Removal of Remazol Brilliant Blue Reactive Dye from Aqueous Solutions Using Watermelon Rinds as Adsorbent

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Removal of Remazol Brilliant Blue Reactive Dyefrom Aqueous Solutions Using Watermelon Rinds asAdsorbentMohd Azmier Ahmada, Norhidayah Ahmada & Olugbenga Solomon Belloab

a School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, NibongTebal, Penang, Malaysiab Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology,Ogbomoso, Oyo State, NigeriaAccepted author version posted online: 29 May 2014.Published online: 03 Feb 2015.

To cite this article: Mohd Azmier Ahmad, Norhidayah Ahmad & Olugbenga Solomon Bello (2015) Removal of RemazolBrilliant Blue Reactive Dye from Aqueous Solutions Using Watermelon Rinds as Adsorbent, Journal of Dispersion Science andTechnology, 36:6, 845-858, DOI: 10.1080/01932691.2014.925400

To link to this article: http://dx.doi.org/10.1080/01932691.2014.925400

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Removal of Remazol Brilliant Blue Reactive Dye fromAqueous Solutions Using Watermelon Rinds asAdsorbent

Mohd Azmier Ahmad,1 Norhidayah Ahmad,1 andOlugbenga Solomon Bello1,2

1School of Chemical Engineering, Engineering Campus, Universiti Sains Malaysia, Nibong Tebal,Penang, Malaysia2Department of Pure and Applied Chemistry, Ladoke Akintola University of Technology, Ogbomoso,Oyo State, Nigeria

GRAPHICAL ABSTRACT

This study examined the feasibility of removing Remazol brilliant blue reactive (RBBR) dye fromaqueous solutions using watermelon rind activated carbon (WRAC) as adsorbent. The surface areaof WRAC prepared is 776.65 m2/g with high pore volume of 0.438 cm3/g, which is comparable tocommercially expensive activated carbon. The effects of contact time, initial dye concentration,pH, and temperature on adsorption of RBBR dye were investigated. Pseudo-first-order, pseudo-second-order, Elovic, and Avrami kinetic models were used to test the experimental data inorder to elucidate the kinetic adsorption process; pseudo-second-order model best fitted the data.Experimental data were analyzed using eight model equations: Langmuir, Freundlich, Temkin,Dubinin–Radushkevich, Radke–Prausnite, Sips, Viet–Sladek, and Brouers–Sotolongo isotherms,and it was found that the Freundlich isotherm model described the equilibrium adsorption databest. The heat of adsorption (DH) indicated the endothermic nature of the process and the negativevalues of free energy (DG) indicated that the adsorption is spontaneous. The mechanism of adsorp-tion was controlled by both film and intraparticle diffusions. The mean free energy obtained fromDubinin–Radushkevich model is 4.972 kJmol�1, indicating that the adsorption of RBBR dye ontoWRAC follows physical adsorption process.

Keywords Adsorption, endothermic, Remazol brilliant reactive dye, watermelon rinds

Received 17 April 2014; accepted 14 May 2014.Address correspondence to Olugbenga Solomon Bello, Department of Pure and Applied Chemistry, Ladoke Akintola University of

Technology, P.M.B 4000, Ogbomoso, Oyo State, Nigeria. E-mail: [email protected] versions of one or more of the figures in the article can be found online at www.tandfonline.com/ldis.

Journal of Dispersion Science and Technology, 36:845–858, 2015

Copyright # Taylor & Francis Group, LLC

ISSN: 0193-2691 print=1532-2351 online

DOI: 10.1080/01932691.2014.925400

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INTRODUCTION

The discharge of textile effluents to water bodies hasraised much concern because of the potential health hazardsassociated with the entry of toxic components into the foodchains of humans and animals caused by this. Syntheticdyes are extensively used for dyeing and printing in textileindustries.[1] The removal of color from textile effluentshas gained much attention over the last few years, not onlybecause of its toxicity, but mainly due to its visibility. Dyescan be classified as follows: anionic, direct, acid, andreactive dyes; cationic basic dyes; and non-ionic dispersedyes.[2] Adsorption process has proven to be one of the bestwater treatment technologies in the world, and activatedcarbon is undoubtedly considered as a universal adsorbentfor the removal of diverse types of pollutants from water.However, in view of the high cost and associated problemsof regeneration, there is a constant search for alternativelow-cost adsorbents. Previously, several researchers hadused various low-cost materials such as clay,[3] narrow-leaved cat tail,[4] fly ash,[5] wood dust,[6] alunite[7] etc.Adsorption has proven to be more versatile and efficientcompared to conventional physicochemical methods ofdye removal.[8] The utilization of waste materials is becom-ing increasingly popular because these wastes representunused materials in many cases and they present seriousdisposal problems. During the past decade, a great deal ofattention has been given to methods of converting thesematerials into useful products.[9] Consequently, manyinvestigators have studied the feasibility of using low-costsorbents such as coconut husk,[10,11] wheat bran,[12] vetiverroots,[13] cotton stalk and its hull,[14] date stones,[15,16] olivewaste cakes,[17] and Posidonia oceanica.[18]

Remazol brilliant blue reactive (RBBR) dye, one of themost important synthetic dyes in the textile industry, is ananthraquinone derivative that represents an importantclass of toxic and recalcitrant organo pollutants.[19,20] Theaim of the present study is to explore the capability ofwater melon (WM) rinds to remove RBBR dye from aque-ous solution under different experimental conditions suchas contact time, pH, initial dye concentration, and solutiontemperature. The adsorption kinetic models, equilibriumisotherm models, and thermodynamic parameters werealso investigated.

