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Journal of Colloid and Interface Science 289 (2005) 42–47www.elsevier.com/locate/jcis

Interaction of Ag(I), Hg(II), and Cu(II) with 1,2-ethanedithiolimmobilized on chitosan:

Thermochemical data from isothermal calorimetry

Eunice F.S. Vieiraa,∗, Antonio R. Cestaria, Elias de B. Santosa, Francisco S. Diasb

a Laboratory of Materials and Calorimetry, Departamento de Química/CCET, Universidade Federal de Sergipe, 49100-000, São Cristóvão,Sergipe, Brazil

b Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, 60455-760, Fortaleza, Ceará, Brazil

Received 21 January 2005; accepted 19 March 2005

Available online 17 May 2005

Abstract

The nature of interactions between metal ions Ag(I), Hg(II), Cu(II) and chitosan derivative of 1,2-ethanedithiol, QTDT, was investiisothermal calorimetry using the membrane breaking technique. Simultaneous determination of thermal effects,Qint, and amount of cationthat interacts,nint, are described. The experimental data have been interpreted in terms of the Langmuir equation to determine theadsorption capacity to form a monolayer,Nmon, and the energy of interaction for a saturated monolayer per gram of QTDT,Qmon. WithNmon andQmon, the molar enthalpy of interaction for formation of a monolayer of anchored cations per gram of QTDT,�monHm, wasdetermined. The�monHm values for Ag(I), Hg(II), and Cu(II) were−60.56,−58.05, and−84.36 kJ mol−1, respectively. Negative valueof �G show the spontaneity of the interaction processes. The least entropically favourable processes, i.e., those which present mo�S values, seem to be compensated by the more favourable enthalpic parameter. 2005 Elsevier Inc. All rights reserved.

Keywords: Isothermal calorimetry; Interaction of metal ions; Adsorption; Chitosan

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1. Introduction

Chitosan [poly(N -deacetyl glucosamine)] is a naturpolymer that has potential application in several areascience[1–14]. It is obtained byN -deacetylation of chitinwhich is the second-most-abundant naturally occurpolymer found in the exoskeleton of marine crustaceansthe second-most-abundant biopolymer next to cellulose[15].Chitosan, like most biopolymers, has the property of reming metal ions from wastewater, and such property has bregarded as an area worthy of further investigation.

The adsorption of metal ions by certain naturally occring biopolymers and synthetic polymers has gained m

* Corresponding author. Fax: +55 79 212 6684.E-mail address: eunice@ufs.br(E.F.S. Vieira).

0021-9797/$ – see front matter 2005 Elsevier Inc. All rights reserved.doi:10.1016/j.jcis.2005.03.043

attention. This area of science has been stimulated by sproblems of great practical importance such as the adstion in soil of highly poisoning cations of some heavyements[16–18]. Amongst the range of contaminants thmay be found in soils, potentially toxic heavy metals areparticular interest because they show a tendency, undermal circumstances, to accumulate in soils and have apersistence time because of their interactions with partlar soil components. Finally, many of the heavy metalsessential, at low concentrations, for plant, animal, andman health, but, at higher concentrations, they can be toChanges in concentration of heavy metals can be sigcantly altered with profiles through a number of biologiand chemical processes. Numerous research efforts are

done to develop methods of removing heavy metal ions andattention has to be given to new efficient alternatives such asremoval by sorption. Soluble heavy metals can be removed

d and

/or

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E.F.S. Vieira et al. / Journal of Colloi

by adsorption on chemically functionalized organic andinorganic supports[19–22].

For the heavy metal ions adsorption task, various tyof chitosan have been explored successfully for years. Tcrease the adsorption capacity, various chemically modchitosans have been prepared[23–29]. Based on the succesof employing Schiff reactions, we introduce an aldehydecould couple the ligand 1,2-ethanedithiol (QTDT) to the sface of chitosan, yielding a QTDT material. The applicatpotential of QTDT as material that has high adsorption prerties toward Ag(I) and Hg(II) ions has been demonstrain the present study.

