## 4 Characterization of Bioadsorbents  
There are several characterization techniques used to understand the physical and chemical properties of materials, their adsorption capacity, and their interactions with pollutants; these include scanning electron microscopy (SEM), the Brunauer-Emmett-Teller method (BET), the Barrett-Joyner-Halenda method (BJH), Fourier transform infrared spectroscopy (FTIR), and energy-dispersive X-ray (EDX), among others. Adsorbents from fruits, vegetables, grains, seeds, herbage, and forage wastes have mainly been characterized by applying FTIR (Tables 1 and 2). This technique makes it possible to identify the molecules and functional groups on the surface of the biomaterials [51] that interact with the contaminants. Some of these correspond to hydroxyl groups generated by the cellulose, hemicellulose, and lignin present in agricultural waste [42]. An SCB-based adsorbent was characterized using this analysis to demonstrate the presence of hydroxyl, C-H, C-O, and -OCH\({}_{3}\) groups as well as other oxygen-containing functional groups on its surface [86]. The structure of mango peel waste was analyzed by FTIR spectroscopy, demonstrating that carboxyl and hydroxyl groups were the main groups responsible for sorption of metals [10,78]. [PERSON] et al. [49] used this characterization method to evaluate the adsorbent obtained from rice husk and lemon juice residues, showing that O-H, N-O, and C-N groups bind to phosphates. All the biowastes and adsorbents reported here have been analyzed using this technique except for peach stones, sunflower leaves/stalks, and several others not characterized by any analytical method (coir pith, corn cob and stalk, soybean, wheat straw, kiwi, tangerine peels, and jackfruit peels). Peach stones were characterized by N\({}_{2}\)/CO\({}_{2}\) adsorption, point of zero charge, and thermal analysis, whereas sunflower residues were characterized by SEM and point of zero charge. The pH at the point of zero charge indicates that above or below that value the net surface charge of the adsorbent is predominantly negative or positive, respectively. Gas adsorption and SEM studies have revealed that peach stone-derived AC presents different pore features and that sunflower leaves/stalks have an agglomerated shape with a porous, heterogeneous, and uneven structure [54,89]. Another, less conventional method is Vibrating-sample magnetometry, which involves the vibration of magnetic materials operating under Faraday's Law, using sensing coils to detect the difference in the voltage variation in proportion to the magnetic moment of the sample [97]. A magnetic peanut husk adsorbent was characterized with this technique, allowing the observation of its properties before and after phosphate adsorption and its recovery using a magnet for practical applications [82]. Other analytical techniques include Zeta potential, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. The first technique was employed to determine the electrostatic affinity for Pb(II) and the dye reactive blue 114 on the surface of magnetic coffee waste and pome peel sorbents [28,83], whereas X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy were used to study the iodine form, oxidation state, and structure of adsorbents derived from mangosteen pericarps, passion fruit finds, red onion peels, and red dragon fruit peels [79]. Brunauer-Emmett-Teller and Barrett-Joyner-Halenda (BET/BJH) are techniques employed to estimate certain surface characteristics of bioadsorbents such as area, pore size, and volume [45]. BET uses the equilibrium adsorption isotherm at the adsorbate boiling point, whereas BJH uses an inverse relation between pore radius and multilayer thickness [98]. BJH was used to determine the micropore and mesopore volume of pineapple waste-based AC, obtaining values of 289 m\({}^{2}\)/g and 253 m\({}^{2}\)/g respectively. Both types of pores enlarge the surface area, resulting in better adsorption because ion interactions occur more smoothly [48].