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Adsorption studies on the removal of Malachite Green from aqueous solutions onto halloysite nanotubes
Applied Clay Science 54 (2011) 34–39
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Research paper
Adsorption studies on the removal of Malachite Green from aqueous solutions onto halloysite nanotubes Gholamreza Kiani a,⁎, Mohammad Dostali a, Ali Rostami a, Ali R. Khataee b a b
School of Engineering-Emerging Technologies, University of Tabriz, Tabriz, Iran Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 16 January 2011 Received in revised form 28 May 2011 Accepted 10 July 2011 Available online 19 August 2011 Keywords: Halloysite nanotubes Nano-adsorbent Textile dye Kinetic model Thermodynamic parameter
a b s t r a c t Halloysite nanotubes (HNTs) were used as nano-adsorbents for removal of the cationic dye, Malachite Green (MG), from aqueous solutions. The adsorption of the dye was studied with batch experiments. The natural HNTs used as adsorbent in this work were initially characterized by FT-IR and TEM. The effects of adsorbent dose, initial pH, temperature, initial dye concentration and contact time were investigated. Adsorption increased with increasing adsorbent dose, initial pH, and temperature. Equilibrium was rapidly attained after 30 min of contact time. Pseudofirst-order, pseudo-second-order and intraparticle diffusion models were considered to evaluate the rate parameters. The adsorption followed pseudo-second-order kinetic model with correlation coefficients greater than 0.999. The factors controlling adsorption process were also calculated and discussed. The maximum adsorption capacity of 99.6 mg g −1 of MG was achieved in pH = 9.5. Thermodynamic parameters of ΔG°, ΔH° and ΔS° indicated the adsorption process was spontaneous and endothermic. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in various fields, such as leather, paper, textile, etc., and their discharge into natural waters causes environmental problems, related to their carcinogenicity, toxicity to aquatic life and undesirable aesthetic aspect (Liu et al., 2011; Rai et al., 2005). Therefore, increasing attention is paid to the removal of dyes from aqueous systems in the last few years. Various methods including aerobic or anaerobic digestion, coagulation, advanced oxidation processes (AOPs) and adsorption (Khataee et al., 2010a, 2010b; Khataee and Kasiri, 2010) have been developed to remove color from dye-containing effluents. These methods vary in effectiveness, cost and environmental impact. Adsorption is a highly efficient and relatively low-cost technique for the treatment of dye contaminated waters. Many kinds of adsorbents such as activated carbon (Aber et al., 2009), silica (Parida et al., 2006), natural polymeric materials (Crini, 2005) and sewage sludge (Rio et al., 2005) have been developed for various applications. However, their operating costs are high. This has led to research for alternative adsorbents. Adsorbents with high specific surface areas, such as mesoporous materials (Juang et al., 2006; Tamai et al., 1999; Yan et al., 2006), microporous materials (Maffei et al., 2006; Wu et al., 2005), carbon nanotubes (Fugestu et al., 2004; Yan et al., 2005) and titanate nanotubes (Lee et al., 2008) have been applied to decrease the dosage of
⁎ Corresponding author. Tel.: + 98 4113393853; fax: + 98 4113294626. E-mail address: [email protected] (G. Kiani). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.07.008
adsorbent. Most recently, various clay minerals tested as unconventional adsorbents for the removal of dyes from aqueous solutions due to their low cost and abundance, and higher specific surface areas (Liu and Zhang, 2007). Clay materials with planar morphology (Lopez Arbeloa et al., 2002; Orthman et al., 2003) and acicular morphology (Alkan et al., 2007; Tahir and Rauf, 2006) have been increasingly gaining attention because they are cheaper than activated carbons and they also have high specific surface area. Natural materials (such as biomass, clay minerals, etc.) and certain waste materials (such as carbon slurry, bottom ash, deoiled soya, red mud, etc.) are classified as low-cost adsorbents, and have been reported in previous studies (Mittal et al., 2005). Among them, clays are considered as promising alternatives. Halloysite is a mineral of the kaolin group, which forms hollow tubular crystals (halloysite nanotubes-HNTs) with a high aspect ratio. The length of HNTs varies in the range of 1–15 μm. HNTs have inner and outer diameters of 10–30 nm and 50–70 nm respectively, depending on the conditions of their formation. The chemical properties of the HNTs outer surface are similar to the properties of SiO2, while the properties of the inner surface can be compared with those of Al2O3 and it can be used as adsorbents (Shchukin et al., 2005). Malachite Green (MG) is removed with difficulty from aqueous solutions. If the solution containing MG is discharged into streams it affects the aquatic life and causes detrimental effects in the liver, gill, kidney, intestine and gonads. In humans, it may cause irritation to the gastrointestinal tract upon ingestion. Skin contact with MG causes irritation redness and pain. Contact with the eye can lead to permanent injury in humans and laboratory animals (Khataee et al., 2009; Kumar et al., 2005).
