Deep eutectic solvents assisted synthesis of MgAl layered double hydroxide with enhanced adsorption toward anionic dyes

Journal Pre-proof Deep eutectic solvents assisted synthesis of MgAl layered double hydroxide with enhanced adsorption toward anionic dyes Na Li, Zhidong Chang (Conceptualization) (Writing - review and editing), Hui Dang (Conceptualization), Yifei Zhan (Conceptualization) (Software), Jingyang Lou (Writing - review and editing), Shan Wang (Writing - review and editing), Sanam Attique (Writing - review and editing), Wenjun Li (Methodology), Hualei Zhou (Methodology), Changyan Sun (Methodology)

PII:

S0927-7757(20)30100-X

DOI:

https://doi.org/10.1016/j.colsurfa.2020.124507

Reference:

COLSUA 124507

To appear in:

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Received Date:

1 September 2019

Revised Date:

23 January 2020

Accepted Date:

23 January 2020

Please cite this article as: Li N, Chang Z, Dang H, Zhan Y, Lou J, Wang S, Attique S, Li W, Zhou H, Sun C, Deep eutectic solvents assisted synthesis of MgAl layered double hydroxide with enhanced adsorption toward anionic dyes, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2020), doi: https://doi.org/10.1016/j.colsurfa.2020.124507

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Deep eutectic solvents assisted synthesis of MgAl layered double hydroxide with enhanced adsorption toward anionic dyes Na Lia, Zhidong Changa,*, Hui Danga, Yifei Zhana, Jingyang Loua, Shan Wanga, Sanam Attiqueb, Wenjun Lia, Hualei Zhoua, Changyan Suna a

Department of Chemistry and Chemical Engineering, School of Chemistry and Biological Engineering, University of Science and Technology Beijing, Beijing

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100083, PR China

Insititute for Composites Science and Innovation (Incs), School of Material Science and Engineering, Zhejiang University, Hangzhou 310027, PR China

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E-mail address: [email protected] (Z. Chang)

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 Corresponding author.

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Graphical abstract

ABSTRACT The chloride (Cl-)-intercalated MgAl layered double hydroxide (LDH) with a three-

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dimensional (3D) hierarchical structure was synthesized via a one-step solvothermal method based on deep eutectic solvents (DESs) including MgCl2·6H2O/urea (molar ratio of 1:2) and AlCl3·6H2O/PEG 200 (molar ratio of 1:3). The as-prepared Mg2Al-Cl LDH acquired high uptake capacities (1051.87, 889.76 and 512.55 mg g-1) for the anionic dyes of methyl orange (MO), congo red (CR) and indigo carmine (IC) at 25°C, which are much higher than those of most reported LDHs in the literature. These high

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uptake capacities are not only associated with electrostatic attraction and hydrogen bonding, but also related to ion exchange between Cl- and dye molecules. The

adsorption kinetics for all studied dyes were well-fitted to the pseudo-second order

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kinetic model well (R2 = 0.999), and the adsorption isotherms all conformed well to the

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Langmuir isotherm model (R2 > 0.97). Thermodynamic studies indicated that the adsorption process was endothermic in nature. In a word, the LDH was prepared with

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DESs using a directive and effective strategy to achieve the desirable Cl- intercalation

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and reduction of the CO32- contamination, which makes the adsorption rapid and highly effective for anionic dyes. Particularly, its micrometre-scale structure facilitates its

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separation from water, giving it extensive application prospect in wastewater treatment. Keywords: deep eutectic solvents, MgAl layered double hydroxide, adsorption,

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anionic dyes

1. Introduction Currently, water pollution has become one of the most genuine environmental problems [1-3]. Organic dyes, as typical water contaminants, pose a significant threat to the environment and human health [4, 5]. Various attempts to alleviate water 2

contamination have been applied, including photo degradation [6], chemical coagulation [7], ion exchange [8], biodegradation [9], catalytic reduction [10-12] adsorption [13] and electro-chemical treatment [14]. Efficient, simple and economical adsorption technology is considered promising [15-19]. Layered double hydroxides (LDHs) are a type of inorganic laminar 2D materials, which are utilized as efficient adsorbents for organic dyes in water [20-23]. LDHs

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consist of positively charged laminates including bivalent ions (e.g., Zn2+, Co2+, Ni2+, Mg2+, etc.), trivalent metal cations (e.g., Mn3+, Al3+, Ga3+, Fe3+, etc.) and negatively

charged interlamellar ions (e.g., CO32-, SO42-, NO3-, OH-, Cl-, etc.), and the chemical

