Sorption Capacity of Carbon-Based Mandarin Orange Peels for Removing Methylene Blue and Ibuprofen from Water

Abstract

Pharmaceutical pollutants in water pose a serious environmental challenge. This research compared the adsorption capacity of mandarin orange peels (MOP) and activated carbon mandarin orange peels (AC-MOP) to adsorb methylene blue (MB) and Ibuprofen (IBF) from an aqueous solution. This is the first study to report on the uptake of Ibuprofen using carbonized mandarin orange peels activated with hydrochloric acid. The biomaterials were characterized using FTIR and SEM. Batch experiments with operational parameters such as pH, contact time, concentration and temperature were investigated for the adsorption of MB and IBF. Isotherms, kinetic calculations and thermodynamic parameters were calculated for the adsorption of MB and IBF. A positive ΔH° suggested the reaction was endothermic, and ΔG° values showed that the sorption process was spontaneous. The isotherm models best fit the Langmuir model with maximum sorption capacities of 74.15 and 78.15 mg/g for MB and IBF, respectively. The adsorption rate for MB was fast and took place within the first 10 min, whilst the removal of IBF was observed at 40 min. The kinetic model evaluation showed that pseudo-second-order was a suitable fit for the mechanism. The re-usability data indicated that the recovery of MB was 70.13%, and IBF was 87.17%. The adsorption capacity of IBF with the carbon-based MOP was higher than that of MB. The results indicated that AC-MOP could be used as an adsorbent for MB and IBF from water. The major advantage of this method is its effectiveness in reducing the concentration of dyes and pharmaceutical pollutants using inexpensive adsorbents.

1. Introduction

Clean drinking water is vital for the survival of living organisms [1]. Water contaminated with metal ions, dyes and pharmaceuticals negatively affects health and the environment [2]. In humans, toxic substances in water damage the skin, eyes and nervous, gastrointestinal and cardiovascular systems [3]. Methylene blue (MB) is a cationic dye that colours cotton and wool products [4]. As a colouring agent, it is also used in surgical procedures to make tissues more visible [5]. MB dye is highly soluble in water, hazardous to aquatic environments and harmful to human health [4]. Ibuprofen (IBF) is crystalline and colourless with a particular odour [6] and reaches the environment via hospital effluents [7]. IBF reduces pain and relieves inflammation. Subsequently, human excretion introduces IBF and its derivatives into water. IBF is also inadequately discharged by hospitals and pharmaceutical industries worldwide [7]. IBF in wastewater has toxic effects even at very low concentrations. Agricultural waste materials have been used as adsorbents to remove pollutants from contaminated water. They include peels of many fruits such as grapefruit [8], pomegranate [9], lemon [10], garlic [5], banana [11], orange [12] and mandarin [13]. Mandarins (Citrus reticulata, family Rutaceae) are one of the most commonly consumed citrus fruits due to their seedless nature, sweetness and easy peeling by hand [14]. Mandarins can prevent human diseases through their anti-inflammatory, antimicrobial and antioxidant activities. They are high in flavonoids, vitamin C and essential oils [15]. Much of the mandarins produced are consumed as juice, and more than 50% are wasted after juice making [16]. The waste is dumped in the environment, creating serious environmental challenges [17].

Several studies have used mandarin orange peels as an adsorbent for various pollutants. A mandarin fruit peel was used to remove Cr(VI) and Pb(II) ions from an aqueous solution via adsorption (Mahmoud and Mohamed 2020) [18]. Pavan et al. (2006) [11] investigated the adsorption of Co(II), Ni(II) and Cu(II) using mandarin peels. The uptake of methylene blue was investigated by Koyuncu et al. (2018) [19] using activated carbon from mandarin peels. Park et al. (2021) [20] used mandarin orange peels as a biosorbent to remove methyl orange and fast green. Mandarin orange peels were also used to remove AB14 dyes in a study by Eldeeb et al. (2022) [21]. Besegatto et al. (2022) [22] used raw mandarin orange peels for the adsorption of methylene blue. Lima et al. (2023) [23] also used pristine mandarin orange peels for the adsorption of methylene blue. In another study, Unugal and Nigiz (2020) [13] carbonized mandarin orange peels using H₂SO₄ to adsorb methylene blue; a maximum capacity of 196 mg/g was obtained. Al-Yousef et al. (2023) [24] used cocoa shells to adsorb Ibuprofen. The calculated Ibuprofen adsorption capacities ranged from 16.67 to 23.81 mg/g for raw biomass and 30.59 to 38.95 mg/g for glycine functionalized biomass. Few studies have used carbonized mandarin orange peels as a potential adsorbent for methylene blue, and there are no investigations for methylene blue adsorption using mandarin orange peels that are carbon-activated with hydrochloric acid.

