Response surface methodology approach for optimization of the removal of chromium(VI) by NH2-MCM-41

Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 860–868

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Response surface methodology approach for optimization of the removal of chromium(VI) by NH2-MCM-41 Julin Cao a, Yunhai Wu b,*, Yanping Jin a, Palizhati Yilihan a, Wenfu Huang a a b

College of Environment, Hohai University, Nanjing 210098, China Key Laboratory of Integrated Regulation and Resources Development of Shallow Lakes, Ministry of Education, Hohai University, Nanjing 210098, China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 23 June 2013 Received in revised form 21 August 2013 Accepted 15 September 2013 Available online 22 October 2013

A central composite design (CCD) combined with response surface methodology (RSM) was employed for maximizing chromium (Cr(VI)) removal from aqueous solution by using amine-functionalized MCM41 (NH2-MCM-41). Four independent variables namely initial pH, metal ion concentration, temperature and adsorbent dosage were investigated. Analysis of variance (ANOVA) of the quadratic model suggested that the predicted values were in good agreement with experimental data. Maximum removal was attained as 98.70% at initial pH 3.5, Cr(VI) concentration of 10 mg/L, temperature 40 8C with an adsorbent dosage of 5 g/L. The synthesized adsorbent was characterized by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The kinetics were evaluated by pseudo-first-order and pseudo-second-order models, pseudo-second-order model was found to describe the process better with a higher correlation. The adsorption data conformed well to Langmuir and Freundlich isotherms. Thermodynamic analysis revealed that the adsorption process was spontaneous and endothermic. The results from adsorption–desorption cycles showed that NH2-MCM-41 held good desorption and reusability. ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Keywords: NH2-MCM-41 Cr(VI) Adsorption Response surface methodology

1. Introduction Heavy metals are highly toxic at low concentrations and can accumulate in living organisms, causing both short-term and longterm adverse effects [1]. Among the various heavy metal ions, chromium is one of the most important heavy metal contaminants in the wastewater of industrial dyes and pigments, film and photography, galvanometry and electric, metal cleaning, plating and electroplating, leather and mining [2]. There exist three oxidation states for chromium in nature, namely Cr(II), Cr(III) and Cr(VI), soluble hexavalent chromium (Cr(VI)) is extremely toxic than the other two species and exhibits carcinogenic effects on biological systems due to strong oxidizing nature [3]. The maximum permissible limit of Cr(VI) in wastewater has been set as 0.05 mg/L by the US Environmental Protection Agency (EPA) and Bureau of Indian Standards (BIS) [4]. Comparing with various conventional treatment methods, such as precipitation, chemical oxidation or reduction, ion exchange, filtration, membrane separation [5] and photocatalytic reduction [6–8], adsorption is regarded as one of the most effective and attractive processes with several advantages associated with no chemical sludge and a high removal efficiency to remove metal

* Corresponding author. Tel.: +86 25 83786697; fax: +86 25 83786697. E-mail address: [email protected] (Y. Wu).

ions [9], besides adsorption is quite popular in terms of convenience, availability, profitability and design. The common sorbents in metal adsorption, including carbon, resins and microalgae have irregular adsorption sites with chemical heterogeneity and reduce the adsorption of metal ions [10]. The adsorption of Cr(VI) by layered double hydroxides and manganese nodule leached residue has been also well studied [11,12]. In recent years much attention has been focused towards the use of various efficient and low-cost adsorbents for the removal of toxic pollutants, new hybrid organic–inorganic mesoporous ordered structures have been lately proposed as heavy metals adsorbents [13]. Mesoporous silicas have exhibited many attractive characterizations, including large surface area, highly ordered structure, controlled pore diameter and incorporation of specific bonding materials [14], moreover, the silica wall surface can be modified with organic groups to tailor their properties and achieve specific purposes [15]. Researchers have succeeded in grafting amino, diamino, triamino, malonamide, carboxyl, dithiocarbamate, humic acid, and imidazole to the mesoporous silica. A series of amine-functionalized mesoporous materials were synthesized by Parida et al. [16–21]. In the literatures, amine-functionalized MCM-41 has been successfully employed to eliminate traces of toxic heavy metal from wastewater [22,23]. In the present work, the objective of the research is to investigate the feasibility of NH2-MCM-41 for the removal of Cr(VI) from aqueous solution and optimize the parameters for

