Journal of Environmental Chemical Engineering 2 (2014) 1870–1880

Kinetic and equilibrium modeling for the adsorptive removal ofmethylene blue from aqueous solutions on of activated fly ash (AFSH)

Sushmita Banerjee a, Gopesh C. Sharma b, M.C. Chattopadhyaya a,Yogesh Chandra Sharma c,*aDepartment of Chemistry, University of Allahabad, Allahabad 211002, IndiabDepartment of Applied Science & Humanities, Invertis University, Bareilly 243123, IndiacDepartment of Chemistry, Indian Institute of Technology (Banaras Hindu University) Varanasi, Varanasi 220105, India

A R T I C L E I N F O

Article history:Received 24 March 2014Accepted 24 June 2014

Keywords:Fly ashMethylene blueAdsorption kineticsIsothermThermodynamics

A B S T R A C T

Fly ash is a byproduct generated during the coal combustion in thermal power plants. Application ofactivated fly ash (AFSH) for removal of a toxic dye, methylene blue (MB) from its aqueous solution hasbeen investigated. Characterization of the adsorbent was carried out with X-ray fluorescence (XRF), X-raydiffractometer (XRD), Fourier transform infrared spectroscopy (FTIR), and scanning electron microscopy(SEM). Surface area of the AFSH was also determined. Batch studies were performed to evaluate influenceof various experimental parameters like initial pH, contact time, initial concentration and temperature onthe removal of MB. Optimum conditions for the removal were found to be as initial pH 9.0, contacttime = 100 min and adsorbent dose = 20 g/L. The surface area of activated fly ash was found to be 58.16 m2/g. The removal followed second order kinetics. The equilibrium data was fitted by Langmuir, Freundlichand Redlich–Peterson isotherm models. Error analysis revealed that Freundlich model fits the data best.Thermodynamic parameters such as change in Gibbs free energy (DG�), enthalpy (DH�), and entropy(DS�) were also calculated. A single-stage batch adsorber design for the MB adsorption onto AFSH wasalso presented based on the Freundlich isotherm model equation. The above findings suggest thatactivated fly ash can be effectively used for discoloration of dye contaminated wastewater.

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1. Introduction

Colorants such as pigments and dyes are widely used in textile,dyeing, paper and pulp, petroleum refining, pharmaceuticals, foodprocessing, fertilizers, tannery, paint and other diverse industries.As such, the effluents of these industries are contaminated withappreciable quantity of dyes which are considered as offensivepollutants because they impart undesirable color, taste and odor towater. In addition to aesthetic unacceptability, colored wateraffects human and marine life adversely due to toxic andcarcinogenic effects [1]. Dyes used in the textile industry aredifficult to remove by conventional waste water treatmentmethods since they are stable to light and oxidizing agents andare resistant to aerobic digestion [2]. Nowadays, thermally stabledyes are introduced which are difficult to degrade after use. Thustreatment of colored wastewater prior to discharge into receivingstreams is essential. Various treatment techniques have been

* Corresponding author. Tel.: +91 542 6702865; fax: +91 542 2876/2368428.E-mail address: ysharma.apc@iitbhu.ac.in (Y.C. Sharma).

http://dx.doi.org/10.1016/j.jece.2014.06.0202213-3437/ã 2014 Elsevier Ltd. All rights reserved.

applied for the removal of dyes from aqueous solutions. The detailsof each technique suggesting merits and demerits of the same havebeen critically reviewed at length by Cooper [3] and Robinson et al.[4]. Removal of dyes from wastewater by adsorption on commer-cial activated carbons is an effective process, but its high cost andpoor regeneration capability confined its large scale application. Inorder to provide a solution for cost effective removal, researchershave attempted to investigate low cost adsorbent materials whichare comparable to commercial activated carbon in terms ofefficiency and adsorption capacity. Fly ash, the byproduct ofthermal power plant is one such suitable adsorbent which isgaining attention as it is generated in large quantities as wastematerial during combustion process globally. It is estimated that500 Mt of fly ash is produced worldwide annually [5]. In India,generation of fly ash from coal based thermal power plants is131 Mt/year and it is expected to increase to 300–400 Mt/year by2016–2017 [6]. The ultimate fate of fly ash is majorly associatedwith its disposal as landfill but this provokes environmental as wellas economical concerns. Hence, it becomes necessary to search foran effective method which answers to these concerns. Applicationof fly ash in different industrial sectors increases markedly, but still

Fig. 1. Molecular structure of methylene blue.

S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880 1871

the generation of fly ash is far greater than its utilization. Therefore,efforts are being directed towards innovative environmentallyfriendly applications which make best possible use of thisabundantly available material. However, probable environmentalimpacts related to fly ash utilization have been extensivelyexplored and are well understood. Vulnerability of heavy metalsleaching from the ashes is relatively low and therefore the risksassociated with the release of heavy metals into the environmentdo not exceed beyond permissible level [7]. Fly ash has potentialuse in wastewater treatment because of its major chemicalcomponents, which are alumina, silica, ferric oxide, calcium oxide,magnesium oxide and carbon, and its physical properties such asporosity, particle size distribution and surface area [8]. Applicationof fly ash for the removal of heavy metals [8,9] organics [10,11] aswell as dyes [12,13] from wastewater has been reported. Raw flyash usually exhibits low adsorption capacity and modification of flyash by physical and chemical treatment would enhance theadsorption capacity, thus enhancing the value for application [14].Taking this into consideration, various researchers have modifiedfly ash using various techniques. Wang et al. [13] conductedcomparative study of dye removal using fly ash treated byconventional chemical, sonochemical and microwave methods.Wang and Zhu [14] tested fly ash after sonochemical treatment forremoval of methylene blue from aqueous solutions. Pengtham-keerati et al. [15] investigated the removal efficiency of phosphateusing HCl and NaOH treated fly ash and reported enhancedefficiency. Nascimento et al. [16] examined the performance ofhydrothermally treated fly ash for adsorption of cations fromaqueous solutions and recently Sahoo et al. [17] evaluated thepotential of alkali modified fly ash for the removal of metals fromacid mine drainage and Polowczyk et al. [18] found enhancedremoval of boron by using agglomerates of fly ash. With focusingonly on dye removal by adsorption, detailed literature reviewsuggested that for last two decade coal based fly ashes as adsorbentmaterials have been meagerly applied. The present investigationdeals with the use of chemically modified fly ash for the removal ofdyes from aqueous solutions.

