Journal of Environmental Management 137 (2014) 81e85
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Removing Cr(VI) from aqueous solutions using a functional ionic liquid-based cross-linked polymer Hejun Gao a, b, Yun Wang c, Liqiang Zheng a, * a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637000, China c Shandong Institute of Metrology, Jinan 250100, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2013 Received in revised form 21 November 2013 Accepted 25 November 2013 Available online 4 March 2014
A novel functional ionic liquid-based cross-linked polymer was synthesized from 1-aminoethyl-3vinylimidazolium chloride hydrochloride and divinylbenzene. The physicochemical properties of the adsorbent were investigated using scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The adsorption capacity of the novel cross-linked polymer with respect to Cr(VI) was investigated using a batch adsorption procedure and the kinetics and thermodynamics of the adsorption process were further investigated. It was found that the adsorption kinetics was well fitted by a pseudo-second-order model and the adsorption isotherms agreed well with the Langmuir model. The maximum adsorption capacity after 5 min at room temperature (25 C) was found to be 391.4 mg/g, which was much better than the most of the previously reported adsorbents. The adsorption process was found to be dominated by electrostatic interactions. The introduction of functional ionic liquid moieties into cross-linked poly(divinylbenzene) polymer constitutes a new and efficient kind of adsorbent. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Cr(VI) Adsorption Ionic liquids Polymers
1. Introduction As one of the world’s most strategic and critical materials, chromium (Cr) is widely used in the metal and chemical industries. With the continued development of these industries, a huge amount of wastewater containing Cr(VI) and Cr(III) is discharged into the environment at a level that nature cannot cope with. Of the two different oxidation states, Cr(VI) is the one which is the most toxic pollutant (Costa, 2003). Cr(VI) is a strong oxidizing agent and potential carcinogen and can cause enormous harm to both human beings and animals. Discharge of Cr(VI) into water is tightly regulated by the Environmental Protection Agency. The maximum permissible limits of Cr(VI) for discharge into potable water, inland surface water, and industrial wastewater are 0.05, 0.1, and 0.25 mg/L, respectively (Hosseini-Bandegharaei et al., 2010; Chen et al., 2011). Many technologies have been applied to remove Cr(VI) from wastewater, including biological treatment (Barrera-Diaz et al., 2012), chemical precipitation (Peng et al., 2004), membrane filtration (Wang and
* Corresponding author. Tel.: þ86 531 88366062; fax: þ86 531 88564750. E-mail address: [email protected] (L. Zheng). http://dx.doi.org/10.1016/j.jenvman.2013.11.055 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
Ge, 2013), ion-exchange (Edebali and Pehlivan, 2010), solvent extraction (Kul and Cetinkaya, 2010), and adsorption technology (Zhao et al., 2013; Yang et al., 2012). The most common one is adsorption technology due to its effectiveness, efficiency, economy and absence of secondary pollution. Many kinds of adsorbent are used to remove Cr(VI) from wastewater, including activated carbon (Liu et al., 2010b), zeolites (Zeng et al., 2010), clays (Mekatel et al., 2012), etc. However, the methods suffer from disadvantages, such as low adsorption capacity and long adsorption time, which limit their application in Cr(VI) wastewater treatment. In recent years, ionic liquids (ILs) have received much attention owing to their unique properties, such as high thermal stability and high ionic conductivity (Welton, 1999). Many IL-based materials have been applied in wastewater treatment. Mahmoud and AlBishri (2011) investigated the adsorption of lead on [NSieOHOMIMþTf2N] and [NSieNH2-OMIMþTf2N], which are hydrophobic ionic liquids on nano-silica. They found that the materials have excellent adsorption capacities with respect to lead (0.5e0.9 and 0.8e1.3 mmol/g, respectively, at pH 1e7). Poursaberi and Hassanisadi (2013) synthesized IL@Fe3O4 nanoparticles from IL-COOH and Fe3O4 and found it to be an effective adsorbent of Reactive Black 5 (maximum adsorption capacity 161.29 mg/g). Lou and Di (2012) prepared an absorbent of macroporous adsorption
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resins functionalized with ionic liquids. The adsorption capacity of flavonoids on this adsorbent reached up to 7 mg/g. The preparation of an effective adsorbent for removing Cr(VI) from aqueous solution is expected to meet the demands required for environmental protection. It is well-known that ammonium groups on the surface of an adsorbent can greatly improve adsorption capacity (Li et al., 2005a; Min et al., 2012). In this work, a cross-linked polymer was obtained by copolymerizing divinylbenzene and 1-aminoethyl-3-vinylimidazolium chloride hydrochloride. The structure and the physicochemical properties of the functional ionic liquid-based cross-linked polymer were characterized. Then, the functional ionic liquid-based cross-linked polymer was used to treat Cr(VI) solutions.
speed of 150 rpm. After reaching equilibrium, 1 mL of each sample was withdrawn and filtered through a 0.45 mm membrane filter (nylon). Then, 0.1 mL of the filtrate was diluted to 50 mL in a colorimetric tube. The concentration of the Cr(VI) ions in the sample was then determined colorimetrically by reaction with 1,5diphenylcarbazide in acid solution. The purple complex solution was analyzed using UVeviseNIR spectroscopy (Hitachi U-4100, Japan) using the lmax at 540 nm. Previous work revealed that the total amount of Cr and Cr(VI) ions was more or less the same, indicating that the Cr(III) ion concentration in the final solution was insignificant (Chen et al., 2011; Gupta and Babu, 2009). Therefore, only the amount of Cr(VI) was measured in our work. The amount of Cr(VI) adsorbed onto the PDVB-IL, qt, (mg/g) was calculated according to the following equation:
2. Materials and methods 2.1. Materials
qt ¼
ðC0 Ct ÞV ; m
(1)
Potassium dichromate (99.8%), 1-vinylimidazole (99%), 2,20 azobis(2-methylpropionitrile) (99%), dimethyl formamide (99%), and 2-chloroethylamine hydrochloride (99%) were purchased from the Beijing Chemical Reagent Co. Divinylbenzene (DVB) (80%) was provided by Aldrich.
where C0 and Ct (mg/L) are the initial concentration of Cr(VI) and at time t, respectively, V is the volume of the solution (L), and m is the mass of PDVB-IL (g).
