Significance Large amount of waste textiles are heaped up, landfilled or incinerated as solid waste year by year, leading to heavy pollution of the environment and waste of the resources. In the face of the worsening energy crisis and environment troubles, the recycling of waste textiles has become a pressing social problem to be tackled. Various ways have been used physically or chemically in the study of the recycling of waste textiles. Among others, hydrothermal degradation is a highly noticeable thermochemical conversion technology featuring environmental friendliness, productive richness, and economy, which can effectively degrade and recycle cellulose textiles, and obtain high value-added hydrothermal products. The green and high-valued recycling of waste textiles are of great economic and social importance for the development of circular economy and reduction of pollution and carbon emission.
Progress New technologies are introduced for hydrothermal degradation of waste cellulose textiles in subcritical (supercritical) environments. With the special properties of subcritical (supercritical) water, cellulose fibers can be degraded hydrothermally into high value-added chemical products such as water-soluble sugar, hydrothermal carbon, and bio-oil. After step-by-step separation, the hydrothermal products can be effectively recovered without pollutant emission. The strong hydrogen bond and stable structure of high crystallinity of cellulose makes it hard to degrade at room temperature, which is the biggest problem in natural cellulose utilization. This paper analyzed the mechanism and reaction process of cellulose fiber hydrothermal degradation, summarized different processes of the hydrothermal degradation and corresponding target products, and concluded the influencing factors of the process and products. In the subcritical (supercritical) water system, acting as a reactant, a catalyst and an organic solvent concurrently, water can effectively break the hydrogen bond and crystal structure, and promote the break of glycosidic bond of cellulose. Under certain hydrothermal conditions, cellulose materials are degraded into substances such as glucose, fructose, 5-HMF (5-hydroxymethylfurfural), and organic acids. The soluble oligomers obtained through hydrolysis will form various carbon containing materials, bio-oils, and a small amount of gases after dehydration, polymerization, condensation, and aromatization. The hydrothermal degradation can be divided into processes of hydrolytic saccharification, hydrothermal carbonization, hydrothermal liquefaction and hydrothermal gasification, with respective products of water-soluble sugar, hydrothermal carbon, bio-oil and gas products. Temperature, time, catalyst, and cellulose structure are important factors affecting the hydrothermal degradation process and degradation products of cellulose. Different reaction conditions result in different degradation rates, products, yields, and properties of the hydrothermal degradation. By adjusting the temperature, catalyst and other parameters, the hydrolysis process can be regulated and the selectivity of target products can be adjusted to achieve different target products.
Conclusion and Prospect Hydrothermal treatment of cellulose fibers can yield high value-added chemical products such as glucose, 5-HMF, lactic acid, and carbon microspheres, while selective degradation of cellulose fibers can achieve effective separation and recycling of cellulose based blended fabrics, demonstrating that hydrothermal degradation technology of cellulose fibers is an effective way for high-valued recycling of waste textiles. However, researches in the area are still in the phase of laboratory exploration, at certain distance from large-scale production, due to the complex process relating to cellulose hydrothermal degradation. Still, the complexity of the hydrothermal degradation process and uncertainty of the decomposition mechanism and pathway of intermediate products, and the catalytic mechanism and regulatory mechanism of catalysts led to low yield of target products. Consequently, additional research should be conducted on the existing issues. With the evolution of the research, the industrial application of hydrothermal degradation technology in waste textiles treatment in the future will achieve significant social and economic benefits.