Chunlai Wuab,
Jing Fan*a,
Juhui Jianga and
Jianji Wangc
aSchool of Environment, Henan Key Laboratory for Environmental Pollution Control, Key Laboratory for Yellow River and Huai River Water Environment and Pollution Control, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, P. R. China.. E-mail: fanjing@htu.cn; Tel: +86-373-3325719
bSchool of Environment Engineering and Chemistry, Luoyang Institute of Science and Technology, Luoyang, Henan 471023, P. R. China
cSchool of Chemistry and Chemical Engineering, Key Laboratory of Green Chemical Media and Reaction, Ministry of Education, Henan Normal University, Xinxiang, Henan 453007, P. R. China
First published on 22nd May 2015
Carbon nanotubes have excellent adsorption property for metal ions. However, they lack selectivity and are difficult to separate from solutions. To resolve these problems, magnetic carbon nanotubes were prepared and functionalized with an imidazolium ionic liquid in this work. The functionalized magnetic carbon nanotube was used to remove Cr(VI) from water. It was found that the removal was highly selective and sensitive. At acidic conditions, 90% of Cr(VI) could be selectively removed on the ppb level with the coexistence of a high concentration of cations like Hg2+, Cd2+ and anions such as NO3− and SO42−. After the adsorption, the material could be collected easily by an external magnet, and then regenerated effectively by using 8% hydrazine hydrate. The high adsorption sensitivity, selectivity and capacity were attributed to the favorable electrostatic attraction, anion exchange affinity and entropy effects. Kinetic analysis indicated that the adsorption process of Cr2O72− was well described by a pseudo second order model. The adsorption isothermal analysis revealed that the adsorption process was endothermic, and could be described by a Langmuir model. In addition, it is interesting to find that unlike the commonly used absorbents, the adsorption capacity of the functionalized material for Cr2O72− increased with increasing temperature.
Adsorption is a promising process for Cr(VI) removal due to its low cost, high efficiency and simple operation.5 The choice of adsorbent is a key point for adsorption techniques, because adsorption capacity, selectivity, and affinity should be taken into consideration. The traditional adsorbents used for Cr(VI) removal include resins,6 active carbons,7 zeolite,8 nanoparticle,9,10 silica gel11 and among others. Compared with conventional adsorbents, carbon nanotubes (CNTs) have excellent adsorption performance and have been widely used for metal adsorption12–14 due to their unique structural, physical and chemical properties.15 However, they have no selectivity for metal ion adsorption, and they are difficult to separate from solutions. To address the separation problem, magnetic carbon nanotubes have been created and utilized for metal ion adsorption.16–18 This is a great progress although poor selectivity still remains a challenge. Considering the fact that adsorption selectivity of magnetic carbon nanotubes would be improved by their surface functionalization,19 the design and development of new surface functionalized magnetic carbon nanotubes are a new trend in Cr(VI) removal.
Ionic liquids (ILs) are composed of organic cations and inorganic or organic anions, and they have a vanishingly small vapor pressure, making them an attractive alternative to volatile organic solvents.20 Therefore, ILs have been used as greener solvents for organic synthesis,21 material preparation,22 catalysis,23,24 separation and extraction.25,26 It has been reported that anionic pollutants have strong affinity with some ILs, and can be extracted by ILs from aqueous solutions.27–29 However, some disadvantages have weakened their applications, for example, high cost and large amount usage of ILs, dissolution of ILs in aqueous solution which may cause water pollution, and the difficulty in the recovery of ILs. In order to overcome these problems, ILs should be supported on the solid surface. This will reduce the consumption of the ILs, avoid their loss and benefit their recycling.30–33
In this work, we report a novel material, 1-hydroxyethyl-2,3-methyl imidazolium chloride ionic liquid functionalized magnetic multi-walled carbon nanotube (Fe3O4/CNT-IL). This material maintains the advantages of both magnetic carbon nanotubes and ILs. In this way, the disadvantages of carbon nanotubes and ILs can be overcome. The IL functionalized magnetic carbon nanotube has been applied to remove Cr(VI) from aqueous solution. It is found that trace Cr(VI) can be adsorbed by Fe3O4/CNT-IL efficiently and selectively due to the strong electrostatic force between anionic Cr2O72− and cationic imidazolium. In deed, 90% of Cr(VI) at 0.028 mg L−1 can be removed, and many cations and anions do not interfere the removal. The adsorption capacity of Cr(VI) is controlled by pH value and temperature of the solution. After the adsorption process, the Fe3O4/CNT-IL can be separated easily by an external magnet and Cr(VI) adsorbed by the material can be recovered by a reduction agent. The material can be regenerated and reused at least four cycles without decrease of adsorption capacity. The removal process is simple and low cost. These findings indicate that the material may be a promising adsorbent for the removal of Cr(VI) from wastewaters on the ppb level.
