Immobilization of Residual Basic Dyes onto ... - SAGE Journals

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Immobilization of Residual Basic Dyes onto Polyamide Ion-exchanger Materials Mohamed Hassen V Baouab1,3*, Mohamed Khalfaoui2, Aghleb Bartegi3 and Robert Gauthier4 (1) Institut Préparatoire aux Etudes d’Ingénieurs de Monastir, Avenue de l’Environnement, 5019 Monastir, Tunisia. (2) Laboratoire de Physique Quantique, Faculté des Sciences de Monastir, 5019 Monastir, Tunisia. (3) Laboratoire de Biochimie et Environnement, Institut Supérieur de Biotechnologie, 5019 Monastir, Tunisia. (4) Laboratoire des Matériaux Polymères et Biomatériaux, UMR CNRS No. 5627, UCB Lyon 1, 43 Boulevard du 11 Novembre 1918, 69622 VilleurbanneCedex, France. (Received 7 February 2005; revised form accepted 26 April 2005)

ABSTRACT: This paper reports the preparation of methacrylic acid-grafted nylon (MAA–nylon) by treating nylon-6,6 fibres with methacrylic acid (MAA) and the use of this modified polyamide as an ion-exchanger for the immobilization of pollutant basic dyes. The grafting of MAA onto nylon-6,6 was demonstrated both by weight uptake and atomic force microscopy. The exchange capacity of MAA–nylon was evaluated by potentiometric titration of the acidic groups. Five MAA–nylon fibres with different degrees of grafting (20–80%) were tested for the adsorption of two basic dyes, viz. Basic Blue 3 and Basic Red 24. Such adsorption was monitored by visible spectroscopy. The adsorption capacity was found to depend on the degree of grafting and on the temperature. The experimental data were fitted using the Langmuir and Freundlich models. However, an improved fit could be obtained by using the Jossens model.

INTRODUCTION In order to limit pollution of the environment, it is necessary to remove dyes which are contained in wastewaters derived from the textile industry. For this reason, new methods such as adsorption onto a solid support have attracted considerable interest as a feasible procedure for removing colour from the effluents. Recently, a number of efficient systems based on the chemical modification of natural substrates or synthetic polymer have been developed in our laboratory (Baouab et al. 2000, 2001, 2004), viz. the cationization of cotton, sawdust or nylon which transforms these materials into potential adsorbents for acid dyes. Another dye type, i.e. basic dyes, generates one of the industry’s major problems. Such dyes, which are used especially for the dyeing of acrylic fibre, are particularly difficult to remove from effluent streams by conventional waste-treatment methods since they are stable to light and to oxidizing agents, and are resistant to aerobic biological treatment (Cooper 1993; McKay 1983; McKay et al. 1986; Miaolin 1992; Shukla and Sakhardande 1991). Since basic dyes generally carry a positive charge, research has been undertaken to explore the potentialities of their immobilization on an anionic support (Suteu et al. 2001). The aim of the present work is to describe the introduction of a high content of carboxylate sites into nylon-6,6 (a process which we refer to as “anionization”) and to describe the *Author to whom all correspondence should be addressed. E-mail: [email protected].

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M.H.V Baouab et al./Adsorption Science & Technology Vol. 23 No. 7 2005

N

(CH3CH2)2N Cl−

N(CH2CH3)2

O +

BB3 CN CH2 O2N

N

N

CH2

−O S CH3 4 + N(CH3)3

N C2H5

BR24 Figure 1. Chemical structures of the selected dyes.

adsorption capacity of the resulting material towards two basic dyes from aqueous solution under various experimental conditions and at different temperatures.

