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Desalination 186 (2005) 199–206 0011-9164/06/$– See front matter © 2005 Elsevier B.V. All rights reserved. *Corresponding author. Development of polyurethane/polyaniline membranes for zinc recovery through electrodialysis F.D.R. Amado a , L.F. Rodrigues Jr. a , M.A.S. Rodrigues b,c , A.M. Bernardes b , J.Z. Ferreira b , C.A. Ferreira a* a Universidade Federal do Rio Grande do Sul, Laboratório de Materiais Poliméricos, Porto Alegre, Brazil Tel. +55 (51) 33169413; Fax +55 (51) 33169414; email: [email protected] b Universidade Federal do Rio Grande do Sul, Laboratório de Corrosão, Proteção e Reciclagem de Materiais, Porto Alegre, Brazil c Centro Universitário Feevale, ICET, Novo Hamburgo, Brazil Received 9 September 2004; accepted 18 May 2005 Abstract In this work, cation exchange membranes were produced by mixing polyurethane (PU) and polyaniline (PAni) doped with p-toluenesulfonic acid (pTSA), and camphorsulfonic acid (CSA). The influences of the polyaniline concentration and the dopant nature on the membranes properties were investigated. Membranes were characterised by swelling and electric conductivity, as well as thermogravimetric (TGA), dynamic mechanical (DMA) and morphological (SEM) analysis. Membranes were used for the treatment of solutions containing zinc by electrodialysis. Results of zinc extraction obtained using these membranes were compared to those obtained using the commercial membrane Nafion 450. Keywords: Polyurethane; Membrane; Electrodialysis; Polyaniline; Metal finishing industry 1. Introduction Wastewaters of the metal finishing industry are considered toxic [1] and therefore they should be carefully treated before their discharge. The sewage discarded by galvanic industries is one of the most widespread sources of non-ferrous and heavy metals [2]. The traditional treatment meth- ods are not efficient, because they generate gal- vanic sludge that is also considered a hazardous waste. The electrodialysis process is becoming a good alternative, when it is compared to the tradi- tional methods of wastewater treatment. The pro- cess presents the advantage of allowing the re- covery and reutilization of water and of chemicals used in the process [3–6]. The principle of electrodialysis involves the removal of ionic components from aqueous solutions through ion exchange membranes doi:10.1016/j.desal.2005.05.019

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Desalination 186 (2005) 199–206

0011-9164/06/$– See front matter © 2005 Elsevier B.V. All rights reserved.

*Corresponding author.

Development of polyurethane/polyaniline membranes for zincrecovery through electrodialysis

F.D.R. Amadoa, L.F. Rodrigues Jr.a, M.A.S. Rodriguesb,c, A.M. Bernardesb,J.Z. Ferreirab, C.A. Ferreiraa*

aUniversidade Federal do Rio Grande do Sul, Laboratório de Materiais Poliméricos, Porto Alegre, BrazilTel. +55 (51) 33169413; Fax +55 (51) 33169414; email: [email protected]

bUniversidade Federal do Rio Grande do Sul, Laboratório de Corrosão, Proteção e Reciclagem de Materiais,Porto Alegre, Brazil

cCentro Universitário Feevale, ICET, Novo Hamburgo, Brazil

Received 9 September 2004; accepted 18 May 2005

Abstract

In this work, cation exchange membranes were produced by mixing polyurethane (PU) and polyaniline (PAni)doped with p-toluenesulfonic acid (pTSA), and camphorsulfonic acid (CSA). The influences of the polyanilineconcentration and the dopant nature on the membranes properties were investigated. Membranes were characterisedby swelling and electric conductivity, as well as thermogravimetric (TGA), dynamic mechanical (DMA) andmorphological (SEM) analysis. Membranes were used for the treatment of solutions containing zinc by electrodialysis.Results of zinc extraction obtained using these membranes were compared to those obtained using the commercialmembrane Nafion 450.