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formulae of LDHs are expressed as [M1-x2+Mx3+(OH)2]x+(An-)x/n·H2O] [24, 25]. Due to

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the exchangeable character of its layer ions, the interlayer can be occupied by dye molecules to purify the water. In general, abundant CO32- ions were inevitably

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intercalated into the interlayer space during the preparation process [26]. According to previous studies, the affinity sequence of various inorganic anions for LDHs is as

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follows: CO32 ≥ SO42 > OH > F > Cl > Br > NO3 > I [27], which brings about

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difficulty for the delamination of CO32- ions by ion exchange. Various organic anions have been captured into the interlayer space of LDHs to improve the adsorption e.g.,

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properties,

2-hydroxyethylammonium

[28],

dodecylsulfate

[29]

and

ethylenediaminetetraacetic acid [30], giving these LDHs outstanding removal performance compared to non-modified LDHs. However, organic anions are possibly released into the water when exchanged with dye molecules [31], thereby leading to the secondary contamination of the water. LDHs with chloride (Cl-) intercalation are 3

desirable due to their relatively weak interlayer bonding, and environmentally friendly nature [32]. As we know, there are few reports regarding the removal of methyl orange (MO), congo red (CR) and indigo carmine (IC) on a Cl--intercalated MgAl LDH at present. Deep eutectic solvents (DESs), which are classified as green solvents, are similar to ionic liquids, which are formed by hydrogen bonding between two or more

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components [33-35]. As the composition of DESs is adjustable, more DESs have been

designed and applied in different areas such as extraction [34, 36], catalysis [37],

electrochemistry [38], etc. During the past few years, scientific researchers have been

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interested in the synthesis of micro/nanostructured materials based on DESs due to their

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peculiar properties. Zhang et al. synthesized two-dimensional (2D/2D) NiS/graphene heterostructure composites through a one-step pyrolysis method in addition to

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sulphidation by using a novel DES (polyethylene glycol 200 (PEG 200)/NiCl2·6H2O)

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as the precursor [39]. Mu’s group synthesized g-C3N4/metal oxide composites by the pyrolysis process based on a ternary DES (FeCl3/urea/melamine) [40]. The approach of

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synthesizing uniformly MgO microcubes with a DES of MgCl2·6H2O/urea has been reported in our previous work [41]. Due to the special nature of eutectic solvents,

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material synthesis involving DESs may generate novel structures, which are hardly generated in conventional solvents. Herein, a novel DES of AlCl3·6H2O/PEG 200 was elaborately designed and was used as a precursor with a DES of MgCl2·6H2O/urea to synthesize LDHs by a one-step solvothermal reaction. The 3D hierarchically structured MgAl LDH with Cl4

intercalation instead of CO32- intercalation was prepared. When evaluated as an adsorbent, its superior adsorption performance for MO, CR and IC was proven. Moreover, the adsorption isotherm, kinetics, thermodynamics and possible adsorption mechanism were elaborated. 2. Materials and methods 2.1. Materials

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Urea (analytical purity) was supplied by the Beijing Chemical Factory, Beijing, China. PEG 200, MgCl2·6H2O, AlCl3·6H2O, MO, IC, CR, rhodamine B (RhB) and methylene blue (MB) were all purchased from the Macklin Biochemical Co., Ltd.,

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Shanghai, China.

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2.2. Preparation of DESs and LDHs

DES-1: In a typical procedure, a mixture of urea (7.40 g) and MgCl2·6H2O (12.52

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g) was ground thoroughly for 5 min at room temperature to form a homogeneous

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solution.

DES-2: A mixture of 9.23 g of AlCl3·6H2O and 22.95 g of PEG 200 (molar ratio of

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1 : 3) was stirred and heated in an oil bath at 50°C to form a homogeneous solution. After that, 15.98, 8.01 and 5.33 g of DES-2 were added respectively to 6 g of DES-

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1 (Mg/Al molar ratio of 1:1, 2:1 and 3:1), stirred vigorously for 5 min and sealed in a 25 mL autoclave (Teflon-lined stainless) heated at 150°C for 6 h. The obtained precipitates were dried at 80°C for 12 h after repeated centrifugation and washing with deionized water and ethanol. According to the molar ratio of Mg/Al, the samples were named as MgAl-Cl LDH, Mg2Al-Cl LDH and Mg3Al-Cl LDH, respectively. 5

