The growth of global population, elevated standards of living in many parts of the world, and fast fashion have led to both an increased demand for textiles and a large accumulation of related surplus/wastes (Barnes et al. 2006; Ding et al. 2018; Määttänen et al. 2021). The textile industry is now one of the major sources of wastes (Bigambo et al. 2019; Ratanakamnuan et al. 2012), producing 40 million tons of waste textiles worldwide each year (Ayed et al. 2021). Compared to incineration or landfilling(Jordan et al. 2019), reuse and recycling are the preferred ways to dispose these textiles, since they reduce the waste of resources and the environmental impact. Especially, white or light-coloured fabrics may be reused to make mops, curtains, etc. In contrast, the dark-coloured ones tend to be incinerated for electricity generation. In fact, up to 70% of all waste textiles is landfilled or incinerated every year. Developing the proper decolorization technology is therefore necessary for improving the utilization of waste textiles.
Cotton is the most common material used for coloured textiles, and more than 50% of cotton textiles are dyed with reactive dyes (Khatri et al. 2015; Tang et al. 2019). In the textile industry, reactive dyes outweigh other dyes (such as acid dyes and direct dyes), because most commercial reactive dyes allow a relatively simple dyeing process, provide bright colours, and cause less damage to the industrial equipment. Most importantly, reactive dyes have good colour fastness by bonding covalently with the active hydroxyl groups in cotton fibres (Hao et al. 2015). Hence, this study focuses on stripping reactive dyes from dark-coloured cotton fabrics.
Most early research on dye decolorization employed tap water, sodium bicarbonate, and six commercially available surfactants as detergents. However, those substances do not really separate the dye from the fibre. Hydrogen peroxide (H2O2) is a common bleaching agent that is cheap and environmentally friendly (Liu et al. 2018; Sang-Hoon Lim 2005; Tang and Sun 2017; Thompson et al. 1993; Topalovic et al. 2010). However, the harsh processing conditions such as high temperature of 90 degrees and high concentration of alkali can lead to extensive water and energy consumption as well as serious chemical damage to the textiles. As effective alternatives, low-temperature and low-alkali decolorizing methods have been proposed for decolorizing cotton (Eren et al. 2014; Jegannathan and Nielsen 2013; Ratanakamnuan et al. 2012). Still, these methods do not completely remove the colour, while they have high energy consumption, cause serious fibre damage, and generate strongly alkaline wastewater and biological pollution (Cai and Evans 2007). Therefore, a remaining crucial issue is how to effectively strip reactive dyes from cotton fabric while minimizing the unwanted effects on cotton fibre and reducing the environmental impact.
Common agents used in traditional decolorizing processes include NaClO2, NaClO, CH2O2, and NaNO3 (Farooq et al. 2013). These chemicals have many obvious disadvantages, including the emission of toxic fumes, high time and energy consumption, reduced fabric strength, and pollution. A previous study examined the reliability of a sequential acid/dithionite/peroxide treatment to strip reactive dyes from cotton (Bigambo et al. 2019). Unfortunately, H2O2 is known to oxidize the alcohol groups of cellulose. This results in a weakening or scission of the glycosidic linkages, followed by depolymerization and limited viscosity (Howitt 1956). A sodium chlorite/potassium permanganate bleaching system could effectively bleach cotton fabric (Abdel-Halim 2012). Compared to the oxidizing agents, reducing agents are also promising for decolorizing cotton fabrics while causing less material degradation, but few related studies have been reported.
In this study, we combined oxidants and reducing agents as decolorizing agents to treat post-consumer waste cotton dyed with vinyl sulfone reactive dyes. Na2S2O4, H2O2, KMnO4, and Na2S2O4-H2O2 were used as decolorizing agents, and their oxidation (reduction) effects on the colour and strength of the fabric were analysed. The decolorizing mechanism was studied by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), and degree of polymerization (DP) measurement. The optimum decolorizing process was identified based on the absorption coefficient/scattering coefficient ratio (K/S), CIE whiteness index, tensile strength, and fabric weight loss after treatment. According to the results, it is possible to obtain high decolorization efficiency and sufficient CIE whiteness while minimizing the tensile strength reduction, fabric weight loss, and waste liquid generation.