Abstract:

Objective Reactive dyeing is known for its complex dyeing process, high energy and water consumption, and large discharge of colored wastewater, which seriously restricts the transformation and upgrading of the printing process. As an effort to reach "emission peak and carbon neutrality", the development of reactive dye cyclic dyeing with energy saving and emission reduction advantages is conducive to promoting the green and low-carbon production in the printing and dyeing industry.

Method The treatment agent contains compounds with bleaching and oxidizing functions. This wastewater treatment agent was selected for dyeing fabric soaping, realizing the low temperature fast soaping of dyeing fabric and recycling of dyeing wastewater. Taking a type of simulated wastewater as the research object and absorbance as the evaluation index, the treatment and reuse process of wastewater was explored. The feasibility of cyclic dyeing was verified by the determination of dye exhaustion rate, fixation rate and color parameters of dyed fabric.

Results The results showed that the decolorization rate of dye solution increased with the increase of temperature, and the higher the temperature the shorter the treatment time (Fig. 2). Under the same treatment time, the higher the treatment agent concentration, the higher the decolorization rate of dye solution, indicating that increasing the concentration of treatment agent was conducive to reducing the number of dye molecules in the waste liquid. In addition, it was found that the same effect could be achieved by prolonging the treatment time when the concentration of treatment agent was low (Fig. 3). When the treatment time was 5 min, the decolorization rate of dye solution with 20 g/L of NaCl was up to 64.7%, which was 21.9% higher than that of without NaCl. Further increasing NaCl concentration to 50 g/L had no significant effect on the decolorization rate of dye solution. Further extending the treatment time to 30 min, NaCl showed little effect on decolorization rate of dye solution (Fig. 6). After treatment at 85 ℃ for 15 min, the maximum absorption wavelength in the visible region disappeared, and the decolorization rate of the dye solution was as high as 99.1% (Fig. 7). When the reactive dye was dyed with recycled water, the dye percentage in the first 30 min was 9.0%-13.9% higher than that of the deionized water at the same time. Increasing the number of dyeing cycles did not change the overall trend of dyeing rate curve (Fig. 8). Compared with the color parameters of the fabric dyed by deionized water, the color of the fabric dyed by cycle was darker, the red and blue light were weakened (Tab. 1).

Conclusion The treatment agent of 3%, the temperature of 85 ℃ and the time of 15 min were appropriate for dealing with 0.07 g/L of C.I. Reactive Red 218 simulated wastewater. The decolorization rate of dyeing wastewater is up to 99.1% after 15 min treatment. Under the same treatment process, the acid condition has the best treatment effect, the neutral condition is the next, and the alkaline condition is the worst. Compared with deionized water, the percentage of reactive dye increased with the extension of dyeing time. During cyclic dyeing, reactive dyes have higher dye exhaustion and fixation rate, and the dyed fabric is darker, and the red and blue light are weakened. This method not only realizes the efficient treatment of dyeing wastewater, but also realizes the reuse of inorganic salts in dyeing residue, which is conducive to energy saving and emission reduction.

Key words: reactive dye, cyclic dyeing, wastewater treatment, decolorization rate, energy saving and emission reduction