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Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions
Accepted Manuscript Title: Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions Author: Lili Jiang Shujun Li Haitao Yu Zongshu Zou Xingang Hou Fengman Shen Chuantong Li Xiayan Yao PII: DOI: Reference:
S0169-4332(16)30236-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.067 APSUSC 32585
To appear in:
APSUSC
Received date: Revised date: Accepted date:
30-11-2015 5-2-2016 5-2-2016
Please cite this article as: L. Jiang, S. Li, H. Yu, Z. Zou, X. Hou, F. Shen, C. Li, X. Yao, Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions
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Lili Jianga,b,*, Shujun Lib, Haitao Yuc, Zongshu Zoua, Xingang Houb, a Fengman Shen , Chuantong Lib, Xiayan Yaob
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a School of metallurgy, Northeastern university, No.3-11, Wenhua Road, Heping District, Shenyang, Liaoning Province, P.R.China. Email: [email protected](Lili Jiang); [email protected](Zongshu Zou); shenfm@ mail.neu.edu.cn(Fengman Shen). b School of Material Science and Technology, Lanzhou University of Technology, Langongping Road, 730050, Lanzhou, Gansu Province, P.R.China. Email: [email protected](Shujun Li); [email protected](Chuantong Li); [email protected](Xiayan Yao); [email protected] (Xingang Hou). c Department of Medical Laboratory, The First Hospital of Lanzhou University, No.1, Donggang Road, Chengguan District, Lanzhou, 730000, Gansu Province, P.R.China. Email: [email protected]. *Corresponding author at: School of Material Science and Technology, Lanzhou University of Technology, Langongping Road, 730050, Lanzhou, Gansu Province, P.R.China. Email: [email protected]. Tel.:+86 0931 2976378; fax:+86 0931 2972702.
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Abstract: The novel functionalization of multi-walled carbon nanotubes (MWCNTs) was synthesized by reacting trimethoxysilylpropanethiol (MPTs), hydrazine, ammonium ferrous sulfate, and ammonium ferric sulfate in sequence as efficient ways to introduce Fe3O4, amino and thiol groups onto the nanotubes sidewalls. The magnetic MWCNTs composite material (N2H4-SH-Fe3O4/o-MWCNTs) was characterized by transmission electron microscopy, field emission scanning electron microscopy, x-ray diffraction, thermo-gravimetric analysis, x-ray photoelectron spectroscopy, fourier transformation infrared spectroscopy and magnetization curve. The results revealed that MPTs and hydrazine were coated on the surface of N2H4-SH-Fe3O4/o-MWCNTs. A series of batch adsorption experiments were conducted to study the experimental conditions, such as pH, contact time, initial concentrations and temperatures, which affected the adsorption process. The adsorption experiment results showed that the maximum equilibrium adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for lead, zinc and phenol was 195.81 mg/g, 169.89 mg/g and 38.97 mg/g at pH 6, respectively. The adsorption isotherm was better fitted by the Freundlich model, and the adsorption kinetics was consistent with pseudo-second order kinetics model. Furthermore, thermodynamic data showed that the adsorption process was spontaneous and exothermic. These results indicated that N2H4-SH-Fe3O4/o-MWCNTs may be promising surface modified materials for removing heavy metal ions and phenol from aqueous solutions. Key words: multi-walled nanotube, adsorption, thermodynamics, dynamics 1
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1. Introduction
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Industrial wastewater containing organic and inorganic pollutants can lead to environmental pollution problems. Among these pollutants, such as phenol, lead, and zinc, frequently co-exist in waste waters from metal plating, dye and painting industries. Heavy metals, such as lead and zinc, enter into the environment and cause health hazards due to their toxicity and bioaccumulation in the living organism [1]. In addition, phenol was found to be highly poisonous to flora and fauna [2]. Removal of heavy metal ions and phenol from environmental samples is an important field of study, and controlling over their concentrations in the environment is especially significant [3-4]. In addition, the removal of heavy metals and phenol from water sources to an acceptable concentration is a current research challenge in water treatment. To date, traditional methods, such as sulfide precipitation [5], ion exchange [6], alum treatment [7], membrane separation, reverse osmosis and iron coagulation [8], have been used to remove or separate heavy metal ions and phenol from aqueous environments. However, filtration and centrifugation techniques are time consuming and separation becomes a difficult issue due to the small size of the ions. Furthermore, their drawbacks such as procedural complexity and high operating cost, have been reported for the existing methods for the removal of lead, zinc, and phenol must be solved. Among these techniques, adsorption is the most attractive due to its simplicity, high efficiency, and its ability to remove multiple low concentration pollutions simultaneously. Therefore, there is still a requirement for the development of efficient adsorbents with high adsorption capacity for lead, zinc and phenol [9]. Recently, carbon-based nano-materials, such as carbon nanotubes (CNTs), have been developed to remove heavy metals from aqueous media, and they are promising materials due to their unique electrical, mechanical, thermal, optical and chemical properties [10-13].CNTs were discovered by Iijima in 1991, and they mainly included single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) [14]. It is well known that the ability of CNTs to establish π-π electrostatic interactions and their large surface areas can facilitate the adsorption of pollutants from water [15]. CNTs have been proven as superior adsorbents for removing organic and inorganic pollutants [16-18]. The adsorption capability of MWCNTs is primarily related to the functional groups, which attaches to the surface of MWCNTs and improves the reactivity and provides active sites [19-20]. Many researchers had reported a number of surface modification strategies, such as amino- [21-23] 2
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and thiol- [24-27] modified MWCNTs to increase the adsorption capacity, selectivity and removal efficiency of heavy metals. Bahramifar et al. had reported that the synthesis of nanocomposites with amino- and thiol-modified MWCNTs can improve the adsorption property for mercury ions from synthetic and real wastewater aqueous solutions due to the added amino and thiol groups [28]. Zhang et al. had developed a novel method for amine functionalized CNTs via combination of mussel inspired chemistry and Michael addition reaction. Results demonstrated that the adsorption capacity of these functionalized CNTs was much improved as compared with pristine CNTs. To authors’ knowledge, there are few reports that use amino and thiol groups to modify MWCNTs, and they have not been used as adsorbents to simultaneously remove heavy metals and phenol. The objective of the present study was to synthesize thiol- and amino-modified MWCNTs and investigate their adsorption capability for phenol and heavy metal from aqueous solutions. In this study, the oxidized MWCNTs were modified by amino and thiol groups. The synthesized adsorbents were characterized by fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Adsorption experiments of Zn(II), Pb(II) and phenol from aqueous solution were performed. The effects of pH, contact time, initial concentrations and temperatures on the adsorption capacity of the modified MWCNTs were investigated. The adsorption data were fitted with the Freundlich and Langmuir isotherm models. The enthalpy, entropy and Gibbs free energy of the functionalized MWCNTs were determined from the experimental data. 2. Experiment and methods 2.1. Materials and reagents
MWCNTs were purchased from Shenzhen Nanotech. Port Co. Ltd., (China). According to the manufacturer, the physicochemical properties of MWCNTs were listed in Table 1. Ultra-pure water was used for the preparation of solution. Lead nitrate, zinc nitrate, and phenol were purchased from Tianjin Fu Rui Technology Co. Ltd., (China). Ammonium ferrous sulfate ((NH4)2Fe(SO4)2•6H2O), ammoniumferricsulfate (NH4Fe(SO4)2•12H2O), nitric acid, sulfuric acid, hydrazine and acetone (purity>99.5%) were purchased from Big Alum Chemical Reagent Factory (China). MPTs was purchased from Beijing Biological Technology (China). 3
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Value 20-40 nm >5 um >97 % <3 % 80-140 m2/g
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Table 1 Physicochemical properties of MWCNTs Physicochemical property diameter length purity ash special surface area
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All of the chemicals were of analytical grade without further purification.
2.2. Characterization techniques
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TEM (type JEM-2010, operating at 200 kv) was used to characterize the morphology of the samples. These samples were dispersed in absolute alcohol by sonication; then a carbon-coated 400 mesh copper grid was immersed in the suspension and air-dried. SEM (JSM-6700F, operating at 0.5-30 kv) was performed to characterize the surface morphology of the adsorbent. FT-IR were recorded using a Nexus-870. All of the samples were prepared by mixing spectroscopic-grade KBr with MWCNTs or modified MWCNTs and they were analyzed in the 400-4000 cm-1 range with a resolution of 2 cm-1. TGA was performed on an thermogravimetric analyzer (NETZSCHSTA-449F3) under N2 atmosphere from 40 to 1200 ℃ at heating rate of 20 ℃/min. X-ray diffraction (XRD, RigakuD/max-2400, operating at 60 kv) was performed to investigate the crystalline phase of the adsorbent. The diffraction patterns were recorded in the 2θ=10-90 o, operating with CuKαradiation (λ=1.5418 Å). The measurements were carried out in air at ambient pressure and temperature. XPS（Escalab 250Xi with a max resolution of 400000 cps and FWHM < 0.5 ev) was applied to determine the surface chemical composition of the samples using AlKα radiation. The XPS spectra were deconvoluted using XPS peak software. The surface atomic percentages were calculated from the corresponding peak areas. The magnetism was measured using a vibrating sample magnetometer (VSM, sensibility between 3-5 × 10-5) manufactured by Nanjing Univ. Instrument Plant, China. Metal ion concentrations were analyzed using an atomic absorption spectrophotometer (HITACHI Z-5000)，and phenol concentrations was analyzed by ultraviolet and visible spectrophotometer (VU-1000). A temperature controlled shaker was used for shaking the aqueous solution containing metal ions and phenol and 4
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temperature controlled water bath (HHS2) was used for maintaining the temperature of aqueous solution. All of the experiment water was performed by ultra-pure water system (Ulupure, UPD-I-20T).
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2.3. Preparation of oxidized MWCNTs
2.4. Preparation of Fe3O4/o-MWCNTs
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MWCNTs (4 g) were placed in a beaker with 120 ml concentrated sulfuric acid and 40 ml concentrated nitric acid to remove impurities and hemispherical caps. Then, the mixture was ultrasonically stirred for 24 h. The suspensions were filtrated, rinsed with deionized water until the pH reached approximately 6, and dried in a vacuum oven at 80 ℃ for 12 h. The obtained product was the oxidized MWCNTs (o- MWCNTs) (as shown in Fig. 1(a)).
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4 g o-MWCNTs, 6.8 g (NH4)2Fe(SO4)2 · 6H2O and 10.4 g NH4Fe(SO4)2 · 6H2O were dissolved in 500 ml ultrapure water and ultrasonically treated for 15 min. Ammonium hydroxide (8 mol/l) was added dropwise until the pH of solution reached 11.The mixture was continuously stirred for 30 min at 50 ℃ until the color changed from black to brown. Finally, the products were collected from the water solution using a magnet and were rinsed 10 times by ultrapure water. The product was dried in a vacuum drying oven at 80 ℃ for 8 h to obtain the Fe3O4/o-MWCNTs product [28] (as shown in Fig. 1(b)). 2.5. Preparation of SH-Fe3O4/o-MWCNTs Fe3O4/o-MWCNTs (200 mg) were dispersed in 150 ml absolute alcohol solution at 25 ℃. Then, 0.5 ml acetic acid and 0.1 ml MPTs was added to the mixed solution. The entire experimental process was protected under nitrogen gas while continually stirred. The reaction time was 24 h at 25 ℃. After the reaction, 50 ml acetone was slowly added to the reaction solution. The product was passed through filter paper and washed with ethanol and ultra-pure water. The product was dried in a vacuum drying oven at 60 ℃ for 12 h. Finally, the SH-Fe3O4/o-MWCNTs were obtained (as shown in Fig. 1(c)). 2.6. Preparation of N2H4-SH-Fe3O4/o-MWCNTs 5
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5 g SH-Fe3O4/o-MWCNTs was suspended in 40 ml absolute alcohol. The mixture was sonicated, stirred vigorously, and then slowly reacted for 5 min after adding hydrazine. Then, the solution was placed in a water bath and stirred for 3 h. Subsequently, the composite material was filtered and washed with ultrapure water for several times. Finally, the product was dried in a vacuum oven for 8 h at 60 ℃. The obtained composite material was N2H4-SH-Fe3O4/o-MWCNTs (as shown in Fig. 1(d)).
