Adsorptive Removal of Hexavalent Chromium from Aqueous Solution Utilizing Activated Carbon Developed from Spathodea campanulata

Abstract

Heavy metal contamination of water sources has emerged as a major global environmental concern, affecting both aquatic ecosystems and human health. Therefore, this study aims to remove hexavalent chromium from an aqueous solution utilizing activated carbon developed from Spathodea campanulata. Chemical treatment with H₃PO₄ followed by thermal activation was employed to enhance the adsorption capability of the precursor material. On the other hand, a full factorial design of 2⁴ including pH (3 and 9), contact time (30 and 60 min), initial chromium concentration (40 and 100 mg/L), and adsorbent dosage of 0.2 and 0.6 g/100 mL was used to optimize the batch-wise adsorption of hexavalent chromium. The characterization results showed that the prepared activated carbon is composed of various functional groups (FTIR), a high specific surface area of 1054 m²/g (BET), morphological cracks (Scanning Electron Microscopy), and a pH point of zero charge of 5.8. The maximum removal efficiency of 96.5% was recorded at optimum working conditions of pH 3, contact time of 60 min, adsorbent dosage of 0.6 g/100 mL, and initial chromium concentration of 40 mg/L. Additionally, kinetics and isotherm studies revealed that the pseudo-second-order model with R² of 0.98 and the Sips model with R² of 0.99 were found to fit the adsorption data better, suggesting homogenous surface and chemisorption. Overall, this research suggests that Spathodea campanulata could be a promising natural source for the development of adsorbents with potential applications in remediating chromium-saturated wastewater at an industrial scale.

1. Introduction

Chromium is a chemical element that exists primarily in two oxidation states: trivalent chromium (Cr (III)) and hexavalent chromium (Cr (VI)). It is widely used in industries such as metallurgy, casting iron, automobiles, refractory, and chemical processes including leather, electroplating, and wood preservation [1]. Industrial activities release hazardous forms of chromium into the environment, which can contaminate water systems and soil [2]. Hexavalent chromium is highly soluble and mobile, making it more detrimental than trivalent chromium [3]. Exposure to chromium can have both positive and negative effects on human health, depending on the dose, exposure time, and oxidation state [4]. Adverse health effects of chromium include bronchial asthma, lung cancer, nasal ulcers, skin allergies, carcinogenicity, and genotoxicity [5]. The World Health Organization (WHO) has suggested a provisional guideline value of 0.05 mg/L for chromium in water. Therefore, efforts have been undergoing to remediate chromium-contaminated environments using various treatment techniques aiming to mitigate its environmental and human health impacts.

Conventional wastewater treatment methods applied for the removal of chromium have certain drawbacks. One of the main drawbacks is the limited efficiency in removing toxic chromium species, especially hexavalent chromium, which can have lethal effects at low concentrations [6]. Another downside is the large amount of sludge formation [7]. Additionally, the longtime requirement and lack of technological advancement limit the application of conventional wastewater treatment techniques [8]. Therefore, advanced wastewater treatment methods such as advanced oxidation processes, electrochemical methods, and membrane filtrations have become the subject of contemporary studies [9]. However, each of these methods has its own set of disadvantages. For instance, advanced oxidation processes may effectively remediate chromium-saturated wastewater; however, the large amount of sludge produced makes its application challenging [10]. Additionally, membrane processes also have their drawbacks. Some challenges include membrane fouling and operating costs. Similarly, electrochemical methods may encounter problems such as electrode fouling and degradation [11]. Hence, the search for cost-effective, efficient, and environmentally friendly wastewater treatment techniques such as adsorption has emerged as a viable option in the modern world [12].

