Valorization of coconut husk into biochar for lead (Pb²⁺) adsorption

3.1 Synthesis of CHBC

The synthesized adsorbent, derived from coconut husks, was prepared through a process that balances environmental sustainability and cost-effectiveness. The raw coconut husks were thoroughly cleaned with distilled and hot water to remove impurities, ensuring minimal contamination. They were then dried at 100°C for 24 h to eliminate moisture and washed with petroleum to make it free from other organic impurities. However, the burning step using petroleum raises concerns regarding environmental sustainability due to carbon emissions and potential introduction of contaminants; hence, a minor amount of petroleum is utilized to wash the impurities. The pyrolysis process, conducted at temperatures of 200°C, 400°C, and 600°C for 20 min each, effectively converted the husks into biochar with improved porosity and adsorption properties. To ensure uniform particle size for enhanced adsorption efficiency, the biochar was finely ground using a mortar and pestle for 10 min. This low-cost preparation method makes use of readily available agricultural waste, aligning with circular economy principles. Optimizing the pyrolysis conditions and exploring alternative activation techniques, such as steam or chemical activation using eco-friendly agents, could further enhance the adsorbent’s performance while maintaining its economic and environmental viability (Figure 1) [30,31,32].

Figure 1

General procedure for CHBC synthesis.

3.2 Preparation of lead nitrate solution

Pb(NO₃)₂ solution was prepared by adding 8.280 g of powdered lead nitrate into a volumetric flask and adding 250 mL of distilled water. This resulted in a 0.1 M solution. Next, 25 mL of the prepared 0.1 M Pb(NO₃)₂ solution was taken and added it to a beaker along with 25 mL of distilled water to convert it into a 0.05 M solution.

3.3 Absorption efficiency study

A 250 mL solution of 0.05 M Pb(NO₃)₂ was made and distributed into five beakers, each containing 50 mL of solution. Subsequently, CHBC was added to each beaker, with amounts of 50 mg (CHBC50), 100 mg (CHBC100), 200 mg (CHBC200), 400 mg (CHBC400), and 500 mg (CHBC500) added to their respective beakers. Following that, the beakers were positioned in an incubator shaker at room temperature for 4 h. At 30-min intervals, samples were extracted from the beakers using a pipette and transferred to Eppendorf tubes for analysis to determine the absorption efficiency of CHBC in the prepared solution (Figure 2) [31,32].

Figure 2

Lead nitrate solution (0.05 M) added with CHBC.

3.4 Temperature study

The procedure for the temperature study mimicked that of the absorption study, with the sole variation being the temperature settings in the incubator shaker [31]. One beaker was placed at room temperature, one beaker at 50°C, another one at 60°C, and all beakers were maintained for 4 h.

3.5 Concentration study

For the concentration study, two different molar concentration solutions of lead nitrate were prepared as follows:

1. A 0.025 M concentration solution of lead nitrate was prepared by combining 25 mL of 0.05 M solution with 25 mL of distilled water.

2. A 0.0125 M concentration solution of lead nitrate was then made by taking 25 mL of 0.025 M solution and adding 25 mL of distilled water.

Subsequently, the prepared molar concentration solutions underwent absorption efficiency testing. This involved placing them in an incubator shaker after adding CHBC.

3.6 pH study

The post-absorbed solution was selected for pH analysis. Samples were extracted from the beaker that exhibited the highest absorption using a pipette and then placed onto litmus paper for pH assessment.

3.7 Adsorption isotherm and kinetic studies

The adsorption isotherm and kinetic studies were performed using various isotherm and kinetic models. The isotherm models include the Langmuir adsorption isotherm, Freundlich adsorption isotherm, Temkin isotherm, Redlich–Peterson (R–P) isotherm, and Hill isotherm models. On the other hand, the kinetic models include the pseudo-first-order kinetic model (PFOM), pseudo-second-order kinetic model (PSOM), Elovich model, and intraparticle diffusion model (IDM). All the aforementioned models were fitted nonlinearly in order to find out the adsorptive parameters for CHBC toward the removal of Pb²⁺ ions. Additionally, a thermodynamic study was carried out to find out the thermodynamic parameters, including enthalpy, entropy, and Gibbs free energy, which will be discussed in detail in Section 4.

