calcined layered double hydroxides

Journal of Molecular Liquids 230 (2017) 344–352

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Adsorption of eriochrome black T from aqueous phase on MgAl-, CoAl- and NiFe- calcined layered double hydroxides: Kinetic, equilibrium and thermodynamic studies Mukarram Zubair a, Nabeel Jarrah a,b,⁎, Mohammad Saood Manzar a, Mamdouh Al-Harthi c,d, Muhammad Daud c,e, Nuhu Dalhat Mu’azu a, Shamsuddeen A. Haladu f a

Department of Environmental Engineering, University of Dammam, 31451 Dammam, Saudi Arabia Department of Chemical Engineering, Mutah University, 61710 Karak, Jordan c Department of Chemical Engineering, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia d Center of Research Excellences in Nanotechnology, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia e Department of Chemical Engineering, University of Engineering and Technology, 25120 Peshawar, Pakistan f Department of Basic Sciences, University of Dammam, 31451 Dammam, Saudi Arabia b

a r t i c l e

i n f o

Article history: Received 4 October 2016 Received in revised form 25 December 2016 Accepted 9 January 2017 Available online 11 January 2017 Keywords: Layered double hydroxides Anionic dye Adsorption Isotherm and kinetic model

a b s t r a c t This work reports the synthesis of MgAl-, CoAl- and NiFe-layered double hydroxides (LDH) with mole ratio of 3:1 via co-precipitation method and calcination at 350 °C under nitrogen for 2 h. The resultant calcined LDHs (CLDHs) were used for the adsorption of Eriochrome black T (EBT) dye from aqueous phase. The influence of pH, contact time, initial EBT concentration, and temperature on the EBT adsorption capacity, kinetics and mechanisms was investigated at bench scale. The optimum pH for the removal of EBT on all the three CLDHs was 2 and the equilibrium adsorption time was 60 min, 30 min and 60 min for CoAl-, MgAl- and NiFe- CLDHs, respectively. Isotherm study showed that adsorption of EBT on all the three CLDHs was best described by Langmuir isotherm model indicating monolayer adsorption behavior. The maximum adsorption capacity was found to be 419.87 mg/g, 540.91 mg/g and 132.49 mg/g for CoAl-, MgAl and NiFe- CLDHs, respectively. The results of kinetic study indicated rapid adsorption behavior for all the three compounds which well fitted pseudo-second order model. The obtained thermodynamic data suggests that the adsorption on CoAl- and MgAl- CLDHs was exothermic in nature, while that of NiFe-CLDH was endothermic in nature. The structure and morphology results obtained after EBT dye adsorption suggest that the adsorption mechanism is associated with electrostatic attraction and hydrogen/or chemical bonding between EBT dye and CLDHs surface functional groups. The high removal efficiency of EBT obtained from this study demonstrated the potential of calcined LDHs as effective adsorbents for the reclamation of wastewater contaminated with toxic pollutants. © 2017 Published by Elsevier B.V.

1. Introduction Anionic and cationic dyes are well known toxic and hazardous pollutants. They are among the groups of chemical substances causing water contamination which can lead to various undesirable consequences in the environment. These dyes are mainly discharged from leather, textile, plastic, paint, pulp, printing & food industries [1]. It has been reported that most industrial dyes are non-biodegradable [2], chemically stable [3], carcinogenic and mutagenic [4], hence, harmful to human health. Moreover, presence of industrial dyes in water bodies also decreases light penetration which affects the photochemical ⁎ Corresponding author: Department of Environmental Engineering, University of Dammam, 31451 Dammam, Saudi Arabia. E-mail address: [email protected] (N. Jarrah).

http://dx.doi.org/10.1016/j.molliq.2017.01.031 0167-7322/© 2017 Published by Elsevier B.V.

