Complexes of carboxymethylcellulose in water. Part 2. Co2+ and Al3+

remediation studies of wastewaters with Co2+,Al3+, Cu2+, VO2+ and Mo6+

Ana Paula Franco a, María Angeles Lobo Recio b, Bruno Szpoganicz c,Aurora López Delgado d, Judith Felcman e,

Ana Lucia Ramalho Mercê a,⁎

a Laboratório de Equilíbrio Químico, Departamento de Química, Universidade Federal do Paraná, CP 19081-81531-990, Curitiba Pr, Brazilb Departamento de Engenharia Sanitária, Universidade Federal de Santa Catarina, UFSC, Trindade-88040-900, Florianópolis SC, Brazil

c Departamento de Química, Universidade Federal de Santa Catarina, UFSC, Trindade-88040-900, Florianópolis SC, Brazild Centro Nacional de Investigaciones Metalurgicas, CENIM, Avda Gregorio del Amo, 8-28040, Madrid, Spain

e Departamento de Química, Pontifícia Universidade Católica do Rio de Janeiro,Rua Marquês de São Vicente, Gávea-22453-900, Rio de Janeiro RJ, Brazil

Received 5 December 2005; received in revised form 27 April 2006; accepted 14 August 2006Available online 6 April 2007

Abstract

The properties of carboxymethylcellulose (CMC) complexes and Co2+ and Al3+ are reported in this work. The complexingpower of CMC was greater to Al3+ than to Co2+, although it was not possible to determine some of the equilibrium constants. Theinfrared (IR) spectroscopy and thermal analysis helped in showing the existence of these complexes in the solid state. The filmsobserved by scanning electron microscopy (SEM) provided a certainty that the chains of the biopolymer were not extensivelybroken by the use of strong mineral acid employed in some of the experimental steps of this study.Two water solutions — bidistilled and deionized water and an Iraí River, Curitiba, PR (Brazil) sample — were obtained by

adding metal salts of Al3+, Co2+, Cu2+, Mo6+(initially in the form of molybdate) and VO2+ to which CMC was later added as aremediation agent. At different times, aliquots of those water samples were analyzed for their metal contents and showed ability tosequester different percentages of each of the metal ions, therefore, rendering the water samples within the Brazilian and Spanishstandards for potable water (varying from b0.3 to 5 mg/L depending on toxicity). The CMC complexes could be recovered bymechanical removal at the pH where these complexes are not very soluble. This process can be applied to municipal wasterwatertreatment plants as CMC is a more cost-effective and non toxic alternative material than commercial employed alum. The metalscan be recycled after the decomplexing process from the recovered solid complexes and with the additional benefit of using CMCthat it will leave no trace of Al3+ ions in the water rising from the use of alum.© 2007 Elsevier B.V. All rights reserved.

Keywords: Wastewater; Cobalt; Aluminum; Carboxymethylcellulose; Complexation equilibria; Remediation; Pickling process; Metal speciation

Hydrometallurgy 87 (2007) 178–189www.elsevier.com/locate/hydromet

⁎ Corresponding author. Tel.: +55 41 3361 3179; fax: +55 41 3361 3186.E-mail address: anamerce@ufpr.br (A.L.R. Mercê).URLs: http://www.quimica.ufpr.br/~anamerce (A.P. Franco), http://www.quimica.ufpr.br/~anamerce (A.L.R. Mercê).

0304-386X/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.hydromet.2006.08.013

1. Introduction

Many industrial processes nowadays generate greatquantities of wastewaters which have an adverseenvironmental impact due to contaminants in thesewaters. There are many possible chemical and biologicalways to treat those wastewaters but the more ecologicallyfriendly solution to remediate metal contaminated waste-waters is recycling.The stainless steel mills are examples of polluting

factories. They generate wastewater in the washing pro-cess, in the final steel production process, the so calledpickling process (Pedrazzini and Giordani, 2000). Basi-cally the method for pickling steel is to remove the oxidelayer from its surface by immersion in a bath of acidicpickling solutions with or without metal ions. The acidicwater is continuously circulated and then neutralized forits acidity with calcium carbonate or hydroxide and re-leased to the environment, though not completely freefrom the metal ions in the solution. These metal ions maybe Co(II) and Mo(VI), among others.Vanadyl ion (VO2+) is normally employed in catalyt-

ic materials. When they are worn out they are sometimestreated as expendable. (Mohebbi et al., 2005).Most surface water treatment plants in some countries

