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Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene
Journal of Alloys and Compounds 587 (2014) 99–104
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene Tianshan Xue a, Yanshan Gao a, Zhang Zhang a, Ahmad Umar b,c,⇑, Xingru Yan d, Xi Zhang d, Zhanhu Guo d, Qiang Wang a,⇑ a
College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, PR China Department of Chemistry, College of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia d Integrated Composites Laboratory, Dan F Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA b c
a r t i c l e
i n f o
Article history: Received 31 August 2013 Received in revised form 13 October 2013 Accepted 19 October 2013 Available online 30 October 2013 Keywords: Wastewater treatment Hydrotalcite Ion exchange Polymer nanocomposite UV absorption
a b s t r a c t In this contribution, we report the removal of acid red 97 (AC97) from simulated dye wastewater by using Zn2Al-NO3 layered double hydroxides (LDHs) adsorbent, and the resource of the LDH adsorbent sludge as nanofiller for polypropylene (PP) for the first time. The obtained Zn2Al-NO3 LDH was analyzed using X-ray diffraction and scanning electron microscopy analysis, confirming the formation of pure and platelike LDH nanoparticles. The effects of adsorption time and initial dye concentration on the removal of AC97 from wastewater were systematically investigated, showing that the Zn2Al-NO3 LDHs is very efficient in removing AC97. The saturated adsorption capacity of water washed and acetone washed Zn2Al-LDHs is 204.4 and 299.5 mg/g, respectively. Finally, the LDH adsorbent sludge was added into PP using a modified solvent mixing method. Thermal gravimetric analysis and ultraviolet (UV) absorption analysis of PP/Zn2Al-AC97 LDHs nanocomposites suggested that the Zn2Al-AC97 LDH can significantly improve the thermal stability and UV shielding ability of PP. This data demonstrated that it is very promising to resource the dye adsorbent sludge as multifunctional nanofiller for polymers. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Although printing and dyeing is an important industry for national economy, the wastewater produced from it has been considered as one of the most important pollutants and its disposal has become a major environmental problem [1–3]. With large volume, high organic content, dark color, complex substances and other features, dye wastewater can directly damage the water bodies and threaten our human health [4]. Therefore, there is an increasing demand for efficient treatment of the dye wastewater [5]. Up to date, there are various methods such as coagulating sedimentation, filtration, electro-coagulation, and adsorption by solid adsorbent have been developed to remove the dyes from wastewaters [6]. Among various water treatment technologies, adsorption has been found to be an effective and inexpensive treatment technology due to its efficiency, simplicity, and applicability. Granular activated carbon [7], activated diatomite [8], and resins [9] are ⇑ Corresponding authors. Address: Department of Chemistry, College of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia (A. Umar), College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, PR China (Q. Wang). Tel.: +86 13699130626. E-mail addresses: [email protected] (A. Umar), qiang.wang.ox@gmail. com, [email protected] (Q. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.158
the most commonly used adsorbents. The main problems of solid adsorption method are the relatively low adsorption capacity and the difficulty to reuse. Layered double hydroxides (LDHs) are a widely used type of anzþ n 3þ ionic clays with the general formula ½M2þ 1x M x ðOHÞ2 ½A z=n mH2O, 2+ 3+ in which M and M are metal positive ions such as Mg2+, Zn2+, Ni2+, Co2+, Cu2+, Ca2+, Mn2+ and A13+, Cr3+, Co3+ or Fe3+, An stands 2 3 for inorganic anions such as CO2 3 , NO3 , F , Cl , Br , I , CrO4 , PO4 , SO2 , or various organic anions, the value of x directly affect the 4 composition of the product, generally [10], the synthesis of pure hydrotalcite need x in a range of 0.17 6 x 6 0.34, and m is the number of water molecules located in the interlayer galleries together with the anions. With the growth of x, the number of crystal water decreases [11–13]. Due to the interlayer anions exchange property and the atomic level distribution of metal cations, LDHs have attracted increasing attention in varies fields such as catalysts, scavengers for pollutants, flame retardants, thermal stabilizers, infrared (IR) absorbers, ultraviolet (UV) absorbers, medical materials and as nanofillers in polymer/LDH nanocomposites [14–17]. In recent years, LDHs have been demonstrated as highly efficient adsorbents for removing anionic dyes from wastewater due to their high anionic exchange capacities and high layer charge densities. Both the features favor strong interaction with anion pollutants [9,18,19]. By utilizing the excellent ion exchange property
