Application of ecological adsorbent in the removal of reactive dyes from textile effluents

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Research ArticleReceived: 23 September 2008 Revised: 14 January 2009 Accepted: 15 January 2009 Published online in Wiley Interscience: 2 April 2009

(www.interscience.wiley.com) DOI 10.1002/jctb.2147

Application of ecological adsorbent in theremoval of reactive dyes from textile effluentsAndressa R. Vasques,a∗ Selene M. Guelli U. de Souza,a Jose A. B. Valleb andAntonio A. Ulson de Souzaa

Abstract

BACKGROUND: The capacity and mechanism of adsorption of the reactive dyes monoazo (RR2) and diazo (RR141), using anew adsorbent with a strong ecological appeal developed from the sludge of the textile effluent treatment process, wereinvestigated. The kinetics and adsorption isotherms were determined at different temperatures and salt concentrations. Afterdetermination of the best experimental conditions for adsorption for both dyes, tests were carried out in fixed-bed adsorptioncolumns.

RESULTS: For both dyes, there was a reduction in the adsorption capacity of the adsorbent developed when the systemoperated at temperatures above 40 ◦C. When 10% (by mass) of sodium chloride was added to the adsorbate RR141 themaximum adsorption increased from 66.67 mg g−1 to 78.74 mg g−1. For both dyes, the addition of sodium sulfate did not favorsignificantly the adsorption. The results obtained for scale-up of the laboratory data for the adsorption columns indicated thatthe operating time with reactive dye diazo is 43.5% longer than that for monoazo.

CONCLUSION: The adsorbent studied was shown to be a very promising alternative in terms of an environmentally friendlyprocess.c© 2009 Society of Chemical Industry

Keywords: adsorption; dyes; adsorbent; biological sludge

NOTATIONCo initial concentration of solute (mg L−1)Ce quantity of solute adsorbed in the fluid phase versus (mg L−1)

or (mmol L−1)E packed bed porosity (adimensional)KL Langmuir equilibrium constant (L mg−1)qe quantity of solute adsorbed in the solid phase (mg g−1 or

mmol g−1)qm maximum quantity of dye adsorbed per unit mass of

adsorbent (mg g−1)R ideal gas constant (atm.L (mol K)−1);RL constant adimensional separation factor, the so-called

equilibrium parameterT temperature (K)tb breakthrough time (min)�H Enthalpy of adsorption (kJ mol−1)

INTRODUCTIONThe discharge of effluents containing residual color, such as dyeseliminated in wastewaters, characteristic of the textile industry,causes damage to the environment since they are toxic to aquaticlife.1 Being of synthetic origin with complex aromatic molecularstructures, the dyes become more stable under environmentalconditions, under the effects of light, pH and microbiologicalattack, and are more difficult to biodegrade.2,3

Despite the existence of several techniques for the treatmentof residual waters, there is no simple process able to ade-quately mineralize colored effluents.4 Therefore, a final polishing

treatment is required, usually involving physical or chemical treat-ment processes, including coagulation, flocculation, biosorptionand biofilms,5,6 ultrafiltration, oxidation (with ozone), Fenton’sreagent, hydrogen peroxide + UV radiation,7,8 electrochemicaldestruction,9 membrane separation, or adsorption usually withactivated carbon.10 – 14 Treatments involving combined processeshave also been cited in the literature,2,7,10 as well as toxicity studieson the effluents before and after the treatments.15

The phenomenon of adsorption occurs because surface atomshave a different position in relation to the atoms in the interior ofthe solid and their coordination number is lower than that of theinner atoms. As a result, the surface atoms have a force towards theinside which must be balanced, that is, in the direction normal tothe surface the field of the elements of the network is not balanced,thus, the molecules adsorbed on the surface are maintained byforces which originate from this surface. The tendency to neutralize

∗ Correspondence to: Andressa R. Vasques, UFSC, Federal University of SantaCatarina, Chemical Engineering and Food Engineering Department, CampusUniversitario Trindade, 88040-900, Florianopolis, Santa Catarina, Brazil.E-mail: andressa [email protected]

a UFSC, Federal University of Santa Catarina, Chemical Engineering andFood Engineering Department, Campus Universitario Trindade, 88040-900,Florianopolis, Santa Catarina, Brazil

b FURB, Chemical Engineering Department, Regional University of Blumenau,Campus II, 89030-000, Blumenau, Santa Catarina, Brazil

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Application of ecological adsorbent in the removal of reactive dyes www.soci.org

this type of action generates a surface energy, which is responsiblefor the adsorption phenomena.

