Adsorption of crystal violet from aqueous solution on activated

Physicochem. Probl. Miner. Process. 48(1), 2012, 253270

Physicochemical Problems of Mineral Processing

www.minproc.pwr.wroc.pl/journal/

ISSN 1643-1049 (print)

Received May 12, 2011; reviewed; accepted July 30, 2011

ADSORPTION OF CRYSTAL VIOLET FROM AQUEOUS SOLUTION ON ACTIVATED CARBON DERIVED FROM GÖLBAŞI LIGNITE Tolga DEPCI *, Ali Riza KUL **, Yunus ONAL ***, Erkan DISLI ****, Salih ALKAN **, Z. Funda TURKMENOGLU * * Yuzuncu Yil University, Faculty of Engineering and Architecture, Department of Mining Engineering, 6580, Van, Turkey ** Yuzuncu Yil University, Faculty of Arts and Science, Department of Chemistry, 6580, Van, Turkey, Tel.: +90 432 225 10 81 ; fax: +90 432 225 11 14, [email protected], (A.R. KUL) *** Inonu University, Faculty of Engineering, Department of Chemical Engineering, 44280 Malatya, Turkey **** Yuzuncu Yil University, Faculty of Engineering and Architecture, Department of Environmental Engineering, 6580, Van, Turkey

Abstract. Activated carbon (AC) was obtained from lignite of the local resource, Gölbaşı – Adıyaman (Turkey) by chemical activation. The Gölbaşı lignite was chosen as the precursor for its availability and low cost. The BET surface area of the activated carbon was found 921 m2/g. The AC was used as an adsorbent for Crystal Violet (CV) in aqueous solution. The adsorption properties of CV onto the activated carbon are discussed in terms of the adsorption isotherms (Langmuir and Freundlich) and the kinetic models (pseudo-first-order, pseudosecond-order and intraparticle diffusion model). It was shown that the experimental results best fitted by the Langmuir model, and the second-order kinetic equation. The thermodynamic parameters show that the adsorption process is endothermic. The experimental results point out that the obtained activated carbon is a viable candidate for sorbent removing CV from aqueous solutions. keywords: crystal violet; activated carbon; Turkish lignite; adsorption

1. Introduction In the textile industry, one of the main problems is removal of dyes and pigments from the wastewaters. It is known that most dyes are toxic, carcinogenic and mutagenic to aquatic organisms, so they have to be removed. Several methods, such as, filtration, coagulation, chemical oxidation, adsorption, etc., are used in order to remove dyes from wastewater (Mohan et al., 2002; Senthilkumaar et al., 2006). Among them, the liquid-phase adsorption process, in which activated carbon is usually

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used, is a promising method for removal of dyes from industrial wastewaters. Activated carbon is of interest because of its relatively great adsorption capacity and the ease of production. Crystal violet is used to colourize diverse products such as paper, leather, fertilizers, anti-freezes, detergents, and also as a component of inks for ball-point pens. It is also used in veterinary medicine, such as a dermatological drug, and for expelling intestinal parasites and fungi from the body (Adak et al., 2005). However, like other common dyes, CV is highly toxic to living organisms. Therefore, it should be removed from the wastewater. As mentioned above, the liquid-phase adsorption process is a promising method for removal of dyes form wastewater. According to the literature survey, recently the following adsorbents were used to remove CV from wastewater: a surfactant-modified alumina (Adak et al., 2005), sludge biomass (Chu and Chen 2002), fly ash (Wang et al., 2005), sepiolite (Eren and Afsin, 2007; Eren et al., 2010), bottom ash, a power plant waste, and de-oiled soya (Mittal et al., 2010), NaOH-modified rice husk (Chakraborty et al., 2010), palm kernel fibre (El-Sayed, 2011). The recent publications on adsorption of crystal violet on activated carbon have been reported by Senthilkumar et al. (2006), Önal (2006), Akmil-Basar (2006), Malarvizhi and Ho, (2010). These authors used activated carbons obtained from agricultural wastes. For instance, Senthikumar et al. (2006) used male flowers of coconut tree, Önal (2006) and Akmil-Basar (2006) used waste apricot, and Malarvizhi and Ho (2010) used wood apple rind as the precursors. Activated carbon is a generic name of family of highly porous, amorphous carbonaceous materials and it cannot be characterized by a structural formula or chemical analysis (McDougall et al., 1980). Activated carbons exhibit an extended surface area, highly developed pore structure (in particular, micropore) and a high surface reactivity (Bansal and Goyal, 2005), so they are generally used to remove hazardous dyes and heavy metals from aqueous solution. Two aims are accomplished in the current research. One is to evaluate lignite of the local resource, Gölbaşı – Adıyaman (Turkey), the original material for production of activated carbon. Since production of activated carbon is rather expensive, different alternative sources of cheap and readily available materials are regarded as the precursors, having in mind to reduce the costs. Besides, it is known that lignite has high ash and high moisture content. In developed countries like the U.S. instead of burning lignite, pyrolysis products are obtained and used as industrial solvents. Therefore, in addition to burning lignite in Turkey, end products like activated carbon should be produced to gain greater added value to Turkish economy. The second purpose of this study is to investigate the adsorption isotherm, kinetics and the thermodynamic parameters of CV adsorption onto activated carbon derived from the Gölbaşı lignite and to compare the adsorption capacity of the activated carbon with the literature data.

