Giving coloured cotton a second life: new BioSusTex research tackles a major barrier to textile recycling
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A new BioSusTex-supported study shows how dark, reactive-dyed cotton waste can be transformed into a white, reusable feedstock while preserving the fibre properties needed to make new textiles.
What happens to a black T-shirt, navy cotton shirt or dark green bedsheet when it reaches the end of its useful life? The cotton itself may still be valuable. The colour, however, can make recycling surprisingly difficult. A new study by researchers from Aalto University published in ACS Omega addresses this problem by developing a two-step process that removes strongly bound reactive dyes from cotton textile waste. The researchers were able to remove up to 99.3% of the colour, producing much white material that retained most of its strength and could successfully be dyed again. The work provides another important contribution from BioSusTex towards a textile sector in which used garments can become raw materials for new ones rather than being discarded.
The hidden problem with recycling coloured clothes
Europe generates millions of tonnes of post-consumer textile waste every year. The newly published paper reports that EU textile consumption reached 19 kg per person in 2022, contributing to 6.95 million tonnes of post-consumer textile waste that year. Globally, textile waste reached an estimated 120 million tonnes in 2024. Recycling this material is not simply a matter of shredding an old garment and spinning it again. Colour is one of the obstacles. When differently coloured cotton textiles are shredded together, the resulting fibres can form unpredictable mixed shades. This restricts the colours and quality of the products that can subsequently be made. Reactive dyes are particularly challenging. They are widely used for cotton because they form strong chemical bonds with cellulose. That makes colours durable through wearing and washing, which is exactly what consumers want from a garment, but also makes the dye difficult to remove when the textile reaches the recycling stage. Removing the original colour can therefore turn a mixed stream of old textiles into a much more flexible raw material.
From dark fabric back towards white cotton
The researchers studied four cotton fabrics in dark green, grey, dark black and navy, each containing mixtures of commonly used reactive dyes. Before testing the recycling process, the fabrics were subjected to ten washing and drying cycles to better represent waste material that had already been used by consumers. They then applied two consecutive treatments.
** The first uses sodium dithionite (Na2S2O4), a reducing agent that attacks the parts of the dye molecules responsible for their colour.
** The second uses hydrogen peroxide (H2O2) under alkaline conditions to remove the remaining colour and help break the chemical connections between dye molecules and cotton.
Rather than relying on one aggressive treatment, the researchers optimised the two stages to achieve effective colour removal while limiting unnecessary chemical and energy use. The visual result is striking. The images presented in the paper show dark green, grey, black and navy fabrics becoming progressively white after the first treatment and approaching pale or white shades after the second.
Up to 99% of the colour removed
The sequential treatment removed between 93% and 99% of the colour from the four tested fabrics. Black cotton showed the highest removal efficiency at 99.3%, while dark green reached 99.2% and navy 98.3%. All of the treated fabrics reached lightness values above 80, giving them a visibly white appearance and making them much more suitable as recycling feedstocks. But making fabric white is only useful if the cotton underneath is still valuable. That is why the team went considerably further than simply measuring colour.
Keeping the cotton useful
Removing a stubborn dye can damage the cellulose chains that give cotton its strength. A recycling process therefore must balance two competing objectives: remove enough colour to make the material useful again, but avoid destroying the fibre strength in the process. After the complete treatment, the fabrics retained approximately 86–92% of their original tensile strength, depending on the sample and direction tested. Overall weight loss remained below 4%. Microscopy showed some localised changes to the fibre surfaces, but the cotton structure remained sufficiently intact for recycling. The researchers also examined the cellulose at a molecular level. The treated cotton had a degree of polymerisation of approximately 1,400–1,700. This is particularly relevant because these values are within or close to ranges already used for dissolving pulp and chemical fibre production. In other words, the treatment did not merely create a cleaner-looking waste material: it produced cellulose with properties that could potentially be processed into new fibres. Chemical analysis provided further evidence. After the two-stage treatment, sulfur from the dyes could no longer be detected and only negligible nitrogen remained. The elemental composition and thermal behaviour of the treated fabrics became much closer to those of undyed cotton.
Can the cotton actually be coloured again?
One of the most practical tests was also one of the simplest: could the stripped fabric be dyed again? The answer was yes. The researchers re-dyed the treated cotton and found that it absorbed and fixed the new dye effectively. Colour strength was close to that of a reference fabric dyed from new cotton, while washing and rubbing fastness remained excellent. This means the treatment did not remove the original colour at the expense of the cotton’s ability to take on a new one. For a circular textile system, that is crucial. An old black garment does not necessarily need to become another black garment. If its original colour can first be removed, the recovered cotton has much greater flexibility: it could potentially be re-dyed, mechanically recycled into new yarns or chemically dissolved and regenerated into entirely new cellulose fibres.
What could this mean for the clothes we buy?
For consumers, textile recycling usually happens out of sight. But technologies such as this could eventually change what happens to garments after we put them into a textile collection bin. Instead of being restricted to low-value applications or being discarded because of their colour, used cotton garments could become a more useful secondary raw material. This could help manufacturers:
** recover more value from existing cotton rather than relying entirely on virgin fibres;
** combine differently coloured waste streams more easily;
** produce recycled textiles in a broader range of new colours; and
** move closer to genuine textile-to-textile recycling, where old clothing becomes new clothing.
The significance is not simply that the colour disappears. It is that enough value remains in the cotton afterwards to give the material another productive life.
Promising results, but industrial questions remain
The researchers are clear that the process is not yet ready to be rolled out directly across industrial textile recycling plants. The current work used a batch process. The authors propose that it could potentially be adapted to continuous textile equipment, with the two treatment stages arranged consecutively, but this still requires further engineering and testing.
Important sustainability questions also remain. Future studies will need to address water and energy consumption, wastewater treatment, recovery of the processing chemicals, process safety and continuous operation. Techno-economic and environmental studies will be needed to determine whether the approach remains attractive when scaled beyond the laboratory.
One practical advantage is that the chemicals used in the process are already widely used in the textile and pulp industries. This could make future integration easier than technologies requiring completely unfamiliar processing systems. The researchers now plan to investigate how the decolourised cotton can be converted into new materials through both mechanical and chemical recycling.
Towards textiles designed for another life
The study highlights an important principle behind the BioSusTex vision: creating a circular textile industry requires more than collecting old clothes. The materials inside those clothes must remain valuable enough to use again. By turning strongly coloured cotton into a white, redyeable material while preserving most of its mechanical and chemical properties, this research addresses one of the practical barriers standing between textile waste and new textile products.
The work was conducted by researchers at Aalto University and acknowledges financial support from the BioSusTex project, alongside Business Finland's TexirC project.
Read the open-access paper: