dye adsorption by leather waste: mechanism diffusion, nature studies, and thermodynamic data

Dye Adsorption by Leather Waste: Mechanism Diffusion, NatureStudies, and Thermodynamic DataJeferson S. Piccin,* Liliana A. Feris, Mrian Cooper, and Mariliz Gutterres

Chemical Engineering Post Graduation Program, Laboratory for Leather and Environmental Studies (LACOURO), FederalUniversity of Rio Grande do Sul, Luiz Englert str., s/no., 90.040-040, Porto Alegre, RS, Brazil

ABSTRACT: Tannery solid waste (leather) is a possible adsorbent of dyecontaminants in wastewater. In this paper the nature and mechanisms of dyesadsorption by chromium-tanned leather waste (CTLW) are proposed anddiscussed on the basis of isotherms, adsorption kinetics, and thermodynamicsof three dyes: Red 357, Black 210, and Yellow 194 in aqueous solutions.Langmuir, BrunauerEmmettTeller (BET) and Henry isotherm modelswere used to fit the adsorption equilibrium data, respectively. The kinetic datawere evaluated using boundary layer mass transfer and intraparticle diffusionmodels. The boundary layer mass transfer coefficient was in the order of 106

and 105 mmin1 for the three studied dyes. However, the intraparticlediffusion were of the order of 108 and 1011 m2min1 for the Red 357 andBlack 210 dyes, respectively, demonstrating that intraparticle diffusion is thepredominant mass transfer mechanism of these dyes. The values of H, S,G, and Ea suggest that adsorption is spontaneous, exothermic, and chemical in nature. The chemical nature is also confirmed byFourier transform infrared (FT-IR) analysis.

1. INTRODUCTIONMany industries, such as leather, food, cosmetics, plastics, andtextiles, use dyes to confer, intensify, or restore the color oftheir products. The world consumption of dyes is estimated tobe around 700 000 tonnes per year.1,2 In these industries,during the processes involved in production usually dyes aredissolved in water. Despite being a contaminant thatsignificantly contributes to the elevation of chemical oxygendemand of wastewaters, at low concentrations they maysignificantly change the color of the water, causing aestheticproblems in water bodies polluted with industrial effluents.Moreover, the biological treatment systems may not be efficientfor the removal of color, when the objective is the wastewaterreuse, especially in tanneries that perform only the leatherfinishing, because in these cases the wastewater has higher dyeconcentrations.Adsorption is one of the most effective methods used for

removal of dyes and other soluble substances from wastewater.Activated carbon is the most widely used adsorbent in variousindustrial sectors. However, especially due to its production andregeneration cost, the use of adsorption in industrial processes,as tanneries, is limited. Therefore, the use of industrial waste asalternative adsorbents has been considered. In this context,solid waste generated in the leather processing operation hasbeen used as alternative adsorbents for the removal of dyes,metals, oils, and surfactants.36 Although a final cleanup ofwater-based effluents using an adsorbent like activated carbon isnot common practice on the industrial scale in the leatherindustry, the use of a solid waste of the process may be arecycling alternative to reduce the cost of disposal this wasteand to reuse the treated water. The ongoing development on

advanced treatment systems enables wastewater regenerationrequirements for segregated or integrated water recycling orreuse and the removal of difficult contaminants to meetenvironmental regulatory standards for discharge.7

Because of the design of fixed bed column adsorptionsystems, the adsorption capacity and mass transfer rate ofsolution for the adsorbent particle are the most importantparameters.8 The adsorption capacity is described by solidliquid equilibrium curves and represented by isotherm models.However, the mass transfer mechanisms for the solution tosolid sorbent can be obtained from the adsorption kineticsusing kinetic models described in the literature. In addition,isotherms and kinetics provide important thermodynamic datafor the characterization of the nature of adsorption.In this paper the effect of temperature on the adsorption of

dye, adsorption isotherm, and adsorption kinetics of leatherwastes were investigated. Isotherm models were proposed, andadsorption mechanisms were evaluated using boundary layermass transfer and intraparticle diffusion models (HSDM). Thethermodynamic parameters were calculated, and the adsorptionnature was evaluated using Fourier transform infrared (FT-IR)analysis.

