A magnetic core-shell dodecyl sulfate intercalated layered double hydroxide nanocomposite for the adsorption of cationic and anionic organic dyes

Applied Clay Science 183 (2019) 105309

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Research paper

A magnetic core-shell dodecyl sulfate intercalated layered double hydroxide nanocomposite for the adsorption of cationic and anionic organic dyes

T

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Yan Lia, , Hao-Yu Bib, Ya-Qin Lianga, Xiao-Ming Maoa, Hui Lia a b

Department of Chemistry, Changzhi University, Changzhi 046011, PR China Department of Biomedical Engineering, Changzhi Medical College, Changzhi 046000, PR China

A R T I C LE I N FO

A B S T R A C T

Keywords: Core-shell structure Layered double hydroxides Magnetic separation Adsorption Organic dyes Cu(II)

A magnetic dodecyl sulfate (DS−) anions intercalated layered double hydroxide (LDH) nanocomposite, Fe3O4@(DS-LDH), was synthesized by coprecipitation-ion exchange method. The Fe3O4@(DS-LDH) nanocomposite was characterized and the results showed that the Fe3O4@(DS-LDH) particles possessed core (Fe3O4)shell (DS-LDH) structures and magnetic properties, and DS− anions were successfully introduced into LDH interlayers. Adsorption of cationic methylene blue (MB) or anionic methyl orange (MO) by Fe3O4@(DS-LDH) were comprehensively clarified with magnetic separation in (MB or MO) sole and (MB + Cu(II) or MO + Cu(II)) binary solutions by adsorption kinetics, thermodynamic and isotherm models. The adsorption of both MB and MO on Fe3O4@(DS-LDH) were governed by the pseudo-second-order kinetic model. The adsorption isotherm data for MB (in sole and binary solutions) and MO in binary solution agreed well with the Freundlich model, and that for MO in sole solution was accorded with the linear model. MB uptake decreased with the addition of Cu(II) ions, whereas the existence of Cu(II) ions enhanced MO removal. According to the adsorption mechanism investigation by XRD and XPS analysis, the main driving force for MB/MO adsorption, the hydrophobic interaction between MB/MO molecules and the three-dimensional hydrophobic region formed by DS−anions in LDH interlayers of Fe3O4@(DS-LDH), was proposed; Cu(II) ions could act as a bridge between Fe3O4@(DS-LDH) and MO, which slightly enhanced MO adsorption; whereas, the adsorption capacity for MB decreased in the presence of Cu(II) ions due to competitive adsorption.

1. Introduction Owing to the fast growth of textile industry, a large amount of organic dyes are discharged into water without proper treatment (Natarajan et al., 2011; Wong et al., 2016). Organic dyes, which have high chemical oxygen demand (COD) values, high chroma, and high salt contents, can reduce light transmission, affect the balance of water ecosystems, and cause many aquatic diseases (Srinivasan and Viraraghavan, 2010; Van der Zee and Villaverde, 2005; Kyzas and Kostoglou, 2014), and some dyes and their metabolites are toxic and harmful to organisms, plants, and human beings (Isa et al., 2007). Therefore, removing dyes from wastewater is of great significance. At present, the main treatment methods for dye wastewater are flocculation and sedimentation (Daneshvar et al., 2004; Dotto et al., 2019), photocatalytic oxidation (Shu and Hsieh, 2006; Blanco et al., 2014), adsorption (Zhu et al., 2016; Hassan and Carr, 2018), biological treatment (dos Santos et al., 2007; Vikrant et al., 2018), and membrane processes (Xu et al., 2018; Ji et al., 2019). Adsorption has attracted the

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most attention because of its high efficiency, simple operation, and low cost (Mouni et al., 2018) and is extensively utilized techniques for the removal of organic dyes in a water solution (Prasad et al., 2018). Currently, finding a cheap adsorbent with high removal ratio has become the focus of research, and layered double hydroxide (LDH) is one of the widely studied nanofunctional materials. LDHs are adsorbent materials with high specific surface area and have wide application prospects in the treatment of hydrophilic anion pollutants in water (Chen et al., 2011). An LDH modified with anionic surfactants can form hydrophobic regions in interlayers and enhance the adsorption of hydrophobic pollutants (Klumpp et al., 2004). However, the separation of LDH adsorbent from treated water after adsorption is always difficult because it has a platelet-like structure which tends to disperse in water (Koilraj and Sasaki, 2016). In contrast, the magnetic Fe3O4 particles can be easily removed from treated water in the presence of an external magnetic field. However, magnetic Fe3O4 particles have low chemical stability and provide poor adsorption capacities due to limited adsorption sites on the surface of each particle

Corresponding author. E-mail address: [email protected] (Y. Li).

https://doi.org/10.1016/j.clay.2019.105309 Received 13 April 2019; Received in revised form 16 September 2019; Accepted 18 September 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

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Scheme 1. Schematic diagram for the preparation of Fe3O4@(DS-LDH).

analytical reagent grade and used as received. Freshly boiled doubledistilled water was used in all experiments.

(Tu et al., 2013). Studies have shown that LDHs are the suitable and easily accessible materials to stabilize the magnetic Fe3O4 particles (Prasad et al., 2018). Therefore, the combination of Fe3O4 particles and modified LDHs is expected to offer highly efficient and easily separable adsorbents. Besides, there are other pollutants being discharged in water environment with the acceleration of urbanization, industrialization, and the influence of agricultural production activities (Wang and Chen, 2009). Heavy metals, which can accumulate in the environment and living organisms and are endocrine disruptor compounds, have been well-detected in all types of environmental water as well (Hussain et al., 2017). Therefore, the influence of heavy metal cannot be neglected in the removal of organic dyes from real wastewater, and the responding study will provide a theoretical basis for the development of efficient adsorbents for the treatment of actual dye wastewater. Copper, like all heavy metals, is potentially toxic and usually exist in environment as an element of heavy metal pollution (Awual and Hasan, 2015; OchoaHerrera et al., 2011). In this paper, Fe3O4@LDH nanocomposites with core-shell structures were prepared by coprecipitation, and then Fe3O4@(DS-LDH) nanocomposites were prepared by ion exchange method. The adsorption behavior of Fe3O4@(DS-LDH) nanocomposites for organic dyes from organic dye-heavy metal combined pollutants was investigated. Cationic methylene blue (MB) and anionic methyl orange (MO) were selected as target dyes, respectively, and Cu(II) was selected as the heavy metal ions. Adsorption behavior (adsorption kinetic, isotherm, and thermodynamic) of the Fe3O4@(DS-LDH) nanocomposites for MB/ MO in the presence/absence of Cu(II) were evaluated through batch experimental method with magnetic separation, and the possible adsorption process and mechanism were discussed so as to provide a theoretical foundation for the treatment of organic dye wastewater.

