This article discusses indirect electrochemical reduction of vat dyes using an iron-triethanolamine complex as a reducing agent. It describes the application and mechanism of indirect electrolysis for dye reduction. Dyeing experiments are conducted under different reduction conditions and the results are compared to a standard sodium dithionite method in terms of color depth, shade, and fastness properties. The new process offers environmental benefits and improved process stability through monitoring of reduction potential in the dye bath.

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Dyes and Pigments xx (2005) 1e8
4 www.elsevier.com/locate/dyepig 61
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8 Potentiostatic studies on indirect electrochemical reduction of vat dyes 65
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10 M. Anbu Kulandainathan a,*, A. Muthukumaran a, Kiran Patil b, R.B. Chavan b 67
11 68
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a
12 Electro Organic Division, Central Electrochemical Research Institute, Karaikudi-630 006, India 69
b
13 Department of Textile Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi-1100016, India 70
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14 Received 10 May 2005; received in revised form 12 July 2005; accepted 10 October 2005 71
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Abstract
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19 76
20 Dispersed vat dyestuffs can be electrochemically reduced by indirect electrolysis using ironetriethanolamine complex as a reducing agent. 77
21 The application and mechanism of indirect electrolysis as a reduction technique are described in detail in this paper. Electrochemically reduced 78
22 vat dye is tested on a laboratory scale in dyeing experiments, and the results of different reduction conditions are discussed. The influence of the 79
23 concentration of the complex-system on the build-up of colour depth, shade and fastness is discussed and compared with samples of the standard 80
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24 dyeing procedure using sodium dithionite as the reducing agent. The new process offers environmental benefits as well as prospects for improved 81
25 process stability, because the state of reduction in the dye-bath can be readily monitored by measuring the reduction potential. 82
26 Ó 2005 Elsevier Ltd. All rights reserved. 83
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29 86
30 1. Introduction a-hydroxyketone which meets requirements in terms of reduc- 87
31 tive efficiency and biodegradability had been tried. However, 88
32 Vat dye is most popular among dye classes used for colora- such compounds are expensive and their use is restricted to 89
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33 tion of cotton, particularly, when high fastness standards are closed systems due to the formation of strong smelling con- 90
34 required to light, washing and chlorine bleaching [1]. Also, densation products in an alkaline solution [7,8]. Some other 91
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35 from the commercial point of view, vat dyes (including indigo) sulphur containing compounds such as hydroxyalkyl sulphi- 92
36 have acquired a large share in the dyestuff market for the col- nate, thiourea, etc., have also been recommended [9,10]. These 93
37 oration of cellulosic fibres. The annual consumption of vat dyes compounds have relatively low amount of sulphur content and 94
38 including indigo is around 33,000 metric tons since 1992 and it also lower equivalent mass which leads to lower sulphur-based 95
39 holds 24% of cellulosic fibre dye market in value terms [2]. salt load in the wastewater. However, in these cases too, it is 96
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40 Vat dyes being water insoluble have to be first converted not possible to dispense with sulphur-based problems totally. 97
41 into water soluble form (leuco dye) by reduction with a strong Fe(OH)2 being a strong reducing agent in alkaline medium, 98
42 reducing agent like sodium dithionate. In its reduced form, the the possibility of using it has also been explored for reducing 99
43 vat dye has substativity towards cellulosic fibres and, after organic dyestuffs. The reducing effect of Fe(OH)2 increases 100
44 absorption, is reoxidised to the original water-insoluble form with increase in pH. However, Fe(OH)2 is poorly soluble in 101
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45 in situ in the fibre [3,4]. The use of sodium dithionate is being an alkaline solution and gets precipitated. It has to be com- 102
46 criticized for the formation of non-environment friendly plexed in order to hold it in solution [11]. A stable complex 103
47 decomposition products such as sulphite, sulphate, thiosulphate with good reducing power is obtained with weaker ligand, 104
48 and toxic sulphur [5,6]. such as gluconic acid. Regarding eco-friendliness, gluconic 105
49 Attempts are being made to replace the sodium dithionite acid can be eliminated in the sewage tank through neutraliza- 106
50 by ecologically more attractive alternatives. In this connection tion with alkali; free Fe(OH)2 can be aerated and converted to 107
51 Fe(OH)3 which acts as a flocculent and brings down wastewa- 108
52 ter load. Tartaric acid has also been tried as a ligand by Chak- 109
53 raborty for complexing Fe(OH)2 in the presence of NaOH for 110
* Corresponding author. Tel.: þ91 4565 227772; fax: þ91 4565 227779.