G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Thus, the present study was devoted to test the ability of HNTs to remove MG which is commonly used in leather, paper and textile industries. The effects of adsorbent dose, initial pH, temperature, initial MG concentration and contact time were investigated. The equilibrium, kinetic data and thermodynamic parameters were processed to understand the adsorption mechanism of MG onto HNTs. 2. Experimental details 2.1. Materials Halloysite (premium grade) was obtained from New Zealand China Clays Ltd (New Zealand). Halloysite was first sieved (125 μm mesh) to remove granules to obtain 42% fine powder (HNT). A TEM image was obtained with a JEOL JEM-2100 (Japan) and Fourier Transform Infrared (FT-IR) spectroscopy was performed on ABB, MB3000 (Canada) spectrometer. X-ray diffraction (XRD) study was carried out with D500 Siemens, X-Ray diffractometer. Analytical reagent grade Malachite Green (MG, C23H25N2Cl) was purchased from Merck Co. (Germany). It has a maximum visible absorbance at a wavelength of 621 nm. MG stock solution was prepared by dissolving an accurately weighed amount of MG in distilled water to achieve a concentration of 500 mg L −1 and subsequently diluted to the required concentrations. 2.2. Adsorption experiments The 100-mL samples containing known concentration of MG and HNTs were added to Erlenmeyer flasks. The flasks were stirred with a rate of 300 rpm in a water bath (Pars Azma, Iran). The pH of the dye solutions was adjusted in the range of 6–10 with 0.1 N HCl or 0.1 N NaOH by using a WTW (pH720, Germany) pH-meter with a combined pH electrode. The pH-meter was standardized with buffers before measurement. At the end of the adsorption period, the solution was centrifuged (Hettich, EBA20, Germany) for 10 min at 5000 rpm. The remaining MG was determined spectrophotometically (PG Instruments T70 Plus, United Kingdom) at λmax = 621 nm using a calibration curve. The amounts of dye adsorbed on HNTs at any time, t, were calculated from the concentrations in solutions before and after adsorption. At any time, the amount of MG adsorbed (mg g−1) (qt), by HNTs was calculated from the mass balance equation as follows: qt = V ðC0 −Ct Þ = W
ð1Þ
where qt is the amount of adsorbed dye on HNTs at any time (mg g −1); C0 and Ct are the concentration of MG before adsorption and after contact time t (mg L −1), respectively; V is the volume of MG solution (L), and W is the mass of HNTs sample used (g) (Alkan et al., 2007; Doğan and Alkan, 2003). In order to determine the best kinetic model which fits the adsorption experimental data, the pseudo-first-order, pseudo-secondorder and intraparticle diffusion models were examined. The linear forms of these models can be described as follows. lnðqe −qt Þ = ln qe −k1 t
ð2Þ
t 1 1 = + t qt qe k2 q2e
ð3Þ
qt = ki t
1=2
+C
ð4Þ
where qe is the amount of the adsorbed pollutant at equilibrium per unit mass of the adsorbent (mg g −1), k1 (min −1), k2 (g mg −1 min −1) and ki (mg g −1 min −1/2) are the rate constants of the adsorption in pseudo-first-order (Eq. (2)), pseudo-second-order (Eq. (3)) and
35
intraparticle diffusion models (Eq. (4)), respectively (Doğan et al., 2004; Taty-Costodes et al., 2003; Vadivelan and Kumar, 2005). 3. Results and discussion The halloysite is predominately tubular with a length of 1–2 μm and an inner diameter of 20–30 nm (Fig. 1a). Fig. 1b shows the X-ray diffraction spectra of HNT and MG adsorbed on HNTs. The diffraction peaks indicates a small amount of quartz in the powder (Fig. 1b). As it can be seen, there is no evident change in the diffraction pattern of HNT after adsorption of MG, suggesting that the main mechanism of removal is adsorption of MG onto the surface of HNTs rather than intercalation in the interlayer of the HNTs. 3.1. Adsorption rate 3.1.1. Effect of contact time and initial dye concentration The effect of initial dye concentration and contact time on the adsorption rate of MG onto HNTs is shown in Fig. 2a. The amount of MG adsorbed at equilibrium increases from 0.2463 to 0.2740 m mol g−1 (89.8–99.9 mg g−1) by increasing the initial MG concentration from 20 to 100 mg L −1. Therefore, adsorption increases with increasing initial dye concentration. The results show that uptake of dye is rapid for the first 20 min reaching equilibrium within about 30 min. 3.1.2. Effect of pH As can be seen in Fig. 2b, the amount of MG adsorbed increased with increasing pH of the solution. MG produces molecular cations in aqueous solution. The adsorption of MG on the HNTs surface is primarily influenced by the surface charge of the adsorbent. The surface of HNTs is positively charged below pH 2.7 while it acquires a negative charge above this pH (Gupta and SainiAli, 2007). The surface of HNTs bears abundant Si–OH and Al–OH, which can ionize in the following form: þ
Hþ
OH−
Si=A1 OH2 ← Si=A1 OH → Si=A1 O
−
Since the surface is mainly occupied by silica, the surface charge will be negatively charged over a wide range of pH values. As a result, HNTs tend to readily bind cations in aqueous solution. 3.1.3. Effect of adsorbent concentration Increasing the adsorbent concentration enhances the amount of adsorbed dye on HNTs (Fig. 3a). Increased adsorbent concentration implies a greater number of possible binding sites. The adsorbent dose was maintained at 0.2 g in all the subsequent experiments, which was considered to be sufficient for the removal of MG. At adsorbent doses greater than 0.3 g, there was little change in either the rate of attaining adsorption equilibrium of the MG, or the amount of adsorbed dye on HNTs. 3.1.4. Effect of temperature The adsorption of MG dye has increased with increasing temperature (Fig. 3b). This is attributed to the increasing rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles, owing to the decrease in the viscosity of the solution, and change the equilibrium capacity of the adsorbent for a particular adsorbate. In addition, a change of the temperature modifies the equilibrium capacity of the adsorbent for a particular adsorbate (Doğan et al., 2004). The equilibrium adsorption capacity was clearly affected by temperature, with the amount of MG adsorbed increasing from 89.25 to 98.2 mg g− 1 when the temperature was raised from 20 to 60 °C. Moreover, increased temperature led to an increase in the rate of dye adsorption, implying a kinetically controlling process, similar to other studies (Alkan et al., 2004). The mobility of
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G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Fig. 1. (a) TEM images of halloysite nanotubes and (b) X-ray diffractograms of I) HNT, II) MG absorbed onto HNTs.
