Ultrasound-assisted adsorption of Congo red from aqueous solution using MgAlCO3 layered double hydroxide

Applied Clay Science 174 (2019) 100–109

Contents lists available at ScienceDirect

Applied Clay Science journal homepage: www.elsevier.com/locate/clay

Research Paper

Ultrasound-assisted adsorption of Congo red from aqueous solution using MgeAleCO3 layered double hydroxide

T

⁎

Weike Zhanga, , Ying Lianga, Jiawei Wanga, Yanrong Zhanga, Zeyu Gaoa, Yanqing Yanga, ⁎ Kai Yangb, a b

College of Environmental Science and Engineering, Taiyuan University of Technology, Jinzhong 030600, China Laboratory for Earth Surface Processes, College of Urban and Environmental Sciences, Peking University, Beijing 100871, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Adsorption Congo red Layered double hydroxide Ultrasonic power Electrostatic force Ion exchange

MgeAleCO3 layered double hydroxide (LDH) was prepared via the hydrothermal method, and used to remove Congo red (CR) from the aqueous solution with the assist of ultrasound. The influence of ultrasonic power on the CR adsorption, together with adsorption kinetics and isotherms were investigated. The mechanism of ultrasound-assisted adsorption was elucidated. The results showed that compared with stirring and shaking, ultrasound-assisted adsorption resulted in a significantly shorter adsorption equilibrium time of 120 min under initial 0.2 g/L CR and 0.2 g/L LDH dosage. The optimal ultrasonic power was 180 W at which a maximum adsorption capacity of 934.43 mg/g was achieved. The adsorption kinetics fitted the pseudo-second order kinetic model well (R2 = 0.9999), and the adsorption isotherms fitted the Langmuir isotherm model well (R2 = 0.998). The adsorption rate remained above 60% after three cycles.

1. Introduction Congo red (CR), an anionic acid dye, has been used widely in food, textile, plastics, paper and printing industries (Dai et al., 2018). Consequently, it has become ubiquitous in industrial wastewater. However, it is difficult to remove CR from wastewater due to its complex structure (Tian et al., 2018). CR can not only pollute aquatic environment, but also cause harmful health effects, such as carcinogenesis, teratogenesis and mutagenesis (Munagapati and Kim, 2017). Conventional wastewater treatment technologies mainly include membrane separation (Xu et al., 2018), coagulation (Hashemian and Foroghimoqhadam, 2014; Xu et al., 2018), biodegradation (Chen et al., 2014; Hashemian and Foroghimoqhadam, 2014), photocatalytic degradation (Chen et al., 2014; Yu et al., 2018), ion-exchange (Yu et al., 2018) and adsorption (Saber-Samandari et al., 2017). Adsorption method is popular in wastewater treatment as featured by its high treatment efficiency, low economic cost, simple operation and strong practicability (Lee et al., 2018; Zheng et al., 2017). Adsorbents play a significant role in adsorption process. Common adsorbents mainly include activated carbon (Pelekani and Snoeyink, 2001), nanoparticles (Li et al., 2017; Lipatova et al., 2018), clay materials (Bulut et al., 2008; Vimonses et al., 2009) and agricultural solid wastes (Sivaraj et al., 2001). Layered double hydroxide (LDH), which is anionic clay with double-layer structure,

⁎

appears to be a promising adsorbent due to its high specific surface area (El Hassani et al., 2017), tunable structure (Chan et al., 2008) and low cost of raw materials. Ultrasonic technology has been attempted to apply in wastewater treatment (Hao et al., 2012; Sharifpour et al., 2018; Wu et al., 2018). The effect of ultrasound on the treatment of organic pollutants in wastewater is attributed to ultrasonic cavitation. In brief, the bubbles existing in the liquid are activated by the ultrasonic field, resulting in a series of dynamic processes such as bubble oscillation, growth, contraction, and even collapse (Khataee et al., 2018). During the process, the energy of ultrasound field is highly centered in the micro cavitation bubbles, and the physical effects such as high pressure, high temperature, acoustic waves, microjets, microstreaming and microturbulence are generated at the instant that the cavitation bubbles collapse (Midathana and Moholkar, 2009). There have been numerous literatures reporting the application of ultrasound-assisted adsorption in wastewater treatment over the past few years. For example, Şayan (Şayan, 2006) studied the adsorption of reactive dye rifacion yellow HE4R onto activated carbon assisted by ultrasound. The author found that the combination of ultrasound and activated carbon was efficient to remove all dye from the textile wastewater. Milenković (Milenkovic et al., 2009) used hazelnut shell-derived granular activated carbon to remove copper (Cu) (II) ions from the aqueous solution with and

Corresponding authors. E-mail addresses: [email protected] (W. Zhang), [email protected] (K. Yang).

https://doi.org/10.1016/j.clay.2019.03.025 Received 8 December 2018; Received in revised form 5 February 2019; Accepted 25 March 2019 0169-1317/ © 2019 Elsevier B.V. All rights reserved.

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Nano (ZS90, Malvern, UK).

without ultrasound assist. The authors demonstrated that sonication increased the adsorption capacity of the activated carbon and promoted intraparticle diffusion. In addition, Milenković (Milenkovic et al., 2013) explored the removal of 4-dodecylbenzene sulfonate (DBS) ions from the aqueous solution by ultrasound-assisted adsorption onto the carbonized corn cob. The adsorption was also improved by sonication. In the present study, the MgeAleCO3 LDH was prepared via the hydrothermal method. We hypothesis that the MgeAleCO3 LDH can effectively remove CR from aqueous solution by ultrasound-assisted adsorption method. Factors influencing the adsorption were studied followed by the elucidation of the adsorption mechanism. The ultrasound-assisted adsorption efficiency of the LDH was also compared with those under conventional adsorption processes (e.g. shaking and stirring).