2.2. Methods
3.1. Characterization of PDVB-IL
2.2.1. Preparation of functional ionic liquid-based cross-linked polymer 1-Vinylimidazole (0.1 mol) and 2-chloroethylamine hydrochloride (0.1 mol) were added into 50 mL acetonitrile. The mixture was stirred and refluxed under a nitrogen atmosphere for 48 h. The resulting solid was washed several times with anhydrous ethanol. Then, the product was dried for 48 h under vacuum and a white solid was obtained. Then, the white product (0.015 mol), DVB (0.00375 mol), and an appropriate amount of 2,20 -azobis(2methylpropionitrile) were dissolved in 50 mL dimethyl formamide under nitrogen. The mixture was stirred at 80 C for 24 h. A yellow solid was collected by filtration and washed with acetone. The new adsorbent of cross-linked polymer (PDVB-IL) was obtained. The preparation process of PDVB-IL is shown in Fig. SM-1.
Scanning electron microscope (SEM) images of the surface of the PDVB-IL are depicted in Fig. SM-2. The surface of the PDVB-IL is rough and irregular in shape. And the specific surface area BET value is 0.124 m2/g. The rough surface can provide a good adsorption site for Cr(VI) to be trapped and adsorbed. The thermal stability of the prepared PDVB-IL was characterized by thermogravimetric analysis (TGA). The TGA plot for the PDVB-IL is shown in Fig. SM-3. The PDVB-IL shows two weight losses, which takes place at ca. 300e410 and 410e500 C. The weight loss of the PDVB-IL at ca. 300e410 C is due to the ILs on the polymer (Cai et al., 2007). The main weight loss occuring between 430 and 500 C is attributed to degradation of the PDVB (Liu et al., 2010a). The amount of IL in the PDVB-IL is about 60 wt.%. The weight loss of PDVB-IL is negligible below 300 C, indicating that the PDVB-IL can be used to treat wastewater at high temperature. In order to confirm the main functional groups present in the adsorbent, FT-IR spectroscopy was used. The infrared spectra of PDVB-IL, PDVB-IL þ Cr(VI) (i.e. after adsorption of Cr(VI) onto PDVBIL), and K2Cr2O7 are shown in Fig. SM-4. In the PDVB-IL spectrum, the peak at 3447 cm1 is attributed to the stretching mode of the NþeH groups. The bands at 1630 and 1552 cm1 are the C]C stretching vibration of the imidazole ring and NþeH in-plane bending vibration, respectively (Pitula and Mudring, 2010). A strong band at 1448 cm1 is assigned to the C]C stretching in the benzenoid rings (Nayak et al., 2010). The peak at 1162 cm1 is the CeN stretching vibration. The above results indicate that the functional ionic liquid-based cross-linked polymer was successfully prepared. After adsorption of Cr(VI) onto the PDVB-IL, the FT-IR adsorption bands undergo obvious shifts and reductions, indicating that the Cr(VI) adsorption corresponds to the possible presence of several functional groups on the adsorbent (Chen et al., 2011). The three bands at 3447 (PDVB-IL), 950 (K2Cr2O7), and 760 cm1 (K2Cr2O7) move to lower wavenumbers in the PDVBIL þ Cr spectrum, indicating that hydrogen bonding interactions could exist between functional groups in the PDVB-IL and Cr(VI) 2 (HCrO 4 and Cr2O7 ). The interesting point is that the strong bands at 1552 and 1162 cm1 disappear in the PDVB-IL þ Cr spectrum, which could be attributed to the electrostatic interaction between 2 the Cr(VI) ions (HCrO 4 and Cr2O7 ) and the functional groups in the
2.2.2. Characterization of the adsorbent The particle size and morphology of the PDVB-IL were characterized using a scanning electron microscope (JSM-7600F, JEOL Ltd., Japan). The specific surface area of the PDVB-IL was characterized using a computer controlled adsorption analyzer (Micromeritics, Gemini V2.0, USA) and expressed using BrunauereEmmetteTeller (BET) theory. Thermogravimetric analyses were carried out using standard equipment (TGA1500 Rheometric Scientific Inc., Piscataway, NJ) to investigate the thermal properties of the samples. The measurements were conducted in an inert atmosphere of nitrogen using 8e10 mg samples, a heating rate of 10 C/min, and a temperature range of ambient to 800 C. Vibrational spectra were obtained using a Fourier transform infrared (FT-IR) spectrometer (VERTEX-70) using KBr pellets and a scanning range of 400e 4000 cm1. 2.2.3. Batch adsorption procedure Batch adsorption was carried out in order to evaluate the adsorption process. Standard Cr(VI) solutions were prepared by dilution of a stock standard solution (1 g/L aqueous chromium(VI) solution). The batch adsorption experiments were carried out as follows. The Cr(VI) solution and PDVB-IL were added to a 50 mL conical flask. The flask was shaken using an oscillator (SHZ-82, Changzhou Shaipu Experimental Instrument Factory, China) at a
3. Results and discussion
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PDVB-IL. The Cr(VI)-loaded spectrum further suggests that the functional groups play a major role in the process of Cr(VI) adsorption on PDVB-IL (Li et al., 2009). 3.2. Effect of pH on adsorption The initial pH value of the aqueous solution is an important parameter in the adsorption process. The Cr(VI) adsorption capacity on PDVB-IL is plotted as a function of the initial pH value in Fig. 1. The results show that the adsorption capacity gradually increases as the pH value of the solution decreases from 12 to 2. The pH value has two kinds of effect on the adsorption of Cr(VI) on PDVB-IL. On the one hand, Cr(VI) in aqueous solution exists in 2 2 2 various forms, such as HCrO 4 , Cr2O7 , CrO4 , etc. The CrO4 anion is 2 predominant in the pH range above 6, while HCrO and Cr 4 2O7 ions are predominant at pH values below 6 (Chen et al., 2011; Anirudhan et al., 2009). The HCrO 4 form only requires one cationic site of PDVB-IL for adsorption at low pH value, while the CrO2 4 form needs two cationic sites of PDVB-IL at high pH values (Yusof and Malek, 2009). On the other hand, with increasing pH, the amount of OHe available is gradually increased and the cationic charges of the amine functional groups (NHþ 3 ) on the surface of the PDVB-IL are consumed. Therefore, the adsorption capacity with respect to Cr(VI) gradually decreases with increasing initial pH. To achieve lower pH values in the solution adjustment with acid is required, however, it is not economical. Therefore, the next experiments were carried out in natural water (pH ¼ 5.6). 3.3. Adsorption kinetics The adsorption kinetics of Cr(VI) on PDVB-IL was studied to investigate the rate at which contamination was removed from aqueous solution. Fig. 2 shows the effect of contact time on the amount of Cr(VI) adsorption for different initial concentrations (50, 100, and 150 mg/L). Fig. 2 shows that the Cr(VI) ions are adsorbed rapidly in the initial 5 min. After that, Cr(VI) adsorption did not change with the contact time. The equilibrium time is very short, which reflects the high accessibility of the Cr(VI) ions to the adsorption sites in the PDVB-IL (Hosseini-Bandegharaei et al., 2010). To investigate the amount of Cr(VI) adsorbed on the surface of the PDVB-IL at any particular time and the amount adsorbed at equilibrium, a pseudo-second-order model is adopted to elucidate the adsorption kinetics. This can be expressed as
Fig. 1. Influence of pH on the adsorption capacity of PDVB-IL towards Cr(VI). (C0 ¼ 100 mg/L, T ¼ 25 C, contact time ¼ 30 min, PDVB-IL ¼ 0.1 g/L).