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of nitrate intercalated LDHs, many anionic dyes such as methyl orange [20], remazol Red 3BS [21], green bezanyl-F2B [22], evans blue [23], acid blue 9 [24], benzopurpurine 4B [25], and acid orangic [26], etc. can be adsorbed and thus removed from the wastewater. However, similar to other types of adsorbent, the resource of the dyes as well as the adsorbents is still a big problem. How to deal with the adsorbent sludge is of great interest to both scientific research and practical application. Since polymer/LDH nanocomposites have been recognized as one of the most promising research field in material chemistry and attracted much attention from both industry and laboratory [27–31], we proposed a new scheme of resource the LDH adsorbent sludge as nanofiller for polymer. The adsorbed dyes molecules can not only increased the compatibility between polymer and LDHs, but also improve the UV adsorption ability. In addition, it is well known that the LDH type nanofiller could improve the thermal stability and tune the rheological property of polymer. In order to prove the viability of using LDHs as efficient adsorbent and the resource of the adsorbent sludge as nanofiller for polymer, in the present work, Zn2Al-NO3 was synthesized and evaluated for the adsorption of an anionic dye acid red 97 (AC97) whose formula is C32H20N4Na2O8S2, as shown in Fig. 1. And the LDH adsorbent sludge Zn2Al-AC97 was added into polypropylene (PP) using a newly developed solvent mixing method by our group. The obtained PP/Zn2Al-AC97 nanocomposites were further characterized and their properties were evaluated in detail. 2. Experimental section 2.1. Synthesis of samples Firstly, Zn2Al-NO3 was synthesized by coprecipitation method. Metal precursor solution containing 7.465 g Zn(NO3)26H2O and 4.7 g Al(NO3)39H2O in 50 ml H2O was added drop-wise into the anion solution containing 2.125 g NaNO3 (0.025 mol) in 50 ml H2O, in the meantime, the pH was kept constant at ca. 10 using a NaOH (4 M) solution. The mixture was aged at room temperature for overnight, followed by centrifuge and washing with water until pH close to 7. The LDH was finally washed with acetone before drying at 65 °C for overnight. The adsorption rates of AC97 over Zn2Al-NO3 were evaluated by placing 0.05 g of the sample in 100 mL of AC97 solution. The initial concentration of AC97 in the solution was 100 mg/L while the initial pH value was about 7. Adsorption was carried out at room temperature with magnetic stirring. The concentration of AC97 was monitored using a visible spectrophotometry (VS) at 500 nm. The resulting LDH sludge was named as Zn2Al-AC97. PP/Zn2Al-AC97 LDH nanocomposites were synthesized using a solvent mixing method. 5 g PP, the acetone washed LDH slurry prepared above and 100 ml xylene were charged into a 250 ml round bottom flask. The amount of LDH added to PP corresponds to 0.2, 0.4, 1, 2, 4, 10 wt%, respectively. The mixture was refluxed at approximately 140 °C for 2 h. After the reflux process was finished, the hot xylene solution containing dissolved PP and highly dispersed LDH nanoparticles was poured into 100 ml hexane (also called a solvent extraction method). The obtained PP/LDH nanocomposites were collected by filtration and dried in vacuum.
Scanning electron microscopy (SEM): SEM analyses were performed on a JEOL JSM 6100 scanning microscope with an accelerating voltage of 20 kV. Powder samples were spread on carbon tape adhered to an SEM stage. Before observation, the samples were sputter coated with a thin platinum layer to prevent charging and to improve the image quality. Thermal gravimetric analysis (TGA): The thermal stability of PP/Zn2Al-AC97 nanocomposites with various LDH loadings was evaluated using a Perkin Elmer diamond TG/DTA analyser from 0 °C to 600 °C with 10 °C/min in the air. Fourier transform infrared spectroscopy (FT-IR): FT-IR spectra were recorded on a FTS 3000 MX FT-IR spectrophotometer in the range of 400–4000 cm1. For each analysis, 100 scans with a resolution of 4 cm1 were collected. UV–Visible: UV–Visible diffuse reflectance spectra were performed on a Lambda 750S UV–Vis spectrometer. The samples were first compressed into thin pellets before testing.
3. Results and discussion 3.1. Characterization of Zn2Al-NO3 LDHs The synthesized Zn2Al-NO3 LDH was first characterized using XRD analysis, as shown in Fig. 2. The 2h peaks close to 11° and 23° are ascribed to the diffractions by basal planes of (0 0 3) and (0 0 6) reflections, denoting their spacings close to 0.78 and 0.38 nm, respectively. The first peak of the doublet close to 2h of 60° is due to direction by planes (1 1 0), and its spacing corresponds to half of the lattice parameter [4]. There were no crystalline by-products such as zinc oxide or metal hydroxides suggesting the purity of synthesized Zn2Al-NO3 LDHs. The morphology of synthesized Zn2Al-NO3 LDH was investigated using FE-SEM. Fig. 3 shows that the original LDH are made up of agglomerated platelet-shaped particles with an average lateral size of about 200 nm. 3.2. Adsorption of AC97 The effect of contact time between the Zn2Al-LDH and AC97 dye solutions is shown in Fig. 4. From which we can see the amount of dye adsorbed increases rapidly within the first 40 min, and then remained nearly the same concentration. An equilibrium state was achieved after 100 min of contact and the adsorption efficiency reached to almost 100% when the concentration of AC97 was 50 mg/L. We can infer that the removal of the adsorbate species is rapid at first and slows down near equilibrium. As it is well known, it is because the large amount of vacant sites availing for adsorption on the surface during the initial time, while the remaining vacant sites are less available for adsorption because of the repulsive forces that occur between the adsorbed and free molecules with time [32].
2.2. Characterization of LDHs and PP/LDH nanocomposites X-ray diffraction (XRD): XRD analysis of Zn2Al-NO3 and PP/Zn2Al-AC97 nanocomposites patterns were recorded on a PANalytical X’Pert Pro instrument in reflection mode with Cu Ka radiation. The accelerating voltage was set at 40 kV with 40 mA current (k = 1.542 Å) at 0.113° s1 from 2° to 70° with a slit size of 1/16 degree.
Fig. 1. Molecular composition of acid red 97.
Fig. 2. XRD analysis of Zn2Al-NO3 LDHs washed with acetone.