activities of marine systems [5,6]. Therefore, effective treatment of wastewater effluents from these industries to remove the dyes in order to meet regulatory requirement for reuse or discharge is highly desirable. Various treatment techniques, like biological [7], coagulation and flocculation [8], filtration [9], membrane separation [10], and ion-exchange [11] have been employed to remove dyes from water bodies. However, these conventional methods have some limitations due to high operational cost, low adsorption efficiency, complexity and less feasible on large scale adsorption [12]. Due to its process simplicity, low cost as well as high removal efficiency, adsorption technique for removal of hazardous pollutants from wastewater has attracted increased interest over the last decades compared to other techniques [13]. Nonetheless, selection of suitable adsorbent and operating conditions has been a challenging step to achieve maximum efficiency in adsorption

M. Zubair et al. / Journal of Molecular Liquids 230 (2017) 344–352

process. Activated carbon [14], zeolite [15], bentonite [16] and ash [17] have been reported for the removal of dyes from aqueous solutions. Yet, due to their low uptake tendency, scientist, engineers and researchers are still looking for ways to develop novel adsorbents that exhibit higher sorption capacity and capture broad range of pollutants with good regeneration capability. Layered double hydroxides (LDH), recognized as anionic clays or hydrotalcite clays, have attracted tremendous attention in various fields such as catalysis [18], water treatment [19], biomedical [20] and polymer nanocomposites [19]. They are represented in the general formula: [M21 − x·M3x·(OH)2]. [An−x/n·mH2O], where M2 and M3 are divalent and trivalent metals ions within the brucite-like layers and An− represents an interlayer anion [21]. The distinctive structure of LDH and their wide flexibility in composition with high anion exchange ability enable them to be effective and versatile adsorbents for the removal pollutants from wastewater. Previous studies demonstrated LDHs and their derivatives as promising low cost adsorbents for the remediation of dyes [22,23]. R Shan et al. [24] reported an efficient adsorption of red dyes on Mg-Al-LDH. Similarly, Ai et al. [25] studied the adsorption performance of an organic dye on Mg-Al-LDH and revealed it to be a highly efficient adsorbent. NiFe-LDH was studied by Saiah et al. [26] and Lu et al. [27] for adsorption of evan blue and methyl orange respectively. They concluded that NiFe-LDH is an effective adsorbent due to its excellent removal efficiency. Thus LDH is a potential future adsorbent for wastewater treatment. To the best of the reviewed literature, the adsorption behavior of EBT on CLDHs has so far not been investigated. Therefore, in the present study, three different layered double hydroxides (CoAl, MgAl and NiFe) were synthesized by co-precipitation method and used for the removal of EBT from aqueous phase. The structures and morphology of the prepared CLDHs were characterized by FTIR, XRD, and SEM. Batch studies were carried out to investigate the influence of parameters such as initial pH, initial dye concentration, contact time and temperature on the removal efficiency of the LHDs. The adsorption mechanism and kinetics were assessed using isotherm and kinetics models. 2. Experimental section

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90 °C for 24 h. The resultant suspension was centrifuged and washed with deionized water and then followed by ethanol washing to remove any impurity. The dense slurry was then dried at 60 °C in vacuum oven overnight. The LDHs were then calcined in a tubular oven for 3 h at 350 °C under nitrogen environment. 2.3. Characterization of CLDHs The synthesized layered double hydroxides were characterized by Fourier transform-IR (FTIR, Nicolet 6700, resolution 4 cm−1), X-ray diffraction (XRD, D8 advance X-ray instrument, wavelength = 0.1542 nm, and 2θ = 2° to 70°), scanning electron microscopy (SEM, SM6460LV(Jeol)) and Brunauer Emmett Teller (BET, Micromeritics, Tristar II series). 2.4. Adsorption studies Batch adsorption experiments were performed to investigate the effect of pH, contact time, initial dye concentration via equilibrium and kinetics studies. About 10 mg of each CLDH was agitated for 10– 120 min at 275 rpm in 30 mL of EBT solution (20–160) mg/L using 50 mL plastic tubes at temperature (25–45 °C). The appropriate pH of the mixture was adjusted using 0.1 mol/L HCl and NaOH solutions. After agitation, the mixture was centrifuged at 3000 rpm for 5 min to separate the CLDH from the residual dye. The final concentration of the dye in the supernatant solution was quantified by Hach Lange spectrophotometer set at a wavelength of 530 nm. The amount of the dye adsorbed on the CLDH qe (mg/g) and percentage removal efficiency were estimated according to Eqs. (1) and (2), respectively.

