Adsorption studies of methylene blue onto activated saw dust: kinetics, equilibrium, and...

Adsorption Studies of Methylene Blue onto

Activated Saw Dust: Kinetics, Equilibrium, and

Thermodynamic StudiesSushmita Banerjee,a Mahesh C. Chattopadhyaya,a Varsha Srivastava,b and Yogesh Chandra Sharmab

aDepartment of Chemistry, University of Allahabad, Allahabad 211 002, Uttar Pradesh, India; [email protected] (forcorrespondence)bDepartment of Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 221005, Uttar Pradesh,India

Published online 00 Month 2013 in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ep.11840

This study is devoted to the application of activated sawdust (ACSD) as adsorbent for the removal of “methylene blue(MB)” from the aqueous solutions. Raw saw dust was acti-vated by a simple and low cost chemical method. After acti-vation, adsorbent was characterized by scanning electronmicroscopy for its surface characteristics. Brunauer-Emmett-Teller surface area and average particle size were deter-mined and found to be 74.23 m2/g and 700 mm, respec-tively. Removal efficiency of adsorbent for MB wasdemonstrated by batch adsorption experiments. pH studyindicates that 9.5 pH is optimum for higher removal of MB.Temperature studies revealed exothermic nature of adsorp-tion process. On studying kinetic models, it was observed thatremoval process is governed by pseudo-first-order kinetics.Intraparticle diffusion study revealed that it is not rate limit-ing step in removal process. Mass transfer coefficient wasalso determined. Value of DG� was found to be negative atall studied temperature which confirms the feasibility of pro-cess. Langmuir’s adsorption isotherm was found to be suita-ble for this system. This study revealed that ACSD can beused as alternates of costly adsorbents for the removal of MBfrom effluents. VC 2013 American Institute of Chemical Engineers

Environ Prog, 00: 000–000, 2013

Keywords: adsorption, dye stuff industries, kinetic study,methylene blue, thermodynamics

INTRODUCTION

Dyes and pigments are widely used in various industriessuch as cosmetics, food coloring, papermaking textile, dying,and printing industry [1]. Effluents of these industries are themajor sources of water pollution. The textile industry pro-duces highly polluted wastewater containing different typesof dyes. Generally, the dyes that are used in the textileindustry are basic dyes, acid dyes, reactive dyes, direct dyes,azodyes, mordant dyes, vat dyes, disperse dyes, and sulfurdyes [2]. Among various dye, methylene blue (MB) is themost commonly used dye. MB is frequently used for dyingcotton, wood, and silk. MB is a water-soluble cationic dyewhich is blue in color (kmax 5 668 nm). Its molecular for-

mula is C16H18N3ClS [(3,7-bis(Dimethyl amino)-phenazathio-nium chloride tetra methyl thionine chloride]. Figure 1depicts the chemical structure of MB as given in Ref. [3].

The extensive use of dyes in dye-manufacturing industriescreates significant problems due to the discharge of coloredwastewater. From an environmental point of view, theremoval of synthetic dyes is of great concern. Most of thedyes cause serious environmental and health problems dueto their chemical stability [4]. Some of dyes can pose muta-genic, teratogenic, and carcinogenic effects [5]. Dyes can alsocause allergic dermatitis and skin irritation [5,6]. Water pol-luted with dye can interrupt photosynthetic processesbecause it can reduce the sunlight penetration to the aquaticenvironment [3]. Due to toxicity of dyes, much attentionshould be paid to treat dyes before discharge.

Many methods such as precipitation, membrane filtration,coagulation, electrochemical reverse osmosis and chemical oxi-dations, aerobic and anaerobic microbial degradation, ionexchange, and adsorption have been used for the removal ofdyes from waste water [7,8]. Most of these methods have vari-ous disadvantages like high operational and maintenance cost[9,10]. So it can not be applied for the removal of dyes fromdye stuff industries. Among various available methods, adsorp-tion is considered to be superior to other techniques and it ismost widely used method because of simplicity of design,operation, and comparable low cost [10]. Application of lowcost adsorbent material can make the adsorption process moreinexpensive. In last few decades, activated carbon is frequentlyused for the removal of dyes from waste water but its highcost and loss in removal efficiency on regeneration alwaysinspire researcher to search of new low cost alternative [7,11].Various adsorbents, such as bamboo activated carbon, sawdust, magnetic nanoparticles, silica, rice husk, waste orangepeel, banana pith, carbon nanotube, clay jute fiber carbon,bentonite, garlic peel, coconut coir, and graphite power havebeen used by several researchers for the removal of dye fromwater and wastewater [12–16]. Saw dust is a by-product of thetimber industry where it is produced from the cutting of wood.It is frequently used as cooking fuel or a packing material.

