Separation and Puriﬁcation Technology 118 (2013) 737–743
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Electrospun nanoﬁbrous rhodanine/polymethylmethacrylate membranes for the removal of heavy metal ions Cheng-Hung Lee a,b, Chang-Lin Chiang b, Shih-Jung Liu b,⇑ a b
Division of Cardiology, Department of Internal Medicine, Chang Gung Memorial Hospital-Linkou, Tao-Yuan, Taiwan Department of Mechanical Engineering, Chang Gung University, Tao-Yuan, Taiwan
a r t i c l e
i n f o
Article history: Received 15 May 2013 Received in revised form 12 August 2013 Accepted 13 August 2013 Available online 23 August 2013 Keywords: Electrospinning Nanoﬁbrous membranes Rhodanine Heavy metal removal
a b s t r a c t In this study we developed rhodanine loaded nanoﬁbrous membranes for the removal of heavy metal ions. Rhodanine and polymethylmethacrylate dissolved in 1,1,1,3,3,3-hexaﬂuoro-2-propanol (HFIP) were electrospun into nanoﬁbrous membranes via an electrospinning process. The morphology of as-spun rhodanine/polymethylmethacrylate nanoﬁbers was examined by scanning electron microscopy. The average diameter of electrospun nanoﬁbers ranged from 840 nm to 1440 nm. The adsorption capability of nanoﬁbrous rhodanine/polymethylmethacrylate membranes was measured and compared with that of bulk rhodanine. The inﬂuence of various process conditions on adsorption efﬁciency was also examined. The experimental results suggested that the electrospun nanoﬁbrous membranes exhibit good Ag (I) and Pb (II) ion uptake capabilities. The metal uptake of nanoﬁbrous membranes increased with the initial metal ion concentrations and decreased with the ﬁltering rate of the solutions. Furthermore, the electrospun membrane could be reused after the recovery process. The empirical results in this study suggested that electrospun rhodanine/polymethylmethacrylate nanoﬁbrous membranes can be a good candidate for the removal of heavy metal ions. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Water pollution by heavy metal ions has become one of the globally environmental problems. Heavy metals are natural components of the earth and cannot be degraded or destroyed. Some heavy metals are essential to maintain the metabolism of the human body; however, at higher concentrations they can lead to poisoning. Long term exposure to other metal ions such as lead (Pb) can result in acute or chronic damage to the brain and the central nervous system on humans. Furthermore, heavy metals are dangerous because they tend to bioaccumulate, i.e., the concentration of a chemical increases in a biological organism over time, compared to the chemical’s concentration in the environment. Heavy metals can enter a water supply by industrial and consumer waste, or even from acidic rain breaking down soils and releasing heavy metals into streams, lakes, rivers, and groundwater. The removal of heavy metal ions has thus become one of the imminent issues for the ecosystem. Presently, various techniques have been developed to remove heavy metal ions from contaminated water, including chemical oxidation  and precipitation , ion exchange , reverse ⇑ Corresponding author. Address: Department of Mechanical Engineering, Chang Gung University, 259, Wen-Hwa 1st Road, Tao-Yuan 333, Taiwan. Tel.: +886 32118166. E-mail address: [email protected]
(S.-J. Liu). 1383-5866/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.seppur.2013.08.020
osmosis , electrochemical applications , biosorption [6,7], adsorption [8,9] and membrane separation [10–16]. Among these techniques, the membrane separation processes have played very important roles in the water puriﬁcation industry. The processes are pressure driven and operate without heating and thus are energetically lower than conventional thermal separation processes. This provides advantages in terms of cost competitiveness. In this study, we developed rhodanine loaded nanoﬁbrous polymethylmethacrylate (PMMA) membranes via electrospinning for the removal of heavy metal ions. Electrospinning is a simple and effective nanofabrication method for preparing