2.2. Preparation of Fe3O4@(DS-LDH) Fe3O4 was prepared by a modified solvothermal method (Deng et al., 2005). FeCl3·6H2O (1.6 g) was dissolved in ethylene glycol solvent (40 mL) at 313 K to form a clear solution, followed by the addition of NaAc·3H2O (4.3 g). The mixture was stirred vigorously for 30 min and then sealed in a stainless-steel reactor with polytetrafluoroethylene liner. The reactor was heated to and maintained at 473 K for 8 h. After cooling down to room temperature, the black product was washed three times at intervals between the distilled water and ethanol and dried at 333 K overnight to obtain Fe3O4 powder. Magnetic core-shell Fe3O4@Mg3Al LDH (abbreviated as Fe3O4@ LDH) nanocomposite was synthesized by non-steady coprecipitation method based on our previous study (Li et al., 2018). Briefly, metallic nitrates with a Mg/Al molar ratio of 3 were dissolved in distilled water to obtain a solution with a total metal ion concentration of 0.5 mol/L. Then, Fe3O4 powder obtained above was dispersed into the mixed nitrate solution under ultrasonic condition to obtain a Fe3O4 nanoparticle suspension (2 g/L). A 5 wt% ammonia/water solution was slowly poured into the suspension under stirring and the pH of the medium was adjusted to 9–10. After complete addition of the solution, the precipitate obtained was aged for 1 h at room temperature. The precipitate was filtered and washed four times with distilled water. The gel-like slurry was sealed and aged at 353 K for 24 h, and Fe3O4@LDH sol was obtained. At last the Fe3O4@LDH sol was dried at 353 K to yield a product of Fe3O4@LDH powder. Additionally, Mg3Al LDH powder was obtained by using the above method without addition of Fe3O4. Fe3O4@(DS-LDH) was prepared by the ion exchange method (as shown in Scheme 1). First, 1 g of Fe3O4@LDH powder was dispersed into 200 mL of distilled water to obtain the Fe3O4@LDH aqueous suspension, and 200 mL of SDS solution was prepared by dissolving the calculated amount of SDS (for theoretical 100% anion exchange) in distilled water. Then, Fe3O4@LDH aqueous suspension was dispersed into SDS solution, and the mixture was oscillated at 333 K in a water bath constant-temperature oscillator for 72 h. The residues were

2. Material and methods 2.1. Chemicals FeCl3·6H2O, NaAc·3H2O, ethylene glycol, ethanol, Mg(NO3)2·6H2O, Al(NO3)3·9H2O, ammonia (NH3·H2O), sodium dodecyl sulfate (SDS), methylene blue (MB), methyl orange (MO), and CuSO4·5H2O were of 2

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Fig. 1. (A) TEM photos, (B) XRD patterns (The inset is the small angle XRD patterns of DS-LDH and Fe3O4@(DS-LDH)), (C) infrared spectra, and (D) zeta potentials of (a) Mg3Al LDH, (b) Fe3O4, (c) Fe3O4@LDH, (d) DS-LDH and (e) Fe3O4@(DS-LDH); (E) hysteresis loops of Fe3O4, Fe3O4@LDH, and Fe3O4@(DS-LDH) at room temperature.

3

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LDH), and V (L) is the suspension volume. At MB/MO concentration of 40 mg/L and temperature of 298 K, (1) the effect of the dosage (0.005–0.1 g/35 mL) of Fe3O4@(DS-LDH) on the removal ratio of MB/MO were investigated, (2) the effect of initial pH value (5.5–11.5) of solution on the adsorption was observed (and the other experiments were carried out at pH 5.5 to avoid Cu(OH)2 formation and precipitation), (3) the change of adsorption capacity (qt) with time (t) was studied, and the adsorption kinetics was determined. The data were expressed as an average of three independent experiments.

centrifuged at 12,000 r/min and washed with distilled water several times for the removal of unreacted SDS molecules. Finally, the precipitate was dried in an oven at 333 K until a constant weight was achieved. In addition, DS-LDH was obtained with the same method, in which Fe3O4@LDH was replaced with Mg3Al LDH. The interference of carbon dioxide during the entire preparation process was reduced by bubbling the dispersion with N2 gas. 2.3. Characterization method The morphology and structures of the samples were analyzed with a Tecnai G2 F20 field emission TEM (FEI Co., U.S.A). Crystal phases were determined by a D/MAX γB X-ray diffractometer (Rigaku Co., Japan) using Cu Kα radiation. The FTIR spectra were collected on an Avatar 380 FTIR spectroscopy (Nicolet-Thermo, USA) over the range of 400–4000 cm−1. The contents of Mg, Al (and Fe) of Mg3Al LDH, Fe3O4@LDH, and Fe3O4@(DS-LDH) were determined by using a Agilent 725 inductively coupled plasma-atomic emission spectroscope (Agilent Technologies Inc., USA), and then the molar ratio of Mg:Al and the mass percent of Fe3O4 can be calculated. The contents of C, N, and S were determined by a CHNS elemental analyzer (Elementar, Vario III EL, Germany). The water contact angle (CA) values were measured using an SL200KB contact angle meter (KINO Industry Co. Ltd., Norcross, GA, USA) at ambient temperature to confirm the surface hydrophilicity of the samples. The sample powder was pressed into a cake, and the mean CA was determined by the average of left and right CAs of a water droplet pipetted onto the cake. The results were the average value of 5 measurements. The BET surface areas of the samples were analyzed from the N2 adsorption-desorption isotherm at 77 K on an Autosorb iQ surface area and porosity analyzer (Quantachrome, USA), and prior to the adsorption-desorption determination, the samples were degassed at 383 K in a N2 flow for 5 h. A JS94H microelectrophoresis instrument (Shanghai Zhongchen Digital Technical Apparatus Co., Ltd., China) was used to measure the zeta potentials of the sample/H2O suspensions (in 0.001 mol/L NaCl solution). The magnetization was measured with a 7307 vibrating sample magnetometer (VSM, Lake Shore Cryotronics Inc. USA). XPS were taken on a PHI-5400 apparatus (Perkin-Elmer Co., USA) with Al Kα beam sourcer to characterize the change in surface composition before and after adsorption.

