the nanoparticles were prepared from the aqueous media containing c-PGA–Fe3+ complexes at a c-PGA concentration of 0.30 mg/mL. Samples for TEM analysis were obtained by placing a drop of colloid dispersion containing the nanoparticles onto a carbon-coated copper grid. It was dried at room temperature and then examined by TEM without any further modification or coating. 2.4.2. Laser light scattering The hydrodynamic diameters of c-PGA complexed with Fe3+ were assessed by using a Zetasizer Nano-ZS instrument (Malvern Instruments Ltd., Worcestershire, UK), at an operating wavelength of 633 nm. The size distribution and the Z-average size of the nanoparticles were measured at 25 °C with an angle detection of 173° in polystyrol cuvettes. The samples were prepared from the reaction mixture, each sample was measured three times and average serial data were calculated. Table 1 Reaction conditions for the formation of c-PGA nanoparticles by ferric ion complexation. Concentration of FeCl3 6H2O solution (mg/mL)
Concentration of
c-PGA (mg/mL) Linear
2.2. Modification Cross-linked c-PGA nanoparticles were prepared by a process described earlier [41]. Briefly, c-PGA was dissolved in water to produce a solution with a concentration of 1 mg/mL. The diamine cross-linker was dissolved in water and the solution was adjusted
1 1 0.1 0.01
Crosslinked 10%
10 10 2 0.2
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2.4.3. Quantitative analysis of iron by atomic absorption spectrophotometry (AAS) An Thermo Unicam Solar 969 model flame atomic absorption spectrometer equipped with deuterium lamp background correction, hollow cathode lamps and air acetylene burner were used for the determination of the ferric ions. The calibration was performed until 5 mg/L. All absorption measurements were performed under the following conditions: wavelengths 248.3 nm, bandpass 0.5 nm, integration time 4 s. Measurements were made using transient height injection mode and three replicate injections for each sample. 2.5. Membrane separation The filters used with the Pellicon XL device, all made of regenerated cellulose, had a cut-off of 10 kD with a membrane area of 50 cm2. The LabScale system has a built in a Cole Parmer peristaltic pump with Sigma–Aldrich PVC laboratory tubing, and it was used to perform the cross-flow membrane technique in the Pellicon XL set-up. The transmembrane pressure during the filtration was controlled and it was 30 psi, monitored by using glycerin-filled pressure gauges (Ashcroft), located downstream of the filtration module. All retentates and permeates were collected and analyzed. Membranes were cleaned by flushing for 15 min with distilled water, followed by recycling with 0.1 M NaOH for 30 min and a final rinse with distilled water. Initially, every new filter was rinsed with deionized water and then with a NaOH rinse program before use. The same procedure was repeated after every filtration and before every new sampling occasion. Membrane integrity and efficiency of every cleaning procedure were verified by control of water permeability. An Amicon, Model 8200 was used as dead-end filtration cell for the membrane separation experiments. The ultrafiltration membrane used was made of regenerated cellulose, and had a cut-off of 10 kDa with a membrane diameter of 63.5 mm. Before use, the membrane was floated skin side down in a beaker of distilled water for at least 1 h. The water was changed three times before use. Integrity of virgin membrane was verified by control of water permeability. After connection of the filtration cell, 150 mL of solution was added to the tank. Compressed air was connected to the cell and a regulator was used to maintain a constant pressure of 2 bar during the filtration. Permeate was collected in a graduated cylinder, and the time was measured with a stopwatch. After use the membrane was rinsed with distilled water of 100 mL, followed for 30 min with 0.1 M NaOH/100 ppm NaOCl (at 25 °C), after which the membrane was flushed with distilled water of 100 mL and kept in the refrigerator. The same cleaning procedure was repeated after every filtration and before every new sampling. Membrane integrity and efficiency of every cleaning procedure were verified by control of water permeability. From the data measured in each experiment, the normalized permeate flux values were calculated as follows:
Permeate fluxðL=m2 h barÞ permeate volume ¼ time membrance area transmembrane pressure
ð1Þ
3. Results and discussion 3.1. Formation of nanoparticles Linear c-PGA and cross-linked c-PGA nanoparticles were used to study the Fe3+-adsorption capacity from aqueous environment. The carboxylic groups of the c-PGA chains can make complex with
Fe3+, and form stable particulate systems, which can be ultrafiltered. Complexation was observed and separated spherical particles or aggregates were obtained, depending on the concentration and the composition of c-PGA/Fe3+. The size and the stability of the particles were investigated as function of these parameters. Solutions of c-PGA–Fe3+ nanoparticles were clear colloid systems and were stable at room temperature for several weeks. Laser light scattering provides valuable information on the size and size distribution of c-PGA nanoparticles complexed with Fe3+ (Fig. 1a). The general trend was observed that the hydrodynamic size of complex particles increased on increase of the c-PGA volume. The deformation ability and hydrodynamic size of particles depends on the rate of Fe3+/c-PGA and intramolecular cross-linked inner structure of biopolymer (Fig. 2). At lower levels of c-PGA volumes, the rate of Fe3+/c-PGA is high, i.e. compact complex particles are formed. These particles have smaller size with low deformability due to the high ferric ion content in the particles (Fig. 2C). On the increase of the c-PGA volume, the ratio of the ferric ions and carboxyl groups of biopolymer changes. Loosely complexed particles with high deformability are formed due to the low rate of Fe3+/c-PGA. These particles have higher clogging affinity and can form a gel layer on the membrane surface during the filtration (Fig. 2B). The c-PGA, as linear biopolymer without any metal ion complexation can be ultrafiltered slowly because of the high deformability of biopolymer (Fig. 2A). The macromolecules form a cake on the membrane and cause clogging therefore low permeate flux can be observed. In our previous work [40] we described that the cross-linking of linear c-PGA results in particles with compact shape, which appears with smaller hydrodynamic diameter and low viscosity values. The hydrodynamic size of particulate complexes depends on the bound Fe3+ content and the inner structure of the biopolymer. Use of cross-linked c-PGA nanoparticles for removal of ferric ions results in more compact particles with smaller size and low deformability (Fig. 2C). Hydrodynamic size of these cross-linked particles varied between 140 and 220 nm, and these values are smaller that hydrodynamic diameter of linear c-PGA–Fe3+ complex particles. Summarizing, the compactness and deformability of c-PGA– Fe3+ complex nanoparticles could be determined by the concentration and ratio of ferric ions and c-PGA, and the cross-linking state of the biopolymer. Optimal development of reaction parameters could result in nanoparticulate complexes, which can be filtered efficiently and fast due to their compact shape, small size and low deformability. The TEM micrograph in Fig. 1b confirmed the nanosize and the structure for dried systems of a c-PGA/Fe3+ complex. The nanoparticles separated into spherical shape in the dried state and in same cases dried as aggregates. The size of these dried particles agrees with the hydrodynamic size results that were measured in the swollen state. 3.2. Effect of feed c-PGA volume on permeate flux and ferric ion removal Fig. 3 shows the effect of the linear c-PGA feed volume and therefore the concentration of the c-PGA–Fe3+ complex nanoparticles on the permeate flux for both investigated membrane techniques at room temperature. For both investigated membrane separation techniques, similar correlation between the permeate flux and the biopolymer concentration was observed: the permeate flux decreased with increasing linear c-PGA concentration. Explanation of this phenomenon is as follows: on the one hand, increasing the biopolymer concentration,
M. Bodnár et al. / Separation and Purification Technology 112 (2013) 26–33
(a)
29
(b)
Fig. 1. Hydrodynamic size () and size distribution (I) (a) of colloid systems containing linear c-PGA–Fe3+ complexes (Vc-PGA = 4, 8, 12, 16 mL, cc-PGA = 10 mg/mL), and TEM micrograph (b) of c-PGA–Fe3+ (Vc-PGA = 8 mL).