To confirm the presence of Fe3O4 nanoparticles inside MWCNT, the material was characterized by a TEM with an accelerating voltage of 200 kV. For the preparation of TEM specimens, the Fe3O4/CNT-IL composite was dissolved in doubly distilled water and dropped onto a carbon coated copper, as shown in Fig. S2.† It was obviously that a few Fe3O4 nanoparticles were located inside the nanotube bundles. Therefore, the material would be stable in acidic solutions and could be separated from aqueous solution conveniently by an external magnet.
In order to confirm that the ionic liquid was really functionalized on the surface of magnetic carbon nanotube, the oxidized carbon nanotube (CNT-COOH), as-prepared magnetic carbon nanotube (Fe3O4/CNT-COOH), ionic liquid functionalized magnetic carbon nanotube (Fe3O4/CNT-IL) and the pure ionic liquid were analyzed by TGA under the protection of N2 at a heating rate of 10° min−1 from 20 to 800 °C. The obtained TGA curves (Fig. S3†) indicated that the ionic liquid was successfully functionalized on the carbon nanotube. It was also found that CNT-COOH had a high thermal stability and showed 16% weight loss during the entire heating cycle, and the magnetic carbon nanotube (Fe3O4/CNT-COOH) prepared by loading Fe3O4 on the surface of carbon nanotube exhibited only 11% weight loss due to the thermal stability of Fe3O4 in N2 atmosphere. However, the ionic liquid functionalized magnetic carbon nanotube (Fe3O4/CNT-IL) exhibited a significant weight loss in the temperature range from 210 °C to 280 °C. Then the weight loss became as slow as CNT-COOH and Fe3O4/CNT-COOH, and a weight loss of 28% was finally observed. This is different from pure ionic liquid which had a significant weight loss within the temperature range from 310 °C to 370 °C. In addition, from the weight loss data of Fe3O4/CNT-COOH and Fe3O4/CNT-IL, it was found that amounts of the ionic liquid on the ionic liquid functionalized magnetic carbon nanotube was about 17%.
The resulting CNT-COOH, Fe3O4/CNT-COOH and Fe3O4/CNT-IL were also characterized by FT-IR spectroscopy analysis. It can be seen from Fig. S4† that CNT-COOH and Fe3O4/CNT-COOH exhibited the characteristic bands at 1577.69 cm−1 and 1574.95 cm−1 resulted from the CC stretching of main structure of multi-walled carbon nanotube,35 and the adsorption at 3432.67 cm−1 and 3401.73 cm−1 resulted from the O–H stretching of carboxylic group. In comparison with CNT-COOH and Fe3O4/CNT-COOH, some new bands were observed on the Fe3O4/CNT-IL samples. The bands at 2854.70 and 2934.77 cm−1 were assigned to the stretching of C–H in –CH3 or –CH2– of the ionic liquid, the bands at 1631.72 cm−1 was assigned to the stretching of –CC– in the imidazolium ring of the ionic liquid, 1108.41 cm−1 was assigned to the stretching of C–O in the ester of the material, and the band at 1725.06 cm−1 was resulted from the CO stretching of ester group. These results indicated that ionic liquid was chemically grafted on the surface of the magnetic carbon nanotubes.
It is known that the materials were protonated under highly acidic conditions, especially at pH <2.0. In this case, Cr(VI) existed in the negative charged form of HCrO4−, thus Cr(VI) could be removed by the protonated carbon nanotubes through electrostatic attraction. Moreover, Cr(VI) could be reduced into Cr(III) at pH <2.0.36 This means that part of Cr(VI) may be reduced by CNT to form Cr(III) at pH <2.0, which also promoted removal of Cr(VI). Consequently, nearly 100% removal percentage was observed, and no difference was found by different adsorbent materials.