Adsorption is an efficient and economical strategy for removing heavy metals from the environment, and biomass-derived activated carbon is a promising adsorbent due to its large surface area, porous structure, and sustainability [13]. Conventional methods for water purification have practical and financial drawbacks, making bio-based adsorbents an innovative and inexpensive alternative for removing micropollutants [13]. Activated carbon is a commonly used adsorbent for the removal of chromium from aqueous solutions [14]. Several studies have investigated the use of different types of activated carbon for this purpose. For example, Medlar Activated Carbon (MAC) and Cucumis Melo Activated Carbon (CMAC) have been found to be effective adsorbents for Cr (VI) removal, demonstrating maximum adsorption capacities of 34.12 mg/g and 54.28 mg/g, respectively [15]. Another study used activated carbon prepared from the pods of the Delonix regime as an eco-friendly adsorbent for hexavalent chromium removal resulting in a maximum removal efficiency of 65% [16]. Additionally, activated carbon derived from Bambara nut shells (BBNSAC) has been shown to have a high adsorption capacity for Cr (VI) ions with a maximum adsorption capacity of 68.00 mg/g [17]. These studies demonstrate the potential of activated carbon as an adsorbent for chromium removal and highlight the importance of selecting the appropriate type of activated carbon for specific applications. Moreover, biobased activated carbon offers advantageous such as cost-effectiveness, availability, and tunable surface properties, making it a promising alternative to commercial activated carbon [18].

The studies on biomass-derived activated carbon for the removal of Cr (VI) reveal several limitations worth noting. These limitations include minimal removal efficiencies, optimal conditions at very low pH of solution, and longer contact time requirements for effective decontamination of Cr (VI). Specifically, while Eichhornia crassipes-derived activated carbon [19] shows a maximum adsorption capacity of 41.53 mg/g, the relatively long contact time of 80 min may limit its practical application in rapid treatment scenarios. Similarly, sunflower cob-based activated carbon achieved a high removal efficiency of 98.8% at a low pH of 1.25 [20], which may not be feasible for most wastewater treatments. Chitosan-modified biochar demonstrated a maximum removal of 92% [21]; however, this lower removal efficiency does not meet standard discharge limits. Additionally, the study on Ceratonia siliqua pod biochar indicated that effective adsorption may require contact times of up to 360 min [22], which could hinder its applicability in industrial processes where shorter treatment times are preferred. Collectively, these factors underscore the need for the careful selection of precursor materials and effective optimization of the adsorption process for more efficient and effective applications in Cr (VI) remediation. In this context, Spathodea campanulata-based activated carbon is chosen for its higher removal efficiency, and the optimization of the adsorption parameters was effectively performed employing a full factorial experimental design.

Spathodea campanulata, also known as African tulip tree, is a plant that has gained popularity for its medical properties and ethnobotanical potential. It has been found to contain various phytochemical constituents, including alkaloids, carbohydrates, saponins, fixed oils and lipids, flavonoids, and phenolics [23]. The plant has been traditionally used for a range of purposes, such as fuel, medicine, environmental applications, and food additives [24]. Recent studies have also shown that spathodea campanulata has the potential for long-distance seed dispersal, making it capable of spreading naturally among islands [25]. Additionally, the plant extract has shown potential for ulcer treatment. Furthermore, the flowers of Spathodea campanulata have exhibited antidepressant activity in animal tests, suggesting their potential as a source for novel antidepressive drugs [26]. The plant has been found to have antioxidant activity and a high content of phenolic compounds, which contribute to its potential as an adsorbent [27]. However, limited attempts have been made to valorize this biomass into an adsorbent preparation. Activated carbon derived from Spathodea campanulata exhibits enhanced dye removal performance due to its high surface area and rich oxygen-containing functional groups. Additionally, its local availability makes Spathodea campanulata-based activated carbon a promising candidate for chromium removal from aqueous solutions, as it provides an environmentally friendly and efficient alternative for wastewater treatment [28,29]. Hence, this research is devoted to developing activated carbon from Spathodea campanulata and evaluating its application in the removal of chromium from aqueous solution. To the best of the authors’ knowledge, no research has been reported on the application of Spathodea campanulata-derived activated carbon for chromium remediation.

2. Materials and Methods

2.1. Adsorbent Preparation

The stem of Spathodea campanulata, collected from Arbaminch, Ethiopia, was utilized in the production of activated carbon. Initially, the raw material was manually reduced in size to facilitate the washing process. The impurities and dirty substances adhering to the material’s surface were eliminated through thorough washing with tap water, followed by rinsing with distilled water. Subsequently, the material was dried in sunlight for one week and then subjected to oven-drying at a temperature of 105 °C for a duration of 24 h. Following this, the dried sample was impregnated with phosphoric acid at a 1:1 ratio with the precursor material, allowing it to soak overnight. The chemically treated precursor material was then thermally activated through carbonization at a temperature of 500 °C for a period of 2 h. Upon the completion of the carbonization process, the adsorbent was removed from the furnace and placed in a desiccator to cool and prevent moisture absorption. The sample was subsequently washed multiple times with distilled water until the pH of the washing water reached 7. Finally, the adsorbent material was oven-dried at a temperature of 105 °C for 24 h and stored in a suitable container for subsequent characterization and batch adsorption studies, as depicted in Figure 1.

