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Al layered double hydroxides for Congo red and Cr(VI) ions
Accepted Manuscript Title: Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions Author: Chunsheng Lei Xiaofeng Zhu Bicheng Zhu Chuanjia Jiang Yao Le Jiaguo Yu PII: DOI: Reference:
S0304-3894(16)30892-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.09.070 HAZMAT 18076
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
3-7-2016 28-9-2016 29-9-2016
Please cite this article as: Chunsheng Lei, Xiaofeng Zhu, Bicheng Zhu, Chuanjia Jiang, Yao Le, Jiaguo Yu, Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.09.070 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.
Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions
Chunsheng Lei a,b, Xiaofeng Zhu b, Bicheng Zhu a, Chuanjia Jiang a, Yao Le a, Jiaguo Yu a,c,*
a
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, PR China, Fax: 0086-27-87879468; Tel.: 0086-27-87871029; E-mail: [email protected] b
College of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, PR China c
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Graphical abstract
1
Highlights Ni/Mg/Al layered double hydroxides (NMA-LDHs) synthesized. NMA-LDHs with hierarchically hollow microsphere structure. Calcined NMA-LDHs have large adsorption capacities for CR and Cr (VI) ions.
Abstract The preparation of hierarchical porous materials as catalysts and sorbents has attracted much attention in the field of environmental pollution control. Herein, Ni/Mg/Al layered double hydroxides (NMA-LDHs) hierarchical flower-like hollow microspheres were synthesized by a hydrothermal method. After the NMA-LDHs was
2
calcination at 600 °C, NMA-LDHs transformed into Ni/Mg/Al layered double oxides (NMA-LDOs), which maintained the hierarchical flower-like hollow structure. The crystal phase, morphology, and microstructure of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Energy-dispersive X-ray spectroscopy elemental mapping, Fourier transform infrared spectroscopy, and nitrogen adsorption−desorption methods. Both the calcined and non-calcined NMA-LDHs were examined for their performance to remove Congo red (CR) and hexavalent chromium (Cr(VI)) ions in aqueous solution. The maximum monolayer adsorption capacities of CR and Cr(VI) ions over the NMA-LDOs sample were 1250 and 103.4 mg/g at 30 °C, respectively. Thermodynamic studies indicated that the adsorption process was endothermic in nature. In addition, the addition of coexisting anions negatively influenced the adsorption capacity of Cr(VI) ions, in the following order: CO32− > SO42− > H2PO4− > Cl−. This work will provide new insight into the design and fabrication of advanced adsorption materials for water pollutant removal. Keywords: Layered double hydroxide; Hierarchical hollow sphere; Adsorption; Congo red; Cr(VI) ion
3
1. Introduction Water is the basis for the survival and development of human society. However, with the accelerated global modernization, large quantities of industrial wastewater containing toxic diazo dyes and heavy metal ions from paper, textile, printing, plastic, and electroplate industries are being discharged into the aquatic environment, which will pose serious risks to human health, living organisms, and the ecosystem. Therefore, exploring efficient and environmentally friendly methods to remove pollutants from wastewater has become an urgent and important task. To date, many common chemical and physical methods have been developed for this purpose [1,2], including adsorption [3,4], biological oxidation [5], photocatalysis [6,7], chemical coagulation [8], and ion-exchange [9]. Among these methods, adsorption is considered to be one of the most promising strategies because of its economic value, simplicity of operation, and high efficiency. The key to applying the adsorption method is the selection and fabrication of adsorbents. Generally, desirable adsorbents should be efficient, stable, low-cost, and environment-friendly. So far, various conventional adsorbents, such as activated carbon, clay, polymers, silicon nanomaterial, and metal oxides, have been studied [10-14]. However, many of the adsorbents have certain deficiencies, including low adsorption capability, low recyclability, and high cost. Hence, developing new adsorbent materials with high surface area, excellent adsorption capability, and low production cost is currently still an important issue for basic research and practical application. Layered
double
hydroxides
(LDHs,
4
with
the
general
formula
of
[M1−xz+Mx3+(OH)2Ax/nn−•mH2O]), a kind of anionic mineral, have been reported as potential adsorbent materials for wastewater treatment because of their layered structure, hierarchical pore structure, high surface area, and interlayer ion exchange capacity [15]. Recently, many easily prepared LDHs were synthesized and applied to the removal of dyes and heavy metal ions, such as Mg/Al-LDH [16], Ca/Fe-LDH [17], and Li/Al-LDH [18]. These LDHs have advantages over commercially available adsorbents in terms of low cost, high adsorption capacity, and non-toxicity. Hence, the employment of LDHs could provide considerable economic and environmental benefits to wastewater treatment. In the meantime, LDH microspheres with porous structures have attracted much interest for their special surface area and structural stability, which have shown their superiority in various applications including adsorption of water pollutants. For example, Yan et al. prepared three different magnetic core shell composites (Zn/Al-LDH, Mg/Al-LDH, and Ni/Al-LDH with Fe3O4 cores) using co-precipitation method and compared their phosphate adsorptive removal properties [19]. Ahmed and Gasser synthesized Mg−Fe−CO3-LDH applied in adsorption of anionic reactive dye [20]. To enlarge host layers and increase the amount of anions in the guest layer, trimetal-similar LDHs have been studied. Saber and Tagaya prepared Zn/Al/Ti-LDH and investigated its intercalation reactions to nanostructures [21]. Kowalik et al. prepared Cu/Zn/Al-LDH and explored its memory effect using in situ XRD [22]. Zhang et al. synthesized Mg/Zn/Al-LDH and investigated its homogeneous cation distribution [23]. Chagas et al. synthesized MgCoAl and NiCoAl LDHs using hydrothermal urea hydrolysis method and studied
5
their structure characterization and thermal decomposition [24]. In this work, triple-metal Ni/Mg/Al-LDHs were designed and prepared using a simple hydrothermal method. After high-temperature calcination, Ni/Mg/Al layered double oxides (LDOs) were obtained and examined as an adsorbent of Congo red (CR) dyes and hexavalent chromium (Cr(VI)) ions. Furthermore, the adsorption kinetics, isotherms, and thermodynamics, as well as the influence of temperature and coexisting anions on the adsorption capacity were investigated.
2. Experimental 2.1. Material Nickel sulfate (NiSO4•6H2O), magnesium sulfate (MgSO4•7H2O), aluminum nitrate [Al(NO3)3•9H2O], urea [CO(NH2)2], potassium dichromate (K2Cr2O7), ethanol, and CR were purchased from Shanghai Chemical Industrial Company. All the reagents were of analytical grade and used without purification. Deionized water was used in the experiment.
2.2. Synthesis Triple-metal Ni/Mg/Al-LDHs were synthesized using a simple hydrothermal method. In detail, NiSO4, MgSO4, Al(NO3)3, and urea were mixed in 80 mL of deionized water with Ni2+:Mg2+:Al3+:urea molar ratio of 1:1:1:6. After being stirred for 1 h, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was kept at 180 °C for 6 h and then cooled to room
6
temperature. The green precipitates were centrifugally separated and washed five times with water and ethanol, respectively. Finally, the washed sample was dried at 80 °C for 12 h and the dried product was labeled as NMA-LDHs. The NMA-LDHs sample was heated at 600 °C for 2 h in a muffle furnace to obtain Ni/Mg/Al-LDOs, which was labeled as NMA-LDOs.
2.3. Characterization The X-ray diffraction (XRD) patterns of the samples were obtained on an X-ray diffractometer (type HZG41 B-PC) using Cu Kα irradiation at a scan rate of 0.05° 2θ s−1. The morphologies of the samples were characterized using a JSM-7500F field emission scanning electron microscope (FE-SEM, JEOL, Japan) at an accelerating voltage of 15 kV and by a JEM-2100F transmission electron microscope (TEM, JEOL, Japan). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping analysis was performed on an EDS instrument (X-MAXn Oxford, UK). The nitrogen adsorption and desorption isotherms were measured using a Micromeritics TriStar П 3020 nitrogen adsorption apparatus (USA) after the samples were subjected to a degassing treatment at 150 °C. The Brunauer−Emmett−Teller (BET) specific surface area (SBET) of each sample was determined by a multipoint BET method using the adsorption data within the relative pressure (P/P0) range of 0.05−0.3. The pore size distributions were evaluated by assuming the cylindrical pore model using the Barrett−Joyner−Halenda (BJH) method [25]. The nitrogen adsorption volume at P/P0 = 1.0 was used to determine the pore volume and average pore size. Fourier transform infrared (FTIR)
7
spectra were recorded with a Shimadzu IRAffinity-1 FTIR spectrometer within the range of 4000−400 cm−1, using KBr as carrier to fix the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. The binding energy was referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon.
