Aalto Researchers Tackle Cotton Recycling’s Color Problem

Color may be standing between an old cotton garment and its next life.
 
Researchers at Aalto University in Finland have developed a two-step process that stripped as much as 99.3 percent of the color from reactive-dyed cotton while preserving most of the material’s strength, potentially addressing a less visible obstacle to textile-to-textile recycling: what to do with dyes already chemically bonded to discarded fibers.
 
The study—published Aug. 13 in the peer-reviewed American Chemical Society (ACS) Omega scientific journal—used sequential reductive and oxidative treatments to turn dark green, gray, black and navy cotton into a lighter, redyeable feedstock. The researchers said removing color before recycling could make recovered cotton more useful for both chemical and mechanical recycling.
 
Color presents a particular problem for cotton recycling because reactive dyes, widely used on cellulosic fibers, form strong covalent bonds with cellulose. Mechanically shredding differently colored cotton can also produce unpredictable mélange shades, limiting the colors and applications available to recyclers. Although considerable research has focused on removing dyes from wastewater, comparatively less has addressed removing them directly from textile substrates, according to the researchers.
 
Ali Tehrani-Bagha, the paper’s corresponding author and a senior university lecturer at Aalto, said the team developed the process specifically for cotton fabrics colored with reactive dyes, which are particularly difficult to remove because they are chemically bonded to cellulose.
 
The researchers first treated the cotton with sodium dithionite, a reducing agent, before bleaching it with hydrogen peroxide under alkaline conditions. The optimized reductive stage used 20 grams per liter of sodium dithionite at 60 degrees Celsius for 60 minutes, followed by an oxidative stage using 1 gram per liter of hydrogen peroxide at 90 degrees Celsius for another 60 minutes. Both stages used a material-to-liquor ratio of 1:20.
 
The distinction between the two stages matters. Sodium dithionite primarily alters the dyes’ chromophores, reducing their visible color without fully breaking the bonds connecting the dyes to cellulose. The hydrogen peroxide treatment then attacks the remaining dye-cotton linkages and removes residual color.
 
Across the four colors, the complete treatment removed between 93.1 percent and 99.3 percent of the color. Black cotton recorded the highest removal rate at 99.3 percent, followed by dark green at 99.2 percent, navy at 98.3 percent and gray at 93.1 percent.
 
The researchers also benchmarked the treatment against previously published color-stripping methods. One earlier sequential treatment of mixed reactive dyes achieved 97.8 percent color removal but used a higher liquor ratio and substantially more hydrogen peroxide, while recording 23.9 percent tensile-strength loss. Aalto’s process reached as much as 99.3 percent color removal with 10.9 percent tensile-strength loss in the comparison. The authors characterized their method as using comparatively less chemistry while having a smaller impact on mechanical properties.
 
Removing dye, however, is only useful to recycling if the process does not destroy the cellulose along with it.
 
The treated fabrics retained between 86 percent and 92 percent of their tensile strength, depending on the sample and direction tested. Total weight loss after both treatments ranged from 2.2 percent to 3.5 percent. Microscopy showed some localized fibrillation after oxidation, while crystallinity changed only slightly.
 
The cellulose did undergo some depolymerization. Its degree of polymerization fell from 3,395 in the undyed reference cotton to between 1,404 and 1,701 following the complete treatment.
 
Tehrani-Bagha said the decline results from some cellulose-chain cleavage during the treatments but should not prevent the material from undergoing further chemical recycling.
 
“A degree of polymerization of approximately 1,400-1,700 remains sufficiently high for further chemical recycling through cellulose dissolution and wet spinning,” he said, adding that some dissolution and spinning processes may actually require a lower, more controlled degree of polymerization.
 
What happens over multiple recycling loops remains unknown. Repeated recycling could cause additional cellulose degradation, Tehrani-Bagha said, but the researchers have not yet investigated multiple cycles and cannot say how many times the material could ultimately be recycled.
 
That trade-off is important. A color-stripping process aggressive enough to create a lighter feedstock but damaging enough to make the cellulose unusable would merely exchange one recycling problem for another.
 
Chemical analysis provided further evidence that the treatment removed more than the cotton’s visible color. Following oxidation, sulfur was no longer detectable in the bulk elemental analysis and nitrogen had fallen to a negligible trace. Carbon, hydrogen and oxygen levels approached those of the undyed cotton reference, according to the study.
 
The researchers also tested whether the stripped material could take color again. The redyed fabrics showed uniform color uptake and washing and rubbing fastness comparable to the reference fabric, according to the study.
 
The experiment stops well short of proving that decolorization can economically operate at textile-recycling scale, however.
 
The study used a batch process on 100 percent cotton woven fabrics supplied by Turkey’s Elyaf Tekstil. Before treatment, the researchers put the dyed fabrics through 10 washing and drying cycles to mimic the cumulative effects of laundering. The four samples contained mixtures of reactive dyes rather than the single dyes frequently examined in previous stripping research.
 
That makes the samples more representative of used textiles than pristine, single-dye laboratory swatches, but it does not replicate the full disorder of an actual post-consumer waste stream, where recyclers can encounter blended fibers, finishes, prints, sewing thread and unknown chemical histories.

The researchers have not yet systematically tested the process on printed or water-repellent textiles. Tehrani-Bagha said chemical binders used in pigment printing and water-repellent finishes could reduce its effectiveness. Cotton containing 4 percent to 5 percent elastane, however, does not hinder the color-stripping process, he said.
 
Nor have the researchers yet established the process’s industrial environmental or economic balance. Tehrani-Bagha said the team does not yet have sufficiently reliable estimates of industrial-scale costs and is conducting a life-cycle assessment that it plans to report in its next publication.

“As with other wet chemical processes in the textile industry, the consumption of water, energy and chemicals is considerable,” he said. Water reuse and chemical recovery could significantly improve both the environmental and economic performance of the process, he added, and will be important considerations in determining its commercial viability.
 
The current process requires two hour-long chemical treatments at 60 degrees and 90 degrees Celsius, respectively, along with multiple rinsing, soaping and neutralization steps. Whether the increased value of a lighter, redyeable recycled feedstock can compensate for those additional processing inputs remains an open question.

The process reported in ACS Omega has so far been developed and evaluated at laboratory scale. The researchers are comparing three laboratory-scale color-stripping approaches, two of which have now been scaled to the kilogram level, Tehrani-Bagha said. He did not specify whether the sequential dithionite-peroxide process reported in the paper is among those two.
 
Although the experiment used a batch process, the researchers see a potential route to continuous production. The treatment “can be conceptually extended to continuous operation” using established textile systems such as pad-batch or continuous washing ranges, according to the paper.
 
Because the chemicals are already common in textile processing, Tehrani-Bagha said the researchers believe the process could ultimately be adapted to existing industrial wet-processing equipment. The next step is demonstrating consistent color removal and fiber-quality retention at larger scale, with chemical consumption, wastewater quality and process control among the principal scale-up considerations.
 
The work was funded in part by Business Finland through the Tackling Textile Circularity Challenges, or TexirC, project. The researchers declared no competing financial interests.
 
For textile recyclers, the question is increasingly not simply whether cotton can be recovered, but what quality of raw material emerges on the other side. A fiber that retains its original color can constrain what comes next. Removing that history could make recycled cotton considerably more flexible, provided doing so does not require more resources than the resulting fiber is worth.