3.8 Instrumentation part

4.1 Preparation of biochar

First, we collected the coconut husks from the local vendors and washed them twice with distilled water and once with hot water to eliminate any impurities present. After that, we kept the husks in an oven at 100°C for 24 h to completely dry them. Then, the dried coconut husks were taken and partly burned using petroleum as a part of slow pyrolysis. The partly burned coconut husk will be placed in a muffle furnace at temperatures of 200°C, 400°C, and 600°C. Each temperature will be maintained for 20-min intervals, totaling 1 h of pyrolysis. This will help to eliminate any impurities which may be present due to the addition of petroleum and also lead to the formation of CHBC. The prepared biochar was then ground for 10 min to make it a fine powder by using a mortar and pestle (Figure 3).

Figure 3

Working procedure for CHBC production.

4.2 Characterization

4.2.1 SEM

Using a scanning electron microscope, the high-resolution, three-dimensional (3D) surface morphology of biochar can be precisely observed. As illustrated in Figure 4, the prepared biochar illustrates a highly porous structure, forming a mesh-like structure which suggests that the biochar offers high porosity for the adsorption of target analyte. For biochar, the formation of numerous micropores is prominent within the temperature range of 400–500°C. On addition of Pb²⁺, the structure undergoes collapse and fragmentation. This variation in structural integrity underscores the participation of surface functional groups in complex formation with Pb²⁺.

Figure 4

SEM images of biochar (a) pre- and (b) post-adsorption.

4.2.2 XRD

XRD was done to analyze the pattern of CHBC both pre- and post-adsorption over a range of 2θ = 0–100°. In Figure 5a, the pre-adsorption XRD pattern (highlighted) indicates the presence of carbonaceous graphite (2θ = 28.496°) and silica (2θ = 40.670°). Pre-adsorbed CHBC suggests that its nature is amorphous due to the lack of sharp peaks. However post-adsorption CHBC, XRD pattern (highlighted) indicates the participation of biochar in complex formation with Pb²⁺ ions (Figure 5b). The complex formation leads to an increase in the crystallinity of biochar from 56.4% to 64.3%. The crystalline behavior of the complexes was attributed toward different kinds of interaction of the functional groups present on the biochar with potentially toxic ions.

Figure 5

XRD patterns of (a) pre-adsorption CHBC and (b) post-adsorption CHBC.

4.2.3 FTIR analysis

Using FTIR spectroscopy, the functional groups present on the surface of both pre- and post-adsorption CHBC were identified. The functional groups present in the pre-adsorption CHBC are shown in Figure 6a. The presence of the –OH functional group is indicated by hydroxyl stretching vibration, in the range between 3,600 and 3,200 cm⁻¹ [35]. The peak between 1,600 and 1,500 cm⁻¹ wavenumbers signifies the stretching vibration of the –COOH functional group and that at 1,642 cm⁻¹ wavenumber indicates the presence of the –NH functional group. However, in Figure 6b, the post-adsorption CHBC graph shows that all the functional groups that are present in the pre-adsorption CHBC participated in complex formation with Pb²⁺ and that all the peaks diminished in post-adsorption CHBC.

Figure 6

FTIR spectra of (a) pre-adsorption CHBC and (b) post-adsorption CHBC.

4.3 Point-of-zero charge (pH_zpc)

The pH_zpc of the adsorbent was evaluated through the salt addition method to analyze its surface properties and the influence of pH on the adsorption mechanism. A 0.01 M KNO₃ solution was used as the background electrolyte, and the initial pH (pH₀) of the solution was adjusted between 2 and 10 using 0.1 M HCl or 0.1 M NaOH. A 100 mL aliquot of the pH-adjusted solution was mixed with 20 mg of adsorbent in an Erlenmeyer flask and stirred using a magnetic shaker for 48 h at room temperature to ensure equilibrium. The final pH (pH_f) was measured, and a plot of pH_f versus pH₀ was constructed. The intersection of the curve with the line pH_f = pH₀ was identified as the pH_zpc.

