Calcined Layered Double Hydroxides for Decolorization of Azo Dye Solutions:Equilibrium, Kinetics, and Recycling Studies

Thaisa P.F. Teixeira,1 Simone I. Pereira,2 Sergio F. Aquino,3,* and Anderson Dias3

1M.Sc. Student, Postgraduate Program in Environmental Engineering, 2Undergraduate Student in Environmental Engineering, and3Department of Chemistry, Campus Morro do Cruzeiro, Federal University of Ouro Preto, Ouro Preto, Brazil.

Received: June 19, 2011 Accepted in revised form: November 25, 2011

Abstract

This work presents results on the sorption of the azo dyes Remazol Yellow GR110 and Remazol Golden YellowRNL onto calcined MgAl-CO3 hydrotalcite under different pH (7 and 11) and temperature (25�C and 40�C)conditions. In addition to isotherm and kinetic data, this work also shows results on two techniques (thermaland ion exchange) for hydrotalcite regeneration. The Langmuir isotherm and the pseudo-second-order kineticsfit best the experimental data for both anionic dyes, resulting in a qmax of 106.3 mg/g (0.18 mmol/g) for RemazolYellow GR110 and a qmax of 657.2 mg/g (1.1 mmol/g) for Remazol Golden Yellow RNL at the best adsorptionconditions (25�C and pH = 7). Results on layered double hydroxides (LDH) characterization (X-ray diffraction,Fourier-transformed infrared Spectroscopy (FTIR) indicate that the azo dyes did not intercalate into the LDH butwere rather adsorbed onto its surface. Thermal recycling reduced the LDH adsorption capacity by 20% to 30%per cycle due to incomplete dye decomposition during the treatment at 500�C. Likewise, the recycling ofhydrotalcite via ion exchange was fairly ineffective, as the efficiencies for dye recovery were 15%, 20% and 30%when chloride, hydroxyl and carbonate anions were respectively used as exchangers.

Key words: adsorption; azo dye removal; calcined layered double hydroxides (LDH); color removal; textileeffluent; wastewater treatment

Introduction

Wastewater from textile industries contains signif-icant amounts of organic dyes, especially of the azo

type, introducing a high color and organic load to the effluent.It is estimated that *15% of dye produced worldwide is lostto the environment due to their incomplete fixation during thestep of fiber dyeing, a release of *1.2 t/d of such compoundsto the environment (Clarke, 1991; Zollinger, 1991). In Brazil,textile effluents are normally treated by the combination ofconventional biological (activated sludge) and physicochem-ical (chemical coagulation) processes. Although these pro-cesses have high efficiency in reducing carbonaceous organicmatter, they are not as efficient in color removal.

An option to complement the conventional biologicaltreatment of textile effluents is the use of a physicochemi-cal step pre- or post-treatment. Several studies have dem-onstrated that hydrotalcites, which are layered doublehydroxides (LDH), are efficient at adsorbing and/or inter-calating anionic substances such as dyes, surfactants, ha-lides, sulfates, nitrates, silicates, chlorides, and polymers

(Cavani et al., 1991; Ulibarri et al., 1995; Barriga et al., 2002;Seida and Nakano, 2002; Cardoso et al., 2003; Lazaridis,2003; Das et al., 2006, 2007; Bouraada et al., 2008, 2009; Lvet al., 2008a, 2008b, 2009).

LDH have the formula [M2þ1� xM3þ

x (OH)2]ðx�nÞþ

x=n �mH2O,where M2 + and M3 + represent divalent and trivalent metalcations; An - is an anion with charge - n; x is the ratio M3 + /(M2 + + M3 + ); and m is the number of moles of water. LDHcontaining Mg2 + and Al3 + as divalent and trivalent cations,respectively, and carbonate as interlayer anions are calledhydrotalcites. These materials have layers consisting of octa-hedral sharing edges, where the vertices are comprised ofhydroxyl groups. At the center of the octahedral are alumi-num and magnesium cations, which provide a positivelycharged structure. Therefore, to counteract the lamellae, in-terlamellar anions are needed.

The heat treatment of LDH leads to the formation of an oxi-hydroxide mixture (calcined LDH) of the constituent cationsfollowing the loss of interlayer carbonate and water. Thecalcined LDH formed can then be placed in contact with ananionic solution, and the LDH containing the anion of interestare obtained through the regeneration of the layered struc-ture. This process of regeneration of the layered structure afterthermal decomposition is called the ‘‘memory effect’’, and itcan be used to remove and recover anionic species from waterand wastewater.

