Chemical Engineering Journal 215–216 (2013) 747–754

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Chemical Engineering Journal

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Acid modified carbon coated monolith for methyl orange adsorption

Willie Cheah a, Soraya Hosseini a, Moonis Ali Khan b, T.G. Chuah a,c, Thomas S.Y. Choong a,c,⇑a Department Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysiab Advance Material Research Chair, Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabiac INTROP, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

h i g h l i g h t s

" Carbon coated monolith (CCM) was modified by nitric acid." Twofolds escalation in acidic sites was observed on modified compared with unmodified CCM." Comparatively 53% higher MO adsorption was observed on modified CCM than CCM." Optimum MO uptake was 132.7 mg/g at equilibration time 4560 min, agitation 200 rpm and temperature 30 �C." Optimum MO elution (73%) was achieved with 1 N NaOH solution.

a r t i c l e i n f o

Article history:Received 3 May 2012Received in revised form 2 July 2012Accepted 2 July 2012Available online 8 July 2012

Keywords:Methyl orangeAcid modified carbon coated monolithAdsorptionDesorptionThermodynamics

1385-8947/$ - see front matter � 2012 Elsevier B.V. Ahttp://dx.doi.org/10.1016/j.cej.2012.07.004

⇑ Corresponding author at: Department Chemical aing, Universiti Putra Malaysia, 43400 UPM Serdang,389466293; fax: +60 386567120.

E-mail address: tsyc2@eng.upm.edu (T.S.Y. Choong

a b s t r a c t

Carbon coated monolith (CCM) was chemically modified by treating with nitric acid. The acid modifiedcarbon coated monolith (ACCM) was then characterized by using various techniques. Two folds increasein acidic sites was observed on ACCM compared to CCM. Surface studies showed mesoporous nature ofACCM. A decrease in ACCM surface area and an increase in pore volume observed after the modification.The ATR-FT-IR studies showed increase in carboxylic groups on ACCM confirming CCM oxidation by nitricacid. The pH studies showed optimum adsorption (88 mg/g) at pH 6 which is very near to pHPZC of ACCM.Contact time studies showed equilibration time in between 4320 and 4560 min for initial MO concentra-tion range 0.05–0.6 g/L. Comparatively 53% higher MO adsorption was observed on ACCM than CCMunder similar experimental conditions. Freundlich model applicability confirms multilayer MO adsorp-tion on ACCM surface. Pseudo-second-order kinetics model was fitted best to the experimental datarevealing chemical nature of adsorption process. The adsorption process is endothermic and spontaneousin nature. Desorption studies showed optimum MO recovery (73%) when 1 N NaOH was used as aneluent.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

As the world population increases, the demands on textileindustry are augmented due to the improving sense of lifestylesand fashion by human beings. Dyes are the coloring agents visiblewith human naked eyes. The presence of dyes not only hampersthe aesthetic quality of water but also affects and alters the aquaticecosystem by reducing the penetration of sunlight and oxygen [1].Methyl orange (MO), a water-soluble azo dye, commonly presentin effluent discharges form textile, food, pharmaceutical, printingand paper manufacturing industries [2]. Due to the toxicity and

ll rights reserved.

nd Environmental Engineer-Selangor, Malaysia. Tel.: +60

).

persistence these discharges can cause serious threat to physico-chemical properties of fresh water and to aquatic life.

Various chemical, biological and physical treatments havebeen utilized to treat the azo dyes [3–7]. Since dyes are resistantto aerobic biodegradation, recalcitrant organic molecules, andstable to oxidizing solutions, adsorption process is proven as areliable and effective act for this treatment [8]. Simplicity inprocess design, ease in operational conditions and economicalaspects are some of the major advantages of adsorption process[9,10].

Activated carbon (AC), an adsorbent, widely used for dyes re-moval from wastewater [11,12]. The well developed pore structureand high internal surface area results in AC’s excellent adsorptionproperties. Furthermore, AC can remove highly odorous dissolvedorganic compounds from industrial effluents. Several studies havereported the utilization ACs for dyes removal form wastewater

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[13–16]. However, regeneration cost and pore blockage are themajor shortcomings of AC for their utility as an adsorbent.