In this investigation, saw dust was used as adsorbent forthe removal of MB from simulated waste water. For removalstudy, usual batch adsorption experiments were carried out.VC 2013 American Institute of Chemical Engineers

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep Month 2013 1

Effect of various important parameters such as initial concen-tration, pH, dose, and temperature were studied for the opti-mization of removal process. Kinetics of removal processwas investigated by applying pseudo-first-order, pseudo-sec-ond–order, and intraparticle diffusion model [17]. Mass trans-fer study was also carried out. Various isotherm models suchas Langmuir, Freundlich, and Tempkin and Pehlyaz wereapplied on resultant data. Thermodynamic study was alsoillustrated for this study.

MATERIALS AND METHOD

Preparation of Adsorbent and CharacterizationFor this investigations, saw dust was collected from a

local saw mill at Allahabad. It was rinsed at least five timesby distilled water to remove adhere impurities. After wash-ing, saw dust was dried in to hot air oven at 100�C. Raw anddried saw dust was modified by simple chemical method forthe enhancement of its adsorptive characteristics. The finepowder of saw dust was treated with 1% perchloric acid inthe ratio of 1:5 (saw dust:perchloric acid, w/v) at 50�C for 4h to remove lignin-based color materials. The saw dust wasfiltered out, washed with distilled water, and was stored inoven for 24 h. After taking the material out of oven, it wasstored in an airtight container for further use. Both the rawand activated saw dust (ACSD) were characterized by scan-ning electron microscopy (SEM). To investigate the presenceof functional groups on surfaces of sawdust and activatedsawdust, Fourier transform infrared spectroscopy (FTIR) (Var-ian 1000 FT-IR, Scimitar Series) of the samples was carriedout. FTIR also helped to investigate the effect of treatmenton the nature and surface characteristics of saw dust. FTIRspectra were recorded in the range of 500–4000 cm21. Thesurface area, particles size, density, and porosity of theadsorbent were also determined. The Brunauer-Emmett-Teller surface area of the ASCD was investigated by using acomputer controlled automated porosimeter (MicromeriticsASAP 2020, V302G single port). Porosity and density ofACSD was measured by the method reported elsewhere [12].

Adsorption StudiesInvestigation of removal efficiency of ACSD for MB was

carried out using batch adsorption experiments. MB wasused as a model cationic dye for the experiments. MB dyewas purchased from local supplier, and was used withoutfurther purification. The stock solution (1000 mg/L) of MBwas prepared by dissolving 1.0 g in 1 L of distilled water.MB solution of various concentration range, viz., 13.37 31022, 20.06 3 1022, 26.74 3 1022, and 33.43 3 1022 mol/Lwere prepared by diluting stock solution with distilled water.Batch adsorption experiments were carried out at 30�C byadding a fixed amount of adsorbent (ACSD) into a 125 mLreagent bottles containing 50 mL dye solution. To investigateadsorption dynamics, the experiments were carried out invarious time intervals to determine the equilibrium time andmaximum adsorption capacity. The pH of the stock solutionswas 3.3. The pH was adjusted using 0.1 M NaOH/0.1 M HClsolutions. The flasks were agitated in a shaker at 30�C with ashaking speed of 100 rpm until the equilibrium was reached.

The ionic strength of the adsorbate solutions was maintainedat 1.0 3 1022 M NaClO4. The adsorbent was separated fromthe aqueous solutions by centrifugation after the equilibriumtime (Remi 24, New Delhi, India). The adsorbent was sepa-rated by centrifugation and the residual concentration of MBin the supernatant solution was analyzed using a UV–Visspectrophotometer at 668 nm. Each experiment was carriedout in duplicate and the average values of duplicate runswere obtained and used for the calculations.

