adsorption/desorption of cationic dye on surfactant modified mesoporous carbon coated monolith:...

Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx

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JIEC-1938; No. of Pages 9

Adsorption/desorption of cationic dye on surfactant modifiedmesoporous carbon coated monolith: Equilibrium, kinetic andthermodynamic studies

Mohamad Rasool Malekbala a, Moonis Ali Khan b, Soraya Hosseini a,Luqman Chuah Abdullah a,c, Thomas S.Y. Choong a,c,*a Department of Chemical and Environmental Engineering, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysiab Chemistry Department, College of Science, King Saud University, Riyadh 11451, Saudi Arabiac INTROP, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia

A R T I C L E I N F O

Article history:

Received 27 August 2013

Accepted 25 February 2014

Available online xxx

Keywords:

Mesoporous carbon coated monolith

Surfactant (F-127)

Methylene blue

Desorption

Regeneration

A B S T R A C T

Surfactant modified carbon coated monolith was used as an adsorbent for methylene blue (MB)

adsorption. Effects of pH, salt, contact time, initial dye concentrations and temperature on dye

adsorption were studied. Higher solution pH favoured MB adsorption. Furthermore, kinetics study

showed that the adsorption could be better represented by the pseudo-second-order model. Linear and

non-linear isotherm studies revealed better fitting of Langmuir model to adsorption data with maximum

monolayer adsorption capacity 388 mg/g. The adsorption was found to be spontaneous and

endothermic. Desorption studies indicate that 0.1 N HCl exhibits higher elution efficiency (82.1%) with

appreciable quantitative MB recovery.

� 2014 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

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Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

1. Introduction

Concerning the hazardous effects of dyes on flora and fauna anincreasing research has been going on worldwide to control or tominimize them. Dyes even in very low concentrations in water areundesirable [1]. Characteristically dyes are stable molecules,resistant to light, heat and biodegradation [2] making conventional(primary and secondary) treatment techniques unsuitable forwater decontamination [3]. Methylene blue (MB), a cationic dye, isusually used as a colouring agent in paper and pulp and textileindustries. Although, MB is not regarded as a highly toxic dye, butstill MB can have various harmful effects on human beings andanimals.

The treatment techniques for removing dyes include coagula-tion and flocculation [4], oxidation or ozonation [5,6], membraneseparation [7], biosorption [8] and adsorption [9]. Adsorption hasan upper hand over the aforementioned processes. Low initial cost,

* Corresponding author at: Department of Chemical and Environmental

Engineering, Universiti Putra Malaysia, 43400 UPM, Seri Kembangan, Selangor,

Malaysia. Tel.: +60 3 89466293; fax: +60 3 86567120.

E-mail addresses: [email protected], [email protected]

(Thomas S.Y. Choong).

Please cite this article in press as: M.R. Malekbala, et al., J. Ind. Eng.

http://dx.doi.org/10.1016/j.jiec.2014.02.047

1226-086X/� 2014 The Korean Society of Industrial and Engineering Chemistry. Publis

ease of operation, simplicity of the design and flexibility are someof the merits of adsorption process. Currently, carbonaceousmaterials such as activated carbon are the most widely usedadsorbents. However, combustion at high temperature, poreblockage and hygroscopicity [10] are some of the demerits ofcarbonaceous adsorbents restricting their practical applicabilityfor dyes removal.

In addition, most of the carbonaceous materials are micropo-rous highly efficient to remove low molecular weight compounds[11–13]. The dyes and pigments molecular dimensions are close toupper limit of micropore size. For efficient removal of largemolecules like dyes and pigments, adsorbents should have a well-developed mesopore structure (pore size of 2–50 nm). Moreover,powder carbonaceous adsorbents cannot be easily regenerated,and can escape through filters, causing handling problems.Therefore, an improved support is required to overcome theproblems related to clogging, dispersion of particles and highpressure drop. At the same time they have low mechanicalstrength that limits their application in certain areas. Studiesshowed use of carbonaceous materials for dyes removal fromaqueous phase [14,15]. However, in some cases the adsorptioncapacity of these adsorbents due to their low mesopore volumewas not high.