Considering these shortcomings, cordierite monolith has beenmodified by making a carbon coat over the surface termed ascarbon coated monolith (CCM). In our previous studies we hadexplored the adsorption properties of methylene blue (MB) andb-carotene on CCMs [17–20]. Previous research revealed the appli-cability of CCM for MO adsorption [18]. In this study, the CCM waschemically treated with nitric acid (HNO3). Nitric acid, a strongacid oxidizer, may enhance acid functionality over CCM surface[21]. The modified CCM was termed as acid modified carbon coatedmonolith (ACCM). The ability of ACCM for MO adsorption wasevaluated by applying various kinetics and thermodynamicsparameters. To check the reusability of the ACCM regenerationstudies were carried out.

2. Experimental

2.1. Materials

Cordierite monoliths (400 cpsi) of channel width 1.02 ± 0.02 mmand wall thickness 0.25 ± 0.02 mm were purchased from BeihaiHaihuang Chemical Packing Co. Ltd., China. Furfuryl alcohol (99%),nitric acid (65%), polyethylene glycol (Mw = 6000) and pyrrol (99%)were purchased from Sigma Aldrich, Malaysia. Methyl orange(MO) with molecular weight – 327.34 g/mol and molecular formula– C14H14N3NaO3S was purchased from Sigma–Aldrich, Malaysia.The reagents and chemicals used during this study were of analyti-cal grade or as specified.

2.2. Preparation of acid modified carbon coated monolith (ACCM)

Furfuryl alcohol (99%), a carbon source and polyethylene glycol(PEG, MW – 6000), a pore former and pyrrol (99%), a binder werehomogeneously mixed. The resultant furfuryl resin was amelio-rated at 20 ± 1 �C by using HNO3 as a catalyst. The acid (0.4 mL)was repeatedly added for an hour at an interval of 10 min each.Due to the exothermic nature of polymerization, the reaction wascarried out in an ice bath. A dip-coating process [22] was used forthe modification. Dried monolith was impregnated for 20 min in aviscous polymerized solution. To avoid channel blockage, the ex-cess solution trap in the monolith channels was purged by usingpressurized air. A light brown monolith was exsiccated by heatingcontinuously at 110 �C for 24 h in an oven. Consequently, themonolith was carbonized in a furnace at 650 �C for a residencetime of 4 h under N2 atmosphere. This eventually produced CCM.The CCM was further modified by treating with HNO3 solution[23]. The CCM was impregnated in 2 N HNO3 (250 mL) solution.The reaction continued at 30 �C for 24 h in a shaker. The treatedmonolith was then rinsed with de-ionized water until it acquiresneutral pH. The acid modified carbon coated monolith (ACCM)was then dried at 65 �C for 24 h in an oven, sealed in polyethylenebags and was kept in a desiccator until they were used forexperiments.

2.3. Characterization of the adsorbent

The textural properties of CCM and ACCM such as surface area,pore volume and pore size were examined by nitrogen (N2) sorp-tion–desorption isotherm at 77 K (Sorptomatic V1.03). To examinethe functional groups present on CCM and ACCM surface and toexamine the functional groups involved in MO adsorption theAttenuated total reflection-Fourier transform-infrared [ATR-FT-IR,Perkin Elmer Spectrum (100 FT-IR spectrometer)] analysis was car-ried out. The surface morphology of CCM and ACCM was studied by

using scanning electron microscopy (SEM) (Hitachi Co., Japan,Model No. S3400N). Solid addition method [24] was used to deter-mine point of zero charge (pHPZC) of ACCM. Potassium nitrate(KNO3, 50 mL) solutions with pH ranging between 2 and 10 wereprepared adjusting pH by using 0.1 M KOH and 0.1 M HNO3 solu-tions. ACCM (0.5 g) was added to each solution and the solutionswere equilibrated in a shaker at 200 rpm under ambient tempera-ture conditions. The pHPZC was described from the point of inter-section curve at abscissa (DpH = 0) for the plot of DpH (thedifference between initial pH and final pH) versus pHi (initialpH). The surface active (acid and base) sites present on CCM andACCM were determined by Boehm’s acid base titration experi-ments [25]. The acidic sites present on CCM and ACCM (0.5 g) wereneutralized by 50 mL 0.1 M solutions of Na2CO3, NaHCO3 andNaOH while, the neutralization of basic sites on CCM and ACCMwere carried out by 50 mL 0.1 M HCl solution. The samples wereequilibrated for 5 days at 200 rpm under ambient temperatureconditions. Afterward, 10 mL of each sample was titrated with0.1 N HCl and 0.1 N NaOH solutions. The titration was carried outin triplicate using a potentiometer.

