Utilization of activated carbon produced from fruit juice industry solid waste for the adsorption of Yellow 18 from aqueous solutions.

Bioresource Technology xxx (2014) xxx–xxx

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Utilization of activated carbon produced from fruit juice industry solid waste for the adsorption of Yellow 18 from aqueous solutions Dilek Angin ⇑ Department of Food Engineering, Faculty of Engineering, University of Sakarya, Esentepe Campus 54187, Sakarya, Turkey

h i g h l i g h t s Fruit waste was transformed into higher-value products such as activated carbon. The sour cherry stones activated carbon was effective in removal of Yellow 18. Both pH and temperature have a strong effect on adsorption capacity of Yellow 18. Yellow 18 adsorption behavior could be described by Langmuir isotherm model. Kinetic studies showed that pseudo-second-order model was suitable for Yellow 18.

a r t i c l e

i n f o

Article history: Received 20 December 2013 Received in revised form 13 February 2014 Accepted 16 February 2014 Available online xxxx Keywords: Solid waste Sour cherry stones Activated carbon Dyestuff Adsorption

a b s t r a c t The use of activated carbon obtained from sour cherry (Prunus cerasus L.) stones for the removal of a basic textile dye, which is Yellow 18, from aqueous solutions at different contact times, pH values and solution temperatures was investigated. The surface area and micropore volume of chemically modified activated carbon were 1704 m2 g1 and 0.984 cm3 g1, respectively. The experimental data indicated that the adsorption isotherms were well described by the Langmuir equilibrium isotherm equation and the calculated adsorption capacity was 75.76 mg g1 at 318 K. The adsorption kinetic of Yellow 18 obeys the pseudo-second-order kinetic model. The thermodynamic parameters were calculated to estimate the nature of adsorption. The activation energy of the system was calculated as 0.71–2.36 kJ/mol. According to these results, prepared activated carbon could be used as a low-cost adsorbent to compare with the commercial activated carbon for the removal of Yellow 18 from wastewater. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Nowadays, dyes have created serious environmental problems with the rapid development of various industries, such as paper, plastic, textile, cosmetic, leather, and coating. It is estimated that more than 70.000 tons of dyes are discharged in effluent from textile and other industries in the world every year (Zhang et al., 2012; Angın et al., 2013a). Colored dye effluents have mainly the complex nature and most of them are toxic to aquatic life, mutagenic and carcinogenic and can cause some health problems (Gerçel et al., 2007). The adsorption method was proven to be more sophisticated and efficient compared to traditional physico-chemical methods such as coagulation, ultrafiltration, ozonation, oxidation, sedimentation, reverse osmosis, flotation etc. for dye removal from wastewater, due to its low-cost, easy availability, simplicity of design ⇑ Tel./fax: +90 264 2955927. E-mail address: [email protected]

and ease of operation (Senthilkumaar et al., 2006; Gerçel et al., 2007). Activated carbons are applied in separation and purification technologies, catalytic processes, biomedical developments, pollution control, and energy storage, among other usages (TreviñoCordero et al., 2013). Activated carbons, because of their large surface area and relatively high adsorption capacity for a wide variety of dyes, have become the most promising and effectively adsorbent (Yang and Qui, 2010). Activated carbon can be produced from any carbonaceous solid precursor, which is largely dependent on its availability, cost and purity. Activated carbon is the most widely used adsorbent for the removal of dyestuff from wastewater because it has an excellent high surface area and high adsorption capacity for organic compounds, but its use is usually limited due to its high cost (Karagozoglu et al., 2007). For this reason, many researchers have investigated more cheaper and efficient activated carbons for the removal of dyes from wastewater such as rice husks (Lin et al., 2013), sugar beet bagasse (Demiral and Gündüzog˘lu, 2010), wood sawdust (Foo and Hameed,

http://dx.doi.org/10.1016/j.biortech.2014.02.100 0960-8524/Ó 2014 Elsevier Ltd. All rights reserved.