nanoﬁbrous membranes with diameters ranging form 5 to 500 nm or higher, which are 102–104 times smaller than those prepared by the traditional method of solution or melt spinning . Su et al.  demonstrated the favorable effect of various metal ions on the electrospinning of chitosan/poly(ethylene oxide) blend nanoﬁbers, while Haider and Park  showed that the electrospun chitosan nanofibers exhibited good erosion stability in water and high adsorption afﬁnity for Cu (II) and Pb (II) ions in an aqueous solution. Teng et al.  developed mesoporous polyvinylpyrrolidone (PVP)/SiO2 composite nanoﬁbers membranes that were functionalized with thioether groups using a combined method of sol–gel and electrospinning process. Their results showed that the composites could be used a highly selective adsorbents for Hg (II) due to the modiﬁcation with thioether groups (-S-). Wang et al.  reported
C.-H. Lee et al. / Separation and Puriﬁcation Technology 118 (2013) 737–743
the successful fabrication of crosslinked poly(ethyleneimine) nanoﬁbrous afﬁnity membranes for removing heavy metal ions via wet-spinning from its aqueous solution with the aid of low loading poly(vinyl alcohol). Li et al.  fabricated zonal thiolfunctionalized silica nanoﬁbers by the sol–gel polymerization of 3-mercaptopropyl trimethoxysilane on the electrospun polycarylonitrile (PAN) nanoﬁbers, followed by removal of PAN nanoﬁbers using dimethyl-formamine dissolution. The zonal silica nanoﬁbers were found to be efﬁcient in removing mercury from aqueous waste streams. Tian et al.  prepared cellulose acetate nonwoven membrane by electrospinning and surface modiﬁcation with poly(methacrylic acid), and the membranes showed high adsorption selectivity for Hg (II). Despite these developed nanoﬁbers showed good afﬁnity for heavy metal ion removal, they required a complex process for the preparation of the ﬁbers. This study prepared rhodanine loaded nanoﬁbrous membrane for removal of heavy metal ions using a simple and effective process. Rhodanine is a heterocyclic molecule that belongs to the sulfur-containing N, O organic compounds. The presence of the thioamide and keto groups leads to a strong ability of the molecule to bind metal ions and is responsible for many biochemical, medical and industrial activities. Song et al.  proposed the polyrhodanine modiﬁed anodic aluminum oxide membranes for the removal of heavy metal ions, and showed that the fabricated membranes have remarkable uptake performance toward Ag (I), Pb (II) and Hg (II) ions. Based on the metal binding property of rhodanine, membranes made of rhodanine can be used as ﬁlters for the removal of heavy metal ions. PMMA is a strong and lightweight material that has good impact strength and a density of 1.17– 1.20 g/cm3, and is easy to be processed. It is also an economical alternative to some engineered polymers such as polyacrylonitrile (PAN), polyvinylidene diﬂuoride (PVDF) or polysulfone (PSU) when extreme strength is not necessary. For the electrospinning of rhodanine loaded nanoﬁbrous polymethylmethacrylate (PMMA) membranes, rhodanine and PMMA were ﬁrst dissolved in 1,1,1,3,3,3-hexaﬂuoro-2-propanol (HFIP). They were then electrospun into nanoﬁber-structured membranes. After electrospinning, the morphology of the electrospun membranes was characterized. Parameters affecting the adsorption property of heavy metal ions, namely Ag (I) and Pb (II) ions, on the nanoﬁbrous membranes were investigated. The recovery of the fabricated membrane for metal ions removal was also examined. 2. Materials and methods 2.1. Materials Rhodanine (2-thioxo-4-thiazolidinone) (Fig. 1) and 1,1,1,3,3,3hexaﬂuoro-2-propanol (HFIP) were purchased from Sigma–Aldrich (Saint Louis, MO, USA). The polymethylmethacrylate (PMMA) used was commercially available materials (PG-33, Chi-Mei Inc., Taiwan) and had a melt ﬂow index of 8.0 g/10 min. For heavy metal ion removal test, silver nitrate (AgNO3), lead nitrate (Pb(NO3)2) and nitric acid were also purchased from Sigma–Aldrich (Saint Louis, MO, USA).
Fig. 1. Molecular formula of rhodanine.