*Corresponding author: Chemistry Department, Federal Universityof Ouro Preto, Campus Morro do Cruzeiro, Ouro Preto, MinasGerais State 35400-000, Brazil. Phone: + 55 31 3559 1837; Fax: + 55 313559 1725; E-mail: sergio@iceb.ufop.br

ENVIRONMENTAL ENGINEERING SCIENCEVolume 29, Number 7, 2012ª Mary Ann Liebert, Inc.DOI: 10.1089/ees.2011.0293

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In this study, calcined LDHs of the type MgAl-CO3 were usedfor the removal of the anionic azo dyes Remazol Yellow GR110and Remazol Golden Yellow RNL from aqueous solutions. Inaddition to obtaining the thermodynamic and kinetic data ofthis process, procedures for recycling the LDHs via thermaldecomposition and ionic exchange were also investigated.

Experimental

Azo dyes used

The dyes Remazol Yellow GR110 and Remazol GoldenYellow RNL are acid reactive azo dyes that have the molec-ular formula C20H22N4O11S3.Na2 and C16H18N4O10S3.Na2 asshown by structures A and B, respectively, in Fig. 1. Both dyeswere kindly provided by a textile industry located in Itabiritocity, MG, Brazil, and were used without purification.

Synthesis of LDH

The LDH were synthesized by the direct method of co-precipitation at constant pH, and Equation (1) represents thetheoretical synthesis of hydrotalcite with a 2/1 Mg/Al ratio.

4 Mg(NO3)2 � 6H2Oþ 2 Al(NO3)3 � 9H2OþNa2CO3

þ 12 NaOH/Mg4Al2(OH)12CO3 � 4H2O

þ 14 NaNO3þ 38 H2O (1)

Equation (1) was used to estimate the amount of reactantsto yield 20 g of the Mg-Al-CO3 LDH type. For this procedure,42.0907 g of Mg(NO3)2$6H2O and 30.7903 g of Al(NO3)3$9H2Owere added in 100 mL of distilled water. This solution wasthen slowly poured into 100 mL of Na2CO3 (1 M). The pH ofthe resulting solution was measured and kept at pH 10 using aNaOH (2 M) solution previously prepared. The procedure ofadding NaOH and monitoring the pH was carried out for 4 hunder continuous stirring at 40�C on a magnetic stirrer withheating. Subsequently, a SOLAB incubator shaker (model SU201250) was used to keep the reaction at 55�C and 200 rpm for20 h.

The resulting suspension was then vacuum filtered anddried at 50�C in an oven to obtain LDH of the MgAl-CO3 type.Part of this material was heat treated in an oven for 4 h at500�C and then kept in a vacuum desiccator. This heat treat-

ment procedure led to the formation of the calcined LDHMg4Al2O7, which is the material used in the subsequent tests.

Azo dye hydrolysis

The azo dye hydrolysis in the laboratory aimed to submitthe dye to the same chemical changes through which it passesduring the dyeing process in a textile industry. To prepare500 mL of 2000 mg/L of hydrolyzed dye solution, 1.00 g ofdye was dissolved in distilled water, and 66.66 g of NaCl wasadded. The resulting mixture was shaken until completedissolution. The solution was heated to 60�C before adding2.50 g of NaOH, and the temperature was kept at 60�C for 1 hbefore adding 3.33 g of Na2CO3. The hydrolyzed dye solutionwas cooled, transferred to a 500 mL flask, and stored in thedark for subsequent adsorption tests.

Adsorption isotherms

Tests to determine the adsorption isotherms were carriedout with the calcined LDH at three different conditions: pH 7and 25�C (A), pH 11 and 40�C (B), and with the hydrolyzeddye solution at pH 12 and 40�C (C). The pH was monitoredand kept at the desired value by adding NaOH 0.01 M orHNO3 0.01 M solutions. These values were chosen becausethey represent the conditions under which the textile effluentmight be submitted throughout the wastewater treatmentsystem; that is, after neutralization for biological treatment(condition A), before neutralization (condition B), and aftermercerization and dye hydrolysis (condition C). In each test,50.00 mg of the calcined LDH was added to 100 mL of dyesolutions (raw or hydrolyzed) at concentrations ranging from10 to 250 mg/L for 24 h using a SOLAB incubator shaker(model SU 201250) at 200 rpm.

