A two-stage reduction–oxidation process turns dark reactive-dyed cotton into light, redyeable feedstock while retaining about 87% of tensile strength, addressing a major bottleneck in textile-to-textile recycling.
Researchers at Aalto University in Finland have developed a chemical process capable of removing up to 99.3% of colour from reactive-dyed cotton waste while largely preserving the cellulose needed for subsequent recycling.
Published in ACS Omega in August 2026, the study addresses a difficult circularity problem: reactive dyes are covalently bonded to cellulose, making coloured cotton harder to convert into consistent, high-quality recycled fibre.
Reduction followed by oxidation
The researchers treated dark green, grey, black and navy cotton containing mixtures of commercial reactive dyes.
The first stage used sodium dithionite (Na₂S₂O₄) as a reducing agent. A subsequent hydrogen-peroxide oxidation removed remaining colour fragments.
Under optimised conditions, colour removal ranged from 93.1% for grey to 99.3% for black, while lightness reached L* values as high as 89.1 and the maximum whiteness index reached 87.6.
Fibre damage remains limited
Effective decolourisation is only commercially useful if the cotton survives treatment.
Average tensile-strength retention was approximately 87.2%, with individual samples generally retaining 86–92%. Maximum material weight loss was 3.5%, while the cellulose maintained a minimum degree of polymerisation of 1,404.
Researchers also successfully redyed the stripped material, with uniform colour uptake. The resulting cotton could therefore potentially enter either mechanical recycling through shredding and blending or chemical recycling into regenerated cellulose fibres.
Colour becomes a recycling variable
The work forms part of Aalto’s TexirC project, which is tackling textile-waste analysis, colour stripping, fibre separation and synthetic-fibre recycling.
For recyclers, removing colour could reduce the need to sort waste into narrow colour streams and produce a more standardised cellulose feedstock.
Industrial viability, however, still depends on chemical recovery, effluent treatment, water and energy consumption, continuous processing and cost per kilogram. The researchers explicitly identify these as scale-up priorities. If those economics work, decolourisation could become an important preprocessing stage connecting post-consumer cotton waste with higher-value fibre-to-fibre recycling.