Please cite this article in press as: Angin, D. Utilization of activated carbon produced from fruit juice industry solid waste for the adsorption of Yellow 18 from aqueous solutions. Bioresour. Technol. (2014), http://dx.doi.org/10.1016/j.biortech.2014.02.100

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D. Angin / Bioresource Technology xxx (2014) xxx–xxx

2012a), walnut shells (Yang and Qui, 2010), cashew nut shell (Senthil Kumar et al., 2010), and plum kernel (Treviño-Cordero et al., 2013). Due to environmental consideration, agricultural and food industries’ wastes are considered to be very important precursor because they are cheap, renewable, safe, available at large quantities and easily accusable sources (Ahmed and Theydan, 2012). These residues are usually disposed of by burning or by deposition in landfills, but conversion to higher-value products such as activated carbon would be preferable. Processing of fruits produces a solid waste of peel/skin, stones or seeds which can be used as precursor for the production of activated carbon. There are lots of fruits containing considerable amounts of stones. One of these fruits is sour cherry (Prunus cerasus L.). Turkey is the largest producer of sour cherry in the world with an annual production of 192,705 tones (Damar and Eksßi, 2012). In present study, adsorption characteristics of high surface area activated carbon (Angin, 2014) obtained from sour cherry stones were demonstrated for the removal of dyestuff which is Yellow 18. 2. Methods 2.1. Materials Sour cherry stones were supplied by the ASYA Fruit Juice and Food Ind. Inc. (Isparta, Turkey), and they were first air dried, then crushed and finally sized, the fraction of particle sizes between 1 and 2 mm was chosen for subsequent studies. The characteristics of sour cherry stones were given in Table 1 (Angin, 2014). The reactive Yellow 18 dye (C.I.13245) with molecular formula of C25H16ClN9Na4O13S4 and molecular weight of 906.12 g mol1 was supplied from SARAR Textile Factory (Eskisehir, Turkey). A stock solution of dye (Yellow 18) was prepared by dissolving an accurate quantity of dye in deionized water.

solids after carbonization were boiled at about 90 °C with 100 mL of 1 N HCl solution for 30 min to leach out the activating agent, filtered and rinsed by warm distilled water several times until the pH value was reached to 6–7. Finally, they were dried at 105 ± 3 °C for 24 h.

2.3. Characterization of activated carbon The contents of carbon, hydrogen, nitrogen and sulfur of the sour cherry stones and activated carbons were measured using a LECO CHNS 932 Elemental Analyzer with ±0.4% accuracy (LECO Instruments, USA). The oxygen contents were calculated by difference. To determine the surface area and pore structure of activated carbon samples, the nitrogen adsorption–desorption isotherms at 77 K were measured by an automated adsorption instrument, Quantachrome, Autosorb-1C with ±0.15% accuracy. The surface area, pore volume and pore size distribution were determined from nitrogen adsorption data by using Quantachrome software. The apparent surface area of nitrogen was calculated by using the BET (Brunauer–Emmett–Teller) equation within the 0.01–0.2 relative pressure range. The micropore volume was determined according to the D–R (Dubinin–Radushkevich) method. The amount of N2 adsorbed at relative pressures near unity (0.99) corresponds to the total amount adsorbed in both the micropores and the mesopores; consequently the subtraction of the micropore volume from the total amount will provide the volume of the mesopores (Yorgun et al., 2009). Surface functional groups were determined by Fourier transform infrared spectra (FTIR) using SHIMADZU IR Prestige 21. Surface morphologies were studied by scanning electron microscopy (SEM). SEM images were performed using JEOL-JSM-6060LV Scanning Electron Microscope.

2.2. Preparation of activated carbon 2.4. Adsorption experiments Chemical activation of sour cherry stones was carried out using zinc chloride (ZnCl2) with an impregnation (ZnCl2:sour cherry stones) ratio of 3:1. About 10 g of the impregnated sample was placed on a ceramic crucible in the tubular reactor (Protherm PTF 12) and heated up to the final activation temperature (700 °C) under the nitrogen flow (100 cm3 min1) at heating rate of 10 °C min1 and held for 2 h at this final temperature. The resulting

Table 1 Characteristics of the sour cherry stones and activated carbon.

