α-Fe2O3 nanocomposite and its application in the removal of acid yellow 17 dye from water

Journal of Hazardous Materials 273 (2014) 127–135

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Synthesis of magnetic activated carbon/␣-Fe2 O3 nanocomposite and its application in the removal of acid yellow 17 dye from water V. Ranjithkumar, S. Sangeetha, S. Vairam ∗ Department of Chemistry, Government College of Technology, Coimbatore 641013, Tamil Nadu, India

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• Activated carbon/␣-Fe2 O3 nanocom• • • •

posite has been prepared by simple pyrolysis route. Iron gluconate in activated carbon matrix was used as precursor. The nanocomposite is magnetic due to the incorporated nano iron oxide in carbon. The nanocomposite acts as a good adsorbent of acid dye following Langmuir isotherm. Magnetic separation can be applied for removing dye adsorbed composite from water.

a r t i c l e

i n f o

Article history: Received 3 December 2013 Received in revised form 1 March 2014 Accepted 16 March 2014 Available online 29 March 2014 Keywords: Composite materials Nanostructures Magnetic properties Adsorption

a b s t r a c t The adsorption of acid yellow 17 dye on activated carbon/␣-Fe2 O3 nanocomposite prepared by simple pyrolytic method using iron(II) gluconate was investigated by batch technique. The composite was characterized by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX), transmission electron microscopy (TEM), and vibrating sample magnetometry (VSM). The size of iron oxide nanoparticles formed from iron(II) gluconate precursor is in the range 5–17 nm. The saturation magnetization (Ms ), remanence (Mr ) and coercivity (Hc ) of the magnetic carbon nanocomposite is 5.6 emu/g, 1.14 emu/g and 448 Oe, respectively. The adsorption data are found to fit well with Langmuir and, fairly well with Freundlich and Tempkin isotherms at higher concentration of dye (40–100 mg/L). Kinetics data indicate that the adsorption of dye follows pseudo-second order kinetics model. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Nowadays, the general type of nanocomposite embedded with organic/inorganic materials is a fast growing area of research. Researchers have given their major effort to obtain the nanoscale structures of nano-composite materials via innovative synthetic approaches and the properties of nano-composite materials

∗ Corresponding author. Tel.: +91 4222432221; fax: +91 4222455230. E-mail address: [email protected] (S. Vairam). http://dx.doi.org/10.1016/j.jhazmat.2014.03.034 0304-3894/© 2014 Elsevier B.V. All rights reserved.

depend not only on the properties of their individual parents but also on their morphology and interfacial characteristics. Many types of carbon nanocomposites are used in several fields like adsorption, batteries etc. Bare metal oxide nanoparticles have plenty of applications in various fields such as environmental, catalysis, batteries, sensors, magnetic storage media and contrast agents in magnetic resonance imaging [1–6]. Different morphologies of nano metal oxides such as nanoflowers, nanorods, nanowires, nanosheets and nanotubes have been reported. Their individual properties have been modified by encapsulating them in carbon materials like activated carbon [7] and carbon nanotube [8,9].

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O Na O

S

CH3

N=N

O

N N Cl

of carbon–metal oxide nanocomposite with uniform morphology and upon its application. In this paper, a simple synthetic route of preparing magnetic activated carbon/␣-Fe2 O3 nanocomposite (AC/␣-Fe2 O3 ) using iron(II) gluconate without using more hazardous chemicals and sustaining severe conditions, and the role of nano metal oxide in carbon matrix on adsorption of acid yellow 17 dye (AYD 17) in aqueous solution are reported.

2. Experimental work 2.1. Materials

Cl O

S

O

Iron(II) gluconate, powder activated carbon (AC) (with surface area 893.83 m2 /g) and AYD17 were purchased from Sigma Aldrich and used as such. All solutions used for the adsorption studies were obtained by diluting the 1000 mg/L of stock AYD 17 using double distilled water.

ONa Fig. 1. Chemical structure of acid yellow 17 dye (AYD 17).

