POTENTIOSTATIC STUDIES ON INDIRECT ELECTROCHEMICAL REDUCTION OF VAT DYEProf. R. B. Chavan* and Kiran Patil, Department of Textile Technology, Indian Institute of Technology, Delhi, Hauz Khas, New Delhi – 1100016 (India) & M. Anbu Kulandainathan, Scientist, Central Electrochemical Research Institute, Karaikudi, Tamil nadu –630006 (India) Abstract Dispersed vat dyestuffs can be reduced by indirect electrolysis using iron-triethanolamine complex. The application of indirect electrolysis as a reduction techniqueis described along with the mechanism. Electrochemically reduced vat dye is tested inlaboratory scale dyeing experiments, and the results of different reduction conditions arediscussed. The influence of the concentration of the complex-system on the build-up ofcolour depth, shade and fastness is discussed and compared with samples of the standarddyeing procedure using sodium dithionite as reducing agent. The new process offersenvironmental benefits and offers the prospects of improved process stability, because thereduction state in the dyebath can be readily monitored by measuring reduction potential.
Introduction Vat dye is one of the most important among the dye classes used for colourationof cotton, particularly, when high fastness standards are required to light, washing andchlorine bleaching.1 Also, from the commercial point of view, vat dyes (including indigo)acquires a large share in the dyestuff market for the colouration of cellulosic fibres. Theannual consumption of vat dyes including indigo is around 33,000 metric tons since 1992and it holds 24% of cellulosic fibre dye market in value terms.2 Vat dyes being water insoluble have to be first converted into water soluble formby reduction with strong reducing agent like sodium dithionate. The use of sodiumdithionate is being criticized for the formation of non-environment friendlydecomposition products such as sulphite, sulphate, thiosulphate and toxic sulphur. 3,4 Attempts are being made to replace the sodium dithionite by ecologically moreattractive alternatives. α-hydroxyketone which meets requirements in terms of reductiveefficiency and biodegradability had been tried. However such compounds are expensiveand their use is restricted to closed systems due to its strong smelling condensationproducts in alkaline solution.5 Some other sulphur containing compounds likehydroxyalkyl sulphinate, thiourea etc, have also been recommended recently. 6 Thesecompounds have relatively low amount of sulphur content and also have lower equivalentmass which leads to lower sulphur based salt load in the waste water. However, in thesecases too, it is not possible to dispense with sulphur based problems totally. Being Fe(OH)2 as a strong reducing agent in alkaline medium, this possibility hasalso been explored for reducing organic dyestuffs. The reducing effect of Fe(OH)2
increases with increase in pH. However, Fe(OH)2 is poorly soluble in alkaline solutionand gets precipitated. It must be complexed in order to hold Fe(OH) 2 in solution7. A stablecomplex with reducing power is obtained with weaker ligands, e.g. gluconic acid.Regarding eco-friendliness, gluconic acid can be eliminated in the sewage tank throughneutralization with alkali; free Fe(OH)2 can be aerated and converted to Fe(OH)3 whichacts as a flocculent and reduces wastewater load. Tartaric acid has also been tried as aligand by Chavan and Chakraborty for complexing Fe(OH)2 in presence of NaOH forreduction and dyeing of cotton with indigo and other vat dyes at room temperature.8Efforts have been made to complex Fe(OH)2 with single and double ligand systems usingtartaric acid, citric acid and gluconic acid which has shown optimistic results. Recently electrochemical reduction of vat, indigo and sulphur dyes is suggested.There are two ways by means which electrochemical reduction is achieved. Direct andindirect electrochemical reduction. In direct electrochemical reduction the chemicalreducing agents are replaced by electrons from electric current, and effluentcontaminating substances can be dispensed with all together.9,10 Although this techniqueis ideal, the stability of reduced dye species is poor affecting the colour yield. In Indirectelectrochemical reduction technique the dye reduction is achieved through a redoxmediator system. Among the various mediator systems suggested in the literature, iron-triethanolamine complex (iron-TEA) seems to be promising. 11,12 Both the electrochemical reduction techniques are not yet commercialized andresearch and development efforts are in progress in this direction. In the present paper attempts are being made to understand the fundamentals ofindirect electrochemical reduction of selected vat dyes using iron-TEA complex as
