Desalination 149 (2002) 101–107

Presented at the International Congress on Membranes and Membrane Processes (ICOM), Toulouse, France,July 7–12, 2002.

0011-9164/02/$– See front matter © 2002 Elsevier Science B.V. All rights reserved

*Corresponding author.

Treatment of textile dye effluents using a new photograftednanofiltration membrane

A. Akbari*, S. Desclaux, J.C. Remigy, P. AptelLaboratoire de Génie Chimique (CNRS UMR 5503), Université Paul Sabatier, 118 route de Narbonne, 31062

Toulouse cedex, FranceTel. +33 (5) 61 55 76 15; Fax +33 (5) 61 55 61 39; email: akbari@chimie.ups-tlse.fr

Received 8 February 2002; accepted 11 March 2002

Abstract

A nanofiltration membrane has been developed by UV-photografting. Sodium p-styrene sulfonate was used forthe modification of a polysulfone ultrafiltration membrane. The membrane cut-off was estimated. The graftedmembranes have been evaluated for the removal of five different dyes with an aim to reuse water in the processhouse. The effect of different parameters such as dye class, pH and the presence of salt was evaluated. It is observedthat the newly developed membranes show acceptable performance both in terms of flux and rejection. Dye retentionwas higher than 97% and hydraulic permeability 0.23–0.28 m3.m–2.d–1 at 0.4 MPa. The influence of pH on theperformance of membranes in terms of fouling and retention was established and compared to a commercial membrane(Desal 5DK).

Keywords: Textile dyes; Nanofiltration; UV-modified membrane; Fouling

1. Introduction

The textile industry uses valuable dyes, whichare clearly visible if discharged into public water-ways. Thus, their disposal creates both an aestheticand environmental wastewater problem. At thesame time, the textile industry continually seeksto conserve water and would economically benefit

from dye recovery. Nanofiltration membranesaddress all these issues [1–4]. First, textile dyesare rejected, recovered and reused. Second, water-way pollution is avoided. And third, reusable wateris produced. Nanofiltration is a separation processin which relatively small organic molecules (MW:200–1000 g/mol) as well as ionic components areretained by a nanoporous membrane. The separationmechanisms and the factors that govern the

102 A. Akbari et al. / Desalination 149 (2002) 101–107

transport through the membrane can be hinderedby the size of the component (sieving mechanism)and/or by the membrane charges (Donnan effect)[5,6].

As for all membrane processes, the majorproblem is permeate flux decline due to the accu-mulation of molecules near the membrane surface.This accumulation, known as concentration pola-risation, leads to an increase in membrane fouling.Fouling occurs strongly with polyamide membranes,because this type of fibre can be dyed by themajority of textile dyes. The studies done for thetreatment of five different textile dye classes (acid,basic, direct, disperse and reactive) by a Desalmembrane which is a thin-film composite membranewith a polyamide active layer, showed that thereis flux decline in all cases [7].

The objective of this paper is the developmentof a new composite nanofiltration membrane lesssensitive to fouling. The membrane was obtainedby photografting [8] a polymer on the surface ofa polysulfone ultrafiltration membrane. Sodiump-styrene sulfonate (SSS) was chosen as a monomer,because the sulfonate group is also found in theanionic dye molecules (as acid, direct and reactivedyes). Thus, the presence of the sulfonate chargeson the membrane surface is expected to repel thedye molecules, increasing rejection and limitingfouling. In addition, polysodium p-styrene sulfonate(PSSS) is relatively insensitive to pH. Watersoluble polyelectrolytes based on polystyrenesulfonic acid and its salts find many applicationssuch as emulsifiers, flocculants, ion exchange resinsand membranes [9].

