The effect of dye chemical structure on adsorption on activated carbon: a comparative study

The effect of dye chemical structureon adsorption on activated carbon:a comparative studyHossam Altaher,a,b,* Tarek E Khalilc and Reda Abubeahd

aDepartment of Chemical Engineering Technology, Yanbu Industrial College, PO Box 30436,Yanbu Industrial City, Saudi ArabiaE-mail: [email protected] Engineering Department, British University in Egypt, Cairo, EgyptcChemistry Department, Faculty of Science, Alexandria University, Ibrahimia, Alexandria,EgyptdDepartment of Chemical Engineering, Faculty of Engineering, El-Minia University, Minia,Egypt

Received: 17 November 2012; Accepted: 4 November 2013

The effect of the molecular structure of dyes Eriochrome Black T and Bromophenol Blue on their adsorptionon the surface of activated carbon manufactured from locally available biosorbent has been studied. Batchexperiments were performed to investigate factors that may affect the adsorption process. The effect ofstirring rate was investigated in the range 0–240 rpm, with an initial concentration of 4–100 mg/l and a stirringtime of 0–400 min. The mechanism and rate of adsorption were investigated for both dyes using pseudo-first-order, pseudo-second-order, intraparticle diffusion, and liquid film diffusion models. The monolayeradsorption capacities for Eriochrome Black T and Bromophenol Blue were found to be 36.5 and 39.68 mg/grespectively. The difference in dye uptake was attributed to the presence of the electron-withdrawing brominegroup in Bromophenol Blue. Results showed that the Langmuir isotherm best fitted the adsorption of the twodyes on the prepared activated carbon. The pseudo-second-order model best fitted the experimental data,and liquid film diffusion and intraparticle diffusion were the controlling adsorption mechanisms.

ColorationTechnology

Society of Dyers and Colourists

IntroductionAmong the different pollutants of aquatic ecosystems, dyesare a large and important group of industrial chemicals forwhich world production is nearly 1 million metric tons ofdye per year [1]. Synthetic dyes originate from a number ofindustries, such as the textile industry, metal plating,packaging, the paper industry [2], the leather industry,cosmetics, the food industry [3], the pharmaceutical indus-try [4], tannery [5], Kraft bleaching [6], hair colouring, andrubber and plastics [7,8]. Among these various sectors, thetextile industry ranks first in terms of usage of dyes forcoloration of fibre [9,10].

Dyes may be classified into many groups. They may beclassified according to their origin into natural or syntheticdyes, according to their chemical structure into 30 sub-groups, and according to usage into 19 subgroups. The mainthree categories are: (1) anionic – direct, acid, and reactivedyes; (2) cationic – all basic dyes; (3) nonionic – dispersedyes. The chemical classes of dyes employed more fre-quently on an industrial scale are azo, anthraquinone,sulphur, indigoid, triphenylmethyl, and phthalocyaninederivatives. Among these, heterocyclic monoazo dispersedyes constitute an important chemical class of textilecolorants and are used in the coloration of hydrophobicsubstrates, with a large share of all the disperse dyes beingused for the coloration of polyester and its blends. A fairlylarge amount of disperse dyes (10–15%) is released in theeffluent during wet processing, and the dyes, being hydro-phobic, resist degradation in an aqueous medium.

Dye contamination in wastewater causes problems inseveral ways. The presence of dyes in water, even in very

low quantities, is highly visible and undesirable. Colourinterferes with penetration of sunlight into waters, retardsphotosynthesis, inhibits the growth of aquatic biota, andinterferes with gas solubility in water bodies [11–14]. Somestudies reported that direct discharge of dyes containingeffluent into a municipal environment may cause allergy,dermatitis, skin irritation, cancer [8], the formation of toxiccarcinogenic breakdown products, and mutagenic andteratogenic effects on aquatic life as well as on humans[3,15]. These problems are heightened by the fact that,formerly, many dyes were made of known carcinogens suchas benzidine, which may be formed as a result of metab-olism [16]. The highest rates of toxicity were found amongbasic and diazo direct dyes [4]. Dye effluent also increasesthe biological oxygen demand (BOD) and causes a lack ofdissolved oxygen to sustain aquatic life.

Large-scale production accompanied with extensiveapplication of synthetic dyes can cause considerable envi-ronmental pollution and represents a serious health risk.Taking into consideration the diverse chemical and phys-ical properties of dyes, investigation of different techniquesfor their removal from wastewater is required. Moreover,the considerable change in physical and chemical proper-ties may affect the response of the dyes from differentgroups to a certain treatment technique.

Colour removal from effluent is one of the most difficultrequirements faced by colour-producing industries. Manymethods have been used for removal of dyes from water andwastewater, such as chemical coagulation, ozonisation,membrane filtration, electrolysis, oxidation, and biodegra-dation. The adsorption process was found to be the mosteffective and non-destructive technique, and is still one of

© 2014 The Authors. Coloration Technology © 2014 Society of Dyers and Colourists, Color. Technol., 130, 1–10 1

doi: 10.1111/cote.12086

the best-known techniques that is widely used for theremoval of dye from aqueous solutions [17].

