# Adsorption of Emerging Pollutant by Pecan Shell-Based Biosorbent
## 3 Results and Discussion
### Effect of pH and the Zero-Charge Point pH  
The charges present on the adsorbent surface, as well as the charges of CPX in aqueous media, are strongly influenced by the pH of the solution [11]. Because of this, the influence of this parameter on the removal of CPX at different pH scales (2-10) was tested, and Figure 5 shows the results. The CPX has an amphoteric characteristic due to the presence of protonatable groups in its molecule; the behavior of CPX varies according to the medium pH. The antibiotic has two protonatable sites that establish dissociation constants (Figure 5a), defined as 5.09 (pKa\({}_{1}\)). Below this value, it behaves as a cation; above 8.8 (pKa\({}_{2}\)), it behaves as an  
Figure 4: Linear plots of the (**A**) pseudo-first order kinetic model, (**B**) pseudo-second order kinetic model, and (**C**) [PERSON] model for ciprofloxacin adsorption by the biosorbent made of pecan shells.  
anion. Among these pH values, ciprofloxacin has a zwitterion characteristic (neutral species); it has both positively charged regions and negatively charged regions in its molecule [19]. In the experiment, the highest CPX removal rate (47%) was between pH 5 and 6; i.e., in the neutral pH range of the antibiotic.  
In addition, the role of the biosorbent surface charge was analyzed using the zero charge point pH test (pH\({}_{\text{PCZ}}\)) and different pH ranges (Figure 5b). The pH\({}_{\text{PCZ}}\) of the tested biosorbent was 7.2. A better anion adsorption occurs below this value because the surface of the material is covered by a positive charge; above this value, the adsorbent has a negative charge, which favors the adsorption of cations [20]. The best range of antibiotic removal is in the region the CPX is in the zwitterion form; this is because below a pH 5 and above a pH 8 both CPX and the biosorbent surface will have the same charge, resulting in an electrostatic repulsion [14].
The results shown in Table 3 demonstrate, through the coefficients of correlation (\(R^{2}\)), that the Freundlich model best fits the experimental data. The best fit to the Freundlich model suggests that adsorption occurs in multiples layers, and that the adsorbate assumes a heterogeneous distribution on the surface of the solid; this results in a non-uniform energy distribution where there is a variety of interactions between the adsorbate and the adsorbent that correspond to a non-ideal, reversible, and predominantly chemical adsorption [21]. The constant \(n\), correlated to adsorption intensity, reveals how the active sites are distributed in terms of energy capacity. Some authors [5] suggests that for values of \(n\) within the range 1 to 10 (1 \(<n<\) 10), as is the case of CPX adsorption by PSB (\(n\) = 3.1184), adsorption occurs in a favorable way; this evidences more energetic active sites (cf. non-uniformity of energy distribution) that are previously occupied to the detriment of less occupied ones. Although the Langmuir model did not result in the best experimental fit, it can still be used to obtain some parameters that the Freundlich isotherm does not inform, such as the maximum adsorption capacity; this was at \(q_{max}\) = \(\pm\)2.6 mg g\({}^{-1}\), indicating that the experiment reached the maximum capacity of the adsorbent [10].