54 E-mail address: manbu123@yahoo.com (M.A. Kulandainathan). reduction and dyeing of cotton with indigo and other vat dyes 111
55 112
56 0143-7208/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. 113
57 doi:10.1016/j.dyepig.2005.10.007 114
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115 at room temperature [12]. Efforts have been made to complex chemicals. The mediator system necessary for indirect electro- 172
116 Fe(OH)2 with single and double ligand systems using tartaric chemical reduction of vat dyes was prepared in an alkaline 173
117 acid, citric acid and gluconic acid which have shown encour- medium from triethanolamine and ferric sulphate, which 174
118 aging results [13]. were analytical grade chemicals. Potentiometric titrations 175
119 Electrochemical reduction of vat, indigo and sulphur dyes were carried out to measure the dye reduction potential using 176
120 is also suggested as an alternative route for ecological and eco- K4Fe(CN)6 as an oxidizing agent which was also an analytical 177
121 nomic reasons. Electrochemical reduction can be achieved by grade chemical. Dye systems investigated were commercial 178
122 direct and indirect electrochemical reduction. In direct electro- products from Atul Ltd.: Novatic Yellow 5G (CI Vat Yellow 179
123 chemical reduction chemical reducing agents are replaced by 2), Novinone Brown RRD (CI Vat Brown 5), Novinone Green 180
124 electrons from electric current, and effluent contaminating FFB (CI Vat Green 1), Novinone Blue RSN (CI Vat Blue 4), 181
125 substances can be dispensed with altogether [14,15]. Although Novinone Blue BO (CI Vat Blue 20), Novinone Brown BR 182
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126 this technique is ideal, the stability of reduced dye species (CI Vat Brown 1), Novinone Black BB (CI Vat Black 36), 183
127 formed is poor thereby affecting the colour yield and the Novinone Brill. Violet RR (CI Vat Violet 1) and Novinone 184
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128 rate limiting step of electrochemical reduction is electron Black CH. 185
129 transfer from the cathode surface to the surface of the micro- 186
130 crystal of the dispersed dye pigment [16e20]. In indirect elec- 2.2. Dyeing procedure and process control 187
131 trochemical reduction technique the reduction of dye is 188
132 achieved through a redox mediator system [21,22]. This pro- 2.2.1. Conventional dyeing 189
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133 cess is mediated by an electron carrier whereby reduction Conventional vat dyed samples were prepared in the labo- 190
134 takes place between surfaces of the electrode and the dye pig- ratory, with a standard procedure using sodium dithionite 191
135 ment instead of direct contact between both surfaces. Among and sodium hydroxide as described elsewhere [13]. 192
136 the various mediator systems suggested in the literature, irone 193
137 triethanolamine complex (ironeTEA) seems to be promising 2.3. Electrochemical dyeing 194
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138 [23e29]. In order to meet all requirements in an economical 195
139 way, a multi-cathode cell with a large number of cathodes, 2.3.1. Design of the electrochemical cell 196
140 electrically connected with one or two anodes, has been sug- The two-compartment electrochemical cell consists of 197
141 gested [30,31]. This configuration allows the operation of a rectangular polyvinyl chloride vessel containing the capacity 198
142 the cell with a maximum area cathode and minimum area an- of 1 l catholyte and 0.1 l anolyte and the separator used was 199
CT
143 ode [32e34]. Very recently, Bechtold et al. have demonstrated 423 Nafion membrane. The anode used was a thin stainless 200
144 that mediator system obtainable by mixing one or more salts steel rod with the surface area of 6 cm2. A three-dimensional 201
145 of a metal capable of forming a plurality of valence states copper wire electrode with a surface area of 500 cm2 served as 202
146 with at least one amino-containing complexing agent and at the cathode. The cathode compartment was thus providing po- 203
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147 least one hydroxyl-containing but amino-devoid complexing rous flow through electrode filling the cathode compartment as 204
148 agent in an alkaline medium has the improved capacity to coil. The catholyte was agitated by an analytical rotator at 205