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Fig. 3. SEM analysis of Zn2Al-NO3 LDHs washed with acetone. Fig. 5. The adsorption amount of AC97 over acetone and water washed LDHs as a function of time.
transfer process. The results obtained from the experiments were used to study the kinetics of AC97 adsorption. The rate kinetics of AC97 adsorption on LDHs was analyzed using pseudo-first-order, pseudo-second-order, and Elovich models, and the results were shown in Fig. 6(b–d) and Table 1. It is clearly shown that the AC97 adsorption by the water washed Zn2Al-LDH and acetone washed Zn2Al-LDH both followed pseudo-second-order kinetics with a high correlation coefficients, R2 = 0.9845 and 0.9891, respectively. Hence, the pseudo-second-order model better represented the adsorption kinetics, suggesting that the rate-limiting step of AC97 onto LDH may be chemisorption. 3.4. Characterization of Zn2Al-AC97 LDHs and PP/Zn2Al-AC97 LDH nanocomposites Fig. 4. AC97 adsorption efficiency in 2 h with different initial dye concentration.
The adsorption isotherms (Fig. 4) of the AC97 onto the LDHs were determined at initial concentrations from 50 to 150 mg/L at room temperature. It was found that the removal percentage of AC97 gradually decreases with the increase in AC97’s initial concentrations. When the initial concentration of AC97 is low, the removal is very fast and reaches equilibrium within less than 40 min. The removal percentage of AC97 decreases with an increase in initial dye concentration. That is because the ratio of initial mole numbers of AC97 to the available surface area is high at a higher initial concentration. For a given adsorbent dose the total numbers of available adsorption sites are fixed thereby adsorbing almost the same amount of dyes [33]. Furthermore, 0.05 g water washed Zn2Al-LDHs and acetone washed Zn2Al-LDHs are thrown into 250 ml (200 mg/L) AC97 solution in order to find out the capacity of adsorption. Fig. 5 shows the adsorption amount of AC97 over water and acetone washed Zn2Al-NO3 LDHs as a function of time. It is obvious that both the adsorption capacity and the adsorption kinetics for acetone washed sample are much faster than those of water washed samples. After 3 h adsorption, the saturated adsorption of water washed Zn2Al-LDHs is 204.4 mg/g, and the value for acetone washed Zn2Al-LDHs is 299.5 mg/g. The larger adsorption capacity is due to acetone wash step can makes LDHs more highly dispersed and have bigger surface area. 3.3. Adsorption kinetics The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass
Fig. 7 shows the FT-IR spectra of Zn2Al-NO3 LDHs, AC97, and Zn2Al-AC97 LDHs. The absorption band around 3446 cm1 shown in the spectrum of the Zn2Al -NO3 LDHs precursor can be assigned to the stretching vibration of the hydroxyl groups of LDH layers and interlayer water molecules. The sharp band around 1384 cm1 corresponds to the stretching vibration of NO3 groups [27]. The peaks at 626 and 544 cm1 can be attributed to Zn-OH and Al-OH vibrations [34]. As shown in Fig. 6b, some characteristic peaks of AC97 were observed in the spectrum, a broad band attributed to the AOH stretching vibration overlapping with the NAH stretching vibration can be observed at around 3404 cm1. The absorption bands at 1619, 1555 and 1499 cm1 correspond to the characteristic vibration bands of phenyl groups. The asymmetric and symmetric stretching vibrations of the ASO 3 group appear at 1192 and 1029 cm1, respectively. Fig. 6c depicts the FT-IR spectrum of Zn2Al-AC97 LDH. The data demonstrates the characteristic features of LDH-like materials together with the characteristic frequencies associated with the presence of AC97 anions. The broad band centered at around 3452 cm1 is due to the OH stretching vibration of interlayer water, hydroxyl groups on the layers and AG282 anions and the NH stretching vibration of AC97 anions. The absorption bands at 1619, 1555 and 1499 cm1 is assigned to the vibration of phenyl groups. The absorption bands of the asymmetric and symmetric stretching vibrations of the ASO 3 group appear at 1192 and 1029 cm1, respectively. The characteristic absorption bands of LDH materials are observed in the low frequency region. The M-OH bending vibration can be observed at 614 and 544 cm1 [34]. The PP/Zn2Al-AC97 LDH nanocomposites were characterized by powder XRD (Fig. 8). In the lower concentration range (0.2–2%), the
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Fig. 6. The adsorption amount of AC97 over acetone and water washed LDHs (a) and the data fitting by pseudo-first-order (b), pseudo-second-order (c) and Elovich (d) models. Table 1 Kinetic constants for AC97 adsorption onto the raw water washed Zn2Al LDH and acetone washed Zn2Al LDH analyzed by pseudo-first-order, pseudo-second-order and Elovich models.
14000
LDH
12000
Pseudo-first-order Pseudo-secondorder Elovich
Equation
Water washed
Acetone washed
dqt/dt = k1(qeqt) dqt/dt = k2(qeqt)2
R2 = 0.9213 R2 = 0.9845
R2 = 0.8014 R2 = 0.9891
dqt/ dt = aexp(bqt)
R2 = 0.9777
R2 = 0.9667
10 % 10000
Intensity
Model
4% 8000
2% 6000
1% 4000
0.4 % 2000
0.2 %
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
2 theta (degree) Fig. 8. XRD analysis of PP/Zn2Al-AC97 LDH nanocomposites.
Fig. 7. FT-IR curves for (a) Zn2Al-NO3 LDHs, (b) AC97 and (c) Zn2Al-AC97 LDHs.