2.2. Electrospinning The electrospinning setup utilized in this study consisted of a syringe and needle (the internal diameter is 0.42 mm), a ground electrode, an aluminum sheet, and a high voltage supply, as shown schematically in Fig. 2. The needle was connected to the high voltage supply, which could generate positive DC voltages and current up to 35 kV and 4.16 mA/125 W respectively. For the electrospinning of rhodanine loaded nanoﬁbrous membranes, a predetermined ratio of rhodanine and PMMA (20 mg/200 mg, w/w) was ﬁrst dissolved in HFIP at three different concentrations of 11%, 12% and 13% by weight, and was then delivered by a syringe pump with a volumetric ﬂow rate of 3, 4, or 5 mL/h. Despite the HFIP was used as the solvent in this study, other solvents such as di- or trichloromethane and dichlorobenzene could also be used. The distance between the needle tip and the ground electrode was 9, 12 and 15 cm, and the positive voltage applied to polymer solutions was set to 15, 17, or 20 kV. Table 1 lists the test trials and the processing parameters. In addition, nanoﬁbrous membrane of virgin PMMA was also prepared as a blank reference. All electrospinning experiments were carried out at room temperature. 2.3. Characterization of nanoﬁbers Electrospun rhodanine/PMMA nanoﬁbrous membranes were observed on a scanning electron microscope (SEM; Hitachi S3000N, Japan) equipped with an energy dispersive X-ray spectroscope (ESEM/EDX) after gold coating. The average diameter of the nanoﬁbers was obtained by analyzing SEM images using a commercial image analysis program (Optimas version 5.22, USA). A Fourier Transform Infrared (FTIR) spectrometry was employed to examine the spectra of electrospun virgin PMMA membranes, rhodanine, and rhodanine/PMMA membranes. The FTIR analysis was conducted on a Bruker Tensor 27 spectrometer at resolution of 4 cm1 and 32 scans. Membrane samples were as pressed KBr disks, and spectra were recorded over the 400– 4000 cm1 range. 2.4. Metal removal capability of the rhodanine/PMMA nanoﬁbrous membranes The metal removal capability of the nanoﬁbrous rhodanine/ PMMA membranes was studied using the device shown schematically in Fig. 3. The membranes used for the test have a diameter of 47 mm and a thickness of 0.15 mm. A predetermined amount of solutions (40 mL) containing different concentrations of heavy metal ions, including Ag (I) and Pb (II), were placed at top of the setup. With the application of vacuum pressure, the solutions were
Fig. 2. Schematically the electrospinning setup.
C.-H. Lee et al. / Separation and Puriﬁcation Technology 118 (2013) 737–743 Table 1 Processing parameters used in the experiments. Test
Percentage of rhodanine (%)
Weight of rhodanine (mg)
Weight of PMMA (mg)
Flow rate (mL / h)
Average ﬁber diameter (lm)
a b c d e f g h i
11 11 11 12 12 12 13 13 13
20 20 20 22 22 22 24 24 24
200 200 200 220 220 220 240 240 240
3 4 5 3 4 5 3 4 5
20 15 17 17 20 15 15 17 20
12 15 9 15 9 12 9 12 15
1.160 0.840 1.230 0.867 1.256 1.300 1.280 1.400 1.440
drawn to pass through the nanoﬁbrous membranes. The ﬁltering rates through the membranes could be adjusted by the throttle value. All tests were carried out at room temperature. After the heavy metal ions being uptaken by the membranes, the solutions were collected by the bottom ﬂask for measurement of the residual metal ion concentrations. The concentrations were measured by an inductively coupled plasma ICP–OES Model Varian 710-ES. All samples were analyzed in triplicate. The uptake capacity (UC) and the adsorptivity of metal ions by the rhodanine/PMMA membrane were calculated by the difference of metal ion concentrations in the collected solutions before and after metal removal experiment:
UC ¼ C 0 C Adsorptivity ¼
ð1Þ C0 C 100 ð%Þ C0
where C0 and C is the concentration of metal ions before and after metal removal experiment in mg/L. To investigate the effect of initial metal ion concentration on uptake capability of the membranes, solutions with various initial concentrations of Ag (I) and Pb (II) ion (from 2 to 10 mg/L) were prepared. After ﬁltering, the uptake of metal ions by the membranes was calculated. In kinetic study, the inﬂuence of ﬁltering rate of the membranes on the metal uptake was examined. The experiments were performed with Ag (I) and Pb (II) ions at initial concentrations of 10 mg/L. The ﬁltering rate through the membranes was adjusted by the throttle value to range between 4 and 16 mL/s (the relevant ﬁltering time ranged between 10 and 2.5 s). The concentrations of uptaken metal ions on the membranes were measured and calculated after the ﬁltering process. To have a comparison, bulk rhodanine powder was employed as a control for heavy metal uptake. Rhodanine powder weighing
Fig. 3. Schematically the device used for the adsorption tests.