This study is the first to report on the uptake of Ibuprofen using carbon-based mandarin orange peels. This study evaluates the activated carbon from mandarin orange peels using a different acid. This study assesses the uptake of methylene blue (MB) and Ibuprofen (IBF) from an aqueous solution using mandarin orange peels (MOP). FTIR spectroscopy and SEM were used to characterize the activated carbon mandarin orange peels (AC-MOP). Operational parameters such as pH, contact time, concentration and temperature were investigated for the adsorption of MB and IBF in batch mode.

2. Chemicals and Methods

2.1. Chemicals

Fresh mandarin orange peels were obtained from a waste site in Vanderbijlpark, Gauteng Province, South Africa. All reagents were used without purification. Ibuprofen (C₁₃H₁₈O₂, >98%) with a molar mass of 206.29 g/mol and methylene blue [(C₁₆H₁₈N₃SCl), 95%] dye with a molar mass of 319.85 g/mol were purchased from Lab-Chem, Founders View, Johannesburg, South Africa. Hydrochloric acid (32%, HCl) and sodium hydroxide (98.5%, NaOH) were acquired from ACE Laboratories, Spartan, Kempton Park, South Africa.

2.2. Preparation of Mandarin Orange Peels

The mandarin orange peels were rinsed with distilled water to remove dust and dried at 60 °C for 24 h. The dried peels were ground to a fine powder. This material was named mandarin orange peels (MOP). The grounded peels were sieved with a 60/200 mesh to obtain roughly 0.8 to 1 mm particle sizes.

2.3. Preparation of Activated Carbon from Mandarin Orange Peels

The MOP powder (15 g) was heated in a furnace at 600 °C for 120 min to produce carbon. The crushed peels were poured into a sample holder and inserted in a catalytic vapour deposition (CVD) furnace under nitrogen to be carbonized at the specified time. The carbon was activated using 0.15 M HCl. The mixture was stirred on a shaker for 4 h, then rinsed with water to remove traces of the HCl. After that, the activated carbon was dried at 40 °C and labelled as activated carbon from mandarin orange peels (AC-MOP). Figure 1 shows the scheme of how the AC-MOP was prepared.

2.4. Characterization of the Adsorbents

A Nicolet iS50 FTIR spectrometer (Madison, WI, USA) was used to determine the functional groups on the surface of the activated carbon (AC) from the MOP adsorbent. A scanning electron microscope (SEM)—JSM IT500 SEM (JEOL Ltd., Tokyo, Japan), operated at 10.0 kV—was used to determine the AC-MOP surface morphology. The surface area was determined using Brunauer–Emmett–Teller (BET) with Micrometrics TriStar II 3020 BET v3.02 (Norcross, GA, USA), and nitrogen was used as the adsorptive gas. A UV-visible spectrophotometer 220 (StellarNet, Inc., Tampa, FL, USA) was used to determine the remaining MB and IBF concentrations after adsorption.

2.5. Batch Adsorption Studies

The pH effect was studied by varying the pH from 2 to 10. The initial contact time was investigated at 1, 5, 10, 25, 40, 60 and 80 min. The concentration effect was studied at 20, 40, 60 and 80 mg/L, while temperature was assessed between 25 and 55 °C. The parameters used were 0.4 g mass of the adsorbent in 50 mL of the standard solution. The adsorption experiments were carried out in triplicate to ensure repeatability. The adsorbents’ pH, concentration and temperature effects were equilibrated at 200 rpm for 2 h. Methylene blue dye (MB) and Ibuprofen (IBF) were used at 100 mg/L as the working solution.