The percentage removal of MB in aqueous solutions wascalculated by following equation:

% Removal of MB5Ci2Ce

Ci

� �3100 (1)

The amount of MB adsorbed per unit mass of the adsorb-ent was determined by following equation:

qe5Ci2Ce

W

� �3V (2)

where qe is amount adsorbed on per unit mass of theadsorbent (mg/g), Ci and Ce are the initial concentration andequilibrium concentration, respectively (mg/L), V (L)is thevolume and W is the mass of adsorbent (g) solution. Desorp-tion studies of the adsorbent were also carried out.

RESULTS AND DISCUSSION

Characterization of Adsorbent (ACSD)To investigate the effect of modification process on sur-

face characteristics of saw dust, SEM of raw and ACSD weretaken (Figures 2a and 2b). SEM micrograph of raw saw dustand ACSD shows some changes which may be during modi-fication process. It is clear from the SEM micrographs ofACSD that modification process enhances the active surfacesite and pores which can be favorable for the MB removal.

FTIR of raw saw dust indicates number of absorptionpeaks showing the complex nature of saw dust, there areseveral functional groups present in saw dust such as car-boxylic acid, amine, amino, amide, and sulfonate groups(Figure 3a). In ACSD, some of the peaks disappear which isdue to treatment with perchloric acid. FTIR of ACSD indicatethe presence of NAH, CAOH, CAOAC, CAH, C@O, andCAX groups (Figure 3b). The absorption band, appearingaround 3300–3500 cm21, indicates the presence of AOHgroups. Peaks in the range of 430–551 cm21 can be attrib-uted due to the bending vibration modes of aromatic com-pounds [18].

Various characteristics of ACSD are given in Table 1. It isclear from Table 1 that the average particles of ACSD are of700 mm. Surface area plays an important role in any removalprocess. Higher surface area can give better removal effi-ciency. Surface area of ACSD was determined to be 74.23m2/g.

Effect of Initial Concentration and Contact TimeEffects of initial concentration of dye solution on the

removal were studied by varying solution concentration from13.37 3 1022 to 33.43 3 1022 mol/L. The influence of MBinitial concentration on the removal percentage dye wasinvestigated and results are shown in Figure 4. It is clearfrom Figure 4 that, MB removal is rapid in the initial stages,that is, for the first 70 min and there after the removal ratebecomes slower and finally attains saturation at 80 min. Thehigher removal at initial stage is due to availability of thelarger number of free adsorption sites in the beginning ofadsorption process. However, as the adsorption process

Figure 1. Chemical structure of MB.

Environmental Progress & Sustainable Energy (Vol.00, No.00) DOI 10.1002/ep2 Month 2013

proceeds, all the available sites are gradually occupied bythe dye species.

It is clear from Figure 4 that removal decreased from 83to 37% by increasing concentration from 13.37 3 1022 to33.43 3 1022 mol/L. Higher removal for the lower concen-tration for a fixed amount of adsorbent dose, can beexplained on the basis that at lower concentrations alladsorbate species can get chance to bind with the adsorbentsites but at higher concentrations, for the same amount ofadsorbent, lower adsorption occurs because of the saturationof the adsorption sites.

Effect of ACSD Dose on the Removal of MBAdsorbent dose is an important parameter in any adsorp-

tion process and it significantly influences the removal ofadsorbate species. The effect of adsorbent dose was studiedby varying the quantity of adsorbent in the range of 10–25g/L by keeping all the other parameters such as initial dyeconcentration, contact time, pH of the solution, rpm, andtemperature constant. It was observed that by increasing theamount of adsorbents, removal of MB significantly increased.When the adsorbent dose increase, the number of active sur-face for adsorption will also increase, as a result increase thepercentage of dye removal from the solution [19]. Thus, anoptimum dose of 25 g/L is selected for all the experiments.

Effect of pH on the Removal of MBIn adsorption studies, solution pH is an important param-

eter. pH of aqueous solutions strongly influences the adsorp-tion capacity. Surface charge, the degree of ionization anddissociation of functional groups are also controlled by thepH of aqueous solutions. Solution pH affects both aqueous

Figure 2. (a) SEM micrographs of raw saw dust. (b) SEM micrographs of ACSD.