24 mg was added to 40 mL of heavy metal solution with Ag (I) and Pb (II) ions at initial concentrations of 10 mg/L. The solution was remained still for 24 h. After then, the concentration of residual metal ion was measured. 2.5. Recovery of the nanoﬁbrous membrane The recovery study was performed with Ag (I) and Pb (II) ions with an initial concentration of 10 mg/L. 40 mL of nitric acid solution was passed through the used membranes at a rate of 16 mL/s to wash out the adsorpted metal ions on the membranes. This is followed by the wash of the nanoﬁbrous membranes with deionized water for neutralization and recondition. To examine the reusability of the recovered rhodanine/PMMA membranes, 40 mL of Ag (I) and Pb (II) ion solution was passed through the recovered membranes at a ﬁltering rate of 16 mL/s. The recovery efﬁciency was calculated according to the following equations:
UCc 100 ð%Þ UCo
where RE is the recovery rate (%), UCo is the uptake of metal ions by the fresh membrane in mg/L, and UCc is the uptake of metal ions after recovery procedure in mg/L. 3. Results and discussion 3.1. Inﬂuence of ﬁber diameters on the uptake capability of electrospun membranes By adopting different processing parameters as listed in Table 1, nanoﬁbers of various diameters could be successfully fabricated. Fig. 4 shows the SEM micrographs of the electrospun rhodanine/ PMMA nanoﬁbers under magniﬁcation of 5000. The diameters of the spun nanoﬁbers ranged between 840 nm and 1440 nm, and the porosity of the nanoﬁbrous matrix was high. Furthermore, the FTIR spectroscopy analysis was performed to verify the rhodanine in the electrospun nanoﬁbrous membranes. Fig. 5a exhibits the FTIR spectra of virgin PMMA nanoﬁbrous membrane, rhodanine and rhodanine loaded membranes. The absorption peaks of rhodanine were observed in electrospun rhodanine/PMMA nanoﬁbers. In the FTIR spectrum of the rhodanine loaded PMMA membrane, the [email protected]
stretching vibration peak at 1710 cm1 was enhanced with the addition of rhodanine monomer. The peak of 1680 cm1 could be assigned to [email protected]
stretching vibration and the absorbance at 1575 cm1 was ascribed to CAN stretching vibration of rhodanine [25,26]. Furthermore, the S peak is clearly shown in the EDX pattern in Fig. 5b, demonstrating that the rhodanine was successfully embedded in the electrospun nanoﬁbers. As a blank reference, the uptake capability of the electrospun virgin PMMA membranes was ﬁrst measured. At the initial metal ion concentration of 10 mg/L, the metal uptake and the adsorptivity value of silver ions were 15.86 mg/m2 and 11.5% respectively,
C.-H. Lee et al. / Separation and Puriﬁcation Technology 118 (2013) 737–743
Fig. 4. SEM images of nanoﬁbers prepared using different processing conditions (test conditions a to i in Table 1).
while the metal uptake and the adsorptivity of lead ions were 14.89 mg/m2 and 10.8% respectively. The uptake capability of the electrospun rhodanine loaded membranes of various ﬁber diameters was measured and shown in Fig. 6. All nanoﬁbrous membranes exhibit superior metal ion uptake capabilities to the virgin PMMA membranes. With the addition of rhodanine, the uptake capability of the nanoﬁbrous membranes is very much enhanced. In addition, the rhodanine/PMMA membranes show higher adsorptions toward silver ions than lead ions. This might be attributed to that the number of sulfur groups is two times more than that of amide groups in rhodanine . The fabricated membrane thus preferentially uptake Ag (I) ions than Pb (II) ions. Furthermore, all membranes prepared using different processing conditions showed comparable uptake capability of heavy metal ions. This can be explained by the fact that due to the nano-sized
Wavenumber (cm )
Metal uptake (mg/m )
(b) Fig. 5. (a) FTIR spectra of virgin PMMA membrane, rhodanine, and rhodanine/ PMMA membranes and (b) EDX pattern of rhodanine/PMMA membranes.
Test Fig. 6. Effect of ﬁber diameter on the uptake capacity of electrospun nanoﬁbrous membranes.