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2.7. Adsorption experiments
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The adsorption capacities of N2H4-SH-Fe3O4/o-MWCNTs for Pb(II), Zn(II) and phenol were evaluated at pH values, contact time, initial concentrations and temperatures. The effects of pH values from 2 to 9 were evaluated by adding 0.1 mol/l HNO3 and 0.1 mol/l NaOH solution to adjust the pH. The initial concentration of Pb(II), Zn(II) and phenol in each solution was 40 mg/l, respectively. 10 mg adsorbents in every 50 ml solution was used for the co-adsorption of Pb(II), Zn(II)and phenol. The conical flasks containing the mixtures were placed on a shaker at 25 ± 0.5 ℃. To be conservative, an equilibration time of 12 h was routinely used. All of the adsorption experiments were performed in triplicate, and the averaged values were reported in this study. The effects of contact time were investigated at the time intervals of 10, 30, 60, 90, 120, 180, 300, 420, 600 and 720 min with an initial concentration of 40 mg/l for Pb(II), Zn(II) and phenol, respectively. 10 mg adsorbents were dispersed into 50 ml solutions at 25 ± 0.5 ℃ under pH 6. To investigate the adsorption isotherm, the initial concentration was changed from 20 to 60 mg/l at 25 ± 0.5 ℃ at pH 6 for 12 h. The adsorbent amount was 10 mg in 50 ml solution. The effects of temperatures on Pb(II), Zn(II) and phenol adsorption were obtained by 10 mg adsorbent in 50 ml solution. The adsorption experiments were performed in flasks containing Pb(II), Zn(II) and phenol concentration of 40 mg/l, respectively. The temperatures varied from 288 k to 308 K at pH 6.0 for 12 h. The liquid after adsorption was pipetted, immediately separated by a magnet from treated solutions, and filtered with 50 µm polytef membranes. The exact concentrations of Pb(II), Zn(II) after filtering the adsorbents were determined using an atomic absorption spectrophotometer and phenol concentration was analyzed by ultraviolet and visible spectrophotometer. The removal rate (%) was calculated using the following equation: 6
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(C0 − Ct ) × 100% C0
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where C0 and Ct (mg/l) were the initial concentration and the concentration at time t of Pb(II), Zn(II) and phenol, respectively. The synthetic routes of the functionalized MWCNTs and the adsorption experiments were shown in Fig. 1.
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Fig. 1. Schematic illustration of N2H4-SH-Fe3O4/o-MWCNTs and adsorption experiment. (a) o-MWCNTs with abundant carboxyl groups on the surface. (b) Fe3O4/o-MWCNTs with co-precipitation method. (c) SH-Fe3O4 /o-MWCNTs with SH groups on the surface. (d) N2H4-SH-Fe3O4/oMWCNTs with amino and thiol groups on the surface. (e) adsorption experiments of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ), and phenol.
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2.8. Role of the components in the N2H4-SH-Fe3O4/o-MWCNTs
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Our main objective in the present work was to develop N2H4-SH-Fe3O4/o-MWCNTs with multi-component, containing MWCNT, Fe3O4, MPTs and hydrazine, to efficiently remove Pb(Ⅱ), Zn(Ⅱ) and phenol, along with an improved removal rate, good selectivity and stability. The role of the individual components in the N2H4-SH-Fe3O4/o-MWCNTs was inferred through various characterization studies and discussed below. The goal of incorporating Fe3O4 was to improve separability from the solution. Furthermore, MPTs may improve the removal rate and adsorption capacity by providing additional binding sites for Pb(Ⅱ), Zn(Ⅱ) and phenol and hydrazine can provide primary amine (–NH2) groups and secondary amine groups (–NH–).
3. Results and discussion 3.1. Morphology and crystal structure characterization Fig.
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N2H4-SH-Fe3O4/o-MWCNTs (c) and a high magnification image of N2H4-SH-Fe3O4/o-MWCNTs (d).
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Fig. 2 shows the TEM images of MWCNT (a), o-MWCNTs (b), and N2H4-SH-Fe3O4/o-MWCNTs (c) and (d). Fig. 2(a) shows that MWCNTs are cylindrical shapes with smooth surfaces and the average diameter is 40 nm with a small amount of catalyst impurities on the surface. Fig. 2(b) shows that oxidized MWCNTs have no impurities, the surface is rough with lots of grooves and the ends are opened. Fig. 2(c) shows that Fe3O4 nanoparticles are uniformly and densely loaded on the surface of o-MWCNTs, and there is a limited amount of vacant areas on the surface. Their size distribution is evaluated using nano measurer software invented Fudan University according to selected TEM images. The latter indicates Fe3O4 size is between 4 nm and 7 nm with a mean diameter of 6 nm. It is noteworthy that no Fe3O4 aggregation is observed on the nanotubes surfaces. Fig. 2(c) indicates that diameter of the N2H4-SH-Fe3O4/o-MWCNTs increases to 50-60 nm, which proves that Fe3O4, MPTs and N2H4 are successfully coated on the surface of o-MWCNTs. Compared with Fig. 2(b), Fig. 2(d) shows the transparent film on the surface of N2H4-SH-Fe3O4/o-MWCNTs is average thickness of 6.5 nm. The apparent evidence indicated that MPTs and hydrazine were modified on the surface of N2H4-SH-Fe3O4/o-MWCNTs.
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Fig. 3. SEM images of MWCNTs (a), o-MWCNTs (b), Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d). The SEM images of MWCNTs, o-MWCNTs, Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs are shown in Fig. 3. Fig .3(a) and (b) reveal that MWCNTs and o-MWCNTs are similar, that they are tubular, curved and highly tangled. The lengths of these nanotubes are approximately 5-10 µm, and the average outer diameter is 20-40 nm. Fig. 3(b) shows that there is slight fragmentation with a length of 100-200 nm, which indicates that MWCNTs are shortened after oxidation treatment. Fig. 3(c) clearly shows that there is a layer of nanoparticles on the surface of the o-MWCNTs. Compared with Fig. 3(b), Fig. 3(c) suggests that Fe3O4 are coated on the surface of o-MWCNTs. Fig. 3(d) shows a white layer on the surface, and the diameter of nanotubes increases to 50-60 nm. The increased diameter revealed that MPTs and hydrazine were deposited on the surface. Fig. 4. XRD patterns of MWCNTs (a), N2H4-SH-Fe3O4/o-MWCNTs (b) and Fe3O4/o-MWCNTs (c). 8
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XRD is the most widely used technique to characterize the crystalline phase of the synthesized materials. Fig. 4 shows the XRD patterns of MWCNTs, Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs. Fig. 4(a) shows that MWCNTs exhibits the typical diffraction peaks at 2θ=25.89 o and 42.78 o, which are assigned to the (002) and (100) planes of the graphite phase (PDF card 41-1487, JCPDS), indicating that a graphite structure existed in this sample [30]. Fig. 4(b) and (c) indicate that the diffraction peaks of MWCNTs become weak, especially at 25.89 o, and they have a lower crystallinity than MWCNTs [31]. Fig. 4(c) shows that the XRD pattern of Fe3O4/o-MWCNTs matches well with the standard cubic phase of Fe3O4 including that the six diffraction peaks (2θ =30.07 °, 35.41 °, 53.39 °, 56.91 °, 62.92 ° and 73.93 °) correspond to the (220), (311), (422), (511), (440), (533) planes (JCPDS No.19-0629) of magnetite, respectively [32] . This illustrates the existence of Fe3O4. The Scherer equation is calculated according to the highest intensity of 35.41 °.The particle size is approximately 6 nm, which is consistent with result of Fig. 2(c). Fig. 4(b) shows that the peaks are weaker, especially at 35.41 °, and broader compared with Fe3O4/o-MWCNTs. Due to the coating of MPTs and hydrazine, the peaks become weak. The amino and thiol groups may have coated on the surface of Fe3O4, which cover the peak of Fe3O4. According to the results of XRD and TEM, the nanoparticles on the surfaces of N2H4-SH-Fe3O4/o-MWCNTs are assigned to Fe3O4.
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3.2. Magnetic characterizations
Fig. 5. Hysteresis loops N2H4-SH-Fe3O4/o-MWCNTs (b)
of
Fe3O4/o-MWCNTs
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The magnetic properties of Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs were measured at 298K using a vibrating sample magnetometer. Fig. 5 shows that the coercivity and remanence of Fe3O4/MWCNTs and N2H4-SH-MWCNTs/Fe3O4 are zero, which reveals their superparamagnetism. The saturation magnetization is 35.85 emu/g and 32.89 emu/g for Fe3O4/o-MWCNTs and N2H4-SH-MWCNTs/Fe3O4, respectively. The large saturation magnetization allow for a fast separation of composites from aqueous solution [33]. The saturation magnetization of N2H4-SH-Fe3O4/o-MWCNTs is slightly less than that of Fe3O4/o-MWCNTs because the surface of N2H4-SH-Fe3O4/o-MWCNTs is covered by 9
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non-magnetic MPTs and hydrazine. 3.3 FT-IR spectra
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Fig. 6. FT-IR spectra of MWCNTs (a) and N2H4-SH-Fe3O4/o-MWCNTs (b)
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The surface functional groups of adsorbent are determined by FT-IR spectra analysis. Fig. 6(a) and (b) show strong peaks at 1560 cm-1 attributed to C=C stretching vibration. Fig. 6(b) shows a strong peak at 599 cm-1 attributed to Fe-O stretching vibration which indicates that Fe3O4 is successfully loaded onto the o-MWCNTs. The appearance of a new peak at 966 cm-1 is assigned to Si-N stretching vibration, which reveals that MPTs and N2H4 are successfully connected. The peaks at 1010 cm-1 and 3421 cm-1 are ascribed to O-H stretching vibration [34]. The oxidized treatment produced hydroxy groups on the outermost surface and defect sites that increased the hydrophilicity of MWCNTs. These oxygen-containing functional groups can readily form H-bonding with polar functional groups (-SH group) on the surface of N2H4-SH-Fe3O4/o-MWCNTs. Moreover, this interaction could be very strong if the oxygen content of the composite was high according to the XPS results (17.55% of oxygen content).The band at 1200-1260 cm−1 is attributed to the stretching vibration of the carboxyl group (−COOH). The band at 2800-3000 cm-1 is associated with symmetric and asymmetric -CH2 stretching vibration. The enhanced intensity of the -CH2 stretching vibration reveals that additional methylene groups from MPTs are attached to the surface of o-MWCNTs. The peak at 2355 cm-1 is associated with -NH2, and a strong peak at 3381 cm-1 may be assigned to the N-H stretching vibration [35]. The appearance of -NH2 and N-H stretching vibrations indicates that amino groups appear on the surface of o-MWCNTs. Fig. 6(b) show that a weak peak at 2513 cm-1, which may be assigned to S-H group. The peak at 871 cm-1 is assigned to the stretching vibration of Si-C. The appearance of S-H and Si-C reveal that MPTs are successfully coated on the surface of o-MWCNTs. Furthermore, a new peak at 1110 cm-1 is attributed to Si-O stretching vibration, which can further prove that MPTs and hydrazine are successfully coated on the o-MWCNTs by connecting with the hydroxy groups on the external surface of o-MWCNT. 3.4 XPS spectra Table 2 10
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The peak, peak area and atomic percentage of N2H4-SH-Fe3O4/o-MWCNTs. Name Energy (ev) Area (%) Atomic (%) C1s 284.8 1.85 73.02 Fe2p 711.54 0.15 5.8 N1s 400.48 0.02 0.86 O1s 530.8 0.44 17.55 S2p 168.95 0.03 1.02 Si2p 101.96 0.04 1.75
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Fig. 7. XPS wide scan of N2H4-SH-Fe3O4/o-MWCNTs (a), C1s (b) ,O1s (c), Fe2p (d),N1s (e),S2p (f) and Si2p (g); the experimental data were fitted with Gaussian components assigned to specific chemical bonds as indicated, and the best fits were plotted in the figures.