Methylene blue, a thiazine class dye is selected as a model dyewhose removal was carried out by adsorption over activated flyash. Over exposure to methylene blue is reported to causeincreased pulse rate, vomiting, shock, heinz body formation,cyanosis, jaundice, quadriplegia, and tissue necrosis in humans[19]. MB enters into the environment during industrial operationsand remains there for longer period due to its complex structuresand thus refrained from physical, chemical and biologicaldegradation. Therefore, most of the wastewater treatment suffersfrom serious setback. Adsorption performance of different type ofdyes including methylene blue over fly ash has been previouslyreported by many researchers but limited studies have beendevoted on describing the adsorption dynamics as well asmechanism. The present study therefore fills in the paucity ofscientific data describing sorption dynamics and mechanistic stepsof adsorbent-adsorbate interaction. In the study, the effect of initialmethylene blue concentration, pH, temperature and contact timefor removal of the dye has been examined. Further, the kinetics andthe mechanistic steps involved in the sorption process were alsobeing evaluated.

2. Materials and methods

2.1. Adsorbate (methylene blue)

All chemicals and reagents used in the experiments andanalyses were of analytical reagent (AR) grade and were used asreceived without further purification. Methylene blue is aheterocyclic aromatic chemical compound and is available as dark

blue crystals. Methylene blue (3,7-bis(dimethylamino)-phenaza-thionium chloride tetramethylthionine chloride, C.I. number,52015) was purchased from British Drug House, Poole, England.Fig. 1 shows the chemical structure of the MB molecule. MB stocksolution (1000 mg/L) was prepared by dissolving 1.0 g of dye in1000 mL of double distilled water, and the working solutions of 5–20 mg/L were prepared daily with required dilution. The concen-tration of the dye in the solutions was determined spectrophoto-metrically at 665 nm. Calibration curves were plotted betweenabsorbance and concentration of the dye in standard solutions.Solution pH was measured using a pH/ion meter (pH meter 335,Systronics, Ahmedabad, India) and adsorption studies were carriedout using UV–vis spectrophotometer (Shimadzu UV–vis Spectro-photometer 2100S, Japan).

2.2. Adsorbent (fly ash)

Fly ash was obtained from Faridabad Thermal Power Plant,Haryana, India. It was derived out of the bituminous coal of LowerGondwana age from Siyal and Gaddi coal mines of Bihar, India. Flyash was washed with sufficient distilled water to remove surfacedust and the soluble inorganic materials which were present in thepores through sedimentation process. The sample was kept foroven-drying at 105 �C for 24 h before use. For activation process,the oven dried adsorbent (500 g) was mixed with 1M HCl solutionin the ratio of (1 g) fly ash to (2 mL) of acid and the mixture wasincubated at 105 �C for 24 h. At the end of the treatment process,the resultant solid products were washed with double distilledwater to get it free from acid and then dried in an oven at 105 �C for24 h. Thus the as prepared activated fly ash (AFSH) was subjected tofurther physical and chemical analysis.

2.3. Characterization of the adsorbent

Chemical compositions of the FA and AFSH were analyzed byenergy dispersive X-ray fluorescence, XRF (thermoscientific ARLuptimx 166). The X-ray diffraction pattern was obtained by adiffractometer (Phillips P.W. 1830) which operated at 40 kV/20 mA,using CuKa radiation with a wavelength of 1.54 Å in the wide angleregion from 20 to 60� on 2u scale. The FT-IR spectral absorptionstudies of the FA and AFSH were carried out to assess the presenceof functional groups by infrared spectrometer (Simadzu/8400S).The pellets of the adsorbent were prepared by intimately mixingthe adsorbent with KBr in ratio of 1:100 respectively. The spectralwavelength was covered in 400–4000 cm�1 range. The surfacefeatures of the FA and FASH was examined through micro-graphstaken by Scanning Electron Microscope (JEOL JSM 840). The surfacearea of the adsorbents was analyzed by TriStat 3000 analyzer(Micomeritics Instrument Corp) using BET method with N2

adsorption at �196 �C. The sample was first degassed at 200 �Cfor 4 h. TriStat 3000 is an automated gas analyzer which containsthree ports and thus makes it valuable to analyze three samples at

Table 1Physico-chemical properties of fly ash.

Chemical characteristics (%) Proximate analysis (%)

Constituents FA AFSH Constituents (%)

Silicon dioxide (SiO2) 43.7 66.2 Ash 77.4Aluminium oxide (Al2O3) 15.7 22.1 Loss on ignition 11.2Iron oxide (Fe2O3) 6.4 7.7 Volatile matter 3.6Calcium oxide (CaO) 9.8 2.2 Fixed carbon 1.9Magnesium oxide (MgO) 0.9 1.1 Moisture 1.2

1872 S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880

a time. Point zero charge of the AFSH was determine according tosolid addition method by using 0.01 M NaCl.