2.3. Characterization The melting point under a N2 atmosphere was tested by differential scanning calorimetry (DSC) (Perkin Elmer TA MDSC2910, USA). Proton nuclear magnetic resonance (1H NMR) spectra were recorded on a Bruker DPX400 spectrometer (Bruker, Switzerland). The microstructure and morphology were characterized by scanning electron microscopy (SEM) (SU8010, Hitachi, Japan) and transmission electron

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microscopy (TEM) (F-20, FEI, USA). The crystallinities of samples were characterized by an X-ray diffraction (XRD) instrument (D/MAX-RB, Rigaku, Japan). The thermal

behaviours of the sample were measured by thermogravimetric analysis (TGA/SDTA

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851e, Mettler Toledo Corporation, Switzerland). Fourier transform infrared (FT-IR)

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spectra of the samples were determined on a Shimadzu IR Affinity-1 spectrometer at a wavelength range of 400-4000 cm-1, Japan. The content of chloride ions in the sample

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was tested by ion chromatography (ICS 5000, Thermo, USA). The concentration was

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measured using a UV-vis spectrophotometer (T9s, Persee). The Brunauer-EmmettTeller (BET) method was conducted for surface area and porosity analysis (ASAP 2460,

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Micrometrics, USA). The elemental contents of the sample were determined with inductively coupled plasma optical emission spectroscopy (ICP-OES 5110, Agilent,

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USA). The elemental status was investigated by X-ray photoelectron spectroscopy (XPS) (EscaLab 250Xi, Thermo, USA). The zeta potentials were determined with a Zetasizer (NaNo ZS 90, Malvern, China). 2.4. Adsorption experiments Typically, MgO power (0.02 g) was added to an aqueous solution of dye (40 mL) 6

in a beaker (100 mL) with gentle stirring for 100 min at 25°C while maintaining a neutral pH unless otherwise stated. Solutions with concentrations of 200 mg L-1 MO, CR and IC were used to conduct the kinetic experiments. High-speed centrifugation of 3 mL of suspension was performed after a fixed interval of time. The adsorption yield () and adsorption capacity [Qt (mg g-1)] were calculated by eq (1) and eq (2), respectively. For adsorption thermodynamics experiments, the Mg2Al-Cl LDH sample

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was added to MO, CR and IC solution (100-600 mg L-1), respectively. The adsorption capacity at equilibrium [Qe (mg g-1)] of the Mg2Al-Cl LDH sample was calculated by eq (3). C0 − Ct ×100% C0 (C0 − Ct )V Qt = m (C0 − Ce )V Qe = m

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 =

(1) (2) (3)

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where C0 is the initial dye concentration (mg L-1), Ct and Ce are the dye concentrations

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at time t and equilibrium (mg L-1), m is the mass of the adsorbent (g), and V is the volume of the aqueous solution (mL).

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3. Results and discussion 3.1. Formation of DESs

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The obtained eutectic solvents of AlCl3·6H2O/PEG 200 are clear and transparent.

The DSC curve (Fig. S1) indicates that the melting point (Tg) of DES is -58°C. Fig. S2 shows the 1H NMR spectra before and after the DES formed. The chemical shift of hydrogen in AlCl3·6H2O increases from 3.33 to 3.36 ppm and the extensive broadening of the proton signal of PEG 200 located at 4.57 ppm appears after DES formation, 7

suggesting the existence of hydrogen bonding in DES. 3.2. Structures and morphologies of LDHs The structures of LDHs with various molar ratios of Mg2+/Al3+ (1:1, 2:1 and 3:1) were evaluated by XRD. As shown in Fig. 1a, the crystal structure of the Mg3Al-Cl LDH sample is comprised of the main phase of LDH (JCPDS file no. 35-0965) and impurity phase of MgCO3 (JCPDS file no. 08-0479), depicting that a large amount of

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carbonate is formed when the molar ratio of Mg2+/Al3+ is high. The as-prepared Mg3Al-

LDH has a basal spacing of 7.65 Å (Fig. 1b), as estimated from Bragg’s law [42]. A series of reflections at 2θ = 11.48°, 23.08°, 34.84°, 39.24°, 46.60°, 60.68° and 61.96°

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of Mg2Al-Cl LDH are attributed to the (003), (006), (012), (015), (018), (110) and (113)

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crystal planes for MgAl LDH (JCPDS file no. 35-0965). Peaks for the planes (003) and (006) imply laudable crystallization. The diffraction peaks of (110) and (113) indicate

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that Mg2+ and Al3+ exist in the host layer. Compared to the Mg3Al LDH sample, the

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increase of the interlayer spacing of Mg2Al-Cl LDH up to 7.70 Å, signifies that Cl- may be introduced into the interlayer space, thus resulting in the interlayer spacing increased.

