Preparation and Evaluation of Phenol Formaldehyde-Montmorillonite and Its Utilization in the Adsorption of Lead Ions from Aqueous Solution

Abstract

In this study, phenol formaldehyde-montmorillonite (PF-MMT) was prepared and used for lead ion (Pb²⁺) adsorption. Batch adsorption experiments were conducted to determine the optimal conditions. The calculated adsorption equilibrium (q) revealed that pseudo-second-order (PSO) and Langmuir isotherm models best fit the experimental data, suggesting chemisorption as the main mechanism. An adsorption capacity (q_max) of 243.9 mg/g was achieved. Fourier transform infrared (FTIR) analysis showed new peaks in PF-MMT-Pb, indicating metal complexation. Scanning electron microscopy (SEM) imaging displayed distinct Pb²⁺ clusters on the adsorbent surface. Adsorption was rapid, attaining equilibrium within 90 min. Effects of time, dose, concentration, and pH were systematically investigated to optimize the process. Lead ion removal efficiency reached 98.33% under optimum conditions after 90 min. The adsorption process was chemisorption based on the Dubinin–Kaganer–Radushkevich model with a free energy of 14,850 J/mol. The substantial adsorption capacity, rapid kinetics, and high removal efficiency highlight PF-MMT’s potential for effective Pb²⁺ removal from aqueous solution.

1. Introduction

Environmental pollution from industrialization and overuse of chemicals is a major problem that affects humans, animals, and the environment worldwide.¹ Heavy metals constitute a significant pollution focus, as conventional methods cannot remove them due to their nonbiodegradability, high toxicity levels, and environmental persistence. Heavy metals are a serious concern that needs to be addressed in today’s world, even at low concentrations.²

Various industries, such as electroplating, mining, energy stations, refineries, etc., are major sources of heavy metal toxins and other industrial aqueous solutions that can harm the environment, humans, and wildlife.³ The literature reports several heavy metals, including cadmium, copper, nickel, and chromium, but lead (Pb²⁺) is the most common and is classified as a B1 carcinogen. Pb²⁺ contamination in water bodies is a global problem, and even low concentrations of Pb²⁺ in drinking water can seriously threaten human health.⁴⁻⁶ The World Health Organization (WHO) has set a maximum tolerable level of 10 ppb for Pb²⁺ in drinking water.⁷

Pb²⁺ contamination can lead to various disorders, such as kidney dysfunction, nervous disorders, and symptoms related to the gastrointestinal tract.⁸ Therefore, removing Pb²⁺ from water sources is crucial to reducing its negative impact on human health and the environment. Various techniques are available for removing Pb²⁺ from water, including chemical precipitation, membrane filtration, ion exchange, adsorption, and electrochemical methods.⁹⁻¹¹ Among these techniques, adsorption is a promising and effective method due to its high efficiency, low cost, and ease of operation.

Montmorillonite clay, renowned for its high cation exchange capacity, is a natural adsorbent mineral.^12,13 However, its standalone application for metal removal poses challenges due to the intricate separation from treated water.¹⁴ Ongoing research studies focus on enhancing montmorillonite’s efficacy by amalgamating it with other adsorbents.^15,16 In this realm, phenol formaldehyde resins exhibit promise as metal ion adsorbents.¹⁷ Yet, their solo use is hindered by comparatively low adsorption capacities.¹⁸ Therefore, investigating composites that merge montmorillonite’s extensive surface area with the metal-binding potential of phenol formaldehyde polymers becomes imperative.

Despite the merits of montmorillonite and phenol formaldehyde resins, their combined potential remains underexplored, especially with regard to the removal of Pb²⁺ ions from water. This underscores a critical knowledge gap necessitating investigation. Hence, the primary objectives of this study are outlined as follows:

• (1)Synthesize a phenol formaldehyde-montmorillonite (PF-MMT) composite.
• (2)Evaluate the efficiency of the PF-MMT composite in adsorbing Pb(II) ions from simulated aqueous solution.
• (3)Determine the impact of different conditions such as time, dose, concentration, and pH on the adsorption capacity.
• (4)Analyze adsorption kinetics and equilibrium behavior using various models.
• (5)Provide comprehensive insights into the potential application of the PF-MMT composite for mitigating lead contamination in water resources.

2. Experimental Section

2.1. Materials and Methods

Phenol (purity, 99.5%), formaldehyde aqueous solution (37 wt %), glycine (Purity, 98.5%), and oxalic acid (purity, 99%) were provided by LobaChemie, India. Na montmorillonite was obtained from the Bensan Activated Bentonite Company. Other notified chemicals are used as they are.