Government and private sectors concentrate in the development of advanced technology for solving some environment problems caused by population growth. In this connection, researchers are synthesizing stable nano-composite materials with good magnetic behavior which have wide applications in catalysis [10], adsorption [11–19] and batteries [20–23]. In addition, uniform morphology of nano oxides also plays a vital role in improvising the electrical, chemical and magnetic properties of nanocomposite with pine-leaf hierarchical structure as reported by Zhang et al. [24]. The methods used to incorporate metal oxide into carbon matrix involve complex procedure such as decomposition of iron(III) nitrate in carbon matrix followed by coating of benzene vapors [6], hydrothermal technique by using activated carbon, graphite powder, iron salts, sodium acetate and glycol [25], electrospinning method using polyacrylonitrile (PAN) and ferric acetylacetonate precursor [26] and coprecipitation method using metal salts and activated carbon under nitrogen atmosphere [10]. Water pollution is a serious issue for human beings and ecosystems. Various media have declared that water pollution is the major reason for the death of more than 14,000 people daily [27]. Every day in India, nearly 580 peoples die due to diarrheal sickness [28]. In China, 90% of cities suffer from some degree of water pollution and nearly 500 million people lack access to safe drinking water [29]. In addition to the acute problems of water pollution in developing countries, developed countries continue to struggle with pollution problems as well. Textile industries release aromatic amines, heavy metals, ammonia, alkali salts and toxic pigments into the nearby water bodies performing a major adverse role in water pollution than other industries do. These untreated toxic chemicals cause chemical and biological changes in our aquatic system, which in turn threaten various species of fish and aquatic plants [30]. Dyeing industries release effluents due to incomplete fixation of dyes onto the fabrics [31]. All water soluble dyes like reactive and acid dyes contaminate the water sources easily. Acid yellow 17 (AYD 17), a monoazo dye whose structure is shown in Fig. 1, is being used in dyeing cotton, wool, silk, leather, paper and it is also a common additive found in ordinary household products such as shampoo, bubble bath, liquid soap [32]. With regard to these issues, many countries have started strict environmental regulations associated with dyestuffs and have several associated industries and research institutes to get cost-effectively feasible wastewater treatment methods. This has paved way for the current researchers to focus on the synthesis

2.2. Preparation of magnetic AC/˛-Fe2 O3 nanocomposite The composite (AC/␣-Fe2 O3 ) was prepared by immersing AC (10 g) in 50 mL of an aqueous solution of iron(II) gluconate hydrate (5 g) and stirring for 6 h. The filtered and dried iron(II) gluconate entrapped AC was heated at 410 ◦ C (gluconate decomposition temperature [33]) for 10 min in the muffle furnace to facilitate the insertion of ␣-Fe2 O3 nanoparticles in carbon matrix. The yield of composite was about 37% after calcination.

2.3. Characterization of magnetic AC/˛-Fe2 O3 nanocomposite Using FT-IR spectrophotometer (Thermo Nicolet, Avatar 370), the functional group vibrations of nanocomposite were recorded. Embedded ␣-Fe2 O3 nanoparticles in carbon matrix were identified by powder XRD with CuK˛ radiation ( = 0.15418 nm) at 40 kV and 40 mA and recorded in the region of 2. Field emission scanning electron microscope (JEOL Model JSM – 6390LV), energy dispersive spectrometer (JEOL Model JED – 2300) and a transmission electron microscope (TEM, Philips CM-200) were used to confirm the morphology and shapes of the magnetic nanocomposite. Magnetic parameters of nanocomposite were measured using vibrating sample magnetometer (Lakeshore VSM 7410). The required pH value of the solutions was adjusted with 0.1 M HCl or NaOH with a pH meter (WTW 340i, Germany) for the measurements.

2.4. Batch adsorption experiments For isotherm adsorption studies, a series of 10 solutions of 100 mL water sample each, and of different concentrations varying from 10 to 100 mg of dye per liter, with 0.1 g of nanocomposite was agitated for 30 min in 500 mL bottles kept in a shaker rotating with an rpm of 150 at three different temperatures 25 ± 1, 40 ± 1 and 50 ± 1 ◦ C. The supernatant solutions were analyzed spectrophotometrically at 402 nm wave length. The concentrations of adsorbed dyes were calculated and applied in Langmuir, Freundlich and Temkin isotherm studies. A batch of 100 mL of 60 mg/L dye solutions was agitated as above for different time intervals 10, 20, 30, 40, 50 and 60 min, at 25 ◦ C and the data were collected for kinetic studies. The effect of pH value on adsorption was investigated by taking the initial pH value varied in the range 1–14 and the initial concentration was fixed at 60 mg/L. All the experiments were repeated thrice for consistent results and the mean values have been used for the investigations of adsorption studies.