However, it was found that at pH = 3–8, the removal percentage of Cr(VI) by ionic liquid functionalized carbon nanotubes (Fe3O4/CNT-IL and CNT-IL) was 40–60% higher than that by carbon nanotubes (CNT and CNT-COOH). This clearly indicated that the removal efficiency could be improved significantly after the functionalization of ionic liquid on carbon nanotube. It is known that when pH >2.0, Cr(VI) existed in the form of Cr2O72−, the oxidation–reduction reaction could be neglected.36 To confirm this point, Cr(III) content in the supernatant after 2 h adsorption at 65 °C was determined by spectrophotometric method.37 However, no Cr(III) was observed in the supernatant within experimental error. This indicates that Cr(VI) was totally adsorbed but not reduced by Fe3O4/CNT-IL, and the removal process was dominated by adsorption. With the increase of solution pH value, the removal percentage of Cr(VI) by carbon nanotubes (CNT and CNT-COOH) decreased rapidly due to the weaker protonation of the materials and the lower positive charge density on these materials. Compared with CNT and CNT-COOH, the ionic liquid functionalized carbon nanotubes (Fe3O4/CNT-IL and CNT-IL) have more active sites, which are attributed to the functionalization of imidazolium cations. Thus more Cr(VI) could be adsorbed by these two ionic liquid functionalized materials, and higher removal percentage was observed. It was also indicated that the removal percentage of Cr(VI) by Fe3O4/CNT-IL was lower than that by CNT-IL (see Fig. 1). The reason is that Fe3O4 in Fe3O4/CNT-IL has no adsorption for Cr(VI). Although Fe3O4/CNT-IL has lower removal efficiency than CNT-IL, the Fe3O4/CNT-IL adsorbent material could be separated easily from solutions by an external magnet after the adsorption process. This makes it very simple to recover and reuse the adsorbent.
On the other hand, in the highly basic solutions, especially at pH >10.0, Cr(VI) existed in the form of CrO42−, which has lower affinity for imidazolium cations than Cr2O72−. Meanwhile, high concentration of OH− might interfere with the adsorption of CrO42−. Therefore, only 10% of Cr(VI) could be removed under such a pH condition. Overall, in order to remove Cr(VI) efficiently by Fe3O4/CNT-IL and to avoid the oxidation of carbon nanotube, pH 3.0 was chosen in the next studies.
(1) |
As shown in Fig. S5,† the removal percentage increased with increasing adsorbent dose and remained constant when the adsorbent dose was increased from 20 mg to 50 mg. As the adsorbent dose was higher than 20 mg, almost 100% Cr(VI) could be removed. This could be explained by the fact that at higher adsorbent dose, more chemically active sites were available to interact with Cr(VI).
Next, the pseudo first order and pseudo second order models expressed by eqn (2) and (3):38
(2) |
(3) |
It can be seen from Table S1† that the pseudo second order model provided better correlation coefficients than the pseudo first order model, and the calculated equilibrium adsorption capacities (qe,cal) from the pseudo second order model agreed better with the experimental values. This suggests that the pseudo second order model is more suitable for describing the adsorption kinetics of Cr(VI) by Fe3O4/CNT-IL. It was reported that adsorption behavior of the pseudo second order model suggested a chemisorption process,39 and this implied that these adsorbents could be applied to remove low concentration of metal ions.40 Thus, the adsorbent developed in this work would be a promising candidate to remove low concentration Cr(VI) from wastewater.
In order to further study the influence of temperature on Cr(VI) removal, the effect of temperature on the removal of different concentrations of Cr(VI) was also investigated. The adsorption data were analyzed by using Langmuir and Freundlich adsorption isotherm models, which are applicable to highly heterogeneous surfaces. From the liner form of Langmuir isotherm,44 the equation is given as:
(4) |
The Freundlich model44 can be presented by
(5) |
Fig. 3b presents the adsorption isotherms of Cr(VI) on the Fe3O4/CNT-IL. The calculated Langmuir and Freundlich constants were summarized in Table S2.† It can be seen that the Langmuir model exhibited relative higher values of regression coefficients than Freundlich model, and the theoretical values of adsorption capacities obtained from Langmuir model were close to the experimental values. These results indicated that the Langmuir model was more suitable to describe the adsorption isotherms.