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XPS is typically considered to be helpful for identifying groups and surface elements of composites. Table 2 shows the peak, peak area and atomic percentage of N2H4-SH-Fe3O4/o-MWCNTs. The data revealed the presence of carbon, oxygen, nitrogen, sulfur, iron and silicon elements on the surface of N2H4-SH-MWCNTs/Fe3O4.The atomic percentage of O, N, S, Fe and Si can reach 17.55 %, 0.86 %, 1.02 %, 5.8 % and 1.75 %, respectively. These data indicated that functional groups, such as amino and thiol groups, were successfully grafted onto the surface of o-MWCNTs. Fig. 7(a) shows the wide scan of N2H4-SH-Fe3O4/o-MWCNTs in which C1s, O1s, Fe2p, N1s, S2p and Si2p are clearly observed. The strong C1s peak at 284.6 ev corresponded to carbon nanotubes, and the photoelectron lines at the binding energies of approximately 530.8 and 711.54 ev are attributed to O1s and Fe2p, respectively [36-37]. The weak peaks at 168.95 ev and 101.96 ev are attributed to S2p and Si2p, which are present in MPTs. The appearance of S2p and Si2p peaks in the N2H4-SH-Fe3O4/o-MWCNTs spectrum is attributed to the –Si-CH2-CH2-CH2-SH bond, in accordance with the structure of N2H4-SH-Fe3O4/o-MWCNTs in Fig. 1(d).The N1s peak is from hydrazine at 400.48 ev. Fig. 7(b) shows that C1s spectrum of N2H4-SH-Fe3O4/o-MWCNTs can be deconvoluted into three Gaussian peaks centered at 284.6 ev, 285.2 ev and 286.6 ev (corresponding to sp2 C=C, C-OH, and C=O [38], respectively). Furthermore, Fig. 7(c) shows the Gaussian peaks are assigned to C=O, C-OH and HO-C=O (530.6 ev, 531.7 ev and 533 ev, respectively). The results of O1s spectra are consistent with C1s spectra. The presence of these oxygen-containing functional groups confirms that the surfaces of MWCNTs are successfully modified by oxidation, which may provide anchors for the 11
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immobilization of Fe3O4 and other groups. It was important to illustrate that these functional groups were successfully coated on the surface of N2H4-SH-Fe3O4/o-MWCNTs. Fig. 7(d) shows that the binding energies at 710.6 and 724.6 ev correspond to Fe2p3/2 and Fe2p1/2 in Fe3O4, respectively [39]. There are no obvious shakeup satellite structures at the higher binding energy sides of both of the main peaks (approximately 718.8 and 729.5 ev), which is characteristic of Fe3O4. The Gaussian peaks assigned to N-H and S-H are also detected at 402.2 and 163.9 ev in Fig. 7(e) and Fig. 7(f), respectively [21]. In addition, the Gaussian peaks of Si-N and Si-O appeared at 102 and 102.6 ev in Fig. 7(g), respectively. The newly appearing Si-O peak can prove that MPTs are coated on the surface of the composite materials, and the results are consistent with the FT-IR result. According to the results of XPS and FT-IR, N2H4-SH-Fe3O4/o-MWCNTs were successfully prepared by the experimental method.
3.5 TGA analysis
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Fig. 8. TGA curves of MWCNTs (a), o-MWCNTs SH-Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d)
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TGA is conducted to quantify the molar mass of the functional groups presented on the surface of the composite materials. Fig. 8 shows the TGA curves of MWCNTs (a), o-MWCNTs (b), SH-Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d). Fig. 8(a) shows the first stage of weight loss (2 wt%) from 220 ℃ to 500 ℃. The second stage of weight loss (2 wt%) is from 700 ℃ to 1100 ℃. The total 4 wt% of weight loss is attributed to the decomposition of amorphous carbon. The initial slight weight loss (0.5 wt%) is ascribed to the elimination of adsorbed humidity. As light weight loss (0.4 wt%) of MWCNT was reported at 550-680 ℃ due to the decomposition of MWCNT. Fig. 8(b) shows that the weight loss is 10.25 wt% from 150 ℃ to 958 ℃. Compared with the weight loss of MWCNTs (4 wt%), the 6.25 wt% weight loss is due to the decomposition of the -COOH and OH groups on the surface of o-MWCNTs. Fig. 8(c) illustrates that the weight loss of 10.4 wt% is consistent with the results of Fig. 8(b). The second weight loss stage in Fig. 8(c) is from 830 ℃ to 940 ℃, and the weight loss is up to 14.6 wt%. Based on the thermogravimetric profile, the result may be concluded that MPTs of 14.6 wt% is grafted onto o-MWCNTs sidewall, which generally corresponds to one thiol group every 113 C atoms. The S and C quantitative 12
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elemental analysis (N2H4-SH-Fe3O4/o-MWCNTs: S 1.02 wt%, C 73.02 wt%) according to the XPS results refers to approximately one thiol group every 190 C atoms, which is in fairly good agreement with previously calculated. Fig. 8(d) shows that there is an obvious weight loss from 940 ℃ to 1100 ℃, and the loss of 6.46% is attributed to the deamination reaction. All of the results indicate that the functional groups are successfully coated on the surface of the MWCNTs, and they are in agreement with the FT-IR and XPS results.
us
3.6. Batch adsorption experiments
V (C 0 − Ce) M
(2)
M
qe =
an
To test the adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs, batch adsorption experiments were performed. The adsorption capacity is calculated using the following equation:
te
d
where qe is the equilibrium adsorption capacity of the adsorbent (mg/g), C0 is the initial adsorbate concentration (mg/l), Ce is the equilibrium concentration of adsorbate (mg/l), V is the volume of solution (l),and M is the weight of adsorbent (g).
Ac ce p
3.6.1 Effects of pH The pH value of the solution was an important factor in the adsorption process, which can affect the structure at the surface of the composite material and the hydrogen ion concentration in the solution [40-41]. Fig. 9 shows the effects of pH on the adsorption capacity and the removal rate of Pb(II), Zn(II) and phenol. Fig. 9(a) shows that the maximum equilibrium adsorption capacity is 199.6 mg/g, 179.8 mg/g and 56.2 mg/g for Pb(II), Zn(II) and phenol, respectively. Fig. 9(a) illustrates that the equilibrium adsorption capacity (qe) of Pb(II) and Zn(II) increases rapidly as pH change from 2.0 to 6.0. Then, qe is constant until pH 9. Moreover, the equilibrium adsorption capacity of phenol remains stable at pH 3. The qe at pH lower than 3 was limited due to the competition of H3O+, which can compete with Pb(II), Zn(II) and phenol for the adsorption sites. The H3O+ ions prevented the target ions from adsorbing on the surface of the composite material. This conclusion was the same with the surface complex formation theory, which proved that as the hydrogen ions increased, competition for the adsorption sites between 13
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protons and metal species was increased [42]. More metal ions were adsorbed with the decrease of positive charges. As pH increased to greater than 6, the equilibrium adsorption capacity slightly changed due to the combined role of adsorption and precipitation of Pb(II) and Zn(II) as Zn(OH)2 and Pb(OH)2, respectively, causing the adsorption studies to be inaccurate (the hydrolysis pH of Zn(II) and Pb(II) was 6.54 and 7.04, respectively) [43]. Fig. 9(b) shows that N2H4-SH-Fe3O4/o-MWCNTs have high removal rates with values of 98% and 84% for Pb(II) and Zn(II), respectively. The removal rate of phenol is only 20%, because of the adverse effects of the -OH groups (on the surface of N2H4-SH-Fe3O4/o-MWCNTs) during the adsorption process. These groups may form hydrogen bonds with water, thus inhibiting the adsorption of phenol. Because the removal rate and adsorption capacity was the highest at pH 6.0, the initial pH of the solution was fixed for all of the subsequent adsorption experiments in this study.
M
Fig. 9. Effects of pH on adsorption capacity (a) and removal rate (b) of Pb(II), Zn(II)and phenol (10 mg N2H4-SH-Fe3O4/o-MWCNTs, 40 mg/l of Pb(II), Zn(II) and phenol, for 12 h).
Ac ce p
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3.6.2 Effects of contact time and adsorption kinetics The effects of contact time on the adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol were studied. Fig. 10 shows that the adsorption capacities of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol markedly increases with the growth of contact time and remain stable after 30 min. The adsorption capacity at time t (qt) is 196 mg/g, 170 mg/g and 37 mg/g for Pb(Ⅱ), Zn(Ⅱ) and phenol at 30 min, respectively. Thus, N2H4-SH-Fe3O4/o-MWCNTs performed excellent adsorption capacity for Pb(II), Zn(II) and phenol. In fact, adsorption equilibrium was achieved at 30 min. This improved adsorption capacity was ascribed to the additional presence of hydrazine and MPTs which can create a network with water molecules, Pb2+, Zn2+, and phenol by hydrogen bonds and van der waals forces.And MWCNTs provides large surface area and a porous environment for the adsorption. Moreover, the increase of Pb(Ⅱ), Zn(Ⅱ) and phenol adsorption may be ascribed to the ionizable surface charges of N2H4-SH-Fe3O4/o-MWCNTs. The isoelectric point was 2.85 for nitric acid oxidized MWCNTs, which indicated that the surface of N2H4-SH-Fe3O4/o-MWCNTs was negatively charged at solution pH>2.85. The negatively charged surfaces of N2H4-SH-Fe3O4/o-MWCNTs favored the adsorption of target ions. 14
Page 14 of 42
ln(qe − qt ) = ln(qe) − k 1t
(3)
ip t
To investigate the adsorption kinetics and adsorption mechanism, two kinetics models, including pseudo-first order and pseudo-second order models, were tested to fit the experimental data. The equation of the pseudo-first order kinetics model is as follows:
us
cr
where qe (mg/g) and qt (mg/g) are the amounts of Pb(II), Zn(II) or phenol adsorbed at equilibrium and at time t, respectively; k1 (1/min) is the pseudo-first order rate constant; and t (min)is time. The equation of pseudo-second order kinetics model is as follows: t 1 t = + 2 q k 2 qe qe
an
(4)
M
where k2 (g/mg/min) is the rate constant of pseudo-second order equation [44-45].
te
d
Fig. 10. Effects of contact time on Pb(II), Zn(II) and phenol adsorption (10 mg N2H4-SH-Fe3O4/o-MWCNTs and 40 mg/l of Pb(II), Zn(II) and phenol at pH 6)
Ac ce p
Fig. 11. Lagergren pseudo-first order kinetics model for the adsorption of Pb(II) (a),Zn(II) (b) and phenol (c). Fig. 12. Pseudo-second order kinetics model for the adsorption of Pb(II) (a),Zn(II) (b) and phenol (c). Table 3 The parameters of pseudo-first order and adsorption of Pb(II), Zn(II) and phenol. adsorbate pseudo-first order qe.exp qe.cal k1 R2 (mg/g) (mg/g) (1/min) Pb(II) 195.81 2.07 0.008 0.836 Zn(II) 170.07 2.27 0.007 0.820 phenol 38.97 2.14 0.008 0.760
pseudo-second order models for pseudo-second order qe.exp qe.cal k2 R2 (mg/g) (mg/g)（g/mg/min） 195.81 196.08 0.357 1 170.07 200 0.025 1 38.97 40 0.025 0.999
Fig. 11 and Fig. 12 show pseudo-first order and pseudo-second order 15
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kinetics models for the adsorption of Pb(II),Zn(II) and phenol. Table 3 illustrates pseudo-first order and pseudo-second order equation parameters calculated from the slope and intercept. It is clearly indicated that the experimental data fit well with pseudo-second order model with a high correlation coefficient (R2).The pseudo-second order model is suitable to describe the adsorption of metal ions and phenol from aqueous solution by N2H4-SH-Fe3O4/o-MWCNTs. The R2 values of pseudo-second order model are one in the adsorption process of Pb(II) and Zn(II), which are higher than that of the pseudo-first order model (0.836 and 0.820). In addition, the calculated values of equilibrium adsorption capacity (qe.cal) from the pseudo-second order model are 196.08, 200, and 40 mg/g for Pb(II), Zn(II) and phenol, respectively. The data are much closer to the experimental data (qe.exp=195.81, 170.07, 38.97 mg/g, respectively) than pseudo-first order model. The rate constant (K2) are 0.357 and 0.025 (g/mg/min) for Pb(II), Zn(II) and phenol. The data illustrated that N2H4-SH-Fe3O4/o-MWCNTs had a faster adsorption rate for Pb(II) than Zn(II) and phenol.