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149 reduce the vat dyes [35,36]. a constant speed to create homogeneous and stable conditions 206
150 Both the electrochemical reduction techniques are not yet in the cathode chamber. The dye-bath of the dyeing apparatus 207
151 commercialized and research and developmental efforts are was circulated through the electrochemical reactor for contin- 208
152 in progress in this direction. Here, the most challenging engi- uous renewal of the reducing capacity of the dye-bath. 209
153 neering task is to achieve a dye reduction rate and a current A schematic drawing of the dye-bath circulation through 210
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154 efficiency, which are high enough to make electrochemical the laboratory dyeing unit and the catholyte circulation 211
155 reaction industrially feasible. through the electrochemical cell is given in Fig. 1. 212
156 In the present study attempts are being made to understand 213
157 the fundamentals of indirect electrochemical reduction of 2.3.2. Determination of reduction potential of vat dyes 214
158 selected vat dyes using ironeTEA complex as a mediator. Here, the reduction potential of the dye is referred as the 215
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159 Essential requirements for the design of electrochemical cell potential developed on the bright platinum electrode vs the ref- 216
160 are suggested. IroneTEAeNaOH molar ratio has been stan- erence electrode when dipped in the dye-bath in which the dye 217
161 dardized to get the dyeing of cotton by indirect electrochemi- gets just solubilised. In order to measure the reduction poten- 218
162 cal reduction technique. Colour yields are compared with tial of vat dyes, a titration was carried out using bright plati- 219
163 those of conventional sodium dithionate method. Repeated num electrode with Hg/HgO/OHÿ as a reference electrode. 220
164 use of dye-bath after dye separation is also explored. A solution of 100 mg vat dye was prepared which was then re- 221
165 duced with 25 ml of 0.1 M NaOH and 100 mg Na2S2O4. This 222
166 2. Experimental solution was then diluted to 50 ml with distilled water and then 223
167 titrated against 0.05 M K4[Fe(CN)6]. The K4[Fe(CN)6] solu- 224
168 2.1. Materials tion was added in the steps of 0.5 ml and the dye solution po- 225
169 tential was measured at every step. The volume of 0.05 M 226
170 The sodium dithionite and sodium hydroxide used for vat K4[Fe(CN)6] solution is then plotted against the dye solution 227
171 dyeing by conventional method were laboratory grade potential in order to find out dye reduction potential. 228
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229 Analytical rotator 286
230 287
231 + - 288
232 289
233 290
234 291
235 292
236 293
237 294
238 295
239 296
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240 Anode: SS Cathode: Cu Peristaltic 297
Anolyte: Catholyte: Pump 1 Dye bath Peristaltic
241 Mediator
298
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(Mediator system 175 ml/min Pump 2
242 100 ml 299
+ dye) 1200ml 175 ml/min
243 300
244 301
245 Electrochemical 302
246 cell 303
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247 304
248 305
Cu wire to measure solution potential w.r.t. Reference electrode
249 306
250 307
Reference electrode (Hg / HgO/OH-)
251 308
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252 Fig. 1. Schematic representation of the electrochemical flow cell. 309
253 310
254 311
2.3.3. Dyeing procedure mediator system should be made as low as possible. High con-
255 312
Selected vat dyes were dyed under the standardized condi- centrations of chemicals in the dye-bath enable more stable re-
256 313
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tions of the mediator system as described in Section 3. While duction conditions and prevent the oxidation of the reduced
257 314
following the electrochemical dyeing technique, the dye-bath dyestuff. However, if we use the mediator solution towards
258 315
was circulated through the cathodic compartment of the elec- its higher concentration end then there will be an eventual in-
259 316
trolytic cell for continuous renewal of the reducing capacity. crease in the loss of chemicals in the recycling loop and also
260 317
All the experiments were performed under potentiostatic con- carried along the fabric at the end of dyeing operation. This
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261 318
ditions (ÿ1050 mV vs Hg/HgO/OHÿ) at room temperature. makes the process uneconomical.