ΔG0 = −RTlnKL | (6) |
lnKL = −ΔH0/RT +ΔS0/R | (7) |
T (K) | 10−3 KL (L mol−1) | ΔG0 (kJ mol−1) | ΔH0 (kJ mol−1) | ΔS0 (J mol−1 K−1) |
---|---|---|---|---|
298 | 2.60 | −19.5 | 61.4 | 273 |
313 | 15.08 | −25.0 | ||
328 | 25.48 | −27.7 |
It is clear that the values of ΔG0 were negative, indicating favorable adsorption of Cr2O72− on Fe3O4/CNT-IL adsorbent. The ΔH0 and ΔS0 values of the adsorption reaction are positive, and TΔS0 is always greater than ΔH0 in value. This suggests that the adsorption of Cr2O72− on Fe3O4/CNT-IL is controlled by entropy changes.45 The positive entropy changes for the adsorption can be explained from the fact that Cr2O72− is well solvated in water, in order to adsorb this anion on the surface of Fe3O4/CNT-IL, the anion has to lose part of its hydration sheath, leading to an entropy increase. Although hydration of Cl−1 released from the ionic liquid in aqueous solution decreases the entropy, the entropy increase resulted from dehydration of Cr2O72− is predominant because of its much bigger volume. This supposition has been verified by the reported hydration entropy data:46 ΔS0 (Cr2O72−) = 261.9 J mol−1 K−1, ΔS0 (Cl−1) = 56.5 J mol−1 K−1. On the other hand, the observed positive ΔH0 values suggest endothermic process of adsorption. This may be due to the dehydration of solvated Cr2O72− and the reduction of the electrostatic interaction between imidazolium cation and Cl−1, which are energy required processes.
Generally, the value of ΔG0 for physical adsorption (−20 to 0 kJ mol−1) is much greater than that for chemical adsorption (−80 to −400 kJ mol−1).38 ΔG0 value (around −25 kJ mol−1) obtained in this work was between the values for physical adsorption and chemical adsorption, which indicates that adsorption of Cr(VI) by Fe3O4/CNT-IL involves both physical and chemical adsorptions.
Temperature/°C | Concentration of Cr(VI)/mg L−1 | Total Cr removal percentage (%) | |
---|---|---|---|
Before adsorption | After adsorption | ||
25 | 0.028 | 0.009 | 68 |
50 | 0.028 | 0.007 | 75 |
70 | 0.028 | 0.003 | 89 |
25 | 0.085 | 0.018 | 79 |
50 | 0.085 | 0.013 | 85 |
70 | 0.085 | 0.007 | 92 |
It was found that ppb level of Cr(VI) could be removed efficiently by Fe3O4/CNT-IL. After adsorption, the concentration of Cr(VI) in the solution was lower than the permitted concentration of Cr(VI) in drinking water advised by USEPA (0.05 mg L−1).3 It was also observed that the removal percentage was influenced by temperature of the system. The relatively high temperature would be beneficial to the removal of Cr(VI), for example, at 25 °C, the removal percentage was 68%, while 21% increase was observed at 70 °C for an aqueous solution containing 0.028 mg L−1 of Cr(VI). This is obviously different from the commonly used absorbents, indicating that the ionic liquid functioned material maintains the property of ionic liquid, and the adsorption ability of Fe3O4/CNT-IL can be modulated significantly by temperature.
Inorganic anions such as Cl−, NO3−, SO42− and PO43− are common coexisting anions with Cr2O72−. The effects of these anions on Cr2O72− adsorption were also investigated. As shown in Fig. 4b, these anions did not interfere with Cr2O72− adsorption even if their concentrations were 2500, 2000, 1300 and 1000 times higher than that (0.076 mmol L−1) of Cr2O72−, respectively. The effect of anions on Cr2O72− adsorption followed the order of Cl− < NO3− < SO42− < PO43−, which is consistent with anion exchange affinity order reported in the literature.47 Obviously, the anion with higher valence, smaller hydrated radius and greater polarizability has stronger competing effect.
The effect of organic anions on Cr2O72− adsorption was presented in Fig. 4c. It is clear that these two organic anions did not interfere with Cr2O72− adsorption even if their concentrations were 2500 and 1300 times higher than that (0.076 mmol L−1) of Cr2O72−, respectively. In addition, the effect of organic anions was found to follow the order: acetate < citrate. This suggests that the anions with higher valence have higher interference.