Although a large body of data has been collected abchitosan and/or chemically modified chitosans, little atttion has been paid to the nature of the bonds of cplexes at the solid–liquid interfaces. Investigations onthermodynamics of cation binding from solutions are fand scattered because of the complexities of solid–soluinterface systems and experimental difficulties. Calorimric techniques have contributed much to our understaing of adsorption phenomena at solid–solution interfacSome of our papers[30–33] develop an improved calormetric titration method to obtain energies evolved atferent surface coverages and the corresponding amousubstance bound. In this survey, isothermal microcaloritry was used to determine enthalpy change upon interacof Ag(I), Hg(II), and Cu(II) with QTDT. The thermal effecoriginating from the interaction,Qint, and the correspondinamount of metals that interacts,nint, were obtained in thesame equilibrium conditions. Langmuir expressions forsorption isotherm were applied to determine the maximadsorption capacity to form a monolayer,Nmon, and the en-ergy of interaction for a saturated monolayer per gramQTDT, Qmon. With Nmon andQmon, the molar enthalpy ointeraction for formation of a monolayer of anchored sver, mercury, and copper per gram of QTDT,�monHm, wasdetermined. Other thermodynamic parameters of the inaction processes are also presented.

2. Materials and methods

Unless stated otherwise, all reagents were of anacal reagent grade. The 1,2-ethanedithiol used in this swas supplied by Merck and used without further purcation. Chitosan powder (degree of deacetylation of 93from fresh Norwegian shrimp shells was a gift from PrimIngredients A.S. (Norway). The chitosan was conditionin a dark air-free flask to prevent a reaction betweenamine groups and the CO2 present in the atmosphere[34].Dimethylformamide from Merck was used as supplied. Atone and methanol (Merck) were purified following procdures available in the literature[35] and kept dry under ni

trogen atmosphere.

The water used throughout the experiments was alwaysdouble-distillated and purged by N2 gas to avoid CO2 disso-

Interface Science 289 (2005) 42–47 43

f

lution. Anhydrous silver, mercury, and copper nitrates (frAcros Organics) were used. Hg(NO3)2 and Cu(NO3)2 wereobtained from Hg(NO3)2·3H2O and Cu(NO3)2·H2O whichwere dehydrated by heating for 8 h in vacuum at 383 K. Taqueous phase contained 0.1 mol dm−3 NaNO3 to maintainconstant ionic strength, and the initial pH was adjustedadding an amount of CO2-free 0.1 mol dm3 HNO3.

2.1. Preparation of the cross-linked chitosan

To prepare the cross-linked chitosan (QTEP), 4.9 cm3 ofepichlorohydrin dissolved in 80 cm3 of dioxan and 800 mof NaOH 0.1 mol dm−3 were added to 20 g of chitosan. Thmixture was kept continuously stirring at 353 K for 6 h. Aftthis procedure, QTEP was filtered off, washed with deiized water and acetone, and dried under vacuum at 33for 16 h. After this treatment, the material was insoluble5 wt% acetic acid.

2.2. Preparation of the 1,2-ethanedithiol chitosan

QTDT chitosan was prepared via Schiff’s base formatbetween the amino groups of chitosan and formaldehyda second step, the reaction with 1,2-ethanedithiol resuin the formation of QTDT. Thus 6.0 g of QTEP, suspendin 90 cm3 of pH 4.0 buffer HOAc/NaOAc 0.2 mol dm−3,was treated with a 37% formaldehyde solution (23.8 c3,0.29 cm3). After stirring at room temperature for 3the material was washed exhaustively with deionizedter, acetone, and dimethylformamid (DMF). The mateQTFA was placed in 90 cm3 of DMF, and 1,2-ethanedithio(7.5 cm3, 89.2 mmol) was added to the reaction mixtuwhich then was stirred, in atmosphere of argon, for 48 h.nally, the product, QTDT, was filtered off and thoroughwashed with DMF, methanol, and acetone. Drying wasformed under vacuum at 333 K for 16 h.

