Adsorption of Cu(II) ions from aqueous solutions on modified chrysotile: Thermodynamic and kinetic studies

Adsorption of Cu(II) ions from aqueous solutions on modified chrysotile: Thermodynamic and kinetic studies

Applied Clay Science 80-81 (2013) 38–45 Contents lists available at SciVerse ScienceDirect Applied Clay Science journal homepage: www.elsevier.com/l...

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Applied Clay Science 80-81 (2013) 38–45

Contents lists available at SciVerse ScienceDirect

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

Research paper

Adsorption of Cu(II) ions from aqueous solutions on modified chrysotile: Thermodynamic and kinetic studies Kun Liu a,⁎, Binnan Zhu a, Qiming Feng a, Qian Wang a, Tao Duan b, Leming Ou a, Guofan Zhang a, Yiping Lu a a b

School of Minerals Processing and Bioengineering, Central South University, Changsha 410083, PR China Joint Laboratory for Extreme Conditions Matter Properties, Southwest University of Science and Technology and Research Center of Laser Fusion, CAEP, Mianyang 621010, PR China

a r t i c l e

i n f o

Article history: Received 11 August 2012 Received in revised form 10 May 2013 Accepted 25 May 2013 Available online xxxx Keywords: Modified chrysotile Copper adsorption Isotherm Thermodynamic parameters Kinetic model

a b s t r a c t The adsorption of Cu(II) ions by modified chrysotile from aqueous solution was investigated under different experimental conditions. The Langmuir and Freundlich equations were introduced to describe the linear forms about the adsorption of Cu(II) ions on the surface of modified chrysotile, and it was found that the adsorption equilibrium process was well described by the Langmuir isotherm model with the maximum adsorption capacity of 1.574 mmol/g at 333 K. The thermodynamic parameters (ΔG0, ΔH0 and ΔS0) for adsorption on modified chrysotile were also determined from the temperature dependence. The influences of specific parameters such as temperature, pH value and initial concentration for the kinetic studies were also examined. The adsorption follows a pseudo-second order rate law. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Copper ions, released into the environment from various industries, are usually present in wastewaters and causing adverse effects, which is of great environmental concern. According to the quantitative assessment of Nriagu and Pacyna (1988), 1198–2944 × 106 kg per year of copper was discharged on land around the world and the amount is increasing with years. Huge amounts of wastewater and sludge containing Cu(II) ions with various concentrations have greatly negative effects on the water environment (Li and Bai, 2005), and cause serious pollution for human foods (Antunes et al., 2003). The adult human body contains 100–150 mg of Cu(II) ions, and an excessive intake of copper by human may result in severe physiological maladies and psychological problems (Chen et al., 2008; Gong et al., 2008; Gupta et al., 2006a; Siao et al., 2007). As a result, the maximum acceptable concentration of Cu(II) ions in drinking water recommended by the world health organization is 1.5 mg/L. In order to reduce Cu(II) ion contamination, various methods have been used to remove copper ions from aqueous solutions. Different routes, such as chemical precipitation, ion exchange, adsorption, membrane filtration, coagulation and flocculation, flotation and electrochemical treatment, have been employed to remove the contaminants (including copper ions) from the wastewater to protect the people and the environment (Fu and Wang, 2011). Among them,

⁎ Corresponding author. Tel.: +86 73188877203. E-mail address: [email protected] (K. Liu). 0169-1317/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.clay.2013.05.014

the adsorption is a recognized method for the removal of soluble contaminants from water due to its high efficiency and easy operation. Many researchers around the world have paid attention to this field and achieved outstanding results. For example, a series of studies accomplished by Gupta et al. have confirmed that the hazardous metal ions (Gupta and Ali, 2004; Gupta and Nayak, 2012; Gupta et al., 2006a, 2010, 2011a), fluoride (Gupta et al., 2007a) and dye (Gupta et al., 2006b, 2006c, 2007b, 2007c, 2009, 2011b; Jain et al., 2003; Mittal et al., 2008, 2009, 2010a, 2010b) could be adsorbed and removed from wastewater by various materials and wastes. Many kinds of adsorbents such as activated carbon (Pesavento et al., 2003; Rao et al., 2006), natural and processed minerals (Weng et al., 2007; Yavuz et al., 2003), inorganic colloids (Subramaniam and Yiacoumi, 2001), industrial solid waste (Agrawal et al., 2004), sawdust (Larous et al., 2005), sewage sludge ash (Pan et al., 2003), olive mill residues (Veglio et al., 2003), blast furnace sludge (Lopez-Delgado et al., 1998), silica and silica gels (Chiron et al., 2003; Dias Filho et al., 1998; Vlasova, 2000) have been developed and tested to adsorb and remove Cu(II) ions. The adsorption efficiency greatly depends on the type of adsorbents. Most of adsorbents present complicated preparation and low capacity for Cu(II) adsorption, which may hinder the practical application of Cu(II) ions removal. Thus, there is a continuing and long-term need to develop more novel adsorbents with low cost and high performance. Of all the above-mentioned adsorbents, the adsorbents prepared from natural minerals are preferred for the removal of Cu(II) ions due to their high efficiency, easy handling, the availability of different adsorbents, and cost effectiveness (Lee and Tiwari, 2012). The most