After the contact time (24 h), which had been previouslydetermined to ensure equilibrium, an aliquot was withdrawnfrom each flask and centrifuged for 20 min using an Excelsamodel 206 BL at 5000 rpm. Because the supernatant and pel-lets were returned to the flasks after the color measurements,the solid/liquid ratio was kept constant throughout the ex-periment. The absorbance of the supernatant was read in anHP8453 spectrophotometer (428 nm for the hydrolyzed sam-ples and 423 nm for the other samples of Remazol YellowGR110; and 410 nm for the hydrolyzed samples and 417 nm

FIG. 1. Chemical structure ofRemazol Yellow GR 110 (a) andRemazol Golden Yellow RNL (b).

686 TEIXEIRA ET AL.

for the other samples of Remazol Golden Yellow RNL). Thedye concentration was then calculated from previously ob-tained external calibration curves.

Adsorption kinetics

Kinetic tests with the calcined LDH were carried out underthe same conditions previously described for the adsorptionisotherms. From the moment of the initial contact of the dyesolution with the calcined LDH, the aliquots were collected inpre-established periods for determining the kinetic curves.Each aliquot was centrifuged for 20 min using an Excelsamodel 206 BL at 5000 rpm, and the absorbance of the super-natant was determined by the procedure previously de-scribed. All collected samples were returned to the flasks afterthe absorbance readings.

Effect of temperature and pH

To study the effects of temperature and pH on the dyeadsorption/intercalation, tests were performed with 50.00 mgof calcined LDH using a SOLAB incubator shaker (model SU201250) at 200 rpm for 24 h with 100 mL of a solution con-taining 25 mg/L of dye at different temperatures (25�C and40�C) and pH (7 and 11), keeping one parameter constant andvarying the other. At the end of the experiment, the absor-bance of the supernatant was read, and the residual dyeconcentration was determined as previously described, thusallowing the determination of the average azo dye removalefficiency for each condition.

LDH recycling capacity

Thermal recycling capacity studies were conducted toevaluate the number of cycles of sorption-calcination-sorptionthe material was able to withstand without significant loss insorption capacity. For this purpose, 20.00 mg of calcined LDHwas added to 100 mL of a solution containing 300 mg/L ofdye and kept at 25�C and pH 7 under continuous mixing(200 rpm in a SOLAB model SU 201250 incubator shaker) for24 h. After the contact time, the final absorbance of the su-pernatant was measured, and the resulting powder wassubmitted to heat treatment for 4 h at 500�C. After this pro-cedure, the powder was weighed and subjected to a newadsorption cycle, keeping constant (at 2/3) the mass ratiobetween the LDH and the azo dye. This procedure, illustratedin Fig. 2, was performed five times.

In addition to thermal treatment, the recycling of used LDH(loaded with dye) was assessed by the ion exchange process.This test consisted of adding 50.00 mg of the oxi-hydroxidemixture to 50 mL of the 2.8 M solutions of chloride, carbonate,and hydroxide anions to recover the adsorbed dye and obtainnew LDH by ion exchange. The tests were carried out at both25�C and 50�C for 24 h, with the mixing speed maintained at200 rpm in a SOLAB incubator shaker (model SU 201250) . Therecovery efficiency was assessed by measuring the superna-tant absorption at the maximum azo dye wavelengths pre-viously mentioned.

LDH characterization

To study the structural properties of the material beforeand after the adsorption experiments, the characterization

techniques of specific surface area and porosity, thermogravimetric analysis (TGA), infrared transmission spectros-copy, X-ray diffraction (XRD) and zeta potential were used asdescribed by Vieira et al. (2009). In addition, the point of zerocharge (PCZ) of the LDHs was determined according to theprocedure described by Aquino et al. (2010), and the pKavalues of both azo dyes were determined by potentiometrictitration.

Results and Discussion

Adsorption isotherms

The adsorption isotherms were obtained from graphs thatrelate the amount of solute adsorbed (mg/g) and the soluteconcentration (mg/L) at equilibrium. The isotherm modelsstudied were Langmuir, Freundlich, and Temkin, accordingto Equations (2)–(4):

qe¼ (qmax KLCe)=(1þKLCe) (2)

qe¼K Cen (3)

qe¼B ln KTþB ln Ce (4)

where q represents the amount of solute adsorbed on the solidphase (mg/g), qmax is the maximum sorption capacity (mg/g), KL is a Langmuir constant related to the affinity betweenadsorbate and adsorbent (L/mg), Ce is the aqueous phaseequilibrium concentration (mg/L), K is the constant of ad-sorption capacity (L/g), n is the constant of adsorption in-tensity, KT is the equilibrium bonding constant (L/mol), and Bis related to the heat of adsorption (Do, 1998; Onal, 2006;Chairat et al., 2008).

