DESALINATION

ELSEVIER Desalination 149 (2002) 101-107

Treatment of textile dye effluents using a new nanofiltration membrane

www.elsevier.com/locate/desal

photografted

A. Akbari*, S. Desclaux, J.C. Remigy, P. Aptel

Laboratoire de G&ie Chimique (CNRS UMR 5503), Universite' Paul Sabatier; 118 route de Narbonne, 31062 Toulouse cedex, France

Tel. +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 for the modification of a polysulfone ultrafiltration membrane. The membrane cut-off was estimated. The grafted membranes have been evaluated for the removal of five different dyes with an aim to reuse water in the process house. The effect of different parameters such as dye class, pH and the presence of salt was evaluated. It is observed that the newly developed membranes show acceptable performance both in terms of flux and rejection. Dye retention was higher than 97% and hydraulic permeability 0.23-0.28 m3.m-?.d-’ at 0.4 MPa. The influence of pH on the performance of membranes in terms of fouling and retention was established and compared to a commercial membrane (Desal SDK).

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

1. Introduction

The textile industry uses valuable dyes, which are clearly visible if discharged into public water- ways. Thus, their disposal creates both an aesthetic and environmental wastewater problem. At the same time, the textile industry continually seeks to conserve water and would economically benefit

from dye recovery. Nanofiltration membranes address all these issues [l-4]. First, textile dyes are rejected, recovered and reused. Second, water- way pollution is avoided. And third, reusable water is produced. Nanofiltration is a separation process in which relatively small organic molecules (MW: 200-l 000 g/mol) as well as ionic components are retained by a nanoporous membrane. The separation

*Corresponding author. mechanisms and the factors that govern the

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

0011-9 164/02/$- See front matter 0 2002 Elsevier Science B.V. All rights reserved PII: SO0 II-9 164(02)00739-7

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

transport through the membrane can be hindered by the size of the component (sieving mechanism) and/or by the membrane charges (Donnan effect)

[Ul. As for all membrane processes, the major

problem 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 the majority of textile dyes. The studies done for the treatment of five different textile dye classes (acid, basic, direct, disperse and reactive) by a Desal membrane which is a thin-film composite membrane with a polyamide active layer, showed that there is flux decline in all cases [7].

The objective of this paper is the development of a new composite nanofiltration membrane less sensitive to fouling. The membrane was obtained by photografting [8] a polymer on the surface of a polysulfone ultrafiltration membrane. Sodium p-styrene sulfonate (SSS) was chosen as a monomer, because the sulfonate group is also found in the anionic dye molecules (as acid, direct and reactive dyes). Thus, the presence of the sulfonate charges on the membrane surface is expected to repel the dye molecules, increasing rejection and limiting fouling. In addition, polysodium p-styrene sulfonate (PSSS) is relatively insensitive to pH. Water soluble polyelectrolytes based on polystyrene sulfonic acid and its salts find many applications such as emulsifiers, flocculants, ion exchange resins and membranes [9].

Table 1 Characteristics of the dyes tested [lo]

2. Experimental

UF membranes were prepared by phase inver- sion (from a collodion kindly provided by Polymem SA, Fourquevaux, France) the distilled water flux was 50 l.h-‘.m-2 at 0.1 MPa. The photo-reactor was a cylindrical chamber in which the membrane was placed at the wall, while the UV lamp was located 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 distilled water and placed in the reactor, the solution of monomer, at suitable concentration (SS), was degassed with nitrogen before irradiation. The membrane was irradiated during a fixed time (Tirr). Then it was removed from the reactor and washed with distilled water. The water flux was measured using distilled water in a cell (Amicon 8050) 50 ml of volume with 13.2 cm2 of active membrane area. For the dye solutions (dye = 100 ppm, volume: 500 ml) a reservoir was used to provide a continuous supply of solution. Filtration was performed at 0.4 MPa under stirring at 580 rpm (Re-5600). The supplier, the chemical structure, the molecular weight and the sign of the dyes tested are given in Table 1.

