Efficient and sustainable bleaching of cotton/spandex integrating UVA radiation and peroxide activator: A study focus on the degradation mechanism of natural pigments in fibres

Abstract

Establishing an energy/water-saving bleaching process is highly expected by textile plants to reduce industrial carbon emission, wastewater discharge and general cost. Therefore, an efficient UVA-assisted N-[4-(triethylammoniomethyl)-benzoyl]- caprolactam-chloride (TBCC) -activated hydrogen peroxide (H2O2) bleaching strategy at ambient temperature for cotton/spandex (C/S) fabric was developed in this study. Results reveal that 7.4 g/L of TBCC-activated H2O2 (30%) achieves a slightly higher whiteness index (WI) on C/S fabrics than those only treated with 60 g/L H2O2 (30%). The treated C/S fabric displays a good dyeability using direct dyes due to the qualified wettability. The mild pH process protects C/S fabrics from damage with a low tensile loss (less than 10%) in warp and weft directions. The WI of C/S fabric mainly depends on the concentration of TBCC/H2O2 couple and radiation time. Moreover, the degradation mechanism of cotton pigment in UVA/TBCC/H2O2 bleaching system was implemented including the identification of effective reactive oxygen species (ROS), tracking of the concentration variation of hydroxyl radical (HO•) under UVA irradiation, and the exploration of degrading kinetics and pathway of morin as the model of pigment impurity. Indicated by the degradation rate constant, both TBCC and UVA plays an accelerating role in enhancing the degradation efficiency. In the radical scavenging experiment, HO• and superoxide radical (O2–•) are two major ROS participating in the degradation. And therein, HO• makes larger contribution than O2–•. HO• concentration has a high positive correlation with radiation duration and TBCC dosage based on the fluorescence labelling method. Confirmed by LC-MS/MS analyses, 2,4,6-trihydroxybenzoic acid (MW: 170.12) and 2,4,-dihydroxybenzoic acid (MW: 154.12) are two major segments degraded from the oxidation and ring-opening of C-ring in morin. Finally, considering energy, water and chemical conservations, the condition i.e. 7.4 g/L of H2O2 (30%) and 20 g/L of TBCC under 30 W UVA radiation for 1 h, is recommended, which significantly increases the WI by 46.5%, and saves about 90% energy and 50% water consumption compared with the conventional waterbath bleaching at 95 °C for 45 min when similar WI enhancement was achieved.

Keywords

• Peroxide activator
• UV radiation
• Degradation mechanism
• Cotton fibre
• Energy conservation