1876-1070/$ – see front matter ß 2013 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jtice.2013.09.011

J. Cao et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 860–868

maximum removal efficiency using central composite design (CCD) under response surface methodology (RSM). RSM is an empirical statistical technology that uses quantitative data obtained from appropriately designed experiments to determine regression model and operating conditions [24]. The use of RSM has been accentuated for developing, improving and optimizing the complex processes and to evaluate the magnitude of various influencing parameters [25], which has widely been applied in chemical engineering and sorption process optimization. CCD is the most frequently used method of RSM, which is suitable for fitting a quadratic surface and helps to optimize the effective parameters with a minimum number of experiments, in addition to analyzing the interaction between parameters [26]. The optimization of Cr(VI) adsorption was performed by varying four independent parameters (initial pH, metal ion concentration, temperature and adsorbent dosage) and a second-order quadratic model was built to predict the response. The sorbent was characterized by Fourier transform infrared spectra (FTIR), X-ray diffraction (XRD) and Energy Dispersive Spectroscopy (EDS). The kinetic and thermodynamic parameters were calculated to determine the adsorption mechanism. The equilibrium data were fitted into Langmuir, Freundlich and Dubinin–Radushkevich equations to determine the correlation between the isotherm models and experimental data. 2. Materials and methods 2.1. Materials MCM-41 was prepared from alkaline synthesis solution. The NH2-MCM-41 was prepared according to the method of Heidari et al. [22]. 2.5 g of calcined MCM-41 was refluxed for 6 h in 50 mL of n-hexane containing 2.5 g of 3-aminopropyltrimethoxisilane at room temperature. The powder of NH2-MCM-41 was filtered, washed with 20 mL of n-hexane and dried at 105 8C for 24 h, then cooled at room temperature overnight. A stock solution (100 mg/L) of Cr(VI) was prepared by dissolving 0.2829 g of 99.9% potassium dichromate (K2Cr2O7) in 1000 mL of distilled water, experimental solution of different concentrations were prepared by diluting the stock solution with suitable volume of distilled water.

solution was 100 mL of 30 mg/L, keeping the temperature at 30 8C. The residual Cr(VI) concentration in the supernatant was determined at predetermined time intervals. After sorption, the solution and solid phase were separated by centrifugation at 8000 rpm for 25 min, the supernatant was analyzed by a UV-spectrophotometer (UV1201 model) at the wavelength of 540 nm. The removal efficiency and adsorption capacity qe (mg/g) were calculated by the following expressions: Removal ð%Þ ¼

qe ¼

The microscopic appearance and chemical properties of MCM41 before and after the amine functionalized were studied by FTIR and XRD. An FTIR spectrometer (JASCO 5300) was used to find the type of functional groups, which the spectra were at 750– 4000 cm1 wavenumber. The XRD patterns were recorded on an X-ray diffractometer (ARL Corporation, Switzerland) using Ni filtered Cu Ka radiation (40 kV and 30 mA) in the 2u range 1–108. EDS observation was performed using a JEOL, JSM-6360LA microscope equipped with an Oxford Links-Isis energy dispersive X-ray analyzer for investigation of the composition of NH2-MCM41. 2.3. Batch adsorption studies The batch experiments were carried out in 250 mL borosil conical flasks by agitating 0.35 g NH2-MCM-41 adsorbent with 100 mL of the Cr(VI) solution at a known initial concentration on a water bath-cum-mechanical shaker in dark condition, stirring the mixture at 130 rpm for 3 h. The initial pH of the solution was adjusted with 0.1 M HCl or NaOH at 3.5. For the adsorption isotherm study, different initial concentrations of Cr(VI) from 10 to 50 mg/L were carried out with various temperatures (25, 30 and 40 8C). For the adsorption kinetics experiments, the reaction Cr(VI)

C0  Ce  100 C0

(1)

