## 3 Results and Discussion
### Adsorption of Lead and Acetaminophen
Additionally, the maximum adsorption capacity for lead ions using Na\({}_{2}\)CO\({}_{3}\)-activated carbon prepared from _Cedrus deodara_ bark was reported to be 236.4 mg g\({}^{-1}\)[32]. In a study focused on activated carbon prepared from rice husk and treated with 5% Na\({}_{2}\)CO\({}_{3}\), the lead adsorption capacity was approximately 0.6 mg g\({}^{-1}\)[23]. Furthermore, the modification of citric acid enhances the adsorption uptake of lead to q\({}_{\rm max}\) = 41.7 mg g\({}^{-1}\)[69]. Citric acid functionalization was also assessed for the adsorption of a representative contaminant of emerging concern, such as acetaminophen [70,71]. Figure 6 portrays the acetaminophen isotherms, and Table 5 summarizes the results achieved for fitting with the most common adsorption models, with the Sips model generally providing the best fit for the experimental data. The untreated char exhibits a relatively low acetaminophen uptake, experimentally around 10 mg g\({}^{-1}\), indicating a poor adsorption performance. In contrast, the activated carbon boosts the maximum q\({}_{\rm max}\) to ca. 80 mg g\({}^{-1}\), showcasing the effectiveness of this activation method in creating a more porous structure with a higher surface area, which facilitates better acetaminophen absorption. The citric acid modification did not substantially alter the acetaminophen efficiency, with a similar maximum experimental adsorption capacity observed. This suggests that citric acid alone does not enhance, and may even hinder, the char's ability to adsorb acetaminophen. Nevertheless, when combined with Na\({}_{2}\)CO\({}_{3}\) activation, the adsorption uptake is enhanced to a value of roughly 80 mg g\({}^{-1}\). The modification of the activated material with citric acid did not improve the \begin{table}
\begin{tabular}{l c c c c c c c} \hline \hline
**Model** & **Parameter** & **Char** & **char-CA-1.0** & **AC** & **AC-CA-0.5** & **AC-CA-1.0** & **AC-CA-1.5** \\ \hline Langmuir & q\({}_{\rm max}\) & 3.59 & 5.86 & 34.67 & 38.74 & 48.04 & 35.93 \\  & K\({}_{\rm L}\) & 0.02 & 0.05 & 2.36 & 2.36 & 1.08 & 1.32 \\  & R\({}^{2}\) & 0.959 & 0.952 & 0.979 & 0.953 & 0.948 & 0.995 \\ \hline Freundlich & K\({}_{\rm F}\) & 0.34 & 1.22 & 16.34 & 20.44 & 17.94 & 15.71 \\  & n\({}_{\rm F}\) & 2.35 & 3.48 & 6.05 & 6.94 & 4.52 & 5.64 \\  & R\({}^{2}\) & 0.999 & 0.944 & 0.907 & 0.947 & 0.932 & 0.874 \\ \hline Sips & q\({}_{\rm max}\) & 5.00 & 7.22 & 35.66 & 50.0 & 60.81 & 35.93 \\  & K\({}_{\rm S}\) & 0.04 & 0.09 & 1.60 & 0.85 & 0.46 & 1.32 \\  & n\({}_{\rm S}\) & 1.46 & 1.49 & 1.35 & 3.17 & 2.01 & 1.00 \\  & R\({}^{2}\) & 0.985 & 0.964 & 0.981 & 0.945 & 0.955 & 0.995 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Model fitting parameters of lead isotherm onto the char and the activated carbon after citric acid modification. adsorption uptake compared to the non-modified sample. This evidence suggests that the mechanism of acetaminophen adsorption does not require the presence of carboxylic acid, which, according to the XPS characterization, is the prime oxygenated group after citric acid functionalization. The literature reports very different adsorption uptakes, strongly affected by the chemical activating agent used and the nature of the carbonaceous precursor material. For example, the activation of Brazil nutshell with ZnCl\({}_{2}\) can create an adsorbent with an uptake as high as 285 mg g\({}^{-1}\)[72]. The activation of spent tea leaves with H\({}_{3}\)PO\({}_{4}\) only retains about 59 mg L\({}^{-1}\)[73]. Or, in the case of the activation of rice with NaOH only, the acetaminophen removal rate is 50 mg g\({}^{-1}\)[74]. Regarding the role played by citric acid in similar carbonaceous materials, the activation of materials from citrus waste with FeCl\({}_{3}\) only reached an acetaminophen yield of 45 mg g\({}^{-1}\)[75].