Characterization of functionalized multiwalled carbon nanotubes and application as an effective filter for heavy metal removal from aqueous solutions
Emad.M. Elsehly1,3,*, N.G. Chechenin1,2, A.V. Makunin1, H.A. Motaweh3, E.A. Vorobyeva1, K.A. Bukunov1,2, E.G. Leksina1, A.B. Priselkova1
1. Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Russia, 2. Faculty of Physics, Lomonosov Moscow State University, Russia, 3. Faculty of Science, Damanhour University, Egypt
*Corresponding author, e-mail:
[email protected] *Present address: : Skobeltsyn Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, Leninskie Gory 1/2, 119234, Russian Federation. Tel.: +79854904995
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Abstract Filtration efficiency of Ni(II) from aqueous solution using pristine and modified MWCNTs filters was investigated as a function of Ni(II) ion concentration, pH, filter mass. MWCNTs were synthesized by CVD method and modified using two complementary treatments, purification (using a mixture of hydrochloric acid and hydrogen peroxide) and functionalization (using nitric acid). The effect and mechanism of each treatment on the structural integrity of pristine MWCNTs has been studied. Morphology of the pristine and modified filters was investigated by Raman Spectrometry (RS), Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS), Fourier Transform Infrared (FTIR) spectrometry and Thermogravimetric analysis. It was found from Raman spectra that the ratio of the intensity of D-band to that of G-band decreased by purification process, and increased by functionalization process. The adsorption mechanism of Ni(II) onto the surface functional groups of modified MWCNTs was confirmed by FTIR spectrum. The filtration results showed that the removal efficiency of Ni(II) is strongly dependent on pH and could reach 85% at pH=8. Also, modified MWCNTs filters can be reused through many cycles of regeneration with high performance. Functionalized MWCNTs filters may be a promising adsorbent candidate for heavy metal removal from wastewater.
Keywords: MWCNTs filters; functionalization; nickel removal; filtration; Raman spectrometry; purification
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1. Introduction Carbon nanotubes (CNTs) have attracted the interest of scientists all over the world because of their special structure and many potential applications. Both singlewalled and multiwalled CNTs have numerous potential applications in basic science and nanotechnology, such as nanoscale electronic devices, field emission transistors, hydrogen storage devices, nanocomposite materials, wastewater treatment membranes [1-3]. MWCNTs are becoming more and more attractive from the practical standpoint. Their relatively low production costs and availability in large quantities are two important advantages over SWCNTs. The hollow and layered nanostructure of CNTs endows them with a characteristically large surface area and a correspondingly high potential sorption capability for heavy metal pollutants. The presence of an excessive amount of heavy metals such as cadmium, chromium, copper, lead, nickel, mercury and zinc in an aqueous environment may result in a major concern due to their toxicity and carcinogenicity, which may cause damage to various systems of the human body [4]. Nickel is a toxic metal ion present in wastewater. More than 40% of nickel produced is used in steel factories, in nickel batteries, and in the production of some alloys, which causes an increase in the Ni(II) burden on the ecosystem and deterioration of water quality [5]. Many methods have been developed and used to remove metal ions from wastewater such as ion exchange, reverse osmosis, and electrodialysis have proven to be either too expensive or inefficient to remove heavy metal ions from aqueous solutions [6]. Increasingly stringent standards regarding the quality of drinking water have stimulated a growing effort in the exploitation of new highly efficient methods. CNT filters, particularly those that are functionalized have been recommended as a potential adsorbent for the removal of heavy metals in contaminated water. Several methods of CNT production currently exist. In the laboratory environment, catalytic chemical vapor deposition (CVD) is preferred over other methods, such as arc discharge and laser ablation, because it produces higher quality CNTs, as well as densely packed vertically aligned arrays. As-prepared MWCNTs materials normally contain a variety of impurities, such as amorphous carbon, metallic catalyst. This problem limits the best performance of CNTs for new applications. For example, hazardous pollutants removal of from aqueous streams 3