(e.g. Canada and Brazil) use aluminum in the form ofalum (aluminum sulphate) often due to its low cost, tohelp remove harmful water borne microorganisms andother particles by causing them to clump together (coag-ulate) into larger particles that then are easily removed bysedimentation and filtration. This process also removesnaturally-occurring organic matter present in water. How-ever, it leaves aluminum, a toxic element at all concentra-tion levels (Silva and Williams, 1991), in the water.Copper is employed in many industrial processes,

such as the wire industry and the brass casting process,leading to its ubiquitous presence in the environment.A renewable, non toxic and reusable complexing

agent is of utmost interest in the recycling of metal ions.Some studies reported in the literature show complexingabilities of some biopolymers to some metal ions (Juangand Shao, 2002; Ngah et al., 2002; Bailey et al., 1999).Although the major studies in the literature have

focused on chitosan and its derivatives (Kartal andImamura, 2005; Jeon and Höll, 2004, 2003; Liu et al.,2002; Ngah et al., 2002) as well as other more powerfulremediation sorbents (Babel and Kurniawan, 2003;Vaughan et al., 2001; Bailey et al., 1999), some authorshave reported on the complexation properties of somenatural and synthetic polymers, including carboxymeth-ylcellulose, and copper (II) (Vasconcelos and Gomes,1997), a crosslinked CMC gel to capture Cu(II) (Hara

et al., 2004) and on a new CMC based resin with whichthe adsorption kinetics were investigated for the cupricion (Zohuriaan-Mehr et al., 2004). All results haveshown CMC as a sequestering agent suitable for copperions, but complete equilibrium studies in aqueous mediainvolving CMC and metal ions are yet to be conducted.Sodium carboxymethylcellulose (CMC) consists of re-peated units of β-1,4 linked anhydroglucose residues,substituted by sodium carboxymethyl groups.In order to propose CMC as a cobalt and aluminum

water remediation agent, it is important to first deter-mine the chemical equilibrium of the formed complexes,such as the strength of the bondings by obtaining thebinding constants, the speciation of these complexesaccording to the pH variations, and whether these com-plexes would prevail in the solid state. It is also desirableto have complexes capable of being further decom-plexed for reuse. So, this work presents some chemicalcharacteristics of the complexes of CMC and Co2+ andCMC and Al3+ and their ability to remediate by com-plexation, some metal ions in different water samples.

2. Materials and methods

The potentiometric titrations (PT), infrared (IR) andultraviolet–visible (UV–Vis) spectroscopy, thermogravi-metry-infrared (TGA-IR) analysis, and scanning electronmicroscopy (SEM) were used to study the complexes ofcarboxymethylcellulose and ionsCo2+ andAl3+ followingthe methodology reported in Noleto et al. (2004) andMercê and Lombardi (2003). Also remediation by com-plexation with CMC of different metal ions in river,double distilled and deionized water samples were con-ducted (Franco and Mercê, 2006).All chemicals used were of analytical-reagent grade

and were used as received. All solutions were madewith bidistilled, deionized, and CO2-free water. Metal(M) solutions were made with chloride salts (Co2+ andAl3+ — Merck, Germany), nitrate and sulfate hydratesalts (Cu2+ — Baker Analyze Reagent, USA,VOSO4·xH2O— Aldrich Chemical, USA, respective-ly) and molybdate salt ((NH4)6Mo7O24·4H2O —Carlo Erba, Italy). Their concentrations were deter-mined with EDTA and proper end-point indicators forCu2+, Co2+ and Al3+ (Schwarzenbach and Flaschka,1969) and with KMnO4 titrations for VO

2+ and Mo6+

(Furman, 1962). The aqueous KOH (Merck, Ger-many) 0.1 mol/L, carbonate-free solution was stan-dardized against potassium hydrogen phthalate (CarloErba, Italy). KCl (Merck, Germany) was the support-ing electrolyte to maintain the ionic strength (Ι) at0.100 mol/L.