Fig. 2. (a) Effect of contact time and initial dye concentration on adsorption of MG onto HNTs from aqueous solutions (T = 20 °C, pH = 8.5) and (b) effect of contact time and initial pH on the removal rate of MG onto HNTs from aqueous solutions (T = 20 °C, [MG]0 = 20 mg L−1, HNTs = 0.2 g).
Fig. 3. (a) Effect of adsorbent dose on adsorption of MG onto HNTs (T = 20 °C, [MG]0 = 20 mg L−1, pH = 8.5) and (b) effect of contact time and temperature on the removal rate of MG onto HNTs from aqueous solutions ([MG]0 = 20 mg L−1, pH = 8.5, HNTS = 0.2 g).
G. Kiani et al. / Applied Clay Science 54 (2011) 34–39 Table 1 Adsorption kinetic parameters of Malachite Green onto HNTs. Temperature (°C) Pseudo-first-order k1, min−1 R2 Pseudo-second-order k2, g/mg min R2 Intraparticle diffusion ki, mg g-1 min1/2 C R2
Table 2 The adsorption parameters of Langmuir at different temperatures.
20
40
60
Temperature (°C)
KL ( L g− 1)
aL (L mg− 1)
RL2
0.0031 0.9100
0.0131 0.8900
0.0155 0.9050
20 40 60
71.42 250 630
0.71 0.25 0.1
0.993 0.983 0.993
9.136 0.9999
7.0475 1
3.8047 1
0.4637 86.406 0.8438
0.3777 95.044 0.9226
0.2670 96.721 0.8292
molecules increases generally with a rise in temperature, thereby facilitating the formation of surface monolayers. The adsorption capacity of MG onto HNTs increases with increase in temperature, suggesting that adsorption is endothermic. 3.2. Kinetics of adsorption From the study of the kinetics of adsorption results are shown in Table 1. The pseudo-second order model well described the experimental data (R 2 N0.99). Similar results have been observed in the adsorption of methylene blue onto HNTs (Zhao and Liu, 2008). In contrast the pseudo-first order model and the intraparticle diffusion models do not describe the adsorption of MG (Fig. 4a). 3.3. Adsorption isotherms The linear form of Langmuir expression is represented by Eq. (5) (El Qada et al., 2006): Ce 1 a = + L Ce KL qe KL
37
ð5Þ
where qe is the solid phase equilibrium concentration (mg g− 1); Ce is the liquid phase equilibrium concentration (mg L − 1), KL and aL are Langmuir constants obtained from the intercept and slope of the straight line of the plot Ce/qevs. Ce. The aL constant is related to the free energy or net enthalpy of adsorption (L mg− 1) (aL α e − ΔH/RT) (Rytwo et al., 2006), and KL is the Langmuir equilibrium constant (L g− 1). Fig. 4b shows the Langmuir plot for the adsorption of MG onto HNTs at 20, 40, and 60 °C. The Langmuir model fits well the experimental data indicating monolayer coverage of the MG at the outer surface of the HNTs particles, and with maximum adsorption capacity (Table 2).
3.4. Adsorption thermodynamics The amount of MG adsorbed at equilibrium at 293.15, 313.15 and 333.15 K was used to obtain thermodynamic parameters. Changes in the free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated using Eqs. (6) and (7): ΔG˚ = −RT lnKL
ð6Þ
ΔG˚ = ΔH ˚–TΔS ˚
ð7Þ
where KL is Langmuir constant when concentration terms are expressed in L mol−1, R is the universal gas constant (8.314 J mol−1 K −1) and T (K) is the temperature. The ΔH° and ΔS° values can be calculated from the slope and intercept of the plot of ΔG°vs.T. The free energy changes (ΔG°) are negative, indicating that adsorption is spontaneous. The decrease in ΔG° value with increasing temperature reveals that adsorption of MG onto HNTs becomes more favorable at higher temperature. The positive value of ΔH° further confirms that adsorption is endothermic. The positive value of ΔS° infers increase of randomness at solid–solution interface during the adsorption of MG on the active sites of HNTs. Moreover, positive value of ΔS° reflects affinity of the adsorbent for MG. lnKL = −ΔH ˚=RT + ΔS ˚=R
ð8Þ
On the basis of a plot of lnKLvs.T − 1 (Eq. (8)), ΔH° can be estimated from the slope and ΔS° from the intercept of what should be a straight line passing through the points. Fig. 5a shows just such a plot with a correlation coefficient of 0.993. The obtained ΔH° and ΔS° values are +18.49 kJ mol − 1 and +78.31 J K − 1, respectively, while the ΔG° is −4.16 kJ mol − 1. This feature may be an indication for monolayer adsorption. The positive value of ΔS° corresponds to an increased degree of freedom in the system as a result of adsorption of the MG molecules (Rytwo et al., 2006). This may reflect the release of hydrated inorganic cations from the HNTs causing an overall increase in entropy analogous, to the dissolution of solid sodium chloride (Seki and Yurdakoc, 2006). Alternatively, structural changes take place as a result of interactions
Fig. 4. (a) Pseudo-second-order plots for the adsorption of MG onto HNTs at various temperatures ([MG]0 = 20 mg L−1, HNTs = 0.2 g, pH = 8.5) and (b) Langmuir plot of adsorption of MG onto HNTs at various temperatures (pH = 8.5, HNTs = 0.2 g, [MG]0 = 20, 60, 100 and 140 mg L−1).