using CNT membranes requires disposing of these impurities, as well as improving the surface chemistry of these CNTs [7]. Nanotube reactivity often is determined by structural defects in the sidewalls that are observed to occur in normal growth processes, and can be introduced under oxidative conditions. These defects can then be used to attach various functional groups to the nanotubes to achieve chemical functionalization [8]. Functional groups such as -OH, -C=O and -COOH could be intentionally introduced onto CNT surfaces by acid oxidation or air oxidation. Those functional groups make CNTs more suitable for heavy metals adsorption, which depends mainly on the specific complexation between metal ions and the hydrophilic functional groups of CNTs [9]. Several attempts have been tried to purify the CNT powders. Gas phase reaction or thermal annealing in air or oxygen atmosphere has been attempted; although the yield of final product was relatively low, the key idea with these approaches is a selective oxidative etching process, based on the fact that the etching rate of amorphous carbons is faster than that of CNTs [10,11]. Since the edge of the CNTs can be etched away as well as carbonaceous particles during the annealing, it is crucial to have a keen control of annealing temperatures and annealing times to obtain high yield, although the yield is also dependent on the purity of the original sample. Liquid-phase reaction in various acids has been tried to remove the transition metals. This process involves repeated steps of filtering and sonications in acidic solution, where the transition metals were melted into the solution. CNTs are usually cut into small lengths and sometimes broken completely. The remaining walls are severely damaged with strong acid solution [12]. Therefore, the choice of acids, immersing time, and temperature are the key factors to have high yield, while maintaining the complete wall of CNTs. Raman Spectrometry (RS) is well known as a powerful tool for the characterization of carbon structures [13]. In situ RS allows monitoring the oxidation kinetics while providing time-resolved information about structural changes during the treatment. The Raman spectrum of MWCNTs is marked by the presence of four bands; two bands refer to undistorted graphite structure, corresponding to the in-plane optical phonon modes, named G- and G′-bands centered approximately at 1580 and 2700 cm−1 respectively. The other two bands are related to the defects in the graphitic sheets, and originate from coherent radial vibrations of six-atom rings, named D- and 4
D″-bands centered approximately at 1350 and 2950 cm−1 respectively. In some reports they refer G′ as 2D and D″ as D+D′, the 2D-band is the second order of the D-band, and is always present even when there is no D-band because no defects are required for the activation of this band. The D and G mode intensities have usually been employed as an indication of chemical modification of CNTs. The ratio of the Raman intensities of the D- and G-bands can be used not only for a qualitative estimate of the quantity of defects in a carbon material, but also as an indication for the purity of these materials. In this paper, MWCNTs were synthesized by CVD method based on the simultaneous injection of a ferrocene–cyclohexane solution through a sprayer into the reaction furnace [14,15]. These MWCNTs were purified and functionalized through two different processes; the first treatment includes the modification of pristine sample using a mixture of HCl and H2O2 and the other process includes the oxidation treatment of purified MWCNTs using nitric acid. A comparative study was performed between the two regimes not only to investigate the efficiency of removing the carbonaceous particles and transition metals, but also to account for the desirable oxygen-containing groups onto the MWCNT surface. Pristine and modified MWCNTs were investigated using RS, thermal analysis, SEM and EDS analysis to study the effect of these treatments. To identify the functional groups produced after oxidation by nitric acid, Fourier transform infrared spectrometry (FTIR) of pristine and modified MWCNTs was employed. To account for the efficiency of functionalized MWCNT filters for heavy metal removal, the removal efficiency of Ni(II) from aqueous solution using pristine and modified MWCNT filters was investigated as a function of Ni(II) ion concentration, pH, MWCNTs filter mass.