Several factors affect adsorption, such as the molecular structureor nature of the adsorbent, the solute solubility, the pH of themedium, the temperature and the molecular diameter of theadsorbate. Activated carbon is widely used in the adsorptionprocess, and can be used in fixed bed columns.2,7,10 Due to thehigh cost of some conventional adsorbents, studies have beendirected toward the use of alternative adsorbents such as bottomash, sludge, red muds, fly ash, blast furnace sludge, etc.4,16,17 Thesematerials have a tendency to remove more inorganic than organiccontaminants.17 Some agricultural products have also been usedas low cost adsorbents, such as sunflower seeds,18 apple pulp,wheat straw,19 eucalyptus,20 native turf,21 etc. However, theseproducts contain the negative charges of cellulose, which repelanionic dyes. They can be modified under conditions of low pH toreduce the ion repulsion, but this means high costs to adjust thepH. Besides these agricultural materials, residual materials haverecently become the object of considerable interest, such as theresidual sludge of biogas, vermiculite and chitosan.22,23

In this study a new adsorbent developed from the residualsludge of a textile effluent treatment process is investigated.24 Inorder to determine the efficiency of the adsorbent in the removalof dyes the kinetics and adsorption isotherms were determined forthe reactive dyes RR2 and RR141, at different temperatures and saltconcentrations. After defining the best experimental conditionsfor adsorption, through the qm (mg g−1) parameter, obtainedthrough the fitting of the Langmuir isotherm for both dyes, thetests were carried out in fixed-bed adsorption columns.

EXPERIMENTALSample preparationThe residual sludge of the textile effluent treatment processwas obtained from a textile finishing factory in the region ofBlumenau, SC, Brazil. The adsorbent was obtained after passingthrough activation stages as described by Ulson de Souza et al.24

The thermal activation stage was carried out at 500 ◦C and afterheating the sample was chemically activated with 1 mol L−1 aceticacid.

The two dyes used as the adsorbates were CI Reactive Red 2(RR2) and CI Reactive Red 141 (RR141). The chemical structures andproperties of these dyes are given in Table 1. Standard solutions ofthe dyes were prepared from a base solution with a concentrationof 2000 mg L−1. The spectrophotometric tests were carried outat the wavelength of maximum absorption (λmax), determined

with a UV-Vis spectrophotometer (Cary 50-Varian, Inc. CorporateHeadquarters, Palo Alto, CA, USA). When necessary, the pH of thesolutions was adjusted with 1 mol L−1 acetic acid or 0.1 mol L−1

sodium hydroxide.

Physico-chemical characterization of the adsorbentSurface area and micropore volumeA small quantity of adsorbent sample was analyzed for adsorptionof N2 (nitrogen) at the temperature of liquid nitrogen in anautomatic physisorption analyzer (Quantachrome Autosorb-1C,Quantachrome Instruments, Boynton Beach, FL, USA). The valuesfor surface area were calculated according to the method describedby Brunauer, Emmett and Teller (BET).25

Scanning electron microscopyThe scanning electron microscopy (SEM) consisted in obtaining amicrograph of the physical structure of the adsorbent and togetherwith the EDS analysis qualitatively and quantitatively identifyingthe chemical composition of the sample using a Philips, modelXL30, scanning electron microscope with energy dispersive X-rayspectroscopy (EDAX).

Metal analysisIn order to classify the adsorbent when in the presence ofheavy metals, according to the concentration limits (mg kg−1)established as acceptable for sludge disposal in landfills andagricultural soils given in the CETESB (Companhia de Tecnologia deSaneamento Ambiental) technical manual P-4230,26 the thermallyand chemically activated adsorbent sample was analyzed in thepresence of lead, cadmium and mercury by atomic adsorptionspectrometry (AAS).

Kinetic testsThe kinetic study is important to determine the time required forthe sample to reach adsorption equilibrium and then to define theoperational conditions for the separation process.

For adsorption kinetics determination, 1 g of adsorbent wasadded to 100 mL of the adsorbate solution (initial concentration300 mg L−1) in 250 mL Erlenmeyer flasks. The pH of the solutionswas adjusted to 4.0 with 0.1 mol L−1 acetic acid and theflasks were then shaken in a Tecnal, model TE 420, shaker(Tecnal Equipamentos para Laboratrios - Piracicaba, SP, Brazil).At regular time intervals aliquots of the samples were removedfor analysis until aqdsorption equilibrium was reached, and the

Table 1. Properties and chemical structures of the reactive dyes used

Dyes Reactive Red 2 Reactive Red 141

Chemical class Monoazo Diazo

Reactive system Dichlorotriazine Monochlorotriazine

Molecular mass (g (gmol)−1) 601.323 1,774.159

Wavelength of maximumabsorption (λm)

538 544

Chemical structure

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www.soci.org AR Vasques et al.

concentration of the remaining solution was determined usingspectrophotometric techniques.