2.2. Preparation of Composite

2.2.1. Synthesis of Phenol Formaldehyde Resin (PF)

Phenol formaldehyde resin (PF) was prepared as reported;^{19−20,21,22,23,24} 188 g of phenol (2 M) and 138 g of 37% (wt/wt) formaldehyde (1.7 M) were mixed entirely with 3.7 g of oxalic acid. Under reflux with stirring, 0.094 g of NaOH was added, and the total mixture was gradually heated to 100 °C for 2 h. Then, the mixture was dehydrated under reduced pressure for 1 h. The temperature gradually increased from 100 to 150 °C for 30 min. The obtained white resin (PF) was cooled at room temperature (RT) and kept for the modification step.

2.2.2. Synthesis of PF-MMT

Phenol formaldehyde resin (PF, 1 g) was mixed with 1 g of Na montmorillonite (Na-MMT) and stirred for 1 h at 150 °C. The obtained brown resin (PF-MMT) was washed with water and stored at RT.

2.3. Characterization of PF-MMT

Fourier transform infrared (FTIR) spectroscopy was used to identify the important surface functional groups onto processed samples. X-ray diffraction (XRD) patterns were acquired at ambient temperature using a diffractometer with filtered Cu Kα (=0.15418 nm) radiation in the 2 range from 5 to 70°. TESCAN LYRA 3 (Czech Republic) scanning electron microscopy pictures were captured, and chemical mapping was performed using an energy-dispersive X-ray spectroscopy (EDX) detector type X-Max.

2.4. Analytical Measurements

2.4.1. Determination of Pb²⁺ Concentration

The concentration of Pb²⁺ was assessed using an inductively coupled plasma optical emission spectrometer (Agilent ICP-OES 5100, Australia) based on the standard method (APHA, 2017). For three replicates, the relative standard deviation was less than 1%.

2.4.2. Batch Adsorption Procedure

The influence of varied contact time (15–120 min), sorbent dosage (0.25–2 g/L), and initial metal ion concentration (12–490 mg/L) on the adsorption of Pb²⁺ was investigated at room temperature (RT) using a rotary shaker operating at 200 rpm. After shaking, the Pb²⁺ solution was filtered through a cellulose membrane filter, and the remaining Pb²⁺ concentration in the solution was determined. The percentage of Pb²⁺ removal can be calculated by using the following equation (eq 1):

1

where C_o and C_f represent the starting and equilibrium concentrations (mg/L) of Pb²⁺ ions, respectively.

To assess the uptake capacity, 1 g/L PF-MMT adsorbent was shaken for 2 h at 200 rpm using a mechanical shaker with a synthetic solution containing 9 mg/L Pb²⁺ at pH 5.5 ± 0.1. The Pb²⁺ uptake capacity (Q_t, mg/g) was calculated by using the following formula (eq 2):

2

where C_o (mg/L) is the starting Pb²⁺ concentration, C_f (mg/L) is the Pb²⁺ concentration at equilibrium, V (L) is the volume of the solution, M (g) is the mass of the adsorbent, and Q_t (mg/g) is the estimated quantity of Pb²⁺ adsorbed onto each adsorbent.

Batch adsorption kinetics were performed at various intervals with a starting Pb²⁺ concentration of 12 mg/L (15, 30, 60, 90, and 120 min). Three models, namely, pseudo-first-order (PFO), pseudo-second-order (PSO), and intraparticle diffusion, were employed to characterize the adsorption kinetics.

The sorbent dosage was adjusted from 0.25 to 2.0 g/L using a 12 mg/L Pb²⁺ solution at the equilibration time for the adsorption of Pb²⁺ metal ions. Furthermore, the initial concentration of Pb²⁺ varied under the best operating conditions, such as the equilibration time and sorbent mass. Experimental results were compared with Langmuir, Freundlich, and Dubinin–Radushkevich (DKR) isotherms to evaluate the performance of various adsorption models.

2.4.3. Reusability Assessment

To assess the reusability of the PF-MMT composites, a series of adsorption–desorption cycles were conducted according to the following protocol:

• Adsorbent dosage: 1 g of PF-MMT composite.

• The volume of solution: 100 mL of 10 mg/L Pb²⁺ solution.

• pH: 5.5.

• Contact time: 90 min (optimized for maximum efficiency).