2.4. Preparation and adsorbate analysis

To prepare MO stock solution (1 g/L) a desired amount of MOwas dissolved in de-ionized water. The stock solution was furtherdiluted to desired MO concentrations. UV–visible spectrophotome-ter (Ultrospec™ 3100 p) at maximum wavelength (kmax) – 464 nmwas used to measure MO concentration.

2.5. Adsorption and desorption studies

Batch process was employed for both adsorption and desorp-tion studies. For adsorption studies, a series of conical flasks(250 mL) containing MO solution (50 mL) of desired initial concen-trations (0.05–0.6 g/L) at pH 6 were equilibrated with 0.5 g of CCMand ACCM in a shaker at 200 rpm under ambient temperature con-ditions. At equilibrium, the adsorbate was filtered and residualconcentration was analyzed using UV spectrophotometer. Theadsorption capacity at equilibrium (qe, mg/g) was determined byusing the following equation:

qe ¼ðCo � CeÞV

mð1Þ

where Co and Ce (mg/L) are the initial and equilibrium concentra-tions of MO, respectively, m (g) is the mass of ACCM, and V (L) isthe volume of adsorbate solution.

The pH studies were carried out in a pH range 5–10. The pH ofthe MO solution (0.25 g/L) was adjusted by adding 0.5 M HCl and0.5 M NaOH solution. The adsorbate was equilibrated by adding0.5 g ACCM in a shaker at 200 rpm for 48 h under ambient temper-ature conditions. Contact time studies were carried out at differentMO concentrations and at predetermined time interval sampleswere filtered and analyzed. The adsorption capacity (qt, mg/g) attime t was determined. Thermodynamic studies were also carriedout by varying reaction temperature from 30 to 50 �C.

For desorption studies, NaOH solution of various strength wasused as an eluent. ACCM was first equilibrated with 0.2 g/L MOsolution in a shaker under ambient temperature conditions. Atequilibration, the ACCM was washed with de-ionized water to re-move unadsorbed traces of MO and was dried in an oven. ACCMwas then treated with eluent to elute MO.

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3. Results and discussion

3.1. Characterization of adsorbents

Boehm titration experiments highlights that compared to basicsites, the number of acidic sites on both CCM and ACCM are higher(Table 1). The acid modification of CCM (ACCM) leads to a two-folds increase in acidic sites. The total number of acidic sites onCCM was 0.5825 mmol/g, escalated to 1.0913 mmol/g on ACCM.The increase in acidic active sites on ACCM was due to the treat-ment of CCM with nitric acid, a strong oxidizing agent, leading tothe rise in oxygen functionalities over ACCM surface [23–28].

The surface area and the pore volume of virgin ACCM and ACCMafter MO desorption were determined using N2 sorption/desorp-tion isotherms. Studies revealed that ACCM in both cases followstype IV isotherm according to IUPAC classification, indicating

mesoporous nature of adsorbent as shown in Fig. 1a and b. TheBET isotherm studies showed decrease in surface area from237 m2/g (CCM) to 232 m2/g (ACCM) while, the pore volume in-creased from 0.29 cc/g (CCM) to 0.36 cc/g (ACCM). The surface areaof ACCM was further reduced to 205 m2/g after MO desorption. Thedecrease in surface area was attributed to the introduction ofstrong oxidizing agent on CCM leading to the destruction of thepore surface. The oxidizing agent introduces oxygen functionalitiesover the surface of CCM leading to obstruction of microporesattributing to decrease in surface area. Similar results for surfacemodification have been described by Chen and Liu [29,30].