mediator. Essential requirements for the design of electrochemical cell are suggested.Iron-TEA-NaOH molar ratio has been standardized to get the dyeing of cotton byindirect electrochemical reduction technique. The colour yields are compared withconventional sodium dithionate method. The repeated use of dye bath after dyeseparation is explored.ExperimentalMaterials.-The sodium dithionite and sodium hydroxide used for vat dyeing byconventional method were laboratory grade chemicals. The mediator system necessaryfor indirect electrochemical reduction of vat dyes was prepared in alkaline medium fromtriethanolamine and ferric sulphate which were analytical grade chemicals. Thepotentiometric titrations were carried out to measure the dye reduction potential by usingK2Fe(CN)6 as an oxidizing agent which was too an analytical grade chemical. The dyesystems investigated were commercial products from Atul Ltd: Novatic Yellow 5G (CIVat Yellow 2), Novinone Brown RRD (CI Vat Brown 5), Novinone Green FFB (CI VatGreen 1), Novinone Blue RSN (CI Vat Blue 4), Novinone Blue BO (CI Vat Blue 20),Novinone Brown BR (CI Vat Brown 1), Novinone Black BB (CI Vat Black 36),Novinone Brill. Violet RR (CI Vat Violet 1) and Novinone Black CH.Dyeing procedure and process controlConventional dyeing.-The conventionally vat dyed samples were prepared in thelaboratory, with a standard procedure using sodium dithionite and sodium hydroxide.8
Electrochemical dyeingDesign of an electrochemical cell.-The electrochemical cell consists of a rectangularpolyvinyl chloride vessel containing 1 l catholyte and 0.1 l anolyte. The anolytes usedwere the same solutions which were being used as the catholytes in every case. Theanode used was thin stainless steel rod with the surface area of 6 cm 2. A three-dimensional copper wire electrode with a surface area of 500 cm2 was served as thecathode. The cathode compartment was thus providing porous flow through electrodefilling the cathode compartment as coil (depth 10cm, front area cm2). The anolyte wasseparated from the catholyte by a diaphragm (cation exchange membrane) to prevent thetwo electrolytes from mixing. The catholyte was agitated by an analytical rotator at aconstant speed to guarantee homogeneous and stable conditions in the cathode chamber.The dyebath of the dyeing apparatus was circulated through the electrochemical reactorfor continuous renewal of the reducing capacity of the dyebath. In accordance withtechnical conditions, the electrolysis experiments did not involve an inert atmosphere. A schematic drawing of the dyebath circulation through the laboratory dyeing unitand the catholyte circulation through the electrochemical cell is given in Fig. 1.
Analytical rotator + - Anode: SS Cathode: Cu Peristatic Peristatic Anolyte: Catholyte: Dyebath Pump 1 Pump 2 Mediator (Mediator system 175 ml/min 175 ml/min 100 ml + dye) 1200ml 1000 ml Electrochemical cell Cu wire to measure solution potential w.r.t. reference electode Reference electrode (Hg / HgO/OH-) Fig. 1. Schematic representation of the electrochemical flow cellDetermination of reduction potential of vat dyes.-Here, the reduction potential of thedye is referred as the potential developed on the bright platinum electrode when dipped inthe dye bath versus the reference electrode at which the dye gets just solubilized. Inorder to measure the reduction of the vat dyes, the titration was carried out using brightplatinum electrode with Ag/AgCl/3M KCl as a reference electrode. A solution of 100mgvat dye was prepared which was reduced with 25ml of 0.1M NaOH and 100mg Na2S2O4.This solution was then diluted to 50 ml with distilled water and then titrated against0.05M K2Fe(CN)6. The K2Fe(CN)6 solution added in the steps of 0.5ml and the dye