Figure 3. (a) FTIR of raw saw dust modified. (b) FTIR of ACSD.

Table 1. Characteristics of ACSD.

Characteristics of adsorbent

Average particle size 700 mmBulk density 0.25 6 0.07 g/cm3

Porosity 74.4 6 7.4%Surface area 74.23 m2/g


chemistry of adsorbate and available active sites of theadsorbents.

To demonstrate the effect of pH on the removal of MBfrom aqueous solutions pH studies were carried out by vary-ing solution pH from 4.5 to 9.5 by keeping other experimen-tal conditions same. The effect of initial pH on the removalof MB in the different pH range of 4.5–9.5 is shown in Figure5. It is clear from that the removal is higher in alkaline range(Figure 4). It was observed that maximum MB removal wasobtained at pH 9.5. Removal of MB increased from 51 to94% by increasing solution pH from 4.5 to 9.5. Rate ofadsorption varies with pH of an aqueous medium becausepH of the medium controls magnitude of the electrostaticcharges that are imparted by ionized dye molecules [20].Removal of MB is lower in the acidic range of pH. Due topresence of higher amount of H1 in lower pH range, surfaceof ACSD become positively charged which hinder theadsorption of cationic species of dye while at higher pHranges adsorbent surface becomes negatively charged whichsupport the adsorption of cationic species [21].

Effect of Temperature on the Removal of MBIn present system, the effect of temperature on the

removal of MB was investigated at three different tempera-tures, viz., 303, 313, and 323 K. Effect of temperature on theremoval of MB is shown in Figures 6a–6d: It can be observedthat when temperature was increased from 303 to 323 K,the removal of MB progressively decreased for all thestudied concentration ranges (Figures 6a–6d). These decreas-ing trends suggest that dyes adsorption is an exothermicprocess.

ADSORPTION KINETICS

Pseudo-First-Order and Pseudo-Second-Order KineticModel

To understand the adsorption process, kinetic study ofany removal process is compulsory. Study of adsorptionkinetics represents the adsorption efficiency. Adsorption ofadsorbate on adsorbent may involve one or more than onesteps such as film diffusion, pore diffusion, surface diffusionand adsorption on the pore surface, or a combination ofmore than one steps. Electrostatic attraction may be involvedin case of rapid adsorption at initial stages. Adsorptionkinetics study was conducted at initial concentrations of MBsolution 13.37 3 1022 mol/L, different dye solution tempera-tures, viz., 303, 313, and 323 K. Optimum dose and pH wereselected for kinetic study. Pseudo-first-order, pseudo–second-order, and intraparticle diffusion model were studied toinvestigate adsorption kinetics [10,19,22,23]. Pseudo-first-order and pseudo-second-order kinetic equation can beexpressed as follows [10,24]:

Pseudo-first-order kinetic model:

log qe2qtð Þ5log qe2k1

2:303t (3)

Pseudo-second-order kinetic model:

t

qt5

1

k2qe2

11

qet (4)

h5k2qe2 (5)

where k1 is the pseudo-first-order rate constant (per min), k2

(g/mg min) is the pseudo-second-order rate constant. qe andqt (mg/g) are the amounts of MB adsorbed per unit weightof the adsorbent at equilibrium and at time t (min), respec-tively. k1 can be calculated from the slope of the plot of“log(qe 2 qt) versus t” (figure not shown). The value of qe

and k2 can be determined by the slope and intercepts of thestraight line of the plots “t/qt versus t,” respectively. h isknown as initial sorption rate (Eq. 3).

