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Article

Adsorption of Phenol from Aqueous Solution Utilizing Activated Carbon Prepared from Catha edulis Stem

by
Meseret Dawit Teweldebrihan
1,2,*,
Mikiyas Abewaa Gnaro
3 and
Megersa Olumana Dinka
1
1
Department of Civil Engineering Sciences, Faculty of Engineering and the Built Environment, University of Johannesburg APK Campus, Auckland Park, Johannesburg 2006, South Africa
2
Institute of Technology, School of Water Resource and Environmental Engineering, Haramaya University, Dire Dawa P.O. Box 138, Ethiopia
3
Department of Chemical Engineering, College of Engineering and Technology, Wachemo University, Hossana P.O. Box. 667, Ethiopia
*
Author to whom correspondence should be addressed.
Environments 2025, 12(9), 314; https://doi.org/10.3390/environments12090314
Submission received: 20 December 2024 / Revised: 18 January 2025 / Accepted: 7 February 2025 / Published: 5 September 2025
(This article belongs to the Special Issue Advanced Technologies of Water and Wastewater Treatment (2nd Edition))
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Abstract

Phenol and its derivatives in water and wastewater are highly toxic and challenging to degrade, posing serious environmental and health risks. Therefore, this research focuses on the removal of phenol from aqueous solutions using activated carbon made from Catha edulis stems. The activation process involved impregnating the Catha edulis stems with phosphoric acid followed by thermal treatment at 500 °C for 2 h. The resulting adsorbent was extensively characterized using various techniques, including Scanning Electron Microscopy (SEM), Fourier Transform Infrared Spectroscopy (FTIR), Brunauer–Emmett–Teller (BET) surface area analysis, and proximate analysis. Batch adsorption experiments were designed using a full factorial approach with four factors at two levels, resulting in 16 different experimental conditions. The characterization results showed that the activated carbon has a high surface area of 1323 m2/g, a porous and heterogeneous structure, and an amorphous surface with multiple functional groups. Under optimal conditions of pH 2, a contact time of 60 min, an adsorbent dosage of 0.1 g/100 mL, and an initial phenol concentration of 100 mg/L, the adsorbent achieved a phenol removal efficiency of 99.9%. Isotherm and kinetics analyses revealed that phenol adsorption fits the Langmuir model and pseudo-second-order kinetics, indicating a uniform interaction and chemisorptive process. This study highlights the effectiveness of Catha edulis stem-based activated carbon as a promising material for phenol removal in water treatment applications.

1. Introduction

Phenol (C6H5OH), a member of the alcohol of the aromatic series, is the simplest phenolic compound that has a sweet aroma and a hydroxyl group attached to an aromatic hydrocarbon; it exhibits solubility in water and color and antioxidant capabilities [1,2]. It is extensively utilized in various industries like oil refining, oil shale processing, and timber, coke, paint, light, medical, and chemical industries [3,4]. The global phenol market size was around USD 23.3 billion in 2020, with a projected growth rate of 4–5% between 2020 and 2027, indicating its significant industrial importance [5,6]. However, phenol and its derivatives in water and wastewater pose serious environmental and human health risks due to their high toxicity and difficulty in degradation [7]. Precisely, exposure to phenol can result in a range of diseases and symptoms, including gastrointestinal problems, skin burns, nerve damage, impaired vision, and damage to the liver and kidneys [8]. Furthermore, phenolic compounds, such as chlorophenols, have been linked to histopathological alterations, genotoxicity, mutagenicity, and even carcinogenic effects, posing significant risks to human health and the ecosystem [9]. Therefore, the World Health Organization (WHO) recommends a maximum phenol concentration of 0.001 mg/L in water, while Spanish legislation limits it to 1 mg/L in wastewater, highlighting the need to control phenolic compounds to protect human health and the environment [10,11].
Conventional wastewater treatment methods like coagulation, flocculation, sedimentation, and filtration effectively remove particulate matter and microorganisms but are less efficient against recalcitrant pollutants like heavy metals and microplastics [12,13]. They often lack cost-effectiveness and may generate toxic byproducts, limiting large-scale applications [14]. In contrast, advanced technologies, such as membrane bioreactors, electrocoagulation, and photocatalytic degradation, achieve superior removal of contaminants of emerging concern and persistent organic pollutants [15,16]. These eco-friendly methods, however, require high initial investments and specialized skills [15]. Additionally, challenges like energy consumption and system integration complexity arise [17]. To address these limitations, adsorption has emerged as a viable, efficient, and cost-effective wastewater treatment option.
Adsorption is a vital wastewater treatment method known for its simplicity, cost-effectiveness, and high efficiency in removing pollutants like dyes and phenolic compounds [18,19]. Commercial activated carbon (CAC) offers high adsorption capacity but is costly and less sustainable [20,21]. Biomass-derived activated carbon (BAC), made from agricultural residues, provides a more economical and eco-friendly alternative with comparable performance and lower environmental impact [22]. BAC effectively removes contaminants such as pentachlorophenol while promoting sustainable waste reuse [20]. For example, BAC from Cassia fistula pods achieved a phenol sorption capacity of 183.79 mg/g [23]. Similarly, Kenaf-derived biochar adsorbed up to 41.1 mg/g of phenol [24], whereas food waste-based biochar reached 14.61 ± 1.38 mg/g [25]. On the other hand, agricultural waste is renewable, widely available, and cost-effective for BAC production. However, BAC performance depends on operational conditions, biomass source, and activation methods. Despite these limitations, BAC remains a sustainable solution for wastewater treatment, balancing efficiency with environmental benefits [26].
Catha edulis (khat), a plant widely cultivated in Ethiopia and traditionally chewed for its stimulant effects, generates significant waste that poses environmental challenges such as nutrient leaching and soil and water contamination [27,28,29]. Recent studies have explored the sustainable utilization of Catha edulis waste, including the extraction of cellulose fibers and nanocrystals for industrial applications and the conversion of Catha edulis stems into activated carbon for fluoride removal from water [29,30,31]. Despite these advancements, no research to date has investigated phenol removal using activated carbon derived from Catha edulis stems. This study aims to fill this gap by preparing activated carbon from Catha edulis biomass and evaluating its efficacy in phenol adsorption. Additionally, this study optimizes phenol adsorption parameters while exploring adsorption kinetics and isotherms to establish the material’s efficiency and applicability.