Ac ce p
te
d
M
3.6.3 Adsorption isotherms To investigate the relationship between the heavy metal ions and phenol adsorbed on N2H4-SH-Fe3O4/o-MWCNTs, two classical adsorption isotherm models, namely, Langmuir [46] and Freundlich models [47] were employed to describe the adsorption of Pb(II), Zn(II) and phenol. The Langmuir isotherm model was valid for monolayer adsorption onto the surface. If ion adsorption followed Langmuir model, then it was described by the following equation [48]: Ce 1 Ce = + qe qm K L qm
(5)
where Ce (mg/l) was the equilibrium concentration of Pb(II), Zn(II) and phenol in the solution; qe (mg/g) was the amounts of Pb(II), Zn(II) or phenol adsorbed at equilibrium; qm (mg/g) was the maximum adsorption capacity, and KL (l/mg) was the effective dissociation constant related to the affinity binding site. The values of qm and KL can be obtained from the intercept and the slope of the linear plot of Ce/qe against Ce [49]. Freundlich isotherm model was based on an exponential distribution of adsorption sites and energies. It was derived from multilayer adsorption and heterogeneous surfaces. The mathematical expression of Freundlich isotherm model is as follows [48]: 16
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ln qe =
1 ln Ce + ln KF n
(6)
cr
ip t
where KF (mmol1-n Ln/kg) is the Freundlich affinity coefficient, and n (unitless) is Freundlich linearity index. The values of n and KF can be obtained by the plots of lnqe against lnCe. The parameters of Langmuir and Freundlich isotherm models were calculated in Table 4, and the curve results of fitting were shown in Fig. 13 and Fig. 14.
us
Fig. 13. Langmuir isotherm model for adsorption of Pb(II) (a), Zn(II) (b), and phenol (c) on N2H4-SH-Fe3O4/o-MWCNTs surface.
an
Fig. 14. Freundich isotherm model for adsorption of Pb(II) (a), Zn(II) (b) and phenol (c) on N2H4-SH-Fe3O4/o-MWCNTs surface.
Ac ce p
te
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M
Table 4 The parameters of Langmuir and Freundlich models for the adsorption of Pb(II), Zn(II) and phenol on N2H4-SH-Fe3O4/o-MWCNTs surface. Adsorbate Langmuir model Freumdlich model 2 qm KL R KF n R2 (mg/g) (l/mg) (mmol1-n Ln/kg) Pb(II) 40 0.96 0.912 296.78 0.3 0.926 Zn(II) 21.28 0.015 0.9681 0.00315 0.165 0.97 phenol 55.56 0.018 0.902 0.5337 0.713 0.921 Fig. 13 and Fig. 14 show the adsorption isotherm models of Pb(II), Zn(II) and phenol on the surface of N2H4-SH-Fe3O4/o-MWCNTs. The parameters of Langmuir and Freundlich isotherm models are summarized in Table 4. Table 4 indicates that the adsorption of Pb(II), Zn(II) and phenol are better fitted to Freundlich isotherm model with higher R2 values than Langmuir isotherm model. The data indicated that the adsorption of Pb(II), Zn(II) and phenol may be interactive and may occur as a multilayer adsorption [50-51]. MWCNTs can aggregate and form bundles by strong van der waals forces in the solution due to their strong hydrophobicity. These bundles lead to the formation of groove, gap, and inner areas. The cluster structure provides heterogeneous adsorption sites for Pb(II), Zn(II) and phenol [49]. The correlation coefficient (R2) of Freundlich isotherm model for phenol is 0.921, which is higher than for Langmuir isotherm model (0.902). Therefore, Freundlich isotherm model correlates with data better 17
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than Langmuir isotherm model, indicating that the adsorption of phenol interacted with itself and N2H4-SH-Fe3O4/o-MWCNTs. A similar phenomenon can be obtained for the adsorption of Pb(II) and Zn(II). The n values are in the 0-1 range, which indicates that adsorption of Pb(II), Zn(II) and phenol on N2H4-SH-Fe3O4/o-MWCNTs is favorable. The n values are smaller than 1, which illustrates a favorable adsorption of heavy metal ions and phenol onto N2H4-SH-Fe3O4/o-MWCNTs.
V (C0 − Ce ) MCe
(7)
M
Kd =
an
us
cr
3.6.4. Adsorption thermodynamics Temperature has a distinct effect on the adsorption of Pb(II), Zn(II) and phenol. The adsorption thermodynamic studies are conducted from 288 K to 308 K with 40 mg/l of Pb(II), Zn(II) and phenol to investigate the thermodynamic parameters. The Gibbs free energy (ΔG) is calculated according to the following equations [52]:
d
∆S 0 ∆H 0 ln Kd = − R RT
te
∆G 0 = ∆H 0 − T ∆S 0
(8) (9)
Ac ce p
where R is the universal gas constant (8.314 J/mol/K), T (K) is the adsorption temperature, and Kd is an equilibrium constant obtained by the initial and the equilibrium concentrations (C0 and Ce) of the target ions. The parameter V (l) is the solution volume, and W (g) is the adsorbent mass. The thermodynamic parameters, such as the Gibbs free energy ( Δ G), enthalpy(ΔH0), and entropy(ΔS0), as well as ciprocal of temperature (1/T) are plotted as 1/T against lnKd in Fig. 15. The values ofΔH0 andΔS0 are evaluated from the slope and intercept. Table 5 shows the values ofΔH0, Δ S0 andΔG0 under the reaction conditions. Table 5 indicates that the absolute values of Gibbs free energy increases as the temperature extends. The negative values of ΔG reveals that the adsorption of Pb(II), Zn(II) and phenol onto N2H4-SH-Fe3O4/o-MWCNTs is spontaneous. The magnitude of ΔG increases as the temperature increases, which indicates that the adsorption is more efficient at higher temperatures. Moreover, the change of ΔG for physisorption occurs between -20 and 0 kJ/mol, while the physisorption and chemisorption occur together over the 18
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range of -20 to -80 kJ/mol, with chemisorption occurring in the range of -80 to -400 kJ/mol [53-56]. TheΔG values indicate that the interactions between Pb(II), Zn(II) and N2H4-SH-Fe3O4/o-MWCNTs could be considered to undergo physisorption and chemisorption mechanisms. However, phenol adsorption is attributed to physisorption. The adsorption mechanism can explain why phenol adsorption capacity is less than that of Pb(II) and Zn(II). These groups, such as thiol and amine groups, on the surface of N2H4-SH-Fe3O4/o-MWCNTs can form chemical bonds with Pb(II) and Zn(II) to increase the adsorption capacity. The values ofΔS are positive, which also indicate that the adsorption process is endothermic. The high values of ΔS imply that a noticeable change in entropy occurs during the adsorption process of Pb(II), Zn(II) and phenol, and the positive values ofΔS indicate a tendency toward a higher disorder at the solid-solution interface during the adsorption. The positive values forΔS0 may be due to the release of water molecules produced by the ion-exchange reaction between the metal ions and the surface functional groups of N2H4-SH-Fe3O4/o-MWCNTs. The positive values reflect the affinity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(II), Zn(II), phenol and an increase in the randomness on the solution interface during the adsorption process [28].
te
Fig.15. Plots of lnKd versus 1/T for Pb(II) (a), Zn(II) (b) and phenol (c)
Ac ce p
Table 5 Thermodynamic constant for the adsorption of Pb(II), Zn(II) and phenol on the surface of N2H4-SH-Fe3O4/o-MWCNTs at different temperatures T/K lnKd ΔH ΔS ΔG （kJ/mol） （J/mol/K） （kJ/mol） Pb(II) 288 k 11.982 -28.819 298 k 12.384 16.819 158.465 -30.403 308 k 12.435 -31.989 Zn(II) 288 k 10.253 -24.485 298 k 10.253 0.92 88.212 -25.367 308 k 10.228 -26.249 phenol 288 k 7.658 -18.253 298 k 7.637 1.221 67.618 -18.929 308 k 7.624 -19.605
4. Conclusions 19
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Acknowledgements
te
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M
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N2H4-SH-Fe3O4/o-MWCNTs were successfully prepared and used for the simultaneous adsorption of Pb(Ⅱ), Zn(Ⅱ) and phenol. TEM images proved that MPTs and hydrazine were coated on the surface of MWCNTs with an average thickness of 6.5 nm. Fe3O4 nanoparticles were uniformly and densely loaded on the surface of o-MWCNTs.. XPS, FTIR, and TGA analysis further proved that MWCNTs were functionalized by thiol and amino groups. The adsorption experiment revealed that the maximum equilibrium adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol reached 195.81 mg/g, 170.07 mg/g and 38.97 mg/g at pH 6, respectively, which proved that the composite was highly efficient. Up to 98% and 84% high removal rates for Pb(II) and Zn(II) could be achieved. The adsorption kinetics indicated that the adsorption process was consistent with pseudo-second order model. The pseudo-second order kinetics model with higher correlation coefficient was suitable to describe the kinetic process of heavy metals and phenol adsorption onto N2H4-SH-Fe3O4/o-MWCNTs. The adsorption isotherm was consistent with Freundlich isotherm model, which was a multilayer adsorption. The thermodynamic values indicated that the adsorption process was spontaneous and exothermic. TheΔG values indicated that interactions between Pb(II), Zn(II) and N2H4-SH-Fe3O4/o-MWCNTs could be considered to undergo physisorption and chemisorption mechanism. However, phenol adsorption was attributed to physisorption. It is hopeful that the composite has a good prospect for application to wastewater treatment.
The authors are grateful for financial support from the National Natural Science Foundation of China (51304101), scientific and research project of institutions of higher learning in Gansu province and open fund of Laboratory of Advanced Processing and Recycling of Non-ferrous Metal in Lanzhou university of technology.
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[38] H.Q. Bi, Y.H. Li, S.F. Liu, P.Z. Guo, Z.B. Wei, C.X. Lv, J.Z. Zhang, X.S. Zhao, Carbon-nanotube-modified glassy carbon electrode for simultaneous determination of dopamine, ascorbic acid and uric acid: the effect of functional groups, Sensor. Actuat. B-chem. 171 (2012) 1132-1140. [39] T. Yang, C. Shen, Z. Li, H. Zhang, C. Xiao, S. Chen, Z.C. Xu, D.G. Shi, J.Q. Li, H.G. Gao, Highly ordered self-assembly with large of Fe3O4 nanoparticles and the magnetic properties, J. Phys. Chem. B 109 (2005) 23233-23236. [40] K. Babaeivelni, A.P. Khodadoust, Adsorption of fluoride onto crystalline titanium dioxide: Effect of pH, ionic strength, and co-existing ions, J. colloid Interf. Sci. 394 (2013) 419-427. [41] J.C. Lazo-Cannata, A. Nieto-Marquez, A. Jacoby, Adsorption of phenol and nitrophenols by carbon nanospheres: Effect of pH and ionic strength, Sep. Purif. Technol. 80 (2011) 217-224. [42] H.A. Mengistu, A. Tessema, M.B. Demlie, T.A. Abiye, O. Roeyset, Surface-complexation modelling for describing adsorption of phosphate on hydrous ferric oxidesurface, Water SA . 41 (2015) 157-167. [43] L. Lv, M.P. Hor, F.B. Su, X.S. Zhao, Competitive adsorption of Pb2+, Cu2+, and Cd2+ ions on microporous titanosilicate ETS-10, J. Colloid Interf. Sci. 287 (2005) 178-184. [44] C.C. Huang, J.C. He, Electrosorptive removal of copper ions from wastewater by using ordered mesoporous carbon electrodes, Chem. Eng. J. 221 (2013) 469-475. [45] J. X. Lin, L. Wang, Comparison between linear and non-linear forms of pseudo-first-order and pseudo-second-order adsorption kinetic models for the removal of methylene blue by activated carbon, Front. Environ. Sci. & Eng. 3 (2009) 320-324. [46] I. Langmuir, Adsoption of gases on plane surface of glass, mica and platinum, J. Am. Chem. Soc. 40 (1918) 1361-1403. [47] M.D. Levan, T. Vermeulen, Binary langmuir and freundlich isotherms for ideal adsorbed solutions, J. Phys. Chem. 85 (1981) 3247-3250. [48] Y. Sağ, Y. Aktay, Application of equilibrium and mass transfer models to dynamic removal of Cr(VI) ions by chitin in packed column reactor, Process Biochem. 36 (2001) 1187-1197. [49] Q. Hu, Z. Xiao, X. Xiong, G. Zhou, X Guan, Predicting heavy metals' adsorption edges and adsorption isotherms on MnO2 with the parameters determined from Langmuir kinetics, J. Environ. Sci-china. 27 (2015) 207-216. [50] P. Baroni, R.S. Vieira, E. Meneghetti, M.G.C. Silva, M.M. Beppu, 24
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Evaluation of batch adsorption of chromium ions on natural and crosslinked chitosan membranes, J. Hazard. Mater. 152 (2008) 1155-1163. [51] K.Y. Foo, B.H. Hameed, Insights into the modeling of adsorption isotherm systems, Chem. Eng. J. 156 (2010) 2-10. [52] H. Al-johani, M.A. Salam, Kinetics and thermodynamic study of aniline adsorption by muli-walled carbon nanotubes from aqueous solution, J. Colloid Interf. Sci. 360 (2011) 760-767. [53] Z.C. Miao, F. Wang, D. Deng, L. Wang, Z.J. Li, Thermodynamics of phenol adsorption on the microsphere based on starch, Adv. Mater. Res. 550-553 (2012) 2629-2632. [54] H. Chen, J. Li, D. Shao, X. Ren, X. Wang, Poly(acrylic acid) grafted multiwall carbon nanotubes by plasma techniques for Co(II) removal from aqueous solution, Chem. Eng. J. 210 (2012) 475-481. [55] F. Wang, Y. Wang, Y. Li, Preparation of 6-diallylamino-1, 3,5-triazine-2,4-dithiol functional nanofilm by electrochemical polymerization technique on aluminum surface, Int. J. Electrochem. Sc. 6 (2011) 793-803. [56] C.C. Liu, Y.S. Li, Y.M. Chen, H.H. Li, M.K. Wang, Removal of methylene blue from aqueous solution using wine-processing waste sludge, Water Sci. and Technol. 65 (2012) 2191-2199.