Adsorption of crystal violet from aqueous solution on activated carbon derived from Gölbaşi lignit

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2. Materials and Methods 2.1 Materials Test solutions (50, 100, 200 and 300 mg/dm3) containing crystal violet (CV) dye were prepared by diluting of stock solution of the dye which was obtained by dissolving weighed quantity of crystal violet (supplied by MERCK) in double distilled water. 2.2 Preparation of activated carbon Activated carbon was prepared from Gölbaşı–Adıyaman (Turkey) lignite which was chosen as the precursor for its availability and low cost. The experimental procedure was based on the study carried out by Onal et al. (2006). Lignite samples were crushed and sieved to -60+40 mesh size fraction. It was mixed with ZnCl2 (lignite/ZnCl2 weight ratio of 1:1) and required amount of distilled water was added on this mixture. Then, this mixture was dried at 378 K in furnace to obtain the impregnated sample. Impregnated sample was heated to the activation temperature of 773 K for 1 hour under N2 flow (100 cm3/min) at the rate of 283 K·min-1. After the activation process, product was cooled down under N2 flow then 0.5 N HCl was added on the samples. This mixture was filtered and it was washed with distilled water several times to remove residual chemicals and chloride ion until filtrate did not give any reaction with AgNO3. Finally, the samples were dried at 378 K for 24h and stored in a desiccator until use. The activated carbon was ground in a standard ring mill and sieved under 400 mesh sizes (-0.038 mm). 2.3. Instrumentation A Tri Star 3000 (Micromeritics, USA) surface analyzer was used to measure nitrogen adsorption isotherm at 77K in the range of 10-6 to 1 relative pressures. Prior to the measurement, the sample was degassed at 400ºC for 2 h. The BET surface area, total pore volume, average pore radius (4V/A by BET), micropore area were obtained from the adsorption isotherms. Mesopore volume was determined by subtracting the micropore volume from total pore volume. The spectral determination of CV concentration in solutions was performed using a Shimadzu UV-VIS spectrophotometer (Model UV-HITACH U-2900, Japan). The zeta potential of the activated carbon was measured by a Zeta Meter 3.0 (Malvern Inc.) equipped with a microprocessor unit. The pH of the test solution was adjusted to the desired value by dropwise addition of dilute NaOH (0.5 %) or HCl (0.1 N). 2.4. Adsorption experiments Adsorption studies were conducted in routine manner by batch technique. A number of stoppered Pyrex glass Erlenmeyer flasks containing a definite volume of solutions of desired concentration were placed in a thermostatic shaking assembly. For

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the studies with activated carbon, 0.1g activated carbon was treated with 50 cm3 of the dye solution at 400 rpm. Adsorption experiment were carried out at different temperatures (298 K, 303 K and 313 K) using four dye solutions which were prepared at 50, 100, 200 and 300 mg/dm3 by using the stock solution (1000 mg/ dm3) at natural pH of the CV solution (pH of 5.8) The amount of dye q (mg/g) adsorbed on activated carbons was calculated by the mass balance equation (1).