Model dye solutions were prepared by dissolving dyes (as supplied) in distilled water at a concentra- tion of 0.1 g/l without adding any auxiliary com- pounds. The concentrations of N$SO, and NaCl in aqueous solution were 0.03 and 0.09 M respec- tively (same ionic strength). The membrane characterisation studies were carried out by a PEG (polyethylene glycol) series with molecular

Dye (supplier)

Acid red 4 (Aldrich) Acid 10 (Aldrich) orange Direct red 80 (Ciba) Disperse blue 56 (Ciba) Reactive 16 (Aldrich) orange

Symbol MW, g/m01

AR4 402.4 A010 496.4 DR80 1372.6 DB.56 305.7 R016 661.5

Structure

Monoazo Monoazo Polyazo Antraquinon Monoazo

Compact formula

CJVWaW C&&Na#7Sz C&&aNa602,S~ CJ%J%Wl C&+Wa#&

Sign

-1 -2 -6 0 -1

kW1, mM

0.25 0.20 0.07 0.33 0.15

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

weights 600, 1000 and 1500 g/m01 at a concen- tration of 1 g/l and by three single salt retention measurements, Na,SO,, NaCl and CaCl,, at three different initial concentrations. The concentration of the feed, the permeate and the retentate was determined either by visible-UV spectrophoto- metry for the dye solution at the maximum absorption wavelength, by conductivity for the salt solution or by Total Organic Carbon Analyser (TOC-5050A) for the PEG solution. Details of definitions and calculations were described in a previous work [6] except the rejection R and concentration factor CF that were calculated according to the relationship:

xlOOand C’F=u C0

where Co, C,] and Cr are the concentration of the feed, permeate and retentate respectively.

3. Results and discussion

3. I. Effect of grafting conditions on permeability

The distilled water permeability of the poly- sulfone membrane (PS) and of the modified polysulfone membranes (PS-g-PSS) is presented in Table 2.

The effect of transmembrane pressure on per- meate flux and dye retention (Fig. 2) shows a quasi-linear increase in flux with the increase in pressure. Flux values are very close to those obtained with pure water, indicating very low concentration polarisation and fouling effect. Dye rejection is total and stable with the pressure increase.

3.4. EfSect of dye class

Water permeability decreases with an increase Permeability ratio and retention of four different of the level of grafting. It is observed that the level dyes, used for dyeing cotton, wool, viscose, nylon of grafting increases both with grafting time and and polyester fibres, are presented in Fig. 3 platted with monomer concentration. For all the experi- against the concentration factor (CF). It is observed ments, M4 membrane grafted in a 4% SS solution that flux and instant retention of dye solutions for 15 min was chosen. are stable with the increase of CF. The retention

3.2. Molecular weight cut-off and salt retention

The molecular weight cut-off (MWCO) value was determined by using a PEG series of increasing molecular weights and three salts. The retention of M4 and Desal 5DK membranes were plotted (Fig. 1 a) vs. the molecular weight of PEG Accord- ing to its conventional definition [ 111, the molecular weight cut-off (Fig. 1 a) of M4 membrane is 1200- 1300 g/mol. Fig. 1 b shows the following salt retention sequence: Na_$O~>NaCI>CaCI,, retention decreases with concentration. These data are typical for a negatively charged membrane [ 121 for which Donnan exclusion plays an important role. It is to be pointed out that the sequence is different and the retention higher for the Desal5DK membrane: Na,SO,>CaCl,>NaCl [ 131.

3.3.Permeate flux and dye rejection versus

pressure

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

Membrane

Grafting [SS], %w/w Condition Tirr, min LP mCBn, I.h~‘.m~2.bar-‘)

MO Ml M2 M3 M4 M5 Desal5DK

- 1 3 3 4 4 - - 10 10 15 IS 20 - 50 19 12 8 5 2 3.6

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

100

90

g 80 d

70

60

100

75

?

2 5o

25

0

m m

l a ’

.

l

l PS-g-P% Lp=S Vh.m2.bar i n Desal SDK Lp=3.6 l/h.m2.bar i

0 500 1000 1500 2000

M.W of PEG (g/mol)

y * Na2S04 + NaCl -A- CaCl2 1

0 0.025 0.05 0.075 0.1 Ionic strength (mol/l)

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

-) Distilled water * DB.56

100

75

50; 5

25

0

0 0.1 0.2 0.3 0.4 0.5 Pressure (MPa)

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

of direct red with a molecular weight of 1373 g/mol and disperse dye, which is not charged but not soluble in water, is 100%. Retention for reactive orange and acid red is lower: this can be attributed to the lower molecular weight and a lower number of charges (Table 1). On the other hand, the fluxes

0.95 a

G 0.9 s

0.85

0.8

99

?

2 98

97

96

+ DR80 m ROl6

. AR4 A DB56

3 CF 5 7

+ ~R80 = ~016 i

. AR4 A DB56 ]

1 3 CF 5

7

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 acid and direct dye solutions are lower than for reactive and disperse dyes.