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Highlights The synthetic method of N2H4-SH-Fe3O4/o-MWCNTs was explored, and amino and thiol groups which can improved the adsorption capacity were successfully grafted on the surface of Fe3O4/o-MWCNTs. The adsorption kinetics can be best described using pseudo-second order kinetics equation and the adsorption isotherms match well with Freundlich model. The maximum adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for lead, zinc and phenol were 199.6 mg/g, 179.8 mg/g and 56.2 mg/g, respectively.
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*Graphical Abstract (for review)
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S0169-4332(16)30236-7 http://dx.doi.org/doi:10.1016/j.apsusc.2016.02.067 APSUSC 32585
To appear in:
APSUSC
Received date: Revised date: Accepted date:
30-11-2015 5-2-2016 5-2-2016
Please cite this article as: L. Jiang, S. Li, H. Yu, Z. Zou, X. Hou, F. Shen, C. Li, X. Yao, Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions, Applied Surface Science (2016), http://dx.doi.org/10.1016/j.apsusc.2016.02.067 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Amino and thiol modified magnetic multi-walled carbon nanotubes for the simultaneous removal of lead, zinc, and phenol from aqueous solutions
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Lili Jianga,b,*, Shujun Lib, Haitao Yuc, Zongshu Zoua, Xingang Houb, a Fengman Shen , Chuantong Lib, Xiayan Yaob
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a School of metallurgy, Northeastern university, No.3-11, Wenhua Road, Heping District, Shenyang, Liaoning Province, P.R.China. Email: [email protected](Lili Jiang); [email protected](Zongshu Zou); shenfm@ mail.neu.edu.cn(Fengman Shen). b School of Material Science and Technology, Lanzhou University of Technology, Langongping Road, 730050, Lanzhou, Gansu Province, P.R.China. Email: [email protected](Shujun Li); [email protected](Chuantong Li); [email protected](Xiayan Yao); [email protected] (Xingang Hou). c Department of Medical Laboratory, The First Hospital of Lanzhou University, No.1, Donggang Road, Chengguan District, Lanzhou, 730000, Gansu Province, P.R.China. Email: [email protected]. *Corresponding author at: School of Material Science and Technology, Lanzhou University of Technology, Langongping Road, 730050, Lanzhou, Gansu Province, P.R.China. Email: [email protected]. Tel.:+86 0931 2976378; fax:+86 0931 2972702.
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Abstract: The novel functionalization of multi-walled carbon nanotubes (MWCNTs) was synthesized by reacting trimethoxysilylpropanethiol (MPTs), hydrazine, ammonium ferrous sulfate, and ammonium ferric sulfate in sequence as efficient ways to introduce Fe3O4, amino and thiol groups onto the nanotubes sidewalls. The magnetic MWCNTs composite material (N2H4-SH-Fe3O4/o-MWCNTs) was characterized by transmission electron microscopy, field emission scanning electron microscopy, x-ray diffraction, thermo-gravimetric analysis, x-ray photoelectron spectroscopy, fourier transformation infrared spectroscopy and magnetization curve. The results revealed that MPTs and hydrazine were coated on the surface of N2H4-SH-Fe3O4/o-MWCNTs. A series of batch adsorption experiments were conducted to study the experimental conditions, such as pH, contact time, initial concentrations and temperatures, which affected the adsorption process. The adsorption experiment results showed that the maximum equilibrium adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for lead, zinc and phenol was 195.81 mg/g, 169.89 mg/g and 38.97 mg/g at pH 6, respectively. The adsorption isotherm was better fitted by the Freundlich model, and the adsorption kinetics was consistent with pseudo-second order kinetics model. Furthermore, thermodynamic data showed that the adsorption process was spontaneous and exothermic. These results indicated that N2H4-SH-Fe3O4/o-MWCNTs may be promising surface modified materials for removing heavy metal ions and phenol from aqueous solutions. Key words: multi-walled nanotube, adsorption, thermodynamics, dynamics 1
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1. Introduction
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Industrial wastewater containing organic and inorganic pollutants can lead to environmental pollution problems. Among these pollutants, such as phenol, lead, and zinc, frequently co-exist in waste waters from metal plating, dye and painting industries. Heavy metals, such as lead and zinc, enter into the environment and cause health hazards due to their toxicity and bioaccumulation in the living organism [1]. In addition, phenol was found to be highly poisonous to flora and fauna [2]. Removal of heavy metal ions and phenol from environmental samples is an important field of study, and controlling over their concentrations in the environment is especially significant [3-4]. In addition, the removal of heavy metals and phenol from water sources to an acceptable concentration is a current research challenge in water treatment. To date, traditional methods, such as sulfide precipitation [5], ion exchange [6], alum treatment [7], membrane separation, reverse osmosis and iron coagulation [8], have been used to remove or separate heavy metal ions and phenol from aqueous environments. However, filtration and centrifugation techniques are time consuming and separation becomes a difficult issue due to the small size of the ions. Furthermore, their drawbacks such as procedural complexity and high operating cost, have been reported for the existing methods for the removal of lead, zinc, and phenol must be solved. Among these techniques, adsorption is the most attractive due to its simplicity, high efficiency, and its ability to remove multiple low concentration pollutions simultaneously. Therefore, there is still a requirement for the development of efficient adsorbents with high adsorption capacity for lead, zinc and phenol [9]. Recently, carbon-based nano-materials, such as carbon nanotubes (CNTs), have been developed to remove heavy metals from aqueous media, and they are promising materials due to their unique electrical, mechanical, thermal, optical and chemical properties [10-13].CNTs were discovered by Iijima in 1991, and they mainly included single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) [14]. It is well known that the ability of CNTs to establish π-π electrostatic interactions and their large surface areas can facilitate the adsorption of pollutants from water [15]. CNTs have been proven as superior adsorbents for removing organic and inorganic pollutants [16-18]. The adsorption capability of MWCNTs is primarily related to the functional groups, which attaches to the surface of MWCNTs and improves the reactivity and provides active sites [19-20]. Many researchers had reported a number of surface modification strategies, such as amino- [21-23] 2
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and thiol- [24-27] modified MWCNTs to increase the adsorption capacity, selectivity and removal efficiency of heavy metals. Bahramifar et al. had reported that the synthesis of nanocomposites with amino- and thiol-modified MWCNTs can improve the adsorption property for mercury ions from synthetic and real wastewater aqueous solutions due to the added amino and thiol groups [28]. Zhang et al. had developed a novel method for amine functionalized CNTs via combination of mussel inspired chemistry and Michael addition reaction. Results demonstrated that the adsorption capacity of these functionalized CNTs was much improved as compared with pristine CNTs. To authors’ knowledge, there are few reports that use amino and thiol groups to modify MWCNTs, and they have not been used as adsorbents to simultaneously remove heavy metals and phenol. The objective of the present study was to synthesize thiol- and amino-modified MWCNTs and investigate their adsorption capability for phenol and heavy metal from aqueous solutions. In this study, the oxidized MWCNTs were modified by amino and thiol groups. The synthesized adsorbents were characterized by fourier transform infrared spectroscopy (FT-IR), thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Adsorption experiments of Zn(II), Pb(II) and phenol from aqueous solution were performed. The effects of pH, contact time, initial concentrations and temperatures on the adsorption capacity of the modified MWCNTs were investigated. The adsorption data were fitted with the Freundlich and Langmuir isotherm models. The enthalpy, entropy and Gibbs free energy of the functionalized MWCNTs were determined from the experimental data. 2. Experiment and methods 2.1. Materials and reagents
MWCNTs were purchased from Shenzhen Nanotech. Port Co. Ltd., (China). According to the manufacturer, the physicochemical properties of MWCNTs were listed in Table 1. Ultra-pure water was used for the preparation of solution. Lead nitrate, zinc nitrate, and phenol were purchased from Tianjin Fu Rui Technology Co. Ltd., (China). Ammonium ferrous sulfate ((NH4)2Fe(SO4)2•6H2O), ammoniumferricsulfate (NH4Fe(SO4)2•12H2O), nitric acid, sulfuric acid, hydrazine and acetone (purity>99.5%) were purchased from Big Alum Chemical Reagent Factory (China). MPTs was purchased from Beijing Biological Technology (China). 3
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Value 20-40 nm >5 um >97 % <3 % 80-140 m2/g
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Table 1 Physicochemical properties of MWCNTs Physicochemical property diameter length purity ash special surface area
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All of the chemicals were of analytical grade without further purification.
2.2. Characterization techniques
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TEM (type JEM-2010, operating at 200 kv) was used to characterize the morphology of the samples. These samples were dispersed in absolute alcohol by sonication; then a carbon-coated 400 mesh copper grid was immersed in the suspension and air-dried. SEM (JSM-6700F, operating at 0.5-30 kv) was performed to characterize the surface morphology of the adsorbent. FT-IR were recorded using a Nexus-870. All of the samples were prepared by mixing spectroscopic-grade KBr with MWCNTs or modified MWCNTs and they were analyzed in the 400-4000 cm-1 range with a resolution of 2 cm-1. TGA was performed on an thermogravimetric analyzer (NETZSCHSTA-449F3) under N2 atmosphere from 40 to 1200 ℃ at heating rate of 20 ℃/min. X-ray diffraction (XRD, RigakuD/max-2400, operating at 60 kv) was performed to investigate the crystalline phase of the adsorbent. The diffraction patterns were recorded in the 2θ=10-90 o, operating with CuKαradiation (λ=1.5418 Å). The measurements were carried out in air at ambient pressure and temperature. XPS（Escalab 250Xi with a max resolution of 400000 cps and FWHM < 0.5 ev) was applied to determine the surface chemical composition of the samples using AlKα radiation. The XPS spectra were deconvoluted using XPS peak software. The surface atomic percentages were calculated from the corresponding peak areas. The magnetism was measured using a vibrating sample magnetometer (VSM, sensibility between 3-5 × 10-5) manufactured by Nanjing Univ. Instrument Plant, China. Metal ion concentrations were analyzed using an atomic absorption spectrophotometer (HITACHI Z-5000)，and phenol concentrations was analyzed by ultraviolet and visible spectrophotometer (VU-1000). A temperature controlled shaker was used for shaking the aqueous solution containing metal ions and phenol and 4
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temperature controlled water bath (HHS2) was used for maintaining the temperature of aqueous solution. All of the experiment water was performed by ultra-pure water system (Ulupure, UPD-I-20T).
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2.3. Preparation of oxidized MWCNTs
2.4. Preparation of Fe3O4/o-MWCNTs
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MWCNTs (4 g) were placed in a beaker with 120 ml concentrated sulfuric acid and 40 ml concentrated nitric acid to remove impurities and hemispherical caps. Then, the mixture was ultrasonically stirred for 24 h. The suspensions were filtrated, rinsed with deionized water until the pH reached approximately 6, and dried in a vacuum oven at 80 ℃ for 12 h. The obtained product was the oxidized MWCNTs (o- MWCNTs) (as shown in Fig. 1(a)).
