The role of anionic substances in removal of textile dyes from solutions using cationic flocculant

The role of anionic substances in removal of textile dyes from solutions using cationic flocculant

Colloids and Surfaces A: Physicochem. Eng. Aspects 214 (2003) 37 /47 www.elsevier.com/locate/colsurfa The role of anionic substances in removal of t...

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Colloids and Surfaces A: Physicochem. Eng. Aspects 214 (2003) 37 /47 www.elsevier.com/locate/colsurfa

The role of anionic substances in removal of textile dyes from solutions using cationic flocculant Rima Jule Zemaitaitiene, Egidija Zliobaite, Rima Klimaviciute, Algirdas Zemaitaitis * Department of Organic Technology, Kaunas University of Technology, Radvilenu 19, LT-3028 Kaunas, Lithuania Received 15 May 2002; accepted 2 July 2002

Abstract The mechanism of dye removal from textile waste water using polyquaternary ammonium salt as a flocculant has been proposed. It was shown that the cationic polymer tends to react with anionic textile finishing chemicals and auxiliaries (anionic detergents, dispersing agents, thickeners), forming intermolecular complexes of different stoichiometry. Under controlled conditions these complexes are able to incorporate the dye ions or molecules and precipitate together. Formation of triple complexes is postulated to be the only possible way for non-ionic dyes to be precipitated. The nature of forces causing the transfer of the dyes to a polymeric matrix has been revealed. The necessity of an anionic polyfunctional compound with a proper chemical structure to play the role of disperse dye carrier is emphasised. The mechanism of incorporation of dye ions into triple complexes, the possibility of competitive reactions, and the problems of stability and reduced cationic functionality of nonstoichiometric complexes are discussed. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Textile dye; Polyelectrolyte; Flocculation; Dual system

1. Introduction Cationic quaternary polyelectrolytes are used in many fields for influencing the stability and coagulation of disperse systems. Most commonly used as flocculation aids, they have received increased attention in water and wastewater treatment in recent years. Several mechanisms [1,2] such as ‘charge patch’ model, ‘bridging’, displacement flocculation, etc. have been proposed to * Corresponding author. Tel./fax: /370-7-456081 E-mail address: [email protected] (A. Zemaitaitis).

explain the destabilisation of colloids and suspensions by polymers. For the cases in which the polymer and the adsorption site are of opposite signs (e.g., cationic polymer/clay system) it is postulated that charge neutralisation is the major mechanism [1]. Textile dye molecules and ions or their aggregates are incomparably smaller than colloidal silica or clay particles. Therefore polymer adsorption on the surfaces loses its importance and the flocculation mechanism might be different. Extensive studies of the interaction between ionic dyes and polycations in solutions have been

0927-7757/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 7 7 5 7 ( 0 2 ) 0 0 4 0 6 - 5

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carried out by Takagishi et al. [3]. Literature review is given in [4]. Methyl Orange and its homologues have in fact been often used as a probe to study the mode and the extent of binding. In those results the importance of both electrostatic and hydrophobic interactions is strongly emphasised. Textile wastes are more intricate systems. Taking into account that pollutants consist of various negatively charged compounds such as thickeners, dispersing agents, anionic detergents, inorganic preserving colloids, etc., their intermolecular complexes (PECs) with cationic polyelectrolytes might be expected to form. Unavoidably increases the expenditure of the cationic polyelectrolyte used. But some anionic compounds in textile wastes are capable to facilitate the flocculation of ionic and disperse dyes as well. In our earlier experiments [5 /8] we have observed that under controllable conditions the polymer and the textile auxiliaries form the optimal structures to incorporate the dyes, leading to a triple complex precipitation. Such an assumption was confirmed by analysis of the purified solutions [5]. The isolated precipitate added to a new portion of the solution exhibited some binding capacity and flocculated additional amounts of the dye and anionic dispersing agent [6]. Surprisingly, the disperse dye which was uncharged seemed to be bound by PECs [5,6,8]. However, no systematic investigation of the binding mechanism has been completed. The electrostatically driven interactions between oppositely charged macromolecules have been known long since [9], but have attracted considerable interest in recent years because of various industrial and environmental applications of PECs. The so-called dual systems involving a step-by-step addition of two oppositely charged polyelectrolytes were very effective in the flocculation of zeolite [10], montmorillonite [11] alumina [12] dispersions, cellulose and cellulose-clay mixtures [13,14]. Addition of preformed PECs [13,15] in some cases has also been found to be a successful method. Removal of the soluble amphiphilic compounds from industrial wastes, including textile dyeing and scouring liquors, by flocculation is also of great interest. Several studies reported the solubilization

of phenols [16,17] and water-insoluble dyes [18,19] by polymer-surfactant complexes and emphasized the role of hydrophobic interactions in these processes. Buchhammer et al. [20] compared the sorption capability of preformed polyelectrolytemicelle complexes and polycation /polyanion complexes for p -nitrophenol or anionic dye model, bromocresol green, and showed that the sorption capability strongly depended on the structure, net charge of the complex particles and increased with increasing molar mass and hydrophobic properties of the components used. But the binding level of the ionic substance was higher than that of p nitrophenol. Dragan et al. [21] examined formation of the tricomponent complexes from preformed nonstoichiometric polycation /anionic dye complexes and polyanions added and postulated the significance of the electrostatic interactions between the negative and the free residual positive charges in the former complexes. Oertel et al. [22] described surface modification of finely dispersed silica materials by polyelectrolytes and provided the evidence that only nonstoichiometric polycomplexes with a great excess of cationic charge exhibited enhanced adsorption of the anionic dyestuff, the minimum value being observed at n/n (molar ratio polyanion/polycation)/1. Wassmer et al. [23], describing the polyelectrolyte titration method, also are of the opinion that the release and displacement of the dye ions might be expected when the stoichiometry 1:1 reaction between the oppositely charged polymers takes place at the equilibrium point. Takagishi et al. [24,25] have provided evidence that insoluble polyion complexes of sodium poly(methacrylate) and piperidinium cationic polymers are capable to bind Methyl Orange and its homologues in water solutions [24] and ethylene glycol [25] as well. The polycomplexes examined corresponded to 1:1 stoichiometry, therefore this matrix, as supposed, should be electrically neutral. Interestingly, the significance of electrostatic interactions that accompany the binding has been postulated. Such controversial results and their interpretation provide the area for a further discussion. It was thus of interest to extend investigations in the field of polycomplex formation and their role in the dye binding processes. The aim was a deeper

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insight into the removal of dyes and anionic uncoloured contaminants from textile wastes by flocculation. To obtain more evidence of the flocculation with the so-called dual system mechanism, the investigations were completed by new experimental data. More detailed fluorescence spectroscopy measurements have been done to extend our previous conclusions.

2. Materials and methods Cationic polymers with relatively short chains (average molecular weight between 17 000 and 18 000), but a high cationic charge density have been chosen as flocculants. The polyquaternary ammonium salts poly(diallyldimethylammonium chloride) (PDADMAC) and poly(vinylbenzyltrimethylammonium chloride) (PVBTMAC) used were of reagent grade. Anionic substances (AN) for PECs formation were of the kinds that usually accompany the dyestuffs in textile wastes. The detergent, potassium palmitate (PP), was of analytical grade. The dispersing agent for disperse dyes, disodium salt of 2,2?-methylen-bis(6naphthalenesulfonic acid) (DNF)*/was purified by precipitation from water /isopropanol 10:1 solution up to 96%. Textile dyes and thickeners */sodium carboxymethylcellulose (CMC), DS/0.77, carboxymethylated starch (CMS), DS /0.36, sodium alginate (ALG)*/ were commercial products and used without purification. Manufacturers and trade names are shown in Table 1. Distilled water was used and had a temperature of 18 /20 8C. Flocculation of dyes has been examined as follows. Dyes were dissolved in water, the solution was transported to the flocculation tubes, and increasing amounts of cationic polymer were added. The solutions were slowly mixed for 30 s and allowed to sediment for 24 h. Samples were taken for analysis from a layer 3 /5 cm below the surface. After filtration through a paper filter, the adsorbance of the supernatant solutions was measured using a KFK-3 colorimeter (Russia) to calculate the residual amount of dyestuff. Precipitation of dyes in the presence of various anionic compounds has been examined in a similar way.