2. Material and methods 2.1. Synthesis of MWCNTs using CVD method The setup for MWCNTs growth is essentially the same as described previously [15]. The CVD reactor is Ø 2.5 cm × 100 cm long quartz tube placed in an automatically temperature controlled oven. Liquid solutions of ferrocene in cyclohexane as well as the support nitrogen gas were supplied into the reactor. The liquid solution evaporation started in the first part of the tube, which is a low 5
temperature zone, and a flow of the support gas saturated with the active vapor components passed into a high temperature zone with a preset temperature 850°C where the active components decomposed. With the described method uniform MWCNTs arrays up to of 20 × 80 mm2 were systematically obtained on a Si-plate. 2.2. Preparation of modified MWCNT and characterization MWCNTs synthesized by the descried method removed from the silicon substrate and crashed to obtain powder form. In first treatment, a 100 mg of pristine MWCNTs was mixed directly with 20 ml of 5 M hydrochloric acid and 20 ml of 50% H2O2 in a 150 ml open flask. Afterwards, the mixture was sonicated in water bath at 60°C for 2 h, using ultrasonic vibrations. At the end of each 30 minutes, the mixture filtered and again 20 ml of hydrochloric acid and 20 ml of H2O2 were added to the slurry. Within the first 30 min, the solution turned green/yellow colored, indicative of iron dissolution. On the last cycle, the solution became colorless. Finally, the resulting suspension was filtered, washed with 500 ml deionized water and dried at 100°C in air for 2 h. In the second treatment, the same amount of pristine MWCNTs was dispersed in 100 ml of nitric acid 60% in a 250 ml flask. Then the mixture solution was sonicated in water bath at 80°C for 6 hours. After that, the solution was filtered and washed several times with deionized water until it reaches pH 7. The functionalized MWCNTs had been dried in oven at 100°C for 12 h. The modified MWCNTs were prepared by combining these treatments. The pristine and modified CNT materials were weighted and analyzed using scanning electron microscope equipped with energy dispersive X-ray spectroscopy to clearly investigate the effectiveness of the purification and oxidation processes. Also, MWCNT samples were analyzed using Raman spectroscopy to account for the structural distortion as a result of the modification methods. Thermogravimetric analysis was performed of the samples to calculate the mass loss after heating which is another characterization method for these treatments. Finally, FTIR spectra of pristine and oxidized MWCNTs were recorded using (IFS 66 V/S) FTIR spectrometer. 2.3. Filter design and Ni(II) stock solution preparation Figure 1 shows the steps for filter design for the adsorption process. Pristine and modified MWCNTs obtained from pervious section were crashed into fine powder and compressed using high pressure piston in the form of circular disk. MWCNTs 6
filters were prepared by sandwiching of compressed pristine and modified MWCNTs between two pieces of glassy fiber membrane with a cotton layer as a membrane substrate, to keep insuring that not any MWCNTs passes through the membrane, and putted into a syringe which act as the filtering system [16]. The1000 ppm Ni(II) stock solution was prepared by dissolving 1.8 g of NiSO4∙7H2O in 1000 ml of distilled water then diluted to the desired concentration. The pH of the stock solutions is adjusted by 0.1 M HNO3 or 0.1 M NaOH. To account for the adsorption mechanism for Ni(II) removal, pH of the stock solutions before and after each filtration process was measured.
Fig. 1 Schematic for the steps of MWCNTs filter design and filtration mechanism
2.4. Experimental procedure All of the batch experiments were performed in 50 ml syringe filtering system. In each experiment, various amounts of MWCNTs, from 0.2 to 0.6 g, were putted into the syringe and the 50 ml of Nickel solution, with initial concentrations of 50-200 ppm, were added. In addition, these variables were examined at different pH varying from 3 to 8. Table 1 shows the experimental parameters and their variations. Before and after each experiment, the nickel concentration was determined in the liquid phase using atomic absorption spectrophotometer. The removal efficiency (R) is defined as follows:
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R(%)
C0 C 100 C0
where CO and C are the nickel concentrations in aqueous solution, initial and after filtration in (mg/l). To evaluate the reversibility of Ni(II) adsorption and filter regeneration, after Ni(II) filtration at pH=8 and concentration 50 ppm, modified MWCNTs with adsorbed Ni(II) were used as a filter to 50 ml of water, NaOH and HNO3 solutions, in order to determine the effect of pH of regeneration solution on desorption process.