MATERIALS AND METHODS

Activated Carbon Preparation

WM fruits were purchased from a local market in ParitBuntar area, Perak, Malaysia. They were washed to removethe dirt and were sliced open, and then the pink flesh wastrimmed from the outer green skin from thick watermelonrind. The thick WM rinds, that is, the peels were then driedto constant weight and stored in an airtight container for

further use. Ten grams of the dried rind was placed in avertical tubular reactor. Then, nitrogen gas was purged intothe reactor to create an inert condition. The flow rate ofnitrogen gas and the heating rate were set at 150 cm3=minand 10�C=min, respectively. The temperature was rampedup from room temperature to 700�C and held for 1 hour.Then, the reactor was cooled down to room temperature.The char produced was stored in an airtight containerfor further treatment. It was then impregnated. Theimpregnation ratio (IR) was calculated using the followingequation

IR ¼wKOH

wchar½1�

where wKOH is dry weight (g) of potassium hydroxide pelletand wchar is dry weight (g) of char. The char and KOHpowder (depending on the IR) were mixed together in deio-nized water in a 250 mL beaker. The mixture was stirredthoroughly before drying it overnight in an oven at 105�Cfor dehydrating purpose. The KOH-impregnated char wasplaced inside the vertical tubular reactor for activationprocess. The system was purged under nitrogen flow ata rate of 150 cm3=min. The temperature was ramped upfrom an ambient temperature to the activation temperatureof 800�C at the heating rate of 10�C=min. Once the desiredactivation temperature was reached, the gas flow wasswitched to carbon dioxide flow at a rate of 150 cm3=minto complete the activation process. Then, the reactor wascooled to room temperature under nitrogen flow. Thesample was washed with 0.1 M HCl. It was further washedwith deionized water several times until the pH of thewashing solution reached 6.5�7. The pH was measuredusing a pH meter (Model Delta 320, Mettler Toledo,China). Filter paper and filter funnel were used in thewashing process. The washed sample was kept in an ovenat 105�C for 12 hours. The dried sample, that is, theactivated carbon (WRAC) was stored in airtight containersfor further characterization and adsorption studies. TheWRAC yield was calculated using the following equation:

Yield ð%Þ ¼ wc

w0�100 ½2�

where wc and w0 are the dry weights of WRAC (g) andprecursor (g), respectively.

Adsorbate Used

RBBB dye was used as an adsorbate to determinethe adsorption performance of the prepared activatedcarbon. The properties of RBBB dye used are listed inTable 1.

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Batch Equilibrium Studies

The effects of initial dye concentration, contact time,solution temperature, and solution pH on the adsorptionuptake for adsorption of RBBB dye on WRAC were studied.Sample solutions were withdrawn at intervals to determinethe residual concentration by a UV–visible spectrophot-ometer at the maximum wavelength of 590 nm. The amountof dye adsorbed at equilibrium, qe(mg=g) was calculated as

qe ¼Co � Ceð Þ

W½3�

where Co and Ce (mg=L) are the liquid-phase concentrationsof initial adsorbate and equilibrium, respectively. V is thevolume of the solution (dm3) and W is the mass (g) of theWRAC used.

Effect of Initial Adsorbate Concentration and ContactTime

A 100 mL portion of RBBB dye solution with knowninitial concentration was poured into a number of 250 mLErlenmeyer flasks. The amount of adsorbent that wasadded into each flask was fixed at 0.1 g. The flasks wereplaced in an isothermal water bath shaker (Model Protech,

Malaysia) at constant temperature of 30�C, with rotationspeed of 120 rpm, until equilibrium point was reached.Samples are withdrawn at intervals to determine theresidual concentration of the dye at 590 nm wavelengthusing a UV–visible spectrophotometer.

Effect of Solution pH

Solution pH was studied by varying the initial pH of thesolution from 2 to 12. The pH was adjusted by 0.1 M NaOHor 0.1 M HCl and measured by using a pH meter. Theadsorbent dosage, rotation speed, solution temperature,and initial dye concentration were fixed at 0.1 g, 120 rpm,30�C and 100 mg=L, respectively.

Adsorption Isotherm Studies

This was carried out by fitting the equilibrium data to theLangmuir, Freundlich, Temkin, Dubinin–Radushkevich,Sips, Vieth–Sladek, Bruoers–Sotolongo, and Radke–Praus-nitz isotherms. The applicability and suitability of theisotherm equation to the equilibrium data were comparedby judging the values of the correlation coefficients, R2

and Dqe. Linear regression was carried out by using Micro-soft Excel spreadsheet with Solver add-in to determine theisotherm parameters.