2.4. Ultrasound-assisted adsorption experiments To investigate the effects of different adsorption processes (viz. ultrasonication, shaking, and stirring) on CR adsorption by the LDH, a set of 100 mL erlenmeyer flasks containing 0.01 g of LDH and 50 mL of 200 mg/L CR solution were placed in an ultrasonic processor (SM900A, Shunmatech, China), a constant-temperature shaker (THZ-82, Guohua, China) and a magnetic mixer (HJ-3, Guohua, China), respectively, for 5 h to reach the adsorption equilibrium. The effect of ultrasonic power on the adsorption was studied at 90, 135, 180, 225 and 270 W. For the adsorption kinetics, the suspensions containing 0.01 g of LDH and 50 mL of CR solution were sonicated by the ultrasonic processor for different absorption periods of 0, 1, 5, 10, 20, 30, 45, 60, 75, 90 and 120 min. The adsorption isotherms of CR adsorption by the LDH were investigated at initial CR concentrations of 30, 60, 90, 120, 150, 180, 210 and 270 mg/L at different temperatures of 303, 323 and 333 K. 0.01 g LDH was added to 50 mL of CR solution in a 100 mL Erlenmeyer flask. The suspensions were placed in the ultrasonic processor for several minutes until the equilibrium was achieved. After adsorption, the solution was centrifuged at 10000 r/min for 10 min. The CR concentration in the supernatant was measured using a UV–visible spectrophotometer (UV-1800PC, Mapada, China) at a wavelength of 500 nm. The dye removal (%), equilibrium adsorption capacity (qe, mg/g), and adsorption capacity at time t (qt, mg/g) are calculated using the following equations:

2. Materials and methods 2.1. Reagents Magnesium nitrate (Mg(NO3)2·6H2O), aluminum nitrate nonahydrate (Al(NO3)3·9H2O), sodium hydroxide (NaOH) and Congo Red (C32H22N6Na2O6S2) were purchased from Damao Chemical Reagent Factory (China). Sodium carbonate anhydrous (Na2CO3) was bought from Tianjin Guangfu Technology Development Co., Ltd. (China). All reagents were of analytical grade, and used without any further purification. 2.2. Preparation of MgeAleCO3 LDH

Dye removal =

The MgeAleCO3 LDH was prepared via the hydrothermal method. 5.12 g Mg(NO3)2·6H2O and 3.75 g Al (NO3)3·9H2O were dissolved fully in 100 mL ultrapure water to obtain solution A. 6.75 g NaOH and 5.30 g Na2CO3 were dissolved fully in 100 mL ultrapure water to obtain solution B. Solutions A and B were added dropwise together to 100 mL ultrapure water in a conical flask placed in a water bath at 80 °C with constant stir. Afterwards, the solution was transferred into the reaction kettle and oven-dried at 100 °C for a 4 h hydrothermal reaction. The solution was then cooled to the room temperature. The white precipitate was rinsed to neutral with ultrapure water and oven-dried at 80 °C for 12 h. The final white solid was ground to fine powder, which was MgeAleCO3 LDH with a molar ratio of Mg/Al = 2:1. In addition, 0.01 g of the as-prepared LDH was added to 50 mL ultrapure water and placed in an ultrasonic machine (SB-5200DTDN, SCIENTZ, China) with an ultrasonic power value of 200 W for 15 min, and then oven-dried at 80 °C for 12 h to obtain the ultrasound-treated LDH (US-LDH).

C0 − Ct × 100% C0

qe =

(C0 − Ce ) V m

qt =

(C0 − Ct ) V m

where C0, Ce and Ct are the initial CR concentration, the equilibrium CR concentration and the CR concentration at time t (mg/L), respectively; V is the volume of CR solution (L); and m represents the amount of LDH adsorbent (g). 2.5. Regeneration test The adsorption of CR onto the LDH during three cycles of the adsorption-thermal treatment was applied to investigate the recyclability of the LDH. The sample, which was separated from the 200 mg/L of CR solution after adsorption, was oven-dried at 90 °C overnight and then calcined at 450 °C for 4 h in a furnace (SX-G07103, Zhonghuan, China). The calcined LDH (CLDH) was recycled for the next run with the same concentration of CR solution.

2.3. Characterization of MgeAleCO3 LDH The surface morphology of the LDH was observed using a scanning electron microscope (SEM) (JSM-7100F, JEOL, Japan) with an operating voltage of 10 kV. The crystal structure and crystal composition of the LDH were identified using an X-ray diffraction (XRD) meter (DX2700, Haoyuan, China) with Cu Kα radiation set at 40 kV and 100 mA, and diffraction angels (2θ) ranging from 5° to 80° at a step size of 8°. The specific surface area and pore size distribution of the LHD were determined by the nitrogen (N2) adsorption–desorption method using a BET specific surface analyzer (Quadrasorb-SI, Quantachrome, USA). The functional groups of the LDH were identified using a Fourier transform infrared (FTIR) spectrometer (Nicolet iS10, Thermo Scientific, USA). The thermogravimetric analysis (TG) and differential scanning calorimeter (DSC) were conducted to study the thermophysical properties of the LDH. Briefly, the LDH sample was heated from 20 to 800 °C at a heating rate of 10 °C min−1 under N2 atmosphere using a simultaneous thermal analyzer (STA 449 F3 Jupiter®, Netzsch, German). And the zeta potential of the LDH was measured by Zetasizer