Fig. 2. Mechanism of the nanoparticle enhanced ultrafiltration process. (A) Large linear biomolecules with large deformability and high adhesion properties, form cake and cause clogging. (B) Biopolymer–metal ion complex particles with low metal ion content, have smaller size, lower deformability and lower clogging properties. (C) Biopolymer–metal ion complex particles with low metal ion content or cross-linked inner structure, rigid spherical particles with no deformability and no clogging.
filtration of more and more viscous solutions was carried out. Viscosity of the colloid systems could be one of the main factors to influence the permeate flux using the membrane separation techniques. On the other hand, laser light scattering measurements provided that hydrodynamic diameter of these particulate complexes increased with increasing the c-PGA concentration. Increasing the c-PGA concentration, the rate of Fe3+/c-PGA decreases and loosely complexed, larger, less compact particles with higher deformability were formed (Fig. 2). These particles can cause slower permeate flux in a more viscous solution, and can make a gel layer on the membrane surface and readily clog the membrane. The rundown of permeate flux curves is similar, they fall during the filtration. In the case of the cross-flow separation technique, where the direction of transmembrane pressure is parallel to the membrane, this reduction is barely detectable. In the case of dead-end membrane filtration, where the direction of transmembrane pressure is perpendicular to the membrane, the decline of the curves is considerable, which reflects the clogging of membrane during the filtration. When the cross-flow technique was compared with the deadend membrane separation using linear c-PGA biopolymer, it can be established that the permeate flux of the c-PGA–Fe3+ complexes proved to be higher in the case of cross-flow membrane separation. The permeate flux of biopolymer free aqueous solution containing FeCl3 6H2O with a concentration of 1 mg/mL was about 115 L/ m2 h bar using cross-flow filtration technique and it was about 45 L/m2 h bar in case of dead-end filtration. The permeate flux of
solutions containing c-PGA–Fe3+ complexes varied between 30 and 70 L/m2 h bar and it reduced barely during the filtration. In the case of dead-end filtration, the permeate flux values varied between 5 and 30 L/m2 h bar and it reduced considerably during the filtration. From Fig. 3 it could be concluded that the separation procedure was faster using cross-flow membrane filtration, and the difference between the two investigated separation techniques is considerable. Summarizing, it can be established that the cross-flow membrane separation was more efficient than the dead-end separation technique using linear c-PGA at a concentration of 10 mg/mL. The c-PGA–Fe3+ complex nanoparticles with relatively high deformability can ultrafilter more efficiently, when the transmembrane pressure is parallel to the membrane. In this case, the cake formation on the membrane surface and the clogging property are negligible. The permeate flux data showed considerable differences on comparison of investigated membrane separation techniques, and some clogging could be observed in case of dead-end separation which was reflected in the fall of the curves. The cross-flow membrane filtration technique combined with the c-PGA biopolymer assures a continuous, rapid and efficient filtration procedure for production of water with ferric ion concentrations recommended for micro-irrigation. The analytical results support the efficiency of the combined nano-membrane separation technique. The analytical measurements confirmed that the linear biodegradable c-PGA is able to adsorb and remove Fe3+ from aqueous
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(a)
(b)
Fig. 3. Effect of feed c-PGA volume on the permeate flux of linear PGA biopolymer–Fe3+ complex at the compositions of the mixture indicated, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b) (cc-PGA = 10 mg/mL).
solution through the use of ultrafiltration processes. Table 2 shows that the Fe3+ concentrations of permeates are adequately low for both the membrane separation technique and the feed c-PGA volume do not influence the ferric ion concentration of permeate and the efficiency of removal. At this ferric ion and c-PGA concentration, 4 mL c-PGA is enough to adsorb and remove the ferric ions in a one step procedure to produce permeates with a concentration lower than 0.1 ppm. If 0 or 2 mL c-PGA solution (c = 10 mg/mL) was added to the FeCl3 solution at a concentration of 1 mg/mL, precipitation was observed, which necessitates centrifugation; however 4 mL or more c-PGA solution was added to the FeCl3 6H2O solution, no precipitation was observed, and a good-quality permeate can be result by using membrane technique. It is necessary to find the optimal ratio of biopolymer and ferric ion to achieve efficient removal and fast filtration in a one-step treatment without any centrifugation. Comparison of the results indicated that it was advisable to combine the c-PGA concentration and cross-flow membrane techniques to enhance the removal of Fe3+ from aqueous environments. 3.3. Effect of cross-linking on permeate flux and ferric ion removal To study the influence of the nanoparticulate formation on the permeate flux, cross-linked c-PGA nanoparticles were prepared and complexed with ferric ions. Fig. 4 shows the effect of the cross-linked c-PGA feed volume on the permeate flux of c-PGA– Fe3+ complex nanoparticles for both investigated membrane techniques at room temperature. The cross-linking process of linear c-PGA macromolecules results in more compact, globular nanoparticles with smaller hydrodynamic size and less deformability [40]. The inner structure and these parameters can influence the permeate flux of nanoparticles complexed with metal ions. For both investigated membrane techniques, tendency between the permeate flux and the concentration of cross-linked biopolymer was similar to the linear c-PGA: the permeate flux decreased with increasing cross-linked c-PGA concentration. Cross-linking of linear macromolecules results in compact particles with a well-defined inner structure and low deformability. The new shape of these particles causes lower viscosity of aqueous solution [40]. Complexation of these particles with ferric ions increases their compact, globular shape, reduces their deformability, and therefore their filtration became faster. In accordance with it, permeate flux of the c-PGA10%–Fe3+ complex nanoparticles was higher compared with the linear c-PGA–Fe3+ complexes. The difference between the permeate flux values became negligible in a function
