| Vat dyes is most popular among dye classes used for coloration of cotton,particular, when high fastness standards are required to light, washing and chlorine bleaching, especially for indigo, are demanded in large.Because a number of jeans are dyed in indigo. But these dyes are indiscerptible in water. Until now, in most industrial vat dyeing process, vat dyes are reduced mainly use sodium dithionite. This process produces large amounts of sodium sulphate and sulphite as by-products which increase the costs for waste water treatment. Hence, many attempts are being made to replace the enviromentally unfavourable sodium dithionte by ecologically. Such as indirect electrochemical, it offered tremendous environmental benefits.In the former research, most people use Fe3+-triethanolamine mononuclear complex as mediators for indirect electrochemical reduction. This paper taking binuclear Fe3+-Ca2+-TEA complex as redox mediator, the introduction of Ca2+-ion into Fe3+-TEA complexes lead to formation of a binuclear Fe3+-Ca2+-TEA and the formal redox potential shifts approximately-0.100V more negatively values,it improved electrochemical reduction efficiency and speed.The electronchemical bechavior of Fe3+-Ca2+-TEA was studied by the cylic voltammetry on glassy carbon electrode. Through fitting the relationship between oxidation, reduction peak current as the square root of scan rate, it presented linear relationship, and showed that Fe3t-Ca2+-TEA complex get electron on the electrod, it was a quasi-reversible diffusion controlled process. After added the indigo, the electrochemical reduction of the complex was a diffusion controlled process. And it was also a quasi-reversible process which believed to followed chemical catalysis mechanism(EC). The mediator got electron from cathode, and then transferred to dye. The copper electrode after the electrochemical processing was characterized by SEM, EDS. The results also supported this process. The Fe(OH)3 precipitation and-OH groups of triethanolamie undergo a coordination reaction according to the analysis of infrared spectroscopy.The Fe3+-Ca2+-TEA complex has been used as redox mediator to discuss the electrochemical reduction of dyeing process by orthogonal and single-elemental experiments. Sodium dithionite has been used as reducing agent to discuss the conventional dyeing method by orthogonal and single-elemetal experiments. The optimum conditions came out in terms of oxidation peak,K/S value, and Current efficiency.Electrochemical reduction:ferric nitiate lOg/L,calcium nitrate 7g/L, triethanolamine 35g/L, sodium hydroxide 9g/L,applied voltage 5V, cathode area of 30cm2, reducing time 50 min.Conventional dyeing:indigo 2g/L, sodium hydroxide 2g/L, Sodium dithionite 4g/L, dyeing time 50s. Oxidation time 3min. The different kinds of vat dyes were used in electrochemical reduction and conventional dyeing process. The same K/S value, washing fastness, rubbing fastness can be obtained by the two dyeing methods. Then, it can be concluded the dyeing effects of the electrochemical reducing and dyeing is the same as the conventional one.Electrochemical reduction use electric energy to reduction vat dyes, it not only reduced the COD value of dyeing waste water, but also lower energy consumption. Therefore, it had obvious environmental protection and energy efficiency.When the operation of electrochemical reduction dyeing has finished,a certain amount of air was blown into the liquorof dye to oxidation leuco. Finally, applying with a pore size of 0.4μm hollow fiber membrane filtration to remove the dye, it can reuse the mediator to the next dyeing, and reduce the consumption of water and chemicals. Compared to conventional dyeing, its economic cost savings of 31%, had obvious economic benefits. At last, it can be concluded the electrochemical reduction is better then the conventional one.
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