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M.R. Malekbala et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx2

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Cordierite monoliths could be utilized to overcome theselimitations as these materials shows a high mechanical strength,good thermal stability, a relatively uniform porosity, uniformflow distribution and low pressure drop [16–18]. Recently,amphiphilic molecules, such as surfactants, have been exten-sively employed for the synthesis of porous carbons. The uniquenature of this category of chemicals allows synthesizingmesoporous materials with large surface areas and uniformpore sizes that are suitable for numerous potential applications.Due to its surface properties, carbonaceous materials synthe-sized by using surfactant provides marked advantages overtypical activated carbon during the adsorption and diffusionprocess [19].

Considering the aforementioned merits of surfactants in thisstudy we had modified cordierite monoliths by using non-ionicsurfactant (F127). The surfactant modified mesoporous carboncoated monolith (MCCM) was then utilized for MB removal formaqueous system. To check the economic feasibility, desorptionstudies were also carried out. The equilibrium and kineticparameters were studied to justify the results.

2. Materials and methods

2.1. Chemical and reagents

Ceramic monoliths (diameter – 25 mm and length – 100 mm)were supplied from Beihai Huihuang Chemical packing Co. Ltd.,GuangXi, China. The chemical composition of monolithic substrateused during the study were SiO2 50.9 � 1%, Al2O3 35.2 � 1%, MgO13.9 � 0.5%, and others <1%. The cell shape of the monolithicchannels was square with channel width – 1.02 � 0.02 mm equiva-lent to a channel density – 62 cell-cm�2 (400 cpsi) and wall thickness– 0.25 � 0.02 mm. The chemicals used for mesoporous carbonsynthesis were furfuryl alcohol (FA; Aldrich, 98%) as carbon source,pyrrole (Py; Aldrich, 98%) as binder, nitric acid (HNO3; Aldrich, 65%) ascarbonization catalyst, ethanol (C2H5OH; Aldrich, 95%) and triblockcopolymer pluronic F127 (Aldrich, 99%) as template(EO106PO70EO106, MW = 12,600).

Cationic dye, Methylene blue (MB), Classification Number52015, chemical formula – C16H18N3SCl, MW – 319.85, lmax –664 nm (measured value) was purchased from Sigma–Aldrich SdnBhd, Malaysia. Reagents and chemicals used were of analyticalgrade or as specified.

2.2. Preparation of adsorbent

A non-ionic surfactant (F127) was first dissolved in10 mL C2H5OH under continuous stirring conditions at 25 8C.Furfuryl alcohol (FA), a carbon source and Py, a binder wasadded thereafter. The resulting mixture was stirred for 30 min inan ice bath. Then, HNO3 (a catalyst) was added and the mixturewas continuously stirred for an hour resulting in a dark redcoloured solution. The polymerization temperature had to bemaintained at around 10 8C. Prior dip-coating, the monolith wasdried at 100 8C for an hour. The bare monoliths were coated byimmersing in the polymerized solution for 24 h. During dip-coating, the fluid mixture was conditioned by cooling in ice bathto slow down the polymerization reaction rate and therebymaintaining contact viscosity. After withdrawal from thecoating solution, excess coating solution was removed frommonolith channels with the evaporation of C2H5OH in a hood.After evaporation, the CCM was heated in oven for 24 h at 100 8Cto allow complete solidification. The samples were thencarbonized at 700 8C for 4 h under purified N2 flow with aheating rate 3 8C/min to decompose the tri-block copolymersand to obtain MCCM.