of cross-linked c-PGA nanoparticles concentration. The permeate flux values varied between 40 and 70 L/m2 h bar in case of crossflow technique, and it was between 15 and 40 L/m2 h bar for dead-end filtration; the difference between these values is considerable. The permeate flux curves are almost horizontal, considerable clogging could not be observed due to the compact particles, which have low deformability, therefore low clogging affinity. The nanoparticles as spheres could move easier under the influence of pressure, therefore the clogging of the membrane during the filtration is not considerable. As compared the investigated membrane techniques, the permeate flux of cross-linked c-PGA–Fe3+ particulate complexes was considerably faster in the case of cross-flow membrane separation technique than for the dead-end filtration. Therefore, the crossflow membrane technique could be applied more efficient from the aspect of permeate flux. It could be established that the application of cross-linked cPGA nanoparticles for ferric ion removal combined with the cross-flow membrane filtration technique provides an opportunity to develop efficient biodegradable nanoparticle-enhanced ultrafiltration, which involves a fast permeate flux and yields permeate with low ferric ion concentration recommended for microirrigation. The analytical results in Table 3 confirm our theory that the cPGA and the cross-linked c-PGA nanoparticles are also suitable to adsorb and remove Fe3+ from aqueous solution through the use of ultrafiltration processes. Both membrane separation technique combined with the cross-linked c-PGA provides adequate results. The feed c-PGA10% volume does not influence the efficiency of removal. In all cases the ferric ion concentrations of permeates are lower than 0.1 ppm, which is required to produce functional water for micro-irrigation. At these determined ferric ion concentrations, 4 mL of cross-linked c-PGA nanoparticles is adequate for efficient removal of Fe3+, and in these cases fast filtration could be obtained. During the cross-linking process, the free carboxyl functional groups of biopolymer decreases. The cross-linking procedure results in new covalent bonds via the carboxyl groups of c-PGA. Due to the decreasing of free functional groups, metal ion adsorption capacity of biopolymer decreases, causing higher ferric ion concentration of permeates. Nevertheless the cross-linked c-PGA nanoparticles produce efficient ferric ion removal, the permeate Fe3+ concentration is much lower than it is necessary for safe and efficient micro-irrigation. The investigated process can result in treated water with low ferric ion concentration. Summarizing the analytical results, the cross-linked c-PGA is also an excellent biodegradable biopolymer for removal of Fe3+ from aqueous solution to produce treated water with a Fe3+ con-
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M. Bodnár et al. / Separation and Purification Technology 112 (2013) 26–33 Table 2 Results of ferric ion removal by linear c-PGA after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 1 mg/mL) pH = 2.6 (mL)
c-PGA
Permeate concentration of Fe3+ (ppb)
(c = 10 mg/mL) (mL)
Cross-flow
Dead-end
100 100 100 100
4 8 12 16
7.7 6.9 11.1 7.4
17.7 8.3 2.5 8.7
centration suitable for micro-irrigation. Nevertheless, in case of a given ferric ion concentration, it is important to find the optimal volume and concentration of cross-linked c-PGA nanoparticles to achieve efficient removal of Fe3+ with fast filtration in a one-step procedure.
3.4. Effect of initial c-PGA concentration on permeate flux and ferric ion removal Analytical results revealed that the linear c-PGA macromolecule and its cross-linked nanoparticles could adsorb and remove ferric ions efficiently from aqueous solution. However, it is necessary to combine the efficiency and the flux results to develop an optimal technique for water treatment. In the first step, ferric ion removal from aqueous solution with a high ferric ion concentration was studied. Using this model solution, it was confirmed that the removal of ferric ion was efficient by the c-PGA or its cross-linked derivative. In the next step, aqueous solutions with low ferric ion concentration were performed. Ferric ion concentration of natural water is lower than 1 mg/mL used in our model aqueous solution. Therefore the linear c-PGA biopolymer at different concentrations were studied as ferric ion remover using aqueous solutions with different, lower ferric ion concentration. Fig. 5 illustrates the effects of the linear c-PGA volume and concentration on the permeate flux of c-PGA–Fe3+ nanoparticles for both investigated membrane separation techniques. For both ferric ion concentrations, no precipitation was observed after pH adjusting. It means that at these given concentrations, 2 mL of biopolymer could adsorb the ferric ions, make stable complexes and could be filtered in a one-step procedure without any centrifugation. Fig. 5 demonstrates that decrease of the c-PGA concentration results in uniform permeate flux curves. The difference between the permeate flux values and curves minimizes, and this trend is
(a)
Table 3 Results of ferric ion removal by cross-linked c-PGA at 10% after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). Cross-linked
FeCl3 6H2O (c = 1 mg/mL) pH = 2.6 (mL)
c-PGA at 10%
100 100 100 100
4 8 12 16
(c = 10 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
9.8 10.9 11.1 11.7
2.1 2.1 4.1 43.5
independent of the membrane filtration processes. The permeate flux values are about 70 and 80 L/m2 h bar for cross-flow filtration at a c-PGA concentration of 2 mg/mL and 0.2 mg/mL, respectively, and varied between 25 and 35 L/m2 h bar for the dead-end membrane separation technique. Comparing the investigated membrane separation techniques, more differences between the permeate flux values can be seen. The permeate flux values are three times larger for cross-flow technique than for dead-end filtration. Consequently, for removal of Fe3+ at lower concentrations, the use of cross-flow technique would be preferred. At high concentration of c-PGA it was concluded that the viscosity of biopolymer solution and the compact shape and deformability of c-PGA–Fe3+ complex nanoparticles determined the permeate flux. Nevertheless, it was demonstrated that the crossflow technique, where the direction of transmembrane pressure is parallel to the membrane, the filtration was more rapid, observation of the pore blocking or cake formation could not be considerable. At lower concentration of biopolymer, the viscosity of solution is similar to the pure water and does not change considerably by increasing the volume of biopolymer; therefore the effect of viscosity is negligible on the filtration speed. The formation of particulate complexes could be the main influencing factor for filtration. It is of fundamental importance to find the optimal biopolymer concentration and volume to achieve effective ferric ion removal with fast membrane filtration in a one-step process. The key is the biopolymer. Knowledge of the Fe3+ concentration of water is essential to develop its treatment with optimal biopolymer parameters. Efficient ultrafiltration could be realized by optimization of the filtration parameters, including the filtration technique, ratio and concentration of biopolymer and ferric ions. Recognition of reaction conditions results in enhancement of the removal efficiency.
(b)
Fig. 4. Effect of feed cross-linked c-PGA concentration on the permeate flux of c-PGA–Fe3+ complex at the compositions of the mixture indicated, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b) (cc-PGA = 10 mg/mL).
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(a)
(b)
(c)
(d)
Fig. 5. Effect of feed c-PGA volume on the permeate flux of linear c-PGA biopolymer–Fe3+ complexes at the c-PGA concentration of 2 mg/mL, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b); and at the c-PGA concentration of 0.2 mg/mL, using cross-flow membrane separation (c), and dead-end membrane separation techniques (d).