2.4. Adsorption studies

The batch experiments were conducted in order to optimize thevarious operating parameters that affect the removal of MB byAFSH. 50 mL dye solution of desired concentration was taken in250 mL of Erlenmeyer flasks at a fixed temperature. The mixturewas mechanically shaken at a constant speed of 180 rpm usingwater bath shaker (Mac-Macro Scientific Works, Pvt. Ltd., NewDelhi, India). The samples were withdrawn after predeterminedtime, and kept it for 24 h for saturation. Thereafter, the adsorbentwas separated from the solution by centrifugation using Remicentrifuge machine (Model R-8CBL). The residual dye concentra-tion was estimated in supernatant spectrophotometrically ondouble beam spectrophotometer at 665 nm. The effect of initial pHon the adsorption process was studied over a pH range of 3.0–10.0,being adjusted using 0.1 M NaOH or 0.1 M HCl. The adsorptionisotherm experiments carried out with dye solutions of differentconcentrations (5–20 mg/L) were shaken with a known amount ofadsorbent (1 g/L) at different temperatures (20�, 30� and 40 �C) tillthe equilibrium was attained. Kinetic investigations were carriedout at different time intervals with three different initial dyeconcentrations keeping temperature, pH, adsorbent dose andshaking speed constant. The residual dye concentrations were

Fig. 2. (a) XRD pattern of FA.

determined from the calibration curve plotted between absor-bance and concentration of dye solution.

The amount of dye adsorbed onto the adsorbent at equilibrium,qe (mg/g) was calculated by using following relationship:

qe ¼ðC0 � CeÞV

W(1)

where C0 and Ce are the initial and equilibrium dye concentrationin mg/L respectively, V is the volume of solution (L) and W is themass of adsorbent (g).

The removal (%) of dye was calculated using the followingequation:

Dyeremovalð%Þ ¼ ðC0 � CeÞ � 100C0

(2)

Blank experiments were also conducted by using dye solutionwithout adsorbent to ensure that no dye was adsorbed onto thecontainer walls and with adsorbent and water only to check that noleaching occurred from the adsorbent, which would ratherinterfere with the measurement of dye concentrations on thespectrophotometer. All adsorption experiments were performed intriplicate and the mean values were taken for the data analysis.

3. Results and discussion

Chemical composition of fly ash samples are provided in Table 1.The major components of fly ashes were mainly oxides of Si and Al,and various other oxides are present in traces. The raw fly ash (FA)and acid activated fly ash (AFSH) were almost of similarcomposition. But, change in constituent’s percentage can benoticed after acid activation, prominent change can be observed forSiO2 and CaO. The decrease in content of CaO is mainly attributedto the chemical reaction between HCl and CaO which probablyenhances solubility of CaO from the fly ash at the same time theSiO2 content enriches from 43.7% to 66.2% after acidic treatment.

The XRD analyses of the samples were presented in Fig. 2. TheXRD of FA (Fig. 2a) reveals the presence of various types of mineralsout of which mullite (alumino silicate) and quartz (silica) are the

(b) XRD pattern of AFSH.

Fig. 3. FTIR spectra (a) AFSH (b) dye loaded AFSH.

S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880 1873

prominent ones. Mullite exhibited strong peaks at 2u values of16.41�, 26.63�, 31.44�, 36.21�, 41.32�, 43�, and 47.1� (with d spacingvalues of 5.406, 3.439, 2.955, 2.61, 2.335, 2.261 and 2.104 Å)similarly presence of quartz can be confirmed by observing strongpeaks at 2u values of 21.76� and 26.88� (with d spacing values of4.158 and 3.41 Å). In addition to this presence of other mineralssuch as hematite, magnetite and lime can be identified by theircharacteristics strong peaks at 2u values of 35.833�, 56.98� and31.96� respectively (d spacing value of 2.633, 1.838 and 2.912). TheAFSH (Fig. 2b) had identical XRD pattern to that of FA, but the peaksof lime (CaO) which were easily distinguished in FA were notproperly discernable in case of AFSH. Comparable observationswere accounted by Pengthamkeerati et al. [20] and Khan et al. [21].

The assessment of surface functional groups was carried out byFTIR technique. The spectra of normal fly ash and activated fly ashdid not reveal much difference, therefore the FTIR of activated flyash and dye loaded activated fly ash were investigated. The spectraof AFSH (Fig. 3a) and dye loaded AFSH (Fig. 3b) displayed that theabsorption bands exhibited at 3372 cm�1 (H��O��H stretching)and 1627 cm�1 (H��O��H bending) due to adsorption of watermolecule. The characteristic peak between 2940 and 2820 cm�1

designated to OH groups attached methyl species. The presence of��CH2 and ��CH3 groups can be identified by the peaks at around1470 cm�1 and 1380 cm�1 respectively. The peaks located at 921and 1162 cm�1 is due SQO stretching and ��NQN�� stretchingindicated by peak at 1636 cm�1, however peaks get shifted to lowerfrequencies after dye adsorption. The adsorption of dye over AFSH

Fig. 4. SEM micrographs of fly ash (a) F

also marked by observing changes in the spectral intensity it can benoticed from the FTIR spectra that transmittance intensityenhances after adsorption of dye molecules. The peak located1743.21 cm�1 is due to the presence of carbonyl functional groupand 1606.85 cm�1 is due to vibrations of aromatic groups whichdiminished afterwards due to dye adsorption. The peak at1140 cm�1 corresponds to Si��O��Si band which shifted to higherfrequencies due to adsorbent–adsorbate interaction. The bands at797, 697, 544 and 512 cm�1 are attributed to quartz [22] andprominent changes have been noticed in the peaks after dyeadsorption. The existence of broad peak can be observed between2800–3500 cm�1 which indicates the presence of hydroxyl groupsover adsorbent surface either in free form or hydrogen bonded or Sibonded. The peaks get altered from their normal position after dyeadsorption indicates about the interaction of Si��OH (silonal)groups with the dye ions and formation of SiO�MB. All of the abovefindings substantiated adsorption of dyes on the adsorbent.