262 319
The catholyte was called as a mediator solution which com-
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263 320
prised triethanolamine, ferric sulphate and sodium hydroxide. Table 1
264 321
The potential prevailing in the catholyte, during electrolysis, Different composition of the mediator solutions used for dyeing experiments
265 322
was measured with a copper wire vs a reference electrode Mediator solution pH Mediator solution composition
266 323
(Hg/HgO/OHÿ). The catholyte was agitated in the catholyte Fe2(SO4)3 (M) TEA (M) NaOH (M)
267 324
compartment itself and was also circulated through the dye-
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268 Group I 325
bath continuously. When the potential achieved in the catho- 1.1 14 0.025 0.30 0.357
269 326
lyte was equivalent to the reduction potential of the dyestuff, 1.2 14 0.03 0.35 0.434
270 327
the dyestuff was introduced into the catholyte compartment. 1.3 14 0.035 0.40 0.500
271 1.4 14 0.04 0.45 0.570 328
In all the experiments 2% of weight of fabric dye was used
272 1.5 14 0.045 0.50 0.643 329
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in the dyeing recipe. The fabric sample was introduced into
273 Group II
330
the dye-bath after 10 min of introduction of dyestuff in order
274 2.1 10.5 0.03 0.35 0.185 331
to allow reduction of the dyestuff. Experiments were carried
275 2.2 12 0.03 0.35 0.225 332
out at a relatively large liquor ratio i.e. 240:1. The dyeing 2.3 13 0.03 0.35 0.275
276 333
was continued in the dye-bath for 1 h with constant agitation 2.4 14 0.03 0.35 0.315
277 334
of the fabric sample with glass rods, while electrolysis and cir- 2.5 14 0.03 0.35 0.375
278 2.6 14 0.03 0.35 0.429
335
culation of the catholyte were in progress. After dyeing the
279 2.7 14 0.03 0.35 0.480 336
sample was withdrawn from the dye-bath, air oxidized, cold
280 337
rinsed, soaped at boil, cold rinsed and air dried. Group III
281 3.1 14 0.025 0.30 0.43 338
282 3.2 14 0.025 0.315 0.43 339
283 2.3.4. Optimization of mediator solution 3.3 14 0.025 0.33 0.43 340
284 In order to make the indirect electrochemical dyeing tech- 3.4 14 0.025 0.345 0.43 341
3.5 14 0.025 0.360 0.43
285 nique eco-friendly and economical, the concentration of the 342
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343 Table 2 400
344 Reduction potential of selected vat dyes 401
345 Dye Reduction potential (V) Dye Reduction potential (V) Dye Reduction potential (V) 402
346 Novatic Yellow 5G ÿ0.818 Novinone Blue RSN ÿ0.880 Novinone Black BB ÿ0.952 403
347 Novinone Brown RRD ÿ0.841 Novinone Blue BO ÿ0.903 Novinone Brill. Violet RR ÿ0.953 404
348 Novinone Green FFB ÿ0.850 Novinone Brown RR ÿ0.935 Novinone Black CH ÿ0.964 405
349 Reduction potential (V vs Hg/HgO/OHÿ). 406
350 407
351 Indirect electrochemical dyeing was carried out with the se- 3. Results and discussion 408
352 lected dyestuffs at different mediator solution concentrations 409
353 in three sets of mediator systems as shown in Table 1. In group 3.1. Reduction potential of vat dyes 410
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354 I, the ferric sulphate to TEA and ferric sulphate to sodium hy- 411