2.3. Characterization of adsorbent

The surface area and pore characteristics which include thepore volume and pore size distribution were analyzed usingSorptomatic V1.03 through N2 adsorption/desorption isotherm at77 K. The surface morphology of MCCM was analyzed by usingscanning electron microscope (SEM LEO 1455) operated at 20.0 kV.The surface functional groups of MCCM before and after MBadsorption were determined by Fourier transform-infrared (FT-IR;Perkin Elmer, model spectrum 100) analysis in range of 4000–600 cm�1.

The functional groups present on the carbon surface wereinvestigated by attenuated total reflection-Fourier transform-infrared (Perkin Elmer, Norwalk) analysis, ranging from 4000 to400 cm�1. For FTIR measurement, a small amount of adsorbent wasadded to KBr powder. Infrared spectra were obtained by scanningthe prepared sample with a spectral resolution of 0.2 cm�1. Dry airwas continuously purged into the spectrometer to get rid of watervapor. A plot of infrared radiation intensity versus the wavenumber known as the infrared spectrum was recorded for samples.

The point zero charge (pHpzc) of MCCM was determined by thesolid addition method [20] using 0.01 N KNO3 solution. Theexperiments were carried out in 250 mL conical flasks containing100 mL of 0.01 N KNO3 solution. The initial pH (pHi) in each flaskwas roughly adjusted between 2 and 10 by adding either 0.1 MKOH or 0.1 M HNO3. The total volume of the solutions was thenadjusted exactly to 50 mL by adding KNO3 solution of samestrength. The pHi of the solutions was then accurately measured.The adsorbent (0.5 g) was added to each flask and the flasks weresecurely capped immediately. The suspensions were then manu-ally shaken and allowed to equilibrate for 48 h with intermittentmanual shaking. The difference between the initial and final pH(pHf) values (DpH = pHi � pHf) was plotted against the pHi. Thepoint of intersection of the resulting curve with abscissa, at whichDpH = 0, gave the pHpzc.

The acidic and basic sites present on MCCM surface weredetermined by acid-base titration experiments [21] .The totalacidic sites were neutralized using alkaline solutions (0.1 N NaOH,0.1 N NaHCO3, 0.1 N Na2CO3, and 0.1 N NaOC2H5) while basic siteswere neutralized with 0.1 N HCl solution. The acidic and basic siteswere determined by adding 50 mL of 0.1 N titrating solution and agram of MCCM to each 250 mL volumetric flasks. The flasks wereslowly agitated in a temperature controlled water bath at 25 8C andwere left for 5 days. Afterward, 10 mL of each sample was titratedwith 0.1 N HCl or 0.1 N NaOH solution.

2.4. Adsorption studies

The MB adsorption experiments onto MCCM were conducted ina set of 250 mL Erlenmeyer flasks. The adsorbate solutions(200 mL) of various initial concentrations (50–400 mg/L) wereprepared. A MCCM (0.5 g) was added to each flask. The flasks werekept in a shaker with an agitation speed of 180 rpm at 25 8C. The pHof the experimental solutions was controlled by the adding HCl(0.1 N) and NaOH (0.1 N) solutions, respectively. The MB concen-tration in the solution before and after adsorption was determinedusing a double beam UV–Vis spectrophotometer (UltrospecTM

3100p) at its maximum wavelength (lmax) – 664 nm. The MBuptake at equilibrium was calculated by Eq. (1).

qe ¼ðCo � CeÞV

m(1)

where qe is the amount MB adsorbed on MCCM at equilibrium (mg/g), Co and Ce are the initial and equilibrium MB concentrations (mg/L), respectively, V is the volume of solution (L) and m is MCCMweight (g).



Fig. 1. FT-IR spectra of MCCM.

M.R. Malekbala et al. / Journal of Industrial and Engineering Chemistry xxx (2014) xxx–xxx 3

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2.5. Desorption and regeneration studies

Batch process was employed for MB desorption and regenera-tion. A MCCM was loaded with 250 mL MB solution of 200 mg/Linitial concentration at pH: 10 and contact time – 48 h. The MBloaded MCCM was washed several times with distilled water toremove unadsorbed MB traces on the surface. Desorptionexperiment was performed at 25 8C for 24 h by using 250 mL of0.05 N each of HCl, H2SO4, H3PO4, ethanol (ETOH), NaCl and NaOHas an eluent. Desorption ratio was calculated from the amount ofMB initially loaded on MCCM and final MB concentration in theeluent. Regeneration studies were conducted by using the sameadsorbent.

