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ElectronParamagneticResonanceandAtomicAbsorptionSpectrometryastoolsfortheinvestigationofCu(II)biosorptionbySargassumfilipendula

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Hydrometallurgy 86

Electron Paramagnetic Resonance and Atomic AbsorptionSpectrometry as tools for the investigation of Cu(II)

biosorption by Sargassum filipendula

Aderval S. Luna a,⁎, Antonio Carlos A. da Costa a,Cristiane A. Henriques a, Marcelo H. Herbst b,1

a Programa de Pós-Graduação em Engenharia Química, Universidade do Estado do Rio de Janeiro, Instituto de Química,Rua São Francisco Xavier, 524, CEP:20550-013, Rio de Janeiro, Brazil

b Departamento de Física dos Sólidos, Instituto de Física, Universidade Federal do Rio de Janeiro, CEP: 21941-590, Rio de Janeiro, Brazil

Received 14 February 2006; received in revised form 12 June 2006; accepted 7 November 2006Available online 17 January 2007

Abstract

A basic investigation into the removal of copper ions from aqueous solutions by Sargassum filipendula was conducted in batchconditions. The influence of different experimental parameters such as initial pH, sorption time, equilibrium conditions, and initialconcentrations of copper ions on copper uptake was evaluated. Results indicated that copper uptake capacity increased from pH 2.0 to3.0, being constant at pH values 4.0 and 5.0. Electron Paramagnetic Resonance proved to be a useful tool for studying the mechanisminvolved in copper biosorption by S. filipendula. For initial copper concentrations smaller than 250 μg mL−1 axial type spectra wereobtained, typical of isolated immobilized copper ions, and a hyperfine structure was observed (A|| =150(2)G, g|| =2.31(1)). For initialcopper concentrations greater than 250 μg mL−1, (qe=53.3 mg g−1), a distorted line, typical of aggregates, was observed, due todipole–dipole magnetic interactions.

The Langmuir model better represented the sorption process, as compared to the model of Freundlich. The process followed asecond-order kinetics, and equilibrium was reached after 10 min of contact between the biomass and the metal solution. Due to itsoutstanding copper uptake capacity (1.30 mmol g−1 biomass), S. filipendula proved to be an excellent biomaterial for accumulatingand recovering copper from industrial solutions. The results show that the biomass has sorption capacities comparable to otherbiomasses and conventional ion-exchange materials.© 2006 Elsevier B.V. All rights reserved.

Keywords: Copper; Biosorption; Kinetics; Equilibrium; Sargassum filipendula; Electron Paramagnetic Resonance

⁎ Corresponding author. Fax: +55 21 2587 7227.E-mail address: asluna@uerj.br (A.S. Luna).

1 Present address: Centro Universitário Estadual da Zona Oeste—UEZO, Rua Manoel Caldeira de Alvarenga, 1203, CEP:23070-200,Rio de Janeiro, RJ, Brazil.

0304-386X/$ - see front matter © 2006 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2006.11.008

1. Introduction

Seaweeds, bacteria, and fungi, including yeasts andvarious aquatic species have been identified as potentialcandidates capable of sorbing toxic metals from aqueoussolutions as well as biotransforming them into less toxicforms, in some cases.

106 A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

Brown seaweeds (Phaeophyceae) constitute an algalgroup containing the characteristic pigment fucoxantine,which is responsible for their brown color. Floatingmasses of Sargassum constitute the Sargasso Sea, andare also very common on the Brazilian Coast. All Sar-gassum species contain floating bubbles, responsible fortheir decreased density, thus contributing to their pre-sence in the marine environment. Quantitatively, themost abundant polysaccharide in the cell wall of brownseaweeds is alginic acid. Alginic acid is a polymercomposed of two uronic acids (β-1,4-D-mannuronic andα-1,4-L guluronic), with molar ratio between the acidsranging from 0.25 to 2.5. Alginic acid is present in theseseaweeds usually as calcium, magnesium, sodium, andpotassium salts, mainly in the cell wall. It is a structuralpolysaccharide with strong ion-exchange properties.Beyond its high metal uptake capacity, this algal genushas been selected for study due to their wide distributionin most tropical countries, which are available at highquantities as a waste biomaterial (da Costa et al., 2001).