179A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

2.1. Potentiometric titrations

The potentiometric titrations were carried out in dupli-cate under an inert atmosphere of water–KOH saturatednitrogen (White-Martins, Brazil) in a water-jacketed ves-sel maintained at 25.0±0.1 °C, (MQBTC 99-20 —Microquímica, Brazil) with CMC (0.10 mmol, Sigma,USA, degree of substitution (DS)=0.70 (kindly deter-mined by Professor Dr. Thomas Heinze — Jena Uni-versität — Germany, as in Saake et al. (2001) andMW=250.000 Da) in the absence and in the presence ofthe metal ions, Co2+ and Al3+. The pKw used was 13.78. Apiston burette (Metrohm AG Herisau/E 274, Switzerland)was used to deliver the titrant, a standard KOH (0.1 mol/L,0.10±0.02 mL)-free CO2, and pH (−log of the molarconcentration of H+) values were directly measured withOrion (Switzerland) glass and Ag–AgCl reference electro-des calibrated with standard HCl and KOH solutions up tothe third digit figure (Martell and Motekaitis, 1992).The values of the binding constants were calculated

using the exact millimole of the added metal ion (M), the

calculated millimole of the ligand taking into accountthe monomeric sugar unit of CMC, the hydrolysis con-stants of Co2+ and Al3+ (Baes and Mesmer, 1976) andthe determined protonation constants of the monomericsugar repeating units (HL) of the biopolymer (Francoand Mercê, 2006; Martell and Smith, 2001) in themathematical model used in Hyperquad microcomputerprogram (Gans et al., 1996). The miniminum percentageof the statistical presence of a species to be considered inthe calculations was 10% of all species present in theequilibria. The species distribution diagrams were ob-tained with HySS (Hyperquad simulation and specia-tion, Alderighi et al., 1999) program using Hyperquadcalculation data as input.

2.2. Ultraviolet–visible spectroscopy (UV–Vis)

The UV–Vis spectra were recorded in aliquots ofabout 3.0 mL of a separate titration in the ratio 1:1 metalto ligand (Hewlett-Packard model 8452-diode arrayspectrophotometer) in the range of 190 to 600 nm.

Fig. 1. a) Potentiometric pH profile of a solution of 0.10 mmol of CMC and Co2+. b) Experimental ( ◊ ) and calculated ( - ) pH curves.

180 A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

Quartz cells of a 1.000 cm path length was used, andwater was the reference.

2.3. Infrared (IR) and Thermogravimetric–Fouriertransform infrared (TGA-FTIR)

Solid complexes of CMC and starting aqueous solu-tions containing the metal ions Co2+ and Al3+ wereobtained as follows. To a magnetic stirred and warmaqueous solution containing 0.10 mmol of CMC, threetimes less millimole of the metal ions was added as solidmetal p.a. salts. KOH (aqueous solution) was used toraise the pH value from 2.0 to 10.0. The aliquots taken atthe various chosen pH values were dried at 40.0 °C forone day in an oven. These solids were employed in theIR spectroscopy and in the thermogravimetric analysis.The IR spectra of solid CMC and the metal complexes

were recorded in an FTIRSpectrometer (MB100, Bomem,DTGS detector) in KBr pellets, scanning 16 times between500 and 5000 cm−1. A quantitative assay was tried byalways employing 1%m/mof sample in 0.1000 g final KBrpellets mass.Thermogravimetry analysis combined with Fourier

transform infrared spectroscopy (TGA-FTIR) was re-corded in a Thermogravimetric Analyser (TGA 7-IRSystem 2000 FTIR-PERKIN ELMER — USA), using250 mg of sample. The experimental conditions were ascanning rate of 5 °C/min, temperature range of 20 to900 °C, under N2 and for the infrared analysis, reso-lution 4.0 cm−1 from 600 to 4000 cm−1.

2.4. Scanning electron microscopy (SEM)

The films were obtained by drying (40 °C, 3 to 5 daysin an oven) approximately 2mL of aliquots of solutions ofCMC in the absence and the presence of the metal ions inthe ligand-to-metal (Co2+ and Al3+) ratio of 3:1 in flat thinpieces of glass. The samples weremetallizedwith gold for1min in aUnion FL 9496BALZERS (model SCD030—Germany) and analyzed by SEM. The micrographs weretaken in a JEOL scanning electron microscope (JSM-

model 6360 LV — Tokyo — Japan), at differentmagnifications and 10 kVof acceleration tension.