K. Liu et al. / Applied Clay Science 80-81 (2013) 38–45

Nomenclature 1/n co ce ct h k KF KL Kd Q0 qe qt R R2 T t

Freundlich isotherm constants initial concentration of Cu(II) ions (mmol/L) equilibrium Cu(II) ion concentration in solution (mmol/L) residual Cu(II) ion concentration in solution at desired time (mmol/L) initial adsorption rate (mmol/(g·min)) equilibrium rate constant of pseudo-second order (g/(mmol·min)) Freundlich isotherm constants (mmol/g) Langmuir isotherm constants (L/mmol) equilibrium constant Langmuir isotherm constants, the maximum adsorption capacity of adsorbent (mmol/g) equilibrium amount of Cu(II) ions on adsorbent (mmol/g) amount of Cu(II) ions on adsorbent at desired time (mmol/g) universal gas constant, 8.314 (J/(mol·K)) correlation coefficient temperature in Kelvin (K) time (min)

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2. Experimental 2.1. Materials and instruments Chrysotile, prepared from asbestos tailing, was supplied by the Xiaobabao asbestos mine, Qilian, Qinghai Province, China, of which percentages in weight for SiO2, MgO, Fe2O3, Al2O3 and H2O were determined as 41.6, 38.6, 2.4, 0.6 and 15.5%, respectively. The purified chrysotile was obtained by soaking, loosening, removing magnetite and impurities, washing in water (Liu et al., 2007). Then the purified chrysotile was dispersed into micro-fibrils by an emulsifying machine at 6000 r/min for 60 min and dried at 383 K for 24 h. The γ-Aminopropyltriethoxysilane (γ-APS) with purity of 98%, and reagent grade copper sulfate, sulfuric acid, acetic acid and ethanol were purchased from Sinopharm Chemical Reagent Co., Ltd, China. The deionized water with electrical resistivity of 18 MΩ cm was used throughout this study. The Cu(II) ion concentration in solution was determined by atomic adsorption spectroscopy (AAS) using a Shimadzu AA-6800 flame atomic adsorption spectrophotometer with the wavelength of 324.8 nm. A scanning electron microscope (SEM, JSM-6360LV) operating with 25 kV accelerating voltage, equipped with energy-dispersive X-ray spectroscopy (EDX, EDAX GENESIS 60S) was used for morphological and chemical composition analysis. 2.2. Modification of chrysotile

