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Biosorption of Direct Red 28 (Congo Red) from AqueousSolutions by Eggshells: Batch and Column StudiesPapita Das Saha a , Shamik Chowdhury a , Madhurima Mondal a & Keka Sinha aa Department of Biotechnology , National Institute of Technology-Durgapur , Durgapur (WB) ,IndiaAccepted author version posted online: 14 Sep 2011.Published online: 28 Dec 2011.

To cite this article: Papita Das Saha , Shamik Chowdhury , Madhurima Mondal & Keka Sinha (2012) Biosorption of Direct Red28 (Congo Red) from Aqueous Solutions by Eggshells: Batch and Column Studies, Separation Science and Technology, 47:1,112-123, DOI: 10.1080/01496395.2011.610397

To link to this article: http://dx.doi.org/10.1080/01496395.2011.610397

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Biosorption of Direct Red 28 (Congo Red) from AqueousSolutions by Eggshells: Batch and Column Studies

Papita Das Saha, Shamik Chowdhury, Madhurima Mondal, and Keka SinhaDepartment of Biotechnology, National Institute of Technology-Durgapur, Durgapur (WB), India

The feasibility of using eggshells as a low-cost biosorbent for theremoval of Direct Red 28 (DR 28) from aqueous solutions was stud-ied in batch and dynamic flow modes of operation. The effect of bio-sorption process variables such as particle size, solution pH, initialdye concentration, contact time, temperature, feed flow rate, andbed height were investigated. Both the Langmuir and Freundlich iso-therm models exhibited excellent fit to the equilibrium biosorptiondata. Optimum pH (6.0), particle size (<250lm), initial dye con-centration (50mg g�1), temperature (313K), and contact time(240min) gave maximum monolayer biosorption capacity of69.45mg g�1 which was higher than those of many sorbent materi-als. Pseudo-second-order kinetic model depicted the biosorptionkinetics accurately. Thermodynamic study confirmed the spon-taneous and endothermic nature of the biosorption process. Break-through time increased with increase in the bed height butdecreased with increase in flow rate. Overall, batch and continuousmode data suggest the applicability of eggshells as an environmentfriendly and efficient biosorbent for removal of DR 28 from aqueousmedia.

Keywords biosorption; breakthrough; Direct Red 28; eggshells;equilibrium; kinetics

INTRODUCTION

In recent years, the discharge of dye contaminated was-tewaters from textiles, tanneries, pharmaceuticals, packedfood industries, pulp and paper, paint, plastics, petroleum,electroplating, and cosmetics industries into aquaticenvironment without adequate treatment reflects a seriousenvironmental problem because of their negativeeco-toxicological effects and bioaccumulation in wildlife(1,2). It is estimated that approximately 700,000 tonnesand 10,000 different types of dyes and pigments are beingproduced annually and a significant proportion of thesedyes enter the environment in wastewater (3). Owing totheir complex molecular structure, dyes are usually verydifficult to be biodegraded, making them hardly eliminated

under natural aquatic environment (4) Therefore, removalof dyes before disposal of the wastewater is extremelyimportant.

Currently, biosorption, defined as the the removal ofmaterials (organic compounds, metal ions, dye molecules,etc.) by inactive, non-living biomass (materials of biologi-cal origin) is considered as an alternative eco-friendly tech-nology to the existing costly water treatment technologiesdue to its low initial cost, simplicity of design, ease of oper-ation, insensitivity to toxic substances, and completeremoval of pollutants even from dilute solutions (5–8).Various low-cost sorbents derived from agricultural wasteor natural materials, have been investigated intensivelyfor removal of dyes from their aqueous solutions (9).

Direct Red 28 (DR 28) is a benzidine-based anionic,diazo dye, prepared by coupling tetrazotized benzidinewith two molecules of napthionic acid (10). It is widelyused in textiles, paper, rubber, and plastic industries (4).It is also used as a laboratory aid in testing for free hydro-chloric acid in gastric contents, in the diagnosis of amyloi-dosis, as an indicator of pH, and also as a histological stainfor amyloid (11). However, the dye has been known tocause an allergic reaction and to be metabolized to benzi-dine, a human carcinogen (12). It is investigated as a muta-gen and reproductive effector. It is a skin, eye, andgastrointestinal irritant and may also affect blood factorssuch as clotting, and induce somnolence and respiratoryproblems (11). Recently, many researchers have investi-gated the use of low-cost materials for DR 28 removalfrom aqueous solutions. Materials such as montmorillonite(13), bentonite (14), rice hull ash (15), flyash (16), activatedred mud (17), rice husk (18), coir pith (19), neem leafpowder (20), orange peel (21), jute stick powder (22), andcattail root (4) have been tested and used for the removalof DR 28 from aqueous solution. However, low sorptioncapacities of these inexpensive sorbents towards DR 28limit their applications in practical field. Hence attemptshave been made in this study to develop a new, inexpensivebiosorbent for removal of DR 28 from aqueous solutionsusing eggshells, a significant solid waste from food proces-sing and manufacturing plants. Traditionally, most of the