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G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Fig. 5. (a) Plot of lnKL vs. T− 1: estimation of thermodynamic parameters for the adsorption of MG onto HNTs and (b) plot of lnksm vs. T −1: estimation of the activation energy, Ea, for the adsorption of MG onto HNTs.
of MG molecules with active groups in the HNTs surface (El Qada et al., 2006). Finally the increased randomness at the solid–solution interface may reflect the extra translational entropy gained by the solvent molecules previously adsorbed on the clay but displaced by the adsorbate species (Tahir and Rauf, 2006). Whatever the explanation for the positive entropy change, it leads to a negative free energy change. Adsorption processes with ΔG° values in the −20 to 0 kJmol − 1 ranges indicate spontaneous physical processes, while those with ΔG° values − 80 to −400 kJ mol − 1 indicate chemisorption (Rytwo et al., 2006). As ΔG° changed from −4.166 to − 4.326 kJ mol − 1 with increase of temperature from 20 to 60 °C, it can be concluded that the adsorption is dominated by physisorption (Tahir and Rauf, 2006), in accordance with the finding that the adsorption is rapid and more spontaneous at higher temperature. Spontaneous adsorption processes are a common feature of many similar studies (Doğan et al., 2006). 3.5. Activation parameters From three of the pseudo-second-order rate constants, k2 (here ksm), each at a different temperature, and by using the Arrhenius
equation (Eq. (9)), it is possible to gain some insight into the type of adsorption. lnksm = ln A–Ea = RT
ð9Þ
Here Ea is the activation energy (J mol −1), ksm the pseudo-secondorder rate constant for adsorption (g mol −1 s −1), A the temperatureindependent Arrhenius factor (g mol −1 s −1), R the gas constant (8.314 J K −1 mol −1), and T the solution temperature (K). The slope of the plot of lnksm vs. T −1 can then be used to evaluate Ea. Low activation energies (5–40 kJ mol −1) are characteristic of physical adsorption, while higher ones (40–800 kJ mol −1) suggest chemisorption (Doğan et al., 2006). In the present study Ea = + 18.28 kJ mol −1 for the adsorption of MG onto HNTs (Fig. 5b), indicating that the adsorption has a low potential barrier corresponding therefore to physisorption. The value is consistent with those found in the literature for the adsorption of dyes onto other adsorbents, e.g., maxilon blue GRL onto sepiolite (34 kJ mol −1) (Doğan et al., 2006), and methylene blue onto perlite (14 kJ mol −1) (Doğan et al., 2004). The clays in nano-scale usually have excellent dispersibility in aqueous-phase and the suspensions have good stability because of the hydrophilic nature of clay minerals. In this work, the MG was adsorbed on halloysite nanotubes aggregated together to millimeter scale particles and were deposited completely within 30 min (Fig. 6). It might be due to the hydrophobic surface of the Malachite Green. This is also an advantage of the application of the natural halloysite nanotubes as a low-cost unconventional nano-adsorbent of MG.
4. Conclusions
Fig. 6. The photos of the MG solution before (left) and after (right) adsorption with HNTs.
Halloysite nanotubes have been proved to be an effective nanoadsorbent for the removal of cationic dye via adsorption from aqueous solution. The equilibrium adsorption was reached within 30 min. A maximum adsorption capacity of 99.6 mg g −1 of Malachite Green was achieved. The adsorption is highly dependent on concentration, pH and temperature. The adsorption of MG by HNTs obeyed pseudo-second-order kinetics with activation energy + 18.28 kJ mol −1, suggesting that the process was physisorption. The best-fit adsorption isotherm was achieved with the Langmuir model, indicating that homogeneous adsorption occurred. The negative values of ΔG° and positive value of ΔH° show that the adsorption is a spontaneous and endothermic. The Malachite Green adsorbed on halloysite nanotubes was aggregated and deposited within 30 min. It is expected that the halloysite nanotubes can be used efficiently as low-cost unconventional nano-adsorbent for the removal of dyes from aqueous solutions.