commonly used are clay minerals, and they generally have high copper adsorption capacity. In the past decades, research have been focused on using different clay minerals and their derivatives as low-cost and effective adsorbents for copper adsorption, including illite (Eloussaief et al., 2009; Turan et al., 2011), hydromica (Vengris et al., 2001), kaolinite (Arai, 2011; Eloussaief et al., 2009), vermiculite (Santos and Masini, 2007), halloysite (Mellouk et al., 2009), bentonite (Bertagnolli et al., 2011) and montmorillonite (Vengris et al., 2001). Most of these researches have indicated that natural products can be good adsorbents for Cu(II) ions. In addition to clay, some kinds of minerals may be a good alternative for Cu(II) ion adsorption process. One of them is chrysotile. Chrysotile is a layered hydrated magnesium silicate, consisting of octahedral sheets of magnesium hydroxide covalently bonded to tetrahedral sheets of silicon oxide (Falini et al., 2004). There is a large number of hydroxyl on the outer surface of chrysotile, which leads to easy grafting and modification on chrysotile surface by functional organic molecules. In recent years, few research works about the adsorption of cations by chrysotile derivatives and modified chrysotile have been reported (Fonseca et al., 2001a, 2001b, 2003). An isotherms of the adsorption of copper on chrysotile, propylamine chrysotile and propylethylenediamine chrysotile, presented by Fonseca et al., indicated that the equilibrium amounts of Cu(II) ions on native and modified chrysotile were about 1, 1.5 and 2 mmol/g, respectively (Fonseca and Airoldi, 2001). It can be considered that native chrysotile and modified chrysotile are efficient adsorbents for Cu(II) ions, and the chrysotile grafted by amino groups is better. However, the detailed thermodynamic and kinetic studies about the adsorption process were scant. In this study, chrysotile was firstly modified by γ-Aminopropyltriethoxysilane in water, which was then used to evaluate the adsorption capacity for Cu(II) ions from aqueous solutions. The results indicated that the modified chrysotile was a kind of promising adsorbent for Cu(II) because of a high adsorption capacity. As the proper focus, the thermodynamics and kinetics about the adsorption of Cu(II) by modified chrysotile, including adsorption isotherm models, thermodynamic parameters, effects of temperature, pH value and initial concentration under kinetic conditions, were investigated in detail.

In a typical procedure, 5 ml of γ-APS was dissolved in 300 ml of water by magnetic stir, to which a small amount of acetic acid was dripped to adjust the pH value to 3.5–4.0. The liquid was stirred under atmosphere at room temperature for 30 min to achieve the hydrolysis of γ-APS. Then, 5 g of chrysotile micro-fibrils was added and stirred under the reflux of solvent at 353 K for 12 h. After cooling the dispersion, the chrysotile was filtered off, washed repeatedly with water, and dried at 353 K for 12 h, to produce modified chrysotile. Fig. 1(a) shows the SEM image of modified chrysotile with a magnification of 20,000 ×. For comparison, the image of modified chrysotile after the adsorption of Cu(II) ions is presented in Fig. 1(b). The results display that the modified chrysotile has typical micro-fibrous morphology which is not changed by Cu(II) ion adsorption. For chemical composition resulted from EDX (Table 1), the element of N can be detected in modified chrysotile because of the grafting of γ-APS on chrysotile surface, and the elements of N and Cu can be identified after the adsorption of Cu(II) ions on modified chrysotile. 2.3. Adsorption of Cu(II) ions on modified chrysotile Adsorption studies of Cu(II) ions were investigated by batch experiments. All batch experiments were carried out in 250 ml conical flasks. 1 g of modified chrysotile was placed in flask containing 100 ml of Cu(II) ion solution at desired concentration and pH value. Then the suspensions were placed in a shaker with invariant rotation rate of 100 rpm and taken for desired time. The solution and adsorbent were separated through filter, and the adsorbent was washed well by water. All the filtrate and washing water were collected carefully and set to the marked volume of 500 ml, which was used to determine the residual Cu(II) ion concentration in solution by AAS. So the concentration of Cu(II) ions on the adsorbent at desired time was calculated using the following mass-balance equation: qt ¼

c0  0:1−ct  0:5 1

ð1Þ

where qt is the amount of Cu(II) ions on adsorbent (mmol/g) at desired time, co is the initial concentration of Cu(II) ions (mmol/L), and ct is the residual Cu(II) ion concentration in solution at desired

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K. Liu et al. / Applied Clay Science 80-81 (2013) 38–45

Fig. 1. SEM images of modified chrysotile before (a) and after (b) the adsorption of Cu(II) ions.

time (mmol/L) that came from the result of AAS. Each value of ct was expressed as an average of three parallel experiments.

(ΔS0) can be calculated from the following Eqs. (4) and (5) (Ho, 2003):