EXPERIMENTAL Materials Nylon-6,6 fabrics were supplied by SITEX (Société Tunisienne de textile, 5000 Monastir, Tunisia) under reference No. 102F34. The extraction of sizing products from the fabrics was conducted as previously described (Baouab et al. 2004). All reagents [methacrylic acid (MAA), methanol, sulphuric acid and sodium peroxydisulphate] were supplied by Aldrich (Sigma-Aldrich Chimie Sarl, Saint-Quentin Fallavier, France) and used without further purification. The two basic dyes used in the experiments were Basic Red 24 (BR24) and Basic Blue 3 (BB3). Both were used in their commercially available form blended with mineral salts to adjust their dyeing power. The chemical structures of these dyes are depicted in Figure 1. It is to be noticed that BR24 possesses a positive charge which is specifically localized on the quaternary ammonium group; in contrast, the positive charge in BB3 is delocalized due to resonance phenomena. Basic Red 24 (BR24) (supplied by Bayer with a purity of 85%) has a molecular weight of 492 g/mol and exhibits a maximum absorbance at a wavelength of 668 nm. Similarly, Basic Blue 3 (BB3) (also supplied by Bayer with a purity of 85%) has a molecular weight of 360 g/mol and exhibits a maximum absorbance at a wavelength of 654 nm. Methods Preparation of MAA–nylon MAA–nylon was prepared as follows. A mixture of methanol and distilled water (500 ml, 40:60 v/v), sodium peroxydisulphate (0.1 g), sulphuric acid (0.5 ml, density = 1.84) and textured nylon-6,6 (5 g) were placed in a 1 l three-necked flask fitted with a dropping funnel, a bar magnet

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and a reflux condenser. The temperature of the mixture was raised to 70°C when 32.5 ml of MAA was added. After heating for the desired reaction time, the sample was removed, washed three times with boiling water and subjected to a repeated Soxhlet extraction with acetone to remove any unreacted MAA. Finally, in order to transform all acidic groups grafted on the nylon into carboxylate groups, the MAA–nylon was treated with 250 ml of sodium hydroxide (0.1 M) over a period of 3 h under agitation at room temperature, then washed thoroughly with distilled water to attain a pH value of 7 and dried at 60°C to constant weight. Four samples of MAA–nylon (I–IV) containing various degrees of grafting depending on the reaction time were prepared. Characterization of MAA–nylon The degree of grafting was calculated from the weight uptake via the relationship: %G =

(m f − m i ) mi

× 100

(1)

where mi and mf are the weight of the dry material before and after treatment, respectively. The percentage grafting (%G) in a given series was found to increase with increasing reaction time, with a degree of grafting of 80% being obtained after 4-h reaction. The original aspect of the nylon fabrics remained unchanged during chemical modification with the exception of the colour which became slightly yellowish. Determination of the exchange capacity, Ca The carboxylate content of MAA–nylon was determined via potentiometric titration methods. Thus, 0.1 g of MAA–nylon initially in the −COO− Na+ form was equilibrated with 10 ml of a 0.2 M HCl solution. After reaction with the carboxylate groups, the remaining acid content in the solution was measured via titration against a 0.1 M NaOH solution. The adsorption capacity, Ca (mequiv/g support), was obtained from the following relationship: Ca =

(V0 − Vg ) m

× M

(2)

where V0 and Vg are the volumes (in ml) of NaOH solution necessary for the neutralization of the remaining acid after exchange with nylon-6,6 (blank) and MAA–nylon, respectively, while M and m are the molarity of the NaOH solution and the mass of the sample (g), respectively. The experimentally determined values of Ca were compared with the determined values of %G and are listed in Table 1. It will be noted that the values arising from the two methods were in agreement for each exchanger. Atomic Force Microscopy (AFM) The microscope used was the Nanoscope III multimode-AFM (Digital Instruments Inc., Santa Barbara, CA, USA) with stereotypes being carried out in tapping mode as described previously (Baouab et al. 2004). Contact with the surface was achieved when the tip had attained the

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TABLE 1. Relationship between the Degree of Grafting, %G, and the Adsorption Capacity, Ca, of MAA–nylon Compounds

Sample

I-MAA–nylon II-MAA–nylon III-MAA–nylon IV-MAA–nylon

Reaction time (h)

%G

1 2 3 4

20 40 60 80

V0 (ml)

19 19 19 19

Vg (ml)

17.5 16.5 15.5 15

Ca (mmol/g) Founda

Calculatedb

1.5 2.5 3.5 4

1.53 2.64 3.46 4.11

a

Based on acid–base titrations. bBased on weight uptake data.