One of the applications of this kind of biomass is thebiosorption of heavy metals from aqueous solution. It canbe considered as an alternative technology in industrialwastewater treatment (Vegliò and Beolchini, 1997). It isbased on the ability of biological materials to accumulateheavy metals from wastewater by either metabolicallymediated or physico-chemical pathways of uptake.

In this context, there are academic and practicalinterests for the study of the biosorption mechanism.Mathematical models can be used for predicting quan-titatively how much a given factor influences metaluptake, but they would be more reliable if they are notarbitrary correlations but rather based on the actualmechanism (Volesky, 2003).

On the other hand, the actual mechanism of bio-sorption is not easily explained because it deals withdifferent types of biomolecules with a highly complexstructure whose various blocks consist of numerousdifferent molecules which, in turn, can display severalbinding sites. For example, one binding site can parti-cipate in different binding mechanisms: carboxyl groupscan engage both in complexation and electrostatic at-traction of metal cations. Consequently, several mechan-isms often act in combination (Volesky, 2003).

A few articles have been published with the aim ofunderstanding how the biosorption mechanism of heavymetal occurs. Particularly in the case of copper ions,Electron Paramagnetic Resonance (EPR) appears as avaluable tool for the study of the adsorptionmechanism ofthese cations by different biomaterials. Using ElectronParamagnetic Resonance (EPR) and Fourier TransformInfrared Absorption (FT-IR), de Carvalho et al. (2001)

reported that the copper ions are incorporated by driedleaves from the Brazilian flora in a strongly axial site. Thesame authors (de Carvalho et al., 2003) proposed a modelfor the sorption site where the copper ions are locatedbetween two adjacent carbon rings in an interstitial sitein the fibers of the same biomass. It is important toemphasize that de Carvalho et al. (2001) studied theincorporation of copper by dried plant leaves, whichpossess polysaccharides different from the ones observedin brown seaweeds. Brown seaweeds contain basically thepolysaccharide alginate while the main polysaccharidesfrom plant leaves are celluloses. This fact markedlyaffects the uptake capacity of the distinct biosorbentmaterials, consequently affecting the mechanisms in-volved in the biosorption by leaves and seaweeds.

Philip et al. (2000) used the EPR to elucidate the siteof interaction of Cu(II) with Bacillus polymyxa. Theyshowed that accumulated copper formed complexeswith oxygen- and nitrogen-containing functional groupswhich may be the carbonyl groups of cell-wall pepti-doglycan or nitrogen atoms of aminosugars or structuralproteins. However, the peptidoglycan nature of the cellwall of Bacillus cells cannot be compared to polysac-charides from brown seaweeds whose chemical com-position is markedly different. Freire-Nordi et al. (2005)used the EPR technique with the purpose of verifyingthe metal adsorption capacity of high molecular weightextracellular polysaccharides produced by the cyano-bacteria Anabaena spiroides. The peptidoglycan com-position of the cell wall of cyanobacteria makes theirexternal structures amenable to biosorption, howeverthrough the interaction of specific cell wall componentsof microbial cells. Nakajima et al. (2001) also used theEPR technique to clarify the binding states of copper inmicrobial cells. The EPR spectra indicated that copperions in the intact and in all the chemically treated cellshave coordination environments with nitrogen andoxygen atoms as donor atoms. Analogously Micrococ-cus luteus cells resemble Bacillus and Anabaena cells interms of chemical composition.

In this work, the use of Sargassum filipendula as abiosorbent for copper ions from aqueous solution wasstudied by Atomic Absorption Spectrometry (AAS)and Electron Paramagnetic Resonance (EPR). The ki-netics and equilibria of the biosorption process wereevaluated and the maximum biosorption capacity of thebiomass, based on dry weight, was determined fromthe equilibrium data measured under optimized condi-tions of pH and shaking rate. Some information aboutthe biosorption mechanism was inferred from EPRanalyses. The novelty of the present work includes theinvestigation of copper biosorption using techniques

107A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

such as EPR, already used for the investigation ofmetals uptake by cellulosic materials such as driedplant leaves or microbial cells, but had not yet beeninvestigated for seaweeds.