2.4. Adsorption isotherm models

ΔG ¼ −RTlnK d

0

Adsorption isotherms of Cu(II) ions on modified chrysotile were determined at 298, 313 and 333 K, respectively, with desired concentration, fixed pH value of 4 and time of 10 h. Eq. (1) can be used to calculate the equilibrium capacity of copper on modified chrysotile (qe, mmol/g), because the time of 10 h is enough for reaction equilibrium, and the ct in the equation can be changed to ce (the equilibrium Cu(II) ion concentration in solution, mmol/L). The Langmuir and Freundlich adsorption isotherm models were used to describe the linear forms about the adsorption of Cu(II) ions on the surface of modified chrysotile. The Langmuir and Freundlich isotherm are expressed as the following Eqs. (2) and (3) (Seader and Henley, 2006): ce 1 1 ¼ þ c qe Q 0 K L Q 0 e

lgq ¼ lgK F þ

ð2Þ

  1 lgC e n

ð3Þ

0

lnK d ¼ −

ΔH ΔS þ RT R

ð4Þ 0

ð5Þ

where Kd is the equilibrium constant, ΔG0 is the free energy of adsorption (kJ/mol), T is the temperature in Kelvin, and R is the universal gas constant (8.314 J/(mol·K)). 2.6. Adsorption kinetics of Cu(II) ions The effects of temperature, pH value and initial Cu(II) ion concentration on adsorption kinetics of Cu(II) ions on modified chrysotile were investigated by batch experiments. Separate flasks were prepared for each time interval and only one flask was taken for desired time. In order to investigate the mechanism of adsorption, pseudosecond order model have been used at different experimental conditions for adsorption processes. Pseudo-second order reaction model is based on adsorption equilibrium capacity, of which the linear form is shown as Eq. (6) (Ho and McKay, 1999), t 1 t ¼ þ ; qt h qe

2

h ¼ kqe

ð6Þ

where ce and qe are the equilibrium Cu(II) ion concentration in solution (mmol/L) and the equilibrium amount of copper on adsorbent (mmol/g), respectively. Q0 (mmol/g) and KL (L/mmol) are Langmuir isotherm constants. KF (mmol/g) and 1/n are Freundlich isotherm constants. The value of Q0 means the maximum adsorption capacity of adsorbent.

where k is the equilibrium rate constant of pseudo-second order (g/(mmol·min)), h is the initial adsorption rate mmol/(g·min), qt and qe are the adsorption quantity of Cu(II) ions on adsorbent (mmol/g) at desired time and equilibrium time, respectively.

2.5. Adsorption thermodynamics

3. Results and discussion

Thermodynamic parameters such as free energy of adsorption (ΔG0), the heat of adsorption (ΔH0) and standard entropy changes

3.1. Adsorption isotherms

Table 1 The composition of modified chrysotile before and after adsorption of Cu(II) ions. Element

C N O Mg Si Fe Cu

Before adsorption of Cu(II) ions

After adsorption of Cu(II) ions

wt.%

at.%

wt.%

at.%

12.62 2.93 2.64 35.92 42.58 3.31 –

23.36 4.65 3.66 33.24 33.78 1.31 –

11.47 2.36 2.81 31.64 41.91 2.44 7.37

22.46 3.96 4.13 30.60 35.09 1.03 2.73

Fig. 2 plots the relationship between the amount of substance of Cu(II) ions adsorbed per unit mass of modified chrysotile (qe, mmol/g) and its equilibrium concentration in the solution (ce, mmol/L) at different temperature of 298, 313 and 333 K. Adsorption equations were obtained by experimental data with Eqs. (2) and (3). The straight-line plots of (ce/qe) against (ce) for Langmuir model and (lgqe) against (lgce) for Freundlich model are presented in Figs. 3 and 4. The isotherm constants were calculated and presented in Table 2. The R2 value of Langmuir-isotherm is higher than that of Freundlich-isotherm, which indicates that the adsorption of Cu(II) ions on modified chrysotile is more consistent with the Langmuir model. This may be due to homogenous distribution of active sites of \NH2 on modified chrysotile surface. The Langmuir constants of KL and Q0 in Table 1 increase with the increasing temperature, pointing out that higher temperature promotes

K. Liu et al. / Applied Clay Science 80-81 (2013) 38–45

Fig. 2. Adsorption isotherm.

the adsorption process. Values of Q0, which is defined as the maximum adsorption quantity of adsorbent at different temperature, have been calculated from the Langmuir plots. The maximum adsorption quantity of modified chrysotile for Cu(II) ions range from 1.357 to 1.574 mmol/g at varied temperature. A brief comparison of adsorption capacities of Cu(II) on various adsorbents obtained from natural mineral resources and biological resources is given in Table 3. Chrysotile modified by γ-Aminopropyltriethoxysilane presents a high adsorption capacity, reflecting a promising future for utilization in Cu(II) ion removal from aqueous solutions.