The ATR-FT-IR spectra of CCM, ACCM and after MO adsorption onACCM are presented in Fig. 2. Band assignments for the spectrum ofCCM indicates that the carbonized carbon contains a number ofatomic groups and structures such as OH, CH2, C@O, CAOAC. Thesignificant bands in the spectra of CCM are those at range 700–3900. A broad peak (CCM) is presented between 3000–2800 cm�1;that is assigned to aromatic ACH3. Indeed the band is overlappingtwo such bands, which are in the spectra of the samples at 2922–2918 and 2845 cm�1 are connected with vas(CAH) and vs(CAH)vibrations (s = symmetric, as = asymmetric). The broad band disap-peared in ACCM and MO adsorption cases. For the samples oxidizedwith HNO3, there is a large decrease in the intensity of the bands at1300 and 1750 cm�1. In this study, the intensity of the band at1750 cm�1 depended on the concentrations of HNO3 used for treat-ment of the CCM. The great intensity of this band occurred when theconcentration of HNO3 for treatment was increased. A band locatedat 1350 cm�1 (acetate ester) is absent from the spectrum MOadsorption. The AOACOACH3 grouping (about 1300–1350 cm�1)

Table 1Surface active sites on CCM and ACCM.

Active sites CCM (mmol/g) ACCM (mmol/g)

Total basic sites 0.0315 0.0151Total acidic sites 0.5825 1.0913Carboxylic 0.1085 0.3901Lactonic 0.1117 0.0890Phenolic 0.3623 0.6121

of acetate ester and methyl groups of aliphatic-CH3 occurred whenthe temperature of treatment was increased [26].

Morphological studies of CCM and ACCM (Fig. S1) illustratesnon-uniform, irregular and porous CCM surface. Modification ofCCM with nitric acid causes smoothening of surface by forming alayer like covering over the surface.

3.2. Effect of pH

Solution pH and ionic strength are the two most influentialoperational factors controlling the adsorption mechanism. Duringthis study, the solution pH was able to restrain the electrostaticinteractions between MO and ACCM system affecting the adsorp-tion efficiency. In fact, the pH dependence for MO will form aninteraction between the dye and its environment as expressed bythe following reactions [19]:

The MO adsorption on ACCM was found to increase with in-crease in initial solution pH from 5 to 6 (Fig. 3) attaining optimumadsorption capacity (88 mg/g) at pH 6. Further increase in pH(above 6) leads to a decrease in MO adsorption. At pH 6, MO occursas quinoid form in aqueous system predominating dispersioninteraction process. The study revealed ACCM’s pHPZC at 6.6(Fig. 4). This shows that above pHPZC the adsorbent surface wasnegatively charged, retarding the diffusion of MO ions on ACCMsurface, confirming the decrease in adsorption above pHPZC wasdue to the repulsive interaction between anionic dye and adsor-bent surface. Similar results for MO adsorption on different adsor-bents were reported elsewhere [31,32].

3.3. Effect of contact time at various initial concentration

The contact time studies are essential to explore the equilibra-tion time for the adsorption process. The contact time studies forMO adsorption on ACCM at various initial MO concentrations werecarried out. The adsorption curves are smooth and continuous. Ini-tially, fast MO adsorption was observed on ACCM confirming chem-ical nature of adsorption process, eventually slows down attainingequilibrium with time [16]. Compared with previous studies [19],the equilibration time for MO adsorption on ACCM was much faster.The equilibration time for initial MO concentration range 0.05–0.6 g/L was in between 4320 and 4560 min (Fig. 5). The adsorptioncapacity increases from 28 to 132.7 mg/g with increase in MO con-centration from 0.05 to 0.6 g/L, respectively. Further, nitrified CCM(ACCM) showed 53% higher MO adsorption capacity compared toCCM [19]. The increase in MO adsorption capacity on ACCM attri-butes to the fact that the CCM oxidation leads to the formation ofcarbon–oxygen surface functional groups as the acid active sitescausing appreciable increase in adsorption.

3.4. Adsorption isotherms

To describe solid–liquid adsorption systems, two isothermsnamely Freundlich and Langmuir have been adopted. Langmuirmodel assume that there are fixed numbers of active sites on the

Fig. 1. N2 sorption–desorption isotherm at 77 K on ACCM virgin (a), ACCM after MO desorption (b).