2.4. Adsorption of CR In the CR adsorption isotherm experiments, 10 mg of the as-prepared sample was added to a series of 250-mL Erlenmeyer flasks filled with 50 mL CR solutions (40−300 mg/L) at natural pH (ca. 7). The flasks were sealed and shaken for 24 h at various temperatures (20, 30, and 40 °C). Afterwards, the flasks were taken out from the shaker, and the samples were removed from the aqueous phase by centrifugation. The concentration of CR remaining in the supernatant solution was obtained using a UV/Vis spectrophotometer (UVMINI-1240, Shimadzu, Japan) at the maximum absorption wavelength of CR (λ = 495 nm). The CR adsorption kinetic experiments were performed by adding 10 mg of the as-prepared sample to a 250-mL Erlenmeyer flask filled with 50 mL of CR solution (100 mg/L) at natural pH (ca. 7). The flask was sealed and agitated at 30 °C. The aqueous samples were taken out at pre-set time intervals, and the concentrations of CR were similarly measured.
8
2.5. Removal of Cr(VI) ions Cr(VI) solutions with various concentrations were obtained by dissolving K2Cr2O7 in distilled water. Adsorption kinetic experiments were carried out by adding 50 mg of as-prepared sample to 100 mL of Cr(VI) solution (100 mg/L), which were kept agitated on a shaker (150 rpm) at 30 °C. At pre-set time intervals, 3 mL aliquots of the mixture was taken out and immediately filtered with a 0.45 µm microfiltration membrane. The Cr(VI) concentration in the filtrate was measured using a UV/Vis spectrophotometer at λ = 540 nm according to the 1,5-diphenylcarbazide method [26]. Adsorption isotherms were measured on the samples using 50 mg of adsorbent and various initial Cr(VI) concentrations (20−100 mg/L). The flasks were sealed and stirred at various temperatures (20, 30, and 40 °C) for 24 h to reach adsorption equilibrium.
2.6. Effect of coexisting anions on the adsorption of Cr(VI) ions The effect of coexisting anions on the adsorption of Cr(VI) ions by the NMA-LDOs sample was investigated at 30 °C. The procedures were the same as those described in Section 2.5, except that different coexisting anions (Cl–, H2PO4–, SO42–, and CO32–) were introduced into the Cr(VI) solution (20 mg/L) by separately adding corresponding sodium salts to various concentrations (0.5, 1, 2, 4, and 20 mmol/L).
9
3. Results and discussion 3.1. Phase structure Fig. 1 shows the XRD patterns of the NMA-LDHs and NMA-LDOs samples, as well as the NMA-LDOs samples after adsorption of CR and Cr(VI) ions. The NMA-LDHs sample has seven major peaks located at approximately 11.2°, 22.9°, 34.8°, 39.2°, 46.8°, 60.9°, and 62.3°, which can be indexed to (003), (006), (012), and (015) planes of Mg−Al hydrotalcite (JCPDS 35-0965), and (018), (110), and (113) planes of takovite (JCPDS 15-0087), respectively. After calcination, the NMA-LDHs sample changes into multi-metal oxide NMA-LDOs, which shows a diffraction characteristic of poor crystallinity. The NMA-LDOs sample has three major peaks located at approximately 36.8°, 43.5°, and 62.9°, which can be indexed to (111), (200), and (220) planes of MgNiO2 (JCPDS 24-0712), respectively. The NMA-LDOs sample does not exhibit the characteristic peaks of alumina oxide on account of the poor crystallinity. After adsorption of CR and Cr(VI) ions, the primary major peaks for NMA-LDOs remain the same. Meanwhile, a strong peak centered at about 22.4° appears, which is a characteristic peak of hydrotalcite. The reappearance of hydrotalcite in the CR and Cr(VI) adsorbed NMA-LDOs sample is a common phenomenon which is referred to as "memory effect" and widely reported in the literature [27,28]. More specifically, the LDOs can combine with anions and water molecules to reconstruct the LDHs layer structure after being added into CR and Cr(VI) solutions. These results verify the successful fabrication of triple-metal Ni/Mg/Al-LDHs and Ni/Mg/Al-LDOs.
10
3.2. Morphology and elemental composition The SEM and TEM images of the NMA-LDHs and NMA-LDOs samples are shown in Fig. 2a−d. The NMA-LDHs sample had hierarchical flower-like architecture, composed of 2−3 µm microspheres with ca. 1.5 µm-sized hollows (Fig. 2a and 2c). The NMA-LDOs sample is also hierarchical flower-like hollow microspheres with the same sizes (Fig. 2b and 2d), suggesting that the microstructures of the prepared samples were not destroyed after calcination. The EDS spectrum in Fig. 2e indicates that the NMA-LDOs sample is mainly composed of Ni, Mg, Al, and O, and the corresponding weight percentages are 18.1%, 11.2%, 10.4%, and 34.3%, respectively. Fig. 2f displays the element mapping images of the NMA-LDOs sample. It can be seen that the Ni, Mg, Al, and O elements have homogenous distributions in the microspheres. This finding provides solid evidence again that Ni/Mg/Al triple metal oxide was successfully prepared.
3.3. Nitrogen adsorption-desorption isotherms Fig. 3 presents the nitrogen isotherms and pore size distribution (PSD) curves of the NAM-LDHs and NMA-LDOs samples. The two isotherms are classified as type IV and they show high adsorption at the high relative pressures, which indicates the presence of mesopores and macropores in the samples. The shape of the two
11
hysteresis loops is of type H3 in the range of 0.5−1.0 P/P0, which is related to the narrow slit-shaped pores formed by the aggregation of nanosheets on the surface of the microspheres. The two PSD curves show wide pore size distributions from 3 to 100 nm with a small peak at 3−4 nm and a broad peak centered at ca. 25 nm. This observation further demonstrates the presence of mesopores and macropores.
The pore structure parameters were calculated and listed in Table 1. The NMA-LDOs sample possesses a larger specific surface area (179 m2 g−1) than the NMA-LDHs sample (101 m2 g−1), which is ascribed to the removal of interlayer water in the calcination process. The hierarchical pore structure and the high surface area of the flower-like hollow microspheres can provide more active adsorption sites and they are conducive to the fast transport of adsorbates in interconnected pore structure systems, thus enhancing the adsorption capacity. 3.4. FTIR spectrum Fig. 4 shows the FTIR spectra of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions. The absorption bands at ca. 3442, 1678, 1640, and 1635 cm−1 are assigned to the O−H stretching vibrations of the hydroxyl groups in the interlayer and surface water molecules [16,29]. The band at ca. 1119 cm−1 is indicative of the vibration of carbonate species [20]. The peak around 1361 cm−1 is associated with the CO32− anion in the interlayer [30]. This peak
was not observed in the FTIR spectrum of the NMA-LDOs sample,
while a new peak centered at ca. 1378 cm−1 is observed after adsorption of CR or
12
Cr(VI) ions onto the NMA-LDOs sample, indicating that CO32− species appear again in the interlayer because the NMA-LDOs samples encounter carbon dioxide in the adsorption process [19,31]. This finding reflects the memory effect of NMA-LDHs, and it is consistent with the XRD analysis. In addition, the peak at ca. 1045 cm−1 is ascribed to the S=O stretching vibration [32,33]. The bands centered at ca. 870 and 581 cm−1 correspond to the stretching vibrations of the Cr−O or Cr−O−Cr bond [34,35]. These results indicate that CR and Cr(VI) ions are adsorbed on the NMA-LDOs sample.