3. Results and discussion 3.1. Characteristics of MgeAleCO3 LDH The SEM image reflected that the LDH was well dispersed with a lamellar structure (Fig. 1). Each layer exhibited a platelet-like structure with a hexagonal shape. This is consistent with the LDH synthesized in Wang (Wang et al., 2013), who successfully prepared the MgeAleCO3 LDH by chemical precipitation and hydrothermal methods. The diameter of the LDH in the present study ranged from 50 to 200 nm, which was averagely smaller than the range of 100–300 nm in Wang (Wang et al., 2013). The XRD pattern of the LDH peaked at (003), (006), (012), (015), (018), (110) and (113) (Fig. 2). All above peaks were narrow with a single crystalline phase, indicating the formation of complete LDH 101

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Fig. 4. The FTIR spectrum of the LDH.

Fig. 1. The SEM image of the LDH.

21.45 nm. According to the IUPAC classification, the adsorption isotherm type of LDH belongs to type V, it is typical for mesoporous materials. And it has the H3 hysteresis loops, which is characteristic for sheets of granular materials (e.g. clay). The FTIR spectrum of the LDH is shown in Fig. 4. A broad band at 3442 cm−1 was ascribed to the OeH stretching vibration (Deák et al., 2018; Extremera et al., 2012). The peak at 1618 cm−1 was associated with the bending vibration of surface-adsorbed and interlayer water molecules (Extremera et al., 2012). The antisymmetric stretching vibration peak caused by the CO32– in the LDH was observed at 1359 cm−1 (Gandara-Loe et al., 2017). In addition, the weak band at 784 cm−1 and 683 cm−1 were due to the MgeOeAl stretching (Yu et al., 2012). The TG and DSC curves of the LDH are shown in Fig. 5. The TG curve revealed that the total mass loss was 42.69% in the range of 20–800 °C. The total mass loss was further divided into three stages, viz. 20–230 °C, 230–430 °C and 430–800 °C. In the first stage, the mass loss was 14.88%, which was related to the evaporation of interlayer and surface-absorbed water molecules from the LDH. The decomposition of OH– and CO32– in the layer occurred in the second stage with a mass loss of 19.88%. The mass loss in the last stage was 7.93%, which was ascribed to the surplus CO32– interlayer (Wang et al., 2013). In the DSC curve, there were two endothermic peaks in the vicinity of 230 °C and 430 °C, corresponding with the evaporation of water molecules and decomposition of OH– and CO32−, respectively. There was another exothermic peak at about 650 °C, which could be associated with the formation of MgeAl oxides (Santos et al., 2017). Above 800 °C, the

Fig. 2. The XRD pattern of the LDH.

structure (Tong et al., 2012). The interlayer spacing (d) of the LDH was 7.55 Å at (003) and 3.77 Å at (006), suggesting that the LDH was in shape-layer (Al Jaafari, 2010). The BET surface area and pore characteristics were calculated according to the N2 adsorption–desorption isotherm and pore size distribution of the LDH (Fig. 3). The LDH had a surface area of 98.63 m2g, a total pore volume of 0.53 cm3/g, and an average pore diameter of

Fig. 3. The N2 adsorption–desorption isotherm and pore size distribution of the LDH. 102

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Fig. 5. The DSC–TG curves of the LDH. Fig. 7. Effect of ultrasonic power on the CR adsorption using Mg-Al-CO3 LDH. Conditions: Initial CR concentration = 0.2 g/L, LDH dosage = 0.2 g/L, contact time = 120 min, T = 298 K. The columns labelled with the same letter were not significantly different at a significance level of p < 0.05 by the least significant difference (LSD) test. The error bars represented one standard deviation (n = 2).

thermal decomposition process was largely completed. 3.2. Effect of different adsorption processes The effect of different adsorption processes on the adsorption of CR by the MgeAleCO3 LDH is shown in Fig. 6. Among these methods, the ultrasound-assisted adsorption had the best performance as evidenced by its shortest adsorption equilibrium time of 2 h. In contrast, the adsorption equilibrium time for either stirring or shaking was 5 h. The difference in adsorption equilibrium was attributed to the dispersion degree of CR. The stirring and shaking methods increased the turbulence between the solid-liquid two-phase via mechanical dispersion. In contrast, the ultrasound-assisted adsorption caused cavitation effects in addition to the mechanical dispersion. The agglomeration formed during the drying process of MgeAleCO3 LDH was broken by ultrasonic cavitation so that the specific surface area was increased and the rate of reaction was promoted accordingly (Milenkovic et al., 2009). In addition, the sole use of ultrasonic without LDH had no effects on the CR adsorption (Fig. 6), suggesting that the main role of ultrasound was to promote the adsorption of CR onto the LDH.

ultrasonic power values. The CR removal rate increased significantly with increasing ultrasonic power from 90 to 180 W (p < 0.05). The value peaked at an ultrasonic power value of 180 W. When the ultrasonic power further increased from 180 to 270 W, the CR removal rate exhibited a decreasing trend. The initial increase in the CR removal rate was mainly due to the predominant mechanical agitation caused by ultrasound at relatively smaller ultrasonic power values. It brings the CR molecules into contact with LDH, so as to improve the CR removal rate. In addition, with the increase of ultrasonic power, the energy in the adsorption system increased as the ultrasound cavitation enhanced, which was beneficial for CR adsorption using LDH (Midathana and Moholkar, 2009). The following reduction in the CR removal rate at ultrasonic power values higher than 180 W could be ascribed to the excessive ultrasonic cavitation, which resulted in the collapse of the cavitation bubbles and desorption of CR from the surface of the LDH (Hamdaoui et al., 2002).