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3.2. Effect of initial ion concentrations on metal uptake The effect of initial ion concentrations on the uptake capability of heavy metal ion for the fabricated rhodanine/PMMA membranes was investigated. Both Ag (I) and Pb (II) ions were selected as the testing metal ions, while ﬁve different initial metal ion concentrations, namely 2, 4, 6, 8, and 10 mg/L, were used for the tests. The ﬁltering rate used was 16 mL/s. Fig. 7 shows the effect of the initial metal ion concentration on the adsorption capability. Clearly, the metal uptake capability of rhodanine/PMMA membrane increased almost linearly with the initial Ag (I) and Pb (II) ion concentrations. For the solutions prepared in this study, the maximum uptake of the rhodanine/PMMA membrane for Ag (I) and Pb (II) ions are 125.7 and 140.2 mg/m2, respectively. At the initial ion concentration of 2 mg/L, the adsorptivity values of silver and lead ions were 55.1% and 44.2% respectively, which were lower than those at higher initial concentrations. The adsorptivity increased somewhat as the initial ion concentrations reached 4 mg/L and above. The highest adsorptivity values were obtained to be 65.1% at 10 mg/L of initial silver ion concentration and 60.4% at 8 mg/L of initial lead ion concentration. During electrospinning, the rhodanine might be completely encapsulated by the PMMA matrix and did not have contacts with the heavy metal ions in the ﬁltering process. This might explain why the adsorptivity of the electrospun ﬁbers was lower that of previous study . Nevertheless, the rhodanine/PMMA membranes developed in this study could effectively remove the heavy metal ions within a very short period of time (less than 10 s). This would provide advantages in terms of low cost and high throughput metal ion ﬁltration. 3.3. Inﬂuence of ﬁltering rates The inﬂuence of ﬁltering rate through the rhodanine/PMMA membranes on the uptake of Ag (I) and Pb (II) ions was investigated for the kinetic study. As shown in Fig. 8, at a low ﬁltering of 4 mL/s, the metal uptake of the membranes reached the maximum. After that, the metal uptake decreased somewhat with the ﬁltering rate of the solutions through the membranes. Song et al.  proposed that the metal ion uptake process by polyrhodanine
Filtering rate (mL/sec) Fig. 8. Inﬂuence of ﬁltering rate through the rhodanine/PMMA membranes on the uptake of Ag (I) and Pb (II) ions.
can be well described by the pseudo-second-order model which assumes that the determining adsorption rate depends on chemical adsorptions. The adsorption of the heavy metal ions onto the rhodanine/PMMA membranes is mainly performed by chemical process involving valence forces via sharing or exchanging electrons. In this study, the higher ﬁltering rates led to shorter interaction time of heavy metal ions and rhodanine in the nanoﬁbers. The metal uptake capability of electrospun rhodanine/PMMA membranes decreased accordingly. Nevertheless, the reduction of adsorption capability with ﬁltering rate is relatively limited. 3.4. Bulk rhodanine versus rhodanine membranes The adsorption capability of nanoﬁbrous rhodanine/polymethylmethacrylate membranes was compared with that of bulk rhodanine. Bulk rhodanine powder (24 mg) was dissolved in the solution (40 mL) containing metal ions as a control. The membranes prepared using the processing condition h in Table 1 was used for the tests. Fig. 9 shows the measured results. For silver ions, the nanoﬁbrous rhodanine/PMMA membranes exhibited inferior metal removal capability to the bulk rhodanine. This is due to the fact that silver ions readily coordinate with the thioamide and sulﬁde functional groups of rhodanine [27,28]. The removal capability of bulk rhodanine in the solution is thus high. Meanwhile, the
Bulk rhodanine Rhodanine membrane
Metal uptake (mg/m )
Metal uptake (mg/g rhodanine)
Metal uptake (mg/m )
ﬁbers all nanoﬁbrous membranes provided abundant surface areas for the adsorption interaction of rhodanine and the metal ions. Fabricated nanoﬁbers of different diameters thus exhibited comparable metal uptake capability. The membrane that exhibited the most uniform ﬁber diameter distribution (fabricated by condition h in Table 1) was selected for all subsequent experiments.
Initial ion concentration (mg/L) Fig. 7. Effect of initial ion concentrations on the uptake capacity of nanoﬁbrous membranes.