ðC 0  C e ÞV M

(2)

where C0 and Ce were the initial and final concentrations of Cr(VI) (mg/L), V was the volume of the solution (mL) and M was the mass of NH2-MCM-41 (g). 2.4. Desorption experiment The regeneration of the adsorbent was performed by the following method: the Cr(VI)-loaded NH2-MCM-41 was separated and stirred with 25 mL of 1 M HNO3 solution for 3 h, after filtration, the adsorbent was washed with deionized water and then dried, the Cr(VI) concentration in the desorption solution was determined to calculate the desorption efficiency. The regenerated adsorbent was further evaluated for its resorption performance and used for six successive cycles in the methods. 2.5. Experimental design and optimization Optimum condition for the adsorption of Cr(VI) was conducted based on a CCD and analyzed using RSM. The effects of the process parameters (initial pH (X1), Cr(VI) concentration (X2), temperature (X3) and NH2-MCM-41 dosage (X4)) were investigated at five levels as summarized in Table 1. The statistical calculation was based on the relationship between the coded (Zi) and the real values (Xi) according to the following equation [27]: Zi ¼

2.2. Physico-chemical characterization

861

Xi  X0 DX i

(3)

where Xi is the real value of the ith independent variable, X0 is the real value of an independent variable at the centre point, and DXi is the step change. Each independent variable was varied over three levels viz. axial points (+ and –), factorial points (+ and –), and the center point. The behavior of the system is explained by the following empirical second-order polynomial model [28]: Yð%Þ ¼ a0 þ

n n n1 X n X X X ai X i þ aii Xi2 þ ai j X i X j þ e i¼1

i¼1

(4)

i¼1 j¼2

where Y is the predicted response, a0 is the constant coefficient, ai, aii and aij are the regression coefficients, Xi and Xj indicate the independent variables in the form of coded values, and e is the random error. Design Expert software (Stat Ease, USA) was used for Table 1 Independent variables and their coded levels for the central composite design. Factors

Initial pH Metal concentration (mg/L) Temperature (8C) Adsorbents dosage (g/L)

Range and levels (coded)

X1 X2 X3 X4

2

1

0

+1

+2

0.5 0 10 0

2.5 10 20 1

5 30 30 3

7.5 50 40 5

10 70 50 7

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regression and graphical analysis of the experimental data. The optimum values of the selected variables were obtained by solving the regression equation and by analyzing the response surface plots. The value of correlation coefficients (R2) indicated the goodness of fit, the significance of the model was evaluated from Fvalue (Fisher variation ratio) and probability value (Prob > F) [29].

3. Results and discussion 3.1. Characterization of NH2-MCM-41 The FTIR spectra were investigated to obtain the information regarding the changes in peak on the functional groups. Fig. 1 shows the FTIR spectra of NH2-MCM-41 before and after adsorption along with the unmodified MCM-41. The spectrum of MCM-41 displayed a broad and strong band (3200–3800 cm1) with peaks at 3446.8 cm1, which could be attributed to the O–H stretching bonds of silanol groups. The vibrations of Si–O–Si could be seen at 1078.2 cm1 (asymmetric stretching) and 858.3 cm1 (symmetric stretching) [30,31]. After amine functionalization, NH2-MCM-41 showed visible absorption bands at 2930.2 cm1 and 1541.1 cm1 corresponding to –CH2 stretching and the bending vibration of N–H group, respectively, demonstrating how changes were induced by grafting of amine functionality to MCM-41. Upon Cr(VI) loading the FTIR spectrum had no significant change except an increase in the intensity of N–H stretching, N–H bending, and Si–O stretching vibrations (Fig. 1), indicating that the structure of NH2-MCM-41 remained relatively intact after adsorbing the metal ions. The low-angle XRD patterns of MCM-41 before and after amine functionalized are shown in Fig. 2, the patterns presented a strong diffraction peak in the low-angle region (2u = 1–38) before and after modification, which associated with the (1 0 0) reflection of hexagonal cell. The parent MCM-41 showed strong (1 0 0) peak and proportional (1 1 0) peak intensities, while the peak (1 0 0) of NH2-MCM-41 was preserved with the other peaks disappeared, which was attributed to the incorporation of amine groups inside the channels of MCM-41. The patterns had confirmed that the presence of the hexagonal structure of the pores and suggested that the modification process did not affect the framework integrity of the ordered MCM-41 [32]. The chemical composition of NH2-MCM-41 after adsorption were analyzed by EDS (Fig. 3), from the EDS spectrum, the expected primary metal elements C, O, Al, Si and Cl elements were recorded. The presence of Cr signals was attributed to the Cr(VI) solution, which could be confirmed that the metal ion was successfully

Fig. 1. FTIR spectra of MCM-41, NH2-MCM-41 and NH2-MCM-41-Cr.

Fig. 2. XRD pattern of MCM-41, NH2-MCM-41 and NH2-MCM-41-Cr.

adsorbed on the functionalized NH2-MCM-41. The mean pore radius, BET surface area and total pore volume of the pure MCM-41 were 3.02 nm, 821 m2/g and 0.94 cm3/g, respectively, for the prepared NH2-MCM-41, they were 2.75 nm, 658 m2/g and 0.78 cm3/g, as determined from nitrogen adsorption–desorption isotherms measured at 196 8C by an ASAP 2010. After adsorption, the mean pore radius, BET surface area and total pore volume of the metal loaded NH2-MCM-41 decreased to 2.47 nm, 578 m2/g and 0.63 cm3/g, respectively. 3.2. Fitting of process models and statistical analysis The CCD model in this study consisted of 16 factorial points, 8 axial points and 6 center points, a total of 30 runs of the CCD experimental design (initial pH (2.5–7.5), metal ion concentration (10–70 mg/L), temperature (20–40 8C) and adsorbent dosage (1.0– 5.0 g/L)) were conducted and the results are presented in Table 2. The analysis of variance (ANOVA) for adsorption study of Cr(VI) ion was used in order to ensure a good model. The results of the second-order response surface model fitting in the form of ANOVA are given in Table 3. The following second-order polynomial model equation described the adsorption percent (Y) of Cr(VI), Y CrðVIÞ ¼ 92:70  13:86X 1  8:27X 2 þ 4:87X 3 þ 13:52X 4 2 2 2 2  8:87X11  2:53X22  1:07X33  13:22X44

 4:13X 1 X 2 þ 2:54X 1 X 4 þ 1:97X 2 X 4

(5)

The larger the magnitude of the F-values and the smaller p-values, the more significant was the corresponding coefficients. The results given in Table 3 showed that this regression was statistically significant at F value of 359.09 and values of p < 0.0001. The ‘‘Lack of Fit’’ F-value was 359.09, a significant lack of fit suggested that there may be some systematic variation unaccounted for in the hypothesized model. This was thought to be due to the exact replicate values of the independent variable in the model that provided an estimate of pure error [33]. P-values less than 0.0500 also indicated high significant regression at 95% confidence level. The high values of coefficient demonstrated a good agreement between the calculated and observed results within the range of experiment, as the values of the determination coefficient (R2 = 0.9970) and adjusted determination coefficient (Adj. R2 = 0.9942) were close to 1. The actual and predicted removal efficiency plot for Cr(VI) is displayed in Fig. 4. It indicated that the predicted values of Cr(VI) removal efficiency obtained from the model and the actual experimental data were in good agreement, providing evidence for the validity of the regression model. The actual values were the measured response data for a particular run arranged by the CCD, and the predicted values were determined by Eq. (5).

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Fig. 3. EDS spectrum of NH2-MCM-41 after adsorption.

3.3. Effect of process variables The best way to identify relation between the factors and response was an examination of surface plots as a function of two factors by holding the other factors at central level [34]. The effect of the process variables on the response factor are shown in the 3D plots (Figs. 5 and 6a–c). 3.3.1. Effect of initial pH Fig. 5a–c shows the combined effect of initial pH with metal ion concentration, temperature and absorbent dosage based on the fitted second-order polynomial equation. The removal efficiency of Table 2 Experimental design in term of coded factors and results of the central composite design. Run order

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30

Coded values

Response (Y(%))

X1

X2

X3

X4

Observed value

Predicted value

1 +1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 2 2 0 0 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 2 2 0 0 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 2 2 0 0 0 0 0 0 0 0

1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 0 0 0 0 0 0 2 2 0 0 0 0 0 0

72.80 48.40 60.50 18.30 81.70 57.20 70.10 30.40 89.80 74.70 84.60 53.10 97.20 83.80 94.30 64.20 84.10 28.20 98.60 64.40 77.30 97.40 8.20 69.30 92.50 92.60 91.90 92.70 92.80 93.00

70.80 46.64 58.63 16.86 80.16 56.30 68.98 28.59 90.16 75.10 84.78 53.19 97.91 84.22 94.61 64.40 84.96 29.51 99.13 66.05 78.70 98.18 12.80 66.88 92.70 92.70 92.70 92.70 92.70 92.70

