One-step bleaching method for animal fiber by persulphate/hydrogen peroxide

Animal fibres are coloured using dyes to satisfy the individual needs; thus, dark animal fibres must be bleached. Compared with frequently used Fe2+-catalysed H2O2 (Fe2+/H2O2) bleaching, persulphate (PDS)/H2O2 bleaching is advantageous owing to its high efficiency, low damage of fibre and environmental friendliness. Herein, different free radical species were precisely controlled by changing the pH of the PDS/H2O2 solution and regulating the existence of NH4+. When NH4+ was absent, sulphate radicals (center dot SO4) and hydroxyl radicals (center dot HO) could be produced at pH = 8-9 and superoxide anions (center dot O2) could be produced at pH = 10. In the presence of NH4+, center dot SO4- and center dot HO could be produced at pH = 8, and center dot O-2(-) was dominant in the solution when pH = 9-10. Black yak wool was used as the research object to demonstrate that the bleaching ability of the generated center dot O-2(-) was stronger than those of the generated center dot SO4- and center dot HO. Compared with that of the commercial product (prepared using Fe2+/H2O2 bleaching), the bleaching time of the decolourised yak wool was reduced by 83.33% (from 24 to 4 h) without the catalysis by heavy metal ions. Meanwhile, the whiteness of the decolourised yak wool (72.31%) was close to that of the commercial product (78.79%). Furthermore, the damage to the cuticle layer and cortex of yak wool was low after bleaching. The breaking strength of the decolourised yak wool was 1.79 cN center dot dtex(-1), which only decreased by 17.13% compared with that of raw yak wool. These results hold great potential in developing environmental-friendly bleaching methods for dark animal fibres with high efficiency and low damage.