Fig. 2. Adsorbed amount of Cr(VI) by PDVB-IL as a function of contact time at different initial concentrations of Cr(VI). (T ¼ 25 C, PDVB-IL ¼ 0.1 g/L).
t 1 t ¼ þ ; qt kq2e qe
(2)
where k is the rate constant of the pseudo-second-order reaction (g/mg min), qt and qe represent the adsorption capacity (mg/g) at time t and equilibrium (respectively), and t (min) is the contact time. The corresponding kinetic parameters are given in Table SM1. The value of the correlation coefficient (R2) for the pseudosecond-order kinetic curve is 0.9999. Moreover, the calculated values of qe are close to the experimental values for pseudo-secondorder kinetics, indicating that the adsorption kinetics is well fitted by a pseudo-second-order model (Ahmad and Kumar, 2011). The pseudo-second-order kinetics indicates that adsorption depends on the adsorbate as well as the adsorbent and involves chemisorption in addition to physisorption. The chemisorption contribution might be the rate limiting step where valency forces are involved via electron sharing or exchange between the adsorbent and the adsorbate. 3.4. Adsorption isotherms It is important to collect and analyze adsorption isotherms, as these can accurately describe how the adsorbate interacts with
Fig. 3. Adsorption isotherms for Cr(VI) at 25, 40 and 55 C. (Contact time ¼ 30 min, PDVB-IL ¼ 0.04 g/L).
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adsorbents and can be used for design purposes. Two well-known adsorption isotherms, the Langmuir and Freundlich models, are used here to describe the adsorption. These isotherms relate the equilibrium adsorption capacity to the equilibrium concentration in the solution (Fig. 3). The Langmuir adsorption model is the most common one to quantify the amount of adsorbate adsorbed on an adsorbent as a function of concentration at a given temperature. The linear form of the Langmuir isotherm is given by
Ce 1 Ce ¼ þ ; qm KL qm qe
(3)
where Ce is the liquid-phase concentration of Cr(VI) at equilibrium (mg/L), qm is the maximum theoretical adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg). The constants qm and KL can be calculated from the intercept and slope of a linear plot of Ce/qe versus Ce. The Freundlich isotherm is the most important multilayer adsorption isotherm for rough surfaces. The linear form of the Freundlich isotherm model can be represented as
log qe ¼
1 log Ce þ log Kf ; n
(4)
where Kf and 1/n are characteristic constants representing the adsorption capacity and adsorption intensity of the system, respectively. The values of Kf and 1/n are obtained from a linear plot of log qe versus log Ce. Table SM-2 presents the results of fitting the data to the Langmuir and Freundlich models. It is seen that the experimental data conforms to both isotherm models at 40 and 55 C. The R2 values for the Langmuir isotherm are larger than those for the Freundlich isotherm model at 25 C, indicating that the Langmuir model is better for fitting the experimental data. This result suggests that there is monolayer coverage of Cr(VI) on the surface of the PDVB-IL at low temperatures and there may be multilayer adsorption present at high temperatures. With an increase in temperature, the qm value is increased, implying that the process of adsorption of Cr(VI) on PDVB-IL is endothermic. The calculated value of qm is 400.0 mg/g at 25 C, which agrees well with the experimental data (qe.exp ¼ 391.4 mg/g). The adsorption capacities of different adsorbents towards Cr(VI) are shown in Table SM-3. It is obvious that the adsorption capacity of PDVB-IL is much better than most of the other adsorbents reported previously. The large adsorption capacity could be due to the strong adsorption affinity of PDVB-IL towards Cr (VI), which is caused by the electrostatic and hydrogen bonding interactions between the anionic form of Cr (VI) and the functional groups in the PDVB-IL. The external surface of PDVB-IL plays an important role for Cr removal. For the Langmuir-type adsorption process, the essential characteristics can be explained using a constant and dimensionless separation factor, RL, which is considered as a more reliable indicator of the adsorption capacity (Venkatesha et al., 2012). The value of RL can be obtained from
RL ¼
1 : 1 þ KL C0
(5)
The RL value indicates whether the adsorption of Cr(VI) on PDVB-IL is unfavorable (RL > 1), linear (RL ¼ 1), favorable (0 < RL < 1), or irreversible (RL ¼ 0) (Chowdhury et al., 2012). The results are shown in Table SM-2. The RL values for all initial concentrations C0 are less than 0.13 at various temperatures, implying favorable adsorption of Cr(VI) on PDVB-IL.