355 droxide molar ratio was kept constant as 1:11.48 and 1:14.3, The reduction potential of vat dyes gives an idea about the 412
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356 respectively as described elsewhere [13]. In group II, the ferric level of difficulty involved in their reduction. Higher the 413
357 sulphate to TEA molar ratio was kept constant at 1:11.48 reduction potential of the dye, higher is the reducing power 414
358 while the ferric sulphate to sodium hydroxide molar ratio required in the dye-bath for its reduction. Thus, from the 415
359 was varied. Whereas in group III, the ferric sulphate to sodium reduction potential values, we can group the dyes under con- 416
360 hydroxide molar ratio was kept constant at 1:14.3 and the fer- sideration in three groups viz. easy to reduce, normal to reduce 417
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361 ric sulphate to TEA molar ratio was varied (Table 2). and difficult to reduce. 418
362 The mediator solution was prepared according to the proce- While following the indirect electrochemical dyeing tech- 419
363 dure described elsewhere [14]. Caustic soda was dissolved in nique, three dyestuffs such as, Green FFB (an easy to reduce 420
364 a small amount of water in which TEA was added. Ferric sul- dye), Blue RSN (a normal to reduce dye) and Violet RR (a 421
365 phate was separately dissolved in a small amount of water and hard to reduce dye) were selected. 422
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366 then added to the TEA/caustic soda mixture with continuous 423
367 stirring with the magnetic stirrer. After the complete dissolu- 424
368 tion of the precipitated iron oxide, the solution was diluted 3.2. Dyeing results 425
369 to the full volume. The solution was subjected to constant stir- 426
370 ring for 90 min. The rates of potential development while carrying out po- 427
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371 tentiostatic electrolysis in the group I, II and III dyeing experi- 428
372 ments with Violet RR dye only are shown in Figs. 2, 3 and 4, 429
2.3.5. Material to liquor ratio experiments
373 respectively. In each dyeing experiment, the dye was intro- 430
As the electrolysis cell was not optimized with regard to
374 duced in the cathodic chamber as soon as the reduction poten- 431
short liquor ratios, experiments were carried out at a relatively
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375 tial of Violet RR had reached i.e. ÿ953 mV. The final dye-bath 432
large material to liquor ratio i.e. 1:240. To understand the ef-
376 potential attained at the end of dyeing in few cases was lower 433
fect of the various materials to liquor ratios on the colour
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377 than that attained during the course of dyeing, especially while 434
depth developed on the fabric samples by indirect electro-
378 dyeing with Green FFB and Blue RSN. 435
chemical dyeing, experiments were also carried out with
379 436
1:120, 1:80 and 1:60 material to liquor ratios by making
380 437
appropriate modifications in the system.