The adsorption characteristics of dyes on various adsor-bents have been extensively investigated for many purposesinvolving separation and purification [18]. The criticalchallenge for applying the adsorption method to dyeremoval is finding a low-cost adsorbent that is readilyavailable and has a high removal capacity for adsorption tocompete successfully with other dye removal techniques.This is the driving force behind further studies attemptingto find an efficient low-cost adsorbent.

Activated carbon is widely used as an adsorbent owing toits high dye uptake, large surface area, and microporousstructure [7]. Activated carbon from vegetable material wasintroduced industrially in the first part of the twentiethcentury and used in sugar refining [19]. The preparationand effective utilisation of activated carbon generated fromnatural plant materials has attracted worldwide attention inview of the large disposal problem detrimental to theenvironment. However, the production of activated carbonis complex and expensive, making this technology econom-ically non-efficient.

Successful representation of the dynamic adsorptiveseparation of solute from solution onto an adsorbentdepends on a good description of the equilibrium separa-tion between the two phases. Isotherms are mathematicalrelationships used to describe the adsorption behaviour of aparticular adsorbent–adsorbate combination. They help inmodelling adsorption behaviour and in calculating theadsorption capacity of materials. Several mathematicalmodels can be used to evaluate experimental data, includ-ing the Langmuir, Freundlich, Dubinin–Radushkevich,Temkin, Frumkin, Harkins–Jura, and Smith isotherm equa-tions. The constant parameters of the isotherm equations forthis adsorption process were calculated by regression usinga linear form of the isotherm equations. Freundlich,Langmuir, and Dubinin–Radushkevich models are the mostcommonly used isotherms. They give valuable informationabout the adsorption process, such as the maximumadsorption capacity, the extent of adsorption (monolayeror multilayer coverage), and the nature of the adsorptionprocess (physical or chemical).

Adsorption kinetics models are helpful in understandingthe adsorption rate, equilibrium time, and mechanismsemployed in the adsorption process. Several kinetic modelshave been proposed to clarify the mechanism of soluteadsorption from an aqueous solution onto an adsorbent, e.g.the pseudo-first-order kinetic model, the pseudo-second-order kinetic model, the intraparticle diffusion model, andthe liquid film diffusion model.

Eriochrome Black T (EBT) is used for dyeing of silk, wool,and multifibres. It is also used as an indicator in complexo-metric titration for determination of hardness. EBT is amember of the azo dye group, which comprises ca. 70% ofall commercially produced dyes used in industry [1]. Largeportions of these azo dyes manage to migrate into theenvironment. The dye and its degradation products, such asnaphthaquinone, are carcinogenic. They are toxic to aquaticorganisms and may cause long-term adverse effects in theaquatic environment. Bromophenol Blue (BPB) is one of thetriphenylmethane dyes. These dyes are considered to bexenobiotic compounds, which are very recalcitrant to

biodegradation. Triphenylmethane dyes are used in manyindustrial processes, such as textile dyeing, paper printing,and food and cosmetic manufacture. They are known to behighly toxic to mammalian cells and mutagenic andcarcinogenic to humans. Although their toxicity is high,they are widely used owing to their relatively low cost,ready availability, and efficacy [20,21].

It is very important to find an efficient and economicadsorbent to treat wastewater that is polluted by these dyesand assess the effect of different factors affecting thetreatment process. The present study was undertaken tostudy the effect of the structure of BPB and EBT onadsorption onto locally prepared activated carbon to reducethe cost of the treatment process. Different factors thatmight affect the adsorption process were studied. Differentequilibrium models were applied to evaluate the adsorptionprocess. The kinetics and mechanisms of reactions wereinvestigated using four models.

Materials and MethodsAdsorbateBPB was obtained from Loba Chemie (India), and EBT fromAcros (USA). The structures of BPB and EBT are shown inFigure 1. A stock solution was prepared from each dye bydissolving an accurately weighed amount (ca. 1 g) of thepowder in 1 l of distilled water. The calibration curves ofthe two dyes were prepared by successive dilution of thestock solutions and measuring the absorbance for thedifferent prepared concentrations at 591 and 503 nm forBPB and EBT respectively.

AdsorbentThe adsorption experiments were conducted with activatedcarbon prepared from date pits by Altaher and Khalil [22].The date pits were washed three times with tap water toremove dirt and foreign materials, followed by washingthree times with distilled water. The date pits were thendried at 105 °C for 24 h in a drying oven. The pits werecooled, crushed, and sieved. The <1 mm fraction wasweighed and mixed with zinc chloride with a mass ratio of1:1 (dry basis). Distilled water was added in a small amount

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Figure 1 Molecular structure of BPB (a) and EBT (b)

Altaher et al. Effect of dye chemical structure on adsorption

2 © 2014 The Authors. Coloration Technology © 2014 Society of Dyers and Colourists, Color. Technol., 130, 1–10

to cover the mixture, which was agitated to ensure homo-geneity. The mixture was carbonised for 2 h at 400 °C in anitrogen atmosphere in a muffle furnace. The formedproduct was washed several times with distilled wateruntil the pH of the washing water was neutral. The formedactivated carbon was filtered through filter paper, dried, andstored in a sealed glass bottle.

