## 3 Results and Discussion
### Adsorption Isotherms  
From the results obtained after chemical, structural and textural characterization, \(\Phi_{5}\)-PPH material was selected as adsorbent for AB113, given that it presents the highest phenyl group surface density (Table 1). Also, to evaluate the effect of the phenyl group on the AB113 adsorption, the pristine PPH material was used as a reference. AB113 adsorption isotherms were investigated in triplicate at 25 \({}^{\circ}\)C and pH 6.5. Figure 3 shows the fitting of the experimental data to the adsorption isotherms for PPH and \(\Phi_{5}\)-PPH respectively. In the case of pristine PPH, the isotherm was fitted to the SIPS isotherm with a saturated adsorption capacity of 0.04 mmol/g (Figure 3A). The adsorption data from \(\Phi_{5}\)-PPH were fitted to a Langmuir isotherm model, showing a saturated adsorption capacity (Q\({}_{\text{o}}\)) of 0.0967 mmol/g and a Langmuir constant (K\({}_{\text{L}}\)) of 613 L/Kg (Figure 3B). Other model fittings are shown as supplementary information as Figure S1. The adsorption effectiveness was also evaluated. The retention effectiveness was close to 100% for \(\Phi_{5}\)-PPH material, when the adsorption capacity was lower than 0.040 mmol/g. From this point it decreases while increasing the amount of AB113 in the solution. For the pure PPH material, the highest retention percentage reached was 55% for the first fit, decreasing in the next points. In the literature, AB113 adsorption on mesoporous activated carbon is reported [44], where the observed adsorption capacities are in the order obtained for \(\Phi_{5}\)-PPH. In this case the adsorption capacities for an activated carbon obtained from a rubber type and for a commercial activated carbon obtained from the Langmuir isotherm (Q\({}_{\text{o}}\)), were 0.014 and 0.011 mmol/g, respectively, although for both activated carbons the \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Sample** & **C** & **N** & **O** & **Si** & **P** & **Zr** & **S** \\ \hline \(\Phi_{5}\)-PPH & 24.50 & 0.50 & 50.07 & 19.55 & 3.52 & 1.86 & - \\ \(\Phi_{5}\)-PPH + AB113 & 29.56 & 0.94 & 46.00 & 18.42 & 2.19 & 1.69 & 0.36 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Atomic concentration (%) of samples \(\Phi_{5}\)-PPH and \(\Phi_{5}\)-PPH + AB113 determined by XPS. specific surface areas are higher than that observed for \(\Phi_{5}\)-PPH with values of 562 and 1168 m\({}^{2}\)/g. Another adsorbent based in red mud obtained a Q\({}_{0}\) = 0.112 mmol/g [45], demonstrating the feasibility of the proposed system to be applied. The difference in the adsorption capacities of PPH and \(\Phi_{5}\)-PPH can be explained by the difference in the intermolecular forces between the different surface groups and the dye monomer. In the case of PPH, the main forces involved are strong hydrogen bonding with the AB113 (Si-OH and P-OH) and the presence of aromatic rings on the surface of phenyl-functionalized \(\Phi_{5}\)-PPH which offer a superior degree of delocalization due to the \(\pi\)-\(\pi\) stacking of the phenyl surface groups of the adsorbent (Figure 4). The different mechanisms of adsorption are also indicated by the fit of sorption data to two distinct isothermal models. The better fit of the \(\Phi_{5}\)-PPH sorption data to the Langmuir isotherm reveals that the sorption sites are identical and energetically homogeneous, suggesting that the coordination between AB113 and the aromatic rings of \(\Phi_{5}\)-PPH is the main mechanism of adsorption involved. Figure 3: AB113 adsorption isotherms in (**A**) PPH and (**B**) \(\Phi_{5}\)-PPH and in relation to mmol AB113 for g of sorbent. However, the better fit of PPH sorption data to the SIPS isotherm indicates that there is more than one type of site involved in this process, each characterized by distinct energies and distinct affinities to adsorb. In this case, it is particularly related to the adsorption on the Si-OH and P-OH groups of PPH. To demonstrate the mechanism described, decreasing a specific area in our material does not limit increased dye adsorption.