3.5. Thermodynamics The effect of temperature on the adsorption of Cr(VI) was studied in order to obtain the relevant thermodynamic parameters, which can further help our understanding of the adsorption mechanism. The standard free energy change (DG0) for adsorption can be calculated from the following equation
DG0 ¼ 2:303RTlog Ke ;
(6)
where R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (K). The thermodynamic parameters standard enthalpy change (DH0) and entropy change (DS0) for the process can be determined from the following equations
Ke ¼
Cad;e ; Ce
log Ke ¼
DS0 DH0 ; 2:303R 2:303RT
DG0 ¼ DH0 T DS0 :
(7)
(8) (9)
where Cad,e is the concentration of adsorbate on the adsorbent at equilibrium (mg/L). The values of DH0 and DS0 can be calculated from the slope and the intercept, respectively, of a linear plot of log Ke against 1/T. Table SM-4 presents the resulting DG0, DH0, and DS0 values for adsorption of Cr(VI) on PDVB-IL. With an increase in temperature, the negative value of DG0 is decreased, indicating that the adsorption behavior is a spontaneous process and the adsorption process is more spontaneous at higher temperatures (Li et al., 2005b). The positive values of DH0 confirm the endothermic nature of the adsorption process, and the positive values of DS0 imply that the adsorbed Cr(VI) represents a certain amount of freedom in the solid/solution interface during the adsorption of Cr(VI) on PDVB-IL lu and Yakup Arica, 2008). This phenomenon is a result (Bayramog of physisorption, which takes place through electrostatic interactions (Ghaedi et al., 2012). This interaction is attributed to the functional groups on the surface of the PDVB-IL.
3.6. Adsorption mechanism From the above results, a mechanism for the adsorption of Cr(VI) onto PDVB-IL can be proposed. When the Cr (VI) ions and water molecules come into contact with the surface of PDVB-IL, the Cr (VI) ions are trapped and adsorbed so that only water molecules can freely come in and out (Fig. SM-5). The intensive positive charge can be formed by ammonium groups on the surface of adsorbent, which can 2 cause a great attraction to the HCrO 4 and Cr2O7 forms of Cr (VI). Therefore, The maximum adsorption capacity after 5 min at room temperature (25 C) was 391.4 mg/g, which is much better than most of the previously reported adsorbents. The other reason for adsorption is mainly attributed to the Cr(VI) ion’s form in aqueous solution, 2 which are greatly affected by the solution’s pH. The HCrO 4 and Cr2O7 forms of Cr(VI), which exist in the solution at pH values below 6, can be effectively adsorbed onto the positive functional surface of PDVB-IL (Miretzky and Cirelli, 2010). At pH > 6, OHe anions will be preferentially adsorbed onto the surface, which consumes the positive charge and covers the adsorption sites (Chen et al., 2011). The adsorption process can be described as follows. The negatively charged Cr(VI) ions get close to the positively charged adsorption sites. Then, the Cr(VI) ions are tightly adsorbed onto the surface of the PDVB-IL by electrostatic and hydrogen bonding interactions.
H. Gao et al. / Journal of Environmental Management 137 (2014) 81e85
4. Conclusions The functional ionic liquid-based cross-linked polymer, PDVB-IL, was successfully prepared by polymerization of 1-aminoethyl-3vinylimidazolium chloride hydrochloride and divinylbenzene. The Cr(VI) adsorptive capacity of the PDVB-IL was measured in solution. The adsorption capacity gradually decreases with an increase in pH. The maximum values of qm are 391.4, 421.6, and 466.6 mg/g at 25, 40, 55 C, respectively. The adsorption thermodynamics show that electrostatic interaction is the dominating role in the adsorption process. This work may provide a new approach for the development of novel kinds of adsorbent with important practical applications. Acknowledgments The authors are grateful to the National Basic Research Program (2009CB930101), the National Natural Science Foundation of China (No. 91127017), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2013.11.055. References Ahmad, R., Kumar, R., 2011. Adsorption of amaranth dye onto alumina reinforced polystyrene. Clean-Soil Air Water 39, 74e82. Anirudhan, T.S., Jalajamony, S., Suchithra, P.S., 2009. Improved performance of a cellulose-based anion exchanger with tertiary amine functionality for the adsorption of chromium (VI) from aqueous solutions. Colloids Surf. A 335, 107e113. Barrera-Diaz, C.E., Lugo-Lugo, V., Bilyeu, B., 2012. A review of chemical, electrochemical and boilogical methods for aqueous Cr (VI) reduction. J. Hazard. Mater. 223, 1e12. lu, G., Yakup Arica, M., 2008. Adsorption of Cr (VI) onto PEI immobilized Bayramog acrylate-based magnetic beads: isotherms, kinetics and thermodynamics study. Chem. Eng. J. 139, 20e28. Cai, Y.Q., Lu, F., Peng, Y.Q., Song, G.H., 2007. Preparation and characterization of amino or carboxyl-functionalized ionic liquids. Chin. Chem. Lett. 18, 21e23. Chen, S., Yue, Q., Gao, B., Li, Q., Xu, X., 2011. Removal of Cr (VI) from aqueous solution using modified corn stalks: characteristic, equilibrium, kinetic and thermodynamic study. Chem. Eng. J. 168, 909e917. Chowdhury, S.R., Yanful, E.K., Pratt, A.R., 2012. Chemical states in XPS and Raman analysis during removal of Cr (VI) from contaminated water by mixed maghemiteemagnetite nanoparticles. J. Hazard. Mater. 235, 246e256. Costa, M., 2003. Potential hazards of hexavalent chromate in our drinking water. Toxicol. Appl. Pharm. 188, 1e5. Edebali, S., Pehlivan, E., 2010. Evaluation of amberlite IRA96 and dowex 18 ionexchange resins for the removal of Cr (VI) from aqueous solution. Chem. Eng. J. 161, 161e166. Ghaedi, M., Sadeghian, B., Pebdani, A.A., Sahraei, R., Daneshfar, A., Duran, C., 2012. Kinetics, thermodynamics and equilibrium evaluation of direct yellow 12 removal by adsorption onto silver nanoparticles loaded activated carbon. Chem. Eng. J. 187, 133e141.