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4 g o-MWCNTs, 6.8 g (NH4)2Fe(SO4)2 · 6H2O and 10.4 g NH4Fe(SO4)2 · 6H2O were dissolved in 500 ml ultrapure water and ultrasonically treated for 15 min. Ammonium hydroxide (8 mol/l) was added dropwise until the pH of solution reached 11.The mixture was continuously stirred for 30 min at 50 ℃ until the color changed from black to brown. Finally, the products were collected from the water solution using a magnet and were rinsed 10 times by ultrapure water. The product was dried in a vacuum drying oven at 80 ℃ for 8 h to obtain the Fe3O4/o-MWCNTs product [28] (as shown in Fig. 1(b)). 2.5. Preparation of SH-Fe3O4/o-MWCNTs Fe3O4/o-MWCNTs (200 mg) were dispersed in 150 ml absolute alcohol solution at 25 ℃. Then, 0.5 ml acetic acid and 0.1 ml MPTs was added to the mixed solution. The entire experimental process was protected under nitrogen gas while continually stirred. The reaction time was 24 h at 25 ℃. After the reaction, 50 ml acetone was slowly added to the reaction solution. The product was passed through filter paper and washed with ethanol and ultra-pure water. The product was dried in a vacuum drying oven at 60 ℃ for 12 h. Finally, the SH-Fe3O4/o-MWCNTs were obtained (as shown in Fig. 1(c)). 2.6. Preparation of N2H4-SH-Fe3O4/o-MWCNTs 5
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5 g SH-Fe3O4/o-MWCNTs was suspended in 40 ml absolute alcohol. The mixture was sonicated, stirred vigorously, and then slowly reacted for 5 min after adding hydrazine. Then, the solution was placed in a water bath and stirred for 3 h. Subsequently, the composite material was filtered and washed with ultrapure water for several times. Finally, the product was dried in a vacuum oven for 8 h at 60 ℃. The obtained composite material was N2H4-SH-Fe3O4/o-MWCNTs (as shown in Fig. 1(d)).
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2.7. Adsorption experiments
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The adsorption capacities of N2H4-SH-Fe3O4/o-MWCNTs for Pb(II), Zn(II) and phenol were evaluated at pH values, contact time, initial concentrations and temperatures. The effects of pH values from 2 to 9 were evaluated by adding 0.1 mol/l HNO3 and 0.1 mol/l NaOH solution to adjust the pH. The initial concentration of Pb(II), Zn(II) and phenol in each solution was 40 mg/l, respectively. 10 mg adsorbents in every 50 ml solution was used for the co-adsorption of Pb(II), Zn(II)and phenol. The conical flasks containing the mixtures were placed on a shaker at 25 ± 0.5 ℃. To be conservative, an equilibration time of 12 h was routinely used. All of the adsorption experiments were performed in triplicate, and the averaged values were reported in this study. The effects of contact time were investigated at the time intervals of 10, 30, 60, 90, 120, 180, 300, 420, 600 and 720 min with an initial concentration of 40 mg/l for Pb(II), Zn(II) and phenol, respectively. 10 mg adsorbents were dispersed into 50 ml solutions at 25 ± 0.5 ℃ under pH 6. To investigate the adsorption isotherm, the initial concentration was changed from 20 to 60 mg/l at 25 ± 0.5 ℃ at pH 6 for 12 h. The adsorbent amount was 10 mg in 50 ml solution. The effects of temperatures on Pb(II), Zn(II) and phenol adsorption were obtained by 10 mg adsorbent in 50 ml solution. The adsorption experiments were performed in flasks containing Pb(II), Zn(II) and phenol concentration of 40 mg/l, respectively. The temperatures varied from 288 k to 308 K at pH 6.0 for 12 h. The liquid after adsorption was pipetted, immediately separated by a magnet from treated solutions, and filtered with 50 µm polytef membranes. The exact concentrations of Pb(II), Zn(II) after filtering the adsorbents were determined using an atomic absorption spectrophotometer and phenol concentration was analyzed by ultraviolet and visible spectrophotometer. The removal rate (%) was calculated using the following equation: 6
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(C0 − Ct ) × 100% C0
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Fig. 1. Schematic illustration of N2H4-SH-Fe3O4/o-MWCNTs and adsorption experiment. (a) o-MWCNTs with abundant carboxyl groups on the surface. (b) Fe3O4/o-MWCNTs with co-precipitation method. (c) SH-Fe3O4 /o-MWCNTs with SH groups on the surface. (d) N2H4-SH-Fe3O4/oMWCNTs with amino and thiol groups on the surface. (e) adsorption experiments of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ), and phenol.
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Our main objective in the present work was to develop N2H4-SH-Fe3O4/o-MWCNTs with multi-component, containing MWCNT, Fe3O4, MPTs and hydrazine, to efficiently remove Pb(Ⅱ), Zn(Ⅱ) and phenol, along with an improved removal rate, good selectivity and stability. The role of the individual components in the N2H4-SH-Fe3O4/o-MWCNTs was inferred through various characterization studies and discussed below. The goal of incorporating Fe3O4 was to improve separability from the solution. Furthermore, MPTs may improve the removal rate and adsorption capacity by providing additional binding sites for Pb(Ⅱ), Zn(Ⅱ) and phenol and hydrazine can provide primary amine (–NH2) groups and secondary amine groups (–NH–).
3. Results and discussion 3.1. Morphology and crystal structure characterization Fig.
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N2H4-SH-Fe3O4/o-MWCNTs (c) and a high magnification image of N2H4-SH-Fe3O4/o-MWCNTs (d).
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Fig. 2 shows the TEM images of MWCNT (a), o-MWCNTs (b), and N2H4-SH-Fe3O4/o-MWCNTs (c) and (d). Fig. 2(a) shows that MWCNTs are cylindrical shapes with smooth surfaces and the average diameter is 40 nm with a small amount of catalyst impurities on the surface. Fig. 2(b) shows that oxidized MWCNTs have no impurities, the surface is rough with lots of grooves and the ends are opened. Fig. 2(c) shows that Fe3O4 nanoparticles are uniformly and densely loaded on the surface of o-MWCNTs, and there is a limited amount of vacant areas on the surface. Their size distribution is evaluated using nano measurer software invented Fudan University according to selected TEM images. The latter indicates Fe3O4 size is between 4 nm and 7 nm with a mean diameter of 6 nm. It is noteworthy that no Fe3O4 aggregation is observed on the nanotubes surfaces. Fig. 2(c) indicates that diameter of the N2H4-SH-Fe3O4/o-MWCNTs increases to 50-60 nm, which proves that Fe3O4, MPTs and N2H4 are successfully coated on the surface of o-MWCNTs. Compared with Fig. 2(b), Fig. 2(d) shows the transparent film on the surface of N2H4-SH-Fe3O4/o-MWCNTs is average thickness of 6.5 nm. The apparent evidence indicated that MPTs and hydrazine were modified on the surface of N2H4-SH-Fe3O4/o-MWCNTs.
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Fig. 3. SEM images of MWCNTs (a), o-MWCNTs (b), Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d). The SEM images of MWCNTs, o-MWCNTs, Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs are shown in Fig. 3. Fig .3(a) and (b) reveal that MWCNTs and o-MWCNTs are similar, that they are tubular, curved and highly tangled. The lengths of these nanotubes are approximately 5-10 µm, and the average outer diameter is 20-40 nm. Fig. 3(b) shows that there is slight fragmentation with a length of 100-200 nm, which indicates that MWCNTs are shortened after oxidation treatment. Fig. 3(c) clearly shows that there is a layer of nanoparticles on the surface of the o-MWCNTs. Compared with Fig. 3(b), Fig. 3(c) suggests that Fe3O4 are coated on the surface of o-MWCNTs. Fig. 3(d) shows a white layer on the surface, and the diameter of nanotubes increases to 50-60 nm. The increased diameter revealed that MPTs and hydrazine were deposited on the surface. Fig. 4. XRD patterns of MWCNTs (a), N2H4-SH-Fe3O4/o-MWCNTs (b) and Fe3O4/o-MWCNTs (c). 8
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XRD is the most widely used technique to characterize the crystalline phase of the synthesized materials. Fig. 4 shows the XRD patterns of MWCNTs, Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs. Fig. 4(a) shows that MWCNTs exhibits the typical diffraction peaks at 2θ=25.89 o and 42.78 o, which are assigned to the (002) and (100) planes of the graphite phase (PDF card 41-1487, JCPDS), indicating that a graphite structure existed in this sample [30]. Fig. 4(b) and (c) indicate that the diffraction peaks of MWCNTs become weak, especially at 25.89 o, and they have a lower crystallinity than MWCNTs [31]. Fig. 4(c) shows that the XRD pattern of Fe3O4/o-MWCNTs matches well with the standard cubic phase of Fe3O4 including that the six diffraction peaks (2θ =30.07 °, 35.41 °, 53.39 °, 56.91 °, 62.92 ° and 73.93 °) correspond to the (220), (311), (422), (511), (440), (533) planes (JCPDS No.19-0629) of magnetite, respectively [32] . This illustrates the existence of Fe3O4. The Scherer equation is calculated according to the highest intensity of 35.41 °.The particle size is approximately 6 nm, which is consistent with result of Fig. 2(c). Fig. 4(b) shows that the peaks are weaker, especially at 35.41 °, and broader compared with Fe3O4/o-MWCNTs. Due to the coating of MPTs and hydrazine, the peaks become weak. The amino and thiol groups may have coated on the surface of Fe3O4, which cover the peak of Fe3O4. According to the results of XRD and TEM, the nanoparticles on the surfaces of N2H4-SH-Fe3O4/o-MWCNTs are assigned to Fe3O4.
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3.2. Magnetic characterizations
Fig. 5. Hysteresis loops N2H4-SH-Fe3O4/o-MWCNTs (b)
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(a)
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The magnetic properties of Fe3O4/o-MWCNTs and N2H4-SH-Fe3O4/o-MWCNTs were measured at 298K using a vibrating sample magnetometer. Fig. 5 shows that the coercivity and remanence of Fe3O4/MWCNTs and N2H4-SH-MWCNTs/Fe3O4 are zero, which reveals their superparamagnetism. The saturation magnetization is 35.85 emu/g and 32.89 emu/g for Fe3O4/o-MWCNTs and N2H4-SH-MWCNTs/Fe3O4, respectively. The large saturation magnetization allow for a fast separation of composites from aqueous solution [33]. The saturation magnetization of N2H4-SH-Fe3O4/o-MWCNTs is slightly less than that of Fe3O4/o-MWCNTs because the surface of N2H4-SH-Fe3O4/o-MWCNTs is covered by 9
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non-magnetic MPTs and hydrazine. 3.3 FT-IR spectra
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Fig. 6. FT-IR spectra of MWCNTs (a) and N2H4-SH-Fe3O4/o-MWCNTs (b)
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The surface functional groups of adsorbent are determined by FT-IR spectra analysis. Fig. 6(a) and (b) show strong peaks at 1560 cm-1 attributed to C=C stretching vibration. Fig. 6(b) shows a strong peak at 599 cm-1 attributed to Fe-O stretching vibration which indicates that Fe3O4 is successfully loaded onto the o-MWCNTs. The appearance of a new peak at 966 cm-1 is assigned to Si-N stretching vibration, which reveals that MPTs and N2H4 are successfully connected. The peaks at 1010 cm-1 and 3421 cm-1 are ascribed to O-H stretching vibration [34]. The oxidized treatment produced hydroxy groups on the outermost surface and defect sites that increased the hydrophilicity of MWCNTs. These oxygen-containing functional groups can readily form H-bonding with polar functional groups (-SH group) on the surface of N2H4-SH-Fe3O4/o-MWCNTs. Moreover, this interaction could be very strong if the oxygen content of the composite was high according to the XPS results (17.55% of oxygen content).The band at 1200-1260 cm−1 is attributed to the stretching vibration of the carboxyl group (−COOH). The band at 2800-3000 cm-1 is associated with symmetric and asymmetric -CH2 stretching vibration. The enhanced intensity of the -CH2 stretching vibration reveals that additional methylene groups from MPTs are attached to the surface of o-MWCNTs. The peak at 2355 cm-1 is associated with -NH2, and a strong peak at 3381 cm-1 may be assigned to the N-H stretching vibration [35]. The appearance of -NH2 and N-H stretching vibrations indicates that amino groups appear on the surface of o-MWCNTs. Fig. 6(b) show that a weak peak at 2513 cm-1, which may be assigned to S-H group. The peak at 871 cm-1 is assigned to the stretching vibration of Si-C. The appearance of S-H and Si-C reveal that MPTs are successfully coated on the surface of o-MWCNTs. Furthermore, a new peak at 1110 cm-1 is attributed to Si-O stretching vibration, which can further prove that MPTs and hydrazine are successfully coated on the o-MWCNTs by connecting with the hydroxy groups on the external surface of o-MWCNT. 3.4 XPS spectra Table 2 10
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The peak, peak area and atomic percentage of N2H4-SH-Fe3O4/o-MWCNTs. Name Energy (ev) Area (%) Atomic (%) C1s 284.8 1.85 73.02 Fe2p 711.54 0.15 5.8 N1s 400.48 0.02 0.86 O1s 530.8 0.44 17.55 S2p 168.95 0.03 1.02 Si2p 101.96 0.04 1.75
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Fig. 7. XPS wide scan of N2H4-SH-Fe3O4/o-MWCNTs (a), C1s (b) ,O1s (c), Fe2p (d),N1s (e),S2p (f) and Si2p (g); the experimental data were fitted with Gaussian components assigned to specific chemical bonds as indicated, and the best fits were plotted in the figures.