Adsorption capacity ¼ qe ¼

C o −C e V W

ð1Þ

Adsorption capacity ¼ qe ¼

C o −C e 100 Co

ð2Þ

2.1. Materials Aluminium(III) nitrate nonahydrate [Al(NO3)3·9H2O], Iron(III) nitrate nonahydrate [Fe(NO3)3·9H2O], magnesium(II) nitrate hexahydrate [Mg(NO3)2·6H2O], cobalt(II) nitrate hexahydrate [Co(NO3)2·6H2O] and nickel(II) nitrate hexahydrate [Ni(NO3)2·6H2O] were purchased from Sigma Aldrich CO. (USA). These chemicals and all other solvents and materials were of high purity as such, they were used without any purification. The characteristics of the EBT dye are listed in Table 1. The dye stock solution of 1000 mg/L concentration was prepared and diluted to the required concentrations using de-ionized water.

where, Co and Ce are the initial and equilibrium concentration (mg/L) of EBT in solution, respectively, qe (mg/g) is the equilibrium adsorption capacity, W (g) is the weight of the adsorbent, and V (L) is the volume of the solution.

2.2. Preparation of Mg\\Al, Co\\Al and Ni\\Fe-CLDH The LDHs (MgAl, CoAl and NiFe) were prepared via co-precipitation synthesis method as described by other authors [28]. Briefly, a 3:1 mol ratio of the precursor salts (M2+: M3+) were dissolved in 80 mL of deionized (DI) water in a reactor equipped with a magnetic stirrer [29]. The solution was stirred vigorously for 2 h at 45 °C. Subsequently, the pH value of the solution was adjusted to 10 ± 0.5 by the addition of 1 M NaOH solution. After stabilizing at the desired pH, the temperature was increased to 90 °C and the reactor was subjected to refluxing at Table 1 Characteristics of Eriochrome black T. Chemical formula

Molecular weight

Color index/type

ƛmax

C20H12N3NaO7S

461.38

14,645/Azo dye

530 nm

Fig. 1. FTIR spectrum of the MgAl-,CoAl- and NiFe- CLDH.

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3. Results and discussion 3.1. Characterization of CLDHs Fig. 1 shows the FTIR spectrum of MgAl, CoAl and NiFe CLDH. The strong characteristic peak at 3446 cm−1 in all the three CLDH was attributed to the stretching vibration mode of the hydroxyl group of the interlayer water molecules and/or hydrogen bonding in the layer of the CLDH surface. The peak at 1636 cm− 1 in MgAl and CoAl and 1646 cm−1 in the NiFe spectrum corresponds to the OH bending vibration of water molecules [30]. The peak in the range of 1390 to 1377 cm−1 on the CLDH spectrum was associated to the vibration mode of carbonate ions [31]. In addition the peaks below 1000 cm−1 were attributed to the M\\O, M\\O\\M and O\\M\\O (M_Mg, Co,

Fig. 2. XRD diffraction patterns of the LDH before calcination.

