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Article

Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based Capsicum annuum

by
Patience Mapule Thabede
*,
Nkululeko Excellent Nkosi
and
Ntaote David Shooto
Natural Sciences Department, Vaal University of Technology, P.O. Box X021, Vanderbijlpark 1900, South Africa
*
Author to whom correspondence should be addressed.
Appl. Sci. 2024, 14(22), 10422; https://doi.org/10.3390/app142210422
Submission received: 10 September 2024 / Revised: 7 November 2024 / Accepted: 8 November 2024 / Published: 13 November 2024
(This article belongs to the Section Surface Sciences and Technology)
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Abstract

:
Toxic metal ions such as cadmium-Cd(II) and chromium-Cr(VI) are toxic, do not degrade easily in nature, and can cause various disorders and diseases in humans. Removing and monitoring Cd(II) and Cr(VI) ions is necessary for improving water quality. This study aimed to produce an adsorbent from activated carbon of Capsicum annuum and assess its ability to adsorb Cd(II) and Cr(VI) ions from water. The results showed that the adsorbent from C. annuum was porous after its conversion to activated carbon. The pH(PZC) of carbon-based Capsicum annuum was acidic, with a pH of 2.68. The highest capacities for both metal ions were observed at a pH of 1 for Cr(VI) and pH 5 for Cd(II), with capacities of 18.38 and 29.48 mg/g, respectively. The Freundlich sorption model proved to be the most suitable method. The adsorption of Cr(VI) and Cd(II) increased when the initial concentrations were raised from 20 and 60 mg/L; thereafter, a decrease was observed. The data showed that the adsorption of Cd(II) was fast and reached a maximum in 40 min, while Cr(VI) increased with time up to 30 min; thereafter, the rate for Cr(VI) decreased, while equilibrium was reached for Cd(II) ions. The temperature effect showed that the adsorption of Cd(II) and Cr(VI) ions reached a maximum at 55 and 45 °C, respectively. The results of enthalpy change (ΔH°) showed that the uptake process was exothermic, while the change in thermodynamic values of Gibbs energy (ΔG°) indicated that the sorption process was spontaneous and achievable. The greatest adsorption capacities for Cd(II) and Cr(VI) ions were 34.34 and 15.24 mg/g, respectively. The activated carbon from C. annuum proved to be effective for the adsorption of Cd(II) and Cr(VI) from wastewater.