The immobilisation of the 1,2-ethanedithiol is outlinin Fig. 1. The QTDT was characterized by determinatof sulfur content using a Thermo Finnigam 1112 Eleme

Fig. 1. Schematic drawing of chemical structures of QTDT.

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me-linetha

in-ual

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tricn-y oneterated

syn-DT.ownhere-

DTnd

44 E.F.S. Vieira et al. / Journal of Colloi

Analyzer, FTIR diffuse reflectance spectroscopy, and tmal analyses. IR spectral data for QTEP, QTFA, and QTwere obtained with the use of a Nicolet Nexus 470 seFTIR spectrophotometer at a resolution of 4 cm−1, by us-ing spectro quality KBr powder. Thermal analyses wperformed using SDT 2960 thermoanalyzer from TA Instments in a dynamic atmosphere of nitrogen (0.33 cm3 min−1)with a heating rate of 0.16 K s−1.

2.3. Calorimetric determinations

All calorimetric determinations were performed in a STARAM C80 mixing calorimeter[36,37], capable of main-taining a baseline of±0.12 µW with a temperature stability of ±0.0001◦C. The experiments were carried out298.15 K with aqueous cation solutions at pH 6. Sampof approximately 50 mg of QTDT were put into the lowpart of the mixing cell closed by a circular PTFE membraInto the upper part of mixing cell, a volume (3.0 cm3) ofcation solution in the concentration range of 1.0 × 10−3 to7.0 × 10−3 moldm−3 was added. After complete stabiliztion of the baseline, a movable rod enables the cation stion to be pushed into the container with the QTDT throuthe PTFE membrane and the interaction cation/QTDT pceeds without stirring. The curve of the detectable caloriter signal (power vs. time) with the same value of basebefore and after the interaction process was evidence

the cation equilibrium conditions were reached. The inte-

Fig. 2. Diffuse reflectance spect

Interface Science 289 (2005) 42–47

t

integration is selected as linear from first to last point, joing two extreme points on the curves. Thus each individexperiment yields a thermal effect,Qr, which was correctedby subtracting the corresponding wetting effect,Qw, of theQTDT in the pure solvent. The thermal effect of membrabreaking for the empty cell was found to be negligible copared toQr and Qw values. After eachQr recording, thecation equilibrium concentration,Ceq, in the supernatanand thus the cation amount that interacts,nint, was deter-mined. Mercury content was determined by complexometitration with dithizone[38], whereas silver and copper cotents were obtained by atomic absorption spectrometra Perkin–Elmer 5000 atomic absorption spectrophotomwith an air-acetylene flame. Each experiment was repein duplicate.

3. Results and discussion

The experimental results demonstrate the successfulthesis of the chitosan derivative of 1,2-ethanedithiol, QTThe idealised reaction can be considered to be that shin Fig. 1. Elemental analysis of sulfur indicated that tchitosan modification procedure with 1,2-ethanedithiolsulted in a product with a sulfur content of 2.54 mmol g−1.The diffuse reflectance spectra of QTEP, QTFA, and QTare shown inFig. 2. Because epichlorohydrin tends to bo

to chitosan using their hydroxyl groups, the final structure

ex-m

gration of the calorimetric curves had been made by thesoftware SETSOFT (SETARAM). The baseline used for the

does not present different groups from the pristine one,cept for the decrease in intensity of the peak at 1599 c−1

ra of QTEP, QTFA, and QTDT.