2. Materials and Methods

2.1. Adsorbent Preparation

The Catha edulis stems were collected from Hossana city in Central Ethiopia. Following collection, the stems were subjected to initial size reduction using an ox. Afterward, the stems were thoroughly washed with distilled water to remove dust and impurities. The washed stems were sun-dried for two weeks, followed by oven drying at 105 °C for 24 h in a Thermo Scientific Heratherm oven (Manufacturer: Thermo Fisher Scientific, Waltham, MA, USA). Once the drying process was complete, approximately 1000 g of the dried stems were powdered using a Wiley Mill Model 4 (Manufacturer: Thomas Scientific, USA), which facilitated further size reduction. The powdered stems were then impregnated with 1000 mL of phosphoric acid (85% purity) in a 1:1 weight-to-volume ratio, chosen based on published articles on the same material to enhance pore formation and surface area enhancement [32]. The samples were then oven-dried again at 80 °C for 4 h in the same oven. After impregnation, the stems were carbonized in a Lenton Muffle Furnace (Model: 502, Manufacturer: Thermal Scientific, Cambridge, UK) at 500 °C for 2 h. Following the carbonization process, the samples were washed multiple times with distilled water to remove residual acid and then oven-dried at 105 °C for an additional 24 h. Finally, the activated carbon was ground to a particle size of 250 μm using a Kuhn Rikon Mortar and Pestle (Manufacturer: Kuhn Rikon, Zurich, Switzerland) and stored in an airtight container for further use.

2.2. Adsorbent Characterization

2.2.1. Proximate Analysis

The proximate analysis of Catha edulis stem-derived activated carbon, including the determination of moisture content (MC), volatile matter (VM), ash content (AC), and fixed carbon (FC), was conducted in accordance with ASTM standards. MC was measured following ASTM D 2867, which involves heating the sample at 105 °C until a constant weight is achieved to determine the percentage of water present in the sample. VM was assessed according to ASTM D 2866, which requires heating the sample at 950 °C to volatilize and burn off organic components. AC was determined using ASTM D 2869, where the sample was heated at 500 °C until all combustible material was oxidized, leaving only inorganic residue. FC was not directly measured by ASTM standards, but it was calculated based on Equation (1) [33,34].
FC (%) = 100 − (MC + VM + AC)

2.2.2. Scanning Electron Microscope (SEM)

The morphology of the Catha edulis stem-based activated carbon, both before and after adsorption, was analyzed using an SEM INSPECT F 50 (Manufacturer: Thermo Fisher Scientific, Breda, The Netherlands). The analysis was conducted under settings of 20 kV, beam size of 7.0, and magnifications of 300× at varying distances to capture a comprehensive view of the surface characteristics and morphology of the sample [35].

2.2.3. Fourier Transport Infrared Radiation (FTIR)

The functional groups in the activated carbon derived from Catha edulis stems were identified using FTIR spectroscopy with the KBr disk method. The FTIR analysis was performed using a Thermo Scientific Nicolet iS50 FTIR Spectrometer (Manufacturer: Thermo Fisher Scientific, USA), with a spectral range of 500–4000 cm−1. A sample preparation procedure involved mixing 1 g of the adsorbent sample with 100 mg of KBr, grinding them to a fine powder, and pressing them into pellets. The FTIR spectra obtained were used to determine and analyze the presence of specific functional groups in the sample before and after pollutant adsorption [36].