Figure 3 shows the adsorption capacity (mg/g) as a func-tion of dye concentration at equilibrium (mg/L) for the threeconditions studied. The adsorption of both dyes was bestdescribed by the Langmuir model, indicating that the ad-sorption occurred in the monolayer and onto a homogeneoussurface. In this case, the adsorbate appears to have interactedwith specific sites through strong links, with the plateau ob-served corresponding to a monolayer of adsorbate, especiallywhen the adsorption was carried out at pH 7, where a higheradsorption capacity was observed (Table 1).

The PCZ was determined at pH 11. At this pH, the potentialzeta of the sorbent material (LDH) was zero (0 mV), whereas

FIG. 2. Scheme of the memory effect property, which al-lows thermal recovery of LDH.

USE OF HYDROTALCITES FOR AZO DYE ADSORPTION 687

the pKa values of the azo dyes were 4 and 6 (two inflectionpoints) for Remazol Yellow GR110; and 3, 3.5, and 6 (threeinflection points) for Remazol Golden RNL (data not shown).These results suggest that at pH 7, the adsorption might haveoccurred due to electrostatic interactions between the LDHs(positively charged) and the dyes (negatively charged due tothe deprotonation of the sulfonic and sulfate acid groups). At

higher pH (11 or 12), the adsorption due to electrostatic in-teraction would be weakened, which might explain the lowerdye removal capacity at such conditions (Table 1).

Figure 3 shows that by increasing the dye concentration theadsorption capacity increased until saturation was reached,indicating that the interaction sites were completely filled.This result is consistent with the adsorption theory, and itreinforces the hypothesis that no intercalation of the dye intothe LDH structure occurred, as will be discussed further usingthe XRD results.

An analysis of Fig. 3 and of the data presented in Table 1demonstrates that the concomitant increase of temperatureand pH resulted in a reduction in the maximum amount ofdye adsorbed. According to the shape of the isotherms andthe parameters RL (RL = 1/[1 + KL$C0]) from the Langmuirmodel, it is concluded that the adsorption isotherms are fa-vorable (0 < RL < 1). For Remazol Yellow GR110, the qmax

was 106.3 mg/g (0.18 mmol/g) at the best conditions found(25�C, pH 7), which is higher than the values reported in theliterature. For instance, Rutz et al. (2008) reported a qmax of13.1 mg/g for the same dye using wasted alumina as an ad-sorbent. For Remazol Golden Yellow RNL, the current studyfound a qmax value of 657.2 mg/g (1.1 mmol/g) at the samebest conditions mentioned before. Al-Degs et al. (2000) ob-served that the Langmuir model also best fitted the experi-mental data during Remazol Golden Yellow adsorption byactivated carbon, and the qmax found varied from 71.4 to111.1 mg/g.

Adsorption kinetics

The kinetic models studied to establish the order of theadsorption process were pseudo-first-order and pseudo-second-order [Equations (5) and (6)], where qe and qt are theamount of dye adsorbed (mg/g) at equilibrium and time t,respectively; k is the adsorption rate constant for the pseudo-first-order (min - 1); and k2 is the adsorption rate constant forthe pseudo-second-order (mg/g/min) (Mane et al., 2007).

qt¼ qe(1� e� kt) (5)

qt¼ qe� [1=(k2 qe tþ 1)] (6)

The curves presented in Fig. 4 best fitted the pseudo-second-order model, which was evaluated by both the cor-relation coefficient of linear regression (R2) and the differencebetween the calculated and measured values of q, as shown inTables 2 and 3. It is observed that the dye removal capacity (qt)

0 20 40 60 80 100 120 140 160 180 200 220 240 260 280 3000

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b

25°C, pH 7 40°C, pH 11 40°C, hydrolyzed Langmuir Freundlich

q e (

mg/

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Ce (mg/L)

FIG. 3. Isotherms of Remazol Yellow GR110 (a) and Re-mazol Golden Yellow RNL (b) adsorption onto LDH underpH 7 and 25�C, pH 11 and 40�C, and hydrolyzed at pH 12and 40�C.