1. Introduction

Several toxic ions such as cadmium and chromium present in water adversely affect the environment and ecosystem [1]. High concentrations of chromium, especially hexavalent chromium [Cr(VI)], are carcinogenic and harmful to aquatic organisms [2]. Chromium accumulates in sediments and water bodies, enters the food chain, and leads to respiratory problems and dermatitis [3]. The World Health Organization (WHO) has recommended a maximum level of 0.05 mg/L for total chromium in drinking water [4]. Chromium concentrations in industrial wastewater are typically between 50–200 mg/L [5].
Cadmium (Cd) is prevalent in rivers and soils worldwide and is carcinogenic [6,7]. Continuous exposure to Cd causes adverse and irreversible neurological damage, osteoporosis, metabolic disorders, kidney failure, and cancer [8]. The limit for Cd(II) concentration set by the WHO and the International Agency for Research on Cancer is below 0.003 mg/L [9]. This pollutant originates from industrial effluents, including electroplating, mining waste, and pigment production [10]. Chromium exists in the environment as Cr(III) and Cr(VI) states, with Cr(VI) being the most toxic. Ingestion, contact, and inhalation are the common routes of chromium entry into the human body. The negative effects of chromium on humans are lung cancer, skin allergies, carcinogenicity, and skin allergies [11]. The presence of heavy metals threatens both the environment and human health and affects the production of food fit for consumption [12]. Researchers are constantly looking for technologies that can effectively adsorb toxic pollutants from the environment, especially from the aquatic environment.
In general, conventional methods have been used to uptake toxic metals and chemicals from the environment [13]. However, these methods have their limitations, such as high costs, possible secondary pollution, and difficulties in some applications [14]. In addition, various analytical methods have been used for quantifying metal ions, namely mass spectrometry, AAS, ICP, and voltammetry methods [15]. Although these techniques are highly sensitive, they are not suitable for on-site detection because they require expensive instruments that are very difficult to obtain [15]. Optical determination and detection of metal ions has become an important field of research as it can be used in several areas, including biomedicine, chemistry, and environmental research [16]. Currently, adsorption is the most efficient method for adsorbing toxic metal ions from water because it offers advantages such as easy handling and selectivity [17,18]. Agriculturally based materials have been used as adsorbents for the removal of different pollutants. This is due to their accessibility, availability, low cost, recyclability, and reusability. Sehar et al. (2021) [19] synthesised sulfur nanoparticles using a precipitation method and cationic surfactants for photocatalytic reduction of Cr(VI). Studies have shown that biodegradation of industrial dyes can be achieved using physicochemical treatment processes [19].
Several studies used plant material converted into activated carbon to adsorb pollutants from wastewater. Carbon from biomass waste or plant materials is recognized as an environmentally friendly adsorbent, with high efficiency and low cost, for use to immobilize pollutants in water [20]. Carbon from agricultural material is a desirable adsorbent due to its high cation exchange, unique physicochemical properties, high specific surface area, and many functional groups [21]. Carbon materials can meet the demand for inexpensive adsorbents for wastewater treatment [22]. Toxic metal ions and carbon can interact with each other in various ways, including through bonding by ion exchange, precipitation with soluble minerals, and complexation of oxygen functions [23].
Researchers have used activated carbon as an effective adsorbent to adsorb dyes, toxic metal ions, and pharmaceutical pollutants. 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]. Lakshmi 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. Tuli et al. (2020) [26] converted tea waste into activated carbon and used it to adsorb methylene blue. Senthilkumar et al. (2024) [26] produced activated carbon from fish waste for the uptake of methylene blue. Mopoung 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 K2CO3 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. Nkosi 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 Aziz et al. (2021) [30]. In another study, Koyuncu (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. annum 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. annum 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 astringency 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 °C and activated with nitric acid (HNO3). 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.

2. Materials and Preparation

2.1. Material and Chemicals

Capsicum annuum was purchased from a local market in Vandebijilpark, Johannesburg, South Africa. Cadmium acetate [Cd(CH3COO)2]-99.95%, potassium dichromate (K2Cr2O7)-99.5%, nitric acid (HNO3)-70%, sodium hydroxide (NaOH) ≥ 98.0%, hydrochloric acid (HCl) ≥ 37.0%, and potassium nitrate (KNO3) ≥ 99.0% were purchased from Merck, South Africa.

2.2. Adsorbents Preparation

2.2.1. Raw Chili Peppers

The chili peppers were washed with ultrapure water and then dried overnight at 45 °C. About 15 g of chili peppers were pulverized to powder. The method used by Shooto and Thabede et al. (2022) [35] was adopted, with modifications, for this study.

2.2.2. Preparation of Carbon from Chili Peppers

The preparation of carbon was performed using the method of Mabalane et al. (2024) [36] with modifications. The chili powder was carbonized at 460 °C with nitrogen gas at 110 L/min at 10 °C/min. Carbonization was carried out for 60 min and then the material was cooled before further use.

2.2.3. Activation of Carbon from Chili Peppers

The carbonized pepper was added to 150 mL of 0.2 M HNO3 for activation. The blend was stirred in a shaker for 180 min. The adsorbent was rinsed with ultrapure water and dried at 50 °C for 20 h. The adsorbent was then used for further analyses. This procedure was adopted from Thabede (2023) [37].