2. Materials and methods

2.1. Seaweed

The brown seaweed S. filipendula (Phaeophyceae,Ectocarpales, Fucales, Sargassaceae) used in this workwas harvested from the sea, sampled, extensively washedwith distilled water to remove particulate material fromtheir surface, and oven-dried at 343 K for 24 h. Onekilogram of biomass was sub-sampled for use in theexperiments. In order to ensure that homogeneous sam-ples were used, standard sampling techniques were ap-plied. Dried biomass was cut, ground in a mortar withpestle, and then sieved. The fraction with 0.3–0.7 mmwas selected for use in the sorption tests.

2.2. Copper solutions

Stock copper solution (1000 μg mL−1) was preparedby dissolving 2.683 g of copper chloride dihydrate(Merck, Darmstadt, Germany) in 100 mL of deionizeddistilled water (DDW) and diluting quantitatively to1000 mL using DDW. Copper solutions of differentconcentrations were prepared by adequate dilution ofthe stock solution with DDW.

2.3. Determination of the copper contents

2.3.1. Atomic Absorption SpectrometryThe concentration of copper in the solutions before

and after the equilibrium was determined by AtomicAbsorption Spectrometry (AAS), using a Perkin-ElmerAAnalyst 300 atomic absorption spectrometer equippedwith deuterium arc background corrector, an air–acety-lene burner, and controlled by IBM personal computer.The hollow cathode lamp was operated at 15 mA and theanalytical wavelength was set at 324.8 nm. Glasswareand polypropylene flasks used were immersed overnightin 10% v/v HNO3 and rinsed several times with DDW.

2.3.2. Electron Paramagnetic ResonanceThe EPR spectra were measured at room temperature

on a Bruker ESP 380-E CW/FT spectrometer operatingin X-Band (9.5 GHz) and 100 kHz frequency modula-tion. The amplitude modulation was kept constant at5.0 G. The g-factors were measured against DPPH(diphenylpicrylhidrazyl radical) standard, g=2.0036.

The spectra were analyzed using WinEPR and simulatedusing SimFonia Bruker shareware programs.

2.4. Batch biosorption studies

Batch biosorption experiments were performed using100 mg of dried biomass added to 25 mL of coppersolution in 500 mL polypropylene flasks. The flaskswere placed on a rotating shaker (Tecnal, Brazil) withconstant shaking of 150 rpm. For the kinetic study, theinitial copper concentration was 20 μg mL−1, the tem-perature was 298 K, and the working pH was that of thesolution (pH=5.0). The sorption time was varied be-tween 3 and 120 min. At predetermined times, the flaskswere removed from the shaker and the solutions wereseparated from the biomass by filtration through filterpaper (Whatman no 40, ashless). The equilibrium iso-therms were determined at similar experimental condi-tions, varying initial copper concentration from 18.4 to1000 μg mL−1 and using a shaking time equal to 2 h.Due to the protonation properties of the carboxyl groupspresent at the cell walls of the seaweed, pH can affect thebiosorption process. Therefore, the effect of initial pHon copper ions uptake was investigated within 2.0–5.0.The initial pH was adjusted using 0.10 mol L−1 HClsolution. No further adjustment on pH was done duringthe experiment. For EPR studies, copper-loaded sea-weed samples were prepared as powders. All biosorp-tion experiments were done in duplicate. Error bars werenot reported, because all the replicate values from theexperiments are presented in the figures.

2.5. Metal uptake

2.5.1. Quantitative determinationThe copper uptake was calculated by the simple con-

centration difference method (Volesky, 2003). The initialconcentration,C0 (μg mL−1), and metal concentrations atany time, Ct (μg mL−1), respectively, were determinedand the metal uptake q (mg metal adsorbed/g adsorbent),was calculated from the mass balance as follows:

q ¼ ðC0−CtÞdV1000w

ð1Þ

where V is the volume of the solution in mL and w is themass of the sorbent in g. Preliminary experiments hadshown that copper adsorption losses to the flask walls andto the filter paper were negligible.

2.5.2. Semi-quantitative evaluation by EPRCopper-loaded seaweed samples of known weight

were measured as powders in Pyrex tubes, and the

Fig. 1. Effect of the initial pH on equilibrium copper ions uptake bySargassum filipendula.