2.2.4. Brunauer–Emmett–Teller (BET) Surface Area

The specific surface area of the activated carbon made from Catha edulis stems was determined through nitrogen physisorption, using the SA-9600 Horiba Surface Area Analyzer (Manufacturer: Horiba, Kyoto, Japan). Sample preparation involved placing 0.4 g of each sample in preparation tubes, which were degassed under vacuum at 100 °C for 2 h to eliminate moisture and impurities. Nitrogen adsorption and desorption isotherms were obtained at room temperature and atmospheric pressure (700 mmHg) to calculate the surface area based on the BET method [37].

2.3. Experimental Design and Batch Adsorption Optimization

The experimental design for phenol adsorption onto Catha edulis stem-based activated carbon was conducted using a full factorial design with four factors at two levels, resulting in a total of 16 experiments. Each experiment was performed in triplicate, and the average value was reported. The independent variables included pH (2 and 10), contact time (30 and 60 min), adsorbent dosage (0.05 and 0.1 g/100 mL), and initial phenol concentration (100 and 200 mg/L). Table 1 presents the independent variables along with their respective values.
Batch adsorption of phenol from an aqueous solution using Catha edulis-based activated carbon was performed in 100 mL of distilled water, adjusted according to the experimental design. A stock solution of 1000 mg/L phenol was prepared by dissolving it in 1 L of distilled water, and the desired concentration of pollutant solution was achieved through dilution. Subsequently, 100 mL of the prepared solution was added to a 250 mL Erlenmeyer flask, and the pH was adjusted using 0.1 M NaOH or HCl, depending on whether the solution was basic or acidic. The appropriate amount of adsorbent was then added to the pH-adjusted solution, and the mixture was shaken at 200 rpm for a specified contact time. After the exposure period, the solution was filtered using Whatman filter paper, and the filtrate was analyzed to determine the residual phenol concentration. The concentration of unadsorbed phenol was measured using a UV-VIS spectrophotometer set at a maximum wavelength of 270 nm. Then, the removal efficiency ( R e ) of phenol by Catha edulis stem-based activated carbon is calculated using Equation (2).
( R e ) ( % ) = C o − C e C o × 100
where C o is the initial phenol concentration (mg/L) and C e is the equilibrium concentration after adsorption (mg/L). On the other hand, the adsorption capacity ( Q e ) was determined using Equation (3).
Q e ( m g g ) = C o − C e m × V
where V is the solution volume (L), and m is the mass of activated carbon used (g). These equations are essential for evaluating the efficiency and capacity of the adsorbent in removing phenol from aqueous solutions [38,39,40].

2.4. Adsorption Isotherm

Adsorption isotherms are mathematical models that describe how adsorbates interact with adsorbents at equilibrium, offering essential insights for applications such as gas purification and environmental remediation. The Langmuir isotherm model assumes monolayer adsorption on a surface with a finite number of identical sites, while the Freundlich isotherm accounts for heterogeneous surfaces and multilayer adsorption. Understanding these isotherms is crucial for designing and optimizing adsorption processes. In this study, the adsorption of phenol onto Catha edulis-based activated carbon was examined under constant conditions: a temperature of 25 °C, an adsorbent dosage of 0.1 g/100 mL, a contact time of 60 min, and a pH of 2, while varying the initial phenol concentration (100, 120, 140, 160, 180, and 200 mg/L). Equations (4) and (5) represent the Langmuir and Freundlich isotherm models, respectively, while Equation (5) presents the separation factor ( R L ), a dimensionless quantity used to evaluate the feasibility of the Langmuir isotherm.
C e Q E = 1 K L Q m a x + C e Q m a x
log Q e = log K F + 1 n log C e
R L = 1 1 + K L × C o
where Q m a x represents the maximum adsorption capacity in mg/g, and K L is a constant related to the affinity of the binding sites in L/mg. For the Freundlich isotherm model, K F (L/mg) is the Freundlich isotherm constant, and 1/n denotes the adsorption intensity, which reflects both the relative distribution of energy and the heterogeneity of the adsorbent sites [32,33,34]. The Temkin isotherm assumes that the heat of adsorption of all molecules in the layer decreases linearly with coverage. The linearized form of the Temkin isotherm is given in Equation (7).
l n   Q E = l n   K T + B R l n C e
where KT is the Temkin isotherm constant related to the adsorption energy (L/mg), B is a constant related to the heat of adsorption (J/mol), R is the universal gas constant (8.314 J/mol·K), and T is the temperature (K). The Dubinin–Radushkevich (D-R) isotherm describes adsorption in microporous adsorbents, assuming that the adsorption sites are energetically heterogeneous. The linearized form of the D-R isotherm is given by Equation (8).
l n   Q E = l n   Q m a x − β ϵ 2
where β is a constant related to the adsorption energy (mol2/J2), and ϵ is the Polanyi potential in J/mol, which is given by Equation (9).
ϵ = R T   l n   ( 1 + 1 C e )