Table 4 Results of ferric ion removal by linear c-PGA after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 0.1 mg/mL) pH = 2.6 (mL)
c-PGA
100 100 100
2 4 8
(c = 2 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
10.4 4.1 20.7
33.2 6.2 4.1
water, the most advantageous quantity (concentration and volume) of the linear and cross-linked c-PGA could be developed. In optimal conditions, compact complex c-PGA–Fe3+ particles with low deformability are formed in a low viscous solution, which could be filtered efficiently and fast. In summary, the biopolymer-based nanoparticle enhanced ultrafiltration technique could be an attractive possibility to achieve good-quality irrigation water.
4. Conclusion 3+
The investigation of the removal of Fe at different ratios using different concentrations of linear c-PGA revealed that ferric ion removal at different compositions could attain concentration values lower than 0.1 ppm recommended for micro-irrigation (Tables 4 and 5). These results supported our theory that the biodegradable c-PGA biopolymer is an excellent absorber for the removal of ferric ion from aqueous solution. Depending on the Fe3+ concentration of
Table 5 Results of ferric ion removal by linear c-PGA after using cross-flow and dead end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 0.01 mg/mL) pH = 2.6 (mL)
c-PGA
100 100 100
2 4 8
(c = 0.2 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
6.2 10.3 12.4
6.1 10.4 8.3
The aim of this research was the removal of Fe3+ from water using biodegradable c-PGA and its cross-linked nanoparticles to produce water with low ferric ion concentration. Our results clearly demonstrated the efficiency of biodegradable c-PGA nanoparticle-enhanced ultrafiltration technique. The c-PGA biopolymer can bind Fe3+ ions and make stable particulate complexes. Optimal parameters could be developed to produce compact complex particles with low deformability and low clogging properties. The compactness and deformability of these particles are the main factors for determination of the speed of membrane filtration. These parameters could be influenced by the rate of compounds, concentration, volume and cross-linking state of biopolymer. It was established that efficient nano-membrane technology can be performed with optimal ferric ion removal and fast filtration using linear or cross-linked c-PGA biopolymer. By combining the filtration parameters, type of membrane technique and the cPGA biopolymer can result in an efficient nano-membrane separation technique for water treatment.
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Contents lists available at SciVerse ScienceDirect
Separation and Purification Technology journal homepage: www.elsevier.com/locate/seppur
Biopolymer-based nanosystem for ferric ion removal from water }thi b, Nóra Harmati b, Zsuzsanna Csikós a, John F. Hartmann c, Magdolna Bodnár a, István Hajdu a, Eszter Ro d d Csaba Balogh , Béla Kelemen , János Tamás b, János Borbély a,e,⇑ a
BBS Nanotechnology Ltd., H-4225 Debrecen, P.O.B. 12, Hungary Department of Water and Environmental Management, University of Debrecen, H-4010 Debrecen, Hungary c ElizaNor Polymer LLC, Princeton Junction, 08550 NJ, USA d Environmental Inspectorate, H-4025 Debrecen, Hungary e Department of Radiology, Medical and Health Science Center, University of Debrecen, H-4032 Debrecen, Hungary b
a r t i c l e
i n f o
Article history: Received 17 December 2012 Received in revised form 24 March 2013 Accepted 25 March 2013 Available online 2 April 2013 Keywords: Poly-gamma-glutamic acid Ferric ion Removal Ultrafiltration Micro-irrigation
a b s t r a c t The removal of ferric ions from aqueous solutions by a nanoparticle-enhanced ultrafiltration technique was investigated. Biodegradable poly-gamma-glutamic acid (c-PGA), a linear biopolymer, and its cross-linked nanoparticles were used to complex the metal ions by forming nanosized spherical particles with more or less deformability. These polymer–metal ion particles were then removed by membrane separation. Two ultrafiltration techniques were studied with the aim of developing a nanoparticleenhanced separation process for the efficient removal of ferric ions from model solutions. The influence of parameters such as the feed c-PGA and Fe3+ concentrations, their proportions and the extent of crosslinking of c-PGA were studied. It was observed that c-PGA could bind and remove ferric ions to produce water with low Fe3+ concentration. These studies show that optimal nano-membrane separation technology can be performed, depending on the environment and quality of treatable water by the combination of the filtration parameters and type of membrane techniques shown here. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Presence of iron in drinking water and water supplies can cause problems. Even at relatively low concentrations, it can be occur as aesthetic and functional problems. Iron can give water a metallic taste, reddish color, turbidity and odor [1,2]. The presence of iron can make the water unusable for food and beverage [3]. In addition, it can precipitate and make clogs for softeners or irrigation systems. Nowadays in agriculture, the water-saving techniques have growing interest, and more attention is given to micro-irrigation systems. Micro-irrigation is a term which describes a family of irrigation systems. It provides irrigation water slowly, directly to the plants. The water discharge patterns are different depending upon specific applications. There is a common property that the average discharge rate per emitter is usually only between 1 L/h and 4 L/h, which is given off dripped to the plants’ root zone through the water distributor system using 2.5 bar pressure as maximum [4– 6]. Micro-irrigation is getting more attention and playing an important role in modern agriculture, due to its many advantages, ⇑ Corresponding author. Address: H-4032 Debrecen, Kiserdo 4, Hungary. Tel./fax: +36 52 541 742. E-mail address: [email protected] (J. Borbély). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.03.043
such as high water application efficiency, ability to irrigate plants independently of the soil type or broken ground, which minimizes soil erosion or energy cost. Micro-irrigation components include flow control equipment, pipes, tubes, emitters, and several accessories. Clogging of these micro-systems is recognized as one of the most important concerns for micro-irrigation. Clogging can be categorized into three major groups, namely, physical [7], biological [8], and chemical clogging [9]. High concentration of metal ions, such as ferric and manganese ions present in irrigation water, as well as calcium and magnesium ions can cause clogging by their precipitation from the water. High ferric ion concentration (c = 2 mg/L) of irrigation water does not disease for the plants, but filter and manifold systems of micro-irrigation equipment can be obstructive because of the precipitation of iron-hydroxide. Therefore, for the micro-irrigation, the water must be treated and remove the ferric ions in order to produce a water with low ferric ion concentration (c = 0.1 mg/L) [10]. Several methods are known for removal of ferric ions from aqueous media, however nowadays there is an increasing interest to find environmentally friendly methods for desalination of water. For removal of metal ions, several natural polymers including chitosan [11,12], cellulose [13,14], alginic acid [15,16] or other natural biopolymers [17,18] have been investigated. The most valuable properties of these biopolymers are their biocompatibility, biodegradability, and flocculating activity for metal ions [19,20].