The procedure was carried out with the two different reactivedyes RR2 and RR141, monoazo and diazo, respectively, and usingdifferent parameters including the presence or absence of salts,sodium chloride and sodium sulfate, in the solution varying thetemperature of the adsorption tests.

Equilibrium studiesIn order to investigate the efficiency of the adsorbent under studythe adsorption isotherms were determined (Ce [mg L−1] – quantityof solute adsorbed in the fluid phase versus qe [mg g−1] – quantityof solute adsorbed in the solid phase) for the different experimentalconditions of temperature and addition of electrolytes to thesolution.

The adsorption isotherms were determined by way of batch testswhere 100 mL of a solution containing different concentrations ofadsorbate (from 100 to 2000 mg L−1) were transferred volumet-rically to 250 mL Erlenmeyer flasks containing 1 g of adsorbent,which were kept under shaking for 6 h in a shaker.

The data obtained from the adsorption experiments were fittedto the Langmuir isotherm:10

qe = qm KL Ce

1 + KLCe(1)

where qm is the maximum quantity of dye adsorbed per unit massof adsorbent and KL is the Langmuir equilibrium constant.

The equilibrium isotherm model is theoretical, based on thehypothesis that the interaction forces between the adsorbedmolecules are negligible and that each site can be occupied byonly one molecule. All of the molecules are adsorbed over a fixedand defined number of active sites.27

The essential characteristics of a Langmuir isotherm can beexpressed in terms of a constant adimensional separation factor,the so-called equilibrium parameter RL. The value of RL indicatesthe type of adsorption isotherm.

Fixed bed testsThe tests were carried out in adsorption columns arranged in series,1.43 cm diameter and 38 cm height, to investigate the influence ofthe process parameters on the removal of reactive monoazo anddiazo dyes. The initial concentration of the dye in the adsorbatesolution used in the continuous tests was 500 mg L−1, and thiswas acidified with 0.1 mol L−1 acetic acid (pH 4.0). 10% of sodiumchloride (NaCl) was added (% by mass) to the adsorbate RR141.Breakthrough curves were obtained for the dyes RR2 and RR141,varying the packed bed height of the adsorbent in the columns(15, 30 and 45 cm) and varying the adsorbate feed flow in thecolumn (8, 12 and 16 mL min−1). Figure 1 shows the schematicdesign of the system of columns for adsorption in a fixed bed.

The adsorption capacity at equilibrium was also determined(column saturation) for the different operational conditions citedabove. Owing to the high adsorption capacity of the adsorbent,the breakthrough curves were obtained until the adsorptionconcentration at the outlet of the column reached 80% of theinlet concentration (C/Co = 0.8).

Figure 1. Adsorption columns and auxiliary equipment.

RESULTS AND DISCUSSIONCharacterization of the adsorbentMoisture content, volatile solids and fixed solidsThe sludge in-natura had a moisture content of approximately 10%on a dry weight basis and a total solids content of 71.13%. Whenactivated at 200 ◦C the composition of fixed solids was 52.55%and of volatile solids was 47.44%. For a temperature of 500 ◦C, thefixed solids were 81.38% and the volatile solids were 18.62%.

Surface Area and Micropore VolumeTable 2 shows the values for surface area, and pore volume andpore size of the adsorbent used.

Analysis of the surface and chemical composition of theadsorbent was carried out by SEM. Figure 2 shows micrographs ofthe surface (magnification 1000×) of the adsorbent under studyas well as a commercial adsorbent (Fitrasorb 200 activated carbon,Calgon, USA).28

From the analysis of these surface images it can be concludedthat the adsorbent under study is a porous solid with an irregularsurface, the presence of active sites and a large quantity of gaps andcavities, thus favoring adsorption. The structure of the adsorbentunder study can be compared with the structure of the commercialactivated carbon (Fitrasorb 200) (Fig. 2(b)).

Figure 3 shows the qualitative analysis through which thechemical elements present in the adsorbent under study wereidentified.

Table 3 shows the results of the quantification of the elementsof the sample analyzed.

