Journal of Molecular Liquids 194 (2014) 130–140

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Journal of Molecular Liquids

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Elimination of anionic dye by using nanoporous carbon prepared from anindustrial biowaste

Fuat Güzel a,⁎, Hasan Sayğılı b, Gülbahar Akkaya Sayğılı a, Filiz Koyuncu aa Department of Chemistry, Faculty of Education, Dicle University, 21280 Diyarbakır, Turkeyb Department of Chemistry, Faculty of Science & Arts, Batman University, 72060 Batman, Turkey

⁎ Corresponding author. Tel.: +90 412 2488377; fax: +E-mail address: fguzel@dicle.edu.tr (F. Güzel).

0167-7322/$ – see front matter © 2014 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.molliq.2014.01.018

a b s t r a c t

a r t i c l e i n f o

Article history:Received 14 November 2013Received in revised form 13 January 2014Accepted 17 January 2014Available online 28 January 2014

Keywords:Tomato wasteNanoporous carbonOrange II adsorptionIsothermKinetics

The preparation of nanoporous carbon from tomato waste (TWNC), and its ability to remove Orange II (OII) dyewere reported. The TWNCwas characterized by Fourier Transform Infrared Spectroscopy (FTIR), Brunauer Tellersurface area (BET), Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD). The effects of initial con-centration, solution pH, adsorbent dosage and temperature were investigated. The kinetic data followed apseudo-first order model. The mechanism of the process was determined from the intraparticle diffusionmodel. The isotherm analysis indicated that the adsorption data could be represented by the Langmuir model.The maximum monolayer adsorption capacity was determined as 312.5 mg g−1 under determined optimumconditions of variables (pH 2.0, adsorbent dosage 0.1 g L−1, contact time 180min and temperature 50 °C). Ther-modynamic study showed that the adsorption was spontaneous and endothermic. The results indicate thatTWNC can be employed as low-cost alternative to expensive commercial activated carbon for treatment of indus-trial wastewater containing OII.

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

The discharge of dye effluents from textile, leather, paper, and plasticindustries into the environment causes severe problems to many formsof life [1]. Azo dyes are the synthetic dyes that are used in many textileindustries. Azo dyes have azo group band (\N_N\) and because oftheir low cost, solubility and stability, they arewidely used inmany tex-tile industries. Azo dyes and their intermediate products are toxic, carci-nogenic and mutagenic to aquatic life [2].

Acid Orange II (p-(2-hydroxy-1-naphthylazo)-benzene sulfonicacid) (OII) is a popular water-soluble monoazo dye that is used for dye-ing a variety ofmaterials such as , cosmetics, wool, silk, cotton and paperindustries. Like most of other azo dyes, it tends to be disposed in indus-trial wastewater and poses a severe health threat to humans [3–5]. It ishighly toxic, and its ingestion can cause eye, skin, mucous membrane,and upper respiratory tract irritations; severe headaches; nausea;water-borne diseases such as dermatitis; and loss of the bone marrowleading to anemia. It has now been well determined that the maincause of its chronic toxicity is the electron-withdrawing character ofthe azo group, which develops an electron deficiency and becomes re-duced to carcinogenic amino compounds. Its consumption can befatal, as it is carcinogenic in nature and can lead to tumors [6,7].Hence, removal of dyes from such wastewaters is a major environmen-tal problem and it is necessary because dyes are hazardous even at lowconcentrations.

90 412 2488257.

ghts reserved.

Color removal from industry or domestic effluents has drawn con-siderable attention in the last few years because of their toxicity and vis-ibility. Various conventional technologies including chemical oxidation,biological treatment, coagulation–flocculation, and membrane process-es are currently effective methods for reducing dye concentrations inwastewater. However, these treatment processes are costly and cannoteffectively be used to treat a wide range of dye containing wastewater.Adsorption techniques for wastewater treatment have become morepopular in recent years owing to their efficiency in the removal of pol-lutants than other conventionalmethods. As a powerful adsorbent, acti-vated carbon (AC) has been widely used for various applications owingto its high surface area and porous features, such as purification ofdrinking water, treatment of exhaust gas and wastewater, support ofcatalyst, gas storage, and electrochemical capacitor and so on. As a resultof environmental compliance in many countries, the need for AC willcontinue growing [8]. However, due to their high production costs,these materials tend to be more expensive than other adsorbents. Thishas led a growing research interest in the production of ACs from re-newable and cheaper precursors. The choice of precursor largely de-pends on its availability, cost, and purity, but the manufacturingprocess and intended applications of the product are also importantconsiderations [9,10]. Therefore, in recent years, various kinds of AChave been prepared from low-cost precursor materials, which are pre-dominantly vegetable wastes, such as orange peels, melon seeds [11],coir pith [12], coconut coir [13], bamboo dust, coconut shell, groundnutshell, rice husk, and straw [14], corncob [15], almond shell [16], palmshell [17], and coconut shells [18,19]. In this paperwe report the adsorp-tion study of nanoporous carbon that is prepared from tomato waste

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131F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

(TW). Tomato is a very abundant and inexpensive material in Mediter-ranean countries. According to the records of the United Nations Foodand Agriculture Organization (FAO), tomato is the most widely grownproduct in fresh vegetables around the world with a productionof 145.6 million tons. Turkey ranks fourth with the production of10 million tons of tomato in the world [20].

In this study,we prepared nanoporous carbon fromTWas a low-costand abundantly available precursor, which is a waste of tomato juiceand paste factories, with chemical activation by zinc chloride as adehydrating agent. Optimum adsorption conditions are determined asa function of pH, TWNC dose, initial OII concentration, contact time,and temperature. The adsorption mechanisms are also evaluated interms of kinetic, equilibrium isotherm and thermodynamic parameters.