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Fig. 2. FT-IR spectra of (a) AC, (b) iron (II) gluconate entrapped AC and (c) AC/␣Fe2 O3 . Fig. 3. Powder XRD patterns of (a) AC and (b) AC/␣-Fe2 O3 .

3. Results and discussion 3.1. FT-IR FT-IR spectra of AC, iron gluconate entrapped carbon and AC/␣Fe2 O3 are shown in Fig. 2(a)–(c) respectively. All the spectra have the characteristic peaks in the range 1100–1200, 1540–1570 and 2320–2370 cm−1 due to C–C in AC confirming that the carbon was not destroyed in composites during pyrolysis. Stretching vibration

noticed at 585 cm−1 in Fig. 2b corresponds to Fe–O of iron(II) gluconate, implying that the iron(II) gluconate is entrapped in AC. The strong absorption peak observed at 592 cm−1 in Fig. 2c, attributed to Fe–O of ␣-Fe2 O3 , reveals the presence of ␣-Fe2 O3 in carbon matrix. These are similar to the reported values [34,35].

Fig. 4. SEM images of AC (a), AC/␣-Fe2 O3 (b and c) and EDX of AC/␣-Fe2 O3 (d).

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Fig. 5. TEM images of AC/␣-Fe2 O3 (a–c) and (d) SAED of AC/␣-Fe2 O3 .

3.2. XRD analysis The XRD patterns of AC and AC/␣-Fe2 O3 nanocomposite are shown in Fig. 3. In this diffraction study, 2 values at 6◦ and 27◦ of AC found in Fig. 3a, are also observed in Fig. 3b and this confirms the presence of carbon in AC/␣-Fe2 O3 nanocomposite. Seven peaks indexed at (1 0 4), (1 1 0), (1 1 6), (1 2 2), (2 1 4), (3 0 0) and (2 0 8) in Fig. 3b match well with JCPDS values (89–8104) of ␣Fe2 O3 corroborating the presence of ␣-Fe2 O3 nanoparticles in the pores of carbon [36,37]. While applying FWHM values measured from PXRD patterns of AC/␣-Fe2 O3 (Fig. 3b) in Scherrer’s formula, D = (K)/(ˇcos) where  is the X-ray wavelength, ˇ is the full width of half maximum (FWHM) of a diffraction peak,  is the diffraction angle and K is the Scherrer’s constant of the order of 0.89, it is found that the crystallite size of ␣-Fe2 O3 in carbon matrix is in the range 5–17 nm.

that the nano size of embedded iron oxide nanoparticles is in the range 5–17 nm. SAED pattern of the AC/␣-Fe2 O3 is shown in Fig. 5d where the appearance of diffraction rings and bright spots represent the higher degree of crystallinity of the particles. This is in good agreement with the reported patterns [38]. 3.5. Magnetic properties Magnetic hysteresis loop of nanocomposite AC/␣-Fe2 O3 is shown in Fig. 6. The saturation magnetization (Ms ), remenance (Mr )

3.3. SEM results The SEM images of the AC and AC/␣-Fe2 O3 nanocomposite are shown in Fig. 4a–c. Fig. 4b and c depict the presence of metal oxide (white aggregates) in the pores of tile like structured AC. EDX spectrum of AC/␣-Fe2 O3 nanocomposite shown in Fig. 4d, indicates the presence of C, Fe and O. The atomic ratio of O to Fe, 3:2 confirms that the material inserted into carbon is Fe2 O3. 3.4. TEM results The TEM images of the AC/␣-Fe2 O3 nanocomposite are shown in Fig. 5a–d. TEM images exhibit the size and shape of magnetic nanocomposite. Fig. 5a and b reveals that the iron oxide nanoparticles are embedded in the pores of carbon matrix. Fig. 5c, indicates

Fig. 6. Magnetization hysteresis loop of AC/␣-Fe2 O3 at room temperature. The inset picture shows the magnetic separation of the AC/␣-Fe2 O3 from water by an ordinary magnet after 10 s.

