Abstract
Prepared orange peel biochar (OPB) was evaluated as a low-cost adsorbent for removing Pb(II) ions from aqueous solutions. The OPB was examined using scanning electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, and Brunauer-Emmett-Teller analysis to identify surface morphology, functional groups, thermal stability, and surface porosity responsible for adsorption, respectively. The biochar showed typical lignocellulosic decomposition behavior and exhibited a microporous surface whose hydroxyl, carbonyl, carboxylate, and phenolic groups allowed effective Pb²⁺ uptake. Batch studies revealed maximum Pb2+ at a pH 5, adsorbent dosage of 0.7 g, contact time of 80 min, adsorbate concentration of 150 mg/L, and temperature of 50 °C. The Langmuir isotherm revealed adsorption maximum capacity of the adsorbent to be 73.5 mg/g, while thermodynamics analysis showed that Pb(II) uptake was endothermic. These results demonstrate that orange-peel biochar can be an effective, environmentally friendly, and renewable adsorbent for lead ions in wastewater treatment.
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Biochar of Orange Peel as an Adsorbent for the Uptake of Lead (II) Ions
Ghaferah H. Al-Hazmi,a Lamia A. Albedair,b Edwin Andrew Ofudje 
,c Moamen S. Refat,d Kholoud K. Alzahrani,e Ali El-Rayyes,f and Emmanuel Kola Oladejo c
Prepared orange peel biochar (OPB) was evaluated as a low-cost adsorbent for removing Pb(II) ions from aqueous solutions. The OPB was examined using scanning electron microscopy, Fourier transform infrared spectroscopy, thermal gravimetric analysis, and Brunauer-Emmett-Teller analysis to identify surface morphology, functional groups, thermal stability, and surface porosity responsible for adsorption, respectively. The biochar showed typical lignocellulosic decomposition behavior and exhibited a microporous surface whose hydroxyl, carbonyl, carboxylate, and phenolic groups allowed effective Pb²⁺ uptake. Batch studies revealed maximum Pb2+ at a pH 5, adsorbent dosage of 0.7 g, contact time of 80 min, adsorbate concentration of 150 mg/L, and temperature of 50 °C. The Langmuir isotherm revealed adsorption maximum capacity of the adsorbent to be 73.5 mg/g, while thermodynamics analysis showed that Pb(II) uptake was endothermic. These results demonstrate that orange-peel biochar can be an effective, environmentally friendly, and renewable adsorbent for lead ions in wastewater treatment.
DOI: 10.15376/biores.21.2.3831-3855
Keywords: Adsorption; Isotherm; Kinetics; Lead (II) ions; Orange peel biochar; Wastewater
Contact information: a: Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia; b: Department of Chemistry, College of Science, Princess Nourah bint Abdulrahman University, P.O. Box 84428, Riyadh 11671, Saudi Arabia; c: Department of Chemical Sciences, Mountain Top University, Ibafo, Ogun State, Nigeria; d: Department of Chemistry, College of Science, Taif University, P.O. Box, 11099, Taif 21944, Saudi Arabia; e: Department of Biology, University College of Umluj, University of Tabuk, Umluj, Tabuk, Saudi Arabia; f: Center for Scientific Research and Entrepreneurship, Northern Border University, 73213, Arar, Saudi Arabia;
* Corresponding author: kolaoladejo7@gmail.com
INTRODUCTION
Water is the most essential resource for human survival, but it is being polluted by waste from industrial, agricultural, and household activities, posing a serious risk to the aquatic ecosystem (John et al. 2019). Industrial activities, including fertilizer production, leather tanning, electroplating, textile manufacturing, sugar processing, metal processing, mining, and municipal waste management release harmful pollutants into water bodies (Raman et al. 2018; Akhtar et al. 2021). Among the contaminants released, toxic metals pose a severe threat to aquatic life, plants, humans, and animals’ health. Their discharge into water systems through industrial actions is a pressing issue, causing irreversible damage to the ecosystems (Vidal et al. 2019; Briffa et al. 2020). Industrial wastewater often contains toxic heavy metal ions including lead, cadmium, arsenic, mercury, and chromium, which are harmful to aquatic life and are threatening to human health (Lupa et al. 2018; Ghori et al. 2019; Balali-Mood et al. 2021). Food crops grown on metal-contaminated soil can become toxic, posing serious health risks to humans who consume them, which can lead to nutritional deficiencies and a range of health problems (Khan et al. 2019).