The scanning electron micrographs at 2000X magnification(Fig. 4a and b) show the morphological structure of fly ash. TheSEM images of fly ash revealed that most of the particles arespherical in shape with a relatively smooth surface. Fig. 4a showssub-angular and spherical particles in raw fly ash with relativelysmooth grains comprises of quartz and iron particles and Fig. 4bshows segregated oblong spherical shaped particles in AFSH, somecrack seems to be generated within the AFSH particles whichenhance surface roughness as well as pore volume of the particles.

The textural properties of FA and AFSH have been summarizedin Table 2. It can be seen from the table that AFSH has much greatersurface area (58.16 m2/g) than that of FA (13 m2/g). This may be dueto surface modification as well as dissolution of CaO from FAparticles by protonated solutions. This observation was supportedby the study of Pengthamkeerati et al. [20] who have also reportedmuch similar surface area (61.84 m2/g) of fly ash after HCltreatment.

3.1. Effect of pH

The solution pH influences surface charge of the adsorbentmaterials; therefore with change in solution pH, adsorptionprocess also gets affected. The alteration of solution pH leads torelease of hydrogen or hydroxyl ions which may further react withthe functional groups available on the active sites of the surface ofthe adsorbent. Before performing analysis over the effect of pH onsorption process it was ensured that change in solution pH does notinflict any effect on the dye structure as well as lmax of dye.

A (b) AFSH at resolution of 2000�.

Table 2Physical properties of fly ash (FA) and activated fly ash (AFSH).

Physical properties FA AFSH

Specific surface area (m2/g) 13.0 58.16Bulk density (g/cm3) 3.51 1.08Total pore volume (cm2/g) 0.006 0.02Average pore volume (cm2/g) 0.035 0.59Average pore diameter (Å) 10.05 22.104Mean diameter (m) 3.8 � 10�4 4.5 �10�5

0 50 10 0 15 0 20 0 25 00.0

0.2

0.4

0.6

0.8

Dye

adso

rbed

(mg/

g)

Con tac t time (min)

5 mg/L10 mg/L20 mg/L

Fig. 6. Effect of initial concentration and contact time for MB removal (pH 9.0,adsorbent dose = 20 g/L, temperature = 20 �C).

1874 S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880

Therefore, all the absorbance data were taken on distinct lmax of664 nm. The sorption results thus obtained were solely owing toadsorption process, not by chemical alteration or degradation ofdye structure. Fig. 5 represents the relationship between initialsolution pH and dye removal. It was clear that the removalpercentage of MB were maximum at basic pH (pH 8.0–9.0), andfurther decreases with reduction in solution pH. Our findings aresupported by earlier workers [7,23]. Therefore, further experi-ments were carried out at the optimum pH of 9.0. The effect of pHon sorption process can be illustrated on the basis of electrostaticforces of interaction that occur between fly ash and dye ions. Forthis, the zero point charge of fly ash was measured (pHZPC = 7.7),revealing that at pH > pHZPC, the adsorbent surface becomesnegatively charged which favors adsorption of cationic dye ionsfrom the bulk. At pH < pHZPC, the dye molecules were protonatedand hydrogen ions start competing with positively charged dyeions for negatively charged adsorption sites resulting in a lowerdye removal. In addition to the repulsion, interaction betweencationic dye ions and positively charged adsorbent surface at lowerpH may be the other reasons for low dye removal.

3.2. Effect of contact time and initial concentrations

The effect of initial concentrations of dye on the removal wasstudied for three initial concentrations of MB viz. 5, 10 and 20 mg/Lrespectively at pH of 9.0 and the results are presented in Fig. 6. Inall cases, the equilibrium reached within 100 min. Removal wasfast in earlier period and then increases sluggishly with theprolongation of contact time. After 100 min., equilibrium wasachieved as no remarkable variation in dye removal was foundbeyond that time. This can be elucidated that during the adsorptionof MB molecules, initially the dye molecules rapidly reached theboundary layer by mass transfer, then they slowly diffused from

Fig. 5. Effect of initial pH for removal of MB (initial concentration = 5 mg/L,adsorbent dose = 20 mg/L, temperature = 20 �C); pHZPC plot of pH final vs. pH initial(adsorbent dosen= 4 g/L; temperature = 20 �C).

boundary layer film onto the adsorbent surface because of theavailability of vacant sites, from there they finally diffused into thepores of adsorbent.

From Fig. 6, it was found that the adsorption of methylene blueincreased from 0.082 to 0.816 mg/g by increasing initial dyeconcentration from 5 to 20 mg/L since the initial dye concentrationprovided necessary driving force to overcome the resistance to themass transfer of dye molecules from liquid phase to solid phase.The increase in initial dye concentration also enhances theinteraction between MB and the adsorbent. Therefore, an increasein initial dye concentration of MB enhances the uptake of dyemolecules due to increase in the driving force.