3.5. XPS spectrum The XPS spectrum of the NMA-LDOs sample was investigated to further verify the chemical composition and chemical status of elements. The survey spectrum in Fig. 5a illustrates that the NMA-LDOs sample is mainly composed of C, O, Ni, Mg, and Al elements. In Fig. 5b, the high-resolution XPS spectrum of Ni 2p presents four peaks at 855.4, 873.1, 861.7, and 879.5 eV, which can be ascribed to Ni 2p3/2 and Ni 2p1/2 of Ni2+ in NiO, and their corresponding satellites, respectively [27]. In Fig. 5c, the peak at 1303.3 eV is indicative of Mg 1s [36]. In Fig. 5d, two peaks at 67.9 eV and 74.0 eV are assigned to Al 2p3/2 and Al 2p1/2 for Al2O3 [37]. The high-resolution XPS spectrum of C 1s in Fig. 5e can be fitted to three peaks at 284.8, 286.4, and 289.0 eV. The main peak at 284.8 eV corresponds to C‒C coordination of surface adventitious carbon. The peak at 286.4 eV is characteristic of C=O from adsorbed CO2 [38]. The peak at 289.0 eV is attributed to carbonate species [39]. The XPS
13
analysis is consistent with the aforementioned EDS spectrum and powerful to demonstrate the trimetal mixed oxides in the NMA-LDOs sample.
3.6. Adsorption kinetics In adsorption studies, adsorption kinetics is important because it can describe the adsorption rate and provide valuable data for understanding the mechanism of sorption reactions [40]. Fig. 6a and 6b show the adsorption kinetics of CR and Cr(VI) ions on the prepared samples, respectively. It can be seen that in all the experiments, adsorption was fast in the initial 50 min. Then, the adsorption rates gradually decreased and equilibrium was reached within 150 min. Obviously, compared with the NMA-LDHs sample, the NMA-LDOs sample has faster adsorption rates and higher adsorption capacities toward both CR and Cr (VI) ions.
The controlling mechanism of adsorption was investigated using the pseudo-first-order [41] and pseudo-second-order kinetic models [42]. The linear forms of these models are respectively as follows: log (qe q t ) logqe
t 1 t 2 q t k 2 qe qe
k1 t 2.303
(1)
(2)
where k1 (min–1) and k2 (g mg–1 min–1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The calculated kinetic model
14
constants (Table 2) indicate that the experimental data fit better with the pseudo-second-order kinetic model than with the pseudo-first-order kinetic model. More
specifically,
the
obtained
correlation
coefficients
(R2)
in
the
pseudo-second-order kinetic model are greater than 0.997, and the normalized root-mean-square errors (RMSE) are smaller than 8.5%. Moreover, the values of qe,cal calculated from the pseudo-second-order kinetic model are very close to the values of qe,exp. Fig. 7a and 7b show the linear plots of t/qt versus t for the adsorption of CR and Cr(VI) ions on the prepared samples, respectively.
3.7. Adsorption isotherms Adsorption isotherms were performed to investigate the adsorption capacity of the samples at different equilibrium adsorbate concentrations. Fig. 8 reveals the adsorption isotherms of CR and Cr(VI) ions at three different temperatures (20, 30, and 40 °C). Langmuir and Freundlich isothermal models were used to describe the adsorption process. The Langmuir model assumes that the adsorbed molecules have no interaction with each other and are localized on the adsorbent surface in a monolayer. The Freundlich isothermal equation is an empirical model based on sorption on heterogeneous surfaces. This model assumes that the stronger binding sites are primarily occupied, and the binding strength decreases with the increasing degree of site occupation.
15
The linear forms of Langmuir and Freundlich isotherm models are respectively expressed as follows [44,45]:
Ce C 1 e qe qmax K L qmax 1 ln qe ln K F ln Ce n
(3)
(4)
where Ce is the equilibrium concentration of CR and Cr(VI) ions in solution (mg/L), qe is the amount of CR and Cr(VI) ions adsorbed at equilibrium (mg/g), qmax is the theoretical maximum adsorption capacity (mg/g), KL is the Langmuir constant (L/mg), KF is the Freundlich constant [(mg/g)•(L/mg)1/n], and 1/n is the heterogeneity factor. The qmax and KL values can be obtained by a plot of Ce/qe versus Ce (see Fig. 9), while t KF and n can be determined by a plot of lnqe versus lnCe. The calculated Langmuir and Freundlich isothermal parameters are summarized in Table 3. The Langmuir isothermal model fits the experimental data better than the Freundlich model in terms of R2. The values of KL (0.116−0.947) are in the range of 0−1, which indicates the favorable uptake of CR and Cr(VI) ions. Further analysis shows that the adsorption capacity of NMA-LDO is higher than that of NMA-LDH at the same equilibrium concentration of CR and Cr(VI) ions. According to the Langmuir isothermal model equation, the theoretical maximum adsorption capacity of CR and Cr(VI) ions on the NMA-LDOs sample is 1250 and 103.4 mg/g at 30 °C, which are approximately four times and twice greater than those on the NMA-LDH sample, respectively. The increased specific surface area after calcination, which produces more active sites for pollutant adsorption, is responsible for the enhanced
16
adsorption capacity. Moreover, the adsorption capacity increases with increasing temperature, indicating that the adsorption is favorable at higher temperature. 3.8. Adsorption thermodynamics The thermodynamics theory assumes that entropy change is the driving force in isolated systems where energy cannot be gained and lost [46]. To further explore the adsorption process, the changes of thermodynamic parameters, such as standard Gibbs free energy change (∆G0), standard enthalpy change (∆H0), and standard entropy change (∆S0), were calculated using Eqs. (5) and (6) [47,48]:
S 0 H 0 ln K L R RT
(5)
G 0 H 0 TS 0
(6)
where R (8.314 J mol–1 K–1) is the universal gas constant, T (K) is absolute temperature, and KL (L/mg) is the Langmuir isotherm constant. The obtained ∆G0, ∆H0, and ∆S0 for the adsorption of CR and Cr(VI) ions on the samples are listed in Table 4. The values of ∆H0 are positive, which indicates that the adsorption processes are endothermic in nature [49]; this finding is consistent with the result that the adsorption capacity of the samples toward CR and Cr(VI) ions increase with increasing temperature (Fig. 8). The increasing temperature will increase the diffusion rate of contaminant molecules across the external boundary layer and in the internal pores of the adsorbent, reduce the viscosity of the solution, and increase the number of active sites due to the construction of some internal bond near the surface of the active site [50,51]. 17
The adsorption capacities of the prepared samples and some previously reported materials for CR and/or Cr(VI) ions are compared (Table 5). It can be observed that the maximum adsorption capacities of the NMA-LDOs sample are significantly higher than those of the reported materials. 3.9. Effects of coexisting anions In general, Cr(VI) ions do not exist alone in industrial wastewater but coexist with other anions such as Cl–, H2PO4–, SO42–, and CO32–. These anions will strongly compete with Cr(VI) ions for the active adsorption sites on the samples [61,62]. Fig. 10 shows the effect of coexisting anions (Cl–, H2PO4–, SO42–, and CO32–) on the adsorption of Cr(VI) ions by the NMA-LDOs sample. The abscissa C is the concentrations of these anions. The ordinate is qanion/q0, where qanion and q0 are the adsorption amounts of Cr(VI) ions at equilibrium in the presence and absence of the anions, respectively. The coexistence of Cl–, H2PO4–, SO42–, and CO32– (20 mmol/L) decreased the equilibrium adsorption amount of Cr (VI) ions by 13.3%, 24.2%, 75.0%, and 85.0%, respectively. The considerable influence of SO42– and CO32– on the Cr(VI) ions adsorption capacity is mainly because the calcined LDHs have a memory effect and the carbonate has a specific affinity in solution [63]. In addition, the comparison of these anions confirms that the divalent ions bond more strongly with the adsorbents than the monovalent anions.