Ni, Fe and Al) vibrations of the CLDH. The FTIR spectrum clearly demonstrates that all the three CLDH exhibited abundant oxygen functionalities. Moreover, the intensity of the characteristic peaks of oxygen containing groups (\\COO and \\OH) is higher in MgAl-CLDH compared to CoAl and NiFe-CLDH. These oxygen functionalities on the surface of CLDH act as active sites and could significantly enhance the adsorption tendency of CLDH [32]. From the BET study, the surface area, pore volume and pore diameter were measured to be (69.5, 41.4, 17.1) m2/g, (0.013, 0.122, 0.037) cm3/g and (0.8, 11.8, 8.6) nm for MgAl, CoAl and NiFe CLDH, respectively. Fig. 2 shows the XRD patterns of MgAl, CoAl and NiFe- CLDHs. The Xray diffraction patterns of MgAl-CLDH exhibit intense reflection peaks at 11.60°, 23.14°, 29.69° and 34.70° with inter layer spacing of 7.61°A, 3.86°A, 3.00°A and 2.77°A respectively. This indicates that MgAl-CLDH was well crystalized with 4R layered structure which is similar to previous reported studies [24]. The diffraction pattern of CoAl-CLDH comprised of peaks at 11.41°, 23.72°, 34.51° and 60.09° demonstrating well prepared crystalline structure [33]. Similarly, NiFe-CLDH diffraction pattern is in good agreement to the structure reported elsewhere by Moeinpour, F. et al. [34]. The pattern showed broad peaks at 12.57°, 29.69°, 39.31° associated to either Ni(OH)2 or Fe(OH)2. However, the lower diffraction intensity suggests that the as-synthesized NiFe- exhibits poor crystalline structure [35]. Fig. 3(a‐c) displays the microstructure of MgAl, CoAl and NiFe-CLDH investigated using scanning electron microscopy, respectively. Obviously, Fig. 3a shows that the CoAl material exhibited small and circular particles resembling fine cotton agglomerates. In contrast, the surface of MgAl displayed in Fig. 3b reveals a non-uniform, irregular cubical

Fig. 3. Scanning electron micrographs of MgAl-,CoAl- and NiFe- CLDH.


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3c, presented a coarse, rutted flowered like particles arranged perpendicular on the surface with sharp and serrated edges.

3.2. Adsorption parameters

Fig. 4. Effect of initial pH. Co = 20 mg/L; T = 25 °C; 10 mg of each CLDH.

appearance similar to the structure reported by Morimoto and Zhang et al. [36]. This irregularity of MgAl-CLDH structure was ascribed to the heat treatment that followed the calcination process. Interestingly, the morphologies of both CoAl and MgAl are similar to those presented earlier by F.L. Theiss. et al. [37]. The structure of NiFe-CLDH as shown in Fig.

3.2.1. Effect of initial pH The effect of initial pH (2–6) on the percentage removal of EBT was studied using 10 mg of each CLDH and initial EBT concentration of 20 mg/L at 275 rpm and 25 °C. The results presented in Fig. 4 show that the percentage removal of EBT increases with decreasing initial pH of the solution from 6 to 2. The maximum percentage removal of EBT obtained was 93.13%, 96.54% and 91.50% attained at pH 2 for CoAl, MgAl and NiFe CLDH, respectively. The increase in the percentage removal of EBT when decreasing the initial pH was associated to the protonation of the oxygen functionalities (\\COO and \\OH) of the CLDH [32] as confirmed from FTIR analysis. Lowering the pH of the solution, transforms the entire surface of the CLDH into positively charged and this leads to a strong interface between the positively charged CLDH surface and the anionic EBT dye via electrostatic attraction [38]. In contrast, Fig. 4 depicts that high pH values (pH N 5) were unfavorable for EBT removal by CoAl, MgAl and NiF CLDH. This is as a result of the existence of extra OH– ions that restrict the EBT anions from adsorption on the active sites (\\COO and OH groups), thus showing remarkable decrease in the percentage removal of EBT to 6.07%, 31.31% and 12.08% for CoAl, MgAl and NiFe at pH 6.0 respectively.

Fig. 5. Effect of contact time (A) CoAl (B) MgAl (C) NiFe; Co = 20–100 mg/L; T = 25 °C; 10 mg; pH = 2.

