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A green, porous and eco-friendly magnetic geopolymer adsorbent for heavy metals removal from aqueous solutions
Journal of Cleaner Production 215 (2019) 1233e1245
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
A green, porous and eco-friendly magnetic geopolymer adsorbent for heavy metals removal from aqueous solutions Ali Maleki*, Zoleikha Hajizadeh, Vajiheh Sharifi, Zeynab Emdadi Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2018 Received in revised form 6 January 2019 Accepted 8 January 2019 Available online 9 January 2019
Geopolymers are a class of synthesized amorphous aluminosilicate materials that can be used as an adsorbent for the removal of heavy metals. In this paper, the bentonite clay was employed to synthesize geopolymer that can remove heavy metals such as Cu(II), Pb(II), Ni(II), Cd(II), and Hg(II) from industrial wastewaters. The Fe3O4 nanoparticles were applied to modify the geopolymer and the use of a geopolymer/Fe3O4 nanocomposite as an efficient and magnetic adsorbent for heavy metals removal from aqueous solution was investigated in this work, for the first time. The influence of different contact time and initial concentrations of metal ions on sorption was examined and the best result was achieved in 2 min contact time in the presence of 0.05 g nanocomposite. The prepared geopolymer and nanocomposite samples were characterized by Fourier transform infrared spectroscopy spectra, fieldemission scanning electron micrograph images, thermogravimetric analysis, energy dispersive X-ray analysis, X-ray diffraction pattern and Brunauer-Emmett-Teller analysis. The prepared magnetic geopolymer base on bentonite clay showed 99%, 99%, 92%, 96% and 92% removal efficiency for the sorption of copper, lead, nickel, cadmium, and mercury ions from industrial wastewaters. The present work includes diverse advantageous such as environmentally-friendly protocol, magnetic separation, inexpensive raw materials, easy and simple conditions and high yields as same as short adsorption times. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Water treatment Geopolymer Adsorbent Bentonite clay Heavy metals Green chemistry
1. Introduction Geopolymer, in general is defined as a solid and stable aluminosilicate material. It was formed by alkali hydroxide or alkali silicate activation of a precursor that is usually supplied as a solid powder (Provis and Deventer, 2009). On a fundamental level, the synthesis of geopolymer comprise dissolution of silica, alumina, and aluminosilicate using alkali solution (Zhuang et al., 2016). In recent years, the waste management via green chemical protocols ~o et al., 2017; Perna and has been rapidly developed (Ascensa Hanzlí cek, 2014). The geopolymer materials are regarded as environmentally friendly due to their low temperature of manufacturing (<100 C) and lower CO2 emission compare to standard cement (Duxson et al., 2007). Although the most study on geopolymer is focused on the field of concrete building materials application, recently they are considered as a suitable replacement for aqueous solutions adsorption application (Duan et al., 2016; Javadian et al., 2015; Qiu et al., 2018).
* Corresponding author. E-mail address: [email protected] (A. Maleki). https://doi.org/10.1016/j.jclepro.2019.01.084 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
In some countries, existing different type of pollution such as heavy metals (Saleh and Gupta, 2014), radionuclide (Gu et al., 2018) and dyes (Mosleh et al., 2018) in wastewater negatively affect the environment. As a result, different materials and nanocomposite such as ZnO/Ag/CdO (Saravanan et al., 2015a,b), ZnO/Ag/Mn2O3 (Saravanan et al., 2015a,b), and activated carbon (Asfaram et al., 2015) were suggested for removal different kinds of environmental pollution. Also, some methods like Fenton oxidation (Karthikeyan et al., 2012) and photodegradation (Saravanan et al., 2013) were used in recent years. The use of heavy metals is increasing in the industry, which leads to environmental pollution (Ahmaruzzaman and Gupta, 2011). In the other word, heavy metals toxicity in the environment is caused by factory emission, national atmospheric deposition, waste water, mining, and livestock manure applications (Wei and Yang, 2010; Xia et al., 2011). Furthermore, in developing countries, due to increase activity in local and farming fields, the large amount of toxic heavy metals penetrated into soil and water (Rahman et al., 2010; Sharma et al., 2007; Solaraj et al., 2010). Therefore, according to the potential toxic effects and the risk of bioaccumulation in an aquatic ecosystem, heavy metals are global
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Table 1 The different effective parameters on synthesized samples. Parameters Type of aluminosilicate Type of alkaline solution The ratio of SiO2/Al2O3
Bentonite NaOH Na2SiO3 2
The ratio of Na2O/Al2O3 Calcination temperature Temperature drying
1 700e800 70e75
Properties
Reference
The bentonite clay was selected for its natural porosity and high surface area. Was selected as an alkali activator due to their more availability and cheap price. The ratio of Si/Al has different effects on geopolymer properties. This ratio was selected according to literature review for adsorption application based on geopolymer as an adsorbent. The ratio of Al2O3/Na2O was chosen according to literature. Chosen as TGA analysis Suitable temperature for geopolymer synthesis based on scholars investigation
Provis and Deventer (2009) Provis and Deventer (2009) Davidovits (2002)
concern (Batvari et al., 2015; David et al., 2012). Heavy metals in aqueous solutions, especially wastewater, can endanger human health (Terra et al., 2008). Copper, cadmium, lead, and mercury are examples of toxic metals that will are ubiquitous in industries (Demirbas et al., 2009; Lalhruaitluanga et al., 2010; Qiu et al., 2018). Some heavy metals are essential nutrients at very small amounts (<5 mg/L). However, at higher dosages, they are toxic and could lead to health problems (Awual et al., 2014). Different materials such as MOF (Li et al., 2018), boron nitride (Yu et al., 2018), ZnO nanoparticles (Jafari et al., 2017), CuO nanoparticles (Dashamiri et al., 2016), active carbon (Jamshidi et al., 2015) and graphene oxide (Robati et al., 2016) can be used as a adsorbents for water contamination. Subsequently, there are many approaches that can be used to remove heavy metals from aqueous solutions. Geopolymer adsorbents are one of the important classes in this regard. There are some advantages to use these approaches, such as low cost, high efficiency and simple production (Azad et al., 2015; Lee and Tiwari, 2013). There are three common aluminosilicate materials that can be used for geopolymers synthesis, which are fly ash, slags, and calcined clay. Calcined clay is favored due to its accessibility and hydrophilicity (Provis and Deventer, 2009). Bentonite as naturally available clay can be accessible inexpensively (Bhattacharyya and Gupta, 2008). Also, due to its unique features such as high surface area, high Al2O3/SiO2 ratio, high amorphous content, chemically stability, high porosity and high capacity of cation-exchange, it can be used in the different applications (Bhatt et al., 2012; Patel et al., 2010; Wang et al., 2009). Nowadays, magnetic nanoparticles have received considerable attentions because of their potential applications in various industries (Maleki et al., 2018a; Hajizadeh and Maleki, 2018b). Due to their superior features such as high surface-to-volume ratio, high stability, low preparation cost, and easy separation are applied in different applications (Maleki et al., 2018b; Maleki et al., 2019a). Moreover, their aggregation problem was resolved by composite with material like silica and polymers (Maleki, 2012, 2013; Maleki et al., 2018c, 2019b). Therefore, diverse nanocomposites were synthesized by scientists in recent years (Maleki, 2018a, 2018b). Owing to the importance of metal adsorption and treatment of industrial wastewater, the magnetic geopolymer nanocomposite was proposed for the first time as a heterogeneous adsorbent. This porous adsorbent was synthesized in two steps. At first, the porous geopolymer was prepared easily, and then, the geopolymer/Fe3O4
Provis and Deventer (2009) This work Davidovits (2002)
nanocomposite was synthesized via the intercalation method (White et al., 2011; Khan et al., 2016). In this process, the geopolymer inner surface hydroxyl groups were hydrogen bonded to the hydroxyl groups of Fe3O4 and magnetic nanoparticles were intercalated into the geopolymer layers. The main objective is to use this geopolymer composite for wastewater purification due to its environmental friendliness, inexpensive production process, easy separation, and high efficiency in the shortest time via green chemistry principles. In other words, the aim of this research is introducing a green and efficient protocol for removal of a wide range of toxic heavy metals from various water and wastewaters by easy and inexpensive magnetic nanocomposites based on natural and available reactants in the shortest time.