4. Conclusion
18
In summary, hierarchical hollow Ni/Mg/Al layered double hydroxides (NMA-LDHs) microspheres were synthesized by a facile hydrothermal method. After calcination at 600 °C, NMA-LDHs transformed into Ni/Mg/Al layered double oxides (NMA-LDOs) with the hierarchical microstructure remaining unchanged. The NMA-LDOs sample presented large adsorption capacities of CR (1250 mg/g) and Cr(VI) ions (103.4 mg/g) when compared to the NMA-LDHs. The adsorption kinetics was demonstrated to follow the pseudo-second-order model. The equilibrium adsorption isotherms could be better described by the Langmuir model. Coexisting anions have different influences on Cr(VI) ions adsorption capacity due to the competitive adsorption on surface and the relative preference of anions. The as-prepared hierarchical hollow NMA-LDOs microspheres are promising adsorbents for the removal of pollutants from wastewater because of their unique hierarchical pore structure, low cost, facile synthesis, and high efficiency.
Acknowledgment This study was partially supported by the NSFC (21433007, 51320105001 and 51272199), 973 Program (2013CB632402), the Fundamental Research Funds for the Central Universities (2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1) and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001).
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[47] I.A. Tan, A.L. Ahmad, B.H. Hameed, Adsorption isotherms, kinetics, thermodynamics and desorption studies of 2,4,6-trichlorophenol on oil palm empty fruit bunch-based activated carbon, Journal of Hazardous Materials 164 (2009) 473-482. [48] C. Zhang, S. Yang, H. Chen, H. He, C. Sun, Adsorption behavior and mechanism of reactive brilliant red X-3B in aqueous solution over three kinds of hydrotalcite-like LDHs, Applied Surface Science 301 (2014) 329-337. [49] J. Yang, M. Yu, T. Qiu, Adsorption thermodynamics and kinetics of Cr(VI) on KIP210 resin, Journal of Industrial and Engineering Chemistry 20 (2014) 480-486. [50] A. Gupta, C. Balomajumder, Simultaneous adsorption of Cr(VI) and phenol onto tea
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Multicomponent
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[60] W. Sun, M. Chen, S. Zhou, L. Wu, Synthesis of hierarchically nanostructured TiO2 spheres with tunable morphologies based on a novel amphiphilic polymer precursor and their use for heavy metal ion sequestration, Journal of Materials Chemistry A 2 (2014) 14004-14013. [61] W. Wang, J. Zhou, G. Achari, J. Yu, W. Cai, Cr(VI) removal from aqueous solutions by hydrothermal synthetic layered double hydroxides: Adsorption performance, coexisting anions and regeneration studies, Colloids and Surfaces A: Physicochemical and Engineering Aspects 457 (2014) 33-40. [62] M. Gan, S. Sun, Z. Zheng, H. Tang, J. Sheng, J. Zhu, X. Liu, Adsorption of Cr(VI) and Cu(II) by AlPO4 modified biosynthetic Schwertmannite, Applied Surface Science 356 (2015) 986-997. [63] M.A. Ulibarri, I. Pavlovic, C. Barriga, M.C. Hermosıń , J. Cornejo, Adsorption of anionic species on hydrotalcite-like compounds: effect of interlayer anion and crystallinity, Applied Clay Science 18 (2001) 17-27.
29
Fig. 1. XRD patterns of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions.
30
Fig. 2. SEM images of NMA-LDHs (a) and NMA-LDOs (b); TEM images of NMA-LDHs (c) and NMA-LDOs (d); EDS spectrum of NMA-LDOs (e) and element mapping images of NMA-LDOs (f).
31
Fig. 3. Nitrogen adsorption−desorption isotherms and the corresponding pore size distribution curves (inset) of NMA-LDHs and NMA-LDOs.
Fig. 4. FTIR spectra of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions.
32
Fig. 5. XPS survey spectrum (a), high-resolution XPS spectra of Ni 2p (b), Mg 1s (c), Al 2p (d), and C 1s (e) of the NMA-LDOs sample. 33
Fig. 6. Variation of adsorption amount with adsorption time for CR (a) and Cr (VI) ions (b) on the prepared samples (T = 30 °C; adsorbent dose = 200 mg/L for CR adsorption and 0.5 g/L for Cr(VI) ion adsorption; initial concentration = 100 mg/L for both CR and Cr(VI) ions). The solid lines represent modelled pseudo-second-order kinetics.
34
Fig. 7. Pseudo-second-order kinetics for adsorption of CR (a) and Cr(VI) ions (b) on the prepared samples.
35
Fig. 8. Adsorption isotherms of CR (a) and Cr(VI) ions (b) on the NMA-LDHs and NMA-LDOs samples at 20, 30, and 40 °C.
36
37
Fig. 9. Langmuir linear plots for adsorption isotherms of CR (a, b) and Cr (VI) ions (c, d) on the NMA-LDHs and NMA-LDOs samples at 20, 30, and 40 °C.
38
Table 1. Textural properties of the samples. dpore
Samples
Vpore
SBET −1
(nm)
(cm g )
(m2 g−1)
NMA-LDHs
23.4
0.59
101
NMA-LDOs
15.7
0.70
179
3
Table 2. Pseudo-first-order and pseudo-second-order kinetic model constants of the as-prepared samples.
Pollutants
Samples
qe,exp (mg/g)
Pseudo-first-order model qe,cal
−2
k1 (10
2
Pseudo-second-order model
RMSE qe,cal
k2 (10−3 g
R2
RMSE
(mg/g)
min–1)
R
NMA-LDHs 262
47
1.2
0.684
88
263
1.0
0.999
7.0
NMA-LDOs 466
139
1.8
0.686
76
476
0.2
0.999
8.5
Cr (VI)
NMA-LDHs 32.5
24.9
4.5
0.986
20
35.3
1.3
0.999
4.0
ions
NMA-LDOs 85.1
64.7
3.7
0.990
19
92.6
0.4
0.997
5.3
CR
a
(%) a (mg/g) mg–1 min–1)
RMSE was normalized to the average of the measured data.
Table 3. Adsorption isotherm parameters of the NMA-LDHs and NMA-LDOs samples. Langmuir isotherm model Freundlich isotherm model Pollutants
Samples
T (K)
qmax
KL
(mg/g) (L/mg)
CR
Cr (VI) ions
R2
KF (mg/g)• 1/n
(L/mg)
n
R2
293
278
0.138
0.992
65
3.0
0.773
NMA-LDHs 303
286
0.385
0.999
114
4.5
0.898
313
303
0.947
0.999
170
6.6
0.931
293
1111
0.116
0.974
256
2.7
0.603
NMA-LDOs 303
1250
0.563
0.999
495
4.7
0.835
313
1385
0.748
0.997
585
6.2
0.934
293
46.3
0.038
0.978
10.1
3.1
0.859
NMA-LDHs 303
49.6
0.039
0.931
4.9
2.2
0.851
313
52.4
0.067
0.947
1.4
1.5
0.837
NMA-LDOs 293
94.3
0.446
0.999
79.6
15.2
0.911
39
(%)
303
103.4
0.576
0.999
67.7
12.4
0.902
313
115.5
0.808
0.999
58.4
10.5
0.895
Table 4. Thermodynamic parameters for adsorption of CR and Cr(VI) ions on the NMA-LDHs and NMA-LDOs samples. Pollutes
Samples
NMA-LDHs CR NMA-LDOs
NMA-LDHs Cr(VI) ions NMA-LDOs
T (K)
∆G0
∆H0
∆S0
(kJ mol−1)
(kJ mol−1)
(kJ mol–1 K−1)
73.5
0.234
71.6
0.228
22.1
0.047
21.7
0.060
293
4.89
303
2.55
313
0.21
293
4.77
303
2.49
313
0.21
293
8.35
303
7.88
313
7.41
293
4.07
303
3.47
313
2.87
Table 5. The adsorption capacities of the NMA-LDOs sample and previously reported materials for CR and Cr(VI) ions. Adsorbents
qmax (mg/g)
References
CR
Cr(VI) ions
Ni/Mg/Al layered double oxides
1250
103.4
This work
α-Fe2O3 nanoparticles
254
17.0
[52]
Mg/Al LDH nanoflakes
585
[53]
Hollow microspheres NiO−SiO2
204
[54]
Hierarchical NiO nanosheets
152
[55]
Hierarchical NiO−Al2O3
357
[56]
Magnetic biochar
77.5
[57]
Multi-wall carbon nanotubes
13.2
[58]
ZIF-67 microcrystals
15.4
[59]
Hierarchically TiO2 spheres
13.4
[60]
40
S0304-3894(16)30892-5 http://dx.doi.org/doi:10.1016/j.jhazmat.2016.09.070 HAZMAT 18076
To appear in:
Journal of Hazardous Materials
Received date: Revised date: Accepted date:
3-7-2016 28-9-2016 29-9-2016
Please cite this article as: Chunsheng Lei, Xiaofeng Zhu, Bicheng Zhu, Chuanjia Jiang, Yao Le, Jiaguo Yu, Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions, Journal of Hazardous Materials http://dx.doi.org/10.1016/j.jhazmat.2016.09.070 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.