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Flocculation is a process that enhance agglomeration or collection of smaller floc particles into larger, more easily sedimentable particles through gentle stirring by hydraulic or mechanical means. The addition of flocculating agents may promote the formation of flocs. Flocculating agents that are usually used for water treatment can be categorized into three major groups, namely, inorganic flocculants [21], organic synthetic polymer flocculants [22,23], hybrid flocculants [24,25] and natural biopolymer flocculants [26– 28]. Recently, biodegradable flocculants have been investigated to minimize the risk for the environment and humans. Flocculation of polyelectrolytes in the presence of bivalent ions is an important process, and is widely used in water treatment technologies. For separation of metal ions, natural c-PGA [29,30] and other natural polymers [15,31] have been investigated. Poly-c-glutamic acid is a water soluble, biodegradable, edible and nontoxic polyanion [32–34]. Therefore, c-PGA and its derivatives have been employed extensively in a variety of commercial applications such as cosmetics, food, medicine, and water treatment [35–38]. c-PGA has appropriate flocculating activity and binds metal ions with high affinity. Therefore, this biopolymer may be useful for water treatment. Optimal pH range, temperature, concentrations and other main factors were determined in different reaction conditions for improving the flocculation activity of c-PGA. In our previous work, formation of complexes of c-PGA with Pb2+ was studied [39]. The size and solubility of these nanoparticles in the dried and swollen states were discussed, and the factors determining the physico-chemical properties of nanoparticles were described. The removal of toxic lead ions from aqueous solution by a combined nano-membrane separation technique was also investigated [40]. It was found that c-PGA could bind and remove more than 99.8% of the lead ions from water through a convenient, low-pressure ultrafiltration technique, resulting in a permeate that satisfied the standard for drinking water recommended by the WHO. The present investigation focuses on the removal of Fe3+ from an aqueous environment by combined nano-membrane technology. Biodegradable linear c-PGA and its cross-linked nanoparticles were used to make complex and remove Fe3+, and different ultrafiltration techniques were compared with the aim of efficient Fe3+ removal with biodegradable systems. The permeate flux and efficiency of Fe3+ removal were studied as a function of the FeCl3 6H2O and c-PGA concentrations, their proportions, the crosslinking of c-PGA and type of membrane separation techniques. 2. Materials and methods 2.1. Reagents
c-PGA (Mw = 400 kDa) was purchased from Vedan Group, Taiwan. Iron (III) chloride hexahydrate was purchased from Sigma–Aldrich Co., Hungary. 2,20 -(Ethylenedioxy)bis(ethylamine) was used as cross-linking agent and 1-(3-(dimethylamino)propyl)-3-ethylcarbodiimide methiodide as coupling agent in the cross-linking process to produce c-PGA nanoparticles. All solutions were prepared with deionized water from a Milli-Q Gradient system (conductivity <0.05 lS/cm).
to pH 5.5 with 0.1 M hydrochloric acid solution. The diamine solution was added to the c-PGA solution and mixed for 30 min at room temperature. After the dropwise addition of carbodiimide solution, the reaction mixture was stirred at 4 °C for 4 h and at room temperature for 20 h. The solution containing cross-linked c-PGA nanoparticles was purified by dialysis for 7 days against distilled water and freeze-dried. 2.3. Binding measurements
c-PGA linear biopolymer and cross-linked c-PGA nanoparticle solutions were used for the removal of Fe3+ from aqueous solution. c-PGA solutions (c = 10 mg/mL, 2 mg/mL, 0.2 mg/mL), crosslinked c-PGA nanoparticle solutions (c = 10 mg/mL) and FeCl3 6H2O solutions (c = 1 mg/mL, 0.1 mg/mL, 0.01 mg/mL) were prepared to study the binding affinity. The c-PGA solution (V = 0, 2, 4, 8, 12 or 16 mL) was added dropwise to the FeCl3 6H2O solution (V = 100 mL, pH = 2.6). The reaction mixture was stirred for 30 min at room temperature. The pH of the reaction mixture was adjusted to 9.0 to facilitate the precipitation of Fe(OH)3 based on the residual free ferric ions. Reaction mixture was stirred, and after that the pH of the reaction mixture was adjusted to 7.0 for the membrane separation. These combined nano-membrane techniques could be effective for water treatment in one step membrane filtration at appropriate reaction conditions, but could require a two-step treatment including centrifugation and membrane separation at not designed reaction parameters. The concentration and quantity of biopolymer have to be calculated according to the ferric ion concentration. Table 1 summarizes the reaction conditions. 2.4. Characterization 2.4.1. Transmission electron microscopy (TEM) A JEOL2000 FX-II transmission electron microscope was used to characterize the size and morphology of the dried nanoparticles. For TEM observation, the nanoparticles were prepared from the aqueous media containing c-PGA–Fe3+ complexes at a c-PGA concentration of 0.30 mg/mL. Samples for TEM analysis were obtained by placing a drop of colloid dispersion containing the nanoparticles onto a carbon-coated copper grid. It was dried at room temperature and then examined by TEM without any further modification or coating. 2.4.2. Laser light scattering The hydrodynamic diameters of c-PGA complexed with Fe3+ were assessed by using a Zetasizer Nano-ZS instrument (Malvern Instruments Ltd., Worcestershire, UK), at an operating wavelength of 633 nm. The size distribution and the Z-average size of the nanoparticles were measured at 25 °C with an angle detection of 173° in polystyrol cuvettes. The samples were prepared from the reaction mixture, each sample was measured three times and average serial data were calculated. Table 1 Reaction conditions for the formation of c-PGA nanoparticles by ferric ion complexation. Concentration of FeCl3 6H2O solution (mg/mL)
Concentration of
c-PGA (mg/mL) Linear
2.2. Modification Cross-linked c-PGA nanoparticles were prepared by a process described earlier [41]. Briefly, c-PGA was dissolved in water to produce a solution with a concentration of 1 mg/mL. The diamine cross-linker was dissolved in water and the solution was adjusted
1 1 0.1 0.01
Crosslinked 10%
10 10 2 0.2