Table 1 Experiment parameters and its variation. Parameter
Variation Low Medium
High
1. Concentration of Ni(II) in aqueous solution (ppm)
50
100
2. pH of Ni(II) aqueous solution
3
6
8
3. pristine and modified MWCNTs filter mass (in g per 50 ml of solution)
0.2
0.4
0.6
200
3. Results and discussion 3.1. Surface analysis of pristine and modified MWCNTs filters Morphology of the pristine and modified MWCNTs was characterized by SEM and EDS analysis. Figure 2a shows the pristine CNT array capped with amorphous and metal impurities produced from the CVD method. With purification treatment using a mixture of (HCl+ H2O2) the sample tubes become clearly seen, without impurities, and with a slight oxidation, Fig. 2b. It is clearly seen from Fig. 2c the effect of nitric acid oxidation which tends to functionalize the MWCNTs walls by oxygen groups. This followed by a clear decrease in the nanotube diameters, Average diameter of pristine and modified CNTs were around 50 and 20 nm, respectively, as determined with scanning electron microscope, which indicates that the functionalization process does not change the bulk structure of the CNTs. The EDS analysis not only offers a quantitative study of the oxygen content of the modified MWCNTs, but also is a measure of metal impurities removal efficiency. MWCNTs as
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prepared contain a fraction of iron metal impurity and a tiny oxygen content due to the synthesis procedure, Fig. 2d. The iron impurity was removed after treating the initial sample with (HCl+H2O2) solution, introducing some oxygen fraction due to small oxidation effect, Fig. 2e. Although the oxidation effect of nitric acid is clear and introduce high oxygen content, this chemical process did not remove completely the iron impurities. Fig. 2f shows that there is still small fraction of metal impurities. Based on the above analysis, the first treatment is more favorable for purification and the second one may lead to high functionalization efficiency.
(a)
(b)
Element CK OK Fe L
(d)
Weight % 95.4 0.84 3.76
(c)
Element CK OK
Weight % 95.43 4.57
Element CK OK Fe L
(f)
(e)
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Weight % 88.75 10.5 0.75
Fig. 2 SEM images showing the surface morphology of pristine MWCNTs (a) HCl+ H2O2 treated (b), oxidation using HNO3 (c). The EDS analysis shows the oxygencontent in each case: pristine MWCNT (d), HCl+ H2O2 treated (e), oxidation using HNO3 (f). 3.2. Raman spectroscopy characterization of pristine and modified MWCNT Raman spectroscopy is a very valuable tool for the characterization of carbonbased nanostructures. Raman spectra of pristine and treated MWCNTs excited with the 632.8 nm laser line are shown in Fig. 3. Each of them consists of four characteristic bands. As discussed before, the D-band is usually attributed to the presence of amorphous or disordered carbon in the CNT samples. The G-band originates from in-plane tangential stretching of the carbon-carbon bonds in graphene sheets. The data obtained from the experiment shows a reduction of the D-band intensity has occurred after (HCl+H2O2) treatment followed by G-band increases. On the other hand, the D-band intensity increases back after nitric acid treatment, but Gband decreases. So, the ID/IG ratio intensity ratio can be used as a qualitative measure of the quantity of defects in MWCNTs before and after treatments. In Fig. 3a, this ratio is 1.1 which indicates on the presence of defects from impurities in synthesized MWCNTs. Modification of these MWCNTs with an aqueous mixture of H2O2 and HCl leads to an appreciable elimination of graphitic nanoparticles. Hence, the ID/IG ratio decreased to reach 0.87, Fig. 3b that accounts for purified MWCNTs with lower defect concentration. In contrast, the ID/IG ratio increased significantly in the case of nitric acid treatment to be 1.35, Fig. 3c, which supports the formation of defects in MWCNTs walls due to oxidation effect and attaching oxygen content like carboxylic groups. Similar results were obtained with Raman spectrum in [17,18]. Also, the other two bands G′ and D″ support our explanation, as G′-band accounts for less defect CNTs, it is clearly seen from Fig. 3, the higher intensity of this band in the purification process using H2O2 and HCl treatment and the lower intensity of G′-band for the case of nitric acid treatment. Finally, D″-band always appeared in defect materials. For pristine and nitric acid oxidation samples, the appearance of this band illustrates the presence of defects. But the appearing of this band with H2O2 and HCl treatment may be from distortions due to oxidation after the treatment. There is also a
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weak band mode approximately at 2450 cm-1, which is the overtone of pristine graphite.