Superb adsorption capacity of hierarchical calcined Ni/Mg/Al layered double hydroxides for Congo red and Cr(VI) ions
Chunsheng Lei a,b, Xiaofeng Zhu b, Bicheng Zhu a, Chuanjia Jiang a, Yao Le a, Jiaguo Yu a,c,*
a
State Key Laboratory of Advanced Technology for Materials Synthesis and
Processing, Wuhan University of Technology, Wuhan 430070, PR China, Fax: 0086-27-87879468; Tel.: 0086-27-87871029; E-mail: [email protected] b
College of Environmental & Safety Engineering, Changzhou University, Changzhou 213164, PR China c
Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah 21589, Saudi Arabia
Graphical abstract
1
Highlights Ni/Mg/Al layered double hydroxides (NMA-LDHs) synthesized. NMA-LDHs with hierarchically hollow microsphere structure. Calcined NMA-LDHs have large adsorption capacities for CR and Cr (VI) ions.
Abstract The preparation of hierarchical porous materials as catalysts and sorbents has attracted much attention in the field of environmental pollution control. Herein, Ni/Mg/Al layered double hydroxides (NMA-LDHs) hierarchical flower-like hollow microspheres were synthesized by a hydrothermal method. After the NMA-LDHs was
2
calcination at 600 °C, NMA-LDHs transformed into Ni/Mg/Al layered double oxides (NMA-LDOs), which maintained the hierarchical flower-like hollow structure. The crystal phase, morphology, and microstructure of the as-prepared samples were characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Energy-dispersive X-ray spectroscopy elemental mapping, Fourier transform infrared spectroscopy, and nitrogen adsorption−desorption methods. Both the calcined and non-calcined NMA-LDHs were examined for their performance to remove Congo red (CR) and hexavalent chromium (Cr(VI)) ions in aqueous solution. The maximum monolayer adsorption capacities of CR and Cr(VI) ions over the NMA-LDOs sample were 1250 and 103.4 mg/g at 30 °C, respectively. Thermodynamic studies indicated that the adsorption process was endothermic in nature. In addition, the addition of coexisting anions negatively influenced the adsorption capacity of Cr(VI) ions, in the following order: CO32− > SO42− > H2PO4− > Cl−. This work will provide new insight into the design and fabrication of advanced adsorption materials for water pollutant removal. Keywords: Layered double hydroxide; Hierarchical hollow sphere; Adsorption; Congo red; Cr(VI) ion
3
1. Introduction Water is the basis for the survival and development of human society. However, with the accelerated global modernization, large quantities of industrial wastewater containing toxic diazo dyes and heavy metal ions from paper, textile, printing, plastic, and electroplate industries are being discharged into the aquatic environment, which will pose serious risks to human health, living organisms, and the ecosystem. Therefore, exploring efficient and environmentally friendly methods to remove pollutants from wastewater has become an urgent and important task. To date, many common chemical and physical methods have been developed for this purpose [1,2], including adsorption [3,4], biological oxidation [5], photocatalysis [6,7], chemical coagulation [8], and ion-exchange [9]. Among these methods, adsorption is considered to be one of the most promising strategies because of its economic value, simplicity of operation, and high efficiency. The key to applying the adsorption method is the selection and fabrication of adsorbents. Generally, desirable adsorbents should be efficient, stable, low-cost, and environment-friendly. So far, various conventional adsorbents, such as activated carbon, clay, polymers, silicon nanomaterial, and metal oxides, have been studied [10-14]. However, many of the adsorbents have certain deficiencies, including low adsorption capability, low recyclability, and high cost. Hence, developing new adsorbent materials with high surface area, excellent adsorption capability, and low production cost is currently still an important issue for basic research and practical application. Layered
double
hydroxides
(LDHs,
4
with
the
general
formula
of
[M1−xz+Mx3+(OH)2Ax/nn−•mH2O]), a kind of anionic mineral, have been reported as potential adsorbent materials for wastewater treatment because of their layered structure, hierarchical pore structure, high surface area, and interlayer ion exchange capacity [15]. Recently, many easily prepared LDHs were synthesized and applied to the removal of dyes and heavy metal ions, such as Mg/Al-LDH [16], Ca/Fe-LDH [17], and Li/Al-LDH [18]. These LDHs have advantages over commercially available adsorbents in terms of low cost, high adsorption capacity, and non-toxicity. Hence, the employment of LDHs could provide considerable economic and environmental benefits to wastewater treatment. In the meantime, LDH microspheres with porous structures have attracted much interest for their special surface area and structural stability, which have shown their superiority in various applications including adsorption of water pollutants. For example, Yan et al. prepared three different magnetic core shell composites (Zn/Al-LDH, Mg/Al-LDH, and Ni/Al-LDH with Fe3O4 cores) using co-precipitation method and compared their phosphate adsorptive removal properties [19]. Ahmed and Gasser synthesized Mg−Fe−CO3-LDH applied in adsorption of anionic reactive dye [20]. To enlarge host layers and increase the amount of anions in the guest layer, trimetal-similar LDHs have been studied. Saber and Tagaya prepared Zn/Al/Ti-LDH and investigated its intercalation reactions to nanostructures [21]. Kowalik et al. prepared Cu/Zn/Al-LDH and explored its memory effect using in situ XRD [22]. Zhang et al. synthesized Mg/Zn/Al-LDH and investigated its homogeneous cation distribution [23]. Chagas et al. synthesized MgCoAl and NiCoAl LDHs using hydrothermal urea hydrolysis method and studied
5
their structure characterization and thermal decomposition [24]. In this work, triple-metal Ni/Mg/Al-LDHs were designed and prepared using a simple hydrothermal method. After high-temperature calcination, Ni/Mg/Al layered double oxides (LDOs) were obtained and examined as an adsorbent of Congo red (CR) dyes and hexavalent chromium (Cr(VI)) ions. Furthermore, the adsorption kinetics, isotherms, and thermodynamics, as well as the influence of temperature and coexisting anions on the adsorption capacity were investigated.
2. Experimental 2.1. Material Nickel sulfate (NiSO4•6H2O), magnesium sulfate (MgSO4•7H2O), aluminum nitrate [Al(NO3)3•9H2O], urea [CO(NH2)2], potassium dichromate (K2Cr2O7), ethanol, and CR were purchased from Shanghai Chemical Industrial Company. All the reagents were of analytical grade and used without purification. Deionized water was used in the experiment.
2.2. Synthesis Triple-metal Ni/Mg/Al-LDHs were synthesized using a simple hydrothermal method. In detail, NiSO4, MgSO4, Al(NO3)3, and urea were mixed in 80 mL of deionized water with Ni2+:Mg2+:Al3+:urea molar ratio of 1:1:1:6. After being stirred for 1 h, the mixture was transferred into a 100 mL Teflon-lined stainless steel autoclave. The autoclave was kept at 180 °C for 6 h and then cooled to room
6
temperature. The green precipitates were centrifugally separated and washed five times with water and ethanol, respectively. Finally, the washed sample was dried at 80 °C for 12 h and the dried product was labeled as NMA-LDHs. The NMA-LDHs sample was heated at 600 °C for 2 h in a muffle furnace to obtain Ni/Mg/Al-LDOs, which was labeled as NMA-LDOs.