Fig. 2. ATR-FT-IR spectra of adsorbent.

750 W. Cheah et al. / Chemical Engineering Journal 215–216 (2013) 747–754

surface of adsorbent [33]. However, each of this active site is onlyavailable for one absorbate molecule to be occupied. Once all theactive site was fully occupied, the surface of the adsorbent willform a homogeneous surface of monolayer adsorption with a de-crease in intermolecular forces between the adsorbate molecules.To explain the behavior of the adsorption on the equilibrium con-dition, the linearized form of Langmuir equation was applied andcan be represented by the following:

Ce

qe¼ 1

qmkLþ Ce

qmð2Þ

Alternatively, Langmuir isotherm can be expressed as:

qe ¼qmkLCe

1þ kLCeð3Þ

where Ce (mg/L) is MO equilibrium concentration in the solution, qe

(mg/g) is the adsorption capacity, qm (mg/g) represents maximumamount of adsorption required to form a homogeneous monolayerat the ACCM and kL (L/mg) denotes Langmuir constant to give an ac-count on adsorption energy. The intercept and slope of the linearplot between Ce/qe versus Ce gives Langmuir constant, kL and qm,respectively.

From a practical point of view, the feasibility of Langmuir iso-therm can be explained in term of the separation factor (RL), adimensionless factor. It can be expressed by the followingequation:

RL ¼1

1þ kLCoð4Þ

If, RL > 1 is the unfavorable adsorption0 < RL < 1 is the favorable adsorptionRL = 1 is the linear adsorptionRL = 0 is the irreversible adsorptionFreundlich isotherm has been used to characterize the hetero-

geneous surface of the adsorbent [34]. It is valid for multilayeradsorption on the surface of the adsorbent where there is possibil-ity that the amount of dye being adsorb on the adsorbent surface isnot constant at a given concentration. The empirical equation is ex-pressed as [35]:

qe ¼ kF C1=ne ð5Þ

While in linearized form can be expressed as:

log qe ¼ log kF þ1n

log Ce ð6Þ

Fig. 3. Effect of pH for MO adsorption on ACCM. (Conditions: initial MO concen-tration – 0.25 g/L, agitation speed – 200 rpm, temperature – 30 �C, contact time –48 h).

Fig. 4. Point of zero charge (pHPZC) plot for ACCM using 0.01 M KNO3.

Fig. 5. Effect of contact time on MO adsorption at various initial concentrations onACCM. (Conditions: agitation speed – 200 rpm, temperature – 30 �C).

Fig. 6. Non-linear isotherm plot for MO adsorption on ACCM. (Conditions: agitationspeed – 200 rpm, temperature – 30 �C).

Table 2Isotherm parameters for MO adsorption on ACCM.

Langmuir constants Freundlich constants

kL (L/g) qm (mg/g) r2 kF (mg/g)(L/mg)1/n n r2

0.018 147.06 0.9692 15.40 2.728 0.9985

W. Cheah et al. / Chemical Engineering Journal 215–216 (2013) 747–754 751

where kF (mg/g) and n are Freundlich constants for the adsorptioncapacity of the adsorbent and the intensity of adsorption, respec-tively. From the linearized equation of Freundlich, the slope of thegraph represents the constant 1/n, while the intercept depict theconstant log kF. In addition, the value of n obtained expressed thefavorability of the adsorption process. To exemplify, value of n > 1shows a favorable adsorption.

A general consensus has been made by comparing the result be-tween Langmuir, Freundlich and experimental data for the adsorp-tion of MO on ACCM. In order to get a better understanding on theadsorption mechanism, the concentration of MO after adsorptionwas theoretically calculated for both the isotherms. The resultswere presented in Fig. 6 while Table 2 listed the isotherm param-eters for Langmuir and Freundlich isotherms.

