# Application of Date Palm Tree Branch-Based Activated Carbon for Aqueous Toxicity Reduction
## 3 Results
### Phenol Adsorption  
After finishing phase 1, we began to investigate the pollution control efficiency of the selected GAC sample for phenol, \(p\)-cresol, and copper adsorption. The phenol adsorption equilibrium isotherm data were first obtained employing the procedure mentioned in the experimental section. The respective adsorption data were fitted to both Langmuir and Freundlich adsorption isotherms. However, the Langmuir adsorption isotherm, i.e., \([C_{e}/(X/M)]=[1/\text{ab}]+[(1/\text{a})\ C_{e}]\), provided a better fit; the respective results are given in Figure 1a. The Langmuir adsorption isotherm constant 'a' was 0.0568 and constant 'b' was 0.066 L/mg. Therefore, in summary, the phenol adsorption model is as follows: q\({}_{e}=[X/M]\) = [abC\({}_{e}/(1+\text{bC}_{e})\)] = [(0.0568)(0.066) C\({}_{e}\)]/[(1+(0.066) C\({}_{e}\))]. [PERSON] et al. [11], who studied methylene blue adsorption equilibrium using date pit-based GAC, also reported that the Langmuir isotherm well described the adsorption equilibrium. [PERSON] et al. [24], who prepared activated carbon from coconut shells using acid treatment and studied its use for the removal of \(\alpha\)-picoline, \(\beta\)-picoline, and \(\gamma\)-picoline from the aqueous phase, also reported that the adsorption equilibrium best fitted to the Langmuir adsorption model. Also, the maximum phenol adsorption capacity for the batch mode was determined to be 56.8 mg/g. Furthermore, Figure 1b provides the pH-dependent adsorption trend for phenol and shows a decreasing phenol adsorption with an increase in pH. We also note a significant and sharp decrease in phenol adsorption above pH 9. Such a pH-dependent phenol adsorption can be explained using changes in phenol and GAC-surface speciation with a change in aqueous pH. Figure 1c provides the phenol aqueous speciation trends and shows that above pH 9, a significant portion of phenol would start to convert from neutral phenol species to anionic phenolate species (considering a phenol p\(K_{a}\) of 9.82). Hence, such a change in the phenol speciation apparently causes the noted decrease in phenol adsorption on the produced GAC sample at higher pH values. Furthermore, the point of zero charge pH\({}_{\text{P2C}}\) for the present GAC was determined to be around 8, which also matches the aforementioned phenol adsorption results; i.e., at pH higher than 8, the surface of GAC would start to be anionic and hence the adsorption of anionic species such as phenolate will be less favorable, as noted in the present work. Figure 1: _Cont._  
Furthermore, the GAC surface functional groups may add to the noted pH-dependent phenol adsorption trend. [PERSON] and [PERSON] [25] identified various acidic functional groups as adsorption sites on the GAC surface. [PERSON] [26], who investigated biomass-based activated carbon (prepared employing phosphoric acid activation), also reported the development of oxygen-containing surface acidic groups including lactonic, carbonyl, phenolic, hydroxylic, and carboxylic groups. We therefore propose that at higher pH values, the conversion of both phenol (to phenolate species) and the GAC surface functional groups to the anionic forms significantly influences the adsorption of the target pollutant, i.e., phenol (total), at the GAC surface sites because of electrostatic repulsion. This effect is certainly more prominent at pH higher than the pK\({}_{\text{a}}\) of phenol (i.e., 9.82). The phenol adsorption kinetics results are provided in Figure 2a. The findings show a sharp uptake of phenol by GAC until 40 min. During this time period, approx. 60% phenol is being adsorbed by GAC. However, after 40 min, the adsorption kinetics seems to be slower. The saturation of surface adsorption sites and macro-pores at an earlier stage may leave mostly the meso-pores and micro-pores available for any further adsorption at a later stage. This may cause a slower phenol uptake by the GAC sample at a later time. However, up to 90% phenol removal does transpire by 175 min.