# Effective Adsorption of Phenoxyacetic Herbicides by Tomato Stem-Derived Activated Carbons
## 3 Results and Discussion  
### Properties of the Obtained Tomato Stem-Derived Activated Carbon (TS-AC)  
Table 2 presents the surface chemistry parameters of the obtained TS-AC, including the contents of acidic and basic functional groups and point of zero charge values. A comparison of KOH and NaOH activation reveals that increasing temperature reduces acidity and increases basic surface functional groups. However, KOH activation produced higher acidic and lower basic surface functional group content than NaOH. Thermogravitetric analysis (TGA) results (Table 3), showing mass losses in the 200-500 \({}^{\circ}\)C, 500-700 \({}^{\circ}\)C, and 700-950 \({}^{\circ}\)C ranges, indicate the thermal decomposition of carboxyl, lactone, and hydroxyl groups, respectively [25]. These TGA findings corroborate the Boehm titration results (Table 2), which revealed a higher concentration of acidic oxygen functional groups in KOH-activated carbons. Conversely, these carbons exhibited a lower concentration of basic functional groups. An SEM-EDS analysis was performed to characterize the surface chemistry of the produced activated carbons. The determined percentage content of elements is presented in Table 4.  
It can be observed that with the increase in activation temperature, the carbon content increases, and the oxygen and calcium content decreases. This applies to both KOH and NaOH used as activating agents. Like other elements such as Mg, Al, Si, or P, their contents are small and practically do not change at different activation temperatures. The SEM images of TS-AC reveal distinct morphological differences influenced by the choice of activating agent and the activation temperature. As illustrated in Figure 1, TS-AC samples activated with KOH and NaOH at 750 \({}^{\circ}\)C exhibit noticeable variations. For reference, the figure also includes an SEM image of the carbonizate obtained at 600 \({}^{\circ}\)C (biochar-600), which served as the precursor for both AC-KOH-750 and AC-NaOH-750 materials--notably, activation with NaOH results in the formation of finer particles, generally smaller than 10 \(\upmu\)m. \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**TS-AC Sample**} & \multicolumn{8}{c}{**Chemical Composition (\% wt.)**} \\ \cline{2-7}  & **C** & **O** & **Ca** & **Mg** & **Al** & **Si** & **P** & **Zn** \\ \hline AC-KOH-550 & 84.3 & 12.7 & 1.7 & 0.4 & 0.1 & 0.1 & 0.4 & 0.2 \\ AC-KOH-650 & 85.5 & 11.5 & 1.6 & 0.4 & 0.1 & 0.1 & 0.5 & 0.3 \\ AC-KOH-750 & 88.6 & 9.2 & 1.2 & 0.4 & 0.1 & 0.1 & 0.4 & - \\ AC-NaOH-550 & 83.0 & 13.5 & 2.2 & 0.4 & 0.1 & 0.1 & 0.4 & 0.2 \\ AC-NaOH-650 & 86.8 & 10.1 & 1.7 & 0.4 & 0.1 & 0.3 & 0.3 & 0.2 \\ AC-NaOH-750 & 88.3 & 9.8 & 1.0 & 0.3 & 0.1 & 0.1 & 0.2 & 0.1 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Surface chemical composition of TS-AC samples determined by SEM-EDS. \begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Activated Carbons**} & **Total Acidic** & **Total Basic Groups** & **Point of Zero** \\  & **Groups (mmol/g)** & **(mmol/g)** & **Charge (pH\({}_{\text{pxc}}\))** \\ \hline AC-KOH-550 & 3.03 & 1.66 & 6.65 \\ AC-KOH-650 & 2.18 & 1.75 & 6.75 \\ AC-KOH-750 & 1.23 & 3.28 & 7.15 \\ AC-NaOH-550 & 1.84 & 2.02 & 7.00 \\ AC-NaOH-650 & 1.23 & 4.18 & 7.20 \\ AC-NaOH-750 & 0.82 & 4.92 & 7.25 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Surface characteristics of the tomato stem-derived activated carbon (TS-AC). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**TS-AC**} & \multicolumn{4}{c}{**Mass Loss (\%)**} \\ \cline{2-5}  & **200–500 \({}^{\circ}\)C** & **500–700 \({}^{\circ}\)C** & **700–950 \({}^{\circ}\)C** & **200–950 \({}^{\circ}\)C** \\ \hline AC-KOH-550 & 3.6 & 5.6 & 5.9 & 15.1 \\ AC-KOH-650 & 2.7 & 4.3 & 4.8 & 11.8 \\ AC-KOH-750 & 1.7 & 2.6 & 3.2 & 7.5 \\ AC-NaOH-550 & 2.3 & 3.9 & 4.4 & 10.6 \\ AC-NaOH-650 & 1.8 & 3.0 & 3.4 & 8.2 \\ AC-NaOH-750 & 1.2 & 2.2 & 2.9 & 6.3 \\ \hline \hline \end{tabular}
\end{table}