For the adsorption of MO by ACCM, the values of RL were in be-tween 0.13 and 0.91 depicting favorable nature of adsorption pro-cess. On the other hand, the value of Freundlich constant n was2.728 (Table 2) again confirming favorable adsorption. The regres-sion coefficient (r2) values as obtained by linearized isothermsshowed better fitting of experimental data to Freundlich isotherm.The applicability of Freundlich isotherm was also confirmed bynon-linearized isotherm. The qm value gave an approximate evalu-ation of MO adsorption on ACCM. It was revealed that the adsorp-tion with ACCM exhibits high adsorption capacity compare withCCM. The qm values for ACCM and CCM are 147.06 and102.04 mg/g [19], respectively. Hence, a 44% increase in MOadsorption was observed on ACCM.

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3.5. Adsorption kinetics and mechanism

Kinetics models are used to reflect a relationship between theadsorption rate and equilibrium time. Through adsorption kinetics,the mechanism for MO uptake rate and the residence time for theadsorption process can be determined. In conjunction, the MOadsorption kinetics on ACCM was analyzed by using Lagergrenfirst-order [36], pseudo-second-order [37], and intra-particle diffu-sion models [38].

Lagergren first-order kinetics is generally expressed as:

dqt

dt¼ k1ðqe � qtÞ ð7Þ

However, after applying the boundary condition at qt = 0�qt andt = 0�t, the integrated formulation equation becomes

logðqe � qtÞ ¼ log qe �k1

2:303� t ð8Þ

where qe and qt are the amounts of adsorbed MO on ACCM at equi-librium and at time t, respectively, k1 is the rate constant. The valueof adsorption capacity, rate constant and correlation coefficient (r2)are determined from the plot log (qe�qt) versus t.

The pseudo-second-order kinetics model is expressed by thefollowing equation:

tqt¼ 1

hþ t

qeð9Þ

where, k2 is rate constant for pseudo-second-order model andh = k2qe

2 is the initial adsorption rate of MO. The slope and interceptof the plot t/qt versus t denotes the rate constant and adsorptioncapacity, respectively.

The parameters as obtained from the linearized plots (figuresnot given) are given in Table 3. As observed, the calculated adsorp-tion capacity qe(cal) for pseudo-second-order model was much morecloser to the experimental adsorption capacity qe(exp) for variousMO concentrations confirming better applicability of pseudo-second-order model compared to pseudo-first-order model. Theseresults are further confirmed by higher regression coefficient (r2)values for pseudo-second-order model compared to pseudo-first-order model (Table 3). Thus, it indicates that the adsorption ofMO on ACCM is a chemisorption process. The adsorption processis likely to occur on the surface site of the adsorbent where therewill be an exchange or sharing of electrons via valance forces.The surface exchange reaction between the adsorbent and adsor-bate will last until the functional sites of the surface are fullyseized. The assumptions used to verify pseudo-second-order is thatthe chemisorption acts as the rate limiting step. The adsorptionprocess happens due to the valence forces between the dye andadsorbent [39]. As the initial MO concentration increases from0.05 to 0.6 g/L, the pseudo-second-order rate constant (k2) de-creases from 27.6 � 10�5 to 4.60 � 10�5 g/mg-min, respectively.This trend is due to lowering in adsorption rate with increase ininitial MO concentration.

Table 3Kinetics parameters for MO adsorption at various initial concentrations on ACCM.

Co (g/L) qe (exp) (mg/g) Lagergren-first-order

qe (cal) (mg/g) k1 � 10�3 (1/min)

0.05 27.0 18.8 2.530.10 44.8 55.2 5.070.20 71.9 66.6 3.220.30 96.5 98.3 4.610.40 108.7 107.8 4.380.50 124.0 111.4 3.450.60 132.7 104.3 2.76

Previous study [19] was used to contemplate MO adsorption be-tween ACCM and CCM. The qe(exp) as determined for MO adsorptionin concentration range 0.05–0.5 g/L on CCM was 15.99–89.50 mg/g[19]. However, during the present study, the qe(exp) obtained for MOadsorption for same concentration range on ACCM was 27.00–124.03 mg/g. It apparently proves that there is a significantincrease in MO adsorption after nitrification treatment has beendone on CCM. Both the studies showed that pseudo-second-orderkinetics model provides a better fit to the experimental data.