3.3. Effect of ultrasonic power Fig. 7 shows the adsorption of CR by the LDH with different

3.4. Adsorption kinetics The effect of contact time on CR adsorption using MgeAleCO3 LDH with initial CR concentrations of 0.15 and 0.2 g/L is illustrated in Fig. 8(a). In order to investigate the dynamic characteristics of the adsorption of CR onto the LDH, the experimental data in Fig. 8(a) were fitted to the pseudo-first order, pseudo-second order and intraparticle diffusion kinetic models. The fitting results and corresponding kinetic parameter values are shown in Fig. 8(b), (c), (d) and Table 1. The equations of pseudo-first order and pseudo-second order kinetic models are as follows (Shan et al., 2014):

lg(qe − qt ) = lg qe −

k1 t 2.303

t 1 1 = + t qt qe k2 qe2 where qe is the equilibrium adsorption capacity (mg/g); qt is the adsorption capacity at time t (mg/g); k1 is the rate constant of the pseudofirst order kinetic (min−1); and k2 is the rate constant of the pseudosecond order kinetic (g/mg·min). The equation of intraparticle diffusion model is expressed as follows (Ofomaja, 2010):

Fig. 6. Effect of different adsorption processes on the CR adsorption using MgAl-CO3 LDH. Conditions: Initial CR concentration = 0.2 g/L, LDH dosage = 0.2 g/L, contact time = 300 min, T = 298 K. Ultrasonic power = 180 W, stirring speed = 300 r/min, shaker speed = 200 r/min. 103

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Fig. 8. (a) Effect of contact time on the CR adsorption using Mg-Al-CO3 LDH with initial CR concentrations of 0.15 and 0.2 g/L. (b) Linear fitting curves with pseudofirst order kinetic model. (c) Linear fitting curves with pseudo-second order kinetic model. (d) Linear fitting curves with intraparticle diffusion kinetic model. Conditions: LDH dosage = 0.2 g/L, contact time = 120 min, T = 298 K.

addition, the theoretical adsorption capacity of the pseudo-second order kinetic equation was 689.66 and 934.58 mg/g, while the actual adsorption amount was 690.16 and 934.43 mg/g, respectively (Table 1). The results further showed that the pseudo-second order model fitted the experimental data well, and the adsorption process was dominated by chemical adsorption (Pooralhossini et al., 2017). Moreover, the fitting results of the intraparticle diffusion kinetic model showed that the adsorption process of CR onto the LDH could be divided into three stages, viz. a fast adsorption phase, a relatively fast adsorption phase and a slow adsorption phase. In the first stage, the adsorption rate was quite fast, and the adsorption process was mainly through the external diffusion which caused the adsorbate went through the boundary layer of the adsorbent. In contrast to the first stage, the adsorption rate decreased in the second stage which was mainly as a result of particle diffusion. The adsorption equilibrium was reached in the third stage as evidenced by the constant adsorbed amount. Additional fitting results also found that the fitting straight lines of three stages did not pass through the origin at different concentrations, which further indicated that the adsorption of CR onto the LDH was controlled by the liquid film diffusion and particle diffusion (Nethaji et al., 2013). In addition, the maximum adsorption capacity of the LDH was 934.43 mg/g, which was significantly higher than the other works that typical literatures on the CR adsorption using MgeAleCO3 LDH, such as Lafi (111.11 mg/g, (Lafi et al., 2016)) and Shan (37.16 mg/g, (Shan et al., 2015)).

Table 1 Parameters of the pseudo-first order, pseudo-second order and intraparticle diffusion kinetic models for the adsorption of CR onto the LDH. Model

Pseudo-first order

Pseudo-second order

Intraparticle diffusion

Parameter

qe,exp qe,cal k1 R2 qe,cal k2 R2 ki,1 R12 ki,2 R22 ki,3 R32

Initial CR concentration (g/L) 0.15

0.2

690.16 528.85 0.0038 0.1845 689.66 0.0026 0.9999 41.298 0.9792 3.6583 0.9857 3.0524 0.7916

934.43 249.05 0.0618 0.8925 934.58 0.0016 0.9999 99.170 0.9989 6.3096 0.9082 1.5989 0.5441

qt = ki t 1/2 + C where ki is the intraparticle diffusion rate constant (mg/g min1/2); and C is a constant related to thickness and boundary layer of the adsorbent. As shown in Fig. 8, when the initial concentration of CR solution was 0.15 and 0.2 g/L, the correlation coefficient of the pseudo-second order kinetic model was 0.9999 and 0.9999, respectively, while the correlation coefficient of the pseudo-first order kinetic model was 0.1845 and 0.8925, respectively. The pseudo-second order kinetic model better described the CR adsorption process by the LDH. In 104

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Table 2 Parameter values of the Langmuir and Freundlich isotherm models for the adsorption of CR onto the LDH. T (K)

Langmuir isotherm

303 323 333

Freundlich isotherm

qm,cal

KL

RL

R2

KF

n

R2

877.193 869.565 826.446

1.036 0.757 0.829

0.0036–0.0302 0.0050–0.0409 0.0045–0.0375

0.999 0.998 0.998

356.018 341 317.669

3.465 3.571 3.547

0.576 0.507 0.589

Fig. 10. Effect of the untreated LDH and ultrasound-treated LDH (US-LDH) on the CR adsorption. Conditions: Initial CR concentration = 0.2 g/L, adsorbent dosage = 0.2 g/L, contact time = 300 min, T = 298 K, adsorption method: stirring. Table 3 BET surface areas and pore characteristics of the LDH and US-LDH.