2.6. Point Zero Charge (pH_PHZ)

The solid addition method was used to obtain pH_(PHZ), according to Ofomaja et al. (2009). Fifty millilitres of 0.1 M KNO₃ was poured into nine containers. The primary pH of the solutions ranged from 1 to 9. Exactly 20 mL of 0.1 M NaNO₃ was added to 0.4 g of AC-MOP. The sample bottles were agitated for 24 h. Afterwards, the final pH of each solution was measured and plotted against the difference between the initial and final pH.

2.7. Regeneration

The reusability and regeneration of the AC-MOP were determined using 0.4 g of the pre-loaded adsorbent. The adsorbed MB and IBF were removed from the AC-MOP surface through agitation four times in 50 mL of 0.2 M HCl at 200 rpm for 120 min and then rinsed several times with ultra-pure water at 200 rpm for 120 min before reuse. The adsorbent was recovered via centrifuge.

2.8. Data Calculations

2.8.1. Isotherms and Kinetic Calculations

The amount of MB dye and IBP adsorbed using the AC-MOP adsorbent was calculated using Equations (1) and (2) to approximate the sorption capacity (q_e) in mg/g and percentage (%) at equilibrium.

$${\mathrm{q}}_{\mathrm{e}}=\frac{\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}\right) \mathrm{V}}{\mathrm{m}}$$

(1)

$$\mathrm{R}\mathrm{e}\mathrm{m}\mathrm{o}\mathrm{v}\mathrm{a}\mathrm{l}(\%)=\frac{\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{e}}\right)\times 100}{{\mathrm{C}}_{\mathrm{i}}}$$

(2)

(C_o) and (C_e) are the initial and final MB dyes and IBF concentrations in the solution (mg/L). (W) is the mass of the adsorbent (g), and (V) denotes the volume of the solution used (mL). Kinetic mechanisms were assessed using the pseudo-first-order (PFO) and pseudo-second-order (PSO) models by Lagergren (1898) [25] using the non-linear Equations (3) and (4), employing KyPlot version 6.0.

$$q_e=q_t(1-e^{-k_{1}t})$$

(3)

$$q_e=\frac{1+k_2{q}_{e}t}{k_2{q}_e^{2}t}$$

(4)

where (q_e) and (q_t) signify the adsorbed MB dye and IBF at equilibrium at the time (t). K₁ is the PFO rate constant (min⁻¹), and K₂ is the PSO rate constant (g.mg/min).

Two isotherm models, (a) Langmuir (Langmuir, 1916) [26] and (b) Freundlich (Freundlich, 1906) [27], were used in non-linear Equations (5) and (6), respectively, using KyPlot version 6.0.

$$q_e=\frac{Q_0{bC}_e}{{1+bC}_e}$$

(5)

$$q_e=k_f{C}_e^{1/n}$$

(6)

2.8.2. Thermodynamics

The thermodynamic parameters [enthalpy ΔH° (KJ/mol)], entropy ΔS° (KJ.mol/K) and energy change ΔG° (KJ/mol) were also calculated. These parameters were calculated at temperatures (25, 35, 45 and 55 °C), using Equations (7) and (8). The thermodynamic equilibrium constant (K_c) value was estimated using Equation (9)

$${ ln K}_c=-\frac{\mathsf{\Delta }{H}^{°}}{RT}-\frac{\mathsf{\Delta }{S}^{°}}{R}$$

(7)

$$\mathsf{\Delta }{G}^{°}=-RT$$

(8)

$${ K}_c=\frac{qe}{Ce}$$

(9)

