Accelerated crystallization of magnetic 4A-zeolite

Journal of Hazardous Materials 358 (2018) 441–449

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Journal of Hazardous Materials journal homepage: www.elsevier.com/locate/jhazmat

Accelerated crystallization of magnetic 4A-zeolite synthesized from red mud for application in removal of mixed heavy metal ions

T

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Wu-Ming Xiea, , Feng-Ping Zhoua, Xiao-Lin Bia, Dong-Dong Chena, Jun Lia, Shui-Yu Sunb, Jing-Yong Liua, Xiang-Qing Chenc a

School of Environmental Science and Engineering, Guangdong University of Technology, Guangzhou, Guangdong 510006, PR China Guangdong Polytechnic of Environmental Protection Engineering, Foshan 528216, PR China c Zhengzhou Research Institute of CHALCO, Zhengzhou, Henan 450041, PR China b

G R A P H I C A L A B S T R A C T

Magnetic 4A-zeolite was synthesized from red mud alone based on the comprehensive technique route and used for mixed heavy metals removal.

A R T I C LE I N FO

A B S T R A C T

Keywords: Red mud Magnetic 4A-zeolite Sodium chloride Accelerated crystallization Adsorption

To cope with the increasing environmental issues of red mud, an integrated technological route for its comprehensive utilization was developed through the extraction of valuable components and the synthesis of magnetic 4A-zeolite. To accelerate the crystallization process of the synthesized 4A-zeolite, sodium chloride (NaCl) was innovatively employed under hydrothermal treatment. The effects of various parameters, including mass ratio of red mud/NaOH, alkali fusion temperature, alkali fusion time and molar ratio of NaCl/Al2O3, were systematically investigated. The results showed that approximately 81.0% Al, 76.1% Si and 95.8% Fe were utilized from red mud using alkali fusion and acid leaching methods. The optimal conditions of the alkali fusion process were determined as: mass ratio of red mud/NaOH = 1/2, alkali fusion temperature of 800 °C, and time of 90 min. Furthermore, when the molar ratio of NaCl/Al2O3 was kept at 1.5, the crystallization time reduced from 240 min to 150 min, and particle size distributions narrowed from 20–100 μm to 1–10 μm. The practical

⁎

Corresponding author. E-mail address: [email protected] (W.-M. Xie).

https://doi.org/10.1016/j.jhazmat.2018.07.007 Received 2 January 2018; Received in revised form 25 June 2018; Accepted 2 July 2018 Available online 06 July 2018 0304-3894/ © 2018 Elsevier B.V. All rights reserved.


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applications in removal of mixed heavy metal ions (Zn2+, Cu2+, Cd2+, Ni2+, and Pb2+) from wastewater indicated that the as-synthesized magnetic 4A-zeolite is a promising candidate for heavy metals adsorption.

1. Introduction

magnetic 4A-zeolite, which can be easily separated by simple magnetic field. The synthesis of magnetic 4A-zeolite based on red mud alone has not yet been reported, and the most common method involves a hydrothermal process but limited by long crystallization time (4 h–13 h) [34–36]. The crystallization process of 4A-zeolite can be described through three steps, including polymerization, depolymerization, and repolymerization of the reactant gels in strong basic conditions. These steps comprise the formation, liberation and growth stages of the nuclei formed in the reactant gel, which were involved multiple equilibrium during the whole crystallization process. Some studies attempted to accelerate the polymerization/depolymerization process by adding various additives, such as TMAOH [37], TEA [38], oxyanions [39] and hydroxyl free radicals [40]. It seems that to improve the polymerization or depolymerization process of the gel can build up the repolymerization process, resulting in accelerated the crystallization. However, some of these additives are expensive and/or toxic to the environment. Herein, sodium chloride (NaCl) with low biotoxicity and cost was innovatively employed in this study to accelerate the crystallization process of 4A-zeolite, resulting in improving the productivity. More importantly, to take full advantage of the constituents of red mud, an integrated route for comprehensive utilization of red mud was conceived, and the final undissolved residue can also be used as a potential by-product of geopolymer without further dealkaline treatment since acid digestion was employed. In our research, the effects of mass ratio of red mud/NaOH, alkali fusion temperature, alkali fusion time and molar ratio of NaCl/Al2O3, were systematically investigated. Finally, the adsorption of mixed heavy metals of the synthesized magnetic 4Azeolite were also studied to extend their applications in terms of converting red mud into useful materials.