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2.4.3. Quantitative analysis of iron by atomic absorption spectrophotometry (AAS) An Thermo Unicam Solar 969 model flame atomic absorption spectrometer equipped with deuterium lamp background correction, hollow cathode lamps and air acetylene burner were used for the determination of the ferric ions. The calibration was performed until 5 mg/L. All absorption measurements were performed under the following conditions: wavelengths 248.3 nm, bandpass 0.5 nm, integration time 4 s. Measurements were made using transient height injection mode and three replicate injections for each sample. 2.5. Membrane separation The filters used with the Pellicon XL device, all made of regenerated cellulose, had a cut-off of 10 kD with a membrane area of 50 cm2. The LabScale system has a built in a Cole Parmer peristaltic pump with Sigma–Aldrich PVC laboratory tubing, and it was used to perform the cross-flow membrane technique in the Pellicon XL set-up. The transmembrane pressure during the filtration was controlled and it was 30 psi, monitored by using glycerin-filled pressure gauges (Ashcroft), located downstream of the filtration module. All retentates and permeates were collected and analyzed. Membranes were cleaned by flushing for 15 min with distilled water, followed by recycling with 0.1 M NaOH for 30 min and a final rinse with distilled water. Initially, every new filter was rinsed with deionized water and then with a NaOH rinse program before use. The same procedure was repeated after every filtration and before every new sampling occasion. Membrane integrity and efficiency of every cleaning procedure were verified by control of water permeability. An Amicon, Model 8200 was used as dead-end filtration cell for the membrane separation experiments. The ultrafiltration membrane used was made of regenerated cellulose, and had a cut-off of 10 kDa with a membrane diameter of 63.5 mm. Before use, the membrane was floated skin side down in a beaker of distilled water for at least 1 h. The water was changed three times before use. Integrity of virgin membrane was verified by control of water permeability. After connection of the filtration cell, 150 mL of solution was added to the tank. Compressed air was connected to the cell and a regulator was used to maintain a constant pressure of 2 bar during the filtration. Permeate was collected in a graduated cylinder, and the time was measured with a stopwatch. After use the membrane was rinsed with distilled water of 100 mL, followed for 30 min with 0.1 M NaOH/100 ppm NaOCl (at 25 °C), after which the membrane was flushed with distilled water of 100 mL and kept in the refrigerator. The same cleaning procedure was repeated after every filtration and before every new sampling. Membrane integrity and efficiency of every cleaning procedure were verified by control of water permeability. From the data measured in each experiment, the normalized permeate flux values were calculated as follows:
Permeate fluxðL=m2 h barÞ permeate volume ¼ time membrance area transmembrane pressure
ð1Þ
3. Results and discussion 3.1. Formation of nanoparticles Linear c-PGA and cross-linked c-PGA nanoparticles were used to study the Fe3+-adsorption capacity from aqueous environment. The carboxylic groups of the c-PGA chains can make complex with
Fe3+, and form stable particulate systems, which can be ultrafiltered. Complexation was observed and separated spherical particles or aggregates were obtained, depending on the concentration and the composition of c-PGA/Fe3+. The size and the stability of the particles were investigated as function of these parameters. Solutions of c-PGA–Fe3+ nanoparticles were clear colloid systems and were stable at room temperature for several weeks. Laser light scattering provides valuable information on the size and size distribution of c-PGA nanoparticles complexed with Fe3+ (Fig. 1a). The general trend was observed that the hydrodynamic size of complex particles increased on increase of the c-PGA volume. The deformation ability and hydrodynamic size of particles depends on the rate of Fe3+/c-PGA and intramolecular cross-linked inner structure of biopolymer (Fig. 2). At lower levels of c-PGA volumes, the rate of Fe3+/c-PGA is high, i.e. compact complex particles are formed. These particles have smaller size with low deformability due to the high ferric ion content in the particles (Fig. 2C). On the increase of the c-PGA volume, the ratio of the ferric ions and carboxyl groups of biopolymer changes. Loosely complexed particles with high deformability are formed due to the low rate of Fe3+/c-PGA. These particles have higher clogging affinity and can form a gel layer on the membrane surface during the filtration (Fig. 2B). The c-PGA, as linear biopolymer without any metal ion complexation can be ultrafiltered slowly because of the high deformability of biopolymer (Fig. 2A). The macromolecules form a cake on the membrane and cause clogging therefore low permeate flux can be observed. In our previous work [40] we described that the cross-linking of linear c-PGA results in particles with compact shape, which appears with smaller hydrodynamic diameter and low viscosity values. The hydrodynamic size of particulate complexes depends on the bound Fe3+ content and the inner structure of the biopolymer. Use of cross-linked c-PGA nanoparticles for removal of ferric ions results in more compact particles with smaller size and low deformability (Fig. 2C). Hydrodynamic size of these cross-linked particles varied between 140 and 220 nm, and these values are smaller that hydrodynamic diameter of linear c-PGA–Fe3+ complex particles. Summarizing, the compactness and deformability of c-PGA– Fe3+ complex nanoparticles could be determined by the concentration and ratio of ferric ions and c-PGA, and the cross-linking state of the biopolymer. Optimal development of reaction parameters could result in nanoparticulate complexes, which can be filtered efficiently and fast due to their compact shape, small size and low deformability. The TEM micrograph in Fig. 1b confirmed the nanosize and the structure for dried systems of a c-PGA/Fe3+ complex. The nanoparticles separated into spherical shape in the dried state and in same cases dried as aggregates. The size of these dried particles agrees with the hydrodynamic size results that were measured in the swollen state. 3.2. Effect of feed c-PGA volume on permeate flux and ferric ion removal Fig. 3 shows the effect of the linear c-PGA feed volume and therefore the concentration of the c-PGA–Fe3+ complex nanoparticles on the permeate flux for both investigated membrane techniques at room temperature. For both investigated membrane separation techniques, similar correlation between the permeate flux and the biopolymer concentration was observed: the permeate flux decreased with increasing linear c-PGA concentration. Explanation of this phenomenon is as follows: on the one hand, increasing the biopolymer concentration,
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(a)
29
(b)
Fig. 1. Hydrodynamic size () and size distribution (I) (a) of colloid systems containing linear c-PGA–Fe3+ complexes (Vc-PGA = 4, 8, 12, 16 mL, cc-PGA = 10 mg/mL), and TEM micrograph (b) of c-PGA–Fe3+ (Vc-PGA = 8 mL).
Fig. 2. Mechanism of the nanoparticle enhanced ultrafiltration process. (A) Large linear biomolecules with large deformability and high adhesion properties, form cake and cause clogging. (B) Biopolymer–metal ion complex particles with low metal ion content, have smaller size, lower deformability and lower clogging properties. (C) Biopolymer–metal ion complex particles with low metal ion content or cross-linked inner structure, rigid spherical particles with no deformability and no clogging.