3. Results and discussion 3.1. Characterization of Fe3O4@(DS-LDH) The TEM images of Mg3Al LDH (a), Fe3O4 (b), Fe3O4@LDH (c), DSLDH (d), and Fe3O4@(DS-LDH) (e) are shown in Fig. 1A. The TEM image of Mg3Al LDH suggested that LDH particles were hexagonal nanoplatelets with particle sizes of 50–100 nm. The morphology of DSLDH particles was similar to that of Mg3Al LDH, but the particle size of the DS-LDH particles increased slightly. Additionally, obvious aggregation of DS-LDH particles can be seen, which may be attributed to the increased particle hydrophobicity (see Table 1). Fe3O4 particles were nearly spherical in shape, with an average particle size of 200–300 nm. The surfaces of Fe3O4@LDH and Fe3O4@(DS-LDH) were rougher than that of pristine Fe3O4, and it was found that the dark spot of Fe3O4 was coated with a thin LDH (or DS-LDH) layer (Fig. 1A (c), and (e)), indicating and the core-shell structures of Fe3O4@LDH and Fe3O4@(DS-LDH) were formed. The XRD patterns for Mg3Al LDH, Fe3O4, Fe3O4@LDH, DS-LDH, and Fe3O4@(DS-LDH) are shown in Fig. 1B. The XRD spectrum of Fe3O4 showed two distinct characteristic diffraction peaks at 29.46° and 35.46°, which confirmed the existence of the Fe3O4 phase (Yan et al., 2015). The XRD spectrum of Mg3Al LDH had a typical diffraction peak at 10.3°. This peak corresponded to the interlayer spacing (d003 = 0.87 nm) and was consistent with the interlayer distance of NO3-LDH (Zhao and Nagy, 2004). As expected, no peaks of impurities were found in XRD spectrum of Fe3O4 and LDH samples, indicating high purity of them. The XRD spectra of Fe3O4@LDH and Fe3O4@(DS-LDH) contained the characteristic diffraction peaks of LDH and Fe3O4, but the peak intensities were obviously weakened, confirming the core-shell structure of Fe3O4@LDH and Fe3O4@(DS-LDH). In addition, no other characteristic peak was detected, so it was considered that Fe3O4@LDH (or Fe3O4@(DS-LDH)) sample was composed of Fe3O4 and Mg3Al LDH (or Fe3O4 and DS-LDH), and the combination was a physical process. The position of the d003 diffraction peak of Fe3O4@LDH was the same as that of Mg3Al LDH, indicating that the addition of Fe3O4 had no effect on the interlayer spacing of Mg3Al LDH. However, the d003 diffraction peaks of DS-LDH and Fe3O4@(DS-LDH) shifted obviously to a low angle compared with the d003 diffraction peak of LDH. This shift indicated

2.4. Adsorption experiments The adsorption of MB/MO in (MB/MO) sole or (MB/ MO + 0.002 mol/L Cu(II)) binary aqueous solution was operated through the following procedures: an appropriate amount of magnetic core-shell Fe3O4@(DS-LDH) nanocomposite and 35 mL sole or binary solution (0–450 mg/L MB/MO) were added into several glass centrifuge tubes. The pH value in each tube was adjusted by adding small amounts of 0.1 mol/L HNO3 or NaOH. All the tubes were shook on a water bath oscillator at 200 r/min and under a constant temperature for a certain time (t) to reach adsorption equilibrium. Then, the solution was separated from Fe3O4@(DS-LDH) by magnetic separation, and the residual concentration of MB or MO was determined by a U-3900 UV–vis spectrophotometer (Hitachi High-Technologies, Tokyo, Japan) at 664 or 464 nm wavelength. The equilibrium adsorption capacity (qe, mg/g) or removal ratio (R%) were calculated according to Eqs. (1) and (2), respectively.

qe =

(c0 − ce ) V m

R% =

c 0 − ce × 100% c0

Table 1 Element contents and contact angles of Fe3O4@LDH and Fe3O4@(DS-LDH). Sample

n(Mg): n(Al)

Fe3O4 Fe3O4@LDH DS-LDH Fe3O4@(DSLDH)

(1) (2)

a

2.70 2.70 2.70

NO3− exchanged quantity b

Contact angle

S

%

degree

5.62 5.20

– 95.13 94.62

30.0 69.1 47.5

wt%

4.16 – 2.71

N 3.61 0.10 0.09

C 0.060 25.32 23.44

a

C element in the interlayer CO32– anions. An n(S):(n(S) + n(N)) molar ratio of the sample was taken as a measure to examine the NO3− exchanged quantity.

where c0 and ce (mg/L) are the initial and equilibrium concentration of MB/MO in solution, respectively, m (g) is the weight of Fe3O4@(DS-

b

4

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are listed in Table 1. As seen from Table 1, the molar ratio of Mg: Al was close to the theoretical value, which confirmed that the LDH synthesized was Mg3Al LDH, and the layered structure of brucite was not destroyed during ion exchange. The mass percentage of Fe3O4 in Fe3O4@LDH (or Fe3O4@(DS-LDH)) was 2.71% (or 4.61%), indicating the successful combination of Fe3O4 and LDH (or DS-LDH). Although freshly boiled double-distilled water was used and the dispersion was bubbled with N2 gas in the entire preparation process, a small amount of CO32– anions were still found in Fe3O4@LDH possibly because of the high affinity between CO32– anions and LDH (Miyata, 1983). The high S contents in DS-LDH and Fe3O4@(DS-LDH) indicated that DS− anions were successfully inserted into LDH interlayers. The water contact angle of Fe3O4@LDH was 30.0°, and the intercalation of DS− anions in LDH interlayers greatly enhanced the hydrophobicity of DS-LDH and Fe3O4@(DS-LDH), whose contact angles were 69.1° and 47.5°, respectively. Therefore, the increased hydrophobic property of Fe3O4@(DSLDH) was likely to be beneficial for the adsorption of MB/MO owing to the apparent hydrophobic-hydrophobic interaction.