Among these pollutants, lead is particularly of great concern because of its widespread industrial use in batteries, paints, alloys, and electroplating processes (Colin et al. 2022). Lead residues released into water bodies can accumulate in aquatic ecosystems, ultimately entering the human food chain and posing great health risks (Mishra et al. 2019). Even at trace levels, lead exposure can lead to severe health effects, such as hypertension, nephritis, constipation, stomach pain, nausea, vomiting, neurological issues, speaking difficulties, renal, and reproductive disorders (Chowdhury et al. 2022; Rahman et al. 2023; Zheng et al. 2024). Therefore, the discharge of lead-containing effluents into water bodies poses a severe risk to both ecosystems necessitating the development of efficient and sustainable removal strategies to ensure safe drinking water and human health protection (Chowdhury et al. 2022).
Conventional treatment methods for lead removal from wastewater include chemical precipitation (Zheng et al. 2024), ion exchange (Jasim and Ajjam 2024), coagulation-flocculation (Benalia et al. 2024), membrane filtration (Azmi 2025), and others. However, they often suffer from high operational costs, incomplete pollutant removal, and secondary pollution generation (Arogundade et al. 2025). The adsorption technique has emerged as a highly effective technique for removing lead ions and other contaminants from wastewater, garnering attention due to its effectiveness and potential for efficient and sustainable remediation (Jjagwe et al. 2021). Despite the numerous advantages offered by adsorption techniques, its high cost of production often limits the full application most especially in developing countries such as Nigeria.
Recently, carbon-rich material, also known as biochar, that is often obtained from biomass pyrolysis, has appeared as a promising low-cost adsorbent to remove pollutants from wastewater because of its porous structure, abundance of functional groups, and high surface area, which enhance its adsorption capacity (Meng et al. 2025). Various by-products of agricultural origin, such as rice husk (Gargiulo et al. 2024), coconut shell (Minh et al. 2023), corn cob (Onyekwere et al. 2024), banana peel (Negroiu et al. 2021), and Manihot esculenta (El-Rayyes et al. 2025), have been successfully examined for the elimination of heavy metals. These eco-friendly adsorbents are abundant and more cost-effective than traditional activated carbon. Their application puts the concept of waste valorization and circular economy into practice (Wang et al. 2022).
Another readily available lignocellulosic agricultural waste with high carbon content that is yet to be explored fully is orange peel. Orange peel is a common agro-waste generated in large quantities by fruit processing industries, and it is mostly disposed of in landfills or burned in open air, which poses serious environmental pollution, such as the generation of greenhouse gases (Afolabi and Musonge 2023). However, it represents a valuable and underutilized source of biomass for biochar production (Michael-Igolima et al. 2023). It contains high amounts of cellulose, hemicellulose, lignin, and functionalized organic compounds that can enhance metal binding when carbonized (Zhang et al. 2022; Mousa et al. 2025). Converting orange peel into biochar not only provides a sustainable waste management strategy but also supports the principles of circular economy by transforming waste into a resource for environmental remediation. Biochars of orange-peel-derived origins have been widely studied for heavy metal removal because of their abundance of oxygen-containing functional groups and lignocellulosic composition. But, many of these findings have focused primarily on achieving high removal efficiencies without establishing internal consistency between experimental adsorption capacity, kinetic transport regimes, isotherm-derived parameters, and thermodynamic behavior. In some cases, the production of the biochar is done at elevated temperature.
The current study investigates the potential of biochar derived from orange peel as an active material for lead ion uptake from aqueous solutions. The study evaluates the impact of process parameters of solution pH, adsorbent dosage, temperature, contact time, and concentration as they relate to the adsorption of the adsorbate. The adsorption process mechanisms were also examined using various adsorption kinetics, isotherms, and thermodynamic investigations.
EXPERIMENTAL
Materials
Fresh orange peels were collected from a fruit store in Ibafo, Ogun State, Nigeria. Thereafter, distilled water was used to thoroughly wash the sample to eliminate dirt and other impurities. After washing, the materials was air-dried for 7 days.
Stock solution of Pb2+ ions were prepared using analytical grade lead (II) nitrate [Pb(NO3)2] procured from Merck, Germany. Hydrochloric acid (HCl: 37%) and sodium hydroxide (NaOH: 98%) were purchased from Sigma-Aldrich, India, and used for pH adjustment.