Keywords: Polyurethane; Membrane; Electrodialysis; Polyaniline; Metal finishing industry

1. Introduction

Wastewaters of the metal finishing industry areconsidered toxic [1] and therefore they should becarefully treated before their discharge. Thesewage discarded by galvanic industries is one ofthe most widespread sources of non-ferrous andheavy metals [2]. The traditional treatment meth-ods are not efficient, because they generate gal-

vanic sludge that is also considered a hazardouswaste. The electrodialysis process is becoming agood alternative, when it is compared to the tradi-tional methods of wastewater treatment. The pro-cess presents the advantage of allowing the re-covery and reutilization of water and of chemicalsused in the process [3–6].

The principle of electrodialysis involves theremoval of ionic components from aqueoussolutions through ion exchange membranes

doi:10.1016/j.desal.2005.05.019

200 F.D.R. Amado et al. / Desalination 186 (2005) 199–206

(IEMs) using an electric field as the driving force.IEMs are used in electrolytic cells because theyallow the selective transport of cations or anions.The transport properties of membranes depend ontheir synthesis parameters such as the polymermatrix nature, crosslinking degree and the fixedionic charges concentration [7].

Different polymeric materials have beenstudied to be applied as membranes. Among them,conducting polymers can be considered, since theyhave some properties that are not found in con-ventional polymers, such as electrochemicalproperties when they are in the doped state [8].

On the other hand, conducting polymers donot present sufficient mechanical properties. Con-ventional polymers, such as high impact poly-styrene [9] and polyurethane [10] have been usedin the form of blends with conducting polymersto improve mechanical properties [11,12]. Theseconventional polymers, associated with poly-aniline doped with different organic acids, can bean alternative for replacing commercial ion ex-change membranes [13,14].

This work aims to investigate the propertiesof PU/PAni composite cationic membranes dopedwith different acids, using them for the treatmentof zinc acid solutions.

2. Experimental

2.1. Materials

Polyurethane aliphatic monodispersed, PUVithane 649 (Rohm and Hass), and aniline(Nuclear) were used to produce the membranes;p-toluenesulfonic acid (pTSA) (Vetec), and cam-phorsulfonic acid (CSA) (Aldrich) were used todope emeraldine base polyaniline. Ammoniumperoxidissulfate (NH4)S2O8 (Synth), HCl, EDTA,H2SO4 and ZnSO4 solutions were prepared withdistilled and deionized water.

2.2. Polyaniline synthesis

Aniline was polymerized according to astandard method [15,16]. Doped polyaniline was

first filtered and then treated with 1.1 M NH4OHto obtain PAni in emeraldine base form. Sub-sequently, polyaniline was doped with p-toluene-sulfonic acid (pTSA) or with camphorsulfonicacid (CSA) both 1.5 M by stirring the solutionfor 24 h.

2.3. Membrane preparation

Suitable amounts of PU and doped PAni weresolubilized in 20 ml N,N-dimethylformamide(DMF), using a Fisaton magnetic stirrer at2000 rpm. The membranes were produced by slowsolvent evaporation for 24 h at room temperatureon a glass plate. Polyaniline concentration ranged10–20 wt.%. The four types of membrane used inthe work are shown in Table 1.

2.4. Membrane characterisation

2.4.1. Swelling

Membranes remained in deionized water atroom temperature for 4 h. The excess of waterwas removed with filter paper and the membraneswere weighed, kept in an oven at 80°C for 6 h,and then weighed again. The water uptake wasdetermined by the weight difference between thewet and the dried membranes. Water absorptionwas expressed in percentage [17].

2.4.2. Electric conductivity

The electric conductivity of membranes wasmeasured according to the four-probe standard

Table 1Nomenclature and characteristics of synthesised mem-branes

Membrane Polyaniline percentage, wt. %

Dopant type

MPU 0 — MPUC 10 10 CSA MPUC 20 20 CSA MPUT 10 10 pTSA MPUT 20 20 pTSA

F.D.R. Amado et al. / Desalination 186 (2005) 199–206 201

method using a Cascade Microtech CS 4-64,associated with a Keithley 2400 Source Meter[18].

2.4.3. Infrared spectroscopy

The samples were prepared with potassiumbromide (KBr) powder. The pure polyaniline waspreviously homogenised in a mill and dried tomake a KBr pellet. All samples were analysedusing a FTIR Perkin Elmer spectrometer modelSpectrum 1000. The spectra were recorded in thespectral range of 400–4000 cm–1.