The values of different kinetic parameters and the correla-tion coefficients (R2) were determined by linear regression.The values of various kinetic parameters for pseudo-first-order and second-order model are given in Table 2. Theexperimental and calculated values of qe were compared andit was observed that there is significant difference betweenexperimental and calculated value of qe for the pseudo-second-order model. While in case of pseudo-first-orderkinetic model, the calculated values of qe were consistentwith experimental values. Further, the higher value of R2 atall temperatures suggest the applicability of pseudo-first-order model and indicate that the experimental data can bewell described by pseudo-first-order model. It suggest thatthe removal of MB by adsorption on ACSD is governed bypseudo-first-order mechanism, and hence the adsorption ofMB on ACSD appears to be controlled by the physisorptionsprocess [25]. The excellent linearity at all studied tempera-tures confirms the applicability of the pseudo-first-order

Figure 4. Effect of initial concentration on the removal of MBby adsorption on ACSD. [Color figure can be viewed in theonline issue, which is available at wileyonlinelibrary.com.]

Figure 5. Effect of solution pH on the removal of MB byadsorption on ACSD. [Color figure can be viewed in theonline issue, which is available at wileyonlinelibrary.com.]


wileyonlinelibrary.com


model for the removal of MB by adsorption on ACSD. Onthe basis of comparison of both models, it can be stated thatremoval of MB by adsorption on ACSD is governed bypseudo-first-order kinetics.

Intraparticle Diffusion StudyThe possibility of intraparticle diffusion was explored

using the intraparticle diffusion model. In any adsorptionprocess, adsorbate species may be transported from the

Figure 6. Effect of temperature on the removal of MB by adsorption on ACSD. (a) 13.37 3 1022 mol/L; (b) 20.06 3 1022 mol/L; (c) 26.74 3 1022 mol/L; and (d) 33.43 3 1022 mol/L. [Color figure can be viewed in the online issue, which is available atwileyonlinelibrary.com.]

Table 2. Values of pseudo-first-order, pseudo-second-order constants at different temperatures.

Pseudo-first-order rate constants Temperature (K) k1 (1/min) qe (Calculated) qe (experimental) R2

303 0.045 5.03 4.16 0.98313 0.059 6.26 3.75 0.99323 0.033 3.60 2.92 0.97

Pseudo-second-order rate constants Temperature (K) k2 (g/ mg min) 3 1023 qe (Calculated) qe (experimental) R2

303 5.12 5.87 4.16 0.99313 3.31 6.28 3.75 0.97323 1.83 6.05 2.92 0.91

Intraparticle diffusion constant Temperature (K) kdiff (mg/g min1/2) R2

303 0.47 0.99313 0.49 0.96323 0.39 0.98



solution on to the solid phase through intraparticle diffusionwhich can be the rate limiting step. The intraparticle diffu-sion model is a commonly used model for identifying thesteps involved during adsorption, which can be described as[23]:

qt5kdiff

ffiffiffiffiffiffiffiffiffiffit1Cp

(6)

where kdiff is the intraparticle diffusion rate constant (mg/gmin1/2), C is the constant depicting the boundary layereffects which can be evaluated from the intercept and slopeof the plot of “qt versus t

1=2” (figure not shown). The valuesof kdiff at different temperatures are given in Table 2. Accord-ing to this model, the plots of “qt versus t1/2” should be lin-ear if intraparticle diffusion is involved in the adsorptionprocess and if these lines pass through the origin then intra-particle diffusion is the rate-controlling step. The linear plotsdo not pass through the origin which indicates that intrapar-ticle diffusion is not the only rate controlling step [26,27].The boundary layer diffusion is also involved in rate control-ling step for the adsorption of MB.

Mass Transfer StudyFor the adsorption of MB at ACSD, mass transfer study

was also carried out using following mass transfer model[28,29]:

lnCt

C02

1

ð11mkÞ

� �5ln

mk

11mk

� �2

11mk

mk

� �bLSst (7)

where “k” is a constant, bL is the coefficient of mass transfer,“m” is the mass of the adsorbent per unit volume, and Ss isthe specific surface area. Values of bL, the coefficient of masstransfer, were calculated at different values of temperature

by the slopes and intercepts of the plots of “ ln Ct

C02 1ð11mkÞ

� �versus t” (Figure 7) and the values coefficient of mass trans-fer are given in Table 3 [28].

The values of m and Ss have been determined as follows:

m5W

V(8)

Ss56m

dpdpð12EpÞ(9)

where Ep is porosity of the adsorbent, dp is the diameter ofadsorbent particle, and dp is the density of adsorbent.