* **

Surface properties

Sour cherry stones

Activated carbon

BET surface area (m2 g1) Micropore area (m2 g1) Total pore volume (cm3 g1) Micropore volume (cm3 g1) Average pore diameter (nm)

41.54 n.d.** 0.0975 n.d. 9.39

1704 1184 1.566 0.984 2.29

Moisture content (wt.%)

2.67 ± 0.23

1.58 ± 0.17

Proximate analysis (wt.%) Volatile Matter Ash Fixed carbon*

78.50 ± 0.64 0.17 ± 0.21 21.33 ± 0.39

14.44 ± 0.28 1.27 ± 0.19 84.29 ± 0.86

Ultimate analysis (wt.%) Carbon Hydrogen Nitrogen Sulphur Oxygen*

48.72 6.41 1.85 n.d. 43.02

80.78 1.87 1.54 n.d. 15.81

By difference. n.d.: not determined.

Adsorption of reactive dyestuff (Yellow 18) onto the produced activated carbon was studied by batch experiments. A fixed amount of activated carbon (0.5 g) and 100 mL of dyestuff solution (100 mg L1) were placed in capped volumetric flasks and shaken at 120 rpm using a temperature controlled water bath with a shaker (GFL) at 24 h. After adsorption, samples were filtered and then the concentration of dyestuff in the supernatant solution was analyzed. All concentrations were measured by using UV spectrophotometer (Shimadzu UV–Vis 1240) at 268 nm. In order to investigate the effect of initial solution pH on the Yellow 18 adsorption, the initial pH values of the solutions (100 mg L1) were adjusted to different values by using dilute NaOH or HCl solutions, measuring them with a pH-meter (WTWinoLab). After adsorption, the pH value providing the maximum dyestuff removal was determined. Also, for the purpose of investigating the effect of adsorbent dosage, batch experiments were carried out at 298 K and at optimum pH value of the solution for 24 h shaking period by adding different amounts of activated carbon (0.1–0.8 g) into each 100 ml Yellow 18 solution (100 mg L1). The removal percentage of dyestuff was calculated according to the following equation:

Removal ð%Þ ¼

C0 Ce 100 C0

ð1Þ

where C0 and Ce (mg L1) are initial and equilibrium concentrations of the dyestuff (Yellow 18), respectively (Yang and Qui, 2010).


3


2.5. Adsorption isotherms Batch adsorption experiments were conducted in a set of conical flasks containing 0.5 g of activated carbon and 100 mL Yellow 18 solutions with different concentrations (50, 100, 150, 200, 250 and 300 mg L1). The flasks were agitated at 120 rpm using a temperature controlled water bath with a shaker (GFL 1086), and maintained at 298, 308 and 318 K for 24 h until the equilibrium was reached. The suspensions were filtered and dyestuff concentrations were measured using a UV spectrophotometer. Yellow 18 uptake at equilibrium, qe (mg g1), was calculated by the following equation:

qe ¼

ðC 0 C e ÞV W

ð3Þ

where qe is the equilibrium dye concentration on the adsorbent (mg g1), Ce the equilibrium dye concentration in the solution (mg L1), Q0 the monolayer adsorption capacity of the adsorbent (mg g1), and KL is the Langmuir adsorption constant (L mg1). When Ce/qe was plotted against Ce, straight line with slope 1/Q0 was obtained indicating that the adsorption of Yellow 18 onto activated carbon. The essential characteristics of the Langmuir isotherm can be expressed in terms of a dimensionless equilibrium parameter (RL) which is defined by:

RL ¼

1 1 þ K L :C 0

ð4Þ

ð6Þ 1

where qs is the D–R monolayer capacity (mg g ), B is a constant related to sorption energy, and e is the Polanyi potential which is related to the equilibrium concentration as follows:

e ¼ RT ln 1 þ

1 Ce

ð7Þ

where R is the gas constant (8.314 J mol1 K1) and T (K) is the absolute temperature. The constant B gives the mean free energy, E, of sorption per molecule of the sorbate when it is transferred to the surface of the solid from infinity in the solution and can be computed using the relationship (Senthil Kumar et al., 2010; Demiral et al., 2008):