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3.2.2. Effect of contact time Fig. 5 (A–C) displays the percentage removal of EBT by CoAl, MgAl and NiFe at different contact times (0 − 120) min and different initial concentrations of EBT. The figure reveals that at all initial dye concentrations, with increase in contact time the percentage removal of EBT increased significantly in the first 30 min for CoAl and NiFe and 10 min for MgAl CLDH. Further increase in contact time shows slow increase in the percentage removal and reached equilibrium within 90 min, 30 min and 90 min for the adsorbents, respectively. The fast adsorption rate within the first 10–30 min is associated to the memory effect of the calcined CLDH [39]. Moreover, MgAlCLDH exhibited up to threefold faster adsorption rate compared to CoAl and NiFe CLDH. This may be due to its high surface area and existence of excessive hydroxyl and carbonate groups on its surface which interact with EBT anions quicker than CoAl and Nife CLDH. 3.2.3. Effect of initial concentration of dye The effect of initial dye concentration (20–160) mg/L on the adsorption capacity of CoAl, MgAl and NiFe were investigated at temperature 25 °C and 10 mg of each CLDH dosage. Fig. 6 shows an increasing EBT adsorption capacity of the CLDH with increase in initial dye concentration. For CoAl and MgAl the adsorption capacity increases from 53.85 mg/g and 54.78 mg/g to 343.44 mg/g and 407.55 mg/g respectively when the dye concentration was increased from 20 to 160 mg/L. This significant improvement in adsorption capacity of CLDH with increase in EBT concentration is attributed to the presence of more EBT anions in the solution which compete with the available binding sites on the surface of CLDH [38]. However, in case of NiFe, the adsorption capacity increased from 51.96 mg/g to 124.65 mg/g when the dye concentration was increased from 20 mg/L to 60 mg/L. Further increase in the concentration to 160 mg/L demonstrated no enhancement in the adsorption capacity of NiFe. This later observation suggests that saturation of the binding sites on the NiFe surface has occurred faster than in the case of CoAl and MgAl [40]. 3.3. Kinetic studies To understand in details the exact dynamics mechanism of the adsorption process, two general kinetics models, namely, pseudo first order and pseudo second order [41] were applied on the experimental

Table 2 Parameters of pseudo first and pseudo second order kinetic models. Pseudo first order

Co CoAl

MgAl

NiFe

20 40 100 20 40 100 20 40 100

Pseudo second order

qe (exp)

qe

k1

R2

qe

k2 × 10−2

R2

54.3 109.53 268.77 56.13 112.83 283.78 53.1 104.37 164.61

150.92 430.56 836.31 4.79 6.72 33.28 141.88 259.56 561.15

0.17 0.19 0.19 1.3 0.09 0.16 0.16 0.16 0.20

0.941 0.927 0.922 0.566 0.662 0.758 0.947 0.856 0.962

65.78 149.25 312.58 56.49 112.35 285.71 67.11 123.45 208.33

0.05 0.01 0.01 2.02 1.72 0.26 0.04 0.04 0.07

0.997 0.991 0.997 0.999 0.999 0.999 0.994 0.998 0.991

data. The linear forms of these two models are given in Eqs. (3) and (4), respectively. t

Pseudo first order equation : ln ðqe −qt Þ ¼ ln qe −k1 Pseudo second order equation :