Zn2Al-AC97 crystalline phase could not be detected, which suggests the well distribution of LDH within PP. With the increase of LDH loading, the characteristic reflection gradually appeared,
suggesting that the LDH nanoplates were successfully introduced into PP matrix [28,35]. In order to study the morphology of obtained PP/Zn2Al-AC97 nanocomposites, SEM analysis for PP/Zn2Al-AC97 nanocomposites with various LDH loadings of 0.2, 2, 4, and 10 wt% was performed, as shown in Fig. 9. Spherical particles were formed for all the samples, which is caused by the rapid precipitation of the polymer composite in hexane. When the loading is low (0.2–1 wt%), the aggregated LDH particles can be seldom seen, which suggests that there is a good dispersion of LDH particles within the PP matrix. However, with the increase of LDH loading from 2 to 10 wt%, the smoothness of the nanocomposite surface declines and more LDH nanoparticles can be clearly seen.
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Fig. 9. SEM images of (a) 0.2 wt%, (b) 2 wt%, (c) 4 wt% and (d) 10 wt% PP/Zn2Al-AC97 nanocomposites.
2.0 0.2 wt% 0.6 wt% 1 wt% 2 wt% 4 wt% 10 wt%
Absorbance(%)
1.6
1.2
0.8
0.4
0.0 300
400
500
600
700
800
wavelength (nm) Fig. 10. UV absorption of PP/Zn2Al-AC97 nanocomposites with various LDH loadings (0.2, 0.6, 1, 2, 4, and 10 wt%).
3.5. Performance tests of PP/LDH nanocomposites The UV absorption capacity of synthesized PP/LDH nanocomposites with various LDH loadings (0.2, 0.6, 1, 2, 4, 10 wt%) were first evaluated, as shown in Fig. 10. With 0.2 wt% LDH loading, the nanocomposite has a very poor UV absorption capacity. However, due to the increase of interlayer intercalated AC97 anions, PP/Zn2Al-AC97 LDH nanocomposites showed excellent UV absorption in the whole range of 200–600 nm. After introducing Zn2AlAC97 LDH, all nanocomposites showed significantly enhanced UV-shielding ability, and the UV absorbance clearly increased with the increase in LDH loadings from 0.2 to 10 wt%. This result clearly demonstrated that the UV shielding ability of PP can be markedly enhanced by introducing small amount of Zn2Al-AC97 LDH.
Fig. 11. TGA curves for PP/Zn2Al-AC97 nanocomposites with different loadings.
The thermal stability of PP/Zn2Al-AC97 nanocomposites with different loadings was tested using TGA (Fig. 11) in air. The 10% weight loss temperature (T0.1) data and 50% weight loss temperature (T0.5) data clearly indicate that the thermal stability of the Zn2Al-AC97 LDH loaded PP nanocomposites is significantly enhanced comparing to pure PP. With the increase of LDH loading from 0.4 to 10 wt%, the T0.5 gradually increased. Comparing to the neat PP whose T0.5 is ca. 361.2 °C. The T0.5 of the sample with 10 wt% Zn2Al-AC97 was increased by 22.1 °C. It is suggested that the barrier properties of the polymer nanocomposites, which include both thermal barrier and transport barrier, are responsible for the observed thermal property enhancement [35,36]. This data suggests that it is promising to use the LDH adsorbent sludge as nanofiller for polymers and has great potential for practical applications.
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4. Conclusions In summary, the Zn2Al-NO3 LDH synthesized by coprecipitation method is a kind of efficient sorbents for AC97. The prepared LDHs have been demonstrated to be highly effective and economic for the removal of the anionic dye AC97 with an adsorption equilibrium time of less than 50 min and maximum adsorption capacity of 299.5 mg/g. The acetone washed Zn2Al-LDHs showed a higher adsorption capacity toward AC97 than water washed Zn2Al-LDHs. The adsorption kinetics followed the pseudo-second-order model, suggesting that the rate-limiting step of AC97 onto LDH may be chemisorption. After adsorption, the possibility of resourcing the LDH adsorbent sludge was demonstrated by synthesizing PP/Zn2Al-AC97 LDH nanocomposites. XRD and SEM analyses indicated that LDH was successfully introduced into PP with a good dispersion. UV analysis demonstrated that the UV shielding ability of PP can be markedly enhanced by introducing small amount of Zn2Al-AC97 LDH. TGA analysis indicated that the thermal stability can be significantly improved specifically the loading is more than 0.4%. Thus, it is very promising to resource the LDH adsorbent sludge as nanofiller for polymers and has great potential for practical applications. Acknowledgements This work is supported by the Fundamental Research Funds for the Central Universities (TD-JC-2013-3), the Beijing Nova Programme (Z131109000413013), the Program for New Century Excellent Talents in University (NCET-12-0787), the National Natural Science Foundation of China (51308045), the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University). References [1] G. Ovejero, A. Rodríguez, A. Vallet, J. García, Chem. Eng. J. 215–216 (2013) 168. [2] G. Ovejero, A. Rodríguez, A. Vallet, J. García, Appl. Catal. B: Environ. 125 (2012) 166.