1 E ¼ pffiffiffiffiffiffi 2B

ð8Þ

The Temkin isotherm model (Mittal et al., 2009; Foo and Hameed, 2012a) contains a factor that explicitly takes into account adsorbing species–adsorbate interactions. This model assumes the following: (i) the heat of adsorption of all the molecules in the layer decreases linearly with coverage due to adsorbent–adsorbate interactions, and that (ii) the adsorption is characterized by a uniform distribution of binding energies, up to some maximum binding energy. The derivation of the Temkin isotherm assumes that the fall in the heat of sorption is linear rather than logarithmic, as implied in the Freundlich equation. The Temkin isotherm has commonly been applied in the following form:

qe ¼ B1 lnðAC e Þ

ð9Þ

Eq. (9) can be linearized as:

qe ¼ B1 lnA þ B1 lnC e

ð10Þ 1

Where KL is the Langmuir constant and C0 is the highest dyestuff concentration (mg L1). The value of RL indicates the type of the isotherm to be either unfavorable (RL > 1), linear (RL = 1), favorable (0 < RL < 1) or irreversible (RL = 0) (Angın et al., 2013a; Mahapatra et al., 2012; Lin et al., 2012). Freundlich isotherm assumes heterogeneous surface energies, in which the energy term in Langmuir equation varies as a function of the surface coverage. The well-known logarithmic form of Freundlich isotherm is given by:

1 log qe ¼ log K F þ log C e n

qe ¼ qs expðBe2 Þ

ð2Þ

where C0 and Ce (mg L1) are initial and equilibrium concentrations of the Yellow 18, respectively, V (L) is the volume of the aqueous Yellow 18, and w (g) is the weight of activated carbon used (Angın et al., 2013a; Mahapatra et al., 2012). The equilibrium data were then fitted to Langmuir, Freundlich, Dubinin–Radushkevick (D–R) and Temkin isotherm models. Langmuir isotherm assumes monolayer adsorption onto a surface containing a finite number of adsorption sites. The linear form of Langmuir isotherm equation (Angın et al., 2013a; Mahapatra et al., 2012; Foo and Hameed, 2011) is represented by the following equation:

Ce 1 Ce ¼ þ qe Q 0 K L Q 0

The isotherm model suggested by Dubinin and Radushkevich (Mittal et al., 2009) has been used to describe the liquid phase adsorption and on the basis of Dubinin–Radushkevich (D–R) equation adsorption energy can be estimated. Assuming that the adsorption in micropores is limited to a monolayer and the Dubinin–Radushkevich equation (Senthil Kumar et al., 2010; Demiral et al., 2008) can be written as:

ð5Þ

where qe is the amount adsorbed at equilibrium (mg g1), Ce the equilibrium concentration of the adsorbate (Yellow 18), and KF and n are Freundlich constants, n giving an indication of how favourable the adsorption process and KF (mg g1) is the adsorption capacity of the adsorbent. KF can be defined as the adsorption or distribution coefficient and represents the quantity of dyestuff adsorbed onto activated carbon adsorbent for a unit equilibrium concentration. The slope 1/n ranging between 0 and 1 is a measure of adsorption intensity or surface heterogeneity, becoming more heterogeneous as its value gets closer to zero. A value of 1/n below 1 indicates a normal Freundlich isotherm while 1/n above 1 is indicative of cooperative adsorption (Angın et al., 2013a; Mahapatra et al., 2012).