3.3. Effect of temperature

The dependence of adsorption capacity of MB on three differenttemperatures (20, 30 and 40 �C) has been investigated. Fig. 7 showsthat the removal percentage increased with the decrease oftemperature which indicated exothermic nature of the adsorptionprocess. This characteristic may be attributed to weakening of thebonds between the dye molecules and the binding sites of theadsorbent or may be due to the either decrease in active sites over

0 50 100 150 200 2500.0

0.1

0.2

0.3

0.4

0.5

Dye

adso

rbed

(mg/

g)

Con tact time (min)

200C 300C 400C

Fig. 7. Effect of temperature for MB removal (pH 9, time = 100 min, adsorbentdose = 20 g/L, initial dye concentration = 5 mg/L).

Table 3Kinetic parameters for the removal of methylene blue by AFSH.

PseudofirstOrdermodel

k1(min�1)

qe (cal)

(mg g�1)qe (exp)

(mg g�1)R2 Pseudo

secondordermodel

k2(g mg�1/min)

qe (cal)

(mg g�1)qe (exp)

(mg g�1)R2 Intra-

particlediffusionmodel

kid,1(mg/gmin0.5)

C1 kid,2(mg/gmin0.5)

C2 Boydkineticmodel

B(s�1)

Di (cm2 s�1)

5 mg/L 0.0415 0.471 0.254 0.971 5 mg/L 0.458 0.348 0.254 0.984 5 mg/L 0.035 0.071 0.017 0.076 5 mg/L 0.84 1.35 �10�6

10 mg/L 0.0437 0.788 0.427 0.9742 10 mg/L 0.0486 0.521 0.427 0.979 10 mg/L 0.044 0.087 0.029 0.134 10 mg/L

0.94 1.71 �10�6

20 mg/L 0.0506 1.706 0.844 0.9542 20 mg/L

0.0514 0.907 0.844 0.981 20 mg/L 0.097 0.106 0.063 0.212 20 mg/L

1.27 2.06 � 10�6

S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880 1875

surface of adsorbent or increase in the thickness of the boundarylayer surrounding the adsorbent with increase in temperature [24].

3.4. Adsorption dynamics

The mechanism of adsorbate–adsorbent interaction can bebetter described by investigating rate expression for adsorption ofMB on fly ash. The rate expression can be determine by analyzingthe adsorption data using three different kinetic models namelyLagergren’s first order [25], pseudo-second order [26] and intra-particle diffusion [27].

The Lagergren first-order kinetic model is as follows [25]:

logðqe � qtÞ ¼ log qe � ð2:303=KadÞ : t (3)

where qt and qe are the amounts of dye adsorbed (mg/g) at anytime t and at equilibrium, respectively, and Kad represents theadsorption rate constant (min�1). Lagergren plot for threedifferent dye concentrations (5, 10 and 20) mg/L were plottedagainst contact time (figure not given). The sorption data for allinitial dye concentrations obey Lagergren model for the first50 min and beyond that the data deviates from its linear form.This indicates that the present kinetic model is inappropriate forpredicting the sorption kinetics of MB onto fly ash. Variousparameters of the Lagergren model have been calculated anddisplayed in Table 3.

Asthefirstorderkineticmodelfailstorepresent thesorptiondata,the data was analyzed using pseudo-second order [26] model:

tqt

¼ 1K2 � q2e

þ tqe

(4)

where K2 (g/mg min) is the second order rate constant which canbe calculated from the intercept of the graph plotted between t/qt

10 20 30 40 50 60 70 80 90 10 0

80

120

160

200

240

280

320

360

t / q

t

t (min)

5 mg/L10 mg/L20 mg/L

Fig. 8. Pseudo second-order plot for the adsorption of MB dye on AFSH at 20 �C fromsolutions with various initial dye solution concentrations, mg/L: 5, 10 and 20.

versus t (Fig. 8), at the same time qe can be calculated from theslope of the same graph and represented in Table 3. Fig. 8represents the pseudo-second order plot, it can be seen thatsorption data maintains its linear profile over the entire period oftime in addition the higher values of correlation coefficients as wellas the values of qe estimated using pseudo-second order modelwere more consistent with the experimental data, suggesting theapplicability of this model for the adsorption process. Further, themodel supports the assumption that chemisorption might beresponsible for sorption process.

In order to resolve the actual mechanistic pathway behind thesorption process of MB, the data was fitted into intra-particlediffusion model suggested by Weber and Morris [26]. This modelhelps to identify various stages of adsorption process (externalmass transfer and pore diffusion) by displaying series of linearplots for the sorption data.

The intraparticle diffusion rate of the dye molecule in particle,Kid can be calculated from the following equation:

qt ¼ Kidt1=2 þ C (5)

where parameter Kid (mg/g min0.5) is the rate constant of intra-particle diffusion and C is the intercept. According to Eq. (5), a plotof qt versus t1/2 should be a straight line with a slope Kid whenadsorption mechanism is governed by intraparticle diffusion.Values of intercept give an idea about the thickness of boundarylayer, the larger the intercept, the greater would be the boundarylayer effect. The intra-particle diffusion plot is depicted in Fig. 9.From the figure it can seen that the plot exhibits multi-linearnature Tables 4 and 5. The presence of two linear curves for thedata of each initial dye concentration indicated that first linear plotcorresponds to external mass transfer or film diffusion up to initialtime period of 50 min. beyond which pore diffusion or intra-

4 5 6 7 8 9 10

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

q t (m

g/g)

t0.5 (min 0.5 )

5 mg/L10 mg/L20 mg/L

Fig. 9. Intra particle diffusion plot for the adsorption of MB dye on AFSH at 20 �Cfrom solutions with various initial dye solution concentrations, mg/L: 5, 10 and 20.