t t t ¼ þ qt k2 q2e qe

ð3Þ ð4Þ

Where, k1 and k2 are the pseudo-1st order and pseudo-2nd order rate constant and qt and qe are the adsorption tendencies of EBT onto the CLDH at time t and equilibrium, respectively. The values of k1 and k2 and qe were calculated from the slope and intercept of the linear plots of Eqs. (3) and (4) and are listed in Table 2. Table 2 clearly shows that for all the three CLDHs, the correlation coefficient (R2) of the pseudo-second order model was higher than those of the pseudo-first order. In fact, the R2 values of pseudo-second order model are found to be N 0.99. This confirms that the pseudo-second order best fits the experimental data compared to the pseudo-first order model. The calculated qe value using pseudo second order model was consistent with experimental qe value [42]. Furthermore, as shown in Fig. 5(A–C), the adsorption rate within the first 10– 30 min is rapid and associated to external surface adsorption, suggesting an important step for the adsorption of EBT on all three CLDHs. Similar results have been reported for the adsorption of methyl orange by magnetic NiFe LDH [27]. 3.4. Isotherm studies Isotherm studies were performed to demonstrate the distribution of adsorbate (EBT) molecules between the water and adsorbent at equilibrium during the adsorption process. In this study, two most commonly used adsorption isotherm models: Langmuir and Freundlich were applied on the experimental data. Langmuir isotherm assumes the homogenous nature of adsorption with equal energy of the entire active adsorption sites [13]. Freundlich isotherm is applicable to heterogeneous surfaces [43]. The non-linear isotherm equations of Langmuir and Freundlich are represented in Eqs. (5) and (6), respectively. qmbC e 1 þ bC e

ð5Þ

Freundlich isotherm model : qe ¼ K f C 1=n e

ð6Þ

Langmuir isotherm model : qe ¼

Fig. 6. Effect of initial concentration; Co = 20–160 mg/L; T = 25 °C; 10 mg; pH = 2.

where, Ce (mg/L) is the equilibrium concentration of the adsorbent, qe (mg/g) is the amount of EBT adsorbed on the CLDHs at equilibrium. The other parameters, b and Kf and n are the constants of the Langmuir and Freundlich models, qm (mg/g) is the maximum adsorption capacity. These parameters can be calculated by fitting the experimental data using non- linear form of the isotherms Eqs. (5–6). The values obtained for all the parameters are summarized in Table 3.

M. Zubair et al. / Journal of Molecular Liquids 230 (2017) 344–352 Table 3 Parameters of Langmuir and Freundlich isotherm models for adsorption of EBT onto CoAl-, MgAl- and NiFe-CLDH. T (K)

Langmuir qmax

Freundlich KL

R2

KF

1/n

R2

0.15 0.15 0.13 0.15 0.05 0.02 0.43 0.49 0.44

0.979 0.941 0.961 0.990 0.984 0.997 0.959 0.940 0.945

66.16 66.16 57.60 78.47 35.60 15.14 55.82 60.07 62.60

0.49 0.49 0.47 0.58 0.64 0.69 0.19 0.20 0.21

0.714 0.606 0.698 0.818 0.875 0.979 0.811 0.796 0.725

(mg/g)

CoAl

MgAl

NiFe

298 308 318 298 308 318 298 308 318

419.87 397.49 383.11 540.91 524.69 437.55 132.49 146.27 165.17

Fig. 7(A–C) displays the combined plots of the equilibrium data, Langmuir and Freundlich isotherm for CoAl, MgAl and NiFe respectively at different temperatures. As depicted in Fig. 7(A–C), for all the three CLDHs at all the investigated temperatures, the Langmuir isotherm model is in better agreement with the equilibrium data compared to the Freundlich model. The values of the correlation coefficient (R2) of the Langmuir isotherm are N0.94 for all the CLDHs which are satisfactory and indicated that adsorption of EBT on the CLDHs fitted well Langmuir isotherm (Table 3). The maximum adsorption capacity calculated using Langmuir isotherm was found to be 419.87 mg/g, 540.91 mg/g and 132.49 mg/g for CoAl, MgAl and NiFe CLDH, respectively. Moreover, Table 3 showed that the correlation coefficients (R2) of Freundlich isotherm were obtained to be b0.9 particularly for CoAl