2.5. Remediation studies

For the remediation assays in aqueous solutions threemain samples were made. A sample was collected from theIraí River–Curitiba–Pr–Brazil, following the procedure inthe literature (Benoit et al., 1997; USEPA, 1996), in theabsence (in natura) and in the presence of addedmetal ionsand a synthetic solution made with distilled water withknown amounts of metal ions (Cu2+, VO2+, Mo6+, Co2+

and Al3+) also added. EDTA or KMnO4 titrations weremade in triplicate and the metal contents found are inTable 2. The detection limits of this methodology wereproven useful since they fell in the range of the Brazilian aswell as the Spanish maximum concentration standards forthosemetal ions in potablewater (b0.3mg/L,Al3+;b3mg/L, VO2+ and Mo6+; and b5 mg/L, Co2+ and Cu2+). At pHaround 5.0, corrected by using either HCl or NaOHaqueous solutions, CMC was added to the aliquots takenfrom the three solutions to reach 1 g/L final concentrationundermagnetic stirring. After 90, 120 and 150min aliquotswere taken and the total metal contents analyzed again. Inthe case of molybdenum, the starting salt used was amolybdate anion, but it is necessary to bear in mind thatafter pH 5–6 it is in fact MoO2

2+, a cation, but it is referredthrough out this text as Mo6+ for simplicity (Franco andMercê, 2006; Mercê et al., 2005).

3. Results and discussion

3.1. Potentiometric titrations and speciation

Potentiometric titrations were carried out in aqueoussolutions of CMC and the metal ions. The system withAl3+ and Fe3+ prevented complete equilibrium results

Table 1Logarithms of the protonation constants of CMC and the bindingconstants of the complexes of CMC and Co2+ (T=25.0 C°,Ι=0.100 mol/L, KCl)

Log K CMC (L) Al3+ Co2+

[HL]/[H]·[L] 11.68±0.01 – –[H2L]/[H]·[HL] 2.98±0.05 – –[ML]/[M]·[L] – 19.5±0.2 10.84±0.06[MOHL2]/[ML2]·[OH] – n.d. 8.29±0.07

n.d.: not detected.

Fig. 2. Species distribution diagram of CMC and Co2+ in the ligand-to-metal ratio of 2:1.

181A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

since insoluble products were produced at early pHvalues of all metal to ligand titration ratios tried.Fig. 1b shows the experimental and calculated potenti-

ometric pH profiles of aqueous solutions of CMC(0.10 mmol) and Co2+ (0.05 mmol). The simulated andthe experimental pH curves after the calculations made,matched, validating the chemical and mathematical mod-els employed.Fig. 1a shows the potentiometric pH profiles of CMC

(0.10 mmol) in the absence and in the presence of Co2+,0.10 and 0.05 mmol (1:1 and 2:1 ligand to metal ratios).The profile of the CMC alone started at pH around 6.7, thecarboxylic group having been already deprotonated at thispH. The following buffer around pH 10 refers to thedeprotonation of the hydroxyl groups of C-6 in the sugarunits of the biopolymer. The protonation constants ofCMC were determined by potentiometric titrations in thepresence of a known amount of strongmineral acid so as tomake all carboxylic groups protonated (pH around 1.9). Itis understood that this assumption refers to the averagedegree of substitution (D.S.) number where the C-6–OHgroups in the monomeric sugar units have not beenacetylated.The protonation equilibria of CMC can be represented

by Eqs. (1) and (2).

H�1CMCþHþ ¼ CMC KL¼½CMC�

½H 1CMC1�½H� ð1Þ

CMCþ Hþ ¼ HCMC KHL ¼ ½HCMC�½CMC�½H� ð2Þ

where HCMC stands for the monomeric sugar unitcompletely protonated (HL). The basic sites consideredare the hydroxyl group in C-6 and the carboxylic group,somewhere in the cyclic structure of the sugar. For sim-plicity the fully protonated species of CMC is representedas HL, and whenever the carboxylic group and the hy-droxyl group of C-6 are deprotonated, they are repre-sented as L and H−1L, respectively.The complexed species found in this work for CMC

and the studied metal ions can be represented by thefollowing Eqs. (3) and (4).