Table 2. Values for surface area, and volume and size of the adsorbentpores

AdsorbentResidual sludge thermically

and chemically activated

Surface area (BET∗) (m2 g−1) 137.61

Total volume of pores (cm3 g−1) 2.76 × 10−1

Micropore volume (cm3 g−1) 5.38 × 10−2

Pore diameter (Å) 8.032

∗ BET : Brunauer–Emmett–Teller.25

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(a) (b)

Figure 2. Micrographs of the adsorbent surfaces: (a) adsorbent, this work; (b) commercial activated carbon Fitrasorb 200.

Figure 3. Qualitative analysis of the adsorbent used.

Table 3. Quantification of the elements of the sample analyzed

Element % Weight

Carbon 21.38

Oxygen 21.61

Sodium 0.72

Manganese 1.6

Aluminum 19.06

Silica 17.88

Phosphorus 10.8

Sulfur –

Chlorine –

Potassium 1.22

Calcium 5.73

Total 100

The data given in Table 3 indicate the presence of theelements carbon, oxygen, sodium, manganese, aluminum, silica,phosphorous, potassium and calcium, these elements beingpresent in the auxiliary products used in the textile finishingprocess and in treatment of the industrial effluents of this sector.

Metal analysisThe metal analysis was carried out using the methodologydescribed earlier. The composition by weight (%) of lead found inthe adsorbent was 10.34 mg kg−1, with 0.22 mg kg−1 of cadmiumand 0.015 mg kg−1 of mercury. The adsorbent thus contains thepotentially toxic elements lead, cadmium and mercury, however,in low concentrations, thus its disposal in industrial landfills wouldnot lead to significant environmental damage according to thelimits established by CETESB.26 These limits are 840 mg kg−1 forlead, 85 mg kg−1 for cadmium and 57 mg kg−1 for mercury.

Kinetics studyAdsorption kineticsThe behavior of the adsorbent regarding the removal of reactivedyes can be compared with the behavior observed by Netpraditet al.17 who used metal hydroxide sludge for the removal ofreactive azo dyes. The composition of the adsorbent they used[Ca2+ (30%), Fe2+ or Fe3+ (15%), Cu2+ (2%), Al3+ (1.5%), Cr3+ orCr6+ (1%), Na+ (0.8%), Ni2+ (0.4%), and Zn2+ (0.3%)] is comparablewith the composition of the adsorbent under study herein, whichis formed basically of metal ions.

When metal ions present in residual sludge are calcined, metaloxides are formed which, in solution, are hydrated forminghydroxide ions, which are responsible for the uptake of positiveions at the adsorbent surface.31 This behavior gives rise to the


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double electric layer reported by Al-Degs et al.12 who reportedthat this layer is formed when two chemically different phasesare placed in contact, with a difference appearing in the electricpotential at the interface.

The formation of this layer is a consequence of the property thatmany materials have to acquire electric charge when in solution.This difference in the potential leads to a charge separation,since the positively or negatively charged surface attracts ionsof opposite charge and repels ions of the same charge. Theseparation of charges causes a distribution of ions close to thesurface and originates the so-called double electric layer, formedby the electrically charged surface, by the counter-ions (ions ofsame charge) and the co-ions (ions of opposite charge) whichare diffuse in the medium. The behavior of the double electriclayer was studied by Stern, who proposed a further layer thatdivides it into two regions. This is called the ‘Stern Layer’ andis located between the charged surface and the solution. Ionslocated outside the Stern layer, constitute the so-called diffuselayer. The potential generated along the dividing line betweenthe fixed and mobile parts of the double layer is called the zetapotential and it has the same value as the Stern potential.

Measurement of the zeta potential reveals the superficialcharge of the adsorbent at a certain pH value, allowing thedetermination of whether the adsorption of cations or anions isfavored under these conditions. Thus, it can be assumed that theadsorbent under study has a positive zeta potential as in the studypresented by Al-Degs et al.12 where the commercial activatedcarbon F-400 was used for the uptake of reactive dyes. Al-Degset al.12 determined that the adsorbent under study has a highsurface basicity with a high affinity for H+ ions (acid activation),leading to the uptake of anions. This behavior can be observed inFig. 4.