Received 23 March 2011; accepted 1 August 2011.Address correspondence to Shamik Chowdhury, Department

of Biotechnology, National Institute of Technology-Durgapur,Mahatma Gandhi Avenue, Durgapur (WB)-713209, India.Tel.: þ919831387640; Fax: þ913432547375. E-mail:chowdhuryshamik@gmail.com

Separation Science and Technology, 47: 112–123, 2012

Copyright # Taylor & Francis Group, LLC

ISSN: 0149-6395 print=1520-5754 online

DOI: 10.1080/01496395.2011.610397

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eggshell waste is disposed in landfills without any pretreat-ment because it has no practical utility (23,24). However,such waste management is not a desirable practice in viewof the environmental odor from biodegradation (23,24).Eggshell has a little developed porosity and pure CaCO3

as an important constituent (25). Due to its intrinsic porestructure in the calcified eggshell, high content of CaCO3

and the availability in abundance (23–25), utilization ofthis waste material as a biosorbent for the treatment ofdye bearing effluents will convert the waste into a usefulmaterial. Therefore, the aim of this work was to evaluatethe feasibility of employing eggshells as low-cost alterna-tive biosorbent for DR 28 removal from aqueous solution.To this end, batch biosorption studies were performed toinvestigate the effect of operational parameters includingparticle size, initial solution pH, contact time, initial dyeconcentration, and temperature. Biosorption kinetics, iso-therms and thermodynamic parameters were evaluatedand reported. Biosorption potential of eggshell in dynamicflow conditions was also investigated and reported.

MATERIALS AND METHODS

Biosorbent Collection and Preparation

Hen eggshells were used in the present investigationand were collected from the institute canteen of NationalInstitute of Technology, Durgapur, India. The shells werefirst thoroughly washed with double-distilled water toremove adhering dirt and any unwanted particles and driedin an oven at 353� 1K for 24 h. The raw biosorbent wasthen crushed and grounded using ball mill and sieved intothe following particle size ranges: >500 mm, 250–500 mmand <250 mm. Finally, it was stored in sterile, air tight glassbottles and used as biosorbent without any pretreatmentfor DR 28 biosorption.

Preparation of Adsorbate Solutions

Direct Red 28 (DR 28) used in this study was of com-mercial quality (CI 22120, MF: C32H22N6Na2O6S2, FW:696.7, kmax: 570 nm) and was used without further purifi-cation. Its molecular structure is shown in Fig. 1. Dye stocksolution (500mgL�1) was prepared by dissolving anaccurately weighed quantity of the dye in double distilledwater. Experimental dye solution of different concentrations

was prepared by diluting the stock solution with suitablevolume of double distilled water. The initial solution pHwas adjusted using 0.1M HCl and 0.1M NaOH solutions.

Biosorbent Characterization

Specific surface area (Ssp, m2 g�1) of the biosorbent par-

ticles were determined by Brunauer, Emmett, Teller (BET)method using a surface area analyzer (QuantachromeNOVA 2200C USA). Fourier Transform Infrared (FTIR)analysis of the biosorbent before and after DR 28 biosorp-tion was carried out to identify the chemical functionalgroups responsible for sorption of dye ions. The FTIRspectra were recorded by Perkin–Elmer Spectrum BX-IIspectrophotometer in the wavenumber range 4000–500 cm�1 at 4 cm�1 spectral resolution. Surface structureof the biosorbent before and after dye biosorption wasexamined using a Hitachi S-3000N scanning electronmicroscope at an electron acceleration voltage of 15 kV.

Batch Biosorption Studies

Batch biosorption experiments were conducted in orderto evaluate the effects of particle size (>500 mm,250–500 mm and <250 mm), solution pH (4.0–10.0), initialdye concentration (10, �100mgL�1), contact time (10–360min), and temperature (293–313K). The batch testswere carried out in 250mL glass-stopperred, Erlenmeyerflasks with 100mL of working volume, with a dye concen-tration of 50mgL�1. A weighed amount (0.3 g) of biosor-bent was added to the solution. The flasks were agitatedat a constant speed of 150 rpm for 4 h in an incubator sha-ker (Model Innova 42, New Brunswick Scientific, Canada)at a constant temperature. Samples were collected from theflasks at predetermined time intervals for analyzing theresidual dye concentration in the solution. The residualamount of dye in each flask was investigated using UV=VIS spectrophotometer (Model Hitachi–2800).