Fig. 3 Raman spectra with ID/IG ratio shown for pristine (a), HCl+ H2O2 treated (b) and HNO3 treated (c) MWCNT samples. 3.3. Functionalization of MWCNT filters and adsorption of heavy metal ions mechanism Nitric acid oxidation with aqueous HNO3 involves the presence of the two ionic species NO3- and H3O+, as well as NO2+, arising from self-dehydration. The exploration of the reaction mechanism was dictated by two steps: i) the fast initial formation of carbonyl groups on the surface; ii) the final production of -COOH groups [19]. At first the attack of a nitrate ion on the reactive carbon atom (reaction a
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in Fig. 4), yields the oxygenated group, and a nitrite ion. The reaction of a nitronium ion is also another mechanism for carbonyl group formation. The nitrosonium byproduct NO+ can then react with water to produce nitrous acid HNO2 and a hydrated proton. These steps compete and account for the initiation of the oxidation reaction. In the second reaction transformation of carbonyl group into phenol occurs, (reaction b), the associated mechanism of this important experimental process is actually very simple. It consists in the protonation of second product by a hydronium and leads to a H-bonded water adduct of [R2COH] +. The third elementary reaction (reaction c) occurs with a second nitrate ion reacting with the carbon atom of the phenol group, leading to the formation of an N2O- group bound to -O-C (R2)-OH. Finally, in (reaction d) one additional hydronium ion H3O+ could assist the spontaneous formation of a -COOH group and the simultaneous enlargement of the vacancy, yielding HNO2 as an end product. The carboxylic function groups change the surface chemistry and the charge of CNTs. This may indicate that the adsorption interaction between the functional groups of oxidized MWCNTs and heavy metal ions in liquid solutions was mainly of ionic interaction nature, which is in agreement with an ion exchange mechanism [16,20].
Fig. 4 MWCNTs functionalization and the adsorption mechanism.
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3.4. MWCNTs purification mechanism using a mixture of HCl and H2O2 Pristine MWCNTs mixed with impurities represented in amorphous carbon and iron catalyst, the later is difficult to be removed by normal purification methods. Hydrochloric acid itself cannot remove most metal impurities from raw MWCNT materials because of the carbon coating. While H2O2 alone has been used in the past to purify carbon nanotubes, the major problem is that MWCNTs are substantially consumed by H2O2, resulting in extremely low yields. Purification technique using a mixture of HCl and H2O2 combines in a single step oxidation involving H2O2 and metal extraction with hydrochloric acid. The mechanism arises from the catalytic effect of iron, that catalyzed the production of hydroxyl radicals (•OH) from H2O2, a more powerful oxidant than H2O2. These radicals attack the carbonaceous shells enclosing iron nanoparticles. The iron impurities become an effective catalyst through its in situ change from chemical state Fe to Fe2+ by the hydrochloric acid action. The mixed H2O2/HCl display high selectivity toward the removal of iron and other nonnanotube impurities; they do not destroy MWCNTs because the iron nanoparticles catalyze the H2O2 attack to the carbonaceous shells but quickly dissolve by acid and diffuse to the solution before they can come in contact with the MWCNTs. 3.5. FTIR spectroscopy For investigations by FTIR spectrometry, the pristine and modified MWCNTs samples were prepared onto KBr crystals by drag and drop method. The MWCNT sample was dispersed in an acetone solution ultra-sonication bath, then MWCNT solution was dragged out using the Pasteur pipette and dropped onto crystal surface. Subsequently, the acetone from KBr surface was evaporated at hot plate at 80°C and a thin film of the investigated sample was obtained. Figure 5 summarizes the behavior of the functional groups observed via FTIR spectroscopy of the pristine and oxidized MWCNT samples before and after filtration. The acid treated MWCNTs shows new peaks in comparison with the FTIR spectrum of the untreated MWCNTs, which lack the hydroxyl and carbonyl groups. The peaks around 3445 cm-1 are assigned to the O–H band in C-OH, and the peaks at 1635 cm-1 are assigned to C=O in – COOH group, also the band at 1150 cm-1 which is the C–O stretch mode associated with ether type groups. This demonstrates that hydroxyl and carbonyl groups have been introduced on the nanotube surface [21]. Similar observations of the 13
FTIR spectra for MWCNT-COOH were reported by Theodore et al. [22] and Wu et al. [23]. It is suggested that the adsorption of heavy metals onto the CNTs are mainly controlled by the strong interactions between the metal ions and hydrophilic surface functional groups, especially carboxyl and hydroxyl groups [24]. Comparison of the FTIR spectra of MWCNTs before and after Ni(II) filtration confirmed this mechanism (Fig. 5). Compared to the original FTIR spectrum, the adsorption of Ni(II) on the functionalized MWCNTs resulted in variations of FTIR peaks which could be attributed to the interactions between metal ions and carboxyl and hydroxyl groups.