G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
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Applied Clay Science j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l a y
Research paper
Adsorption studies on the removal of Malachite Green from aqueous solutions onto halloysite nanotubes Gholamreza Kiani a,⁎, Mohammad Dostali a, Ali Rostami a, Ali R. Khataee b a b
School of Engineering-Emerging Technologies, University of Tabriz, Tabriz, Iran Department of Applied Chemistry, Faculty of Chemistry, University of Tabriz, Tabriz, Iran
a r t i c l e
i n f o
Article history: Received 16 January 2011 Received in revised form 28 May 2011 Accepted 10 July 2011 Available online 19 August 2011 Keywords: Halloysite nanotubes Nano-adsorbent Textile dye Kinetic model Thermodynamic parameter
a b s t r a c t Halloysite nanotubes (HNTs) were used as nano-adsorbents for removal of the cationic dye, Malachite Green (MG), from aqueous solutions. The adsorption of the dye was studied with batch experiments. The natural HNTs used as adsorbent in this work were initially characterized by FT-IR and TEM. The effects of adsorbent dose, initial pH, temperature, initial dye concentration and contact time were investigated. Adsorption increased with increasing adsorbent dose, initial pH, and temperature. Equilibrium was rapidly attained after 30 min of contact time. Pseudofirst-order, pseudo-second-order and intraparticle diffusion models were considered to evaluate the rate parameters. The adsorption followed pseudo-second-order kinetic model with correlation coefficients greater than 0.999. The factors controlling adsorption process were also calculated and discussed. The maximum adsorption capacity of 99.6 mg g −1 of MG was achieved in pH = 9.5. Thermodynamic parameters of ΔG°, ΔH° and ΔS° indicated the adsorption process was spontaneous and endothermic. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthetic dyes are widely used in various fields, such as leather, paper, textile, etc., and their discharge into natural waters causes environmental problems, related to their carcinogenicity, toxicity to aquatic life and undesirable aesthetic aspect (Liu et al., 2011; Rai et al., 2005). Therefore, increasing attention is paid to the removal of dyes from aqueous systems in the last few years. Various methods including aerobic or anaerobic digestion, coagulation, advanced oxidation processes (AOPs) and adsorption (Khataee et al., 2010a, 2010b; Khataee and Kasiri, 2010) have been developed to remove color from dye-containing effluents. These methods vary in effectiveness, cost and environmental impact. Adsorption is a highly efficient and relatively low-cost technique for the treatment of dye contaminated waters. Many kinds of adsorbents such as activated carbon (Aber et al., 2009), silica (Parida et al., 2006), natural polymeric materials (Crini, 2005) and sewage sludge (Rio et al., 2005) have been developed for various applications. However, their operating costs are high. This has led to research for alternative adsorbents. Adsorbents with high specific surface areas, such as mesoporous materials (Juang et al., 2006; Tamai et al., 1999; Yan et al., 2006), microporous materials (Maffei et al., 2006; Wu et al., 2005), carbon nanotubes (Fugestu et al., 2004; Yan et al., 2005) and titanate nanotubes (Lee et al., 2008) have been applied to decrease the dosage of
⁎ Corresponding author. Tel.: + 98 4113393853; fax: + 98 4113294626. E-mail address: [email protected] (G. Kiani). 0169-1317/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2011.07.008
adsorbent. Most recently, various clay minerals tested as unconventional adsorbents for the removal of dyes from aqueous solutions due to their low cost and abundance, and higher specific surface areas (Liu and Zhang, 2007). Clay materials with planar morphology (Lopez Arbeloa et al., 2002; Orthman et al., 2003) and acicular morphology (Alkan et al., 2007; Tahir and Rauf, 2006) have been increasingly gaining attention because they are cheaper than activated carbons and they also have high specific surface area. Natural materials (such as biomass, clay minerals, etc.) and certain waste materials (such as carbon slurry, bottom ash, deoiled soya, red mud, etc.) are classified as low-cost adsorbents, and have been reported in previous studies (Mittal et al., 2005). Among them, clays are considered as promising alternatives. Halloysite is a mineral of the kaolin group, which forms hollow tubular crystals (halloysite nanotubes-HNTs) with a high aspect ratio. The length of HNTs varies in the range of 1–15 μm. HNTs have inner and outer diameters of 10–30 nm and 50–70 nm respectively, depending on the conditions of their formation. The chemical properties of the HNTs outer surface are similar to the properties of SiO2, while the properties of the inner surface can be compared with those of Al2O3 and it can be used as adsorbents (Shchukin et al., 2005). Malachite Green (MG) is removed with difficulty from aqueous solutions. If the solution containing MG is discharged into streams it affects the aquatic life and causes detrimental effects in the liver, gill, kidney, intestine and gonads. In humans, it may cause irritation to the gastrointestinal tract upon ingestion. Skin contact with MG causes irritation redness and pain. Contact with the eye can lead to permanent injury in humans and laboratory animals (Khataee et al., 2009; Kumar et al., 2005).
