Comparison of activated carbon and bottom ash for removal of reactive dye from aqueous solution

Bioresource Technology 98 (2007) 834–839

Comparison of activated carbon and bottom ash for removal of reactive dye from aqueous solution Ali Rıza Dinc¸er *, Yalc¸ın Gu¨nesß, Nusret Karakaya, Elc¸in Gu¨nesß Department of Environmental Engineering, Trakya University, Corlu, Tekirdag, Turkey Received 2 January 2006; received in revised form 16 March 2006; accepted 19 March 2006 Available online 11 May 2006

Abstract The adsorption of reactive dye from synthetic aqueous solution onto granular activated carbon (GAC) and coal-based bottom ash (CBBA) were studied under the same experimental conditions. As an alternative to GAC, CBBA was used as adsorbent for dye removal from aqueous solution. The amount of Vertigo Navy Marine (VNM) adsorbed onto CBBA was lower compared with GAC at equilibrium and dye adsorption capacity increased from 0.71 to 3.82 mg g1, and 0.73 to 6.35 mg g1 with the initial concentration of dye from 25 to 300 mg l1, respectively. The initial dye uptake of CBBA was not so rapid as in the case of GAC and the dye uptake was slow and gradually attained equilibrium. Ó 2006 Elsevier Ltd. All rights reserved. Keywords: Adsorption; Activated carbon; Bottom ash; Reactive dye; Waste material

1. Introduction Most industries use dyes and pigments to color their products. Discharge of dye-bearing wastewater into natural streams and rivers from textile, paper, carpet, and printing industries poses a severe problem, as dyes impart toxicity to aquatic life and are damaging the aesthetic nature of the environment. Many of the dyes used in industry are stable to light and oxidation, as well as resistant to aerobic digestion (Gupta et al., 2003a). Most dyes are considered to be non-oxidizable substances by conventional biological and physical treatment because of their complex structure and large molecular size (Weber and Morris, 1962). The adsorption process provides an attractive alternative treatment, especially if the adsorbent is inexpensive and readily available. Bottom ash and fly ash are produced at rates of 12–20% by weight of the original coal when coal is burned to produce steam for energy generation (Chen et al., 1991). Granular activated carbon is the most popular *

Corresponding author. Tel.: +90 282 6529475/76; fax: +90 282 6529372. E-mail address: [email protected] (A.R. Dinc¸er). 0960-8524/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2006.03.009

adsorbent and has been used with great success. However, GAC is expensive and its regeneration and reuse makes it more costly (McKay, 1982; Choy et al., 1999). A number of non-conventional, low cost adsorbents have been tried for dye removal. These include bottom ash (Mukhtar et al., 2003), fly ash (Kao et al., 2000; Janos et al., 2003), coir pith (Namasivayam and Kavitha, 2002), cassava peel (Rajeshwarisivaraj et al., 2001), cotton (Bouzaida and Rammah, 2002), orange peel (Sivaraj et al., 2001), baggasse fly ash (Gupta et al., 2003b), cellulose-based wastes (Annadurai et al., 2002), sewage sludge (Pan et al., 2003), kaolinite (Ghosh and Bhattacharyya, 2002), zeolite (Meshko et al., 2001), wheat straw (Robinson et al., ¨ zacar and S 2002), sawdust (O ß engil, 2005; Garg et al., 2003), charfines (Mohan et al., 2002), oil shale and olive mill waste (Abu El-Shar et al., 1999). The waste material ‘Bottom ash’ is an undesired collected material procured from thermal power generation plants after combustion of coke (Mittal et al., in press). It has been successfully used as potential adsorbent for the removal of metals (Gupta et al., 2004) as well as dyes (Arevalo et al., 2002). Adsorption capacity depends not only on the porous structure of the adsorbent but also on