2. EXPERIMENTAL SECTION

2.1. Adsorbate. Three commercial dyes used were suppliedby the Business Leather Unit of Lanxess Company. Information

Received: September 4, 2012Accepted: February 12, 2013Published: February 21, 2013

Article

pubs.acs.org/jced

2013 American Chemical Society 873 dx.doi.org/10.1021/je301076n | J. Chem. Eng. Data 2013, 58, 873882

pubs.acs.org/jced

on the C.I. Acid Red 357 (Red 357, an azo-dissulphonated Cr-organic-complex dye, CAS No. 57674-14-3, and purity of 55%), C.I. Acid Black 310 (Black 210, an amine dissulfonatedtriazo organic-dye, CAS No. 99576-15-5, and purity of 70 %),and C.I. Acid Yellow 194 (Yellow 194, an azo-dissulphonatedCo-organic-complex dye, CAS No. 85959-73-5, and purity of50 %) were obtained from the American Association of TextileChemists and Colorists (AATCC), the United States Environ-mental Protection Agency (EPA), and previous work.9 Figure 1shows their optimized chemical structure using ChemBio 3D11.0.1 software and others characteristics. Solutions ofcommercial dyes with approximately 800 mgL1 wereproduced for the experiments, and these correspond to aninitial concentration of 375 mgL1, 465 mgL1, and 555mgL1 of Yellow 194, Red 357, and Black 210 dyes,respectively, calculated based on the purity of the dye products.2.2. Adsorbent Preparation. Leather waste samples from

chromium-tanned leather shaving operation were obtainedfrom a local tannery (Portao/RS, Brazil). The so-calledadsorbent chromium-tanned leather waste (CLTW) wasdried, ground, and sieved according to Piccin et al.9 Table 1shows the physical-chemical characteristics of the adsorbentused.2.3. Sorption Experiments. Equilibrium adsorption and

kinetics studies were carried out by batch conditions at differenttemperatures [(15 to 45) C] according Piccin et al.9 The dyeconcentrations were determined by UVvis spectrophotometryusing standard curves obtained considering the purity degree ofthe dyes. All adsorption experiments were performed at pH 2.5and in duplicate, and data were considered satisfactory with acoefficient of variation of less than 2.5 %. The Giles et al.10

classification was adopted to evaluate the behavior ofequilibrium data, and Langmuir, BrunauerEmmettTeller(BET), and Henry isotherm models were proposed to correlateexperimental data.9 From kinetic data, film fluid in boundarylayer mass transfer and intraparticle diffusion models wereproposed, and the mechanisms of adsorption were checked.2.4. FT-IR Analysis. The adsorbent samples were dried to

constant weight at 105 C, before and after the adsorptionprocess. Afterward, the samples were analyzed by infraredspectroscopy (Perkin-Elmer, Spectrum 1000, USA), in therange of (4000 to 400) cm1, using transmittance spectrumwith a potassium bromide disc (1 part of adsorbent for 20 partsof KBr).11,12

3. RESULTS AND DISCUSSION3.1. Adsorption Isotherms. Figure 2 shows the effect of

temperature on adsorption equilibrium of leather dyes inCTLW. According Giles et al.,10 it is shown that Red 357 dyeadsorption isotherm is apparently of the H2 type, traditionalshape observed in studies of alternative adsorbents. In this case,the H2 type isotherm was represented by the Langmuirmodel with satisfactory adjustment to the experimental data. Itwas the same with adsorption of dyes on chromium containingleather wastes,3,6 thermo-chemical modified leather wastes,12

chitosan beads,13 and Spirulina platensis.14

Nonconventional equilibrium data are observed for Black210 and Yellow 194, corresponding to H3 and C1 isothermtypes, respectively. Black 210 adsorption by CTLW are of H3and L3 types, corresponding to temperatures less than 25 Cand more than 35 C, respectively. However, Yellow 194 dyeadsorption shows C1 isotherm type. In literature, H3 andL3 types isotherms have been satisfactorily represented by the

type of BET model, and it was the same with pentaclorophenoladsorption by carbonized pine bark,15 dyes in groundwater byclay,16 and biosorption for color removal of pulp mill effluentby fungal biomass.17 Moreover, linear isotherms of C1 type

Figure 1. Optimized three-dimensional chemical structures of theleather dyes Red 357 (a), Black 210 (b), and Yellow 194 (C).