Fig. 5 FTIR spectra of pristine MWCNTs (a), modified MWCNTs before filtration (b) and modified MWCNTs after filtration (c).
3.6. Thermal stability of pristine and modified MWCNTs The study of the thermal degradation of materials is of major importance since it can, in many cases; determine the upper temperature limit of use for a material. It is well known that different structural forms of carbon can exhibit different oxidation behavior. For example, disordered or amorphous carbons tend to be oxidized at around 500°C, because of their lower activation energies for oxidation or due to the presence of a large number of active sites. On the other hand, a well graphitized structure starts to oxidize at a relatively higher temperature between 600 and 700°C,
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depending on type of CNTs. Differences regarding the oxidation behavior for the pristine, HCl+H2O2 treated and oxidized MWCNT samples can be clearly seen in Fig. 6. Pristine nanotubes are thermally stable up to a temperature of 400°C. The residue of 1% can be attributed to impurities remaining in the sample after the production process Fig. 6, curve a. In comparison to this result the oxidized MWCNTs starts to decompose at lower temperature. The thermal degradation of HNO3-treated MWCNTs in the temperature range between 200 to 400°C is caused by the decomposition of the carboxylic groups attached to the surface during the nitric acid treatment. A weight loss of 15% in this temperature region reveals the successful functionalization of carbon nanotubes and formation of oxygen containing groups, Fig. 6, curve c. Thermal degradation in the range between the 400°C and 500°C may be explained by the elimination of hydroxyl functionalities, attached to the MWCNT walls. Finally, at the temperatures higher than 500°C, the observed degradation corresponds to the thermal oxidation of the remaining disordered carbon. It is worth adding that the a high thermal stability observed for MWCNTs treated with mixture of HCl and H2O2, curve b in Fig. 6, which has a plateau in weight loss up to 500°C, is a feature of more pure CNTs obtained by such treatment. The residue of 2% can be attributed to small oxidation effect. This finding is in agreement with Raman results and allows us to conclude that this specific treatment is the most effective for both purification and mild oxidation of the as-received material.
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Fig. 6 The weight loss as a function of temperature for pristine MWCNT (a), HCl+ H2O2 treated (b) and HNO3 treated (c) samples. 3.7. Effect of Ni(II) concentration in aqueous solution on the removal efficiency A comparative study of the filtration efficiency of pristine and modified MWCNT filters is performed for Ni(II) removal as a function of its concentration in aqueous solution. From Fig. 7 one can see as the concentration of Ni(II) aqueous solution increase the removal efficiency decreases. At 200 ppm concentration, R is only 18.5% for 0.4 g pristine MWCNTs (P-MWCNT) but for 0.4 g modified MWCNTs (MMWCNT) the efficiency is 28%; whereas at low concentration of 50 ppm, R of MMWCNT is around 53%. This may indicate that the adsorption interaction between M-MWCNT and Ni(II) ions was mainly of ionic interaction nature which is in agreement with an ion exchange mechanism, as illustrated in Fig. 4. High concentrations of Ni(II) limits its transfer to M-MWCNT surfaces, this in accordance with the reports [25,26], which shows low adsorption efficiency for heavy metal ions at higher concentrations.
Fig. 7 Variation in removal efficiency with concentration of Ni(II) at pH=6.