Table 1Equilibrium uptake (qe) and fraction of copper removed (Xa, %) bySargassum filipendula at different initial metal concentrations (shakingtime=2 h and pH 5.0)

C0

(μg mL−1)298 K

qe (mg g−1) Xa (%)

18.4 4.3 (0.1) a 89.1 (1.6)36.9 8.3 (0.2) 90.1 (1.8)92.2 21.3 (0.5) 92.4 (1.7)184 40.0 (0.8) 87.0 (1.9)277 53.3 (1.3) 77.0 (1.5)369 57.9 (1.7) 62.8 (1.6)585 70.1 (1.9) 47.9 (1.7)780 76.2 (2.0) 39.1 (1.4)878 77.4 (2.1) 35.3 (1.7)a Figures in parenthesis are standard deviations obtained for trip-

licate measurements of the parameter for two identical samples (n=6).

108 A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

amount of immobilized copper ions in each sample wasestimated by double-integration of the EPR signal.

3. Results and discussion

The influence of initial pH and copper concentra-tion on biosorption of copper ions by S. filipendulawas investigated. The results were expressed as theamount of copper ions adsorbed on dried algae at anytime (q, mg g−1) or at equilibrium (qe) and the con-centration of copper ions that remained in solution atthe equilibrium (Ce, μg mL−1). The fraction of copperions adsorbed (Xa, %) was also reported.

Xa;% ¼ 1−Ce

C0

� �� 100 ð2Þ

3.1. Influence of initial pH

It is well known that the pH of the medium affects thesolubility of metal ions and the concentration of the

Fig. 2. EPR spectra of copper loaded Sargassum filipendula.

counter ions on the functional groups of the biomass cellwalls, so pH is an important parameter on biosorption ofmetal ions from aqueous solutions.

S. filipendula presents a high content of ionizablegroups (carboxyl groups from mannuronic (pKa=3.38)and guluronic (pKa=3.65) acids) on the cell wall poly-saccharides, which makes it, at least in theory, veryliable to the influence of the pH. As shown in Fig. 1, theuptake of copper(II) depends on pH at low values,increasing with the increase in pH from 2.0 to 3.0and then reaching a plateau in the range 4.0–5.0.Similar results were reported in the literature fordifferent Cu(II)-biomass systems (Sağ et al., 1998;Matheickal and Yu, 1999). At pH values lower than3.0, copper (II) removal was inhibited, possibly as aresult of the competition between hydrogen and cop-per ions on the sorption sites, with an apparent pre-ponderance of the hydrogen ions, which restricts theapproach of metal cations as a consequence of the

Fig. 3. Adsorption isotherm of copper ions by Sargassum filipendulaat 298 K and pH 5.0.

Fig. 4. Linearized Langmuir adsorption isotherm of copper(II) ions bySargassum filipendula at 298 K and pH 5.0.

Table 2Freundlich and Langmuir adsorption constants associated withadsorption isotherms of copper(II) ions on Sargassum sp. at 298 Kand pH 5.0

Freundlich constants Langmuir constants

KF n R Q0 (mg g−1) KL (L g−1 mol−1) R

5.7 (1.0) a 2.3 (0.2) 0.908 82.6 (1.2) 1654 (217) 0.998a Figures in parenthesis are standard deviations (n=20).

109A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

repulsive force. As the pH increased, the carboxylategroups in S. filipendula would be exposed, increasingthe negative charge density on the biomass surface,increasing the attraction of metallic ions with positivecharge and allowing the biosorption onto the cellsurface.

The experiments were carried out with initial pHvalues lower than 6.0 since insoluble copper hydroxidestarts precipitating from the solutions at higher pHvalues, making true sorption studies impossible. At theend of the experiments, pH slightly increased, reachinga maximum value of 6.0.

3.2. Effect of the initial copper concentration onbiosorption equilibrium

Identification of coordination sites is a fundamentalstep in elucidating the mechanism of cupric ions sorptionin S. filipendula biomass. The Electron ParamagneticResonance technique is a powerful tool for investigating

Fig. 5. Linearized Freundlich adsorption isotherm of copper(II) ions bySargassum filipendula at 298 K and pH 5.0.

the immediate environment of the paramagnetic ion. TheEPR spectral parameters are sensitive to changes in thesymmetry of this environment and allow the study of theion state in a host as a function of structural arrangementof functional groups. For example, the EPR spectra aresensitive to the uptake of copper, as presented in Fig. 2.For initial copper concentration of 18.4 μg mL−1

(qe=4.3 mg g−1) the spectrum is of axial-type, typicalof isolated immobilized copper ions, and the hyperfinestructure is clearly observed. On the other hand, forinitial copper concentration of 277 μg mL−1 (qe=53.3 mg g−1) a distorted line, typical of aggregates, isobserved, due to dipole–dipole interactions, precludingthe observation of hyperfine interactions.