Table 4Isotherm parameters and error analysis values for the removal of methylene blue by AFSH.

Dye Model 20 �C 30 �C 40 �C

Methylene blue Langmuirq0 14.281 11.637 10.418KL (L/mg) 0.878 0.913 0.944RL 0.054 0.052 0.049R2 0.984 0.962 0.981HYBRID 2486.683 2966.271 3271.113

Methylene blue FreundlichKF (mg/g) (L/mg)1/n 0.557 0.263 0.196n 4.341 2.067 1.644R2 0.9917 0.9961 0.999HYBRID 28.801 27.653 32.669

Methylene blue Redlich–PetersonKR (L g �1) 12.1 9.5 7.1aR (L mg�1) 24.7 64.56 91.21b 0.36 0.32 0.212R2 0.989 0.995 0.998HYBRID 50.952 52.826 48.634

1.0

1.2

1.4

1.6

1.8

t

5 mg/L10 mg/L20 mg/L

1876 S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880

particle diffusion dominates as designated by second linear plot.The deviation in linearity from the origin has been observed whichindicates that intra-particle diffusion is not the sole ratedetermining step. The Kid values for different dye concentrationshave been calculated and presented in Table 3. The majorlimitation that comes into play in the intraparticle diffusion studyis that it fails to express about the rate controlling steps involved inthe dye sorption process. Therefore, in order to establish the ratelimiting step concerning dye sorption process the data werefurther analyzed by the kinetic expression suggested by Boyd et al.[28]:

F ¼ 1 � 6p2

� �exp½�Bt� (6)

where F is the fraction of solute adsorbed at different times t and Btis a mathematical function of F and is represented as follows [28]:

F ¼ qqa

(7)

where, q and qa represents the amount adsorbed (mg/g) at anytime t and at infinite time (in the present study 100 min).

Thus, the value of Bt can be calculated for each value of F usingRichenberg table [29]. The calculated Bt values were plottedagainst time as shown in Fig. 10. The linearity of this plot willprovide useful information to distinguish between film diffusionand intraparticle-transport-controlled rate of adsorption. FromFig. 10, it can be observed that at all concentrations, the Bt versus tplot deviates from linearity and did not pass through origin. Itreflects that for the studied concentration range, film diffusion orexternal mass transport mainly govern the sorption process andcan be considered as rate limiting. The slope of the Boyd plot can beused to calculate Bt values for the determination of effectivediffusion coefficient, Di (cm2/s) using the relation:

Table 5Values of different thermodynamic parameters of methylene blue adsorption onAFSH at various temperatures.

Temperature(K)

Standard Gibbs freeenergy change DG�

(kJ/mol)

Standardenthalpy changeDH� (kJ/mol)

Standard entropychange DS� (J/mol K)

293 �4.839 �20.79 �72.11303 �3.193313 �3.132

Bt ¼ p2Di

r2(8)

where, r represents the radius of the particle which is in the rangeof 0.0059–0.0037 cm calculated by sieve analysis and assumingspherical particles. The calculated Di values at different initial dyeconcentrations are given in Table 6. The average Di values wereestimated to be 1.71 �10�6 cm2/s. As reported by Singh et al. [30],intraparticle diffusion can be a rate-limiting step in adsorptionprocess when value of Di is of the order of 10–11 cm2/s. In thepresent investigation the values of Di calculated which are in theorder of 10�6. These values are 5 orders of magnitude greater thanvalue referred by Singh et al. [30]. This also indicates that externalmass transfer is the rate-controlling step.

3.5. Adsorption isotherm

Investigation of adsorption isotherm is one of the mostimportant parts of adsorption study as it reveals the interactionbetween the adsorbent and adsorbate. Furthermore, it gives anidea about the adsorption capacity of the adsorbent. In the presentinvestigation, the sorption equilibrium data were examined usingtwo parametric (Langmuir and Freundlich model) and threeparametric (Redlich Peterson model) isotherm expressions. Thedetails of each model were discussed as follows:

10 20 30 40 50 60 70 80 90 10 00.0

0.2

0.4

0.6

0.8

B

Time (min)

Fig. 10. Boyd kinetic plot for the adsorption of MB dye on AFSH at 20 �C fromsolutions with various initial dye solution concentrations: 5, 10 and 20 mg/L.

Table 6Comparison of Langmuir adsorption capacities of different adsorbents for the removal of methylene blue.

S.N. Adsorbents Adsorption capacity (mg/g) Temperature (�C) Adsorption isotherm Reference

1 Fly ash A 6.0 22 Langmuir [7]2 Fly ash 1.9 30 Langmuir and Freundlich [14]3 ZnCl2 activated rice husk 9.7 – – [39]4 Magnetic rectorite 31.2 – – [40]5 Sugar extracted spent rice biomass 8.1 25–45 Langmuir [41]6 Zeolite NaA 64.8 30–70 Langmuir [42]7 KOH activated POME sludge 23.5 – Langmuir [43]8 ZnCl2 activated POME sludge 22.4 – Langmuir [44]9 Commercial activated carbon 22.3 – Langmuir [44]10 Magnetic graphene-carbon nanotube composite 65.8 10–40 Langmuir [44]11 Activated fly ash 14.28 20 Freundlich Present study

S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880 1877

Langmuir isotherm model [31] is expressed as follows:

Ce

qe¼ 1

KLq0þ Ce

qe(9)

where q0 is maximum adsorption capacity (mg/g) and KL (valuesfor Langmuir constant related to the energy of adsorption (L/mg))[24]:

RL ¼ 11 þ KLC

(10)

A dimensionless constant separation factor (RL) of Langmuirisotherm was used to determine the favorability of the adsorptionprocess. RL is defined as Eq. (10); the values of RL indicate the typeof isotherm to be irreversible (RL = 0), favorable (0 < RL< 1), linear(RL = 1) or unfavorable (RL > 1).