The pH value of the solution is important to the adsorptionprocess, particularly in relation to the adsorption capacity. Oxidesgenerate a positive or negative superficial charge when insuspension. This charge is proportional to the solution pH. Auseful index that indicates whether the surface is likely to becomenegatively or positively charged as a function of the pH is the pHvalue at which the net electric charge of the surface is zero. Thisvalue is called the zero point charge (pHZPC). A pH value lowerthan the pHZPC indicates that the superficial charge is positiveand therefore the adsorption of anions is favored. For pH values

Figure 4. Double layer formation according to the Stern model.

higher than the pHZPC the superficial charge is negative and theadsorption of cations is favored.12

Therefore, it is possible to establish a relation with the studyreported by Al-Degs et al.12 in which the commercial activatedcarbon F-400 was used for reactive dye uptake. The pHzpc valuefor this carbon was found to be 7.2 and the pH values measured atequilibrium for the three reactive dyes studied were 5.5, 5.0 and5.0, respectively, for the yellow, black and red reactive dyes. LowpH values strongly indicate that the activated carbon F-400 has apositive charge in the external layer (at equilibrium pH < pHzpc)during the adsorption process. This positive charge attracts thenegative portions of the reactive dyes.

This behavior was also observed by Netpradit et al.29 whoobtained a pHzpc of around 8.7, favoring the uptake of the negativecharge of the reactive dyes.

Figure 5 shows comparative results of the kinetic studies for thedyes (a) RR2 and (b) RR141 carried out at different temperatures.

Comparing the two dyes at 25 ◦C it can be observed that thereis greater adsorbent efficiency for the removal of dye RR141.The concentration of dye remaining at equilibrium for the dyeRR2 at this temperature was 0.031 mmol L−1 and for the dyeRR141 was only 0.0022 mmol L−1. This result indicates that dyeswith a larger quantity of charges have a greater tendency to beadsorbed by the adsorbent due to the electrical attraction of thedyes for the positively charged sites of the adsorbent surface. Thestructures of RR141 and RR2 have 8 and 2 sulfonic groups (−SO3

−),

Figure 5. Comparative kinetics for the dyes: (a) RR2; and (b) RR141 atdifferent temperatures.


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Figure 6. Comparative kinetics for the dye RR2 in the presence of(a) sodium chloride and (b) sodium sulfate.

respectively. Thus, the dye RR141, with a larger quantity of negativecharges, has a greater attraction force for the positive charges ofthe electric layer on the adsorbent surface, having therefore agreater exchange capacity than the dye RR2, this behavior havingpreviously been observed by Netpradit et al.29 The equilibriumtime for these kinetic tests did not exceed 5.5 h (330 min) beingonly 3.7 h (220 min) for the dye RR141 at the temperatures 25 and40 ◦C.

Figure 6 shows the comparative kinetics for the dye RR2 in thepresence of (a) sodium chloride and (b) sodium sulfate.

The equilibrium time for the kinetic tests shown in Fig. 6 andFig. 7 did not exceed 5.5 h (330 min), being 5 h (300 min) for thedye RR141 in the presence of 10% (by mass) of sodium chlorideand 4 h (240 min) for RR2 in the complete absence of salt.

In Fig. 6(a) it can be observed that with the addition of sodiumchloride (NaCl) to the adsorbate RR2, in different concentrations(1%, 5% and 10% m : m), there was inhibition of the adsorptioncapacity of the adsorbent. This effect increases with increase in theNaCl concentration. When 10% NaCl is added to the solution, theadsorption capacity is reduced by approximately 77.64% relativeto the equilibrium concentration obtained when no salt speciesis added to the solution. This behavior could be due to thelarge increase in the number of positively charged electrolytes,being repelled when in contact with the positive charges on theadsorbent surface, thus inhibiting the negative ions of the dyefrom approximating its surface. The addition of sodium sulfate(Fig. 6(b)) gives the solution a high concentration of sulfate ions,which neutralize the cations of the adsorbent surface. This leadsto the anions of the dye becoming closer to the adsorbent surface,

Figure 7. Comparative kinetics for the dye RR141 in the presence of(a) sodium chloride and (b) sodium sulfate.

enabling a lower adsorption inhibition effect in comparison withthe chloride ion. A significant influence of the anion is observedin the adsorption capacity inhibition process, in the case understudy [Cl−] and [SO4

−2], this effect being much greater in the caseof the former, as reported by Netpradit et al.29

For the dye RR141, the addition of sulfate ions to the solutionreduced the adsorption significantly, since this dye has a greaternumber of anions interacting with the positive ions of the sodiumand impeding the approximation of the anionic dye to theadsorbent surface. This behavior can be seen in Fig. 7(b), whichshows the comparative kinetics for the dye RR141 in the presenceor absence of sodium sulfate in the solution. The addition ofsodium chloride (10%) (Fig. 7(a)) favored adsorption in the caseof the dye RR141; this being more anionic than dye RR2 the saltcations can bind to the adsorbate with negative charge, reducingthe ionic repulsion and thus increasing the quantity of moleculesadsorbed on the surface.