solution potential measured at every step. The ml of 0.05M K2Fe(CN)6 solution is thenplotted against the dye solution potential in order to find out dye reduction potential.Dyeing Procedure.-Selected vat dyes were dyed under the standardized conditions of themediator system as described in results and discussion section. While following theelectrochemical dyeing technique, the dyebath was circulated through the cathodiccompartment of the electrolytic cell for continuous renewal of the reducing capacity. Allthe experiments were performed under potentiostatic conditions (-1050 mV) at roomtemperature. The catholyte was called as a mediator solution which was comprised oftriethanolamine, ferric sulphate and sodium hydroxide. The potential prevailing in thecatholyte, while electrolysis, was measured with a copper wire versus a referenceelectrode (Hg/HgO/OH-). The catholyte was agitated in the catholyte compartment itselfand was also circulated through the dyebath continuously. When the potential achieved inthe catholyte was equivalent to the reduction potential of the dyestuff, the dyestuff wasintroduced into the catholyte compartment. In all the experiments 2% on weight of fabricdye was used in the dyeing recipe. The fabric sample was introduced into the dyebathafter ten minutes of introduction of dyestuff in order to allow reduction of the dyestuff.The experiments were carried out at a relatively long liquor ratio i.e. 240:1. The dyeingwas continued in the dyebath for one hour with constant agitation of the fabric samplewith glass rods by hand, while electrolysis and circulation of the catholyte was on going.After dyeing the sample was withdrawn from the dye bath, air oxidized, cold rinsed,soaped at boil, cold rinsed and air dried.
Optimization of mediator solution.-In order to make the indirect electrochemicaldyeing technique ecological and economical, the concentration of the mediator systemshould be as low as possible. High concentrations of the chemicals in the dyebath bestowmore stable reduction conditions and prevent oxidation of the reduced dyestuff. However,if we use the mediator solution towards its higher concentration end then there will beeventual increase of the losses of the chemicals in the recycling loop and also carriedalong the fabric at the end of dyeing operation. This makes the process uneconomical. The indirect electrochemical dyeing was carried out with the selected dyestuffs atdifferent mediator solution concentrations in three sets of mediator systems as shown intable I. In the group I, the ferric sulphate to TEA and ferric sulphate to sodium hydroxidemolar ratio was kept constant 1:11.48 and 1:14.3 respectively as followed in the reference13. In the group II, the ferric sulphate to TEA molar ratio was kept constant at 1:11.48while the ferric sulphate to sodium hydroxide molar ratio was varied. Whereas in groupIII, the ferric sulphate to sodium hydroxide molar ratio was kept constant at 1:14.3 andthe ferric sulphate to TEA molar ratio was varied. The mediator solution was prepared according to the procedure followed inreference 14. The caustic soda was dissolved in a small amount of water in which TEAwas added. The ferric sulphate was separately dissolved in a small amount of water andthen added to the TEA/caustic soda mixture with continuous stirring with the magneticstirrer. After complete dissolution of the precipitated iron oxide, the solution was dilutedto the full volume. The solution was kept under constant magnetic stirring for 90 minutes. Table I. Composition of the mediator solutions
Group IIIMediator Ferric pH Mediator solution composition soluion sulphate : index TEA molar ratio3.1 1:10 14 12 g/l Fe2(SO4)3 + 44.757 g/l TEA + 17.16 g/l NaOH3.2 1:10.5 14 12 g/l Fe2(SO4)3 + 46.994 g/l TEA + 17.16 g/l NaOH3.3 1:11 14 12 g/l Fe2(SO4)3 + 49.23 g/l TEA + 17.16 g/l NaOH3.4 1:11.48 14 12 g/l Fe2(SO4)3 + 51.47 g/l TEA + 17.16 g/l NaOH3.5 1:12 14 12 g/l Fe2(SO4)3 + 53.708 g/l TEA + 17.16 g/l NaOHMaterial to liquor ratio experiments.- As the electrolysis cell was not optimized withregard to short liquor ratios, the experiments were carried out at a relatively long materialto liquor ratio i.e. 1:240. To see the effect of the various material to liquor ratios on thecolour dept developed on the fabric samples by indirect electrochemical dyeing,experiments were also carried out with 1:120, 1:80 and 1:60 material to liquor ratios bymaking appropriate modifications in the system.Recycling of mediator solution experiments.- After the dyeing experiment, theremaining dye and reduced mediator were oxidized by bubbling air though the solution toform insoluble dye.