For Pb–R-BC, the pH_zpc was found to be 6.0. When the pH is lower than the pH_zpc, the surface of the adsorbent acquires a positive charge due to the protonation process, which results in decreased adsorption of Pb²⁺ as H⁺ ions compete for the available active sites. Conversely, as the pH increases, H⁺ ion concentration decreases, leading to surface deprotonation and an increase in available binding sites. At pH values above pH_zpc, the adsorbent surface acquires a negative charge, enhancing electrostatic attraction and facilitating the adsorption of cationic Pb²⁺ ions (Figure 7).

Figure 7

pH_zpc for the adsorbent toward the removal of Pb²⁺ ions.

4.4 Removal of Pb²⁺ ions

For the removal of Pb²⁺ ion from the water sample, initially 50 mL of 0.05 M Pb(NO₃)₂ solution was taken in five separate beakers followed by the addition of 50 mg (CHBC50), 100 mg (CHBC100), 200 mg (CHBC200), 400 mg (CHBC400), and 500 mg (CHBC500) of CHBC in the beakers, respectively. After that, the reaction mixture was maintained at room temperature for 4 h in an incubator shaker. The sample for analysis was collected after every 30 min to monitor the adsorption progress. The obtained sample was spun at 8,000 rpm for 10 min in order to perform the spectroscopic examination, and the supernatant was used to analyze the reaction’s progress. The adsorption results signify that 500 mg of BC would show the maximum adsorption efficiency in comparison to other doses. In addition, the influence of other factors, such as temperature, pH, and concentration of metal ions, was also examined.

In order to evaluate the effect of concentration of potentially toxic ions, varying concentrations of lead nitrate were prepared, i.e., 0.05 M, 0.025 M, and 0.0125 M, and the analysis was performed using 500 mg of CHBC under ambient conditions, which indicates that CHBC500 showed the best adsorption at 0.0125 M Pb²⁺.

Additionally, the effects of temperature on CHBC500 adsorption on Pb²⁺ were examined at three distinct temperature levels: room temperature, 50°C, and 60°C. Room temperature produced the best results. From all the optimization studies, maximum adsorption was found under optimal conditions, which include 500 mg of CHBC dose, 0.0125 M Pb²⁺ ions, and pH 6 at ambient temperature (Table 2).

4.5 Adsorption isotherm study

4.5.2 Freundlich isotherm model

As demonstrated by Eq. 3, the Freundlich isotherm model states that a logarithmic decrease in the enthalpy of adsorption correlates with an increase in the number of occupied sites [45]. The adsorption of heterogeneous surfaces is the fundamental process underlying this isotherm. The Freundlich equation is represented as follows:

(3) ln q e = I n k F + 1 n ln c e

Here, “k _F” stands for the Freundlich constant and “n” is the parameter indicating the linearity and type of the adsorption process. The notion of linear adsorption is that chemical interactions take place when n = 1, and physical interactions predominate when n > 1 [35]. The value of a competitive adsorption process is less than one, whereas the value of a cooperative adsorption process is larger than one. With a correlation constant of R ² = 0.8, the 1/n number derived from the study’s results suggests that the adsorbents’ surface appears more unequal (Table 3, Figure 8).

Figure 8

(a) Isotherm study: Langmuir isotherm, Freundlich isotherm, Temkin isotherm, Hill isotherm, and R–P isotherm models; (b) kinetic study: PSOF, PSOM, Elovich model, and IDM.

4.6 Adsorption kinetic study

4.7 Pseudo-second-order kinetics

According to the PSOM, the square of the number of accessible sites determines how quickly sorption occurs [35]. A mathematical description is provided by Eq. 6

Then, the values of q _e (adsorption equilibrium) (mg·g⁻¹) and k ₂ (constant) (g mg·min⁻¹) were determined using the slope and intercept of the graph q _t vs t. Table 4 provides details on these values with a correlation coefficient of R ² = 0.866 (Table 4, Figure 8b).

4.8 Thermodynamic parameters

Thermodynamic parameters, including Gibbs free energy (ΔG°), enthalpy (ΔH°), and entropy (ΔS°), play a crucial role in evaluating the spontaneity and heat exchange associated with the adsorption process 130. These parameters can be determined using the equilibrium constant (KCK_CKC), which reflects the adsorbent’s capacity to retain the adsorbate and the extent of its mobility in the solution. The experimental thermodynamic data were calculated at varying temperatures.