that the interlayer spacing increased to about 3.08 nm. The expanded spacing was consistent with the intercalation of DS− anions in LDH interlayers of Mg3Al LDH and Fe3O4@LDH. Given that the end-to-end length of dodecyl sulfate is 2.08 nm and the LDH layer thickness is 0.48 nm (Zhao and Nagy, 2004), it can be proposed that DS− anions were intercalated into the interlayer region of LDH as a horizontal orientation, i.e., DS− anions intercalated into LDH interlayers formed micelles, which can arrange to interpenetrate as a bilayer, as shown in Scheme 1. Additionally, as seen from the inset in Fig. 1B, the decreased reflective intensity of Fe3O4@(DS-LDH) indicated the crystallinity decreased after combination DS-LDH with Fe3O4. Mg3Al LDH, Fe3O4, Fe3O4@LDH, DS-LDH, and Fe3O4@(DS-LDH) were characterized also through FTIR spectroscopy, and the results are shown in Fig. 1C. A strong and wide absorption peak appeared in all the samples from 3400 cm−1 to 3500 cm−1, which was attributed to the OeH stretching vibration of the hydroxyl groups in the layers (and the stretching vibration of water molecules in LDH interlayers). The bending vibration absorption band of the surface hydroxyl groups was also recorded at ~1620 cm−1. In the spectrum of Fe3O4, a peak at ~576 cm−1 was observed from the FeeO lattice vibration of Fe3O4. In the spectrum of Mg3Al LDH, the peak at ~1380 cm−1 was assigned to the characteristic vibration absorption peak of the NO3− ion (Wu et al., 2005), and the peaks at 400–900 cm−1 were due to the stretching vibration and bending vibration of M–O and M–OH. The characteristic absorption peaks of LDH and Fe3O4 appeared in the spectrum of the Fe3O4@LDH spectrum, implying that the Fe3O4 nanoparticles were successfully assembled with the LDH phase. In the spectra of DS-LDH and Fe3O4@(DS-LDH), the CeH stretching vibration absorption peak at 2850–2960 cm−1 (Yang et al., 2006), the –CH2 shearing vibration absorption peak at 1465 cm−1, the –CH3 symmetrical bending vibration absorption peak at ~1380 cm−1, and the –SO4 vibration absorption peak at ~1205 cm−1 were observed, indicating that DS− ions had been successfully loaded on LDH and Fe3O4@LDH. The specific surface areas of Mg3Al LDH, Fe3O4, Fe3O4@LDH, DSLDH, and Fe3O4@(DS-LDH) were determined and the results were 78.2, 57.6, 47.9, 15.8, and 28.7 m2·g−1, respectively. The addition of DS− anions can enhance the hydrophobicity (Table 1) of the samples and thereby promoted the aggregation of the particles (Fig. 1A (d)) and decreased the specific surface areas. The zeta potentials of LDH, Fe3O4, Fe3O4@LDH, DS-LDH, and Fe3O4@(DS-LDH) at the range of pH 5–11 were determined, and the results are shown in Fig. 1D. In essence, a positive zeta potential value may indicate a positive surface charge of the sample, and vice versa. The electronic interaction between the negatively charged Fe3O4 and positively charged LDH at the experimental pH range was sufficient to induce stable self-assembly of the two components (Chen et al., 2011) and formed the positively charged coreshell Fe3O4@LDH (Fig. 1A (c)). For DS-LDH and Fe3O4@(DS-LDH), the surface charges decreased or even became negative, suggesting the presence of DS− anions exchanged with NO3− anions in LDH interlayers (and adsorbed onto the surface of the samples). The magnetization curves of Fe3O4, Fe3O4@LDH, and Fe3O4@(DSLDH) at room temperature are shown in Fig. 1E. The saturation magnetization values of Fe3O4, Fe3O4@LDH, and Fe3O4@(DS-LDH) were 73.5, 41.1, and 25.0 emu/g, respectively, and all the coercivity values were 0, indicating good superparamagnetism of the three samples. The saturation magnetization value of Fe3O4@LDH was lower than that of Fe3O4 possibly due to the existence of non-magnetic LDH shell (Fig. 1A(c)), and the saturation magnetization can be further reduced by the existence of DS− anions in LDH interlayers of Fe3O4@(DS-LDH) (Fig. 1B). It can be obviously seen that Fe3O4@(DS-LDH) was attracted quickly (30 s) toward the magnet (inset in Fig. 1E), and the magnetic separation experiment showed that Fe3O4@(DS-LDH) can be used as a magnetic adsorbent to remove pollutants in actual dyeing wastewater treatment. The molar ratio of Mg:Al, the mass percentage of Fe3O4 and N/C/S elements, and the contact angles of Fe3O4@LDH and Fe3O4@(DS-LDH)

3.2. Effect of adsorbent dosage The optimization of adsorbent dosage is very beneficial to practical application. Fig. S1 showed that with the increase of Fe3O4@(DS-LDH) dosage (0.005–0.1 g/35 mL), the removal ratio of MB/MO increased first (0.005–0.05 g/35 mL) and then tended to be stable (0.05–0.1 g/ 35 mL). The initial increase in removal ratio was due to the additional adsorption sites of Fe3O4@(DS-LDH). However, the removal ratio was basically unchanged when a high dosage of Fe3O4@(DS-LDH) was used, due to the limited availability of MB/MO in the medium. Therefore, in the subsequent experiments, the dosage of Fe3O4@(DS-LDH) was set as 0.05 g/35 mL. 3.3. Effect of initial solution pH The influence of initial solution pH (5.5–11.5) on the removal of MB/MO in sole and binary solutions by Fe3O4@(DS-LDH) was studied to gain further insight into the adsorption process, and the results are shown in Fig. S2. The adsorption capacities of MB in both sole and binary solutions increased and then decreased with increasing pH. By contrast, the adsorption capacities of MO in both sole and binary solutions decreased gradually with increasing pH. In the pH range of 5.5–10.0, Fe3O4@(DS-LDH) was negatively charged (Fig. 1D), and the absolute charge value increased with increasing pH (Fig. 1D), therefore, the electrostatic attraction force between MB cations and Fe3O4@(DSLDH) increased with increasing pH and the adsorption capacity of MB increased. At high pH (> 10.0) values, the structure of Mg3Al LDH was partially destroyed, and thus the adsorption capacity of MB decreased. When Cu(II) ions were added, the competitive adsorption of Cu(II) and MB ions occurred, especially at low pH values, and the adsorption capacity in binary solution was smaller than that in sole solution. Conversely, the electrostatic repulsion force between Fe3O4@(DS-LDH) and MO anions increased with increasing pH, therefore, the adsorption capacity decreased. When Cu(II) ions were added, they acted as bridges between Fe3O4@(DS-LDH) and MO anions, and the adsorption capacity of MO increased. However, Cu(OH)2 gradually precipitated at high pH value, and the bridging effect was weakened, so the adsorption capacity decreased. Additionally, the adsorption capacity of MO in binary solution was larger than that in sole solution at the same pH, which confirmed the contribution of Cu(II) ions to MO adsorption. 3.4. Adsorption kinetic The adsorption kinetic model predicts not only the adsorption rate but also the reaction mechanism and evaluates the adsorption efficiency between Fe3O4@(DS-LDH) and MB/MO. The adsorption capacity of MB/MO in sole and binary solutions on Fe3O4@(DS-LDH) as a function 5

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24 MB

22

22

MB+Cu

20

MO

18

18 MO+Cu

qt (mg/g)

qe (mg/g)

20

16

16 14

MB MB+Cu(II) MO+Cu(II) MO

14

12

12

10

10 0

20

40

60

2

80

3

4

5 1/2

6

7

8

9

1/2

t (min )

t (min) Fig. 2. Effect of time on the adsorption capacity of Fe3O4@(DS-LDH) for MB/ MO in sole and binary solutions.