Journal of Chemical & Engineering Data Article

dx.doi.org/10.1021/je301076n | J. Chem. Eng. Data 2013, 58, 873882874

were represented by the Henry model, as observed forphosphate adsorption by loess modified with zinc,18 theadsorption of Reactive Black and Reactive Yellow hydrolyzeddyes by diatomaceous earth19 and the adsorption of MethyleneBlue, Reactive Black, and Reactive Yellow dyes by calcineddiatomite.20 A previous study reported by Piccin et al.9

presented the Langmuir (eq 1), BET (eq 2), and Henry (eq3) isotherm models to predict the equilibrium data of Red 357,Black 210, and Yellow 194 dyes adsorption by CTLW,respectively. Table 2 shows the observed parameters, thecorrelation coefficient (R2), and average relative errors (AREs)for the adsorption of tannery dyes by CLTW, according to theselected models for each one of them.

=+

qq k C

k C1em L e

L e (1)

= +

qq k C

k C k C k C(1 )(1 )eBET 1 e

2 e 2 e 1 e (2)

=q k Ce H e (3)

High coefficients of determination (R2 > 0.95) exhibited inTable 2 indicate that more than 95 % of the variability of qe as afunction of Ce increase can be explained by the proposedmodels. Moreover, low AREs demonstrate that the fractionerror of qe distributed across the entire concentration range islower than 10 %. Therefore, the isotherms models proposed foreach dye has a good theoretical correlation with experimental

Table 1. Characterization of Chromium-Tanned LeatherWaste (CTLW)

parameterdeterminedvaluea method

moisture (%) 7.8 0.8 ASTM D3790-79ashes (%, D.B.) 9.0 0.4 ASTM D2617-06total carbon (%, D.B.) 37.1 2.5 instrumental (Shimadzu

SSM-5000A)total chromium(%, D.B.)

2.5 0.1 ABNT NBR 11054

particle diameter (mm) 0.98 0.22 screeningdensity (kgm3) 1450.2 37.0 picnometryaMean standard deviation, n = 3.

Figure 2. Adsorption isotherms of Red 357 (a), Black 210 (b), and Yellow 194 (c) in different temperature conditions.

Table 2. Adsorption Isotherm Parameters of Tannery DyeAdsorption by CTLW

parameter 15 C 25 C 35 C 45 C

Red 357KL (Lmg

1) 0.218 0.142 0.112 0.090QM (mgg

1) 218.8 232.0 239.9 250.6R2 0.980 0.975 0.953 0.973ARE (%) 3.7 5.7 5.4 6.0

Black 210K1 (Lmg

1) 2.907 0.522 0.042 NDK2 (Lmg

1)103 2.10 1.70 1.50 NDQBET (mgL

1) 43.2 108.8 138.1 NDR2 0.994 0.998 0.996 NDARE (%) 4.857 2.494 3.711 ND

Yellow 194KH (Lg

1) 2.172 1.909 1.100 0.993R2 0.986 0.985 0.988 0.997ARE (%) 5.2 3.6 4.7 1.4



data of adsorption equilibrium and, thus, can be used torepresent the equilibrium curves. The solid lines traced inFigure 2 represent the correlation of the proposed models toequilibrium experimental data.For the Red 357 adsorption isotherm (Figure 2a), the