2.2. Adsorbent Characterization

The Spathodea campanulata-based activated carbon was characterized using various techniques. Fourier transform infrared spectroscopy (FTIR) was used to detect functional groups attached to the adsorbent material along the wavelengths of 4000 to 400 cm⁻¹. Brunauer–Emmett–Teller (BET) analysis was used to estimate the specific surface area of the activated carbon at the degassing temperature and time of 200 °C and 1 h, respectively. Scanning Electron Microscopy (SEM) was utilized to examine the surface morphology of the spathodea campanulata-based activated carbon. Finally, the pH point of zero charge aimed at investigating the charge density of the activated carbon was performed through the mass addition method reported by Abewaa et al. [30].

2.3. Batch Adsorption Experimental Design

Full factorial design involving four factors at two levels was used to optimize chromium adsorption from aqueous solution. These operating parameters are pH (3 and 9), contact time (30 and 60 min), initial Cr concentration (40 and 100 mg/L), and adsorbent dosage (0.2 and 0.6 g/100 mL). On the other hand, the removal efficiency is a response variable. Normally, a full factorial design comprising four factors at two levels results in 16 experimental runs. However, the experimental runs were performed in triplicate, and the results are presented as the average value.

Table 1. Full factorial experimental design for adsorption of hexavalent chromium from aqueous solution.

Parameter	Unit	Lower (−)	Higher (+)
pH	-	3	9
Contact time	min	30	60
Initial Cr concentration	mg/L	40	100
Adsorbent dosage	g/100 mL	0.2	0.6

shows the full factorial design for the adsorption of chromium from aqueous solution.

2.4. Batch Adsorption Experiments

This study involved batch adsorption experiments using a potassium dichromate (K₂Cr₂O₇) solution. The pH-adjusted solution was mixed with a specific dose of activated carbon and agitated for a predetermined time. After agitation, the solution was filtered through Whatman No. 42 filter paper to separate the adsorbent from the liquid phase. The filtrate was then analyzed to determine the residual chromium concentration. To measure the unadsorbed chromium, the filtrate was mixed with 1, 5-diphenylcarbazide and analyzed using UV-VIS spectrophotometry at a maximum wavelength of 540 nm. The adsorption capacity and removal efficiency were then calculated using Equations (1) and (2), where V (mL), m (g), C_i (mg/L), and C_f (mg/L) represent the volume of the solution, the mass of the adsorbent, the initial chromium concentration, and the final chromium concentration, respectively [31].

$$Q_e \left(\mathrm{m}\mathrm{g}/\mathrm{g}\right)=\left({\displaystyle \frac{C_i-C_f}{m}}\right)\times V$$

(1)

$$R_e\left(\%\right)=\left({\displaystyle \frac{C_i-C_f}{C_i}}\right)\times 100$$

(2)

2.5. Adsorption Isotherm

Adsorption isotherm models are mathematical models describing the distribution of adsorbate molecules between the adsorbent and liquid phases at equilibrium. Additionally, adsorption isotherm models are used to analyze the interaction between the adsorbate ions and the surface of the adsorbent material. In this study, the optimum operating parameters’ values (pH of 3, contact time 60 min, and adsorbent dosage of 0.6 g/100 mL) and varied chromium concentrations of 40–100 mg/L are utilized to carry out the isotherm studies. Then, Freundlich’s and Langmuir’s isotherm models were used to analyze the nature of the adsorption at equilibrium. Equations (3) and (4) are formulas for the Langmuir and Freundlich isotherm models, respectively.

$${\displaystyle \frac{Q_{max}{K}_L{C}_e}{1+K_L{C}_e}}$$

(3)

Q_e = K_FC_e^1/n

(4)

Additionally, the adsorption dimensionless factor constant (R_L) that is used to estimate Langmuir isothermal feasibility is indicated by Equation (5).