G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Thus, the present study was devoted to test the ability of HNTs to remove MG which is commonly used in leather, paper and textile industries. The effects of adsorbent dose, initial pH, temperature, initial MG concentration and contact time were investigated. The equilibrium, kinetic data and thermodynamic parameters were processed to understand the adsorption mechanism of MG onto HNTs. 2. Experimental details 2.1. Materials Halloysite (premium grade) was obtained from New Zealand China Clays Ltd (New Zealand). Halloysite was first sieved (125 μm mesh) to remove granules to obtain 42% fine powder (HNT). A TEM image was obtained with a JEOL JEM-2100 (Japan) and Fourier Transform Infrared (FT-IR) spectroscopy was performed on ABB, MB3000 (Canada) spectrometer. X-ray diffraction (XRD) study was carried out with D500 Siemens, X-Ray diffractometer. Analytical reagent grade Malachite Green (MG, C23H25N2Cl) was purchased from Merck Co. (Germany). It has a maximum visible absorbance at a wavelength of 621 nm. MG stock solution was prepared by dissolving an accurately weighed amount of MG in distilled water to achieve a concentration of 500 mg L −1 and subsequently diluted to the required concentrations. 2.2. Adsorption experiments The 100-mL samples containing known concentration of MG and HNTs were added to Erlenmeyer flasks. The flasks were stirred with a rate of 300 rpm in a water bath (Pars Azma, Iran). The pH of the dye solutions was adjusted in the range of 6–10 with 0.1 N HCl or 0.1 N NaOH by using a WTW (pH720, Germany) pH-meter with a combined pH electrode. The pH-meter was standardized with buffers before measurement. At the end of the adsorption period, the solution was centrifuged (Hettich, EBA20, Germany) for 10 min at 5000 rpm. The remaining MG was determined spectrophotometically (PG Instruments T70 Plus, United Kingdom) at λmax = 621 nm using a calibration curve. The amounts of dye adsorbed on HNTs at any time, t, were calculated from the concentrations in solutions before and after adsorption. At any time, the amount of MG adsorbed (mg g−1) (qt), by HNTs was calculated from the mass balance equation as follows: qt = V ðC0 −Ct Þ = W
ð1Þ
where qt is the amount of adsorbed dye on HNTs at any time (mg g −1); C0 and Ct are the concentration of MG before adsorption and after contact time t (mg L −1), respectively; V is the volume of MG solution (L), and W is the mass of HNTs sample used (g) (Alkan et al., 2007; Doğan and Alkan, 2003). In order to determine the best kinetic model which fits the adsorption experimental data, the pseudo-first-order, pseudo-secondorder and intraparticle diffusion models were examined. The linear forms of these models can be described as follows. lnðqe −qt Þ = ln qe −k1 t
ð2Þ
t 1 1 = + t qt qe k2 q2e
ð3Þ
qt = ki t
1=2
+C
ð4Þ
where qe is the amount of the adsorbed pollutant at equilibrium per unit mass of the adsorbent (mg g −1), k1 (min −1), k2 (g mg −1 min −1) and ki (mg g −1 min −1/2) are the rate constants of the adsorption in pseudo-first-order (Eq. (2)), pseudo-second-order (Eq. (3)) and
35
intraparticle diffusion models (Eq. (4)), respectively (Doğan et al., 2004; Taty-Costodes et al., 2003; Vadivelan and Kumar, 2005). 3. Results and discussion The halloysite is predominately tubular with a length of 1–2 μm and an inner diameter of 20–30 nm (Fig. 1a). Fig. 1b shows the X-ray diffraction spectra of HNT and MG adsorbed on HNTs. The diffraction peaks indicates a small amount of quartz in the powder (Fig. 1b). As it can be seen, there is no evident change in the diffraction pattern of HNT after adsorption of MG, suggesting that the main mechanism of removal is adsorption of MG onto the surface of HNTs rather than intercalation in the interlayer of the HNTs. 3.1. Adsorption rate 3.1.1. Effect of contact time and initial dye concentration The effect of initial dye concentration and contact time on the adsorption rate of MG onto HNTs is shown in Fig. 2a. The amount of MG adsorbed at equilibrium increases from 0.2463 to 0.2740 m mol g−1 (89.8–99.9 mg g−1) by increasing the initial MG concentration from 20 to 100 mg L −1. Therefore, adsorption increases with increasing initial dye concentration. The results show that uptake of dye is rapid for the first 20 min reaching equilibrium within about 30 min. 3.1.2. Effect of pH As can be seen in Fig. 2b, the amount of MG adsorbed increased with increasing pH of the solution. MG produces molecular cations in aqueous solution. The adsorption of MG on the HNTs surface is primarily influenced by the surface charge of the adsorbent. The surface of HNTs is positively charged below pH 2.7 while it acquires a negative charge above this pH (Gupta and SainiAli, 2007). The surface of HNTs bears abundant Si–OH and Al–OH, which can ionize in the following form: þ
Hþ
OH−
Si=A1 OH2 ← Si=A1 OH → Si=A1 O
−
Since the surface is mainly occupied by silica, the surface charge will be negatively charged over a wide range of pH values. As a result, HNTs tend to readily bind cations in aqueous solution. 3.1.3. Effect of adsorbent concentration Increasing the adsorbent concentration enhances the amount of adsorbed dye on HNTs (Fig. 3a). Increased adsorbent concentration implies a greater number of possible binding sites. The adsorbent dose was maintained at 0.2 g in all the subsequent experiments, which was considered to be sufficient for the removal of MG. At adsorbent doses greater than 0.3 g, there was little change in either the rate of attaining adsorption equilibrium of the MG, or the amount of adsorbed dye on HNTs. 3.1.4. Effect of temperature The adsorption of MG dye has increased with increasing temperature (Fig. 3b). This is attributed to the increasing rate of diffusion of the adsorbate molecules across the external boundary layer and in the internal pores of the adsorbent particles, owing to the decrease in the viscosity of the solution, and change the equilibrium capacity of the adsorbent for a particular adsorbate. In addition, a change of the temperature modifies the equilibrium capacity of the adsorbent for a particular adsorbate (Doğan et al., 2004). The equilibrium adsorption capacity was clearly affected by temperature, with the amount of MG adsorbed increasing from 89.25 to 98.2 mg g− 1 when the temperature was raised from 20 to 60 °C. Moreover, increased temperature led to an increase in the rate of dye adsorption, implying a kinetically controlling process, similar to other studies (Alkan et al., 2004). The mobility of
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G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Fig. 1. (a) TEM images of halloysite nanotubes and (b) X-ray diffractograms of I) HNT, II) MG absorbed onto HNTs.
