# Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based _Capsicum annuum_
## 4 Results
### Adsorption Data
#### 4.3.2 Contact Time Effect of Activated Carbon  
Sorption rate is crucial in water treatment because it gives information about the reaction paths and the mechanism of the adsorption reaction [57]. Different time intervals were used to determine the equilibrium sorption capacity of Cd(II) and Cr(VI) ions. Figure 7 shows the uptake rate for Cd(II) and Cr(VI) ions on the activated carbon. The uptake of Cd(II) and Cr(VI) is rapid at the beginning of the contact time. As the time increased, the sorption capacity also increased, and thereafter the adsorption equilibrium for Cd(II) ions was attained after 40 min. In contrast, the uptake of Cr(VI) ions decreased after 30 min. This indicates that a contact time of 40 min was sufficient for the sorption of Cd(II) and Cr(VI) ions in this study. It can be assumed that the sorption of Cd(II) and Cr(VI) ions by \(C\). _annuum_ occurs by intraparticle diffusion and the uptake reaction mechanisms [50]. The rate of Cd(II) and Cr(VI) ions adsorption by the \(C\). _annuum_ adsorbent was high in the first  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**adsorption Models**} & \multirow{2}{*}{**Parameters**} & \multicolumn{2}{c}{**Pollutants**} \\ \cline{3-4}  & & **Cd(II)** & **Cr(VI)** \\ \hline Langmuir & \(Q_{o}\) (mg/g) & 24.92 & 12.94 \\  & \(B\) (L/mol) & 0.324 & 0.213 \\  & \(R^{2}\) & 0.846 & 0.890 \\  & \(RMSE\) & 12.87 & 6.42 \\  & \(MPSD\) & 27.43 & 15.09 \\ Freundlich & \(K_{F}\) & 11.121 & 1.050 \\  & \(N\) & 1.563 & 1.414 \\  & \(R^{2}\) & 0.972 & 0.985 \\  & \(RMSE\) & 9.42 & 6.42 \\  & \(MPSD\) & 4.42 & 3.58 \\ \hline Experimental & \(q_{e}\) (mg/g) & 34.34 & 15.24 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Isotherms for the adsorption of Cd(II) and Cr(VI).  
Figure 6: Concentration effect of adsorption by activated carbon.  
30 min, resulting in 70-80% uptake. Complete adsorption equilibrium was obtained within 40 min for Cd(II), while for Cr(VI) ions, uptake reduced drastically due to slow kinetics; this is because Cr(VI) ions might have penetrated the adsorbent and reached saturation.  
Three kinetic models (PFO, PSO, and IPD) were analyzed to evaluate the influence of the sorption of Cd(II) and Cr(VI) (see Table 2). The model that best describes the results was chosen based on the \(R^{2}\) values, which are close to 1. Table 2 shows that the activated carbon matched the PFO model. In addition, the calculated sorption capacity was nearer to the actual experimental sorption capacity, with \(R^{2}\) values close to 1. The PSO and IPD models could not explain the sorption process. The PFO model suggests that the rate-limiting step of the sorption process is associated with the sorption of the adsorbent used [58]. Therefore, the uptake of Cd(II) and Cr(VI) ions by the activated carbon is due to van der Waal forces [58]. This also shows that the uptake process of Cd(II) and Cr(VI) ions by activated carbon is due to an endothermic reaction [59]. The literature suggests that PFO occurs when the attachment of adsorbing materials to sorption sites is the rate-limiting step [60]. The reason why PSO could not fit might imply that the rate of uptake slowed down to a greater extent than expected based on the filling of adsorption sites [60].  
\begin{table}
\begin{tabular}{c c c c} \hline \hline \multirow{2}{*}{**Kinetic Models**} & \multirow{2}{*}{**Parameters**} & \multicolumn{2}{c}{**Pollutants**} \\ \cline{3-4}  & & **Cd(II)** & **Cr(VI)** \\ \hline PFO & \(q_{e}\) (mg/g) & 32.46 & 26.40 \\  & \(K_{1}\) (min\({}^{-1}\)) & 0.078 & 0.074 \\  & \(R^{2}\) & 0.988 & 0.965 \\ PSO & \(q_{e}\) (mg/g) & 19.25 & 20.09 \\  & \(K_{2}\) (g\(\cdot\)mg/min) & 1.004 & 1.006 \\  & \(R^{2}\) & 0.877 & 0.824 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Kinetics for the sorption of Cd(II) and Cr(VI).  
Figure 7: Time effect of adsorption by activated carbon.