3. Experimentations

3.1. Batch Experiments

A solution (200 mg/L) was prepared from cadmium acetate [Cd(CH3COO)2] and potassium dichromate (K2Cr2O7). The effect of time was studied at different intervals from 1 to 80 min with a standard solution concentration of 100 mg/L at a pH of 5.5. A working solution of 20 mL was added to 10 mg of the adsorbent in sealed bottles. The concentration effect using 20–100 mg/L at a time of 60 min was investigated with 20 mL of the indicated concentrations added to sealed bottles containing 10 mg of activated carbon. The pH effect was investigated at pH values of 1–9 with 20 mL of the standard solution, which was then placed in bottles with 10 mg of the carbon. The temperature effect was investigated at 303, 308, 318, 328, and 338 K under similar experimental conditions. For each parameter, a shaker was used to shake the working standards and adsorbents at 200 rpm. Afterwards, the bottles were taken out and the adsorbents were allowed to settle overnight before being filtered. For the validity and correctness of the results, the adsorption experiments were conducted in triplicate.
The calculation of adsorption capacity was achieved by using Equation (1), where qe is capacity in mg/g of Cd(II) and Cr(VI) ions.
q e = ( C o − C e )   V W
The starting and ultimate concentrations are denoted by Co and Ce, respectively, in mg/L. The volume of the solution and the mass of the adsorbent are indicated by V (in mL) and W (in mg), respectively. The non-linear graphs for the fitting of adsorption can be viewed in the Supplementary Materials.

3.2. Error Determination

Statistical significance was assessed and estimated for the isotherm models. This was obtained by two error functions, namely the (MPSD) Marquart’s percentage standard deviation [38] and the (RMSE) root mean square error [39], as indicated in Equations (2) and (3), respectively.
M P S D = 100   1 N − p ∑ i = 1 N   ( q e ,   e x p − q e ,   c a l q e ,   e x p ) 2  
R M S E = 1 N − p ∑ i = 1 N ( q e ,   e x p − q e ,   c a l ) 2  
where qe,exp describes the experimental uptake and qe,cal refers to the calculated uptake. N and p denote the number of parameters and data points, respectively.

3.3. pH at Point Zero Charge

A 20 mL solution containing 0.02 M potassium nitrate was added to the sample bottles. The starting pH of each solution was altered to pH values from 1–9. Then, 10 mg of adsorbent was added to the sample bottles. The mixture was shaken for 48 h. The final pH was calculated and used to determine the pH(PZC).

3.4. Analytical Methods

SEM images were determined using a JSM IT500 SEM (JEOL Ltd., Tokyo, Japan). The FTIR spectrum was obtained with a 400-Perkin Elmer (Waltham, MA, USA) instrument to determine the functional groups on the activated carbon. Thermal analyses were conducted using a Perkin Elmer TGA 4000 thermogravimetric (purchased from PerkinElmer, South Africa) analyzer at a heating rate of 10 °C/min using nitrogen. The pH value at the charge zero point pH(PZC) was determined using a pH meter. The concentration of Cr(VI) and Cd(II) ions was determined using an iCAP 7000 plus series ICP-OES spectrometer, Thermo Fisher Scientific, Waltham, MA, USA)

4. Results

4.1. Characterization

4.1.1. FTIR Analysis

FTIR investigation was performed to identify and understand the interactions between the activated carbon and the functional groups on the adsorbent (Figure 1). The activated carbon from C. annuum showed characteristic bands at 3266, 2923, 2842, 1749, 1592, 1438, 1365, 1156, and 722 cm−1. These wavenumbers correspond to the (-OH) stretching, (-CH) stretching of the two bands of (-CH3) and (-CH2), the ketonic group (-COOH), and the carbonyl group (-C=O). The bands around 1300 and 1000 cm−1 are -C-O-C and -C-O vibrations of alcohols and carboxylic acids, respectively [40]. Thus, the FTIR spectrum of activated carbon before adsorption of Cd(II) and Cr(VI) clearly shows that the outer surface of the adsorbent has a hydroxyl group that can be deprotonated to bind Cd(II) ions and protonated to accept Cr(VI). The other functional groups are rich in oxygen, which can also promote the sorption of Cd(II) ions. These results demonstrate the active functionality of activated carbon for absorption of Cd(II) and Cr(VI) ions.

4.1.2. SEM Analysis

The surface of the activated adsorbent was analyzed with an SEM and Figure 2 shows the characterization of the adsorbent. The prepared activated carbon exhibits a dense and uniform wrinkled morphology. Figure 2A,B indicate different areas on the adsorbent surface. Figure 2A shows an image that has wrinkles, and the other part of the image shows pores. Figure 2B indicates wrinkles that have more cavities and pores on most of the adsorbent’s surface. The prepared adsorbent exhibits porosity and can provide adequate adsorption capacity for Cd(II) and Cr(VI) ions [41]. The presence of the cavities suggests that the adsorbent may have a better capacity to adsorb contaminants since they can diffuse into these voids and settle on the active sites [42].