3. Results and Discussion

3.1. Characterization of the Adsorbents

3.1.1. Functional Groups on MOP and AC-MOP

The surface functional groups of the MOP and AC-MOP adsorbents were obtained using FTIR spectroscopy; the results can be viewed in Figure 2. The MOP showed more peaks than the AC-MOP. The intense and broad peak at 3286 cm⁻¹ resembles the -OH stretching vibrations of cellulose, hemicellulose, lignin and the absorbed water [19]. The two bands at 2920 and 2851 cm⁻¹ were due to -CH₃ and -CH_2, respectively. The ketonic group -C=O was detected at 1759 cm⁻¹ [28]. The 1640 and 1600 cm⁻¹ doublet peaks indicated -NH₂ [29]. The band at 1001 cm⁻¹ is attributed to the C-O stretching vibration of ether and ester [30]. After carbonization of the MOP, most of the peaks disappeared whilst others blue-shifted, and others had less intensity. The peak at 3301 cm⁻¹ is assigned to the -OH group. The peak detected at 2915 cm⁻¹ is attributed to the aliphatic -C-H stretching vibrations in lignin, cellulose and hemicelluloses. The -C=O group was observed at 1709 cm⁻¹. The 1577 cm⁻¹ peak represented the presence of COOH vibrations [30]. The absorption peak at 1046 cm⁻¹ was due to the C-O stretch. The disappearance and shifting of the OH, C=O, C-O and NH₂ functional groups’ peaks suggest an interaction between AC-MOP and the pollutants [31].

3.1.2. Morphology Determination of MOP and AC-MOP

Figure 3 shows MOP (Figure 3a,b) and AC-MOP (Figure 3c,d) surface morphologies. The surface characterization of MOP in Figure 3a,b shows morphological variations before converting the adsorbent to activated carbon. The MOP indicates a filamentous morphology with longitudinal strips or fibres with grooves between irregular fibres. The SEM images shown in Figure 3c,d for the AC-MOP revealed a smoother and rough surface in certain areas after activation. Pores were also observed. A similar observation was recorded during the activation of coconut for the adsorption of congo red, methylene blue and neutral red dyes [32]. The activation process resulted in numerous pores and cavities formed under high temperatures [32].

To determine the changes in the elemental components before and after the activation of carbon, the EDS spectra of the MOP and AC-MOP were analyzed and are shown in Figure 4a,b, respectively. The results indicated that both adsorbents contained different percentage weights of carbon, oxygen and potassium (

Table 1. The surface element analysis of MOP and AC-MOP adsorbents.

Element	MOP	AC-MOP
C	55.07	88.59
O	44.78	9.99
K	0.16	0.19
Mg		0.13
Cl		0.40
Ca		0.71
Total	100	100

Gray color = N/A (not applicable for the specific material).

) before and after activation. Oxygen (O) and carbon (C) are the main components of the MOP and AC-MOP, which are characteristic of plant-based materials [33]. The AC-MOP contained high levels of carbon and potassium because these elements form part of the MOP. Other elements with trace amounts in the AC-MOP included magnesium (Mg) and calcium (Ca) as well as potassium (K). The observed chloride (Cl) traces were from the activation agent (HCl).

3.1.3. Physicochemical Characterization of MOP and AC-MOP

The BET analysis of the MOP and AC-MOP are shown in

Table 2. Physicochemical characterization of MOP and AC-MOP.

Adsorbent	Surface Area (m2/g)	Pore Width (nm)	Pore Size (cm2/g)	Point Zero Charge
MOP	1.430	2.566	0.0034	3.31
AC-MOP	5.478	3.412	0.0089	5.29