The amount of dye adsorbed per unit biosorbent (mgdye per g biosorbent) was calculated according to a massbalance on the dye concentration using Eq. (1):

qe ¼ðC0 � CeÞV

mð1Þ

where C0 is the initial dye concentration (mgL�1), Ce is theequilibrium dye concentration in solution (mgL�1), V isthe volume of the solution (L), and m is the mass of the bio-sorbent in g.

The percent removal (%) of dyes was calculated usingthe following equation:

Removal ð%Þ ¼ C0 � Ce

C0� 100 ð2Þ

FIG. 1. Molecular structure of Direct Red 28 (DR 28).

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Continuous Biosorption Studies

Continuous flow biosorption experiments were conduc-ted in a glass column (3 cm internal diameter and 50 cmheight) at room temperature. A known quantity of thebiosorbent was packed into the glass column. A poroussheet was attached at the bottom of the column in orderto support the biosorbent bed and to ensure uniform inletflow and a good liquid distribution into the column. Thetop of the bed was covered by a layer glass beads (1mmin diameter) in order to avoid the loss of biosorbent andalso to ensure a closely packed arrangement. Dye sol-ution of known concentration (50mgL�1) at an initialpH of 6.0 was pumped to the column in a down-flowdirection by a peristaltic pump (PP-EX204C, Miclins,India). The effect of feed flow rate (v) (5–15mLmin�1)and bed height (h) (2–10 cm) were studied. Dye solutionat the outlet of the column was collected at regular timeintervals and the concentration of DR 28 in the effluentwas analyzed using UV=VIS spectrophotometer (ModelHitachi–2800). Operation of the column was stoppedwhen the effluent DR 28 concentration exceeded 99.5%of its initial concentration.

Statistical Analysis

In order to ensure the reproducibility of results, all thebiosorption experiments were performed in triplicate andthe results are presented as means of the replicates alongwith standard deviation (represented as error bars). Micro-soft Excel 2007 program was employed for data processing.Linear regression analyses were used to determine slopesand intercepts of the linear plots and for statistical analysesof the data.

THEORY

Biosorption Isotherms

The Langmuir, Freundlich, and Dubinin–Radushkevich(D-R) isotherms were used to describe the equilibriumbiosorption data.

Langmuir Isotherm

The Langmuir adsorption isotherm describes that theuptake occurs on a homogeneous surface by monolayeradsorption without interaction between adsorbed mole-cules and is commonly expressed as (26,27):

Ce

qe¼ Ce

qmþ 1

KL qmð3Þ

where qe (mg g�1) and Ce (mgL�1) are the solid phase con-centration and the liquid phase concentration of adsorbateat equilibrium respectively, qm (mg g�1) is the maximumbiosorption capacity, and KL (Lmg�1) is the biosorption

equilibrium constant. The constants KL and qm can bedetermined from the slope and intercept of the plotbetween Ce=qe and Ce.

Freundlich Isotherm

The Freundlich isotherm is applicable to non-ideal bio-sorption on heterogeneous surfaces and the linear form ofthe isotherm can be represented as (26,28):

log qe ¼ log KF þ 1

n

� �log Ce ð4Þ

where qe is the equilibrium dye concentration on biosor-bent (mg g�1), Ce is the equilibrium dye concentration insolution (mgL�1), KF (mg g�1) (L g�1)1=n is the Freundlichconstant related to biosorption capacity, and n is theheterogeneity factor. KF and 1=n are calculated from theintercept and slope of the straight line of the plot log qeversus log Ce.

Dubinin-Radushkevich (D-R) Isotherm

The Dubinin-Radushkevich (D-R) isotherm modelenvisages about the heterogeneity of the surface energiesand has the following formulation (26,29):

ln qe ¼ ln qm � be2 ð5Þ

e ¼ RT ln 1þ 1

Ce

� �ð6Þ

where qm is the maximum biosorption capacity, b is a coef-ficient related to the mean free energy of biosorption(mmol2 J�2), e is the Polanyi potential (Jmmol�1), R isthe gas constant (8.314 Jmol�1K�1), T is the temperature(K), and Ce is the adsorbate equilibrium concentration(mgL�1). The D-R constants qm and b can be determinedfrom the intercept and slope of the plot between lnqe ande2. The constant b gives an idea about the mean free energyE (kJmol�1) of biosorption per mole of the adsorbatewhen it is transferred to the surface of the solid frominfinity in the solution and can be calculated using therelationship (29):

E ¼ 1ffiffiffiffiffiffi2b

p ð7Þ

If the magnitude of E is between 8 and 16 kJmol�1, the bio-sorption process is supposed to proceed via chemisorption,while for values of E< 8 kJmol�1, the biosorption processis of physical nature (29).