The kinetics data was also used to analyze the intra-particle dif-fusion for MO adsorption on ACCM. This approach was based onWeber and Morris model [38]. The model was characterized bythe following equation:

qt ¼ kit1=2 þ I ð10Þ

where, ki and I are the intra-particle diffusion rate constant (mg/g min1/2) and the intercept, respectively. The plot of qt versus t1/2

produces the value of ki as the slope and I as the intercept.There are few assumptions proposed by Weber and Morris in

order to describe the intra-particle diffusion. The adsorption pro-cess will involve:

(1) The transport of the adsorbate molecule from the bulk solu-tion to the boundary layer surrounding the adsorbent (filmdiffusion).

(2) The diffusion of the adsorbate from the boundary layer tothe external surface of adsorbent (bulk diffusion).

(3) The diffusion of the adsorbate from the surface to the inter-nal pores of the adsorbent (intra-particle diffusion or porediffusion).

(4) The uptake of adsorbate to the active site of the adsorbentvia several mechanisms such as chemisorption, physisorp-tion, ion exchange, complexation, precipitation or chelation.

The overall adsorption rate is controlled by the slowest adsorp-tion step which is also known as rate-limiting step. Generally, theinitial part of the adsorption is rapid due to external diffusion (li-quid phase mass transfer rate) while the latter part will be therate-limiting step (intra-particle mass transfer rate). Thus, intra-particle diffusion is one of the factors that affect the rate of attain-ment for the adsorption to reach equilibrium state.

Fig. 7 illustrates Weber and Morris plot for MO adsorption onACCM. As presented in Fig. 7, it reveals that there are few stepsplaying a significant role in the adsorption mechanism due to themulti-linearity correlation. The deviation of the straight line fromthe origin shows that intra-particle diffusion is not only the mech-anism involving in the adsorption process [40].

Initially, the adsorption rate is very high resulted from the filmdiffusion. As MO molecule start to adsorb on the external surface ofACCM, there is immediate utilization on the active sites of theadsorbent. As time passed by, the process of adsorption was con-trolled by intra-particle diffusion. Since the rate of adsorptionbeing slow down, this region is also known as the rate limitingstep. Finally the plateau segment indicates the equilibrium state

Pseudo-second-order

r2 qe (cal) (mg/g) k2 � 10�5 (g/mg min) r2

0.994 27.4 27.6 0.9990.951 52.1 10.8 0.9980.989 79.3 6.71 0.9970.941 97.1 6.57 0.9990.972 109.8 6.35 0.9990.971 123.4 4.69 0.9980.987 133.3 4.60 0.998

0

35

70

105

140

0 20 40 60 80 100

qt

t0.5 (min)

0.05 g/L 0.1 g/L 0.2 g/L 0.3 g/L0.4 g/L 0.5 g/L 0.6 g/L

Fig. 7. Weber and Morris plot for MO adsorption on ACCM. (Conditions: agitationspeed – 200 rpm, temperature – 30 �C).

Table 4Intra-particle diffusion parameters for MO adsorption at various initial concentrationson ACCM.

Co (g/L) ki (mg/g min1/2) I r2

0.05 1.053 0.466 0.9760.10 2.310 1.114 0.9370.20 3.151 1.677 0.9650.30 4.120 1.967 0.9760.40 4.581 2.639 0.9820.50 4.957 2.761 0.9880.60 5.097 2.995 0.994

Table 5Thermodynamics parameters for MO adsorption on ACCM.

Temp. (K) DG0 (kJ/mol) DH0 (kJ/mol) DS0 (J/mol-K)

303 �9.785313 �9.798 14.25 13.96323 �10.060

Fig. 8. Effect of various NaOH concentrations for desorption of MO from ACCM.(Conditions: Co – 0.2 g/L, agitation speed – 200 rpm, temperature – 30 �C).

W. Cheah et al. / Chemical Engineering Journal 215–216 (2013) 747–754 753

for the adsorption process. At here the maximum MO adsorptionon ACCM was occurred. The equilibrium stage at the end wasdue to the decrease in number of active sites available for theadsorption as well as low concentration of MO in the bulk solution.