Red mud is a by-product of alumina production issued during alkaline-leaching of bauxite ores using the Bayer process. Approximately 0.8–1.5 tons of red mud are generated per ton of alumina production depending on the used bauxite ores and operating conditions [1]. The disposal of red mud in landfills and sea created large negative environmental impact. The safe disposal and storage of red mud are currently under international focus and better reduction of large quantities is by reusing it [2,3]. Recent developments in red mud explored a range of industrial, environmental and engineering processes, such as adsorption of toxic gasses and heavy metals [4–8], catalysis [9,10], soil remediation [11], and framework for sewage sludge conditioning [12]. However, due to the high alkalinity of red mud, neutralization process has to be employed by seawater [13], fungus [14], or CO2 [15] before utilization, preventing its large-scale practical application. Thus, new approaches are needed for reducing large quantities of red mud. On the other hand, red mud could be considered as a reusable secondary resource instead of waste since it contains potential valuable compounds, such as Al2O3, SiO2 and Fe2O3. With the development of technology, various strategies have been investigated for recovering the major components of red mud. For example, Li et al. [16] used a high gradient superconducting magnetic separation (HGSMS) system to separate the extremely fine red mud particles (< 100 μm) into the high and low iron content fractions. Li et al. [17] have successfully synthesized uniform hierarchical porous γ-AlOOH microspheres via a hydrothermal route using NaAlO2 leached from red mud in the presence of urea. However, most of the researches were just focused on the recovery of monocomponent from red mud, and then little attention has been paid for the other components in the residue. Therefore, to develop a comprehensive technique routine for taking full advantage of red mud components especially for Al2O3, SiO2 and Fe2O3 as secondary resource would be of inherent economic value. Among the methods employed for recycling valuable components from red mud, the synthesis of zeolite attracted a great deal of attention [18–22]. Among the different minerals possessing adsorbent properties, zeolite is promising for metal purification [23]. Zeolites are usually considered multiporous, referring to crystalline aluminosilicate or silica polymorph based on corner sharing TO4 (T = Si and Al) tetrahedral. These form a three-dimensional four connected frameworks with uniformly sized pores of molecular dimensions [24]. Due to its unique pore size and framework structure, 4A-zeolite has widely been used in adsorbents [25,26], catalysis [27,28], and detergent builders [29,30]. Therefore, the synthesis of 4A-zeolite has attracted increasing interest. However, the costs of the raw materials based on pure chemical agents are relatively high for 4A-zeolite synthesis. Hence, various waste products from certain industries are being explored as ingredients to reduce production costs. Due to high SiO2 and Al2O3 contents, kaolin and coal fly ash are used as the principal raw materials to reduce the cost of production and to reuse the waste materials [31–33]. In this context, red mud can be considered as a suitable alternative source due to its high silicon and aluminum contents. Although previous studies succeeded to synthesis 4A-zeolite red mud as raw materials, only Al2O3 and SiO2 have been reutilized from red mud. This generated new contaminations since other elements remained in undissolved residuals. Moreover, the separation of the synthesized 4A-zeolite powder from the complex heterogeneous system remains a major challenge for largescale industrial applications. Therefore, Fe2O3 as a major constituent of red mud could also be reutilized as main Fe3+ resources of magnetic particles for combination with the synthesized 4A-zeolite to form

2. Experimental 2.1. Materials and reagents The red mud used in this study was obtained from Zhengzhou, Henan Province, China. The chemical compositions of red mud were analyzed by X-ray fluorescence (XRF) (Table 1), and found to contain mainly Al2O3, SiO2, Fe2O3 and Na2O. Hydrochloric acid (HCl, analytical grade), sodium hydroxide pellets (NaOH, analytical grade) and ferrous chloride (FeCl2, analytical grade) were used to prepare magnetic 4Azeolite. The water used for preparing all solutions was obtained from Milli-Q deionized water system. 2.2. Synthesis of magnetic 4A-zeolite Three steps were employed to produce magnetic 4A-zeolite from red mud. The first step was based on reutilizing Al, Si and Fe components from red mud by using alkali fusion and acid leaching methods. Different mass ratios of red mud/NaOH were prepared, placed in a graphite crucible and calcined in a muffle furnace. The alkali fusion Table 1 Chemical compositions of red mud (wt.%).

Red mud a b

442

Al2O3

SiO2

Fe2O3

Na2O

CaO

Othersa

LOIb

16.56

17.75

23.96

2.08

21.11

8.01

9.83

K2O, TiO2, SO3, MgO, MnO, and ZrO2. LOI: Loss on ignition.


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size were analyzed via CO2 adsorption at 0 °C on a Micromeritics ASAP 2020 analyzer with samples outgassed at 300 °C for 3.0 h under vacuum before measurements.