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Removing Cr(VI) from aqueous solutions using a functional ionic liquid-based cross-linked polymer Hejun Gao a, b, Yun Wang c, Liqiang Zheng a, * a
Key Laboratory of Colloid and Interface Chemistry, Shandong University, Ministry of Education, Jinan 250100, China Chemical Synthesis and Pollution Control Key Laboratory of Sichuan Province, China West Normal University, Nanchong 637000, China c Shandong Institute of Metrology, Jinan 250100, China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 July 2013 Received in revised form 21 November 2013 Accepted 25 November 2013 Available online 4 March 2014
A novel functional ionic liquid-based cross-linked polymer was synthesized from 1-aminoethyl-3vinylimidazolium chloride hydrochloride and divinylbenzene. The physicochemical properties of the adsorbent were investigated using scanning electron microscopy, thermogravimetric analysis, and Fourier transform infrared spectroscopy. The adsorption capacity of the novel cross-linked polymer with respect to Cr(VI) was investigated using a batch adsorption procedure and the kinetics and thermodynamics of the adsorption process were further investigated. It was found that the adsorption kinetics was well fitted by a pseudo-second-order model and the adsorption isotherms agreed well with the Langmuir model. The maximum adsorption capacity after 5 min at room temperature (25 C) was found to be 391.4 mg/g, which was much better than the most of the previously reported adsorbents. The adsorption process was found to be dominated by electrostatic interactions. The introduction of functional ionic liquid moieties into cross-linked poly(divinylbenzene) polymer constitutes a new and efficient kind of adsorbent. Ó 2014 Elsevier Ltd. All rights reserved.
Keywords: Cr(VI) Adsorption Ionic liquids Polymers
1. Introduction As one of the world’s most strategic and critical materials, chromium (Cr) is widely used in the metal and chemical industries. With the continued development of these industries, a huge amount of wastewater containing Cr(VI) and Cr(III) is discharged into the environment at a level that nature cannot cope with. Of the two different oxidation states, Cr(VI) is the one which is the most toxic pollutant (Costa, 2003). Cr(VI) is a strong oxidizing agent and potential carcinogen and can cause enormous harm to both human beings and animals. Discharge of Cr(VI) into water is tightly regulated by the Environmental Protection Agency. The maximum permissible limits of Cr(VI) for discharge into potable water, inland surface water, and industrial wastewater are 0.05, 0.1, and 0.25 mg/L, respectively (Hosseini-Bandegharaei et al., 2010; Chen et al., 2011). Many technologies have been applied to remove Cr(VI) from wastewater, including biological treatment (Barrera-Diaz et al., 2012), chemical precipitation (Peng et al., 2004), membrane filtration (Wang and
* Corresponding author. Tel.: þ86 531 88366062; fax: þ86 531 88564750. E-mail address: [email protected] (L. Zheng). http://dx.doi.org/10.1016/j.jenvman.2013.11.055 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.
Ge, 2013), ion-exchange (Edebali and Pehlivan, 2010), solvent extraction (Kul and Cetinkaya, 2010), and adsorption technology (Zhao et al., 2013; Yang et al., 2012). The most common one is adsorption technology due to its effectiveness, efficiency, economy and absence of secondary pollution. Many kinds of adsorbent are used to remove Cr(VI) from wastewater, including activated carbon (Liu et al., 2010b), zeolites (Zeng et al., 2010), clays (Mekatel et al., 2012), etc. However, the methods suffer from disadvantages, such as low adsorption capacity and long adsorption time, which limit their application in Cr(VI) wastewater treatment. In recent years, ionic liquids (ILs) have received much attention owing to their unique properties, such as high thermal stability and high ionic conductivity (Welton, 1999). Many IL-based materials have been applied in wastewater treatment. Mahmoud and AlBishri (2011) investigated the adsorption of lead on [NSieOHOMIMþTf2N] and [NSieNH2-OMIMþTf2N], which are hydrophobic ionic liquids on nano-silica. They found that the materials have excellent adsorption capacities with respect to lead (0.5e0.9 and 0.8e1.3 mmol/g, respectively, at pH 1e7). Poursaberi and Hassanisadi (2013) synthesized IL@Fe3O4 nanoparticles from IL-COOH and Fe3O4 and found it to be an effective adsorbent of Reactive Black 5 (maximum adsorption capacity 161.29 mg/g). Lou and Di (2012) prepared an absorbent of macroporous adsorption
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resins functionalized with ionic liquids. The adsorption capacity of flavonoids on this adsorbent reached up to 7 mg/g. The preparation of an effective adsorbent for removing Cr(VI) from aqueous solution is expected to meet the demands required for environmental protection. It is well-known that ammonium groups on the surface of an adsorbent can greatly improve adsorption capacity (Li et al., 2005a; Min et al., 2012). In this work, a cross-linked polymer was obtained by copolymerizing divinylbenzene and 1-aminoethyl-3-vinylimidazolium chloride hydrochloride. The structure and the physicochemical properties of the functional ionic liquid-based cross-linked polymer were characterized. Then, the functional ionic liquid-based cross-linked polymer was used to treat Cr(VI) solutions.
speed of 150 rpm. After reaching equilibrium, 1 mL of each sample was withdrawn and filtered through a 0.45 mm membrane filter (nylon). Then, 0.1 mL of the filtrate was diluted to 50 mL in a colorimetric tube. The concentration of the Cr(VI) ions in the sample was then determined colorimetrically by reaction with 1,5diphenylcarbazide in acid solution. The purple complex solution was analyzed using UVeviseNIR spectroscopy (Hitachi U-4100, Japan) using the lmax at 540 nm. Previous work revealed that the total amount of Cr and Cr(VI) ions was more or less the same, indicating that the Cr(III) ion concentration in the final solution was insignificant (Chen et al., 2011; Gupta and Babu, 2009). Therefore, only the amount of Cr(VI) was measured in our work. The amount of Cr(VI) adsorbed onto the PDVB-IL, qt, (mg/g) was calculated according to the following equation:
2. Materials and methods 2.1. Materials
qt ¼
ðC0 Ct ÞV ; m
(1)
Potassium dichromate (99.8%), 1-vinylimidazole (99%), 2,20 azobis(2-methylpropionitrile) (99%), dimethyl formamide (99%), and 2-chloroethylamine hydrochloride (99%) were purchased from the Beijing Chemical Reagent Co. Divinylbenzene (DVB) (80%) was provided by Aldrich.
where C0 and Ct (mg/L) are the initial concentration of Cr(VI) and at time t, respectively, V is the volume of the solution (L), and m is the mass of PDVB-IL (g).