Fourier transform infrared and scanning microscopyFourier-Transform Infrared (FTIR) spectra of the preparedactivated carbon before and after adsorption were recordedon a Bruker Tensor 37 infrared spectrophotometer, (BrukerScientific Instruments). The spectrum of the preparedactivated carbon was recorded in the range 4000–400 cm�1 as potassium bromide (KBr) pellets, which wereprepared by grinding 200 mg of carbon samples with 0.5 gof KBr. The resolution was 4 cm�1, and the signal-to-noiseratio ranged from 5000:1 to 45000:1.

The surface morphology of the prepared activated carbonsamples was examined using scanning electron microscopy(SEM) on a JEOL-JSM 5300 instrument (Japan) at differentmagnifications.

Adsorption studiesAll experiments were conducted at room temperature(20 � 2 °C). The shaking speed for all experiments wasalso constant and set at 250 rpm, except for when studyingthe effect of stirring rate. Detailed experimental conditionsare summarised in Table 1. For every experiment, a blankwas measured. The experiments were conducted twice, andthe average was determined. Equilibrium dye uptake (qe orqt) was determined on the basis of the initial and finalconcentration of the dye.

Adsorption isothermsTo study the effect of mass on the adsorption process, a50 ml aliquot of 115–170 mg/l of the dye solution wasplaced in 125 ml Erlenmeyer flasks. The pH values of allsolutions were kept constant at normal pH achieved whendissolving the solid dye in distilled water. Different massesof adsorbent were added to the flasks. The mixture wasagitated for 6 h using a Yellow Line shaker (IKA, Germany),and the final concentration of the dye was measured.

Kinetic studyThe effect of time was studied by adding accuratelyweighed amounts of adsorbents to flasks containing 50 mlof the dye solution at normal pH (the pH of the dye solutionwithout adjustment) and having the same previouslymentioned concentration. The masses added to all flasks

were the same. The flasks were agitated for 6 h, and thefinal concentration of the dye was measured.

Effect of initial concentrationThe effect of the initial concentration was studied in asimilar manner to the change in mass experiment, but themass of adsorbent was kept constant and the concentrationin the flasks was changed from 4 to 100 mg/l. A quantity of50 ml of every prepared concentration of the dye solutionunder investigation was placed in a conical flask. Pre-weighed activated carbon (according to Table 1) was addedto each flask, and the flasks were covered. The final dyeconcentration of the dye solution was measured using anultraviolet-visible spectrophotometer (UV mini-1240; Shi-madzu, Japan).

Adsorption isothermsLangmuir isothermThis model assumes that the removal of adsorbate takesplace on a specific homogeneous surface by monolayeradsorption [23]. It also assumes that the coverage ofadsorbate on the surface of adsorbent is of equal energy ofadsorption. The bonding to the sorption sites can be eitherchemical or physical, but must be strong enough to preventdisplacement of the sorbate molecules. The linear form ofthis model is given by the equation:

Ce=qe ¼ 1=ðqmkLÞ þ Ce=qm ð1Þwhere Ce (mg/l) is the equilibrium concentration, qe (mg/g) isthe dye uptake at equilibrium, qm (mg/g) is themaximumdyeuptake, and kL (l/mg) is a constant related to adsorption rate.The favourability of the adsorption is given by the dimen-sionless separation factor RL, which is given by the equation:

RL ¼ 1=ð1þ kLC0Þ ð2Þwhere C0 (mg/l) is the initial dye concentration. The valueof the separation factor indicates the favourability of theadsorption process as follows: unfavourable, RL > 1; linear,RL = 1; favourable, 0 < RL < 1; irreversible, RL = 0.

Freundlich isothermThis is an empirical model that can be applied to non-idealsorption on heterogeneous surfaces as well as multilayersorption [24]. Its linear form is given by the equation:

lnqe ¼ lnkf þ 1=nðlnCeÞ ð3Þwhere qe and Ce have the same designation as in theLangmuir equation, kf is a constant related to the bondingenergy – kf [mg/g (l mg)1/n] can be defined as the adsorptioncoefficient, and 1/n identifies the adsorption intensity of dye

Table 1 Range of variables for batch experimentsa

Experiment Initial concenration Contact time Adsorbent dose Stirring rate Particle size pH Volume

Effect of initial concentration EBT: 4–88 BPB: 5–100 360 0.16 250 <250 Normal 50Effect of adsorbent mass EBT: 115 BPB: 170 360 0.02–0.26 250 <250 Normal 50Effect of time EBT: 115 BPB: 170 0–400 0.16 250 <250 Normal 50Effect of stirring rate EBT: 115 BPB: 170 240 0.16 0–450 <250 Normal 50

a Concentration in mg/l; mass in g; time in min; volume in ml, adsorbent particle size in lm; stirring rate in rpm.



onto adsorbent. If the value of exponent n is greater than 1,then the adsorption represents a favourable adsorptioncondition.