2. Materials and methods 2.1. Materials and instruments Bentonite nanoclay (Bentonite Aldrich 682659) was used as a primary source for the synthesis of geopolymer in this work. All chemicals such as iron (II) chloride, iron (III) chloride, sodium hydroxide, sodium silicate, and ammonium hydroxide were of analytical grade and purchased from Merck and Aldrich. Fourier transform infrared (FT-IR) spectroscopy spectra were recorded on a Shimadzu IR-470 spectrometer by the method of KBr pellets. Fieldemission scanning electron micrograph (FE-SEM) images were taken with TESCAN MIRA3 and VEGA TESCAN. X-ray diffraction (XRD) pattern of the nanocomposite was recorded with PhilipsCW1730. The thermogravimetric analysis (TGA) was taken by Bahr-STA 504 instrument under air atmosphere. Energy-dispersive X-ray (EDX) analysis was recorded with a Numerix DXPeX10P. Brunauer-Emmett-Teller (BET) analysis was achieved by micromeritics ASAP 2020. The concentration of adsorbed heavy metals was determined using atomic absorption spectrometry (Shimadzu AA-6300).
2.2. Preparation parameters The synthesized geopolymer specimens were prepared based on many parameters, summarized in Table 1.
Table 2 Different amounts of Fe3O4 nanoparticles for the preparation of target samples. Entry Number Number Number Number Number Number
0 1 2 3 4 5
Type of sample
Amount of Fe3O4 nanoparticles (g)
Ratio of geopolymer/Fe3O4
Geopolymer Nanocomposite Nanocomposite Nanocomposite Nanocomposite Nanocomposite
0 0.16 0.14 0.12 0.08 0.03
e 2.5 2.8 3.3 5 13
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Fig. 1. The TGA and DTA curve of bentonite clay.
2.2.1. Preparation of the geopolymer Initially, for the preparation of the alkali activator, 2.0 g of sodium hydroxide was added to 50 mL of distilled water (1 M NaOH solution). After that, 1.4 g of sodium silicate was added to the mixture. The solution was stirred at room temperature until the content was completely dissolved. Then, 1.2 g of calcined bentonite nanoclay was added to the alkali activator solution. The mixture was stirred for 5e7 min up to a slurry solution was obtained. The resulting mixture was placed in an open beaker and dried in a conventional heating oven for 72 h at 75 C. The geopolymer uniform powder was achieved by trituration with a mortar and pestle. 2.2.2. Preparation of Fe3O4 nanoparticles At first, 3.24 g of FeCl2 and 6.48 g of FeCl3 was added to the
100 mL deionized water. The mixture was stirred under nitrogen gas flow at 60 C. To prepare iron oxide, 7 mL solution of ammonium hydroxide was added dropwise during 20 min to the initial mixture. The pH of the final mixture was controlled in the range of 9e10 and stirred for 1 h and then the black precipitate was separated by the external magnet. Finally, magnetic nanoparticles were washed with distilled water 3 times and dried in an oven at 70 C for 24 h. 2.2.3. Preparation of the nanocomposite The geopolymer/Fe3O4 as a magnetic nanocomposite was prepared via the intercalation method. Initially, 0.4 g of geopolymer was added to a 20 mL deionized water and the mixture was stirred up to a slurry solution was obtained. Then different amounts of Fe3O4 nanoparticles were added to the geopolymer powder and the
Fig. 2. The FT-IR spectra of: a) bentonite, b) calcined bentonite, c) geopolymer, d) nanocomposite.
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homogenous mixture was achieved after 30 min of mixing. The product was dried in an oven at 70 C for 24 h. The resulting powders will be tested for their adsorptive capabilities. As indicated in Table 2, multiple amounts of Fe3O4 nanoparticles were used to prepare different magnetic nanocomposite.
2.2.4. Adsorption experiments The sorption of heavy metal on bentonite nanoclay based geopolymer and nanocomposite was investigated in the case of copper, nickel, lead, cadmium, and mercury ions, represented by Cu(NO3)2, Ni(NO3)2, Pb(NO3)2, Cd(NO3)2.4H2O, and Hg(NO3)2.4H2O. The nanocomposite was washed several times with distilled water before adsorption process to reach pH ¼ 7. The metal solutions were prepared at different concentrations of 2600, 3000, and 5000 ppm. Then, 0.05 g of the geopolymer (Entry 0, Table 2), and nanocomposite samples (Entries 1e5, Table 2) are mixed in 10 mL of ion solution (5 mg/mL) at room temperature without stirring at (2, 10, 30, 60, and 120 min). Then, the solution was filtered, and the concentrations of the adsorbed metal ions were determined using
atomic absorption spectrometry. Tables S2eS16 in the supplementary data show the result of the adsorption of the metal ions at different contact time and concentration of metal solution. The percentage of removal efficiency can be calculated using the following equation, eq. (1): RE¼ (C0-Ceq/C0) 100%
(1)
Where RE is as removal efficiency, C0 is initial concentration of metal ions (ppm), and Ceq is the concentration of the remaining ions in the metal solution (ppm).
3. Results and discussion 3.1. Characterization of the geopolymer nanocomposites 3.1.1. Bentonite thermal analysis According to Fig. 1, the TGA plots show a mass loss between 80 and 200 C, which is due to removal of moisture water. The mass
Fig. 3. The XRD pattern of: (a) geopolymer/Fe3O4 nanocomposite (b) the reference patterns.
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Fig. 4. The FE-SEM images of: a) geopolymer, b) Fe3O4, c) geopolymer/Fe3O4 at 1 kX magnification, d) geopolymer/Fe3O4 at 200 kX.
Fig. 5. EDX analysis of: a) fresh geopolymer/Fe3O4, b) the recycled nanocomposite, c) the original SEM image where EDX mapping was executed and corresponding elemental mapping of EDX patterns for the nanocomposite.
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loss registered between 400 and 700 C is related to the loss of water due to dehydroxylation of clay (Alabarse et al., 2011; Yurdakoç et al., 2008). In the DTA curve, the broad peak between 650 and 750 C is attributed to the decomposition of carbonate (Ayari et al., 2005).