At the optimized conditions of sorption, the bestparameters obtained by solving the spin Hamiltonianconsidering the 63Cu (69% n.a.) and 65Cu (31% n.a.)were: A|| =150(2)G, g|| =2.31(1), A⊥=16(2)G e g⊥=2.06(1). These values indicate that the most probablesorption sites consist of carboxylate moieties (Peisachand Blumberg, 1974) of alginic acid functional groupsunits, that is, β-1,4-D-manuronic and α-1,4-L-guluronicacids.

Table 1 compares the equilibrium uptakes and thefraction of copper adsorbed obtained at 298 K anddifferent initial copper concentrations using AAS.Regarding the influence of the initial concentration ofcopper ions, the equilibrium sorption capacity of thebiomass increased with the increase in the initial copperions concentration up to 1000 μg mL−1, while thefraction of copper adsorbed presented the oppositetrend. The difference between bulk and surface metalions concentration is one of the driving-forces to over-come the resistances to adsorption process. In the ab-sence of mass-transfer resistances (as observed in ourexperiments), surface and bulk concentrations are iden-tical, so the increase in initial concentration of copperions will enhance the adsorption process, as noticed inTable 1. As it can be seen in Fig. 3, negligible increase inthe values of qe are observed for equilibrium concentra-tions greater than 400 μg mL−1, suggesting that abovethis level of solute, solid–liquid equilibrium is probably

Table 3Copper(II) adsorption capacities of reported sorbents

Adsorbent qmax

(mmol g−1)References

Granulated activatedcarbon, F-400

0.03 Muraleedharan et al. (1995)

Rhizopus arrhizus 0.25 Volesky (2003)Pseudomonas aeruginosa 0.29 Chang et al. (1997)Phanerochaetechrysosporium

0.42 Say et al. (2001)

Rhizopus arrhizus 0.53 Sağ et al. (1998)Pre-treated Eckloniaradiata

1.11 Matheickal and Yu (1999)

Pre-treated Durvillaeapotatorum

1.30 Matheickal et al. (1999)

Chlorella vulgaris 1.40 Mehta and Gaur (2001)Ulothrix zonata 2.77 Nuhoglu et al. (2002)Sargassum sp. 1.30 This study

Fig. 7. Influence of sorption time on copper(II) ions uptake by Sar-gassum filipendula at 298 K and pH 5.0 (C0=18.4 μg mL−1).

110 A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

limited by the diffusion of the copper ions towards thenegative charged metal-sequestering sites on the surfaceof the seaweed; in fact, the algae surface does notdisplay sufficient free sites for metal uptake, beingsaturated. This is also indicated by the line shape of theEPR spectra of biomass loaded with initial copper(II)concentrations higher than 250 μg mL−1 (qe=53.3 mgg−1) that present distorted broad lines typical of mag-netic dipole–dipole interactions, usually found in aggre-gates (Fig. 2).

3.3. Equilibrium modeling

Modeling the equilibrium data is fundamental forindustrial application of biosorption. It gives informationfor comparison among different biomaterials underdifferent operational conditions, designing and optimiz-ing operating procedures (Benguella andBenaissa, 2002).

Fig. 6. Effect of the sorption time on Cu-loaded biomass EPR spectra.

In order to examine the relationship between sorbed(qe) and aqueous concentration (Ce) at equilibrium, thesorption isotherm models of Langmuir (4) and Freun-dlich (5) were tested for fitting our data. The Langmuirmodel assumes that (i) the solid surface presents a finitenumber of identical sites which are energetically uni-form; (ii) there are no interactions between adsorbedspecies, meaning that the amount adsorbed has noinfluence on the rate of adsorption; (iii) a monolayer isformed when the solid surface reaches saturation. On theother hand, Freundlich model assumes that enthalpy ofadsorption is independent of the amount adsorbed andthe empirical Freundlich equation, based on sorption onheterogeneous surface, can be derived assuming a loga-rithmic decrease in the enthalpy of adsorption with theincrease in the fraction of occupied sites.