temperature and alkali fusion time were investigated from 400 °C to 800 °C and 30 min to 300 min, respectively. After completion of the alkali fusion process, the samples were then added into a water leaching reactor by keeping the leaching process at around 50 °C and string for 30 min. And then, the solution was immediately separated from the mixture by filtration and used as zeolite precursors. Due to their chemical properties, iron components could not react with NaOH under the alkali fusion conditions. Therefore, they can be reutilized by acid leaching (1 M L−1 HCl) as Fe3+ resources to produce magnetic particles. The second step dealt with the synthesis of 4A-zeolite. Red mud prepared pure 4A-zeolite was synthesized under hydrothermal treatment at the molar ratio of SiO2/Al2O3:Na2O/SiO2:H2O/Na2O as 1.6:1.5:30 according to a previously published procedure with a little modified [41]. A precalculated NaCl was added together with the precursor gel with continuous stirring for 30 min. After the dissolution of NaCl, the mixture was transferred to a Teflon-lined stainless-steel autoclave and crystallized by thermal treatment under autogenous pressure and static conditions at 95 °C for 240 min. The final step dealt with the formation of magnetic 4A-zeolite according to the procedure reported by Alireza [42]. In brief, the synthesized 4A-zeolite was suspended in 200 mL nitrogen purged water and then 30 mL of 1:2 of Fe3+:Fe2+ molar ratio supplied by FeCl2 solution was added dropwise in a glove box under nitrogen to prepare the magnetic particles incorporated 4A-zeolites. The pH of the solution was adjusted either by adding hydrochloric acid 0.1 M (HCl) or sodium hydroxide 0.1 M (NaOH) to keep the pH at 11. The composites were obtained by filtration and washing with distilled water until the pH of the washed solution reached 7. They were subsequently dried in an oven overnight at 95 °C followed by annealing under N2 for 6 h at 450 °C. To explore the practical applications of the synthesized magnetic 4A-zeolite, adsorption experiments were performed to remove mixed heavy metal ions (Zn2+, Cu2+, Cd2+, Ni2+, and Pb2+) in aqueous solutions. All the reactions mentioned above could be summarized as follows, and the experimental parameters are listed in Table 2.

Al2O3(s) +2NaOH(s) → Na2Al2O4(s) + H2 O(g )

(1)

SiO2(s) +2NaOH (s) → Na2SiO3(s) + H2 O(g )

(2)

2.4. Batch sorption experiments 2.4.1. Adsorption isotherms Adsorption isotherms of mixed heavy metals on the adsorbent were also carried out by a batch equilibration technique. Adsorbent (0.1 g) was added to 100 mL centrifugal tubes, which was then filled with 50 mL of mixed heavy metal solution with different initial ion concentrations (50, 100, 200, 300, 400, 500, and 800 mg L−1) at adjusted pH of 4. The centrifugal tubes were shaken for 24 h at 25 ± 0.5 °C in a constant temperature oscillator. The suspensions were then centrifuged, and the resulting supernatants were collected to determine the equilibrium ion concentrations. The adsorption equilibrium results were fitted with Langmuir and Freundlich models, the most frequently used to investigate the equilibrium between adsorbent and adsorbate [43]. The mathematical expressions were presented as follows:

Langmuir sorption isotherm Freundlich

FeCl 2(aq) +2FeCl3(aq) +8NaOH(aq) → Fe3 O4(s) +8NaCl (aq) +4H2 O(l)

(5)

model: qe = KF Cen

(6) (7)

2.4.2. Adsorption kinetics experiments The adsorption kinetics experiments were performed in a batch reactor (250 mL) at 25 ± 0.5 °C with continuous stirring at 300 rpm. The bath contained 100 mL of mixed heavy metal ions (pb2+, Zn2+, Cu2+, Ni2+, and Cd2+) dissolved in 100 mg L−1 at the initial pH values of 4 added to 0.1 g of the magnetic 4A-zeolite of the prepared red mud. Aliquots of the supernatant (0.5 mL) were withdrawn at different time intervals (from 5 to 300 min) and the total sampling volume retained at 5% of the total solution volume. The supernatants were filtered off with 0.45 μm filter. The filtrates were acidified with 2% HNO3 to decrease the pH value and prevent any precipitation before ICP-MS measurement. The adsorbent was finally separated by a magnet. The sorption kinetic defining the efficiency of sorption of mixed heavy metal ions was checked by pseudo-first-order and pseudo-second-order [44], defined as follows:

(3) (4)

isotherm

KL Ce 1 + KL Ce

Where Ce (mg L−1) and qe (mg g−1) are the metal ion concentration in the solution and adsorbed metal ion on the adsorbent at equilibrium, qm (mg g−1) and KL (L mg−1) are the Langmuir constants, KF and n are the Freundlich isotherm parameters related to the adsorption capacity and intensity of adsorption, respectively.

96NaAlO2(aq) +96Na2SiO3(aq) +312H2 O(l) → Na 96Al 96Si96 O384 ⋅216H2 O(s ) +192NaOH(aq)

Fe2 O3(s) +6HCl (aq) → 2FeCl3(aq) +3H2 O(l)

sorption

model: qe = qm

2.3. Analytical methods

Table 2 . Experimental parameters for the alkali fusion stage.