Fig. 3. Intraparticle diffusion kinetics of MB/MO on Fe3O4@(DS-LDH) in sole and binary solutions.

of contact time (t) at pH 5.5 is shown in Fig. 2. All the MB/MO adsorption showed a fast initial adsorption rate, which was followed by a relatively slow adsorption, and the adsorption equilibrium was achieved within 80 min. Therefore, adsorption time of 90 min was selected in other adsorption experiment. The initial rapid adsorption can be attributed to the adsorption of MB/MO onto the Fe3O4@(DS-LDH) surface, and the subsequent slow one was due to the adsorption in the in-gallery of Fe3O4@(DS-LDH), which was the dominant mechanism. The kinetic data in Fig. 2 were used for fitting to pseudo-first-order (Lagergren, 1898) and pseudo-second-order equations (Gosset et al., 1986), whose linear forms can be expressed as Eqs. (3) and (4), respectively.

adsorbent. A large C value indicates a large effect of boundary layer on adsorption. Every plot in Fig. 3 did not pass through the origin, showing that the intraparticle diffusion was not the only rate-controlling step (Bhattacharyya and Sharma, 2004). Additionally, every plot showed a double linearity, which indicated the adsorption process had two steps (Özcan et al., 2006), which was consistent with the result obtained by fitting with the pseudo-second-order equation. The first step described the distribution of MB/MO through the solution to the external surface of Fe3O4@(DS-LDH), and the second step was attributed to the partition of MB/MO molecules into LDH interlayer regions. The kint and C values for the adsorption were calculated from the slopes of the plots in Fig. 3 and are presented in Table S2. The diffusion rate constant kint,1 was larger than kint,2 due to the increase of the diffusion amount of MB/MO to Fe3O4@(DS-LDH) and diffusion resistance C1 was less than C2, indicating that MB/MO diffused quickly on the surface of Fe3O4@(DSLDH), and the main rate-controlling step in the adsorption process was intraparticle diffusion.

ln(qe − qt ) = −k1 t + ln qe

(3)

t t 1 = + qt qe k2 qe2

(4)

where qt and qe (mg/g) are the MB/MO adsorption capacity on Fe3O4@(DS-LDH) at time t (min) and equilibrium, respectively, and k1 (1/min) and k2 (g/(mg·min)) are the rate constants of the pseudo-firstorder and pseudo-second-order equations, respectively. The fitting data of qe,cal (mg/g), k1 (or k2), and correlation coefficient (R2) are shown in Table S1. The R2 values of the pseudo-secondorder expression were higher than those of the pseudo-first-order expression, and the qe,cal values obtained by fitting to pseudo-secondorder equations were close to the experimental results (qe,exp). Therefore, the adsorption of MB/MO on Fe3O4@(DS-LDH) in sole and binary solutions were governed by the pseudo-second-order kinetic model; that is, the adsorption rate was affected by MB/MO concentration and Fe3O4@(DS-LDH) dose, and the adsorption might occur through two steps. As shown in Table S1, the qe,exp values for different adsorbate followed the order MB > MB + Cu(II) > MO + Cu (II) > MO, which indicated the adsorption capacity of Fe3O4@(DSLDH) for MB was larger than that for MO in sole solution due to the negatively charge of Fe3O4@(DS-LDH) (Fig. 1D) and the former decreased but the latter increased with the addition of Cu(II) ions. To further clarify the rate-determining, the experimental results were analyzed using intraparticle diffusion equation (Eq. 5) (Weber and Morris, 1963), and the results are shown in Fig. 3.

qt = k int t 1/2 + C

3.5. Adsorption isotherm The adsorption isotherm, describing the relationship between the mass of adsorbate per unit mass of adsorbent and the aqueous adsorbate concentration at a constant temperature at equilibrium, generates a series of parameters whose values represent the affinity and surface properties of the adsorbent, and then the adsorption isotherm plays an important role in the design of adsorption process. The adsorption isotherms of MB and MO on Fe3O4@(DS-LDH) were studied under the optimized conditions: adsorbent dosage of 0.05 g/35 mL, contact time of 90 min and initial solution pH 5.5. Fig. 4 shows the adsorption isotherm of MB/MO on Fe3O4@(DS-LDH) at a constant temperature (298, 308, or 318 K). As can be seen from Fig. 4A and B, the adsorption capacities of Fe3O4@(DS-LDH) for MB/MO in both sole and binary solutions increased with increasing initial concentration of MB/MO at first and then appeared (and kept) constant with further increasing concentration of MB/MO at the same temperature. Additionally, the adsorption capacities of MB in both sole and binary solutions and MO in binary solution increased with increasing temperature (298–318 K), suggesting that the adsorption processes were endothermic in nature. On the contrary, the adsorption capacity of MO in sole solution decreased with increasing temperature, indicating that the adsorption process was exothermic. The adsorption behavior and mechanism were further explored by

(5) 1/2

where kint (mg/(g·min )) is the intraparticle diffusion adsorption rate constant, and C is a constant describing the boundary layer of an 6

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adsorption, and n > 1 denotes a favorable adsorption; Kd (L/g) is the distribution coefficient indicating the ratio between the adsorption capacity (qe, mg/g) and the equilibrium concentration (ce, mg/L) of MB/MO, and a high Kd value indicates an efficient adsorption, a is the linear regression constant. The maximum adsorption capacities (qm,exp, mg/g) observed in the experiment, the fitted constants for the three models, and the regression coefficients are summarized in Table S3. From Table S3, the equilibrium data for MB in sole and binary solutions and MO in binary solution were fitter with Freundlich model (R2 > 0.991) than others and the value of n (> 1) indicated that the adsorption is favorable; the equilibrium data for MO in sole solution were fitter with the linear model (R2 > 0.998) than others and the value of n in the Freundlich model (R2 > 0.941) was close to 1, indicating it was a partitioning adsorption. The KF values for MB (or MO) in both sole and binary solutions increased (or decreased) with increasing temperature, indicating the adsorption capacity for MB (or MO) increased (or decreased) with increasing temperature. At the same temperature, the KF value for MB (or MO) in sole solution was larger (or smaller) than that for MB (or MO) in binary solution, indicating that the adsorption capacity of Fe3O4@(DS-LDH) for MB (or MO) decreased (or increased) with the addition of Cu(II) ions, which was accorded with Figs. 2-4.