filtration of more and more viscous solutions was carried out. Viscosity of the colloid systems could be one of the main factors to influence the permeate flux using the membrane separation techniques. On the other hand, laser light scattering measurements provided that hydrodynamic diameter of these particulate complexes increased with increasing the c-PGA concentration. Increasing the c-PGA concentration, the rate of Fe3+/c-PGA decreases and loosely complexed, larger, less compact particles with higher deformability were formed (Fig. 2). These particles can cause slower permeate flux in a more viscous solution, and can make a gel layer on the membrane surface and readily clog the membrane. The rundown of permeate flux curves is similar, they fall during the filtration. In the case of the cross-flow separation technique, where the direction of transmembrane pressure is parallel to the membrane, this reduction is barely detectable. In the case of dead-end membrane filtration, where the direction of transmembrane pressure is perpendicular to the membrane, the decline of the curves is considerable, which reflects the clogging of membrane during the filtration. When the cross-flow technique was compared with the deadend membrane separation using linear c-PGA biopolymer, it can be established that the permeate flux of the c-PGA–Fe3+ complexes proved to be higher in the case of cross-flow membrane separation. The permeate flux of biopolymer free aqueous solution containing FeCl3 6H2O with a concentration of 1 mg/mL was about 115 L/ m2 h bar using cross-flow filtration technique and it was about 45 L/m2 h bar in case of dead-end filtration. The permeate flux of
solutions containing c-PGA–Fe3+ complexes varied between 30 and 70 L/m2 h bar and it reduced barely during the filtration. In the case of dead-end filtration, the permeate flux values varied between 5 and 30 L/m2 h bar and it reduced considerably during the filtration. From Fig. 3 it could be concluded that the separation procedure was faster using cross-flow membrane filtration, and the difference between the two investigated separation techniques is considerable. Summarizing, it can be established that the cross-flow membrane separation was more efficient than the dead-end separation technique using linear c-PGA at a concentration of 10 mg/mL. The c-PGA–Fe3+ complex nanoparticles with relatively high deformability can ultrafilter more efficiently, when the transmembrane pressure is parallel to the membrane. In this case, the cake formation on the membrane surface and the clogging property are negligible. The permeate flux data showed considerable differences on comparison of investigated membrane separation techniques, and some clogging could be observed in case of dead-end separation which was reflected in the fall of the curves. The cross-flow membrane filtration technique combined with the c-PGA biopolymer assures a continuous, rapid and efficient filtration procedure for production of water with ferric ion concentrations recommended for micro-irrigation. The analytical results support the efficiency of the combined nano-membrane separation technique. The analytical measurements confirmed that the linear biodegradable c-PGA is able to adsorb and remove Fe3+ from aqueous
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(a)
(b)
Fig. 3. Effect of feed c-PGA volume on the permeate flux of linear PGA biopolymer–Fe3+ complex at the compositions of the mixture indicated, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b) (cc-PGA = 10 mg/mL).
solution through the use of ultrafiltration processes. Table 2 shows that the Fe3+ concentrations of permeates are adequately low for both the membrane separation technique and the feed c-PGA volume do not influence the ferric ion concentration of permeate and the efficiency of removal. At this ferric ion and c-PGA concentration, 4 mL c-PGA is enough to adsorb and remove the ferric ions in a one step procedure to produce permeates with a concentration lower than 0.1 ppm. If 0 or 2 mL c-PGA solution (c = 10 mg/mL) was added to the FeCl3 solution at a concentration of 1 mg/mL, precipitation was observed, which necessitates centrifugation; however 4 mL or more c-PGA solution was added to the FeCl3 6H2O solution, no precipitation was observed, and a good-quality permeate can be result by using membrane technique. It is necessary to find the optimal ratio of biopolymer and ferric ion to achieve efficient removal and fast filtration in a one-step treatment without any centrifugation. Comparison of the results indicated that it was advisable to combine the c-PGA concentration and cross-flow membrane techniques to enhance the removal of Fe3+ from aqueous environments. 3.3. Effect of cross-linking on permeate flux and ferric ion removal To study the influence of the nanoparticulate formation on the permeate flux, cross-linked c-PGA nanoparticles were prepared and complexed with ferric ions. Fig. 4 shows the effect of the cross-linked c-PGA feed volume on the permeate flux of c-PGA– Fe3+ complex nanoparticles for both investigated membrane techniques at room temperature. The cross-linking process of linear c-PGA macromolecules results in more compact, globular nanoparticles with smaller hydrodynamic size and less deformability [40]. The inner structure and these parameters can influence the permeate flux of nanoparticles complexed with metal ions. For both investigated membrane techniques, tendency between the permeate flux and the concentration of cross-linked biopolymer was similar to the linear c-PGA: the permeate flux decreased with increasing cross-linked c-PGA concentration. Cross-linking of linear macromolecules results in compact particles with a well-defined inner structure and low deformability. The new shape of these particles causes lower viscosity of aqueous solution [40]. Complexation of these particles with ferric ions increases their compact, globular shape, reduces their deformability, and therefore their filtration became faster. In accordance with it, permeate flux of the c-PGA10%–Fe3+ complex nanoparticles was higher compared with the linear c-PGA–Fe3+ complexes. The difference between the permeate flux values became negligible in a function
of cross-linked c-PGA nanoparticles concentration. The permeate flux values varied between 40 and 70 L/m2 h bar in case of crossflow technique, and it was between 15 and 40 L/m2 h bar for dead-end filtration; the difference between these values is considerable. The permeate flux curves are almost horizontal, considerable clogging could not be observed due to the compact particles, which have low deformability, therefore low clogging affinity. The nanoparticles as spheres could move easier under the influence of pressure, therefore the clogging of the membrane during the filtration is not considerable. As compared the investigated membrane techniques, the permeate flux of cross-linked c-PGA–Fe3+ particulate complexes was considerably faster in the case of cross-flow membrane separation technique than for the dead-end filtration. Therefore, the crossflow membrane technique could be applied more efficient from the aspect of permeate flux. It could be established that the application of cross-linked cPGA nanoparticles for ferric ion removal combined with the cross-flow membrane filtration technique provides an opportunity to develop efficient biodegradable nanoparticle-enhanced ultrafiltration, which involves a fast permeate flux and yields permeate with low ferric ion concentration recommended for microirrigation. The analytical results in Table 3 confirm our theory that the cPGA and the cross-linked c-PGA nanoparticles are also suitable to adsorb and remove Fe3+ from aqueous solution through the use of ultrafiltration processes. Both membrane separation technique combined with the cross-linked c-PGA provides adequate results. The feed c-PGA10% volume does not influence the efficiency of removal. In all cases the ferric ion concentrations of permeates are lower than 0.1 ppm, which is required to produce functional water for micro-irrigation. At these determined ferric ion concentrations, 4 mL of cross-linked c-PGA nanoparticles is adequate for efficient removal of Fe3+, and in these cases fast filtration could be obtained. During the cross-linking process, the free carboxyl functional groups of biopolymer decreases. The cross-linking procedure results in new covalent bonds via the carboxyl groups of c-PGA. Due to the decreasing of free functional groups, metal ion adsorption capacity of biopolymer decreases, causing higher ferric ion concentration of permeates. Nevertheless the cross-linked c-PGA nanoparticles produce efficient ferric ion removal, the permeate Fe3+ concentration is much lower than it is necessary for safe and efficient micro-irrigation. The investigated process can result in treated water with low ferric ion concentration. Summarizing the analytical results, the cross-linked c-PGA is also an excellent biodegradable biopolymer for removal of Fe3+ from aqueous solution to produce treated water with a Fe3+ con-
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M. Bodnár et al. / Separation and Purification Technology 112 (2013) 26–33 Table 2 Results of ferric ion removal by linear c-PGA after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 1 mg/mL) pH = 2.6 (mL)
c-PGA
Permeate concentration of Fe3+ (ppb)
(c = 10 mg/mL) (mL)
Cross-flow
Dead-end
100 100 100 100
4 8 12 16
7.7 6.9 11.1 7.4
17.7 8.3 2.5 8.7
centration suitable for micro-irrigation. Nevertheless, in case of a given ferric ion concentration, it is important to find the optimal volume and concentration of cross-linked c-PGA nanoparticles to achieve efficient removal of Fe3+ with fast filtration in a one-step procedure.