Mþ L ¼ ML ML ¼ ½ML�½M�½L� ð3Þ

MLþ OH ¼ MOHL MOHL ¼ ½MOHL�½ML�½OH� ð4Þ

where L stands for the ligand (anhydroglucose unit) ingeneral, as it was not possible to make a distinction

among the binding sites involved in the complexationusing the present experimental conditions; M = Co2+ orAl3+; ML is the complexed species of one mole of theligand and one mole of the metal ion and MOHL is theequivalent of ML with one hydroxyl group in thecoordination sphere of the metal ion. In the profiles ofCMC and Al3+ (Fig. not shown) early formation ofinsoluble products prevented the calculation of furtherspecies but ML.Eq. (5) represents the generic hydrolysis equation for

the hydrolytic species employed in the calculations(Baes and Mesmer, 1976):

YMXþ þ ZOH� ¼ MYðOHÞð�zþyxÞZ KH ¼ ½MYðOHÞðzyxÞZ �

½MX �Y½OH �Zð5Þ

The logarithms of the binding constants for ML are19.5±0.2,10.63±0.03, 12.08±0.02 and 15.89±0.03, forM = Al3+, Cu2+, VO2+ and Mo6+ respectively. Taking alook at the formation constants calculated forML species(refer to Table 1) the descending order of stability foundis Al3+ and Fe3+NMo6+NVO2+NCo2+NCu2+ (Francoand Mercê, 2006). It can be assumed that CMC bindspreferentially with the oxocations, in spite of their biggersize, than with the smaller cations Cu2+ and Co2+.As stated before, the systems of CMCwith either Fe3+

or Al3+ presented formation of insoluble products at thevery beginning of the experiments. Although some of thestability constants could not be calculated, this behaviorindicates a very high affinity between CMC and thesemetal ions, since the complexes formed very early in theequilibria, even when there is still the presence of strongmineral acid, which needs to be added in order to preventthe metal hydrolysis. This is an indication that the bindingconstants, if possible to be calculated, would be higherthan those calculated for Mo6+ and VO2+.The species distribution diagrams obtained for CMC

and Co2+ (Fig. 2) showed the existence of thecomplexed species in a broad range of pH values, pHaround 2 until 11.

3.2. Ultraviolet–visible titrations

The UV–Vis spectra of CMC alone and with themetal ions Co2+ and Al3+ (Figs. 3–5, respectively) wereobtained in aqueous solutions with varying pH valuesby the addition of a base, following the species distri-bution diagrams. It was possible to monitor the for-mation of the complexed species in the ultraviolet (Al3+)and ultraviolet and visible (Co2+) regions.

182 A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

The comparison of UV–Vis spectra of a pure substancein the absence and in the presence of metal ions helps toidentify modifications in the characteristic bands of thepossible chelating groups in the search of complexation. InFig. 3 (CMC alone) the UV spectra show maximum ab-sorption in 191 nm due to the carbonyl groups in CMC. Asthe pH values are higher than 3, where there is thedeprotonation of the carboxylic group, this maximum ab-sorption was a little higher and also shifted towards greaterwavelength values. After pH 10 it can be seen in the spectrathe beginning of the deprotonation of the hydroxyl group in

the C-6 of the sugar monomers of CMC as there is anothershift in the maximum absorption, 193 nm (Lever, 1968).The charge transfer (CT) bands due to d-d transitions

of aquocobalt (II) are near 300 nm and almost coin-cidental from pH 2.01 to 8.00. When this metal ion iscomplexed to CMC these absorption bands change themaximum intensity and shift to higher wavelengths asthe pH is raised. The d-d absorption in the visible region(4T2g6

4T1g) also changes the maximum intensity in theregion of 590 to 690 nm (Lever, 1968) as the complexesare being formed with pH raise from 2.10 to 10.00.

Fig. 3. UV spectra of an aqueous solution of CMC (10−5 mol/L) with varying pH values.