2.2. Methods
3.1. Characterization of PDVB-IL
2.2.1. Preparation of functional ionic liquid-based cross-linked polymer 1-Vinylimidazole (0.1 mol) and 2-chloroethylamine hydrochloride (0.1 mol) were added into 50 mL acetonitrile. The mixture was stirred and refluxed under a nitrogen atmosphere for 48 h. The resulting solid was washed several times with anhydrous ethanol. Then, the product was dried for 48 h under vacuum and a white solid was obtained. Then, the white product (0.015 mol), DVB (0.00375 mol), and an appropriate amount of 2,20 -azobis(2methylpropionitrile) were dissolved in 50 mL dimethyl formamide under nitrogen. The mixture was stirred at 80 C for 24 h. A yellow solid was collected by filtration and washed with acetone. The new adsorbent of cross-linked polymer (PDVB-IL) was obtained. The preparation process of PDVB-IL is shown in Fig. SM-1.
Scanning electron microscope (SEM) images of the surface of the PDVB-IL are depicted in Fig. SM-2. The surface of the PDVB-IL is rough and irregular in shape. And the specific surface area BET value is 0.124 m2/g. The rough surface can provide a good adsorption site for Cr(VI) to be trapped and adsorbed. The thermal stability of the prepared PDVB-IL was characterized by thermogravimetric analysis (TGA). The TGA plot for the PDVB-IL is shown in Fig. SM-3. The PDVB-IL shows two weight losses, which takes place at ca. 300e410 and 410e500 C. The weight loss of the PDVB-IL at ca. 300e410 C is due to the ILs on the polymer (Cai et al., 2007). The main weight loss occuring between 430 and 500 C is attributed to degradation of the PDVB (Liu et al., 2010a). The amount of IL in the PDVB-IL is about 60 wt.%. The weight loss of PDVB-IL is negligible below 300 C, indicating that the PDVB-IL can be used to treat wastewater at high temperature. In order to confirm the main functional groups present in the adsorbent, FT-IR spectroscopy was used. The infrared spectra of PDVB-IL, PDVB-IL þ Cr(VI) (i.e. after adsorption of Cr(VI) onto PDVBIL), and K2Cr2O7 are shown in Fig. SM-4. In the PDVB-IL spectrum, the peak at 3447 cm1 is attributed to the stretching mode of the NþeH groups. The bands at 1630 and 1552 cm1 are the C]C stretching vibration of the imidazole ring and NþeH in-plane bending vibration, respectively (Pitula and Mudring, 2010). A strong band at 1448 cm1 is assigned to the C]C stretching in the benzenoid rings (Nayak et al., 2010). The peak at 1162 cm1 is the CeN stretching vibration. The above results indicate that the functional ionic liquid-based cross-linked polymer was successfully prepared. After adsorption of Cr(VI) onto the PDVB-IL, the FT-IR adsorption bands undergo obvious shifts and reductions, indicating that the Cr(VI) adsorption corresponds to the possible presence of several functional groups on the adsorbent (Chen et al., 2011). The three bands at 3447 (PDVB-IL), 950 (K2Cr2O7), and 760 cm1 (K2Cr2O7) move to lower wavenumbers in the PDVBIL þ Cr spectrum, indicating that hydrogen bonding interactions could exist between functional groups in the PDVB-IL and Cr(VI) 2 (HCrO 4 and Cr2O7 ). The interesting point is that the strong bands at 1552 and 1162 cm1 disappear in the PDVB-IL þ Cr spectrum, which could be attributed to the electrostatic interaction between 2 the Cr(VI) ions (HCrO 4 and Cr2O7 ) and the functional groups in the
2.2.2. Characterization of the adsorbent The particle size and morphology of the PDVB-IL were characterized using a scanning electron microscope (JSM-7600F, JEOL Ltd., Japan). The specific surface area of the PDVB-IL was characterized using a computer controlled adsorption analyzer (Micromeritics, Gemini V2.0, USA) and expressed using BrunauereEmmetteTeller (BET) theory. Thermogravimetric analyses were carried out using standard equipment (TGA1500 Rheometric Scientific Inc., Piscataway, NJ) to investigate the thermal properties of the samples. The measurements were conducted in an inert atmosphere of nitrogen using 8e10 mg samples, a heating rate of 10 C/min, and a temperature range of ambient to 800 C. Vibrational spectra were obtained using a Fourier transform infrared (FT-IR) spectrometer (VERTEX-70) using KBr pellets and a scanning range of 400e 4000 cm1. 2.2.3. Batch adsorption procedure Batch adsorption was carried out in order to evaluate the adsorption process. Standard Cr(VI) solutions were prepared by dilution of a stock standard solution (1 g/L aqueous chromium(VI) solution). The batch adsorption experiments were carried out as follows. The Cr(VI) solution and PDVB-IL were added to a 50 mL conical flask. The flask was shaken using an oscillator (SHZ-82, Changzhou Shaipu Experimental Instrument Factory, China) at a
3. Results and discussion
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PDVB-IL. The Cr(VI)-loaded spectrum further suggests that the functional groups play a major role in the process of Cr(VI) adsorption on PDVB-IL (Li et al., 2009). 3.2. Effect of pH on adsorption The initial pH value of the aqueous solution is an important parameter in the adsorption process. The Cr(VI) adsorption capacity on PDVB-IL is plotted as a function of the initial pH value in Fig. 1. The results show that the adsorption capacity gradually increases as the pH value of the solution decreases from 12 to 2. The pH value has two kinds of effect on the adsorption of Cr(VI) on PDVB-IL. On the one hand, Cr(VI) in aqueous solution exists in 2 2 2 various forms, such as HCrO 4 , Cr2O7 , CrO4 , etc. The CrO4 anion is 2 predominant in the pH range above 6, while HCrO and Cr 4 2O7 ions are predominant at pH values below 6 (Chen et al., 2011; Anirudhan et al., 2009). The HCrO 4 form only requires one cationic site of PDVB-IL for adsorption at low pH value, while the CrO2 4 form needs two cationic sites of PDVB-IL at high pH values (Yusof and Malek, 2009). On the other hand, with increasing pH, the amount of OHe available is gradually increased and the cationic charges of the amine functional groups (NHþ 3 ) on the surface of the PDVB-IL are consumed. Therefore, the adsorption capacity with respect to Cr(VI) gradually decreases with increasing initial pH. To achieve lower pH values in the solution adjustment with acid is required, however, it is not economical. Therefore, the next experiments were carried out in natural water (pH ¼ 5.6). 3.3. Adsorption kinetics The adsorption kinetics of Cr(VI) on PDVB-IL was studied to investigate the rate at which contamination was removed from aqueous solution. Fig. 2 shows the effect of contact time on the amount of Cr(VI) adsorption for different initial concentrations (50, 100, and 150 mg/L). Fig. 2 shows that the Cr(VI) ions are adsorbed rapidly in the initial 5 min. After that, Cr(VI) adsorption did not change with the contact time. The equilibrium time is very short, which reflects the high accessibility of the Cr(VI) ions to the adsorption sites in the PDVB-IL (Hosseini-Bandegharaei et al., 2010). To investigate the amount of Cr(VI) adsorbed on the surface of the PDVB-IL at any particular time and the amount adsorbed at equilibrium, a pseudo-second-order model is adopted to elucidate the adsorption kinetics. This can be expressed as
Fig. 1. Influence of pH on the adsorption capacity of PDVB-IL towards Cr(VI). (C0 ¼ 100 mg/L, T ¼ 25 C, contact time ¼ 30 min, PDVB-IL ¼ 0.1 g/L).