Fig. 2. (a) Effect of contact time and initial dye concentration on adsorption of MG onto HNTs from aqueous solutions (T = 20 °C, pH = 8.5) and (b) effect of contact time and initial pH on the removal rate of MG onto HNTs from aqueous solutions (T = 20 °C, [MG]0 = 20 mg L−1, HNTs = 0.2 g).
Fig. 3. (a) Effect of adsorbent dose on adsorption of MG onto HNTs (T = 20 °C, [MG]0 = 20 mg L−1, pH = 8.5) and (b) effect of contact time and temperature on the removal rate of MG onto HNTs from aqueous solutions ([MG]0 = 20 mg L−1, pH = 8.5, HNTS = 0.2 g).
G. Kiani et al. / Applied Clay Science 54 (2011) 34–39 Table 1 Adsorption kinetic parameters of Malachite Green onto HNTs. Temperature (°C) Pseudo-first-order k1, min−1 R2 Pseudo-second-order k2, g/mg min R2 Intraparticle diffusion ki, mg g-1 min1/2 C R2
Table 2 The adsorption parameters of Langmuir at different temperatures.
20
40
60
Temperature (°C)
KL ( L g− 1)
aL (L mg− 1)
RL2
0.0031 0.9100
0.0131 0.8900
0.0155 0.9050
20 40 60
71.42 250 630
0.71 0.25 0.1
0.993 0.983 0.993
9.136 0.9999
7.0475 1
3.8047 1
0.4637 86.406 0.8438
0.3777 95.044 0.9226
0.2670 96.721 0.8292
molecules increases generally with a rise in temperature, thereby facilitating the formation of surface monolayers. The adsorption capacity of MG onto HNTs increases with increase in temperature, suggesting that adsorption is endothermic. 3.2. Kinetics of adsorption From the study of the kinetics of adsorption results are shown in Table 1. The pseudo-second order model well described the experimental data (R 2 N0.99). Similar results have been observed in the adsorption of methylene blue onto HNTs (Zhao and Liu, 2008). In contrast the pseudo-first order model and the intraparticle diffusion models do not describe the adsorption of MG (Fig. 4a). 3.3. Adsorption isotherms The linear form of Langmuir expression is represented by Eq. (5) (El Qada et al., 2006): Ce 1 a = + L Ce KL qe KL
37
ð5Þ
where qe is the solid phase equilibrium concentration (mg g− 1); Ce is the liquid phase equilibrium concentration (mg L − 1), KL and aL are Langmuir constants obtained from the intercept and slope of the straight line of the plot Ce/qevs. Ce. The aL constant is related to the free energy or net enthalpy of adsorption (L mg− 1) (aL α e − ΔH/RT) (Rytwo et al., 2006), and KL is the Langmuir equilibrium constant (L g− 1). Fig. 4b shows the Langmuir plot for the adsorption of MG onto HNTs at 20, 40, and 60 °C. The Langmuir model fits well the experimental data indicating monolayer coverage of the MG at the outer surface of the HNTs particles, and with maximum adsorption capacity (Table 2).
3.4. Adsorption thermodynamics The amount of MG adsorbed at equilibrium at 293.15, 313.15 and 333.15 K was used to obtain thermodynamic parameters. Changes in the free energy (ΔG°), enthalpy (ΔH°) and entropy (ΔS°) were calculated using Eqs. (6) and (7): ΔG˚ = −RT lnKL
ð6Þ
ΔG˚ = ΔH ˚–TΔS ˚
ð7Þ
where KL is Langmuir constant when concentration terms are expressed in L mol−1, R is the universal gas constant (8.314 J mol−1 K −1) and T (K) is the temperature. The ΔH° and ΔS° values can be calculated from the slope and intercept of the plot of ΔG°vs.T. The free energy changes (ΔG°) are negative, indicating that adsorption is spontaneous. The decrease in ΔG° value with increasing temperature reveals that adsorption of MG onto HNTs becomes more favorable at higher temperature. The positive value of ΔH° further confirms that adsorption is endothermic. The positive value of ΔS° infers increase of randomness at solid–solution interface during the adsorption of MG on the active sites of HNTs. Moreover, positive value of ΔS° reflects affinity of the adsorbent for MG. lnKL = −ΔH ˚=RT + ΔS ˚=R
ð8Þ
On the basis of a plot of lnKLvs.T − 1 (Eq. (8)), ΔH° can be estimated from the slope and ΔS° from the intercept of what should be a straight line passing through the points. Fig. 5a shows just such a plot with a correlation coefficient of 0.993. The obtained ΔH° and ΔS° values are +18.49 kJ mol − 1 and +78.31 J K − 1, respectively, while the ΔG° is −4.16 kJ mol − 1. This feature may be an indication for monolayer adsorption. The positive value of ΔS° corresponds to an increased degree of freedom in the system as a result of adsorption of the MG molecules (Rytwo et al., 2006). This may reflect the release of hydrated inorganic cations from the HNTs causing an overall increase in entropy analogous, to the dissolution of solid sodium chloride (Seki and Yurdakoc, 2006). Alternatively, structural changes take place as a result of interactions
Fig. 4. (a) Pseudo-second-order plots for the adsorption of MG onto HNTs at various temperatures ([MG]0 = 20 mg L−1, HNTs = 0.2 g, pH = 8.5) and (b) Langmuir plot of adsorption of MG onto HNTs at various temperatures (pH = 8.5, HNTs = 0.2 g, [MG]0 = 20, 60, 100 and 140 mg L−1).