maximum oscillation amplitude. In contrast to the contact mode, the siliceous tip which was in intermittent contact with the surface did not modify the state of the surface. The micrograph depicted in Figure 2(a) shows that the untreated nylon fibre had a smooth surface. However, its cross-sectional area was not constantly circular but exhibited flattening distributed randomly all along the fibre, probably due to the extrusion and texturing operations undertaken. A fibre in the solvent without MAA presented the same aspect as the untreated fibre. In contrast, the AFM images of MAA–nylon [Figures 2(b) and (c)] exhibited a very significant increase in surface roughness of the grafted fibre which was attributed to the presence of grafted anionic sites. Adsorption of basic dyes The affinity of MAA–nylon anion-exchanger for the adsorption of basic dyes was determined by stirring 0.1 g of the prepared material and 100 ml of an aqueous solution with an initial dye concentration (C0) mechanically in an Ahiba Nuance® laboratory machine (Salvis AG, Reussbühl, Switzerland) for 2 h at a given temperature. The suspension was then filtered and the concentration (Ce) of the remaining dye in the filtrate was determined using an Uvikon 941 Plus® spectrophotometer (Kontron Instruments, Milan, Italy) at the maximum absorbance wavelength. The solute concentration in the solid phase, Ye, was then deduced by difference with Ce. Adsorption isotherms, Ye versus Ce, were determined varying two parameters, i.e. the %G value of the MAA–nylon material and the temperature of the dye solution. RESULTS AND DISCUSSION It is worthy of note that, under the chosen operating conditions, untreated nylon-6,6 exhibited no affinity towards the dyes studied in accord with the literature for basic dyes (Peters 1975). Mechanism of graft copolymerization Graft copolymerization onto textile fibres offers intriguing possibilities because grafting usually leads to the addition of properties associated with the side chains over and above the main chain properties. Chemical grafting involves the formation of active centres upon the nylon backbone.

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(a)

4

6 µm X 2.000 µm/div Z 699.999 nm/div

2 pa66.100

(b)

2 4 6

maa(8).200

X Z

µm

2.000 µm/div 699.999 nm/div

(c)

2 4 maa(80).100

6 µm

X Z

2.000 µm/div 699.999 nm/div

Figure 2. AFM views of (a) untreated nylon-6,6 fibre, (b) I-MAA–nylon (%G = 20) and (c) III-MAA–nylon (%G = 60).

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Once these centres are formed, the polymer chains which start to grow on them lead to the formation of branches. According to Abdelmoula et al. (1997), the mechanism of graft copolymerization may be written as follows: •

Thermal dissociation The S2 O 82 − ion dissociates thermally according to the equation: S2 O 28 − → 2SO •4 −

•

2 H2 O

2HSO −4 + 2HO •

Initiation The HO • radical thus formed may initiate the production of nylon radicals: HO • + nylon → nylon • + H 2 O

•

Propagation Once the nylon radicals are produced, the monomer adds to them to give a graft polymer: nylon • + MAA → nylon − MAA •

•

Termination Termination can occur either via chain transfer or combination reactions involving the growing chain radical: nylon − MAA • + MAA → nylon − MAA + MAA • nylon − MAA • + • AAM − nylon → graft copolymer

Treatment of coloured waters by grafted nylon The equilibrium distribution between the III-MAA–nylon support and the solution at 20°C for the dyes tested is represented by the isotherms depicted in Figure 3, where the limiting values (adsorption capacities), Yref, for the dyes were 1620 mg/g for BR24 and 694 mg/g for BB3. This indicates that III-MAA–nylon exhibited a much greater affinity for BR24 relative to BB3. The values listed relate to the weight of the commercial dye including added salts. Data for the untreated fabric are not given since there was little affinity between the tested dyes and nylon. Effect of the degree of grafting The influence of the degree of grafting is shown in Figure 4 which presents the curves of Yref versus %G for the adsorption of the two tested dyes at 20°C. Firstly, it will be observed that the adsorption capacity increased with the degree of grafting up to a value of 60% but that above this value the adsorption capacity towards both dyes decreased spectacularly. This result can in terms of interpreted in terms of the transformation of the highly grafted support into a very swollen gel in aqueous medium and in the presence of dyes. Under these circumstances, dye diffusion to anionic sites on the support surface would become difficult