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This is consistent with the theoretical results, which verified that the attraction of monovalent Cl- is less to the laminate than that of bivalent CO32-. Subsequently, the

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contents of Mg2+, Al3+ and Cl- in the Mg2Al-Cl LDH (the product is digested with cold concentrated HNO3) were determined by ICP-OES (Mg2+ and Al3+) and ion chromatography (Cl-). The results showed that the molar ratio of Mg2+ : Al3+ : Cl- is 1.93 : 1 : 0.84, wherein the Mg/Al molar ratio of the Mg2Al-Cl LDH sample is close to that (2.00) of the precursor involved in the synthesis. Furthermore, lattice parameters 8

of the as-prepared Mg2Al-Cl LDH are determined to be a = 3.05 Å and c = 23.11 Å, which are attributed to the peak positions of the (003) and (110) planes of the XRD pattern (calculation processes are introduced in the supporting information). For the MgAl-Cl LDH sample, the diffraction peaks are basically consistent with the peaks of the Mg2Al-Cl LDH. However, the adsorption performance of the MgAl-Cl LDH sample for the mentioned dyes is lower than that of Mg2Al-Cl LDH in the adsorption

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experiment (Fig. S3), which is because the adsorption sites of MgAl-Cl LDH are

covered by the excess aluminium that may exist in an amorphous form. Therefore, the

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structure and properties of Mg2Al-Cl LDH are presented in subsequent research.

Fig. 1. Full XRD patterns (a) and detailed XRD patterns from 10 to 13° (b) of the

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Mg2Al-Cl LDH sample.

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Fig. 2a-c give the high and low-magnification SEM and TEM images of the Mg2AlCl LDH product. It displays 3D hierarchical architecture (diameters are ca. 1-10 μm) is assembled by numerous nanosheets, which facilitates its separation from water. As displayed in Fig. 2b, the TEM reveals that the thickness and size of the nanosheets are approximately 30 and 600 nm, respectively. Moreover, the energy dispersive spectrometry (EDS) and elemental mapping manifest that Mg, Al, O and Cl elements 9

are distributed homogeneously on the surface of the Mg2AlCl-LDH architecture, and it is also depicted that Cl- has been introduced into the interlayer space of Mg2Al-Cl LDH

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as observed in Fig. 2d-f. In addition, XRD patterns also validate the above results.

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Fig. 2. SEM images of the Mg2Al-Cl LDH sample at low magnification (a) and high magnification (b); TEM image (c); EDS spectrum (d); and element mapping images (e,

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f).

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3.3. Thermal behaviour and textural properties

Fig. 3. Thermal curves of the Mg2Al-Cl LDH sample. The thermal behaviour of the as-prepared Mg2Al-Cl LDH sample is provided in Fig. 3. Obviously, two steps of weight loss are observed as the calcining temperature 10

increases from 25 to 1000°C. At 25-255°C, the first step appears in the curve with a weight loss of 9.23%, which is attributed to the loss of interlayer and surface absorbed water. Subsequently, in the range of 255-1000°C, a significant endothermic peak centred at 410°C appears with approximately 30.38% relative loss, corresponding to the elimination of the chloride ions and collapse of the MgAl layered structure. The final products after 900°C treatment are MgAl2O4 (JCPDS file no. 21-1152) and MgO

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(JCPDS file no. 45-0946), as determined by XRD (Fig. S4). The N2 adsorptiondesorption isotherm curve of the Mg2Al-Cl LDH product and the accompanying pore size distribution curve are depicted in Fig. 4 and its inset. A type IV isotherm with a

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type-H3 hysteresis loop is observed via the IUPAC classification, inferring the

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formation of pores by the stacking of nanosheet building blocks. The surface area and total volume of pores for Mg2Al-Cl LDH are 26.64 m2 g-1 and 0.12 cm3 g-1 by the BET

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method, respectively. Additionally, the pore sizes are mainly in the range of 2-3 and 5-

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10 nm with an average pore diameter of 26.34 nm (Fig. 4 inset), implying well-built

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mesoporous structures.