Fig. 4. UV–Vis spectra of a separate titration in the ratio 1:1metal-to-ligand, in aqueous solutionswith varying pHvalues for the CMC (10−1mol/L) andCo2+.

183A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

Fig. 5 presents the UV–Vis spectra of CMC and Al3+

(10−5 mol/L ligand and metal) where one can see abroadening of the absorption around 195–200 nm (car-bonyl group) as the pH values go from pH 3.5 to 7.6.This behavior is a strong indication of complexation byCMC (Lever, 1968).

3.3. Infrared spectroscopy

In plain monosaccharides, the primary hydroxylsubstituents are the most chemically reactive. The IRspectra in this work were mainly an attempt to quantita-tively monitor the decrease as the complexation oc-curred in the –OH stretching band, around 3400 cm−1.Even aware of the water adsorption ability of the solidcomplexes and KBr pellets in almost all obtained IRspectra with CMC and the metal ions studied, theintensity of the –OH band decreased with the increaseof pH. In addition, in these IR studies both regioncharacteristic of –OH groups, around 3400–3600 cm−1

and carbonyl group around 1760, 1425 and 1600 cm−1,were investigated. The region of 1300–1500cm−1 coversthe scissoring and wagging type of bending modes of –CH2 and –CH3 groups, as well as hydroxyl planebending. This –CH2 and –CH3 group bands help inunderstanding the formation of hydrogen bonds of theC-6–OH group in the sugar moiety of CMC. Also theregion of 1640–1650 cm−1 refers to water in aqueousderived solutions of carbohydrates, and 1384–1390 cm−1

refers to inorganic compounds (Maltlhouthi and Koening,1996).

IR assays both in aqueous and in the solid complexesof the biopolymer alone showed no significant differ-ences in the main absorption characteristic bands ofCMC showing that the drying process in this work didnot promote artifacts.Figs. 6 and 7 show the quantitative IR spectra of the

obtained solids of CMC and Co(II) and Al(III), respec-tively. By looking at the spectra as the pHvalues are raisedthe complexation can be followed by the decrease in theregion of deformations of the –OH group in C-6 (primarycarbon) in 3600 cm−1 as well as at intermolecular hydro-gen bonds in 3450 cm−1, C = O axial deformations of thecarboxylic group in 1760 cm−1 and symmetrical and anti-symmetrical stretching in 1425 and 1600 cm−1(Liang andMarchessault, 1959). The decrease in those bands can beattributed to a decrease in the intermolecular hydrogenbonds, therefore indicating that these Lewis basic sites arebeing complexed to the Lewis acids, the metal ions, eithervia the –OH of C-6 and/or the carboxylic groups. Theexceptions observed can be explained by the hygroscopicbehavior of some obtained complexes (Fig. 6, CMC andCo2+, in the pH values 4 to 5 and 5 to 6; Fig. 7 CMC andAl3+, only in the pH value 4 to 5). The bands of thecarbonyl groups (1760 and 1425 cm−1) also presented adisplacement and in more basic pH values, these bandscompletely disappeared indicating that all availablecarboxylic groups are involved in the metal complexation(Liang and Marchessault, 1959).The IR spectra confirmed the maintenance of the com-

plexed species in the solids extracted from the aqueoussystems.

Fig. 5. UV–Vis spectra of a separate titration in the ratio 1:1metal-to-ligand, in aqueous solutionswith varying pHvalues for theCMC (10−5mol/L) andAl3+.

184 A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

3.4. Thermal analysis

The TGA-IR results were very similar to thoseobtained by IR separately and discussed earlier. Thepercentage of mass loss in the thermal spectra of CMCalone under the conditions described in experimentalsection presented a loss of adsorbed water (endothermictransition) and change in conformation and breaking ofchains from 20 to 275 °C (exothermic transition),respectively. Further on, there are more breaks of chainand the final oxidative process, at 570 °C, leaving a 37%of total mass at 920 °C. The solid metal complexes ofCMC and Co2+ and Al3+, have shown a different ther-mal pattern including endothermic and exothermic ther-mal transitions due to a loss of adsorbed water and

breaking of chains, respectively, in the temperaturerange of 20 to 135 °C. At higher temperatures there arechanges in conformation as well as break of chains from273 to 394 °C. Greater mass losses begin to happen after400 °C until the final oxidative thermal process above485 °C for either Co2+ and Al3+ (Hanna et al., 1999;Hatakeyama andQuinn, 1995). AlthoughCMCpresentedhigher resistance to thermal degradation than Co2+ andAl3+ complexes, these latter complexes still degraded at avery high temperature.