2. Materials and methods

2.1. Materials

TWwas collected from tomato paste factory in Adana city of Turkey.Firstly, the TWwaswashed and dried in an air oven at 70 °C for 24 h andthen crushed and sieved to the desired particle size (between 177 μmand 400 μm) for using in the chemical activation experiment. Zinc chlo-ride (purchased from Sigma-Aldrich) of purity 99.9%was used as chem-ical activator in preparation of TWNC. OII (purchased from Fluka) wasused as adsorbate. The general characteristics are given in Table 1 [21].The dye stock solution was prepared by dissolving accurately weigheddye in distilled water to a concentration of 1000 mg L−1. The experi-mental solutions of the desired concentrationswere obtainedby succes-sive dilutions. All chemicals used were of analytical grades.

2.2. Preparation of TWNC

30 g of TW (on a dry basis) was impregnated with 30 g zinc chloride(TW/ZnCl2 weight ratio of 1:1) and 5mL of distilledwater was added tothis mixture for obtained slurry. Then, this slurry was dried at 105 °Cfor 12 h in an air oven. The impregnated sample was placed in astainless-steel horizontal reactor (7 cm diameter and 100 cmlength), and then heated to the activation temperature of 500 °Cfor 1 h under nitrogen atmosphere (99.99%) flow (100 mL min−1)at a heating rate of 10 °Cmin−1. It was cooled down to room temperatureunder nitrogen flow and then carbonized product was added on 0.2 N500 mL hydrochloric acid solution and boiled for 1 under reflux. Thismixture was filtered and washed with hot distilled water at severaltimes to remove residual chemicals and chlorine until filtrated solutiondid not give any reactionwith 0.1N silver nitrate. The yieldwas calculated

Table 1General characteristics of Orange II dye.

Chemical structure

Molecular formula C16H11N2NaO4S Molecular weight (g/mol) 350.32C.I. number 15510 Molecular volume (Ǻ3) 232C. I. name Acid Orange II Molecular surface area

(Å2/molecule)a279

Chemical class Anionic dye Width (Ǻ)a 7.3Chromophore Monoazo Length (Ǻ)a 13.6Ionization Acidic Depth (Ǻ)a 2.3Dye content (%) 85λmax (nm) 485

a By referenced [21].

as the ratio of the dryweight of resultant activated carbon to theweight ofthe air-dried of the raw precursor. The obtained TWNC was dried at105 °C for 12 h and ground and sieved between 177400 μm. Finally, theTWNC was stored in desiccators for further use in adsorptionexperiments.

2.3. Characterization of TWNC

The physical and chemical characteristics of the activated carbon, in-cluding: proximate and ultimate analysis, total pore volume, mesoporevolume, micropore volume, surface area, average pore diameter, pointof zero charge (pHpzc), and surface functional group analyses were de-termined using standard analytical procedures.

The proximate analysis was conducted according to ASTM D3173-3175 standards [22] and the results were expressed in terms of mois-ture, ash, volatile matter, and fixed carbon contents. The Elemental An-alyzer (Thermo Scientific Flash 2000, CHNSAnalyzer, Italy)was used forultimate analysis, and the results were expressed in terms of carbon, ni-trogen, hydrogen and sulfur element contents. The content of oxygenwas calculated as difference to 100%.

The surface physicalmorphologies of TWand TWNCbefore and afterOII adsorption were identified by using SEM technique (JEOL JSM-6335F, USA).

The XRD patterns of the TW and TWNC were collected on an X-raypowder diffractometer (Bruker, D8 Discovery EVA, Germany). Texturalcharacteristics were determined by N2 adsorption–desorption isothermsmeasured at−196 °C (Micromeritics, ASAP 2020). Prior to the measure-ment, the TWNCwas outgassed at 320 °C under nitrogen flow for 6 h. Thenitrogen adsorption–desorption data were recorded at liquid nitrogentemperature (−196 °C) and was measured over a relative pressure(P/Po) range from approximately 10−6 to 1. The specific surface area(SBET) was determined by means of the standard BET equation [23,24]applied in the relative pressure range from0.05 to 0.35 [25]. This study as-sumes that the cross-sectional area of a nitrogen molecule is 0.162 nm2.The external surface area (including only mesopore, Sext), micropore vol-ume (Vμ) and micropore area (Sμ) were calculated by t-plot method. Thetotal pore volume (VT) was estimated by converting the amount of nitro-gen gas adsorbed (expressed in cm3g−1 STP) at a relative pressure of 0.95to liquid volume of the nitrogen adsorbate [26]. The mesopore volume(Vm) was determined by subtracting the micropore volume from thetotal pore volume while the microporosity fraction (Vμ (%) = Vμ / VT)and mesoporosity fraction (Vm (%) = Vm / VT) were based on the totalpore volume. The average pore diameter (Dp) was estimated from theBET surface area and total pore volume (Dp = 4VT / SBET) assuming anopen-ended cylindrical pore model without pore networks [27]. Thisstudy assumes that micropores are less than 2 nm wide, mesopores are2–50 nm wide and macropores are more than 50 nm wide [25,26]. Thepore size distribution was determined by using Barrett–Joyner–Halenda(BJH) method (Harkins and Jura model, FAAS corrections) [28].