Cr(VI) increased with increase pH ranging from 2.5 to 3.5, when pH > 3.5, the removal efficiency of Cr(VI) decreased significantly at any fixed metal ion concentration, temperature and absorbent dosage. The pH dependence of Cr(VI) adsorption can be related to the type and ionic state of functional groups present on the adsorbent and chromium speciation in the solution. The Cr(VI) in the solution exists in the form of oxy anions such as HCrO4, Cr2O72 and CrO42. The dominant species at low pH (1 < pH < 3) is HCrO4, which leads to electrostatic attraction between positively charged adsorbent surface and negatively charged chromium species [35], hence the maximum adsorption (91.8%) occurs under acidic conditions (pH 3.5). With the increase in pH of the solution, the degree of protonation of the amine groups of NH2-MCM-41 reduces gradually; furthermore, CrO42 is dominant at high pH (pH > 6.0), the competition of the OH with the bichromate anion for the adsorption sites could lead to a reduction in the percentage adsorption of Cr(VI) [36]. The other reasons for low adsorption at high pH could be due to the damage of the typical siliceous hexagonal structure of MCM-41 above pH 8 [37], and each of these bivalent ions (CrO42, Cr2O72) would neutralize twice the number

Table 3 Analysis of variance (ANOVA) of the response surface quadratic model for Cr(VI) adsorption. Source

Sum of squares

Degrees of freedom

Mean square

F-value

Prob > F

Model X1 X2 X3 X4 X1X2 X1X3 X1X4 X2X3 X2X4 X3X4 X12 X22 X32 X42

17891.29 4612.05 1641.76 569.40 4387.51 273.08 1.89 103.53 4.31 62.02 0.28 2155.87 175.31 31.15 4790.48

14 1 1 1 1 1 1 1 1 1 1 1 1 1 1

1277.95 4612.05 1641.76 569.40 4387.51 273.08 1.89 103.53 4.31 62.02 0.28 2155.87 175.31 31.15 4790.48

359.09 1295.95 461.32 160.00 1232.85 76.73 0.53 29.09 1.21 17.43 0.077 605.78 49.26 8.75 1346.08

<0.0001 <0.0001 <0.0001 <0.0001 <0.0001 <0.0001 0.4773 <0.0001 0.2887 0.0008 0.7846 <0.0001 <0.0001 0.0098 <0.0001

Residual Lack of fit Pure error Cor total

53.38 53.38 0.000 17944.67

15 10 5 29

3.56 5.34 0.000

37.68

<0.0001

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3.5. Adsorption kinetics and isotherms 3.5.1. Kinetic study Kinetics of the adsorption process is vital in wastewater treatment, as it provides essential information on the reaction pathways and the solute uptake rate. The batch experimental data of kinetic study were applied to two frequently used adsorption kinetic models, namely pseudo-first-order and pseudo-secondorder models. The linear forms of pseudo-first-order [44] and pseudo-second-order equations [45] are expressed as: logðqe  qt Þ ¼ logqe  t 1 t ¼ þ qt k2 q2e qe Fig. 4. Correlation of actual and predicted removal efficiency for Cr(VI).

of sites available on the surface with univalent charge (HCrO4) [38]. 3.3.2. Effect of metal ion concentration As shown in Figs. 5a and 6a and b, it was obvious that the removal efficiency of Cr(VI) decreased gradually with the increase in metal ion concentration. Higher initial Cr(VI) concentration provided less driving force to overcome various mass transfer resistances of the metal ion from the aqueous to the solid phase resulting in higher collision between Cr(VI) and NH2-MCM-41 surface increasing the uptake of Cr(VI) [37,39]. In addition, the extent of adsorption comes down for a fixed adsorbent content at high metal ion concentration due to decreased number of available adsorption sites on the adsorbent surface [40]. 3.3.3. Effect of temperature Figs. 5b and 6a and c present the effects of temperature on the removal efficiency of Cr(VI), which were observed that the adsorption of Cr(VI) increased with increasing temperature. The enhancement in removal efficiency with increasing in temperature might be due to the increased diffusion rate of the metal ions across the external boundary layer and into the internal pores of the adsorbent [41]. Moreover, there may be an increase in the number of active adsorption sites due to the bond rupture of function groups on the sorbent surface, leading to the enhance of sorption [42]. 3.3.4. Effect of adsorbent dosage The effect of adsorbent dosage was found to be the highest among studied variables on metal ion removal based on the ANOVA results (Table 3). The percentage of Cr(VI) removal increased from 42.7 to 98.6%, when the adsorbent dosage was increased from 1.0 to 5.0 g/L (Figs. 5c and 6b and c). The increase in percent adsorption with an increase in the dose of the adsorbent might be due to availability of more surface area with more functional groups at a higher mass of adsorbent [35,43]. 3.4. Confirmation experiments The predicted optimal conditions based on the RSM were initial pH 3.5, adsorbent dose 5.0 g/L, metal concentration of 10 mg/L, temperature 40 8C, reaction time 3 h and stirring speed 130 rpm, the maximum removal efficiency of Cr(VI) was 99.54%. Confirmatory experiments were conducted with the parameters as suggested to check the accuracy of the optimum set of parameters, and the percent removal was found to be 98.70%. The experimental value closely agreed with the result obtained from RSM, which validated the findings of response surface optimization.