4.1.3. Thermal Analysis

Figure 3 shows the thermogravimetric analysis (TGA) and the derived thermal analysis (DTA) of the adsorbent. The thermal profile of the activated carbon shows two main stages of mass loss from room temperature to 800 °C. The first stage, due to moisture loss of 2.4%, was observed between 30 and 120 °C [35]. The results are comparable to those of Simonovska et al. (2016) [43], who used chili stalks as an adsorbent. The largest weight loss of 38.91% between 228 and 520 °C was observed as the major decomposition step due to the degradation of lignocellulosic materials, including the loss of volatiles. Similar results were obtained by Park et al. (2022) [44] during their study of thermal analysis for pepper stems. After this step, the remainder of the activated carbon was observed between 550 and 800 °C. The DTA profile of the activated carbon showed three distinct peaks at 68.67 °C, 228.73 °C, and 354.42 °C. The main degeneration stage was detected at 453.07 °C.

4.2. Physicochemical Analysis

4.2.1. Effect of pH at Point Zero Charge

The diagram of pHi in relation to pHf is shown in Figure 4. The pH at point zero charge value for the adsorbent was 2.68, indicating acidic results. When the pH is larger than pH(PZC), the sorbent surface is negatively charged. This favors the adsorption of cations, suggesting a higher uptake of Cd(II) [45]. On the other hand, the surface of the activated carbon was positively charged when the pH of the solutions was below pH(PZC), meaning that anions could be adsorbed. These results agree with the pH results.

4.2.2. Effect of pH

pH is the critical parameter for the detection and uptake of metal ions, and the acidity of the sorbent solution also plays a crucial role in the formation of metal chelates [46]. To establish the effect of pH at which the activated carbon can remove Cd(II) and Cr(VI) ions, a pH range between 1.0 and 9.0 was used. Figure 5 shows the influence of the pH value on the absorption of Cd(II) and Cr(VI) ions. The highest absorption was observed at pH 1 for Cr(VI) and then a decrease was observed as the pH increased from 3 to 9. This is because Cr(VI) is ionic in all pH ranges, specifically as HCrO4− and Cr2O72− at a pH < 6 and CrO42− at a pH > 6 [47]. The activated carbon and the Cr(VI) ions existed as opposite charges, which ultimately favored the interaction at an acidic pH [48]. At alkaline pH, Cr(VI) and the adsorbent tend to be negatively charged, leading to a repulsion interaction and thus lower sorption. A different trend was observed for the uptake of Cd(II), which showed that the highest capacity was reached at pH 5. These results indicate that the adsorption of Cd(II) ions was low in an acidic medium. With increasing pH, the adsorption of Cd(II) ions increased. At neutral pH and above, the uptake of Cd(II) ions decreased due to competition from the hydroxyl group [49]. Adsorbents are usually affected by the specific pH solution. In other studies, the adsorption efficiency increased with increasing pH because of the reduced electrostatic repulsions between the adsorbent functional surface and adsorbate. This favoured the adsorption of Cd(II). A similar observation was reported in Awual et al. (2020) and Rahman et al. (2020) [50,51].