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XPS is typically considered to be helpful for identifying groups and surface elements of composites. Table 2 shows the peak, peak area and atomic percentage of N2H4-SH-Fe3O4/o-MWCNTs. The data revealed the presence of carbon, oxygen, nitrogen, sulfur, iron and silicon elements on the surface of N2H4-SH-MWCNTs/Fe3O4.The atomic percentage of O, N, S, Fe and Si can reach 17.55 %, 0.86 %, 1.02 %, 5.8 % and 1.75 %, respectively. These data indicated that functional groups, such as amino and thiol groups, were successfully grafted onto the surface of o-MWCNTs. Fig. 7(a) shows the wide scan of N2H4-SH-Fe3O4/o-MWCNTs in which C1s, O1s, Fe2p, N1s, S2p and Si2p are clearly observed. The strong C1s peak at 284.6 ev corresponded to carbon nanotubes, and the photoelectron lines at the binding energies of approximately 530.8 and 711.54 ev are attributed to O1s and Fe2p, respectively [36-37]. The weak peaks at 168.95 ev and 101.96 ev are attributed to S2p and Si2p, which are present in MPTs. The appearance of S2p and Si2p peaks in the N2H4-SH-Fe3O4/o-MWCNTs spectrum is attributed to the –Si-CH2-CH2-CH2-SH bond, in accordance with the structure of N2H4-SH-Fe3O4/o-MWCNTs in Fig. 1(d).The N1s peak is from hydrazine at 400.48 ev. Fig. 7(b) shows that C1s spectrum of N2H4-SH-Fe3O4/o-MWCNTs can be deconvoluted into three Gaussian peaks centered at 284.6 ev, 285.2 ev and 286.6 ev (corresponding to sp2 C=C, C-OH, and C=O [38], respectively). Furthermore, Fig. 7(c) shows the Gaussian peaks are assigned to C=O, C-OH and HO-C=O (530.6 ev, 531.7 ev and 533 ev, respectively). The results of O1s spectra are consistent with C1s spectra. The presence of these oxygen-containing functional groups confirms that the surfaces of MWCNTs are successfully modified by oxidation, which may provide anchors for the 11
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immobilization of Fe3O4 and other groups. It was important to illustrate that these functional groups were successfully coated on the surface of N2H4-SH-Fe3O4/o-MWCNTs. Fig. 7(d) shows that the binding energies at 710.6 and 724.6 ev correspond to Fe2p3/2 and Fe2p1/2 in Fe3O4, respectively [39]. There are no obvious shakeup satellite structures at the higher binding energy sides of both of the main peaks (approximately 718.8 and 729.5 ev), which is characteristic of Fe3O4. The Gaussian peaks assigned to N-H and S-H are also detected at 402.2 and 163.9 ev in Fig. 7(e) and Fig. 7(f), respectively [21]. In addition, the Gaussian peaks of Si-N and Si-O appeared at 102 and 102.6 ev in Fig. 7(g), respectively. The newly appearing Si-O peak can prove that MPTs are coated on the surface of the composite materials, and the results are consistent with the FT-IR result. According to the results of XPS and FT-IR, N2H4-SH-Fe3O4/o-MWCNTs were successfully prepared by the experimental method.
3.5 TGA analysis
M
Fig. 8. TGA curves of MWCNTs (a), o-MWCNTs SH-Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d)
(b),
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TGA is conducted to quantify the molar mass of the functional groups presented on the surface of the composite materials. Fig. 8 shows the TGA curves of MWCNTs (a), o-MWCNTs (b), SH-Fe3O4/o-MWCNTs (c) and N2H4-SH-Fe3O4/o-MWCNTs (d). Fig. 8(a) shows the first stage of weight loss (2 wt%) from 220 ℃ to 500 ℃. The second stage of weight loss (2 wt%) is from 700 ℃ to 1100 ℃. The total 4 wt% of weight loss is attributed to the decomposition of amorphous carbon. The initial slight weight loss (0.5 wt%) is ascribed to the elimination of adsorbed humidity. As light weight loss (0.4 wt%) of MWCNT was reported at 550-680 ℃ due to the decomposition of MWCNT. Fig. 8(b) shows that the weight loss is 10.25 wt% from 150 ℃ to 958 ℃. Compared with the weight loss of MWCNTs (4 wt%), the 6.25 wt% weight loss is due to the decomposition of the -COOH and OH groups on the surface of o-MWCNTs. Fig. 8(c) illustrates that the weight loss of 10.4 wt% is consistent with the results of Fig. 8(b). The second weight loss stage in Fig. 8(c) is from 830 ℃ to 940 ℃, and the weight loss is up to 14.6 wt%. Based on the thermogravimetric profile, the result may be concluded that MPTs of 14.6 wt% is grafted onto o-MWCNTs sidewall, which generally corresponds to one thiol group every 113 C atoms. The S and C quantitative 12
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elemental analysis (N2H4-SH-Fe3O4/o-MWCNTs: S 1.02 wt%, C 73.02 wt%) according to the XPS results refers to approximately one thiol group every 190 C atoms, which is in fairly good agreement with previously calculated. Fig. 8(d) shows that there is an obvious weight loss from 940 ℃ to 1100 ℃, and the loss of 6.46% is attributed to the deamination reaction. All of the results indicate that the functional groups are successfully coated on the surface of the MWCNTs, and they are in agreement with the FT-IR and XPS results.
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3.6. Batch adsorption experiments
V (C 0 − Ce) M
(2)
M
qe =
an
To test the adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs, batch adsorption experiments were performed. The adsorption capacity is calculated using the following equation:
te
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where qe is the equilibrium adsorption capacity of the adsorbent (mg/g), C0 is the initial adsorbate concentration (mg/l), Ce is the equilibrium concentration of adsorbate (mg/l), V is the volume of solution (l),and M is the weight of adsorbent (g).
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3.6.1 Effects of pH The pH value of the solution was an important factor in the adsorption process, which can affect the structure at the surface of the composite material and the hydrogen ion concentration in the solution [40-41]. Fig. 9 shows the effects of pH on the adsorption capacity and the removal rate of Pb(II), Zn(II) and phenol. Fig. 9(a) shows that the maximum equilibrium adsorption capacity is 199.6 mg/g, 179.8 mg/g and 56.2 mg/g for Pb(II), Zn(II) and phenol, respectively. Fig. 9(a) illustrates that the equilibrium adsorption capacity (qe) of Pb(II) and Zn(II) increases rapidly as pH change from 2.0 to 6.0. Then, qe is constant until pH 9. Moreover, the equilibrium adsorption capacity of phenol remains stable at pH 3. The qe at pH lower than 3 was limited due to the competition of H3O+, which can compete with Pb(II), Zn(II) and phenol for the adsorption sites. The H3O+ ions prevented the target ions from adsorbing on the surface of the composite material. This conclusion was the same with the surface complex formation theory, which proved that as the hydrogen ions increased, competition for the adsorption sites between 13
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protons and metal species was increased [42]. More metal ions were adsorbed with the decrease of positive charges. As pH increased to greater than 6, the equilibrium adsorption capacity slightly changed due to the combined role of adsorption and precipitation of Pb(II) and Zn(II) as Zn(OH)2 and Pb(OH)2, respectively, causing the adsorption studies to be inaccurate (the hydrolysis pH of Zn(II) and Pb(II) was 6.54 and 7.04, respectively) [43]. Fig. 9(b) shows that N2H4-SH-Fe3O4/o-MWCNTs have high removal rates with values of 98% and 84% for Pb(II) and Zn(II), respectively. The removal rate of phenol is only 20%, because of the adverse effects of the -OH groups (on the surface of N2H4-SH-Fe3O4/o-MWCNTs) during the adsorption process. These groups may form hydrogen bonds with water, thus inhibiting the adsorption of phenol. Because the removal rate and adsorption capacity was the highest at pH 6.0, the initial pH of the solution was fixed for all of the subsequent adsorption experiments in this study.
M
Fig. 9. Effects of pH on adsorption capacity (a) and removal rate (b) of Pb(II), Zn(II)and phenol (10 mg N2H4-SH-Fe3O4/o-MWCNTs, 40 mg/l of Pb(II), Zn(II) and phenol, for 12 h).
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3.6.2 Effects of contact time and adsorption kinetics The effects of contact time on the adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol were studied. Fig. 10 shows that the adsorption capacities of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol markedly increases with the growth of contact time and remain stable after 30 min. The adsorption capacity at time t (qt) is 196 mg/g, 170 mg/g and 37 mg/g for Pb(Ⅱ), Zn(Ⅱ) and phenol at 30 min, respectively. Thus, N2H4-SH-Fe3O4/o-MWCNTs performed excellent adsorption capacity for Pb(II), Zn(II) and phenol. In fact, adsorption equilibrium was achieved at 30 min. This improved adsorption capacity was ascribed to the additional presence of hydrazine and MPTs which can create a network with water molecules, Pb2+, Zn2+, and phenol by hydrogen bonds and van der waals forces.And MWCNTs provides large surface area and a porous environment for the adsorption. Moreover, the increase of Pb(Ⅱ), Zn(Ⅱ) and phenol adsorption may be ascribed to the ionizable surface charges of N2H4-SH-Fe3O4/o-MWCNTs. The isoelectric point was 2.85 for nitric acid oxidized MWCNTs, which indicated that the surface of N2H4-SH-Fe3O4/o-MWCNTs was negatively charged at solution pH>2.85. The negatively charged surfaces of N2H4-SH-Fe3O4/o-MWCNTs favored the adsorption of target ions. 14
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ln(qe − qt ) = ln(qe) − k 1t
(3)
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To investigate the adsorption kinetics and adsorption mechanism, two kinetics models, including pseudo-first order and pseudo-second order models, were tested to fit the experimental data. The equation of the pseudo-first order kinetics model is as follows:
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cr
where qe (mg/g) and qt (mg/g) are the amounts of Pb(II), Zn(II) or phenol adsorbed at equilibrium and at time t, respectively; k1 (1/min) is the pseudo-first order rate constant; and t (min)is time. The equation of pseudo-second order kinetics model is as follows: t 1 t = + 2 q k 2 qe qe
an
(4)
M
where k2 (g/mg/min) is the rate constant of pseudo-second order equation [44-45].