The Langmuir Isotherm

This model depends on the assumption that inter-molecular forces decrease rapidly with distance andconsequently help to predict the existence of monolayercoverage of the adsorbate on the outer surface of adsorbent.The linear form of the Langmuir isotherm equation[21] isgiven by:

Ce

qe¼ 1

QoKLþ 1

QoCe ½4�

where Ce is the equilibrium concentration of the adsorbate(mg=L), qe is the amount of adsorbate adsorbed per unitmass of adsorbent (mg=g), Qo is the maximum monolayeradsorption capacity of the adsorbent (mg=g), and KL isthe Langmuir adsorption constant related to the free energyof adsorption (L=mg). The constant values are evaluatedfrom the intercept and slope of the linear plot of the experi-mental data of (Ce=qe) versus Ce. The essential characteris-tics of the Langmuir equation can be expressed in terms ofdimensionless separation factor, RL, defined as:

RL ¼1

ð1þ KLCoÞ½5�

where Co is the highest initial dye concentration while RL

value implies that the adsorption is unfavorable (RL> 1),linear (RL¼ 1), favorable (0<RL< 1), or irreversible(RL¼ 0).

TABLE 1Properties of Remazol brilliant blue reactive dye

Properties

Chemicalname

Disodium 1-amino-9, 10-dioxo-4-[3-(2sulfonatooxyethylsulfonyl)anilino]anthracene-2-sulfonate

Commonname

Remazol brilliant blue R

Generic name Reactive blue 19CAS number 2580-78-1Color index

number61200

Ionisation ReactiveMaximum

wavelength590 nm

Empiricalformula

C22H16N2Na2O11S3

Molecularweight

626.54 g=mol

Chemicalstructure

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The Freundlich Isotherm

The Freundlich model is an empirical equation basedon the adsorption of heterogeneous surface or surfacesupporting sites of varied affinities. It is assumed that thestronger binding sites are occupied first and the bindingstrength decreases with the increasing degree of site occu-pation. The Freundlich isotherm[22] is expressed as

log qe ¼ log Kf þ ð1=nÞ log Ce ½6�

where qe is amount of adsorbate adsorbed per unit mass ofadsorbent (mg=g); Kf is the Freundlich isotherm constant(mg=g).(L=mg)1=n), which indicate the relative adsorptioncapacity of the adsorbent related to the bonding energy;Ce is the equilibrium concentration of the adsorbate (mg=L)and nF is the heterogeneity factor representing the deviationfrom linearity of adsorption and is also known as the Freun-dlich coefficient. If the plot of (log qe) against (log Ce) gavestraight line, it indicates that the Freundlich isotherm fitsthe adsorption data. Other constants can be calculated fromthe slope (1=n) and the intercept (log Kf) of the linear plot ofexperimental data. The slope of 1=n ranging between 0 and 1is a measure of adsorption intensity, becoming more hetero-geneous as its value gets closer to zero.

The Temkin Isotherm

The Temkin isotherm contains a factor that explicitlytakes into account the adsorbent–adsorbate interactions.This model assumes that the heat of adsorption of all themolecules in the layer would decrease linearly with cover-age due to adsorbent–adsorbate interactions. The Temkinmodel[23] is expressed as:

qe ¼ B ln At þ B ln Ce ½7�

where B is the RT=b constant related to the heat of adsorp-tion (L=mg); qe is the amount of adsorbate adsorbed atequilibrium (mg=g); Ce is the equilibrium concentrationof adsorbate (mg=L); T is the absolute temperature; R isthe universal gas constant (8.314 J=mol K); and At is theequilibrium binding constant (L=mg). A graph of the plotof qe versus ln Ce will yield the value of slope (B) as wellas the value of intercept (At).

The Dubinin–Radushkevich Isotherm

The Dubinin–Radushkevich isotherm is an empiricalmodel for the adsorption of subcritical vapors onto micro-pore solids following a pore filling mechanism. It is appliedto distinguish the physical and chemical adsorption forremoving a molecule from its location in the sorption spaceto the infinity, which can be express as[24]

qe ¼ qs exp �BDR ee2� �

½8a�

where e can be correlated

e ¼ RT ln 1þ 1

Ce

� �½8b�

E ¼ 1ffiffiffiffiffiffiffiffiffiffiffi2BDR

p ½8c�

where R, T, and Ce represent the gas constant (8.314 J=mol K),absolute temperature (K), and adsorbate equilibriumconcentration (mg=L), respectively. The value obtainedfor BDR was then used to estimate free energy E of the sorp-tion per molecule of the sorbate when it is transferred to thesurface of the solid from the infinity in the solution. A plotof lnqe versus e2 will yield a straight line where BDR and qs

are obtained from the slope and the intercept.

The Sips Isotherm

The Sips isotherm model is a combined form of theLangmuir and Freundlich expressions deduced forpredicting the heterogeneous adsorption systems andcircumventing the limitation of the rising adsorbateconcentration associated with the Freundlich isothermmodel.[25] At high adsorbate concentration, it predictsmonolayer adsorption characteristics of Langmuirisotherm, while in low adsorbate concentration, it reducesto Freundlich isotherm. The Sips model[25] is expressed as

qe ¼Qm ksCeð Þms

1þ ksCeð Þms½9�

where ks is the Sips isotherm model constant and ms is theSips isotherm model exponent.