Langmuir theory (eq 1) is observed when the adsorbent surfaceand the adsorptions sites are energetically homogeneous andonly one adsorption occurs by site.13 This behavior indicatesthat high adsorption capacities are observed in the solid phasewith low concentrations of solute in the liquid phase, due to thehigh affinity of the dye molecules for the leather surface.Therefore, Table 2 shows an increase in qm values and decreasein kL values with temperature increase from (15 to 45) C forRed 357 adsorption. The behavior of these parameters indicatesthat higher temperatures increase the maximum adsorptioncapacity, which can only be achieved with higher liquidconcentrations. The observed values of qm in this study arehigher than those reported in other research for the adsorptionof dyes for alternative and conventional adsorbents, as the caseof Reactive Red (C.I. 18286) by leather wastes (56 mgg1 to163 mgg1) and activated carbon (48 mgg1),6 FD&C 40 bySpirulina platensis (225.2 mgg1 at 25 C and pH = 4),14 andmethylene blue and crystal violet by clay (50.8 and 57.8,respectively).16 An increase in the maximum adsorptioncapacity and decrease in kL constant with the increase intemperature was observed with dye adsorption on FD&C Red40 and C.I. Reactive Black 13 dyes by chitosan.13,21

Concerning the linear isotherm shape, as it is observed forisotherm data from Yellow 194 dye in Figure 2c, the adsorptioncapacity increases linearly with an increase in equilibriumconcentration. Henrys law applies when the relationship ofequilibrium between concentrations on the fluid phase andsolid phase are linear (C1 type) and proportional to Henrys

constant (kH). In this case, the number of sites is very superiorto the number of adsorbate molecules. Henrys law applies tothe adsorption on a uniform surface at sufficiently lowconcentrations so that all molecules are isolated from theirnearest neighbors. This suggests that occurs a hydrophobicinteraction between the adsorbent and adsorbate,10 causing alinear increase in adsorption capacity. The linear relationshipbetween the fluid phase and adsorbed phase equilibriumconcentrations, with a constant of proportionality, which isequal to the adsorption equilibrium constant known as Henrysconstant (kH). An increase in temperature from (15 to 45) Ccauses a decrease in kH values of 2.172 Lg

1 for 0.993 Lg1.This indicates that adsorption capacities are approximately 2.2times superior with reduction in temperature from (45 to 15)C. At high temperatures, weak physical chemical interactiontakes place between the dye and the adsorbent due to reductionin hydrogen bonds and van der Waals interactions, reducing theadsorption capacity.14,22

For the Black 210 equilibrium data, Figure 2b shows that theBET isotherm model predict satisfactorily experimental data.The BET isotherm is an extension of the Langmuir theory formonolayer adsorption to multilayer adsorption, when theincrease of adsorbateadsorbate interaction and multilayerformation occurs due to secondary adsorption at a givensite.12,23 The multilayer formation may occur due to a change inorganizational form of dye molecules arranged on the surface ofthe adsorbent, in horizontal to vertical alignment, or due tosolubility reduction caused by superficial hydrophobic inter-actions between the adsorbate and the adsorbent.10,15,17,23

These phenomena can be complemented by chemicalinteractions between adsorbate in solution and one that hasbeen adsorbed, since the Black 210 dye has R-NH2 group,

Figure 3. Adsorption kinetics of Red 357 (a), Black 210 (b), and Yellow 194 (c) dyes in different temperature conditions.



which under acidic conditions can be protonated, resulting inadditional adsorption of dye in solution.Therefore, for the Black 210 dye adsorption, there is a strong

influence of temperature on the equilibrium sorption. Theincrease of temperature favor the formation of monolayers,causing an increase in the qBET value of 42.3 mgg

1 to 138.1mgg1, which were superior than the values observed for qBETof methylene blue and crystal violet dyes present ingroundwater by clay (44 mgg1 and 47 mgg1, respectively)16

and pentachlorophenol by carbonized bark (0.84 mgg1).15,23

As the temperature increase occurs, an increase of free volumeand a decrease of the interaction between solvents and solidsurfaces are observed, exposing a higher number of adsorptionsites, which are favorable to adsorption.However, the values of k1 and k2 decrease with increasing