Preparation Of Orange Peel Biochar
The cleaned and oven-dried orange peels (105 °C for 24 h) were then reduced to powder using a mortar and pestle. The powdered orange peel was converted into biochar through pyrolysis in a muffle furnace under limited oxygen at 500 °C for 2 h. The biochar was repeatedly washed with distilled water to eliminate residual ash and soluble impurities, then oven-dried again (105 °C) and stored in airtight containers for further use.
Characterization of the Biochar
Fourier transform infrared (FT-IR) analysis was performed to detect the metal ion binding functional groups available on the surface of the biochar. The FT-IR analysis was conducted using an FT-IR spectrometer (PerkinElmer Spectrum 2, USA) within the range of 4000 to 400 cm⁻¹. Finely ground samples were mixed with KBr powder and compressed into pellets before analysis.
To observe the surface morphology of the biochar by scanning electron microscopy (SEM), the samples were first coated with a thin layer of gold to improve conductivity. The SEM device was a JEOL JSM-5600 (Tokyo, Japan). The Brunauer–Emmett–Teller (BET) technique was employed for determined the pore size distribution and surface area using a Quantachrome NOVA 2200C instrument (USA).
Thermogravimetric investigation (TGA 4000, PerkinElmer, Haarlem, Netherlands) was performed under a nitrogen atmosphere from room temperature to 800 °C at a heating rate of 10 °C/min to evaluate the thermal stability characteristics of the orange peel.
The point of zero charge (pHpZc) of the OPB was measured using the pH-drift technique. A total of 25 mL of NaCl solutions (0.01 M) were adjusted in the pH range of 2 to 8, and thereafter, 5 mg of biochar was added and shaken for 12 h at 150 rpm. Thereafter, the pH was recorded again and the changes between the first and the last pH; (ΔpH = pHf – pHo) were plotted against pHo to determine the pHpZc.
Preparations of Lead Solutions
A total of 1.60 g of Pb(NO₃)₂ was dissolved in 1000 mL of distilled water for the preparation of 1000 mg/L stock solution of Pb²⁺. Different working solutions in the range of 10 to 150 mg/L were prepared via serial dilution of the stock solution. The adjustment of pH of the solution was made using 0.1 M HCl or NaOH.
Batch Adsorption Studies
Batch sorption studies were conducted in triplicate, and the average values were recorded to estimate the efficiency of OPB for Pb²⁺ removal. In each run, 0.7 g of OPB was contacted with 50 mL of Pb²⁺ solution (150 mg/L) in a conical flask, and the solution pH was adjusted as desired. Thereafter, agitation of the mixture on a rotatory orbital shaker at a constant speed of 100 rpm was done for a predetermined contact time within the range of 10 to 120 min at room temperature. After the attainment of equilibrium, the content was filtered, and the residual lead content in the filtrate was determined using an Atomic Absorption Spectrophotometer (AAS) (Jenway 7325, UK). The percentage removal (%) and adsorption capacity (qₑ, mg/g) were determined by adopting Eqs. 1 and 2, respectively,
where C0 and Ce are Pb2+ concentrations at initial and equilibrium stages in mg/L, and V and m are the volume of the solution (L) and OPB mass (g), respectively.
Adsorption Modeling
Adsorption kinetics
The sorption kinetics were characterized using linearized forms of pseudo-first-order, pseudo-second-order, and intraparticle diffusion kinetics models (Eqs. 3 to 5) (Chen et al. 2019; Mustapha et al. 2021; El Hammari et al. 2022; Ademoyegun et al. 2022; Ofudje et al. 2024). These models are used to describe the rate of Pb2+ ions adsorption onto the OPB surface.
In the above equations, qe and qt represent the quantities of Pb2+ adsorbed at equilibrium (mg/g) and at time t (min), respectively. The k1 and k2 represent the pseudo-first and pseudo-second order adsorption rate constants in min−1 and (g/mg min) respectively, kp stands for rate constant of intra-particle diffusion (mg/g/min1/2), and Ci represents the boundary layer thickness effect (intercept). The best model fit is determined by the highest correlation coefficient (R²) and lowest Sum of squared error (SSE) value, indicating the most accurate description of adsorption kinetics under the studied conditions. The SSE values for adsorption capacity were calculated using Eq. 6 (Ofudje et al. 2023),
where N represents the number of data points.