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1800

1600

1400

Ye (mg/g)

1200

1000

800

600

400

200

0 0

200

400

600

800

1000

Ce (mg/l) Figure 3. Adsorption isotherms at 20°C of () BR24 and () BB3 onto III-MAA–nylon (%G = 60).

so that the electrostatic attraction between the surface and the dye molecules would diminish. For this reason, degrees of grafting greater than 60% were not employed for the supports studied. Secondly, the quantity of BR24 attached to the support increased in a virtually linear fashion for degrees of grafting in the range 20–60%. The data listed in Table 2 indicate that the ratio between the maximum concentration of adsorbed dye (mmol/g), as calculated for the pure dye molecules, and the adsorption capacity (Y/Ca) was constant. In addition, ca. 0.80 of the anionic sites on the solid support were occupied by one dye molecule. Thirdly, the data in Table 2 indicate that when the degree of grafting of BB3 was 20% the adsorption capacity was 0.66, and for high %G it seems that not all the carboxylate groups could be employed for adsorption. Thus, at the degree of grafting of 60%, the adsorbed dye occupied ca. one-half of the number of sites, suggesting that one dye molecule interacted with two adsorption sites on the solid support. Thus, adsorption was more effective at low concentrations with the approach of a dye molecule to an adsorption site becoming increasingly difficult as the dye concentration increased. This was probably due to steric hindrance and to the activity of the positive charges on the dye. Indeed, the extent of approach between the adsorbate and adsorbent in the adsorption process depends on the stability of the positive charge in the ionized form of the dye.

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1800

1600

1400

1200

Yref (mg/g)

1000

800

600

400

200

0 0

20

40

60

80

100

%G Figure 4. Plots of the adsorption limit (adsorption capacity), Yref, versus the degree of grafting, %G, for () BR24 and () BB3, respectively.

TABLE 2. Comparison between the Adsorption Capacity, Ca, and the Quantity of Pure Dye Adsorbed, Ya, at 20°C

Samples

I-MAA–nylon II-MAA–nylon III-MAA–nylon IV-MAA–nylon aY

=

%G

20 40 60 80

Ca (mmol/g)

1.5 2.5 3.5 4

Yref × purity Molecular weight

.

Quantity of dye adsorbed (mmol/g) BR24 Yref (mg/g)

Y (mmol/g)

Y/Ca

BB3 Yref (mg/g)

Y Y/Ca (mmol/g)

694 1188 1620 420

1.20 2.05 2.80 0.72

0.80 0.82 0.80 0.18

425 610 694 288

1.00 1.44 1.63 0.68

0.66 0.57 0.47 0.17

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The positive charge in BB3 does not possess the same degree of activity as that in BR24 as it is not localized and it is distributed between the ammonium and oxonium groups. However, the positive charge of BR24 is localized on the ammonium groups alone and is therefore capable of exerting a considerable attractive force on approaching anions. On this basis, it would be expected that the adsorption affinity between BR24 and MAA–nylon would be strong. All the isotherm curves attained a plateau level which, according to Giles et al. (1974a,b), corresponds to the formation of a dye monolayer on the accessible surfaces of the fibres with adsorption taking place via an ionic mechanism. The molecules of BR24 and BB3 are aligned, respectively, in a perpendicular and a parallel manner to the adsorbent surface. With both dyes, ionic interactions are initially established between the positive groups of the dye and the carboxylate groups of MAA–nylon. Effect of temperature The results of experiments to determine the effect of temperature on the adsorption of BR24 onto III-MAA–nylon are presented in Figure 5 where the limiting values (adsorption capacities), Yref, for the dye were 1620, 1493, 1188 and 950 mg/g at temperatures of 20°C, 40°C, 60°C and 80°C, respectively. This parameter had a similar effect on the adsorption of both tested dyes. The decrease in the adsorption capacity of MAA–nylon with increasing temperature noted above may

1800 1600 1400

Ye (mg/g)

1200 1000 800 600 400 200 0 0

200

400

600

800

1000

Ce (mg/l) Figure 5. Influence of temperature on the adsorption of BR24 onto III-MAA–nylon (%G = 60). Data points refer to the following temperatures: , 20°C; , 40°C; , 60°C; , 80°C.