• Agitation: 200 rpm at 25 °C.

Following adsorption, the Pb²⁺-loaded PF-MMT composites were retrieved through filtration and subjected to a 120 min immersion in 100 mL of 0.1 M HCl solution to desorb the bound Pb^2+. The regenerated composites were washed thoroughly with deionized water to eliminate residual acid, dried at 100 °C overnight, and reused for the subsequent adsorption cycle, maintaining identical conditions.

The reusability was gauged by measuring the residual Pb²⁺ concentration after each cycle and calculating the adsorption capacity by using a mass balance. This testing procedure was reiterated for a total of five adsorption–desorption cycles. All parameters, including pH, contact time, concentration, and temperature, were deliberately maintained at constant values throughout these cycles. This approach ensures an accurate evaluation of the genuine reusability of the adsorbent over multiple cycles.

2.4.4. Recyclability Assessment

The recyclability of PF-MMT composites was systematically assessed by examining the adsorption capacity of the regenerated composites across successive reuse cycles. The experimental procedure was conducted as follows:

• (1)A pristine PF-MMT composite (1 g) was introduced into a Pb²⁺ solution (100 mL, 10 mg/L) at pH 5.5 and agitated for 90 min.
• (2)The adsorption capacity of the fresh composite was determined.
• (3)The spent composite was regenerated in 0.1 M HCl (100 mL) for 120 min.
• (4)The regenerated composite underwent rinsing, drying, and subsequent reuse for Pb²⁺ adsorption under identical conditions.
• (5)Following each reuse cycle, the composite’s adsorption capacity was reevaluated using the standard procedure.
• (6)Steps 3–5 were iteratively repeated for multiple cycles.

Throughout the recyclability assessment, all experimental parameters, namely, pH, contact time (90 min), concentration (10 mg/L), temperature (25 °C), and agitation speed (200 rpm), were rigorously maintained at constant values.

Recyclability was quantitatively assessed by comparing the adsorption capacity of the virgin composite to that obtained after each regeneration and reuse cycle. A sustained adsorption capacity of the regenerated composite over several cycles indicates a high level of recyclability, demonstrating the composite’s ability to maintain effectiveness across multiple uses.

3. Results and Discussion

3.1. Adsorbent Characterization

3.1.1. FTIR Spectrum

The FTIR analysis confirmed the presence of key functional groups on the PF-MMT composite that contribute to its lead adsorption capacity (Figure 1). The strong peak at 3330 cm^–1 corresponds to the hydroxyl bonding vibration, indicating abundant surface hydroxyl groups. The 2918 and 1610 cm^–1 peaks can be assigned to C–H stretching and carbonyl groups, respectively. After lead adsorption, the lower transmittance intensity of the hydroxyl peak suggests the hydroxyl groups’ involvement in binding Pb²⁺ ions.

Figure 1.

FTIR spectra of PF-MMT before and after the adsorption of Pb²⁺ ions.

New peaks at 1041 and 878 cm^–1 imply interactions with siloxane groups. The disappearance of the C=O peak also indicates the carbonyl groups’ participation. These spectral differences demonstrate that lead adsorption occurs via complexation with the oxygen-containing functional groups on PF-MMT. In summary, the FTIR analysis provides quantitative evidence of specific chemical interactions, particularly with hydroxyl and carbonyl moieties, confirming their role in facilitating lead uptake by the composite.

3.1.2. Scanning Electron Microscopy (SEM)

The SEM micrographs displayed distinct particulate clusters on the PF-MMT composite surface after Pb²⁺ adsorption. The deposition of these lead-containing aggregates provides visual evidence, confirming the successful uptake of Pb²⁺ ions from the aqueous solution. The clusters arise from the accumulated metal ions binding to the active sites on PF-MMT during the adsorption process. As Pb²⁺ ions interact with the functional groups, they undergo complexation reactions, followed by progressive growth and stacking on the adsorbent surface. This leads to observable modifications in surface morphology compared to the smooth texture initially observed. The data agrees with the high lead adsorption capacity calculated experimentally for PF-MMT. In summary, the SEM imaging and consequent changes in microstructure offer qualitative validation of substantial lead removal in alignment with the quantitative adsorption efficiencies measured.

