Decolorization of cellulose-based post-consumer textile waste in a sequence involving sodium hydroxide, chlorine dioxide, and hydrogen peroxide

Every year, the world discards mountains of clothing. Much of this waste ends up in landfills or is burned, a fate that is increasingly difficult to accept as the fashion industry churns out more garments than ever before. A promising solution lies in turning these old clothes back into new ones, a process known as fiber-to-fiber recycling. However, this circular dream faces a stubborn obstacle: color. Most discarded textiles are a chaotic mix of fabrics dyed in countless shades, and to make them useful again, that color must be stripped away. The challenge is not just removing the dye, but doing so without destroying the fabric itself. The material at the heart of this puzzle is cellulose, the natural fiber found in cotton and the regenerated fibers used in viscose. These fibers are long chains of molecules that give cloth its strength, but they are also delicate; the very chemicals needed to bleach away the dye can sometimes snap these chains, turning strong thread into weak, useless mush.

Researchers at Nouryon, a chemical company in Sweden, set out to solve this balancing act. They focused on a specific type of waste: a mixture of post-consumer cotton and viscose garments that had been sorted, shredded, and prepared for treatment. Their goal was to find a sequence of chemical steps that could wash the color out of these mixed fabrics while keeping the fibers intact enough to be spun into new yarn. They tested a three-step process involving sodium hydroxide, chlorine dioxide, and hydrogen peroxide. The first step, using sodium hydroxide, acts like a strong soap that helps loosen the dye from the fibers. The second step introduces chlorine dioxide, a powerful bleaching agent, and the final step uses hydrogen peroxide to finish the job and brighten the material. The researchers discovered that this combination could transform dark, mixed-color waste into a bright, clean material, but the exact conditions of each step mattered immensely.

The journey began with the alkaline treatment. When the researchers treated the shredded fabric with sodium hydroxide at high temperatures, the color began to lift. They found that adding a specific amount of the chemical—150 kilograms for every tonne of fabric—was enough to remove a significant portion of the dye without causing excessive damage. This initial wash raised the brightness of the cotton from a dull 10 percent to about 40 percent, and the viscose from 7 percent to nearly 50 percent. The liquid that drained away during this stage was dark with the removed dyes, proving that the color was being successfully separated from the fibers. Crucially, the researchers observed that while the cotton fibers lost some of their internal strength during this process, the viscose fibers remained surprisingly stable, holding their structure well even under the chemical assault.

Next came the chlorine dioxide stage, which served as the heavy lifter for removing the remaining color. The researchers adjusted the acidity of the water in this step, finding that a slightly acidic environment was the sweet spot. At this level, the chlorine dioxide stripped away the rest of the dye, pushing the brightness of the cotton up to 74 percent and the viscose to 77 percent. This stage was particularly important because it allowed the team to control the strength of the fibers. By fine-tuning the acidity, they could prevent the chemical from breaking down the cellulose chains too aggressively. This meant they could target a specific level of fiber strength, ensuring the material would be robust enough for future use. The process was so effective that the final product looked like a clean, white sheet of paper, a stark contrast to the colorful, mangled scraps that started the process.

The final step involved hydrogen peroxide, a common bleaching agent, but with a twist. The researchers added magnesium to the mix, a move that acted as a stabilizer. In the absence of magnesium, the hydrogen peroxide can sometimes become too aggressive, breaking down the fibers and releasing organic waste into the water. With magnesium present, the bleaching became more controlled. The fibers retained more of their original strength, and the amount of dissolved organic material in the wastewater dropped significantly. This was a vital finding, as it showed that the process could be made cleaner and more efficient. The magnesium did not make the fabric whiter than it would have been without it, but it protected the fabric's integrity and reduced the environmental burden of the treatment.

Throughout the experiment, the team also looked closely at what remained in the final product. They found that some colored fibers persisted, but when they analyzed these stubborn bits, they discovered they were not cotton or viscose at all. Instead, they were polyester, likely from the sewing threads used to stitch the original garments. This contamination is a known issue in recycling, as polyester does not dissolve in the same way cellulose does. The researchers noted that while this is a hurdle, it is one that can be managed with better sorting or separation techniques before the bleaching process begins. The study concluded that the three-step sequence was a viable path forward. It successfully turned a complex, mixed waste stream into a bright, high-quality raw material, proving that the color of our discarded clothes does not have to be the end of their story.