LDH US-LDH

Surface area (m2g)

Total pore volume (cm3/g)

Average pore diameter (nm)

98.63 108.80

0.53 0.67

21.45 24.58

capacity (qe) is described by the adsorption isotherm which can be used to investigate the relevant adsorption mechanism. The Langmuir and Freundlich isotherm models have been commonly used to analyze the adsorption experimental data. The two models can be expressed as follows (Nethaji et al., 2013):

Ce 1 C = + e qe qm KL qm

ln qe =

1 ln Ce + ln KF n

where Ce is the equilibrium concentration of CR (mg/L); qe and qm represent the equilibrium adsorption capacity (mg/g) and the maximum adsorption capacity (mg/g), respectively; and KL, KF and n are the Langmuir and Freundlich constants, respectively. In addition, there is a separation factor (RL) of the Langmuir isotherm model. The values of RL and n indicate the feasibility of adsorption that includes 0 < RL < 1 (Mahmoodian et al., 2015) and 2 < 1/n < 10 (Chen et al., 2011) for favorable adsorption. RL is calculated as follows (Chen et al., 2011; Mahmoodian et al., 2015):

Fig. 9. (a) Adsorption isotherms of CR onto the LDH at different temperatures of 303, 323 and 333 K. (b) Linear fitting curves with the Langmuir isotherm model at the three different temperatures. (c) Linear fitting curves with the Freundlich isotherm model at the three different temperatures. Conditions: Initial CR concentration = 0.2 g/L, LDH dosage = 0.2 g/L, contact time = 120 min.

1 1 + KL C0

3.5. Adsorption isotherms

RL =

When the adsorption reaches equilibrium, the relationship between the equilibrium concentration (Ce) and the equilibrium adsorption

The CR adsorption experimental data and fitting results are shown in Fig. 9. The calculation results of the coefficients are listed in Table 2. 105

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Fig. 11. (a): The SEM image of the LDH. (b) and (c): The SEM images of the US-LDH.

Intensity (a.u.)

After adsorption (7.76 Å)

Before adsorption (7.55 Å)

10

20

30

40

50

60

70

80

2θ (°) Fig. 14. The XRD pattern of the LDH and LDH-CR.

Fig. 12. The FTIR spectra of the LDH and LDH-CR.

The adsorption isotherms were well represented by the Langmuir isotherm model (R2 = 0.998): the adsorption of CR onto the LDH was in monomolecular type which means only a layer of adsorbate molecules can be adsorbed onto the surface of the adsorbent. Both the values of RL and n reflected that the adsorption of CR onto the LDH was favorable in an available condition. 3.6. Ultrasound-assisted adsorption mechanism In order to explore the effect of ultrasonic cavitation on the structure of the LDH, the LDH was subjected to ultrasonic treatment prior to the adsorption test. Compared with the untreated LDH, the ultrasoundtreated LDH (US-LDH) exhibited better adsorption performance with the adsorption rate improved by 33% (Fig. 10). This may be attributed to the micro-jetting formed by the ultrasound-generated cavitation bubbles which inhibited the agglomeration of the nanoparticles, and therefore improved the specific surface area and pore size of the USLDH (Table 3). Moreover, the SEM images showed that the structure of the US-LDH was more dispersed and looser than the untreated LDH (Fig. 11). The FTIR spectra of the LDH before and after the CR adsorption are shown in Fig. 12. Two obvious additional peaks at 1045 and 1174 cm−1 were observed on the curve of the LDH-CR. The two bands were ascribed to the stretching of sulfonate group from the CR molecules (Huang et al., 2017), indicating that the CR molecules had been successfully adsorbed by the LDH. The CO32– in the layer of the LDH was replaced by the SO3− of the CR molecules via ion exchange (Li et al., 2016), resulting in the removal of CR from the aqueous solution. In conclusion, the mechanism of the ultrasound-assisted adsorption of CR from the aqueous solution by the MgeAleCO3 LDH were mainly divided into the following three parts. Above all, the agglomeration of

Fig. 13. The Zeta potential of the LDH.

Table 4 The results of the adsorption of CR onto the LDH under different initial pH condition. pHinitial

pHequilibrium

Dye removal (%)

qe (mg/g)

2.37 4.11 6.46 7.64 9.80

4.68 7.45 7.56 7.68 8.35

87.76 89.03 90.80 90.41 83.96

901.80 914.82 933.05 928.96 862.73

106

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Fig. 15. Schematic illustration of mechanism of CR adsorption onto the LDH.

