## 3 Results and Discussion  
### Characterization of the Adsorbents  
Table 2 lists the physical properties and elemental composition of the adsorbents. SW submitted to a direct charring rendered a lowest yield due to pyrolysis of residual low molecular weight sugars, peptides, and amine compounds. It was also expected that pyrolysis released gases such as CO, CO\({}_{2}\), H\({}_{2}\)O, H\({}_{2}\), and CH\({}_{4}\)[41]. Phosphoric acid acted as an activator which affected the total porosity of the samples. Further, AC2 having an adsorbent-to-phosphoric acid ratio of 1:3 rendered a lower porosity as compared to AC1 having a 1:2 ratio. This indicates that low levels of phosphoric acid are desirable for creating a porous structure in SW. Previous attempts to produce activated charcoals using other activated agents such as NaOH and ZnCl\({}_{2}\) failed giving rise to a massive degradation of SW and very low yields (\(<\)40%) [24]. Moreover, the pyrolysis and activation processes increased the true density of the adsorbents, but this variable was not reflected in a major increase of bulk density. Therefore, pyrolysis, per se, did not lead to a great increase in total porosity (Table 2), but rather decreased the total pore volume of SW, which was reflected on a lower S\({}_{\text{BET}}\) (Table 3). Further, the commercial material (CAC) showed the lowest density and largest moisture content. The pyrolysis process reduced the amount of carbon and triggered the complete disappearance of N, probably due to the formation of volatile compounds such as NO, NO\({}_{2}\), and N\({}_{2}\)O\({}_{5}\)[42]. In fact, heating at 500 \({}^{\circ}\)C and acidic activation promoted the evolution of H and O from SW to H\({}_{2}\)O and H\({}_{2}\), rather than CO, CO\({}_{2}\), and hydrocarbons leading to a net reduction of oxygen [43]. Upon activation, the carbon content slightly increased, whereas the oxygen content was reduced accordingly. These changes are certainly related to breaking of the least stable chemical bonds present in the adsorbents, promoting the removal of some atoms as gaseous compounds and consequently, contributed to the rearrangement of the carbon structure. Interestingly, the acidic activation caused a major increase in the content of phosphorous most probably in form of phosphate moieties, whereas the content of calcium as CaCO\({}_{3}\) and CaO was reduced. On the other hand, CAC essentially exhibited carbon and oxygen, and traces of other elements as compared to the shrimp-based adsorbents. The Boehm titration quantifies functional groups on the surface of each adsorbent based on a selective neutralization using a series of alkalis and acid solutions. These results suggest that acidic moieties prevailed on the surface of most adsorbents except for SW and CM (Table 3). In fact, the process of pyrolysis increased the total alkalinity of SW due to the reduction of surface phenol groups. This phenomenon was more remarkable for CM possibly due to the partial transformation of CaCO\({}_{3}\) to CaO. Likewise, a subsequent activation with H\({}_{3}\)PO\({}_{4}\) removed any trace of alkaline functional groups due to the increase in phenol, carboxylic, and phosphoric moieties, especially when the starting material was CM. In fact, the acidity of AC2 was attributed only to the phosphoric and phenol moieties. Further, chemical activation caused a pH decrease and increase in conductivity. This phenomenon is mainly explained by the emerging presence of phosphate and phenol moieties. Conversely, CAC exhibited phenol and alkaline moieties at a 4:1 ratio, but this adsorbent was less acidic than the activated materials (i.e., AC1 and AC2) due to the lack of phosphate moieties. Thus, CV adsorption in this commercial adsorbent is mostly explained by the largest surface area which in turn was responsible for the large moisture content.