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be attributed to the enhanced magnitude of the reverse (desorption) step in the mechanism. This suggests that the interactions between the modified nylon substrate and the dye molecules must be quite reversible under these circumstances. This is possibly due to the exothermic effect of the surrounding aqueous medium during the adsorption process. For BR24 and BB3, respectively, the limiting adsorption value at 80°C was ca. 58% and ca. 57% of that observed at ambient temperature. This parameter appears to have a greater influence than for cationized nylon (Baouab et al. 2004), thereby indicating that a desorption process is possible as previously mentioned (Baouab et al. 2000, 2001, 2004). Adsorption isotherms The three widely accepted equilibrium adsorption isotherm models, i.e. those of Langmuir (1918), Freundlich (1926) and Jossens (Jossens et al. 1978; Weber and Mathews 1976), were used to evaluate the sorption behaviour of the examined modified nylon. All the parameters associated with these methods are defined below in the Nomenclature section. Analysis via the Langmuir isotherm The Langmuir isotherm may be expressed by the following equation: Ye =

QbC e 1 + bC e

or

Ce Ye

=

1 Qb

+

Ce

(3)

Q

According to equation (3), a plot of Ce / Ye versus Ce should produce a straight line. If the experimental data fit this plot with a relatively good correlation coefficient, this would show that the data are described correctly by the Langmuir relationship (Figure 6). The Langmuir constants are summarized in Table 3 from which it is seen that there was good agreement between the experimental values of Yref and the calculated value of Q. When measured at different temperatures, the Langmuir equilibrium constant KL = Qb can be used to determine the enthalpy of adsorption, ∆H, via the Clausius–Clapeyron equation:  ∆H  K L = A exp −   RT 

or

log K L =

− ∆H 2.303R

×

1 T

+ log A

(4)

Hence, a plot of log KL versus 1/T (K) (Figure 7) yields a slope equal to −∆H / 2.303R allowing ∆H to be calculated. The enthalpies associated with the adsorption of an initially solvated dye molecule onto the solid support at various %G values are listed in Table 3. Figure 8 shows that, at a high degree of grafting, the system BR24/MAA–nylon was less stable energetically than the BB3/MAA–nylon system. In contrast to BR24, the BB3 molecule has a symmetric planar structure and anchors onto a surface in a parallel manner. This suggests that the probable establishment of supplementary hydrogen bonding between the electrophilic groups of the dye and the nucleophilic groups of MAA–nylon could reinforce the electrostatic attraction. However, the positive charge of BB3 is dispersed throughout its structure rather than located at a particular point, so that repulsion between the π-electrons of the aromatic rings of the dye and the negatively charged MAA–nylon may not be effective in this system. With BR24, where the positive charge is localized on the molecule and where anchorage occurs perpendicularly, the sorption process was found to be slightly exothermic. This is ascribed to repulsion between the π-electrons of the aromatic rings of the BR24 and the negatively charged

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1.2

1.0

Ce /Ye (g/l)

0.8

0.6

0.4

0.2

0.0 0

200

400

600

800

1000

1200

Ce (mg/l) Figure 6. Influence of temperature on the Langmuir isotherms for the adsorption of BR24 onto III-MAA–nylon (%G = 60). Data points refer to the following temperatures: , 20°C; , 40°C; , 60°C; , 80°C.

MAA–nylon rather than to any contribution arising from hydrogen bonding with the electrostatic attraction involved in the adsorption process. This means that the adsorption enthalpy of the BR24/MAA–nylon system becomes less and less exothermic as the extent of grafting increases. At low values of %G, the adsorption sites are spaced and the adsorption process is simply governed by electrostatic interaction. Under these circumstances, the adsorption enthalpies of the two dyes are close. It should also be noted that BR24 has a greater affinity for MAA–nylon relative to BB3, but that the BB3/MAA–nylon system is more stable than the BR24/MAA–nylon system. The values of the enthalpies indicate that heat was liberated during the adsorption process in all cases, thereby limiting the adsorption of these dyes by MAA–nylon at high temperatures. Such results are similar to those found in the adsorption of acid dyes onto cationized nylon (Baouab et al. 2004). Analysis via the Freundlich isotherm When heterogeneous surface energies are involved, the Freundlich equation may be used in the general form: Ye = PC1e / n

or

log Ye = log P + 1/ n log C e

(5)