2.3. Characterization The X-ray diffraction (XRD) patterns of the samples were obtained on an X-ray diffractometer (type HZG41 B-PC) using Cu Kα irradiation at a scan rate of 0.05° 2θ s−1. The morphologies of the samples were characterized using a JSM-7500F field emission scanning electron microscope (FE-SEM, JEOL, Japan) at an accelerating voltage of 15 kV and by a JEM-2100F transmission electron microscope (TEM, JEOL, Japan). Energy-dispersive X-ray spectroscopy (EDS) elemental mapping analysis was performed on an EDS instrument (X-MAXn Oxford, UK). The nitrogen adsorption and desorption isotherms were measured using a Micromeritics TriStar П 3020 nitrogen adsorption apparatus (USA) after the samples were subjected to a degassing treatment at 150 °C. The Brunauer−Emmett−Teller (BET) specific surface area (SBET) of each sample was determined by a multipoint BET method using the adsorption data within the relative pressure (P/P0) range of 0.05−0.3. The pore size distributions were evaluated by assuming the cylindrical pore model using the Barrett−Joyner−Halenda (BJH) method [25]. The nitrogen adsorption volume at P/P0 = 1.0 was used to determine the pore volume and average pore size. Fourier transform infrared (FTIR)
7
spectra were recorded with a Shimadzu IRAffinity-1 FTIR spectrometer within the range of 4000−400 cm−1, using KBr as carrier to fix the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using an ultra-high vacuum VG ESCALAB 210 electron spectrometer equipped with a multichannel detector. The binding energy was referenced to the C 1s peak at 284.8 eV of the surface adventitious carbon.
2.4. Adsorption of CR In the CR adsorption isotherm experiments, 10 mg of the as-prepared sample was added to a series of 250-mL Erlenmeyer flasks filled with 50 mL CR solutions (40−300 mg/L) at natural pH (ca. 7). The flasks were sealed and shaken for 24 h at various temperatures (20, 30, and 40 °C). Afterwards, the flasks were taken out from the shaker, and the samples were removed from the aqueous phase by centrifugation. The concentration of CR remaining in the supernatant solution was obtained using a UV/Vis spectrophotometer (UVMINI-1240, Shimadzu, Japan) at the maximum absorption wavelength of CR (λ = 495 nm). The CR adsorption kinetic experiments were performed by adding 10 mg of the as-prepared sample to a 250-mL Erlenmeyer flask filled with 50 mL of CR solution (100 mg/L) at natural pH (ca. 7). The flask was sealed and agitated at 30 °C. The aqueous samples were taken out at pre-set time intervals, and the concentrations of CR were similarly measured.
8
2.5. Removal of Cr(VI) ions Cr(VI) solutions with various concentrations were obtained by dissolving K2Cr2O7 in distilled water. Adsorption kinetic experiments were carried out by adding 50 mg of as-prepared sample to 100 mL of Cr(VI) solution (100 mg/L), which were kept agitated on a shaker (150 rpm) at 30 °C. At pre-set time intervals, 3 mL aliquots of the mixture was taken out and immediately filtered with a 0.45 µm microfiltration membrane. The Cr(VI) concentration in the filtrate was measured using a UV/Vis spectrophotometer at λ = 540 nm according to the 1,5-diphenylcarbazide method [26]. Adsorption isotherms were measured on the samples using 50 mg of adsorbent and various initial Cr(VI) concentrations (20−100 mg/L). The flasks were sealed and stirred at various temperatures (20, 30, and 40 °C) for 24 h to reach adsorption equilibrium.
2.6. Effect of coexisting anions on the adsorption of Cr(VI) ions The effect of coexisting anions on the adsorption of Cr(VI) ions by the NMA-LDOs sample was investigated at 30 °C. The procedures were the same as those described in Section 2.5, except that different coexisting anions (Cl–, H2PO4–, SO42–, and CO32–) were introduced into the Cr(VI) solution (20 mg/L) by separately adding corresponding sodium salts to various concentrations (0.5, 1, 2, 4, and 20 mmol/L).
9
3. Results and discussion 3.1. Phase structure Fig. 1 shows the XRD patterns of the NMA-LDHs and NMA-LDOs samples, as well as the NMA-LDOs samples after adsorption of CR and Cr(VI) ions. The NMA-LDHs sample has seven major peaks located at approximately 11.2°, 22.9°, 34.8°, 39.2°, 46.8°, 60.9°, and 62.3°, which can be indexed to (003), (006), (012), and (015) planes of Mg−Al hydrotalcite (JCPDS 35-0965), and (018), (110), and (113) planes of takovite (JCPDS 15-0087), respectively. After calcination, the NMA-LDHs sample changes into multi-metal oxide NMA-LDOs, which shows a diffraction characteristic of poor crystallinity. The NMA-LDOs sample has three major peaks located at approximately 36.8°, 43.5°, and 62.9°, which can be indexed to (111), (200), and (220) planes of MgNiO2 (JCPDS 24-0712), respectively. The NMA-LDOs sample does not exhibit the characteristic peaks of alumina oxide on account of the poor crystallinity. After adsorption of CR and Cr(VI) ions, the primary major peaks for NMA-LDOs remain the same. Meanwhile, a strong peak centered at about 22.4° appears, which is a characteristic peak of hydrotalcite. The reappearance of hydrotalcite in the CR and Cr(VI) adsorbed NMA-LDOs sample is a common phenomenon which is referred to as "memory effect" and widely reported in the literature [27,28]. More specifically, the LDOs can combine with anions and water molecules to reconstruct the LDHs layer structure after being added into CR and Cr(VI) solutions. These results verify the successful fabrication of triple-metal Ni/Mg/Al-LDHs and Ni/Mg/Al-LDOs.
10
3.2. Morphology and elemental composition The SEM and TEM images of the NMA-LDHs and NMA-LDOs samples are shown in Fig. 2a−d. The NMA-LDHs sample had hierarchical flower-like architecture, composed of 2−3 µm microspheres with ca. 1.5 µm-sized hollows (Fig. 2a and 2c). The NMA-LDOs sample is also hierarchical flower-like hollow microspheres with the same sizes (Fig. 2b and 2d), suggesting that the microstructures of the prepared samples were not destroyed after calcination. The EDS spectrum in Fig. 2e indicates that the NMA-LDOs sample is mainly composed of Ni, Mg, Al, and O, and the corresponding weight percentages are 18.1%, 11.2%, 10.4%, and 34.3%, respectively. Fig. 2f displays the element mapping images of the NMA-LDOs sample. It can be seen that the Ni, Mg, Al, and O elements have homogenous distributions in the microspheres. This finding provides solid evidence again that Ni/Mg/Al triple metal oxide was successfully prepared.
3.3. Nitrogen adsorption-desorption isotherms Fig. 3 presents the nitrogen isotherms and pore size distribution (PSD) curves of the NAM-LDHs and NMA-LDOs samples. The two isotherms are classified as type IV and they show high adsorption at the high relative pressures, which indicates the presence of mesopores and macropores in the samples. The shape of the two
11
hysteresis loops is of type H3 in the range of 0.5−1.0 P/P0, which is related to the narrow slit-shaped pores formed by the aggregation of nanosheets on the surface of the microspheres. The two PSD curves show wide pore size distributions from 3 to 100 nm with a small peak at 3−4 nm and a broad peak centered at ca. 25 nm. This observation further demonstrates the presence of mesopores and macropores.