0 1 Ag(I)
Fig. 9. Metal ions uptake capacity of the bulk rhodanine and nanoﬁbrous rhodanine membranes.
C.-H. Lee et al. / Separation and Puriﬁcation Technology 118 (2013) 737–743
study can be used for ﬁltration of various heavy metal ions from contaminated water.
Removal efficiency (%)
4. Conclusions 80
Cycles Fig. 10. Removal efﬁciency of metal ions in recovered nanoﬁbrous membranes.
ﬁltering time of the solutions through the rhodanine membranes is short. The metal uptake of the rhodanine membranes is thus inferior to that of bulk rhodanine. On the other hand, the nanoﬁbrous membranes showed much higher uptake values than the bulk rhodanine for lead ions. Pearson  reported that the Pb (II) ions preferably bind to amide groups than sulfur groups. In addition, the disparity in cation radius and interaction energy of the heavy metal ions can also inﬂuence the different uptake capability of the membranes to different metal ions. The enhanced metal uptake performance of the nanoﬁbrous membrane for lead ions is attributed to the increased surface area of nanoﬁbers which provides more metal binding sites compared to bulk rhodanine. 3.5. Recovery of nanoﬁbrous rhodanine/PMMA membranes The recovery of the fabricated nanoﬁbrous membranes for the removal of Ag (I) and Pb (II) ions were investigated. To recover the metal binding property, the used rhodanine/PMMA membrane was washed with nitric acid solution at a concentration of 3% (0.68 mol/L). When the acid solution passed through the membrane, the adsorbed metal ions were substituted with abundant H+ ions. After being washed by the nitric acid, the membranes were washed several times with deionized water to neutralize the membranes. After the recovery process, the metal ion uptake capability of the nanoﬁbrous membrane was evaluated. The experimental result in Fig. 10 suggests that the metal removal capability of the recovered membrane exhibited comparable metal ions removal capability to those of the fresh ones. Even after ﬁve times of recovery, the rhodanine/PMMA membrane still showed up to 96.0% and 95.3% of Ag (I) and Pb (II) ions uptake efﬁciency respectively. Obviously the rhodanine/PMMA membranes developed in this study showed good reusability for the removal of heavy metal ions. To examine whether the rhodanine had been washed away from the nanoﬁbrous membranes, the n-methyl-2-pyrrolidinone (NMP) solvents were added into the collected solutions. All solutions remained optically transparent, which conﬁrmed that no rhodanine was washed off the membranes . This might explain why the used membranes readily recover their metal-binding properties by acid treatment and washing process. Despite the rhodanine/PMMA nanoﬁbrous membrane works inferior to AAO membranes  for heavy metal ion removal, it is fabricated by a much simpler process. This provides advantages in terms of large quantity and cost effectiveness for industrial use. All these suggest that the rhodanine/PMMA nanoﬁbrous membranes developed in this
In this study, we have developed rhodanine loaded nanoﬁbrous membranes for the removal of heavy metal ions. Rhodanine and polymethylmethacrylate dissolved in 1,1,1,3,3,3-hexaﬂuoro-2-propanol (HFIP) were electrospun into nanoﬁbrous membranes via an electrospinning process. The average diameter of electrospun nanoﬁbers ranged from 840 nm to 1440 nm. All electrospun nanoﬁbrous membranes exhibit good Ag (I) and Pb (II) ion uptake capabilities. The metal uptake of nanoﬁbrous membranes increased with the initial metal ion concentrations and decreased with the ﬁltering rate of the solutions. Furthermore, the electrospun membrane could be reused after the recovery process. The empirical results in this study suggest that electrospun rhodanine/ polymethylmethacrylate nanoﬁbrous membranes can be a good candidate for the removal of heavy metal ions. References  A. Heidari, H. Younesi, Z. Mehraban, Removal of Ni(II), Cd(II), and Pb(II) from a ternary aqueous solution by amino functionalized mesoporous and nanomesoporous silica, Chem. Eng. J. 153 (2009) 70–79.  