381 438
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382 1000 439
Solution Potential (-mV vs Hg/HgO/OH-)
383 2.3.6. Recycling of mediator solution experiments 440
384 After completing the dyeing experiment, the remaining dye 441
385 and reduced mediator were oxidized by bubbling air though 900
442
386 the solution to form an insoluble dye. 443
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387 The oxidized dye particles can be segregated out through 444
800
388 the vacuum filtration using G3 type of porcelain membrane. 445
389 The clear filtered out mediator solution was recycled further 446
390 to carry out dyeing experiments. 700 Mediator index: 1.1
447
Mediator index: 1.2
391 448
Mediator index: 1.3
392 2.3.7. Dyed samples evaluation 600 Mediator index: 1.4
449
393 Results of the dyeing experiments were characterized by Mediator index: 1.5
450
394 colour measurement in the form of K/S and CIELab coordi- 451
395 nates with the help of spectrophotometer X4000 (Jaypak) us- 500 452
-50 0 50 100 150 200 250 300 350
396 ing D65 light source, 10 viewing angle. 453
Time (min)
397 Dyed samples were also evaluated for wash fastness 454
398 (SOURCES: IS 764; 1979), light fastness (SOURCE: IS Fig. 2. Dye-bath potential development with varying ferric sulphate concentration 455
399 2454; 1985) and rubbing fastness (SOURCE: IS 766; 1988). (group I) electrolysis with Violet RR dye. 456
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457 Table 3 514
Mediator index:2.1 Comparison of colour yield and colour coordinates of conventional and
458 1000 515
Solution Potential (-mV vs Hg/HgO/OH-)
Mediator index:2.2 electrochemical dyeing with Green FFB
459 Mediator index:2.3
516
460 Mediator solution Final dye-bath potential K/S L* a* b* 517
900 Mediator index:2.4 index (V vs Hg/HgO/OHÿ)
461 Mediator index:2.5 518
462 With dithionite* 15.4626 35.44 ÿ35.20 ÿ0.98 519
Mediator index:2.6
800 1.1* ÿ0.860 5.1460 55.37 ÿ39.92 ÿ2.67
463 Mediator index:2.7 1.2* ÿ0.880 4.7258 53.39 ÿ32.58 ÿ6.24
520
464 1.3* ÿ0.870 5.6865 52.95 ÿ38.25 ÿ3.72 521
465 700 1.4* ÿ0.874 3.3465 58.97 ÿ33.46 ÿ4.71 522
466 1.5* ÿ0.996 4.1874 57.42 ÿ36.84 ÿ3.27 523
467 600 Initial dye-bath potential before starting the electrolysis in each case was 524
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468 ÿ0.350 V vs Hg/HgO/OHÿ. 525
469 526
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500
470 527
471 cell due to the deposition of a block material as a hard layer on 528
0 50 100 150 200 250 300 350
472 Time (min)
the cathode. As can be seen from the corresponding K/S values 529
473 in Table 3, the colour depth appears to be increasing with an in- 530
474 Fig. 3. Dye-bath potential development with varying sodium hydroxide crease in NaOH concentration above pH 13. 531
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475 concentration (group II) during electrolysis with Violet RR dye. There is no effect of the TEA concentration variation in the 532
476 mediator solution in the given range of group III experiments 533
477 on the colour depth and shade. Also the solution potential de- 534
478 From the variation of the CIELab coordinates from the velopment rate is found to be unaffected (Fig. 4). However, at 535
479 group I experiments as shown in Table 3, it becomes clear 44.75 g/l TEA, the iron complex becomes unstable and ferric 536
that the colour depth and shade did not show strong depen-
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480 hydroxide is found to be precipitating. 537
481 dence on the iron salt concentration in the mediator solution. 538
482 However, the depth appears to be good around 12e14 g/l fer- 539
ric sulphate concentration. The potential development rate ap- 3.2.1. Influence of MLR on colour yield
483 540
pears to be increasing with increase in ferric sulphate All the dyeing experiments for mediator solution optimiza-
484 541
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concentration as can be seen in Fig. 2. tion were carried out at 1:240 material to liquor ratio. The
485 542
In case of group II experiments, the solution potential could depth of the dyed samples was very low as compared to the
486 543
not be developed above ÿ953 mV vs Hg/HgO/OHÿ while conventionally dyed samples which were dyed at 1:30 material
487 544
working up to pH 13. Therefore dyeing was not possible during to liquor ratio. In order to investigate the possibilities of im-
488 545
working with NaOH concentration below 11 g/l in the mediator proving the colour depth, dyeing experiments were carried
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489 546
solution. During electrolysis of the solution with pH below 13, out with Blue RSN dye at 1:240, 1:120, 1:80 and 1:60 material
490 547
the complex formed was found to be unstable and caused addi- to liquor ratios using 1.2 mediator index solution.