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Fig. 4. Linearized Freundlich isotherms obtained from Cu (II) ion adsorption on modified chrysotile.

were obtained from the slop and intercept of a plot of lnKd against 1/T (Fig. 5). The calculated parameters are presented in Table 4. The negative value of ΔG0 means the feasibility of the reaction and the spontaneous nature of the adsorption of Cu(II) ions by modified chrysotile. The positive value of ΔH0 indicates adsorption reaction to be in agreement with the endothermic nature of interaction and governs the possibility of physical adsorption (Arivoli et al., 2007; Hema and Arivoli, 2007). Higher temperature results in more negative value of ΔG0, indicating that increased temperature is favorable for the adsorption of Cu(II) ions.

3.2. Thermodynamic evaluation of the process 3.3. Influence of process variables Thermodynamic parameters, including free energy of adsorption (ΔG0), the heat of adsorption (ΔH0) and standard entropy changes (ΔS0), were evaluated by Eqs. (4) and (5). The equilibrium constant Kd (L/g) was obtained by multiplying the Langmuir constants Q0 and KL (Aksu and Isoglu, 2005; Yao et al., 2010). The temperature were 298, 313 and 333 K. The Gibbs free energy indicates the degree of spontaneity of adsorption process and the higher negative value represents a more energetically favorable adsorption. ΔH0 and ΔS0

3.3.1. Effect of the temperature Fig. 6(a) shows the effect of the adsorption temperature on the retention of copper, at temperature levels of 298, 313 and 333 K. Series of experiments have been carried out to investigate the effect of temperature, with a constant concentration of 50 mmol/L and pH value of 4. Fig. 6(a) displays the adsorption capacity of Cu(II) ions increased with time and achieved a maximum value at 60 min. Afterwards, the adsorption capacity reached a constant value, which pointed out that no more Cu(II) ions were further adsorbed by modified chrysotile. To ensure the achievement of adsorption equilibrium, the adsorption experiments were carried out for 180 min, although the equilibration time was found to be about 60 min. It can be indicated that the adsorption of the Cu(II) ions at varied temperatures are increased instantly at initial 10 min and then keep increasing gradually until the equilibrium is reached. Although the time to reach equilibrium at different temperatures is basically the same, the adsorption quantity of Cu(II) ions at 333 K in 10 min is significantly more than those of 298 and 313 K. Moreover, the maximum adsorption capacity of Cu(II) ions increased from 1.082 mmol/g at 298 K to 1.360 mmol/g

Table 2 Langmuir and Freundlich adsorption constants obtained at different temperature. Temperature (K)

Fig. 3. Linearized Langmuir isotherms obtained from Cu(II) ion adsorption on modified chrysotile.

298 313 333

Langmuir constants

Freundlich constants 2

Q0

KL

R

KF

1/n

R2

1.357 1.477 1.574

0.120 0.192 0.289

0.988 0.993 0.995

0.240 0.336 0.452

0.393 0.352 0.303

0.969 0.947 0.945

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K. Liu et al. / Applied Clay Science 80-81 (2013) 38–45

Table 3 Several adsorbents derived from natural products with high adsorption capacities of Cu(II) ions.

Table 4 Thermodynamic parameters for the adsorption of Cu(II) ions by modified chrysotile. ΔG0 (kJ/mol)

ΔH0(kJ/mol)

ΔS0(kJ/mol·K)

24.096

0.123

Adsorbent

Adsorption capacity (mmol/g)

References

Mauritanian clay Modified rice husk Hazelnut shell activated carbon Modified montmorillonite by OMHP Chitosan Dried sunflower leaves Pecan shell carbon Montmorillonite Modified chrysotile by γ-APS treated sugar beet pulp Modified montmorillonite by ethylenediamine Hazelnut shell activated carbon Papaya seed

0.74 0.86 0.92 1.19 1.25 1.40 1.48 1.52 1.57 1.87 1.94

Ely et al. (2011) Ye et al. (2012) Demirbas et al. (2009) Abou-El-Sherbini and Hassanien (2010) Schmuhl et al. (2001) Benaissa and Elouchdi (2007) Shawabkeh et al. (2002) Kozak et al. (2010) This study Altundogan et al. (2007) Kozak et al. (2010)

three mechanisms (Deng et al., 2003; Fonseca and Airoldi, 2001; Parida et al., 2012). Firstly, an ion-exchange process takes place, in which the proton in the residual hydroxyl groups is exchanged with the Cu(II) ions in solution; secondly, the amino groups attached to the surface of chrysotile complex Cu(II) ions by using the available basic nitrogen atoms; thirdly, the protonated amino group complex or adsorb Cu(II) ions by chelating or ion-exchange. These three mechanisms can be described by Eqs. (7), (8) and (9):

3.05 3.32

Milenkovic et al. (2009) Hadi et al. (2011)

2Si–O–Mg–OH þ Cu

at 333 K, which means that with increasing temperature the adsorption of Cu(II) ions increases.

298 K

313 K

333 K

−12.618

−14.696

−16.944

\NH2 þ Cu



þ

3.3.2. Effect of the pH value Fig. 6(b) shows that the adsorption of Cu(II) ions increased with increasing pH value from 2 to 4, with a constant concentration of 50 mmol/L and temperature of 313 K. The pH value is an important parameter in the process of adsorption. Although the diagram of distribution of Cu(II) species as function of pH value (with concentration of 1 × 10−2 mol/L) showed the precipitate as Cu(OH)2 would generate at pH value above 6 (Wang and Qin, 2005), actually the insoluble precipitation of Cu(OH)2 would occur at the pH value of above 4–5 (Aksu and Isoglu, 2005). Consequently, no experiments were carried out at pH value greater than 4. Below pH value of 4, the dominate species of copper is Cu(II) that mainly involved in the adsorption process. Fig. 6(b) indicates that the adsorption of Cu(II) ions at different pH values are increased rapidly at initial 10 min and then keep increasing gently to reach equilibrium. Higher pH value results in more adsorption quantity, i.e. the maximum adsorption capacity of Cu(II) ions increased from 0.614 mmol/g at pH value of 2 to 1.212 mmol/g at pH value of 4, which means higher pH value favors the adsorption of Cu(II) ions on modified chrysotile. The effect of pH can be explained by considering the residual hydroxyl groups, dominant amino groups and protonation of amine groups on the surface of modified chrysotile, which are proposed as

Fig. 5. A Plot of lnKd against 1/T for Cu(II) ion adsorption by modified chrysotile.



þ

¼ ðSi–O–Mg–OÞ2 Cu þ 2H 2þ

¼ \NH2 …Cu orð\NH2 Þ2 þ Cu þ

þ



\NH2 þ H ¼ \NH3 andð\NH3 Þ2 þ Cu



ð7Þ

¼ ð\NH2 Þ2 …Cu 2þ

¼ ð\NH2 Þ2 …Cu



ð8Þ

þ

ð9Þ

þ 2H

For ion-exchange mechanism, the lower the pH value, the higher the H+ concentration in solution. High concentration of H+ can promote the competitive adsorption between H3O+ and Cu2+ at residual hydroxyl groups, as well favor the equilibrium displacement in the opposite direction (see Eq. (7)) to inhibit the ion-exchange process of Cu(II). For complex mechanism, the main interactive process is the formation of the complexes in which the lone pair of the basic nitrogen atoms of the amine groups is bonded to the Cu2+. As it is well known, higher pH environment promotes the basic properties of amine groups, which facilitates the complexity of Cu(II) by amine

Fig. 6. Influence of process variables on the adsorption kinetics of modified chrysotile, including the effects of temperature (a), pH value (b) and initial concentration of Cu(II) ions (c).