2.4.4. Thermogravimetric analysis (TGA)

The membranes were analysed using a TAInstruments thermogravimetric analyser modelTGA 2050 in the temperature range 25–1000°Cunder N2 atmosphere. The heating rate was20°C/min for all samples.

2.4.5. Dynamic mechanical analysis (DMA)

The membranes were analysed using a TAInstruments Dynamic Mechanical Analyser modelDMA 2980 in the temperature range 150–200 °C.The heating rate was 5°C/min at a frequency of1 Hz. The samples dimensions were 200 mm ×70 mm × 0.26 mm.

2.4.6. Morphology

Scanning electron micrographs of membranesurfaces were taken by a Philips XL20 microscopeafter sputtering the samples with gold.

2.4.7. Electrodialysis

Electrodialysis experiments were carried outin a three-compartment laboratory cell. A platinumplate was used as anode and a stainless steel plateas cathode. An AMT (Asahi Glass Company)anionic membrane was used. The cationic mem-branes were the synthesised MPUT10, MPUT20,MPUC10, MPUC20 and the commercial Nafion450 (DuPont de Nemours Co.). The exposed area

of the membranes was 10 cm2. 0.1 M H2SO4 wasused in the cathodic and anodic compartments,while 0.1 M ZnSO4 0.1M H2SO4 was used in theintermediate compartment. Fig. 1 presents ascheme of the used cell.

Mechanical stirring was carried out in all com-partments with a capacity of 200 ml each. Themembranes remained in their respective solutionsfor 24 h. A pseudo-stationary state was achievedwith a pre-electrodialysis for 20 min. After this,solutions were replaced by new ones and theexperiment was restarted [9,19]. Solutions wereprepared with distilled and deionized water. Acontinuous current of 100 mA was applied in allexperiments. The temperature was kept at 25°Cand the experiment time was 240 min. Zincconcentration was determined by complexometryusing EDTA [20,21].

3. Results and discussion

3.1. Swelling

Table 2 presents the results obtained by thick-ness and swelling measurements of the cathionicmembranes.

The synthesised membranes absorbed a higheramount of water than the Nafion 450 membrane,due to their porous polyurethane matrix. Thisdifference can also be associated with thehydrophilic behaviour of SO3 from the dopantacid. The MPUC membrane presented the greatest

Fig. 1. Electrodialysis cell scheme.

202 F.D.R. Amado et al. / Desalination 186 (2005) 199–206

swelling because its dopant (CSA) is more hydro-philic than the other (pTSA) [9]. It can similiarlybe observed that as the polyaniline concentrationin the membrane increases, the swelling does also,independently of the acid used.

3.2. Electric conductivity

Table 3 shows the conductivity of synthesisedmembranes and pure PAni doped with differentorganic acids.

The decrease of electric conductivity in themembranes, compared to pure polymers, resultsfrom the low dispersion of conducting polymerinto the matrix, which is insulating [9,18]. Spir-ková et al [22] measured the conductivity ofpolyurethane/polyaniline anisotropic blends. For1%, 5% and 10% of PAni, conductivity valuesranging 10–8–10–12 S cm–1 were found.

It is observed in Table 3 that the acid type usedas polyaniline dopant influences the membraneconductivity, but the polyaniline concentrationdoes not have an expressive effect on the electricconductivity.

3.3. Infrared spectroscopy

To verify the incorporation of PAni in poly-meric matrix, samples of PAni/CSA and of PU,as well as membrane samples, were analysed.Fig. 2 shows the FTIR spectrum of PAni/CSA,PU and MPUC20 membranes.