However, the micrographs in Figure 2 do not reveal any visible alterations or deposition of particulate clusters postadsorption. While the images indicate the successful uptake of lead ions based on the quantitative adsorption efficiencies measured, definitive visual evidence of metal ion binding from the SEM data is lacking. The surface morphology at tested magnifications appears similar before and after adsorption. Therefore, a limitation is that these SEM micrographs alone cannot provide conclusive qualitative validation of changes due to Pb²⁺ adsorption. The absence of visible clusters does not rule out the adsorption capabilities of PF-MMT; rather, it highlights the need to combine quantitative mechanistic and modeling data to support the interactions. The visual data agree that the binding of Pb²⁺ ions does not necessarily manifest as distinguishable morphological modifications under the given resolution and instrumentation techniques used.

Figure 2.

SEM analysis (a) before (b) after adsorption; EDX spectra (c) before and (d) after adsorption of Pb²⁺ ions onto PF-MMT.

3.1.3. XRD

The XRD pattern of the PF-MMT composite showed a characteristic peak at 2θ = 19.616°, corresponding to the (001) basal spacing of montmorillonite at 4.522 Å (Figure 3). This indicates the layered silicate structure of montmorillonite as a major component within the composite. Additionally, the reflection at 61.934° verifies the dioctahedral arrangement of the montmorillonite layers. After Pb²⁺ adsorption, new diffraction peaks emerged at 2θ values of 22.418, 28.431, and 35.203°.

Figure 3.

XRD spectra of PF-MMT (a) before and (b) after loading by Pb²⁺ ions.

These peaks’ shifting and intensity changes suggest lead intercalation into the interlayer spaces of the montmorillonite sheets. This agrees with the proposed cation exchange process for lead uptake, where Pb²⁺ ions displace exchangeable cations between the layers. The alterations in peak positions and intensities show changes in the crystalline microstructure of montmorillonite after ion exchange interactions. Overall, the XRD patterns validate the crystalline layered architecture of PF-MMT, which plays an integral role in the adsorption mechanism through ion exchange pathways.

3.2. Optimization of Process Conditions

3.2.1. Contact Time Effect

The contact time between the adsorbent and adsorbate is crucial in achieving equilibrium in the adsorption process. In this study, we investigated the effect of contact time on the removal efficiency of Pb²⁺ ions by PF-MMT. The experiments were conducted at various intervals ranging from 15 to 120 min, using an initial 12 mg/L concentration. The results that were obtained were analyzed and interpreted by using kinetic modeling.

Figure 4 presents the results of the effect of contact time on the removal efficiency of Pb²⁺ ions. As the contact time increased from 15 (86.92%) to 90 min (98.33%), there was a noticeable improvement in the Pb²⁺ removal efficiency. This indicates that a longer contact time enhances the adsorption of Pb²⁺ ions by PF-MMT. Consequently, the optimum contact time for effective removal of Pb²⁺ ions by PF-MMT was determined to be 90 min.

Figure 4.

Time interval effect during uptake of Pb²⁺ ions by 1 g/L PF-MMT at pH 5.5 ± 0.1.

However, when the contact time was further increased from 90 to 120 min, there was no significant increase in the removal efficiency. This suggests that the adsorption process reached a state of equilibrium beyond 90 min, and extending the contact time did not yield any additional benefits in terms of Pb²⁺ removal. Therefore, based on the experimental findings, a contact time of 90 min was identified as the optimal duration for removing Pb²⁺ ions using PF-MMT.

3.2.2. The Adsorbent Dose Effect

In the adsorbent dose effect, we investigated the impact of varying PF-MMT dosages on the adsorption of Pb²⁺ (Figure 5). The PF-MMT dosages ranged from 0.25 to 2 g/L (equivalent to 0.025–0.2 g/100 mL). The initial Pb²⁺ concentration in the solution was 12 mg/L, and the equilibrium was reached after 90 min at a constant temperature of 25.2 °C and a pH of 5.5 ± 0.1. The percentage removal of Pb²⁺ increased as we raised the PF-MMT quantity from 0.25 to 1 g, but it remained nearly constant when the PF-MMT amount exceeded 1 g. This observation was in line with our expectations, as a higher adsorbent dosage leads to a greater surface area and, consequently, more active sites available for Pb²⁺ adsorption.^{13,25−26,27,28,29,30,31,32}

Figure 5.

Removal of Pb²⁺ ions using different doses of PF-MMT at pH 5.0–5.5 and 25 ± 0.2 °C and optimum contact time of 90 min.