The surface chemistry of TWNC was analyzed by FTIR spectroscopy,Boehm's titration, and pH of the point of zero change (pHpzc) methods.Surface functional groups were detected using the pressed potassiumbromide (KBr) pellets containing 5% of carbon sample by FTIR spec-trometer (PerkinElmer spectrum 100, USA) in the scanning range of4000–450 cm−1. The quantification of the basic and acidic groups onthe surface of the TWNC was performed according to the Boehms titra-tion method [29] using analytical grade reagents. The combined influ-ence of all the functional groups of activated carbon determines pHpzc,i.e., the pH at which the net surface charge on carbon was zero. ThepHpzc of the TWNCwas determined by themethod described by Preethiand Sivasamy [15]. The difference between the initial pH (pHi) andΔpH(pHi−pHf) values was plotted against the pHi. The point of intersectionof the resulting curvewith abscissa, where pHwas zero, gives the pHpzc.

Adsorptive properties of the TWNCwere preliminarily characterizedby measuring both iodine and methylene blue numbers. The iodinenumber andmethylene blue number tests were conducted as described

132 F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

in ASTM [30] and the China National Standards [31]. The iodine number(mg of iodine adsorbed/g of carbon, IN) and methylene blue number(mg of methylene blue adsorbed/g of carbon, MN) are considered as ameasure of adsorption capability of activated carbon. Normally, iodinenumber denotes the amount of micropore (less than 10 Å in diameter)and methylene blue number (equal or greater than 15 Å in diameter)denotes the amount of mesopore of activated carbon [32,33].

Some measured chemical and physical characteristics of the TWNCand TW are given in Tables 2, 3 and 4.

2.4. Batch adsorption experiments

Adsorption experiments were carried out in 100 mL flasks and thetotal volume of the reaction solution was kept at 50 mL. The flaskswere shaken at 120 rpm for the required time in a water bath shaker(Daihan-WSB-30, Korea). The effects of various operating parameters,pH of the solutions (2–9), adsorbent dosage (0.1–0.9 g), initial con-centration of dye (75–200 mg L−1), contact time (5–270 min) andtemperature (20–50 °C), on the adsorption of OII were studied. Thetemperature was controlled using an isothermal shaker. After each ad-sorption process, the samples were centrifuged (5000 rpm, 10 min)for solid–liquid separation and the residual dye concentration in solu-tion was analyzed by a UV–vis spectrophotometer (PerkinElmer-Lamda 25, USA) at 484 nm. In the present study, adsorption isothermswere carried out by using several solutions with different concentra-tions. The amount of dye adsorbed onto per gram of adsorbent (qe)was calculated by using Eq. (1).

qe ¼C0−Ceð ÞV

mð1Þ

where C0 and Ce are initial and equilibrium OII concentrations, respec-tively (mg L−1), V is OII solution volume (L), andm is themass of adsor-bent (g).

For kinetic studies, 0.1 g of TWNC was contacted with 50 mL of OIIconcentrations 75–200 mg L−1 by using water bath shaker at 25 °C.The agitation speedwas kept constant at 120 rpm. At predetermined in-tervals of time, solutions were analyzed for the final concentration ofOII. The amount of adsorption qt (mg g−1), at time t (h), was calculatedby:

qt ¼C0−Ctð ÞV

Wð2Þ

where Ct (mg L−1) is concentrations of OII at time t.TWNC was separated from solution by centrifugation and then

dried. Itwas agitatedwith 50mLof distilledwater at different pH values(2–12) for the predetermined contact time of the adsorption process.The desorbed OII in the solution was separated by centrifugation and

Table 2Proximate and ultimate analyses of TW and TWNC.

Proximate analysis (wt.%) Ultimate analysis (wt.%)

TW TWNC TW TWNC

Moisture 2.95 7.48 C 59.84 72.93Ash 1.58 1.47 H 8.79 3.49Volatile matter 82.67 22.73 S 0.26 0.33Fixed carbona 12.80 68.32 N 4.08 3.42Burn off – 78.63 Oa 27.03 19.83Yield – 21.37Chemical recovery(ZnCl2 %)

– 96.42

a By difference.

analyzed as before. The desorption efficiency (D%) was calculatedusing Eq. (3):

D% ¼ CdCa

� 100 ð3Þ

where Cd is the desorbed amount (mg L−1) and Ca is the adsorbedamount (mg L−1).

2.5. Error analysis

Nonlinear Chi-square test (χ2) and average relative error (ARE) tocheck consistency of adsorption kinetic and isothermmodels were per-formed in addition to determination the linear regression correlationcoefficient (R2). The expressions of the error functions are given below:

χ2 ¼XN

i¼1

qe;exp−qe;cal� �2

qe;expð4Þ

ARE ¼ 100N

XN

i¼1

qe;exp−qe;cal���

���qe;exp

24

35i

ð5Þ

where qe,exp and qe,cal represent the amount of OII (mg g−1) determinedas experimentally and calculated adsorbed, respectively. N is the num-ber of observations in the regressionmodel. Isotherm and kinetic exper-iments were carried out at least twice to check reproducibility of theresults and the average results were used in the error analysis.

3. Results and discussion

3.1. Characterization of the TWNC

SEM micrographs of TW and TWNC before and after adsorption areshown in Fig. 1. As can be seen from Fig. 1a, the surface texture of theTWwas regular and undulating with only a few pores available on thesurface. However, after ZnCl2 activation treatment, many various sizesof pores in a honeycomb can be observed on the sample surface asshown in Fig. 1b. During activation process, the ZnCl2–carbon reactionoccurred, which enhance the pore development thus, the surface areaand porosity increased. In addition, almost heterogeneous type ofpores structure was distributed on the TWNC surface. It is clear that,TWNC appears to have numbers of poreswhere, there is a good possibil-ity for dye to be trapped and adsorbed into these pores. SEM imagesshowed bright dark color on the surface (Fig. 1b). The surface after ad-sorption was turned to light color (Fig. 1c). This may be due to the ad-sorption of OII on the surface.