3. Results and discussion

3.1. Characterization of MCCM

Fig. 1 showed FT-IR spectra of MCCM before and after MBadsorption, and after MB desorption. The C–H stretching bandswere appeared between 2900 and 2700 cm�1. The vibration peaksat 2991 and 2784 cm�1 assigned to asymmetric and symmetricstretching of CH2 groups belong to saturated C–H in alkane andaldehyde groups [22,23], respectively were observed. Aromaticbands show absorptions in the regions 1600–1550 cm�1 and1500–1400 cm�1 due to C–C stretching vibrations in the aromaticring. The bands at 1567 and 1416 cm�1 belongs to ring stretchingvibrations [24]. An intense peak was observed at 1182 cm�1

assigned to C–O stretching vibrations of phenolic group [22,23].Peaks in region 956–770 cm�1 ascribed to the vibration of C–H outof plane mode which further confirms the existence of aromaticstructures of carbon basal plane [25]. Metal oxide stretching bandswas also revealed at low frequency of the spectrum [26]. The FT-IRspectrum of MCCM after MB adsorption showed nearly the samecharacteristics as presented in FT-IR spectrum of MCCM beforeadsorption. As shown, the adsorption is probably a physicalprocess or, if it is chemical, the adsorbent–adsorbate bond energy

Fig. 2. Optical images, (a) bare cordierite, (b) cordierite after coating with mesoporous ca

materials of MCCM adsorbent.


is very low [26,27]. In addition, after the loading of MB on MCCM anew peak appeared at 683 cm�1 and it disappeared afterdesorption. This peak is due to C–N or C–S groups of MBconfirming the attachment of dye on the MCCM. Differentmechanisms are proposed for MB adsorption; (1) the cationiccentre N+ of MB can make favourable interactions with the p-electron cloud of aromatic side chains (2) p–p interactionsbetween p aromatic ring donors of MB and p acceptor groups inthe adsorbent [28].

An optical image of MCCM for bare monolith and carbon coatedmonolith are illustrated in (Fig. 2a,b), respectively. SEM images formesoporous carbon coated on honeycomb cordierite aftercarbonization show relatively uniform surface and the coverageof carbonic materials inside channels (Fig. 2c, d). High magnifica-tion image (Fig. 2e) revealed the network of carbonaceousmaterials which linked together making an interconnectedworm-like framework of porous structures. Surface studies ofMCCM showed surface area (BET) and total pore volume 849 m2/gand 0.3 cm3/g, respectively. N2 adsorption/desorption isothermplot (Fig. 3) revealed applicability of type IV isotherm for the

rbon; SEM images of (c, d) inside channels, (e) high resolution of network carbonic



Fig. 3. Nitrogen adsorption/desorption isotherm plot of MCCM.

Table 1Surface active sites on MCCM.

Active sites Values (milliequiv./g)

Total acidic sites 0.6612

Carboxylic 0.1651

Lactonic 0.1277

Phenolic 0.3684

Total basic sites 0.0205


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synthesized sample according to IUPAC classification. This typereports on the bimodal materials with micro/mesopores. Thehorizontal branch near the saturation pressure (p/po = 0.9)indicates all the mesoporous are filled with liquid adsorbate.Fig. 4 exhibited pore-size distribution of MCCM determined usingthe BJH-methods. The pore-size distributions of sample revealed abroad distribution with a mean size of 21 nm. The Boehm acid basetitration experiment showed dominance of total acidic sites overthe MCCM surface (Table 1).