. The results showed that the surface area of the MOP was 1.43 m²/g with a pore width of 2.57 nm and pore size of 0.0034 cm²/g. On the other hand, the surface area, pore width and pore size of AC-MOP were 5.48 m²/g, 3.41 nm and 0.0089 cm²/g, respectively. Thus, the AC-MOP adsorbent experienced an increase of 383% in surface area, 133% in pores and 261% in pore size compared to the MOP adsorbent. Table 2 shows that the BET surface area of AC-MOP was greater than that of the MOP. This was ascribed to the formation of cavities on the surface of the AC-MOP [34]. The results also indicated that the values for pore sizes and width were higher for AC-MOP than for MOP. It was, therefore, expected that AC-MOP would adsorb more than MOP. The point zero charge of MOP and AC-MOP is indicated in Table 2. The results show that the pH(pzc) is 3.31 for MOP and 5.29 for AC-MOP. The ΔpH values were negative before the pH values of 3.31 and 5.29 for MOP and AC-MOP, respectively. This showed that MOP and AC-MOP surfaces had a negative charge, and at pH values above pH(pzc), the adsorbent surfaces developed positive ΔpH values. Eldeeb et al. (2022) [21] incorporated triethylenetetramine (TETA) in mandarin orange peels for the adsorption of acid brown 14 dye. Their study showed a BET-specific surface area of 5.88 m²/g and a monolayer volume of 1.3519 cm³/g. The total pore volume of 0.017 cm³/g, a mean pore diameter of 11.715 nm (mesopores), a mesosurface area of 6.1776 m²/g and a mesopore volume of 0.01835 cm³/g were attained. The adsorption capacity was found to be 416.67 mg/g. The adsorption capacity of mandarin orange peels was investigated for methylene blue by Lima 2023 [23]. They obtained a BET-specific surface area of 0.9632 m²/g and an average pore diameter of 1.603 nm. Koyuncu et al. (2018) [19] used mandarin orange peels to prepare nanoporous carbon via chemical activation with H₃PO₄ using microwave radiation. They reported a specific surface area of 1021 m²/g for removing methylene blue. Mandarin orange peel biochar was used for the adsorption of methyl orange (Park) [20]. He obtained a surface area of 8.5 m²/g and a pore volume of 0.016 cm³/g. Mandarin orange peels were used by Yilmaz et al. (2022) [35] to adsorb acid Red 35 dye. The prepared adsorbent had a maximum adsorption capacity of 476.19 mg/g; the BET-specific surface area was 5.65 m²/g, the total pore volume value was 0.0175 cm³/g and the mean pore diameter was 11.74 nm.

3.2. Batch Studies

3.2.1. Adsorption pH Effect for MB and IBF Removal

Figure 5 shows the pH effect at pH 2, 4, 6, 8 and 10. The pH of the solution is one of the most vital parameters in adsorption because it influences the pollutant and adsorbent materials’ speciation and charge densities [36]. A similar trend was observed for both pollutants. Higher adsorption representing the maximum capacity was observed for MB and IBF at pH 6. Lower sorption capacities were observed at lower pH due to the repulsion force between the adsorbent and pollutants and competition for active sites [37]. As pH increased, greater sorption capacities were observed, and the maximum sorption capacity for MB and IBF were 60.96 and 71.37 mg/g, respectively.

3.2.2. Adsorption Temperature Effect for the Removal of MB and IBF

Temperature is an important parameter in the adsorption process. The effect of adsorption temperature for MB and IBF with AC-MOP was carried out at three temperatures of 35, 45 and 55 °C with 100 mg/L concentrations. The temperature effect on the removal capacity with AC-MOP for MB dye and IBF is illustrated in Figure 6. The temperature plots show that MB and IBF had different adsorption trends. It was observed that a rise in temperature solution from 25 to 45 °C resulted in higher uptake for MB; after that, a decrease in capacity was observed. On the other hand, the adsorption for IBF increased from 25 to 45 °C, but at higher temperatures, the capacity remained constant. The high uptake of MB and IBF at 45 °C could be attributed to lowered activation energy required, increased kinetics between the adsorbate molecules and the surface of the adsorbent as well as change in the textural properties of the AC-MOP [38].

The determination of entropies (ΔS°) and enthalpies (ΔH°) for the slope and intercept—respectively—including the Gibbs free energy (ΔG°) of MB and IBF is shown in

Table 3. Thermodynamic results for the adsorption of MB and IBF.

Thermodynamic Parameters	MB	IBF
ΔS° (kJ/mol/K)	0.3923	0.1445
ΔH° (kJ/mol)	60.2224	70.1356
ΔG° (kJ/mol)−25 °C	−2.3445	−6.5656
35 °C	−3.6532	−4.3412
45 °C	−5.6723	−5.8844
55 °C	−8.1223	−2.7723

. ΔS° values were positive, offering the freedom of randomness of MB and IBF in the solution during the process. The positive ΔH° suggests that the reaction was endothermic. The positive ΔH° values may have resulted from the dissociation of water [34]. Generally, thermodynamic parameters showed that enthalpy is the driving force for the deprotonation of the AC-MOP surface. The ΔH° data of this work were similar to those of Sudrajat et al. (2021) [38]. The negative ΔG° values signify that the sorption process was spontaneous and exothermic.