3.1.2. Spectroscopic analysis FT-IR spectroscopy was applied to study the structure of
Table 3 Texture properties of geopolymer sample and nanocomposite sample 5. Physical properties
Geopolymer sample
Nanocomposite (Sample no. 5)
Surface area (m2/g) Pore size (nm) Total pore volume of pores (cm3/g)
1.13 14.28 0.004
2.32 13.76 0.008
Table 4 Metal ions efficiency in 2 min contact time and in the presence of nanocomposite 5. Metal ions
Removal efficiency of different metal ions (%RE)
2600 (ppm) 3000 (ppm) 5000 (ppm)
Copper ~99 ~99 97e98
Lead ˃ 99 ˃ 99 80e94
Nickel 92e94 90e94 80e93
Cadmium 96e97 90e91 86e87
Mercury ~92 88e90 81e93
synthesized geopolymer and nanocomposite (Fig. 2). As can be seen in the FT-IR spectrum of bentonite (Fig. 2a), the bands at 400 and 500 cm1 are related to the SieOeSi bending vibration and octahedral AleOeSi bending. The strong peak at ~1100 cm1, attributed to SieO as a silicate structure in the plane stretching vibration. Furthermore, the absorption bands at 2800 cm1 and 2900 cm1 correspond to the stretching vibration of aliphatic CeH groups due to maybe presence of organic components and impurity in the bulk bentonite. Also, the stretching of the OH group of bentonite has appeared around 3400 cm1. The calcined bentonite spectrum (Fig. 2b) shows all the major peaks of bentonite, but, after the calcination and removal all organic impurities the absorption bands at around 2800 cm1 and 2900 cm1 were disappeared. The abovementioned peaks all appeared in the spectra of geopolymer and nanocomposite. The band around 3400 cm1 refers to stretching vibrations of the hydroxyl groups from iron oxide and hydroxyl group of geopolymer. Moreover, The peak at around 1400 cm1 is related to the presence of carbonate (Ferone et al., 2013). The Fe3O4 characteristic peak at around 550 cm1 was overlapped with the bending vibration of AleOeSi.
3.1.3. Crystallographic analysis As shown in Fig. 3a, the geopolymer/Fe3O4 has the crystalline peaks at different diffraction angles (2q). The observed peaks for the aluminum oxide are similar to the characteristic data of aluminum oxide (JCPDS card No. 01-075-0278). Also, the peak at
Scheme 1. Physical adsorption mechanism by geopolymer/Fe3O4 natural nanocomposite.
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2Ɵ ¼ 24.7 and 34.4 are attributed to silicon oxide (JCPDF card No. 00-027-0605). The other crystalline peaks are related to magnetite iron oxide nanoparticles with cubic structure (JCPDS card No. 01088-0315). 3.1.4. Morphological analysis The morphology, pore surface and possible aggregation of the particles in the produced materials were investigated by FE-SEM images (Fig. 4). Fig. 4a and b shows the geopolymer and Fe3O4 nanoparticles surface morphology. Also, geopolymer/Fe3O4 nanocomposite morphology was studied in two different magnifications (Fig. 4c and d). FE-SEM images of nanocomposite show the dispersion of cubic iron oxide nanoparticles on the surface of the geopolymer. However, the morphology of the geopolymer shows that the surface of the materials is compact. Meanwhile, the loading of Fe3O4 on geopolymer surface is not homogeneous, therefore, the surface is readily available for adsorption even post-magnetization. 3.1.5. EDX analysis The results of the EDX analyses of geopolymer/Fe3O4 nanocomposite are shown in Fig. 5. It confirms the presence of Al, Si, Na, Fe and O atoms and elements in the nanocomposite (Fig. 5a). The presence of all above elements in the recovered and reused nanocomposite after the first treatment of adsorption confirmed the
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stability of geopolymer/Fe3O4 nanocomposite and physical adsorption process (Fig. 5b). Furthermore, Fig. 5c shows the original SEM image and the corresponding executed EDX elemental mapping with uniform distribution of Fe3O4 nanoparticles in the nanocomposite. 3.1.6. BET analysis The surface area, pore size and pore volume of geopolymer and nanocomposite were studied (Table 3). It can be seen that the surface area and pore volume of nanocomposite were increased compared to the geopolymer. Although, the pore size of nanocomposite slightly decreased due to the loading of the Fe3O4 nanoparticle. Nevertheless, the pore size of the nanocomposite was acceptable enough for the heavy metals adsorption. 3.2. Adsorption test As can be seen in Table 4, the highest efficiency in the case of copper adsorption is 2600 and 3000 ppm, at an efficiency of ~99%. However, at a concentration of 5000 ppm, its efficiency is reduced to ~97e98%. In the case of lead adsorption, the efficiency for 2600 and 3000 ppm exceed 99% while, is reduced to ~80e94% at 5000 ppm, except in the case of nanocomposite 5, whose efficiency exceeds 99%. In the case of nickel ion, the efficiency falls within
Fig. 6. Langmuir isotherm in different metal ions for sample 0.
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92e94% at a concentration of 2600 ppm, at a concentration of 3000 ppm, to 90e94%, and at a concentration of 5000 ppm, to 80e93%, except in the case of nanocomposite number 5, whose efficiency exceeds 99%.
The highest efficiency for cadmium and mercury correspond to 2600 concentrations are 96e97% and 92%, with a minimum efficiency of 5000 in the range of 86e87%, and 81e93%. In this current ion, the adsorption decreases with increasing concentrations. Also,
Fig. 7. Langmuir isotherm in different metal ions for sample 1.
Fig. 8. Langmuir isotherms in different metal ions for sample 2.
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the selectivity of the nanocomposite was examined in the presence of copper, nickel and cadmium ions solution. As a result, the desirable adsorption about 60% was achieved for all ions. Overall, the results confirmed that higher efficiency is at the first moments of experimental work and contact time did not influence the adsorption process. Using a different value of Fe3O4 on the geopolymer as
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nanoparticles have no specific effect on the efficiency of the adsorption, however, it positively affects the magnetization of the sample and easy separation of nanocomposite from heavy metals solution. The purpose of this research is to simplify the adsorption process by using the novel magnetic nanocomposites. The indicated condition, confirm the high performance of nanocomposite at
Fig. 9. Langmuir isotherms in different metal ions for sample 3.
Fig. 10. Langmuir isotherms in different metal ions for sample 4.
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ambient temperature without any stirring or changed in the pH of the solution under normal atmospheric conditions. As the result, the nanocomposite 5 and 2 min contact time were chosen for analyses and adsorption study. Moreover, it should be mentioned that this nanocomposite could not adsorb chromium ions.
solution. So, after the adsorption process and removing the nanocomposite, the concentration of Naþ ion in the remaining ions solutions was determined by atomic absorption spectrometry analysis. The absence of Naþ ions confirmed that the ion exchange was not occurred.
3.2.1. Mechanism of adsorption Due to the use of porous nanocomposite and the short contact time, the physical mechanism was suggested (Scheme 1). According to high adsorption efficiency, the possible ions exchange process could be confirmed by the presence of Naþ ion in the metal ion
3.2.2. Adsorption isotherms An adsorption isotherm is important towards the design of an adsorption system. A measured adsorption isotherm has been fitted with Langmuir isotherm equation at different metal ions concentration (2600, 3000, and 5000 ppm).
Fig. 11. Langmuir isotherms in different metal ions for sample 5.