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Biosorption Kinetics

The data obtained from the batch biosorption experi-ments at different temperatures were used to evaluate thekinetics of the biosorption process. Three kinetic modelswere tested to obtain the rate constants and elucidate thecontrolling mechanism of the biosorption process, namely,pseudo-first-order, pseudo-second-order, and intraparticlediffusion models.

Pseudo-First-Order Kinetic Model

The linear form of Lagergren’s pseudo-first-orderequation is given as (30,31):

logðqe � qtÞ ¼ log qe �k1

2:303t ð8Þ

where qt and qe are the amount of dye adsorbed at time tand at equilibrium (mg g�1) and k1 (min�1) is the rate con-stant of the equation calculated from the slope of the linearplot of log(qe� qt) versus t.

Pseudo-Second-Order Kinetic Model

The pseudo-second-order kinetic model of Ho andMcKay (30,32) can be represented in the following form:

t

qt¼ 1

k2q2eþ 1

qet ð9Þ

where k2 (gmg�1min�1) is the biosorption rate constant.A plot of t=qt versus t yields a straight line with a slopeof 1=qe. The value of k2 is determined from the interceptof the plot.

Intraparticle Diffusion Model

The intraparticle diffusion equation is given as follows(30,33):

qt ¼ kit0:5 þ C ð10Þ

where ki is the intraparticle diffusion rate constant (mg g�1

min�0.5) and C (mg g�1) is a constant that gives the infor-mation regarding the thickness of the boundary layer.

Activation Energy and Thermodynamic Parameters

The activation energy Ea for DR 28 biosorption ontoeggshells was calculated by the Arrhenius equation (34):

ln k ¼ lnA� Ea

RTð11Þ

where k is the rate constant, A is the Arrhenius constant, Ea

is the activation energy (kJmol�1), R is the gas constant(8.314 Jmol�1K�1) and T is the temperature (K). Ea canbe determined from the slope of a plot of lnk versus 1=T.

Thermodynamic behavior of biosorption of DR 28 oneggshells was evaluated by the thermodynamic parameters– Gibbs free energy change (DG0), enthalpy (DH0) andentropy (DS0). These parameters were calculated usingthe following equations (34,35):

DG0 ¼ �RT lnKC ð12Þ

KC ¼ Ca

Ceð13Þ

DG0 ¼ DH0 � TDS0 ð14Þ

where KC is the equilibrium constant, Ca is the equilibriumdye concentration on the biosorbent (mgL�1) and Ce is theequilibrium dye concentration in solution (mgL�1). A plotof DG0 versus temperature, T will be linear with the slopeand intercept giving the values of DH0 and DS0.

RESULTS AND DISCUSSION

Biosorbent Characterization

The specific surface area of the biosorbent particles (Ssp,m2 g�1) obtained by BET measurements) and their sizes arelisted in Table 1. The Ssp values increased as the particlesize decreased. However, the relatively large surface areaof the biosorbent particles indicates that there was a goodpossibility for the dye molecules to be trapped and interactwith the ionizable functional groups (i.e., carboxyl,hydroxyl, and amino) onto the surface of the biosorbent.

The FTIR spectra of the biosorbent before and after dyebiosorption are shown in Fig. 2(a)–(b). It appears fromFig. 2(a) (before biosorption) that strong band in theregion of 3000–3500 cm�1 reflect O–H stretching vibration,showing the presence of hydroxyl group on the surface ofthe biosorbent. The peak at 2919 cm�1 indicates the –CHstretching vibration (32). The peak around 1730 cm�1

represents the C=O stretching vibration of the carboxylgroups. The peak around 1417 cm�1 can be stronglyassociated with the presence of carbonate minerals withinthe eggshell matrix (24). The two observable peaks atabout 712 cm�1 and 875 cm�1, can be attributed to thein-plane deformation and out-plane deformation modes,

TABLE 1BET specific surface areas for the different biosorbent

particle size ranges

Size range (mm)BET specific surfacearea (Ssp) (m

2 g�1)

<250 10.82250–500 9.27>500 8.03

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respectively, in the presence of calcium carbonate (24).After DR 28 biosorption (Fig. 2b), the characteristichydroxyl band at 3270 cm�1 shifts to 3290 cm�1. Inaddition, the carbonate peak at 1417 cm�1 shifts to thelower frequency (1405 cm�1) after dye uptake. Also, thecarbonate peaks at 875 cm�1 and 712 cm�1 shifts to880 cm�1 and 690 cm�1, respectively. This shift in peaksafter biosorption of DR 28 indicates a chemical interactionbetween DR 28 molecules and the hydroxyl and carbonategroups onto the eggshell surface.