X-ray diffraction is utilized to investigate the crystalline structure ofthe samples. As shown in Fig. 2, there were very broad diffraction peaksin both of the samples. TW shows binary peak between 2θ = 10° and26° ranges. TWNC exhibits in wide 2θ ranges (between 2θ = 10° and20°). The XRD pattern reveals the amorphous state of the obtained sam-ples. Activated carbons which are achieved from agricultural wastesshow usually amorphous structure. Possible functional groups whichexist on the activated carbon surface may complicate formation of thecrystallization [34]. According to the FTIR results, TWNC has a multi-functional surface; hence these functional groups complicate the crys-tallization of the sorbent. XRD results confirm this situation.

Nitrogen adsorption is a standard technique widely used for the de-termination of porosity of carbonaceous adsorbents. Fig. 3a shows thenitrogen isotherm at−196 °C for the TWNC. As seen from Fig. 3a, it ex-hibits adsorption isotherm of type IV according to IUPAC [24]. The typeIV isotherm characteristically shows the simultaneous presence ofmicro and mesopore. The initial part of the isotherm follows the samepath like the corresponding type II isotherm and therefore the result

Table 3Textural and adsorptive characteristics of TWNC.

SBET(m2 g−1)

Sμ(m2 g−1)

Sext(m2 g−1)

VT(cm3 g−1)

Vμ(cm3 g−1)

Vμ(%)

Vm(cm3 g−1)

Vm(%)

Dp(nm)

IN(mg g−1)

MN(mg g−1)

722.17 345.85 376.32 0.476 0.201 42.23 0.275 57.77 2.64 592.88 134.80

(a)

(c)

(b)

133F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

of monolayer–multilayer adsorption on the mesopore walls [35]. TheBET surface area (SBET), external surface area (mesopore surface area,Sext), micropore surface area (Sμ), total pore volume (VT), microporefraction, mesopore fraction and average pore diameter (Dp) results ob-tained by applying the BET equation and t-method to nitrogen adsorp-tion at −196 °C are listed in Table 3. The pore size distribution isshown in Fig. 3b. It can be found from Fig. 3b that the sample exhibitsmultimodal distribution in both the micropore and mesopore domainsbut does not have macropores. This was confirmed by the microporeand mesopore fraction values in Table 3. Porosity results suggest thatthe TWNCbecause of having developedmesopore is suitable for dye ad-sorption. This idea is also supported by the values of IN and MN values(Table 3). Therefore, the high BET surface area of the TWNC rendersthem to be suitable as effective adsorbent for the removal of both airand wastewater pollutants.

The FTIR spectra of TWNC before and after adsorption were takento determine the frequency changes in the functional groups of theadsorbent and compared with each other to confirm the adsorptionof the dye (Fig. 4). In the spectra of native TWNC, the band observedat 3245.68 cm−1 present bonded \OH group on the surface. Thisband disappeared after adsorption, indicating that \OH groupsplay an important role in the adsorption. The bands observed at2922.96 and 2853.11 cm−1 correspond to symmetric \CH2 vibra-tion and \CH2 stretching vibration, respectively. These peaks disap-peared after adsorption. The peak at 1741.68 cm−1 is the indicativeof C_O stretching of carboxylic acids, which disappeared after adsorp-tion. The peak observed at 1592.65 cm−1 is assigned to aromatic C_Cstretching. This peak shifted to 1573.25 cm−1 with a significance dif-ference of 19.4 cm−1 after adsorption. The peaks located at 1374.41 and1154.55 cm−1 present lactone groups on the TWNC surface. The bandsthat appeared at 1090.63 and 718.72 cm−1 indicated Si\O stretching vi-brations. New bands appeared in the fingerprint region at 1114.28,1002.49 and 869.20 cm−1 on the TWNC surface after adsorption. Thesebands confirm the adsorption of OII on TWNC surface. The probable bind-ing mechanism of the OII (at pH 2) is given in Fig. 5.

The surface acidity and basicity are important criteria describing thesurface chemistry of the carbon adsorbents. The combined influence ofall the functional groups of TWNC determines pHpzc. Fig. 6a shows thatat 5.30, ΔpH = 0. Therefore the pHpzc of TWNC is 5.30. Analysis of thedata presented in Table 4 shows that TWNC exhibits an acidic behavior,with the surface acidity of 1.33meq g−1 with themaximum compositionof phenolic group (0.26meq g−1)with traces of lactonic (0.53 meq g−1)and carboxylic (0.54 meq g−1) groups, and 0.95 meq g−1 as surfacebasicity. Also, FTIR spectra confirm the presence of the carboxylic andphenolic groups.

3.2. Effect of various parameters for OII adsorption on TWNC

3.2.1. Effect of solution pHThe aqueous solution pH has been reported to present a significant

influence on the adsorptive uptake of dye molecules due to its impacton both the surface binding-sites of the adsorbent and the ionization

Table 4Surface chemical characteristics of the TWNC.

Carboxylic(meq g−1)

Phenolic(meq g−1)

Lactonic(meq g−1)

Total acidity(meq g−1)

Total basicity(meq g−1)

pHpzc

0.54 0.26 0.53 1.33 0.95 5.30

process of the dye molecule [36]. The effect of initial pH on adsorptionwas determined at different pH values (2–9). The pH values of solutionswere adjusted by dropwise addition of 0.1 M HCl and 0.1 M NaOH solu-tion. In this study, initial dye concentration, shaking time, temperatureand the amount of adsorbent were fixed at 200 mg L−1, 270 min,25 °C and 0.1 g, respectively. Fig. 6a shows the effect of pHonOII remov-al. It was observed that adsorption capacity decreased by increasing pH

Fig. 1. SEM micrographs of raw TW (a), TWNC (b), and TWNC after OII adsorption(c) (magnifications: 1000×).