Based on the above analysis, it is clear that the selectivity of carbon nanotube was greatly improved by the functionalization of ionic liquid, and Cr2O72− could be selectively adsorbed by Fe3O4/CNT-IL in the coexistence of cations such as Hg2+, Cd2+, Cu2+ and anions such as Cl−, NO3−, SO42−, PO43−, acetate and citrate.
Adsorbent | Dosage of adsorbent (mg mL−1) | Concentration of Cr(VI) (mg L−1) | Equilibrium time (h) | Adsorption amount qm (mg g−1) | Ref. |
---|---|---|---|---|---|
Oxidized multi walled carbon nanotubes | 0.1 | 6 | 65 | 4.26 | 48 |
Aactivated carbon | 1 | 44 | 10 | 90.99 | 49 |
Activated carbon | 0.8 | 50 | 48 | 23.5 | 50 |
Activated carbon coated with quaternized poly(4-vinylpyridine) | 1 | 226 | 24 | 53.7 | 51 |
Multi-walled carbon nanotubes | 10 | 0.1 | 12 | — | 52 |
Ce3+ doped ZnFe2O4 | 0.5 | 60 | 72 | 57.24 | 53 |
Fe3O4/CNT-IL | 1 | 80 | 12 | 55.43 | This work |
The following two steps were used to prepare ionic liquid functionalized magnetic multi-walled carbon nanotubes. In the first step, ionic liquid was grafted on the CNT-COOH via esterification reaction. For this purpose, CNT-COOH was added into SOCl2 (30 mL), and the solution was refluxed for 1 h. Excess of SOCl2 was removed by rotary evaporator under reduced pressure, then 30 mL of anhydrous THF was added, followed by the addition of 1.0 g of 1-hydroxyethyl-2,3-methyl imidazolium chlorine ionic liquid. The mixture was stirred for 12 h at room temperature, and the product was collected, and then washed by THF, anhydrous alcohol and deionized water for several times until no absorbance in filtrate was observed. Thereafter, the material was dried under vacuum for 24 h at 60 °C. In the second step, ionic liquid functionalized carbon nanotube was magnetized using a procedure adapted from Goh et al.54 In brief, 2.98 g of FeCl3·6H2O and 1.53 g of FeSO4·7H2O were dissolved in 100 mL of deionized water, and then 1.0 g of the CNT-IL was added into the solution. The mixture was sonicated for 1 h and the air in the inner core of multi-walled carbon nanotubes was extracted by a vacuum pump to facilitate the uptake of the iron solution into the carbon nanotubes. The solution was stirred vigorously for 12 h at 70 °C under N2 condition, and the pH value was adjusted to 11–13 using ammonium hydroxide. The system was kept refluxed at the boiling point of the solution for 2 h. Finally, the ionic liquid functionalized magnetic carbon nanotube was collected by an external magnet. To clean the surface of the Fe3O4/CNT-IL, the material was washed with dilute hydrochloric acid and deionized water, then washed with anhydrous alcohol for several times, and dried under vacuum for 24 h at 60 °C.
(8) |
Footnote |
† Electronic supplementary information (ESI) available: Kinetic parameters for the pseudo first order and pseudo second order model for Cr(VI) adsorption onto Fe3O4/CNT-IL (Table S1). Langmuir and Freundlich parameters for Cr(VI) adsorption on Fe3O4/CNT-IL (Table S2). XRD patterns of (a) CNT-COOH and (b) Fe3O4/CNT-IL (Fig. S1). The TEM images of Fe3O4/CNT-IL (Fig. S2). The TGA curve of (a) CNT-COOH, (b) Fe3O4/CNT-COOH, (c) Fe3O4/CNT-IL and (d) ionic liquid under the protection of N2 (Fig. S3). FT-IR spectrum of (a) CNT-COOH, (b) Fe3O4/CNT-COOH and (c) Fe3O4/CNT-IL (Fig. S4). Effect of adsorbent dosage on Cr(VI) removal by Fe3O4/CNT-IL (Fig. S5). Adsorption–desorption cycle of Fe3O4/CNT-IL for Cr(VI) (Fig. S6). See DOI: 10.1039/c5ra06026e |
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