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Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Adsorption of acid red from dye wastewater by Zn2Al-NO3 LDHs and the resource of adsorbent sludge as nanofiller for polypropylene Tianshan Xue a, Yanshan Gao a, Zhang Zhang a, Ahmad Umar b,c,⇑, Xingru Yan d, Xi Zhang d, Zhanhu Guo d, Qiang Wang a,⇑ a
College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, PR China Department of Chemistry, College of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia Promising Centre for Sensors and Electronic Devices (PCSED), Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia d Integrated Composites Laboratory, Dan F Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA b c
a r t i c l e
i n f o
Article history: Received 31 August 2013 Received in revised form 13 October 2013 Accepted 19 October 2013 Available online 30 October 2013 Keywords: Wastewater treatment Hydrotalcite Ion exchange Polymer nanocomposite UV absorption
a b s t r a c t In this contribution, we report the removal of acid red 97 (AC97) from simulated dye wastewater by using Zn2Al-NO3 layered double hydroxides (LDHs) adsorbent, and the resource of the LDH adsorbent sludge as nanofiller for polypropylene (PP) for the first time. The obtained Zn2Al-NO3 LDH was analyzed using X-ray diffraction and scanning electron microscopy analysis, confirming the formation of pure and platelike LDH nanoparticles. The effects of adsorption time and initial dye concentration on the removal of AC97 from wastewater were systematically investigated, showing that the Zn2Al-NO3 LDHs is very efficient in removing AC97. The saturated adsorption capacity of water washed and acetone washed Zn2Al-LDHs is 204.4 and 299.5 mg/g, respectively. Finally, the LDH adsorbent sludge was added into PP using a modified solvent mixing method. Thermal gravimetric analysis and ultraviolet (UV) absorption analysis of PP/Zn2Al-AC97 LDHs nanocomposites suggested that the Zn2Al-AC97 LDH can significantly improve the thermal stability and UV shielding ability of PP. This data demonstrated that it is very promising to resource the dye adsorbent sludge as multifunctional nanofiller for polymers. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Although printing and dyeing is an important industry for national economy, the wastewater produced from it has been considered as one of the most important pollutants and its disposal has become a major environmental problem [1–3]. With large volume, high organic content, dark color, complex substances and other features, dye wastewater can directly damage the water bodies and threaten our human health [4]. Therefore, there is an increasing demand for efficient treatment of the dye wastewater [5]. Up to date, there are various methods such as coagulating sedimentation, filtration, electro-coagulation, and adsorption by solid adsorbent have been developed to remove the dyes from wastewaters [6]. Among various water treatment technologies, adsorption has been found to be an effective and inexpensive treatment technology due to its efficiency, simplicity, and applicability. Granular activated carbon [7], activated diatomite [8], and resins [9] are ⇑ Corresponding authors. Address: Department of Chemistry, College of Science and Arts, Najran University, P.O. Box 1988, Najran 11001, Saudi Arabia (A. Umar), College of Environmental Science and Engineering, Beijing Forestry University, 35 Qinghua East Road, Haidian District, Beijing 100083, PR China (Q. Wang). Tel.: +86 13699130626. E-mail addresses: [email protected] (A. Umar), qiang.wang.ox@gmail. com, [email protected] (Q. Wang). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.10.158
the most commonly used adsorbents. The main problems of solid adsorption method are the relatively low adsorption capacity and the difficulty to reuse. Layered double hydroxides (LDHs) are a widely used type of anzþ n 3þ ionic clays with the general formula ½M2þ 1x M x ðOHÞ2 ½A z=n mH2O, 2+ 3+ in which M and M are metal positive ions such as Mg2+, Zn2+, Ni2+, Co2+, Cu2+, Ca2+, Mn2+ and A13+, Cr3+, Co3+ or Fe3+, An stands 2 3 for inorganic anions such as CO2 3 , NO3 , F , Cl , Br , I , CrO4 , PO4 , SO2 , or various organic anions, the value of x directly affect the 4 composition of the product, generally [10], the synthesis of pure hydrotalcite need x in a range of 0.17 6 x 6 0.34, and m is the number of water molecules located in the interlayer galleries together with the anions. With the growth of x, the number of crystal water decreases [11–13]. Due to the interlayer anions exchange property and the atomic level distribution of metal cations, LDHs have attracted increasing attention in varies fields such as catalysts, scavengers for pollutants, flame retardants, thermal stabilizers, infrared (IR) absorbers, ultraviolet (UV) absorbers, medical materials and as nanofillers in polymer/LDH nanocomposites [14–17]. In recent years, LDHs have been demonstrated as highly efficient adsorbents for removing anionic dyes from wastewater due to their high anionic exchange capacities and high layer charge densities. Both the features favor strong interaction with anion pollutants [9,18,19]. By utilizing the excellent ion exchange property