2.5. Adsorption Kinetics

Adsorption kinetics refers to the rate at which adsorbates adhere to adsorbents, crucial for applications in environmental, chemical, and pharmaceutical fields, particularly in water treatment and drug purification [41,42]. The pseudo-first-order (PFO) and pseudo-second-order (PSO) models are two primary frameworks used to describe these kinetics. The PFO model assumes that the rate of adsorption is proportional to the number of unoccupied sites, while the PSO model posits that the rate is proportional to the square of the number of unoccupied sites [43,44]. This comprehensive understanding aids in the effective design and optimization of adsorption systems. In this research, adsorption kinetics was studied at varying contact times of 20, 30, 40, 50, 60, and 70 min, with other parameters kept constant: adsorbent dosage of 0.1 g/100 mL, initial phenol concentration of 200 mg/L, and a pH of 2. Equations (10) and (11) represent the mathematical models for the PFO and PSO kinetics, respectively.
log Q e − Q t = log Q e − K 1 × t 2.303
where Q t (mg/g) is the amount of phenol adsorbed per unit mass of the adsorbent at any time t (minutes), Q e (mg/g) is the equilibrium adsorption capacity, and K 1 (min)−1 is the PFO equilibrium rate constant.
t Q t = t Q e + 1 K 2 Q e 2
where Q t (mg/g) is the amount of phenol adsorbed per unit mass of the adsorbent at any time t (min), Q e (mg/g) is the equilibrium adsorption capacity, and K 2 (g·mg−1·min−1) is the PSO equilibrium rate constant.

3. Results and Discussions

3.1. Adsorbent Characteristics

3.1.1. Proximate Analysis

The activated carbon prepared from Catha edulis exhibits a moisture content of 3.5%, which is favorable as it indicates minimal water retention, leading to higher adsorption efficiency and better performance in practical applications. The volatile matter content is recorded at 19.9%, suggesting that some organic residues remain after the carbonization process. While this is relatively high, it also indicates that there may be room for further activation to enhance the carbon’s adsorption capacity. With an ash content of 9.6%, the activated carbon has a moderate level of inorganic residues, which could potentially obstruct its pores and reduce its effectiveness. However, the fixed carbon content of 67% is relatively good, reflecting a substantial amount of carbon available for adsorption. This study’s proximate analysis results are superior compared to those reported by [45] for sodium hydroxide surface-modified cassava activated carbon. The authors found values of 6.57 ± 0.20% for moisture content, 49.17 ± 0.46% for volatile matter, 5.00 ± 0.34% for ash content, and 45.83 ± 0.47% for fixed carbon. Overall, while the carbon shows promise, optimizing the production process to reduce volatile matter and ash content could further improve its quality and adsorption capabilities.

3.1.2. FTIR

FTIR was used to study the change in functional groups of the adsorbent before and after adsorption, as shown in Figure 1. Catha edulis is a natural fiber that is mainly composed of cellulose, lignin, and wax. FTIR spectrum of activated adsorbent would contain a lot of bands at different parts of the absorption feature. As shown in Figure 2, the activated carbon primarily included the following functional groups: primary aliphatic alcohols, aliphatic hydrocarbons, carboxylic groups, alkynes substituted, aliphatic primary amines, and inorganic phosphorus. Moreover, the activated carbon clearly shows the band at 3317.65 cm−1 because of the stretching frequency of –OH. When extended to the next peak in some parts, a little bit of shift was observed for the spectra results of the aromatic ring from 1616.88 to 1317.81 cm−1. The adsorption band of the activated carbon in the range of 1247.75 and 1027.01 cm−1 was shifted and reflected the stretching of C-O of the methoxy group (-OCH3) of the aromatic ring of lignin. The hydroxyl and carboxyl groups, as aliphatic and phenolic extractives, could contribute to the sorption of MB. The hydroxyl and carboxylic groups were mentioned as the most efficient groups for the sorption of methylene blue. After adsorption, the C-O bond was shifted from 1026.31 to 1027.01 cm−1, not only in this band but also in other spectra at 1616.88, 1317.81, 1247.75, 773.83, and 519.16 cm−1, respectively [46].