Fig. 2. XRD profiles of TWNC and TW.

134 F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

from 2 to 9. The maximum adsorption was observed at pH 2.0, indicat-ing that the adsorptionwas strongly pH dependent. The pHpzc valuewasdetermined as 5.30 (Fig. 6a). At pH b pHpzc, the carbon surface has a netpositive charge, while at pH N pHpzc the surface has a net negativecharge [37]. The pHpzc value indicated that the surface of TWNC is pos-itively charged at pH less than 5.30 and negatively charge at pH valuesabove 5.30. At pH2, a significantly high electrostatic attraction exists be-tween negatively charged OII ions and TWNC surface. Therefore, pH 2.0was considered to a more effective pH, and it was used for further stud-ies. The lower adsorption at alkaline pH is because of the presence of ex-cess OH− ions competing with the dye anions for the adsorption sites.At low pH, the electrostatic attraction is significantly high, which causesan increase in dye adsorption due to the increase in the number of pro-tonated groups, such as carboxylic (― CO― OH2+), phenolic (― OH2+)and chromenic groups on the surface of the activated carbon [38,39].As the pH of the system increases, the number of negatively chargedsites increases and the number of positively charged sites decreases. Anegatively charged surface site on the adsorbent did not favor theadsorption of anionic dye due to electrostatic repulsion. In this study,the electrostatic attraction and repulsion reactions expected to occurat the solid/liquid interface in acidic and alkaline media are given

Fig. 3. Nitrogen adsorption (filled symbols–desorption (empty symbols) isotherms(a) and pore size distribution (b) for the TWNC.

Eqs. (6) and (7), respectively. A similar result was observed for theadsorption of various textile dyes on some agricultural waste and acti-vated carbon [40–42].

TWACð ÞOH→HþTWACð ÞOHþ2 þ OIIð Þ−→ TWACð ÞOHþ2−−−− OIIð Þ− ð6Þ

TWACð ÞOH →OH−

TWACð ÞO− þH2Oþ OIIð Þ−→ TWACð ÞO−↔ OIIð Þ− ð7Þ

3.2.2. Effect of adsorbent doseThe sorption of OII onto TWNC was investigated with changing the

quantity of adsorbent from 0.1 to 0.9 g L−1. For this investigationTWNC was contacted with 275 mg L−1 OII solutions (50 mL) for270 at 25 °C. As presented in Fig. 6b, with the increasing amount of ad-sorbent, qe (mg g−1) values, which are the amount of OII adsorbed perunit weight of adsorbent at equilibrium, were decreased. This may beattributed to the decrease in total adsorption surface area available toOII resulting from overlapping or aggregation of adsorption sites [43].

3.2.3. Effect of contact time and initial concentrationThe initial concentration provides an important driving force to

overcome the mass transfer resistances of all of the molecules betweenthe aqueous and solid phases [44]. The adsorption of OII on TWNC atvarious initial concentrations at pH2 and 25 °Cwas studied as a functionof contact time in order to determine the necessary adsorption equilib-rium time. Fig. 6c shows the effects of contact time and initial concentra-tion. The adsorption at different OII concentrations is rapid at the initialstages and then gradually decreases with the progress of adsorptionuntil the equilibrium is reached. The rapid adsorption at the initial con-tact time can be attributed to the availability of the positively chargedsurface of TWNC and the slow rate of OII adsorption is probably due tothe slow pore diffusion of the dye ions into the bulk of the solution tothe TWNC surface. As shown in Fig. 6c, the required time to reach equi-librium was 180 min and selected as contact time for further experi-ments. In the adsorption process, initially dye molecules have to firstencounter the boundary layer effect and then it has to diffuse fromboundary layerfilm onto adsorbent surface and thenfinally, it has to dif-fuse into the porous structure of the adsorbent. This phenomenon willtake a relatively longer contact time. However, the adsorbed amountof OII was smaller at lower initial concentrations and greater at higherinitial concentrations, it was also seen that an increase in initial concen-trations resulted in increased dye uptake. The adsorption capacity atequilibrium (qe) increased from 11.54 to 25.55mg g−1 with an increasein the initial OII concentrations from 75 to 200 mg L−1.

3.2.4. Effect of temperatureThe effect of solution temperature on the OII adsorption processwas

examined by varying the adsorption temperature from 20 to 50 °C,while other operating parameterswere kept constant. The effect of tem-perature on OII adsorption is shown in Fig. 6d. The adsorption wasfound to increase from 185.0 to 312.5 mg g−1 with an increase in tem-perature. This suggests that the adsorption process is endothermic innature. This may be due to the increase in the dye mobility to penetrateinside the sample pores at high temperature. Besides, it might also bedue to the increase in chemical interaction between the adsorbate andsurface functionalities of adsorbent [45].

3.2.5. Effect of ionic strengthThe ionic strength of the solution is one of the factors that control

both electrostatic and non-electrostatic interactions between the adsor-bate and the adsorbent surface [46]. Different concentrations (0–2M) ofNaCl solution were used to determine the effect of ionic strength on theadsorption of OII. Experimental conditions such as; initial dye concen-tration, adsorbent dose, shaking time, temperature and pH were fixedat 200 mg L−1, 0.1 g, 270 min, 25 °C and 2.0, respectively. The effect of

Fig. 4. FTIR spectra of TWNC and TWNC after adsorption of OII.