Dubinin–Radushkevich isothermThis model is used to predict the nature of the adsorptionprocess as physical or chemical by calculating the sorptionenergy [25]. It assumes that the sorption curve is related tothe porous structure of the sorbent. The model is repre-sented by the following equation:

lnqe ¼ lnqm � B€2 ð4Þwhere qe is the amount of adsorbate adsorbed onto theadsorbent surface (mg/g) at equilibrium, qm represents themaximum dye uptake by the adsorbent (mg/g), and B is aconstant related to the mean free energy of adsorption permole of adsorbate as it is transferred to the surface of thesolid from infinite distance.

The Polanyi potential is given by:

€ ¼ RT lnð1þ 1=CeÞ ð5Þ

E ¼ 1=ð2BÞ0:5 ð6Þwhere R is the ideal gas constant (kJ/mol K) and T is thetemperature (K). If E < 8 kJ/mol, the adsorption process isphysical, if 8 < E < 16 kJ/mol, the adsorption process is ionexchange, and if E > 16 kJ/mol, the adsorption process ischemical.

Kinetic modelsTo examine the mechanism of adsorption of dyes on thesurface of activated carbon, some of the commonly usedkinetic models were applied. These kinetic models arepseudo-first-order, pseudo-second-order, and intraparticlediffusion models.

Pseudo-first-order modelThe linear form of this model is given by [26]:

lnðqe � qtÞ ¼ lnqe � k1t ð7Þwhere qe (mg/g) and qt (mg/g) are the amounts of dyeadsorbed on the surface of the adsorbate at equilibrium andat any time t (min) respectively, and k1 (1\min) is the rateconstant of the pseudo-first-order adsorption.

Pseudo-second-order modelThe linear form is given by [27]:

t=qt ¼ 1=k2qe þ t=qe ð8Þwhere k2 (g/mg min) is the rate constant of the pseudo-second-order equation, qe (mg/g) is the maximum dyeuptake, and qt (mg/g) is the amount adsorbed at time t (min).

Intraparticle diffusion modelThis model considers the intraparticle diffusion as the rate-controlling step for adsorption [28]. It is given by thefollowing equation:

qt ¼ kidt0:5 þ C ð9Þ

where qt (mg/g) is the amount adsorbed at time t (min) andkid (mg/g min0.5) is the rate constant for the intraparticlediffusion model.

Liquid film diffusion model (Boyd model)The film diffusion mass transfer rate equation is given bythe following equation [2]:

ln 1� ðqt=qeÞ½ � ¼ �Bt ð10Þwhere B (min�1) is liquid film diffusion constant, and qt andqe are the dye uptake at any time and at equilibriumrespectively.

A plot of ln(1 � qt/qe)–t should be a straight line with aslope B if the liquid film diffusion is the rate-limiting step.

Results and DiscussionEffect of adsorbent massIt is clear from Figure 2 that, for both BPB and EBT, as theadsorbent mass increased, the percentage of adsorptionincreased by virtue of an increase in the available activeadsorption sites. On the other hand, for EBT, the dye uptakedecreased with an increase in the adsorbent mass, whichcan be attributed to the high amount of adsorbent, leadingto unsaturated adsorption sites. The behaviour of BPB wasquite different. Results of this experiment indicated thegreat effect of adsorbent dose on the uptake of dye BPB. Asshown in Figure 2, the value of q increased with an increasein adsorbent dose until the optimum dose of 0.2 g/100 ml ofsolution and remained almost constant even with anincrease in the adsorbent mass. The results suggest that,for dye adsorption, an appropriate adsorbent should be usedto lower the adsorbent cost. The optimum dose of adsorbentdepends on the dye concentration in the solution. It must betaken into consideration that, even though a higher adsor-bent dose gives more adsorption sites for the adsorbate, it iscommonly found that, when the adsorbent concentration istoo high, some adsorption sites may be blocked by physicalor spatial hindrance, causing a decrease in the availabilityof some sorption sites [29].

The observed higher dye uptake in the case of BPBrelative to EBT can be explained in terms of comparing the

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Figure 2 Effect of adsorbent mass on removal of dye; qe is theuptake of the adsorbate; q is the percentage removal of adsorbate;the initial concentrations were 4–88 mg/l for EBT and 5–100 mg/lfor BPB [Colour figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]



chemical structure of both dyes. In the case of BPB there arefour bromo substituent groups on the benzene rings; theseelectron-withdrawing groups deactivate the benzene ringsby decreasing the electron density on the rings through aresonance-withdrawing effect, and hence the electron den-sity on the bromine atoms increases and becomes moreactive in binding with the H atom of the hydroxyl groups onthe activated carbons, forming H bonds. This is in agree-ment with previous studies that attributed the high adsorp-tion activity of activated carbon to surface functionalgroups. It has been shown that, owing to the presence of apolar –OH group, a phenol molecule is able to form H bondswith surface functional groups. On the other hand, thearomatic ring of the molecule determines its ability to havehydrophobic interactions. The optimum doses for EBT andBPB were found to be 0.16 and 0.2 g/100 ml respectively.