3.1.3. BET Surface Area

The BET surface area of Catha edulis-based activated carbon, determined to be 1323 m2/g, indicates a highly porous structure that is particularly advantageous for removing phenol from aqueous solutions. This extensive surface area enhances the material’s adsorption capacity, enabling it to effectively capture and retain phenol molecules. Compared to other activated carbons, such as the one reported by Schwantes et al. with a surface area of 0.92 m2/g [47], this high BET surface area suggests that Catha edulis-based activated carbon can offer efficient phenol removal, making it a promising choice for water treatment applications where effective contaminant removal is crucial. Moreover, this study’s BET surface area is approximately two times the one reported by Sulaiman et al., which was 653.93 m2/g [48].

3.1.4. SEM

The adsorbent underwent SEM analysis to examine its surface morphology. SEM analysis was performed both before and after the adsorption process. The purpose of this analysis was to understand how phenol adsorption altered the physical structure and surface characteristics of the adsorbent material. Figure 2 presents the results of the SEM analysis, showcasing the surface morphology of the adsorbent. Comparing the images obtained before and after adsorption, notable changes in the surface features can be observed. Specifically, it is evident that the porosity of the material decreased following phenol adsorption, which indicates successful deposition of the target pollutant onto the adsorbent material [49]. This result is in good agreement with the SEM images of water hyacinth-based activated carbon reported by [46,50].

3.2. Batch Adsorption Performances

The batch adsorption performance of Catha edulis stem-based activated carbon for phenol removal is significantly influenced by several key factors: pH, adsorbent dosage, initial phenol concentration, and contact time, as shown in Table 2. The data show that acidic conditions (pH 2) generally yield higher removal efficiencies compared to basic conditions (pH 10). This trend is likely due to the increased protonation of phenol molecules in acidic environments, which enhances their affinity for the activated carbon. Phenol tends to exist as a neutral species in acidic conditions, promoting easier adsorption by the activated carbon’s surface, which has more electrostatic attraction toward the phenol molecules [51]. The highest removal efficiency of 99.9% was achieved at pH 2 with a dosage of 0.1 g/100 mL and a 60 min contact time at an initial phenol concentration of 100 mg/L. The variation in removal efficiency with pH highlights the importance of optimizing this parameter for effective phenol adsorption. Additionally, the adsorbent dosage and contact time play crucial roles in determining the removal efficiency. Higher adsorbent dosages (0.1 g/100 mL) generally provide more active sites, leading to greater phenol uptake [52]. Longer contact times (60 min) allow for more extended interaction between the phenol molecules and the activated carbon, further enhancing adsorption [53]. However, at higher initial phenol concentrations (200 mg/L), the competition for adsorption sites increases, slightly reducing the efficiency [54]. The interplay of these factors suggests that optimal phenol removal can be achieved by carefully adjusting the adsorbent dosage, contact time, and pH to match the specific conditions of the treatment process.

3.3. Interaction Effects

3.3.1. pH and Adsorbent Dosage

pH measures whether a solution is acidic (pH < 7) or basic (pH > 7). This factor is very essential in the adsorption experimental activity, especially for dye adsorption. The pH of the intermediate pH would control the magnitude of electrostatic charges that are imparted by the ionized dye molecules. Above all, the adsorption increment will vary with the pH of an aqueous in between pH 2 and 10. The effect of pH on phenol adsorption capacity was studied, and the obtained results are displayed clearly in Figure 3. The optimal removal efficiency at pH 2 was obtained at 99.9%. The response of phenol removal as a variable of pH and adsorbent dosage in various experimental processes is shown in Figure 3. The increment observed in phenol removal efficiency was because of the increase in adsorbent dosage, which increased the surface area, and more porosity was found than predicted [55]. The pH variable has shown a significant effect as a cause of electrostatic attraction force at pH 2.

3.3.2. Contact Time and Adsorbent Dosage

Figure 4 illustrates the interaction between the dosage of biosorbent and the duration of contact on the efficiency of phenol adsorption, using activated carbon derived from Catha edulis stem. The graph depicts a positive correlation between the amount of adsorbent used and the duration of contact, leading to higher removal rates of phenol. Typically, extending the contact time enhances the efficacy of removing contaminants from water or industrial wastewater by increasing the availability of active binding sites. Moreover, prolonging the contact duration enhances the likelihood of the adsorbate molecules adhering to the surface of the adsorbent material [56,57]. The highest adsorption percentage observed was 95.4%, achieved under conditions with an optimized contact time and adsorbent dosage, a pH of 3.32, and an initial phenol concentration of 150 mg/L.

3.3.3. Contact Time and pH

The three-dimensional representation in Figure 5 illustrates the interaction effect between pH and contact time on the removal efficiency of phenol. Notably, this interaction demonstrates a positive impact on the removal efficiency of the phenol. During this analysis, the adsorbent dosage remained constant at 0.075 g/100 mL, while the pH varied from 2 to 10 and the contact time ranged from 30 to 60 min. Examining the graph, it becomes evident that the simultaneous variation in pH and contact time enhances the efficacy of phenol removal. Such a phenomenon can be attributed to the combined influence of pH and contact time on the availability of active binding sites and the chemical nature of the adsorbent surface [23]. The maximum removal efficiency, obtained through the interaction effect of pH and contact time, reached 83.4%. This highlights the significance of optimizing both parameters in tandem to achieve the highest possible removal efficiency.