A.R. Dinc¸er et al. / Bioresource Technology 98 (2007) 834–839

its chemical structure. It is for this reason that, in order to characterise the adsorption capacity of a material, determining its surface area and pore size distribution would not be enough, it being often that carbons having the same surface area exhibit different adsorptive behaviors (Walker et al., 1996). There was no literature on dye (VNM) adsorption on coal-based bottom ash with which data may be compared. In the present study, the effectiveness of CBBA to remove dye from aqueous solutions was explored and compared with that of commercial GAC. The effect of pH, initial dye concentration and shaking time on the adsorption of VNM onto GAC and CBBA were investigated using batch experiments. 2. Methods 2.1. Materials The particle size distribution of bottom ash was determined by sieving the samples manually shaking with stainless steel mesh screens with openings of standard 0.074, 0.125, 0.177, 0.25 and 0.59 mm ASTM sieves. The particle size distribution of the bottom ash was in the range of: <0.074 mm—26.9%, 0.074–0.125 mm—17.8%, 0.125– 0.177 mm—14.1%, 0.177–0.250 mm—14.7%, 0.25–0.59 mm—25.9%, >0.59 mm—0.6%. The granular activated carbon with a size range between 0.25 mm and 2.0 mm was used in the batch experiments. Surface areas of the samples were 400–1000 m2 g1 and 1.77 m2 g1 for GAC and CBBA, respectively. The surface area of bottom ash was measured by a Quantachrome Autosorb-1 surface area analyzer. The chemical composition of the coal-based bottom ash was measured using ICP spectroscopy. Composition of the collected coal-based bottom ash was 32.35% SiO2, 12.00% Al2O3, 7.28% Fe2O3, 4.54% MgO, 9.83% CaO, 3.14% Na2O, 1.83% K2O, 0.55% TiO2, 0.06% MnO, 7.53% SO3, 0.13% Ba, <0.005% Cd, <0.02% Cu, 0.04% Cr, <0.005% Co, 0.04% Ni, <0.02% Pb, <0.02% Sb, and 0.10% Zn. The loss on ignition was found to be 20.5% by weight. Bottom ash was alkaline in nature with a pH of 300 ml water + 10 g ash as 9.63. The coal-based bottom ash used in this study was collected as a homogeneous 15 kg sample from the coal combustion chamber in C ¸ orlu. 2.2. Equilibrium studies The adsorption of Vertigo Navy Marine (VNM) from aqueous solution onto CBBA and GAC was performed using batch experiments conducted in this study. Coalbased bottom ash and granular activated carbon were used in the batch experiments without any pretreatment. Vertigo Navy Marine (C.I Blue222) is a cationic dye broadly used in the textile industry. The dye Vertigo Navy Marine was obtained from GOTEK Chemical Company in Turkey.

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The aqueous solution was prepared by dissolving dyestuff in deionised water to produce a stock solution of 500 mg l1 at pH 7. Solutions of the required concentrations were prepared by successive dilution of the stock solution. The adsorption studies were carried out at different dye concentrations (25, 50, 75, 100, 150 and 300 mg l1) to investigate the adsorptive capacity of the selected adsorbents (GAC and CBBA). The experiments were carried out at fixed adsorbent dose (10 g/300 ml) in the aqueous solution at 25 °C temperature. The samples were taken at the end of the desired contact time, i.e., 5, 15, 30, 45, 60 and 90 min, to determine the effect of shaking time. A control flasks with only the adsorbent in 300 ml of deionized water was used simultaneously under the same conditions. The flasks were agitated at room temperature (25 °C) at 250 rpm (SL350 Nu¨ve). The effect of solution pH was studied by performing the adsorption experiments at three different pH levels; 4.0, 7.0 and 9.0. The pH of the solution was adjusted with 1 N HCl or NaOH solution by using a WTW340 pH meter. After 90 min contact time, the equilibrium pHeq was recorded and the dye concentration was determined. All further studies for the sake of comparison on both the adsorbents were carried out at a pH of 9.0 (CBBA) and 4.0 (GAC). The dye solution was separated from the adsorbent by centrifugation (CN180 Nu¨vefuge) at 2000 rpm for 45–60 min. The residual concentration of the dye was determined using a calibration curve prepared at the corresponding optimum wavelength (kmax) of 612 nm using a thermospectronic AQUAMATE spectrometer. Langmuir and Freundlich isotherms were employed to determine the adsorption capacity of the adsorbents. 3. Results and discussion The major components of the bottom ash were silica, iron, alumina, magnesium and SO3 along with other compounds present at low concentrations. Typical values of BET (N2, 77 k) surface areas for commercial activated carbons were usually between 400 and 1500 m2 g1 (Abu ElShar et al., 1999). Low surface area (1.77 m2 g1) of the adsorbent (CBBA) tested showed that bottom ash particles did not have many micropores. The adsorption process of dye by different adsorbents was affected by various parameters such as pH, contact time and concentration of dye solution. Fig. 1 shows the effect of pH and initial dye concentration on adsorption by GAC and CBBA. Evidently pH significantly affected the extent of adsorption of dye over both the adsorbents. It was observed that the dye uptake by GAC was higher at lower pH. At lower pH the surface of the adsorbents becomes positively charged and this would facilitate sorption of the colour cation probably by exchange sorption (Mohan and Karthikeyan, 2004). The variances in solid phase aggregation of dye on activated carbon is probably due to interactions between the cationic