Figure 9 shows typical Freundlich plots of log Ye versus log Ce which, according to Fritz and Schlunder (1981), can be resolved into two straight lines. The magnitude of the exponent n gives

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TABLE 3. Langmuir Constants and Enthalpy for the Adsorption of Dyes onto MAA–nylon

Dye

BR24

%G

20

40

60

80

BB3

20

40

60

80

Temperature (°C)

Yref (mg/g)

∆H (kJ/mol)

Langmuir constants Q (mg/g)

KL (l/g)

b (l/mg)

20 40 60 80

694 555 460 350

714 555 476 357

49.26 35.71 24.57 18.65

0.07 0.06 0.05 0.05

20 40 60 80

1188 1005 744 560

1250 1000 769 588

36.36 30.30 21.23 11.49

0.03 0.03 0.03 0.02

20 40 60 80

1620 1493 1188 950

1666 1500 1250 1000

58.14 26.73 35.33 41.66

0.03 0.02 0.03 0.04

20 40 60 80

420 400 381 346

435 417 384 357

23.42 15.57 21.50 15.62

0.02 0.04 0.06 0.04

20 40 60 80

425 350 275 200

434 357 277 204

31.94 13.96 11.86 13.67

0.07 0.04 0.04 0.08

20 40 60 80

610 538 400 312

625 535 400 333

46.29 22.27 14.12 4.80

0.07 0.04 0.03 0.01

20 40 60 80

694 606 502 396

714 625 526 416

49.26 23.4 10.25 5.85

0.07 0.04 0.02 0.01

20 40 60 80

288 240 183 145

294 250 192 153

34.01 7.36 4.36 2.37

0.11 0.02 0.02 0.02

−14.12

−16.12

−3.60

−3.88

−12.11

−30.94

−31.06

−37.03

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1.8 1.7 1.6

log KL

1.5 1.4 1.3 1.2 1.1 1.0 2.7

2.8

2.9

3

3.1

3.2

3.3

3.4

3.5

103/T (K−1) Figure 7. Plots of log KL versus the reciprocal temperature at the following degrees of grafting for BR24: , 20; , 40; , 60; , 80.

%G 0

20

40

60

80

−5 −10

∆H (kJ/mol)

−15 −20 −25 −30 −35 −40 Figure 8. Behaviour of the adsorption enthalpy as a function of %G for () BR23 and () BB3, respectively.

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3.3

3.2

3.1

log Ye

3.0

2.9

2.8

2.7

2.6 1.0

1.5

2.0

2.5

3.0

log Ce Figure 9. Influence of temperature on the Freundlich isotherms for the adsorption of BR24 onto III-MAA–nylon (%G = 60). Data points refer to the following temperatures: , 20°C; , 40°C; , 60°C; , 80°C.

an indication of the favourability and capacity of the adsorbent/adsorbate system. According to Treybal (1987), values of n > 1 represent favourable adsorption conditions. In the present case, the exponent n was in the range 1.07 < n < 7.03 in all cases, thereby showing that the adsorption process was favourable. Analysis via the Jossens isotherm Although Figure 6 shows that the adsorption data accord well with the Langmuir isotherm, Figure 9 indicates that this is not the case with the Freundlich equation. Weber and Mathews (1976) postulated the use of a general isotherm model, viz. the Jossens isotherm, which takes both the Langmuir and Freundlich expressions into account. This isotherm may be represented by the following equation: Ye =

iC e 1 + j(C e ) m

(6)

Since the expression contains three unknowns, viz. i, j and m, the data were evaluated by iteration using a computer program (MicrocalTM OriginTM 5.0 Professional; Northampton, MA 01060 USA). The results are presented in Table 3.