3.4. Adsorption Isotherm

Adsorption isotherms are critical for assessing the capacity of the adsorbent. In this study, the Langmuir, Freundlich, Temkin, and Dubinin–Radushkevich (D-R) models were used to evaluate the adsorption of phenol onto the activated carbon of Catha edulis stem. The Langmuir model, which assumes monolayer adsorption on a finite number of identical sites, showed a high R2 value of 0.95, indicating a strong fit to the experimental data as shown in Figure 6. This model provided a maximum Langmuir adsorption capacity Qmax of 208.33 mg/g as shown in Table 3, which represents the maximum amount of phenol that can be adsorbed per gram of adsorbent. The Langmuir constant KL of 0.453 L/mg reflects the affinity of the adsorption sites for the adsorbate [58], while the separation factor (RL) of 0.0216 suggests favorable adsorption conditions [59]. In comparison, the Freundlich model, which describes adsorption on a heterogeneous surface, had a lower R2 value of 0.93, indicating a less precise fit, as shown in Figure 7. The Freundlich constant KF of 89.63 L/mg signifies a high adsorption capacity of the adsorbent for phenol, suggesting that the activated carbon effectively removes the pollutant from the solution [60]. Additionally, the Freundlich parameter n of 3.69 and 1/n of 0.2708 indicate that the adsorption process is favorable but not perfectly uniform across the surface [32,58]. The Temkin isotherm fit, shown in Figure 8, resulted in a binding energy constant (B) of 14.38 J/mol and an adsorption equilibrium constant (A) of 9.67 L/g, indicating a moderate strength of interaction between phenol and the activated carbon surface. The Dubinin–Radushkevich (D-R) isotherm fit is shown in Figure 9, demonstrating a better fit compared to the Temkin and Freundlich isotherm models, though it is slightly lower than that of the Langmuir isotherm. The higher R2 value for the Langmuir model suggests that the adsorption process on the activated carbon of Catha edulis stems aligns more closely with monolayer adsorption, indicating a relatively homogeneous surface with minimal interaction between adsorbate molecules. These findings are consistent with previous studies on biomass-based adsorbents [61].

3.5. Adsorption Kinetics

The adsorption of phenol onto Catha edulis stem-based activated carbon was evaluated using both pseudo-first-order and pseudo-second-order kinetic models, as shown in Figure 10 and Figure 11, respectively. The pseudo-first-order model, with a rate constant K1 of 0.0278 min−1 and an R2 value of 0.993, indicates a good fit to the experimental data but shows limitations. The calculated equilibrium adsorption capacity Qe from this model was 32.1 mg/g, which is significantly lower than the experimental value of 205.3 mg/g, as shown in Table 4. This discrepancy suggests that the pseudo-first-order model may not fully capture the adsorption dynamics, likely because it does not account for the potential involvement of chemisorption or more complex interactions that occur at the solid–liquid interface. In contrast, the pseudo-second-order model demonstrated a higher rate constant K2 of 0.5999 g/mg·min and a superior R2 value of 0.9997, indicating an excellent fit to the data. The calculated Qe from this model, 203.87 mg/g, is much closer to the experimental value of 205.3 mg/g, signifying that the pseudo-second-order model provides a more accurate description of the adsorption process. The higher accuracy of the pseudo-second-order model suggests that the adsorption is likely controlled by chemical interactions and that the adsorption sites on the activated carbon are utilized more effectively, fitting the observed data better than the pseudo-first-order model [62,63].
The mechanism underlying the adsorption of phenol removal utilizing Catha edulis stem-derived activated carbon encompasses a confluence of electrostatic attraction, surface adsorption, and chemisorption processes [63]. Under acidic pH conditions (pH 2), phenol molecules undergo protonation, which amplifies their polarity and augments electrostatic interactions with the negatively charged surface of the activated carbon [64]. The substantial surface area, porous morphology, and the presence of functional groups such as hydroxyl (-OH), carboxyl (-COOH), and methoxy (-OCH3) within the activated carbon facilitate both physical and chemical adsorption; these functional groups engage in the formation of hydrogen bonds, van der Waals forces, and potentially ionic interactions with phenol molecules [65]. The adsorption phenomenon adheres to the Langmuir isotherm model, signifying monolayer adsorption characterized by homogenous interaction sites, whereas pseudo-second-order kinetics implies that chemisorption predominates in the adsorption process [66,67]. Fourier Transform Infrared (FTIR) Spectroscopy further substantiates the participation of surface functional groups, as evidenced by shifts in bonding frequencies that were detected post-phenol adsorption, thereby indicating substantial chemical interactions [68,69].