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of nitrate intercalated LDHs, many anionic dyes such as methyl orange [20], remazol Red 3BS [21], green bezanyl-F2B [22], evans blue [23], acid blue 9 [24], benzopurpurine 4B [25], and acid orangic [26], etc. can be adsorbed and thus removed from the wastewater. However, similar to other types of adsorbent, the resource of the dyes as well as the adsorbents is still a big problem. How to deal with the adsorbent sludge is of great interest to both scientific research and practical application. Since polymer/LDH nanocomposites have been recognized as one of the most promising research field in material chemistry and attracted much attention from both industry and laboratory [27–31], we proposed a new scheme of resource the LDH adsorbent sludge as nanofiller for polymer. The adsorbed dyes molecules can not only increased the compatibility between polymer and LDHs, but also improve the UV adsorption ability. In addition, it is well known that the LDH type nanofiller could improve the thermal stability and tune the rheological property of polymer. In order to prove the viability of using LDHs as efficient adsorbent and the resource of the adsorbent sludge as nanofiller for polymer, in the present work, Zn2Al-NO3 was synthesized and evaluated for the adsorption of an anionic dye acid red 97 (AC97) whose formula is C32H20N4Na2O8S2, as shown in Fig. 1. And the LDH adsorbent sludge Zn2Al-AC97 was added into polypropylene (PP) using a newly developed solvent mixing method by our group. The obtained PP/Zn2Al-AC97 nanocomposites were further characterized and their properties were evaluated in detail. 2. Experimental section 2.1. Synthesis of samples Firstly, Zn2Al-NO3 was synthesized by coprecipitation method. Metal precursor solution containing 7.465 g Zn(NO3)26H2O and 4.7 g Al(NO3)39H2O in 50 ml H2O was added drop-wise into the anion solution containing 2.125 g NaNO3 (0.025 mol) in 50 ml H2O, in the meantime, the pH was kept constant at ca. 10 using a NaOH (4 M) solution. The mixture was aged at room temperature for overnight, followed by centrifuge and washing with water until pH close to 7. The LDH was finally washed with acetone before drying at 65 °C for overnight. The adsorption rates of AC97 over Zn2Al-NO3 were evaluated by placing 0.05 g of the sample in 100 mL of AC97 solution. The initial concentration of AC97 in the solution was 100 mg/L while the initial pH value was about 7. Adsorption was carried out at room temperature with magnetic stirring. The concentration of AC97 was monitored using a visible spectrophotometry (VS) at 500 nm. The resulting LDH sludge was named as Zn2Al-AC97. PP/Zn2Al-AC97 LDH nanocomposites were synthesized using a solvent mixing method. 5 g PP, the acetone washed LDH slurry prepared above and 100 ml xylene were charged into a 250 ml round bottom flask. The amount of LDH added to PP corresponds to 0.2, 0.4, 1, 2, 4, 10 wt%, respectively. The mixture was refluxed at approximately 140 °C for 2 h. After the reflux process was finished, the hot xylene solution containing dissolved PP and highly dispersed LDH nanoparticles was poured into 100 ml hexane (also called a solvent extraction method). The obtained PP/LDH nanocomposites were collected by filtration and dried in vacuum.
Scanning electron microscopy (SEM): SEM analyses were performed on a JEOL JSM 6100 scanning microscope with an accelerating voltage of 20 kV. Powder samples were spread on carbon tape adhered to an SEM stage. Before observation, the samples were sputter coated with a thin platinum layer to prevent charging and to improve the image quality. Thermal gravimetric analysis (TGA): The thermal stability of PP/Zn2Al-AC97 nanocomposites with various LDH loadings was evaluated using a Perkin Elmer diamond TG/DTA analyser from 0 °C to 600 °C with 10 °C/min in the air. Fourier transform infrared spectroscopy (FT-IR): FT-IR spectra were recorded on a FTS 3000 MX FT-IR spectrophotometer in the range of 400–4000 cm1. For each analysis, 100 scans with a resolution of 4 cm1 were collected. UV–Visible: UV–Visible diffuse reflectance spectra were performed on a Lambda 750S UV–Vis spectrometer. The samples were first compressed into thin pellets before testing.
3. Results and discussion 3.1. Characterization of Zn2Al-NO3 LDHs The synthesized Zn2Al-NO3 LDH was first characterized using XRD analysis, as shown in Fig. 2. The 2h peaks close to 11° and 23° are ascribed to the diffractions by basal planes of (0 0 3) and (0 0 6) reflections, denoting their spacings close to 0.78 and 0.38 nm, respectively. The first peak of the doublet close to 2h of 60° is due to direction by planes (1 1 0), and its spacing corresponds to half of the lattice parameter [4]. There were no crystalline by-products such as zinc oxide or metal hydroxides suggesting the purity of synthesized Zn2Al-NO3 LDHs. The morphology of synthesized Zn2Al-NO3 LDH was investigated using FE-SEM. Fig. 3 shows that the original LDH are made up of agglomerated platelet-shaped particles with an average lateral size of about 200 nm. 3.2. Adsorption of AC97 The effect of contact time between the Zn2Al-LDH and AC97 dye solutions is shown in Fig. 4. From which we can see the amount of dye adsorbed increases rapidly within the first 40 min, and then remained nearly the same concentration. An equilibrium state was achieved after 100 min of contact and the adsorption efficiency reached to almost 100% when the concentration of AC97 was 50 mg/L. We can infer that the removal of the adsorbate species is rapid at first and slows down near equilibrium. As it is well known, it is because the large amount of vacant sites availing for adsorption on the surface during the initial time, while the remaining vacant sites are less available for adsorption because of the repulsive forces that occur between the adsorbed and free molecules with time [32].
2.2. Characterization of LDHs and PP/LDH nanocomposites X-ray diffraction (XRD): XRD analysis of Zn2Al-NO3 and PP/Zn2Al-AC97 nanocomposites patterns were recorded on a PANalytical X’Pert Pro instrument in reflection mode with Cu Ka radiation. The accelerating voltage was set at 40 kV with 40 mA current (k = 1.542 Å) at 0.113° s1 from 2° to 70° with a slit size of 1/16 degree.
Fig. 1. Molecular composition of acid red 97.
Fig. 2. XRD analysis of Zn2Al-NO3 LDHs washed with acetone.