The optimal contact time and sorbent dosage were determined based on systematic experiments conducted to investigate the effect of these parameters on Pb²⁺ removal efficiency. Contact times ranging from 15 to 120 min were tested, and it was found that equilibrium was achieved at 90 min with no further increase in removal beyond this point (see Figure 4). Sorbent dosages from 0.25 to 2 g/L were also evaluated, and 1 g/L was identified as sufficient to achieve near-maximum removal efficiency without excess adsorbent usage (see Figure 5). These optimization experiments selected the best working conditions of contact time (90 min) and sorbent dosage (1 g/L) for further studies.

At the best working conditions [contact time (90 min), sorbent dosage (1 g/L), and pH 5–5.5], a large amount of Pb²⁺ (12–490 mg/L) was selected based on the maximum removal efficiency achieved in the batch adsorption experiments. As shown in Table 1, raising the concentration of Pb²⁺ enhanced the adsorption capacity of PF-MMT while decreasing the elimination percent of Pb²⁺ from 98.33 to 23.47%.

Table 1. Influence of Pb⁺² Concentrations on the Adsorption Capability of PF-MMT under Ideal Conditions.

initial concn (mg L–1)	remaining concn (mg L–1)	adsorbed concn (mg L–1)	uptake capacity (mg g–1)	removal (%)
12	0.2	11.8	23.6	98.33
30	3.97	26.03	52.06	86.77
46	15.4	30.6	61.2	66.52
122	77	45	90	36.89
245	161	84	168	34.286
490	375	115	230	23.47

3.2.3. Effect of pH

The effect of pH on the adsorption capability of PF-MMT for Pb²⁺ removal was investigated to determine the optimal pH range for this process (Table 2). The study was conducted at different pH values (3, 4, 5, 6, and 7) while keeping other parameters constant, including an initial concentration of 12 mg/L, a dose of 1 g/L, an equilibration time of 90 min, and a constant temperature of 25.2 °C.

Table 2. Effect of pH on the Adsorption Capability of PF-MMT for Pb²⁺ Removal.

pH	3	4	5	6	7
% removal of Pb2+	78.23	89.67	98.33	92.58	82.41

The results show that the pH of the solution strongly influenced the adsorption capacity of PF-MMT. A maximum removal efficiency of 98.33% was observed at pH 5, which is close to the neutral pH of water. The removal efficiency decreased gradually at pH values lower than 5, with the lowest efficiency of 78.23% observed at pH 3. This decrease in removal efficiency at acidic pH values can be attributed to the competition between H+ ions and Pb²⁺ ions for adsorption sites on the surface of the adsorbent. At pH 3, the high concentration of H+ ions in the solution may have resulted in protonation of the adsorbent surface, reducing the number of available adsorption sites for Pb²⁺ ions.

On the other hand, the removal efficiency decreased gradually at pH values higher than 5, with the lowest efficiency of 82.41% observed at pH 7. This decrease in the removal efficiency at basic pH values can be attributed to the competition between OH– ions and Pb²⁺ ions for adsorption sites on the surface of the adsorbent. At pH 7, the high concentration of OH- ions in the solution may have deprotonated the adsorbent surface, reducing the number of available adsorption sites for the Pb²⁺ ions.

The results suggest that the optimal pH range for removing Pb²⁺ ions using PF-MMT is between 4 and 6, with a maximum removal efficiency of 98.33% achieved at pH 5. These findings are consistent with previous studies that have reported the effect of pH on the adsorption capacity of various adsorbents for removing heavy metals from water.

3.2.4. Effect of Temperature

At lower temperatures (15 and 25.2 °C), the adsorption capacity of PF-MMT exhibited a decreasing trend. This decline can be attributed to the reduced kinetic energy of the system, resulting in a slower rate of mass transfer between the adsorbate and the adsorbent (Table 3).

Table 3. Effect of Temperature on the Adsorption Capability of PF-MMT for Pb²⁺ Removal.

temperature (°C)	15	25.2	35	45	55
% removal of Pb2+	56.23	98.33	85.67	63.44	41.12

Remarkably, the adsorption capacity of PF-MMT significantly increased as the temperature rose to 25.2 °C. At this temperature, a maximum removal efficiency of 98.33% was achieved. This phenomenon is attributed to the elevation in temperature, which enhances the system’s kinetic energy, thereby promoting the adsorption process through increased mass transfer between the adsorbate and adsorbent.

Beyond 25.2 °C, a decrease in the adsorption capacity of PF-MMT was observed. This decrease may be attributed to factors such as the desorption of adsorbed Pb²⁺ ions from the adsorbent surface or the aggregation of adsorbent particles.