Fig. 2. Adsorbed amount of Cr(VI) by PDVB-IL as a function of contact time at different initial concentrations of Cr(VI). (T ¼ 25 C, PDVB-IL ¼ 0.1 g/L).
t 1 t ¼ þ ; qt kq2e qe
(2)
where k is the rate constant of the pseudo-second-order reaction (g/mg min), qt and qe represent the adsorption capacity (mg/g) at time t and equilibrium (respectively), and t (min) is the contact time. The corresponding kinetic parameters are given in Table SM1. The value of the correlation coefficient (R2) for the pseudosecond-order kinetic curve is 0.9999. Moreover, the calculated values of qe are close to the experimental values for pseudo-secondorder kinetics, indicating that the adsorption kinetics is well fitted by a pseudo-second-order model (Ahmad and Kumar, 2011). The pseudo-second-order kinetics indicates that adsorption depends on the adsorbate as well as the adsorbent and involves chemisorption in addition to physisorption. The chemisorption contribution might be the rate limiting step where valency forces are involved via electron sharing or exchange between the adsorbent and the adsorbate. 3.4. Adsorption isotherms It is important to collect and analyze adsorption isotherms, as these can accurately describe how the adsorbate interacts with
Fig. 3. Adsorption isotherms for Cr(VI) at 25, 40 and 55 C. (Contact time ¼ 30 min, PDVB-IL ¼ 0.04 g/L).
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adsorbents and can be used for design purposes. Two well-known adsorption isotherms, the Langmuir and Freundlich models, are used here to describe the adsorption. These isotherms relate the equilibrium adsorption capacity to the equilibrium concentration in the solution (Fig. 3). The Langmuir adsorption model is the most common one to quantify the amount of adsorbate adsorbed on an adsorbent as a function of concentration at a given temperature. The linear form of the Langmuir isotherm is given by
Ce 1 Ce ¼ þ ; qm KL qm qe
(3)
where Ce is the liquid-phase concentration of Cr(VI) at equilibrium (mg/L), qm is the maximum theoretical adsorption capacity (mg/g), and KL is the adsorption equilibrium constant (L/mg). The constants qm and KL can be calculated from the intercept and slope of a linear plot of Ce/qe versus Ce. The Freundlich isotherm is the most important multilayer adsorption isotherm for rough surfaces. The linear form of the Freundlich isotherm model can be represented as
log qe ¼
1 log Ce þ log Kf ; n
(4)
where Kf and 1/n are characteristic constants representing the adsorption capacity and adsorption intensity of the system, respectively. The values of Kf and 1/n are obtained from a linear plot of log qe versus log Ce. Table SM-2 presents the results of fitting the data to the Langmuir and Freundlich models. It is seen that the experimental data conforms to both isotherm models at 40 and 55 C. The R2 values for the Langmuir isotherm are larger than those for the Freundlich isotherm model at 25 C, indicating that the Langmuir model is better for fitting the experimental data. This result suggests that there is monolayer coverage of Cr(VI) on the surface of the PDVB-IL at low temperatures and there may be multilayer adsorption present at high temperatures. With an increase in temperature, the qm value is increased, implying that the process of adsorption of Cr(VI) on PDVB-IL is endothermic. The calculated value of qm is 400.0 mg/g at 25 C, which agrees well with the experimental data (qe.exp ¼ 391.4 mg/g). The adsorption capacities of different adsorbents towards Cr(VI) are shown in Table SM-3. It is obvious that the adsorption capacity of PDVB-IL is much better than most of the other adsorbents reported previously. The large adsorption capacity could be due to the strong adsorption affinity of PDVB-IL towards Cr (VI), which is caused by the electrostatic and hydrogen bonding interactions between the anionic form of Cr (VI) and the functional groups in the PDVB-IL. The external surface of PDVB-IL plays an important role for Cr removal. For the Langmuir-type adsorption process, the essential characteristics can be explained using a constant and dimensionless separation factor, RL, which is considered as a more reliable indicator of the adsorption capacity (Venkatesha et al., 2012). The value of RL can be obtained from
RL ¼
1 : 1 þ KL C0
(5)
The RL value indicates whether the adsorption of Cr(VI) on PDVB-IL is unfavorable (RL > 1), linear (RL ¼ 1), favorable (0 < RL < 1), or irreversible (RL ¼ 0) (Chowdhury et al., 2012). The results are shown in Table SM-2. The RL values for all initial concentrations C0 are less than 0.13 at various temperatures, implying favorable adsorption of Cr(VI) on PDVB-IL.