Considering the previous discussion regarding the formationof a double electric layer, the presence of electrolytes in theadsorptive medium interferes with the formation of this electriclayer, which can be expanded when the solution has a highconcentration of sulfate ions that neutralize the cations at theinterface of the diffuse layer. The anions of the dye thus approachthe adsorbent surface with a lower intensity, hindering theadsorption of the dye.


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In general, when NaCl and Na2SO4 were added to the dyesolution, the dye uptake was reduced. For the dye RR2, thecolor removal decreased with increasing NaCl concentration. Thecolor removal for the dye RR141 was inhibited by the additionof sodium sulfate when compared with the addition of NaCl,because SO4

−2 ions have a stronger negative charge than Cl−

ions. It was also evidenced that the removal of dyes with largerchains is mainly dependent on the electrical attraction. The SO4

−2

ions may compete strongly with SO3− ions for binding sites on

the adsorbent under study. This result is similar to the removal ofreactive dyes reported by Natpradit et al.29

The authors suggest that this behavior is due to the interactionbetween the surface and the added solutes, which may block someof the active sites for the adsorption of the dye molecules. TheCl− ions of the sodium chloride and the SO4

−2 ions of the sodiumsulfate may interfere with the electrostatic attraction between theSO3

− ions of the dyes and the positive charges of the double layerof adsorbent under study. Futhermore, the higher the valence ofthe anions, the higher the interference in the adsorption of anionicdyes.

Equilibrium studiesThe adsorption isotherms indicate that the adsorbent caneffectively adsorb the impurities present and the requiredpurification can be obtained, as well as providing a maximumestimate of the adsorption capacity.29

The isotherms were adjusted by Langmuir29 and Freundlich29

following the methodology presented by Netpradit et al.17

However, in this study, better results were found for the linearcorrelation coefficient R2 of the Langmuir isotherm, indicating thatthis isotherm better represented the behavior of the adsorbent

under study, being comparable with the kinetics behavior foundpreviously.

Table 4 gives the equilibrium parameters of the Langmuirisotherm for the different experimental conditions.

On evaluating the parameters given in Table 4, obtainedthrough the linearization of the Langmuir equation (Equation (1)),it can be verified that the maximum adsorption capacity of theadsorbent decreased according to the increase in the temperatureof the adsorption tests, for both dyes. When working with differentconcentrations of sodium chloride added to the solution with theadsorbate RR2, the maximum adsorption capacity according tothe Langmuir equation (qm) reduced according to the increasein the concentration of this salt. For the tests carried out withthe addition of sodium chloride good correlation coefficients(0.907–0.987) were obtained for both dyes.

When different masses of sodium sulfate were added to thesolution with the adsorbate RR2, the maximum adsorption capacityincreased in relation to the addition of NaCl. For the dye RR141there was a slight reduction in the qm (mg g−1) in comparisonwith the adsorption tests carried out in the presence of NaCl. InTable 4 the best adsorption conditions can be identified (highestqm (mg g−1) value). For the dye RR2, the best adsorption conditionswere 25 ◦C in the complete absence of salt in the solution, andfor the dye RR141, the best adsorption conditions were 25 ◦C inthe presence of 10% (by mass) of sodium chloride in the solution.There was a drop of 58.81% in the adsorption capacity of dye RR2when 10% of sodium chloride was added to the solution and therewas an increase of 15.33% for the dye RR141 when 10% of sodiumchloride was added to the adsorbate.

With the determination of the KL constant (L mg−1) it ispossible to determine the enthalpy of adsorption �H, using

Table 4. Equilibrium parameters of the Langmuir isotherm for different experimental conditions

Langmuir parameters

Dyes Temperature (◦C) KL (L mg−1) qm (mg g−1) R2

RR2 25 0.113 53.476 0.928

40 0.275 31.746 0.872

60 0.077 24.876 0.954

RR141 25 0.209 66.670 0.997

40 0.199 65.789 0.976

60 0.117 51.020 0.984

Concentration of NaCl (% mass)∗

RR2 1 0.104 45.450 0.907

5 0.366 30.120 0.930

10 0.409 22.026 0.931

RR141 1 0.210 44.248 0.987

5 0.091 47.619 0.967

10 0.077 78.740 0.984

Concentration of sodium sulfate (% mass) (∗)

RR2 1 0.051 47.393 0.985

5 0.291 39.850 0.965

10 0.191 39.526 0.991

RR141 1 0.347 52.356 0.912

5 0.750 40.161 0.862

10 0.004 66.667 0.960

∗ The tests with salt were carried out at 25 ◦C.