Table 2Thickness and swelling of PU sample, synthesised andcommercial membranes

*supported membrane

Table 3Electric conductivity of pure PAni doped with differentorganic acids and synthesised membranes

σ, S/cm PAni/CSA 1.9×10–1 PAni/p-TSA 2.3×10–1 MPUC 10 2.1×10–8

MPUT 10 4.7×10–9 MPUC 20 2.3×10–8 MPUT 20 5.2×10–9

3500 3000 2500 2000 1500 1000

1645

1140

1731

3430

294833

70

1075

1554

1034

1122

1299

146715

601732

MPUC20

PU

PAniCSA

Wave number (cm-1)

The polyaniline spectrum is similar to the onedescribed in the literature [10,23]. Peaks at 1467cm–1 and 1560 cm–1 are related to benzenoid andquinoid rings respectively. Peak at 1299 cm–1 isassociated with an angular deformation of the C–N group. The doping of polyaniline can be ob-served at 1122 cm–1 in the formation of the pola-rons H+N=Q=NH+. Peak at 1034 cm–1 shows S=Ogroups associated with the sulfonic acid used asdopant.

In the polyurethane spectrum, different peakswere observed. Peak at 2948 cm–1 corresponds to

Fig. 2. FTIR spectrum of PAni/CSA, PU sample andMPUC20 membrane.

F.D.R. Amado et al. / Desalination 186 (2005) 199–206 203

an angular deformation of CH3. At 1731 cm–1 thereis a peak attributed to the stretching of C=Ogroups. Peaks at 1645 cm–1 and 1554 cm–1 cor-respond to N2H stretching. Peaks at 1075 cm–1 and1140 cm–1 are associated with the stretching ofC–O–C groups [24]. According to the poly-urethane used, a small displacement can occur inthese peaks.

In the membrane spectrum, peaks of PAni andPU spectra are observed, demonstrating the incor-poration of PAni into the polyurethane matrix.Some of the peaks are overlapped, as the stretchingof N–H groups at approximately 3430 cm–1 [10].

3.4. Thermal behaviour

3.4.1. Thermogravimetric analysis (TGA)

The thermogravimetric analysis of MPUC10and MPUC20 membranes compared to PU samplewithout polyaniline, is presented in Fig. 3a. Thethermogram shows that pure polyurethane pre-sents a first thermal event that beginning at around250°C, and a second event starting at around400°C, due to the decomposition of the polymericchain. The thermal behavior of MPUC10 and

200 400 600 800 10000

20

40

60

80

100

120 MPU MPUC10 MPUC20

Wei

gth

(%)

Temperature200 400 600 800 1000

0,0

0,2

0,4

0,6

0,8

1,0

1,2

1,4

Temperature (0C)

DTG

(%m

in-1)

MPUC MPUC

MPUC20 membranes is similar to PU, but thedegradation begins at around 200°C, due to themixture with polyaniline.

It should be emphasized that when the amountof polyaniline increases, the residue at 1000°Cdoes also, probably due to aromatic rings withstable bounds in the main chain of polymer thatare not decomposed at high temperatures in N2atmosphere.

Fig. 3b shows the derivative weight (DTG) asa function of the temperature where the decom-position speed is at a maximum. It is possible toverify two peaks for the PU sample at 341°C and408°C and others two peaks for the MPUC10membrane at 357°C and 410°C, regarding theuncured and cured polyurethane respectively. Inthe MPUC20 membrane, only one intermediatepeak can be observed at 368°C, referring to theinteraction between the polyurethane and poly-aniline chains, which causes the decrease of thestorage modulus, according to Fig. 4 [25].

3.4.2. Dynamic mechanical analysis (DMA)

Fig. 4 shows that the MPUC10 membrane pre-sents a similar behaviour to PU. MPUC20 mem-

Fig. 3. (a) Thermogravimetric analysis of MPUC10 and MPUC20 membranes and PU sample; (b) Differential thermalanalysis of MPUC10 and MPUC20 membranes and PU sample (atm N2 and heating rate of 20°C/min).

MPU MPUC10 MPUC20

(a) (b)

204 F.D.R. Amado et al. / Desalination 186 (2005) 199–206

brane presents storage modulus smaller than othersamples. This might have happened due to the in-crease of elasticity of polyurethane promoted bypolyaniline making the membrane more flexible.This effect could not be visualized in the mem-branes that contain 10% of polyaniline.

3.5. Morphology

Fig. 5 shows that the polyaniline insertedmembrane MPUC20 is noticeable, since PU isporous, and with the polyaniline dispersion in intothe polymeric matrix modifies the membranes

0

200

400

600

800

1000

1200

Stor

age

Mod

ulus

(MPa

)

-150 -100 -50 0 50 100 150Temperature (°C)

––––––– MPU–––––– MPUC 10–––––– MPUC 20

Fig. 4. Dynamic mechanical analy-sis of the MPUC10 and MPUC20membranes and PU sample.