4.3. Adsorption Data

4.3.1. Initial Concentration Effect of Activated Carbon

The capacity of activated carbon for the uptake of Cd(II) and Cr(VI) ions was assessed based on the sorption rate at different concentrations. The sorption of Cd(II) and Cr(VI) at different initial concentrations is displayed in Figure 6. The diagrams show that the adsorption capacity increases as the concentration of the Cd(II) and Cr(VI) in the solution increases. As the concentration of the two pollutants increased, the sorption capacity of the adsorbent increased due to the driving force between the liquid and the solid phases [52,53]. Activated carbon from C. annuum presented potential for the sorption efficiency of Cd(II) and Cr(VI) ions. Despite this, when the concentration increased above 60 mg/L, the adsorption sites on the carbon became saturated and a decrease in adsorption was observed.
The sorption isotherm analyses show the correlation between the pollutant concentrations in the water and the amount of adsorbate on the adsorbent when both phases are in equilibrium [54]. Two sorption isotherms were used to estimate the interaction of Cd(II) and Cr(VI) ions with the activated carbon and are displayed in Table 1. The data show that the adsorption process obeys the Freundlich isotherm model. The Freundlich model suggests that a multilayer deposition on the sorbent surface is the cause of the sorption process [55]. The value of Kf also suggests that Cd(II) and Cr(VI) ions are coordinated with the adsorption sites of the activated carbon [39]. Freundlich sorption constants were calculated from the isotherm with a coefficient of R2 of 0.972 for Cd(II) and 0.985 for Cr(VI). The sorption capacities of the adsorbent were 15.24 for Cr(VI) and 34.34 mg/g for Cd(II). Langmuir’s non-linear regression did not fit well with the experimental data. This could be because the Langmuir equation does not provide a mechanism for understanding sorption phenomena; it is usually used to estimate the maximum uptake from experimental data [56]. The MPSD and RMSE results for Cd(II) and Cr(VI) on activated carbon indicate that the reduced error values are obtained in the Freundlich model.

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 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 R2 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 R2 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].

4.3.3. Temperature Effect of Activated Carbon

The temperature effect on adsorption was investigated for 60 min at 200 rpm and the results are indicated in Figure 8. The adsorption of Cd(II) and Cr(VI) ions reached a peak as the temperature increased to 45 °C. Beyond this point, temperature increases did not influence the adsorption process. This may be due to fewer active sites available for the adsorption of Cd(II) and Cr(VI) ions [61]. This ultimately led to equilibrium for Cr(VI) ions, while for Cd(II) there was a slight decline [61]. Bassareh et al. (2023) [62] had similar results in the sorption of lead ions from wastewater with activated carbon.
The thermodynamic parameters for Cr(VI) and Cd(II) ions are displayed in Table 3. The data were determined at temperatures of 298, 308, 318, 328, and 338 K. The adsorption process for Cr(VI) and Cd(II) ions shows that ∆H° was negative. This suggests that the sorption process is exothermic [63]. Positive value of ∆S° suggests the escape of solvated H2O from the Cr(VI) and Cd(II) ions into the solution during sorption [64]. The change in Gibbs energy ∆G° values for Cr(VI) and Cd(II) ions were negative at all temperatures, indicating that the reaction was spontaneous and exothermic [65]. This also indicates the physical nature of the process [66]. The values of ∆G° decreased with increasing sorption temperature, suggesting removal of pollutants, rather than their uptake, at high temperatures [67].

4.3.4. Proposed Sorption Mechanism

FTIR Results After Adsorption of Cd(II) and Cr(VI)

The FTIR spectrum (Figure 9) was used to predict the possible uptake of Cd(II) and Cr(VI) ions. Based on the FTIR results, -OH, -CH from -CH3, -CH from -CH2, -COOH, -C=O, -CO, and -COC were present on the adsorbent surface. However, these functional groups shifted to higher wavenumbers, while others shifted to lower numbers. This shift in wavenumbers indicates the involvement of functional groups as binding sites for Cd(II) and Cr(VI) ions during the sorption process [68]. The peak intensity of the -C=O, -OH, and -COOH on the activated carbon differed after the uptake of the pollutants, suggesting that a complexation reaction between Cd(II) ions and functional groups had occurred [69]. Previous research suggests that the adsorption of pollutants by the adsorbent C. annuum occurs through various mechanisms controlled by processes such as electrostatic attraction, ion exchange, π-interactions, complexation, and hydrogen bonding between the pollutants and the oxygen-containing groups on the surface, namely carboxyl, hydroxyl, and carboxylate [70,71].

XRD Results After Adsorption of Cd(II) and Cr(VI)

The XRD pattern in Figure 10 shows the activated carbon before adsorption and after the adsorption of Cd(II) and Cr(VI). The broad peak corresponding to the (002) plan is observed between 8.09 and 26.80°, which is due to cellulosic material [67]. After adsorption of Cd(II), the peak is observed between 10.53 and 26.57° and shifted to 17.42 and 29.22° after the adsorption of Cr(VI). The activated carbon before adsorption also exhibits sharp peaks due to crystallinity at 27.05, 29.61, 32.19, and 38.96. However, after the adsorption of Cd(II) and Cr(VI), the sharp peaks disappeared. The disappearance of crystalline peaks after adsorption confirms that the adsorbent contains mainly amorphous compounds as a result of the decomposition of cellulose, indicating an amorphous carbon structure [72]. This may be due to the adsorption of Cd(II) and Cr(VI) on the carbonized C. annuum [73].