$$R_L={\displaystyle \frac{1}{1+K_L{C}_e}}$$

(5)

where Q_max is the maximum Langmuir adsorption capacity, K_L is the Langmuir constant related to the adsorption capacity, and K_F and 1/n are Freundlich constants related to the adsorption capacity and intensity [32]. Moreover, the Temkin [33], Sips [34], and Redlich–Peterson isotherm [35] models are utilized to further elucidate the adsorption mechanism and are represented by Equations (6)–(8), respectively. These models provide insights into the adsorption process by describing various interactions and capacities of the adsorbent–adsorbate system.

$$Q_e={\displaystyle \frac{R_T}{b}}\mathrm{l}\mathrm{n}{(K}_T{C}_e)$$

(6)

$$Q_e={\displaystyle \frac{Q_{max}({K{C}_e)}^n}{(1+{K{C}_e)}^n}}$$

(7)

$$Q_e={\displaystyle \frac{K_R{C}_e}{1+a_R{C_e}^g}}$$

(8)

where the universal gas constant, R, has a value of 8.314 joules per mole per Kelvin (J/mol·K), while T indicates the absolute temperature in Kelvin (K). The parameter b refers to the Temkin constant, which is related to the adsorption heat and is expressed in joules per mole (J/mol). K_T is the Temkin isotherm equilibrium binding constant, with units of liters per milligram (L/mg). K is the Sips isotherm constant (L/mg), while n is the dimensionless heterogeneity parameter. Lastly, K_R and a_R are the Redlich–Peterson isotherm constants, measured in liters per gram (L/g) and liters per milligram (L/mg), respectively, with g being a dimensionless exponent ranging between 0 and 1.

2.6. Adsorption Kinetics

Adsorption kinetics examines the rate at which the adsorption occurs at varied contact time while keeping other operating parameters at fixed values. Pseudo-first-order (Equation (9)), pseudo-second-order (Equation (10)), and intraparticle diffusion (Equation (11)) models are among various adsorption kinetics models developed to describe the contact time dependence of the adsorption process. During the adsorption kinetics study of chromium adsorption onto Spathodea campanulata-derived activated carbon, fixed values of adsorbent dosage (0.6 g/100 mL), initial chromium concentration of 40 mg/L, pH of 3, and varied contact time of 10, 20, 30, 40, 50, and 60 min were used.

$${Q_t=Q_e (1-e}^{-K1t})$$

(9)

$$Q_t={\displaystyle \frac{{Q_e}^2{K}_{2}t}{1+Q_e{K}_{2}t}}$$

(10)

$$Q_e={K_{d}t}^{1/2}+C$$

(11)

In Equation (6), K₁ (g/(mg min)) refers to the pseudo-first-order constant, Q_e is the equilibrium adsorption capacity in mg/g, and t (min) represents the contact time. Additionally, K₂ and K_d are the pseudo-second-order and intraparticle diffusion constants, respectively, whereas C is the concentration [36].

2.7. Adsorption Thermodynamics

The energy and feasibility of the adsorption process are assessed through adsorption thermodynamics by varying the system’s temperature. The thermodynamic study of hexavalent chromium adsorption onto Spathodea campanulata-derived activated carbon involved temperature variations ranging from 25 to 65 °C (25, 35, 45.55, and 65 °C). The changes in Gibbs free energy (ΔG^O), enthalpy (ΔH^O), and entropy (ΔS^O) were calculated using the Van’t Hoff equation, as described in Equations (12)–(15). In these equations, R is the universal gas constant (8.314 J/mol·K), K_C is the thermodynamic constant, and T represents the absolute temperature (K). Additionally, C_e is the equilibrium concentration of hexavalent chromium, and Q_e denotes the amount of dye adsorbed on the adsorbent at equilibrium (mg/g) [35,36].

$${∆G}^O=-R_T\ln K_C$$

(12)

$$K_C={\displaystyle \frac{Q_e}{C_e}}$$

(13)

$$\ln K_C={\displaystyle \frac{{∆S}^O}{R}}-{\displaystyle \frac{{∆H}^O}{R_T}}$$

(14)

$${∆G}^O={∆H}^O-T{∆S}^O$$

(15)

3. Results and Discussion

3.1. Adsorbent Characteristics

The pH point of zero charge indicates the condition at which the surface charge density of an adsorbent material is equal to zero. It provides information about the attraction and repulsion between the adsorbent and adsorbate. The pHpzc of activated carbon of spathodea campanulata is evaluated to be 5.8 as shown in Figure 2. This suggested that the surface of the adsorbent is dominated by positive charges below pHpzc and that negative charges dominate the surface of the adsorbent above a pH of 5.8. Normally, it is easier to adsorb cations on negatively charged surfaces and anions on positively charged surfaces [37]. Hence, the maximum removal efficiency of chromium recorded at a pH of 3 supports the pHpzc concept.