135F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

ionic strength is shown in Fig. 6e. As can be seen, the adsorbed amountis increased as the concentration of salt is increased up to 0.4 M. Thiscould be attributed to the salt, which does not affect the interaction be-tween the dye molecules and sorption sites. However, a slight decreasein adsorbed amounts after this concentration is attributed to competi-tive adsorption between dye and increasing chloride ions with increas-ing of salt concentrations on the positively charged adsorbent at pH 2.

3.3. Kinetic studies

Adsorption kinetic studies are important in the treatment of aqueouseffluents because they provide valuable information on the mechanismof the adsorption process. The adsorption kinetics of TWNC for OII at theinitial concentration from 75 to 200 mg L−1 is shown in Fig. 6c. In thisstudy, the kinetic data were analyzed using two kinetic models:pseudo-first order [47] and pseudo-second order [48] kinetic models.The best-fit model was selected based on R2, ARE and χ2 values.

The linear form of the pseudo-first order kinetic model equation is:

log qe−qtð Þ ¼ logqe−k1

2303t ð8Þ

where k1 (min−1) is the pseudo-first order sorption rate constant, qeand qt (mg g−1) are the amount of the adsorbate adsorbed per gramof adsorbent at equilibrium and any time, respectively. The values of

S O

O

NN

OH2

O COOH2

O

HHH

Fig. 5. Binding mechanism for the

k1 and qe for OII adsorption by TWNC were determined from the plotof log(qe − qt) versus 1/t (Fig. 7a).

The linear form of the pseudo-second order kinetic model equationis:

tqt

¼ 1k2q

2eþ 1qe

t ð9Þ

where k2 (g mg−1 min−1) is the pseudo-second order sorption rateconstant. Plots of t/qt versus t for all experimental concentrations gavestraight lines (Fig. 7b), and values of qe and k2 were calculated fromthe slope and intercept, respectively.

The values of kinetic parameters and R2, ARE and χ2 values calculat-ed from both kinetic models are listed in Table 5. As seen in Table 5, thepseudo-first order kinetic model has the higher R2 and lower ARE andχ2 values than the pseudo-second order kinetic model. However, qe,calvalues determined for the pseudo-first order kineticmodelweremainlyclose to the qe,exp values. These indicate that the adsorption of OII ontoTWNC complies with the pseudo-first order kinetic model.

3.4. Adsorption mechanism

Thepseudo-first order and pseudo-secondorder kineticmodels can-not identify the diffusion mechanism and rate controlling steps that af-fect the adsorption process, thus intraparticle diffusion model based on

S O

O

NN

OH2

COOH2

O

C

O

H2OOH

adsorption of OII onto TWNC.

ΔΔpH

pHi

qe (m

g/g

)

(a) (b)

(d)

(e)

(c)

Fig. 6.Effects of solution pH andpHpzc (a), adsorbent dose (b), initial dye concentration and contact time (c), solution temperature (d) and ionic strength (e) on the adsorption of OII onto TWNC.

136 F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

the theory proposed byWeber andMorris [49]was tested. According tothis theory, the intraparticle diffusion equation is expressed as follows:

qt ¼ kidt1=2 þ C ð10Þ

where kid is the intraparticle diffusion rate constant (mg g−1 min−1/2)and C is the thickness of the boundary layer. If themechanismof adsorp-tion process follows the intraparticle diffusion, the plot of qt versus t1/2

would be a straight line and the kid and C can be calculated from theslope and intercept of the plot, respectively.

Fig. 7c shows the adsorbed amount versus t1/2 for different initialconcentrations. As seen from Fig. 7c, plots are not linear over thewhole time range,which indicates thatmore than onemode of sorption,

involved in the OII adsorption. As already mentioned, the adsorptionmechanism for any adsorbate removal by an adsorption process maybe assumed to involve the following four steps: (i) bulk diffusion;(ii) film diffusion; (iii) pore diffusion or intraparticle diffusion; and(iv) adsorption of dye on the sorbent surface [50]. It has been demon-strated in the literature that the first step could be “neglected” if a suf-ficient speed of stirring was used. For intraparticle diffusion plots, thefirst portion indicates a boundary layer effect at the initial stage of theadsorption. The second portion of linear curve (shown in Fig. 7c) isthe gradual adsorption stage where intraparticle diffusion is the ratelimiting step. In some cases, the third portion exists, which is the finalequilibrium stage where intraparticle diffusion starts to slow downdue to the extremely low adsorbate concentrations left in the solutions

Fig. 7. The plots of pseudo first-order (a), pseudo second-order (b) and intraparticle diffusion(c) kinetic models at various concentrations for OII adsorption onto TWNC.

Table 5Kinetic parameters for OII adsorption onto TWNC at different initial concentrations.

C0 (mg L−1) 75 125 150 175 200

Pseudo-first orderqe,exp (mg g−1) 11.54 15.44 17.86 21.41 25.55qe,cal (mg g−1) 8.68 14.28 17.41 21.06 24.45k1 (min−1) 0.0193 0.0159 0.0161 0.0196 0.0128R2 0.9981 0.9849 0.9965 0.9975 0.9980χ2 0.13 0.09 0.01 0 0.05ARE 1.19 0.94 0.31 0.20 0.54

Pseudo-second orderqe,cal (mg g−1) 12.78 18.59 23.47 29.85 32.89k2·103 (g mg−1 min−1) 3.43 1.11 0.66 0.53 0.52R2 0.9883 0.9845 0.9965 0.9914 0.9804χ2 0.71 0.64 1.76 3.33 2.11ARE 3.54 2.27 3.49 0.81 3.19

Table 6Intraparticle diffusion parameters for OII adsorption onto TWNC at different initialconcentrations.

