# Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based _Capsicum annuum_
## 5 Conclusions  
In this study, _C. annuum_ was used to produce activated carbon for the effective uptake of Cd(II) and Cr(VI) ions from water. The activated carbon provided ample sites for the sorption of both pollutants due to its porous nature and several functional groups. The Freundlich model fits the experimental data well. The maximum sorption capacities for Cd(II) and Cr(VI) ions were 15.24 and 34.34 mg/g, respectively. The activated carbon exhibited high kinetic performance, resulting in the sorption of Cd(II) and Cr(VI) within 30 and 40 min, respectively. The adsorption of Cd(II) and Cr(VI) indicates that the uptake process was exothermic and that the reaction was spontaneous. We conclude that the main mechanisms responsible for the adsorption of Cd(II) and Cr(VI) on the activated carbon are precipitation, ion exchange, complexation, electrostatic attraction, and hydrogen bonding. The activated carbon from C. _annuum_ performed better in adsorbing Cd(II) and Cr(VI) ions from an aqueous solution than other similar materials. Activated carbon prepared from C. _annuum_ has proven to be effective in removing Cd(II) and Cr(VI) from an aqueous solution. Although activated carbon has been used to remove metal ions using different adsorbents, the use of _C. annuum_ as an adsorbent still needs to be explored for other pollutants, such as pharmaceuticals and dyes. Factors such as compatibility with industrial conditions and cost need to be investigated to ensure the applicability of _C. annuum_, as this study is only  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Adsorbents**} & \multicolumn{2}{c}{\(\boldsymbol{q_{(max)}\) (mg/g)} } \\ \cline{2-3}  & **Cr(VI)** & **References** \\ \hline Cauliflower stem & 64.10 & [79] \\ _Capsicum annuum_ & 34.34 & This study \\ _Etucalyptus_ tree bark & 30.6 & [80] \\ Rice straw & 17.47 & [81] \\ _Rumex abyssinicus_ & 19.35 & [82] \\ Rice husk & 9.97 & [83] \\ White tea residue & 9.11 & [84] \\ Sludge & 7.00 & [85] \\ \hline \hline \multirow{2}{*}{**Adsorbents**} & \multicolumn{2}{c}{\(\boldsymbol{q_{(max)}\) (mg/g)} } \\ \cline{2-3}  & **Cd(II)** & **References** \\ \hline Rice husk & 28.96 & [86] \\ _Capsicum annuum_ & 15.24 & This study \\ Rice straw & 14.97 & [13] \\ Corn straw & 12.40 & [87] \\ Corn stalk & 12.10 & [88] \\ Attrapulgite & 10.38 & [89] \\ Buffalo weed & 9.70 & [90] \\ Sewage sludge & 0.87 & [91] \\ \hline \hline \end{tabular}
\end{table}
Table 4: Adsorbents used to prepare carbon for the sorption of Cr(VI) and Cd(II) ions.  
based on a laboratory-scale investigation. Further research is required to fill these gaps and aid in the development of cost-effective materials for the adsorption of different pollutants.  
The following supporting information can be downloaded at: [[https://www.mdpi.com/article/10.3390/app142210422/s1](https://www.mdpi.com/article/10.3390/app142210422/s1)]([https://www.mdpi.com/article/10.3390/app142210422/s1](https://www.mdpi.com/article/10.3390/app142210422/s1)), Supplementary material for the fitting for isotherms and kinetics is included.  
Conceptualization, P.M.T. and N.D.S.; methodology, P.M.T. and N.D.S.; software, N.D.S.; validation, P.M.T.; formal analysis, N.E.N.; investigation, N.E.N.; resources, P.M.T. and N.D.S.; data curation, N.E.N.; writing--original draft preparation, N.E.N.; writing--review and editing, P.M.T.; visualization, N.D.S.; supervision, P.M.T. and N.D.S.; project administration, P.M.T. and N.D.S. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is available upon request.  
We want to thank the Vaal University of Technology for allowing us to use its facilities.  
The authors declare no conflicts of interest.