1

1

where B1 = RT/b, with b (J mol ), A (L g ), R (8.314 J mol K1) and T (K) are Temkin constant related to heat of sorption, equilibrium binding constant, gas constant and absolute temperature, respectively (Demiral et al., 2008). 2.6. Thermodynamic parameters Three thermodynamic parameters, i.e. change in the Gibbs free energy (DG°), enthalpy (DH°), and entropy (DS°), were calculated to evaluate the thermodynamic feasibility and the nature of the adsorption process. DG° can be calculated according to the following equation:

DG ¼ RTlnK

ð11Þ 1

1

where R is the gas constant (8.314 J mol K ), T is the temperature (K), and K is the thermodynamic equilibrium constant of the adsorption process, reflecting dyestuff distribution between the solid and liquid phases at equilibrium. Equilibrium constant (K) was estimated as:

K¼

qe Ce

ð12Þ

According to the van’t Hoff equation:

lnK ¼

DS DH R RT

ð13Þ


4


The values of DH° (kJ mol1) and DS° (J mol1 K1) were evaluated from the slope and intercept of van’t Hoff plots (Huang et al., 2014; Lin et al., 2013). 2.7. Adsorption kinetics Adsorption kinetic experiments were conducted by contacting 0.5 g adsorbent with 100 mL dyestuff solution (100 mg L1) at 298, 308, 318 K and at optimum pH value for 24 h continuous shaking. The concentration of dyestuff in supernatant was determined at different time intervals. In order to analyze the kinetic mechanism of adsorption process, the experimental data were fitted in the pseudo-first-order, pseudo-second-order, and intra-particle diffusion models which are described as Eqs. (14)–(16); Pseudo-first-order equation:

log ðqe qt Þ ¼ log qe

k1 t 2:303

ð14Þ

Pseudo-second-order equation:

t 1 t ¼ þ qt k2 qe qe

ð15Þ

Intra-particle diffusion equation: 1

qt ¼ kp t 2 þ C

ð16Þ 1

Where qt and qe (mg g ) are amounts of Yellow 18 adsorbed over a given period of time t and at equilibrium, respectively; t is the adsorption time (min); k1 (min1), k2 (g mg1min1), and kp (mg g1 min1/2) are the adsorption rate constants of the pseudofirst-order adsorption, the pseudo-second-order adsorption and the intra-particle diffusion, respectively; and C (mg g1) is a constant in the intra-particle diffusion equation, corresponding to the thickness of boundary layer (Huang et al., 2014; Angın et al., 2013b; Lin et al., 2012; Demiral and Gündüzog˘lu, 2010). 3. Results and discussion 3.1. Characterization of activated carbon The results of proximate and ultimate analysis and surface properties of the sour cherry stones and activated carbon are given in Table 1. The carbon content increased after activation process, and the hydrogen, nitrogen and oxygen contents indicated the opposite change trend. Since the sulfur content of activated carbon was below the detection limit, the activated carbon could be used in adsorption and purification process. This is due to the release of volatiles during carbonization that results in the elimination of non-carbon species and enrichment of carbon (Aygün et al., 2003). Also, activation process led to an increase in fixed carbon content while decrease in volatile matter content. The ash content of activated carbon slightly increased by activation process. The porosity has a strong effect on the adsorption properties of the activated carbon. The specific surface area of activated carbon was found to be 1704 m2 g1 and a good deal of the material (63%) consisted of micropores. The nitrogen adsorption–desorption isotherms and pore size distribution of activated carbon obtained from sour cherry stones are presented in Supplementary Figs. S1a and S1b. The isotherms of activated carbon can be properly classified as a mixture of type I and type IV isotherms. According to the International Union of Pure and Applied Chemistry (IUPAC) classification, type I isotherm can be associated with microporous structure while type IV isotherm exhibited by the mixture of microporous and mesoporous material (Angın et al., 2013a; Sütçü and Demiral, 2009; Foo and Hameed, 2011). A