Table 5 The Langmuir coefficients for different samples in different metal ions. Samples
0 1 2 3 4 5
Cu(II)
Pb(II)
Ni(II)
Hg(II)
Cd(II)
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
0.985 0.986 0.986 0.983 0.983 0.983
0.0122 0.0129 0.0146 0.0097 0.0101 0.0102
502.18 502.92 495.18 511.38 508.01 510.29
0.935 0.935 0.897 0.916 0.917 0.994
0.0015 0.0016 0.0009 0.0009 0.0008 0.051
543.42 539.65 551.03 550.96 552.98 1227.2
0.849 0.864 0.845 0.843 0.822 0.912
0.00482 0.00599 0.00203 0.00192 0.00237 0.0513
288.44 256.62 391.9 397.74 343.25 1227.2
0.812 0.803 0.807 0.796 0.806 0.788
0.0028 0.0025 0.0022 0.0018 0.0027 0.0013
304.79 313.54 335.67 350.0 305.74 385.33
0.868 0.840 0.843 0.843 0.841 0.848
0.0023 0.0015 0.0014 0.0014 0.0015 0.0014
398.61 419.22 430.25 430.23 422.02 433.34
Table 6 Comparison the efficiency of geopolymer/Fe3O4 nanocomposite with other reported works. Entry
Adsorbent
L/S ratio
Temp. ( C)
Time (h)
Ref.
1 2 3 4 5 6
Carbon nanotube Chitosan-GLA Sawdust Active carbon Mag/CS Geopolymer/Fe3O4
5 1 40 5 1 5
r.t. r.t. 23 C 20 C r.t. r.t.
4 1.5 24 3 6 0.033
Stafiej and Pyrzynska (2007) Wang Ngah and Fatinathans (2008) Ya et al. (2001) ska et al. (2017) Kołodyn Horst et al. (2016) This work
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In the Langmuir model, the adsorption of metal ions occurs on the homogeneous surfaces and the active sites of the adsorbent. Adsorption is a single layer interaction between the species adsorbed on it. The Langmuir equation, eq. (2) is: Ce/Qe ¼ 1/qmKL þ 1/qm (Ce) (Langmuir, 1918)
(2)
Where qm (mg/g) is the maximum adsorption capacity and KL (L/mg) is the Langmuir constant calculated from the plot 1/qe to 1/ Ce. Qe (mg/g) is the amount of adsorbed metal, and Ce (mg/L) is concentration of the metal equilibrium. Figs. 6e11 shows the Langmuir isotherm graph for the 3000 ppm solution of different ions in the presence of different nanocomposites. Additionally, Table 5 tabulates the Langmuir coefficients for the different samples (with different Fe3O4 nanoparticles) and metal ions. The adsorption capacity was calculated using the following equation, eq. (3). qm¼ (C0-Ceq)V/m
(3)
Where C0 is initial concentration of metal ions (ppm), Ceq is the concentration of the remaining ions in the metal solution (ppm), V
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is the volume of heavy ions solution (L) and m is the weight of adsorbent (g). Then, the adsorption isotherm constant (qm and KL) were calculated using the slope of the equation of a line. The correlation coefficients (R2) were determined to be 0.78e0.99 for the different metal ions. The R2 for the Langmuir isotherms are near 1, the monolayer adsorption is obtained, and the process is exceptionally desirable. According to Figs. 6e11 and Table 5, the desirable rates of the samples were 2˂3˂4˂0˂1˂5. Moreover, in the case of nickel, the R2 in sample 5 exceeds 0.9, being close to 1. However, for Mercury and Cadmium, the correlation coefficients are less than 0.9, and not close to 1. Therefore, the metal ions order according to Langmuir model are Cu(II)>Pb(II)>Ni(II)>Hg(II)>Cd(II), and sample 5 with 0.03 g of Fe3O4 nanoparticles reported better adsorbent capacity. 3.3. Comparison the efficiency of geopolymer/Fe3O4 nanocomposite with literature reports The performance of geopolymer/Fe3O4 nanocomposite was investigated. As can be seen in Table 6, several adsorbents have been used for the adsorption of heavy metal ions. Also, they have shown high adsorption capacity. But, most of them suffer from disadvantages such as long contact time and difficult recycling and
Fig. 12. The FT-IR spectra of: a) geopolymer/Fe3O4, b) geopolymer/Fe3O4 in alkaline solution, c) geopolymer/Fe3O4 in acid solution.
Fig. 13. a) Recycling diagram of geopolymer/Fe3O4 nanocomposite, b) The FT-IR spectra of fresh nanocomposite and recycled nanocomposite.
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recovering of adsorbent. Therefore, the results of the present work are more reliable than previous ones. 3.4. Stability of nanocomposites in different pH The stability of the nanocomposite was examined in both acidic and alkaline solutions. The durability of geopolymer/Fe3O4 nanocomposite in alkaline solution was evaluated by the dispersion of geopolymer/Fe3O4 nanocomposite (0.05 g) in 10 mL sodium hydroxide (0.1 M). The nanocomposite was stable in alkaline solution and this result was confirmed by FT-IR analysis (Fig. 12b). In addition, the stability of the nanocomposite in pH ¼ 1 was checked by adding 0.05 g of the nanocomposite in 10 mL of hydrogen chloride solution 0.1 M (5 mg/mL). The characteristic band of sodium carbonate in 1400 cm1 should disappear in acid solution due to decomposition of Na2CO3 to Na2O and CO2. However, the feature band of Na2O was around 1400 cm1 and appear in the same region with carbonate (Wu et al., 2016). As can be seen in Fig. 12c, the geopolymer/Fe3O4 was also stable in acidic solution. 3.5. Reusability and stability of magnetic geopolymer nanocomposite The reusability of the nanocomposite was studied for copper ion adsorption (3000 ppm and 10 min). Firstly, for desorption process and removing adsorbed metal ions from recycled nanocomposite, geopolymer/Fe3O4 nanocomposite was washed with deionized water, ammonium chloride (4 M) and sodium hydroxide (0.2 M) (Krishnamurti et al., 1999., Lata et al., 2015). Then, magnetic nanocomposite was dried and reused at least 4 times without any significant lose in adsorption (Fig. 13a). Furthermore, the stability of recycled geopolymer/Fe3O4 nanocomposite was confirmed by FT-IR spectra (Fig. 13b). 4. Conclusions Magnetic nanocomposite based on geopolymer has been successfully synthesized in simple and mild conditions. This nanocomposite has excellent adsorption in the case of copper, lead, and nickel, cadmium, and mercury ions from industrial wastewaters at room temperature without being stirred or changed pH in normal atmospheric conditions. According to the adsorption result and Langmuir model, the copper ion was best adsorbed. Composites synthesized in this work can be effectively applied for wastewater treatment, heavy metal ions removal in various purposes especially for cleaner industrial production and inexpensive waste management. Also, the effect of Fe3O4 nanoparticles was investigated. The result shows that the different amounts of Fe3O4 nanoparticles have no specific effect on the efficiency of adsorption, while it has a positive effect on the magnetization and easy separation. This study has the most significant advantages such as using fast, inexpensive and eco-friendly material, easy preparation method, and simplicity in application, magnetic separation of the adsorbent and high efficiency at ambient condition. Acknowledgement The authors thank the Iran University of Science and Technology for partial financial support by the Research Council and the reviewers for their valuable comments and suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.01.084.