Effect of initial concentration of adsorbateThe effect of the initial concentration of the two inspecteddyes on the adsorption process is illustrated in Figure 3.The figure shows that the initial dye concentration plays avital role in the dye uptake. The high dye concentration canresult in a high concentration gradient between the solutionand adsorbent, which acts as a driving force to transfer thedye molecules from the bulk solution to the particle surface,thus overcoming the mass transfer resistance between thesolution and the adsorbent. The two adsorption systemshad different behaviour. In the case of EBT an increase inthe initial concentration from 4 to 88 mg/l resulted in anincrease in removal efficiency to 53%, followed by adecrease in this efficiency to a value of 14%. This may beattributed to the effect of steric hindrance. Increasing theconcentration of these bulky molecules will increasethe steric hindrance, which affects their adsorption on thesurface of activated carbon. On the other hand, increasingthe initial concentration of BPB from 5 to 100 mg/l resultedin a continuous increase in the removal efficiency up to avalue of 82.5%. This phenomenon may indicate that the

adsorbent did not reach saturation owing to the low contactopportunity with low dye concentration. Also, a highconcentration of dye works as a driving force for theadsorption process. Increase in the concentration leads toan increased driving force. These results clearly indicate thehigh affinity of the adsorbent towards BPB compared withEBT, taking into consideration the maximum achievedremoval of EBT (53%) compared with the 82.5% for BPB.This is supported by the monolayer adsorption capacitiespresented in Table 2. This monolayer adsorption capacity is39.68 mg/g for BPB compared with 36.496 mg/g for EBT.

Adsorption equilibriumAdsorption equilibrium provides important information forevaluating the applicability of sorption processes as a unitoperation. The parameters of the developed isothermsexpress the surface properties and affinity of the sorbentat a fixed temperature and pH. Thus, an accurate mathe-matical description of the equilibrium isotherm is essentialto the effective design of sorption systems [30].

By comparing the correlation coefficient values obtainedfrom Langmuir, Freundlich, and Dubinin–Radushkevichisotherm models (presented in Table 2) and fitting themodels to the experimental data (depicted in Figures 4a and4b), it can be concluded that the Langmuir isotherm modelis the most suitable for fitting the data obtained for the twodyes. Based on the R2 values (0.971 for EBT and 0.997 forBPB), the linear form of Langmuir appears to produce thebest fitting for the two systems. Moreover, the qm valuespredicted from the Langmuir model were closer to theexperimental qe values. The maximum adsorption capaci-ties (qm) for EBT and BPB are 36.5 and 39.7 mg/g respec-tively. These high adsorption capacity values indicate theefficiency of the prepared activated carbon for the removalof both dyes. The basic assumption of the Langmuir modelis that the adsorbate forms a monolayer on the surface of theadsorbent. This means that only one dye molecule can beadsorbed on one adsorption site and the intermolecularforces decrease with distance. It is also assumed that theadsorbent surface is homogeneous in character and pos-sesses identical and energetically equivalent adsorptionsites. The best fit of equilibrium data in the Langmuirisotherm expression confirms monolayer adsorption of thetwo dyes onto activated carbon, as well as the homogeneousdistribution of active sites on the surface. The results alsodemonstrate no interaction and transmigration of dyemolecules in the plane of the neighbouring surface. The

Table 2 Adsorption parameters for the applied adsorption models

Isotherm Parameters EBT BPB

Langmuir R2 0.9709 0.9966qm 36.496 39.683kL 0.075 0.145RL 0.107 0.039

Freundlich R2 0.8667 0.9321n 2.627 8.361kf 2.304 3.786

Dubinin–Radushkevich R2 0.983 0.8452qm 31.387 35.88B 3 9 10�5 1 9 10�5

E 0.129 0.223

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Figure 3 Effect of initial dye concentration on adsorption process;qe is the uptake of the adsorbate; q is the percentage removal ofadsorbate; the dose of adsorbent was 3.2 g/l [Colour figure can beviewed in the online issue, which is available at wileyonlineli-brary.com.]



RL values for EBT and BPB are 0.107 and 0.039 respectively.These values indicate that both systems are favourable.However, as the value of BPB is nearer to zero, it is morefavourable.

Referring to Table 2 for information about data fitting tothe Dubinin–Radushkevich model, the regression coeffi-cients are considerably higher for EBT, indicating acceptablefitting. The value for BPB is moderate. However, the exper-imental values of the adsorption capacities for the systems arevery far from the calculated values, which means that thisisotherm cannot be used to model the adsorption process.