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Using the so-called dual systems the cationic polymer was added first followed by AN after 5/7 s, and mixing continued for half a minute. The concentrations, mole numbers and their ratios are related to the base units of the polymers and anionic compounds used. For determination of the optimal molar relations of the PEC constituents, the isomolar series of experiments were performed. Dyes were dissolved in water to a concentration of 0.11 g dm 3. Aliquots (90 cm3) of the dye solutions were transported to 150 cm3 tubes. Decreasing amounts of 6/103 M cationic polymer (10, 9, 8,. . .,0 cm3) and increasing amounts (0, 1, 2,. . .,10 cm3) of equimolar anionic substance (AN) solution were added, so that the total molar concentration of both reagents was constant and the concentration of the commercial dye during precipitation experiments was 0.1 g dm 3. Fluorescence spectra were taken using a Hitachi MPF-4 (Japan) type fluorescence spectrometer. 8Tolylamino-1-naphthalenesulfonic acid (TNS) and naphthalene (Nf) were used as fluorescence probes. Simultaneously they represented an anionic and a disperse dye model, respectively. The fluorescent properties of DNF were exploited for examining its performances during a triple complex formation. The TNS and Nf fluorescence was exited at l/380 and 280 nm, respectively. The relative fluorescence intensity (I/I0) was determined as the ratio of fluorescence intensity of TNS solutions in the presence of other chemicals and that of TNS alone.

3. Results and discussion Some experimental data extracted from the previous works [5 /7] and supplemented by the new ones are shown in the summary Table 2 and Table 3. Data in Table 2 illustrate the best flocculation results for the dye/anionic substance/polyquaternary ammonium salt systems achieved by varying the concentration of PDADMAC. The amount of the residual dye characterizes the optimal dye removal level. From the data shown in Table 2 it is clear that in general a greater amount of polyquaternary salt is needed to remove the

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Table 1 Polymers, textile dyes and model substances Substance

Symbol/trade name

Manufacturer

Poly(diallyldimethylammonium chloride) Poly(vinylbenzyltrimethylammonium chloride) Sodium carboxymethylcellulose, DS/0.77 Sodium carboxymethylstarch, DS/0.36 Sodium alginate Potassium palmitate Disodium salt of 2,2?-methylen-bis(6-naphthalene sulfonic acid) 8-Tolylamino-1-naphthalenesulfonic acid Naphthalene C.I.Disperse Blue 73 C.I.Disperse Blue 7 C.I.Disperse Blue 3 C.I.Disperse Yellow 211 C.I.Acid Blue 78

PDADMAC PVBTMAC CMC CMS ALG/Manutex PP DNF/Dispergator NF

Reachim (Russia) Biolar (Latvia) VNIISS Vladimir (Russia) Emslandstarke (Germany) Alginate Industry (UK) Riedel-de-Haen Reachim (Russia)

TNS Nf Terasil blau R Dispersnyj sine zelenyj Dispersnyj sinij K Terasil Gelb 4G Kislotnyj chisto goluboj antraquinononovyj Kationnyj zheltyj 4Z

NIOPIK (Russia) Riedel-de-Haen Ciba Tambov chemical plant (Russia) Tambov chemical plant (Russia) Ciba Rubezhansk chemical plant (Ukraine) Rubeschansk chemical plant (Ukraine)