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A.R. Dinc¸er et al. / Bioresource Technology 98 (2007) 834–839 7

100 90

Removal Efficiency (%)

6

qe (mg/g)

5 4 3 pH4 A.Carbon pH7 A.Carbon

2

80 70 60 50 40 25

30

50

pH9 A.Carbon

75

20

pH4 Bottom ash

100

pH7 Bottom ash

1

10

pH9 Bottom ash

150 300

0

0 0

50

100

150

200

250

300

0

350

20

40

100

Fig. 2. Variation of dye removal efficiency with time at different dye concentrations. Adsorbent dose, 10 g bottom ash/300 ml, pH = 9, T = 25 °C.

120

100

Removal Efficiency (%)

groups on the dye molecule and the carboxyl, carbonyl, lactonic and quinone(acidic) and basic functional groups within the activated carbon structure (Walker and Weatherley, 2001). The minimum dye color removal for GAC was observed at the neutral pH. At neutral pH, there is a possibility of oxidation of the surface oxygen complexes present on the surface, which may impart positive charge to the granular activated carbon surface (Mohan et al., 2002). Maximum uptake of VNM was observed at pH 4.0 on GAC (Fig. 1). When the initial pH of the dye solution was increased from 4.0 to 9.0, dye adsorption by GAC at equilibrium decreased 6.35 to 5.03 mg g1. Fig. 1 also shows that the effect of pH on the adsorptive removal of VNM by CBBA. The increase in initial pH increased the amount of dye adsorbed by CBBA. At higher pH, the surface of CBBA particles may get negatively charged, which enhances the positively charged dye cations through electrostatic force of attraction. With increasing basicity of the medium the polarity of the electrical double layer on both silica and alumina undergoes a change from positive to negative and therefore enhances the uptake of the positive dye molecule (Gupta et al., 2003a). Lower adsorption of VNM at acidic pH is provable due to the presence of excess of H+ ions competing with the dye cations for the adsorption sites. The maximum dye removal by CBBA was observed at pH 9. The uptake of dye by CBBA at equilibrium increased from 3.53 to 3.82 mg g1 with the increase of pH from 4 to 9. Percent adsorption efficiency of GAC and CBBA at equilibrium decreased with increase in initial dye concentration. The removal efficiency of VNM by CBBA and GAC at different contact time are shown in Figs. 2 and 3. The amount of dye adsorbed increased with increase in agitation time. In the case of GAC, the initial rate of dye uptake was rapid and it rapidly attained equilibrium for low initial dye concentrations. Similar results have been reported in

80

Time (min)

Co (mg/l)

Fig. 1. Effects of initial pH and concentrations on adsorption by coalbased bottom ash and activated carbon.