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Fig. 3. SEM analysis of Zn2Al-NO3 LDHs washed with acetone. Fig. 5. The adsorption amount of AC97 over acetone and water washed LDHs as a function of time.
transfer process. The results obtained from the experiments were used to study the kinetics of AC97 adsorption. The rate kinetics of AC97 adsorption on LDHs was analyzed using pseudo-first-order, pseudo-second-order, and Elovich models, and the results were shown in Fig. 6(b–d) and Table 1. It is clearly shown that the AC97 adsorption by the water washed Zn2Al-LDH and acetone washed Zn2Al-LDH both followed pseudo-second-order kinetics with a high correlation coefficients, R2 = 0.9845 and 0.9891, respectively. Hence, the pseudo-second-order model better represented the adsorption kinetics, suggesting that the rate-limiting step of AC97 onto LDH may be chemisorption. 3.4. Characterization of Zn2Al-AC97 LDHs and PP/Zn2Al-AC97 LDH nanocomposites Fig. 4. AC97 adsorption efficiency in 2 h with different initial dye concentration.
The adsorption isotherms (Fig. 4) of the AC97 onto the LDHs were determined at initial concentrations from 50 to 150 mg/L at room temperature. It was found that the removal percentage of AC97 gradually decreases with the increase in AC97’s initial concentrations. When the initial concentration of AC97 is low, the removal is very fast and reaches equilibrium within less than 40 min. The removal percentage of AC97 decreases with an increase in initial dye concentration. That is because the ratio of initial mole numbers of AC97 to the available surface area is high at a higher initial concentration. For a given adsorbent dose the total numbers of available adsorption sites are fixed thereby adsorbing almost the same amount of dyes [33]. Furthermore, 0.05 g water washed Zn2Al-LDHs and acetone washed Zn2Al-LDHs are thrown into 250 ml (200 mg/L) AC97 solution in order to find out the capacity of adsorption. Fig. 5 shows the adsorption amount of AC97 over water and acetone washed Zn2Al-NO3 LDHs as a function of time. It is obvious that both the adsorption capacity and the adsorption kinetics for acetone washed sample are much faster than those of water washed samples. After 3 h adsorption, the saturated adsorption of water washed Zn2Al-LDHs is 204.4 mg/g, and the value for acetone washed Zn2Al-LDHs is 299.5 mg/g. The larger adsorption capacity is due to acetone wash step can makes LDHs more highly dispersed and have bigger surface area. 3.3. Adsorption kinetics The mechanism of adsorption depends on the physical and chemical characteristics of the adsorbent as well as on the mass
Fig. 7 shows the FT-IR spectra of Zn2Al-NO3 LDHs, AC97, and Zn2Al-AC97 LDHs. The absorption band around 3446 cm1 shown in the spectrum of the Zn2Al -NO3 LDHs precursor can be assigned to the stretching vibration of the hydroxyl groups of LDH layers and interlayer water molecules. The sharp band around 1384 cm1 corresponds to the stretching vibration of NO3 groups [27]. The peaks at 626 and 544 cm1 can be attributed to Zn-OH and Al-OH vibrations [34]. As shown in Fig. 6b, some characteristic peaks of AC97 were observed in the spectrum, a broad band attributed to the AOH stretching vibration overlapping with the NAH stretching vibration can be observed at around 3404 cm1. The absorption bands at 1619, 1555 and 1499 cm1 correspond to the characteristic vibration bands of phenyl groups. The asymmetric and symmetric stretching vibrations of the ASO 3 group appear at 1192 and 1029 cm1, respectively. Fig. 6c depicts the FT-IR spectrum of Zn2Al-AC97 LDH. The data demonstrates the characteristic features of LDH-like materials together with the characteristic frequencies associated with the presence of AC97 anions. The broad band centered at around 3452 cm1 is due to the OH stretching vibration of interlayer water, hydroxyl groups on the layers and AG282 anions and the NH stretching vibration of AC97 anions. The absorption bands at 1619, 1555 and 1499 cm1 is assigned to the vibration of phenyl groups. The absorption bands of the asymmetric and symmetric stretching vibrations of the ASO 3 group appear at 1192 and 1029 cm1, respectively. The characteristic absorption bands of LDH materials are observed in the low frequency region. The M-OH bending vibration can be observed at 614 and 544 cm1 [34]. The PP/Zn2Al-AC97 LDH nanocomposites were characterized by powder XRD (Fig. 8). In the lower concentration range (0.2–2%), the
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Fig. 6. The adsorption amount of AC97 over acetone and water washed LDHs (a) and the data fitting by pseudo-first-order (b), pseudo-second-order (c) and Elovich (d) models. Table 1 Kinetic constants for AC97 adsorption onto the raw water washed Zn2Al LDH and acetone washed Zn2Al LDH analyzed by pseudo-first-order, pseudo-second-order and Elovich models.
14000
LDH
12000
Pseudo-first-order Pseudo-secondorder Elovich
Equation
Water washed
Acetone washed
dqt/dt = k1(qeqt) dqt/dt = k2(qeqt)2
R2 = 0.9213 R2 = 0.9845
R2 = 0.8014 R2 = 0.9891
dqt/ dt = aexp(bqt)
R2 = 0.9777
R2 = 0.9667
10 % 10000
Intensity
Model
4% 8000
2% 6000
1% 4000
0.4 % 2000
0.2 %
0
5 10 15 20 25 30 35 40 45 50 55 60 65 70
2 theta (degree) Fig. 8. XRD analysis of PP/Zn2Al-AC97 LDH nanocomposites.