2. Experimental

UF membranes were prepared by phase inver-sion (from a collodion kindly provided by PolymemSA, Fourquevaux, France) the distilled water fluxwas 50 l.h–1.m–2 at 0.1 MPa. The photo-reactorwas a cylindrical chamber in which the membranewas placed at the wall, while the UV lamp waslocated at the centre. It was a polychromatic lamp,UV Hanau Heraeus TQ 150 (Hg medium pressure).The experimental method was as follows: the as-cast UF membrane was washed with distilledwater and placed in the reactor, the solution ofmonomer, at suitable concentration (SS), wasdegassed with nitrogen before irradiation. Themembrane was irradiated during a fixed time(Tirr). Then it was removed from the reactor andwashed with distilled water. The water flux wasmeasured using distilled water in a cell (Amicon8050) 50 ml of volume with 13.2 cm2 of activemembrane area. For the dye solutions (dye =100 ppm, volume: 500 ml) a reservoir was usedto provide a continuous supply of solution. Filtrationwas performed at 0.4 MPa under stirring at580 rpm (Re~5600). The supplier, the chemicalstructure, the molecular weight and the sign ofthe dyes tested are given in Table 1.

Model dye solutions were prepared by dissolvingdyes (as supplied) in distilled water at a concentra-tion of 0.1 g/l without adding any auxiliary com-pounds. The concentrations of Na

2SO

4 and NaCl

in aqueous solution were 0.03 and 0.09 M respec-tively (same ionic strength). The membranecharacterisation studies were carried out by a PEG(polyethylene glycol) series with molecular

Table 1Characteristics of the dyes tested [10]

Dye (supplier) Symbol MW,

g/mol

Structure Compact formula Sign [dye],

mM

Acid red 4 (Aldrich) AR4 402.4 Monoazo C17H13N2NaO5S –1 0.25

Acid orange10 (Aldrich) AO10 496.4 Monoazo C16H10N2Na2O7S2 –2 0.20

Direct red 80 (Ciba) DR80 1372.6 Polyazo C45H26N10Na6O21S6 –6 0.07

Disperse blue 56 (Ciba) DB56 305.7 Antraquinon C14H10N2O4Cl 0 0.33

Reactive orange 16 (Aldrich) RO16 661.5 Monoazo C20H17N3Na2O11S3 –1 0.15

A. Akbari et al. / Desalination 149 (2002) 101–107 103

weights 600, 1000 and 1500 g/mol at a concen-tration of 1 g/l and by three single salt retentionmeasurements, Na

2SO

4, NaCl and CaCl

2, at three

different initial concentrations. The concentrationof the feed, the permeate and the retentate wasdetermined either by visible-UV spectrophoto-metry for the dye solution at the maximumabsorption wavelength, by conductivity for thesalt solution or by Total Organic Carbon Analyser(TOC-5050A) for the PEG solution. Details ofdefinitions and calculations were described in aprevious work [6] except the rejection R andconcentration factor CF that were calculatedaccording to the relationship:

( )( ) 1001(%) ×−=tC

tCR

r

p and

( )0C

tCCF r=

where C0, C

p and C

r are the concentration of the

feed, permeate and retentate respectively.

3. Results and discussion

3.1. Effect of grafting conditions on permeability

The distilled water permeability of the poly-sulfone membrane (PS) and of the modifiedpolysulfone membranes (PS-g-PSS) is presentedin Table 2.

Water permeability decreases with an increaseof the level of grafting. It is observed that the levelof grafting increases both with grafting time andwith monomer concentration. For all the experi-ments, M4 membrane grafted in a 4% SS solutionfor 15 min was chosen.

Table 2Distilled water permeability of the PS and PS-g-PSS membranes (T = 25°C)

Membrane M0 M1 M2 M3 M4 M5 Desal 5DK

Grafting

Condition

[SS], %w/w

Tirr, min

—

—

1

10

3

10

3

15

4

15

4

20

—

—

Lpmean, l.h–1

.m–2

.bar–1

) 50 19 12 8 5 2 3.6

3.2. Molecular weight cut-off and salt retention

The molecular weight cut-off (MWCO) valuewas determined by using a PEG series of increasingmolecular weights and three salts. The retentionof M4 and Desal 5DK membranes were plotted(Fig. 1a) vs. the molecular weight of PEG. Accord-ing to its conventional definition [11], the molecularweight cut-off (Fig. 1a) of M4 membrane is 1200–1300 g/mol. Fig. 1b shows the following saltretention sequence: Na

2SO

4>NaCl>CaCl

2, retention

decreases with concentration. These data are typicalfor a negatively charged membrane [12] for whichDonnan exclusion plays an important role. It is tobe pointed out that the sequence is different andthe retention higher for the Desal 5DK membrane:Na

2SO

4>CaCl

2>NaCl [13].