k1 t 2:303

(6)

(7)

where qe and qt are the adsorption capacity (mg/g) at equilibrium and at time t (min), respectively, k1 (min1) and k2 (g/(mg min)) are the rate constants. It is evident from Fig. 7 that the linear plots of log(qe  qt) vs. t and t/qt with t show the applicability of pseudo-first-order and pseudo-second-order equations. The estimated kinetic models and related parameters with linear regression coefficient (R2) are shown in Table 4, the values of R2 showed that pseudo-secondorder model better fitted to the experimental data than pseudofirst-order model. The theoretical qe value (7.6734 mg/g) was closer to the experimental value (7.5244 mg/g). The adsorption kinetics followed the pseudo-second-order equation, indicating chemical sorption as the rate-limiting step of adsorption mechanism [46,47]. 3.5.2. Adsorption isotherms In order to optimize the adsorption process parameters, Langmuir, Freundlich and Dubinin–Radushkevich (D–R) isotherm models were used to analyze the experiment data. The Langmuir model is valid for monolayer sorption on surfaces containing a finite number of identical sorption sites and represented by the following equation [48]: qe ¼

qm K L C e 1 þ K LCe

(8)

where qm is the maximum adsorption capacity (mg/g) and KL is the Langmuir constant (L/g). Freundlich isotherm can be used for non-ideal multilayer adsorption that involves heterogeneous sorption, and is expressed by equation as follows [49]: 1=n

qe ¼ K f Ce

(9)

where Kf is the Freundlich constant related to the adsorption capacity and n is Freundlich exponent. The D–R isotherm is applied to determine the nature of the adsorption processes as either physical or chemical which is expressed by the following equations [50]: qe ¼ qm expðK e2 Þ 

e ¼ RTln 1 þ

1 E ¼ pffiffiffiffiffiffiffi 2K

1 Ce



(10)

(11)

(12)

where K is the Dubinin–Radushkevich constant related to the free energy of adsorption (mol2/KJ2), e is the Polanyi potential,

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Fig. 5. Response surface plots for combined effect of initial pH and metal ion concentration (a), initial pH and temperature (b), initial pH and adsorbent dosage (c) on the removal efficiency of Cr(VI).

T is the solution temperature (K), R is the gas constant (8.314 J/(mol K)) and E is the mean free energy of sorption (KJ/mol). The isotherm parameters and linear regression coefficient (R2) values are shown in Table 5. The results indicated that Freundlich model employed to describe the heterogeneous systems fitted well compared to Langmuir model. Furthermore, the maximum adsorption capacity of Cr(VI) by NH2-MCM-41 was 38.55 mg/g occurring at the highest studied temperature of 40 8C. Lam et al.

[51] also studied a similar amino functionalized MCM-41 for the selective adsorption of Cr2O72 and Cu2+, the maximum adsorption capacity of NH2-MCM-41 for Cr2O72 was 94.6 mg Cr/g, which also proven that the adsorbent displayed a higher adsorption capacity than others [52,53]. Table 5 also presents that n is greater than 1.0 at all temperatures, indicating that Cr(VI) is favorably adsorbed by NH2-MCM-41 at studied conditions. The values of E (12.79– 13.67 KJ/mol) indicated that the adsorption process was a chemical adsorption.

