Being exposed to ambient air pollution is a severe threat to human health causing around 4.2 million deaths every year ^[1][2][3][4][1,2,3,4]. Particulate matter (PM) is the main air pollutant in urban environments that causes severe health problems ^[2][3][5][2,3,5]. These small particles, solids or liquid aerosols, come from a variety of sources, both natural and anthropogenic, such as fossil fuel combustion, construction dust, and industrial processes ^[6][7][6,7]. Mainly, two types are defined based on particle diameter: PM10 are particles with a diameter of 10 µm or less and PM2.5 are particles with a diameter of 2.5 µm. These particles, especially PM2.5, can penetrate deep into the lungs, leading to severe health issues ^[3][5][3,5].

One way to combat air pollution is to address the problem at its source and reduce emissions from polluting activities. Another way is to filter polluted air using active or passive filtration technology ^{[8][9][10][11][12]}[8,9,10,11,12]. Various methods and systems have been introduced to actively filter polluted outdoor air ^[8][11][12][8,11,12]. Among these are solutions that make use of the air-purifying properties of plants. One such method is the use of ‘moss walls’ ^[11][12][11,12]. These systems, illustrated in Figure 1a, draw in polluted air through ventilation fans installed inside a hollow wall that is covered with ‘moss filters’, passing the stale air through the moss and the substrate on which the moss grows. After filtering, the purified air is released back into the environment. Due to the absence of a developed root system mosses have developed a very high surface area of densely packed leaves to absorb water and nutrients compared to other plants ^[13][14][13,14]. This allows them to trap air pollutants, such as PM, in a very efficient way, similar to manmade filters [15]. Research performed in 2019 at the University of Antwerp has shown that the filtration efficiency of moss panels is 43.1% for PM10 and 22.8% for PM2.5 [16].

To use living moss as part of a filter setup in an air purification system, it needs a breathable and moisture-retaining substrate on which it can grow. Currently, in the moss wall that is the subject of this study, a synthetic microfiber cloth is used for this purpose, see Figure 1b ^[11][16][11,16]. However, this material is highly environmentally polluting as it contains and releases microplastics to the environment, i.e., pieces of plastic less than 5 mm in diameter, that are not degradable and are harmful to various ecosystems [17]. The exposure of microplastics to humans, whether through ingestion or inhalation, can cause potential health risks. These risks include medical conditions, such as gastrointestinal issues, chronic pulmonary diseases, fibrosis in the lungs, and an elevated risk of lung cancer ^[18][19][18,19].

To mitigate the entry of microplastics from the substrate of the moss wall into the environment, this study aims to lay the foundations for a bio-based alternative to microfiber cloth, utilizing excess moss from the system as a raw material. By using saturated moss from the filters and/or excess moss (after cleaning), the system will become more self-sustaining with reduced dependence on external inputs. Therefore, this solution not only addresses immediate environmental concerns but also promotes the circularity of the moss wall.

2. Biocomposite Materials

Biocomposites are materials consisting of a reinforcing phase and a matrix surrounding the reinforcing phase, where at least one of the two is derived from natural sources. They can be categorized based on the type of matrix and fibers they employ, resulting in two main classifications: partially biodegradable and completely biodegradable. Within the category of completely biodegradable biocomposites, bio-fibers are utilized in combination with matrices composed of biodegradable polymers. However, currently, the market for fiber reinforcement is still dominated by synthetic fibers, resulting in mainly partially biodegradable solutions [20]. In many applications, natural fibers are a good alternative to synthetic fibers that are derived from petroleum or other non-renewable resources. This is supported by extended research on the mechanical properties of natural fibers, which exhibit promising similarities to traditional fibers but still have room for further improvement [21]. They are considered to be more environmentally friendly as they are made from renewable resources. Moreover, they have low CO₂ emissions during processing, are abundantly available and cheap, and can be recycled or biodegraded at the end of their life [22]. By utilizing these materials, biocomposites can offer a sustainable solution to the disposal management issues faced by certain countries, addressing waste accumulation while promoting resource efficiency [23]. Much progress has already been made in developing methods to make biocomposites using different combinations of natural fibers and binders. In this respect, several recent studies have explored the use of moss-based biocomposites for constructing thermal insulation plates in buildings ^[24][25][26][24,25,26]. In these studies, Sphagnum moss was used as the main component in combination with various binders, viz., animal glue, liquid glass, and urea-formaldehyde. Biocomposites for thermal insulation applications have also been produced from hemp and flax shives with potato starch as a binder. The aggregate–binder ratio was chosen to be the lowest possible so the sample could withstand its self-weight, resulting in a ratio of 1:3. Starch was prepared using the casting method, whereafter the mixture was compressed and left to dry [21]. Starch is also commonly used in the packaging industry for the preparation of bioplastics. As shown by Marichelvam et al. (2019), the tensile properties of starch are suitable for the production of packing materials, and the preparation method is straightforward [27]. Furthermore, several papers have investigated the use of metakaolin in combination with corn starch to create a biofilm. This combination results in an improvement in the mechanical tensile strength and hydric properties of the obtained biocomposites ^[28][29][28,29]. The use of wood and paper pulp as binders in biocomposites is currently also being researched [30]. Fuentes et al. (2021) developed biodegradable pots based on different waste materials such as, among others, used paper, wheat flour, and corn-waste flour [31]. Additionally, a combination of wooden fibers and peat moss has been used for the design of biodegradable pots, containing 80% wooden fibers and 20% peat moss, to improve the uptake of water [32]. To conclude, the growing interest in biocomposites offers numerous opportunities to develop sustainable materials for diverse applications. Although there is a lot of ongoing research on natural fibers and biodegradable polymers to replace petroleum-based products, the development of fully biodegradable biocomposites is still in its early stages. Moreover, the integration of moss fibers alongside other natural fibers and biodegradable polymers to develop biocomposites, other than for insulation purposes, remains a relatively unexplored topic. In the broader context of continuous research on moss walls for urban air purification, and with the aim of advancing the state of the art in (moss-based) biocomposites, this study therefore explores the feasibility of creating a new fully biodegradable, moss-based biocomposite. In the first instance, the objective is to develop a material that uses excess moss from the system as a raw material and that can support its own weight while preserving its structural integrity. Consequently, this preliminary investigation stands as a meaningful contribution to ongoing research efforts and advancing the current state of the art.