comparison of the significant parameters of activated carbon with the carbonized sour cherry stones showed best improvement of the BET surface area, total pore volume, implying pore development and widening of the existing pores during activation process. FTIR analysis results of sour cherry stones and activated carbon are given in Supplementary Fig. S2. The mentioned in figure, while the bands about 1985–2462 cm1 which were generally attributed to symmetric or asymmetric stretching of aliphatic band in –CH, –CH2 or –CH3 were observed as much stronger at produced activated carbons, they were not seen in the FTIR spectrum of the sour cherry stones. The aliphatic functional groups were formed, possibly due to the extraction of H element and OH groups from the aromatic rings during the impregnation and heat treatment stages as a result of the dehydration effect of ZnCl2 (Angın et al., 2013b). The bands about 1020.34 cm1 show the C–O–C stretching (alcohols, ethers or phenols) and O–H deformation vibrations (Foo and Hameed, 2011). These bands decreased by activation process as compared to the raw material. These results indicated that the carbonyl groups and substitution in aromatic rings were affected by thermal effect (Angin, 2014). In order to examine the surface morphology, the sour cherry stones and activated carbon were subjected to scanning electron microscopy (SEM). Supplementary Fig. S3 illustrates the SEM images. Significant differences were observed between the surface topography of sour cherry stones and activated carbon. SEM image of sour cherry stones showed the absence of micropores, as supported by the BET results (41.54 m2g1). Also, activated carbon exhibit an even, homogeneous, highly porous and well pronounced array of honey-combed structures, indicating good possibility for the dyes to be trapped and adsorbed (Angın, 2014; Demiral and Demiral, 2008). 3.2. Effect of initial pH In general, initial pH value may enhance or depress the dyes uptake, inter-correlated to the changes of adsorbent surface properties and dye chemistry (Foo and Hameed, 2012b; Benadjemia et al., 2011). The effect of initial pH value on the removal of dyestuff was investigated at the pH range of 1.0–12.0 for activated carbon (Supplementary Fig. S4). As shown in Fig. S4, adsorption increased from 4.43 mg g1 (22.16% removal) to 19.68 mg g1 (98.39% removal) with decrease of pH from 6.0 to 2.0 and after pH 6.0, no significant difference in the amount of dyestuff adsorbed was observed on proceeding further till pH 12. Low pH leads to an increase in H+ ion concentration in the system and the surface of the activated carbon acquires positive charge by absorbing H+ ions. As the activated carbon surface is positively charged at low pH, a significantly strong electrostatic attraction appears between the positively charged activated carbon surface and anionic dye molecule leading to maximum adsorption of dyestuff (Senthil Kumar et al., 2010; Mittal et al., 2009). Thus pH 2.0 was selected as the optimum pH value and used for all further experiments. Similar results were reported for the adsorption of dyestuff using sunflower seed shells (Osma et al., 2007), sepiolite (Öztürk and Bektasß, 2006), thermally activated coir pith carbon (Kavithaa and Namasivayam, 2008), safflower seed cake biochar (Angın et al., 2013b) and cashew nut shell (Senthil Kumar et al., 2010). 3.3. Effect of adsorbent dosage The effect of dosage of activated carbon on the percentage removal of Yellow 18 is shown in Supplementary Fig. S5. The percentage removal of Yellow 18 increased from 38.35% to 98.81% as the activated carbon dosage was increased from 0.1 to 0.8 g/ 100 mL. This was the result of the increasing the number of available adsorption sites by increasing the adsorbent dosage (Wang



5

et al., 2010). When the activated carbon dosage was 0.5 g/100 mL, the removal percentage of Yellow 18 reached to 97.46%. The results also indicated that the removal efficiency beyond 0.5 g/100 mL was still increasing. But, this increase was negligible. Therefore, the 0.5 g/100 mL was chosen as the optimum dosage and used for further adsorption experiments. 3.4. Adsorption isotherms The adsorption isotherm describes how the adsorbed molecules distribute between the liquid phase and the solid phase when the adsorption process reaches an equilibrium state (Yang and Qui, 2010). The analysis of the isotherm data by fitting them to different isotherm models is an important step to find the suitable model that can be used for design purpose (Lin et al., 2013; Li et al., 2013). In this study, four isotherm models, namely the Langmuir, Freundlich, Dubinin–Radushkevich (D–R) and Temkin isotherm were used for describing the results. Comparison of isotherm models for Yellow 18 adsorption onto activated carbon at different solution temperatures was shown in Fig. 1. Also, the fitting results, i.e. isotherm constants and correlation coefficients are shown in Table 2. For Langmuir adsorption isotherm, RL values (Eq. (4)) for the investigated dye-adsorbent system, were found to be between 0.039 and 0.147 at different solution temperatures and confirmed the activated carbon was favourable for adsorption of Yellow 18 under the conditions used in this study. Also, the Langmuir isotherm parameters, Q0 and KL, were found to have increased with increasing temperature. The maximum adsorption capacity increased from 50.76 to 75.76 mg g1 as the temperature increased from 298 to 318 K. High temperatures caused to increase the kinetic energy of the dye and therefore enhanced the mobility of the dye ions (Karagozoglu et al., 2007). The correlation coefficients were 0.9905, 0.9858 and 0.9892 for 298, 308 and 318 K, respectively. The experimental results were also found to be consistent with Freundlich model. Magnitude of the exponent, 1/n, gives an indication of the favourability of adsorption (Yang and Qui, 2010; Mahapatra et al., 2012). Values of 1/n were found to be Temkin > Freundlich > Dubinin–Radushkevich. The results revealed that the equilibrium data fitted better by the two-parameter models rather than three-parameter models.