60

80

60

Co=25 mg/l Co=50 mg/l Co=75 mg/l Co=100 mg/l Co=150 mg/l Co=300 mg/l

40

20

0

0

10

20

30

40

50

60

70

80

90

100

Time (min) Fig. 3. Variation of dye removal efficiency with time at different dye concentrations. Adsorbent dose, 10 g A.Carbon/300 ml, pH = 4, T = 25 °C.

literature on the extent of removal of dyes (Kanan and Sundaram, 2001). In the case of CBBA, the shape of the uptake plot was different to that of GAC. The initial dye uptake was not so rapid as in the case of activated carbon and dye uptake was slow and gradually attained equilibrium. This may be due to the fact that the coal-based bottom ash have not much micro and macroporous structure. Contrary to GAC, CBBA possesses low BET surface area, which is responsible for its observed low adsorption capacity. Dye removal efficiency was higher for low dye concentrations because of availability of unoccupied binding sites on the adsorbents. Percent color removal decreased with

A.R. Dinc¸er et al. / Bioresource Technology 98 (2007) 834–839 1.2 1

y = 0.3096x + 0.2564 2

R = 0.6127

0.8

logq e

increasing dye concentrations because of nearly complete coverage of the binding sites at high dye concentrations. As shown in Figs. 2 and 3, when the dye concentration was increased from 25 to 300 mg l1, percent dye adsorption at equilibrium decreased from 95% to 42% for CBBA and 96% to 71% for GAC. Larger amounts of dyes were removed by the CBBA and GAC in the first 20 min of contact time. At 25 mg l1 dye concentration, the equilibrium time was reached within less than 45 min. A longer time was needed to reach equilibrium when the initial dye concentration was above 25 mg l1 for CBBA. As a result, time required for equilibrium adsorption of CBBA was much longer than that of GAC. From Figs. 1–3, it was evident that the amount of VNM adsorbed per unit mass of adsorbent increased with the increase in C0, although percentage VNM removal decreased with the increase in C0. Both adsorbents were very effective in removing the dye in all the pH values at low initial dye concentration. But, the dye adsorption by GAC and CBBA was affected by pH at high initial dye concentration. Although the adsorption capacity of CBBA was lower than that of GAC, CBBA could be an attractive adsorbent for the removing of dye containing wastewaters.

0.6 y = 0.319x - 0.1892 2 R = 0.9708

0.4 0.2

Bottom Ash A.Carbon

0 -1

-0.5

0

60 y = 0.249x + 6.525

Ce/qe (g/l)

50

R2 = 0.9073

40 y = 0.1531x + 0.4642 R2 = 0.9979

30

0.5

1

1.5

2

2.5

3

-0.2

logCe -0.4

Fig. 5. Freundlich isotherm plots for the adsorption of Vertigo Navy Marine.

Table 1 Isotherm constants for the Langmuir and Freundlich isotherms Adsorbent

3.1. Adsorption isotherms Figs. 4 and 5 present the Langmuir and Freundlich isotherm plots for Vertigo Navy Marine adsorption on the CBBA and GAC. The distribution of dye between the adsorbent and dye solution, when the system is at equilibrium, is important to obtain the capacity of the GAC and CBBA. Table 1 summarizes the Q0 and KL values for the Langmuir isotherm, the Kf and n values for the Freundlich isotherm and the correlation coefficients for the two isotherms.

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Coal-based bottom ash (CBBA) Granular activated carbon (GAC)

Freundlich isotherm

Langmuir isotherm

Kf

n

R2

Q0 (mg g1)

KL (l mg1)

R2

0.65

3.29

0.61

4.02

0.038

0.91

1.81

3.1.3

0.97

6.53

0.329

0.99

The Langmuir isotherm is represented by the following equation (Namasivayam et al., 2001): C e =qe ¼ 1=ðQ0  K L Þ þ ð1=Q0 Þ  C e ;