The BET specific surface area of Spathodea campanulata-derived activated carbon was determined to be 1054 m²/g. This surface area is higher compared to the surface areas of various biomass-based activated carbons, such as water hyacinth (203.83 m²/g) [38], eucalyptus globulus (253.25 m²/g), and Onopordum heteracanthum (5.73 m²/g) [39]. The high surface area of this adsorbent material is attributed to the chemical activation, thermal treatment, and nature of the precursor material. Fundamentally, activated carbon having a higher surface area is effective in removing pollutants from water and wastewater due to enough active sites being present on the surface of the adsorbent [29,40]. The surface morphology of spathodea campanulata-derived activated carbon was analyzed using SEM, and the micrograph is presented in Figure 3. The adsorbent material is found to be porous with cracks and cavities resulting from chemical and thermal activations carried out to enhance the adsorption performance of the activated carbon. Normally, an activated carbon appearing porous enhances the removal of the intended material through the attachment of the adsorbate on the pores observed.

The FTIR analysis results of Spathodea campanulata-derived activated carbon are shown in Figure 4. The analysis showed that the adsorbent material is composed of various functional groups that could make it a potential candidate for the removal of chromium from water and wastewater. Specifically, the peak observed at 2426 cm⁻¹ is attributed to the carbon–carbon double bond of the precursor material. Additionally, the peak found at 2108 cm⁻¹ corresponds to the carbon hydrogen bond (C-H). On the other hand, the carbon oxygen (C=O) functional group is indicated by the peak observed at 1710 cm⁻¹. The stretching vibration of carbon–oxygen functional groups is shown by the peak observed at 1658 cm⁻¹. Moreover, the peaks observed at 1154 and 1029 cm⁻¹ would indicate the C-O-C bond. Finally, the P=O is indicated by the peak observed at 481 cm⁻¹ [41].

3.2. Batch Adsorption Performance

The results of the batch adsorption performance of the chromium adsorption onto activated carbon of Spathodea campanulata are presented in

Table 2. Batch adsorption result of chromium removal onto Spathodea campanulata activated carbon.

Run	pH	Initial Chromium Concentration (mg/L)	Adsorbent Dosage (g/100 mL)	Contact Time (min)	Actual Removal Efficiency (%)	Predicted Removal Efficiency (%)
1.	3	40	0.6	30	81.5	81.85
2.	9	40	0.2	60	52.6	52.30
3.	9	40	0.6	60	86.5	87.00
4.	3	40	0.2	30	47.4	47.15
5.	9	100	0.2	60	41.6	41.60
6.	3	40	0.6	60	96.5	95.77
7.	9	100	0.6	60	76.6	76.30
8.	9	100	0.6	30	62.1	62.38
9.	9	40	0.2	30	38.1	38.37
10.	3	100	0.6	30	71.4	71.15
11.	9	100	0.2	30	28.1	27.68
12.	3	100	0.2	30	36.4	36.45
13.	3	100	0.6	60	84.9	85.08
14.	3	40	0.2	60	60.9	61.08
15.	3	100	0.2	60	49.9	50.38
16.	9	40	0.6	30	73.1	73.08

. As a result, the maximum chromium removal efficiency of 96.5% was attained at the optimum operating conditions of pH 3, contact time of 60 min, initial chromium concentration of 40 mg/L, and adsorbent dosage of 0.6 g/100 mL. At optimum working conditions, it was able to reduce the amount of chromium from 40 to 1.4 mg/L. On the other hand, the minimum removal efficiency recorded throughout the experiment was determined to be 28.1%. The huge variation observed between the minimum and maximum removal efficiencies indicates the significance of the effects of the operating parameters. On the other hand, the experimentally determined removal efficiencies closely align with the predicted values obtained through the Response Surface Methodology (RSM), thereby validating the theoretical model and confirming its reliability for accurately representing the system’s performance

The effectiveness of removing chromium from aqueous solution using activated carbon derived from Spathodea campanulata is compared to other biosorbents made from lignocellulosic, agricultural leftovers, and woody materials. The results of this comparative study, in conjunction with the recent literature, suggest that Spathodea campanulata-derived activated carbon is superior in its ability to remove chromium from a water solution. This superiority is likely due to the properties of the precursor material and the affinity between the target pollutant and the adsorbent materials. Consequently, the activated carbon produced can serve as a viable alternative adsorbent for the elimination of toxic and persistent heavy metals, such as chromium.