3.5. Scanning electron microscopy

An analysis of the films formed by CMC and CMCand its complexes by scanning electron microscopy

Fig. 6. IR spectra of various pH extracted solid samples of CMC alone (0.1 m mole) and with metal ions Co2+, in the ratio ligand to metal of 3:1.a) 500–2000 cm−1 b) 2000–3800 cm−1.

185A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

(SEM) was performed. The obtained micrographs of allfilms in either acidic or basic pH were homogeneous anduniform, showing that the complexation steps as well asthe drying method did not break the chains of the bio-polymer so as to compromise the stability and the filmformation capacity (Walker et al., 2003).

3.6. Remediation results

The solutions employed in the remediation studiespresented metal concentrations in the two water solu-tions in Table 2 after the addition of salts of the metalions studied. It was not possible to detect (n.d.) anymetal ion concentration in the Iraí River water with noaddition of these metal ions (Iraí River water in natura)with the analytical procedures employed.

The concentrations of these metal ions were chosen soas to be as close as possible to ordinary contaminatedwastewater of steelmaking industries. In particular theAl3+

concentration was estimated taking into account a possiblecontamination in a water treatment plant employing alum.After addition of an appropriate amount of CMC to

make 1 g/L final concentration in each of two solutionstested, proper aliquots were taken at intervals of 90, 120and 150 min. These chosen intervals of time were basedon Brazilian municipal water treatment plants which needto keep water for approximately 2 h to be treated withalum. The metal contents left in the solutions wereproperly analyzed (as described in Section 2) and arepresented in Table 3.Fig. 8 shows three bar diagrams: a) the initial

concentrations of Iraí River and double distilled and

Fig. 7. IR spectra of various pH extracted solid samples of CMC alone (0.1 mmol) and with metal ions Al3+, in the ratio ligand-to-metal of 3:1. a)500–2000 cm−1 b) 2000–3800 cm−1.

186 A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

deionized water solutions with addedmetal ions b) remedi-ation results by addition of CMC after 90, 120 and 150minin the Iraí River water and c) in the distilled and deionizedwater.The metal ions in general were all complexed to

CMC, some more efficiently than others. Such is the caseof Mo6+, which after 150 min of contact with CMC, stillleft 50% of its metal content in the distilled and deion-ized water. On the contrary, Co2+ virtually complexed tocompleteness in both samples studied, considering thedetection limits of the analytical technique employed.Mo6+ binds strongest to CMC among all metal ionsstudied (Franco and Mercê, 2006) but it seems that in thepresence of competing ions and ligands (for examplehumic acids in the river water) the best complexation ofCMC occurs with Co2+. This can be explained by takinginto account that only at pH higher than 5.5–6.0, does thecation Mo6+ exist and that before this value the anionmolybdate is the major species present. As the exper-iment was carried out in more acidic pH values to mimicthe wastewaters of the steel pickling process and watertreatment, the complexation with molybdenum was hin-dered by the presence of the anion moiety in the equi-libria along with the competition with other metal ions.The presence of humic substances in the river water is

also playing a role in the equilibria favouring complex-ation with copper ion (Zhou et al., 2005). This preferenceleft the other metal ions in higher concentration and free tocomplex with CMC. In the double distilled and deionizedwater this effect was not observed and Cu(II) virtuallycomplexed completely to CMC. The other metal ionsshowed indistinct complexation with CMC in the absenceor in the presence of humic substances.