3.2. Effect of pH

The pH is an important parameter affecting both aqueouschemistry and surface charge of the adsorbent. The effect of pH onMB adsorption onto MCCM was studied within pH range 2–11under the specified experimental conditions (Fig. 5). The MBadsorption on MCCM from aqueous phase was highly dependenton solution pH. The adsorption capacity was minimum at pH: 2(92 mg/g) and attaining optimum value (240 mg/g) at pH: 10. Theincrease in pH leads to increase the number of OH� ions in aqueousphase enhancing MB adsorption capacity. Under highly acidic pH

Fig. 4. Pore size distribution of MCCM.


conditions, H+ may compete with MB ions (cationic dye) foroccupying adsorption sites on absorbent surface, thereby decreas-ing MB adsorption. Also, a change in solution pH affects the natureof the surface charge on the adsorbent. A negative chargedeveloped on the surface oxides of adsorbent in basic mediumresulting in comparatively higher cationic dye adsorption than inbasic solution [29,30]. Optimum MB adsorption under highly basicconditions on various adsorbents was reported by other research-ers [31,32]. The observed MCCM pHzpc was 5.5 (Fig. 6). AtpH < pHPZC, MCCM surface is positively charged while, atpH > pHPZC, the surface is negatively charged. This observationalso confirms optimum MB adsorption at higher pH values.

3.3. Effect of ionic strength

Wastewater streams containing dyes generally containssignificant quantities of salts, thus, the effect of electrolyte onMB removal needs to be investigated. The effect of ionic strengthon MB adsorption was studied at 200 mg/L initial MB concentra-tion at 25 8C and pH: 10. As revealed in Fig. 7, MB adsorption onMCCM increased on addition of small molar quantities of NaCl. Thepresence of electrolyte such as NaCl, MgCl2, CaCl2 may havedifferent effects in aqueous solution. These salts dissociates inwater to provide positive and negative ions (Na+, Mg2+,Ca2+ andCl�). The neutralization of MCCM surface charge may occur due tocompeting of the resulted ions with MB for surface adsorption. Atlow salt concentrations, electrostatic repulsions are predominantso that high salt retentions are obtained. The retention factor ofNaCl is low when NaCl concentration is high with reduction inelectrostatic interactions. This can be explained by the screeningeffect due to addition of NaCl. Theoretically, the electrostatic forcesbetween the adsorbent surface and adsorbate ions are attractive

Fig. 5. Effect of pH on MB adsorption onto MCCM.



Fig. 6. Point of zero charge (pHpzc) of MCCM.

Fig. 8. Effect of contact time on MB adsorption at various initial concentrations onto

MCCM.


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and an increase in ionic strength will decrease the adsorptioncapacity. Conversely, when the electrostatic forces are repulsive,an increase in ionic strength will increase adsorption [33,34]. Fig. 7shows the adsorption of positively charged dye molecules onnegatively charged mesoporous carbon increased with increase inNaCl concentration. The higher NaCl concentration created a lowretention factor that decreases MB dissociation to their ionic formsfor adsorption, therefore, the adsorption capacity would beconstant at higher NaCl concentrations.

3.4. Effect of contact time at various initial MB concentrations

Contact time studies for MB adsorption on MCCM at variousinitial concentrations were carried at pH: 10 and temperature –298 K. The MB adsorption on MCCM was initially fast as largenumbers of vacant surface sites were available for MB adsorption[35,36] and then became slower attaining equilibrium as near theequilibrium the remaining vacant sites were difficult to beoccupied probably due to slow pore diffusion of the solutemolecules on the solid phase from bulk phase (Fig. 8). The amountof MB adsorbed at equilibrium (qe) increased from 95 to 381 mg/gas the initial concentration increased from 50 to 400 mg/L. Thisindicates that the initial MB concentration plays an important rolein the MB adsorption onto MCCM. Hence, higher initial MBconcentration will enhance MB uptake. The equilibration timeranged between 660 and 4180 min with increase in MBconcentration from 50 to 400 mg/L. Similar equilibration time

Fig. 7. Effect of ionic strength on MB adsorption (Co of MB – 200 mg/L).