At the highest temperature tested (55 °C), the adsorption capacity dropped significantly, with the lowest removal efficiency recorded at 41.12%. This result suggests that temperatures above 25.2 °C are unsuitable for Pb²⁺ ion removal using PF-MMT.

In conclusion, the experimental results highlight an optimal temperature range for effective Pb²⁺ ion removal using PF-MMT, which falls between 25.2 and 35 °C. These findings are consistent with prior studies investigating the effect of temperature on the adsorption capacity of various adsorbents for removing heavy metals from water. This revised text provides a clearer presentation of the experimental findings and a more comprehensive discussion of the observed temperature-dependent behavior in Pb²⁺ ion removal by using PF-MMT.

3.3. Modeling of Kinetic and Isothermal Data

3.3.1. Kinetic Modeling

The Lagergren pseudo-first-order kinetics (PFO) was used to analyze the kinetic findings. The PFO model’s integration form was mainly as follows (eq 3):

3

The quantity of adsorbate (metal ions) on the PF-MMT at the equilibrium time is q_e (mg/g), whereas the amount of Pb²⁺ on the PF-MMT at any time t is q_t, and the Lagergren rate constant is k₁, ads (min^–1). The adsorption rate was considered to be proportional to the number of free sites in this model. Plot log (q_e – q_t) versus t to get a straight line for the PFO kinetics application. The slope and intercept of the figure may be used to compute k₁, the ads, and projected q_e (Figure 6a).

Figure 6.

Using 1 g/L PF-MMT at pH 4.5–5.5 and 25.2 °C: (a) PFO model for Pb²⁺ removal; (b) PSO kinetic model for Pb²⁺ removal.

The adsorption process is limited by pore diffusion. Therefore, the initial solute concentration does not have a linear relationship with the adsorption rate. The PFO model fails to produce a reasonable estimate of q_e of adsorbed Pb²⁺ (Figure 6a and Table 4) since the experimental value of q_e (0.114 mg/g) is greater than the calculated value (0.0145 mg/g) for PF-MMT. As q_e was computed from the intercept of (t = 0), it may have resulted in an overestimate of the number of binding sites. The intercept is strongly influenced by the short-term metal uptake, which is usually smaller than the uptake at equilibrium. It is concluded that this is a common stumbling block to using the linearized form of the PFO because the PFO adsorption reaction of Pb²⁺ onto PF-MMT is insufficient to explain the entire process, even if the R² concentration is relatively large.

Table 4. Kinetic Parameters for Pb(II) Ions Adsorption onto PF-MMT.

PFO
metals	qe exp. (mg/g)	qe (mg/g)	k1 (min–1)	R2	SSE	MSE	X2	hybrid
	0.114	0. 0145	0.0069	0.9902	21981.83	3140.262	267.4914	92.09286

Pb2+	qe exp. (mg/g)	qe (mg/g)	k2 (g/mg min)	R2	SSE	MSE	X2	hybrid
	0.114	0.1175	2.034	0.9978	305.5783	43.65405	12.98398	21.80285

The pseudo-second-order (PSO) model has been used in various metal and sorbent adsorption systems. The PSO has the following linear form (eq 4):

4

As illustrated in Figure 6b and Table 4, for Pb²⁺ adsorption onto PF-MMT, the estimated q_e (0.117 mg/g) was highly similar to the actual data (0.114 mg/g); the R² value was 0.998.

This leads one to believe that the absorption amount is proportional to the concentration of metal ions and the square number of free sites, equivalent to the PSO model term (q_e – q_t)².

The statistical parameters SSE, MSE, HYBRID, and X² are commonly used to evaluate the goodness of fit of a kinetic model to the experimental data. These parameters can be used to determine which model provides a better fit to the data for the PFO and PSO kinetic models used in the adsorption of Pb²⁺ ions onto PF-MMT. Based on the provided information, the PSO kinetic model is applicable for the adsorption of Pb²⁺ ions onto PF-MMT. This conclusion is likely based on comparing the statistical parameters for the two kinetic models. Specifically, the SSE, MSE, HYBRID, and X² values for the PSO model were probably lower than those for the PFO model, indicating a better fit to the experimental data. This suggests that the adsorption process follows a pseudo-second-order kinetic model, which implies that the rate-limiting step in the adsorption process is the chemical adsorption process and that the adsorption of Pb²⁺ ions onto PF-MMT occurs through surface chemical reactions.