C.I.Basic Yellow 12

disperse or ionic dyes from their water solutions in the presence of various anionic compounds compared with the behaviour of PDADMAC alone. Such an amount arises if the AN concentration increases. But in some cases (e.g., for the basic dye and Disperse Yellow 211) addition of both PDADMAC and DNF is necessary. The anionic detergent, potassium palmitate, assists a complete decolouring of C.I.Disperse Blue 7 solution. Also, CMC in conjunction with a cationic polymer enhances the removal of acid dye. Only 3% residue of C.I.Acid blue 78 remains after precipitation with the accurately determined dual CMC/PDADMAC system, while the polyquaternary salt alone used at a concentration of 24 mg dm 3 removed 83% of the dyestuff, and its overdosing could not help to achieve better results, because restabilisation occurred. Moreover, the presence of AN in dye solutions supports the formation of larger and better-settling flocs. Assuming that the dyestuff, the cationic flocculant and AN precipitate together, one might expect a simultaneous removal of anionic contaminants in large amounts shown in Table 2. Unfortunately, the systems are very sensitive to variations in the composition, and an excellent flocculation in neutral solutions can be observed only within a narrow range of the molar

ratios of both reagents, which is not much wider than the optimal region for the cationic polyelectrolyte alone. Dependence of the flocculation behaviour on the polyanion(anion)/polycation molar ratio has been examined varying such a ratio in the frame of the total constant isomolar concentration of both reagents as shown in Fig. 1. The n/n  ratios listed in Table 3 corresponded to the best flocculation results (minimal dyestuff residuals in supernatant liquor) obtained in such a way. The results can be summarised as follows. It was observed that dual systems helped to remove textile dyes regardless of their charge. Anionic dye (C.I.Acid Blue 78), non-ionic disperse dyes (C.I.Disperse Blue 3, 7 and 73) and some of cationic dyes (C.I.Basic Yellow 12) precipitated in the same manner. The main difference in their flocculation behaviour was the value of the optimal n/n molar ratio of both reagents needed: in general the anionic dye was removed by addition of a greater amount of cationic polymer (n /n decreases) than the basic dye which carries the positive charge itself. For flocculation of anionic and disperse dyes in neutral solutions in all cases, except systems with PP, an excess of the cationic charge in the polymeric loci

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Table 2 Dependence of the residual dye on its initial concentration in the solution, concentrations of AN and PDADMAC added Dye

Cdye (mg dm 3)

AN

CANa (mg dm3)

CPDADMAC (mg dm 3)

C.I.Disperse Blue 7

25 50 50 100 100 100 100 100 100 100

DNF DNF CMC DNF DNF CMC CMC CMS ALG PP

/ / 7.2 / 56.6 22.3 62.5 166.0 53.5 97.2

4.8 9.1 15.5 24.0 58.3 36.4 63.1 48.6 53.4 43.7

2 1 5 2 4 5 5 6 4 0

C.I.Disperse Blue 73

25 50 100 100 100

DNF DNF DNF DNF DNF

/ / / / /

3.0 6.5 11.6 13.7 15.8

0 0 19 0 68

C.I.DisperseYellow 211

25 25 50 100

/ DNF DNF DNF

/ 15.5 30.6 59.0

10.7 10.7 20.3 40.5

80 0 4 0

C.I.Acid Blue 78

100 100 100 100

/ DNF CMC CMS

/ 70.8 53.7 116.2

24.0 48.6 68.0 63.2

17 25 3 21

C.I.Basic Yellow 12

100 100

/ DNF

/ 99.1

50.0 29.2

100 35

Residual dye (%)

a

In the concentrations of AN given not shown are the amounts of DNF, which accompany the disperse dyestuffs according to the commercial dye formula.