The pore structure parameters were calculated and listed in Table 1. The NMA-LDOs sample possesses a larger specific surface area (179 m2 g−1) than the NMA-LDHs sample (101 m2 g−1), which is ascribed to the removal of interlayer water in the calcination process. The hierarchical pore structure and the high surface area of the flower-like hollow microspheres can provide more active adsorption sites and they are conducive to the fast transport of adsorbates in interconnected pore structure systems, thus enhancing the adsorption capacity. 3.4. FTIR spectrum Fig. 4 shows the FTIR spectra of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions. The absorption bands at ca. 3442, 1678, 1640, and 1635 cm−1 are assigned to the O−H stretching vibrations of the hydroxyl groups in the interlayer and surface water molecules [16,29]. The band at ca. 1119 cm−1 is indicative of the vibration of carbonate species [20]. The peak around 1361 cm−1 is associated with the CO32− anion in the interlayer [30]. This peak
was not observed in the FTIR spectrum of the NMA-LDOs sample,
while a new peak centered at ca. 1378 cm−1 is observed after adsorption of CR or
12
Cr(VI) ions onto the NMA-LDOs sample, indicating that CO32− species appear again in the interlayer because the NMA-LDOs samples encounter carbon dioxide in the adsorption process [19,31]. This finding reflects the memory effect of NMA-LDHs, and it is consistent with the XRD analysis. In addition, the peak at ca. 1045 cm−1 is ascribed to the S=O stretching vibration [32,33]. The bands centered at ca. 870 and 581 cm−1 correspond to the stretching vibrations of the Cr−O or Cr−O−Cr bond [34,35]. These results indicate that CR and Cr(VI) ions are adsorbed on the NMA-LDOs sample.
3.5. XPS spectrum The XPS spectrum of the NMA-LDOs sample was investigated to further verify the chemical composition and chemical status of elements. The survey spectrum in Fig. 5a illustrates that the NMA-LDOs sample is mainly composed of C, O, Ni, Mg, and Al elements. In Fig. 5b, the high-resolution XPS spectrum of Ni 2p presents four peaks at 855.4, 873.1, 861.7, and 879.5 eV, which can be ascribed to Ni 2p3/2 and Ni 2p1/2 of Ni2+ in NiO, and their corresponding satellites, respectively [27]. In Fig. 5c, the peak at 1303.3 eV is indicative of Mg 1s [36]. In Fig. 5d, two peaks at 67.9 eV and 74.0 eV are assigned to Al 2p3/2 and Al 2p1/2 for Al2O3 [37]. The high-resolution XPS spectrum of C 1s in Fig. 5e can be fitted to three peaks at 284.8, 286.4, and 289.0 eV. The main peak at 284.8 eV corresponds to C‒C coordination of surface adventitious carbon. The peak at 286.4 eV is characteristic of C=O from adsorbed CO2 [38]. The peak at 289.0 eV is attributed to carbonate species [39]. The XPS
13
analysis is consistent with the aforementioned EDS spectrum and powerful to demonstrate the trimetal mixed oxides in the NMA-LDOs sample.
3.6. Adsorption kinetics In adsorption studies, adsorption kinetics is important because it can describe the adsorption rate and provide valuable data for understanding the mechanism of sorption reactions [40]. Fig. 6a and 6b show the adsorption kinetics of CR and Cr(VI) ions on the prepared samples, respectively. It can be seen that in all the experiments, adsorption was fast in the initial 50 min. Then, the adsorption rates gradually decreased and equilibrium was reached within 150 min. Obviously, compared with the NMA-LDHs sample, the NMA-LDOs sample has faster adsorption rates and higher adsorption capacities toward both CR and Cr (VI) ions.
The controlling mechanism of adsorption was investigated using the pseudo-first-order [41] and pseudo-second-order kinetic models [42]. The linear forms of these models are respectively as follows: log (qe q t ) logqe
t 1 t 2 q t k 2 qe qe
k1 t 2.303
(1)
(2)
where k1 (min–1) and k2 (g mg–1 min–1) are the pseudo-first-order and pseudo-second-order rate constants, respectively. The calculated kinetic model
14
constants (Table 2) indicate that the experimental data fit better with the pseudo-second-order kinetic model than with the pseudo-first-order kinetic model. More
specifically,
the
obtained
correlation
coefficients
(R2)
in
the
pseudo-second-order kinetic model are greater than 0.997, and the normalized root-mean-square errors (RMSE) are smaller than 8.5%. Moreover, the values of qe,cal calculated from the pseudo-second-order kinetic model are very close to the values of qe,exp. Fig. 7a and 7b show the linear plots of t/qt versus t for the adsorption of CR and Cr(VI) ions on the prepared samples, respectively.
3.7. Adsorption isotherms Adsorption isotherms were performed to investigate the adsorption capacity of the samples at different equilibrium adsorbate concentrations. Fig. 8 reveals the adsorption isotherms of CR and Cr(VI) ions at three different temperatures (20, 30, and 40 °C). Langmuir and Freundlich isothermal models were used to describe the adsorption process. The Langmuir model assumes that the adsorbed molecules have no interaction with each other and are localized on the adsorbent surface in a monolayer. The Freundlich isothermal equation is an empirical model based on sorption on heterogeneous surfaces. This model assumes that the stronger binding sites are primarily occupied, and the binding strength decreases with the increasing degree of site occupation.
15
The linear forms of Langmuir and Freundlich isotherm models are respectively expressed as follows [44,45]:
Ce C 1 e qe qmax K L qmax 1 ln qe ln K F ln Ce n
(3)
(4)
where Ce is the equilibrium concentration of CR and Cr(VI) ions in solution (mg/L), qe is the amount of CR and Cr(VI) ions adsorbed at equilibrium (mg/g), qmax is the theoretical maximum adsorption capacity (mg/g), KL is the Langmuir constant (L/mg), KF is the Freundlich constant [(mg/g)•(L/mg)1/n], and 1/n is the heterogeneity factor. The qmax and KL values can be obtained by a plot of Ce/qe versus Ce (see Fig. 9), while t KF and n can be determined by a plot of lnqe versus lnCe. The calculated Langmuir and Freundlich isothermal parameters are summarized in Table 3. The Langmuir isothermal model fits the experimental data better than the Freundlich model in terms of R2. The values of KL (0.116−0.947) are in the range of 0−1, which indicates the favorable uptake of CR and Cr(VI) ions. Further analysis shows that the adsorption capacity of NMA-LDO is higher than that of NMA-LDH at the same equilibrium concentration of CR and Cr(VI) ions. According to the Langmuir isothermal model equation, the theoretical maximum adsorption capacity of CR and Cr(VI) ions on the NMA-LDOs sample is 1250 and 103.4 mg/g at 30 °C, which are approximately four times and twice greater than those on the NMA-LDH sample, respectively. The increased specific surface area after calcination, which produces more active sites for pollutant adsorption, is responsible for the enhanced
16
adsorption capacity. Moreover, the adsorption capacity increases with increasing temperature, indicating that the adsorption is favorable at higher temperature. 3.8. Adsorption thermodynamics The thermodynamics theory assumes that entropy change is the driving force in isolated systems where energy cannot be gained and lost [46]. To further explore the adsorption process, the changes of thermodynamic parameters, such as standard Gibbs free energy change (∆G0), standard enthalpy change (∆H0), and standard entropy change (∆S0), were calculated using Eqs. (5) and (6) [47,48]:
S 0 H 0 ln K L R RT
(5)
G 0 H 0 TS 0
(6)
where R (8.314 J mol–1 K–1) is the universal gas constant, T (K) is absolute temperature, and KL (L/mg) is the Langmuir isotherm constant. The obtained ∆G0, ∆H0, and ∆S0 for the adsorption of CR and Cr(VI) ions on the samples are listed in Table 4. The values of ∆H0 are positive, which indicates that the adsorption processes are endothermic in nature [49]; this finding is consistent with the result that the adsorption capacity of the samples toward CR and Cr(VI) ions increase with increasing temperature (Fig. 8). The increasing temperature will increase the diffusion rate of contaminant molecules across the external boundary layer and in the internal pores of the adsorbent, reduce the viscosity of the solution, and increase the number of active sites due to the construction of some internal bond near the surface of the active site [50,51]. 17
The adsorption capacities of the prepared samples and some previously reported materials for CR and/or Cr(VI) ions are compared (Table 5). It can be observed that the maximum adsorption capacities of the NMA-LDOs sample are significantly higher than those of the reported materials. 3.9. Effects of coexisting anions In general, Cr(VI) ions do not exist alone in industrial wastewater but coexist with other anions such as Cl–, H2PO4–, SO42–, and CO32–. These anions will strongly compete with Cr(VI) ions for the active adsorption sites on the samples [61,62]. Fig. 10 shows the effect of coexisting anions (Cl–, H2PO4–, SO42–, and CO32–) on the adsorption of Cr(VI) ions by the NMA-LDOs sample. The abscissa C is the concentrations of these anions. The ordinate is qanion/q0, where qanion and q0 are the adsorption amounts of Cr(VI) ions at equilibrium in the presence and absence of the anions, respectively. The coexistence of Cl–, H2PO4–, SO42–, and CO32– (20 mmol/L) decreased the equilibrium adsorption amount of Cr (VI) ions by 13.3%, 24.2%, 75.0%, and 85.0%, respectively. The considerable influence of SO42– and CO32– on the Cr(VI) ions adsorption capacity is mainly because the calcined LDHs have a memory effect and the carbonate has a specific affinity in solution [63]. In addition, the comparison of these anions confirms that the divalent ions bond more strongly with the adsorbents than the monovalent anions.
