## 3 Obtaining Waste-Based Bioadsorbents
### Activated Carbon Production  
The synthesis of AC from low-cost waste materials and its subsequent use as a biosorbent has become more common. Generally, AC implementation requires pre-treatment before application to contaminated waters; the use of microwaves [31], ultrasound techniques [32], or chemical agents for biochar activation have all shown promising results. Furthermore, the use of different chlorides such as NaCl, KCl, and CaCl\({}_{2}\) has resulted in AC with better adsorption capacity [25; 33]. The common way to activate biowaste-based carbon is after carbonization, although in certain cases the activation is carried out before. This can be physical (e.g., pressurization, pyrolysis, or gasification) and/or chemical (e.g., by using acid, alkali, or salt compounds) depending on the nature of the waste and its further application [34; 35; 36]. The removal of heavy metals with AC using seed and grain wastes has been widely explored, as in the case of the buttons (female flowers) that are one of the most abundant residues in coconut plantations (comprising about 55-95% of remains). This coconut waste was evaluated by [PERSON] and [PERSON] [37] for the adsorption of different heavy metals from industrial effluents. The buttons were treated with sulfuric acid and placed inside a graphite tube in a furnace to generate AC; the collected carbonized material showed high metal removal performance (100%). Spent coffee waste is another material commonly used to adsorb a wide range of contaminants present in fresh water and wastewater through carbon activation by NaOH [38]. AC from fruit waste such as papaya peel can be an effective adsorbent to remove lead from contaminated water. This biowaste was dried (105 \({}^{\circ}\)C) and carbonized, followed by a chemical activation process using H\({}_{3}\)PO\({}_{4}\) as the oxidant agent [39]. Concerning AC derived from forage and herbage sources (e.g., cereal byproducts and palm tree leaves), these must first pass through a drying process, then be soaked in acid solution, and finally carbonized into a particle size between 300-425 \(\upmu\)m. This has the aim of adjusting the kinetic model at 56 \({}^{\circ}\)C in order to effectuate the capacity to remove heavy metals such as Pb(II) and Cr(VI) from aqueous solutions [40; 41]. The AC sources employed to remove dyes and other industrial pollutants comprise a long list of several types of vegetable wastes. Peant shells have been transformed into AC through chemical treatment with H\({}_{3}\)PO\({}_{4}\) and used to adsorb acid yellow 36 [42]. The same chemical modification was performed with _Acacia erioloha_ seed biochar to increase its surface area and porosity and thereby improve the adsorption yield for methylene blue (MB) and iodine [43]. Other authors processed coffee waste with KOH and pyrolysis to obtain granular AC into calcium alginate beads for the treatment of dye contaminants, finding that the material could be used for up to seven cycles [44]. The efficiency of H\({}_{3}\)PO\({}_{4}\) as a carbon activator was compared with that of ZnCl\({}_{2}\) in the context of siriguela seeds and cocoa shells for the adsorption of bovine serum albumin (BSA) and \(\alpha\)-lactalbumin (\(\alpha\)-Lac), which are food industry wastes; H\({}_{3}\)PO\({}_{4}\) was more effective for \(\alpha\)-Lac, while ZnCl\({}_{2}\) performed better in BSA removal [45]. Coir with subjected to a carbonization treatment at 700 \({}^{\circ}\)C was used in a single and a multi-component system to adsorb congo red, rhodamine-B, and acid violet [46; 47]. Another fruit-based biochar that has been widely studied is that resulting from pineapple waste (crowns, leaves and stems). These residues were subjected to pyrolysis and permeated with ZnCl\({}_{2}\) to turn them into AC with a prominent adsorption capacity for dyes (e.g., MB) thanks to a large surface area (914.67 m\({}^{2}\)/g) [48].