Adsorption Isotherm
The Langmuir isotherm (Eq. 7) assumes monolayer adsorption on homogeneous adsorbent sites, and is given as (Mustapha et al. 2021; Ademoyegun et al. 2022):
The dimensionless separation factor RL from the Langmuir isotherm model indicates adsorption favorability and was estimated using Eq. 8 below (Din et al. 2025),
where qe, Ce and C0 are as previously defined, Qmax defines the maximum adsorption capacity (mg/g), and KL is the Langmuir adsorption constant (L/mg).
The Freundlich isotherm model (Eq. 9) describes sorption on heterogeneous surfaces (Aderibigbe et al. 2024; Ofudje et al. 2024),
where the Freundlich constant is given as KF (mg/g), which is related to the adsorption capacity, and the dimensionless constant that reflects the adsorption intensity is given as n. Insight into the nature of the sorption process is provided by the value of n, where n = 1 implies linear adsorption, n < 1 suggests favorable adsorption, and n > 1 suggests weak adsorption.
The Dubinin-Radushkevich (D-R) isotherm (Eq. 10) assesses adsorbate-adsorbent interactions, estimates adsorption free energy, and distinguishes between chemical and physical adsorption mechanisms based on the Polanyi potential theory. It is useful for analyzing adsorption across concentrations and understanding adsorbent porosity (Ghosh et al. 2023). Equation 10 is given as,
where qe is as previously defined, qo denotes maximum adsorption capacity (theoretical) (mg/g), and K is the D-R constant that reflects the adsorption energy (mol²/J²).
The mean adsorption energy, E, indicates the adsorption process type, and it is calculated as:
The E values classify the mechanism, where 0 to 8 kJ/mol implies physisorption, and E > 16 kJ/mol implies chemisorption, according to Ofudje et al. (2023).
Adsorption thermodynamics
The parameters from thermodynamic analysis, which include enthalpy (ΔH), Gibbs free energy (ΔG), and entropy (ΔS), were determined using Eqs. 12 and 13 (Bouguettoucha et al. 2015; Ogundiran et al. 2022; Kori et al. 2024; Din et al. 2025),
where T is the absolute temperature (K), ρ is the water density (g/L), R is the universal gas constant, and KD denotes the equilibrium constant.
Reusability and Regeneration Studies
The reusability and regeneration of orange peel biochar were examined to assess its potential for repeated applications in lead ion removal. Following each adsorption cycle, the biochar was desorbed using a 0.1 M acetic acid solution, subsequently cleansed using distilled water, and then dried for 12 h in an oven at 105 °C. The regenerated biochar was used in the subsequent experiments under constant parameters, such as a biochar dosage of 1 g, Pb2+ concentration of 150 mg/L, a pH of 5, a contact time of 80 min, and a temperature of 50 °C. This procedure was conducted for five consecutive cycles, and the recovery efficiency (%RE) was estimated from Eq. 14:
Effect of Competing Ions
To assess the applicability of OPB under realistic water condition, competitive adsorption experiments were performed in the presence of representative background electrolytes and co-contaminants commonly found in natural and wastewater systems. Batch adsorption tests were done at the optimized conditions (pH 5, Pb²⁺ = 150 mg/L, OPB dosage = 0.7 g, contact time = 80 min, 50 °C), with the addition of Mg²⁺ and Ca²⁺ (50 mg/L each), carbonate (CO₃²⁻), sulfate (SO₄²⁻), chloride (Cl⁻), and natural organic matter (NOM, 10 mg/L as humic acid).
RESULTS AND DISCUSSION
The Role of Pb2+ Initial Concentration and Agitation Time
The impact of agitation time (10 to 120 min) as well Pb2+ initial concentration (10 to 150 mg/L) was studied, while keeping other parameters constant (Fig. 1). It was noted that during the first 30 min, the amount adsorbed increased rapidly because of the large amount of unfilled binding sites on OPB’s surface that easily attract Pb2+ molecules (Ofudje et al. 2024).
Nonetheless, as the reaction continues, the sites become occupied, and the adsorption rate slows down, gradually reaching equilibrium (Ofudje et al. 2024). This is evidenced with a maximum adsorption of 69.6 mg/g at Pb2+ concentration of 150 mg/L and 80 min.
At lower concentrations, adsorption capacities were reduced compared to higher concentrations, indicating that higher concentrations increase the driving force for mass transfer, enabling more Pb2+ ions to react with available adsorption sites of the biochar surface (Mandal et al. 2021).