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3.8. Effect of pH of Ni(II) aqueous solution The solution pH is one of the dominant parameters controlling the filtration efficiency. At a pH ≤ 8, it is known that Ni2+ is the predominant nickel species which can be present in deionized water and the Ni2+ removal is mainly accomplished by sorption process. Ni(II) removal by P-MWCNT and M-MWCNT filters as a function of pH ranging from 3 to 8, for 50 ppm concentration and 0.4 filter mass, is given in Fig. 8. The removal of Ni(II) by M-MWCNTs filter increased from 22% at pH=3 to 85% at pH=8. The solution pH affected the surface charge of M-MWCNT, the degree of ionization, and the speciation of the surface functional groups. In low pH range (35), the surface of M-MWCNTs became positively charged and Ni(II) ions could be hardly adsorbed on the surface of oxidized MWCNTs because the Ni(II) ions are hydrated cations. Therefore, the removal efficiency of Ni(II) ions was very small in this range. But at higher pH of (6-8), more Ni(II) ions would be taken up, this results similar to the reports [27,28]. In conclusion, the increase in Ni(II) removal as the pH increases could be explained on the basis of a decrease in competition between protons and Ni(II) ions for the same adsorption sites and by the decrease in positive surface charge, which resulted in a lower electrostatic repulsion between the surface and Ni(II) ions. Similar results obtained by [16,29], these studies showed maximum removal efficiency of Fe(II) and Ni(II) at pH range of 8. Fig. 9 shows the pH evolution during the filtration process by M-MWCNTs. The points below the solid line, which indicate a decrease in solution pH during filtration process, were observed at pH of (6-8). This could be explained by the ion exchange mechanism model. Since the functional groups such as carboxylic acid (–COOH) are present on the surface site of CNTs, fig. 4, H+ in the functional groups exchange with Ni2+ in the aqueous solution and thus lead to a decrease of pH of the solution [30]. No change in pH is observed for the points close to the solid line.
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Fig. 8 Variation in removal efficiency with pH of Ni(II) aqueous solution.
Fig. 9 The change of Ni(II) aqueous solution pH during filtration process by MMWCNT.
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3.9. Effect of MWCNT filter mass Figure 10 shows an increase of the removal efficiency of Ni(II) with the increase of MWCNTs filter mass. The removal efficiency of Ni(II) by M-MWCNT filter could reach 65% for 0.6 g filter mass at pH = 6 and 50 ppm concentration. This phenomenon implied that the filtration depended on the availability of binding sites. The oxidization treatment had evident impact on the MWCNTs removal efficiency of Ni(II). It is also clear that the removal efficiency of M-MWCNTs was higher than that of P-MWCNTs. M-MWCNT filters had larger specific surface area than P-MWCNT ones. It is known that oxidation of carbon surface can offer not only a higher specific surface area, but also a larger number of oxygen-containing functional groups, which increases the ion exchange capacity of carbon materials.
Fig. 10 Variation in removal efficiency of Ni(II) with MWCNTs filter mass at pH=6 and C=50 ppm. 3.10. Adsorption efficiency of Ni(II) and contact time The adsorption efficiency is correlated to the contact time between Ni(II) ions in the solution and the CNTs in the filter. The contact time (tc) can be roughly estimated in our filtration experiment as the thickness of the filter (hf) divided by the solution flow rate (fs) through the filter as follow:
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tc
hf fs
hf hs / t s
h f .t s Vs / A
,
where hs is the solution height in the filtration syringe, ts is the total time required for filtration, Vs is the volume of the solution (50 cm3) with the cross-section A of the syringe, containing the solution, which is the same area as the filter. In our experiment, Ni(II) solution concentration was 50 ppm with pH =6, filter mass was 0.6 g, thickness and active area of the filter were 0.4 cm and 3cm2 respectively. Filtration time, solution flow rate, contact time and removal efficiency of Ni(II) for PMWCNTs and M-MWCNT filters are listed in Table 2. Contact time of 0.96 min was required to remove 42% of Ni(II) by P-MWCNT filter. Even at 0.6 min contact time, the removal efficiency by M-MWCNT filter was high and could reach 65%. Also the flow rate of the solution through the filter in the case of M-MWCNT filter was higher than P-MWCNT one. The short contact time of the M-MWCNT filter, further supports the application of M-MWCNTs as effective sorbents [29]. Table 2 The filtration time, solution flow rate, contact time and the removal efficiency of Ni(II) for P-MWCNTs and M-MWCNT filters. Filtration time Flow rate Contact time Removal efficiency (min) (cm/min) (min) (%) P-MWCNT 40 0.42 0.96 42 M-MWCNT
25
0.67
0.60
65
3.11. Effect of solution pH on the filter regeneration To regenerate the MWCNT-based filters we applied a treatment by HNO3 and NaOH solutions in addition to pure water treatment. Figure 11 shows the recovery of modified MWCNT filter at three different pH. It is apparent that the recovery percentage due to Ni(II) desorption from the filter increased with decreasing pH. H+ is the main compound lead Ni(II) being recovered from adsorbent surface. At high and neutral pH, the concentration of H+ is lower than acid solutions, so the filter recovery is lower. Therefore, HNO3 solution was selected as the regeneration solution for effective Ni(II) desorption. This recovery also indicates that ion exchange was involved in the adsorption mechanism. The recovery of 0.6 g of M-MWCNT filter after absorbing 65% of 50 ppm Ni(II) concentration by 50 ml HNO3 solution is low cost process. Economically the costs for recovery are not higher than the new MMWCNTs cost, making the technology cost neutral. 50 ml HNO3 costs 0.025 $ which is very low compared with new CNT filter which is about 10 $/ g. So, M-MWCNTs
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are low-cost adsorbent and offer a promising technique for industrial wastewaters cleanup.