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Fig. 7. Kinetic models of pseudo-first-order (a) and pseudo-second-order (b) for 60 mg/L AYD 17 solution onto AC and AC/␣-Fe2 O3 at 25 ◦ C (volume of AYD 17: 100 mL, adsorbent: 0.1 g, pH: 5, shaking time: 10–60 min and agitation: 150 rpm).

Table 1 Rate constants of pseudo-first order and pseudo-second-order kinetic models for adsorption of AYD 17 onto AC and nanocomposite AC/␣-Fe2 O3 . Adsorbent

AC AC/␣-Fe2 O3

Pseudo-first-order rate constants

Pseudo-second-order rate constants

qe ,exp (mg/g)

k1 (min−1 )

qe ,cal (mg/g)

R2

k2 (g/(mg min))

qe ,cal (mg/g)

R2

0.013 0.048

7.880 27.04

0.9269 0.8019

0.0154 0.0019

55.6 66.7

0.9999 0.9899

and coercive force (Hc ) values were found to be 5.6 emu/g (pure ␣Fe2 O3 – 65 emu/g), 1.14 emu/g and 448 Oe (pure ␣-Fe2 O3 – 204 Oe) respectively. It also shows that there is a decrease in Ms and an increase in Hc of AC/␣-Fe2 O3 when compared to bare ␣-Fe2 O3 [39]. This observation reveals that the metal oxide is embedded within carbon. Furthermore, the ferromagnetic nature of the nanoparticles is characterized by the ratio of remanence to saturation magnetization (Mr /Ms ), for nanocomposite AC/␣-Fe2 O3 (Mr /Ms = 0.20), the low remanence value of less than 25% clearly indicates that the ␣-Fe2 O3 nanoparticles are in a super-paramagnetic state at room temperature. This magnetic behavior was used to separate dye adsorbed nanocomposite from water as displayed in inset picture of Fig. 6. It is observed that AC/␣-Fe2 O3 which is attracted by a magnet making the solution clear which could be easily removed by pipet or decanted off. Therefore, it can be used as a magnetic adsorbent to remove pollutants from aqueous solutions. 3.6. Adsorption study 3.6.1. Kinetic study Fig. 7 shows the comparison of pseudo-first-order and pseudosecond order kinetic studies of 100 mL of 60 mg/L of acid yellow dye (AYD 17) adsorbed at pH 5 by using two adsorbents (0.1 g) namely AC and AC/␣-Fe2 O3 . The kinetic equations used are: Pseudo-first-order model : log(qe − qt ) = log qe − Pseudo-second order model :

t 1 t = + qt qe k2 q2e

k1 t 2.303

(1) (2)

where qe and qt are the amounts of AYD 17 adsorbed (mg/g) at equilibrium and at any time t (min), t is the adsorption time (min), k1 (min−1 ) and k2 (g mg−1 min−1 ) are the pseudo-first and pseudo-second orders rate constants respectively. The kinetic constants obtained by linear regression for the two models (Fig. 7) are summarized in Table 1. Correlation coefficients R2 values for Pseudo-second order model are found to be closer to unity and the

57.9 58.0

Table 2 Isotherm constants for the adsorption of AYD 17 onto nanocomposite AC/␣-Fe2 O3 at various temperatures. Isotherm studies