Intraparticle diffusion

C0 (mg L−1) 75 125 150 175 200kid (mg g−1 min−1/2) 0.6663 1.1238 1.3346 1.3973 2.0180R2 0.9787 0.9889 0.9943 0.9801 0.9978

137F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

[51]. The values of kid and C were obtained from the second linear por-tion. Table 6 shows the calculated intraparticle diffusion parameters forthe adsorption process. With reference to Fig. 7c, there was a linear rela-tionship with three stages over a period of time, but they did not pass

through the origin. It suggested that the adsorptionmechanism is rathera complex process and the intraparticle diffusion is involved in the over-all rate of adsorption process but it is not the only rate-controlling step.

3.5. Equilibrium studies

An adsorption isotherm describes the relationship between theamount of adsorbate taken up by the adsorbent and the adsorbate con-centration remaining in solution. The shape of the isotherm is the firstexperimental tool used to diagnose the nature of the adsorption phe-nomenon. The isotherms have been classified by Giles et al. [52] intofour main groups: L, S, H and C. According to the cited classification,the isotherm for the OII displayed an L curve pattern (Fig. 6d). The iso-therm had a concavity toward the abscissa axis, which indicated thatas more sites in the substrate are filled, it becomes more difficult for afresh solute molecule to find a vacant site [53,54].

In this study, two commonly used isotherms i.e. Langmuir [55] andFreundlich [56] were applied to fit the experimental equilibrium iso-therm data.

The Langmuir model assumes a monolayer sorption onto a surfaceand no interaction between the adsorbed molecules, even on adjacentsites. The linear forms of the Langmuir (Eq. (11)) adsorption isothermcan be expressed as follows:

Ceqe

¼ 1qmb

þ 1qm

Ce ð11Þ

where Ce is the equilibrium concentration (mg L−1), qe is the amount ofadsorbate (mg g−1), qm is qe for complete monolayer adsorption capac-ity (mg g−1), and b is the equilibrium adsorption constant (L mg−1).From the slope and intercept of straight portion of the linear plot obtain-ed by plotting Ce/qe against Ce (Fig. 8a), the values of the Langmuir con-stants were calculated.

The essential characteristics of the Langmuir isotherm can beexpressed in terms of dimensionless constant separation factor, RLgiven by [57]:

RL ¼1

1þ bC0ð12Þ

0 30 60 90 120 150 1800.0

0.2

0.4

0.6

0.8

1.0

1.2

Ce (mg/L)

Ce/

qe

(g/L

)

0.4 0.8 1.2 1.6 2.0 2.41.5

1.7

1.9

2.1

2.3

2.5

2.7

293 K

303 K

313 K

323 K

293 K

303 K

313 K

323 K

In Ce

In q

e

(a)

(b)

Fig. 8. The Langmuir (a) and Freundlich (b) linear adsorption isotherms of OII onto TWNCat different temperatures.

Table 7Isotherm parameters obtained for adsorption of OII onto TWNC at different temperatures.

Temperatures (°C)

20 30 40 50

Langmuirqm (mg g−1) 185.00 222.22 277.78 312.50b (L mg−1) 0.027 0.059 0.065 0.078RL 0.051 0.018 0.024 0.022R2 0.9708 0.9928 0.9686 0.9984χ2 0.37 2.21 1.35 0.80ARE 0.40 0.71 0.67 0.42

FreundlichKF (mg g−1(L mg−1)1/n) 22.60 37.14 49.24 76.101/n 0.44 0.39 0.31 0.28R2 0.8558 0.8965 0.9054 0.9155χ2 8.87 3.27 7.45 0.91ARE 2.05 1.12 1.38 0.45

138 F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

where b is the Langmuir constant and C0 is the highest initial dye concen-tration (mg L−1). The parameter RL shows whether a sorption system isfavorable if (0 b RL b 1) or unfavorable (RL N 1) in batch processes. If theRL value is zero and unity, the type of isotherm is irreversible and linear,respectively. Since RL values were obtained in range between 0 and 1,at 20, 30, 40 and 50 °C, it can be said that the adsorption is favorable atoperation conditions studied.

The Freundlich model was developed to present multilayer adsorp-tion on a heterogeneous surface. The linear forms of the Freundlich(Eq. (13)) adsorption isotherm can be expressed as follows:

lnqe ¼ lnK F þ1n

lnCe ð13Þ

where KF (mg g−1(L mg−1)1/n) is the Freundlich constant and taken asan indicator of adsorption capacity, and 1/n is a measure of the adsorp-tion intensity. From the slope and intercept of straight portion of the lin-ear plot obtained by plotting lnqe against lnCe (Fig. 8b), the values of theFreundlich constants were calculated.

The values of isotherm parameters R2, ARE and χ2 values calculatedfrom both isotherm models are listed in Table 7. As seen in Table 7, theLangmuir model has the higher R2 and lower ARE and χ2 values thanthose of the Freundlich model. This indicates that the adsorption of OIIonto TWNC complies with the Langmuir isotherm model. The maxi-mum monolayer adsorption capacity is found as 312.5 mg g−1 at50 °C. This high adsorption capability depends on the presence of suit-able surface-active groups on the surface as well as the porosity ofTWNC. Results suggest that the mesopore-dominant TWNC is suitablefor dye adsorption. Thus, due to the length of the OII which is 1.36 nm[21]; it is easily transported into the TWNC pores (Dp is 26.44 Å). Asseen from Table 7, the constants qm and b increased with an increasein temperature, this indicates that the adsorption density was higherat higher temperatures.