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Fig. 4. N2 adsorption/desorption isotherms with the corresponding pore-size distribution (inset) of the Mg2Al-Cl LDH product. 3.4. Adsorption studies Five dyes (100 mg L-1) with different electronegativities: MO, CR and IC are electronegative, RhB and MB are electropositive were selected as adsorbates to investigate the adsorption probability of the Mg2Al-Cl LDH product. As shown in Fig.

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5a, the removal efficiencies for MO, CR and IC reached 99%, but the removal efficiencies were only 9% and 6% for RhB and MB solutions, respectively, suggesting

that only electronegative materials can be effectively removed by Mg2Al-Cl LDH. To

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determine that the removal process of the three dyes on the Mg2Al-Cl LDH is

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adsorption rather than photocatalysis, the removal experiments were performed in natural light and in dark simultaneously (concentration is 200 mg L-1, 25°C and natural

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pH). After 30 minutes of adsorption, the concentration of the solution was measured.

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The results of the control group were shown in Fig. S5. It is found that the removal capacities for MO, CR and IC were basically the same with and without light, suggesting

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that the influence of photolysis can be ignored. Consequently, MO, CR and IC are used as target pollutants to evaluate the adsorption behaviour of the Mg2Al-Cl LDH in the

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present work.

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Fig. 5. (a) The digital photographs of the prior and after adsorption of the 3D Mg2AlCl LDH architectures towards MO, CR, IC, RhB and MB solution at 25°C. (b) Removal efficiencies of the Mg2Al-Cl LDH sample towards five organic dyes.

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3.4.1 Effect of the contact time and initial pH

The dye solution (200 mg L-1) of MO, CR and IC were employed to assess the

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adsorption performance of the Mg2Al-Cl LDH sample. As observed in Fig. 6a, for all

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of the studied dyes, the adsorption capacity (Qt) was determined as a function of time. It is apparent that the adsorption capacities of Mg2Al-Cl LDH toward the three dyes are

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higher, especially for MO and CR. Additionally, equilibrium can be achieved in approximately 30 min, indicating the fast removal rates of the mentioned dyes.

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The effect of the pH of the solution on the removal efficiencies of Mg2Al-Cl LDH

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for MO, CR and IC dyes (200 mg L-1) are determined. Fig. S6 displays the changes in the removal efficiencies of MO, CR and IC on Mg2Al-Cl LDH with initial pH values of 4-12. The adsorption capacities of Mg2Al-Cl LDH toward MO, CR and IC demonstrate an increasing trend at pH values of 4-6 and decrease at pH values of 8-12. The surface charge of the adsorbent is susceptible to pH fluctuations, which can affect the adsorption performance. Therefore, the zeta potentials of the adsorbent under 13

various pH values are investigated. As shown in Fig. S7, the point of zero charge (pHPZC) of the Mg2Al-Cl LDH is 10.53. Positive surface charges of the Mg2Al-Cl LDH sample when the pH is below the pHPZC, confirm the binding of negatively charged MO, CR, and IC molecules. However, the negative surface charge of absorbent when the pH is above the pHPZC disrupts the adsorption of MO, CR and IC due to the electrostatic repulsion. Moreover, in an alkaline environment, excessive OH- inhibits the adsorption

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of the adsorbent for anionic dyes, consequently decreasing the adsorption capability in the pH range of 8-12 to different degrees. In the pH range of 10-12, the adsorption

other interactions, not just electrostatic attraction.

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3.4.2 Adsorption kinetics

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capacity is still above 300 mg g-1, suggesting that the adsorption process is affected by

Different kinetic models are generally employed for checking the adsorption

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method of adsorbents, namely the (i) pseudo-first-order kinetic model, (ii) pseudo-

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second-order kinetic model and (iii) intraparticle diffusion model. The linearized forms and meanings of the mentioned kinetic models are shown in Table S1. Linear plots of

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ln (Qe-Qt) and t/Qt versus t for the removal of the studied dyes by the as-prepared Mg2Al-Cl LDH sample are depicted in Fig. 6b, c. Parameters and the theoretical

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adsorption capacity for each kinetic model are obtained from the intercept and slope fitting, respectively. From the calculated correlation coefficients (R2) of the linear fitted curves (Table S2), it is found that the pseudo-second-order kinetic model (R2 = 0.999) fits the experimental observations much better than the pseudo-first-order kinetic model for all of the studied dye solutions, and the theoretical adsorption values (Qe,cal) are in 14

close proximity to the experimental data (Qe,exp). On the basis of the above analysis, it is indicated that the adsorption of MO, CR and IC adsorption on the Mg2Al-Cl LDH

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sample can be described by the pseudo-second-order model.