Fig. 7. FT-IR curves for (a) Zn2Al-NO3 LDHs, (b) AC97 and (c) Zn2Al-AC97 LDHs.
Zn2Al-AC97 crystalline phase could not be detected, which suggests the well distribution of LDH within PP. With the increase of LDH loading, the characteristic reflection gradually appeared,
suggesting that the LDH nanoplates were successfully introduced into PP matrix [28,35]. In order to study the morphology of obtained PP/Zn2Al-AC97 nanocomposites, SEM analysis for PP/Zn2Al-AC97 nanocomposites with various LDH loadings of 0.2, 2, 4, and 10 wt% was performed, as shown in Fig. 9. Spherical particles were formed for all the samples, which is caused by the rapid precipitation of the polymer composite in hexane. When the loading is low (0.2–1 wt%), the aggregated LDH particles can be seldom seen, which suggests that there is a good dispersion of LDH particles within the PP matrix. However, with the increase of LDH loading from 2 to 10 wt%, the smoothness of the nanocomposite surface declines and more LDH nanoparticles can be clearly seen.
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Fig. 9. SEM images of (a) 0.2 wt%, (b) 2 wt%, (c) 4 wt% and (d) 10 wt% PP/Zn2Al-AC97 nanocomposites.
2.0 0.2 wt% 0.6 wt% 1 wt% 2 wt% 4 wt% 10 wt%
Absorbance(%)
1.6
1.2
0.8
0.4
0.0 300
400
500
600
700
800
wavelength (nm) Fig. 10. UV absorption of PP/Zn2Al-AC97 nanocomposites with various LDH loadings (0.2, 0.6, 1, 2, 4, and 10 wt%).
3.5. Performance tests of PP/LDH nanocomposites The UV absorption capacity of synthesized PP/LDH nanocomposites with various LDH loadings (0.2, 0.6, 1, 2, 4, 10 wt%) were first evaluated, as shown in Fig. 10. With 0.2 wt% LDH loading, the nanocomposite has a very poor UV absorption capacity. However, due to the increase of interlayer intercalated AC97 anions, PP/Zn2Al-AC97 LDH nanocomposites showed excellent UV absorption in the whole range of 200–600 nm. After introducing Zn2AlAC97 LDH, all nanocomposites showed significantly enhanced UV-shielding ability, and the UV absorbance clearly increased with the increase in LDH loadings from 0.2 to 10 wt%. This result clearly demonstrated that the UV shielding ability of PP can be markedly enhanced by introducing small amount of Zn2Al-AC97 LDH.
Fig. 11. TGA curves for PP/Zn2Al-AC97 nanocomposites with different loadings.
The thermal stability of PP/Zn2Al-AC97 nanocomposites with different loadings was tested using TGA (Fig. 11) in air. The 10% weight loss temperature (T0.1) data and 50% weight loss temperature (T0.5) data clearly indicate that the thermal stability of the Zn2Al-AC97 LDH loaded PP nanocomposites is significantly enhanced comparing to pure PP. With the increase of LDH loading from 0.4 to 10 wt%, the T0.5 gradually increased. Comparing to the neat PP whose T0.5 is ca. 361.2 °C. The T0.5 of the sample with 10 wt% Zn2Al-AC97 was increased by 22.1 °C. It is suggested that the barrier properties of the polymer nanocomposites, which include both thermal barrier and transport barrier, are responsible for the observed thermal property enhancement [35,36]. This data suggests that it is promising to use the LDH adsorbent sludge as nanofiller for polymers and has great potential for practical applications.
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4. Conclusions In summary, the Zn2Al-NO3 LDH synthesized by coprecipitation method is a kind of efficient sorbents for AC97. The prepared LDHs have been demonstrated to be highly effective and economic for the removal of the anionic dye AC97 with an adsorption equilibrium time of less than 50 min and maximum adsorption capacity of 299.5 mg/g. The acetone washed Zn2Al-LDHs showed a higher adsorption capacity toward AC97 than water washed Zn2Al-LDHs. The adsorption kinetics followed the pseudo-second-order model, suggesting that the rate-limiting step of AC97 onto LDH may be chemisorption. After adsorption, the possibility of resourcing the LDH adsorbent sludge was demonstrated by synthesizing PP/Zn2Al-AC97 LDH nanocomposites. XRD and SEM analyses indicated that LDH was successfully introduced into PP with a good dispersion. UV analysis demonstrated that the UV shielding ability of PP can be markedly enhanced by introducing small amount of Zn2Al-AC97 LDH. TGA analysis indicated that the thermal stability can be significantly improved specifically the loading is more than 0.4%. Thus, it is very promising to resource the LDH adsorbent sludge as nanofiller for polymers and has great potential for practical applications. Acknowledgements This work is supported by the Fundamental Research Funds for the Central Universities (TD-JC-2013-3), the Beijing Nova Programme (Z131109000413013), the Program for New Century Excellent Talents in University (NCET-12-0787), the National Natural Science Foundation of China (51308045), the Key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University). References [1] G. Ovejero, A. Rodríguez, A. Vallet, J. García, Chem. Eng. J. 215–216 (2013) 168. [2] G. Ovejero, A. Rodríguez, A. Vallet, J. García, Appl. Catal. B: Environ. 125 (2012) 166.
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