The value of RL for adsorption of OII indicated that the adsorptionwas favorable (RL b 1). The high values of KF indicated that TWNC hada higher adsorption capacity and affinity for OII. The Freundlich intensi-ty parameter, 1/n, indicates the deviation of the adsorption isothermfrom linearity. If 1/n = 1, the adsorption is linear i.e. the adsorptionsites are homogeneous and there is no interaction between the adsorbedspecies. If 1/n b 1, the adsorption is favorable; the adsorption capacityincreases and new adsorption sites appear. If 1/n N 1, the adsorption isunfavorable; the adsorption bonds become weak and the adsorptioncapacity decreases [58]. The values of 1/n for OII being less than 1 indicatefavorable adsorption.

3.6. Adsorption thermodynamics

Thermodynamic parameters such as change in free energy (ΔG0),the change in enthalpy (ΔH0) and the change in entropy (ΔS0) were cal-culated to evaluate the thermodynamic feasibility and the nature of theprocess. ΔG0, ΔH0 and ΔS0 were calculated by using Eqs. (14) and (15),respectively.

ΔG0 ¼ −RT lnK ð14Þ

lnK ¼ ΔS0

R−ΔH

0

RTð15Þ

where K obtained from the Langmuir isotherm equation that equals toqmb (L mg−1), R and T are gas constant (8.314 J mol−1 K−1) and absolutetemperature (K), respectively. By plotting a graph of lnK versus 1/T fromEq. (15), the values ΔH0 and ΔS0 can be estimated from the slopes andintercepts, respectively (figure not shown).

The calculated values of thermodynamic parameters are listed inTable 8. The negative ΔG0 values for the four temperatures illustratedthat the adsorption process was feasible and spontaneous. The values

Table 8Thermodynamic parameters for OII adsorption onto TWNC.

ΔH0 (kJ mol−1) ΔS0 (J mol−1 K−1) −ΔG0 (kJ mol−1)

20 °C 30 °C 40 °C 50 °C

33.66 130.0 3.92 7.18 7.29 8.08

139F. Güzel et al. / Journal of Molecular Liquids 194 (2014) 130–140

of ΔG0 increased with the increasing temperature, this indicated thatthe adsorption was more beneficial at higher temperature. The positivevalue of ΔH0 indicates that the process is endothermic in nature. Thepositive value of ΔS0 demonstrated the increased randomness whichdisplayed good affinity between OII and the surface of TWNC duringthe adsorption process.

3.7. Desorption studies

Desorption studies help to elucidate the mechanism of adsorptionand recycling of the spent carbon and the dye. To make the adsorptionprocess more economical, it is necessary to re-generate the spentcarbon and dye. For this purpose, in order to remove the adsorbed OIIfrom TWNC surface, various pH ranges were studied (Fig. 9). As thepH of desorbing solution was increased, the percent desorption in-creased from 1.62% at pH 2 to 12.12% at pH 12. As the pH of the systemincreases, the number of negatively charged sites increased. A negative-ly charged site on the TWNC at high pHs studied favors desorption ofdye anions due to the electrostatic repulsion [12]. At pH 12, a signifi-cantly high electrostatic repulsion exists between the negativelycharged surface and dye molecules.

4. Conclusion

The characteristics of TWNC were determined and found to have asurface area, total pore volume, average pore diameter, methyleneblue and iodine numbers of 722.17 m2 g−1, 0.476 cm3 g−1 and26.44 Å, 134.34 mg g−1 and 592.48 mg g−1, respectively. It includesboth micro and mesopores. Percentages of micropore and mesoporevolume fractions are 42% and 58% respectively. An adsorption study ofOII from aqueous solution onto TWNC was performed where the effectof various parameterswas tested and kinetic and isothermmodels weresuggested. The adsorption of OII was the best at pHs below of the pHpzcof TWNC (5.30). Adsorption isotherm data were well fitted to theLangmuir adsorption equilibrium model. Maximum OII adsorption ca-pacity of 312.5 mg g−1 was obtained at optimum operating conditionsof pH 2, 0.1 g L−1 adsorbent dose and 3 h contact time. The separation

Fig. 9. Desorption studies for the re

factor RL lies well in between 0 and 1. The Freundlich constant (1/n)fell between 0 and 1.0 which indicated that the process is favorable.The kinetics of the adsorption followed the pseudo-first ordermodel. The results of the intraparticle diffusion model suggestedthat intraparticle diffusion was not the only rate controlling step.The calculated thermodynamic parameters, namely ΔG0, ΔH0, andΔS0 showed that adsorption was spontaneous and endothermicunder examined conditions. The desorption tests showed the maxi-mum OII released of 12.12% at pH 12.

The present study concludes that the TWNC could be employed aslow-cost adsorbent as alternatives to commercial activated carbon forthe removal of acidic dyes from water and wastewater; it contributesfor the implementation of sustainable development in both the tomatoproduction and environmental protection.

Acknowledgments

The authors acknowledge the Scientific Research Fund of Dicle Uni-versity for financial support (Project No: 12-ZEF-95).

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Elimination of anionic dye by using nanoporous carbon prepared from an industrial biowaste1. Introduction2. Materials and methods2.1. Materials2.2. Preparation of TWNC2.3. Characterization of TWNC2.4. Batch adsorption experiments2.5. Error analysis

3. Results and discussion3.1. Characterization of the TWNC3.2. Effect of various parameters for OII adsorption on TWNC3.2.1. Effect of solution pH3.2.2. Effect of adsorbent dose3.2.3. Effect of contact time and initial concentration3.2.4. Effect of temperature3.2.5. Effect of ionic strength

3.3. Kinetic studies3.4. Adsorption mechanism3.5. Equilibrium studies3.6. Adsorption thermodynamics3.7. Desorption studies

4. ConclusionAcknowledgmentsReferences