Table 1. Langmuir Isotherm Parameters for Remazol Yellow GR110 and Remazol Golden Yellow

RNL Adsorption onto Calcined Layered Double Hydroxides Under Different Conditions

Dye type Parameter 25�C, pH 7 40�C, pH 1140�C, pH 12

(with hydrolyzed dye solution)

Remazol Yellow GR110 qmax 106.27 62.54 85.98KL 0.1672 0.1664 0.5227RL 0.0234 0.0235 0.0076R2 0.9107 0.9073 0.9821

Remazol Golden Yellow RNL qmax 657.2 209.6 135.5KL 0.4005 0.0918 0.3144RL 0.0003 0.8976 0.0743R2 0.9554 0.8134 0.9776

688 TEIXEIRA ET AL.

was reduced when its initial concentration was lower, prob-ably due to mass transfer limitations, as a smaller amount ofdye in the solution implies a decreased driving force for ad-sorption. In contrast, the tests carried out with the lower dyeconcentration rendered a higher percentage of dye removalbecause the amount of calcined LDH used was kept constant;hence, the LDH/dye ratio was higher.

Table 4 shows the efficiencies of Remazol Yellow G110 andRemazol Gold Yellow RNL removal by adsorption onto LDHin terms of the pH and temperature values studied. It can be

seen that the higher the temperature and pH, the smaller thepercentages of dye removal. These results are consistent withthose obtained in the isotherm study (Fig. 3 and Table 1). Thelower removal at higher pH can be attributed to a competitionbetween the dye and the hydroxyl ion for the adsorption sitesand to the PCZ and the zeta potential of the LDH being *11.In turn, the decrease in adsorption efficiency at higher tem-perature is probably due to the adsorption phenomenon be-ing exothermic (Pavan et al., 2000; Crepaldi et al., 2002a; Reiset al., 2004).

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a b

FIG. 4. Kinetic curves of the adsorption of Remazol Yellow GR110 (a) Remazol Golden Yellow RNL (b) under conditions A(pH 7 and 25�C), B (pH 11 and 40�C), and C (hydrolyzed and 40�C). P1 to P7 are the initial azo dye concentrations:P1 = 250.0 mg/L; P2 = 125.0 mg/L; P3 = 100.0 mg/L; P4 = 75.0 mg/L; P5 = 50.0 mg/L; P6 = 25.0 mg/L; and P7 = 10.0 mg/L.

USE OF HYDROTALCITES FOR AZO DYE ADSORPTION 689

LDH recycling capacity

During the hydrotalcite thermal regeneration, there was aloss in the removal capacity of Remazol Yellow GR110 andRemazol Golden Yellow RNL of *20% and 30% per cycle,respectively. The initial dye removal (first cycle) was relativelylow due to the lower LDH/dye ratio (2/3) employed in the test.

After five cycles, the loss in adsorption capacity was *80%to 90% for both dyes, and the dye removal efficiency wasbelow 5%. These results were expected because the thermalanalysis indicated that only *30% of the dye mass was lost at500�C (data not shown), that is, the dyes used do not com-pletely decompose at 500�C. Therefore, the residual dye ad-sorbed accumulated onto the LDH surface along the cycles ofregeneration-calcination, precluding the adsorption of newdye molecules. A reduction in recycling capacity has also beenreported in LDH studies removing polyethylene terephthal-ate (10% loss after 5 cycles) and vanadate and arsenate (50%loss after 2 cycles), according to Crepaldi et al. (2002b) andKovanda et al. (1999). Thermal regeneration at temperatureshigher than 500�C is not possible, as the high temperaturewould irreversibly alter the LDH structure.

With regard to the ion exchange recovery procedure, theresults showed that low recovery efficiencies were obtainedwith all anionic solutions tested. In the best scenario, only 30%of dye adsorbed onto the LDH was recovered for a simple ionexchange with sodium carbonate at 25�C. The recovery effi-ciency was according to the following anion order:CO3

2� >OH� > Cl� . This order was expected (Miyata, 1983),but the recovery efficiencies were fairly low for all anions tes-ted. Because the adsorption process is exothermic, an increaseddesorption at higher temperature was expected. However, theincrease in temperature from 25�C to 50�C did not enhance dyerecovery either. These results strengthen the hypothesis thatthe dye removal was not due to an intercalation process.