3.6. Comparative Analysis

Comparative analysis indicated that Catha edulis stem-based activated carbon was an efficient adsorbent for phenol removal. Moreover, it is very competitive in terms of adsorption capacity compared to different materials reported in the literature, as depicted in Table 5. Although the capacities of some adsorbents, such as oak wood activated carbon [70], are a little higher, the material prepared from Catha edulis outperforms or is on par with many others, including petroleum pitch activated carbon [71], olive stone activated carbon [72], and several advanced composites such as graphene oxide-bentonite [56]. This work underlines the excellent possibility of using Catha edulis stem, which is a renewable and underutilized biomass, as an inexpensive and sustainable adsorbent for effective environmental remediation.

3.7. Limitations of the Work

This study offers valuable insights into the properties and adsorption potential of Catha edulis-based activated carbons (ACs) but has several limitations. Due to financial and time constraints, detailed proximate and ultimate analyses (organic and inorganic components) were not conducted, limiting the understanding of material composition and transformation. While FTIR and SEM analyses were performed, the limited data restricted the depth of interpretation regarding the material’s characteristics. Furthermore, nitrogen adsorption–desorption isotherms and pore size distribution data were insufficient for a thorough evaluation of pore structure and its impact on adsorption. X-ray diffraction (XRD) analysis was not carried out, missing a critical insight into the crystalline structure. Additionally, this study did not include a comprehensive optimization of adsorbent preparation conditions, limiting its practical applicability. Despite these limitations, this study provides a solid foundation for future research.

4. Conclusions

This study explored the efficient removal of phenol from aqueous solutions using activated carbon derived from Catha edulis stems, emphasizing its potential for use in environmental remediation. Chemical and thermal modifications were applied to enhance the sorption capacity of Catha edulis stems, with successful results demonstrated through various experimental approaches. Scanning Electron Microscope (SEM) analysis confirmed the porous structure of the activated carbon, which significantly enhances its adsorption properties. The maximum phenol removal efficiency of 99.9% was achieved under optimal conditions, which included a pH of 2, a contact time of 60 min, an adsorbent dosage of 0.1 g/100 mL, and an initial phenol concentration of 100 mg/L. The Langmuir isotherm model best fits the experimental data, indicating the adsorption process follows a monolayer adsorption model. This study also emphasizes the underutilized potential of Catha edulis stems as a sustainable biomass for environmental applications. In conclusion, Catha edulis stem-based activated carbon is a promising, inexpensive, and renewable material for environmental remediation and has potential for future industrial applications.

Author Contributions

Conceptualization, M.D.T. and M.A.G.; methodology, M.D.T.; software, M.D.T.; validation, M.D.T., M.A.G. and M.O.D.; formal analysis, M.D.T.; investigation, M.D.T.; resources, M.D.T.; data curation, M.D.T.; writing—original draft preparation, M.D.T.; writing—review and editing, M.D.T.; visualization, M.A.G.; supervision, M.O.D.; project administration, M.D.T.; funding acquisition, M.D.T. 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

All data generated are included in the main body of the manuscript.

Acknowledgments

We would like to thank Addis Ababa Science and Technology University, Adama Science and Technology, and Wachemo University for facility support.

Conflicts of Interest

The authors declare no conflicts of interest.