The SEM micrographs of the biosorbent material beforeand after dye biosorption are illustrated in Fig. 3(a–b).These micrographs clearly indicate the appearance of themolecular cloud over the surface of dye-loaded biosorbent,which was absent on the rough and porous structure of thebiosorbent before loading with dye.

Effect of Particle Size

In the first stage of the biosorption studies, the effect ofparticle size on DR 28 biosorption by eggshells was inves-tigated. Results are shown in Fig. 4. The results illustratedin Fig. 4 indicate that the amount adsorbed at equilibriumincreased by decreasing the particle size. As biosorption is asurface phenomenon, this can be attributed to the relation-ship between the effective specific surface area of thebiosorbent particles (Ssp, m

2 g�1) obtained by BET mea-surements) and their sizes (Table 1). The Ssp valuesincreased as the particle size decreased and, as a conse-quence, the saturation capacity per unit mass of biosorbentincreased. The large external surface area for small parti-cles removes more dye than the large particles (36,37). Asimilar observation has been reported by Mittal et al. forbiosorption of DR 28 onto bottom ash and deoiled soya(11). A size fraction of <250 mm was selected for all furtherstudies.

Effect of pH

The initial pH of the dye solution is an important moni-toring parameter in dye biosorption, influencing the surfacecharge of the biosorbent, the degree of ionization of the dyepresent in the solution and the dissociation of functional

FIG. 2. FTIR spectra of eggshell (a) before DR 28 biosorption (b) after

DR 28 biosorption.

FIG. 3. SEM micrograph of eggshell (a) before DR 28 biosorption (b)

after DR 28 biosorption.

FIG. 4. Effect of particle size on biosorption of DR 28 by eggshells

(experimental conditions: initial dye concentration: 50mgL�1, biosorbent

dose: 0.3 g=0.1L, agitation speed: 150 rpm, temperature: 303K, contact

time: 5 h).

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groups on the active sites of the biosorbent. DR 28 is anexample of diazo dye, and the initial solution pH influencesits molecular structure in aqueous systems (38). Therefore,in the present investigation, the effect of pH on the removalefficiency of DR 28 was studied at different pH rangingfrom 4.0 to 10.0. Figure 5 presents the effect of pH onDR 28 biosorption on eggshells. Evidently, pH signifi-cantly affects the extent of biosorption of DR 28. As seenin Figure 5, with increase in pH of the solution the percent-age removal of dye increases till pH 6.0 but with furtherincrease in pH, the percentage removal of dye drops signifi-cantly. Since maximum removal is obtained at pH 6.0, allfurther studies were carried out at pH 6.0. DR 28 is ananionic dye, which exists in aqueous solution in the formof negatively charged ions. As a charged species, the degreeof its biosorption onto the biosorbent surface is primarilyinfluenced by the surface charge on the biosorbent, whichin turn is influenced by the solution pH. At low pH values,protonation of the functional groups present on the biosor-bent surface easily takes place. The surface of the biosor-bent becomes positively charged, and this increases thebiosorption of the negatively charged dye ions throughelectrostatic forces of attraction. As the pH of the dye sol-ution increases, a proportional decrease in biosorptiontakes place due to the successive deprotonation of positivecharged groups on the biosorbent and electrostatic repul-sion between negatively charged sites on the biosorbentand dye anions. Similar result was reported for biosorptionof DR 28 on neem leaf powder (20).

Effect of Initial Dye Concentration and Contact Time

The biosorption process is greatly influenced by theinitial concentration of the adsorbate, as the adsorptive

reactions are directly proportional to the concentration ofthe adsorbate. Therefore, the effect of different initial dyeconcentration on biosorption of DR 28 onto eggshellswas investigated. As evident from Fig. 6, with increase ininitial dye concentration the percentage removal of dyedecreased and showed little decrease at higher concentra-tions. Thus it can be explained that all biosorbents havea limited number of active sites, which become saturatedat a certain concentration (40). It was also interesting tonote that the biosorption capacity at equilibrium increasedwith increase in initial dye concentration (not shown inFig. 6). This is a result of the increase in the driving forcefrom the concentration gradient (41). In addition, if theconcentration of DR 28 in solution is higher, the activesites of eggshell are surrounded by much more DR 28 ions,and the adsorption phenomenon occurs more efficiently.So the value of qe increases with the increase of initialDR 28 concentrations (42). The increase of the loadingcapacity of eggshell with increasing initial DR 28 concen-tration may also be due to higher interaction betweenDR 28 and adsorbent. A similar trend was observed forthe biosorption of DR 28 by clay materials (10).