LDH characterization

The calcined LDH had 21.3 m2/g of surface area and0.07977 cm3/kg of total porosity, and the material could becharacterized as mesoporous. The surface area is small com-pared with active carbons, but it resulted in relatively highadsorption capacities, as previously described (section 3.1).Because this work shows that the adsorption of RemazolYellow azo dyes by LDHs was a surface-related phenomenon,it is expected that color removal increases with the increase ofthe LDH dose. From the adsorption capacities determined, aremoval of 75% of Remazol Yellow GR110 from a 100 mg/Lsolution would require an LDH dose of *0.7 g/L; whereas aremoval of 99.99% of such azo dye could be accomplishedwith a higher dose of *0.95 g/L.

Table 2. Kinetic Parameters of Remazol Yellow

G110 Adsorption onto Calcined Layered Double

Hydroxide Under Different Conditions

Pseudo-second-order

C (mg/L)qe exp

(mg/g) qe (mg/g) K2 (g/mg/min) R2

25�C, pH 7250 111.78 148.37 0.00002 0.9523125 117.58 128.53 0.00004 0.9902100 80.08 87.64 0.00005 0.974475 86.34 95.97 0.00006 0.992850 90.8 97.66 0.00007 0.995425 47.08 49.33 0.00029 0.995710 19.28 18.13 0.00135 0.9881

40�C, pH 11250 50.49 47.10 0.00182 0.9799125 48.53 50.66 0.00031 0.9818100 49.52 55.31 0.00025 0.995975 39.72 44.50 0.00025 0.976550 24.58 26.60 0.00044 0.984525 32.62 37.48 0.00027 0.988410 16.20 15.44 - 0.0081 0.9399

40�C, pH 12 (hydrolyzed dye solution)250 83.09 89.85 0.00016 0.9704125 43.97 47.15 0.00033 0.9891100 68.49 71.17 0.00013 0.908875 46.14 42.48 0.00027 0.885850 47.46 49.36 0.00017 0.922725 29.99 30.77 0.00046 0.964910 20.00 20.49 0.00429 0.9997

Table 3. Kinetic Parameters of Remazol Golden

Yellow RNL Adsorption onto Calcined Layered

Double Hydroxide Under Different Conditions

Pseudo-second-order

C (mg/L)qe exp

(mg/g) qe (mg/g) K2 (g/mg/min) R2

25�C, pH 7250 162.2 161.3 3.20 · 10 -4 0.9999125 87.2 84.8 3.67 · 10 -4 0.9975100 68.6 67.7 4.62 · 10 -4 0.998775 70.2 67.4 4.76 · 10 -4 0.996850 50.6 47.8 5.45 · 10 -4 0.994125 29.6 28.5 7.73 · 10 -4 0.995810 11.6 11.7 1.31 · 10 -3 0.9981

40�C, pH 11250 227.6 229.9 1.30 · 10 -4 0.9995125 143.0 148.4 9.98 · 10 -5 0.9988100 88.2 90.2 2.66 · 10 -4 0.999575 76.4 77.1 2.21 · 10 -4 0.998650 53.0 52.9 4.46 · 10 -4 0.999025 37.4 37.1 4.95 · 10 -4 0.997510 12.6 12.2 8.52 · 10 -4 0.9887

40�C, pH 12 (hydrolyzed dye solution)250 51.0 51.4 6.0 · 10 -4 0.9995125 50.0 54.5 1.3 · 10 -4 0.9963100 45.0 53.5 6.7 · 10 -5 0.979975 28.8 28.3 4.5 · 10 -4 0.993350 21.6 20.3 8.8 · 10 -4 0.989925 11 10.9 2.2 · 10 -3 0.998310 6.6 6.7 1.9 · 10 -3 0.9970

Table 4. Removal Efficiency of Azo Dye Adsorption

Under the Different Conditions Tested

% Removal

pHTemperature

(�C)Remazol Yellow

GR110Remazol Golden

Yellow RNL

7 25 54.0 – 2.0 64.5 – 2.011 25 48.0 – 2.0 55.0 – 2.07 40 44.0 – 2.0 53.0 – 2.011 40 38.0 – 2.0 52.2 – 2.0

690 TEIXEIRA ET AL.

Figure 5 shows the XRD for the calcined LDH submitted tothe process of adsorption using Remazol Yellow GR110 andRemazol golden Yellow RNL. The basal spacing and parame-ters calculated from the XRD spectra show that a lamellarstructure was obtained. Although no chemical analysis wascarried out to confirm the composition of the LDH obtained [asindicated in Equation (1)], the XRD data show that the basalspacing given by the peaks (003), (006), (009) and calculatedaccording to Palmer et al. (2009), the net parameters (a = 3.0 A;C = 23.6 A) calculated according to Perez-Ramirez et al. (2001),and the average particle size (t = 24.9 nm) calculated accordingto Zhao et al. (2009) are typical of an LDH of the MgAl-CO3 type.