Langmuir isotherm qe ¼ Q0dKLdCe

1þ KLdCeð4Þ

Freundlich isotherm qe ¼ KFdC1=ne ð5Þ

where Ce is the metal concentration in the liquidphase at equilibrium, KL the Langmuir constant, qethe metal uptake at equilibrium, Q0 the maximummetal uptake capacity, and KF and n the Freundlichparameters.

The corresponding constants and the coefficients ofcorrelation (R) associated with each linearized form ofboth models (Figs. 4 and 5) are presented in Table 2. Theresults indicate that Langmuir isotherm best fits theexperimental data over the experimental range studied,since it presents the greater correlation coefficient. Asshown in Fig. 3, the predicted profiles are in goodagreement in all ranges of Ce evaluated.

A comparison of Cu(II) uptake capacity betweenS. filipendula (Q0=82.6 mg g−1) and other biosorbents

Table 4Comparison between adsorption rate constants, qe estimated and coefficients of correlation associated with the Lagergren pseudo first-order and withthe pseudo second-order kinetic models (C0=18.4 μg mL−1, w=0.100 g, V=25 mL, pH 5.0, agitation rate 150 rpm)

First-order kinetic model Second-order kinetic model qe,exp(mg g−1)

k1,ads(min−1) qe (mg g−1) R k2,ads(g mg−1 min−1) qe (mg g−1) R

0.150 (0.011) a 1.21 (0.35) 0.967 0.349 (0.043) 4.30 (0.03) 1.000 4.26

a Figures in parenthesis are standard deviations (n=10 first-order kinetic model; n=20 second-order kinetic model).

111A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

reported in the literature is given in Table 3. Although adirect comparison is not possible due to the varyingexperimental conditions employed, in general, the Cu(II) uptake capacity of Sargassum sp. is higher than thatof most of the other biosorbents.

3.4. Biosorption kinetics and mechanism of copper ions

For initial copper concentrations of 18.4 μg mL−1

(qe=4.3 mg g−1) the spectrum is of axial-type, typicalof isolated immobilized copper ions; the hyperfinestructure is clearly observed and the spectra could besimulated. On the other hand, for 780 μg mL−1 initialcopper concentration, (qe=76.2 mg g−1), a distortedline typical of aggregates is observed, due to dipole–dipole magnetic interactions, precluding the observa-tion of hyperfine structure (McGarvey, 1966). Oneshould note that for 277 μg mL−1 initial concentration(qe=53.3 mg g−1), the line shape of the EPR spectrumis a sum of at least two contributions, isolated Cu(II)species and copper aggregates.

At the optimized conditions of sorption used for theEPR measurements, that is, 18.4 μg mL−1 Cu(II) initialconcentration, the best parameters obtained by solvingthe spin Hamiltonian considering the 63Cu (69% n.a.)and 65Cu (31% n.a.) were: A|| =150(2)G, g|| =2.31(1),A⊥=16(2)G, g⊥=2.06(1), Fig. 2.

Fig. 8. Linearized pseudo-second-order kinetic model for copper(II)ions uptake by Sargassum filipendula at 298 K.

These values are different from those observed forionic Cu(II) in solution (eg. [Cu(H2O)6]

2+ aquocom-plex, A|| =128G, g|| =2.40, Lewis et al., 1966), that is,they indicate a higher degree of covalence for the sorbedcopper species.

The main parameters of an axial divalent copper EPRspectrum are those measured in the parallel region,where the hyperfine splitting parameters are usually wellresolved. The magnitudes of A|| and g|| are correlated tothe nature of the ligands, in terms of covalency, so thesevalues can be used to assign structure (Kivelson andNeiman, 1961). Such correlation is the basis of the well-known Peisach and Blumberg (P and B) plots (Peisachand Blumberg, 1974).

As discussed above, the biomass samples investigat-ed in this work mainly comprise the M(I), M(II)-alginate-based structural polysaccharides, that is, onlyoxygen binding sites are expected. Moreover, the ion-exchange of copper species will occur in differentbinding sites with a range of water molecules in order tomaintain the electroneutrality. This will result in thebroadening of the EPR lines, primarily due to g and Aanisotropy.

