From coloured cotton waste to clean recycling feedstock: new BioSusTex research simplifies textile decolourisation
A new BioSusTex-supported study shows that reactive-dyed cotton waste can be transformed into a white, recyclable feedstock using a simpler one-bath process that reduces water and energy use while preserving the quality of the cotton fibres.
Colour helps make our clothes attractive and durable. But once a garment reaches the end of its life, that same colour can become a major obstacle to recycling. Reactive dyes, widely used to colour cotton, form strong chemical bonds with the fibre. This helps clothing keep its colour through repeated washing, but it also makes those dyes difficult to remove when we want to turn old cotton into new textiles.
A new open-access study published in RSC Advances tackles this challenge by comparing two ways of removing colour from dyed cotton waste. The researchers found that a simplified one-bath process can produce a white, uniform cotton feedstock while using less water and energy than a conventional two-bath approach. The research, supported by the BioSusTex project, brings textile recycling another step closer to recovering the value of coloured cotton instead of sending it to landfill, incineration or lower-value applications.
Why does colour make cotton harder to recycle?
Global textile waste is currently estimated at around 120 million tonnes per year and could exceed 150 million tonnes by 2030. Less than 1% is recycled into new textiles. One reason is that recycling coloured material is more difficult than recycling clean, white cotton. Residual dyes can affect the appearance and quality of recycled fibres, make it harder to blend recycled cotton with virgin material and interfere with chemical recycling processes in which cellulose is dissolved and regenerated into new fibres. A high-whiteness feedstock gives recyclers much more freedom. Once most of the original colour has been removed, the cotton can potentially be mechanically recycled, blended into new yarns or chemically processed into regenerated cellulose fibres. This makes decolourisation an important step towards true textile-to-textile recycling.
Two processes, one important difference
The researchers worked with industrially dyed black, navy, dark green and grey 100% cotton fabrics, each containing mixtures of reactive dyes rather than a single laboratory dye. They compared two processes using sodium carbonate and sodium dithionite, chemicals selected using Safe and Sustainable by Design considerations. In the two-bath process, the cotton first undergoes an alkaline treatment using sodium carbonate. The fabric is then washed, rinsed and transferred to a separate bath containing sodium dithionite for the second colour-removal step. The one-bath process removes this intermediate stage. Instead of discarding the first treatment liquid, sodium dithionite is added directly to the same bath after the temperature is reduced. This may sound like a relatively small process change, but it removes an entire washing and treatment stage.
Turning dark fabrics into white recycling material
Both methods achieved high levels of colour removal. The two-bath processes reached up to 99% decolourisation, while the one-bath process achieved around 97% or more for most tested fabrics, with lightness values approaching or exceeding the target required for a high-quality recycling feedstock. Importantly, the one-bath process generally produced white and more uniformly decolourised fabrics than the two-bath approach. Not every dye behaved in exactly the same way. Grey fabric proved particularly challenging because one of its dyes contained copper, which increased its stability and made complete removal more difficult. This highlights an important reality for textile recycling: real garments contain complex combinations of dyes, and recycling technologies must be able to deal with this diversity rather than only ideal laboratory samples.
Using less water and energy
The strongest advantage of the one-bath process was not only its colour-removal performance, but its greater resource efficiency. Because the same liquid is used for both stages, the process eliminates the intermediate washing step and avoids the need for a second decolourisation bath. For producing one kilogram of white cotton feedstock, the study estimated:
· 134.7 kg of water for the one-bath process, compared with 243.1 kg for the two-bath process;
· 6.73 kWh of electricity, compared with 8.84 kWh;
· 0.045 kg of soap, compared with 0.088 kg.
The amounts of the main decolourising chemicals remained essentially the same. The one-bath approach also generates a single main wastewater stream rather than two separate treatment streams, simplifying wastewater management. The authors note that future systems could potentially recover and reuse some of the chemicals from this wastewater, further improving resource efficiency. For consumers, the process itself may remain invisible. But technologies like this could help make recycled cotton less resource-intensive and easier to produce at industrial scale.
Removing the dye without destroying the cotton
Colour removal is only useful if the fibre underneath remains valuable. The researchers therefore examined much more than appearance. They measured tensile strength, weight loss, cellulose structure, chemical composition, thermal behaviour and fibre morphology. The results showed that the cotton retained very good mechanical performance. After the full treatments, tensile strength retention was above 94% for most fabrics, while even the lowest result remained above 88%. Microscope images also showed that the fibres retained their characteristic cotton structure, with only minor surface changes and no substantial damage from either the one-bath or two-bath process.
At the molecular level, some shortening of cellulose chains occurred during the high-temperature alkaline stage. However, the later reductive treatment caused very little additional degradation, and the crystalline structure of cellulose remained preserved. Chemical analysis also showed that sulfur associated with the dyes was no longer detectable after treatment, while the thermal behaviour of the decolourised cotton moved closer to that of undyed cotton.
Together, these findings indicate that the process removes much of the unwanted dye while retaining the characteristics needed for further recycling.
From the laboratory towards industrial scale
A particularly encouraging part of the study was the first scale-up of the one-bath process. After laboratory optimisation, the researchers tested the method in a larger air-bath digester. The scaled-up samples matched or exceeded the laboratory results for lightness, decolourisation and whiteness, while some fabrics also showed lower weight loss. The samples were uniformly decolourised even without strong agitation, suggesting that the process has potential for larger-scale operation. This does not mean the technology is already ready for widespread industrial use. Further development is still required, including environmental and economic assessment, chemical and water recovery, and testing on more complex textile waste streams. But demonstrating that the process still performs when moved beyond very small laboratory samples is an important step towards practical implementation.
What could this mean for the clothes we wear?
For consumers, the long-term goal is simple: more of the cotton already in circulation could become new textile material instead of waste. A successful decolourisation process could help recyclers combine waste fabrics of different colours into a cleaner, more consistent raw material. This could increase the range and quality of products that can be made from recycled cotton. Instead of an old black shirt being restricted to dark or low-value recycled products, its colour could first be removed. The recovered cotton could then potentially be spun into new yarns through mechanical recycling, blended with other fibres, recoloured for new textile products, or dissolved and regenerated into new cellulose fibres through technologies such as Lyocell or Ioncell. The paper concludes that the treated cotton is already suitable for mechanical recycling, while further adjustment of its cellulose properties could also support chemical recycling.
The next step towards textile-to-textile circularity
The research team now highlights several priorities for future work. These include life-cycle and economic assessments at industrial scale, adaptation of the process to cotton-polyester blends and other types of dyes, integration with mechanical spinning and chemical fibre-regeneration processes, and recovery and reuse of water, chemicals and dyes. These next steps are essential. A technically effective recycling process also needs to be environmentally sound, economically realistic and compatible with the complex mixture of materials found in real textile waste. The study nevertheless demonstrates an important principle: recycling cotton does not necessarily require increasingly complex processing. In this case, combining two treatment stages into one bath improved colour removal while reducing resource use and simplifying the process. That is precisely the kind of practical improvement needed to move textile circularity from ambition towards industrial reality.
The work was carried out by researchers at Aalto University and was supported by the BioSusTex project (Grant Agreement No. 101135372), funded by the European Union, together with the Business Finland-funded TexirC project.
Read the open-access paper:Comparative study of decolorization processes for dyed cotton waste: enabling circularity through white feedstock, RSC Advances, 2026. DOI: 10.1039/d6ra05247a