(4000 min) for MB adsorption on mesoporous carbon was reportedelsewhere [37].

3.5. Adsorption kinetics

Kinetic models, pseudo first-order [38] and pseudo-second-order [39] were used to determine the adsorption behaviour suchas adsorption type, rate and adsorption capacity of the system. Thebest model was selected by the determination of R2 and comparingadsorption capacity values at equilibrium (qe,exp) and calculated(qe,cal).

The pseudo first-order model expressed as:

logðqe � qtÞ ¼ log qe �k1t

2:303(2)

where, k1 is a pseudo-first-order rate constant determined byplotting log(qe � qt) versus t (figures not given).

The pseudo second-order kinetic is given as:

t

qt

¼ 1

k2q2e

þ t

qe

(3)

where, k2 is a pseudo-second-order rate constant determined fromthe intercept and the slope of plot between t/qt versus t (figures notgiven).

The parameters obtained from the models are presented inTable 2. The pseudo-first-order model did not showed goodregression coefficient (R2) results for the entire concentrationrange while, pseudo-second order model showed higher R2 valuesfor the entire concentration range. Moreover, the qe,exp and qe,cal

values for MB on MCCM were nearer (Table 2) confirming theapplicability of pseudo-second-order model for the entire concen-tration range. The applicability of pseudo-second-order modelpredicts chemisorption process.

The pseudo-first-order and pseudo-second-order kinetic mod-els could not identify the diffusion mechanism. Thus, the kineticresults were then analyzed by using Weber and Moris intra-particle diffusion model [40]. The plot of qt versus t1/2 usuallyshows more than one linear portion (Fig. 9). As observed, the intra-particle diffusion plots were not linear over the whole time range,implying that more than one process affecting the adsorption. Thisstudy was in good agreement with previously reported study onMB adsorption [41].



Table 2Kinetics parameters for MB adsorption onto MCCM.

Pseudo-first-order Pseudo-second-order

Co (mg/L) qe,exp (mg/g) qe,cal (mg/g) k1� 10�4 (1/min) R2 qe,cal (mg/g) k2� 10�4(g/mg-min) R2

50 95.7 81.5 108.2 0.99 97.1 2.7 0.99

100 193.6 174.5 85.2 0.98 196.1 0.9 0.99

200 279.7 228.6 32.2 0.96 285.7 0.3 0.99

300 345.4 283.5 20.7 0.93 357.1 0.2 0.99

400 381.5 308.2 18.4 0.90 400.0 0.1 0.99

Table 3Isotherm parameters for MB adsorption onto MCCM.

Langmuir constants Freundlich constants

b (L/g) qm (mg/g) R2 KF (mg/g)(L/mg)1/n 1/n R2

24.4 388.4 0.99 64.4 0.36 0.91


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3.6. Adsorption isotherm

Adsorption isotherms are used to illustrate the distribution ofadsorbate between solid and solution phase at equilibrium. Anoptimized data of adsorption process design can be achieved withthe help of isotherm studies. Freundlich [42] and Langmuir [43]isotherm models were applied to evaluate the adsorption data.

Freundlich model is an empirical equation that describesadsorption on heterogeneous surface through a multilayeradsorption mechanism [44]. Freundlich model in linearized formis given as:

log qe ¼ log KF þ1

nlog Ce (4)

Langmuir model assumes a monolayer adsorption onto theadsorbent surface. Furthermore, it has a free-energy change for alladsorption sites considering no adsorbate–adsorbate interaction.In linearized form Langmuir model is given as:

Ce

qe

¼ 1

bqm

þ 1

qm

� Ce (5)

where, Ce (mg/L) and qe (mg/g) are the equilibrium concentrationand the adsorption capacity at equilibrium state, respectively. Theparameters KF [(mg/g) (L/mg)1/n] and n are Freundlich constantsobtained by plotting log qe versus log Ce (Figure not given). TheLangmuir isotherm constants b (L/mg) and qm (mg/g) are obtainedfrom the intercept and slope of the plot between Ce/qe versus Ce