K. Liu et al. / Applied Clay Science 80-81 (2013) 38–45

43

groups (Ambundo et al., 2000). For protonation mechanism, the low pH value favors the protonation of the amine groups to form + \NH+ 3 . Lower pH value, more \NH3 will be conversed from \NH2 groups (Udaybhaskar et al., 1990). On the one hand, the electrostatic repulsion between the Cu2+ and the surface of modified chrysotile increases with the formation of more \NH+ 3 sites on the surface at lower pH value (Banerjee and Chen, 2007); on the other hand, with the decrease of solution pH, the reaction in Eq. (9) proceeds to the left, resulting in a decrease of the number of complexes. All abovementioned effects would result in the reduction of Cu(II) ion adsorption on modified chrysotile with decreasing solution pH values, and the increasing adsorption at higher pH value. 3.3.3. Effect of the initial concentration Initial concentration is a pivotal parameter because it determines the capacity of an adsorbent for given adsorbent dosage. The effect of initial concentration was researched on Cu(II) ion removal from aqueous solutions by varying the concentration of Cu(II) ions from 10 to 100 mmol/L, while keeping a temperature of 313 K and a pH value of 4. Fig. 6(c) shows that the maximum adsorption capacity of Cu(II) ions adsorbed increased from 0.647 to 1.415 mmol/g. This may be explained by a promotion in the driving force of the concentration gradient with the increasing initial Cu(II) ion concentration to overcome all mass transfer resistance of Cu(II) ions between the aqueous and solid phases. Thus, as the conclusion drawn by Chen et al., a higher initial concentration of Cu(II) ions will enhance the adsorption capacity (Chen et al., 2005). The maximum adsorption capacity of modified chrysotile increased with an increasing initial Cu(II) ion concentration, however, the percentage removal of Cu(II) ions showed an opposite trend. Calculated from the experimental results, when the initial concentration increased from 10 to 100 mmol/L, the percentage removal of Cu(II) ions decreased from 64.7% to 14.2%. Many researchers have reported the similar observation for the copper adsorption by various adsorbents, which indicates that the use of a proper amount of adsorbents will ensure an effective and economical removal of Cu(II) from water. Meanwhile, the adsorption quantity displays a rapid increase at initial 10 min and then shows a slow growth to reach nearly equilibrium. This illustrates that the adsorption of Cu(II) ions onto modified chrysotile are fast, which will contribute to a rapid removal of copper contaminant in aqueous solution.

Fig. 7. Pseudo-second order plots for Cu(II) ion adsorption using modified chrysotile, including different temperature (a), different pH value (b) and different initial concentration of Cu(II) ions (c).

including temperature, pH value and initial concentration, were investigated and found having the influence on the adsorption process. The adsorption of Cu(II) ions for all conditions was rapid at an initial stage of 10 min and then kept on increasing gradually until the equilibrium was reached. The adsorption capacities of Cu(II) ions on modified chrysotile were increased with increasing temperature, pH value and initial concentration. The pseudo-second order kinetic reaction model was used to fit the batch adsorption kinetics and found an excellent correlation with the adsorption of Cu(II) ions from aqueous solution on modified chrysotile.

3.4. Adsorption kinetics

Acknowledgment

In order to predict of adsorption kinetic models of Cu(II) ions, pseudo-second order kinetic models was applied to data (Eq. (6)). The effects of the temperature, pH value and initial Cu(II) ion concentrations were investigated. The equilibrium rate constants of pseudosecond order were determined by plotting t/qt against t. Fig. 7 shows the kinetics plots. The kinetic constants and correlation coefficients were presented in Table 5. The experimental data showed excellent compliance with the pseudo second-order kinetic model in terms of good correlation coefficients and very close values between qt (amount of Cu(II) ions on adsorbent at 180 min, acquired by experiments) and qe (equilibrium amount of Cu(II) ions, calculated from the model).

This work was financially supported by the National Natural Science Foundation of China (No. 51104180); the Hunan Provincial Natural Science Foundation of China (No. 13JJ4015); the China Postdoctoral Science Foundation (No. 2013M530362); the Open Foundation of Joint Laboratory for Extreme Conditions Matter Properties, Southwest

4. Conclusion This paper presented the thermodynamic and kinetic data for adsorption of Cu(II) ions from aqueous solution on modified chrysotile. The experimental data were correlated better by the Langmuir isotherm model. The maximum adsorption capacities at different temperature from 298 to 333 K, with a constant pH value of 4, were in range of 1.357–1.574 mmol/g. The reaction for adsorption of Cu(II) ions by modified chrysotile was found to be endothermic in nature, which was resulted from thermodynamic parameters obtained from the investigation of adsorption process. The experimental parameters,

Table 5 The pseudo-second order kinetic model rate constants at different experimental conditions. Parameters

qe (mmol/g)

Temperature (K) 298 1.077 313 1.226 333 1.349 pH value 2 3 4

0.600 1.180 1.226

Initial concentration (mmol/L) 10 0.635 50 1.226 100 1.422

h (10−2 mmol/g·min)

k (g/mmol·min)

R2

28.715 34.901 142.984

0.247 0.232 0.786

0.999 0.999 0.999

42.928 23.463 34.901

1.193 0.168 0.232

0.999 0.999 0.999

4.110 34.904 49.329

0.102 0.232 0.244

0.981 0.999 0.999

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