ð1Þ

where qe and Ce are defined as the amount of dye adsorbed (mg g1) and equilibrium liquid-phase concentration (mg l1), respectively. KL is a direct measure of the intensity of the sorption (l mg1), and Q0 is a constant related to the area occupied by a monolayer of adsorbate, reflecting the maximum adsorption capacity (mg g1). From the data of Ce/qe vs Ce, Q0 and KL can be determined from the slope and intercept. The essential characteristics of Langmuir equation can be expressed in terms of a dimensionless separation factor RL, which is defined by McKay et al. (1989), as RL ¼ 1=ð1 þ K L C 0 Þ

20 Bottom ash

10

0

A.Carbon

0

50

100

150

200

Ce (mg/l) Fig. 4. Langmuir isotherm plots for the adsorption of Vertigo Navy Marine.

where C0 is any adsorbate concentration at which the adsorption is carried out. Favourable adsorption is indicated by 0 < RL < 1 (Namasivayam et al., 2001). The Langmuir isotherm constants were found to be Q0 = 4.02 mg g1, KL = 0.038 l mg1 for CBBA (R2 = 0.91), and Q0 = 6.53 mg g1, KL = 0.329 l mg1 for GAC (R2 = 0.99). The RL values were found to be between 0.01 and 0.51 for dye concentrations of 25, 50, 75, 100, 150 and 300 mg l1 (data not shown in Table 1). The RL

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values between 0.01 and 0.51 show favourable adsorption of VNM on GAC and CBBA. The Freundlich isotherm, as an empirical equation has been shown to be satisfactory for low adsorbate concentrations. The Freundlich equation was linearized as follows: Logðqe Þ ¼ logðK f Þ þ 1=n logðC e Þ

ð2Þ

Kf and n are constants incorporating all factors affecting the adsorption process such as adsorption capacity and intensity, respectively. Values of Kf and n were calculated from the intercept and slope of the plots of log qe versus log Ce. From the slope and intercept of the best-fit lines, the following Freundlich isotherm constants were found for GAC and CBBA. Kf = 0.65 mg g1, n = 3.29 for CBBA (R2 = 0.61) and, Kf = 1.81 mg g1, n = 3.13 for GAC (R2 = 0.97). The Freundlich exponent n between 3.13 and 3.29 indicates favourable adsorption for which 1 < n < 10 (Sivaraj et al., 2001; Namasivayam et al., 2001). Applicability of these isotherm equations was compared by calculating a correlation coefficient, R2. It can be seen that the Langmuir model (R2 = 0.99) yields a little better fit than the Freundlich model (R2 = 0.97) for the adsorption of VNM onto GAC. The adsorption isotherm data were fitted well to Langmuir istherm (R2 = 0.91) for the adsorption of VNM onto CBBA (Table 1). 4. Conclusions The results indicated that CBBA could be employed as low cost alternatives to GAC in wastewater treatment. CBBA appeared to be a promising adsorbent for the removal of reactive dyes from aqueous solutions. The extent of dye removal increased with decreased initial concentration of the dye and also increased with increased contact time. Adsorption of VNM onto GAC occured faster and reached higher equilibrium levels as compared to the CBBA. The GAC had a higher adsorption capacity than CBBA at lower and higher dye (VNM) concentrations. Acknowledgement This study was partly financed by a research project grant from Trakya University (Grant No: 546), CorluTekirdag, Turkey. References Abu El-Shar, W.Y., Gharaibeh, S.H., Mahmoud, S., 1999. Removal of dyes from aqueous solutions using low-cost sorbents made of solid residues from olive-mill wastes and solid residues from refined Jordanian oil shale. Environ. Geol. 39 (10), 1090–1098. Annadurai, G., Juang, R.S., Lee, D.J., 2002. Use of cellulose wastes for adsorption of dyes from aqueous solutions. J. Hazardous Mater. B92, 263–274. Arevalo, E.F., Stichnothe, H., Thorning, J., Calmano, W., 2002. Evaluation of a leaching process coupled with regeneration/recycling of extractant for treatment of heavy metal contaminated solids. Environ. Technol. 23, 571–581.

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