q  C 0  C V / W

(1)

where, C0 (mg/ dm3) is the initial dye concentration and C(mg/ dm3) is unadsorbed dye concentration in solution at time t, V (l) and W (g) is the volume of the solution and the weight of the dry activated carbons used respectively. The effect of pH on the CV adsorption on the activated carbon was investigated by batch technique which was carried out at 50 mg/L initial dye concentration with 0.1 g/50 cm3 adsorbent dosage at 293 K for 60 min of equilibrium time and a constant stirring speed of 400 rpm. The pH values were adjusted in the range of 2 – 10. The effect of stirring rate was also investigated by varying the stirring speed between 100 and 500 rpm for 60 min at a constant dye concentration (50 mg/ dm3) at pH of 5.8. In order to determine the kinetic parameters, the stoppered pyrex glass Erlenmeyer flasks containing 500 ml volume of solutions of 50 mg/L concentration were placed in a thermostatic shaking assembly. The AC sample of 1.0 g was put into the dye solution and the mixture was stirred at 400 rpm at 298 K at pH 5.8. Every 10 minutes, the samples of 5 cm3 were taken by micropipette without ceasing the system. The dye solution was filtrated with Whattman fitler paper. Concentration of the dye (CV) was determined using the UV/VIS spectrophotometer. Thermodynamic parameters were determined from the experiments carried out at the different temperatures, for the different dye concentrations. 3. Results and discussion 3.1. Characterization of activated carbons In order to characterise porosity of the activated carbon, nitrogen adsorption at 77oK, which is the standard method for characterisation of adsorbents, was applied. The N2 adsorption isotherm of the activated carbon is given in Figure 1. It can be classified as Type 1 - characteristic of micropore solids, according to the IUPAC classification (Sing et al., 1985). Table 1 presents the porous structure parameters obtained by applying the BET equation to N2 adsorption on the activated carbon, at 77 K. It shows that the activated carbon has remarkable surface area, primarily contributed by micropores.


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3

Specific volume adsorbed (cm /g)

350 300 250 200 150 100 50 0 0

0.2

0.4

0.6

0.8

1

1.2

Relative Pressure (P/Po )

Fig. 1. Adsorption isotherm of nitrogen on the activated carbon, at 77K Table 1. Total surface area (SBET ), areas of micro- and meso-pores (Smicro and Smezo) and the pore volumes (Vmicro and Vmeso) of the prepared activated carbon SBET m2/g

Smicro m2/g

Smezo m2/g

Vt cm3/g

Vmicro cm3/g

Vmeso cm3/g

Dp* nm

921

812

109

0.476

0.427

0,049

2.11

*

Dp denotes the average pore diameter determined from the BET equation; Vt denotes the total pore volume.

3.2. Effects of temperature and the dye concentration The effects of temperature and the dye (CV) concentration on adsorption properties of the activated carbon are illustrated in Figure 2. It can be seen the percentage of dye removal at the adsorption equilibrium, showing a temperature-dependent character. It increases with the temperature raise which means that the adsorption process is endothermic. Sahu et al. (2009) and Singh et al. (2011) mentioned that the number of binding sites for the dye molecules on the adsorbent surface may be increased by the temperature raise. In addition, Almeida et al. (2009) mentioned that increase of the dye removal depending on the temperature can be explained by the increase of the mobility of the dye molecules. On the contrary, the adsorption capacity of the activated carbon decreases with increasing the dye concentration. This may indicate that the adsorption is limited by number of the available active sites (Gupta et al., 1988) not sufficient for the high initial concentration of the dye (CV).

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Dye Removal (%)

100

50 298 K 303 K 313 K 0 0

100

200

300

400

Initial dye concentration (mg/L) Fig. 2. Effect of temperature and initial dye concentration on equilibrium adsorption of crystal violet onto the activated carbon (0.1 g AC/50 cm3, pH of 5.8, 400 rpm)

3.3. Effect of contact time on the adsorption From an economical point of view, the contact time required to reach equilibrium is an important parameter in waste water treatment. In order to investigate effect of contact time, the experiments were carried out for 10, 20, 30, 40, 50 and 60 min, using the fixed adsorbent dosage of 0.1 g at the natural pH (pH of 5.8) and the dye concentration of 50 mg/dm3. The results are shown in Fig. 3. It reveals that the equilibrium adsorption percentage increases with increasing the contact time and approaches to the equilibrium after about 50 min Therefore 60 min. was accepted as optimal time for adsorption of CV on the activated carbon.

