The fashion industry has a considerable environmental impact, especially due to the increased generation of waste textiles as a result of fast fashion business models. Although fiber-to-fiber recycling processes are being developed, such a process is in reality a downcycling process, in which the mechanical properties of the textile fibers are impoverished with each cycle. Thus, new alternatives are required to completely close the fashion loop through chemically recycling textile fibers unfit for other types of recycling or resale due to their poor quality. We have evaluated the possibility of using acid hydrolysis to directly depolymerize the cotton fibers in waste textiles to produce a glucose solution, which could subsequently be used for the production of chemicals or fuels. Although a one-step procedure with sulfuric acid was unable to deliver high glucose production, it was possible to achieve a glucose yield over 90% through a two-step procedure, in which concentrated and dilute sulfuric acid were combined to exploit the benefits of both concentrations. Glucose concentrations around 40 g/L were achieved by increasing the solids loading in the two-step process, which might be sufficiently high for the fermentation of the solution into high-value products. Thus, this study demonstrates that it would be possible to chemically recycle (cellulose-based) waste textiles via acid hydrolysis, which, if correctly designed, could avoid the need to use enzymes to achieve high conversion efficiencies.

Large amounts of cellulose-based waste textiles are generated every year, yet little is done to recycle this waste. Alternatives such as fiber-to-fiber recycling, where a significant part of the value of the waste textiles is recovered, are attractive possibilities. In this study, we have investigated the viability of using hydrated zinc chloride (ZnCl₂·4H₂O) as a solvent and swelling agent to convert cotton waste textiles (the most abundant cellulose-based waste textile) into a dissolving pulp that can be used as raw material for the production and spinning of viscose fibers. The solvent produced an accessible dissolving pulp and exhibited excellent recyclability, maintaining good dissolving power even after repeated recycling. The dissolving pulp was subsequently used to produce viscose dope, a spinning solution which was spun and cut into viscose staple fibers. The viscose dope exhibited good properties (moderate filter clogging value and gamma number), and the resulting staple fibers were strong and of good quality (high linear density, elongation, and tenacity). These results illustrate the potential of using hydrated zinc chloride for the production of viscose grade dissolving pulp from cotton waste textiles.

The fashion industry is becoming one of the largest emitters worldwide due to its high consumption of raw materials, its effluents, and the fact that every garment will eventually contribute to the vast amount of waste being incinerated or accumulating in landfills. Although fiber-to-fiber recycling processes are being developed, the mechanical properties of the textile fibers are typically degraded with each such recycle. Thus, tertiary recycling alternatives where textiles are depolymerized to convert them into valuable products are needed to provide end-of-life alternatives and to achieve circularity in the fashion industry. We have developed a method whereby cotton waste textiles are depolymerized to form a glucose solution, using sulfuric acid as the sole catalyst, with a high yield (>70%). The glucose solution produced in this process has a high concentration (>100 g/L), which reduces the purification cost and makes the process industrially relevant. This method can be applied regardless of the quality of the fibers and could therefore process other cellulosic fibers such as viscose. The glucose produced could subsequently be fermented into butanediol or caprolactam, precursors for the production of synthetic textile fibers, thus retaining the value of the waste textiles within the textile value chain.

Recycling of textile fibers is a mandatory step in the life-cycle of fabrics to avoid incineration or landfilling. After fiber-to-fiber recycling, cellulose fiber saccharification opens the door to the production of biochemical building blocks through microbial fermentation. This study documents for the first time the production of optically pure D-lactic acid (D-LA), one of the precursors of polylactic acid (PLA), from post-consumer waste viscose. Enzymatic hydrolysis was applied to white and colored waste viscose to produce two glucose-rich solutions that were used as carbon source for the genetically modified Pediococcus acidilactici ZP26. Afterward, batch and fed-batch cultivations were conducted with very similar results for the control and both waste viscose hydrolysates (white and colored). The strain was able to produce D-LA without any signs of inhibition by possibly toxic compounds present in the recycled textiles (additives, dyes, and other contaminants). The highest D-LA concentration and yield was reached in the batch reactors with colored waste viscose at 42.4 g/L and 0.97 g/g, respectively. In the fed-batch cultivations, the best result was achieved with the white waste solution, with 59.5 g of D-LA (40.8 g/L), and a yield on total glucose of 0.70 g/g. Notwithstanding the need for more research in the utilization of saccharified waste viscose as carbon source for bioprocesses, this is an important proof of concept of an application of streams from tertiary recycling of cellulosic fibers in the production of platform biochemicals. Graphical Abstract

Viscose turns recalcitrant towards hydrolysis after use. Chemical pretreatment does not increase hydrolysis yield, as opposed to other biomass feedstocks.