The circular bioeconomy (circular economy + biological resources) creates a sustainable system, turning waste into valuable resources. In wastewater treatment, it optimizes water reuse, recover nutrients, generate renewable energy, and develop valuable products [1]. Annually, approximately 380 km3 of global wastewater is produced, with treatment rates varying (developing countries: ~8%, developed countries: ~70%) [2]. High treatment costs (e.g., Membrane Bioreactor= ∼600 USD/(m3/d), China 2018) [3] drive the search for cost-effective alternatives. In developed and rural areas, filtration/biofiltration technologies, evaluating raw/modified agro-industrial residues as filter beds (e.g., sugarcane bagasse, rice husks), are gaining traction [4]. A review of the last five years on SCOPUS with keywords "biosorbents" and "agro-industrial residues" identified approximately 250 relevant research studies (Fig. 1).
Despite effective removal of contaminants (organic matter, nutrients, heavy metals, etc.) by filters/biofilters (up to 100%), challenges lie in recovering and sustainably managing spent materials (post-adsorbents). Less than 1% of SCOPUS publications explore applications for spent materials. Common methods for post adsorbent use include regeneration through chemical/thermal/biological processes, microwave irradiation, etc. However, these processes face industrial-scale limitations due to high energy requirements or acids/bases/chelates/supercritical fluids consumption [5]. Regenerated adsorbents, though less effective (up to 80%), are typically reused for the same contaminant removal [6]. They can also be applied to other contaminants removal, but careful treatment or disposal is fundamental to avoid secondary environmental contamination [7]. Incineration for biochar production offers an alternative, enhancing soil properties and serving as fertilizer if previously used for nutrient removal [5, 7].
Furthermore, post-adsorbents can serve as super capacitors and catalyst/catalyst support. For instance, biochar initially used to remove heavy metals may exhibit an enhanced catalytic effect or transform into a super capacitor, replacing expensive nanomaterials like carbon nanotubes [5, 6]. Discovering a secondary use for spent adsorbents would reduce treatment costs. However, it's important to ensure that adsorbed contaminants are not released [7]. To prevent release, spent adsorbents could be "encapsulated," serving as reinforcement for composites [8].
Over the past five years, approximately 4,150 studies have employed natural fibers as reinforcements for polymeric composites, yet none have delved into the potential of post-adsorbents. Their incorporation could enhance the properties of composites owing to the biota present in the material of the filtering bed [9]. The biological treatment of the reinforcement offers advantages, as microorganisms produce enzymes that eliminate undesirable substances such as waxes, lignin, pectic compounds, and lignocellulosic materials from fibers [9, 10]. These microorganisms reduce the fiber hydrophobicity and increase roughness, improving thus the interlocking of reinforcement-matrix, thereby strengthening the mechanical properties of the composite [9].
Despite these benefits, biological treatment poses challenges, including prolonged processing times and the difficulty of obtaining specific microorganisms/enzymes at local market [9]. Utilizing microorganisms from wastewater for simultaneous treatment (wastewater) and biological modification (reinforcement) could potentially overcome these issues.
In composites, the matrix also plays a pivotal role. Currently, studies are exploring polymerizable thermoplastic resin matrices, such as acrylic-based resins [11, 12]. Acrylic-based resins exhibit strong adhesion to different substrates, resistance to aging, light stability, effective pigment binding, easy application, and cost-effectiveness, making them advantageous for composites [13]. Within the framework of the circular bioeconomy, it has been recognized that OPEFBF can be efficiently applied in several areas such as composting, water treatment, and composite reinforcement, etc. Nevertheless, it is important to highlight that, to date, post-adsorbents from biofilters have not been explored as potential reinforcements. Furthermore, considering the promising alternative provided by acrylic resins, this study seeks to evaluate the properties of the resulting composites (Fig. 2).
Figure 2.