The Vieth–Sladek Isotherm

The Vieth–Sladek isotherm is given by the followingequation[26]

qe ¼ KvsCe þqmBVSCe

1þ BVSCe½10�

where qe is the adsorbed amount at equilibrium (mg=g); Ce

is the adsorbate equilibrium concentration (mg=L); Qm ismaximum adsorption capacity; and KVS and BVS areVieth–Sladek constants. The Vieth–Sladek isotherm is usedfor estimating diffusion rates in solid materials fromtransient sorption.

The Brouers–Sotolongo Isotherm

The Brouers–Sotolongo model[27] is expressed as:

qe ¼ Qm 1� exp �kBS Ceð ÞaBS� �� �

½11�

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where kBS and aBS are Brouers–Sotolongo constants.The exponent is a measure of the width of the sorptionenergy distribution and energy heterogeneity of the sorbentsurface.

The Radke–Prausnitz Isotherm

The Radke–Prausnitz model[28] isotherm can be repre-sented as

qe ¼kRPQmCe

ð1þ kRPCeÞmRP½12�

where qe is the adsorbed amount at equilibrium (mg=g),mRP is the Radke–Prausnitz maximum adsorption capacity(mg=g), Ce is the adsorbate equilibrium concentration(mg=L), KRP is the Radke–Prausnitz equilibrium constant,and mRP is the Radke–Prausnitz model exponent. TheRadke–Prausnitz model will be converted to another modelin a certain condition. At low concentration, if the value ofthe exponent’s mRP is equal to unity, the Radke–Prausnitzmodel is reduced to the Langmuir model. While, ifliquid-phase concentration is high, the Radke–Prausnitzmodel is converted to the Freundlich model.

BATCH KINETIC STUDIES

This procedure of batch kinetic studies is similar to thatof batch equilibrium studies. The difference is that therequired amount of adsorbent–adsorbate solution wastaken at preset time intervals and the concentration of thesolution was measured. The amount of adsorption at timet, qt (mg=g), was calculated using Equation (13)

qt ¼Co � Ctð ÞV

W½13�

where Co and Ct (mg=L) are the liquid-phase concentrationsof adsorbate at initial and at any time t, respectively. V isthe volume of the solution and W is the mass of adsorbentused. The adsorption kinetics of dye on adsorbent wasinvestigated using pseudo-first-order, pseudo-second-order,Avrami, and Elovich models, respectively.

Pseudo-First-Order Kinetic Model

The pseudo-first-order kinetic model equation isgenerally expressed as follows[29]:

ln qe � qtð Þ ¼ lnqe � k1 t ½14�

where qe is the amount of adsorbate adsorbed at equilib-rium (mg=g) qt is the amount of solute adsorb per unitweight of adsorbent at time (mg=g), k1 is the rate constantof pseudo-first-order sorption (1=h). A plot of ln (qe – qt)versus t gives a straight line with the slope of k1 and theintercept of ln qe.

Pseudo-Second-Order Kinetic Model

The pseudo-second-order equation can be expressed as[30]

t

qt¼ 1

k2q2e

þ 1

qet ½15a�

The constant k2 is used to calculate the initial sorption rate,h, at t¼ 0, as follows:

h ¼ k2q2e ½15b�

Thus, the rate constant k2, initial adsorption rate h, andpredicted value of qe can be calculated. The linear plot oft=qe versus t gives 1=qe is the slope and 1=h is the intercept.

The Elovich Kinetic Model

The simplified Elovich equation is expressed as[31]

qt ¼1

bln abð Þ þ 1

bln t ½16�

where a is the initial desorption rate (mg=(g min)) and b isthe desorption constant (g=mg) during any experiments. Aplot of qt versus ln t gave a linear relationship with slope of1=b and an intercept of (1=b) ln (ab). The 1=b value reflectsthe number of sites available for adsorption whereas thevalue of 1=b ln (ab) indicates the adsorption quantity whenln t is equal to zero.

The Avrami Kinetic Model

The Avrami equation is used to verify specific changesof kinetic parameters as functions of the temperature andreaction time. It is also an adaptation of the kinetic thermaldecomposition modeling.[32]

The Avrami kinetic model is expressed as

qt ¼ qe 1� exp � KAV tð Þ½ �nAVf g ½17�

where qt is the adsorption fraction at time t; kAv is theadjusted kinetic constant; and nAv is another constant,which is related to the adsorption mechanism. n valuecan be used to verify possible interactions of the adsorptionmechanisms in relation to the contact time and thetemperature.

Validity of Kinetic Model

The applicability and fitting of the isotherm equation tothe kinetic data was compared by judging the R2 values andthe normalized standard deviation Dqt (%) calculated fromEquation (18). The normalized standard deviation, Dqt

(%), was used to verify the kinetic model used to describethe adsorption process. It is defined as

Dq ¼ 100

ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiPðqexp � qcalÞ=qexp

2n� 1

s½18�

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where n is the number of data points, qexp and qcal (mg=g)are the experimental and calculated adsorption capacityvalues, respectively. Lower value of Dqt indicates good fitbetween experimental and calculated data.