Table 3. Comparative analysis of different biobased activations applied for chromium adsorption.

S.No	Activated Carbon Precursor Material	Maximum Removal Efficiency (%)	Adsorption Capacity (mg/g)	Reference
1.	Coffee husk	97.65	23.2	[42]
2.	Spent coffee grounds	95	187.6	[43]
3.	Parthenium hysterophorus	90	1	[44]
4.	Water hyacinth	90.4	6.02	[38]
5.	Amaranthus retroflexus	81	108.14	[45]
6.	Garcinia kola hull	96.25	3.86	[46]
7.	Spathodea campanulata	96.5	10.65	This work

presents the findings of a literature search, comparing the effectiveness of different biomass-derived activated carbons for chromium removal to the research conducted in this study.

3.3. Analysis of Variance (ANOVA) and Fit Summary

The adsorption of Cr (VI) from aqueous solutions using activated carbon derived from Spathodea campanulata was analyzed via ANOVA, as summarized in

Table 4. ANOVA for linear model.

Source	Sum of Squares	df	Mean Square	F-Value	p-Value
Model	6357.95	4	1589.49	9580.47	<0.0001	significant
A–pH	308.00	1	308.00	1856.45	<0.0001
B–Initial chromium concentration	457.96	1	457.96	2760.31	<0.0001
C–Adsorbent dose	4816.36	1	4816.36	29,030.12	<0.0001
D–Contact time	775.62	1	775.62	4674.98	<0.0001
Residual	1.82	11	0.1659
Cor Total	6359.77	15

. The batch adsorption data exhibited a linear regression pattern, as shown in

Table 5. Model summary statistics.

Source	Std. Dev.	R2	Adjusted R2	Predicted R2	PRESS
Linear	0.4073	0.9997	0.9996	0.9994	3.86	Suggested
2FI	0.4806	0.9998	0.9995	0.9981	11.83
Quadratic					*	Aliased

*: PRESS value could not be computed or is unreliable due to aliasing in the quadratic model.

. All independent factors were found to significantly impact adsorption, with p-values below 0.05. The Model F-value of 9580.47 confirms the statistical significance of the model, indicating only a 0.01% chance that such a large F-value could occur due to random noise. Furthermore, the ANOVA demonstrated a strong correlation between the predicted and adjusted data sets, with the Predicted R² of 0.9994 closely aligning with the Adjusted R² of 0.9996 (difference < 0.2). Additionally, the adequacy precision, which measures the signal-to-noise ratio, exceeded the desirable threshold with a ratio of 299.080. This suggests that the model effectively navigates the design space.

Equation (16) expresses the relationship in terms of coded factors, allowing predictions of the response based on specified levels of each factor. Analysis of Equation (16) reveals that the pH and initial chromium concentration negatively impact the chromium removal efficiency, whereas the adsorbent dosage and contact time exert a positive influence on Cr (VI) adsorption:

Cr (VI) Removal efficiency (%) = +61.72 − 4.39A − 5.35B + 17.35C + 6.96D

(16)

Here, A represents pH, and B denotes initial chromium concentration, while C and D signify the adsorbent dosage and contact time, respectively.

3.4. Interaction Effects

3.4.1. pH and Adsorbent Dosage

The 3D representation of the interaction effect of pH and adsorbent dosage is shown in Figure 5 using version 13. Fundamentally, the interaction between the adsorbent dosage and pH plays a significant role in the efficiency of chromium removal. In this study, the combined effect of pH and adsorbent dosage is found to negatively affect chromium removal. Moreover, the combined effect of pH and adsorbent dosage resulted in the maximum percentage removal of chromium ions at a lower pH and higher adsorbent dosage. A similar finding is reported in multiple papers. For instance, Beksissa et al. reported that the maximum removal efficiency of chromium is attained at a higher adsorbent dosage [47]. Similarly, Behera et.al. observed that the adsorbent dose had a synergistic effect on chromium removal efficiency, while pH had an antagonistic effect [48].