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G. Kiani et al. / Applied Clay Science 54 (2011) 34–39
Fig. 5. (a) Plot of lnKL vs. T− 1: estimation of thermodynamic parameters for the adsorption of MG onto HNTs and (b) plot of lnksm vs. T −1: estimation of the activation energy, Ea, for the adsorption of MG onto HNTs.
of MG molecules with active groups in the HNTs surface (El Qada et al., 2006). Finally the increased randomness at the solid–solution interface may reflect the extra translational entropy gained by the solvent molecules previously adsorbed on the clay but displaced by the adsorbate species (Tahir and Rauf, 2006). Whatever the explanation for the positive entropy change, it leads to a negative free energy change. Adsorption processes with ΔG° values in the −20 to 0 kJmol − 1 ranges indicate spontaneous physical processes, while those with ΔG° values − 80 to −400 kJ mol − 1 indicate chemisorption (Rytwo et al., 2006). As ΔG° changed from −4.166 to − 4.326 kJ mol − 1 with increase of temperature from 20 to 60 °C, it can be concluded that the adsorption is dominated by physisorption (Tahir and Rauf, 2006), in accordance with the finding that the adsorption is rapid and more spontaneous at higher temperature. Spontaneous adsorption processes are a common feature of many similar studies (Doğan et al., 2006). 3.5. Activation parameters From three of the pseudo-second-order rate constants, k2 (here ksm), each at a different temperature, and by using the Arrhenius
equation (Eq. (9)), it is possible to gain some insight into the type of adsorption. lnksm = ln A–Ea = RT
ð9Þ
Here Ea is the activation energy (J mol −1), ksm the pseudo-secondorder rate constant for adsorption (g mol −1 s −1), A the temperatureindependent Arrhenius factor (g mol −1 s −1), R the gas constant (8.314 J K −1 mol −1), and T the solution temperature (K). The slope of the plot of lnksm vs. T −1 can then be used to evaluate Ea. Low activation energies (5–40 kJ mol −1) are characteristic of physical adsorption, while higher ones (40–800 kJ mol −1) suggest chemisorption (Doğan et al., 2006). In the present study Ea = + 18.28 kJ mol −1 for the adsorption of MG onto HNTs (Fig. 5b), indicating that the adsorption has a low potential barrier corresponding therefore to physisorption. The value is consistent with those found in the literature for the adsorption of dyes onto other adsorbents, e.g., maxilon blue GRL onto sepiolite (34 kJ mol −1) (Doğan et al., 2006), and methylene blue onto perlite (14 kJ mol −1) (Doğan et al., 2004). The clays in nano-scale usually have excellent dispersibility in aqueous-phase and the suspensions have good stability because of the hydrophilic nature of clay minerals. In this work, the MG was adsorbed on halloysite nanotubes aggregated together to millimeter scale particles and were deposited completely within 30 min (Fig. 6). It might be due to the hydrophobic surface of the Malachite Green. This is also an advantage of the application of the natural halloysite nanotubes as a low-cost unconventional nano-adsorbent of MG.
4. Conclusions
Fig. 6. The photos of the MG solution before (left) and after (right) adsorption with HNTs.
Halloysite nanotubes have been proved to be an effective nanoadsorbent for the removal of cationic dye via adsorption from aqueous solution. The equilibrium adsorption was reached within 30 min. A maximum adsorption capacity of 99.6 mg g −1 of Malachite Green was achieved. The adsorption is highly dependent on concentration, pH and temperature. The adsorption of MG by HNTs obeyed pseudo-second-order kinetics with activation energy + 18.28 kJ mol −1, suggesting that the process was physisorption. The best-fit adsorption isotherm was achieved with the Langmuir model, indicating that homogeneous adsorption occurred. The negative values of ΔG° and positive value of ΔH° show that the adsorption is a spontaneous and endothermic. The Malachite Green adsorbed on halloysite nanotubes was aggregated and deposited within 30 min. It is expected that the halloysite nanotubes can be used efficiently as low-cost unconventional nano-adsorbent for the removal of dyes from aqueous solutions.
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