ADSORPTION THERMODYNAMICS

The experimental data obtained from batch adsorptionstudies performed earlier were analyzed by using thethermodynamic equations as expressed by Equations(19a–c).

DG ¼ �RT ln KL ½19a�

LnK ¼ DS

R� DH

RT½19b�

DG was calculated using Equation (19a). The values of DHand DS can be obtained, respectively, from the slope and theintercept of Van’t Hoff plot of ln KL versus 1=T. Values ofKL may be calculated from the relation ln qe=Ce at differentsolution temperatures of 30�C, 45�C, and 60�C, respect-ively. Arrhenius equation has been applied to evaluate theactivation energy of adsorption representing the minimumenergy that reactants must have for the reaction to proceed,as shown by the following relationship:

ln k2 ¼ ln A� Ea

RT½19c�

where k2 is the rate constant obtained from the pseudo-second-order kinetic model, (g=mg h), Ea is the Arrheniusactivation energy of adsorption (kJ=mol), A is theArrhenius factor, R is the universal gas constant(8.314 J=mol K), and T is the absolute temperature. Whenln k2 is plotted against 1=T, a straight line with slope of –Ea=R is obtained.

ADSORPTION MECHANISM

The adsorption mechanisms of RBBB dye on the adsorb-ent were investigated using intraparticle diffusion modelrepresented by Equation (20). The applicability and fittingof the model throws more light on the mechanism of RBBBdye adsorption onto the WRAC prepared.

Intraparticle Diffusion Model

Intraparticle diffusion model[33] is expressed as shown inEquation (20)

qt ¼ kpit12 þ Ci ½20�

where Ci is the intercept and kpi (mg=g h1=2) is the intrapar-ticle diffusion rate constant, which can be evaluated fromthe slope of the linear plot of qt versus t1=2. The qt is theamount of solute adsorb per unit weight of adsorbent per

time, (mg=g), and t1=2 is the half adsorption time, (g=h mg).The intercept of the plot reflects the boundary layereffect. The larger the intercept, the greater the contributionof the surface sorption in the rate-controlling step. If theregression of qt versus t1=2 is linear, and it passes throughthe origin, then intraparticle diffusion is the sole rate-limiting step. If the linear plots at each concentration donot pass through the origin, it indicates that the intraparti-cle diffusion is not the only rate-controlling step.[34]

RESULTS AND DISCUSSIONS

Characterization of Activated Carbon Produced

Fourier Transform Infrared Spectroscopy

Figure 1 shows a long bandwidth around 3861�3626 cm�1 on WR and 3861� 3738 cm�1 on WRAC repre-senting the O–H stretching vibration of hydroxyl functionalgroups with hydrogen bonding. Other major peaks detectedat bandwidths of 2287 cm�1, 1712� 1665 cm�1, 1531 cm�1,and 1010 cm�1 are assigned to C�C stretching of alkynegroup, stretching of lactones, ketones and carboxylic anhy-drides, C=C of aromatic ring and C–O group stretching inether or phenol group. The disappearance of C=C of aro-matic ring in the AC sample indicates that these functionalgroups were thermally unstable. Aromatic group disap-peared due to the oxidative degradation of aromatic ringsduring chemical impregnation and heating stages. Themedium peaks around 686� 590 cm�1 found on the spec-trum were due to the presence of C�C and H–C–H bendingfunctional groups (Table 2).

Scanning Electron Micrographs

Scanning electron micrographs of WR and WRAC areshown in Figure 2. WRAC shows many pores in the formof holes on its surface (Figure 2). The well-developed poresresulted to the large surface area and porous structure ofthe activated carbon produced. Similar results wereobtained in the adsorption of RBBB dye using coconutshell-based activated carbon.[35]

Proximate Analysis

Table 3 reports the proximate analysis results of RawWR, WR char, and WRAC. As shown in Table 3, thecarbon content in the WRAC increased significantlycompared to both char WR and raw WR materials dueto its high carbonization temperature. Moisture andvolatile matter decreased after carbonization. The highfixed carbon content after activation makes WRAC agood adsorbent for dye removal.[36] This is due to thepyrolytic effect at high temperature at which most of theorganic substances get degraded and discharged both asgas and as liquid vapors leaving a material with highcarbon content.[37]

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Surface and Pore Characteristics

Table 4 shows the Brunauer, Emmett, and Teller(BET) surface area; mesopore surface area; total porevolume; and average pore diameter of different precur-sors of the WR produced. The surface area of WRACis 776.65 m2=g with a high pore volume of 0.438 cm3=g.

This is similar to the result obtained during the prep-aration of activated carbon from rubber seed coat havinga surface area of 782.1 m2=g and a pore volume of0.494 cm3=g.[36] The average pore diameter is 3.74 nm,the value is in the marcopore region according to IUPACclassification.[38]

FIG. 1. FTIR spectra, a) WR, b) WR char, and c) WRAC.