Fig. 17. The XRD pattern of the LDH-CR1~ LDH-CR4. Fig. 16. Regeneration of the LDH after the CR adsorption. The columns labelled with the same letter were not significantly different at a significance level of p < 0.05 by the least significant difference (LSD) test. The error bars represented one standard deviation (n = 2).

change significantly under pH 8.27, but it began to decline with the increase of pH (> 8.27). It furthermore illustrated the electrostatic attraction was crucial for the adsorption of CR onto the LDH. At last, inside the LDH, the SO3− of the CR molecules displace the interlayer CO32– through ion exchange (Li et al., 2016; Shan et al., 2015). It is also approved by the results of the XRD, and the XRD pattern of the LDH before and after the CR adsorption are shown in Fig. 14. As can be seen from the figure, little changes took place in the layer spacing of LDH. According to the Bragg's Law (2dsinθ = nλ), the layer spacing of LDH (7.55 Å) and LDH-CR (7.76 Å) were calculated, suggesting that the ion exchange between CO32– and SO3−. Overall, the principal benefits of ultrasound-assisted adsorption were the reduction in adsorption time and promotion of adsorption rate for CR onto the MgeAleCO3 LDH. The schematic illustration of the adsorption mechanism is displayed in

LDH in aqueous solution was broken by ultrasonic cavitation, resulting in the increase of specific surface area of LDH and the enhancement of reaction probability between LDH and CR. Then, on the surface of the LDH, the CR molecules were attracted to the metal cations (viz. Mg2+ and Al3+) on the laminates by electrostatic force. As can be seen in Fig. 13, the isoelectric point (pHzpc) of the LDH was 8.27, which means the LDH had a positive surface charge under pH 8.27. In other words, the surface of LDH was positive in CR solution. The results of the adsorption of CR onto the LDH under different initial pH condition were reported in the Table 4. It would appear that the adsorption didn't 107

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Acknowledgements This study was supported by the Key R&D Projects of Shanxi Province (Social Development Field, 201803D31049), the Natural Science Foundation of Shanxi Province (201801D221341), Scientific and Technological Innovation Programs of Higher Education Institutions in Shanxi (STIP, 2016147) and the National Natural Science Foundation of China (21706179). References Al Jaafari, Abdullah I., 2010. Controlling the morphology of nano-hybrid materials. Am. J. Appl. Sci. 7 (2), 171–177. Bulut, E., Ozacar, M., Sengil, I.A., 2008. Equilibrium and kinetic data and process design for adsorption of Congo Red onto bentonite. J. Hazard. Mater. 154 (1–3), 613–622. Chan, Y.-N., Juang, T.-Y., Liao, Y.-L., Dai, S.A., Lin, J.-J., 2008. Preparation of clay/epoxy nanocomposites by layered-double-hydroxide initiated self-polymerization. Polymer 49 (22), 4796–4801. Chen, H., Zhao, J., Wu, J., Dai, G., 2011. Isotherm, thermodynamic, kinetics and adsorption mechanism studies of methyl orange by surfactant modified silkworm exuviae. J. Hazard. Mater. 192 (1), 246–254. Chen, R., Wang, W., Zhao, X., Zhang, Y., Wu, S., Li, F., 2014. Rapid hydrothermal synthesis of magnetic CoxNi1−xFe2O4 nanoparticles and their application on removal of Congo red. Chem. Eng. J. 242 (8), 226–233. Dai, L., Zhu, W., He, L., Tan, F., Zhu, N., Zhou, Q., He, M., Hu, G., 2018. Calcium-rich biochar from crab shell: an unexpected super adsorbent for dye removal. Bioresour. Technol. 267, 510–516. Deák, A., Csapó, E., Juhász, Dékány I., Janovák, L., 2018. Anti-ulcerant kynurenic acid molecules intercalated Mg/Al-layered double hydroxide and its release study. Appl. Clay Sci. 156, 28–35. El Hassani, K., Beakou, B.H., Kalnina, D., Oukani, E., Anouar, A., 2017. Effect of morphological properties of layered double hydroxides on adsorption of azo dye Methyl Orange: a comparative study. Appl. Clay Sci. 140, 124–131. Extremera, R., Pavlovic, I., Pérez, M.R., Barriga, C., 2012. Removal of acid orange 10 by calcined Mg/Al layered double hydroxides from water and recovery of the adsorbed dye. Chem. Eng. J. 213 (12), 392–400. Gandara-Loe, J., Jacobo-Azuara, A., Silvestre-Albero, J., Sepúlveda-Escribano, A., RamosFernández, E.V., 2017. Layered double hydroxides as base catalysts for the synthesis of dimethyl carbonate. Catal. Today 296, 254–261. Gao, Z., Sasaki, K., Qiu, X., 2018. Structural memory effect of Mg-Al and Zn-Al layered double hydroxides in the presence of different natural humic acids: process and mechanism. Langmuir 34 (19), 5386–5395. Hamdaoui, O., Naffrechoux, E., Tifouti, L., Pétrier, C., 2002. Effects of ultrasound on adsorption-desorption of p-chlorophenol on granular activated carbon. Ultrason. Sonochem. 10 (2), 109–114. Hao, L., Wang, R., Liu, J., Liu, R., 2012. Ultrasound-assisted adsorption of anionic nanoscale pigment on cationised cotton fabrics. Carbohydr. Polym. 90 (4), 1420–1427. Hashemian, S., Foroghimoqhadam, A., 2014. Effect of copper doping on CoTiO3 ilmenite type nanoparticles for removal of Congo red from aqueous solution. Chem. Eng. J. 235 (1), 299–306. Huang, G., Sun, Y., Zhao, C., Zhao, Y., Song, Z., Chen, J., Ma, S., Du, J., Yin, Z., 2017. Water-n-BuOH solvothermal synthesis of ZnAl-LDHs with different morphologies and its calcined product in efficient dyes removal. J. Colloid Interface Sci. 494, 215–222. Khataee, A., Arefi-Oskoui, S., Samaei, L., 2018. ZnFe-Cl nanolayered double hydroxide as a novel catalyst for sonocatalytic degradation of an organic dye. Ultrason. Sonochem. 40 (Pt A), 703–713. Lafi, R., Charradi, K., Djebbi, M.A., Ben Haj Amara, A., Hafiane, A., 2016. Adsorption study of Congo red dye from aqueous solution to Mg-Al-layered double hydroxide. Adv. Powder Technol. 27 (1), 232–237. Lee, S.H., Tanaka, M., Takahashi, Y., Kim, K.W., 2018. Enhanced adsorption of arsenate and antimonate by calcined Mg/Al layered double hydroxide: investigation of comparative adsorption mechanism by surface characterization. Chemosphere 211, 903–911. Li, B., Zhang, Y., Zhou, X., Liu, Z., Liu, Q., Li, X., 2016. Different dye removal mechanisms between monodispersed and uniform hexagonal thin plate-like MgAl-CO32−-LDH and its calcined product in efficient removal of Congo red from water. J. Alloys Compd. 673, 265–271. Li, Q., Zhan, Z., Jin, S., Tan, B., 2017. Wettable magnetic hypercrosslinked microporous nanoparticle as an efficient adsorbent for water treatment. Chem. Eng. J. 326, 109–116. Lipatova, I.M., Makarova, L.I., Yusova, A.A., 2018. Adsorption removal of anionic dyes from aqueous solutions by chitosan nanoparticles deposited on the fibrous carrier. Chemosphere 212, 1155–1162. Mahmoodian, H., Moradi, O., Shariatzadeha, B., Salehf, T.A., Tyagi, I., Maity, A., Asif, M., Gupta, V.K., 2015. Enhanced removal of methyl orange from aqueous solutions by poly HEMA–chitosan-MWCNT nano-composite. J. Mol. Liq. 202, 189–198. Midathana, V.R., Moholkar, V.S., 2009. Mechanistic studies in ultrasound-assisted adsorption for removal of aromatic pollutants. Ind. Eng. Chem. Res. 48 (15), 7368–7377. Milenkovic, D.D., Dasic, P.V., Veljkovic, V.B., 2009. Ultrasound-assisted adsorption of copper(II) ions on hazelnut shell activated carbon. Ultrason. Sonochem. 16 (4), 557–563.