R.R. Navarro, S. Wada, K. Tatsumi, Heavy metal precipitation by polycation– polyanion complex of PEI and its phosphonomethylated derivative, J. Hazard. Mater. 123 (2005) 203–209.  S.-H. Song, B.-Y. Yeom, W.S. Shim, S.M. Hudson, T.-S. Hwang, Synthesis of biocompatible CS-g-CMS ion exchangers and their adsorption behavior for heavy metal ions, J. Ind. Eng. Chem. 13 (2007) 1009–1016.  Y. Benito, M.L. Ruiz, Reverse osmosis applied to metal ﬁnishing wastewater, Desalination 142 (2002) 229–234.  A.J. Chaudhary, S.M. Grimes, Mukhtar-ul-Hassan, Simultaneous recovery of copper and degradation of 2,4-dichlorophenoxyacetic acid in aqueous systems by a combination of electrolytic and photolytic processes, Chemosphere 44 (2001) 1223–1230.  M.F. Sawalha, J.R. Peralta-Videa, J. Romero-Gonzalez, J.L. Gardea-Torresdey, Biosorption of Cd(II), Cr(III), and Cr(VI) by saltbush (Atriplex canescens) biomass: thermodynamic and isotherm studies, J. Colloid Interface Sci. 300 (2006) 100–104.  A. Sari, D. Mendil, M. Tuzen, M. Soylak, Biosorption of Cd(II) from aqueous solution by red algae (Ceramium virgatum): equilibrium, kinetic and thermodynamic studies, J. Hazard. Mater. 157 (2008) 448–454.  M. Choi, J. Jang, Heavy metal ion adsorption onto polypyrrole-impregnated porous carbon, J. Colloid Interface Sci. 325 (2008) 287–289.  J.-Y. Lee, T.-S. Kwon, K. Baek, J.-W. Yang, Adsorption characteristics of metal ions by CO2-ﬁxing chlorella sp. HA-1, J. Ind. Eng. Chem. 15 (2009) 354–358.  H. Bessbousse, T. Rhalou, J.-F. Verchere, L. Lebrun, Removal of heavy metal ions from aqueous solutions by ﬁltration with a novel complexing membrane containing poly(ethyleneimine) in a poly(vinyl alcohol) matrix, J. Membr. Sci. 307 (2008) 249–259.  S. Zhang, F. Cheng, Z. Tao, F. Gao, J. Chen, Removal of nickel ions from wastewater by Mg(OH)2/MgO nanostructures embedded in Al2O3 membranes, J. Alloys Compd. 426 (2006) 281–285.  R.A. Kumbasar, Selective extraction and concentration of chromium(VI) from acidic solutions containing various metal ions through emulsion liquid membranes using Amberlite LA-2, J. Ind. Eng. Chem. 16 (2010) 829–836.  R. Qu, Chemical modiﬁcation of waste poly(p-phenylene terephthalamide) ﬁbers and its binding behaviors to metal ions, Chem. Eng. J. 181–182 (2012) 458–466.  T. Lin, Polypyrrole-coated electrospun nanoﬁbre membranes for recovery of Au(III) from aqueous solution, J. Membr. Sci. 303 (2007) 119–125.  S.-Y. Park, Preparation of the electrospun chitosan nanoﬁbers and their applications to the adsorption of Cu(II) and Pb(II) ions from an aqueous solution, J. Membr. Sci. 328 (2009) 90–96.  M. Yigitoglu, Selective removal of Cr(VI) ions from aqueous solutions including Cr(VI), Cu(II) and Cd(II) ions by 4-vinly pyridine/2-hydroxyethylmethacrylate monomer mixture grafted poly(ethylene terephthalate) ﬁber, J. Hazard. Mater. 166 (2009) 435–444.  Z.M. Huang, Y.Z. Zhang, M. Kotaki, S. Ramakrishna, A review on polymer nanoﬁbers by electrospinning and their applications in nanocomposites, Compos. Sci. Technol. 63 (2003) 2223–2253.  P. Su, C. Wang, X. Yang, X. Chen, C. Gao, X.X. Feng, J.Y. Chen, J. Ye, Z. Gou, Electrospinning of chitosan nanoﬁbers: the favorable effect of metal ions, Carbohydr. Polym. 84 (2011) 239–246.  S. Haider, S.Y. Park, Preparation of the electrospun chitosan nanoﬁbers and their applications to the adsorption of Cu (II) and Pb (II) ions from an aqueous solution, J. Membr. Sci. 328 (2009) 90–96.
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