Fig. 5. Microscopy (A) PU and (B) MPUC20 membrane surface.

superficial morphology [26]. The differencebetween the polyurethane matrix and the thepolymeric matrix the pores seem to be filled upby PAni.

The morphologic difference between the ma-trixes probably has an influence in the ions trans-port; a porous structure has as an advantage of anelectric resistance lower than the one of the densemembrane, due to higher swelling. On the otherhand, the dense structure of the membrane onlyallows the passage of ions with the water of itshydration layers, but presents a higher mechanicalstrength than porous structure [3,27].

F.D.R. Amado et al. / Desalination 186 (2005) 199–206 205

Fig. 6. Zn2+ percent extraction for the MPUC20, MPUT20,MPUC10, MPUT10 and Nafion 450 using 10 mA/cm2

and 240 min.

Fig. 7. Electric resistance of MPUC20, MPUT20,MPUC10, MPUT10 and Nafion 450 membranes mea-sure during the electrodialysis in 0.1 M ZnSO4.

As the dopant acid structure does not influenceFTIR, TGA and DMA analysis and morphology,those results are not presented here.

3.6. Electrodialysis

Electrodialysis experiments were evaluated interms of percent extraction and electric resistance.

3.6.1. Percent extraction

Fig. 6 presents the zinc percent extraction forMPUC20, MPUT20, MPUC10, MPUT10 andNafion 450 membranes.

An increase of the amount of PAni (10–20%)raises the zinc percent extraction due to an increaseof sulfonic group’s concentration.

The kind of acid used as polyaniline dopanthas a slight effect on the zinc transport just in theconcentration range of 10%. This result will befurther studied, because the structures of the usedacids are different and they showed interactionswith the studied ion.

3.6.2. Electric resistance

Fig. 7 shows the electric resistance of mem-branes as a function of time. It can be observedthat an increase of the polyaniline amount in themembranes causes a noticeable decrease in theelectric resistance. This can be attributed to a

higher number of groups SO3 in the membrane,causing a lower electric resistance for the system.

It is not possible to observe that the kind ofdopant acid causes significant modifications in thetransport and electric resistance of the membranefor the range of 20% of polyaniline. Neverthelessin the range of 10%, it can be observed that thetransport for the MPUT10 membrane is higherthan MPUC10 shown in Fig. 7, where the mem-brane MPUT10 presents lower electric resistance.

The synthesised membranes presented cha-racteristics related to transport and resistancesimilar to the commercial membrane Nafion 450,which demonstrates the usage possibility ofconducting polymers and conventional polymerscomposite membranes in the industrial effluentstreatment as these membranes present the advan-tage of having an easy production and a low cost.

4. Conclusion

The increase in the polyaniline concentrationcauses an increase in the swelling and zinctransport, due to an increase of SO3 group’sconcentration. The type of dopant acid seems tohave an influence on the properties of zinc

206 F.D.R. Amado et al. / Desalination 186 (2005) 199–206

transport through the membranes, even though thepresent different structures.The acid structure usedas dopant also has significant influences on theswelling due to the hydrofilicity difference, but itdoes not affect the electric conductivity.

The electric resistance decreases with the in-crease of the polyaniline concentration also dueto an increase of SO3 groups concentration, whichis important for reducing the system electric resist-ance.

The results of thermal analysis showed thatthe PU/PAni membranes can be processed at thetemperature of approximately 200°C, and fromthis temperature the degradation of membranesbegins.

The infrared spectroscopy results have shownthe polyaniline in polymeric matrix, because thecharacteristic bands of PAni are present in thespectra of the membranes, especially the peak at1034 cm–1 regarding the group S=O.

The results obtained by this work indicate thatPU/PAni membranes can represent an alternativeto commercial membrane for the metal finishingindustry.

Acknowledgements

The authors wish to thank CNPq (Edital CT-HIDRO 03/2002) and FAPERGS for financialsupport.

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