3.5. Thermodynamics The effect of temperature on the adsorption of Cr(VI) was studied in order to obtain the relevant thermodynamic parameters, which can further help our understanding of the adsorption mechanism. The standard free energy change (DG0) for adsorption can be calculated from the following equation
DG0 ¼ 2:303RTlog Ke ;
(6)
where R is the universal gas constant (8.314 J/mol K) and T is the absolute temperature (K). The thermodynamic parameters standard enthalpy change (DH0) and entropy change (DS0) for the process can be determined from the following equations
Ke ¼
Cad;e ; Ce
log Ke ¼
DS0 DH0 ; 2:303R 2:303RT
DG0 ¼ DH0 T DS0 :
(7)
(8) (9)
where Cad,e is the concentration of adsorbate on the adsorbent at equilibrium (mg/L). The values of DH0 and DS0 can be calculated from the slope and the intercept, respectively, of a linear plot of log Ke against 1/T. Table SM-4 presents the resulting DG0, DH0, and DS0 values for adsorption of Cr(VI) on PDVB-IL. With an increase in temperature, the negative value of DG0 is decreased, indicating that the adsorption behavior is a spontaneous process and the adsorption process is more spontaneous at higher temperatures (Li et al., 2005b). The positive values of DH0 confirm the endothermic nature of the adsorption process, and the positive values of DS0 imply that the adsorbed Cr(VI) represents a certain amount of freedom in the solid/solution interface during the adsorption of Cr(VI) on PDVB-IL lu and Yakup Arica, 2008). This phenomenon is a result (Bayramog of physisorption, which takes place through electrostatic interactions (Ghaedi et al., 2012). This interaction is attributed to the functional groups on the surface of the PDVB-IL.
3.6. Adsorption mechanism From the above results, a mechanism for the adsorption of Cr(VI) onto PDVB-IL can be proposed. When the Cr (VI) ions and water molecules come into contact with the surface of PDVB-IL, the Cr (VI) ions are trapped and adsorbed so that only water molecules can freely come in and out (Fig. SM-5). The intensive positive charge can be formed by ammonium groups on the surface of adsorbent, which can 2 cause a great attraction to the HCrO 4 and Cr2O7 forms of Cr (VI). Therefore, The maximum adsorption capacity after 5 min at room temperature (25 C) was 391.4 mg/g, which is much better than most of the previously reported adsorbents. The other reason for adsorption is mainly attributed to the Cr(VI) ion’s form in aqueous solution, 2 which are greatly affected by the solution’s pH. The HCrO 4 and Cr2O7 forms of Cr(VI), which exist in the solution at pH values below 6, can be effectively adsorbed onto the positive functional surface of PDVB-IL (Miretzky and Cirelli, 2010). At pH > 6, OHe anions will be preferentially adsorbed onto the surface, which consumes the positive charge and covers the adsorption sites (Chen et al., 2011). The adsorption process can be described as follows. The negatively charged Cr(VI) ions get close to the positively charged adsorption sites. Then, the Cr(VI) ions are tightly adsorbed onto the surface of the PDVB-IL by electrostatic and hydrogen bonding interactions.
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4. Conclusions The functional ionic liquid-based cross-linked polymer, PDVB-IL, was successfully prepared by polymerization of 1-aminoethyl-3vinylimidazolium chloride hydrochloride and divinylbenzene. The Cr(VI) adsorptive capacity of the PDVB-IL was measured in solution. The adsorption capacity gradually decreases with an increase in pH. The maximum values of qm are 391.4, 421.6, and 466.6 mg/g at 25, 40, 55 C, respectively. The adsorption thermodynamics show that electrostatic interaction is the dominating role in the adsorption process. This work may provide a new approach for the development of novel kinds of adsorbent with important practical applications. Acknowledgments The authors are grateful to the National Basic Research Program (2009CB930101), the National Natural Science Foundation of China (No. 91127017), Specialized Research Fund for the Doctoral Program of Higher Education of China (No. 20120131130003) and the Shandong Provincial Natural Science Foundation, China (ZR2012BZ001). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jenvman.2013.11.055. References Ahmad, R., Kumar, R., 2011. Adsorption of amaranth dye onto alumina reinforced polystyrene. Clean-Soil Air Water 39, 74e82. Anirudhan, T.S., Jalajamony, S., Suchithra, P.S., 2009. Improved performance of a cellulose-based anion exchanger with tertiary amine functionality for the adsorption of chromium (VI) from aqueous solutions. Colloids Surf. A 335, 107e113. Barrera-Diaz, C.E., Lugo-Lugo, V., Bilyeu, B., 2012. A review of chemical, electrochemical and boilogical methods for aqueous Cr (VI) reduction. J. Hazard. Mater. 223, 1e12. lu, G., Yakup Arica, M., 2008. Adsorption of Cr (VI) onto PEI immobilized Bayramog acrylate-based magnetic beads: isotherms, kinetics and thermodynamics study. Chem. Eng. J. 139, 20e28. Cai, Y.Q., Lu, F., Peng, Y.Q., Song, G.H., 2007. Preparation and characterization of amino or carboxyl-functionalized ionic liquids. Chin. Chem. Lett. 18, 21e23. Chen, S., Yue, Q., Gao, B., Li, Q., Xu, X., 2011. Removal of Cr (VI) from aqueous solution using modified corn stalks: characteristic, equilibrium, kinetic and thermodynamic study. Chem. Eng. J. 168, 909e917. Chowdhury, S.R., Yanful, E.K., Pratt, A.R., 2012. Chemical states in XPS and Raman analysis during removal of Cr (VI) from contaminated water by mixed maghemiteemagnetite nanoparticles. J. Hazard. Mater. 235, 246e256. Costa, M., 2003. Potential hazards of hexavalent chromate in our drinking water. Toxicol. Appl. Pharm. 188, 1e5. Edebali, S., Pehlivan, E., 2010. Evaluation of amberlite IRA96 and dowex 18 ionexchange resins for the removal of Cr (VI) from aqueous solution. Chem. Eng. J. 161, 161e166. Ghaedi, M., Sadeghian, B., Pebdani, A.A., Sahraei, R., Daneshfar, A., Duran, C., 2012. Kinetics, thermodynamics and equilibrium evaluation of direct yellow 12 removal by adsorption onto silver nanoparticles loaded activated carbon. Chem. Eng. J. 187, 133e141.
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