In order to establish the equilibrium time for maximumuptake and to know the kinetics of the biosorption process,DR 28 biosorption on eggshells was also investigated as afunction of contact time. DR 28 showed a fast rate of bio-sorption during the first 60min of the dye-biosorbent con-tact. The fast biosorption rate may be explained by anincreased availability in the number of active binding siteson the adsorbent surface. At higher contact time the rate ofbiosorption decreases, gradually leading to equilibrium.This decline is due to the saturation of the binding sitesand less availability of biosorption sites. The equilibriumwas achieved within 240min. After this equilibrium period,

FIG. 5. Effect of pH on biosorption of DR 28 by eggshells (experimental

conditions: initial dye concentration: 50mgL�1, biosorbent dose: 0.3 g=

0.1L, agitation speed: 150 rpm, temperature: 303K, contact time: 5 h).

FIG. 6. Effect of initial dye concentration on biosorption of DR 28 by

eggshells (experimental conditions: biosorbent dose: 0.3 g=0.1L, agitation

speed: 150 rpm, pH: 6.0, temperature: 303K, contact time: 5 h).

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the amount of dye adsorbed did not show time-dependentchange. Similar results have been reported in literature forbiosorption of DR 28 on other biosorbents (4,38).

Effect of Temperature

Temperature is an important design parameter affectingthe biosorption process. Therefore, batch biosorptionexperiments were carried out at different temperatures ran-ging from 293 to 313K. Data obtained from the experi-ments are presented in Fig. 7. As shown in Fig. 7, thedye removal increased with rise in temperature from 293to 313K at the same dye concentration. The increase inpercentage removal of dye at increasing temperatures couldbe explained by the increased affinity of binding sites fordye molecules. An increase in temperature also results inan increase in mobility of the dye molecules and a decreasein the retarding forces acting on the molecules. These resultin the enhancement in the dye binding capacity of the bio-sorbent. The observed trend in increased dye removalcapacity with increasing temperature suggests that biosorp-tion of DR 28 by eggshells is kinetically controlled by an

endothermic process. A similar trend was reported for bio-sorption of DR 28 onto CaCl2 modified bentonite (38).

Biosorption Isotherms

In the present investigation, the isotherm study of DR28 dye was conducted at different temperatures. TheFreundlich, Langmuir, and Dubinin-Radushkevich (D-R)isotherm models were used to describe the equilibrium bio-sorption data. The parameters and correlation coefficientsobtained from the plots of Langmuir (Ce=qe versus Ce),Freundlich (logqe versus logCe) and D-R (lnqe versus e2)(figures not shown) are listed in Table 2. Based on the cor-relation coefficients, the applicability of the isotherms wascompared (Table 2). The experimental results indicate thatbiosorption of DR 28 onto eggshells follows both Freun-dlich and Langmuir models. From Table 2, the monolayerbiosorption capacity qm increases with increase in tempera-ture. The Langmuir constant, KL, also increases withincrease in temperature. Seen overall, the information thusobtained specifies an endothermic nature of the existingprocess. In addition, the magnitude of the Freundlich con-stant n gives a measure of favorability of biosorption. Thevalues of n between 1 and 10 (i.e., 1=n less than 1) repre-sents a favorable biosorption process. For the presentstudy the value of n also presented the same trend repre-senting a beneficial biosorption process.

In order to determine the type of biosorption, the equi-librium data were also tested with the D–R isothermmodel. The estimated values of E for the present study werefound to be >8 kJmol�1 at all temperatures (Table 2)which implies that biosorption of DR 28 on eggshells ischemical ion exchange (29).

Biosorption Kinetics

For evaluating the biosorption kinetics of DR 28 mole-cules, the pseudo-first-order and pseudo-second-order kin-etic models were used to fit the experimental data. Thevalues of pseudo-first-order rate constants, k1 and qe, werecalculated from slopes and intercepts of the plots oflog(qe� qt) versus t (figure not shown). The k1 values, thecorrelation coefficients R2, and theoretical and experi-mental equilibrium biosorption capacity qe are given in

FIG. 7. Effect of temperature on biosorption of DR 28 by eggshells

(experimental conditions: initial dye concentration: 50mgL�1, biosorbent

dose: 0.3 g=0.1L, agitation speed: 150 rpm, pH: 6.0, contact time: 5 h).