Nanocomposite AC/␣-Fe2 O3 298 K

313 K

Langmuir, whole concentration range (10–100 mg/L) 200.00 333.30 qm (mg/g) b (L/mg) 0.0770 0.0450 R2 0.6544 0.6572 Langmuir, lower concentration range (10–40 mg/L) qm (mg/g) 38.460 41.670 b (L/mg) 0.5420 0.4710 2 R 0.3678 0.3736 Langmuir, higher concentration range (40–100 mg/L) qm (mg/g) 71.430 83.330 0.9000 0.8000 b (L/mg) 0.9228 0.9744 R2 Freundlich, whole concentration range (10–100 mg/L) 21.130 21.630 KF (mg/g) (L/mg)1/n n 2.1700 2.0000 2 0.6586 0.7008 R Freundlich, lower concentration range (10–40 mg/L) 15.740 16.600 KF (mg/g) (L/mg)1/n 2.9500 2.8300 n R2 0.3352 0.3451 Freundlich, higher concentration range (40–100 mg/L) 1/n 41.400 41.500 KF (mg/g) (L/mg) 5.7300 4.4700 n 0.8214 0.9336 R2 Temkin, whole concentration range (10–100 mg/L) 5.8900 3.8000 A (L/mg) 14.860 18.920 B 0.7400 0.8135 R2 Temkin, lower concentration range (10–40 mg/L) 79.430 75.860 A (L/mg) 4.3500 4.8000 B 0.2492 0.2577 R2 Temkin, higher concentration range (40–100 mg/L) 125.89 22.390 A (L/mg) 8.7500 13.230 B 2 0.8473 0.9522 R

323 K 333.30 0.0430 0.6658 40.000 0.4960 0.3704 83.330 0.7500 0.9506 21.280 1.9100 0.7108 16.290 2.9700 0.3354 41.110 4.0100 0.9079 3.4700 20.000 0.8148 102.32 4.4300 0.2436 16.980 14.370 0.9354

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Fig. 8. Adsorption isotherms for higher concentration range of AYD 17 onto AC/␣-Fe2 O3 at different temperatures: Langmuir (a–c), Freundlich (d–f) and Temkin (g–i) (volume of AYD 17: 100 mL, initial concentration of AYD 17: 10–100 mg/L, adsorbent: 0.1 g, pH: 5, shaking time: 30 min and agitation: 150 rpm).

calculated qe values agree well with the experimental values (qe,exp ) [40]. 3.6.2. Isotherm study Adsorption capacity and adsorption behavior of the prepared adsorbent (AC/␣-Fe2 O3 ) were evaluated by isotherm studies. Langmuir, Freundlich and Temkin equations [41–43] were utilized to suggest the adsorption isotherms at 25 ± 1, 40 ± 1 and 50 ± 1 ◦ C. The linear form of Langmuir equation at ambient temperature is represented by the following Eq. (3): 1 2 1 = + qe qm bqm Ce

(3)

where Ce (mg/L) is equilibrium concentration of AYD 17 in solution, qe (mg/g) is the adsorption capacity of AYD 17 adsorbed at equilibrium, qm (mg/g) is the maximum amount of AYD 17 adsorbed per unit mass of adsorbent required for monolayer coverage of the surface and b (L/mg) is the adsorption heat constant. The slope and intercept of linear plots of 1/Ce against 1/qe yield the values of 1/(bqm ) and 1/qm for Eq. (3) [10]. The values of qm , b and R2 calculated from the linear forms are given in Table 2, and these indicate that Langmuir adsorption isotherm is found more suitable only to higher concentrations (40–100 mg/L and R2 range: 0.9228–0.9744) showing good linearity in the plots (Fig. 8a–c). Further, the R2 values found in the range 0.3678–0.3736 for the lower concentration range (10–40 mg/L) and 0.6544–0.6658 for whole concentration range (10–100 mg/L) show that Langmuir isotherm is not obeyed. Some of the adsorbents whose qm values have been reported in the literature along with AC/␣-Fe2 O3 are listed in Table 3. Analysing the values, it is understood AC/␣-Fe2 O3 has comparatively good qm value, 71.43 mg/g considering the cost of production and

availability of adsorbent. Hence it is reasonable to suggest that this nanocomposite is an efficient and low cost adsorbent. The Freundlich isotherm model was also used to study the results of AYD 17 adsorption on adsorbent AC/␣-Fe2 O3 (Fig. 8d–f). The Freundlich model can be expressed by the following Eq. (4): Log qe = log KF +