4. Conclusion
18
In summary, hierarchical hollow Ni/Mg/Al layered double hydroxides (NMA-LDHs) microspheres were synthesized by a facile hydrothermal method. After calcination at 600 °C, NMA-LDHs transformed into Ni/Mg/Al layered double oxides (NMA-LDOs) with the hierarchical microstructure remaining unchanged. The NMA-LDOs sample presented large adsorption capacities of CR (1250 mg/g) and Cr(VI) ions (103.4 mg/g) when compared to the NMA-LDHs. The adsorption kinetics was demonstrated to follow the pseudo-second-order model. The equilibrium adsorption isotherms could be better described by the Langmuir model. Coexisting anions have different influences on Cr(VI) ions adsorption capacity due to the competitive adsorption on surface and the relative preference of anions. The as-prepared hierarchical hollow NMA-LDOs microspheres are promising adsorbents for the removal of pollutants from wastewater because of their unique hierarchical pore structure, low cost, facile synthesis, and high efficiency.
Acknowledgment This study was partially supported by the NSFC (21433007, 51320105001 and 51272199), 973 Program (2013CB632402), the Fundamental Research Funds for the Central Universities (2015-III-034), Self-determined and Innovative Research Funds of SKLWUT (2015-ZD-1) and the Natural Science Foundation of Hubei Province of China (No. 2015CFA001).
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Fig. 1. XRD patterns of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions.
30
Fig. 2. SEM images of NMA-LDHs (a) and NMA-LDOs (b); TEM images of NMA-LDHs (c) and NMA-LDOs (d); EDS spectrum of NMA-LDOs (e) and element mapping images of NMA-LDOs (f).
31
Fig. 3. Nitrogen adsorption−desorption isotherms and the corresponding pore size distribution curves (inset) of NMA-LDHs and NMA-LDOs.
Fig. 4. FTIR spectra of the NMA-LDHs and NMA-LDOs samples, and the NMA-LDOs samples after adsorption of CR and Cr(VI) ions.
32
Fig. 5. XPS survey spectrum (a), high-resolution XPS spectra of Ni 2p (b), Mg 1s (c), Al 2p (d), and C 1s (e) of the NMA-LDOs sample. 33
Fig. 6. Variation of adsorption amount with adsorption time for CR (a) and Cr (VI) ions (b) on the prepared samples (T = 30 °C; adsorbent dose = 200 mg/L for CR adsorption and 0.5 g/L for Cr(VI) ion adsorption; initial concentration = 100 mg/L for both CR and Cr(VI) ions). The solid lines represent modelled pseudo-second-order kinetics.
34
Fig. 7. Pseudo-second-order kinetics for adsorption of CR (a) and Cr(VI) ions (b) on the prepared samples.
35
Fig. 8. Adsorption isotherms of CR (a) and Cr(VI) ions (b) on the NMA-LDHs and NMA-LDOs samples at 20, 30, and 40 °C.
36
37
Fig. 9. Langmuir linear plots for adsorption isotherms of CR (a, b) and Cr (VI) ions (c, d) on the NMA-LDHs and NMA-LDOs samples at 20, 30, and 40 °C.
38
Table 1. Textural properties of the samples. dpore
Samples
Vpore
SBET −1
(nm)
(cm g )
(m2 g−1)
NMA-LDHs
23.4
0.59
101
NMA-LDOs
15.7
0.70
179
3
Table 2. Pseudo-first-order and pseudo-second-order kinetic model constants of the as-prepared samples.
Pollutants
Samples
qe,exp (mg/g)
Pseudo-first-order model qe,cal
−2
k1 (10
2
Pseudo-second-order model
RMSE qe,cal
k2 (10−3 g
R2
RMSE
(mg/g)
min–1)
R
NMA-LDHs 262
47
1.2
0.684
88
263
1.0
0.999
7.0
NMA-LDOs 466
139
1.8
0.686
76
476
0.2
0.999
8.5
Cr (VI)
NMA-LDHs 32.5
24.9
4.5
0.986
20
35.3
1.3
0.999
4.0
ions
NMA-LDOs 85.1
64.7
3.7
0.990
19
92.6
0.4
0.997
5.3
CR
a
(%) a (mg/g) mg–1 min–1)
RMSE was normalized to the average of the measured data.
Table 3. Adsorption isotherm parameters of the NMA-LDHs and NMA-LDOs samples. Langmuir isotherm model Freundlich isotherm model Pollutants
Samples
T (K)
qmax
KL
(mg/g) (L/mg)
CR
Cr (VI) ions
R2
KF (mg/g)• 1/n
(L/mg)
n
R2
293
278
0.138
0.992
65
3.0
0.773
NMA-LDHs 303
286
0.385
0.999
114
4.5
0.898
313
303
0.947
0.999
170
6.6
0.931
293
1111
0.116
0.974
256
2.7
0.603
NMA-LDOs 303
1250
0.563
0.999
495
4.7
0.835
313
1385
0.748
0.997
585
6.2
0.934
293
46.3
0.038
0.978
10.1
3.1
0.859
NMA-LDHs 303
49.6
0.039
0.931
4.9
2.2
0.851
313
52.4
0.067
0.947
1.4
1.5
0.837
NMA-LDOs 293
94.3
0.446
0.999
79.6
15.2
0.911
39
(%)
303
103.4
0.576
0.999
67.7
12.4
0.902
313
115.5
0.808
0.999
58.4
10.5
0.895
Table 4. Thermodynamic parameters for adsorption of CR and Cr(VI) ions on the NMA-LDHs and NMA-LDOs samples. Pollutes
Samples
NMA-LDHs CR NMA-LDOs
NMA-LDHs Cr(VI) ions NMA-LDOs
T (K)
∆G0
∆H0
∆S0
(kJ mol−1)
(kJ mol−1)
(kJ mol–1 K−1)
73.5
0.234
71.6
0.228
22.1
0.047
21.7
0.060
293
4.89
303
2.55
313
0.21
293
4.77
303
2.49
313
0.21
293
8.35
303
7.88
313
7.41
293
4.07
303
3.47
313
2.87
Table 5. The adsorption capacities of the NMA-LDOs sample and previously reported materials for CR and Cr(VI) ions. Adsorbents
qmax (mg/g)
References
CR
Cr(VI) ions
Ni/Mg/Al layered double oxides
1250
103.4
This work
α-Fe2O3 nanoparticles
254
17.0
[52]
Mg/Al LDH nanoflakes
585
[53]
Hollow microspheres NiO−SiO2
204
[54]
Hierarchical NiO nanosheets
152
[55]
Hierarchical NiO−Al2O3
357
[56]
Magnetic biochar
77.5
[57]
Multi-wall carbon nanotubes
13.2
[58]
ZIF-67 microcrystals
15.4
[59]
Hierarchically TiO2 spheres
13.4
[60]
40