Dye Removal (%)

100 90 80 70 60 50 0

20

40

60

Contact Time (min.)

Fig. 3. Effect of contact time on equilibrium adsorption of crystal violet on the activated carbon, AC (0.1 g AC/50 cm3, pH of 5.8, 400 rpm, 298 K)


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3.4. Effect of stirring rate In order to determine the stirring rate effect on adsorption, the experiments were carried out varying the stirring speed between 100 and 500 rpm at dye concentration of 50 mg/ dm3 and stirring time of 60 min at natural pH of 5.8. Figure 4 shows the effect of stirring rate on the CV adsorption. As seen, the percentage of the dye removal increases depending on the increase in stirring rate. This result is very compatible with the studies done by Garg et al. (2004) and El-Sayed (2011). They claimed this situation depends on decreasing the thickness of the diffusion layer around the adsorbent surface due to the stirring rate. However, it is clearly seen in Figure 4 that the difference in the percentage increase is not very sufficient after 400 rpm value, so 400 rpm was considered as optimal stirring rate for the adsorption of the CV. Also, up to now, this issue is fixed in our experience for dye adsorption.

Dye Removal (%)

100

80

60

40 0

200

400

600

Stirring Rate (rpm) Fig. 4. Effect of stirring rate on adsorption of crystal violet on the activated carbon

3.5. Effect of pH The pH of the distilled water used in the present study is 6.4 and after addition of the CV to obtain dye solution, pH of the solution was measured as 5.8 due to the cationic properties of CV. Then, the activated carbon was added the dye solution and adsorption experiment was started. The starting and the final pH of the mixture were noted and these values were noted as 4.13 and 4.48, respectively. This behaviour was also observed in the literature and it was explained with the ion exchange between the carbon surface and the cationic form of the dye molecule (Kurbatov et al., 1951; Kadirvelu et al., 2005; Ho, 2005; Malarvizhi and Ho, 2010). The effect of the pH of solution on CV adsorption is shown in Fig. 5. The adsorption of CV on the activated carbon increases with increasing the pH of solution. After the pH range of 6 and 8, the difference in the percentage of the dye removal was not recorded at very significant amount. Similar trend of pH effect was observed in the

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recent studies about adsorption of CV to be done by Jain and Jayaram (2010), Mittal et al., (2010), Chakraborty et al., (2010) and Singh et al., (2011). Thus, natural pH of the starting CV solution of 5.8 was considered as operational pH for the adsorption of the CV due to the considering of the cost and simplicity. Besides, it is clearly seen that the adsorption capacity of the activated carbon decreases with increasing the dye concentration. This observation supports the previous result which was presented in the effects of the dye concentration on adsorption part.

Dye Removal (%)

100

80

60 50 mg/L 100 mg/L 40 0

5

pH

10

15

Fig. 5. Effect of solution pH on adsorption of crystal violet on the activated carbon

Zeta Potential (mV)

0 0

2

4

6

8

10

12

-20

-40

-60 Equilibrium pH

Fig. 6. Zeta potential of the activated carbon

Literature survey shows that pH of solution affect of the surface properties of the adsorbent and ionization/dissociation of the adsorbate molecule (Saeed et al., 2010; Chakraborty et al., 2010; Mittal et al., 2010). As mentioned, pH of the solution affects