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E.F.S. Vieira et al. / Journal of Colloi

Fig. 3. DTA curves of unmodified chitosan, QTEP, QTFA, and QTDT

(δ(N–H)) characteristic of chitosan, indicating a partialvolvement of the amino groups in the cross-linking reactiHowever, the spectrum of QTFA does not exhibit thesorption band due to (δ(N–H)) suggesting the formation othe imine. Probably, the absorption due to C=N stretchingwhich is weak in aliphatic Schiff’s base is masked by amI at 1655 cm−1. The spectrum of QTDT is similar to thQTFA spectrum. The peak due toν(S–H) vibration is notdetected, probably either because of the weak signal ocause 1,2-ethanedithiol is bonded to chitosan, accordinthe schematic of the chemical structure of QTDT inFig. 1A.

The DTA curves of the chitosans are shown inFig. 3. Thefirst endothermic DTA peaks, from 25 to about 120◦C, arerelated to loss of water physically adsorbed on the materThe other peaks enough good distinctions of the mateone another. Two exothermic peaks are observed for Qat 150 and 300◦C, and three exothermic peaks are obserfor QTFA and QTDT at 160, 240, and 270◦C. However, thedistinction between the QTFA and QTDT chitosans isserved by the differences between the relative amplitudethe peaks of those materials from 240 and 270◦C.

Calorimetric procedure was used to elucidate the tmodynamic features for complexing reactions of the mions with QTDT. Interaction energies originating from tinteraction of the cations with the QTDT,Qint = Qr − Qw,were calculated with bothQr andQw normalised for 1 g ofQTDT. Figs. 4 and 5show the values ofnint andQint plottedas a function ofCeq and provide evidence that the adsotion has the shape of a Langmuir isotherm. Quantitatioa Langmuir isotherm requires the use of an ideal solutwhich implies that only monolayer adsorption occurs andinteractions exist between molecules at adjacent sites[39].The following linearized form equations were used:

(1)(Ceq/nint) = (1/KNmon) + (Ceq/Nmon),

(2)(Ceq/Qint) = (1/KQmon) + (Ceq/Qmon),

whereNmon is the maximum adsorption capacity to form

monolayer,Qmon is the energy of interaction for a saturatedmonolayer per gram of QTDT, andK is a parameter of affin-ity that includes the equilibrium constant. Plots ofCeq/nint

Interface Science 289 (2005) 42–47 45

-

Fig. 4. Cation amount that interacts,nint, as a function of equilibrium concentration,Ceq.

Fig. 5. Interaction energies,Qint, from the interaction of the cations witthe QTDT as a function of equilibrium concentration,Ceq.

againstCeq and Ceq/Qint againstCeq give straight lines,the slopes and intercepts of which correspond toNmon orQmon andK , respectively. The applicability of the Langmumodel to the adsorption data is made by the calculationthe theoreticalnint andQint values from the values ofCeq,Nmon, Qmon, andK . The good agreement of the experimetal and calculated values assures that the Langmuir mcan be used to explain the adsorption data satisfactorilyquantitatively compare the applicability of the model, merelative deviations in relation to the experimental (exp) acalculated (calc) values ofnint andQint were calculated fromEq.(3), as follows[40]:

(3)100× 1

n

n∑

i=1

∣∣∣∣Vi(exp) − Vi(calc)

Vi(exp)

∣∣∣∣,

wheren is the number of experimental data andV is thevalue ofnint or Qint. The Langmuir model was found to be

46 E.F.S. Vieira et al. / Journal of Colloid and Interface Science 289 (2005) 42–47

Table 1Results of maximum adsorption capacity to form a monolayer,Nmon, energy of interaction for a saturated monolayer per gram of QTDT,Qmon, and thermo-dynamic parameters for the interaction of Ag(I), Hg(II), and Cu(II) with QTDT in aqueous solutions

Nmon(10−4 mol g−1)

−Qmon(J mol−1)

−�monHm(kJ mol−1)

−�satHm(kJ mol−1)

lnK −�G

(kJ mol−1)−�S

(J K−1 mol−1)

Hg(II) 5.18 30.07 58.05 56.29 4.73 11.72 155.37

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Ag(I) 5.40 32.70 60.56Cu(II) 2.44 20.58 84.36

in correlation with the experimental data with mean reladeviations varying from 0.23 to 1.75%.