References

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Environments 12 00314 g001
Figure 1. FTIR analysis of the prepared adsorbent before and after adsorption.
Figure 1. FTIR analysis of the prepared adsorbent before and after adsorption.
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Figure 2. SEM images of Catha edulis-based activated carbon: (A) (before adsorption) and (B) (after adsorption).
Figure 2. SEM images of Catha edulis-based activated carbon: (A) (before adsorption) and (B) (after adsorption).
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Figure 3. Interaction effects of pH and adsorbent dosage on phenol adsorption.
Figure 3. Interaction effects of pH and adsorbent dosage on phenol adsorption.
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Figure 4. Three-dimensional representation of interaction effects of contact time and adsorbent dosage on phenol adsorption.
Figure 4. Three-dimensional representation of interaction effects of contact time and adsorbent dosage on phenol adsorption.
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Figure 5. Interaction effects of pH and contact time on phenol adsorption.
Figure 5. Interaction effects of pH and contact time on phenol adsorption.
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Figure 6. Langmuir isotherm model for adsorption of phenol onto Catha edulis activated carbon.
Figure 6. Langmuir isotherm model for adsorption of phenol onto Catha edulis activated carbon.
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Figure 7. Freundlich isotherm model for adsorption of phenol onto Catha edulis activated carbon.
Figure 7. Freundlich isotherm model for adsorption of phenol onto Catha edulis activated carbon.
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Figure 8. Temkin isotherm model fit for phenol adsorption onto Catha edulis activated carbon.
Figure 8. Temkin isotherm model fit for phenol adsorption onto Catha edulis activated carbon.
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Figure 9. Dubinin–Radushkevich (D-R) isotherm fit for phenol adsorption onto Catha edulis activated carbon.
Figure 9. Dubinin–Radushkevich (D-R) isotherm fit for phenol adsorption onto Catha edulis activated carbon.
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Figure 10. Pseudo-first-order kinetics for adsorption of phenol onto Catha edulis activated carbon.
Figure 10. Pseudo-first-order kinetics for adsorption of phenol onto Catha edulis activated carbon.
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Figure 11. Pseudo-second-order kinetics model for adsorption of phenol onto Catha edulis activated carbon.
Figure 11. Pseudo-second-order kinetics model for adsorption of phenol onto Catha edulis activated carbon.
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Table 1. Experimental design for adsorption of phenol onto Catha edulis stem-based activated carbon.
Table 1. Experimental design for adsorption of phenol onto Catha edulis stem-based activated carbon.
Variable Unit Lower (−)Higher (+)
pH-210
Contact time min3060
Adsorbent dosage g/100 mL0.050.1
Phenol concentration mg/L100200
Table 2. Batch adsorption performances of Catha edulis-based activated carbon for phenol adsorption.
Table 2. Batch adsorption performances of Catha edulis-based activated carbon for phenol adsorption.
RunpHAdsorbent Dosage (g/100 mL)Initial Phenol Concentration (mg/L)Contact Time (min)Removal Efficiency (%)
1100.12006084.8
2100.11006087.5
320.052003066.9
4100.051006072.1
5100.052006069.4
620.11003084.9
720.052006077.3
820.12003082.3
9100.12003074.4
10100.11003077.1
1120.11006099.9
1220.12006092.8
1320.051003069.6
1420.051006079.9
15100.051003061.6
16100.052003058.9
Table 3. Langmuir and Freundlich isotherm model parameters.
Table 3. Langmuir and Freundlich isotherm model parameters.
Isotherm Parameters Values
Langmuir R20.95
Qmax (mg/g)208.33
KL (L/mg)0.453
RL0.0216
Freundlich R20.93
n3.69
1/n0.2708
KF (L/mg)89.63
Temkin A (L/g)89.58
B (J/mol0.271
R20.94
Dubinin–Radushkevich (D-R)K (mol2/J2)2.04 × 10−7
Qmax (mg/g)156.
R20.63
Table 4. Pseudo-first-order and pseudo-second-order kinetics parameters for adsorption of phenol onto Catha edulis activated carbon.
Table 4. Pseudo-first-order and pseudo-second-order kinetics parameters for adsorption of phenol onto Catha edulis activated carbon.
Kinetics Model Parameters Values
Pseudo-first-order R20.993
K10.0278 min−1
Qe32.1 mg/g
Pseudo-second-order R20.9997
K20.5999 g/mg·min
Qe203.87 mg/g
Table 5. Phenol adsorption using different adsorbents.
Table 5. Phenol adsorption using different adsorbents.
S.NoAdsorbent Maximum Adsorption Capacity (mg/g)Reference
1.Oak wood activated carbon250[70]
2.Petroleum pitch activated carbon 189.96[71]
3.Olive stone activated carbon 120[72]
4.Graphene oxide-bentonite nanocomposites46.43[56]
5.Alumina-pillared clay30.61[57]
6.Sewage sludge-derived activated carbon 122.72[73]
7.Silica/calcium alginate nanocomposite100.55[74]
8.Catha edulis-based activated carbon 208.5This work
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Teweldebrihan, M.D.; Gnaro, M.A.; Dinka, M.O. Adsorption of Phenol from Aqueous Solution Utilizing Activated Carbon Prepared from Catha edulis Stem. Environments 2025, 12, 314. https://doi.org/10.3390/environments12090314

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Teweldebrihan MD, Gnaro MA, Dinka MO. Adsorption of Phenol from Aqueous Solution Utilizing Activated Carbon Prepared from Catha edulis Stem. Environments. 2025; 12(9):314. https://doi.org/10.3390/environments12090314

Chicago/Turabian Style

Teweldebrihan, Meseret Dawit, Mikiyas Abewaa Gnaro, and Megersa Olumana Dinka. 2025. "Adsorption of Phenol from Aqueous Solution Utilizing Activated Carbon Prepared from Catha edulis Stem" Environments 12, no. 9: 314. https://doi.org/10.3390/environments12090314

APA Style

Teweldebrihan, M. D., Gnaro, M. A., & Dinka, M. O. (2025). Adsorption of Phenol from Aqueous Solution Utilizing Activated Carbon Prepared from Catha edulis Stem. Environments, 12(9), 314. https://doi.org/10.3390/environments12090314

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