3.5. Thermodynamic parameters Adsorption thermodynamics such as Gibbs free energy change (DG°), enthalpy change (DH°), and entropy change (DS°) provide an insight into the mechanism and adsorption behavior of an


6


Table 2 Adsorption isotherm constants for adsorption of Yellow 18 onto activated carbon at different solution temperatures. Isotherms

Solution temperature (K)

Langmuir 298 308 318

Constants Q0 (mg g1)

KL (L mg1)

50.76 58.82 75.76

0.121 0.188 0.499

0.9905 0.9858 0.9892

KF (mg/g) (L/mg)1/n

R2

0.3831 0.4368 0.5607

9.986 12.319 22.951

0.8787 0.8246 0.9846

qs (mg g1)

B (mol2 kj2)

E (kj mol1)

R2

38.18 44.48 43.81

1.0 0.8 0.09

298 308 318

298 308 318

0.027–0.142 0.017–0.096 0.007–0.039

R2

1/n (L mg1)

Freundlich

D–R

RL

Temkin 298 308 318

0.71 0.79 2.36

0.8949 0.9160 0.8702

A(L g1)

B1

R2

1.892 2.248 5.939

9.4023 11.978 15.427

0.9605 0.9568 0.9761

Table 3 Thermodynamic parameters for the adsorption of Yellow 18 onto activated carbon. T (K)

DG° (kJ mol1)

DH° (kJ mol1)

DS° (J mol1K1)

298 308 318

5.05 ± 0.54 6.15 ± 0.35 8.62 ± 0.42

47.91 ± 14.80

177.01 ± 47.55

isolated system. Its original concept assumes that energy cannot be gained or lost, which entropy change is the driving force (Lin et al., 2013). Generally, free energy (DG°) values between 20 kJ mol1 and 0 kJ mol1 suggests a physisorption process; whilst DG° values in range of 80 kJ mol1 to –400 kJ mol1 suggests a chemisorption process (Huang et al., 2014). In Table 3, the overall free energy change during the adsorption process was negative for the experimental range of temperatures, corresponding to a spontaneous physical process of Yellow 18 adsorption onto activated carbon. When the temperature increases from 293 to 318 K, the magnitude of free energy change shifts to high negative value suggested that the adsorption was more favorable at high temperature. The positive value of DH° (47.91 ± 14.80) indicates that the adsorption is endothermic (Foo and Hameed, 2012b). This is in accordance with the aforementioned result depicted by the isotherms (Table 2); that the adsorption capacity increased with increasing temperature. Meanwhile, the value of DS° was 177.01 ± 47.55 J mol1 K1, which indicated the increasing of randomness at the solid–solution interface with the loading of Yellow 18 molecules onto the external and internal surfaces of the carbonaceous substance (Table 3).