The oxidized dye particles can be segregated out through the vacuum filtration byusing G3 type of porcelain beaker. The clear filtered out mediator solution was recycledfurther to carry out dyeing experiments.Dyed samples evaluation.- The results of the dyeing experiments were characterized bycolour measurement in the form of K/S and CIELab coordinates with the help ofspectrophotometer X4000 (Jaypak) using D65 light source, 10O viewing angle. The dyed samples were also evaluated for Wash fastness (SOURCES: IS 764;1979), light fastness (SOURCE: IS 2454; 1985) and rubbing fastness (SOURCE: IS 766;1988).Results and discussionsReduction potential of vat dyes.- The reduction potential of vat dyes gives idea aboutthe level of difficulty involved in their reduction. Higher the reduction potential of thedye, higher is the reducing power required in the dyebath for its reduction. Thus, from thereduction potential values, we can group the dyes under consideration in three groups viz.easy to reduce, normal to reduce and difficult to reduce. Table II. Reduction potential of selected vat dyes Dye Reduction Dye Reduction Dye Reduction Potential Potential Potential (-mV) (-mV) (-mV)
Novatic 818 Novinone 880 Novinone 952Yellow 5G Blue RSN Black BBNovinone 841 Novinone 903 Novinone 953Brown RRD Blue BO Brill. Violet RRNovinone 850 Novinone 935 Novinone 964Green FFB Brown RR Black CHWhile following the indirect electrochemical dyeing technique, three dyestuffs, GreenFFB as a easy to reduce dye, Blue RSN as a normal to reduce dye and Violet RR as ahard to reduce dye were selected.Dyeing results The rate of potential development while potentiostatic electrolysis in the group I,II and III dyeing experiments with Violet RR dye only are shown in fig. 2, 3 and 4respectively. In each dyeing experiment, the dye was introduced in the cathodic chamberas soon as the reduction potential of Violet RR was reached i.e. -953mV. The finaldyebath potential achieved at the end of dyeing in few cases was lower than that ofachieved during the course of dyeing, especially while dyeing with Green FFB and BlueRSN.
1000 900 Solution Potential (-mV) 800 Mediator index:3.1*** Mediator index:3.2*** 700 Mediator index:3.3*** Mediator index:3.4*** 600 Mediator index:3.5*** 500 -20 0 20 40 60 80 100 120 140 160 Time (min) Fig. 4. Dyebath potential development with varying TEA concentration (Group III) during electrolysis with Violet RR dye. From the variation of the CIELab coordinates from the group I experiments asshown in table III, it becomes clear that the colour depth and shade did not show strongdependence on the iron salt concentration in the mediator solution. However the depthappears to be good around 12 to 14 g/l ferric sulphate concentration. The potentialdevelopment rate appears to be increasing with increase in ferric sulphate concentrationas can be seen from figure II. Table III. Comparison of colour yield and colour coordinates of conventional and electrochemical dyeingMediator solution Final dyebath K/S L* a* b* index potential (- mV)
2.3*** 584 Dyeing not possible2.4*** 987 2.0706 54.97 25.45 -26.872.5*** 993 2.2632 53.53 25.15 -26.662.6*** 1001 3.1007 48.72 26.54 -30.032.7*** 1001 4.4808 43.71 28.11 -30.843.1*** 1002 2.8823 49.71 26.59 -30.503.2*** 992 3.5124 47.04 27.51 -30.793.3*** 997 3.9246 45.32 27.49 -31.023.4*** 1001 3.0499 47.82 22.68 -28.253.5*** 995 3.1660 48.79 26.99 -28.83Foot Note: * with Green FFB ** with Blue RSN *** with Violet RRInitial dyebath potential before electrolysis in each case was around -350 mV. In case of group II experiments, the solution potential could not be developedabove -953 mV while working up to pH 13. Therefore dyeing was not possible duringworking with NaOH concentration below 11 g/l in the mediator solution. Whileelectrolysis with pH below 13, the complex was found to be unstable and causedadditional problems in the proper functioning of the electrochemical cell due to ablackish material deposition in the form of a hard layer on the cathode. As can be seenfrom the corresponding K/S values in table III, the colour depth appears to be increasingwith NaOH concentration above pH 13. There has not been any effect of the TEA concentration variation in the mediatorsolution in the given range of the group III experiments on the colour depth and shade.Also the solution potential development rate found to be unaffected (fig.4). However at