(Figure not shown).Table 3 summarized the results obtained by apply Freundlich

and Langmuir isotherm models to the isotherm data. Compara-tively higher regression coefficient (R2) value for Langmuir model

Fig. 9. Intra-particle diffusion plot for MB adsorption on MCCM at various initial

concentrations.


showed better fitting of the model to experimental data. Thisconfirms monolayer coverage of MB on to MCCM surface and alsohomogeneous distribution of active sites on the adsorbent surface.The applicability of Lagmuir model was further confirmed by non-linear isotherm plot (Fig. 10). The observed maximum monolayersaturation capacity (qmax) was 388 mg/g which was comparativelyhigher than that reported qmax values for MB adsorption ofmodified clay [45].

The essential feature of Langmuir isotherm can be expressed byseparation factor (RL), a dimensionless constant, can be repre-sented as:

RL ¼1

1 þ bC0(6)

where, C0 is the initial MB concentration (mg/L). In general, anisotherm can be irreversible (RL = 0), favourable (0 < RL < 1), linear(RL = 1), or unfavourable (RL > 1) [46]. For MB adsorption onMCCM, RL falls in range of favourable adsorption process (i.e.0 < RL < 1).

3.7. Adsorption thermodynamics

The temperature has a pronounced effect on the adsorptioncapacity of the adsorbents [47]. Increasing the temperature is

Fig. 10. Non-linear isotherm plots.



Fig. 11. Effect of temperature on MB adsorption onto MCCM at various initial

concentrations.

Table 4Thermodynamic parameters for MB adsorption onto MCCM.

DS8 (J/mol K) DH8 (kJ/mol) DG8 (kJ/mol)

293 K 303 K 313 K

50.8 21.9 �22.5 �24.6 �25.9


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known to increase the rate of diffusion of the adsorbate moleculesacross the external boundary layer and in the internal pores of theadsorbent particle, decrease the viscosity of the solution andchange the equilibrium capacity of the adsorbent for a particularadsorbate [48]. Fig. 11 depicted MB adsorption at variousconcentrations (50–400 mg/L) on MCCM at different temperatures(298, 308 and 318 K). An increase in MB adsorption on MCCM withincrease in temperature was observed. The adsorption capacity at400 mg/L initial MB concentration increased from 388 to 440 mg/gwith increase in temperature from 298 to 318 K, indicatingendothermic nature of process. Similarly at various MB concen-trations the increment in adsorption capacity was observed(Fig. 11). The enhancement in the adsorption capacity might bedue to possibility of an increase of number of active sites for theadsorbent as well as an increase in the mobility of the adsorbatemolecules [49].

Various thermodynamics parameters such as standard enthal-py change (DH8), standard entropy change (DS8) and Gibb’s freeenergy change (DG8) were evaluated. Van’t Hoff equation was usedto determine DH8 and DS8 values. The equation is given as:

ln b ¼ DS�

R�DH�

RT(7)

where, R (8.314 J/mol K) is the universal gas constant, T (K) is theabsolute temperature and b is the Langmuir constant. A plot of ln b

against 1/T was used to obtain DS8 and DH8 (Figure not shown). TheDG8 can be calculated using the relation below:

G� ¼ �RT ln b (8)

Table 5Comparison of the maximum adsorption capacity of MB onto various adsorbents.