must be preserved (n /n  B/1). These findings indicate that active electrostatic interactions between dyes and the polymeric chain are of great importance. A notable influence of pH on the binding of textile dyes by cationic polymer in the presence of various AN is to be stressed. In alkaline medium the range of optimal n/n  molar ratios becomes wider, but for flocculation the amounts of polyquaternary salt greater than in neutral solutions are needed. The influence of the acid is more positive, especially for the systems which contain polysaccharides. When pH decreases to 2, as is shown in Table 3, an effective precipitation of anionic and disperse dyes can be observed when almost all possible combinations of polyquaternary ammonium salt and CMC or CMS are used. This behaviour cannot be attributed to the possi-

bilities provided by the cationic polymer itself, because addition of acid does not change markedly its flocculative capacity. It is reasonable to suggest that interaction between polyquaternary salt and AN takes place in dye solution, and the structural peculiarities in structure of their PECs are responsible for such a favourable dye precipitation. According to theory [9], the main forces responsible for the PECs formation between oppositely charged polyelectrolytes are Coulombic. In acidic medium, ionisation of anionic carboxygroups is strongly suppressed and solubility of polysaccharides in water solutions markedly reduced. Obviously, in these conditions (pH :/2) the nonstoichiometric intermolecular complexes (NPECs) are formed. Such highly permeable species preserve an appreciable amount of free quaternary ammonium groups. Dye anions can

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Table 3 Optimal molar ratios of IPC substances in removal of various dyes and their dependence on pH Dye

pH

IPC substances

n  /n 

Residual dye (%)

C.I.Disperse Blue 3

6.5 /7.0 11.7 6.8 /7

CMC/PDADMAC CMC/PDADMAC CMC/PDADMAC

0.35 /0.52 0.3 /0.53 0.49 /0.53

6 /8 6 /8 2 /10

C.I.Disperse Blue 7

2.0 1.8 1.5

CMC/PDADMAC CMC/PDADMAC CMC/PDADMAC

0.18 /2.7 0.25 /1.67 0.1 /10.0

5 /8 25 /30 100

C.I.Disperse Blue 7

11.7 6.7 2.0

CMS/PDADMAC CMS/PDADMAC CMS/PDADMAC

0.43 /0.54 0.8 /1.2 0.67 /4.0

6 /7 4 /6 6 /10

C.I.Disperse Blue 7

11.7 7 1.9 1.6

ALG/PDADMAC ALG/PDADMAC ALG/PDADMAC ALG/PDADMAC

0.43 /0.54 0.78 /0.85 1.08 /1.78 2.03 /8.9

6 /8 4 /10 5 /9 5 /10

C.I.Disperse Blue 7

12.7 7 2.7

DNF/PDADMAC DNF/PDADMAC DNF/PDADMAC

0.25 /0.67 0.55 /0.71 0.67 /1.28

6 /9 4 /10 8 /12

C.I.Disperse Blue 7

12.0 7 2.1 6.9

CMC/PVBTMAC CMC/PVBTMAC CMC/PVBTMAC PP/PDADMAC

0.25 /0.45 0.37 /0.47 0.67 /1.7 1.1 /1.4

5 /15 6 /10 6 /11 0 /5

C.I.Acid Blue 78

7 2.4

CMC/PDADMAC CMC/PDADMAC

0.33 /0.45 0.1 /1.86

3 /16 1 /17

C.I.Acid Blue 78

7 2.0

CMS/PDADMAC CMS/PDADMAC

0.43 /0.67 0.2 /4.0

21 /30 2 /19

C.I.Acid Blue 78

11.7 7 2.4

DNF/PDADMAC DNF/PDADMAC DNF/PDADMAC

0.28 /0.58 0.89 /1.2 0.43 /1.0

31 /40 26 /37 25 /35

C.I.BasicYellow 12

6.5 /7

DNF/PDADMAC

1.5 /2.5

35 /50

C.I.Disperse Blue 7

easily reach them and be fixed on the positively charged sites in NPECs. It should be pointed out that most favourable for dye binding are polysaccharide derivatives with the irregularly distributed charge in their chains (CMC, CMS). For the regular macromolecules of sodium alginate a higher amount of acid is needed to obtain a desired liability of the intermolecular bonds. As a consequence, an excellent precipitation using the PDADMAC/ALG system can be observed in a very wide region of the molar ratios of both reagents used at pH 1.6. In such conditions, formation of PDADMAC/CMC complexes, as viscosity measurements [7] have shown, is inconceivable, and all the dye remains in solution. The