RR
491 548
tional problems in the proper functioning of the electrochemical Values of K/S and CIELab coordinates of dyeing experi-
492 549
ments are given in Table 4 which show marginal improvement
493 550
in colour depth of the dyed samples with shorter material to
494 551
liquor ratios.
495 552
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496 1000 553
Solution Potential (-mV vs Hg/HgO/OH-)
497 3.2.2. Recycling of mediator solution 554
498 Dyeing without effluent i.e. dye-bath recycling seems to 555
900
499 be very attractive in the present environment conscious era. 556
500 557
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501 800 558
Mediator index:3.1
502 Table 4 559
Mediator index:3.2 Comparison of colour yield and colour coordinates of conventional and
503 700 560
Mediator index:3.3 electrochemical dyeing Blue RSN
504 Mediator index:3.4
561
505 Mediator solution Final dye-bath potential K/S L* a* b* 562
600 Mediator index:3.5
index (V vs Hg/HgO/OHÿ)
506 563
507 With dithionite 9.1666 35.94 6.62 ÿ38.03 564
500 1.1 ÿ0.875 2.6641 52.92 ÿ3.29 ÿ27.93
508 565
1.2 ÿ0.900 3.6045 48.44 0.58 ÿ32.87
509 1.3 ÿ0.890 2.2319 55.17 ÿ2.25 ÿ28.24 566
-20 0 20 40 60 80 100 120 140 160
510 1.4 ÿ0.905 2.5897 52.71 0.23 ÿ31.25 567
Time (min)
511 1.5 ÿ0.983 1.9702 56.10 ÿ1.10 ÿ27.58 568
512 Fig. 4. Dye-bath potential development with varying TEA concentration Initial dye-bath potential before starting the electrolysis in each case was 569
513 (group III) during electrolysis with Violet RR dye. around ÿ0.350 V vs Hg/HgO/OHÿ. 570
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571 Table 5 Table 7 628
572 Comparison of colour yield and colour coordinates of conventional and Regeneration of mediator solution 629
electrochemical dyeing with Violet RR Recycling step Fresh solution Absorbance
573 630
574 Mediator solution Final dye-bath potential K/S L* a* b* addeda (ml) Before oxidation After filtration 631
index (V vs Hg/HgO/OHÿ) and filtration
575 632
576 With dithionite 15.73 23.10 16.93 ÿ23.13 Original 0 1.384 0.072 633
1.1 ÿ0.922 0.2810 77.92 10.16 ÿ9.76
577 1 30 1.533 0.027 634
1.2 ÿ0.996 3.0499 47.82 22.82 ÿ28.25 2 41 1.403 0.050
578 1.3 ÿ0.994 3.2752 47.87 26.70 ÿ30.07 635
3 28 1.417 0.079
579 1.4 ÿ0.979 2.4236 52.75 26.42 ÿ27.63 4 42 1.420 0.070
636
580 1.5 ÿ0.1001 2.9228 49.78 26.78 ÿ29.23 637
a
2.1 ÿ0.539 Dyeing not possible Fresh solution was added to make the total volume to 1 l before starting
581 each experiment.