1 log Ce n

(4)

where KF (mg1−(1/n) L1/n g−1 ) is the adsorption capacity of the adsorbent and 1/n is another constant related to the surface homogeneity. From the slope and intercept of linear plots of log qe against log Ce , the values of 1/n and log KF for Eq. (4) [10] were obtained and they are given in Table 2 along with the correlation coefficients. These values show that the Freundlich plots deviate from linearity for the whole concentration range and also for lower concentration range 10–40 mg/L, indicating that AYD 17 was adsorbed unfavorably by adsorbents at the whole and lower concentration range studied. Despite, it fits very well with the experimental data that are obtained at the higher concentration range 40–100 mg/L (R2 range: 0.8214–0.9336). Freundlich isotherm constants, KF and n for the higher concentration range, studied at different temperatures indicate that n values are greater than unity and R2 values of composite as listed in Table 2, suggesting that AYD 17 was adsorbed favorably by adsorbent AC/␣-Fe2 O3 with higher concentration at high temperature [44,45]. The Temkin isotherm assumes that the fall in the heat of adsorption is linear rather than logarithmic, as implied in the Freundlich equation. The Temkin isotherm is used in the following form: qe =

RT ln ACe b

(5)

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Table 3 Adsorption capacity for the removal of various acid dyes from aqueous solutions by some low cost adsorbents. Acid dyes

Adsorbents

qm (mg/g)

Reference

Acid yellow 17 Acid yellow 17 Acid yellow 17 Acid yellow 17 Acid yellow 36 Acid yellow 36 Acid orange 10 Acid orange 52 Acid red 14 Acid red 57 Acid red 114 Acid red 274

AC/␣-Fe2 O3 composite Commercial activated carbon Non-living aerobic granular sludge Calcined alunite Waste carbon slurries Rice husk carbon Activated carbon prepared from sugarcane bagasse Banana peel Soy meal hull Neurospora crassa Activated pongam seed shells Enteromorpha prolifera

71.43 83.33 133.3 151.5 211 86.9 5.780 21 109.89 113.6 204 238.1

This study [44] [45] [47] [48] [49] [50] [51] [52] [53] [54] [55]

The linear form of Temkin isotherm equation is qe = B ln A + B ln Ce

(6)

where B = RT/b, T is the absolute temperature in K, R is the universal gas constant, 8.314 J mol−1 K−1 , A is the equilibrium binding constant (L/mg) and B is related to the heat of adsorption. The linear forms of Temkin isotherm plots for higher concentration range (40–100 mg/L) composite at different temperatures are presented in Fig. 8g–i. Temkin constants A, B and R2 values for lower (10–40 mg/L), higher (40–100 mg/L) and whole concentrations (10–100 mg/L) are given in Table 2. The results show fit fairly well with the higher concentration (R2 range: 0.8473–0.9522). Looking abreast of all models applied to the adsorption experiment data, it is inferred that Langmuir’s model exhibits a better fit to the adsorption data for the nanocomposite AC/␣-Fe2 O3 adsorbent, at the temperature of 25, 40 and 50 ◦ C showing R2 values 0.9228–0.9744 in the higher concentration range (40–100 mg/L). This means that at lower concentration range (10–40 mg/L), the dye molecules are unable to diffuse through the shells of carbon, whereas at higher concentration range, diffusion happens rapidly and complex with ␣-Fe2 O3 particles. At 40 ◦ C, diffusion further improves leading to adsorption to both carbon particles and to ␣-Fe2 O3 , and attains equilibrium. Moreover, the adsorption has occurred at finite number of definite localized sites that are identical and equivalent [46]. Hence, this isotherm study substantiates homogeneous and monolayer adsorption mechanism. Temkin model is found to be fairly good fitting to the data at 40 ◦ C for higher concentration range (40–100 mg/L) showing R2 value, 0.9522 which indicates a uniform distribution of binding energies over the population of surface binding adsorption sites, supporting the homogeneous adsorption mechanism. 3.6.3. Effect of pH The adsorptions of AYD 17 (60 mg/L; 100 mL) onto adsorbent (0.1 g) studies were conducted at different pH (1, 3, 5, 8, 10, 12 and 14) by using 0.1 M NaOH/HCl appropriately. The removal of AYD 17 decreases gradually with increasing pH for AC and AC/␣-Fe2 O3 as shown in Fig. 9. The adsorption by AC/␣-Fe2 O3 remains constant through pH 8–12 and this may be due to the fact that the anionic dyes replace the hydroxyl ions in the pores of carbon described by

a ligand exchange mechanism [31]. Hence, basic pH is not suitable for dye uptake because of the competition between dye molecules and hydroxyl ions to enter the adsorbent with Lewis acid active sites [12,44]. 3.6.4. Effect of temperature The thermodynamic parameters such as standard enthalpy change (H◦ , kJ mol−1 ), standard entropy change (S◦ , J mol−1 K−1 ) and standard Gibbs free energy change (G◦ , kJ mol−1 ) were calculated by using Eqs. (7) and (8): G◦ = −RT ln K Log K =

