## 6 Challenges and Future Perspectives  
Agricultural waste has been demonstrated to be one of the most interesting materials for water remediation due to its ability to adsorb heavy metals, dyes, nutrients, pharmaceuticals, and other contaminants. Nevertheless, several variables that represent a challenging perspective in the near future need to be considered during the recovery process from water sources. One of these is the lifetime and final disposal of the biomaterial, which could negatively impact both the environment and the cost of the procedures. The most common disposal method, due to its low cost and reliability, is chemical neutralization of spent bioadsorbents, which are later placed in a landfill. Despite being a simple mechanism, this entails the possibility of secondary contamination of the soil, groundwater, surface water, and air, especially if the landfill lacks appropriate leachate and gas collection systems [108]. Another popular strategy to discard used biomaterials is incineration, which results in high reduction of the biomass at the cost of possible secondary pollution through the emission of gases and fly ash that may contain trace amounts of the adsorbed contaminants [109]. However, there are other heat treatments used to extend the lifecycle of adsorbents or regenerate biochars, such as hydrothermal modification, gasification, and pyrolysis; regardless of the benefits of recycling byproducts, high costs represent a considerable limitation for the industry [109].  
Microbial degradation has been studied as a recovery method for exhausted bioadsorbents. Unfortunately, it has several disadvantages that make it an unsuitable alternative at industrial scales, for example, its long degradation time, high sediment production, and the high sensitivity of the processes [110]. On the other hand, the chemical extraction of contaminants with sulfuric acid or EDTA, among others, displays satisfactory results in recovering and recycling both heavy metals and extractants [111, 112]. A fairly new strategy with promising outcomes has recently been studied, although it requires further research; this is the biosynthesis of nanomaterials using bioadsorbents as raw material [113].  
Other challenges related to the use of bioadsorbents include the wide range of contaminants and the hydrological dynamics existing in different water sources. In this review, only four contaminant categories have been taken into consideration; however, it is necessary to explore other bioadsorbents that have been used to remove personal care and other lifestyle products such as galaxolide, tonalide [114, 115, 116], saccharin, sucrose, and caffeine [117, 118, 119, 120]. Other known groups that form the growing list of emerging contaminants are chemical synthesizers and intermediates, corrosion inhibitors, flame retardants, plasticizers, per- and polyfluorinated compounds, biocides, hormones, resistance genes, etc. [121, 122, 123, 124, 125]. These emerging contaminants make more crucial the development of future research on the adsorption of these contaminants through the implementation of agricultural residues. Finally, the behavior of bioadsorbents in the treatment of other water sources, including groundwater, surface water, marine water, and different environmental matrices such as landfill leachates and industrial gas emissions, is another matter requiring further investigation. The current methodologies and protocols for the manufacturing of biowastes should be improved, with the aim of enhancing the adsorption process and favoring the implementation of bioadsorbents by further reducing costs.