3.6. Adsorption kinetics Adsorption kinetic describes the controlling mechanism of adsorption processes which in turn governs the mass transfer and equilibrium time (Mestre et al., 2011). The experimental data of Yellow 18 adsorption onto activated carbon at different time intervals were examined using pseudo-first-order, pseudosecond-order and intra-particle diffusion kinetic models. The fit of these models was checked by each linear plot of log (qeqt) versus t (Fig. 2a), (t/qt) versus t (Fig. 2b) and qt versus t1/2 (Fig. 2c) at the temperatures of 298, 308 and 318 K, respectively and by comparing to the regression coefficients for each expression. The rate constants, calculated equilibrium uptakes and the corresponding correlation coefficients were given in Table 4. The

Fig. 2. Linear kinetic plots for the adsorption of Yellow 18 onto activated carbon (a) pseudo-first-order, (b) pseudo-second-order, (c) intra-particle diffusion model.


D. Angin / Bioresource Technology xxx (2014) xxx–xxx Table 4 Kinetic parameters for the adsorption of Yellow 18 onto activated carbon at different solution temperatures. Solution temperature (K)

298

308

318

qe,exp (mg g1)

19.49 ± 0.108

19.64 ± 0.048

19.84 ± 0.024

Pseudo-first-order k1 (min1) qe,calc (mg g1) R2 Pseudo-second-order k2 (g mg1 min1) qe,calc (mg g1) R2 Intra-particle diffusion kp (mg g1 min1/2) C (mg g1) R2

7

necessary to thank instructor Halil Ibrahim Balkul and Dr. Serpil Ozturk for proofreading of the current manuscript on behalf of ‘‘post-eding office of Sakarya University for academic publications’’. Appendix A. Supplementary data

0.0023 9.24 0.9495

0.0037 8.84 0.9818

0.0051 7.75 0.9595

0.0029 16.08 0.9967

0.0025 18.15 0.9975

0.0029 19.31 0.9994

0.3584 8.64 0.9567

0.4353 9.24 0.9786

0.4570 10.35 0.9371

correlation coefficients of pseudo-first-order kinetic model were low. Moreover, a big difference between the experimental and calculated equilibrium adsorption capacity (qe) was observed, indicating a poor fit of pseudo-first-order model to the experimental data. At all studied temperatures, the straight lines with extremely high correlation coefficients (>0.9967) were obtained. In addition, the calculated qe values also agreed with the experimental data in the case of pseudo-second-order kinetic model. Therefore, the adsorption kinetic could be approximated more favorably by pseudo-second-order kinetic model for Yellow 18 adsorption. Similar results were also reported by other researchers (Angın et al., 2013b; Lin et al., 2013; Demiral and Gündüzog˘lu, 2010; Senthil Kumar et al., 2010). The pseudo-first-order and pseudo-second-order kinetic models cannot identify the diffusion mechanism. Therefore, the kinetic results were also analyzed by using the intra-particle diffusion kinetic model (Gerçel et al., 2007). According to the intraparticle-diffusion model, a plot of qt versus t1/2 should be linear if intra-particle-diffusion is involved in the adsorption process, and if this line passes through the origin the intra-particle diffusion is the rate controlling step. When the plot does not pass through the origin, this is indicative of some degree of boundary layer control and this further shows that the intra-particle diffusion is not the only rate limiting step, but also other kinetic models may control the rate of adsorption, all of which may be operating simultaneously (Demiral and Gündüzog˘lu, 2010; Senthil Kumar et al., 2010). As can be seen from Fig. 2c, the linear plots at each temperature did not pass through the origin, which indicates that the intra-particle diffusion was not only a rate controlling step. 4. Conclusion This study demonstrated the feasibility of sour cherry stones as a promising precursor for the production of activated carbon with a noticeable adsorption capacity for the removal of Yellow 18. The results just showed initial pH value and solution temperature had effects on the adsorption capacity. The kinetic and isotherm model analyses revealed that the experimental data well fitted to the pseudo-second-order model and Langmuir isotherm. The adsorption process was found to be controlled by external mass transfer at the earlier stages and by intra-particle diffusion at the later stages. The thermodynamic parameters indicated a spontaneous and endothermic adsorption. Acknowledgements I would like to thank ASYA Fruit Juice and Food Ind. Inc. (Isparta, Turkey) for providing sour cherry stones. Also, it would be

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