3.3.Permeate flux and dye rejection versuspressure

The effect of transmembrane pressure on per-meate flux and dye retention (Fig. 2) shows aquasi-linear increase in flux with the increase inpressure. Flux values are very close to thoseobtained with pure water, indicating very lowconcentration polarisation and fouling effect. Dyerejection is total and stable with the pressureincrease.

3.4. Effect of dye class

Permeability ratio and retention of four differentdyes, used for dyeing cotton, wool, viscose, nylonand polyester fibres, are presented in Fig. 3 plattedagainst the concentration factor (CF). It is observedthat flux and instant retention of dye solutionsare stable with the increase of CF. The retention

104 A. Akbari et al. / Desalination 149 (2002) 101–107

60

70

80

90

100

0 500 1000 1500 2000

M.W of PEG (g/mol)

R (

%)

PS-g-PSS Lp=5 l/h.m².bar

Desal 5DK Lp=3.6 l/h.m².bar

a

0

25

50

75

100

0 0.025 0.05 0.075 0.1

Ionic strength (mol/l)

R (

%)

Na2SO4 NaCl CaCl2

b

Fig. 1. PEG (a) and salt (b) retention (T = 25°C, TMP =0.4 MPa, Re = 5600).

0

6

12

18

24

0 0.1 0.2 0.3 0.4 0.5

Pressure (MPa)

Flu

x (

l/h

.m²)

0

25

50

75

100

R (%

)

Distilled water DB56

Fig. 2. Flux and dye retention vs. pressure (T = 25°C,Re = 5600, [dye] = 100 ppm).

0.8

0.85

0.9

0.95

1

1 3 5 7CF

Ls/

Lp

DR80 RO16

AR4 DB56

a

96

97

98

99

100

1 3 5 7CF

R (

%)

DR80 RO16

AR4 DB56

b

Fig. 3. Permeability ratio (a) and dye retention (b) vs.concentration factor (TMP = 0.4 MPa, T = 25°C, [dye] =100 ppm, Re = 5600).

of direct red with a molecular weight of 1373 g/moland disperse dye, which is not charged but notsoluble in water, is 100%. Retention for reactiveorange and acid red is lower: this can be attributedto the lower molecular weight and a lower numberof charges (Table 1). On the other hand, the fluxes

of acid and direct dye solutions are lower thanfor reactive and disperse dyes.

3.5. Effect of pH

Figs. 4, 5 show the effect of pH variation onthe permeability ratio and dye rejection as a functionof CF for the Desal 5DK and M4 membranes. Asshown by these data, pH has no influence on thegrafted membrane flux or on retention. On thecontrary, Desal membrane, which is a thin-filmcomposite membrane with a polyamide activelayer is very sensitive to pH. It is observed thatwhen decreasing the pH from 6 to 3, the permeateflux decreases by roughly 20%. This decline hasbeen attributed to the affinity of dyes for poly-amide, which leads to the fouling of the membrane[7].

A. Akbari et al. / Desalination 149 (2002) 101–107 105

0.52

0.64

0.76

0.88

1

1 3 5 7CF

Ls/

Lp

Desal 5 pH=3 Desal 5 pH=6

PS-g-PSS pH=6 PS-g-PSS pH=3

a

92

94

96

98

100

1 3 5 7CF

R (

%)

Desal 5 pH=3 Desal 5 pH=6PS-g-PSS pH=3 PS-g-PSS pH=6

b

Fig. 4. Influence of pH on the permeability ratio (a) and retention (b) of acid red 4 for the grafted and Desal 5 membranes(TMP = 0.4 MPa, T = 25°C, Re = 5600, [dye] = 100 ppm).