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BATCH EQUILIBRIUM STUDIES

Effect of Contact Time and Initial Dye Concentration

The contact time between the synthetic dye and theadsorbent is an important parameter in the adsorptionprocess. Figure 3 shows the effect of contact time betweenWRAC and RBBR dye with initial concentrations of 25�500 mg=L. The amount of RBBR dye molecules adsorbedby WRAC increases rapidly within the first 6 hours forconcentrations (25�200 mg=L). At higher concentrations(400�500 mg=L), the contact time increases to 22 hoursbefore the equilibrium is reached. Thus, equilibrium stateswere achieved at 6 and 22 hours, respectively. Previousadsorption studies show that the removal of the adsorbatespecies is rapid at first and slows near the equilibrium.[39,40]

This observation may be associated with the existence of alarge number of vacant sites on the surface of WRAC thatare available for adsorption during the initial stage.However, the remaining vacant surface sites are lessavailable for adsorption with time due to the repulsive forcesthat occur between the adsorbed and free molecules.[40]

FIG. 2. SEM micrographs of a) WR (� 1000); b) WRAC (� 1000).

TABLE 2FTIR spectrum band assignments for WR, WR char, and WRAC

Assignment

Band positions (cm�1)

WR WR char WRAC

O–H stretching of hydroxyl group 3861� 3626 3861� 3738 3861� 3738C�C stretching of alkyne group 2287� 2098 2106 2287C=O stretching of lactones, ketones, and carboxylic anhydrides 1712 1712� 1665 1728C=C of aromatic ring 1531 1531 —C–O groups stretching in ester, ether or phenol group 1010� 987 1006 1010–C�C–H–C–H bending functional group 686 597 590

TABLE 4Surface area and pore characteristics of the samples

Sample

BETsurface

area(m2=g)

Mesoporesurface

area(m2=g)

Totalpore

volume(cm3=g)

Averagepore

diameter(nm)

WR 5.93 — — —WR char 114.39 57.50 0.125 3.68WRAC 776.65 418.87 0.438 3.74

WR: Watermelon rind; —: not available.

TABLE 3Proximate analysis of different precursors of WR

Sample

Proximate analysis (%)

Moisture Volatile matter Fixed carbon Ash

WR 10.55 72.60 12.11 4.74WR char 6.84 38.17 50.34 4.65WRAC 4.20 28.60 62.68 4.52

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Effect of pH on Adsorption of RBBR Dye onto WRAC

pH exerts a profound influence on dye adsorption. Theamount of dye removed was maximum, that is, 97.20% atpH 2, and it decreased sharply to 69.94% at pH 6, there-after the decrease was gradual until pH 12 (41.78%)(Figure 4). Higher adsorption of RBBR dye at acidic pHis due to the electrostatic attraction between the negativelycharged dye molecule and the positively charged WRACsurface. As surface charge density decreases with anincrease in the solution pH, the WRAC surface becomesnegatively charged, thereby repelling the negativelycharged dye molecule, this resulted in a decrease in the rateof dye adsorption at basic pH. We reported a similar trendin the adsorptive removal of RBBR dye using cocoa podhusks.[41]

Effect of Solution Temperature on Adsorption of RBBRDye onto WRAC

The effect of temperature on adsorption capacity wasinvestigated by measuring the adsorbed amount at threedifferent temperatures: 30�C, 45�C, and 60�C; the results

are presented in Figure 5. It is clearly seen that with theincrease of temperature, the adsorption capacity of WRACincreases from 16.38 to 202.76 mg=g, from 16.97 to226.35 mg=g, and from 17.95 to 243.74 mg=g at 30�C,45�C, and 60�C, respectively. This observation reveals thatRBBR dye removal is an endothermic process in which themobility of large dye molecules increases with temperature.The increase of qe with an increase in temperature is also anindication of higher penetration of dye molecules into thepores at higher temperatures resulting in increased numberof available sites for adsorption. A similar trend wasobserved in the removal of basic Red using bentonite.[42]

ADSORPTION STUDIES

Adsorption Isotherms

The adsorption data obtained at different initial dyeconcentrations were fitted into eight different isothermmodels. The plots are presented in Figure 6. The Freun-dlich isotherm model fitted the adsorption data best havingthe highest R2 and the lowest Dqe values. (Table 5) A nF

value greater than 1 also indicates that the adsorption isfavorable. The Qm value of 333.33 mg=g obtained for theLangmuir isotherm model was compared with otherisotherm Qm values, this comparison shows that the Sipsisotherm has a similar Qm value with the Langmuirisotherm (Table 5). A comparison of the adsorptioncapacity of WRAC with other Qm values obtained fromother adsorbent is reported in Table 6. Table 6 shows thatWRAC used in this work has the highest adsorptioncapacity, and it range within the best and the most efficientadsorbents for RBBR dye removal.

BATCH KINETIC STUDIES

Figure 7 shows the plot of four different kinetic modelsused to explain the adsorption data, the pseudo-second-order kinetic models fit well with experimental data when

FIG. 3. The effect of contact time and initial RBBR dye concentration

on WRAC at 30�C.