250

(A)

qe (mg/g)

200

150

MB, 298K MB, 308K MB, 318K MB+Cu, 298K MB+Cu, 308K MB+Cu, 318K

100

50

0 0

20

40

60

80

100

120

140

ce (mg/L)

250

(B) 200

3.6. Adsorption thermodynamics The influence of temperature on adsorption process can be reflected by thermodynamic parameters, namely, Gibbs free energy change (ΔGo), enthalpy change (ΔHo), and entropy change (ΔSo), and can be calculated by the following equations (Zhao et al., 2010):

qe (mg/g)

150

MO, 298K MO, 308K MO, 318K MO+Cu, 298K MO+Cu, 298K MO+Cu, 298K

100

50

KD =

ln KD = −

0 0

20

40

60

80

100

ΔG o

120

fitting the data in Fig. 4 by Langmuir, Freundlich, and linear models. The Langmuir isotherm assumes that a monolayer adsorption of MB/ MO molecules on the surface of Fe3O4@(DS-LDH) and all the active sites of Fe3O4@(DS-LDH) present equal affinity to MB/MO molecules. The Freundlich isotherm considers that adsorption occurs on heterogeneous surfaces. The linear isotherm indicates a partitioning adsorption mechanism. The Langmuir (1916), Freundlich (1906), and linear (You et al., 2002) model equations can be represented by the following equations, respectively.

qe = K d ce + a

1 ln ce n

ΔH o 1 ΔS o ⋅ + R T R

= −RT ln KD =

ΔH o

(10)

−

T ΔS o

(11)

where KD (L/g) is the distribution ratio, ΔH (kJ/mol) and ΔS (J/mol/ K) are determined from the slope and intercept of the plot of lnKD versus 1/T through Eq. (10), respectively, and ΔGo (kJ/mol) is directly calculated from Eq. (11). The relative thermodynamic parameters for adsorbing MB (and MO) in sole and binary solutions onto Fe3O4@(DSLDH) are listed in Table S4. The negative values of ΔGo at all investigated temperatures suggested that the adsorption occurred spontaneously. Meanwhile, the positive ΔHo value for MB in sole and binary solutions indicated that the adsorption process was endothermic, which was accorded with the fact that the adsorption capacity for MB increased with increasing temperature (Fig. 4). Conversely, the negative ΔHo value for MO in sole and binary solutions demonstrated that the adsorption process was exothermic, which were in agreement with the adsorption capacity for MO decreased with increasing temperature (Fig. 4).

Fig. 4. Adsorption isotherms of Fe3O4@(DS-LDH) for (A) MB and (B) MO in sole and binary solutions at different temperatures.

ln qe = ln KF +

(9)

o

ce (mg/L)

ce 1 c = + e qe KL qm qm

c 0 − ce V × ce m

o

3.7. Adsorption mechanisms

(6)

Studying the adsorption mechanism is helpful to the understanding of the interaction between adsorbent and adsorbate in wastewater treatment and how this interaction can be used to optimize the adsorption process. The XRD patterns and XPS spectra were used to clarify the adsorption mechanisms. The XRD patterns of Fe3O4@(DS-LDH) before and after MB/MO adsorption are shown in Fig. 5. The XRD patterns of Fe3O4@(DS-LDH) after adsorption had no obvious changes compared with that before adsorption. However, the d003 diffraction peak of the sample moved slightly toward a small angle, and the interlayer space increased from 3.08 nm to 3.15 nm (or 3.22 nm) after the adsorption of MB (or MO). The increased basal spacing was due to the adsorption of MB/MO into

(7) (8)

where ce (mg/L) is the equilibrium MB/MO concentration in the solution, qe (mg/g) is the adsorption capacity of MB/MO at equilibrium, qm (mg/g) is the maximum adsorption capacity, KL (L/mg) is the Langmuir adsorption equilibrium constant related to the affinity between Fe3O4@(DS-LDH) and MB/MO; KF ((mg/g)/(mg/L)1/n) is the Freundlich equilibrium constant describing the adsorption capacity of Fe3O4@(DS-LDH) (the larger the KF value, the greater the adsorption capacity); n is an empirical constant related to the strength of 7

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d003 = 3.08 nm d003 = 3.22 nm

Cu 2p

(A) Intensity

Fe3O4@(DS-LDH) Fe3O4@(DS-LDH) + MO

d003 = 3.15 nm

2.0

2.5

3.0

3.5

4.0

Intensity (a.u.)

Intensity

Fe3O4@(DS-LDH) + MB

Fe 3 O 4 @ (DS-HTlc)+MB

4.5

2θ (degree)

Fe3O4@(DS-LDH)

Fe 3O 4 @ (DS-HTlc)+MO Fe 3O 4 @ (DS-HTlc)+Cu(II)+MB

Fe 3 O 4 @ (DS-HTlc)+Cu(II)+MO

Fe3O4@(DS-LDH) + MB

Fe 3 O 4 @ (DS-HTlc)

Fe3O4@(DS-LDH) + MO 920

930

940

950

960

970

B.E. (eV)

10

20

30

40 2θ (degree)

50

60

70 Cu2p

(B)

Fig. 5. Powder XRD patterns of Fe3O4@(DS-LDH) before and after MB/MO adsorption (The inset is the small angle XRD patterns).

934.2

Intensity (a.u.)