Si and Al levels in the filtrate were determined by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) manufactured by PerkinElmer with an internal standard method after the nitric acid digestion in Mars microwave digestion instrument. X-ray diffraction (XRD) patterns were conducted at 25 °C in a Bruker D8 Advance X-ray diffractometer using monochromatized Cu/Kα radiation (40 kV,40 mA). The sample was scanned with a step size of 0.02° and accounting time of 0.2 s per step. X-ray fluorescence (XRF) spectra were recorded on a Shimazu EDX-70000 X-ray fluorescence spectrometer. Fourier transform infrared (FT-IR) analysis was conducted on a Bruker Tensor 27 spectrometer. FT-IR spectra in transmittance mode were recorded in the range of 400–4000 cm−1 at a resolution of 4 cm−1 using the KBr pressed disk technique. Magnetic measurements were performed at room temperature by using Lake Shore Value stream mapping (VSM) 7400 with a maximum magnetic field of 1 T. Scanning electron microscopy (SEM) analysis was carried out on HITACHI S4800 machine. Mastersizer 2000 was employed to determine the particle size distributions of the synthesized 4Azeolite. The Zeta potential of the gel obtained from the precursor solution was measured by PALS Zeta Potential Analyzer. The surface area and pore 443

Samples

Mass Ratio of Red Mud/NaOH

Alkali Fusion Temperature (oC)

Alkali Fusion Time (min)

AF-1 AF-2 AF-3 AF-4 AF-5

2/1 1.5/1 1/1 1/1.5 1/2

600 600 600 600 600

90 90 90 90 90

AF-6 AF-7 AF-8 AF-9 AF-10

1/2 1/2 1/2 1/2 1/2

400 500 600 700 800

90 90 90 90 90

AF-11 AF-12 AF-13 AF-14 AF-15

1/2 1/2 1/2 1/2 1/2

600 600 600 600 600

30 60 90 120 150


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Pseudo-first-order kinetics

Pseudo-second-order

model: log(qe − qt ) = log qe −

kinetics

model:

k1 t 2.303 (8)

t 1 t = + qt qe k2 × qe2

(9) −1

where qe and qt are the amounts of heavy metals adsorbed (mg g ) at equilibrium and at any time t (min), respectively, k1 is the pseudo-firstorder kinetics model rate constant (min−1) and k2 is the pseudo-secondorder kinetics model rate constant (g mg−1 min−1). 3. Results and discussion 3.1. Effects of alkali fusion process on reutilizing Al and Si from red mud In alkali fusion process, three key reutilization factors (mass ratio of red mud/sodium hydroxide(NaOH), alkali fusion temperature, and alkali fusion time) were systematically examined to obtain the optimal fusion conditions. The crystalline structure obtained from XRD was used as the criteria to evaluate the fusion results and the extraction efficiency of chemical components after the filtration process were also investigated by XRF. To reutilize major components of Al and Si issued from red mud, different mass ratios of red mud/NaOH (2/1, 1.5/1, 1/1, 1/1.5, 1/2) were carried out (Fig. 1A). The results suggested that different mass ratio of red mud/NaOH obviously affected the extraction process. Quantities of Na2SiO3 and NaAlO2 as major composites of 4A-zeolite were significantly improved as NaOH was added since the diffractions in the XRD patterns became intensified. The low mass ratio of red mud/ NaOH at 2/1 or 1.5/1 could form the structure of Na2SiO3. However, no formation of NaAlO2 was detected. This indicated that the mass of the

Fig. 2. Crystalline-growth curves of the synthesized 4Azeolite under different molar ratios of NaCl/Al2O3.

used NaOH was insufficient. The increase in mass ratio of red mud/ NaOH to 1/1.5 transformed NaAlO2 into a single phase, meaning that the structure of NaAlO2 can be generated at high alkalinity. As can be seen in the XRD patterns, as of NaOH (red mud/NaOH = 1/2) increased, higher and pure phases of Na2SiO3 and NaAlO2 were observed. Considering that excess alkali can be employed during the next stage of crystallization, mass ratio of red mud/NaOH at 1/2 was selected to be used during the process of alkali fusion.

Fig. 1. (A) XRD patterns of alkali fusion reaction products with different mass ratios of red mud/NaOH; (B) XRD patterns of alkali fusion reaction products with different alkali fusion temperatures; (C) XRD patterns of alkali fusion reaction products with different alkali fusion time; (D): a-c: principal chemical compositions of red mud after the filtration process; d: principal chemical compositions of red mud after the acid leaching process. 444


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when temperature further rose (800 °C), the recovery efficiency of Al2O3 and SiO2 could reach to 61.6% and 60.0%, respectively (Fig. 1Db). Besides, as the more Al2O3 and SiO2 were extracted, the enrichment of Fe2O3 could be obtained. It is worth noting that high purity of Fe2O3 in red mud is beneficial for the acid leaching process. Hence, 800 °C was employed as the optimal alkali fusion temperature to achieve the desired composites. Alkali fusion time is also an important factor affecting the extraction process. The alkali fusion process was carried out under different fusion periods ranging from 30 min to 150 min with the increment of 30 min (Fig. 1C). Single and pure structures were obtained as fusion time arose. However, at the initial fusion time (< 60 min), the remaining composites of quartz and aluminum oxide indicated that the fusion reaction was still incomplete. Higher and pure phases of Na2SiO3 and NaAlO2 can be observed, indicating that the contents of soluble silicate and aluminate salt increased by alkali fusion time reaching 90 min. Because the products of alkali fusion reaction underwent little change at fusion time (> 90 min), the time period of alkali fusion within 90 min was selected as the optimal fusion time. According to discussion mentioned above, the optimal conditions of the alkali fusion stage were: mass ratio of red mud/NaOH at 1/2, 800 °C alkali fusion temperature, and 90 min alkali fusion time. After completion of alkali fusion process, the soluble silicate and aluminate salt were separated from undissolved substances through washing by deionized water and filtration. Undissolved substances were subsequently reutilized to recover the Fe2O3 components using 1 M L−1 HCl solutions at the liquid to solid ratio of 10:1. Fig. 1D-d shows the main chemical composites of the residues after filtering. It can be seen that 81.0% Al, 76.1% Si, and 95.8% Fe were extracted from red mud by alkali fusion and acid leaching. XRF measurements confirmed that valuable components in red mud could be extracted at relatively low energy and costs compared to the previous work [45].