The values of coefficient of mass transfer at different tem-peratures have been given in Table 3. It is clear from Table 3that value of mass transfer coefficient is higher at 303 K. Onincreasing temperature from 303 to 323 K, value of bL

decreased from 1.59 3 1022 to 0.07 3 1022 (cm/s) whichshows the decreased rate of removal at higher temperature.This study elucidate that rate of transfer of MB on ACSD ishigher at 303 K but as we increase the temperature of MBsolution, rate of transfer of MB molecule started to decreaseresulting in decreased removal efficiency at higher tempera-tures. Thus, lower temperatures favor removal of MB in pres-ent system.

ADSORPTION ISOTHERMS

For designing any adsorption system, it is necessary toevaluate the data using different adsorption isotherm todetermine the adsorption capacity. Adsorption isothermsgive information about the nature of the solute–surface inter-action and the relationship between the mass of adsorbateadsorbed per unit weight of adsorbent and the liquid-phaseequilibrium concentration of the adsorbate. Adsorption

isotherm study was conducted at different initial concentra-tions of MB solution, viz., 13.37 3 1022, 20.06 3 1022, 26.743 1022, and 33.43 3 1022 mol/L and different temperatures303, 313, and 323 K. The experiments of isotherm studieswere carried out at optimum dose and pH as ascertainedexperimentally. The isotherm results were analyzed usingLangmuir, Freundlich, and Tempkin isotherms.

Langmuir IsothermLangmuir isotherm is based on the assumptions that

adsorption takes place at specific homogeneous sites withinthe adsorbent [28–30] and adsorbed molecules do not showany interaction. No further adsorption is possible onceadsorbate molecules occupy the adsorption sites. Accordingto this isotherm, the adsorption energy is distributed homo-geneously over the entire coverage surface.

The linearized expression of Langmuir model can beexpressed as follows [27]:

Ce

qe5

1

Q0b1

Ce

Q0(10)

where qe (mg/g) is the amount adsorbed at equilibrium, Ce

(mg/L) is the equilibrium concentration of the solute. Q0

(mg/g) and b (L/mg) are known as Langmuir’s constantswhich are related to the capacity and energy of adsorption,respectively. The values of different Langmuir’s parameterswere calculated from the plot “Ce/qe versus Ce” (figure notgiven) and values of Langmuir’s constant is given in Table 4.

Freundlich IsothermThe Freundlich isotherm is commonly used to describe the

adsorption characteristics of multilayer and heterogeneous

Figure 7. Mass transfer plots for the removal of MB byadsorption on ACSD at different temperatures. [Color figurecan be viewed in the online issue, which is available atwileyonlinelibrary.com.]

Table 3. Values of mass transfer coefficient for the removalof MB by adsorption on ACSD.

Temperature (K)Coefficient of mass

transfer (bL 3 1022 cm/s)

303 1.59313 1.31323 0.07



surfaces. The logarithmic form of the equation is expressed asfollows [30]:

log qe5log Kf11

nlog Ce (11)

where Kf is the Freundlich constant denoting adsorptioncapacity (mg/g) and 1/n is the adsorption intensity (L/mg).Ce is the residual concentration of solute remaining in thesolution (mg/L), qe is the amount of adsorbate adsorbed bya unit mass of adsorbent at equilibrium (mg/g). Value of Kf

and 1/n were calculated by the slopes and intercepts of theplots of “log Ce versus log qe” (figure not given). Values ofFreundlich constants are given in Table 4.

Tempkin IsothermAccording to Tempkin isotherm, the energy of the adsorp-

tion will decrease linearly with coverage.Tempkin isotherm can be expressed in its linear form as

follows [31]:

qe5RT

bln ACe (12)

qe5B1ln A1B1ln Ce (13)

where B1 is the Tempkin constant related to the heat ofadsorption, A is the equilibrium binding constant correspond-ing to the maximum binding energy (L/mg), and R is the gasconstant (8.314 J/mol/K). The constants A and B1 can bedetermined from the intercept and the slope of the linear plotof experimental data of qe versus lnCe (figure not given). Thevalues of the constants A and B1 are listed in Table 4.

