## 3 Results and Discussion
### Adsorption Isotherms and Kinetics
\begin{table}
\begin{tabular}{l l l l l} \hline \hline \multirow{2}{*}{**Parameter**} & **AP** & \multicolumn{2}{c}{**PAP**} \\ \cline{2-5}  & **Cr(VI)** & **Cd(II)** & **Cr(VI)** & **Cd(II)** \\ \hline Freundlich & & & & \\ \hline R\({}^{2}\) & 0.837 & 0.999 & 0.710 & 0.911 \\ \hline K\({}_{\rm F}\)(mg\({}^{1-(1/n)}\)(dm\({}^{3}\))\({}^{1/n}\)g\({}^{-1}\)) & 0.2201 & 0.2712 & 0.3669 & 0.2952 \\ \hline
1/n & 0.7392 & 0.8861 & 0.7047 & 0.9684 \\ \hline Dubinin–Radushkevich & & & & \\ \hline R\({}^{2}\) & 0.993 & 0.828 & 0.918 & 0.8305 \\ \hline q\({}_{\rm d}\) [mg/g] & 5.78 & 6.91 & 6.02 & 7.50 \\ \hline K\({}_{\rm d}\) [mol\({}^{2}\)/k\({}^{2}\)] & \(-\)0.0001 & 0.0000 & \(-\)0.0006 & \(-\)0.0007 \\ \hline Langmuir & & & & \\ \hline R\({}^{2}\) & 0.6326 & 0.7885 & 0.4803 & 0.0850 \\ \hline K\({}_{1}\) & 0.0104 & 0.0077 & 0.0139 & 0.00421 \\ \hline Temkin & & & & \\ \hline R\({}^{2}\) & 0.958 & 0.9412 & 0.8335 & 0.9023 \\ \hline B & 2.2488 & 4.0309 & 2.5940 & 4.7241 \\ \hline K\({}_{\rm t}\) & 0.1413 & 0.1549 & 0.2001 & 0.1854 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Parameters of sorption isotherm models. Figure 5: Pseudo-second-order kinetic rate model for AP (**A,B**) and PAP (**C,D**), Cr(VI) (**A,C**) and Cd(II) (**B,D**) adsorption. \begin{table}
\begin{tabular}{c c c c c c c c c c} \hline \hline  & \multicolumn{6}{c}{**AP**} & \multicolumn{6}{c}{**PAP**} \\ \cline{2-10}  & \multicolumn{6}{c}{**Cr(VI) concentration [mg/dm\({}^{3}\)]**} \\ \cline{2-10}  & 50 & 100 & 150 & 200 & 300 & 50 & 100 & 150 & 200 & 300 \\ \cline{2-10}  & \multicolumn{6}{c}{PSO} \\ \hline qe & 1.4430 & 2.4084 & 4.4075 & 4.7665 & 7.5937 & 0.5624 & 1.8898 & 5.2575 & 5.3936 & 7.4058 \\ \hline k\({}_{2}\) & 0.0127 & 0.0822 & 0.3779 & 0.0939 & 0.0082 & 0.2353 & 0.1326 & 0.2197 & 0.0762 & 0.0154 \\ \hline R\({}^{2}\) & 0.9652 & 0.9815 & 1.0000 & 0.9956 & 0.9904 & 0.8931 & 0.8499 & 0.9998 & 0.9951 & 0.9969 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Results obtained for the pseudo-second-order rate kinetic model. \begin{table}
\begin{tabular}{l l l} \hline \multirow{2}{*}{**Biochar Material**} & \multicolumn{2}{c}{**Sorption Capacity (mg/g)**} \\ \cline{2-3}  & \multicolumn{2}{c}{**Cr(VI)**} & \multicolumn{2}{c}{**Cd(II)**} \\ \hline PAP & 7.5 & 12 \\ \hline Coconut shells & 2 [26] \\ \hline \end{tabular}
\end{table} 
Table 6: Comparison of sorption capacities of biochar derived from various biosorbents. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Biochar Material**} & \multicolumn{6}{c}{**Sorption Capacity (mg/g)**} \\ \cline{2-11}  & \multicolumn{2}{c}{**Cr(VI)**} & \multicolumn{2}{c}{**Cd(II)**} \\ \hline PAP & 7.5 & 12 \\ \hline Coconut shells & 2 [26] \\ \hline \end{tabular}
\end{table} 
Table 7: Comparison of sorption capacities of biochar derived from various biosorbents. \begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \multirow{2}{*}{**Biochar Material**} & \multicolumn{6}{c}{**Sorption Capacity (mg/g)**} \\ \cline{2-11}  & \multicolumn{2}{c}{**Cr(VI)**} & \multicolumn{2}{c}{**Cd(II)**} \\ \hline PAP & 7.5 & 12 \\ \hline Coconut shells & 2 [26] \\ \hline \end{tabular}
\end{table} 
Table 5: Comparison of sorption capacities of biochar derived from various biosorbents.