Fig. 3. Particle size distributions of the terminated synthesized 4A-zeolite under different molar ratios of NaCl/Al2O3. Table 3 Zeta Potential, SBET and Average Pore Size of the as-synthesized 4A-zeolite. Samples

Zeta Potential (mV)

SBET (m2 g−1)

Average Pore Size (nm)

a* b* c* d* e* f*

4.06 3.66 1.44 −10.06 −25.38 −39.00

524.14 502.23 430.66 303.45 220.43 105.72

0.41 0.40 0.37 0.41 0.39 0.42

* : a: n(NaCl): n(Al2O3) = 2: 1; b: n(NaCl): n(Al2O3) = 1.5: 1; c: n(NaCl): n (Al2O3) = 1: 1; d: n(NaCl): n(Al2O3) = 1: 1.5; e: n(NaCl): n(Al2O3) = 1: 2; f: NaCl free.

3.2. Effects of sodium chloride (NaCl) on crystallization process of 4Azeolite

Fig. 1B shows the XRD patterns of the reaction products under different alkali fusion temperatures from 400 °C to 800 °C with the increment of 100 °C. The single and pure phases could be observed as the temperature increased. At temperature < 500 °C, lower XRD intensities of Na2SiO3 and NaAlO2 can be attributed to the limited reaction energies. As the fusion temperature > 600 °C, the XRD intensities of Na2SiO3 and NaAlO2 enhanced, meaning that the amounts of Na2SiO3 and NaAlO2 could gradually increase under such temperature conditions. Although the XRD phases of the products have a little change

As mentioned above, although the previous experiments yielded quality and worthy results, the traditional hydrothermal performance of 4A-zeolite synthesis often suffers from long crystallization time (4 h–13 h). Therefore, NaCl as environmental-friendly and low costly electrolyte was used to accelerate the crystallization. The relative crystallinity and particle size distributions of the synthesized 4A-zeolite, under various molar ratios of NaCl/Al2O3 and different crystallization time (from 30 min to 240 min) were investigated at several stages of the synthesis process. The relative crystallinity of

Fig. 4. SEM images of the terminated synthesized 4A-zeolite with and without NaCl: (a) n(NaCl): n(Al2O3) = 1: 2, (b) n(NaCl): n(Al2O3) = 1: 1.5, (c) n(NaCl): n (Al2O3) = 1: 1, (d) n(NaCl): n(Al2O3) = 1.5: 1, (e) n(NaCl): n(Al2O3) = 2: 1, (f) NaCl free. 445


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Fig. 5. (A) XRD patterns and (B) FT-IR spectra of magnetic 4A-zeolite compared with the synthesized 4A-zeolite and pristine ferroferric particles.

on solution-mediated transport [46], solution-solid phase transformation [47], hydrogel transformation [36], and autocatalytic nucleation [48] were proposed. Though the crystallization mechanism of zeolite is still unclear, it is a generally agreed that zeolites are typically synthesized in strongly basic medium, where a high concentration of hydroxide ions (OH−) would assist in mineralization of silicate and aluminate species in the reactant gels [49]. This crystallization process can be divided into three steps. The first step consisted of polymerization and formation of amorphous gel via making Si, AleOeSi, Al bonds. Second, depolymerization and forming of soluble aluminosilicates/silicates will form via breaking Si, AleOeSi, Al bonds. Third, repolymerization and remaking of Si, AleOeSi, Al bonds will occur around the hydrated cation species through condensation reaction, comprising the nucleation and crystal growth stage of the crystallization. The nuclei should uniformly be distributed in the gel and subsequently released into the liquid phase as the gel depolymerized. As the nuclei were released into the liquid phase, they grow rapidly at the expense of reactive species dissolved in the liquid phase. This would lower the concentration of these reactive species in the liquid phase and accelerates depolymerization of the gel, further increasing depolymerization of the gel number of released nuclei. The presence of electrolyte impacted stability of the gel. It seems that the added of NaCl accelerated the process of crystallization and influenced the stability of the gel obtained from the precursor solution, accelerating depolymerization. Decaying Zeta potential with continuous addition of NaCl confirmed this hypothesis (Table 3). Lower Zeta potential meant instable gel. Due to limited solubility of aluminosilicates formed during depolymerization process and expense of reactive species in liquid phase, further increase in NaCl (n(NaCl)/n(Al2O3) = 2.0) showed no marked changed in crystallization. Therefore, the molar ratio of NaCl/Al2O3 at 1.5 was selected as the optimum condition according to the analysis mentioned above.

