Abstract:

Objective The yak wool fibers are inherent in colors primarily including black, purple and brown, failing to meet the demand for multi-color varieties for applications. Hence, the fibers need to be bleached prior to dyeing with target colors. However, the traditional bleaching process has limitations in oxidant selection and elusive mechanism remained in conventional yak wool decolorization. Herein, the bleaching capacity and environmental impact of homogeneous advanced oxidation processes with three typical oxidants were compared. Meanwhile, the mechanism of hydrogen peroxide(H₂O₂)-driven efficient decolorization was also investigated.

Method Inspired by the homogeneous advanced oxidation technology for organic pollutants degradation, a comparative study was conducted to examine the effects of three typical oxidants H₂O₂, peracetic acid (PAA), and potassium peroxymonosulfate (PMS) on decolorization of yak wool fibers using ferrous sulfate as the catalyst. Using the whiteness index and breaking strength as evaluation indicators, the most suitable oxidant was identified and the decolorization parameters were optimized. The reactive oxygen species were identified and analyzed using the electron spin resonance(EPR) technology and quenching experiment. Finally, the environmental impact of the decolorization process was assessed via the life cycle assessment (LCA).

Results The results of single-factor experiments indicated that hydrogen peroxide is the most suitable green oxidizing agent for decolorizing yak wool. The optimal decolorization conditions were determined to be an H₂O₂ concentration of 25 g/L, a temperature of 60 ℃, and a solution pH value of 8.5. Under these conditions, the whiteness of yak wool fibers reached up to 68%, with a fiber breaking strength loss of approximately 12%. Mechanistic analysis of the H₂O₂ decolorization process revealed the presence of hydroxyl radicals (·OH) and singlet oxygen (¹O₂) in the system. Further quenching experiments demonstrated that ·OH and ¹O₂ are the main reactive oxygen species responsible for decolorization, with ·OH playing the primary role and ¹O₂ having a secondary effect. Scanning electron microscope images revealed that yak wool fibers treated with H₂O₂ or PAA for decolorization exhibited surface damage, including roughened cuticle scales and unclear scale edges. Fibers treated with PMS showed even more severe damage, with cuticle scales becoming indistinct or completely detached in some areas. LCA results showed that the PMS decolorization process had the highest environmental impacts on global warming potential, terrestrial ecotoxicity, non-carcinogenic human toxicity, and fossil resource scarcity, while both PMS and PAA decolorization processes had significant negative impacts on environmental ecology and human health. Therefore, compared to the other two oxidants, decolorization using H₂O₂ was more environmental-friendly and low-carbon.

Conclusion Inspired by the advantages of the homogeneous advanced oxidation technology for organic pollutants degradation, three typical oxidants including hydrogen peroxide, peracetic acid, and hydrogen persulfate were screened and compared for yak wool fiber decolorization using ferrous sulfate as the catalyst. The results revealed that hydrogen peroxide is the most suitable oxidizing agent for practice. The optimal decolorization condition was identified to be H₂O₂ concentration of 25 g/L, temperature of 60 ℃, and solution pH value of 8.5. After being decolorized by hydrogen peroxide, the whiteness of yak wool fibers reached 68%, and the loss of strength was relatively small. The decolorization mechanism of hydrogen peroxide bleaching system showed that both the hydroxyl radical and singlet oxygen contributed to the yak wool fiber decolorization, in which hydroxyl radical played the primary role and singlet oxygen served a secondary function. Life cycle assessment showed that, compared to PMS and peracetic acid, the use of H₂O₂ for decolorization had the lowest environmental impact, which aligns better with the green and low-carbon development trend for yak wool decolorization processes.

Key words: yak wool, decolorization oxidant, hydrogen peroxide, life cycle assessment, homogeneous advanced oxidation, decolorization