SEM Images After Adsorption of Cd(II) and Cr(VI)

Figure 11A,B shows the surface morphologies determined by SEM of the activated carbon after the adsorption of Cd(II) and Cr(VI). The SEM images of the prepared carbon show a different morphology after the adsorption of Cd(II) and Cr(VI). Both images display pore structures that could not be seen clearly. The images show that the surface of the adsorbent no longer displays wrinkles but rather that the surface is rough. The total change in the surface morphology after the adsorption process may have contributed to the sorption capacity of the adsorbent.

Proposed Adsorption Mechanism of Cd(II) and Cr(VI)

Figure 12 shows the proposed uptake mechanism of both metal ions. Functional groups, electrical charge, and the structure of the adsorbate and the adsorbent determine the uptake mechanism [74]. The structure of the adsorbate and the functional groups on the adsorbent are the two main factors that determine the adsorption mechanism [74]. Yuan et al. (2022) [75] reported that when toxic metal ions are adsorbed under basic conditions, functional groups of the activated carbon, such as -COH and -COOH, are deprotonated by the removal of H+ and form anionic structures, namely -COO- and -CO-, which ultimately promote the uptake of cations. This is shown in Equations (4) and (5) [75]. In addition, cadmium can also react with OH− to form a precipitate of Cd(OH)2, as shown in Equation (6).
Cd2+ + 2C-COOH → (C-COO)2Cd + 2H+
Cd2+ + C-OH + H2O → C-OCd+ + H3O+
2OH− + Cd2+ → Cd(OH)2↓
At a pH value of 2, Cr(VI) is usually present as HCrO4− or Cr2O72− [76]. The functional groups on the adsorbent surface are protonated in acidic conditions (pH = 1). Therefore, the HCrO4− or Cr2O72− under acidic conditions interact electrostatically with positively charged protonated groups during the adsorption process [77,78]. Once the adsorption has occurred, the Cr-O compound forms hydrogen bonds with the hydroxyl groups [78]. Consequently, the sorption of Cr(VI) by the adsorbent can be explained by electrostatic attraction and hydrogen bond formation.

4.3.5. Comparative Studies

Table 4 shows the sorption capacities of activated carbon from different materials compared to activated carbon from chili peppers. The table shows that activated carbon from C. annuum performs better than other adsorbents, suggesting that it has great potential for removing Cd(II) and Cr(VI) ions from an aqueous solution.

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 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.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/app142210422/s1, Supplementary material for the fitting for isotherms and kinetics is included.

Author Contributions

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.

Funding

This research received no external funding.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data is available upon request.