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Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro
A green, porous and eco-friendly magnetic geopolymer adsorbent for heavy metals removal from aqueous solutions Ali Maleki*, Zoleikha Hajizadeh, Vajiheh Sharifi, Zeynab Emdadi Catalysts and Organic Synthesis Research Laboratory, Department of Chemistry, Iran University of Science and Technology, Tehran, 16846-13114, Iran
a r t i c l e i n f o
a b s t r a c t
Article history: Received 22 April 2018 Received in revised form 6 January 2019 Accepted 8 January 2019 Available online 9 January 2019
Geopolymers are a class of synthesized amorphous aluminosilicate materials that can be used as an adsorbent for the removal of heavy metals. In this paper, the bentonite clay was employed to synthesize geopolymer that can remove heavy metals such as Cu(II), Pb(II), Ni(II), Cd(II), and Hg(II) from industrial wastewaters. The Fe3O4 nanoparticles were applied to modify the geopolymer and the use of a geopolymer/Fe3O4 nanocomposite as an efficient and magnetic adsorbent for heavy metals removal from aqueous solution was investigated in this work, for the first time. The influence of different contact time and initial concentrations of metal ions on sorption was examined and the best result was achieved in 2 min contact time in the presence of 0.05 g nanocomposite. The prepared geopolymer and nanocomposite samples were characterized by Fourier transform infrared spectroscopy spectra, fieldemission scanning electron micrograph images, thermogravimetric analysis, energy dispersive X-ray analysis, X-ray diffraction pattern and Brunauer-Emmett-Teller analysis. The prepared magnetic geopolymer base on bentonite clay showed 99%, 99%, 92%, 96% and 92% removal efficiency for the sorption of copper, lead, nickel, cadmium, and mercury ions from industrial wastewaters. The present work includes diverse advantageous such as environmentally-friendly protocol, magnetic separation, inexpensive raw materials, easy and simple conditions and high yields as same as short adsorption times. © 2019 Elsevier Ltd. All rights reserved.
Keywords: Water treatment Geopolymer Adsorbent Bentonite clay Heavy metals Green chemistry
1. Introduction Geopolymer, in general is defined as a solid and stable aluminosilicate material. It was formed by alkali hydroxide or alkali silicate activation of a precursor that is usually supplied as a solid powder (Provis and Deventer, 2009). On a fundamental level, the synthesis of geopolymer comprise dissolution of silica, alumina, and aluminosilicate using alkali solution (Zhuang et al., 2016). In recent years, the waste management via green chemical protocols ~o et al., 2017; Perna and has been rapidly developed (Ascensa Hanzlí cek, 2014). The geopolymer materials are regarded as environmentally friendly due to their low temperature of manufacturing (<100 C) and lower CO2 emission compare to standard cement (Duxson et al., 2007). Although the most study on geopolymer is focused on the field of concrete building materials application, recently they are considered as a suitable replacement for aqueous solutions adsorption application (Duan et al., 2016; Javadian et al., 2015; Qiu et al., 2018).
* Corresponding author. E-mail address: [email protected] (A. Maleki). https://doi.org/10.1016/j.jclepro.2019.01.084 0959-6526/© 2019 Elsevier Ltd. All rights reserved.
In some countries, existing different type of pollution such as heavy metals (Saleh and Gupta, 2014), radionuclide (Gu et al., 2018) and dyes (Mosleh et al., 2018) in wastewater negatively affect the environment. As a result, different materials and nanocomposite such as ZnO/Ag/CdO (Saravanan et al., 2015a,b), ZnO/Ag/Mn2O3 (Saravanan et al., 2015a,b), and activated carbon (Asfaram et al., 2015) were suggested for removal different kinds of environmental pollution. Also, some methods like Fenton oxidation (Karthikeyan et al., 2012) and photodegradation (Saravanan et al., 2013) were used in recent years. The use of heavy metals is increasing in the industry, which leads to environmental pollution (Ahmaruzzaman and Gupta, 2011). In the other word, heavy metals toxicity in the environment is caused by factory emission, national atmospheric deposition, waste water, mining, and livestock manure applications (Wei and Yang, 2010; Xia et al., 2011). Furthermore, in developing countries, due to increase activity in local and farming fields, the large amount of toxic heavy metals penetrated into soil and water (Rahman et al., 2010; Sharma et al., 2007; Solaraj et al., 2010). Therefore, according to the potential toxic effects and the risk of bioaccumulation in an aquatic ecosystem, heavy metals are global
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Table 1 The different effective parameters on synthesized samples. Parameters Type of aluminosilicate Type of alkaline solution The ratio of SiO2/Al2O3
Bentonite NaOH Na2SiO3 2
The ratio of Na2O/Al2O3 Calcination temperature Temperature drying
1 700e800 70e75
Properties
Reference
The bentonite clay was selected for its natural porosity and high surface area. Was selected as an alkali activator due to their more availability and cheap price. The ratio of Si/Al has different effects on geopolymer properties. This ratio was selected according to literature review for adsorption application based on geopolymer as an adsorbent. The ratio of Al2O3/Na2O was chosen according to literature. Chosen as TGA analysis Suitable temperature for geopolymer synthesis based on scholars investigation
Provis and Deventer (2009) Provis and Deventer (2009) Davidovits (2002)
concern (Batvari et al., 2015; David et al., 2012). Heavy metals in aqueous solutions, especially wastewater, can endanger human health (Terra et al., 2008). Copper, cadmium, lead, and mercury are examples of toxic metals that will are ubiquitous in industries (Demirbas et al., 2009; Lalhruaitluanga et al., 2010; Qiu et al., 2018). Some heavy metals are essential nutrients at very small amounts (<5 mg/L). However, at higher dosages, they are toxic and could lead to health problems (Awual et al., 2014). Different materials such as MOF (Li et al., 2018), boron nitride (Yu et al., 2018), ZnO nanoparticles (Jafari et al., 2017), CuO nanoparticles (Dashamiri et al., 2016), active carbon (Jamshidi et al., 2015) and graphene oxide (Robati et al., 2016) can be used as a adsorbents for water contamination. Subsequently, there are many approaches that can be used to remove heavy metals from aqueous solutions. Geopolymer adsorbents are one of the important classes in this regard. There are some advantages to use these approaches, such as low cost, high efficiency and simple production (Azad et al., 2015; Lee and Tiwari, 2013). There are three common aluminosilicate materials that can be used for geopolymers synthesis, which are fly ash, slags, and calcined clay. Calcined clay is favored due to its accessibility and hydrophilicity (Provis and Deventer, 2009). Bentonite as naturally available clay can be accessible inexpensively (Bhattacharyya and Gupta, 2008). Also, due to its unique features such as high surface area, high Al2O3/SiO2 ratio, high amorphous content, chemically stability, high porosity and high capacity of cation-exchange, it can be used in the different applications (Bhatt et al., 2012; Patel et al., 2010; Wang et al., 2009). Nowadays, magnetic nanoparticles have received considerable attentions because of their potential applications in various industries (Maleki et al., 2018a; Hajizadeh and Maleki, 2018b). Due to their superior features such as high surface-to-volume ratio, high stability, low preparation cost, and easy separation are applied in different applications (Maleki et al., 2018b; Maleki et al., 2019a). Moreover, their aggregation problem was resolved by composite with material like silica and polymers (Maleki, 2012, 2013; Maleki et al., 2018c, 2019b). Therefore, diverse nanocomposites were synthesized by scientists in recent years (Maleki, 2018a, 2018b). Owing to the importance of metal adsorption and treatment of industrial wastewater, the magnetic geopolymer nanocomposite was proposed for the first time as a heterogeneous adsorbent. This porous adsorbent was synthesized in two steps. At first, the porous geopolymer was prepared easily, and then, the geopolymer/Fe3O4
Provis and Deventer (2009) This work Davidovits (2002)
nanocomposite was synthesized via the intercalation method (White et al., 2011; Khan et al., 2016). In this process, the geopolymer inner surface hydroxyl groups were hydrogen bonded to the hydroxyl groups of Fe3O4 and magnetic nanoparticles were intercalated into the geopolymer layers. The main objective is to use this geopolymer composite for wastewater purification due to its environmental friendliness, inexpensive production process, easy separation, and high efficiency in the shortest time via green chemistry principles. In other words, the aim of this research is introducing a green and efficient protocol for removal of a wide range of toxic heavy metals from various water and wastewaters by easy and inexpensive magnetic nanocomposites based on natural and available reactants in the shortest time.