Fig. 1. Influence of Pb2+concentration and agitation time on the sorption capacity of OPB
Impact of Solution pH
The solution pH influences both the adsorbent’s surface charge and the ionization degree of the adsorbate in the adsorption of lead ions. Figure 2 shows the percentage removal variation of Pb2+ by OPB across different pH values ranging from 2 to 8, while other variables were kept constant.
Fig. 2. Influence of solution pH on the adsorption of Pb2+ onto OPB
It was noted that the adsorption efficiency rose steadily from 49.6% at pH 2 to 77.8% at pH 5, after which it declined to 45.2% at pH 8. This behavior can be because of the electrostatic interactions between the surface of the adsorbent and the molecules of the metal ion (Ngueagni et al. 2025). This behavior is consistent with the value of the zero-point charge measured (pHpZc = 3.5), suggesting the OPB surface to be positively charged at a pH lower than 3.5 and negatively charged when above it. At lower pH, the OPB’s surface is highly protonated because of high H⁺ ions concentration, resulting in an electrostatic repulsion of the Pb2+ on the surface of the biochar (Ngueagni et al. 2025). However, with a rise in the pH, the surface charge gradually becomes less positive, enhancing electrostatic attraction and Pb2+ uptake.
Optimum adsorption efficiency was observed at pH 5, with a percentage removal of 77.8 %. Above pH 5, the adsorption efficiency decreases because of the increase in the formation of Pb(OH)2, which reduces the adsorbent’s affinity for the positively charged metal ions (Singh et al. 2023). Ngueagni et al. (2025) also reported the same observation, confirming the pH-dependent nature of the adsorption process.
Influence of Temperature
The effect of temperature on the sorption of Pb²⁺ by the OPB was investigated over the 25 to 65 °C range while other variables were kept constant as presented in Fig. 3. As seen in the plot, the percentage removal increased from 54 % at 25 °C to a maximum of 82% at 55 °C. At the initial level, increasing temperature enhances the mobility of Pb²⁺ ions and improves their diffusion into the internal pores of the biochar.
Fig. 3. Influence of temperature on the sorption of Pb2+ by OPB
Adsorption Kinetics
The linearized plots for the sorption kinetics of Pb²⁺ onto OPB are shown in Figs. 4 to 6, and the parameter values are provided in Table 1. The three models applied suitably explain the adsorption process, as evidence from the high values of coefficient of determination (R²).
Fig. 4. Pseudo-first-order kinetic plot for Pb2+ sorption by OPB
Fig. 5. Pseudo-second-order kinetic plot for Pb2+ sorption by OPB
Fig. 6. Intraparticle diffusion plot for Pb2+ sorption by OPB
Table 1. Kinetics Parameters for Pb2+ Sorption by OPB
The pseudo-second-order showed the highest value. In addition, the near agreement between the experimental adsorption capacity (Qₑ, exp.) and the calculated equilibrium capacities (Qₑ, cal.) further support the suitability of the pseudo-second-order model. The value of R² obtained affirmed that intraparticle diffusion also contributes to the sorption process, but it is not the only rate-limiting step, as plots do not pass through the origin (Fig. 6). This deviation implies that external mass transfer, boundary layer diffusion, and interactions taking place on the surface, such as hydrogen bonding and ion exchange, also play key roles (Arogundade et al. 2025). Both the pseudo–first-order and pseudo–second-order kinetic models, as well as the intra-particle diffusion model, showed a good fit for lead ion adsorption onto the biochar, with high R² values (R² > 0.9). This combination of findings implies that the sorption rate is mainly controlled by diffusion processes and that interactions between the molecules of the adsorbate occur within the material’s porous structure (Hubbe et al. 2019). The cited work also showed that a mistake had been made during the original derivation of the pseudo-second order model. The fact that the expression happens to fit many data sets for adsorption onto porous materials appears to have been a matter of luck.
Adsorption Isotherms
The equilibrium data for the adsorption of Pb2+ onto orange peel biochar (OPB) were evaluated using three prominent isotherm models as depicted graphically in Figs. 7 to 9, with their corresponding parameters presented in Table 2.
The Langmuir isotherm gave a reasonable fit to the experimental data, yielding a maximum adsorption capacity (Qₘₐₓ) of 73.5 mg/g with R2 value of 0.956. With this, the Langmuir model is seen as an effective saturation model, assuming a finite number of adsorption sites under fixed solution pH and temperature. The value of the separation factor (RL) calculated using the highest initial Pb²⁺ concentration (C0 = 150 mg/L) was found to be 0.059, confirming that Pb²⁺ adsorption on the biochar is favourable. The Freundlich model exhibited an excellent fit to the equilibrium data. Among the isotherm models, the Freundlich model demonstrated the best fit for the adsorption data, as indicated by its high coefficient of determination (R2). This suggests that Pb2+ adsorption occurs on a heterogeneous surface that contains multiple types of binding sites with varying affinities. The value of the Freundlich dimensionless parameters (1/n), which is less than 1, indicates a strong adsorption intensity, suggesting that the sorption is favourable.