3.4. Effect of initial c-PGA concentration on permeate flux and ferric ion removal Analytical results revealed that the linear c-PGA macromolecule and its cross-linked nanoparticles could adsorb and remove ferric ions efficiently from aqueous solution. However, it is necessary to combine the efficiency and the flux results to develop an optimal technique for water treatment. In the first step, ferric ion removal from aqueous solution with a high ferric ion concentration was studied. Using this model solution, it was confirmed that the removal of ferric ion was efficient by the c-PGA or its cross-linked derivative. In the next step, aqueous solutions with low ferric ion concentration were performed. Ferric ion concentration of natural water is lower than 1 mg/mL used in our model aqueous solution. Therefore the linear c-PGA biopolymer at different concentrations were studied as ferric ion remover using aqueous solutions with different, lower ferric ion concentration. Fig. 5 illustrates the effects of the linear c-PGA volume and concentration on the permeate flux of c-PGA–Fe3+ nanoparticles for both investigated membrane separation techniques. For both ferric ion concentrations, no precipitation was observed after pH adjusting. It means that at these given concentrations, 2 mL of biopolymer could adsorb the ferric ions, make stable complexes and could be filtered in a one-step procedure without any centrifugation. Fig. 5 demonstrates that decrease of the c-PGA concentration results in uniform permeate flux curves. The difference between the permeate flux values and curves minimizes, and this trend is
(a)
Table 3 Results of ferric ion removal by cross-linked c-PGA at 10% after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). Cross-linked
FeCl3 6H2O (c = 1 mg/mL) pH = 2.6 (mL)
c-PGA at 10%
100 100 100 100
4 8 12 16
(c = 10 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
9.8 10.9 11.1 11.7
2.1 2.1 4.1 43.5
independent of the membrane filtration processes. The permeate flux values are about 70 and 80 L/m2 h bar for cross-flow filtration at a c-PGA concentration of 2 mg/mL and 0.2 mg/mL, respectively, and varied between 25 and 35 L/m2 h bar for the dead-end membrane separation technique. Comparing the investigated membrane separation techniques, more differences between the permeate flux values can be seen. The permeate flux values are three times larger for cross-flow technique than for dead-end filtration. Consequently, for removal of Fe3+ at lower concentrations, the use of cross-flow technique would be preferred. At high concentration of c-PGA it was concluded that the viscosity of biopolymer solution and the compact shape and deformability of c-PGA–Fe3+ complex nanoparticles determined the permeate flux. Nevertheless, it was demonstrated that the crossflow technique, where the direction of transmembrane pressure is parallel to the membrane, the filtration was more rapid, observation of the pore blocking or cake formation could not be considerable. At lower concentration of biopolymer, the viscosity of solution is similar to the pure water and does not change considerably by increasing the volume of biopolymer; therefore the effect of viscosity is negligible on the filtration speed. The formation of particulate complexes could be the main influencing factor for filtration. It is of fundamental importance to find the optimal biopolymer concentration and volume to achieve effective ferric ion removal with fast membrane filtration in a one-step process. The key is the biopolymer. Knowledge of the Fe3+ concentration of water is essential to develop its treatment with optimal biopolymer parameters. Efficient ultrafiltration could be realized by optimization of the filtration parameters, including the filtration technique, ratio and concentration of biopolymer and ferric ions. Recognition of reaction conditions results in enhancement of the removal efficiency.
(b)
Fig. 4. Effect of feed cross-linked c-PGA concentration on the permeate flux of c-PGA–Fe3+ complex at the compositions of the mixture indicated, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b) (cc-PGA = 10 mg/mL).
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M. Bodnár et al. / Separation and Purification Technology 112 (2013) 26–33
(a)
(b)
(c)
(d)
Fig. 5. Effect of feed c-PGA volume on the permeate flux of linear c-PGA biopolymer–Fe3+ complexes at the c-PGA concentration of 2 mg/mL, using cross-flow membrane separation (a), and dead-end membrane separation techniques (b); and at the c-PGA concentration of 0.2 mg/mL, using cross-flow membrane separation (c), and dead-end membrane separation techniques (d).
Table 4 Results of ferric ion removal by linear c-PGA after using cross-flow and dead-end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 0.1 mg/mL) pH = 2.6 (mL)
c-PGA
100 100 100
2 4 8
(c = 2 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
10.4 4.1 20.7
33.2 6.2 4.1
water, the most advantageous quantity (concentration and volume) of the linear and cross-linked c-PGA could be developed. In optimal conditions, compact complex c-PGA–Fe3+ particles with low deformability are formed in a low viscous solution, which could be filtered efficiently and fast. In summary, the biopolymer-based nanoparticle enhanced ultrafiltration technique could be an attractive possibility to achieve good-quality irrigation water.
4. Conclusion 3+
The investigation of the removal of Fe at different ratios using different concentrations of linear c-PGA revealed that ferric ion removal at different compositions could attain concentration values lower than 0.1 ppm recommended for micro-irrigation (Tables 4 and 5). These results supported our theory that the biodegradable c-PGA biopolymer is an excellent absorber for the removal of ferric ion from aqueous solution. Depending on the Fe3+ concentration of
Table 5 Results of ferric ion removal by linear c-PGA after using cross-flow and dead end membrane separation techniques (made of cellulose, MWCO of 10 kDa). FeCl3 6H2O (c = 0.01 mg/mL) pH = 2.6 (mL)
c-PGA
100 100 100
2 4 8
(c = 0.2 mg/mL) (mL)
Permeate concentration of Fe3+ (ppb) Cross-flow
Dead-end
6.2 10.3 12.4
6.1 10.4 8.3
The aim of this research was the removal of Fe3+ from water using biodegradable c-PGA and its cross-linked nanoparticles to produce water with low ferric ion concentration. Our results clearly demonstrated the efficiency of biodegradable c-PGA nanoparticle-enhanced ultrafiltration technique. The c-PGA biopolymer can bind Fe3+ ions and make stable particulate complexes. Optimal parameters could be developed to produce compact complex particles with low deformability and low clogging properties. The compactness and deformability of these particles are the main factors for determination of the speed of membrane filtration. These parameters could be influenced by the rate of compounds, concentration, volume and cross-linking state of biopolymer. It was established that efficient nano-membrane technology can be performed with optimal ferric ion removal and fast filtration using linear or cross-linked c-PGA biopolymer. By combining the filtration parameters, type of membrane technique and the cPGA biopolymer can result in an efficient nano-membrane separation technique for water treatment.
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