A Study Of Electrochemical Reduction Of Vat Dyes

The traditional chemical reduction and dyeing technology is fairly mature and its operation is relatively simple,but this method produces great economic burden and serious environment pollution,because of addition of large amounts of chemicals in the reduction process.Instead,the electrochemical process minimizes the consumption of chemicals,reduces the cost of production and diminishes the discharge of wastewater,making it a promising alterative to chemical reduction of vat dyes.Therefore,the electrochemical techniques would be of wide prospects in future dyeing industry.In this paper,the indirect and direct electrochemical reduction behavior of indigo was studied by cyclic voltammetry and electrolysis experiments.The reduction mechanism and the catalytic properties of indigo on the carbon felt electrode was discussed.Moreover,we gave a preliminary analysis of the effects of ultrasound on the electrochemical reduction of indigo using the ultrasound probe and bath technical.Finally,surface structure and specific surface area of the carbon felt were investigated by SEM and BET,respectively.According to all research above,the following conclusions can be drawn:(1)A conventional two-compartment glass H-cell,separated by a Nafion-324 membrane,was used for indirect electrolysis of indigo on carbon felt to explore the effects of different parameters on current efficiency(CE),and the optimized process conditions were obtained:the initial concentration of indigo is 0.07 mot·L-1,the current density is 1 A·dm2,the reaction temperature is 50℃,and the thickness of carbon felt is 10mm.The results show that high CE(78.45%)can be successfully achieved under optimized conditions and the current density is found to be the main factor.In addition,under the optimized experimental conditions above,a scale-up electrolysis experiment with a plate and frame cell was carried out at a flow rate of 160 L·h-1 and the total CE is 57.24%.(2)Direct electrochemical reduction of indigo is thermodynamically feasible,and the main reason that the effective direct electrochemical reduction can’t be accomplished lies in the kinetics(poor contact between dispersed indigo and electrode).The study also found that a significant increase on CE of the middle stage of indirect electrolysis is mainly due to the mechanism of radicals.(3)The cyclic voltammograms of indigo on different electrodes(glassy carbon and carbon felt)showed that the magnitude of reductive peak current density of Fe(III)at the carbon felt electrode has increased significantly,which has a 20 to 30 folds enhancement compared to the voltammetric responses generated on the gassy carbon electrode.The SEM data indicated that the carbon felt was composed of numerous carbon fibers and a large number of pores among the fibers were formed.Furthermore,the apparent surface area of carbon felt was detected by BET as 7289 cm2·g-1.In short,such a porosity surface structure would provide much more electrochemical active sites,enhance the electrochemical performance of electrochemical reduction of indigo and improve the current density of the electrochemical reduction of indigo.(4)Different electrode materials(stainless steel mesh,copper foam and carbon felt)were used in electrolysis experiments of indigo with ultrasound and without ultrasound under the same experimental conditions,respectively.The results indicated that ultrasound can significantly improve CE.A further study of the effects of ultrasound on the electrochemical reduction of indigo was given using ultrasound probe technical,and it was found that under ultrasound conditions,the Fe3+ reduction process was very significant,and accelerated electron transfer between Fe2+ and indigo to increase the reduction rate of indigo,which explained ultrasonic technical how to play a major role in the reduction process of indigo.Moreover,we gave an analysis of particle sizes of indigo with ultrasound and without ultrasound treatment by Mastersizer 2000.The results demonstrated that larger particles of solid indigo can be effectively broken down and smaller particles with a size between 1 and 10 μm were created,that is,a more uniform distribution of the particles can be obtained with ultrasound.