The linearized form of Freundlich isotherm model [32] isexpressed as under [24]:

logqe ¼ logKF þ 1nðlogCeÞ (11)

where qe is the amount of dye adsorbed per unit of adsorbent atequilibrium time (mg/g), Ce is equilibrium concentration of dye insolution (mg/L). KF and n are Freundlich isotherm constants whichindicate the capacity and the intensity of the adsorptionrespectively.

Redlich–Peterson isotherm model [33]:

lnKRCe

qe� 1

� �¼ lnaR þ blnCe (12)

-3 -2 -1 0 1 2

-1.8

-1.6

-1.4

-1.2

-1.0

-0.8

-0.6

-0.4

ln q

e

ln Ce

200C 300C 400C

Fig. 11. Freundlich isotherm plot at different temperatures for the removal of MB byAFSH (pH 9, time = 100 min, adsorbent dose = 20 g/L).

where KR is the R–P isotherm constant (L g�1), aR the R–P isothermconstant (L mg�1) and b the exponent which lies between 0 and 1,Ce the equilibrium liquid phase concentration (mg L�1). Since theRedlich–Peterson isotherm equation contains three unknownparameters KR,aR and b, therefore, the parameters of the equationswere determined by minimizing the distance between theexperimental data points and the theoretical model predictionswith the “Solver” add-in function of the Microsoft Excel program.The R–P equation is a combination of the Langmuir and Freundlichequations. In the limit, as the exponent b tends to zero, theequation becomes more Freundlich, or heterogeneous, and as theexponent b tends to 1, the equation approaches the ideal Langmuircondition [34].

It is, however, well known that investigating differentparameters of a non-linear equation from its linear form fails torepresent accurate values of the parameter under study thereforethe error functions of non-linear regression basin were employedfor obtaining the best-fit isotherm model to the experimentalequilibrium data. Thus hybrid fractional error function (HYBRID)has been employed in this study to find out the most suitableisotherm model which can represent the experimental data. Thiserror function was developed by Porter et al. [35] in an attempt toimprove the fit of the sum of the squares of the errors at lowconcentrations by dividing it by the measured value. It alsoincludes the number of degrees of freedom of the system – thenumber of data points, n, minus the number of parameters, p, of theisotherm equation – as a divisor [34]:

-5 -4 -3 -2 -1 0 1 20

1

2

3

4

5

ln [(

K R*C e/q

e) -

1]

ln Ce

200 C300 C400 C

Fig.12. Redlich–Peterson isotherm plot at different temperatures for the removal ofMB by AFSH (pH 9, time = 100 min, adsorbent dose = 20 g/L).

Fig. 13. Schematic illustrating a design of single-stage batch adsorber.

1878 S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880

100n � p

Xn

i¼1

ðqe; means � qe; calcÞ2qe; means

" #(13)

The Langmuir, Freundlich, and R–P isotherm constants weredetermined from the plots of Ce/qe versus Ce, ln qe versus lnCe

(Fig. 11) and ln(KRCe/qe� 1) versus ln Ce (Fig. 12) respectively atthree different temperatures of 20�, 30� and 40 �C. The best-fitequilibrium model was determined based on the linear squaredregression correlation coefficient R2. From Table 3, it was observedthat the equilibrium sorption data were very well represented byFreundlich isotherm followed by Redlich–Peterson isotherm andthe Langmuir model with high correlation coefficient values.According to the Freundlich theory, n reveals whether theadsorption is favorable, when n < 1, it is unfavorable adsorption;when n = 1, it is linear adsorption; when n > 1, it is favorableadsorption. The value of n was above 1, which meant theadsorption process was favorable. Further, very high R2 values(>0.99) showed that in addition to Freundlich isotherm, theequilibrium data can also be represented by the Redlich–Petersonmodel. The value of constant b is near to 0, and these indicate theisotherm is approaching the Freundlich form. The values of the R–Pconstants were converted into the values of the Freundlichconstants. KR/aR approximated the Freundlich constant KF. As aresult, the Freundlich and Redlich–Peterson adsorption modelsseem to provide the best fit and indicate heterogeneous nature ofsorption process. Furthermore, by comparing the result of errorfunction, the low values of HYBRID for Freundlich and R–P modelindicate the suitability of these two isotherm models overLangmuir model for the better representation of equilibrium data.

3.6. Thermodynamics studies

Thermodynamic studies for the present adsorption of methy-lene blue onto ASFH were undertaken to explicate the mechanisminvolved. Different thermodynamic parameters such as change instandard free energy (DG�), enthalpy (DH�) and entropy (DS�) were

estimated by using Eq. (12)[24]

logKc ¼ DS�

2:303R

� �� DH�

2:303RT

� �(14)

DG� ¼ �RTlnKc (15)

where R (8.314 J/mol K) is the gas constant, T (K) is the absolutetemperature, and Kc (L/g) is the standard thermodynamicequilibrium constant defined by qe/Ce. The values DH� and DS�

were calculated from the slopes and intercepts of the plot log Kc

versus 1/T (graph not given). The various thermodynamicparameters at the three temperatures studied are given in Table 3.The values of DG� were found to be negative at all temperatureswhich indicated about the spontaneous nature of the MBadsorption onto AFSH. The decrease of DG� values with thedecrease in temperature implies that lower temperature favors dyesorption. The negative value of DH� suggests that adsorption of MBover AFSH is exothermic in nature. The negative value of DS�

suggested that the adsorption of dye ions is controlled by enthalpy.Moreover the magnitude of DG� and DH� also indicates about thetype of sorption process. The DG� values between �20 and 0 kJ/molattributed to physisorption whereas chemisorption is in the rangeof �80 to 400 kJ/mol [36]. In present study the values of DG� lieswithin the range of physisorption process. Likewise the magnitudeof DH� if lies between 2.1–20.9 kJ/mol the adsorption process canbe accounted as physical while chemical adsorption generally fallsinto a range of 80–200 kJ/mol [37], in this case DH� valuecorresponds to physisorption range. Thus MB sorption onto AFSHis physical in nature.