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the Clausius–Clapeyron equation30 (Equation (2)). The standardenthalpy of adsorption may be determined through the slope ofthe straight line obtained from the linear regression of ln (KL) versus1/T . The enthalpy of adsorption was −11.1383 kJ mol−1 for RR2and −13.9966 kJ mol−1 for RR141. The negative sign obtained forthe adsorption enthalpy for RR2 and RR141 indicates exothermicadsorption in which heat is released during the adsorption process.This behavior was observed in the kinetic tests (Fig. 5(a) and (b))and equilibrium tests (Table 4), where the adsorption capacitydecreased as a function of the increase in temperature. Also, itis possible to conclude from the low enthalpy values obtained[<2000 kJ mol−1]30 that the adsorption process is of a physicalnature and occurs due to a difference in the energy and/orattractive forces, leading to molecules being physically capturedby the adsorbent. These interactions have a long reach, however,they are weak.

KL = A exp

(−�H

RT

)(2)

The adsorption isotherms for the two reactive dyes at constanttemperature (60 ◦C) are shown in Fig. 8(a). In order to confirm themechanism of dye adsorption on the adsorbent the isothermswere plotted again in Fig. 8(b) in which it is possible to observethat the adsorbent needs greater amounts of RR2 (mol dye g−1

adsorbent) than RR141 since RR2 contains less sulfonic groupsthan RR141. Therefore, it can be concluded that the mechanism isdependent on the amount of charge of the reactive dyes.

For the best adsorption conditions for both dyes, RL variedbetween 0.02 and 0.75 as shown in Table 4, indicating that theseisotherms are favorable.

In Table 5 the removal percentages are given for the dyes RR2and RR141 for the best experimental adsorption conditions atinitial dye concentrations of 100 and 500 mg L−1.

From the data on the removal percentages given in Table 5 itcan be verified that the adsorbent efficiently removed both dyesstudied.

Tests in continuumBreakthrough curves were obtained for the dyes RR2 and RR141under the best adsorption conditions shown in Table 5. The packedbed porosity had a value ε = 0.42.

The continuum curves were obtained for dye RR2 at an operatingtemperature of 25 ◦C for different bed heights and flows, and forthe dye RR141 at 25 ◦C with the addition of 10% (m : m) of sodiumchloride (NaCl) to the feed solution of the column, also withdifferent flows and bed heights. The breakthrough times for theseconditions are shown in Table 6.

The results obtained for the breakthrough curves show that anincrease in the feed flow leads to a greater volume of solution perunit of time through the column, and therefore, a shorter operating

Figure 8. Adsorption Isotherms for the adsorption of two reactive dyes onthe adsorbent in different units: (a) mg of dye g−1 of adsorbent versusmg L−1 and (b) mmol of dye g−1 of adsorbent versus mol L−1 (adsorbentdose: 1 g per 100 mL; system pH: 4.0; temperature: 60 ◦C).

time for both dyes studied, that is, the adsorbent saturates morequickly with an increase in flow.

For dye RR2, the column saturation time when it was operatedwith a fixed bed height of 15 cm and flow of 16 mL min−1, wasonly 55 min, and for a flow of 8 mL min−1 the time was 3.1times longer. For dye RR141, the saturation times under the sameconditions were 160 and 320 min, respectively. When the columnwas operated with dye RR2 solution the operating time wasreduced compared with the operating time of columns with dyeRR141 solution for the different flows, since the quantity of soluteadsorbed at equilibrium qe (mg g−1), obtained through fitting theLangmuir isotherm, was 53.48 mg g−1 for RR2 and 78.74 mg g−1

for RR141.Guelli U. Souza et al.32 studied the dye Basic Green 4 in fixed

bed columns packed with granular activated carbon for differentfeed flows. For a feed flow of 3.2 mL min−1 the breakthrough timewas two times higher than that for a flow of 6.0 mL min−1.