The calculated parameters of different isotherm modelsand their linear correlation coefficients are summarized inTable 4. The Langmuir adsorption capacity for MB is esti-mated to be 4.58; 3.79; 3.67 mg /g at different temperatures,respectively. Adsorption capacity of adsorbent material wasfound to be decreased by increasing temperature (Table 4).Decreasing trend of adsorption capacity of adsorbent onincreasing temperature indicates that the removal is favorableat lower temperature. Decrease in Langmuir adsorptioncapacity by increasing temperature shows that when the tem-perature increased, the amount of dye on adsorbentdecreased at equilibrium, which indicates that the desorptionprocess took place at higher temperatures.

The value of linear correlation coefficients (R2) for theremoval of methylene was found to be 0.997 at 303 and 313K while at 323 K it was 0.972. The value of R2 at all tempera-tures indicated that the experimental data well correlatedwith the Langmuir equation. Applicability of Langmuir modelconfirms that the adsorbent surface is covered by monolayer

coverage of dye molecules at the homogenous distributionof adsorption sites.

Freundlich capacity was determined to be 3.71, 3.60, and1.69 mg/g at 303, 313, and 323 K, respectively.

The value of Freundlich adsorption capacity Kf was alsohigher at lower temperature and decreases with increase intemperature. Decreasing values of Freundlich capacity fur-ther indicated that adsorption of MB on ACSD is exothermicprocess and higher adsorption capacity can be achieved atlower temperature. As shown in Table 4, the values of 1/nare between 0 and 1, confirms that the adsorption processesare favorable. A high value of n shows the effective adsorp-tion over the entire studied concentration range. The valueof R2 for MB removal was found to be 0.985, 0.998, and0.988 at 303, 313, and 323 K, respectively. The results ofFreundlich isotherm model showed that adsorption is favor-able at all studied temperatures. The Freundlich isotherm fitsquite well with the experimental data. The value of B1 forTempkin isotherm model was found to be 0.24mg/g at 303K. Decreasing trend of adsorption capacity for all three iso-therm model supports for exothermic nature of MB adsorp-tion on ACSD.

Adsorption capacities of some of the adsorbents whichhave been used for the removal of MB from aqueous solu-tions are tabulated in Table 5. Cylindrical graphene–carbonnanotube hybrid showed 81.97 mg/g adsorption capacity[32]. Adsorption capacity of silica nanosheets was reported tobe 12.66 mg/g [36]. Adsorption capacity of various types ofactivated carbons such as Hazelnut shell activated carbon,Coir pith carbon, and Almond shell activated carbon wasreported to be 8.82, 5.87, and 1.33 mg/g, respectively[39,42,43]. In this study, adsorption capacity of ACSD wasfound to be 4.58 mg/g which is quite significant.

THERMODYNAMIC STUDY

Thermodynamic study of any removal processes is neces-sary to demonstrate the feasibility of the process. Differentthermodynamic parameters, viz., standard free energy (DG0),enthalpy (DH0), and entropy (DS0) were determined usingthe following equations [28,30]:

Kc 5CAc

Ce(14)

DG05 2RT ln Kc (15)

DH 05RT2T1

T22T1

� �ln

Kc2

Kc1

� �(16)

DS05ðDH 02DG0Þ

T(17)

where CAc and Ce are the equilibrium concentrations ofadsorbate species on adsorbent (mg/L) and in the solution

Table 4. Values of Langmuir, Freundlich, and Tempkin isotherm parameters at different temperatures.

Temperature (K) Q0 (mg/g) b (L/mg) R2

Langmuir’s parameters 303 4.58 1.55 0.997313 3.79 0.45 0.998323 3.67 0.36 0.972

Freundlich’s parameters Kf (mg/g) 1/n R2

303 3.71 0.051 0.985313 3.60 0.016 0.998323 1.69 0.175 0.988

Tempkin parameters B1 (mg/g) A (L/mg) R2

303 0.24 8.9676E 1 14 0.999313 0.09 1.02123E 1 35 0.991323 0.53 237.9975917 0.992


(mg/L), respectively. The negative value of DG0 suggests thefeasibility and the spontaneous nature of the adsorption. Ingeneral, the values of DG0 in between 0 and 220 kJ/molindicate that the adsorption process is physisorption, whilethe values in between 280 and 2400 kJ/mol correspond tochemisorption [44,45]. The values of DG0 suggest the adsorp-tion is a physisorption process. Negative value of DH0 con-firms the exothermic nature of removal process (Table 6).