TABLE 2Isotherm constants for biosorption of DR 28 onto eggshells at different temperatures

T (K)

Langmuir isotherm parameters Freundlich isotherm parameters D-R isotherm parameters

qm(mg g�1)

KL

(Lmg�1) R2KF (mg g�1)(Lmg�1)1=n n R2

qm(mg g�1)

b(mmol2 J�2)

E(kJmol�1) R2

293 64.274 0.817 1.000 25.157 4.197 0.999 56.218 3.98� 10�9 11.208 0.937303 67.819 1.429 1.000 28.346 4.540 0.999 58.937 3.59� 10�9 11.801 0.935313 69.457 1.986 0.999 30.406 4.882 0.999 60.518 3.24� 10�9 12.422 0.928

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Table 3. The R2 values in Table 3 suggests that biosorptionof DR 28 onto eggshells does not follow pseudo-first-orderkinetics. In addition, the theoretical and experimental equi-librium biosorption capacities, qe obtained from these plotsvaried widely, confirming that the pseudo-first-order modelwas not appropriate for describing the biosorption kineticsof DR 28 onto eggshells. On the contrary, the kinetic datashowed excellent fit to the pseudo-second-order equation atall temperatures studied. The plot of t=qt against t at differ-ent temperatures is shown in Fig. 8. The pseudo-second-order rate constant k2, the calculated qe values and the cor-responding linear regression correlation coefficients valuesR2 are given in Table 3. From Table 3, it is evident that thecalculated qe values agree with experimental qe values well,and also, the correlation coefficients for the pseudo-second-order kinetic plots at all the studied temperatures arehigher (R2> 0.99). It can thus be easily concluded thatthe ongoing reaction proceeds via a pseudo-second-ordermechanism rather than a pseudo-first-order mechanism.A similar phenomenon was also observed in the biosorp-tion of DR 28 on deoiled soya (11).

The biosorption data were further analyzed using theintraparticle diffusion model (Eq. 10). According to

Eq. (10), if a plot of qt versus t0.5 is linear and passesthrough the origin, then intraparticle diffusion was the solerate-limiting step. In the present study, the plots of qt ver-sus t0.5 were linear at all temperatures, but the plots did notpass through the origin (Fig. 9), suggesting that the bio-sorption involved intraparticle diffusion, but that was notthe only rate-controlling step. The values of C are usefulin determining the boundary thickness: a larger C valuecorresponded to a greater boundary layer diffusion effect.The C values (7.265–2.176mg g�1) decreased with theincreasing temperature (293–313K) indicating higher tem-peratures abated the boundary layer diffusion effect. The kivalues calculated from Fig. 9 were 5.18, 5.22, and5.38mg g�1 min�0.5 at 293, 303, and 313K, respectively.ki increased with increasing temperature suggesting theendothermic nature of the existing biosorption process.

Activation Energy and Thermodynamic Parameters

From the pseudo-second-order rate constant k2(Table 3), the activation energy Ea for biosorption of DR28 on eggshells was determined using the Arrhenius equa-tion (Eq. 11). By plotting lnk2 versus 1=T, Ea was obtainedfrom the slope of the linear plot (figure not shown). The

TABLE 3Kinetic parameters for biosorption of DR 28 onto eggshells

T (K) qexp (mg g�1)

Pseudo-first-order kinetic model Pseudo-second-order kinetic model

qcal (mg g�1) k1 (min�1) R2 qcal (mg g�1) k2 (gmg�1min�1) R2

293 62.861 47.016 0.018 0.954 63.209 0.003 0.999303 65.537 49.497 0.026 0.950 65.915 0.007 0.999313 67.264 52.936 0.038 0.958 67.684 0.012 0.999

FIG. 8. Pseudo-second-order kinetic plots for biosorption of DR 28

onto eggshells at different temperatures.

FIG. 9. Weber and Morris plots for biosorption of DR 28 onto eggshells

at different temperatures.

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value of Ea for DR 28 biosorption on eggshells was 51.85kJmol�1. The magnitude of Ea provides information onthe nature of the biosorption process, that is, whether itis physical or chemical, with values of Ea less than 40 kJmol�1 corresponds to physisorption and higher values rep-resent the chemical reaction process (29). In the presentstudy, the value of the activation energy suggests thatDR 28 biosorption onto eggshells is chemisorption.

The Gibb’s free energy (DG0) for biosorption of DR 28onto eggshells at all temperatures was obtained from Eq.(12) and are listed in Table 4. The values of DH0 andDS0 were determined from the slope and intercept of theplot of DG0 versus T (Fig. 10) and are also listed inTable 4. Negative values of DG0 indicate the thermodyna-mically feasible and spontaneous nature of the dye biosorp-tion process (43,44). The increase in DG0 values withincrease in temperature shows an increase in feasibility ofbiosorption at higher temperatures. The positive value ofDH0 implies that the biosorption phenomenon is endo-thermic. A positive value of DS0 reflects the affinity ofthe biosorbent towards the adsorbate species. In addition,the positive value of DS0 suggests increased randomnessat the solid=solution interface (45).

Column Studies

In order to evaluate the effectiveness of eggshells forcontinuous mode dye biosorption, column experimentswere performed. The effects of feed flow rate and bedheight were studied.