Themolecular size of the dyeplays an important role in theadsorption process because the adsorbate molecules mustenter themicropores of the adsorbent to be adsorbed. The dyeuptake would be lower for an adsorbate with a largemolecular volume [31]. The dye uptake is strongest whenthe pores are just enough to permit the molecules to enter.This may be one factor of increasing the uptake of dye BPBhaving amolecular size of 3.42 nm2 comparedwith 3.48 nm2

for EBT.

Kinetic studyChemical kinetics describes the mechanism of a certainphysical/chemical process, whereas equilibrium gives noinformation about the mechanism and reaction rate. Study

of the kinetics of sorption processes is also important forevaluating optimum conditions (including equilibriumtime) for batch adsorption processes. Thus, different kineticmodels were applied to investigate these two importantissues. Figure 5a illustrates the change in dye uptake withtime. It is clear from the figure that the dye uptake increaseswith increasing shaking time until equilibrium is reachedafter 240 min. Initially, the sorption sites at the surface ofthe adsorbent were vacant, and could easily adsorb thedyes. As time went on, the adsorption sites becamesaturated, the concentration gradient decreased, and con-sequently the adsorption rate decreased. This is clear fromgradual increase in dye uptake with time as equilibrium isapproached, which may be attributed to the reduction inavailable sites for adsorption. At this point of equilibrium,the rate of dye adsorption is equivalent to the rate of dyedesorption to the solution. Some differences can beobserved between the two sorbate systems (Figure 5a). Ascan be seen from the figure, BPB was initially more rapidlyadsorbed than EBT. Figure 5a indicates that rapid adsorp-tion occurred within the first 2 min for both adsorptionsystems. In this period, the adsorbate diffuses from the bulksolution to the external film surrounding the adsorbentsurface. Following this fast adsorption process, there is agradual adsorption stage until equilibrium is attained.

In order to investigate the controlling mechanism ofadsorption, the pseudo-first-order and pseudo-second-ordermodels were applied. The pseudo-first-order model consid-ers the rate of occupation of adsorption sites to beproportional to the number of occupied sites, whereas thepseudo-second-order model considers the rate of occupa-tion of adsorption sites to be proportional to the square ofthe number of occupied sites. The k1, qe, and correlationcoefficient (R2) values for the pseudo-first-order equationare given in Table 3. The R2 values are relatively high forboth EBT and BPB. However, the experimental dye uptakevalues of the two dyes are far from the calculated valuesobtained from the pseudo-first-order model, which suggeststhat the latter should be ruled out as a mechanism for theadsorption process [32]. The pseudo-second-order model isbased on the assumption that the rate-limiting step may bechemisorption, which involves valence forces by sharing orelectron exchange between the adsorbent and adsorbate.The greater mobility of EBT compared with BPB, on thebasis of comparing their k2 values, may be attributed tomolecular weight. The small molecules diffuse faster andenter the pores more easily than the larger molecules. Infact, the pseudo-second-order kinetic model has a better fitfor the data. The experimental values of the dye uptake forboth EBT and BPB agree well with that obtained from thetheoretical pseudo-second-order model. The results inTable 3, supported by the high values of R2 obtained fromthe linear plot for both dyes, suggest the applicability of thepseudo-second-order model for the two adsorption systems.The adsorption of the two dyes is probably due to a surfaceexchange reaction until the surface functional sites are fullyoccupied, followed by diffusion of the dyes into theactivated carbon pores for further interaction.

The prediction of the rate-limiting step is important inthe adsorption processes. It influences design and scale-upof the process. The adsorption process may be controlled byseveral steps, including diffusion of the adsorbate from the

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Figure 4 Theoretical against practical values for the adsorption of(a) EBT and (b) BPB [Colour figure can be viewed in the onlineissue, which is available at wileyonlinelibrary.com.]



bulk of the solution to the thin film surrounding the particle,diffusion of the adsorbate from the film to the surface of theadsorbent, diffusion from the surface to the macro-, micro-,andnanopores, anduptakeof the adsorbate. The rate-limitingstep may be one or a combination of the second and thirdsteps. The intraparticle diffusion model and film diffusionmodels were applied because the pseudo-first-order andpseudo-second-order models cannot identify the diffusionmechanism.