temperature. Like kL in the Langmuir model, k1 represents theinverse of the equilibrium concentration in the liquid when theadsorption capacity reaches half the capacity of monolayeradsorption (or f(1/k1) = 0.5qBET, where f is the function of theBET isotherm). In other words, the k1 increase indicates thatsmaller equilibrium concentrations are necessary to saturate themonolayer. This implies a more rectangular isotherm curve tothe achievement of monolayer saturation, as it is clearly seen inFigure 2b.The k2 values represent the inverse of the concentration

value when the isotherm becomes a vertical line (Cs = 1/k2)and are associated with superficial solubility of dye.16 In general,the solubility of the dyes is also increased with highertemperatures. Therefore, adsorbateadsorbate interactions arereduced. Thus, a temperature reduction from (45 to 15) Creduces Cs values from 666.6 mgL

1 to 476.2 mgL1,indicating that high adsorption capacity by multilayer formationis obtained with lower equilibrium concentrations, favoring

adsorption. The Cs in the adsorption of crystal violet dye ingroundwater by clay was 116 mgg1, whereas the presence ofNH4Cl resulted in reduction of Cs from 137 mgg

1 to 116mgg1.16 Besides, the Cs value of pentachlorophenoladsorption by carbonized bark was 5.43 mgg1.15,23 A decreasein k1 and k2 values indicates that adsorption of Black 210 isfavored by low temperatures.

3.2. Adsorption Kinetics. Figure 3 shows the effect oftemperature on the adsorption kinetics of dyes by chromiumtanned leather waste. The adsorption capacity of Yellow 194increases rapidly during the first hour of adsorption and thenremains nearly constant after it, suggesting that equilibrium wasquickly reached. For temperatures of (35 and 25) C, althoughthe adsorption capacity of Red 357 is initially lower than that ofYellow 194 dye, after (30 to 60) min, the adsorption capacity ofRed 357 dye is higher than that of the Yellow 194. Highertemperatures are associated with a greater adsorption capacityof Red 357 dye compared to Yellow 194. Experiments haveshown a clear trend to increased absorption capacity of the Red357 and Black 210 dyes after 120 min, meaning thatequilibrium adsorption capacity has not been reached.Adsorption rates involve the following mechanisms: (i) liquid

bulk diffusion; (ii) convection in mass boundary layersurrounding the particle, or liquid film diffusion; (iii)intraparticle diffusion (surface or pore diffusion); (iv) availableadsorption sites. However, bulk diffusion and adsorptionreaction are typically instantaneous, and the mass transfer iscontrolled only by convection in the boundary layer orintraparticle diffusion separately or simultaneously.24,25

In the case of liquid mass transfer, or external convection, themodel assumes for short periods of time, that the diffusion stepdoes not affect the adsorption rate. The model can be describedby the liquid linear driving force (LDF), according eq 4:24,25

Figure 4. Plotting of the liquid film diffusion model: (a) Red 357; (b) Black 210; (c) Yellow 194.



= qt

k a C Cdd

( )f e (4)

where q is the dye concentration on the solid phase (mgg1), kfis the mass transfer coefficient on the boundary layer(mmin1), a is the specific surface area (m2g1), and C andCe are the dye concentrations in the liquid phase (mgm

3).Using appropriate initial conditions, the solution of eq 4 isgiven according to eq 5.

=

C CC C

k awVtln e

0 ef p

(5)

Plotting the left side of eq 5 versus the time will providelinearized data (Figure 4), with angular coefficient equal to kfa,for the range where convection limits the rate of adsorption.When the internal resistance is predominant in the mass

transfer process, the intraparticle diffusion can be representedby the HSDM model, according to eq 6.24,26,27

=

+

qt

DQR R

QR

2s

2

2(6)

Using appropriate boundary and initial conditions, thesolution of the model is given according to eq 7.