Fig. 18. The SEM image of the CR-adsorbed LDH after three-times calcination recycle.

Fig. 15.

3.7. Adsorbent recyclability As shown in Fig. 16, the CR adsorption rate by the LDH dropped to about 60% after three cycles. Notably, the adsorption rate reached up to 99% after the first use and decreased with recycle times. The increase in the adsorption values can be explained by the calcination which is beneficial to improve the special surface area and total pore volume of the LDH. And the CR is an organic pollutant, containing some groups, which is thought to affect the crystal growth and arrangement of the regenerated LDH that contribute to the increase of specific surface area (Gao et al., 2018). In addition, there were two main reasons why the adsorption rate decreased with increasing number of cycles. On the one hand, the adsorption sites of the LDH were still occupied by some CR residue after calcination. Therefore, the adsorption sites were not completely released. On the other hand, the layered structure of the LDH collapsed and was unable to return to its original lamellar shape after multiple calcinations. The phase composition of the samples that different recycle of the LDH after the CR adsorption was observed by XRD, as shown in Fig. 17. In the curve of Recycle 1, the characteristic peaks of LDH begins to disappear, and replaced with the characteristic peaks of MgeAl oxide. As the increase of cycle number, the intensity of the MgeAl oxide's characteristic peaks is enhanced greatly, revealing the LDH is undergoing a phase transition into a spinel structure. The structure of the LDH became molten and formed a grain boundary so that it could not provide sufficient adsorption sites for CR. The SEM image of the CR adsorbed-LDH after calcination (LDH-CR-C) is showed in Fig. 18.

4. Conclusions The MgeAleCO3 LDH was effective in the removal of CR from aqueous solution as assisted by ultrasound. The adsorption equilibrium was reached within 120 min, which was 3 times shorter than the stirring and shaking adsorption processes. The maximum adsorption capacity of the LDH was 934.43 mg/g, and the adsorption rate for CR was above 90%. The ultrasound-assisted adsorption mechanism was mainly attributed to electrostatic force and ion exchange. The adsorption followed the pseudo-second order kinetic model and Langmuir isotherm model well. The LDH was easily regenerated by calcination, with an adsorption rate for CR above 60% after three cycles. 108