The D–R isotherm model gave insight into the nature of the sorption mechanism. The mean free energy of adsorption (E) was calculated to be 0.495 kJ/mol, which is well below the threshold of 8 kJ/mol for physisorption (Mohrazi and Ghasemi-Fasaei 2023; Arogundade et al. 2025). This low value indicates that the uptake of Pb²⁺ onto OPB also proceeds through physical adsorption mechanisms (Dubinin 1960; Mohrazi and Ghasemi-Fasaei 2023). Therefore, the equilibrium behavior is accredited to weak van der Waals forces, electrostatic attraction, and pore-filling effects rather than specific chemical bonding. The comparison of the adsorption capacities from different adsorbents for the removal of Pb2+ from wastewater is presented in Table 3. This comparison reinforces the suitability of OPB as a competitive and effective adsorbent for lead-contaminated wastewater.
Fig. 7. Langmuir isotherm for the sorption of Pb2+ onto OPB
Fig. 8. Freundlich isotherm for the sorption of Pb2+ onto OPB
Fig. 9. Dubinin–Radushkevich isotherm for the sorption of Pb2+ onto OPB
Table 2. Parameters from the Isotherm Analysis of the Sorption of Pb2+ onto OPB
Table 3. Comparison of Various Adsorbents’ Adsorption Capacities for Pb2+ Uptake by OPB
Thermodynamics Studies
Figure 10 illustrates the plot of the adsorption thermodynamics, while the parameters are presented in Table 4. The Gibbs free energy (ΔG°) with negative value indicates that Pb²⁺ adsorption on OPB was found to be a spontaneous process under all investigated temperatures.
The value of ΔG° gave an insight into the adsorption nature, with values ranging from −20 to 0 kJ/mol reflecting physisorption, while values in the range of −80 and −400 kJ/mol are indicative of chemisorption (Sireesha et al. 2025). Given that the range of ΔG° from this study falls in the range of physisorption, it can be inferred that the sorption of Pb2+ is also governed by physical sorption process. The positive enthalpy change (ΔH°) value confirms that the adsorption process is endothermic, which implies that energy input is required for effective adsorption (Ogundiran et al. 2022). The positive entropy (ΔS°) value infers that during adsorption, there is an increase in randomness at the solid-liquid interface, which indicates greater disorder in the system as Pb2+ is adsorbed onto the surface of the biochar.
Fig. 10. Thermodynamic plot for the uptake of Pb2+ onto OPB
Table 4. Parameters of Thermodynamic Analysis for the Sorption of Pb2+ by OPB
Regeneration and Reusability Studies
The desorption behaviour of Pb²⁺ from OPB was evaluated over five consecutive adsorption–desorption cycles as shown in Fig. 11. It was observed that the recovery efficiency was 75.2% in the first cycle and gradually decreased to 65.1% by the fifth cycle. The steady decline observed across the cycles may arise from partial blocking of some active sites on the adsorbent surface during each adsorption cycle (Ofudje et al. 2023). The ability of the adsorbent to maintain efficiencies above 65% after five cycles indicates that OPB possesses a good level of stability and reusability. This supports its potential application as a low-cost and regenerable adsorbent for Pb²⁺ removal in wastewater treatment systems.
Fig. 11. Thermodynamic plot for the adsorption of Pb2+ onto OPB
Adsorption Mechanism
The combined kinetic modelling, isotherm analysis, and thermodynamic results suggest that the uptake of Pb²⁺ onto the OPB proceeds through a multistep mechanism involving both physical and chemical interactions (Fig. 12).
Fig. 12. Schematic representation of the adsorption mechanism of Pb2+ onto OPB
These interactions allow Pb²⁺ ions to interact with the functional groups on the OPB surface. The contribution of physical uptake is reflected in the Dubinin–Radushkevich (D–R) isotherm, where the mean free adsorption energy (E = 0.495 kJ/mol) indicates that a portion of the process involves physisorption through weak electrostatic attractions. The equilibrium modelling revealed that both the Langmuir and the Freundlich models fit the data well. This combination supports a dual mechanism where Pb²⁺ ions first attach to high-affinity sites through complexation, followed by additional uptake into pores and lower-affinity regions through physical interactions. Thermodynamic results from free energy and enthalpy changes also corroborated the fact that the process is controlled by physisorption.