0.8

0.85

0.9

0.95

1

1 3 5 7 9CF

Ls/

Lp

AO10 pH=10.3 AO10 pH=6

a

80

85

90

95

100

1 3 5 7 9

CF

R (

%)

AO10 pH=10.3 AO10 pH=6

b

Fig. 5. Influence of pH on the permeability ratio (a) and retention (b) of acid orange 10 for the grafted M4 membrane(TMP = 0.4 MPa, T = 25°C, Re = 5600, [dye] = 100 ppm).

3.6. Effect of salt

Preliminary studies were done to determinethe performance of the membrane in the treatmentof solutions composed of sodium sulphate orsodium chloride and dyes. A reactive dye and anacid dye were chosen since they are widely usedto colour wool and nylon fibres and the processrequires large amount of salt and causes seriousenvironmental problems. The dye concentrationwas set at 100 ppm and concentrations of Na

2SO

4

and NaCl were 0.03 and 0.09 M respectively(same initial ionic strength). The applied pressurewas 0.4 MPa for all the experiments but theeffective driving pressure used to calculate Ls isthe applied pressure minus the transmembraneosmotic pressure. The results are presented in

Figs. 6–8. It is observed that while the flux is onlylittle affected (Ls/Lp are in the range of 0.87–1.0for all the solutions data not shown), dye retentionis notably lower in the presence of salt. The decreaseis less pronounced in presence of NaCl (Fig. 8)than in presence of Na

2SO

4 (Figs. 6, 7). At the same

time, the retention for the two salts remainedidentical whether the salt was alone or mixed withthe dyes. The decrease in retention was more pro-nounced for acid orange (AO10) which has thelowest molecular weight.

These data can be qualitatively explained asfollows: for the dyes, adding salt leads to a largeincrease in ionic strength, the membrane chargeis shielded and the Donnan exclusion effect is thenconsiderably reduced. However, the steric effect

106 A. Akbari et al. / Desalination 149 (2002) 101–107

0

25

50

75

100

0 400 800 1200 1600Time (min)

R (

%)

RO16 Na2SO4 0.03M

RO16 (mix.) Na2SO4 (mix.)

Fig.6

0

25

50

75

100

0 400 800 1200 1600Time (min)

R (

%)

AO10 Na2SO4 0.03MAO10 (mix.) Na2SO4 (mix.)

Fig.7

0

25

50

75

100

0 400 800 1200 1600Time (min)

R (

%)

AO10 NaCl 0.09M

AO10 (mix.) NaCl (mix.)

Fig.8

Figs. 6–8. Retention for solutions containing dye and salt(M4 membrane, TMP = 0.4 MPa, T = 25°C, Re = 5600 andpH = 6).

remains and the dye rejection is now more inaccordance with the rejection curve obtained withPEG (Fig. 1a). For the salts, the addition of arelatively low amount of dyes does not modifythe ionic strength, as salt retention is not modified.

4. Conclusions

The present work demonstrated that the mem-branes developed, based on UV-photopolymeri-sation of sodium p-styrene sulfonate monomer onan UF membrane, can be successfully used forthe treatment of dye effluents. The results showedthat the grafted membrane is far less sensitive tofouling than polyamide membrane. Experimentsdone at different pH values also showed that pHhas no longer an influence on the retention or fluxof the dye solution. The evaluation of the salteffect showed that, in the practice, it would bepreferable to change the electrolyte from a divalentanion to a monovalent anion.

Future work is still needed to complete thecharacterisation of the grafted membranes tomodel the filtration performance more quanti-tatively. It is also planned to optimise both thesupport and the photografting process.

Acknowledgements

This study was been financially supported bya doctoral grant from the government of Iran, bySociété Française d’Exportation des RessourcesEducatives (SFERE) and by Région Midi-Pyrénées.

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