DESORPTION EXPERIMENTS

Reuse of exhausted adsorbent for some other cycle canreduce the cost of treatment process which makes the treat-ment process cost effective. For this, first, the ACSD was con-tacted at a randomly selected high dye concentration of 1500mg/L and the solution was shaken to attain equilibrium.After equilibrium, the adsorbent was separated from solutionby centrifugation and dried in oven at 80�C. Residual dyeconcentration in the aliquot was determined by UV-Visiblespectrophotometer. After separation from the solution, thedye-laden adsorbent was allowed to contact with double dis-tilled water (50 mL) at different pH and 0.1 M sodium phos-phate (pH 7) for the predetermined equilibrium time ofadsorption. Thereafter, the amount of dye desorbed was cal-culated by determining the concentration of desorbed dye inthe liquid. The amount of the dye desorbed (%) was calcu-lated as follows:

Desorption ð%Þ5 Mass of dye desorbed

Mass of dye adsorbed

� �3100 (18)

Further, after desorption, adsorbent was dried and againcontacted with dye solution of 1500 mg/L and similar proce-dure was repeated for another cycle desorption. It was inter-esting to note that in first cycle, the adsorbent displayed 82%desorption. The desorption cycle was repeated thrice. Thedye displayed 82%, 68%, and 47% desorption in the threecycles, respectively. These results show that the regeneratedadsorbent (ACSD) can be confidently reused at least for two

cycles. The reuse of the adsorbent will help in reducingamount of sludge generation in the process of removal.

CONCLUSIONS

Following conclusion can be drawn from this study1. Adsorption of cationic dye, MB on ACSD has been stud-

ied. ACSD was prepared by simple and economicallyviable method.

2. SEM study depicts the effect of activation process of sawdust and that the surface of activated adsorbent becamemore suitable for adsorption of dye.

3. Removal of MB was higher at lower concentration anddecreased by increasing concentration of MB insolution.

4. Alkaline medium was found to be more efficient for theremoval of MB by adsorption on ACSD. Temperaturestudy revealed the exothermic nature of removalprocess.

5. Kinetics of removal process was investigated and the lin-earity of plots at all studied temperatures, confirmed theapplicability of the pseudo-first-order model.

6. Values of coefficient of mass transfer suggested that theadsorbent displayed a favorable removal.

7. On analysis of equilibrium data by different isothermmodel, viz., Langmuir, Freundlich, and Tempkin models,it was observed that Langmuir is fitting well for thisstudy.

8. Studies of various thermodynamic parameters indicatedthe feasibility and spontaneity of removal process.

9. ACSD has potential for the removal of MB from aqueoussolutions it can be proved an economically viable alter-nate for costly adsorbent.

10. The desorption results indicated that the dye could bereused at least twice before disposal.

ACKNOWLEDGMENT

One of the authors (SB) is thankful for CSIR UGC for pro-viding Junior Research Fellowship.

Table 5. Comparison of adsorption capacity of ACSD with other adsorbents.

AdsorbentsAdsorption

capacity (mg/g) References

Cylindrical graphene–carbon nanotube hybrid 81.97 [32]Orange peel 18.6 [33]Exfoliated graphene oxide 17.3 [34]Active carbon 16.43 [35]Silica nanosheets 12.66 [36]Polyaniline nanotubes base/silica composite 10.31 [37]Raw beech sawdust 9.78 [38]Hazelnut shell activated carbon 8.82 [39]Sugar extracted spent rice biomass 8.13 [40]Water hyacinth root Powder 8.04 [41]Coir pith carbon 5.87 [42]Almond shell activated carbon 1.33 [43]ACSD 4.58 Present study

Table 6. Value of different thermodynamic parameters for the removal of MB by adsorption on ACSD at different temperatures.

Temperature (K) DG� (kcal/mol) DH� (kcal/mol) DS� (cal/mol/K)

303 20.95 29.31 20.83313 20.68323 20.21


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