3.4.2. Initial Chromium Concentration and Adsorbent Dosage

The interaction between adsorbent dosage and initial chromium concentration plays a significant role in determining the efficiency of chromium removal [49,50]. In this, the combined effects of the initial chromium concentration and adsorbent dosage are examined at a constant pH value of 6 and contact time of 45 min as shown in Figure 6. Normally, a lower initial chromium concentration and higher adsorbent dosage increase the removal efficiency of chromium. In this particular study, the combined effect of the initial chromium concentration and adsorbent dosage is determined to positively affect the adsorption process.

3.4.3. Adsorbent Dosage and Contact Time

The interaction effect of adsorbent dosage and contact time on the removal efficiency of chromium using spathodea campanulata-derived activated carbon is presented in Figure 7. As indicated in Figure 7, the removal efficiency of chromium is positively impacted by the combined effects of adsorbent dosage and contact time. Normally, increasing the contact time increases the removal efficiency of pollutants from water or wastewater due to the increment of active sites available for adsorption. Likewise, increasing the contact time increases the possibility that the adsorbate will be attached to the specific surface of the adsorbent material [51,52,53]. The maximum removal efficiency recorded as the contact time interacts with adsorbent dosage while holding the pH at 6 and initial chromium concentration of 70 mg/L was determined to be 89.6%.

3.5. Adsorption Isotherm

The nonlinear curve fit of the Langmuir, Freundlich, Sips, Temkin, and Redlich–Peterson isotherm models provides a comprehensive analysis of the adsorption of chromium onto Spathodea campanulata-based activated carbon, as shown in Figure 8. The Langmuir isotherm model, with an R² of 0.98, suggests a good fit, indicating that the adsorption occurs on a homogeneous surface with identical adsorption sites. The Q_max of 10.65 mg/g represents the maximum adsorption capacity, while the K_L value of 0.85 indicates a strong adsorption affinity between the adsorbate and adsorbent. The R_L value of 0.029, which is less than 1, signifies favorable adsorption conditions, supporting the efficiency of the process. The Freundlich model, with an R² of 0.94, describes adsorption on a heterogeneous surface. The K_F value of 6.45 (mg/g) (L/mg) reflects the adsorption capacity, and the 1/n value of 0.14 confirms that the adsorption is favorable, meaning chromium binds efficiently to the activated carbon surface. This suggests effective and efficient chromium removal from aqueous solutions. The Temkin model, with an R² of 0.96, accounts for the heat of adsorption and the interactions between the adsorbate and adsorbent, with A = 110.11 and B = 1.296 indicating the magnitude of the adsorption energy and the relationship between adsorption and temperature. The Sips model, which provides the best fit with the highest R² of 0.99 and a low reduced chi-square of 0.135, combines aspects of both the Langmuir and Freundlich models, suggesting a system with both homogeneous and heterogeneous sites. The Q_max of 11.339 mg/g, K = 0.868, and n = 0.723 indicate efficient and favorable adsorption. Finally, the Redlich–Peterson model, with an R² of 0.989, also fits the data well, with constants K = 12.250, a = 1.345, and b = 0.345, which describe the adsorption equilibrium and the system’s non-ideality. Overall, the Sips isotherm model offers the most accurate description of the chromium adsorption process, as it best accounts for the complex adsorption behavior observed on the activated carbon surface. The adsorption of chromium may involve both electrostatic interactions and surface complexation with functional groups on the activated carbon surface, such as oxygen-containing groups. This is consistent with the behavior observed in other studies on metal ion adsorption [54,55,56].

3.6. Adsorption Kinetics

The data obtained from batch adsorption experiments conducted at varied contact times while holding the other parameters at their respective values are fed into the nonlinear equation of the pseudo-first-order model (Figure 9), whereas the linear form of the pseudo-second-order kinetic models and the resulting plot are shown in Figure 10. The kinetics analysis revealed that the pseudo-first-order kinetics model resulted in an R² of 0.90, K₁ of 0.048, and Q_e calculated of 6.9 mg/g. Additionally, the error analysis of the pseudo-first-order kinetics model resulted in a reduced chi-square of 0.02. On the other hand, the pseudo-second-order kinetics fit resulted in an R² of 0.98, K₂ value of 0.007, Q_e calculated value of 8.4 mg/g, and residual sum of squares of 0.2.