44.75 g/l TEA, the iron complex became instable and ferric hydroxide was found to beprecipitating.Influence of MLR on colour yield. - All the dyeing experiments for mediator solutionoptimization were carried out at 1:240 material to liquor ratio. The depth of the dyedsamples was very low as compared to the conventionally dyed samples which were dyedat 1:30 material to liquor ratio. In order to investigate the possibilities to improve thecolour depth, dyeing experiments were carried out with Blue RSN dye at 1:240, 1:120,1:80 and 1:60 material to liquor ratio using 1.2 mediator index solution. Table IV. Effect of ML Ratios on colour yield and CIELab coordinates MLR Final dyebath K/S L* a* b* potential (-mV)1:240 900 3.6045 48.44 0.58 -32.871:120 958 2.9914 50.85 -0.72 -30.431:80 950 3.0879 50.49 -1.65 -29.251:60 965 4.0401 46.55 -1.01 -29.91The K/S and CIELab coordinates of the dyeing experiments are given in table IV whichshows marginal improvement in colour depth of the dyed samples with shorter materialto liquor ratios.
Recycling of mediator solution.-Dyeing without effluent i.e. dyebath recycling seemsto be very attractive in today’s environment conscious era. In order to reuse to themediator solution with different dyes, the residual dye has to be removed from the dyeliquor. However, this is very straightforward especially in case of vat dyes which becomeinsoluble in the dyebath on oxidation. In the laboratory dyeing experiments, this can beachieved with air bubbling through the dye bath. The insoluble oxidized dye particleswere removed by vacuum filtration with G3 porcelain filter. The mediator solution used for the recycling experiments was composed of 12 g/lFe2(SO4)3 + 51.372 g/l TEA + 17.372 g/l NaOH (1.2 index mediator solution). Table Vshows the absorbance of the dyebath before oxidation and filtered out oxidized dye bathat 620 nm. The difference in the absorbances shows a high degree of dye removal. Thefiltered mediator solution was again used for the next dyeing experiment. However, therewas a loss of 4-5% of the mediator solution throughout the process which was required tobe replenished with fresh mediator solution which is also given in table V. Thus thedyebath was recycled four times. Blue RSN dye was used during each dyeingexperiment. Table V. Regeneration of mediator solution Recycling Fresh Solution Absorbance step Added Before Oxidation & After (ml) filtration Filtration Original 0 1.384 0.072 1 30 1.533 0.027 2 41 1.403 0.050 3 28 1.417 0.079
4 42 The CIElab coordinates of the dyed samples (table VI) for four recycling dyeingexperiments, carried out under the similar experimental conditions shows no effect on thecolour depth and shade. There was also no effect on the rate of development of thesolution potential since the curves in the fig. 5. Thus, reproducibility of the dyeing resultswere confirmed along with the electrochemical regeneration of the reducing agent.Table VI. Colour yield and CIELab coordinates for repeated dyeing with regenerated mediator solutionRecycling Final dyebath K/S L* a* b* step potential (-mV)Original 995 2.4954 52.95 0.62 -31.061 965 2.3803 53.82 -0.97 -29.352 950 2.8766 51.25 -0.55 -30.313 958 2.6414 52.39 -0.39 -29.674 953 3.0783 50.65 -1.09 -30.50