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Fig. 6. Response surface plots for combined effect of metal ion concentration and temperature (a), metal ion concentration and adsorbent dosage (b), adsorbent dosage and temperature (c) on the removal efficiency of Cr(VI).

3.6. Thermodynamics To calculate the thermodynamic activation parameters such as enthalpy (DH0), entropy (DS0) and Gibbs free energy (DG0), the following equations were applied [54]:

DG0 ¼ RTlnK C

KC ¼

C Ae Ce

(13)

(14)

lnK C ¼

DS0 R



DH 0 RT

(15)

where KC is the equilibrium constant, CAe and Ce (mg/g) indicate the metal ion amounts on adsorbent phase and adsorbate phase, respectively. DH0 and DS0 were obtained from the slope and intercept of van’t Hoff plot of ln KC versus 1/T. The thermodynamic parameters for the adsorption of Cr(VI) on NH2-MCM-41 at different temperatures are listed in Table 6. The DG0 indicated the degree of spontaneity of the adsorption process, these negative values of DG0 decreased with temperature

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Table 4 Kinetic parameters for the adsorption of Cr(VI) on NH2-MCM-41 at 40 8C. Pseudo-second-order

Pseudo-first-order 1

k1 (min 0.0089

)

2

qe (mg/g) 0.7724

R 0.9957

k2 (g/(mg min)) 0.0560

R2 0.9998

qe (mg/g) 7.6734

Table 5 Isotherm parameters for the adsorption of Cr(VI) on NH2-MCM-41. Temperature (8C)

25 30 40

Langmuir

Freundlich

D–R

qm

KL

R2

Kf

n

R2

qm

K

R2

E

27.55 32.03 38.55

0.1092 0.1104 0.0772

0.9698 0.9844 0.9834

3.015 2.945 3.413

1.405 1.253 1.342

0.9658 0.9766 0.9833

10.87 10.97 10.73

0.0641 0.0535 0.0372

0.9006 0.8815 0.8617

12.79 13.06 13.67

Table 6 Thermodynamic parameters for the adsorption of Cr(VI) on NH2-MCM-41. Temperature (8C)

DG0 (KJ/mol)

DH0 (KJ/mol)

DS0 (J/mol K)

25 30 40

2.33 2.49 2.83

7.6

33.31

increasing, indicating that the adsorption process was spontaneous in nature and higher temperature could obviously promote the removal of Cr(VI). This result was in agreement with the temperature study. The positive value of DH0 suggested that the adsorption process was endothermic, and the positive value of DS0 reflected the affinity of Cr(VI) for the NH2-MCM-41 [55].

Fig. 8. Desorption efficiency of NH2-MCM-41 in serial recycle tests.

3.7. Desorption and regeneration studies The regeneration of the adsorbent was studied in order to make the adsorption process more economical and feasible. The adsorption reversibility of Cr(VI) onto NH2-MCM-41 was studied using 1 M HNO3 solution, the results in Fig. 8 revealed that the desorption efficiency was 64% after six cycles. The mechanism of desorption could be due to the ion-exchange of Cr(VI) ion with the H+ ion on the surface of the NH2-MCM-41 [56]. The adsorption– desorption cycle result demonstrated that the NH2-MCM-41 could be reused up to 6 times without a significant change in the amount of adsorption for Cr(VI) ion.

4. Conclusions

Fig. 7. Kinetic plots of pseudo-first-order (a) and pseudo-second-order (b) for adsorption of Cr(VI) on NH2-MCM-41 at 40 8C.

The results of the present studies showed that NH2-MCM-41 prepared by synthesis method was an efficient adsorbent for the heavy metal chromium removal. The use of response surface methodology involving central composite design for optimization of process parameters was studied. Experiments were performed as a function of initial pH, metal ion concentration, temperature and absorbent dosage. At optimum adsorption conditions, the predicted removal efficiency of Cr(VI) by NH2-MCM-41 reached 99.54%. The adsorption was found to follow the pseudo-secondorder model from the kinetic studies. The equilibrium data fitted well to both Langmuir and Freundlich models with the maximum adsorption capacity of 38.55 mg/g. The calculated thermodynamic parameters indicated the feasibility, endothermic and spontaneous nature of the adsorption process. The sequential

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