Fig. 6. Magnetization curves of the magnetic 4A-zeolite compared with the pristine ferroferric particles. The insert table showed the magnetic parameters of the magnetic 4A-zeolite and pristine ferroferric particles.

4A-zeolite was defined as the ratio of the integrated intensity of five most intense peaks in XRD patterns (2θ = 21.70°, 24.02°, 27.16°, 29.98° and 34.22° in Fig. 5A) of the sample obtained at different crystallization time respect to a well-crystallized standard sample obtained under the same conditions. The additive of NaCl significantly decreased the crystallization time and led to narrower particle size distributions (PSDs) (Figs. 2 and 3, respectively). As the molar ratio of NaCl/Al2O3 increased from 1/2 to 1.5/1, the crystallization rate of 4A-zeolite dramatically accelerated, reducing the crystallization time from 240 min to 150 min. These were equivalent to 1.6–5.2 folds shorter than those recorded previously. Besides, PSDs of the synthesized 4A-zeolite could also be narrowed from 20–100 μm to 1–10 μm. Due to smaller particle size, larger surface area of the synthesized 4A-zeolite (enlarged from 152.72 m2 g−1 to 502.23 m2 g−1) with an average pore size of 0.41 nm were obtained (Table 3). On the other hand, although the addition of NaCl can influence the crystallization time and PSDs, the morphology of the synthesized 4A-zeolite did not change during crystallization (Fig. 4). To explain the crystallization of zeolite, various mechanisms based

3.3. Characterization of synthesized magnetic 4A-zeolite X-ray diffractogram was used to identify the crystal components in the synthesized magnetic 4A-zeolite. Fig. 5A illustrates the XRD patterns of the synthesized 4A-zeolite with and without magnetic particle. The intensities of the pristine ferroferric particle were also detected as the criteria to evaluate success of the synthesis. The peaks of the synthesized 4A-zeolite agreed well with expected sample peaks given in relevant literatures [29–31,50]. Also, the XRD patterns of the 446


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Fig. 7. (A) and (B) is the comparison of isothermal sorption model for mixed heavy metals removal onto the synthesized magnetic 4A-zeolite under different initial ion concentrations; (C) and (D) is the comparison of kinetic model for mixed heavy metals removal onto the synthesized magnetic 4A-zeolite under different contact times. Table 4 The calculated parameters of pseudo-first-order kinetic model and pseudo-second-order kinetic model for the sorption of mixed heavy metals onto the synthesized magnetic 4A-zeolite. C0 (mg g−1)

100

Metal ions

Zn2+ Cu2+ Cd2+ Ni2+ Pb2+

qe,

exp

43.65 39.07 55.33 38.74 99.84

(mg g−1)

Pseudo-first-order Kinetic model: log(q e−qt )=logq e− k1 (min−1)

qe,

1.13 × 10-2 2.00 × 10-3 3.91 × 10-3 1.42 × 10-2 6.22 × 10-3

25.26 26.92 41.08 31.56 2.061

cal.

(mg g−1)

k1 t 2.303

Pseudo-second-order Kinetic model:

R2

k2 (g mg−1 min−1)

qe,

0.968 0.843 0.950 0.941 0.992

1.02 × 10−3 6.90 × 10−4 3.68 × 10−4 8.56 × 10−4 4.76 × 10−2

45.45 35.59 56.50 41.15 100.00

cal.

t 1 = qt k2 × q2 e

(mg g−1)

+

t qe

R2 0.999 0.999 0.997 0.996 1.000

hydroxyl groups to form the magnetic composite. The broad bands at 3446.4, 1652.9 and 1437.5 cm−1 were attributed to water of hydration when compared with the main adsorption bands of synthesized zeolite reported in literatures [51,52]. The magnetic properties of the synthesized magnetic 4A-zeolite were evaluated by VSM at room temperature and compared with the pristine ferroferric particles (Fig. 6). The magnetic parameters are presented in the Insert Table. For both ferroferric particles and synthesized magnetic 4A-zeolite, the increase in external magnetic field intensity raised the magnetization, then saturation magnetizations reached to 71.56 and 66.02 emu g−1, respectively. The decrease in synthesized magnetic 4A-zeolite on saturation magnetizations was attributed to the effects of dispersion of non-magnetic volume of the

synthesized magnetic 4A-zeolite did not show any notable change in 4A-zeolite diffraction peaks. This suggested that obvious damage in zeolite framework did not take place, and the magnetic particles could be found within the XRD features of the synthesized 4A-zeolite. FT-TR spectrum also confirmed this consequence. In particular, FTIR spectrum of magnetic 4A-zeolite illustrated the overlapping in the absorption bands of the synthesized 4A-zeolite and magnetic particles from 750 to 500 cm−1 (Fig. 5B). Well-defined band of FeeO vibrations of the magnetic particles can be seen at 578.5 cm−1. By comparing the spectra of the samples containing magnetic particles, the intensity of strong and broad absorption from 1700 to 1000 cm−1 decreased, indicating possible formation of electrostatic forces and hydrogen bonding between synthesized 4A-zeolite and magnetic particles with 447


