# Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based _Capsicum annuum_
## 1 Introduction
Chemical activation has been shown to offer better characteristics than physical activation processes because of the simplicity, low temperatures required, shorter activation times, and the development of porous materials as adsorbents [24]. [PERSON] et al. (2009) [25] used rice husk and converted it to activated carbon for the removal of Indigo Carmine dye. Their results showed that the maximum adsorption capacity was 13.97 mg/g. [PERSON] et al. (2020) [26] converted tea waste into activated carbon and used it to adsorb methylene blue. [PERSON] et al. (2024) [26] produced activated carbon from fish waste for the uptake of methylene blue. [PERSON] et al. (2024) [27] prepared carbon from sugarcane leaves and rice straw and activated it by using KMnO4. Activated carbon from chili straw was prepared by activation with K2 CO3 and applied for the sorption of methyl orange and methylene blue dyes [28]. The surface properties of red pepper waste-based activated carbon were investigated for supercapacitor electrodes using ZnCl2 as an activator. [PERSON] et al. (2024) [29] produced an Fe3O4 carbon composite from chili pepper for the removal of paracetamol, methylene blue, and nickel ions from wastewater. Chili stems were used as an adsorbent to remove methylene blue from an aqueous solution by [PERSON] et al. (2021) [30]. In another study, [PERSON] (2022) [31] used _C. annuum_ and converted it into activated carbon to adsorb methanol yellow and methylene blue. Agricultural materials have been used as carbon adsorbents for the removal of different types of water pollutants. This is because of their accessibility, availability, low cost, reusability, and recyclability [32]. The investigation of the adsorption of _C. annuum_ was conducted because this material contains high levels of capsaicin and secondary alkaloids [33]. These are exceptional sources of carbon and are valuable because they have oxygen functional groups, aromatic rings, and amine groups. These properties make _C. annuum_ an excellent adsorbent, and it is also expected to increase the sorption capacity. Several adsorbents have been used for the removal of Cr(VI) and Cd(II) ions. This includes using agricultural materials for preparing activated carbon. However, there are limited studies that report using chili peppers for adsorption for the uptake of metal ions from wastewater. No study has been conducted on removing Cd(II) and Cr(VI) simultaneously using activated carbon from C. _annuum_ from an aqueous solution. _Capsicum annuum_ has different parts, including calyx, capsicum glands, seeds, placenta, endocarp, mesocarp, exocarp, and stalk [34]. In this study, all parts of _C. annuum_ were used except the stalk. Waste from _C. annuum_ cannot be used as fertilizer or feed because of its astingency and pungency. Chemical treatments can alter the cell surface by masking the groups and exposing metal ion binding sites. In this research, carbon was produced from _C. annuum_ at 460 \({}^{\circ}\)C and activated with nitric acid (HNO\({}_{3}\)). This activated carbon was analyzed and the adsorption capacities for Cd(II) and Cr(VI) ions were investigated. The behavior of Cd(II) and Cr(VI) ions was investigated by considering the reaction time, concentration of Cd(II) and Cr(VI) ions, temperature, and pH value. In addition, the sorption kinetics, isotherms, and thermodynamics were analyzed to explain the uptake of Cd(II) and Cr(VI) ions. The objectives of this study were (i) to convert _C. annuum_ to carbon and activate it with nitric acid and (ii) to determine the uptake capacity of the produced carbon-based adsorbent. This study highlights how _C. annuum_ can be used to produce activated carbon and its role in the sorption of Cd(II) and Cr(VI) ions, contributing to knowledge about the effective removal of toxic metal ions in wastewater treatment.