Fe3O4@(DS-HTlc)+Cu(II)+MB

LDH interlayers of Fe3O4@(DS-LDH), that is, MB/MO adsorption occurred mainly within the in-gallery of the sample. As we all know, adsorption forces, which are present at the interface between adsorbent and adsorbate, including van der Waals interaction, hydrophobic bonding, hydrogen bonding, charge transfer, ion bonding, ligand exchange, dipole interaction, and chemical bond force, and these forces accounts for the thermal changes in the adsorption process, therefore, the adsorption mechanism can be inferred by determining the thermodynamic parameters of adsorption at the interface. Von Oepen et al. (1991) summarized the range of enthalpy change caused by various adsorption forces (van der Waals force: 4–10 kJ/mol; hydrophobic force: about 5 kJ/mol); Hydrogen bond: 2–40 kJ/mol; Coordination exchange: about 40 kJ/mol; Dipole force: 2–29 kJ/mol; Chemical bond: > 60 kJ/mol). In this study, ΔHo values were < 25.73 kJ/mol (Table S4), indicating that no strong interaction force was present in these processes, and the main adsorption mechanism of Fe3O4@(DS-LDH) for MB/MO may be due to hydrophobic interaction and van der Waals force. The XPS was employed to further confirm the surface state and chemical status of Fe3O4@(DS-LDH) before and after MB/MO adsorption in sole and binary solutions. The Cu 2p low and high resolution XPS spectra obtained for Fe3O4@(DS-LDH) are displayed in Fig. 6A-C. As seen from Fig. 6A-C, a new peak 934.2 eV (or 933.2 eV) appeared after MB (or MO) adsorption in binary solution, indicating Cu(II) formed outer sphere surface complexes with surface hydroxyl groups of LDH and the different Cu 2p peaks after MB and MO adsorption indicated chemical environments of Cu(II) adsorbed by Fe3O4@(DS-LDH) in MB + Cu(II) and MO + Cu(II) binary solutions were different. In addition, as seen from Fig. 6A and C, a new peak at 953.3 eV appeared after adsorption in MO + Cu(II) binary solution, showing the formation of CueO bonds, which can be attributed to the electrostatic attraction of Cu(II) ion and sulfonate of MO, therefore, Cu(II) ions acted as bridges between Fe3O4@(DS-LDH) and MO anions, and promoted the adsorption for MO, which was consistent with the result in 3.3. Effect of initial solution pH The high-resolution XPS spectra of C 1 s and N 1 s of Fe3O4@(DSLDH) before and after adsorption are shown in Fig. S3(A and B). The C 1 s peaks (Fig. S3A) of all the samples positioned at 284.8 ± 0.1 eV and 286.3 ± 0.1 eV were assigned to C=C/C–C and CeO bonds, respectively, which were attributed to DS− anions in the interlayers of LDH. The binding energies of 289.4 eV for C 1 s denoted the existence of CO32– anions, which were in good accordance with the result shown in Table 1, demonstrating some CO32– anions existed in the Fe3O4@(DSLDH) samples both before and after adsorption.

920

930

940

950

960

B.E.(eV)

933.2

Cu 2p

Intensity (a.u.)

(C)

Fe 3O 4@(DS-HTlc)+Cu(II)+MO 953.3

920

930

940

950

960

B.E. (eV) Fig. 6. Low-resolution XPS spectra (A) of Cu 2p of Fe3O4@(DS-LDH) before and after adsorption in sole and binary solutions and high resolution XPS spectra of Cu 2p of Fe3O4@(DS-LDH) after adsorption of MB (B) and MO (C) in binary solutions.

In Fig. S3B, the 399.6 eV and 407.1 eV binding energies for N 1 s of Fe3O4@(DS-LDH) before adsorption were attributed to NeO bond and NO3− anions, respectively, and indicated that NO3− anions in the interlayers of Fe3O4@LDH were not completely replaced by DS− anions, i.e., NO3− anions exchanged quantity was < 100% (as shown in Table 1). After MB/MO adsorption, the corresponding peak (407.1 eV) 8

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declined considerably or disappeared completely, indicating that the remaining NO3− anions were replaced by some anions: (i) for MB adsorption in sole solution (Fe3O4@(DS-LDH) + MB), the anions were mainly Cl− anions (from MB solution); (ii) for MB adsorption in binary solution (Fe3O4@(DS-LDH) + MB + Cu(II)), the anions can be Cl− (from MB solution) and SO42− anions (from Cu(II) solution); (iii) for MO adsorption in sole solution (Fe3O4@(DS-LDH) + MO), the anions were mainly MO anions; (iv) for MO adsorption in binary solution (Fe3O4@(DS-LDH) + MO + Cu(II)), the anions can be MO and SO42− anions (from Cu(II) solution). Additionally, the characteristic N 1 s peaks of Fe3O4@(DS-LDH) after adsorption at 400.0 ± 0.1 eV attributed to the CeN bonds and the peak at 401.7 eV ascribed to the C–N+ bond indicated that MB/MO were adsorbed onto Fe3O4@(DS-LDH). Based on the above analysis, the adsorption of MB/MO by Fe3O4@(DS-LDH) mainly occurred through the following ways: (i) the hydrophobic force between MB/MO molecules and the hydrophobic DS− anions which existed mainly in LDH interlayers of Fe3O4@(DSLDH); (ii) ion exchange between MO anions and NO3− anions in LDH interlayers of Fe3O4@(DS-LDH); (iii) the electrostatic attraction between negatively charged Fe3O4@(DS-LDH) and MB cations (and Cu(II) ions), therefore, the adsorption capacity for MB decreased in the presence of Cu(II) ions due to competitive adsorption; (iv) the electrostatic attraction between negatively charged Fe3O4@(DS-LDH) and Cu(II) ions at first, and then the attraction between Cu(II) ions and MO anions, therefore, the adsorption capacity for MO increased with the addition of Cu(II) ions.

250

Fe3O4@(DS-LDH)

(A)

qe (mg/g)

200

DS-LDH 150

100

50

Fe3O4 Fe3O4@LDH LDH

0 0

30

60

90

120

150

ce (mg/L)

Fe3O4@(DS-LDH)

200

(B)

DS-LDH

qe (mg/g)

150

3.8. Comparison with raw materials and other clayey adsorbents The adsorption isotherms of 0.05 g/35 mL Fe3O4, LDH, Fe3O4@ LDH, DS-LDH, and Fe3O4@(DS-LDH) for MB at 298 K are shown in Fig. 7A, and those for MO are shown in Fig. 7B. The adsorption capacity of the materials for MB had the following order: Fe3O4@(DS-LDH) > DS-LDH > Fe3O4 > Fe3O4@LDH > LDH, and that for MO was: Fe3O4@(DS-LDH) > DS-LDH > LDH > Fe3O4@LDH > Fe3O4. At the experimental pH 5.5, Fe3O4 (LDH) had negative (positive) charge, which had a significant electrostatic repulsion force with MO anions (MB cations), so the adsorption capacity of Fe3O4 (LDH) for MO (MB) was extremely low. For MB (MO) adsorption, the surface positive charge of Fe3O4@LDH was smaller than that of LDH (Fig. 1D), so the electrostatic repulsion (attraction) force between Fe3O4@LDH and MB (MO) ions was smaller, therefore, the adsorption capacity of Fe3O4@ LDH was larger (smaller) than that of LDH. DS-LDH and Fe3O4@(DSLDH) can efficiently adsorb MO (MB), mainly due to the “effective dissolution” of MB (MO) in the three-dimensional hydrophobic region formed by DS− anions in LDH interlayers. The high adsorption capacity of negatively charged Fe3O4@(DS-LDH) and positively charged DS-LDH for MB/MO confirmed that the “dissolution” in the hydrophobic region was the main reason for the efficient adsorption. Additionally, the larger adsorption quantity of Fe3O4@(DS-LDH) for MB/MO than that of DS-LDH was maybe due to its larger specific surface area. Compared with the previous reported clayey adsorbents such as kaolinite (Sarma et al., 2011), montmorillonite (Sarma et al., 2011), zeolite (Wang and Zhu, 2006), acid-activated kaolin (Gao et al., 2016), Fe3O4/ZnCr-LDH (Chen et al., 2012), this work has demonstrated that Fe3O4@(DS-LDH) was effffective in the removal of MB/MO dye from the aqueous phase and could remove cationic MB and anionic MO, simultaneously, since previous literatures mainly focused on the adsorption of single dye. The intercalation of dodecyl sulfate expanded the interlayer of LDH and converted the surface of Fe3O4@LDH from hydrophily to hydrophobicity, providing new adsorption sites for MB and MO.

