## 1 Introduction  
Growing concern over the negative impact of the buildup of microplastics in the environment is related to the disposal of single-use plastics (e.g., air filters derived from synthetic polymer fibers). More recently, the development of sustainable air filters has been based on biomaterial platforms including hemicellulose, chitin, and chitosan, which represent a more sustainable approach due to the lower environmental impact upon disposal [1; 2]. Cellulose is a natural biopolymer that is derived from lignocellulosic biomass sources such as flax, hemp, wood, and cotton [3]. Fax (_Linum ustiatssimum_ L.) fiber belongs to the best fiber family, which is sourced from the inner bark of the flax plant stem. These fibers are known for their high strength and rigidity, and, unlike synthetic fibers, they show little stretching under load [4]. Flax fibers (FFs) exhibit a composite-like hierarchical structure, deriving their properties from micro- and nano-level configurations, making them excellent candidates for creating biodegradable composites. The anatomy of a flax stem includes layers like bark, phloem, xylem, and a central void, with fiber bundles located along the outer surface. FFs can range in length from 80 to 150 cm, where the primary components are cellulose, hemicellulose, and lignin, along with the presence of residual oils [5; 6]. Studies have shown that air filters made from biodegradable fibers have comparable filtration efficiency to traditional synthetic filters but may require some pretreatment to achieve the desired fiber properties, such as durability and filtration effectiveness [7]. In this regard, a wide range of biological, physical, and chemical oxidation processes have been studied [8; 9; 10]. Comparison of these methods indicates that chemical oxidation processes are potentially more efficient and cost-effective. One drawback accompanying chemical treatment with alkali agents such as sodium hydroxide (NaOH) relates to the generation of large quantities of wastewater that represent hazardous caustic waste to the environment and human health [8; 11]. Several chemical treatments have been applied to natural fibers. For example, the use of NaOH involves immersion of the fibers in an alkali solution to remove fiber components such as lignin or impurities to yield cellulose-enriched products with greater surface area [12]. Alkali treatment can improve the wettability and adhesion of fibers, as well as enhance their mechanical properties [13]. However, excessive alkali treatment of fibers can lead to degradation and reduced tensile strength. By comparison, the acid treatment process involves exposure of fibers to acid media, such as aqueous sulfuric or hydrochloric acids. In turn, acid treatment can improve fiber flexibility and fiber strength, and alter the surface area and fiber porosity [14]. However, chemical treatment with acids can also damage fibers and release toxic byproducts into the environment. Additional types of chemical treatments include bleaching and dyeing [15; 16; 17; 18]. Bleaching can improve fiber whiteness and cleanliness, while dyeing can add color and aesthetic appeal. The choice of chemical treatment for the processing of natural fiber filter media should be based on a balance between the desired properties and sustainable processing to minimize potential negative environmental and health impacts. Optimization of treatment conditions (reagent concentration, temperature, and processing time) can reduce adverse effects while maximizing desired outcomes to obtain natural fibers with desired properties for tailored applications. In this regard, researchers have directed research toward the development of sustainable fiber treatment methodologies to address the limitations of conventional chemical processes [8]. The Fenton process is an advanced oxidation reaction (AOR) that has attracted attention due to its efficiency, cost-effectiveness, and potential for green chemical processing.