3.5. Effect of pH

Figs. 4, 5 show the effect of pH variation on the permeability ratio and dye rejection as a function of CF for the Desal5DK and M4 membranes. As shown by these data, pH has no influence on the grafted membrane flux or on retention. On the contrary, Desal membrane, which is a thin-film composite membrane with a polyamide active layer is very sensitive to pH. It is observed that when decreasing the pH from 6 to 3, the permeate flux decreases by roughly 20%. This decline has been attributed to the affinity of dyes for poly- amide, which leads to the fouling of the membrane

[71.

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

1 l . . * . . 100; .

A-+ l

0.88 1. . a

I

. . b . 98

B, . * n

,~ ~~ .._ _~~ A m. a $0.76 ,

.

2 . l ** l 5 96 1

I 0.64 l Desal 5 pH=3 m Desal 5 pH=6 94 I l Desal 5 pH=3 n Desal 5 pH=6 I

0.52 I A PS-g-P% pH=6 . PS-g-PSS pH=3 ~ . PS-g-PSS p-H=3 r I”~-g-PSS pH==$I

92 I_ _ . ..~~___ __~ ._ _..,

1 3 CF

5 7 1 3 CF 5 7

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).

1

0.95

;" 0.9 r

0.85

0.8

1

l

.

a 100

4 l . . n

b

. . . 95

. ? 2 90

85 --~~ ~:- + A010 pH=l0.3 n A010 pH=6 ~ , l A010 pH=10.3 n A010 pH=6 i

80

3 c5F 7 9 1 3 5 7 9 CF

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).

3.4. EfSect of salt

Preliminary studies were done to determine the performance of the membrane in the treatment of solutions composed of sodium sulphate or sodium chloride and dyes. A reactive dye and an acid dye were chosen since they are widely used to colour wool and nylon fibres and the process requires large amount of salt and causes serious environmental problems. The dye concentration was set at 100 ppm and concentrations of Na,SO, and NaCl were 0.03 and 0.09 M respectively (same initial ionic strength). The applied pressure was 0.4 MPa for all the experiments but the effective driving pressure used to calculate Ls is the applied pressure minus the transmembrane osmotic pressure. The results are presented in

Figs. 6-8. It is observed that while the flux is only little affected (LdLp are in the range of 0.87-l .O for all the solutions data not shown), dye retention is notably lower in the presence of salt. The decrease is less pronounced in presence of NaCl (Fig. 8) than in presence of Na,SO, (Figs. 6,7). At the same time, the retention for the two salts remained identical whether the salt was alone or mixed with the dyes. The decrease in retention was more pro- nounced for acid orange (AOlO) which has the lowest molecular weight.

These data can be qualitatively explained as follows: for the dyes, adding salt leads to a large increase in ionic strength, the membrane charge is shielded and the Donnan exclusion effect is then considerably reduced. However, the steric effect

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

100 I* IA

75

? e50 /O d

25

0 i

0

100 ‘4 IA

75 I

q- I

Z50 i

25 1

lo 0 4

0

6 . .

. A . Fig.6

00 m m

l R016 0 Na2S04 0.03M

A ROl6 (mix.) 0 Na2S04 (mix.)

400 800 1200 1600 Time (min)

. . Fig.7

. A

0 0 0 0 0

l A010 0 Na2S04 0.03M A A010 (mix.) 0 Na2S04 (mix.) (

7

400 Time8 gin) ?

1200 1600

:

l

Fig:8

l A010 q NaCl 0.09M

A A010 (mix.) 0 NaCl (mix.)

0 00 @

400 800 1200 1600 Time (min)

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

remains and the dye rejection is now more in accordance with the rejection curve obtained with PEG (Fig. la). For the salts, the addition of a relatively low amount of dyes does not modify the 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 on an UF membrane, can be successfully used for the treatment of dye effluents. The results showed that the grafted membrane is far less sensitive to fouling than polyamide membrane. Experiments done at different pH values also showed that pH has no longer an influence on the retention or flux of the dye solution. The evaluation of the salt effect showed that, in the practice, it would be preferable to change the electrolyte from a divalent anion to a monovalent anion.

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

Acknowledgements

This study was been financially supported by a doctoral grant from the government of Iran, by SociCte Franqaise d’Exportation des Ressources Fducatives (SFERE) and by Region Midi-Pymnees.

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