The intraparticle diffusion model results, shown in Figure 11, reveal key insights into the adsorption mechanism of hexavalent chromium onto Spathodea campanulata stem-based activated carbon. The plot does not pass through the origin, indicating that the intraparticle diffusion model does not exclusively control the adsorption rate. Instead, the multilinear plot highlights a multi-step adsorption process. The first linear region corresponds to external mass transfer or film diffusion, where hexavalent chromium ions migrate toward the adsorbent surface. The second region reflects slower intraparticle diffusion into the pores, identified as the rate-limiting step of the process. The slope of this second linear portion (intraparticle diffusion coefficient, k_d) was calculated to be 0.058, while the R² value for this portion was 0.96, demonstrating a good fit. The intercept value of 5.95 indicates a significant contribution of surface adsorption. A higher intercept suggests a dominant role of surface adsorption over pore diffusion [57]. The connectivity of the two linear regions indicates a smooth transition between the mechanisms, emphasizing the importance of both the adsorbent’s surface properties and pore structure in removing hexavalent chromium. Comparing the three kinetics models, the pseudo-second-order model provides a good fit with the adsorption data. This suggests that the adsorption behavior of these materials is influenced by the concentration of the adsorbate and the number of available adsorption sites on the adsorbent surface. Hence, the rate of adsorption is controlled by both the adsorbate and the adsorbent, and the nature of the adsorption is more likely chemisorption [58,59,60].

3.7. Adsorption Thermodynamics

Thermodynamics parameters for the adsorption of hexavalent chromium onto Spathodea campanulata-based activated carbon unveil useful information on the nature of the adsorption process, as shown in Figure 12. The positive ${∆H}^O$ of 39.89 kJ/mol indicates that the adsorption process is endothermic; thus, with increasing temperature, the adsorption capacity increases since the system absorbs heat. The positive ${∆S}^O=$ 145.99 J/(mol·K) is indicative of increased randomness at the solid–liquid interface during the adsorption process, improving system disorder. The consistently negative $∆$G^O values ($∆G$^O = −3.62 kJ/mol, −5.08 kJ/mol, −6.54 kJ/mol, −8.00 kJ/mol, and −9.46 kJ/mol at 298 K, 308 K, 318 K, 328 K, and 338 K, respectively) confirm the spontaneous nature of the adsorption, emphasizing its thermodynamic feasibility under the studied conditions. Collectively, these parameters indicate that the adsorption of hexavalent chromium onto Spathodea campanulata-derived activated carbon is a spontaneous, endothermic process driven by increased disorder and is enhanced at elevated temperatures, making it a promising approach for chromium remediation [60].

4. Conclusions

In this work, activated carbon was prepared from the stem of Spathodea campanulata through chemical treatment followed by thermal activation. Accordingly, the Spathodea campanulata-derived activated carbon appeared to be composed of multiple functional groups (FTIR), morphological cracks with a porous structure (SEM), a high specific surface area of 1054 m²/g, and pHpzc of 5.8. These characteristics highlight the applicability of the adsorbent material in removing toxic and persistent pollutants such as chromium. On the other hand, the maximum and minimum removal efficiencies recorded in this study were found to be 96.5 and 28.1%. The adsorption isotherm study was investigated using the Langmuir and Freundlich models in which the Langmuir isotherm with a higher R² of 0.98 is found to fit the data better, suggesting monolayer and homogenous surface interaction. Additionally, the kinetics study revealed that the pseudo-second-order with the highest R² of 0.98 was determined to describe the adsorption process best, showing its chemisorption nature. Finally, this study highlighted that Spathodea campanulata-derived activated carbon can be utilized as an effective adsorbent for the removal of chromium from an aqueous solution. However, this study has limitations, including the use of synthetic Cr (VI) solutions rather than actual wastewater, which may contain competing ions affecting adsorption performance. Additionally, this study focused on a limited range of parameters, and the long-term stability and regeneration capacity of the adsorbent were not explored. Future research should prioritize the regeneration and reuse of the adsorbent, evaluate its performance in complex wastewater matrices, and assess its scalability for large-scale applications.