The values of ki and I for various initial MO concentrations arepresented in Table 4. As can be observed, the intra-particle diffu-sion rate, ki increases with the increase in initial MO concentration.This result from higher number of MO molecules present in thesolution which largely attain the active sites on ACCM surface.The values of I also revealed the capaciousness of thickness inboundary layer. As indicated in Table 4, an increase in I valueswas observed with increase in initial MO concentration. This attri-butes to greater effect of the boundary layer on high MO concen-tration [41]. Thus, thicker the boundary layer surroundingadsorbent eventually higher the uptake of MO.

3.6. Effect of temperature

The MO adsorption on ACCM was further examined through dif-ferent temperature conditions. If the adsorption capacity increasesas the temperature increases, it means the adsorption is via chemi-sorption. In contrary, if the adsorption decreases with increase in

temperature, it may due to physical adsorption [17,42]. Theadsorption process was conducted under isothermal conditionsat 30, 40 and 50 �C. As the outcome, when the temperature in-creases from 30 to 50 �C, the adsorption capacity also increasesfrom 124 to 221 mg/g for the same amount of initial MO concen-tration (0.5 g/L). This shows a 78% increase in the adsorptioncapacity. Thus, it also proves that the adsorption mechanism isvia chemisorption and agrees with the result as determined inpseudo-second-order kinetics model. The free energy of adsorp-tion, DG0, was determined using the following equation:

DG0 ¼ �RT ln K ð11Þ

The values of standard enthalpy change (DH0) and standard en-tropy change (DS0) were determined by using the followingequation:

ln K ¼ DS0

R� DH0

RTð12Þ

where T (K) is the absolute temperature, R is the universal gas con-stant (8.134 J/mol-K) and K is standard thermodynamic equilibriumconstant.

K ¼ CAe

Ceð13Þ

where CAe and Ce (mg/L) are the equilibrium concentrations ofadsorbate on ACCM and in solution, respectively.

The plot of ln K versus 1/T (Van’t Hoff plot) (figure not given)gives the values of these thermodynamics parameters. The nega-tive values of DG0 indicate that the process is spontaneous andthe spontaneity increases with increase in temperature (Table 5).On the other hand, positive value of DS0 indicates an increased dis-orderness at the adsorbent surface during adsorption process.Moreover, positive value of DH0 proves that the adsorption ofMO on ACCM is an endothermic process.

3.7. Desorption process

Desorption studies elucidate the recovery on MO from MCCM.Desorption process was conducted by in NaOH solution of variousstrengths. Since ACCM was treated with HNO3 and the adsorptionof MO on ACCN was very low at higher pH values (Fig. 3), this im-plies that basic medium could be used for desorption process [43].The maximum adsorption capacity obtained before desorption

754 W. Cheah et al. / Chemical Engineering Journal 215–216 (2013) 747–754

process was 74 mg/g. Fig. 8 illustrates the effects of NaOH concen-trations on MO desorption. It is observed that the MO desorptioncapacity increased with the increased in NaOH concentrations.The highest amount of desorption (54 mg/g) was found when 1 NNaOH was used as the eluents. It may be suggested that the posi-tive charge of ACCM was shielded by NaOH molecule under alka-line condition which eventually increases the desorptionefficiency. Moreover, the abundance of oxide groups on ACCM sur-face were attached by strong activation agent (NaOH) which super-sede MO molecule thus, enhances the desorption activity.

4. Conclusions

The performance of CCM was enhanced by oxidizing the adsor-bent with nitric acid. It was proved that the adsorption and desorp-tion capacity increases tremendously for ACCM compare withCCM. The ACCM characterization studies showed reduction in sur-face area and domination of mesopores. The optimum adsorptionwas observed at pH 6. Isotherm studies showed better fit to Fre-undlich isotherm model. The kinetics studies showed applicabilityof pseudo-second-order model. Weber and Morris plot verified theadsorption mechanism was due to multi-linearity correlation.Thermodynamics data obtained indicates that MO adsorption onACCM was endothermic and spontaneous. Furthermore, MOadsorption on ACCM was via chemisorption. Desorption studiesshowed 73% MO recovery with 1 N NaOH solution.

Acknowledgements

The authors would like to gratefully acknowledge UniversitiPutra Malaysia and MOHE (Project Number: 03-04-10-803FR) forthe financial support of this work.

Appendix A. Supplementary material

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.cej.2012.07.004.

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