| In view of the presence of carbonyl and amino groups in the indigo molecule,hydrogen bonds are easily generated inside and between the molecules.Therefore,from the perspective of chemical reduction,the reduction of indigo requires a stronger reducing agent.Sodium dithionite has become the preferred reducing agent for indigo reduction due to its strong reducing power and high cost performance.However,it is easy to produce a large amount of sulfur-containing wastewater during its use.Although the reduction with sodium borohydride as the reducing agent will not cause environmental problems,the reduction rate is slow at room temperature and the efficiency is low.Therefore,external thermal energy is often needed to raise the temperature of the system and additional metal salts are needed as catalysts.These make the reduction process cumbersome,and the heating operation and the heat of water liberation of sodium borohydride are not conducive to the controllability of the reaction performance of the system.In a series of new reduction methods,electrochemical reduction has advantages such as clean and efficient due to the use of electrons that can be quickly transferred for reduction.Therefore,this paper combines electrochemical technology with the catalytic sodium borohydride reduction of indigo system to improve the efficiency of sodium borohydride reduction of indigo system,improve system controllability,and simplify the dyeing process.According to the conductivity and p H value of the complex solution,the oxidation-reduction potential of the reducing solution,and the K/S value of dyed cloth samples,the metal salts,ligands and anolyte solutions suitable for the system were selected in sequence.The results show that copper ion becomes the preferred metal ion because it can not only promote the release of hydride ion in sodium borohydride,but also combine with indigo carbonyl oxygen more stably;Sodium gluconate is a preferred ligand because of its ability to form a stable complex with copper ions and the high conductivity of the complex solution;It is advisable to use copper sulfate as the main component of the anolyte to supplement copper ions in the catholyte in real time.In addition,the weak acidity of the anodic copper sulfate solution can relieve the excessively high p H of the catholyte caused by the consumption of water during the hydrolysis of sodium borohydride,and will not cause strong interference to the p H of the catholyte.Cyclic voltammetry was used to study the electrochemical behavior of copper ions,and the reaction mechanism was derived:the anolyte copper sulfate enters the cathode chamber,so that there are more divalent copper ions in the cathode chamber than borohydride,and the divalent copper ions react with the borohydride to obtain the reducing substance cuprous hydride after the electrons obtained at the cathode are reduced.The copper ion in the cuprous hydride combines with the indigo carbonyl oxygen to enhance the polarity of the double bond in the carbonyl group,which facilitates the attack of H~–on the carbonyl carbocation,thereby completing the hydrogenation reduction of indigo.Through single factor experiment and response surface experiment to explore the influence of the concentration of the main components in the catholyte on the absolute value of the reduction potential and the K/S value of dyed fabric samples.The results showed that the influence of sodium hydroxide concentration and the interaction between sodium borohydride and sodium hydroxide concentration were extremely significant items for the K/S value of dyed fabric samples and the absolute value of reduction potential of the reducing solution.The optimal concentrations of each component obtained are as follows:sodium borohydride 0.5 g/L,sodium hydroxide 2.5 g/L and copper sulfate 0.65g/L.Under this condition,the absolute value of the reduction potential was 968,and the K/S value was 11.92.EDS and XRD analysis of the deposit on the anode side of the cotton diaphragm after electrolysis showed that the atomic percentage of copper was about 10%,and its main forms were pure copper and cuprous oxide with different crystal structures.The weakly acidic copper sulfate anode solution can dissolve the cuprous oxide,which to a certain extent avoids the clogging of the cotton cloth diaphragm by the deposits,so that the utilization of copper ions in the anode solution will not be significantly reduced by the influence of the deposits.The effects of copper sulfate concentration in anolyte,applied voltage and reduction time on K/S value of dyeing cloth samples were investigated by single factor and orthogonal test.The results showed that there was a strong interaction between the reduction time and the other two variables,the copper sulfate concentration in anolyte and the applied voltage.In all single items,the influence of anode copper sulfate concentration,applied voltage and reduction time gradually became smaller.The optimized process conditions were:copper sulfate concentration in anolyte 32 g/L,applied voltage 13 V and reduction time 30 min.Under this condition,the K/S value was 12.37,which was higher than the K/S value of 11.81 for traditional sodium borohydride reduction dyeing and 9.97for copper ion-catalyzed sodium borohydride reduction dyeing.For the three reduction dyeing processes of sodium borohydride electrocatalytic reduction,sodium borohydride reduction and sodium borohydride reduction,the color and light stability of electrocatalytic reduction dyeing and sodium borohydride reduction dyeing fabric samples are not much different;The soaping fastness and rubbing fastness of the three processes are basically the same;The electrocatalytic reduction of sodium borohydride and the reduction of copper ion catalyzed by sodium borohydride correspond to very little copper content on the dyed fabric samples,which were 0.04%and 0.03%,respectively.This showed that the copper ion only activated the indigo carbonyl oxygen,and almost never dyed the fabric along with the leuco after H~–completes the attack on the carbonyl carbon.By adding only 50%of the initial use amount of sodium borohydride to the cathodic reducing solution that has been dyed on cotton fabrics,the reduction solution can be recycled.When the number of cycles of the cathodic reducing solution was less than 3,the corresponding K/S value was above 10,indicating that the cathodic reducing solution in the electrochemical reduction system of sodium borohydride had a certain cycle dyeing performance.Use the above sodium borohydride electrochemical reduction system to explore the influence of different metal ions on the reduction of indigo when using copper sulfate in the cathode or anode respectively.The results showed that with copper sulfate as the main component of the anolyte,the K/S value obtained without any metal ions in the catholyte was 9.285.The reduction effects of using calcium ion,aluminum ion and zinc ion were worse than that without any metal ion.The corresponding K/S value of these metal ions was only between 8～9.The dyeing effect was best when copper ions were used in both the cathode and the anolyte,which corresponds to a K/S value of 9.617.With copper sulfate as the main component of the catholyte,the effects of using copper ions,aluminum ions and zinc ions in the anolyte were equivalent,and the corresponding K/S values were9.617,9.528 and 9.403 respectively.The calcium ion had the worst effect,with a K/S value of 5.107.For the catholyte and anolyte using the same metal ion,only the copper ion system could reduce the dye. |
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