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and NiFe-CLDH. Similarly, the values of 1/n b 1 demonstrate the Langmuir isotherm while 1/n N 1 reveals the cooperative adsorption [44]. The values of 1/n in Table 3 is found b1, confirming that the adsorption of EBT on CLDH could be well described by Langmuir isotherm model. The results showed that the adsorption of EBT on all CLDHs is attributed to monolayer surface adsorption with electrostatic attraction and chemical/or hydrogen bonding between adsorbate and adsorbent [27]. 3.5. Thermodynamic studies Thermodynamic studies were performed at three temperatures (298 K, 308 K and 318 K). Thermodynamic parameters such standard free energy (ΔG)(kJ/mol), standard entropy (ΔS)(J/molK) and standard enthalpy (ΔH)(kJ/mol) were calculated using Eqs. (7–8) [45] ΔG ¼ −RT ln K d

ð7Þ

ΔS ΔH − R RT

ð8Þ

lnK d ¼

where, R is the universal gas constant, T (K) is the absolute solution temperature and Kd is the thermodynamic equilibrium constants that is estimated according to the method of Xin et al. [46] by plotting ln(qe/Ce) vs qe and extrapolate qe to zero. The values of ΔS and ΔH were calculated from the intercept and the slope of the linear plot of LnKd vs 1/T (Fig. 8) and are presented in Table 4.

Fig. 7. Adsorption isotherm of EBT on CoAl (A), MgAl (B) and NiFe (C) at 25 °C, 35 °C and 45 °C.

350


3.6. Comparison with conventional adsorbents Table 5 shows the maximum adsorption capacity exhibited by conventional adsorbents previously, reported in the literature. The comparison results demonstrated that the CLDHs produced in this work showed a comparatively higher removal capacity of EBT dye, indicating them to be effective and potential adsorbents for reclamation of water contaminated with anionic dyes. Moreover, compared to un-calcined MgAl-LDH [48], the maximum adsorption capacity reported (Table 5) was 128 mg/g, which is relatively much lower than the calcined LDH prepared in this study. This is attributed to the efficient method of synthesis that facilitates abundant formation of oxygen functional groups on the LDHs surface and increase in the surface area after calcination of LDHs [49,50]. 3.7. Adsorption mechanism

Fig. 8. Plot of LnKd vs 1/T for the valuation of thermodynamic parameters.

Table 4 Thermodynamic parameters for adsorption of EBT onto MgAl-, CoAl- and NiFe-CLDH.

CoAl

MgAl

NiFe

T (K)

Kd

ΔG (kJ/mol)

298 308 318 298 308 318 298 308 318

51.28 44.89 38.60 74.02 23.51 8.68 30.92 40.51 48.46

−9.75 −9.74 −9.65 −10.66 −8.08 −5.71 −8.50 −9.47 −10.26

ΔH (kJ/mol)

ΔS (J/mol K)

−11.17

−4.73

−84.47

−247.77

17.72

88.11

As shown in Table 4, the negative values of ΔG at all the temperatures indicated the EBT adsorption on CoAl, MgAl and NiFe as spontaneous and thermodynamically feasible. In case of CoAl and MgAl, the negative values of Δ H suggested that the adsorption of EBT on CoAl and MgAl is exothermic in nature and consequently, decreased the adsorption capability with increase in temperature. This is in agreement with increase in ΔG with increasing temperature. The negative values of ΔS indicated the greater order of reaction during the adsorption of EBT on to CoAl and MgAl [47]. However, for NiFe, the positive values of Δ H showed that the adsorption of EBT is endothermic in nature and results in increase in adsorption capacity with increasing temperature (Table 4). The positive values of ΔS demonstrated that randomness increased at the solid-solution interface during the adsorption of EBT on NiFe. This may be attributed to the enlargement of pore size or activation of NiFe surface at higher temperature [46].