PDADMAC/DNF complexes are still less influenced by changes in the acidity of the solutions, because DNF is the strong aromatic sulfonic acid derivative. Addition of a neutral salt also results in formation of NPECs. However, the removal of dyes is not so effective and the higher concentrations of the salt or cationic flocculant are needed (Fig. 1). This situation can be understood assuming that the screening of charges, both positive and negative, by inorganic ions takes place in solution. Another goal of the study was to evaluate the role of hydrophobicity of dyes and polymers in the flocculation behaviour in the solutions in the presence of AN. The versatile conclusions can be

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Fig. 1. Residual C.I.Disperse Blue 7 vs. PDADMAC/CMC molar ratio: in dist. water ( /); in 0.01N NaOH (m); in 0.01N HCl (^); in 0.1N KCl (k). Total amount of polyelectrolytes added 6/10 4 M.

made. The behaviour of the piperidinium polymer PDADMAC has been compared with that of the more hydrophobic PVBTMAC in the systems with CMC. The former, as Table 3 shows, seems to be better because of a more extended flocculation region in the acid medium and greater optimal n/ n values in neutral or alkaline solutions. Probably the intramolecular interactions between the aromatic rings in the side chains cause a steric hindrance for the penetration or fixation of dye ions or small molecules. Hydrophobicity of the dyestuffs was not so important as expected. Two disperse diaminoanthraquinone dyes of the similar structure differing only in the amount of /OH groups have been examined, and data are listed in Table 3. The flocculation region of the more hydrophobic C.I.Disperse Blue 3 is slightly wider, and a smaller amount of the cationic polymer for its precipitation is needed in relation to C.I.Disperse Blue 7. There is a certain disagreement among the results of dye precipitation, obtained using dual polycation/polyanion(anion) systems in the presence of urea. Some results deny the role of hydrophobic interactions in these processes. Unexpectedly [6], addition of urea, known in terms of classical interpretations as a ‘destroyer’ of hydrophobic bonds accelerated the removal of C.I.Disperse Blue 7 by PDADMAC/DNF. To clarify this inconsistency, further experiments to evaluate the

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role of various additives (water-structure makers and water-structure breakers) are in progress. It is important also to discuss the mode of transfer of dye ions and uncharged molecules from water environment to the polymer chain and their incorporation into PECs. Fluorescence spectroscopy threw some light upon these phenomena. In the related experiments naphthalene was used as a disperse dye model, and TNS as an anionic dye model. Fluorescence spectra were been obtained for individual Nf, TNS, DNF, their dual Nf/DNF, TNS/DNF, TNS/CMC and triple systems made with an increasing amount of the cationic polymer. Some spectra are shown in Figs. 2 and 3. Fig. 2 illustrates a decrease of Nf fluorescence intensity (emission band at l/336 nm) caused by addition of DNF (curve 2). This behaviour can be explained by interaction of the aromatic rings of both chemicals, which are hydrophobic in nature. When increasing amounts of non-fluorescencing PDADMAC are added to the solution (curves 3, 4), the fluorescence spectra do not return to the former state of the pure naphthalene in water (curve 1), they show any replacement or repush of Nf back to the water environment. Simultaneously decreases the fluorescence band at l /414 nm, which can be attributed to DNF. These findings imply the formation of a Nf/DNF/PDADMAC triple complex. Obviously DNF preserves its

Fig. 2. Fluorescence spectra of Nf, Nf/DNF and Nf/DNF/ PDADMAC in water solution. Concentrations in base units related to ionogenic groups: [Nf]/4/10 5; [DNF]/6/ 10 5; [PDADMAC] /6/10 5.