BET Analysis
The BET analysis gave the pore-size distribution, as revealed in Fig. 13. It showed that the orange peel biochar structure is dominated by micropores (< 2 nm), with a transition toward small mesopores as the pore diameter increased in biochars derived from lignocellulosic material. Such a distribution contributed meaningfully to a high specific surface area (72.1 m²/g), which favored the adsorption of contaminants. This aligns with a literature report on orange-peel biochar systems, where KOH activation effectively improves pore volume and microporosity (Zhang et al. 2022). The high density of narrow pores around 0.25 to 0.35 nm is likely attributed to the rearrangement of carbon domains during thermal activation, similar to those properties reported for citrus-based biochars by Nandiyanto et al. (2025).
Fig. 13. The pore size distribution analysis of orange peel biochar
Discussion on TGA
The thermogravimetric analysis (TGA) curve of orange peel showing multistage thermal degradation pattern characteristic is depicted in Fig. 14. An early small weight loss was seen below 120 °C, which is attributed to the evaporation of physically adsorbed moisture (Kordoghli et al. 2023; Mousa et al. 2025). This accounted for 5 to 10% mass loss and is consistent with the lignocellulosic biomass nature of citrus residues. A steady weight loss from 120 °C to around 250 °C reflects the decomposition of hemicellulose, which begins at lower a temperature because of its amorphous and thermally unstable nature. A huge weight-loss was seen around 250 and 350 °C, indicating hemicellulose and cellulose degradation (Kordoghli et al. 2023; Mousa et al. 2025). This sharp decline is characteristic of citrus peels, where cellulose depolymerization and the breakdown of pectin-rich cell walls lead to the release of some volatiles. The maximum rate of devolatilization is evidence from the sharpest inflection point in this region. Above 350 °C, the curve displayed a slower, and gradual mass loss up to 450 °C, which was indicative of the decomposition of lignin components (Kordoghli et al. 2023). Lignin is known to degrade within a wide range of temperature due to its heterogeneous aromatic structure, that enhanced the formation of a stable carbonaceous char. Beyond 450 °C, the remaining weight is assigned to carbon and ash content.
Fig. 14. TGA analysis of orange peel biochar
FT-IR Investigation
The orange peel biochar’s FT-IR spectra before and after Pb²⁺ sorption, as shown in Fig. 15, reveal distinct changes in the surface functional groups, confirming the participation of oxygen-containing moieties in Pb²⁺ uptake. The spectrum of OPB displays a small peak corresponding to –OH stretching at 3350 cm⁻¹, which is typical of hydroxyl and phenolic groups commonly reported in citrus-derived biochars (Zhang et al. 2022). After Pb²⁺ uptake, this peak became more intense and shifted slightly to 3388 cm⁻¹, indicating ion exchange or coordination interactions between the surface hydroxyls and Pb²⁺. The peaks seen at 1653 cm⁻¹ before and 1638 cm⁻¹ after Pb uptake are attributed to carbonyl/carboxylate and aromatic C=C groups, as well as shifts after metal sorption. This information is consistent with metal–carboxyl complexation reported for Pb²⁺ sorption on lignocellulosic biochars (Mousa et al. 2025). The bands observed at 1162 cm⁻¹ before and 1077 to 1338 cm⁻¹ after are assigned to C–O stretching and the intensity stats to decrease after adsorption, suggesting their involvement in metal binding (Nandiyanto et al. 2025). The bands seen at 550 to 610 cm⁻¹ region are assigned to metal–oxygen (Pb–O) vibrations, affirming the formation of inner-sphere Pb²⁺ complexes on the biochar surface.
Fig. 15. The FT-IR analysis of orange peel biochar before (a) and after (b) Pb2+ adsorption
SEM Analysis
Prior to adsorption (Fig. 16a), the biochar surface displayed a rough, layered, and irregular morphology with fractured sheet-like structures.
Fig. 16. The SEM images of OPB prior (a) and after Pb2+ sorption (b)
This is typical of biochar derived from lignocellulosic biomass due to heterogeneous carbon formation during thermal degradation. The presence of visible pores suggests the availability of surface features that may facilitate pollutant interaction. Following Pb²⁺ exposure (Fig. 16b), no substantial alteration in surface morphology is observed.