=

=

q

q nn

DtR

16 1

exptn

e

1

22 2 s

p2

(7)

Suziki28 and Qiu et al.24 reports that, for short periods oftime, when qt/qe is less than 0.3, eq 7 approximates to eq 8:

= q

qA

DstR

t

e p2

(8)

where Rp is the mean radius of the particles and A equal to 3.38and 4.24 for the linear isotherm and for the rectangularisotherm, respectively. Plotting qt versus t

1/2 (Webber andMorris plot, Figure 5) it shows multilinearity portions; an initialand curved behavior indicates the convective resistance, and thefinal linear portions, relative to solid diffusion effects.2,2931

Figures 4 and 5 shows that Red 357 and Yellow 194 dye havelinear portions starting of the origin throughout the experimentfor both propose the mass transfer mechanisms. This suggeststhat the two proposed models are able to explain the kinetics ofadsorption and the adsorption rate is controlled simultaneouslyby fluid-film mass transfer and intraparticular diffusion.However, by adsorption of Black 210, the sharp curvature inthe Figure 4b and linear portion starting throughout theexperiment of the Weber and Morris plot (Figure 5b) suggestthat the adsorption rate mechanisms is of intraparticulardiffusion. The resistance mechanism, boundary layer con-vection, or intraparticle diffusion are evaluated by nondimen-sional Biot number (NBi), calculated from eq 9.

32,33

=N

k R C

D qBif p 0

s p o (9)

where Rp is the particle radius (m), p is the adsorbent density(kg3m3), and q0 is the adsorption capacity (mgg

1)considering Ce = C0. Table 3 shows the values obtained forthe convection coefficient, the effective diffusion inside the solidparticle, calculated according to eqs 5 and 7, respectively, anddimensionless Biot number.

Figure 5. Plotting of intraparticle diffusion model: (a) Red 357; (b) Black 210; (c) Yellow 194.



The data in the Table 3 show that, in magnitude, the soliddiffusion of the dye in the adsorbent is Yellow 194 > Red 357 Black 210, while the convection coefficient is on the order of105 mmin1 to 106 mmin1 for the three dyes. The highdiffusivity values of the Yellow 194 dye makes that theequilibrium is reached within a few minutes, while the Red 357dye equilibrium is reached in a few hours and the Black 210 dyetakes days or even weeks, as can be evaluated in Figure 3. Thekf values observed were inferior that for the adsorptionherbicides and pentaclorophenol by activated charcoal (104

mmin1 and 103 mmin1, respectively)32,34 and pesticide byacid treated oil shale ash (102 mmin1).35 However, the Dsvalues of the Yellow 194 and Red 357 dyes were superior toother alternative adsorbents, such as FD&C Red 40 dye bychitosan (1010 m2min1) and pesticide by acid treated oilshale ash (109 m2min1). This may be because the stirring ofthe system and the high concentration of dye used in this study,which causes an increase in solution viscosity. These differencescan be explained, because while the first is affected principallyby the stirring of the system, that is not identical in the differentstudies, both could also have affected by initial dyeconcentration, which was higher in this present work, reducingviscosity and causing a decrease in external and internal masstransfer coefficients.32,34

In relation to the effect of temperature on the diffusioncoefficient, Table 3 shows that for the three dyes the increase intemperature favored intraparticle diffusion due the increase inDs values. Similarly, for the dyes Red 357 and Yellow 194,increase in temperature led to increase in external convectioncoefficient (kf), reducing resistance to mass transfer inboundary layer. In general, the convection and intraparticlediffusion coefficients increase due the increase of adsorbatemolecular diffusion in the solvent at greater temper-atures.8,32,34,36 In addition, increasing temperature reduces theviscosity of a solution, facilitating penetration of the adsorbatein the solid sorbent.35

The decrease in Biot number indicates that as temperaturerises the thickness of the boundary layer surrounding theadsorbent and the mass transport resistance of the adsorbate inthe boundary layer is reduced. According Cooney,37 for NBi 30, there is reasonablycomplete dominance of intraparticle resistance. Therefore, the

external mass transfer control predominates in the adsorptionprocess of dye Yellow 194, while both mechanisms are active inthe adsorption of Red 357 dye and the intraparticular diffusioncontrols the mass transfer dye Black 210, due the low values ofsolid diffusion coefficients.

3.3. Adsorption Thermodynamics. Adsorption thermo-dynamics was determined using the thermodynamic equili-brium coefficients obtained at different temperatures for thethree dyes, to verify possible adsorption mechanisms.The Gibbs free energy change (G) is associated to

spontaneity of the process, and it is calculated according toeq 10.