S ◦ 2.303R

(7) −

H ◦

(8)

2.303 RT

where R is the gas constant, K is the equilibrium constant (obtained from Langmuir equation), and T is the temperature (K) [11].

Fig. 9. Effect of initial pH on the adsorption of AYD 17 onto AC (a) and AC/␣-Fe2 O3 (b) at 25 ◦ C (volume of AYD 17: 100 mL, initial concentration of AYD 17: 60 mg/L, Adsorbent: 0.1 g, shaking time: 30 min and agitation: 150 rpm). Data were presented on graphs with means and standard deviations (S.D.). The bars represent the standard error of the mean.

Table 4 Thermodynamic parameters of AYD 17 adsorption onto AC and nanocomposite AC/␣-Fe2 O3 at various temperatures. Adsorbent

Temperature (K)

b (L/mg)

Log K

G◦ (k J mol−1 )

H◦ (k J mol−1 )

S◦ (J mol−1 K−1 )

AC

298 313 323

1.500 0.423 0.210

−0.1761 0.3737 0.6778

1.0048 −2.2394 −4.1918

63.18

208.7

AC/␣-Fe2 O3

298 313 323

0.900 0.800 0.750

0.0458 0.0969 0.1249

−0.2611 −0.5808 −0.7727

5.859

20.54

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[3]

[4]

[5]

[6]

[7]

[8] [9] [10] Fig. 10. Van’t Hoff plot for the adsorption of AYD 17 on AC and AC/␣-Fe2 O3 , (initial concentration of AYD 17: 60 mg/L, volume: 100 mL, pH: 5, shaking time: 30 min and agitation: 150 rpm).

The values of S◦ and H◦ were calculated from the slope and intercept of the Van’t Hoff linear plot of log K versus 1/T (Fig. 10). The negative values of G◦ show the spontaneity of adsorption and the positive values of H◦ (Table 4) confirm the endothermic nature of adsorption. The positive values of S◦ show the increased randomness at the solid/solution interface during the adsorption of AYD 17 [12].

[11]

[12]

[13]

[14]

[15]

4. Conclusions In this paper, a successful synthetic method to prepare a stable magnetic AC/␣-Fe2 O3 nanocomposite without using highly toxic chemicals is reported. This technique is also suitable for preparing similar carbon–metal oxide nanocomposite using transition and inner-transition metal salts. AC/␣-Fe2 O3 nanocomposite possesses excellent magnetic properties and ␣-Fe2 O3 in carbon matrix is in nano-size with uniform morphology. Moreover, they are attracted by permanent magnet in water which makes easy way to separate water from carbon adsorbed with dye. Hence this magnetic composite would be a promising material in adsorption technology. The pseudo-second-order model fits well with the adsorption of AYD 17. Isotherm study reveals that Langmuir isotherm is obeyed to a better extent in the concentration range 40–100 mg/L and adsorption follows monolayer and homogeneous mechanism. Acidic pH is suitable to remove AYD 17 in aqueous solution. The positive values of S◦ indicate the feasibility of solid/solution interface adsorption and that of H◦ show that the adsorption by nanocomposite is endothermic. Therefore, from practical point of view, the AC/␣Fe2 O3 nanocomposite material would be a promising magnetic adsorbent for the removal of acid dyes from its aqueous solution. Acknowledgement

[16]

[17]

[18]

[19]

[20] [21]

[22]

[23]

[24]

[25]

We thank the All India Council for Technical Education, New Delhi (grant-in-aid No. 8023/BOR/RID/RPS-012/2009-10), for financial support to this work.

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