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Fig. 3. Fluorescence spectra of DNF, DNF/TNS and DNF/ TNS/PDADMAC in water solution. Concentrations in base units related to ionogenic groups: [DNF]/6/10 5; [TNS]/ 4/10 5; [PDADMAC1]/3.2/10 5; [PDADMAC2]/ 4.8/10 5.

negative charge and tends to react with the cationic polyquaternary salt, bringing the nonionic dyestuff itself in a pair and playing the role of a carrier. Such is the essence of the proposed mechanism of the flocculation of non-charged textile dyes. Not all kinds of dispersing agents used in textile are able to act as the carriers. Therefore, the presence of an anionic polyfunctional compound with apolar highly hydrophobic sites (for DNF */ aromatic rings) in an optimal amount for PEC formation with a cationic flocculant is of great importance. This conclusion is supported by observations illustrated in Table 2, when disperse dyes with different commercial additives were to be precipitated. Commercial products of Disperse Blue 7 and Disperse Blue 73 consist of a coloured pigment and DNF almost in equal parts. Such dyes were successfully precipitated using appropriate amounts of PDADMAC alone. For removal of Disperse Yellow 211 dye containing a dispersing agent of another kind, both cationic polymer and DNF were needed. It might be also expected that the anionic detergent was able to act in a similar way as DNF due to an interaction of its hydrocarbon chains and disperse dye molecules. Such an expectation was indirectly confirmed by excellent clarification results obtained using pal-

mitate (Table 2) and a minimum of the cationic polymer added. CMC, CMS, ALG probably cannot serve as carriers, but their interaction with the cationic polymer helps to reduce the stability of the PECs formed. In general, most of the substances that are able to form ionic bonds with a cationic polymer lower the charge of its chains and solubility in water and reduce the probability of restabilisation, if the concentration of polyelectrolyte undesirably increases. Unexpectedly, fluorescence measurements have confirmed that dye anions can be brought to the reaction place in a similar manner as uncharged molecules. Notable changes in the fluorescence spectra of DNF in the presence of TNS (anionic dye model) have been observed (Fig. 3). It seems that repulsion of their uniform charges does not hinder a hydrophobic interaction between the nonpolar parts of the dyestuff and the anionic substance. Once PDADMAC is added to TNS/ NF solution, the intensity of the band at l /404 nm, which can be attributed to TNS/NF, decreases and a new fluorescence band arises at l/496 nm. However, the latter band is shifted to the shorter waves and its intensity is always lower in relation to the fluorescence of TNS/PDADMAC at the same concentration of the cationic polymer. Therefore the assumption of the triple TNS/

Fig. 4. Relative fluorescence intensity of TNS in the systems TNS/DNF/PDADMAC (m) and TNS/CMC/PDADMAC (k) vs. AN concentration. [TNS] /4/10 5; [PDADMAC] /6/ 10 5.

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DNF/PDADMAC complex formation seems to be reasonable. One might also expect the anionic dyes to be attached and bound to the cationic polymer without any carrier. But they inevitably must compete

(a) Nf DNF l (Nf DNF)

ions found new binding sites (most probably hydrophobic) or remained in the netting of PEC and precipitated together. Fluorescence data might be summarised by the following schemes:

PDADMAC

l

(b) TNSDNF l (TNS DNF)

(c) TNSCMC

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[(Nf DNF)+PDADMAC];

PDADMAC

l

[(TNS DNF)PDADMAC];