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the adsorption of CV. This observation can be explained by considering the zeta potential of the activated carbons. Therefore, in the present study, the zeta (ζ) potential variation of the activated carbon was investigated and this variation is given in Fig. 6. Zeta potential increases in the negative direction with increasing the pH and isoelectrical point (IEP), which represents no net electrical charge of surface at the specific pH, is approximately 2.8. If pH values is lower than the IEP (pH of 2.8), the surface of activated carbon surface has positive charged and protonation of the functional groups easily occurs on it. At this range, due to same charge of the cationic dye and the activated carbon, electrostatic repulsion takes place between them. Also in high acidic medium, lower adsorptive removal of the dye was due to the presence of excess hydrogen ions competing with the positively charged dye molecules (Mall et al., 2005; Wang et al., 2010; El-Sayed, 2011). As the pH of the dye solution increases, the surface of the activated carbon has negative charge due to the successive deprotonation of positive charged groups on the surface of activated carbon, and negative charge density on the surface increases. Therefore the electrostatic attraction between the negatively charged sites of the adsorbent and the positively charged dye molecules (=N+(CH3)2) occurs (Chakraborty et al., 2010; Mittal et al., 2010). Depending on this feature, the adsorption of the CV on the activated carbon increases with increasing the pH of the solution. 3.6. Adsorption isotherms Adsorption isotherms are mathematical models which describe distribution of the solute among two phases, i.e., the liquid and the adsorbed phases. In the present study, results of the adsorption at different temperatures were fitted with the Langmuir and Freundlich models which earlier have been found applicable to many dye sorption processes (Onal et al., 2006; Akmil-Başar 2006; Wang et al., 2010; Malarvizhi and Ho, Y.S., 2011; El-Sayed 2011; Ayed , 2011). The linearised Langmuir isotherm can be expressed by the following equation (Langmuir, 1918):

C e / q e  1 / Q0 b  C e / Q0

(2)

where Ce is the equilibrium solute concentration in the liquid phase(mg/ dm3), qe is the adsorption capacity (mg/g), b (dm3/mg) and Q0 (mg/g) are the temperature-dependent Langmuir isotherm constants, where Q0 signifies the theoretical monolayer capacity, i.e., the maximum amount that can be adsorbed. The Langmuir equation is applicable to homogeneous adsorption, i.e., when each sorbate molecule has equal sorption activation energy. The Freundlich isotherm describes heterogeneous and reversible adsorption not restricted to formation of a monolayer. The Freundlich equation transformed to the linear form is as follows (Freundlich, 1906):

log qe  log k f  log Ce / n

(3)

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where, kf (dm3/g) is the Freundlich constant and 1/n (dimensionless) is the heterogeneity factor. Langmuir plots (Ce/qe versus Ce) and Freundlich plots (log qe versus log Ce) for CV adsorption at different temperatures are depicted in Figs. 7 and 8, respectively. And the isotherm parameters calculated by applying the commonly accepted linear regression procedure to the linear representation of the isotherms, are summarized in Table 2. 8

1.9

log q e

Ce/qe (g/L)

6

4

1.6

298 K

2

298 K

303 K

303 K

313 K

313 K

0

1.3

0

50

100 150 Ce (mg/L)

200

250

Fig. 7. Langmuir isotherm plots for adsorption of CV onto activated carbon at different temperatures

-1.5

0

1.5

3

log Ce

Fig. 8. Freundlich isotherm plots for adsorption of CV onto AC at different temperatures

Table 2. The fitted isotherm parameters for adsorption of CV onto the AC at different temperatures. 298 Langmuir Q0 (mg/g) b * 10-2 (L/mg) R2 Freundlich kf (L/g) n R2

Temperature (K) 308 318

60.8 0.17 0.971

61.2 0.15 0.988

65.8 0.075 0.995

10.64 26.67 0.354

8.08 25.81 0.641

6.72 28.52 0.878

Comparing quality of fitting the Langmuir and Freundlich isotherms in terms of the correlation coefficient, R2 , one may see that adsorption of CV on the activated carbon is quite well consistent with the Langmuir model, but not with the Freundlich one. The similar result was obtained in recent studies by done Wang et al. (2010), Malarvizhi and Ho (2011), El-Sayed (2011), Keyhanian et al. (2011). They found that the equilibrium adsorption data to be obtained for the adsorption of CV on different adsorbents were well described by the Langmuir model. Besides, it is seen from Table 2, the adsorption capacity (Q0) - determined from the Langmuir model fit, increases with temperature raise showing that the adsorption process is endothermic. It may be concluded that raise in temperature accelerates transportation of dye molecules from solution to the adsorbent surface.


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In order to determine whether the adsorption system is favourable or not, ‘RL’ named as dimensionless separation factor, which is obtained from Langmuir model (Weber and Chakravorti, 1974; Hall et al., 1966) is defined by the following equation

R L  1 /(1  bCo )

(4)

where RL is a dimensionless separation factor, Co is the initial dye concentration and b is Langmuir constant. The feasibility of the reactions are explained using the value of RL (RL > 1 -unfavourable, RL = 1 -linear, 0