With theNmon andQmon values listed inTable 1, the mo-lar enthalpy of interaction for formation of a monolayeranchored cations per gram of QTDT,�monHm, was directlyobtained by means of the expression:

(4)�monHm = Qmon/Nmon.

Inspection ofFig. 5 shows an energy of saturation, Qsat.Assuming thatQsat is the maximum energy for the adsortion process,�satHm values were calculated as:�satHm =Qsat/Nmon. As can be seen inTable 1, excluding copperthe values of�satHm are satisfactorily in agreement withe�monHm values. Recent studies[41,42]and other experiments in progress in our laboratory have demonstratedvalues derived from saturation energies and fit of the Lamuir isotherm provide similar description of the adsorptproperties. The negative interaction enthalpies are consiwith results obtained from titration calorimetry for adsotion on thiol-modified silica gel[30–33].

A prominent fact upon inspection ofTable 1is that themodified chitosan QTDT may be used for stable bondof a transition metal. Interactions between the Lewis-accations and the sulfur-donor atoms attached to chitosanreflected by�monHm values in the sequence Cu(II)> Ag(I)> Hg(II). However, based on our previous studies[30–33],it was established that this thermodynamic quantity musonly considered to evaluate the acid–base interaction wthe same coverage is achieved for adsorption of all cationwas assumed because, in lower coverage, interactionswith the most energetic sites, yielding greater�monHm val-ues than those at a higher coverage. The values of maxiadsorption capacity to form a monolayer,Nmon, decrease inthe order Ag(I)> Hg(II) > Cu(II), indicating that 21.3 and20.4% of the attached ligands 1,2-ethanedithiol are availfor adsorption with Ag(I) and Hg(II), respectively, whereonly 9.6% of the coverage is achieved for interactionCu(II). Thus it is more coherent to consider the interactenthalpies at different amounts of coverage of the sur(i.e., �intHm = Qint/nint), whose values can be comparin the same surface-coverage range. InFig. 6, �intHm val-ues are plotted against the covered fraction of the surfθ = nint/nsulf, wherensulf is the amount of sulfur per gramof QTDT. It is clear that interactions at lower coverage yi

higher enthalpy change values, which decrease with increasing coverage. In this case,�intHm values present a similarsequence to that found for�monHm values.

58.30 4.56 11.30 165.2163.32 5.13 12.72 240.29

t

t

r

,

Fig. 6. Interaction enthalpies at different amounts of coverage of the sur�intHm, as a function of the covered fraction of the surface,θ , for Ag(I),Hg(II), and Cu(II).

Other thermodynamic parameters of the interaction pess, including free energy and entropy were determinedK used in Eq.(2) by:

(5)�G = −RT lnK,

(6)�S = (�monHm − �G)/T ,

whereR is the gas constant. However, it must be emphasthat if K is a thermodynamic equilibrium constant, it muonly be a function of temperature. FromFig. 6, it is clearthat the�intHm values are not constant with the coveraWe hope that the development of this study may promptther interest in this aspect. The calculated results are listTable 1. The small but negative�G values are indicative othe spontaneous nature of the interaction processes. Enhas been defined as the degree of chaos of the systemthe negative values of this parameter reflect the adsorpof Ag(I), Hg(II), and Cu(II). It is interesting to observe ththe least entropically favourable processes, i.e., those wpresent more negative�S values, seem to be compensaby the more favourable enthalpic parameter.

Acknowledgments

The authors gratefully acknowledge Primex IngredieA.S. (Norway) for both the high-quality chitosan sample a

-the continuous support to our research group. A.R.C. andE.B.S. acknowledge the Brazilian agency CNPq for fellow-ships.

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E.F.S. Vieira et al. / Journal of Colloi

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