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mud as the source of SiO2, Al2O3 and Fe3+ was carried out. During alkali fusion stage, the maximum dissolubility of SiO2 and Al2O3 occurred at mass ratio of red mud/NaOH of 1/2, alkali fusion temperature of 800 °C, and alkali fusion time of 90 min. For crystallization process, NaCl as an environmental-friendly and low-cost electrolyte could be mixed with the gel solution to significantly accelerate the crystallization process by keeping the molar ratio of n(NaCl)/n(Al2O3) at 1.5. This reduced the crystallization time from 240 min to 150 min and narrowed particle size distributions from 20–100 μm to 1–10 μm. Additionally, the contents of Fe2O3 in red mud were recovered after the alkali fusion stage to produce magnetic particles growth on the 4Azeolite surface through co-precipitation process formed during synthesized magnetic 4A-zeolites. This could be used as adsorbents to remove mixed heavy metals from wastewater. In sum, the synthesized magnetic 4A-zeolite from waste materials would have huge potential applications in sorption for heavy metal removal at low-cost and high simplicity separation.

Table 5 The calculated parameters of Langmuir isotherm model and Freundlich isotherm model for the sorption of mixed heavy metals onto the synthesized magnetic 4A-zeolite. Metal

Zn2+ Cu2+ Cd2+ Ni2+ Pb2+

Langmuir isotherm q e=qm KL Ce /(1+KL Ce)

Freundlich isotherm q e=KF Cne

qm,L (mg g−1)

KL (L mg−1)

R2

KF (mg g−1) (L mg−1)1/n

n

R2

331.46 136.33 131.96 119.70 116.81

8.34 × 10−2 0.50 × 10−2 1.81 × 10−2 1.05 × 10−2 0.80 × 10−2

0.971 0.973 0.957 0.959 0.980

91.21 5.27 22.33 12.72 8.63

0.22 0.47 0.27 0.33 0.38

0.779 0.961 0.930 0.947 0.903

synthesized 4A-zeolite phase in the composite magnetic 4A-zeolite. By applying a reverse external magnetic field, the magnetization achieved a reverse saturation. The magnetic hysteresis loops of the synthesized magnetic 4A zeolite exhibited approximately S-shaped curves with high remanent magnetizations (11.98 emu g−1) and coercivity (−89.34 Oe) when compared to pristine ferroferric particles (10.12 emu g−1 of remanent magnetizations and −109.36 Oe of coercivity). The remarkable similarities in shapes of the curves and magnetic parameter values suggested that the synthesized ferroferric particles were formed and kept their main features. In other words, they did not present any modification in their magnetic properties when incorporated onto 4Azeolite surface.