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Fig. 6. Effect of the contact time effects on dye adsorption (a); fitting curves of the pseudo-first (b) and pseudo-second order kinetic models (c) as well as intra-particle

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diffusion kinetic model (d) (concentration is 200 mg L-1, 25 °C and natural pH). The linear fitting of the intraparticle diffusion model for MO, CR and IC removal

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by the Mg2Al-Cl LDH sample is shown in Fig. 6d and the corresponding parameters are calculated from intercept and slope of the fitted lines and are summarized in Table S3. Clearly, for the removal of all of the studied dyes by the Mg2Al-Cl LDH, two linear portions are observed, which suggests that the adsorption process can be affected by two rate-determining steps. Initially, the high concentration of the adsorbates and the 15

numerous active sites on the outer surface of the Mg2Al-Cl LDH accelerate the molecular transport towards and from the adsorbent surface, resulting in the appearance of steeper lines. Additionally, a more moderate slope follows, which means that the dye molecules (MO, CR and IC) diffuse into the interior with a slow removal rate as all of external active sites of the Mg2Al-Cl LDH sample are nearly occupied. In addition, the

process is involved in the MO, CR and IC adsorption. 3.4.3 Adsorption isotherms

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fitted line does not go through the origin, and it can be inferred that more than one

Adsorption isotherms for MO, CR and IC removal on the 3D hierarchically

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structured Mg2Al-Cl LDH are subsequently analysed (Fig. 7). As can be seen, the Qe,exp

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903.42 and 499.11 mg g-1, respectively.

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(maximum experimental uptake capacity) values for MO, CR and IC are 1030.13,

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Fig. 7. Langmuir (a), Freundlich (b), Temkin (c) and D-R (d) isotherms of the Mg2AlCl LDH sample for MO, CR and IC removal at 25°C. Generally, observations for the mentioned dyes were analysed by utilizing four isotherm models, including the Langmuir, Freundlich, Temkin and DubininRadushkevich (D-R) models, and Table S4 shows the related nonlinear equations and the definition of parameters for each model. The adsorption experimental data and

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fitting results of each model for MO, CR and IC adsorption onto the Mg2Al-Cl LDH are shown in Fig. 7. The values of relevant parameters and the correlation coefficients (R2) are presented in Table 1. From the value of R2, the experimental isotherms for all

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of the studied anionic dyes are well represented by the Langmuir isotherm model (R2 >

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0.97), implying the uniform nature of the surface and the mono-layer coverage of dye molecules on the Mg2Al-Cl LDH surface. Moreover, the Qe,cal (theoretical maximum

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value) values of the Mg2Al-Cl LDH toward MO, CR and IC are 1051.87, 889.76 and

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512.55 mg g-1, which are close to the experiment results, which are deduced from Langmuir isotherm model. Compared with other reported LDH adsorbents, it has a

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significant adsorption capacity for anionic dyes, especially for MO (> 1000 mg g-1), which is listed in Table S5. Furthermore, for the Langmuir-type adsorption, the existing

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affinity between the Mg2Al-Cl LDH and dyes can be determined by the RL (separation factor). It can be expressed as follows: RL = 1⁄(1+KL C0 )

(4)

The shapes of the isotherms are classified irreversible (RL = 0), linear (RL = 1), unfavorable (RL > 1) and favorable (0 < RL <1). After being calculated, the values of 17

RL are in the ranges of 0.00928-0.00389 for MO, 0.00659-0.00276 for CR and 0.03720.00851 for IC, suggesting the favorable adsorptions of the three mentioned anionic dyes on the Mg2Al-Cl LDH. Additionally, the R2 value obtained by the D-R isotherm model is less than that calculated by the Langmuir model, and it exhibits the same trend that the values of Qe,cal for MO and CR are higher than for IC. Therefore, the average free energy E (Table S3) is calculated, which indicates the type of adsorption process

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such as chemical adsorption, physical adsorption, etc. As seen in Table 1, the values of E for all of the studied dyes are lower than 8 kJ mol-1, indicating that the adsorption

process is dominated by physical mechanisms, such as hydrogen bonding and

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electrostatic interaction.