2. Materials and methods 2.1. Materials and instruments Bentonite nanoclay (Bentonite Aldrich 682659) was used as a primary source for the synthesis of geopolymer in this work. All chemicals such as iron (II) chloride, iron (III) chloride, sodium hydroxide, sodium silicate, and ammonium hydroxide were of analytical grade and purchased from Merck and Aldrich. Fourier transform infrared (FT-IR) spectroscopy spectra were recorded on a Shimadzu IR-470 spectrometer by the method of KBr pellets. Fieldemission scanning electron micrograph (FE-SEM) images were taken with TESCAN MIRA3 and VEGA TESCAN. X-ray diffraction (XRD) pattern of the nanocomposite was recorded with PhilipsCW1730. The thermogravimetric analysis (TGA) was taken by Bahr-STA 504 instrument under air atmosphere. Energy-dispersive X-ray (EDX) analysis was recorded with a Numerix DXPeX10P. Brunauer-Emmett-Teller (BET) analysis was achieved by micromeritics ASAP 2020. The concentration of adsorbed heavy metals was determined using atomic absorption spectrometry (Shimadzu AA-6300).
2.2. Preparation parameters The synthesized geopolymer specimens were prepared based on many parameters, summarized in Table 1.
Table 2 Different amounts of Fe3O4 nanoparticles for the preparation of target samples. Entry Number Number Number Number Number Number
0 1 2 3 4 5
Type of sample
Amount of Fe3O4 nanoparticles (g)
Ratio of geopolymer/Fe3O4
Geopolymer Nanocomposite Nanocomposite Nanocomposite Nanocomposite Nanocomposite
0 0.16 0.14 0.12 0.08 0.03
e 2.5 2.8 3.3 5 13
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Fig. 1. The TGA and DTA curve of bentonite clay.
2.2.1. Preparation of the geopolymer Initially, for the preparation of the alkali activator, 2.0 g of sodium hydroxide was added to 50 mL of distilled water (1 M NaOH solution). After that, 1.4 g of sodium silicate was added to the mixture. The solution was stirred at room temperature until the content was completely dissolved. Then, 1.2 g of calcined bentonite nanoclay was added to the alkali activator solution. The mixture was stirred for 5e7 min up to a slurry solution was obtained. The resulting mixture was placed in an open beaker and dried in a conventional heating oven for 72 h at 75 C. The geopolymer uniform powder was achieved by trituration with a mortar and pestle. 2.2.2. Preparation of Fe3O4 nanoparticles At first, 3.24 g of FeCl2 and 6.48 g of FeCl3 was added to the
100 mL deionized water. The mixture was stirred under nitrogen gas flow at 60 C. To prepare iron oxide, 7 mL solution of ammonium hydroxide was added dropwise during 20 min to the initial mixture. The pH of the final mixture was controlled in the range of 9e10 and stirred for 1 h and then the black precipitate was separated by the external magnet. Finally, magnetic nanoparticles were washed with distilled water 3 times and dried in an oven at 70 C for 24 h. 2.2.3. Preparation of the nanocomposite The geopolymer/Fe3O4 as a magnetic nanocomposite was prepared via the intercalation method. Initially, 0.4 g of geopolymer was added to a 20 mL deionized water and the mixture was stirred up to a slurry solution was obtained. Then different amounts of Fe3O4 nanoparticles were added to the geopolymer powder and the
Fig. 2. The FT-IR spectra of: a) bentonite, b) calcined bentonite, c) geopolymer, d) nanocomposite.
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homogenous mixture was achieved after 30 min of mixing. The product was dried in an oven at 70 C for 24 h. The resulting powders will be tested for their adsorptive capabilities. As indicated in Table 2, multiple amounts of Fe3O4 nanoparticles were used to prepare different magnetic nanocomposite.
2.2.4. Adsorption experiments The sorption of heavy metal on bentonite nanoclay based geopolymer and nanocomposite was investigated in the case of copper, nickel, lead, cadmium, and mercury ions, represented by Cu(NO3)2, Ni(NO3)2, Pb(NO3)2, Cd(NO3)2.4H2O, and Hg(NO3)2.4H2O. The nanocomposite was washed several times with distilled water before adsorption process to reach pH ¼ 7. The metal solutions were prepared at different concentrations of 2600, 3000, and 5000 ppm. Then, 0.05 g of the geopolymer (Entry 0, Table 2), and nanocomposite samples (Entries 1e5, Table 2) are mixed in 10 mL of ion solution (5 mg/mL) at room temperature without stirring at (2, 10, 30, 60, and 120 min). Then, the solution was filtered, and the concentrations of the adsorbed metal ions were determined using
atomic absorption spectrometry. Tables S2eS16 in the supplementary data show the result of the adsorption of the metal ions at different contact time and concentration of metal solution. The percentage of removal efficiency can be calculated using the following equation, eq. (1): RE¼ (C0-Ceq/C0) 100%
(1)
Where RE is as removal efficiency, C0 is initial concentration of metal ions (ppm), and Ceq is the concentration of the remaining ions in the metal solution (ppm).
3. Results and discussion 3.1. Characterization of the geopolymer nanocomposites 3.1.1. Bentonite thermal analysis According to Fig. 1, the TGA plots show a mass loss between 80 and 200 C, which is due to removal of moisture water. The mass
Fig. 3. The XRD pattern of: (a) geopolymer/Fe3O4 nanocomposite (b) the reference patterns.
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Fig. 4. The FE-SEM images of: a) geopolymer, b) Fe3O4, c) geopolymer/Fe3O4 at 1 kX magnification, d) geopolymer/Fe3O4 at 200 kX.
Fig. 5. EDX analysis of: a) fresh geopolymer/Fe3O4, b) the recycled nanocomposite, c) the original SEM image where EDX mapping was executed and corresponding elemental mapping of EDX patterns for the nanocomposite.
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loss registered between 400 and 700 C is related to the loss of water due to dehydroxylation of clay (Alabarse et al., 2011; Yurdakoç et al., 2008). In the DTA curve, the broad peak between 650 and 750 C is attributed to the decomposition of carbonate (Ayari et al., 2005).