Fig. 11 Effect of solution pH on desorption of Ni(II) from modified MWCNTs filters.
4. Conclusions In order to meet specific requirements for particular applications (e.g. water treatment applications), purification and functionalization of MWCNTs are essential features. The efficiency of P-MWCNTs- and M-MWCNTs-based filters to remove Ni(II) from aqueous solution was investigated. It was noted that, the key factors favoring the removal efficiency are high pH and low initial concentration. The removal efficiency of M-MWCNTs filter is higher than that of P-MWCNTs one, suggesting that functionalized MWCNTs filters are more efficient for Ni(II) removal and possess good potential for heavy metal removal from aqueous solutions. A detailed methodology for the purification and functionalization of MWCNT via two different processes is presented. The Raman spectroscopy is used as a quantitative method for evaluating the purity and chemical functionalization of MWCNTs. The purity and the oxidation of MWCNTs was characterized by the intensity ratio of Dband to G-band (ID/IG), which was 1.1, 0.87 and 1.35 for pristine, (HCl + H2O2) and nitric acid treated samples, respectively. Functionalization of MWCNTs by various 21
functional groups and the adsorption of Ni(II) onto these groups has been confirmed using FTIR spectroscopy. The MWCNTs surface morphology was greatly changed after modification in addition to the attachment of functional groups. This study gives evidence of two complementary processes that not only removing impurities from pristine MWCNTs but also functionalizing them with oxygen containing groups, which enhances the performance of MWCNTs as a novel filter technology towards water treatment applications with high removal efficiency and regeneration possibility. Acknowledgements The help Mr. N.B. Akimov on different stages of the work is highly acknowledged. The work is supported by the Program of MSU Development and Russian Foundation for Basic Research (RFBR) (grants # 14-02-01230a and # 14-02-31147 mol_a). E. Elsehly is thankful for the financial support from the Ministry of High Education (missions sector) in Egypt. References [1] C. Klumpp, K. Kostarelos, M. Prato, A. Bianco, Functionalized carbon nanotubes as emerging nanovectors for the delivery of therapeutics, Biochem. Biophys. Acta 1758 (2006) 404-412. [2] M. Endo, M.S. Strano, P.M. Ajayan, Potential applications of carbon nanotubes, Top Appl. Phys. 111 (2008) 13-62. [3] J.M. Schnorr, T.M. Swager, Emerging applications of carbon nanotubes, Chem. Mater 23 (2011) 646-657. [4] P.W. Purdom, Environmental Health, second ed., Academic Press, New York, (1980). [5] M. Ajmal, R.A.K. Rao, R. Ahmad, J. Ahmad, Adsorption studies on citrus reticulate: removal and recovery of Ni(II) from electroplating wastewater, J. Hazard. Mater B 79 (2000) 117-131. [6] P. Ganesan, R. Kamaraj, G. Sozhan, S. Vasudevan, Oxidized multiwalled carbon nanotubes as adsorbent for the removal of manganese from aqueous solution, Environmental Science and Pollution Research 20 (2013) 987-996. 22
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