Acknowledgements This work is financially supported by the Key Science and Technology Special Project of Guangdong Province (No. 2015B010110004), and the Science and Technology Program of Guangzhou (No. 2014Y2-00214). References [1] Y. Hua, K.V. Heal, W. Friesl-Hanl, The use of red mud as an immobiliser for metal/ metalloid-contaminated soil: a review, J. Hazard. Mater. 325 (2017) 17–30. [2] Y. Liu, R. Naidu, Hidden values in bauxite residue (red mud): recovery of metals, Waste Manage. 34 (2014) 2662–2673. [3] H.I. Gomes, W.M. Mayes, M. Rogerson, D.I. Stewart, I.T. Burke, Alkaline residues and the environment: a review of impacts, management practices and opportunities, J. Clean. Prod. 112 (2016) 3571–3582. [4] Y. Wang, Y. Cheng, M. Yu, Y. Li, J. Cao, L. Zheng, H. Yi, Methane explosion suppression characteristics based on the NaHCO3 /red-mud composite powders with core-shell structure, J. Hazard. Mater. 335 (2017) 84–91. [5] I. Smičiklas, S. Smiljanić, A. Perić-Grujić, M. Šljivić-Ivanović, M. Mitrić, D. Antonović, Effect of acid treatment on red mud properties with implications on Ni(II) sorption and stability, Chem. Eng. J. 242 (2014) 27–35. [6] R.N. Collins, M.W. Clark, T.E. Payne, Solid phases responsible for MnII, CrIII, CoII, Ni, CuII and Zn immobilization by a modified bauxite refinery residue (red mud) at pH 7.5, Chem. Eng. J. 236 (2014) 419–429. [7] G. Lopes, L.R.G. Guilherme, E.T.S. Costa, N. Curi, H.G.V. Penha, Increasing arsenic sorption on red mud by phosphogypsum addition, J. Hazard. Mater. 262 (2013) 1196–1203. [8] I. Smičiklas, S. Smiljanić, A. Perić-Grujić, M. Šljivić-Ivanović, D. Antonović, The influence of citrate anion on Ni(II) removal by raw red mud from aluminum industry, Chem. Eng. J. 214 (2013) 327–335. [9] N.I. Bento, P.S.C. Santos, T.E. de Souza, L.C.A. Oliveira, C.S. Castro, Composites based on PET and red mud residues as catalyst for organic removal from water, J. Hazard. Mater. 314 (2016) 304–311. [10] S.C. Kim, S.W. Nahm, Y. Park, Property and performance of red mud-based catalysts for the complete oxidation of volatile organic compounds, J. Hazard. Mater. 300 (2015) 104–113. [11] X. Kong, Y. Guo, S. Xue, W. Hartley, C. Wu, Y. Ye, Q. Cheng, Natural evolution of alkaline characteristics in bauxite residue, J. Clean. Prod. 143 (2016). [12] H. Zhang, J. Yang, W. Yu, S. Luo, L. Peng, X. Shen, Y. Shi, S. Zhang, J. Song, N. Ye, Mechanism of red mud combined with Fenton’s reagent in sewage sludge conditioning, Water Res. 59 (2014) 239–247. [13] M. Johnston, M.W. Clark, P. McMahon, N. Ward, Alkalinity conversion of bauxite refinery residues by neutralization, J. Hazard. Mater. 182 (2010) 710–715. [14] Y. Qu, B. Lian, B. Mo, C. Liu, Bioleaching of heavy metals from red mud using Aspergillus niger, Hydrometallurgy (2013) 71–77. [15] R.C. Sahu, R.K. Patel, B.C. Ray, Neutralization of red mud using CO2 sequestration cycle, J. Hazard. Mater. 179 (2010) 28–34. [16] Y. Li, J. Wang, X. Wang, B. Wang, Z. Luan, Feasibility study of iron mineral separation from red mud by high gradient superconducting magnetic separation, Phys. C: Supercond. 471 (2011) 91–96. [17] J. Li, L. Xu, P. Sun, P. Zhai, X. Chen, H. Zhang, Z. Zhang, W. Zhu, Novel application of red mud: facile hydrothermal-thermal conversion synthesis of hierarchical porous AlOOH and Al2O3 microspheres as adsorbents for dye removal, Chem. Eng. J. 321 (2017) 622–634. [18] D.T.N. Quyen, L.C. Loc, H.K.P. Ha, T.H.N. Dang, N. Tri, N.T.T. Van, D.T.N. Quyen, L.C. Loc, H.K.P. Ha, T.H.N. Dang, Synthesis of adsorbent with zeolite structure from red mud and rice husk ash and its properties, International Conference on Chemical Engineering, Food and Biotechnology, (2017), p. 20034. [19] S. Xue, F. Zhu, X. Kong, C. Wu, L. Huang, N. Huang, W. Hartley, A review of the characterization and revegetation of bauxite residues (red mud), Environ. Sci.

3.4. Performance of synthesized magnetic 4A-zeolite towards removal mixed heavy metals The application of the synthesized magnetic 4A-zeolite was performed as adsorbents to remove the mixed heavy metals (Zn2+, Cu2+, Ni2+, Pb2+, and Cd2+) from wastewater. For all metal ions, the Langmuir model represented a better fit of the experimental data than the Freundlich model (comparing the R2 values). The adsorption isotherms of magnetic 4A-zeolite by red mud prepared are shown in Fig. 7A–B. The parameters obtained from the plots, including qm, KL and R2 for Langmuir model, as well as KF, n and R2 for Freundlich model are listed in Table 5. The good agreement between the Langmuir plots and the experimental data suggested that a monolayer coverage of metal ions was deposited on the outer surface of the adsorbent. According to the experimental results, the sorption sequence on magnetic 4A-zeolite could be arranged in the following order: Pb2+ > Cd2+ > Zn2+ > Ni2+ > Cu2+. This could be attributed to the crystal structure, dehydration energy, and hydrated radii of the metal ions [53,54]. A 100 mg L−1 solution of each heavy metal ions was used to study the adsorption kinetics. The log(qe-qt) and t/qt were plotted against time (min) to determine the pseudo-first-order rate constant k1 (Fig. 7C) and pseudo-second-order rate constant k2 (Fig. 7D), respectively. The result indicated that the pseudo-second-order model was suitable for describing the equilibrium of mixed heavy metals on the synthesized magnetic 4A-zeolite. The regression correlation coefficients were > 0.99 and calculated qe values were similar to the experimental values (Table 4). Heavy metal uptake could be attributed to different mechanisms of ion-exchange, as well as the adsorption process by which the ions not only move through the pores of the zeolites but also through channels of the lattice [55,56]. The maximum equilibrium adsorption capacity obtained for Pb2+ was 100.00 mg g−1, which decreased to 56.50 mg g−1 for Cd2+, 45.45 mg g−1 for Zn2+, 41.15 mg g−1 for Ni2+, and 35.59 mg g−1 for Cu2+. 4. Conclusions A systematic study of the synthesized magnetic 4A-zeolite using red 448


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