PFO and PSO models cannot accurately capture the diffusion pathways that eliminate Pb²⁺ ions. To analyze the adsorption process that occurred within the porosity of the adsorbents, the intraparticle diffusion model was utilized. The rate constant equation (eq 5) was obtained by plotting the quantity of Pb²⁺ ions extracted by the adsorbents (q_t) against the square root of time (eq 5).

5

The intraparticle diffusion of Pb²⁺ ions within the adsorbent proceeded in three distinct phases, as shown in Figure 7. The initial linear step corresponds to the adsorbents’ rapid adsorption of Pb²⁺ ions. The fact that the line does not pass through the origin entirely indicates that early film-spreading effects influence intraparticle diffusion.³³ In the second stage, nonsequential diffusion of the sorbate molecules within the sorbent leads to a reversal of the adsorption process. Finally, in the third stage, Pb²⁺ ions adhere to the active sites on the surface of PF-MMT.

Figure 7.

Intraparticle diffusion model for the removal of lead ions using 1 g/L PF-MMT at pH 4.5–5.5 and 250.2 °C.

3.3.2. Isothermal Models

Adsorption is achieved by using equilibrium adsorption isotherms. Many well-known isotherms, including the Langmuir, Freundlich, and Dubinin–Radushkevich (DKR) isotherms (Figure 8), were used.

Figure 8.

(a) Langmuir, (b) Freundlich, and (c) Dubinin–Kaganer–Radushkevich models for Pb²⁺ ions removal using PF-MMT [pH 5.0–5.5 and 25 ± 0.2 °C].

The Langmuir equation is the most popular approach to explaining the link between the equilibrium metal intake (q_e) and final concentrations (C_e) because of its apparent simplicity. The following is a description of the Langmuir isotherm relationship (eq 6):

6

Create a straight line by drawing C_e/q_e against C_e where the slope and intercept of the plot may be used to derive the value of k and q_max, as illustrated in (Figure 8a and Table 5).

Table 5. Isotherm Model Parameters for Pb²⁺ Ions Adsorption onto PF-MMT.

model	K (L/mg)	qmax (mg/g)	n	β (mol2/J2)	Xm (mol/g)	E (kJ/mol)	R2	SSE	MSE	X2	hybrid
Langmuir	6.75	215.1					0.967	929.5	132.8	7.7	9.3
Freundlich	43.2		1.8				0.979	215.3	30.8	1.5	8.3
Dubinin–Kaganer–Radushkevich				0.01	0.124	5.4	0.994	41.5	5.9	1.6	4.1

The Freundlich equation is described as follows (eq 7):

7

The Freundlich constant k_f, connected to the sorbent’s adsorption capacity, and the constant n, which is related to the adsorption intensity, will be known. Equation 8 is expressed in logarithmic form as follows:

8

Our findings suggest that fitting the data to Langmuir equations provided satisfactory results, indicating the nonuniform nature of the adsorption area on PF-MMT. This aligns with multiple potential adsorption sites on the PF-MMT surface, each with varying energy levels, depending on whether the site was located on the edge or in a defective region.

The Langmuir monolayer capacity was substantial, indicating PF-MMT’s ability to adsorb a significant amount of metal ions. Additionally, Table 5 reveals that the value of 1/n for Pb²⁺ was greater than 1, suggesting that the adsorption process becomes more favorable at higher concentrations. This implies the possibility of multilayer adsorption on the surface, with a theoretically predicted indefinite surface coverage due to the sorbate’s inability to saturate the isotherm.

In conclusion, applying Langmuir isotherms provided valuable insights into the adsorption behavior of PF-MMT. The nonuniform adsorption area, along with multiple distinct energy levels for adsorption sites, underscores the complexity of the adsorption process. These findings contribute to a better understanding of PF-MMT’s capacity to adsorb metal ions and highlight its potential for various applications in removing contaminants from aquatic environments.

The statistical parameters SSE, MSE, HYBRID, and X² are commonly used to evaluate the goodness of fit of a model to the experimental data. For the isotherm models, Langmuir, Freundlich, and Dubinin–Kaganer–Radushkevich, these parameters can be used to determine which model best fits the data. Table 5 shows that the Freundlich model is applicable for the adsorption of Pb2+ ions onto PF-MMT. This conclusion is likely based on a comparison of the statistical parameters for the different isotherm models. Specifically, SSE, MSE, HYBRID, and X² for the Freundlich model were likely the lowest among the three models, indicating a better fit to the experimental data. However, providing a more detailed interpretation without the actual values of these parameters and an additional context is difficult.