1000 900 Solution Potential (-mV) 800 Mediator Index: 1.2** 700 Original First Recycle Second Recycle 600 Third Recycle Fourth Recycle 500 0 20 40 60 80 100 120 Time (min) Fig. 5. Effect of mediator recycling on the dyebath potential developmentFastness results.- The few selected dyed samples with indirect electrochemical dyeingtechnique were tested for washing fastness, light fastness, wet and dry rubbing fastness.The results are shown in table VII. All the fastness properties appear to be equivalentwith the conventionally dyed samples using sodium dithionite as a reducing agent. Table VII. Comparison of fastness properties of electrochemical and conventional dyeing Reduction/Mediator Washing Light Rubbing fastness system fastness fastness Wet DryWith dithionite * 4-5 5 4 5With dithionite ** 4-5 5 4 5With dithionite *** 4-5 5 4 4-5
12 g/l Fe2(SO4)3 +51.372 g/l TEA + 4-5 4-5 4-5 517.372 g/l NaOH (MLR1:240) *12 g/l Fe2(SO4)3 +51.372 g/l TEA + 4 4-5 4 4-517.372 g/l NaOH (MLR1:240) **12 g/l Fe2(SO4)3 +51.372 g/l TEA + 4-5 5 4 4-517.372 g/l NaOH (MLR1:240) ***12 g/l Fe2(SO4)3 +51.372 g/l TEA + 4 4-5 4-5 517.372 g/l NaOH (MLR1:60)**12 g/l Fe2(SO4)3 +51.372 g/l TEA + 19.2 4 4-5 4-5 5g/l NaOH *** * with Green FFB ** with Blue RSN *** with Violet RRConclusionsPotentiostatic experiments revealed that iron-TEA complexes show a sufficientlynegative redox potential in alkaline solution which is sufficient for indirectelectrochemical reduction of vat dyes. The continuous regeneration of the reducing agent
is possible by cathodic reduction. However, colour depth appears to be poor with theimplemented electrochemical system as compared to the conventional vatting techniqueusing sodium dithionite as a reducing agent. Better results may be possible withsophisticated electrochemical systems and optimization of the mediator system.Experiments with shorter material to liquor ratios have shown better colour depths.Fastness properties appear to be equivalent with the conventional dyeing technique withgood light, washing and rubbing fastnesses. Iron-TEA complex regenerable reducing system allows reuse of the dyebathchemicals after replenishing it with the lost amount of chemicals during the oxidation andfiltration stage of indirect electrochemical dyeing. Thus the system offers enormousecological advantages. Recycling experiments also shows the capability of reproducingthe same depth and shade while working with the same dye. 1. design of suitable electrochemical cell for electrochemical dyeing of cotton with dyes on laboratory scale is suggested. 2. molar concentrations of mediator system were optimized for development of reduction potential suitable for reduction of vat dyes 3. It was possible to get the electrochemical dyeing of cotton with selected vat dyes under optimized conditions. However the depth of dyeing is much weaker compared to conventional dyeing with sodium dithionate as reducing agent. 4. It was possible to regenerate the mediator system and its repeated use for dyeing.
5. Further work is essential to improve the dye uptake and establish the economic and environment benefits of the electrochemical dyeing system.AcknowledgementsWe are obliged to Prof. A. K. Shukla, Director, Central Electrochemical ResearchInstitute for providing their laboratory facilities to carry out this research work.REFERENCES 1. D. Phillips, JSDC, 12, 183 (1996). 2. Albert Roessler & Xiunan Jin, Dyes and pigments, 59, 223 (2003). 3. JR. Aspland, Textile Chem Coor, 22, 24 (1992). 4. Jose Cegarra, Publio Puente, Jose valldeperas, ‘The dyeing of textile Material’, Texilia, Italy, 355 (1992). 5. E.Marte, Textile Praxis Int, 44, 7, 737 (1989). 6. U. Baimgartev, Rev Prog Coloration, 17, 29 (1987). 7. B. Somet, Melliand Textilberichte, Vol. 76 (1995), p161 8. R. B. Chavan and J. N. Chakraborty, J.N. Chakraborty PhD thesis, Indian Institute of Technology, 2002 9. Dr. Wolfgang Schrott, ITB International textile Bulletin, May (2000). 10. T. Bechtold, US patent No. US5244549, 1993-09-14
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