3.1.2. Spectroscopic analysis FT-IR spectroscopy was applied to study the structure of
Table 3 Texture properties of geopolymer sample and nanocomposite sample 5. Physical properties
Geopolymer sample
Nanocomposite (Sample no. 5)
Surface area (m2/g) Pore size (nm) Total pore volume of pores (cm3/g)
1.13 14.28 0.004
2.32 13.76 0.008
Table 4 Metal ions efficiency in 2 min contact time and in the presence of nanocomposite 5. Metal ions
Removal efficiency of different metal ions (%RE)
2600 (ppm) 3000 (ppm) 5000 (ppm)
Copper ~99 ~99 97e98
Lead ˃ 99 ˃ 99 80e94
Nickel 92e94 90e94 80e93
Cadmium 96e97 90e91 86e87
Mercury ~92 88e90 81e93
synthesized geopolymer and nanocomposite (Fig. 2). As can be seen in the FT-IR spectrum of bentonite (Fig. 2a), the bands at 400 and 500 cm1 are related to the SieOeSi bending vibration and octahedral AleOeSi bending. The strong peak at ~1100 cm1, attributed to SieO as a silicate structure in the plane stretching vibration. Furthermore, the absorption bands at 2800 cm1 and 2900 cm1 correspond to the stretching vibration of aliphatic CeH groups due to maybe presence of organic components and impurity in the bulk bentonite. Also, the stretching of the OH group of bentonite has appeared around 3400 cm1. The calcined bentonite spectrum (Fig. 2b) shows all the major peaks of bentonite, but, after the calcination and removal all organic impurities the absorption bands at around 2800 cm1 and 2900 cm1 were disappeared. The abovementioned peaks all appeared in the spectra of geopolymer and nanocomposite. The band around 3400 cm1 refers to stretching vibrations of the hydroxyl groups from iron oxide and hydroxyl group of geopolymer. Moreover, The peak at around 1400 cm1 is related to the presence of carbonate (Ferone et al., 2013). The Fe3O4 characteristic peak at around 550 cm1 was overlapped with the bending vibration of AleOeSi.
3.1.3. Crystallographic analysis As shown in Fig. 3a, the geopolymer/Fe3O4 has the crystalline peaks at different diffraction angles (2q). The observed peaks for the aluminum oxide are similar to the characteristic data of aluminum oxide (JCPDS card No. 01-075-0278). Also, the peak at
Scheme 1. Physical adsorption mechanism by geopolymer/Fe3O4 natural nanocomposite.
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2Ɵ ¼ 24.7 and 34.4 are attributed to silicon oxide (JCPDF card No. 00-027-0605). The other crystalline peaks are related to magnetite iron oxide nanoparticles with cubic structure (JCPDS card No. 01088-0315). 3.1.4. Morphological analysis The morphology, pore surface and possible aggregation of the particles in the produced materials were investigated by FE-SEM images (Fig. 4). Fig. 4a and b shows the geopolymer and Fe3O4 nanoparticles surface morphology. Also, geopolymer/Fe3O4 nanocomposite morphology was studied in two different magnifications (Fig. 4c and d). FE-SEM images of nanocomposite show the dispersion of cubic iron oxide nanoparticles on the surface of the geopolymer. However, the morphology of the geopolymer shows that the surface of the materials is compact. Meanwhile, the loading of Fe3O4 on geopolymer surface is not homogeneous, therefore, the surface is readily available for adsorption even post-magnetization. 3.1.5. EDX analysis The results of the EDX analyses of geopolymer/Fe3O4 nanocomposite are shown in Fig. 5. It confirms the presence of Al, Si, Na, Fe and O atoms and elements in the nanocomposite (Fig. 5a). The presence of all above elements in the recovered and reused nanocomposite after the first treatment of adsorption confirmed the
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stability of geopolymer/Fe3O4 nanocomposite and physical adsorption process (Fig. 5b). Furthermore, Fig. 5c shows the original SEM image and the corresponding executed EDX elemental mapping with uniform distribution of Fe3O4 nanoparticles in the nanocomposite. 3.1.6. BET analysis The surface area, pore size and pore volume of geopolymer and nanocomposite were studied (Table 3). It can be seen that the surface area and pore volume of nanocomposite were increased compared to the geopolymer. Although, the pore size of nanocomposite slightly decreased due to the loading of the Fe3O4 nanoparticle. Nevertheless, the pore size of the nanocomposite was acceptable enough for the heavy metals adsorption. 3.2. Adsorption test As can be seen in Table 4, the highest efficiency in the case of copper adsorption is 2600 and 3000 ppm, at an efficiency of ~99%. However, at a concentration of 5000 ppm, its efficiency is reduced to ~97e98%. In the case of lead adsorption, the efficiency for 2600 and 3000 ppm exceed 99% while, is reduced to ~80e94% at 5000 ppm, except in the case of nanocomposite 5, whose efficiency exceeds 99%. In the case of nickel ion, the efficiency falls within
Fig. 6. Langmuir isotherm in different metal ions for sample 0.
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92e94% at a concentration of 2600 ppm, at a concentration of 3000 ppm, to 90e94%, and at a concentration of 5000 ppm, to 80e93%, except in the case of nanocomposite number 5, whose efficiency exceeds 99%.
The highest efficiency for cadmium and mercury correspond to 2600 concentrations are 96e97% and 92%, with a minimum efficiency of 5000 in the range of 86e87%, and 81e93%. In this current ion, the adsorption decreases with increasing concentrations. Also,
Fig. 7. Langmuir isotherm in different metal ions for sample 1.
Fig. 8. Langmuir isotherms in different metal ions for sample 2.
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the selectivity of the nanocomposite was examined in the presence of copper, nickel and cadmium ions solution. As a result, the desirable adsorption about 60% was achieved for all ions. Overall, the results confirmed that higher efficiency is at the first moments of experimental work and contact time did not influence the adsorption process. Using a different value of Fe3O4 on the geopolymer as
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nanoparticles have no specific effect on the efficiency of the adsorption, however, it positively affects the magnetization of the sample and easy separation of nanocomposite from heavy metals solution. The purpose of this research is to simplify the adsorption process by using the novel magnetic nanocomposites. The indicated condition, confirm the high performance of nanocomposite at
Fig. 9. Langmuir isotherms in different metal ions for sample 3.
Fig. 10. Langmuir isotherms in different metal ions for sample 4.
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ambient temperature without any stirring or changed in the pH of the solution under normal atmospheric conditions. As the result, the nanocomposite 5 and 2 min contact time were chosen for analyses and adsorption study. Moreover, it should be mentioned that this nanocomposite could not adsorb chromium ions.
solution. So, after the adsorption process and removing the nanocomposite, the concentration of Naþ ion in the remaining ions solutions was determined by atomic absorption spectrometry analysis. The absence of Naþ ions confirmed that the ion exchange was not occurred.
3.2.1. Mechanism of adsorption Due to the use of porous nanocomposite and the short contact time, the physical mechanism was suggested (Scheme 1). According to high adsorption efficiency, the possible ions exchange process could be confirmed by the presence of Naþ ion in the metal ion
3.2.2. Adsorption isotherms An adsorption isotherm is important towards the design of an adsorption system. A measured adsorption isotherm has been fitted with Langmuir isotherm equation at different metal ions concentration (2600, 3000, and 5000 ppm).
Fig. 11. Langmuir isotherms in different metal ions for sample 5.