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3.4.4 Adsorption thermodynamic

The effect of the temperature (298 K, 308 K and 318 K) on MO, CR and IC

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adsorption was also investigated at natural pH. As shown in Fig. 8a-c, the removal

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capabilities for MO, CR and IC on Mg2Al-Cl LDH rise with increasing temperature, depicting that the adsorption is an endothermic process. Moreover, thermodynamic

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parameters including ∆G° (free energy change), H° (enthalpy change) and S° (entropy change) were calculated from the intercept and slope of the linear fitting plot

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of the van’t Hoff equation (see the supporting information), which is shown in Fig. 8d. Table 2 lists all the related calculated thermodynamic parameters. The negative values of ∆G° and positive values of ∆S° imply that the removal processes of MO, CR and IC on the Mg2Al-Cl LDH under the studied temperatures are both spontaneous and endothermic. Meanwhile, ∆H° > 0 further demonstrates the endothermic nature of the 18

as-prepared Mg2Al-Cl LDH for the adsorptions of MO, CR and IC. Additionally, the values of ∆H° for the adsorption of all of the dyes are lower 40 kJ mol-1, suggesting the physical purification process for the removal of the mentioned dyes on the Mg2Al-Cl

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LDH sample.

Fig. 8. Adsorption isotherms for MO (a), CR (b) and IC (c) adsorption on the Mg2Al-

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Cl LDH; (d) plots of the van’t Hoff equation for MO, CR and IC removal.

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3.4.5 Adsorption mechanisms of the Mg2Al-Cl LDH sample for anionic dyes

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Fig. 9. The SEM images and corresponding EDS spectrums of the Mg2Al-Cl LDH sample after adsorption MO (a, d), CR (b, e) and IC (c, f).

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As shown in Fig. 9a-c, the SEM images of LDHs after adsorption has no

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significant changes compared with before adsorption (Fig. 2b). The all corresponding EDS spectrums (Fig. 9d-f) display that the additional peaks of N and S which origin

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from the organic dyes are detected and Cl is disappeared, indicating that MO, CR and

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IC molecules has been adsorbed on the Mg2Al-Cl LDH sample and Cl- has been exchanged by dye anions. To evaluate the possible removal mechanisms of the Mg2Al-

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Cl LDH product for MO, CR and IC, the adsorbents prior to and after adsorption were characterized by FT-IR, XPS and XRD analysis. The FT-IR spectra of Mg2Al-Cl LDH

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prior to and after dye removal are shown in Fig. 10a. The adsorption peaks in the range of 500-800 cm-1 are associated with M-O, M-O-M and O-M-O lattice vibrations (M = Mg or Al) [43]. The weak band at 1613 cm-1 is assigned to the H-O-H bending vibration of interlayer water molecules. The significant peak at 1105 cm-1 is ascribed to the aromatic ring of MO, CR and IC molecules [44] and the peak at 1030 cm-1 is assigned 20

to the S=O vibrations [45]. The above analysis reveals that the mentioned dyes were adsorbed onto the surface of the Mg2Al-Cl LDH product. Similar to our previous report [36], the –OH on the surface of Mg2Al-Cl LDH can form hydrogen bonds with -N-H or –N=N of the three dyes, which facilitates the removal of the related dyes. Moreover, the high zeta potential value of the adsorbent solution is determined (18.6 mV) at natural pH, which contributes to the adsorption of anionic dye molecules on the Mg2Al-

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Cl LDH surface by electrostatic interactions. Furthermore, the XPS spectra of the

Mg2Al-Cl LDH sample prior to and after the removal of the three dyes were tested, which are displayed in Fig. 10b. Three dominant elements including Mg, O, and Al are

-p

detected in all of the spectra, whereas the S 2p and N 1s peaks are observed, and the Cl

re

2s and Cl 2p peaks disappeared after dye adsorption on Mg2Al-Cl LDH. This indicates that the Cl- in the Mg2Al-Cl LDH are exchanged by dye molecules in the adsorption

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process, which is agreement with the EDS results (Fig. 9d-f). Fig. 10c depicts the XRD

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patterns of Mg2Al-Cl LDH prior to and after dye adsorption. For MO adsorption, the peak of the (003) plane at 11.48° is split into three peaks at 7.32°, 11.14° and 14.48°

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after dye removal. The peak at 7.32° is ascribed to the expanded interlayer from 7.7 to 12.07 Å by anion exchange between the Cl- and MO molecule. The peak at 11.14° after