4. Conclusions

Based on the data obtained by potentiometric andUV–Vis titrations in aqueous solution and IR, TG-IRand SEM in the obtained solid complexes between

CMC and some metal ions, it was possible to concludethat the complexation occurs either via both Lewis basicgroups in the monomeric sugar unit of CMC–C-6hydroxyl and the carboxylic groups, or in only onedepending on the conditions and on the metal ionemployed.The data of the potentiometric titrations provided the

speciation according to the variation of pH. Thedistribution diagram provided useful information onthe pH values where there is the existence of thecomplexed species, helping the remediation step to betaken as efficiently as possible. In the case of CMC andCo2+, within pH 2 to 11 there is the presence of at leastone complexed species.The TG and IR results have shown the maintenance

of the complexes in the solid state and also showed thateven at high temperatures, normally employed inindustrial processes, very few breaks in the sugar chainshappened. The scanning electron microscopy providedthe morphology of the obtained films of the metalcomplexes with CMC.The CMC treated water samples showed that this

renewable biopolymer is a very good remediation agent as

Table 2Total metal concentrations obtained for the two water solutions afteraddition of solid starting p.a. salts of Al3+, Co2+, Cu2+, Mo6+ and VO2+

Total metalconcentration

Iraí River water[mol/L]

Distilled and deionizedwater [mol/L]

Cu a 1.4×10−4 8×10−5

Co a 1.2×10−4 6×10−5

Al a 1.34×10−3 1.62×10−3

VOb 6.9×10−4 5.2×10−4

MOb 3.7×10−4 2.5×10−4

a EDTA titration employing proper end point indicators (Schwar-zenbach and Flaschka, 1969).b KMnO4 titrations (Furman, 1962).

Table 3Total metal concentrations and percentage of reduction compared tothe initial concentrations in Table 2, in Iraí River and distilled anddeionized waters after 90, 120 and 150 min of contact with CMC(1 g/L) in triplicate

Time (min) /total metalconc.

Iraí Riverwater ±1×10−5 [mol/L]

Reduction(%) a

Distilled anddeionizedwater ±1×10−5 [mol/l]

Reduction(%) a

90 Cu b 10×10−5 31 6×10−5 19Co b 2×10−5 83 n. d. 100Al b 4.0×10−4 70 8×10−5 95VOc 2.6×10−4 62 1.7×10−4 66Mo c 3.3×10−4 10 2.1×10−4 17

120 Cu b 9×10−5 32 4×10−5 50Co b 2×10−5 85 n. d. 100 d

Al b 4.0×10−4 70 8×10−5 95VOc 9×10−5 86 9×10−5 83Mo c 3.1×10−4 16 1.7×10−4 34

150 Cu b 9×10−5 36 n. d. 100 d

Co b n. d. 100 n. d. 100 d

Al b 1.0×10−4 92 1×10−5 99VOc 9×10−5 86 9×10−5 83Mo c 2.5×10−4 32 1.2×10−4 50

a red. (%)=percentage of reduction=(initial concentration of addedmetal ion−concentration after addition of CMC)/ initial concentrationof added metal ion. n.d. = not detected.b EDTA titration employing proper end point indicators (Schwarzenbachand Flaschka, 1969).c KMnO4 titrations (Furman, 1962).d Virtually 100%, considering the detection limit of the titrationsemployed (Schwarzenbach and Flaschka, 1969; Furman, 1962).

187A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

the concentrations of the metal ions left in the solutionsafter 150 min of contact with 1 g/L CMC were reduced toonly traces for the majority of metal ions studied. Even if

the remediation process with CMC may not be so cost-effective when compared to ordinary precipitation employ-ing alum, the main advantage of its use is that the

Fig. 8. Graphic bars of the a) initial concentration of the metals added to Iraí River and distilled and deionized waters, and the comparison of b) the IraíRiver and c) distilled and deionized initial waters with aliquots obtained after 90, 120 and 150 min of remediation with 1 g/L of CMC M2+, M3+ orM6+ stands for M2+, M3+, or M6+, respectively.

188 A.P. Franco et al. / Hydrometallurgy 87 (2007) 178–189

complexes can be recovered, decomplexed and all reagentsreused, leaving no trace of Al3+ in the treated water.

Acknowledgement

The authors wish to thank Fundação Araucária —Paraná— Brasil, CNPq, Professor Dr. Thomas Heinze,University of Jena — Germany, and The MicroscopyCenter of UFPR, Paraná — Brasil, Vera ReginaPionteke and Professor Dr. Ney Pereira Mattoso Filho.

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