According to the intraparticle model, if this model isinvolved in the adsorption process, the plot of dye uptake qtagainst t0.5 should be represented by a straight line. If thisline passes through the origin, intraparticle diffusion is therate-limiting step. However, if the line intersects the dyeuptake axis, other kinetic models may control the rate ofadsorption together with intraparticle diffusion. The valueof the intercept gives an indication of the thickness of theboundary layer. The larger the intercept, the greater is theboundary layer. The linear plot of the intraparticle modeldid not pass through the origin, probably owing to thedifferences in mass transfer rates in the initial and finalstages of adsorption. This implies that intraparticle diffu-sion was not the only rate-limiting mechanism in theadsorption process. The value of the intercept shown inTable 3 for BPB is greater than that for EBT, indicating adifference in boundary layer thickness of the two systems.On the other hand, investigating the fitting of the experi-mental data to the liquid film diffusion model (shown inFigure 5b) indicates a much better fitting of the data. The R2

values for EBT and BPB are 0.985 and 0.992 respectively.Comparing these values with those of intraparticle diffusion(0.983 and 0.968 respectively) indicates that the liquid filmdiffusion model is better applied to explain the adsorptionmechanism. Figure 5b shows that the straight lines of the

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Eriochrome BT

Bromophenol B

y = –0.0118x – 0.2602

R 2 = 0.9853

y = –0.0076x – 0.4383

R 2 = 0.9928

–4

–3.5

–3

–2.5

–2

–1.5

–1

–0.5

0

0 100 200 300 400

ln [1

–(q t

/qe)

]

Time, min

EBT

BPB

(a)

(b)

Figure 5 (a) Effect of time on the uptake of dyes EBT and BPB; (b) liquid film diffusion model fitting of the experimental data [Colour figurecan be viewed in the online issue, which is available at wileyonlinelibrary.com.]

Table 3 Kinetic parameters for adsorption of dyes on activatedcarbon

Kinetic model Parameters EBT BPB

Pseudo-first-order R2 0.9408 0.9705k1 0.014 0.0082qe 15.407 27.159

Pseudo-second-order R2 0.9825 0.9885k2 1.67 9 10�3 7.05 9 10�4

qe 18.48 43.86Intraparticle diffusion R2 0.983 0.9683

kid 0.7845 1.4943C 4.2 13

Film diffusion R2 0.985 0.992B 0.26 0.438



two adsorption systems do not pass through the origin,which suggests the engagement of both diffusion models incontrolling the adsorption process. This is borne out by theresults in the following section. The agitation rate has asignificant effect on the adsorption process. Increasing theagitation rate will decrease the resistance of diffusion of theadsorbate molecules through the thin layer surrounding theadsorbent particles.

Effect of stirring rate on adsorption of dyesFigure 6 represents the effect of the shaking speed on theuptake of dyes EBT and BPB. It is clear from the figure thatan increase in shaking speed from 50 to 150 rpm results inan increase in the adsorption capacities for both dyes.Further increase in agitation speed does not result in anysignificant increase in dye uptake. Increasing the speed ofagitation causes the thickness of the adsorbent boundarylayer to decrease, which lowers the resistance for transfer ofdyes to the surface of the adsorbent owing to the turbulencethat would be produced by high speed. On account of this,the adsorbent molecules are forced towards the surface ofthe adsorbent, facilitating the adsorption process [33]. Thisresult agrees well with that obtained from the intraparticlediffusion model, indicating that BPB has a thicker boundarylayer, which implies a higher effect of agitation speed onthis system, as illustrated in Figure 6.

Dye uptake by the prepared activated carbonTable 4 compares the maximum adsorption capacity ofvarious activated carbons prepared from different precur-sors with that for the activated carbon in the presentresearch. It can be seen that the activated carbon preparedin the present study has a very high adsorption capacitycompared with some activated carbons, e.g. 800-fold forBPB and 368-fold for EBT compared with that prepared byIqbal and Ashiq [34], a higher capacity compared withactivated carbons prepared by Dada et al. [35], and acomparable adsorption capacity to activated carbons pre-pared by Ladhe et al. [36–38]. The surface structure of theprecursor, together with preparation conditions (time ofburning, type of activator, and its ratio to the precursor),would have a pronounced effect on the surface functionalgroups of the produced activated carbon and the nature ofthe pores on this surface. These two properties are veryimportant in determining the adsorption capacity of theactivated carbon.

Surface analysisFourier transform infrared spectral analysisInfrared spectroscopy provides qualitative informationabout the characteristic functional groups on the surfaceof the prepared activated carbon sample. The adsorptivecapacity of the adsorbent is also influenced by its surfacechemical structure. The surface of carbon materials is, ingeneral, rich in a variety of surface functional groups,among which the C–O groups are predominant [39]. Thesurface functional groups were found to be partiallyresponsible for physicochemical adsorption on activatedcarbon [40].

The FTIR spectrum of the prepared activated carbon isshown in Figure 7. The absorption at 3432 cm�1 wasattributed to m(O–H) vibrations of hydroxyl groups orsurface-bonded water. The band appearing at 1628 cm�1

is assigned to C=C stretching vibrations. The band locatedaround 1378 cm�1 could be attributed to m(C–O) vibrationsof carboxylate groups. A brief summary assignment ofspecific functional groups and their frequencies of vibrationto the observed FTIR bands of the activated carbon is shownin Table 5.