PDADMAC

l

[TNSPDADMACCMC] l [(PDADMACCMC) TNS];

with the other anionic compounds (detergents, thickeners, dispersing agents) if the latter are present in solutions. If the sequence of components added is changed, the competitive reactions are seen very clearly. Emission of TNS in pure water is negligible, but it becomes considerable, as is shown in Fig. 4, in the presence of a cationic polymer. That supports the assumption about an occurrence of their interaction and change of the TNS environment to a less polar one [26]. When adding increasing amounts of DNF or CMC to TNS and PDADMAC solution a certain increase in the relative fluorescence intensity of TNS has been observed up to a certain point. Probably such a behaviour is connected with conformational arrangements of the PDADMAC macromolecules and hydrophobization of the PECs formed. Further on there is a steep decrease in I /I0. It can be related to a displacement and repush of the fluorescent dye ions to the water environment. In spite of TNS release, it can be precipitated to 40/ 50% using PDADMAC and DNF or CMC in relevant molar ratios and concentrations similar to those for acid dye in Table 3. Such an inconsistency might be explained supposing that the dye

here: * */Coulombic forces; / / */hydrophobic forces; / / */dispersion forces, interpenetration. Probably the electrostatic interactions are needed only to transfer the dye ions to the nearest environment of the cationic polymer, later even their mechanical incorporation into PECs accompanied by weak Van der Waal’s dispersion forces seems to be a satisfactory reason for dye particles to be precipitated. The conformational compatibility and adaptation of macromolecular chains and the dye in such processes are certainly of great importance. There is the probability that the forces moving in such ‘steric complexes’ based on interpenetration and adaptation cannot be designated as ‘hydrophobic’ in terms of their classical interpretation. Then the explanation of the flocculation behaviour in the presence of urea might be found. However, such an assumption needs more detailed studies. Based on the above considerations, textile dye binding by a cationic flocculant in the presence of various anionic substances is a multi-step process. The first step might be the interaction caused by non-polar forces between dye particles and certain anionic textile wastewater contaminants such as

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dispersing agents, anionic detergents. As a result, the disperse dye/AN pair behaves as an anionic particle. For this reason, for effective anionic or disperse dye binding, free and easily accessible cationic groups in NPECs must be preserved. The next step is the electrostatic interactions between cationic polyelectrolyte and various anionic substances, including dye, and the main moving forces there are Coulombic. The competitive reactions between AN and dye anions can be controlled varying the pH and ionic strength of the solutions. Dye ions and molecules exhibit only a negligible affinity towards the anionic polysaccharides examined. Obviously the role of the latter consists in inserting the new easily adaptable non-polar moieties in PEC’s and reducing stability of the total system. For understanding the flocculation mechanism of the basic yellow dye, more experimental data are required. It should be admitted that its precipitation results using a dual system were not very good (Table 2), and for other basic dyes they were not satisfactory at all. Taking into account a less hydrophilic nature of the basic yellow 12 as compared to that of the other dyes, its interaction with hydrophobic sites of DNF is also possible.

4. Conclusions An active participation of various anionic textile wastewater contaminants in dye removal using cationic flocculant is postulated. Flocculation is mainly the result of a triple complex formation. Electrostatic interactions are responsible for transferring dye particles from the water environment to the polymer chain. For this reason, for effective binding free, dissociated and easily accessible cationic groups in the polymer must be preserved. Incorporation of dye ions or molecules into triple complexes occurs probably due to Coulombic and non-polar forces. Chemicals with apolar highly hydrophobic sites (presumably aromatic rings, hydrocarbon chains), which preserve their negative charge during interaction with dye ions or molecules, facilitate their transfer from water environment to the polymer.

For effective flocculation of disperse dyes the presence of such species in solutions is indispensable. The role of anionic polysaccharide derivatives often used as textile printing thickeners probably consists in inserting new easily adaptable nonpolar moieties for dye binding and reducing the stability of the total system. Interpolymeric complex formation, competitive reactions between polysaccharides and dye anions and thus the flocculation efficiency can be easily controlled varying the pH and ionic strength of the solutions.

Acknowledgements Ciba AG, Lithuanian Textile Institute and NIOPIK, Moscow, are gratefully acknowledged for supplying dyes and chemicals. The authors wish to thank Prof. H. Ho¨cker, Aachen, for helpful advice in our work.

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