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Fig. 10. Effects of contact time on Pb(II), Zn(II) and phenol adsorption (10 mg N2H4-SH-Fe3O4/o-MWCNTs and 40 mg/l of Pb(II), Zn(II) and phenol at pH 6)
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Fig. 11. Lagergren pseudo-first order kinetics model for the adsorption of Pb(II) (a),Zn(II) (b) and phenol (c). Fig. 12. Pseudo-second order kinetics model for the adsorption of Pb(II) (a),Zn(II) (b) and phenol (c). Table 3 The parameters of pseudo-first order and adsorption of Pb(II), Zn(II) and phenol. adsorbate pseudo-first order qe.exp qe.cal k1 R2 (mg/g) (mg/g) (1/min) Pb(II) 195.81 2.07 0.008 0.836 Zn(II) 170.07 2.27 0.007 0.820 phenol 38.97 2.14 0.008 0.760
pseudo-second order models for pseudo-second order qe.exp qe.cal k2 R2 (mg/g) (mg/g)（g/mg/min） 195.81 196.08 0.357 1 170.07 200 0.025 1 38.97 40 0.025 0.999
Fig. 11 and Fig. 12 show pseudo-first order and pseudo-second order 15
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kinetics models for the adsorption of Pb(II),Zn(II) and phenol. Table 3 illustrates pseudo-first order and pseudo-second order equation parameters calculated from the slope and intercept. It is clearly indicated that the experimental data fit well with pseudo-second order model with a high correlation coefficient (R2).The pseudo-second order model is suitable to describe the adsorption of metal ions and phenol from aqueous solution by N2H4-SH-Fe3O4/o-MWCNTs. The R2 values of pseudo-second order model are one in the adsorption process of Pb(II) and Zn(II), which are higher than that of the pseudo-first order model (0.836 and 0.820). In addition, the calculated values of equilibrium adsorption capacity (qe.cal) from the pseudo-second order model are 196.08, 200, and 40 mg/g for Pb(II), Zn(II) and phenol, respectively. The data are much closer to the experimental data (qe.exp=195.81, 170.07, 38.97 mg/g, respectively) than pseudo-first order model. The rate constant (K2) are 0.357 and 0.025 (g/mg/min) for Pb(II), Zn(II) and phenol. The data illustrated that N2H4-SH-Fe3O4/o-MWCNTs had a faster adsorption rate for Pb(II) than Zn(II) and phenol.
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3.6.3 Adsorption isotherms To investigate the relationship between the heavy metal ions and phenol adsorbed on N2H4-SH-Fe3O4/o-MWCNTs, two classical adsorption isotherm models, namely, Langmuir [46] and Freundlich models [47] were employed to describe the adsorption of Pb(II), Zn(II) and phenol. The Langmuir isotherm model was valid for monolayer adsorption onto the surface. If ion adsorption followed Langmuir model, then it was described by the following equation [48]: Ce 1 Ce = + qe qm K L qm
(5)
where Ce (mg/l) was the equilibrium concentration of Pb(II), Zn(II) and phenol in the solution; qe (mg/g) was the amounts of Pb(II), Zn(II) or phenol adsorbed at equilibrium; qm (mg/g) was the maximum adsorption capacity, and KL (l/mg) was the effective dissociation constant related to the affinity binding site. The values of qm and KL can be obtained from the intercept and the slope of the linear plot of Ce/qe against Ce [49]. Freundlich isotherm model was based on an exponential distribution of adsorption sites and energies. It was derived from multilayer adsorption and heterogeneous surfaces. The mathematical expression of Freundlich isotherm model is as follows [48]: 16
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ln qe =
1 ln Ce + ln KF n
(6)
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where KF (mmol1-n Ln/kg) is the Freundlich affinity coefficient, and n (unitless) is Freundlich linearity index. The values of n and KF can be obtained by the plots of lnqe against lnCe. The parameters of Langmuir and Freundlich isotherm models were calculated in Table 4, and the curve results of fitting were shown in Fig. 13 and Fig. 14.
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Fig. 13. Langmuir isotherm model for adsorption of Pb(II) (a), Zn(II) (b), and phenol (c) on N2H4-SH-Fe3O4/o-MWCNTs surface.
an
Fig. 14. Freundich isotherm model for adsorption of Pb(II) (a), Zn(II) (b) and phenol (c) on N2H4-SH-Fe3O4/o-MWCNTs surface.
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Table 4 The parameters of Langmuir and Freundlich models for the adsorption of Pb(II), Zn(II) and phenol on N2H4-SH-Fe3O4/o-MWCNTs surface. Adsorbate Langmuir model Freumdlich model 2 qm KL R KF n R2 (mg/g) (l/mg) (mmol1-n Ln/kg) Pb(II) 40 0.96 0.912 296.78 0.3 0.926 Zn(II) 21.28 0.015 0.9681 0.00315 0.165 0.97 phenol 55.56 0.018 0.902 0.5337 0.713 0.921 Fig. 13 and Fig. 14 show the adsorption isotherm models of Pb(II), Zn(II) and phenol on the surface of N2H4-SH-Fe3O4/o-MWCNTs. The parameters of Langmuir and Freundlich isotherm models are summarized in Table 4. Table 4 indicates that the adsorption of Pb(II), Zn(II) and phenol are better fitted to Freundlich isotherm model with higher R2 values than Langmuir isotherm model. The data indicated that the adsorption of Pb(II), Zn(II) and phenol may be interactive and may occur as a multilayer adsorption [50-51]. MWCNTs can aggregate and form bundles by strong van der waals forces in the solution due to their strong hydrophobicity. These bundles lead to the formation of groove, gap, and inner areas. The cluster structure provides heterogeneous adsorption sites for Pb(II), Zn(II) and phenol [49]. The correlation coefficient (R2) of Freundlich isotherm model for phenol is 0.921, which is higher than for Langmuir isotherm model (0.902). Therefore, Freundlich isotherm model correlates with data better 17
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than Langmuir isotherm model, indicating that the adsorption of phenol interacted with itself and N2H4-SH-Fe3O4/o-MWCNTs. A similar phenomenon can be obtained for the adsorption of Pb(II) and Zn(II). The n values are in the 0-1 range, which indicates that adsorption of Pb(II), Zn(II) and phenol on N2H4-SH-Fe3O4/o-MWCNTs is favorable. The n values are smaller than 1, which illustrates a favorable adsorption of heavy metal ions and phenol onto N2H4-SH-Fe3O4/o-MWCNTs.
V (C0 − Ce ) MCe
(7)
M
Kd =
an
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cr
3.6.4. Adsorption thermodynamics Temperature has a distinct effect on the adsorption of Pb(II), Zn(II) and phenol. The adsorption thermodynamic studies are conducted from 288 K to 308 K with 40 mg/l of Pb(II), Zn(II) and phenol to investigate the thermodynamic parameters. The Gibbs free energy (ΔG) is calculated according to the following equations [52]:
d
∆S 0 ∆H 0 ln Kd = − R RT
te
∆G 0 = ∆H 0 − T ∆S 0
(8) (9)
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where R is the universal gas constant (8.314 J/mol/K), T (K) is the adsorption temperature, and Kd is an equilibrium constant obtained by the initial and the equilibrium concentrations (C0 and Ce) of the target ions. The parameter V (l) is the solution volume, and W (g) is the adsorbent mass. The thermodynamic parameters, such as the Gibbs free energy ( Δ G), enthalpy(ΔH0), and entropy(ΔS0), as well as ciprocal of temperature (1/T) are plotted as 1/T against lnKd in Fig. 15. The values ofΔH0 andΔS0 are evaluated from the slope and intercept. Table 5 shows the values ofΔH0, Δ S0 andΔG0 under the reaction conditions. Table 5 indicates that the absolute values of Gibbs free energy increases as the temperature extends. The negative values of ΔG reveals that the adsorption of Pb(II), Zn(II) and phenol onto N2H4-SH-Fe3O4/o-MWCNTs is spontaneous. The magnitude of ΔG increases as the temperature increases, which indicates that the adsorption is more efficient at higher temperatures. Moreover, the change of ΔG for physisorption occurs between -20 and 0 kJ/mol, while the physisorption and chemisorption occur together over the 18
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range of -20 to -80 kJ/mol, with chemisorption occurring in the range of -80 to -400 kJ/mol [53-56]. TheΔG values indicate that the interactions between Pb(II), Zn(II) and N2H4-SH-Fe3O4/o-MWCNTs could be considered to undergo physisorption and chemisorption mechanisms. However, phenol adsorption is attributed to physisorption. The adsorption mechanism can explain why phenol adsorption capacity is less than that of Pb(II) and Zn(II). These groups, such as thiol and amine groups, on the surface of N2H4-SH-Fe3O4/o-MWCNTs can form chemical bonds with Pb(II) and Zn(II) to increase the adsorption capacity. The values ofΔS are positive, which also indicate that the adsorption process is endothermic. The high values of ΔS imply that a noticeable change in entropy occurs during the adsorption process of Pb(II), Zn(II) and phenol, and the positive values ofΔS indicate a tendency toward a higher disorder at the solid-solution interface during the adsorption. The positive values forΔS0 may be due to the release of water molecules produced by the ion-exchange reaction between the metal ions and the surface functional groups of N2H4-SH-Fe3O4/o-MWCNTs. The positive values reflect the affinity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(II), Zn(II), phenol and an increase in the randomness on the solution interface during the adsorption process [28].
te
Fig.15. Plots of lnKd versus 1/T for Pb(II) (a), Zn(II) (b) and phenol (c)
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Table 5 Thermodynamic constant for the adsorption of Pb(II), Zn(II) and phenol on the surface of N2H4-SH-Fe3O4/o-MWCNTs at different temperatures T/K lnKd ΔH ΔS ΔG （kJ/mol） （J/mol/K） （kJ/mol） Pb(II) 288 k 11.982 -28.819 298 k 12.384 16.819 158.465 -30.403 308 k 12.435 -31.989 Zn(II) 288 k 10.253 -24.485 298 k 10.253 0.92 88.212 -25.367 308 k 10.228 -26.249 phenol 288 k 7.658 -18.253 298 k 7.637 1.221 67.618 -18.929 308 k 7.624 -19.605
4. Conclusions 19
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Acknowledgements
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N2H4-SH-Fe3O4/o-MWCNTs were successfully prepared and used for the simultaneous adsorption of Pb(Ⅱ), Zn(Ⅱ) and phenol. TEM images proved that MPTs and hydrazine were coated on the surface of MWCNTs with an average thickness of 6.5 nm. Fe3O4 nanoparticles were uniformly and densely loaded on the surface of o-MWCNTs.. XPS, FTIR, and TGA analysis further proved that MWCNTs were functionalized by thiol and amino groups. The adsorption experiment revealed that the maximum equilibrium adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for Pb(Ⅱ), Zn(Ⅱ) and phenol reached 195.81 mg/g, 170.07 mg/g and 38.97 mg/g at pH 6, respectively, which proved that the composite was highly efficient. Up to 98% and 84% high removal rates for Pb(II) and Zn(II) could be achieved. The adsorption kinetics indicated that the adsorption process was consistent with pseudo-second order model. The pseudo-second order kinetics model with higher correlation coefficient was suitable to describe the kinetic process of heavy metals and phenol adsorption onto N2H4-SH-Fe3O4/o-MWCNTs. The adsorption isotherm was consistent with Freundlich isotherm model, which was a multilayer adsorption. The thermodynamic values indicated that the adsorption process was spontaneous and exothermic. TheΔG values indicated that interactions between Pb(II), Zn(II) and N2H4-SH-Fe3O4/o-MWCNTs could be considered to undergo physisorption and chemisorption mechanism. However, phenol adsorption was attributed to physisorption. It is hopeful that the composite has a good prospect for application to wastewater treatment.
The authors are grateful for financial support from the National Natural Science Foundation of China (51304101), scientific and research project of institutions of higher learning in Gansu province and open fund of Laboratory of Advanced Processing and Recycling of Non-ferrous Metal in Lanzhou university of technology.
References
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Highlights The synthetic method of N2H4-SH-Fe3O4/o-MWCNTs was explored, and amino and thiol groups which can improved the adsorption capacity were successfully grafted on the surface of Fe3O4/o-MWCNTs. The adsorption kinetics can be best described using pseudo-second order kinetics equation and the adsorption isotherms match well with Freundlich model. The maximum adsorption capacity of N2H4-SH-Fe3O4/o-MWCNTs for lead, zinc and phenol were 199.6 mg/g, 179.8 mg/g and 56.2 mg/g, respectively.
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*Graphical Abstract (for review)
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Figure.1 Click here to download high resolution image
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Figure.2 Click here to download high resolution image
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Figure.3 Click here to download high resolution image
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Figure.4 Click here to download high resolution image
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Figure.5 Click here to download high resolution image
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Figure.6 Click here to download high resolution image
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Figure.7 Click here to download high resolution image
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Figure.8 Click here to download high resolution image
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Figure.9 Click here to download high resolution image
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Figure.10 Click here to download high resolution image
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Figure.11 Click here to download high resolution image
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Figure.12 Click here to download high resolution image
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Figure.13 Click here to download high resolution image
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Figure.14 Click here to download high resolution image
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Figure.15 Click here to download high resolution image
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