Table 5 The Langmuir coefficients for different samples in different metal ions. Samples
0 1 2 3 4 5
Cu(II)
Pb(II)
Ni(II)
Hg(II)
Cd(II)
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
R2
KL
Qm
0.985 0.986 0.986 0.983 0.983 0.983
0.0122 0.0129 0.0146 0.0097 0.0101 0.0102
502.18 502.92 495.18 511.38 508.01 510.29
0.935 0.935 0.897 0.916 0.917 0.994
0.0015 0.0016 0.0009 0.0009 0.0008 0.051
543.42 539.65 551.03 550.96 552.98 1227.2
0.849 0.864 0.845 0.843 0.822 0.912
0.00482 0.00599 0.00203 0.00192 0.00237 0.0513
288.44 256.62 391.9 397.74 343.25 1227.2
0.812 0.803 0.807 0.796 0.806 0.788
0.0028 0.0025 0.0022 0.0018 0.0027 0.0013
304.79 313.54 335.67 350.0 305.74 385.33
0.868 0.840 0.843 0.843 0.841 0.848
0.0023 0.0015 0.0014 0.0014 0.0015 0.0014
398.61 419.22 430.25 430.23 422.02 433.34
Table 6 Comparison the efficiency of geopolymer/Fe3O4 nanocomposite with other reported works. Entry
Adsorbent
L/S ratio
Temp. ( C)
Time (h)
Ref.
1 2 3 4 5 6
Carbon nanotube Chitosan-GLA Sawdust Active carbon Mag/CS Geopolymer/Fe3O4
5 1 40 5 1 5
r.t. r.t. 23 C 20 C r.t. r.t.
4 1.5 24 3 6 0.033
Stafiej and Pyrzynska (2007) Wang Ngah and Fatinathans (2008) Ya et al. (2001) ska et al. (2017) Kołodyn Horst et al. (2016) This work
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In the Langmuir model, the adsorption of metal ions occurs on the homogeneous surfaces and the active sites of the adsorbent. Adsorption is a single layer interaction between the species adsorbed on it. The Langmuir equation, eq. (2) is: Ce/Qe ¼ 1/qmKL þ 1/qm (Ce) (Langmuir, 1918)
(2)
Where qm (mg/g) is the maximum adsorption capacity and KL (L/mg) is the Langmuir constant calculated from the plot 1/qe to 1/ Ce. Qe (mg/g) is the amount of adsorbed metal, and Ce (mg/L) is concentration of the metal equilibrium. Figs. 6e11 shows the Langmuir isotherm graph for the 3000 ppm solution of different ions in the presence of different nanocomposites. Additionally, Table 5 tabulates the Langmuir coefficients for the different samples (with different Fe3O4 nanoparticles) and metal ions. The adsorption capacity was calculated using the following equation, eq. (3). qm¼ (C0-Ceq)V/m
(3)
Where C0 is initial concentration of metal ions (ppm), Ceq is the concentration of the remaining ions in the metal solution (ppm), V
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is the volume of heavy ions solution (L) and m is the weight of adsorbent (g). Then, the adsorption isotherm constant (qm and KL) were calculated using the slope of the equation of a line. The correlation coefficients (R2) were determined to be 0.78e0.99 for the different metal ions. The R2 for the Langmuir isotherms are near 1, the monolayer adsorption is obtained, and the process is exceptionally desirable. According to Figs. 6e11 and Table 5, the desirable rates of the samples were 2˂3˂4˂0˂1˂5. Moreover, in the case of nickel, the R2 in sample 5 exceeds 0.9, being close to 1. However, for Mercury and Cadmium, the correlation coefficients are less than 0.9, and not close to 1. Therefore, the metal ions order according to Langmuir model are Cu(II)>Pb(II)>Ni(II)>Hg(II)>Cd(II), and sample 5 with 0.03 g of Fe3O4 nanoparticles reported better adsorbent capacity. 3.3. Comparison the efficiency of geopolymer/Fe3O4 nanocomposite with literature reports The performance of geopolymer/Fe3O4 nanocomposite was investigated. As can be seen in Table 6, several adsorbents have been used for the adsorption of heavy metal ions. Also, they have shown high adsorption capacity. But, most of them suffer from disadvantages such as long contact time and difficult recycling and
Fig. 12. The FT-IR spectra of: a) geopolymer/Fe3O4, b) geopolymer/Fe3O4 in alkaline solution, c) geopolymer/Fe3O4 in acid solution.
Fig. 13. a) Recycling diagram of geopolymer/Fe3O4 nanocomposite, b) The FT-IR spectra of fresh nanocomposite and recycled nanocomposite.
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recovering of adsorbent. Therefore, the results of the present work are more reliable than previous ones. 3.4. Stability of nanocomposites in different pH The stability of the nanocomposite was examined in both acidic and alkaline solutions. The durability of geopolymer/Fe3O4 nanocomposite in alkaline solution was evaluated by the dispersion of geopolymer/Fe3O4 nanocomposite (0.05 g) in 10 mL sodium hydroxide (0.1 M). The nanocomposite was stable in alkaline solution and this result was confirmed by FT-IR analysis (Fig. 12b). In addition, the stability of the nanocomposite in pH ¼ 1 was checked by adding 0.05 g of the nanocomposite in 10 mL of hydrogen chloride solution 0.1 M (5 mg/mL). The characteristic band of sodium carbonate in 1400 cm1 should disappear in acid solution due to decomposition of Na2CO3 to Na2O and CO2. However, the feature band of Na2O was around 1400 cm1 and appear in the same region with carbonate (Wu et al., 2016). As can be seen in Fig. 12c, the geopolymer/Fe3O4 was also stable in acidic solution. 3.5. Reusability and stability of magnetic geopolymer nanocomposite The reusability of the nanocomposite was studied for copper ion adsorption (3000 ppm and 10 min). Firstly, for desorption process and removing adsorbed metal ions from recycled nanocomposite, geopolymer/Fe3O4 nanocomposite was washed with deionized water, ammonium chloride (4 M) and sodium hydroxide (0.2 M) (Krishnamurti et al., 1999., Lata et al., 2015). Then, magnetic nanocomposite was dried and reused at least 4 times without any significant lose in adsorption (Fig. 13a). Furthermore, the stability of recycled geopolymer/Fe3O4 nanocomposite was confirmed by FT-IR spectra (Fig. 13b). 4. Conclusions Magnetic nanocomposite based on geopolymer has been successfully synthesized in simple and mild conditions. This nanocomposite has excellent adsorption in the case of copper, lead, and nickel, cadmium, and mercury ions from industrial wastewaters at room temperature without being stirred or changed pH in normal atmospheric conditions. According to the adsorption result and Langmuir model, the copper ion was best adsorbed. Composites synthesized in this work can be effectively applied for wastewater treatment, heavy metal ions removal in various purposes especially for cleaner industrial production and inexpensive waste management. Also, the effect of Fe3O4 nanoparticles was investigated. The result shows that the different amounts of Fe3O4 nanoparticles have no specific effect on the efficiency of adsorption, while it has a positive effect on the magnetization and easy separation. This study has the most significant advantages such as using fast, inexpensive and eco-friendly material, easy preparation method, and simplicity in application, magnetic separation of the adsorbent and high efficiency at ambient condition. Acknowledgement The authors thank the Iran University of Science and Technology for partial financial support by the Research Council and the reviewers for their valuable comments and suggestions. Appendix A. Supplementary data Supplementary data to this article can be found online at https://doi.org/10.1016/j.jclepro.2019.01.084.
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