# Application of Date Palm Tree Branch-Based Activated Carbon for Aqueous Toxicity Reduction
## 3 Results
### \(p\)-Cresol Adsorption  
The \(p\)-cresol adsorption equilibrium data from the present work were also fitted to both Langmuir and Freundlich adsorption isotherms. Similar to the phenol results, the Langmuir adsorption isotherm provided a better fit for \(p\)-cresol adsorption, as shown in Figure 3a. The value of the respective Langmuir isotherm constant 'a' was 0.083 and that of constant 'b' was 0.112 L/mg. This also yielded the maximum \(p\)-cresol adsorption capacity for the batch mode at 83 mg/g. On the other hand, the maximum phenol adsorption capacity for the batch mode was determined to be 56.8 mg/g, which indicates that \(p\)-cresol adsorbs better on the respective GAC surface. In fact, the other \(p\)-cresol adsorption results, reported later in this section, also support this observation. Figure 2: (**a**) Results from the phenol adsorption kinetics experiment (the initial phenol and GAC were 21 mg/L and 2000 mg/L, respectively). (**b**) Phenol adsorption: data fitting to first-order kinetics (the initial phenol and GAC were 21 mg/L and 2000 mg/L, respectively). Figure 3: Cont. Furthermore, the pH-dependent \(p\)-cresol adsorption trend as given in Figure 3b also shows a decreasing \(p\)-cresol adsorption with an increase in pH. Figure 3c provides the \(p\)-cresol aqueous speciation trends and shows that above pH 10, a significant portion of \(p\)-cresol would start to convert from neutral \(p\)-cresol species to anionic cresolate species (considering a \(p\)-cresol pK\({}_{\text{a}}\) of 10.26). Also, the speciation of GAC surface functional groups, as discussed earlier, may add to the noted pH-dependent \(p\)-cresol adsorption trend. Hence, we propose that at higher pH values, the conversion of both \(p\)-cresol (to cresolate species) and the GAC surface functional groups to the anionic forms significantly influences the adsorption of \(p\)-cresol (total) because of electrostatic repulsion. Figure 4a provides the \(p\)-cresol adsorption kinetics results and indicates significant \(p\)-cresol uptake up to 60 min (i.e., 72%), after which the adsorption kinetics seems to be slower. The respective kinetic data fitting results, as given in Figure 4b, show a first-order kinetics with a rate constant equal to 0.0165 1/min. This value is somewhat higher than the aforementioned phenol adsorption rate constant of 0.0138 1/min. [PERSON] et al. [27], who prepared activated carbon from agricultural by-products, also noted lower phenol adsorption compared to 2,3,4-trichlorophenol and ascribed it to the preferential adsorption of the latter. [PERSON] and [PERSON] [28], who prepared activated carbon from apricot stone shells by chemical treatment with phosphoric acid, also reported lower phenol adsorption compared to substituted phenols. We suggest that the differences in the stereochemistry of the two pollutants, i.e., phenol and \(p\)-cresol, may explain the aforementioned differences in their adsorption kinetics trends. Figure 3: (**a**) Langmuir adsorption isotherm parameter fitting for \(p\)-cresol adsorption modeling. (**b**) Results of \(p\)-Cresol removal vs. pH (GAC: 3000 mg/L; initial \(p\)-cresol concentration: 100 mg/L). (**c**) Calculated qualitative \(p\)-cresol speciation trends with respect to aqueous pH (using pK\({}_{\text{a}}\) 10.26). Furthermore, findings from the \(p\)-cresol continuous flow adsorption column study, completed at an initial \(p\)-cresol concentration of 100 mg/L and a flow rate of 4 mL/min, showed that a \(C/C_{o}\) of 0.10 is reached at 120 min and a C/C\({}_{o}\) of 0.84 at 915 min (Figure 5). Figure 4: (**a**) Results from the \(p\)-cresol adsorption kinetics experiment (the initial \(p\)-cresol and GAC were 21 mg/L and 2000 mg/L, respectively). (**b**) \(p\)-cresol adsorption: data fitting to first-order kinetics (the initial \(p\)-cresol and GAC were 21 mg/L and 2000 mg/L, respectively).