On the other hand, the IR spectra of activated carbon afteradsorption of EBT and BPB are more or less similar to eachother. Their spectra are also similar to that of activatedcarbon before adsorption. However, the intensity of thepeaks is either minimised or shifted slightly, indicating thata binding process is taking place at the surface of theadsorbents. By comparing the spectra of activated carbonand of activated carbon in the presence of the two dyes, aswell as the spectra of the dyes, it is clear that the surface –

OH and C–O groups are the main functional groupsresponsible for adsorption. This can also be explained interms of the disappearance of peaks of N�N, S=O, S–O,N–O, and C–N characteristics for EBT and S=O, S–O, and C–Brcharacteristics for BPB after the adsorption process, asindicated in Table 5.

Normalisation of spectra eliminates path length variationand also reduces the differences between each single mea-surement of the same sample. The spectra are normalised tothe most intense band or at the same integrated intensity in agiven spectral region. The C=C band is used as an internalstandard for normalisation. Normalisation allows for aneffective comparison across a heterogeneous set of samples.

Scanning electron microscopeAn SEM is a type of electron microscope that images asample by scanning it with a high-energy beam of electrons

0

5

10

15

20

25

30

35

40

0 100 200 300 400 500

q, m

g/g

Stirring rate, rpm

EBT

BPB

Figure 6 Effect of the stirring rate on the uptake of dyes EBT andBPB [Colour figure can be viewed in the online issue, which isavailable at wileyonlinelibrary.com.]

Table 4 Comparison of the adsorption capacity of the preparedactivated carbon with that of other activated carbons

Number

Adsorption capacity, mg/g

ReferenceEBT BPB

1 �0.099 �0.050 [34]2 0.8419 [35]3 27.10027 [36]4 46.512 [37]5 33.333 [38]6 36.5 39.68 Present study



in a raster scan pattern. The electrons interact with theatoms that make up the sample, producing signals thatcontain information about the sample’s surface topography,composition, and other properties such as electrical con-ductivity.

The pore structure of the raw date pits and preparedactivated carbon is shown in Figures 8 and 9 respectively.The availability of pores and internal surface is necessaryfor an effective adsorbent. The sorbent surface demonstratesa well-developed and uniform surface, forming an ordinaryporous structure with a macroporous character, which isresponsible for the highly developed surface area of thismaterial. This can serve as evidence for the high adsorptioncapacity observed for this prepared activated carbon.Comparing the micrographs for both raw date pits andprepared activated carbon, it is possible to observe well-developed pores of cylindrical shape in the case of activatedcarbon, represented by points A and B in Figure 9. Otherpores are formed that have a deformed cylindrical shape, asindicated by points C and D in Figure 9. The pores thatwere originally cylindrical have become partially flattened

cylinders after burning. This deformation shape may beexplained by fusion of the walls of two or more adjacentpores. Also, some foreign material can be seen inside the

Table 5 Significant FTIR bands of the prepared activated carbon

Functional groups

Assignment

Band position m, cm�1

Activatedcarbon

Activated carbon inpresence of EBT

Activated carbon inpresence of BPB EBT BPB

O–H stretching vibration 3432 3437 3440 3444 3474Aliphatic C–H stretching vibration 2922 2923 2923Aromatic C–H Stretching vibration 2974 3093C=C stretching vibration 1628 1630 1633 1620 1628In-plane bending vibration of C–H 1435 1380 1379C–O stretching vibration 1000–1160 1050–1160 1060–1163 1053–1099 1160–1194N�N stretching vibration 1596S=O stretching vibration 1476 1446S–O stretching vibration 790 811N–O stretching vibration 1341C–N stretching vibration 1221–1278C–Br stretching vibration 628

3778

.75

3698

.27

3431

.74

2921

.71

2354

.76

2128

.53

1628

.38

1434

.39

1377

.90

1163

.13

1115

.94

1054

.87

897.

57

610.

6456

1.73

435.

20

500100015002000250030003500

Wavenumber, cm–1

9294

9698

100

Tra

nsm

ittan

ce, %

Figure 7 FTIR spectra of the activated carbon

30 kV ×7500 1 µm 016852

Figure 8 Scanning electron micrograph of raw date pits



pores, as observed at points E and F in Figure 9. Thepresence of such foreign material may lower the adsorptioncapacity of the prepared activated carbon towards the testedadsorbates.

ConclusionThe structure of the dye and its molecular weight werefound to have a significant effect on the adsorption process.BPB, having four bromine groups in its structure, was foundto have a higher adsorption capacity. The adsorptioncapacity was found to be influenced by the initial concen-tration of the dye in the aqueous solution, the contact time,the mass of the adsorbent, and the rate of stirring. TheLangmuir adsorption isotherm and pseudo-second-ordermodel were found to describe the data very well. Themonolayer adsorption capacities for EBT and BPB were 36.5and 39.68 mg/g respectively.

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B

C

D

EF30 kV ×1000 10 µm

9.55 µm

856

12.05 µm A

Figure 9 Scanning electron micrograph of activated carbon [Colourfigure can be viewed in the online issue, which is available atwileyonlinelibrary.com.]



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The effect of dye chemical structure on adsorption on activated carbon: a comparative study