Sustainable natural colouration of khadi cotton: natural dyes, bio-mordants, resource-efficient processing and circularity perspectives

The transition towards more sustainable textile production has renewed scientific interest in natural colorants and low-impact coloration technologies. Textile wet processing, particularly dyeing and finishing, is associated with substantial water, energy and chemical requirements, making coloration an important target for cleaner production strategies (Hussain, 2018; Khattab, 2020; Haji, 2020; Uddin, 2022). Natural colourants obtained from plants, microorganisms, animals, minerals and agro-industrial residues have consequently attracted renewed attention because of their renewable origin, chemical diversity and potential for biodegradability and waste valorization (Pizzicato, 2023; Salauddin Sk M., 2021; Repon, 2024a; Vankar, 2000). However, natural origin alone does not establish environmental superiority. Extraction conditions, mordant selection, dye–fibre affinity, water and energy requirements, colour reproducibility, fastness and scalability must be evaluated together (Pizzicato, 2023; Salauddin Sk M., 2021; Benli, 2024; Mansour, 2018; Phan, 2021). Khadi cotton provides a distinctive context for this transition because its hand-spun and handwoven production system is strongly associated with decentralized manufacturing, artisanal knowledge, rural employment and cultural identity. Previous life-cycle research on the khadi-handloom system has demonstrated the importance of integrating environmental, socioeconomic and cultural dimensions when assessing its sustainability (Bhalla, 2018). This review critically examines natural colouration of cotton with particular emphasis on its relevance to khadi. The chemistry and application of important natural colorant classes, including indigoids, anthraquinones, flavonoids, tannins and curcuminoids, are discussed together with the growing use of agricultural and food-processing residues as dye resources (Pizzicato, 2023; Salauddin SK M., 2021; Repon, 2024; Vankar, 2000; Phan, 2020). Attention is given to madder indigo & pomegranate peels because of their historical significance, chemical diversity and demonstrated applicability to cellulosic textiles (Vankar, 2008; Ajmal, 2014; Otaviano, 2023; Li, 2022). The roles of conventional mordants, Bio-mordants and fibre modification strategies are critically assessed, with emphasis on their effects on colour strength, shade development and Colourfastness (Benli, 2024; Repon, 2024b; Thakker, 2022; Rahman, 2023; Shahmoradi Ghaheh, 2021; Nazir, 2022; Hosen, 2021). Conventional exhaust dyeing is compared with resource-efficient approaches such as ultrasound-assisted dyeing, surface modification, padding and pad-steam processing (Vankar, 2007; Zia, 2019; Cai, 2022; Khattak, 2023; Baig, 2021; Wang, 2022). Pad-dry-steam processing is identified as a potentially valuable technology for decentralized khadi production because controlled liquor application can reduce the volume of dye liquor requiring heating and discharge while potentially improving shade reproducibility. Nevertheless, the associated drying and steaming energy must be included in any quantitative sustainability assessment. Colour strength, CIELAB coordinates, colour difference and washing, rubbing and light fastness are identified as essential indicators for evaluating the technological viability of naturally colored khadi (Salauddin Sk M., 2021; Repon, 2024a; Ma, 2020). The review further examines circular-economy opportunities associated with agricultural residues, rural livelihood generation and small-scale production (Bhalla, 2018; Agnhage, 2017). Major research gaps include inconsistent extraction procedures, insufficient chemical characterization of natural colourants, incomplete reporting of colourimetric data, limited standardization of fastness testing, inadequate resource balances, insufficient pilot-scale validation and limited integrated life cycle and techno-economic assessments. The review proposes a research framework in which colorant chemistry, extraction, fibre modification, application technology, colour performance, durability, resource consumption, environmental impact and socioeconomic feasibility are optimized simultaneously. The future of sustainable khadi coloration therefore lies not simply in replacing synthetic dyes with natural dyes, but in combining traditional textile knowledge with modern colour science, process engineering and life cycle thinking.