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adsorption and at 11.48° prior to adsorption are considered to be the same peak. Based on the previous reports [27], the additional peak at 14.48° (interlayer is 6.06 Å) may be ascribed to the contraction of adjacent layers. For CR and IC adsorption, the peak of the (003) plane of the Mg2Al-Cl LDH before adsorption at 11.48° shift to a small angle of 10.76 and 11.32° after adsorption (Fig. 10c), which is corresponding to the interlayer 21

spacing is expanded from 7.7 Å to 8.22 Å and 7.81 Å calculated by Bragg’s law [42] (Fig. 10d). Compared to MO adsorption, there is no adjacent layers contraction for CR

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and IC adsorption, and the schematic illustration is shown in Fig. 11.

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Fig. 10. FT-IR spectra (a), XPS spectra (b), full XRD patterns (c) and detailed XRD

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patterns in the range from 5 to 30° (d) prior to and after the dye adsorption.

22

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Cl-LDH sample for MO, CR and IC.

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4. Conclusions

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Fig. 11. The schematic illustration of the possible removal mechanism of the Mg2Al-

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In this work, the 3D hierarchically structured MgAl LDH with Cl- intercalation was synthesized by a facile solvothermal method based on the DESs of MgCl2·6H2O/urea

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and AlCl3·6H2O/PEG 200. Compared with CO32- intercalated MgAl-LDH and other metal layer LDHs in previous studies, it showed excellent removal performance for

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anionic dyes such as MO, CR and IC from aqueous solutions. The theoretical maximum adsorption quantities for MO, CR and IC on Mg2Al-Cl LDH architecture were 1051.87, 889.76 and 512.55 mg g-1 at 25°C at natural pH, respectively. The adsorption mechanism of Mg2Al-Cl LDH for MO, CR and IC is mainly dominated by the hydrogen bonding, electrostatic attraction and anion exchange between the interlayer Cl- and 23

mentioned molecules. The appropriate pore structure also provided more adsorption sites for dye molecules. Moreover, the thermodynamic properties, adsorption isotherm and adsorption kinetics were proposed in detail. It revealed that the interlayer spacing of Mg2Al-Cl LDH can be expanded after dye adsorption. Unlike the cases of CR and IC, there are also some adjacent layers that could be squeezed besides the expansion for the intercalation of MO molecules. In summary, the LDH prepared by solvothermal

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synthesis with DESs is the most effective method to achieve the admirable Clintercalation and reduce the CO32- contamination, and it possesses the excellent

-p

adsorptivity for anionic dyes.

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Credit Author Statement Zhidong Chang: Conceptualization, Writing - Review & Editing. Hui Dang: Conceptualization. Yifei Zhan: Conceptualization, Software. Jingyang Lou: Writing - Review & Editing. Shan Wang: Writing - Review & Editing. Sanam Attique: Writing - Review & Editing. Wenjun Li: Methodology. Hualei Zhou: Methodology. Changyan Sun: Methodology.

Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments This project is supported by the National Natural Science Foundation of China (Grant No. 21276022). 24

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Table 1 Isothermal parameters for the removal of the mentioned dyes by the Mg2Al-Cl LDH sample.

D-R

CR

IC

0.427 1051.869 0.978 6.037 529.751 0.863 24.279 17.22 0.917 940.881 0.825 0.797

0.603 889.758 0.977 8.582 527.302 0.901 234.68 27.599 0.938 839.81 0.834 0.912

0.259 512.553 0.974 6.562 239.054 0.773 18.701 38.132 0.84 476.656 0.933 0.52

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Temkin

KL (L mg ) Qe,cal (mg g-1) R2 n KF (L g-1) R2 A (L mg -1) b (J mol-1) R2 Qe,cal (mg g-1) R2 E (kJ mol-1)

MO

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Freundlich

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Langmuir

Parameters

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Isotherm models

Table 2 Thermodynamic parameters of the Mg2Al-Cl LDH sample for MO, CR and IC adsorption.

∆G° (kJ mol-1)

(kJ mol-1)

(J mol-1 K-1)

298 K

308 K

318 K

R2

21.53

83.20

-3.26

-4.10

-4.93

0.952

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MO

∆S°

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∆H°

Dyes

9.20

40.33

-2.81

-3.21

-3.62

0.988

IC

7.73

33.53

-2.26

-2.60

-2.93

0.974

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CR

33