3.2.3. Concentration Effect on the Adsorption of MB and IBF

Figure 7 shows the adsorption capabilities of AC-MOP for MB and IBP in the solution at various initial concentrations. The trend from the plot indicates an increasing uptake with increased pollutant concentrations. This means that MB and IBP uptake on AC-MOP depended on the concentration. A similar observation was obtained in the Mabungela et al. (2022) [30] study. This type of observation is explained by Statorori et al. (2019) [39], stating that the increase in concentration is due to the high mass transfer resistance of pollutants in the liquid and solid phases. The maximum sorption capacities for MB and IBF were 73.74 and 77.23 mg/g, respectively.

The Langmuir and Freundlich models estimated the isotherms in

Table 4. Isotherms for the uptake of MB and IBF.

Models	MB	IBF
Langmuir Qo (mg/g)	74.1538	78.1524
b (L/mol)	0.3094	0.5997
R2	0.9987	0.9980
Freundlich N	1.6514	1.5827
Kf	0.2577	0.2369
R2	0.7870	0.8973
Exp qe (mg/g)	73.7411	77.2327

. The Langmuir model suggests the process is achieved via monolayer adsorption of pollutants on the material, and the sorption is homogeneous [39]. This model also proposes that the material has binding sites, and every site is accountable for binding a single pollutant with no interaction between the adsorbed ions [40]. The data showed that regression coefficients obtained for MB and IBF on AC-MOP were 0.9987 and 0.9975, respectively, and the best fit was the Langmuir model.

3.2.4. Time Effect on the Adsorption of MB and IBF

The MB and IBF sorption rate with AC-MOP was assessed between 1 and 80 min, as shown in Figure 8. Figure 8 displayed a similar trend, showing the adsorption capacities of AC-MOP increased as contact time increased. Phase 1 was ascribed to a faster adsorption rate occurring between 1 and 25 min in the initial stages. This was attributed to the readily available functional groups such as -CO -OH, -COC, -NH₂ and -CH=CH and pores on the AC-MOP surface, which allowed for greater adsorption. A similar trend was observed by Chikri et al. 2020 [41]. Stage 2 involves a slower increase due to the saturation of the active sites as time passes. AC-MOP reached equilibrium after 40 min.

Two kinetic models were evaluated using PFO and PSO to determine the adsorption mechanisms of MB and IBF with AC-MOP. The results in

Table 5. Kinetics for uptake of MB and IBF.

Models	MB	IBF
PFO qe (mg/g)	52.7645	68.8334
k1 (min−1)	2.2898	2.5522
R2	0.8995	0.8507
PSO qe (mg/g)	62.7934	75.3956
k2 (g.mg.min−1)	1.0079	1.0044
R2	0.9977	0.9922
Exp qe (mg/g)	65.8978	72.4432

show that the pseudo-second-order projected capacity figures are more similar to experimental capacity data than the pseudo-first-order. The regression coefficient for the pseudo-second-order for both pollutants is close to 1 compared to the pseudo-first-order. According to the regression coefficient data, the pseudo-second-order better fits the mechanism than the pseudo-first-order. The results of the k₂ parameter indicated a higher adsorption rate for MB than IBF. This agrees with the results obtained in the time effect. The PSO model proposes that the sorption of MB dye and IBF with AC-MOP possibly occurred through electron transfer [42].

4. Reusability and Regeneration Studies of AC-MOP

The regeneration or reusability of adsorbents is crucial in determining the sorption method’s efficacy [43]. AC-MOP regeneration was investigated using 0.4 g of the used (IBF and MB), loaded AC-MOP. The results in Figure 9 show the regeneration process, which was repeated four times. A recovery of 70.13% for MB was obtained at the beginning of the regeneration, followed by 64.12% in the first cycle, 50.37% in the second cycle, 41.12% in the third cycle and 38.35% in the last. The removal efficiency of AC-MOP for MB decreased as the number of recycles increased. The reduction in the removal efficiency may be related to the loss of the binding or active sites during the reuse [44]. Although the adsorption effectiveness decreased with increasing recycling numbers, AC-MOP was able to remove methylene blue. Optimizing the pH solution is important in increasing the adsorption capacity, is useful for the desorption process and should be considered to improve the regeneration cycle [45]. A similar procedure was used to determine the reusability for IBF, and the recovery was 87.54, 85.17, 74.87 and 65.41%, respectively, with a maximum recovery of 50.58%. These results showed that the adsorptive power of AC-MOP decreased after each cycle. This is due to the inability of the adsorbent to desorb MB and IBF from the openings and functional groups during regeneration.