The analysis of EPR data, taking into account thediscussion above, indicate that the most probablesorption sites consist of carboxylate moieties of alginicacid functional groups units, that is, β-1,4-D-manuronicand α-1,4-L-guluronic acids. Considering that these sitesare monovalent, each Cu(II) may bind to two carbox-ylate moieties, presumably of different blocks in thealginate structure.

Fig. 6 shows a series of EPR spectra of copper-loadedsamples prepared with initial copper concentration of18.4 μg mL−1, but measured at different sorption times.It clearly shows that the intensity of the EPR lineincreases as a function of sorption time. Moreover,semi-quantitative analysis of the data shows an uptakeof about 80% of copper ions after 3 min, in accordancewith AAS quantitative measurements showed below.

As can be seen in Fig. 7, for an initial concentrationof copper ions equal to 18.4 μg mL−1 the rate ofremoval of copper ions is extremely rapid in the first10 min, but, it decreases significantly and approaches

112 A.S. Luna et al. / Hydrometallurgy 86 (2007) 105–113

zero (equilibrium). The fast biosorption kinetics ob-served is typical for biosorption of metals involving noenergy-mediated reactions, where metal removal fromsolution is due to purely physico-chemical interactionsbetween biomass and metal solution (Aksu, 2001). Oncemore, it was confirmed that results obtained from EPRspectroscopy corroborate AAS measurements.

The shape of the q versus time curves is similar tothat reported by different authors concerning otherCu(II)-biomass systems (Chang et al., 1997; Mehtaand Gaur, 2001; Nuhoglu et al., 2002; Sağ and Aktay,2002).

3.5. Kinetic modeling

Two different kinetic models were used to adjust theexperimental data of Cu2+ biosorption on S. filipendula.The pseudo first-order Lagergren model,

logðqe−qÞ ¼ log qe−k1;ads2:303

t ð6Þ

where qe(mg g−1) and q(mg g−1) are the amounts ofadsorbed metal ions on the biosorbent equilibrium andat any time t, respectively, and k1,ads is the Lagergrenrate constant, and the pseudo-second order model (Fer-nandez et al., 1995; Ho et al., 1996),

tq¼ 1

k2;adsdq2eþ 1qe

t ð7Þ

where k2,ads is the rate constant of second order bio-sorption (g mg−1 min−1).

The Lagergren first-order rate constant (k1,ads) and qedetermined from the model indicated that this modelfailed to estimate qe since the experimental values of qediffered from those estimated (Table 4). By plotting t/qagainst t (Fig. 8), a straight line could be obtained,allowing the determination of the second-order rateconstant (k2,ads) and qe values. The coefficient of corre-lation for the second order kinetic model was equal toone and the estimated value of qe also agreed with theexperimental one. Both facts suggest that the sorption ofcopper(II) ions follows the pseudo-second-order kineticmodel, which relies on the assumption that chemicalbiosorption may be the rate-limiting step. Similar resultswere obtained in a previous work conducted at threedifferent temperatures (Antunes et al., 2003).

4. Conclusions

The EPR technique was employed to study thesorption of Cu(II) by S. filipendula in different condi-

tions of pH, initial Cu(II) concentration, and sorptiontime. From a qualitative focus, the correlation betweenthe hyperfine parameters obtained by solving the spinHamiltonian and the comparison with the hexaaquocop-per(II) complex hyperfine parameters showed that theimmobilized copper(II) species share some covalencewith the biopolymer host. Taking into consideration thenature of the alginate structural polysaccharide and thedifferent combinations of the alginic acid monomer unitsin polymeric blocks, the EPR results indicate that theimmobilization of Cu(II) ions occurs through cation-exchangeable carboxylate anions of alginate monomers.EPR was also used in semi-quantitative measurementsand fully corroborates the results from AAS.

From the AAS measurements, we concluded that theLangmuir adsorption model well described the biosorp-tion equilibrium of copper (II) ions on S. filipendula in thestudied conditions. Biosorption of copper(II) ions ontobiomass followed a second-order adsorption kinetics.

The results confirmed that S. filipendula is a potentialbiomaterial to remove copper ions with a high bio-sorption capacity: 1.30 mmol g−1. This value can becompared with those observed for other marine micro-algae and it is considerably higher than the valuesobtained with the majority of the biosorbents.

Acknowledgments

A. C. A. C. and A. S. L. would like to thank UERJ,through Prociência Program. M.H.H. thanks FAPERJfor an Associate Research Fellowship.

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