Effect of Feed Flow Rate

Flow rate is an important parameter in evaluating theperformance of a biosorption process, particularly for con-tinuous treatment of wastewater on industrial scale. There-fore, the effect of flow rate on DR 28 biosorption byeggshells was investigated by varying the flow rate from 5to 15mLmin�1 and keeping the initial dye concentration(C0¼ 50mgL�1) and bed height (h¼ 2 cm) constant. Theeffect of flow rate on breakthrough performance at theabove operating conditions is shown in Fig. 11(a). It canbe seen from the figure that the biosorption efficiencywas higher at lower flow rate. This can be explained bythe fact that at lower flow rate, the residence time of thedye ions is more and hence they get more time to capture

TABLE 4Activation energy and thermodynamic parameters for

biosorption of DR 28 onto eggshells

Ea

(kJmol�1)DG

(kJmol�1)DH

(kJmol�1)

DS(Jmol�1

K�1)

51.85 293K 303K 313K 68.10 258.25�7.61 �10.56 �12.79

FIG. 11. (a) Effect of flow rate on breakthrough curve for biosorption of

DR 28 onto eggshells (h¼ 2 cm; C0¼ 50mgL�1) (b) Effect of bed height

on breakthrough curve for biosorption of DR 28 onto eggshells

(v¼ 5.0mL min�1; C0¼ 50mgL�1).

FIG. 10. Plot of Gibb’s free energy change versus temperature for DR 28

biosorption onto eggshells.

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the available reaction sites of the biosorbent (46). The dyeions also have more time to diffuse into the pores of thebiosorbent through intra-particle diffusion. As the flowrate increases, the residence time of the dye solution inthe column decreases. The contact time of the dye ions withthe biosorbent is very short and hence they do not haveenough time to capture the reaction sites on the biosorbentsurface or diffuse into the pores of the biosorbent, leavingthe column before equilibrium occurs (47).

Effect of Bed Height

In order to find out the effect of bed height on the break-through curve, the dye solution having influent DR 28 con-centration 50mgL�1 was passed through the column at aflow rate of 5.0mLmin�1 by varying the bed height.Figure 11(b) represents the performance of breakthroughcurves at bed heights of 2 cm, 6 cm, and 10 cm. FromFigure 11(b), the breakthrough time increased with increas-ing bed depth from 2 to 10 cm. As the bed height increases,the dye molecules had more time to contact with the bio-sorbent that resulted in higher removal efficiency of DR28 in the column (47). So a higher bed column results ina decrease in the solute concentration in the effluent atthe same time (47).

Comparison of Eggshell with Other Sorbents

Table 5 summarizes the comparison of the maximumDR 28 biosorption capacities of various sorbents includingeggshell. A direct comparison is difficult due to the varying

experimental conditions employed in those studies. How-ever, from Table 5 it can be concluded that eggshell hashigher biosorption capacity of DR 28 than many of theother reported biosorbents.

CONCLUSION

In this study, eggshell was tested and evaluated as apossible biosorbent for removal of DR 28, a hazardousanionic dye from its aqueous solution using a batch experi-mental set-up. The biosorption studies were carried out asa function of particle size, solution pH, contact time, initialdye concentration, and temperature. The following conclu-sions are made based on the results of present study:

� The biosorption was highly dependent on initialdye concentration, reaction temperature and sol-ution pH. Percentage removal of the dye moleculedecreased with increase in the initial dyeconcentration.

� DR 28 biosorption onto eggshells followed boththe Freundlich and Langmuir isotherm models.The maximum monolayer adsorption capacitywas 69.45mg g�1 at pH¼ 5.0, initial dye con-centration¼ 50mgL�1, temperature¼ 313K, andcontact time¼ 240min. According to theDubinin-Radushkevich (D-R) isotherm model,biosorption of DR 28 onto eggshells was chemi-sorption.

� The biosorption kinetics study followed thepseudo-second-order kinetic model.

� Intra-particle diffusion was not the solerate-controlling factor.

� Thermodynamic parameters such as change inGibbs free energy (DG0), enthalpy (DH0), andentropy (DS0) were estimated. Thermodynamicanalysis revealed that the removal of DR 28 fromaqueous solution by eggshells was a spontaneousand endothermic process.

� The breakthrough curves were significantly affec-ted by both flow rate and bed height. The break-through time increased with the decrease of theflow rate. The same effect was observed whenthe bed depth was increased.

Taking into consideration all the above obtained results, itcan be concluded that eggshell is a promising biosorbentfor DR 28 and it can be suggested for the removal ofDR 28 from industrial wastewater.

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TABLE 5Comparison of DR 28 biosorption capacity of eggshells

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