Acknowledgments

We want to thank the Vaal University of Technology for allowing us to use its facilities.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. FTIR spectrum of the activated carbon.
Figure 1. FTIR spectrum of the activated carbon.
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Figure 2. SEM images (A,B) of activated carbon before adsorption.
Figure 2. SEM images (A,B) of activated carbon before adsorption.
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Figure 3. TGA and DTA profile of activated carbon.
Figure 3. TGA and DTA profile of activated carbon.
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Figure 4. pH at the point zero charge by activated carbon.
Figure 4. pH at the point zero charge by activated carbon.
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Figure 5. pH at the point zero charge by activated carbon.
Figure 5. pH at the point zero charge by activated carbon.
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Figure 6. Concentration effect of adsorption by activated carbon.
Figure 6. Concentration effect of adsorption by activated carbon.
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Figure 7. Time effect of adsorption by activated carbon.
Figure 7. Time effect of adsorption by activated carbon.
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Figure 8. Temperature effect of adsorption by activated carbon.
Figure 8. Temperature effect of adsorption by activated carbon.
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Figure 9. FTIR spectrum of activated carbon after the sorption of Cd(II) and Cr(VI).
Figure 9. FTIR spectrum of activated carbon after the sorption of Cd(II) and Cr(VI).
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Figure 10. XRD spectra of AC before adsorption and AC after adsorption of Cd(II) and after adsorption of Cr(VI).
Figure 10. XRD spectra of AC before adsorption and AC after adsorption of Cd(II) and after adsorption of Cr(VI).
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Figure 11. SESEM images (A,B) of activated carbon after adsorption.
Figure 11. SESEM images (A,B) of activated carbon after adsorption.
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Figure 12. The proposed sorption mechanism of Cd(II) and Cr(VI) ions.
Figure 12. The proposed sorption mechanism of Cd(II) and Cr(VI) ions.
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Table 1. Isotherms for the adsorption of Cd(II) and Cr(VI).
Table 1. Isotherms for the adsorption of Cd(II) and Cr(VI).
Adsorption ModelsParametersPollutants
Cd(II)Cr(VI)
LangmuirQo (mg/g)24.9212.94
B (L/mol)0.3240.213
R20.8460.890
RMSE12.876.42
MPSD27.4315.09
FreundlichKF1.1211.050
N1.5631.414
R20.9720.985
RMSE9.426.42
MPSD4.423.58
Experimentalqe (mg/g)34.3415.24
Table 2. Kinetics for the sorption of Cd(II) and Cr(VI).
Table 2. Kinetics for the sorption of Cd(II) and Cr(VI).
Kinetic ModelsParametersPollutants
Cd(II)Cr(VI)
PFOqe (mg/g)32.4626.40
K1 (min−1)0.0780.074
R20.9880.965
PSOqe (mg/g)19.2520.09
K2 (g·mg/min)1.0041.006
R20.8770.824
IPDC (mg/g)6.2464.234
Ki (g/g·min1/2)0.0570.067
R20.8450.811
Experimentalqe (mg/g)33.1427.48
Table 3. Thermodynamic parameters for the adsorption of Cd(II) and Cr(VI).
Table 3. Thermodynamic parameters for the adsorption of Cd(II) and Cr(VI).
ParametersPollutants
Cd(II)Cr(VI)
∆H° (KJ mol−1)−2.15−3.23
∆S° (KJ mol−1K−1)0.05670.0245
∆G° (KJ mol−1) 298 K −5.91−7.93
308 K−4.24−4.31
318 K−3.74−4.21
328 K−2.53−1.56
338 K−1.90−1.13
Table 4. Adsorbents used to prepare carbon for the sorption of Cr(VI) and Cd(II) ions.
Table 4. Adsorbents used to prepare carbon for the sorption of Cr(VI) and Cd(II) ions.
Adsorbentsq(max) (mg/g)References
Cr(VI)
Cauliflower stem64.10[79]
Capsicum annuum34.34This study
Eucalyptus tree bark30.6[80]
Rice straw17.47[81]
Rumex abyssinicus19.35[82]
Rice husk9.97[83]
White tea residue9.11[84]
Sludge7.00[85]
Adsorbentsq(max) (mg/g)References
Cd(II)
Rice husk28.96[86]
Capsicum annuum15.24This study
Rice straw14.97[13]
Corn straw12.40[87]
Corn stalk12.10[88]
Attapulgite10.38[89]
Buffalo weed9.70[90]
Sewage sludge0.87[91]
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Thabede, P.M.; Nkosi, N.E.; Shooto, N.D. Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based Capsicum annuum. Appl. Sci. 2024, 14, 10422. https://doi.org/10.3390/app142210422

AMA Style

Thabede PM, Nkosi NE, Shooto ND. Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based Capsicum annuum. Applied Sciences. 2024; 14(22):10422. https://doi.org/10.3390/app142210422

Chicago/Turabian Style

Thabede, Patience Mapule, Nkululeko Excellent Nkosi, and Ntaote David Shooto. 2024. "Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based Capsicum annuum" Applied Sciences 14, no. 22: 10422. https://doi.org/10.3390/app142210422

APA Style

Thabede, P. M., Nkosi, N. E., & Shooto, N. D. (2024). Effective Uptake of Cadmium and Chromium from Wastewater Using Carbon-Based Capsicum annuum. Applied Sciences, 14(22), 10422. https://doi.org/10.3390/app142210422

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