100

50

LDH Fe3O4@LDH Fe3O4

0 0

30

60

90

120

150

ce (mg/L) Fig. 7. Adsorption isotherms of Fe3O4, LDH, Fe3O4@LDH, DS-LDH, and Fe3O4@(DS-LDH) for (A) MB and (B) MO.

synthesized by coprecipitation-ion exchange method and used to remove cationic (MB) and anionic (MO) dyes with or without Cu(II) from aqueous solution. The samples were characterized by TEM, XRD, FTIR, ICP-AES, elemental analysis, contact angle, specific surface area, zeta potential, and VSM. The adsorption properties of Fe3O4@(DS-LDH) for MB and MO with or without Cu(II) were investigated and compared under the conditions of the adsorbent dosage, initial solution pH, contact time, dye concentration, and temperature. Three kinetic models and three isotherm models were fitted to the experimental data. The XRD patterns and XPS spectra were used to clarify the adsorption mechanisms. The major conclusions of this study were as follows: (1) The adsorption capacity of MB tended to increase and then decrease with increasing pH. The adsorption capacity of MO decreased with increasing pH. (2) The adsorption capacity of MB decreased slightly, whereas that of MO increased slightly, after Cu(II) ions were added at pH 5.5. (3) The adsorption of MB/MO in sole and binary solutions on Fe3O4@(DS-LDH) were governed by the pseudo-second-order kinetic model. (4) The adsorption of MB/MO proceeded through the following steps: the distribution of MB/MO through the solution to the external surface and then the partitioning of MB/MO molecules into the ingallery of Fe3O4@(DS-LDH). Intraparticle diffusion is the main ratecontrolling step.

4. Conclusions An effective magnetic core-shell adsorbent, Fe3O4@(DS-LDH), was 9

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(5) The adsorption isotherm data for MB in sole and binary solutions and MO in binary solution agreed well with the Freundlich model, and the data for MO in sole solution were accorded with the linear model. (6) The adsorption thermodynamics showed that the adsorption process for MB (MO) was endothermic (exothermic) and spontaneous. (7) The hydrophobic force between MB/MO molecules and LDH interlayer hydrophobic region of Fe3O4@(DS-LDH) was the main reason for the efficient adsorption.

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Declaration of Competing Interest We declare that we assume no financial and personal relationships with other people or organizations that can inappropriately influence our work. No professional or other personal interests of any nature or kind exist in any product, service, and/or company that can be construed to influence the position presented in or the review of the manuscript entitled. Acknowledgements This work was supported by the Natural Science Foundation of Shanxi Province of China(201701D121034), the Key Research and Development Projects of Shanxi Province of china (201803D121100), the Fund for Shanxi 1331 Project Key Subjects Construction, China, and the Support Plan for Innovative Research Team of Changzhi University, China. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.clay.2019.105309. References Awual, M.R., Hasan, M.M., 2015. Colorimetric detection and removal of copper(II) ions from wastewater samples using tailor-made composite adsorbent. Sensors Actuators B Chem. 206, 692–700. Bhattacharyya, K.G., Sharma, A., 2004. Azadirachta indica leaf powder as an effective biosorbent for dyes: a case study with aqueous Congo red solutions. J. Environ. Manag. 71 (3), 217–229. Blanco, J., Torrades, F., Morón, M., Brouta-Agnésa, M., García-Montaño, J., 2014. PhotoFenton and sequencing batch reactor coupled to photo-Fenton processes for textile wastewater reclamation: feasibility of reuse in dyeing processes. Chem. Eng. J. 240, 69–475. Chen, C., Gunawan, P., Xu, R., 2011. Self-assembled Fe3O4-layered double hydroxide colloidal nanohybrids with excellent performance for treatment of organic dyes in water. J. Mater. Chem. 21, 1218–1225. Chen, D., Li, Y., Zhang, J., Li, W., Zhou, J., Shao, L., Qian, G., 2012. Effificient removal of dyes by a novel magnetic Fe3O4/ZnCr-layered double hydroxide adsorbent from heavy metal wastewater. J. Hazard. Mater. 243, 152–160. Daneshvar, N., Sorkhabi, H.A., Kasiri, M.B., 2004. Decolorization of dye solution containing Acid Red 14 by electrocoagulation with a comparative investigation of different electrode connections. J. Hazard. Mater. 112, 55–62. Deng, H., Li, X.L., Peng, Q., Wang, X., Chen, J.P., Li, Y.D., 2005. Monodisperse magnetic single-crystal ferrite microsphere. Angew. Chem. Int. Ed. 44, 2782–2785. dos Santos, A.B., Cervantes, F.J., van Lier, J.B., 2007. Review paper on current technologies for decolourisation of textile wastewaters: perspectives for anaerobic biotechnology. Bioresour. Technol. 98, 2369–2385. Dotto, J., Fagundes-Klen, M.R., Veit, M.T., Palácio, S.M., Bergamasco, R., 2019. Performance of different coagulants in the coagulation/flocculation process of textile wastewater. J. Clean. Prod. 208, 656–665. Freundlich, H.M.F., 1906. Over the adsorption in solution. J. Phys. Chem. 57, 385–470. Gao, W., Zhao, S., Wu, H., Deligeer, W., Asuha, S., 2016. Direct acid activation of kaolinite and its effffects on the adsorption of methylene blue. Appl. Clay Sci. 126, 98–106. Gosset, T., Trancart, J.L., Thevenot, D.R., 1986. Batch metal removal by peat kinetics and thermodynamics. Water Res. 20, 21–26. Hassan, M.M., Carr, C.M., 2018. A critical review on recent advancements of the removal of reactive dyes from dyehouse effluent by ion-exchange adsorbents. Chemosphere 209, 201–219. Hussain, A., Maitra, J., Khan, K.A., 2017. Development of biochar and chitosan blend for heavy metals uptake from synthetic and industrial wastewater. Appl Water Sci 7 (8), 4525–4537. Isa, M.H., Lang, L.S., Asaari, F.A.H., Aziz, H.A., Ramli, N.A., Dhas, J.P.A., 2007. Low cost

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