The standard Gibbs free energy change (ΔG°) was determined using the following equations:

where R is the universal gas constant (J·mol⁻¹), and T represents the absolute temperature (K). The equilibrium constant (K _C) is defined as the ratio of equilibrium concentration of the adsorbate on the adsorbent (q _e) to its equilibrium concentration in the solution (C _e):

Furthermore, the van’t Hoff equation was employed to determine the enthalpy and entropy changes:

A linear plot of ln K _C versus 1/T (Figure 9a) enables the calculation of ΔH° and ΔS° from the slope and intercept, respectively. The absolute values of ΔG° decrease with increasing temperature, indicating that the adsorption process becomes less thermodynamically favorable at elevated temperatures. The negative enthalpy change (ΔH° = −23743.3 kJ·mol⁻¹) confirms that the adsorption process is exothermic. Moreover, a positive ΔS° (ΔS° = 52.86) value indicates an increase in disorder at the solid–liquid boundary during adsorption, reflecting a greater level of randomness within the system. These results align with earlier research where poly(methyl methacrylate)/graphene oxide and its nanocomposite successfully adsorbed the malachite green dye, further highlighting the exothermic characteristic of the process. Apart from this, in order to check the potential of CHBC for lead removal, an anti-interference study was conducted in the presence of other potentially competing metal ions, including Fe²⁺, Fe³⁺, Ni²⁺, Co²⁺, Zn²⁺, Cd²⁺, Hg²⁺, Mn²⁺, Cu²⁺, and K⁺ (Figure 9). The results demonstrated that the absorption response of the CHBC and PB²⁺ remained largely unaffected by the presence of these interfering ions, indicating a strong affinity and selective interaction between CHBC and Pb²⁺ ions. Furthermore, adsorption studies confirmed that CHBC exhibits high selectivity for Pb²⁺ removal, reinforcing its potential as an effective adsorbent for lead-contaminated water treatment. The minimal impact of competing metal ions suggests that the adsorption mechanism is predominantly driven by specific binding interactions and surface characteristics, making CHBC a promising material for selective Pb²⁺ remediation (Figure 9b) [45,49].

Figure 9

(a) Thermodynamic parameter analysis for the adsorbent to eliminate lead ion; (b) interference study of the competing ions.

4.9 Regeneration and reusability of spent adsorbents for Pb²⁺ removal

The recovery and repurposing of used adsorbents are essential for evaluating the reversibility of the adsorption process and improving its cost efficiency. In this research, several desorbing agents, such as acids, bases, and salts at varying concentrations, were tested for their effectiveness in regenerating the biochar. Of all the agents tested, 0.1 M NaOH was identified as the most effective desorbing agent. The efficiency of NaOH in regenerating the adsorbents can be attributed to the predominant presence of anionic lead species at pH > 7, which exhibit lower affinity for negatively charged adsorbent surfaces. Given the zero-point charge values of biochar, their surfaces become negatively charged above pH 7, facilitating the desorption of Pb²⁺ species under basic conditions. A 500 mg sample of used biochar was added to a 100 mL conical flask, followed by the introduction of 20 mL of 0.1 M NaOH and 20 mL of distilled water. The mixture was stirred at a temperature of 30°C for 15 min and then allowed to settle. The supernatant was filtered, and the adsorbent was rinsed with distilled water until a neutral pH was achieved. The adsorbent was then dried at 105°C and allowed to cool to room temperature before being utilized again for adsorption experiments. This regeneration process was repeated multiple times, and the Pb²⁺ removal efficiency was assessed after each cycle. The results indicate a gradual decline in Pb²⁺ removal efficiency with successive regeneration cycles. However, the removal efficiency remained above 80% even after five regeneration cycles, demonstrating the potential reusability of biochar. The percentage removal of Pb²⁺ ions across multiple regeneration cycles was as follows: 56.53%, 54.0%, 54.3%, 52.6%, and 51.2% (for cycles 1–5, respectively).