= G RT kln D (10)

where kD is the thermodynamic equilibrium constant, R is theuniversal gas constant (8.314 Jmol1K1), and T is thetemperature (K). kD was obtained from the slope of initiallinear portion of qe vs Ce,

38 and converted to a dimensionlessconstant according Milonjic.39

Gibbs free energy is the difference between adsorptionenthalpy (H) and adsorption entropy (S), at constanttemperature. Thus, by applying this concept to eq 12, vantHoffs (eq 11) can be used to determine the thermochemicalparameters through the plot of ln kD vs 1/T obtaining a angularcoefficient equal to H/R and linear coefficient equal to S/R.

= + k HRT

SR

ln D (11)

Moreover, the diffusion coefficients are represented as anexponential function of temperature, according to theArrhenius equation.27,28,40

=

D D

ERT

exps 0a

(12)

where Ea is the activation energy (kJ mol1) and D0 is the

diffusion at a temperature of 0 K. Then, Ea and D0 aredetermined by regression of the linearized form of theArrhenius equation, by plotting ln(Ds) vs 1/T, obtaining alinear coefficient equal to ln(D0) and a slope equal to Ea/R.Figures 6 and 7 shown vant Hoffs and Arrhenius plots for Red357, Black 210, and Yellow 194 dyes adsorption. Table 4 showsGibbs free energy change (G), enthalpy (H), and entropy(S), activation energy (Ea) obtained through eqs 10, 11, andlinearized form of eq 12 and respective R2 of the plots.

Table 3. Convective and Intraparticle Diffusion Coefficientsand Biot Number of Tannery Dye Adsorption by CTLW

dye/temperature kf Ds

C mmin1 R2 m2min1 R2 Bi

Red 35715 4.41106 0.888 1.36109 0.983 6.7825 3.98106 0.849 2.87109 0.994 2.7535 4.41106 0.963 4.89109 0.989 1.7345 4.95106 0.979 1.01108 0.994 0.91

Black 21015 7.11106 0.852 5.621012 0.951 147.525 4.46106 0.812 3.251011 0.956 90.835 4.77106 0.811 6.151011 0.981 50.3

Yellow 19415 1.33105 0.921 1.48108 0.937 0.4025 1.39105 0.935 1.63108 0.985 0.4435 1.58105 0.929 2.16108 0.968 0.6545 2.19105 0.977 3.56108 0.981 0.60

Figure 6. vant Hoff plot of tannery dye adsorption by CTLW.



Negative G values indicate that the adsorption process ismore favorable and spontaneous at low temperatures, notrequiring external energy from outside of the system.14

However, for Black 210 dye adsorption, a significant increasein G values as temperature increases leads to a decrease in theadsorption driving force, reducing adsorption.13,30 Negative Hvalues indicate that the adsorption process is exothermicbecause there is a decrease in system enthalpy. A decrease inenthalpy indicates that the Black 210 dye adsorption is mostaffected by temperature. Negative S values indicated that dyesare arranged in ordered manner in the solid phase. Therefore,adsorption causes a reduction in system disorder. For Red 357and Black 210 dyes, although internal mass transfer resistancewas important, activation energy (Ea) values were higher than50 kJmol1. These values suggest that the adsorption processhas a chemical nature.29 However, for Yellow 194 dye,intraparticle diffusion can be neglected, for it is not influencedby temperature. Thus, a low Ea value is observed.3.4. IR Spectroscopy. Figure 8 shows FT-IR analysis of

leather waste adsorbent before and after adsorption of dyes.Before adsorption, the peaks near (3500 to 3300) cm1

represent the symmetric stretching vibration of the OHgroup that overlaps the vibration of the NH group. Absorptionpeaks for the sample in 1960 cm1 may result from thestretching vibration of CH. The structure of the proteinmaterial is demonstrated by the signals in 1659 cm1 in relationto the carbonyl group (CO) and in 1547 cm1 to the NHgroup. Moreover, the peak at 1239 cm1 is attributed to acombination of NH bending and CN stretching and thepeak of 1127 cm1 to CO stretching.11,12 After theadsorption process, the presence of a peak near (1042 to

1033) cm1 was observed due the vibration of SO3+ group.