FIG. 4. Effect of pH on adsorption of RBBR dye onto WRAC.

FIG. 5. The effect of solution temperature on adsorption of RBBR

dye onto WRAC.

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FIG. 6. Plots of a) Langmuir, b) Freundlich, c) Temkin, d) Dubinin–Radushkevich, e) Radke–Prausnite, f) Sips, g) Vieth–Sladek, and h) Brouers–

Sotolongo isotherms for RBBR dye adsorption on WRAC.

TABLE 5Isotherm values for RBBR dye adsorption unto WRAC at 30�C

WRAC-RBBR

Langmuir Freundlich TemkinDubinin–

Radushkevich SipsVieth–Sladek

Bruouers–Sotolongo

Radke–Prausnitz

Qm 333.33 1=nF 0.722 B 54.741 BDR 0.0089 Qm 332.14 KVS 0.139593 Qm 325.727 KRP 0.002KL 0.0051 KF 0.15 A 0.0999 E 4.9772 Ks 0.004 Qm 391.68 kBS 0.006 Qm 327.39R2 0.9664 R2 0.998 R2 0.9267 qs 7.4953 ms 0.2727 bVS 0.002 a BS 0.787 mRP 1.031RL 0.282 nF 1.385 R2 0.8478 R2 0.982 R2 0.973 R2 0.971 R2 0.965Dqe 12.215 Dqe 4.544 Dqe 8.481 Dqe 6.154 Dqe 6.174 Dqe 13.821 Dqe 8.491 Dqe 5.987

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compared to the other kinetic models. The data reported inTable 7) showed that the RBBR dye adsorption parametersgave good agreement with the pseudo-second-order kineticmodel with R2(0.983) value close to unity and low values ofDqt, (2.797%). The rate coefficient, k2, of pseudo-second-order was found to decrease with the increase in initialdye concentration. It indicates that as the initial concen-tration increases the electrostatic interaction decreases onthe site, thus lowering the dye affinity toward WRAC.

However, Elovich desorption constant, bEl, increases asthe initial dye concentration increases. On the other hand,the Elovich constant, aEl, decreases with the increase ininitial dye concentration. This indicates that the adsorptionprocess involves more than one mechanism. The findingswere in agreement with the several other studies on theadsorption of RBBR dye onto polyaniline=extracellularpolymeric substances,[45] fly ash,[46] and Scenedesmusquadricauda,[47] respectively.

ADSORPTION THERMODYNAMICS

The activation energy was found to be 5.57 kJmol�1

(Table 8). The magnitude of the activation energy may givean idea about the type of sorption. Two main types ofadsorptions may occur: physical and chemical. Physisorp-tion process normally has activation energy of5–40 kJ mol�1, while chemisorptions have higher activationenergy of 40–800 kJ mol�1.[48] The value of Ea for theadsorption of RBBR dye onto WRAC is within the rangeof physisorption. This value is consistent with the valuesreported in the literature.[49] The Gibbs free energy change,(DG), is the fundamental criterion of spontaneity of achemical reaction. Generally, DG for physisorption is lessthan that for chemisorptions. For physisorption, the changein free energy is between 0 and 20 kJ mol�1, for chemisorp-tions it is in the range of 20–400 kJmol�1.[50] The values of

TABLE 6Comparison of maximum monolayer adsorption capacities

of RBBR dye unto various adsorbents

Adsorbents Qm (mg=g) References

Polyaniline=chitosan 303.03 [43]Polyaniline doped p-toluene

sulfonic acid28.27 [44]

Polyaniline camphor sulfonic acid 42.00 [44]Polyaniline=EPS 361.82 [45]Fly ash 135.70 [46]Scenedesmus quadricauda 45.70 [47]Periwinkle shell activated carbon 312.64 [35]WRAC 333.33 This study

EPS: extracellular polymeric substances.

FIG. 7. Linearized plots of a) pseudo-first-order, b) pseudo-second-order, c) Avrami and d) Elovich kinetic model for RBBR dye adsorption onto

WRAC at 30�C.

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DG (�13.32 to �15.12 kJ mol�1) (Table 8) for the adsorp-tion process studied were within the range of physisorption.The value of enthalpy change (DH) shows the exothermic orendothermic nature of the adsorption process which may beeither physisorption or chemisorption. In this study, thevalue of DH (4.99 kJmol�1) for the adsorption processis less than 40 kJmol�1 indicating that the reaction isendothermic. It equally confirms that the interactionfollows physisorption mechanism.