5. Post Adsorption Results

5.1. FTIR Analysis

FTIR analysis of AC-MOP compared the adsorption of MB and IBF before and after. This was undertaken to identify the functional groups responsible for the adsorption of MB and IBF. The FTIR spectra of MB and IBF after adsorption are shown in Figure 10. The absorption due to the -OH of AC-MOP shifted to 3387 cm⁻¹ after sorption of MB, whilst after the adsorption of IBF, the peak shifted to 3334 cm⁻¹_. These peaks showed a decrease in intensity after adsorption. The ketonic group -C=O moved to 1701 cm⁻¹ after the adsorption of MB and to 1696 cm⁻¹ after the adsorption of IBF. The peak due to -COOH vibrations shifted to 1572 cm⁻¹ after the adsorption of MB and to 1567 cm⁻¹ after the sorption of IBF. The -C-O peak shifted to 1023 and 1018 cm⁻¹ after the adsorption of MB and IBF, respectively. The shifting of peaks to lower and higher wavenumbers suggests that the functional groups -OH, -C=O, -COOH and -C-O were involved in the adsorption processes [34].

5.2. SEM Analysis

The SEM images of AC-MOP after adsorption of MB and IBF were used to assess the stability of the adsorbents (Figure 11a,b). The morphology of AC-MOP changed after the adsorption of MB and IBF, especially after the adsorption of IBF in Figure 11b. The surface of AC-MOP after IBF adsorption is rougher than before the adsorption. AC-MOP appeared similar before and after MB adsorption with grooves and cavities.

6. Adsorption Mechanism of IBF and MB with AC-MOP

Based on the results from the FTIR spectroscopy, a possible adsorption mechanism of MB and IBF with the AC-MOP adsorbent is presented in Figure 12. The interaction between MB and IBF on AC-MOP can be governed using van der Walls interactions of hydrophobic interactions (due to cellulose and hemicellulose), π–π interaction and hydrogen bonds [45,46]. In addition, because the AC-MOP capacity is high in acidic conditions (pH 5), hydrogen bonding and possible electrostatic interactions occur between the carbon materials and pollutants [47]. The MB and IBF uptake process might also be briefly described as a multi-step process involving their transfer from a liquid phase to the solid phase of the adsorbent [48]. The diffusion on the inner and outer surfaces and pores of AC-MOP was finally loaded onto the active sites of AC-MOP [49].

7. Conclusions

Mandarin orange peels activated with carbon (AC-MOP) were used to adsorb methylene blue and Ibuprofen from an aqueous solution. FTIR spectroscopy showed that the -C-O, -C=O, -OH and -NH₂ functional groups were modified substantially compared to those in MOP, which may have increased adsorption capacity. BET data of AC-MOP indicated a higher surface area of 5.48 m²/g than that of MOP with 1.43 m²/g. The results showed that carbonization and activation of MOP changed its structure. Batch sorption studies established the pH dependency of adsorption, with the optimal pH for both pollutants being 6.0. Thermodynamics data showed that the sorption process was spontaneous and exothermic. The adsorption data followed the Langmuir model for both pollutants, suggesting that the adsorption was homogeneous on the surface of AC-MOP. The adsorption kinetics best fitted PSO compared to the PFO. The overall sorption capacities for methylene blue and Ibuprofen were 73.74 mg/g and 79.82 mg/g, respectively. The optimum times for methylene blue and Ibuprofen sorption were 10 and 40 min, respectively. Few studies have reported on mandarin peels as potential adsorbents. This research has shown that methylene blue and Ibuprofen adsorption can be achieved using an activated carbon-based mandarin orange peel treated with hydrochloric acid. Some of the major advantages of using waste materials as adsorbents are that they are abundantly available and made up of lignin and cellulose, which can potentially absorb dyes and other pollutants. Other advantages include simple technique, minimum processing, adsorption of pollutants (dyes, pharmaceutical and metal ions) and easy regeneration.