Applied Clay Science 174 (2019) 100–109

W. Zhang, et al.

Sharifpour, E., Khafri, H.Z., Ghaedi, M., Asfaram, A., Jannesar, R., 2018. Isotherms and kinetic study of ultrasound-assisted adsorption of malachite green and Pb(2+) ions from aqueous samples by copper sulfide nanorods loaded on activated carbon: Experimental design optimization. Ultrason. Sonochem. 40 (Pt A), 373–382. Sivaraj, R., Namasivayam, C., Kadirvelu, K., 2001. Orange peel as an adsorbent in the removal of acid violet 17 (acid dye) from aqueous solutions. Waste Manag. 21 (1), 105–110. Tian, C., Feng, C., Wei, M., Wu, Y., 2018. Enhanced adsorption of anionic toxic contaminant Congo Red by activated carbon with electropositive amine modification. Chemosphere 208, 476–483. Tong, D.S., Liu, M., Li, L., Lin, C.X., Yu, W.H., Xu, Z.P., Zhou, C.H., 2012. Transformation of alunite residuals into layered double hydroxides and oxides for adsorption of acid red G dye. Appl. Clay Sci. 70 (6), 1–7. Vimonses, V., Lei, S., Jin, B., Chow, C.W.K., Saint, C., 2009. Kinetic study and equilibrium isotherm analysis of Congo Red adsorption by clay materials. Chem. Eng. J. 148 (2–3), 354–364. Wang, X., Bai, Z., Zhao, D., Chai, Y., Guo, M., Zhang, J., 2013. New synthetic route to MgAl-CO3 layered double hydroxide using magnesite. Mater. Res. Bull. 48 (3), 1228–1232. Wu, Y., Han, Y., Tao, Y., Fan, S., Chu, D.T., Ye, X., Ye, M., Xie, G., 2018. Ultrasound assisted adsorption and desorption of blueberry anthocyanins using macroporous resins. Ultrason. Sonochem. 48, 311–320. Xu, J., Xu, D., Zhu, B., Cheng, B., Jiang, C., 2018. Adsorptive removal of an anionic dye Congo red by flower-like hierarchical magnesium oxide (MgO)-graphene oxide composite microspheres. Appl. Surf. Sci. 435, 1136–1142. Yu, X.-Y., Luo, T., Jia, Y., Xu, R.-X., Gao, C., Zhang, Y.-X., Liu, J.-H., Huang, X.-J., 2012. Three-dimensional hierarchical flower-like Mg-Al-layered double hydroxides: highly efficient adsorbents for As(v) and Cr(vi) removal. Nanoscale 4 (11), 3466–3474. Yu, Z., Xu, C., Yuan, K., Gan, X., Zhou, H., Wang, X., Zhu, L., Zhang, G., Xu, D., 2018. Template-free synthesis of MgO mesoporous nanofibers with superior adsorption for fluoride and Congo red. Ceram. Int. 44 (8), 9454–9462. Zheng, Y., Zhu, B., Chen, H., You, W., Jiang, C., Yu, J., 2017. Hierarchical flower-like nickel(II) oxide microspheres with high adsorption capacity of Congo red in water. J. Colloid Interface Sci. 504, 688–696.

Milenkovic, D.D., Bojic, A., Veljkovic, V.B., 2013. Ultrasound-assisted adsorption of 4dodecylbenzene sulfonate from aqueous solutions by corn cob activated carbon. Ultrason. Sonochem. 20 (3), 955–962. Munagapati, V.S., Kim, D.-S., 2017. Equilibrium isotherms, kinetics, and thermodynamics studies for Congo red adsorption using calcium alginate beads impregnated with nano-goethite. Ecotoxicol. Environ. Saf. 141, 226–234. Nethaji, S., Sivasamy, A., Mandal, A.B., 2013. Preparation and characterization of corn cob activated carbon coated with nano-sized magnetite particles for the removal of Cr (VI). Bioresour. Technol. 134 (2), 94–100. Ofomaja, A.E., 2010. Intraparticle diffusion process for lead(II) biosorption onto mansonia wood sawdust. Bioresour. Technol. 101 (15), 5868–5876. Pelekani, C., Snoeyink, V.L., 2001. A kinetic and equilibrium study of competitive adsorption between atrazine and Congo red dye on activated carbon: the importance of pore size distribution. Carbon 39 (1), 25–37. Pooralhossini, J., Ghaedi, M., Zanjanchi, M.A., Asfaram, A., 2017. Ultrasonically assisted removal of Congo Red, Phloxine B and Fast green FCF in ternary mixture using novel nanocomposite following their simultaneous analysis by derivative spectrophotometry. Ultrason. Sonochem. 37, 452–463. Saber-Samandari, S., Saber-Samandari, S., Joneidi-Yekta, H., Mohseni, M., 2017. Adsorption of anionic and cationic dyes from aqueous solution using gelatin-based magnetic nanocomposite beads comprising carboxylic acid functionalized carbon nanotube. Chem. Eng. J. 308, 1133–1144. Santos, R.M.M.D., Gonçalves, R.G.L., Constantino, V.R.L., Santilli, C.V., Borges, P.D., Tronto, J., Pinto, F.G., 2017. Adsorption of Acid Yellow 42 dye on calcined layered double hydroxide: effect of time, concentration, pH and temperature. Appl. Clay Sci. 140, 132–139. Şayan, E., 2006. Optimization and modeling of decolorization and COD reduction of reactive dye solutions by ultrasound-assisted adsorption. Chem. Eng. J. 119 (2–3), 175–181. Shan, R.-R., Yan, L.-G., Yang, K., Yu, S.-J., Hao, Y.-F., Yu, H.-Q., Du, B., 2014. Magnetic Fe3O4/MgAl-LDH composite for effective removal of three red dyes from aqueous solution. Chem. Eng. J. 252, 38–46. Shan, R.-R., Yan, L.-G., Yang, Y.-M., Yang, K., Yu, S.-J., Yu, H.-Q., Zhu, B.-C., Du, B., 2015. Highly efficient removal of three red dyes by adsorption onto Mg-Al-layered double hydroxide. J. Ind. Eng. Chem. 21 (1), 561–568.

109