3.7. Adsorption mechanism

The main components of fly ash are silica and alumina as itsfunctional oxides groups. Therefore, chemical speciation of thesetwo groups influences the dye sorption process. The surface

0 2 4 6 8 100

20

40

60

80

100

120

140

160

180

200

220M

ass

of A

dsor

bent

(g)

Effluent Volume, C0 (L)

90 % 80 % 70 % 60 %

Fig. 14. Design plot obtained for adsorbent mass against volume of effluent treatedfor various percentage of MB dye removal, with Freundlich isotherm using initialMB dye concentration of 10 mg/L.

S. Banerjee et al. / Journal of Environmental Chemical Engineering 2 (2014) 1870–1880 1879

charges of silica and alumina get highly affected by solution pH.The silica as well as alumina acquire positive charge in protonatedsolutions and get converted into SiO� and Al2O3

� in alkalinemedium. The pHZPC of silica is 1.7 [38] while that of alumina is 7.9[39], thus the surface of fly ash above the respective pHZPC of thesetwo oxides developed negative charge density, thus providesuitable adsorption sites for the binding of dye ions on theadsorbent surface. Similarly the pHZPC of fly ash in this study hasbeen investigated as 7.7, hence at pH beyond 7.7, the surface of flyash becomes more negatively charged which further facilitates theuptake of cationic dye from the aqueous phase over the adsorbent.The following proposed reaction mechanism as briefed by Sahooet al. [17] for removal of heavy metal has been adopted in thepresent investigation to explore the scavenging process of dyemolecules from aqueous phase:

SiOH þ OH� pH>1:7 SiO� þ H2O (16)

AlOH þ OH� pH>7:9 AlO� þ H2O (17)

SiO� þ MBþ Basic medium SiO � MB (18)

AlO� þ MBþ Basic mediumAlO � MB (19)

3.8. Design of single-stage batch adsorber from sorption equilibriumdata

Adsorption isotherms give an idea about the designing of singlebatch adsorption system [40]. A schematic diagram of single-stagebatch adsorber is shown in Fig. 13. In this system the best fitisotherm, Freundlich model was applied for the design of batchadsorber. The fundamental reason after designing an adsorber is toreduce the initial concentration of MB of from C0 (mg/L) to Ce (mg/L) for which the total dye solution is V in liter. The quantity ofadsorbent used for this design was M (g) with the time in which theamount adsorbed changes from q0 (mg/g) to qt (mg/g). Theadsorption of dye over the adsorbent attains mass balanceequation with the time when t = 0, q0 = 0; as the time proceedt ! t1, q0! qe i.e. amount of MB loaded over solid phase is equal toMB removed from the liquid phase by adsorption. This can berepresented by a mass balance equation which can be representedas:

VðC0 � CeÞ ¼ Mðqe � q0Þ ¼ Mqe (20)

Equilibrium isotherm study exhibited the sorption of MB ontoASFH follows a Freundlich isotherm model, therefore aboveEq. (10) is further reassembled as:

MV

¼ ðC0 � CeÞqe

¼ ðC0 � CeÞKFC

1=ne

(21)

Fig. 14 portrayed series of plots that demonstrated relationshipbetween mass of adsorbent required for the treatment of effluentsof different volumes, ranging from 1 L to 10 L to achieve dyeremoval efficiency of 60, 70, 80 and 90% with initial MBconcentration of 10 mg/L as a baseline data for the design singlestage batch adsorption system.

3.9. Comparison of AFSH with other adsorbent

A comparative study in terms of sorption temperature,adsorption equilibrium and uptake capacity has been carriedout with other sorbents was summarized in Table 6. It is obviousfrom Table 6 that the uptake capacity of AFSH for MB is ofcomparable order of magnitude or greater than that of manyadsorbent materials. The variation in dye uptake capacity can beobserved with the change of sorbent material which may be due tothe differences in structure, porosity, functional groups and surfacearea of each adsorbent.

4. Conclusions

The results of the present investigation show that acid activatedfly ash (AFSH) has high potential for adsorption of MB fromaqueous solutions. The amount of dye uptake was found toincrease with an increase in initial dye concentration, contact time,pH but was found to decrease with increase in temperature. Thereaction mechanism was elucidated and removal followed secondorder kinetics and adsorption of MB on AFSH is a two step process:a rapid adsorption of dye onto the external surface followed bypore diffusion. The experimental data were represented by theFreundlich and Redlich-Peterson isotherm models. The maximumadsorption capacity of AFSH from Langmuir model was found to be14.28 mg/g. Thermodynamic parameters DG�, DH� and DS� weredetermined and showed that adsorption of MB on AFSH wasfeasible, spontaneous and exothermic under laboratory scale. Thus,in brief, AFSH can be considered as promising candidate forscavenging of dye stuff from aqueous solutions.

Acknowledgement

One of the authors (SB) is thankful to CSIR, New Delhi, forawarding Junior Research Fellowship.

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