The mechanism of adsorption was further evaluated by FTIR and SEM analysis of the CLDH after EBT adsorption. Fig. 9 shows the FTIR spectrum of MgAl, CoAl and NiFe CLDH after EBT adsorption. The FTIR spectrum clearly indicates that after adsorption of EBT, the characteristic peak in the range of 1370–1390 cm−1 which corresponds to the \\COO groups changed to very low intensity compared to the unadsorbed CLDH spectrum (Fig. 1). Similarly, the intensity of the hydroxyl groups at 3446 cm−1 is also lowered or even disappeared in MgAl and CoAl spectrum, respectively. The decrease in the intensity of the oxygen functionalities (\\COO and\\OH) after adsorption of EBT dye on all the three CLDH spectrum clearly suggests that the adsorption of EBT dye on CLDH surface was associated to the strong interaction between EBT and positively charged surface of the CLDH via electrostatic attraction and covalent/hydrogen bonding. Similar behavior was also reported after adsorption of dyes on some CLDH surfaces [47]. The micrographs of MgAl, CoAl and NiFe after adsorption of EBT are shown in Fig. 10. It can be observed in the micrograph of CoAl-CLDH that after adsorption, the cotton like surface of CoAl- seemed to be covered with EBT clusters indicating effective adsorption on its active sites. For MgAl-CLDH, after adsorption of the EBT dye, the irregular, uneven surface changed to smooth patterns with very few numbers of irregular cubical appearances. This indicates that the EBT molecules are entirely adsorbed within the interlayer surface of MgAl-CLDH through strong hydrogen bonding. Similarly, some clusters of EBT dye are also found in the micrograph of the adsorbed-NIFe-CLDH. 4. Conclusion In this study, layered double hydroxides (LDH) were prepared via co-precipitation method. Batch adsorption experiments were carried out to evaluate the performance of the CLDHs for the removal of Eriochrome Black T (EBT) from aqueous phase. The optimum removal

Table 5 Adsorption capacity and parameters of EBT on conventional adsorbents. Adsorbent

pH

Isotherm/Kinetic

qm(mg/g)

References

Activated carbon from rice hulls Activated charcoal Maize stem MgAl-LDH Magnetite/silica/pectin NPs Magnetite/ pectin nanoparticles Calcined MgAl-LDH Calcined CoAl-LDH Calcined NiFe-LDH

2

Freundlich/2nd order

2 2–4 2.0 2.0

−/− 60/25

Langmuir/2nd order Langmuir/2nd order −/− Sip/2nd order

160 4.71 167.84 128 65.35 72.35

[51] [52] [53] [48] [54] [54]

2.0 2.0 2.0

30/25 60/25 60/25

Langmuir/2nd order Langmuir/2nd order Langmuir/2nd order

540.91 419.87 132.49

This study This study This study

Time, Temp


351

adsorption isotherm models could be better described by Langmuir isotherm model indicating mono-layer adsorption. The maximum adsorption capacity of CoAl-, MgAl and NiFe- CLDHs for EBT was found to be 419.87 mg/g, 540.91 mg/g and 132.49 mg/g respectively. The adsorption capacity of CoAl- and MgAl-CLDH decreases with increase in solution temperature whereas for NiFe-CLDH, the adsorption capacity increases with increase in temperature. The mechanism for the adsorption of EBT onto calcined layered double hydroxides was governed by electrostatic interaction followed by hydrogen and chemical bonding with the functional groups on the surface of CLDHs. The results showed that the synthesized CLDHs are effective adsorbents for EBT in aqueous solutions.

5. Acknowledgements The author would like to thank College of Engineering of University of Dammam and Chemical Engineering Department of King Fahd University of Petroleum and Minerals for their laboratory support in this research work. Fig. 9. FTIR spectrum of LDH after EBT adsorption.

References of EBT was achieved at pH of 2 and equilibrium was attained at 60 min, 30 min and 60 min for CoAl-, MgAl- and NiFe- CLDH respectively. The kinetic experimental data well fitted pseudo-second order model. The

Fig. 10. Scanning electron micrographs of MgAl-,CoAl- and NiFe- CLDH after adsorption of EBT dye.

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