Effect of Competing Ions
As displayed in Fig. 17, the presence of divalent alkaline earth metals significantly suppressed Pb²⁺ uptake. The Pb²⁺ removal decreased from 78% in single-solute systems to 52% and 55% in the presence of Ca²⁺ and Mg²⁺, respectively. This suppression can be attributed to direct competition for negatively charged surface functional groups such as –OH and –COO⁻ and increased ionic strength, which lessens electrostatic attraction (Wu et al. 2021; Xu et al. 2022).
Fig. 17. Effect of competing cations, anions, and natural NOM on Pb²⁺ uptake efficiency by orange peel biochar under optimized adsorption conditions
Anionic species also reduced Pb²⁺ uptake behavior. Chloride and sulfate ions reduced Pb²⁺ removal to 47% and 44%, respectively, due to aqueous Pb–Cl and Pb–SO₄ complex formation, which lowers the concentration of free Pb²⁺ available for adsorption. Carbonate displayed a stronger inhibitory effect (38%), which may be because of the formation of stable Pb–carbonate species and surface site blocking (Chen et al. 2017; Shi et al. 2018). Surprisingly, the natural organic matter (NOM) induced the most pronounced reduction with Pb²⁺ uptake reducing to 33%. This effect can be ascribed to complexation of Pb²⁺ by humic substances in solution and pore/site blocking on the OPB surface, consistent with previous reports for biochar-based adsorbents (Song et al. 2025). These competition findings showed that although Pb²⁺ removal by OPB is reduced in complex matrices, substantial removal efficiency is retained, indicating that OPB performance extends beyond ideal laboratory conditions and remains relevant for practical water treatment applications.
CONCLUSIONS
This work evaluated the effectiveness and sustainability of biochar produced from orange peel (OPB) for the sorption of Pb2+ from aqueous solution, and the following deductions were derived:
- Orange peel biochar was found to have a porous, heterogeneous surface capable of adsorbing Pb²⁺ from aqueous solution, with adsorption performance strongly impacted by solution pH, contact time, temperature, and initial metal concentration.
- The Langmuir model indicated a maximum adsorption capacity of 73.5 mg/g, while the Freundlich model suggested a heterogeneous nature of the biochar surface.
- The low mean adsorption energy obtained from the Dubinin–Radushkevich model, combined with thermodynamic data, shows that Pb²⁺ sorption was governed mainly by physical adsorption mechanisms, including weak electrostatic interactions and pore-filling effects.
- Thermodynamic analysis revealed that the process is endothermic and spontaneous, with enhanced haphazardness at the solid-solution interface.
- The characterization results confirmed the presence of functional groups and a porous structure of the OPB which supports the adsorption process.
- Overall, the orange peel biochar (OPB) presents a low-cost, renewable and efficient material for removing Pb2+ from contaminated water, making it a promising option for sustainable remediation applications. Further investigations on fixed-bed column experiments, dynamic modeling, practical deployment and scale-up are needed for engineered water-treatment systems. In addition, testing at environmentally relevant low-concentration ranges (µg/L to low mg/L), along with validated QA/QC protocols, are important future work.
ACKNOWLEDGMENTS
Princess Nourah bint Abdulrahman University Researchers Supporting Project number (PNURSP2026R76), Princess Nourah bint Abdulrahman University, Riyadh, Saudi Arabia. The authors extend their appreciation to the Deanship of Scientific Research at Northern Border University, Arar, KSA, for funding this research work through the project number “NBU-FFR-2026-2985-03”
Authors Contributions
Ghaferah H. Al-Hazmi and Ali El-Rayyes: Contributed Resources, Software, Funding acquisition; Lamia A. Albedair: Project administration, Contributed Resources, Writing – Review & Editing; Edwin Andrew Ofudje: Conceptualization, Investigation, and supervised the work; Moamen S. Refat: Visualization, Validation, Writing – Review & Editing; Kholoud K. Alzahrani: Formal analysis, Software, Writing – Review & Editing; Oladejo Emmanuel Kola: Methodology, Data curation, Writing – Original Draft; All authors: Approval of final manuscript.
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Article submitted: November 26, 2025; Peer review completed: January 18, 2026; Revised version received: January 28, 2026; Further revised version accepted: February 16, 2026; Published: March 9, 2026.
DOI: 10.15376/biores.21.2.3831-3855