638
ÿ0.545
F
2.2 Dyeing not possible
582 2.3 ÿ0.584 Dyeing not possible
639
583 2.4 ÿ0.987 2.0706 54.97 25.45 ÿ26.87 640
OO
584 2.5 ÿ0.993 2.2632 53.53 25.15 ÿ26.66 641
585 2.6 ÿ0.1001 3.1007 48.72 26.54 ÿ30.03 642
586 2.7 ÿ0.1001 4.4808 43.71 28.11 ÿ30.84 643
3.1 ÿ0.1002 2.8823 49.71 26.59 ÿ30.50
587 3.2 ÿ0.992 3.5124 47.04 27.51 ÿ30.79 Table 8 644
588 3.3 ÿ0.997 3.9246 45.32 27.49 ÿ31.02 Colour yield and CIELab coordinates for repeated dyeing with regenerated 645
PR
589 3.4 ÿ0.1001 3.0499 47.82 22.68 ÿ28.25 mediator solution 646
590 3.5 ÿ0.995 3.1660 48.79 26.99 ÿ28.83 Recycling step Final dye-bath potential K/S L* a* b* 647
591 Initial dye-bath potential before starting the electrolysis in each case was (V vs Hg/HgO/OHÿ) 648
592 around ÿ0.350 V vs Hg/HgO/OHÿ. Original ÿ0.995 2.4954 52.95 0.62 ÿ31.06 649
593 1 ÿ0.965 2.3803 53.82 ÿ0.97 ÿ29.35 650
2 ÿ0.950 2.8766 51.25 ÿ0.55 ÿ30.31
ED
594 3 ÿ0.958 2.6414 52.39 ÿ0.39 ÿ29.67
651
595 4 ÿ0.953 3.0783 50.65 ÿ1.09 ÿ30.50 652
596 653
597 654
Table 6
598 655
CT
Effect of ML ratios on colour yield and CIELab coordinates
599 656
MLR Final dye-bath potential K/S L* a* b*
600 (V vs Hg/HgO/OHÿ)
657
601 658
1:240 ÿ0.900 3.6045 48.44 0.58 ÿ32.87
602 1:120 ÿ0.958 2.9914 50.85 ÿ0.72 ÿ30.43
659
E
603 1:80 ÿ0.950 3.0879 50.49 ÿ1.65 ÿ29.25 Table 9 660
604 1:60 ÿ0.965 4.0401 46.55 ÿ1.01 ÿ29.91 Comparison of fastness properties of electrochemical and conventional dyeing 661
RR
605 Reduction/mediator Washing Light Rubbing fastness 662
606 system fastness fastness Wet Dry 663
607 a 664
With dithionite 4e5 5 4 5
608 With dithioniteb 4e5 5 4 5 665
609 With dithionitec 4e5 5 4 4e5 666
CO
610 0.025 M Fe2(SO4)3 þ 4e5 4e5 4e5 5 667
1000 0.345 M TEA þ 0.45 M
611 668
Solution Potential (-mV vs Hg/HgO/OH-)
NaOH (MLR 1:240)a
612 669
0.025 M Fe2(SO4)3 þ 4 4e5 4 4e5
613 900 0.345 M TEA þ 0.45 M 670
614 NaOH (MLR 1:240)b 671
UN
615 0.025 M Fe2(SO4)3 þ 4e5 5 4 4e5 672
800
616 0.345 M TEA þ 0.45 M 673
NaOH (MLR 1:240)c
617 Mediator Index:1.2
674
0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5
618 700 Original 0.345 M TEA þ 0.45 M 675
619 First Recycle NaOH (MLR 1:60)a 676
620 600 Second Recycle 0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5 677
621 Third Recycle 0.345 M TEA þ 0.45 M 678
NaOH (MLR 1:60)b
622 Fourth Recycle 679
500 0.025 M Fe2(SO4)3 þ 4 4e5 4e5 5
623 0.345 M TEA þ 0.45 M 680
624 NaOH (MLR 1:60)c 681
0 20 40 60 80 100 120
625 a
With Green FFB. 682
Time (min)
626 b
With Blue RSN. 683
c
627 Fig. 5. Effect of mediator recycling on the dye-bath potential development. With Violet RR. 684
DYPI2176_proof Š 11 November 2005 Š 6/8 Š e-annotations