ADSORPTION MECHANISM

Figure 8 shows the plot of the experimental dataobtained at 30�C. The plots are not linear over the wholetime range; however, they exhibit multi-linearity revealingthe existence of three successive adsorption steps[50] (similartrends were obtained at 45�C and 60�C—figure not shown).The first stage is faster than the second, and it is attributedto the external surface adsorption referred to as the

boundary layer diffusion. Thereafter, the second linear partis attributed to the intraparticle diffusion stage; this stage isthe rate controlling step. The third linear portion is referredto as the equilibrium stage. It is imperative to note that thesecond linear plots did not pass through the origin, indicat-ing that the intraparticle diffusion is involved in the adsorp-tion process, but it is not the only rate-controlling step.Other mechanisms such as film diffusion, complexation,or ion-exchange could also control this adsorptionprocess.[51] Table 9 shows the intraparticle model constantsfor the adsorption of RBBR dye onto WRAC. The kp

values and C (the intercept) values for all initial concen-tration were found to increase from first stage of adsorptiontoward the third stage. The increase in dye concentrationresults in an increase in the driving force thereby increasing

TABLE 7Kinetic model constant values for RBBR dye adsorption onto WRAC at 30�C

Model Kinetic parameters Initial RBBR dye concentration (mg=L)

Pseudo-first-order 25 50 100 200 400 500qeexp (mg=g) 16.38 30.23 56.92 100.76 182.04 202.76k1 (min�1) 0.512 0.463 0.378 0.355 0.336 0.301qe cal (mg=g) 13.19 26.59 50.24 92.90 171.01 193.14R2 0.972 0.952 0.975 0.984 0.964 0.963Dqt (%) 12.97 24.646 19.86 9.643 15.65 18.03

Pseudo-second-order qecal (mg=g) 13.77 24.17 41.26 71.90 129.15 135.71k2 (min�1) 0.224 0.089 0.052 0.023 0.011 0.009h (mg=g min) 66.82 111.71 152.81 249.90 399.25 438.06R2 0.983 0.980 0.989 0.994 0.996 0.987Dqt (%) 2.797 8.646 9.586 11.643 9.165 12.033

Elovich a El (mg=g min) 21.04 5.33 2.098 0.759 0.337 0.243bEl (g mg�1) 2.398 4.832 9.334 17.64 32.80 37.47R2 0.952 0.940 0.936 0.968 0.943 0.937Dqt (%) 15.797 23.646 14.586 18.643 28.165 17.033

Avrami nAV 0.531 0.583 0.571 0.657 0.699 0.753kAV (min�1) 0.100 0.067 0.060 0.037 0.028 0.019R2 0.969 0.946 0.923 0.978 0.949 0.953Dqt (%) 5.064 20.264 26.796 27.483 18.397 57.232

TABLE 8Thermodynamic parameters for RBBR dye adsorption

onto WRAC

Activatedcarbon

DH(kJ=mol)

DS(J=mol K)

Ea

(kJ=mol)

–DG(kJ=mol)

303 K 318 K 333 K

WRAC 4.99 10.62 5.57 13.32 14.42 15.12FIG. 8. Plots of intraparticle diffusion model for RBBR dye

adsorption onto WRAC at 30�C.

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the dye diffusion rate. The constant, C, was found toincrease with the increase in initial dye concentration, indi-cating the boundary layer effect.[52]

CONCLUSIONS

In this study, the amount of RBBR dye adsorbed perunit WRAC mass increased with the increase in initialdye concentration and temperature. Adsorption of RBBRdye revealed that the pseudo-second-order model bestrepresented the adsorption kinetic data. Out of the eightmodels used in the study, the Freundlich isotherm best fit-ted the equilibrium adsorption data. The mean free energyof adsorption suggests that the adsorption of RBBR dyeonto WRAC can be characterized as physisorption pro-cess. Adsorption mechanism implies that the process iscontrolled by intraparticle diffusion; however, that wasnot the only rate-controlling step. Thermodynamic para-meters revealed that the adsorption of RBBR dye ontoWRAC was endothermic and spontaneous. This study con-cluded that WRAC is an appropriate adsorbent for remov-ing synthetic dye from wastewater.

FUNDING

The financial support in the form of grants from USM;the three-month USM-TWAS Visiting Researcher Fellow-ship, FR number: 3240268492, awarded to OlugbengaSolomon Bello; and the accumulated leave granted toOlugbenga Solomon Bello by his home institution to utilizethe fellowship is thankfully recognized.

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TABLE 9Intraparticle diffusion model constant for adsorption of RBBR dye onto WRAC at 30�C

Activatedcarbon

RBBR initialconcentration (mg=L) 25 mg=L 50 mg=L 100 mg=L 200 mg=L 400 mg=L 500 mg=L

WRAC Kp1(mg=gh1=2) 20.4215 30.7692 51.3161 97.3774 113.8681 114.4751Kp2 (mg=gh1=2) 2.0197 4.0320 5.06467 9.1324 20.2741 23.3321Kp3 (mg=gh1=2) 0.0005 0.0008 0.0009 0.0010 0.0012 0.0012C1 1.0564 1.8288 3.5793 5.4507 9.1326 4.9944C2 8.5754 16.4201 29.4806 50.1370 85.3568 88.0632C3 17.2303 32.0228 58.0536 109.8445 202.8806 237.5055(R1)2 0.8565 0.8676 0.8323 0.8765 0.8667 0.9665(R2)2 0.9456 0.7759 0.7662 0.7731 0.8245 0.8581(R3)2 0.9997 0.9976 0.9986 0.9998 0.9766 0.9788

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