Table 5. Removal percentages for the dyes RR2 and RR141, for the best adsorption conditions at initial dye concentrations of 100 and 500 mg L−1

Adsorbate Temp. (◦C) Co (mg L−1) Ce (mg L−1) qe (mg g−1) %Removal RL

RR2 25 500 57.14 44.29 88.57 0.1313

100 0.274 9.97 99.73 0.9693

RR141 with 10%by mass of NaCl

25 500 21.37 47.86 95.72 0.3783

100 2.80 9.72 97.20 0.8230


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Table 6. Breakthrough time (tb) and quantity of dye removed by the adsorbent at equilibrium (qe) for the dyes RR2 and RR141

DyeFlow

(mL min−1)Bed height

(cm)tb

(min)qe

(mg g−1) DyeFlow

(mL min−1)Bed height

(cm)tb

(min)qe

(mg g−1)

RR2 8 15 170 13.70 RR141 8 15 320 38.69

30 290 19.00 30 530 48.56

45 430 19.55 45 820 57.70

12 15 100 16.41 12 15 210 41.73

30 165 27.20 30 360 49.25

45 210 21.51 45 490 44.70

16 15 55 13.42 16 15 160 39.98

30 100 17.08 30 290 45.82

45 190 18.60 45 330 42.37

Figure 9. Breakthrough curves for the dyes (a) RR2 (T = 25 ◦C) and(b) RR141 (T = 25 ◦C; 10% NaCl) for different bed heights with a feedflow of 16 mL min−1.

Figure 9 shows breakthrough curves for the dyes (a) RR2 and(b) RR141, for different bed heights and a fixed flow of 16 mLmin−1.

According to Fig. 9, on the breakthrough curve obtained atdifferent bed heights it can be observed that an increase in thecritical height of the bed led to slower saturation of the columnowing to it having a greater quantity of adsorbent, thus increasingthe operating time of the adsorbent. For a bed height of 45 cmand fixed flow of 12 mL min−1 the saturation time for the dye RR2was 52.38% longer when compared with that for a bed heightof 15 cm, and for the dye RR141 under the same conditions, the

saturation time was 57.14% longer than that for a bed heightof 15 cm. The equilibrium adsorption capacity for the dye RR2increased on increasing the height of the column packing from15 to 30 cm in proportion 1 : 1.7 for a fixed flow of 12 mL min−1,and for an increase in flow from 8 to 16 mL min−1 there were nosignificant variations in this parameter.

The experimental results were compared with data in theliterature reported by Guelli U. Souza et al.,32 showing a highadsorption capacity of the adsorbent under study.

CONCLUSIONSWhen tests were carried out at 25 ◦C, the dye RR141 was moreeasily adsorbed by the adsorbent than the adsorbate RR2. This isdue to the different chemical structures of the two dyes.

Through kinetic tests it was verified that when sodium chlorideis added to the adsorbate RR2, at different concentrations, thereis a reduction in the adsorption capacity of the adsorbent forconcentrations above 5%. In the case of the dye RR141, theaddition of NaCl increased the equilibrium adsorption capacityfrom 44.25 mg g−1 (1% by mass of NaCl) to 78.74 mg g−1 (10% bymass of NaCl).

For both dyes the addition of sodium sulfate did not favoradsorption of the dyes under study, since there is a lower quantityof conducting ions present in the adsorbates RR2 and RR141 inthe presence of 1, 5 and 10% sodium sulfate in the solution.

From the equilibrium tests carried out it was possible toconclude that the best experimental conditions for the adsorptionof RR2 were a temperature of 25 ◦C in the complete absence ofsalt addition to the solution. For the dye RR141, best experimentalconditions were a temperature of 25 ◦C in the presence of 10% bymass of sodium chloride in the solution. The results for qm (mg g−1)obtained through the fitting of the Langmuir isotherm were53.48 mg g−1 and 78.74 mg g−1, for RR2 and RR141, respectively.

High color removal efficiency of the adsorbent was verifiedfor both dyes studied, where the percentage removal for dyesRR2 and RR141 under the best adsorption conditions, at initialconcentrations of 100 and 500 mg L−1, were 88.57% and 99.73%,respectively, for RR2, and 95.72% and 97.20%, respectively, forRR141.

The equilibrium adsorption capacity for the dye RR2 increasedwith increase in the packed column height from 15 to 30 cm inthe proportion 1 : 1.7 with a fixed flow rate of 12 mL min−1, andfor an increase in the flow from 8 to 16 mL min−1 there were nosignificant variations in this parameter.


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The parameters determined in the tests in continuum can beused to design the column in prototypes and at industrial scale.

The adsorbent studied was shown to be a very promisingalternative in terms of an environmentally friendly process, aimingat reduction of the environmental impact of textile industry dyeson water resources.

ACKNOWLEDGEMENTSThe authors are grateful to Coordenacao de Aperfeicoamento dePessoal de Nıvel Superior – CAPES, Brazil, for financial support bymeans of a grant and to FINEP for support through the projectINOTEXTIL.

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Application of ecological adsorbent in the removal of reactive dyes from textile effluents

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