The XRD results also showed there was a regeneration ofthe LDH from the calcined material, but there was no inter-calation of the dye molecules. The calculated basal spacing(d003) of 7.8 A is in accordance with the intercalation of car-bonate ions (Kloprogge et al., 2001), which is a ubiquitouscontaminant of aqueous solutions. Higher spacing values of*20 A would be expected if the dyes were intercalated be-tween the layers of the material, which would result in aleftward shift of a d(003) peak.

The Fourier-transformed infrared spectroscopy (FTIR)spectra (Fig. 6) show the characteristic bands of both dyes andLDH before and after the adsorption tests. It can be seen thatthe dyes used in this study had similar FTIR spectra due totheir similar structure and the presence of the same acid andazo groups. The spectra highlights are the bands near1400 cm - 1 related to O = S – SO3H stretching; 2900 cm - 1 and1170 cm - 1 related to the HO – SO3H and O = S – SO3

- groups,respectively; 1460 cm - 1 specific to the azo group N = N;1000 cm - 1 and 1500 cm - 1 related to the N = N sulfonic acidsand sulfonate groups, respectively; and bands of *700 cm - 1

related to the aromatic rings of both dye molecules.

Figure 6 also shows that the LDH used for the adsorption ofRemazol Yellow GR110 had traces of the dye according to thebands between 1100 cm - 1 and 1500 cm - 1 related to the sul-fonic acid and sulfate groups. Likewise, the LDH used forRemazol Golden Yellow RNL adsorption showed character-istic bands of the azo dye of *1200 and 1700 cm - 1. Theseresults confirm the TGA data, which indicated that thermaltreatment is not a good option for LDH regeneration. Theseresults also suggest that the color removal was due to dyeadsorption rather than its intercalation.

Conclusions

The results presented in this paper show that adsorption ofthe anionic azo dyes Remazol Yellow GR110 and RemazolGolden Yellow RNL onto LDH is possible, and it followed theLangmuir isotherm and the pseudo-second-order model.Thermal recycling resulted in a loss of adsorption capacity, asthe recovery efficiency was reduced by 20%–30% per cycledue to incomplete dye decomposition during the heat treat-ment at 500�C. LDH recycling by ionic exchange was notpossible because the dyes were not intercalated into the

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(200)

d

FIG. 5. X-ray diffraction of the materials: (a) MgAl-CO3, (b)calcined MgAl-CO3, (c) calcined MgAl-CO3 after contactwith a solution of Remazol Yellow GR110, (d) calcinedMgAl-CO3 after contact with a solution of Remazol GoldenYellow RNL.

4000 3000 2000 10000

5

10

15

20

25

30

35a

b

LDH LDH + GR110 GR110

Tra

nmita

nce

wave number

4000 3000 2000 10000

10

20

30 LDH + RNL RNL LDH

Tra

nsm

itanc

e

wave number

FIG. 6. Infrared spectrum of LDH samples after the ad-sorption tests with Remazol Yellow GR110 (a) and RemazolGolden Yellow RNL (b).

USE OF HYDROTALCITES FOR AZO DYE ADSORPTION 691

lamellae structure, as confirmed by the XRD results. Despitethe nonoccurrence of dye intercalation, the LDHs proved to bea good azo dye adsorbent because the qmax values obtained atthe best conditions studied (pH = 7 and T = 25�C) were 106.3and 657.2 mg/g for Remazol Yellow GR110 and RemazolGolden Yellow RNL, respectively. Such adsorption capacitiesare higher than those reported in the literature for adsorbentsthat have higher porosity and surface area.

Acknowledgments

The authors would like to thank UFOP, CNPq, FAPEMIG,and CAPES for their financial support.

Author Disclosure Statement

No competing financial interests exist.

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