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2500

2000

Ye (mg/g)

1500

1000

500

0 0

100

200

300

400

500

600

700

800

900

1000

Ce (mg/l) Figure 10. Comparison of theoretical isotherms with the experimental data at 20oC for the adsorption of BR24 onto III-MAA–nylon (%G = 60). Experimental data are depicted by open circles while the various lines refer to the application of the following isotherm equations: —— , Langmuir; _ _ _ , Freundlich; - - -, Jossens.

The corresponding theoretical plots arising from the application of the Langmuir, Freundlich and Jossens isotherm equations to the experimental data for the adsorption of BR24 onto III-MAA–nylon at 20°C are depicted in Figure 10 to enable an assessment to be made of their ability to correlate experimental results over a wide range of concentration. It will be seen from this figure that the weaknesses of the Langmuir isotherm are highlighted in the region of monolayer coverage, the Freundlich isotherm does not appear to be able to characterize the adsorbent/adsorbate system over the whole range of concentrations, whereas the Jossens isotherm is in good agreement with the experimental data at high concentration. Effect of isotherm shape Weber and Chakravorti (1974) have considered that the isotherm shape can used to predict whether the adsorption is “favourable” or “unfavourable”. If the process is considered to be Langmuirian in type, the isotherm shape can be classified by the term “r”, viz. a dimensionless constant separation factor, defined by Hall et al. (1966) as: r =

1 1 + bC ref

(7)

Figure 11 shows typical shape factor plots (a dimensionless solid-phase concentration, q, versus a dimensionless liquid-phase concentration, X) for the adsorption of the two studied dyes onto

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1.0

0.8

q = Ye /Yref

0.6 r=1 0.4

0.2

0.0 0

0.2

0.4

0.6

0.8

1.0

X = Ce /Cref Figure 11. Dimensionless concentration isotherms as a function of the separation factor for the adsorption at 20°C of () BR24 and () BB3 onto III-MAA–nylon (%G = 60).

III-MAA–nylon. Both systems showed favourable adsorption, i.e. 0 < r < 1 with “r” being near zero.

CONCLUSIONS A strong acid ion-exchange nylon was obtained by grafting MAA onto nylon-6,6 in a mixture of methanol and distilled water in the presence of sodium peroxydisulphate as an initiator at 70°C. The factors affecting basic dye removal from aqueous solution employing MAA–nylon were studied. MAA–nylon was found to be an excellent adsorbent for both dyes with high adsorptive capacities being observed, viz. 1620 and 694 mg dye/g MAA–nylon for Basic Red 24 and Basic Blue 3, respectively. Comparison of the theoretical Langmuir and Jossens isotherms with the experimental data gave good agreement. Both isotherms were shown to be favourable and the effect of the degree of grafting was significant. The influence of the temperature was studied and enthalpies of adsorption were determined. However, the Freundlich isotherm was found to be incapable of characterizing the adsorbent/adsorbate systems under the experimental conditions employed. Because of the availability and low cost of nylon-6,6, its behaviour as an anion-exchanger seems to offer a potential for the environmental clean-up of wastewater from the operation of dye works. Future work will involve fitting the experimental adsorption isotherms to a statistical physics model (Khalfaoui et al. 2002a, 2003). This should provide a better understanding and interpretation of the adsorption process at a molecular level as been achieved for the adsorption of acid dyes onto cationized cotton (Khalfaoui et al. 2002b).

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NOMENCLATURE A b Ce C0 Cref ∆H i j KL m n P q Q R r X Ye Yref

pre-exponential factor in the Clausius–Clapeyron equation constant related to the energy of adsorption (l/g) dye concentration in solution at equilibrium (mg/l) initial dye concentration in solution (mg/l) highest fluid-phase concentration (mg/l) enthalpy of adsorption (kJ/mol) constant in Jossens isotherm (l/g) constant in Jossens isotherm (l/mg) Langmuir equilibrium constant (l/g) constant in Jossens isotherm adsorption intensity measure of adsorption capacity [mg(mg/l)1/n/g] dimensionless solid-phase concentration at equilibrium dye concentration at monolayer coverage (mg/g) universal gas constant [kJ/(mol K)] dimensionless constant separation factor dimensionless liquid-phase concentration at equilibrium dye concentration at equilibrium (mg/g) maximum solid-phase dye concentration (mg/g)

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