Additionally, visible changes are observed in peaks of (1239 and1127) cm1. Such behaviors are caused by changes in themolecular composition of the groups possibly by dyeelectrostatic interaction with the amino group of the leathersorbent, proving the chemical nature of adsorption.

4. CONCLUSIONSChromium-tanned leather waste was used as alternativeadsorbents of Red 357, Black 210, and Yellow 194 dyes inaqueous solutions at different temperatures. The maximummonolayer adsorption capacity of Red 357 and Black 210 dyeswere 250.6 mgg1 at 45 C and 138.1 mgg1 at 35 C,respectively, and the temperature reduction led to a decrease ofmonolayer saturation. The observed values for respectiveisotherms were superior to adsorption of others dyes byconventional or alternative adsorbents. Moreover, for the Black210 dye adsorption, a nonconventional isotherm, the reductionin temperature favored the formation of multilayers at lowerequilibrium concentrations, and the superficial solubility at 15C was of 476.2 mgL1. Yellow 194 adsorption isotherms didnot provide formation of mono- or multilayers, and theadsorption capacity at equilibrium for a liquid concentration of100 mgL1 decreased from 217.2 mgg1 to 99.3 mgg1 with atemperature increase of (15 to 45) C.The solid diffusion coefficients (Ds) were in the order of 10

8

m2min1, 109 m2min1, and 1011 m2min1 for Red 357,Black 210, and Yellow 194 dyes, and the temperature reductionlead to an increase of diffusion coefficients. Convectivecoefficients (kf) were in the order of 10

5 to 106 mmin1

for all dyes and conditions studied. A Biot number (NBi) greaterthan 0.5 shows that both mechanisms of mass transfer(convection and diffusion) control the adsorption process ofRed 357. However, for Black 210 with NBi > 30 and for Yellow194 with NBi < 0.5 it was found that solid diffusion andboundary layer mass transfer have complete dominance overthe mass transfer, respectively.Negative values of G, H, and S suggest a spontaneous,

exothermic, and favorable adsorption process for three studieddyes. Moreover, thermodynamic studies of activation energy(Ea) and FT-IR analysis suggest that dye adsorption has achemical nature.The results observed for the adsorption capacity and mass

transfer rates in the order of magnitude of other conventionaland nonconventional adsorbents clearly demonstrated the

Figure 7. Arrhenius plot of tannery dye adsorption by CTLW.

Table 4. Thermodynamics Parameters of Tannery DyeAdsorption by CTLW

parameters T/C Red 357 Black 210 Yellow 194

Isotherm Thermodynamic DataG (kJmol1K1) 15 9.25 11.57 1.86

25 8.66 10.01 1.6035 8.43 4.49 0.2445 8.24 ND 0.02

R2 0.952 0.920 0.920H (kJmol1) 18.6 112.9 24.9S (kJmol1K1) 0.033 0.35 0.07

Kinetics Thermodynamic DataR2 0.996 0.940 0.927Ea (kJmol

1) 23.6 89.4 22

Figure 8. FT-IR spectra of: (a) CLTW; (b) CLTW + Red 357; (c)CLTW + Black 210; (d) CLTW + Yellow 194.



efficiency of leather waste to dyes removal present in tannerywastewater, allowing survival of an industrial byproduct beforethe disposal in landfills of hazardous waste or by othertraditional method.

AUTHOR INFORMATIONCorresponding Author*Tel.: +55 51 3308 3954; Fax: +55 51 3308 3277. E-mailaddress: [email protected] (J. S. Piccin), [email protected] (M. Gutterres).FundingThe authors would like to thank CNPq (National Council ofScience and Technological Development) for the financialsupport by Edict CTAGRO No. 40/2008 and the BusinessLeather Unit of Lanxess Company for the technical support.NotesThe authors declare no competing financial interest.

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