Adsorbent Experimental conditions

Co (ppm) pH

MCCM (F127) 50–400 10

Anaerobic granular sludge 20–300

Multiporous palygorskite 100 6.5

MCCM (PEG) 50–480 10

Acid Activated Algerian Bentonite 10–100 4.7

Carbon nanotubes 40 7

Poly NAN ospheres 30–70 7

Clay honeycomb monoliths 10–100 –


The thermodynamic parameters are listed in Table 4. Thenegative values of DG8 indicate that the adsorption of MB ontoMCCM is spontaneous and the positive value of DH8 confirms thatthe adsorption process is an endothermic. The positive value of DS8reflecting the affinity of the adsorbent material towards dyes [50].Similar observations were reported for MB adsorption on differentadsorbents [51,52]. The adsorption capacities of various adsor-bents used for MB removal were compared and given in Table 5.From Table 5, it can be seen that the capacities of adsorbents foradsorbing MB range from 3 to 388 mg/g. The adsorption capacity ofMCCM (PEG) is less compared to MCCM (F127) synthesized in thiswork. An increase of 3 fold in MB adsorption capacity by usingMCCM (F127) indicated the positive effect of surfactant on thetextural properties of MCCM for other applications.

3.8. Desorption of MB

3.8.1. Effect of various eluents on MB desorption

Fig. 12 revealed the amount of MB desorbed by variouschemical agents as eluents. The MB elution by using 0.05 N HCl wasoptimum (39%) followed by H2SO4 (0.05 N), H3PO4 (0.05 N), ETOH(0.05 N), NaOH (0.05 N) and NaCl (0.05 N). The elution efficiencywas further investigated at various HCl concentrations (Fig. 13).The maximum MB elution (82.1%) was obtained with 0.1 N HClsolution. Further increase in HCl concentration leads to decrease inelution efficiency. This might be due to deterioration of adsorptionsite on MCCM surface at higher HCl concentrations.

3.8.2. Regeneration studies

Regeneration of MCCM is an important step in order to checkthe economic feasibility of adsorption process. The regenerationstudies were carried out using 0.1 N HCl solution as it givesoptimum MB elution (82.1%). The regeneration studies werecarried out in batch mode for five successive cycles (Fig. 14).Results showed 21% drop in the adsorption capacity for secondcycle. This decrease in adsorption capacity might be caused due tothe decomposition or damage caused by acidic solution to certainadsorption sites or functional groups present over MCCM surface[54]. The decrease in the adsorption capacity was about 15% for thethird cycle. The adsorption capacity remains almost stagnant(250 mg/g) for the consecutive cycles (up to fifth cycle) showingthat the adsorbent could be reused without any further lose inadsorption capacity.

Adsorption capacity (mg/g) References

T (8C)

25 388 This work

25 212.77 [3]

25 134.77 [2]

30 121 [17]

20 56.34 [53]

25 46.2 [12]

25 20 [9]

– 3 [16]



Fig. 14. Regeneration studies of MCCM.

Fig. 12. Desorption of MB from MCCM by various eluents.

Fig. 13. Desorption of MB from MCCM by using HCl as an eluent.


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4. Conclusions

The present investigation showed that MCCM mesoporousitycould enhance MB adsorption capacity. Initial MB concentration,solution pH and temperature showed profound influence onadsorption process. Optimum MB adsorption was observed at pH:10. The adsorption of MB on MCCM was a monolayer adsorption asconfirmed by the applicability of Langmuir model. The kineticsstudies suggested applicability of pseudo-second-order model.Adsorption of MB on the MCCM is favourably influenced by anincrease in the temperature of the operation. The free energy (DG8),enthalpy (DH8), and entropy (DS8) terms were determined, and thenegative values of DG8 indicated that MB adsorption process is aspontaneous and the positive DH8 value confirmed that theadsorption process was endothermic in nature. The elution of MBform MCCM was optimum with 0.1 N HCl confirming occurrenceion-exchange mechanism. The regeneration studies by 0.1 N HClshowed 21 and 15% loss in MB adsorption capacity for first andsecond cycle thereafter, the adsorption capacity remain almoststagnant.

Acknowledgements

The authors would like to gratefully acknowledge Ministry ofEducation (MOE), Malaysian Government and Universiti PutraMalaysia (UPM) for the financial support of this work (via vot:9416900).

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