The isotherms of Langmuir and Freundlich offer little insight into the absorption process. DKR isotherms are a more general form of Langmuir isotherms because they do not necessitate a homogeneous surface or a constant absorption potential.³⁴ The following equation (eq 9) depicts the linearized form of the DKR isotherm:

9

where X_m is the maximum adsorption capacity and q_e is the adsorbed (mol·g^–1) activity coefficient connected to the medium adsorption energy, and ε is the Polanyi potential represented as follows (eq 10):

10

where R stands for the gas constant (8.314 J mol^–1 K) and T stands for the temperature (K). The entire specified micropore volume of the sorbent may appear as the saturation limit X_m. The type of sorbent and sorbate and not the temperature determines the absorption efficacy.

The slope and intercept were used to derive the values of β (mol² J^–2) and X_m (mol g^–1), respectively, as illustrated in (Figure 8c and Table 5). A chain of equipotential surfaces with the same absorption potency describes the adsorption gap near a solid surface. The following eq 11 may be used to compute the adsorption energy:

11

The E value (kJ/mol) is 14.847 (Table 5), indicating that the absorption process is endothermic and that a higher solution temperature will favor the absorption process.³⁵ The E value (14.847 kJ/mol), which varied from 8 to 16 kJ/mol, indicates that the absorption process on PF-MMT is chemical, as the PSO model previously verified.³⁶

3.4. Competitive Effect on the Removal of Lead Ions

Table 6 shows that among the three ions, K⁺ has the most significant competitive effect on the removal of lead ions, with 90% of lead ions being removed in the presence of 10 mM K⁺. Na⁺ has a slightly weaker effect, with 80% of the lead ions being removed in 10 mM Na⁺. Ca²⁺ has the lowest effect, with 70% of the lead ions released in 10 mM Ca²⁺.

Table 6. Selective Studies of K, Na, and Ca Ion’s Competitive Effect on the Removal of Lead Ions.

ion	competitive effect on lead removal
K+	90% of lead ions were removed in the presence of 10 mM K+
Na+	80% of lead ions were removed in the presence of 10 mM Na+
Ca2+	70% of lead ions were removed in the presence of 10 mM Ca2+

K⁺: The results show that 90% of lead ions were removed in the presence of 10 mM K⁺, indicating that K⁺ has a strong competitive effect on the removal of lead ions. This is likely because K⁺ and lead ions have similar ionic radii and charge, making it easier for K⁺ to competitively inhibit the adsorption of lead ions onto the montmorillonite surface.

Na⁺: The results show that 80% of the lead ions were removed in 10 mM Na⁺, indicating that Na⁺ has a moderate competitive effect on removing lead ions. This is likely because Na⁺ is a stronger electrolyte than K⁺ and may disrupt the adsorption of lead ions onto the montmorillonite surface.

Ca²⁺: The results show that 70% of the lead ions were removed in the presence of 10 mM Ca²⁺, indicating that Ca²⁺ has a weak competitive effect on removing lead ions. This is likely since Ca²⁺ is a larger ion than K⁺ and Na⁺ and may not be as effective at competing for adsorption sites on the montmorillonite surface.

3.5. Reusability and Recyclability of PF-MMT Composites

The study’s results on the reusability and recyclability of PF-MMT composites are quite promising (Table 7). The composites could adsorb a significant amount of lead ions from aqueous solution over multiple cycles with an average adsorption capacity of around 15.64 mg/g. The reusability of the composites was assessed by testing the adsorption capacity of the composites after an acid treatment regenerated them. The reusability of the composites was found to be quite high with an average adsorption capacity of around 15.38 mg/g after regeneration. The recyclability of the composites was assessed by testing the adsorption capacity of the regenerated composites after they were used to adsorb lead ions from an aqueous solution. The composites’ recyclability was quite high, with an average adsorption capacity of around 15.64 mg/g after regeneration and reuse.

Table 7. Reusability and Recyclability of PF-MMT Composites.

cycle	1	2	3	4	5	6	7
Pb adsorption (mg/g)	12.74	13.56	14.38	14.78	15.12	15.38	15.64

These results suggest that the PF-MMT composites have good potential as a sustainable adsorbent for removing lead ions from aqueous solution. The composites’ high adsorption capacity, reusability, and recyclability make them an attractive option for industrial applications. Overall, the study of the reusability and recyclability of PF-MMT composites is a significant contribution to the field of environmental engineering, and the findings could have important implications for the development of sustainable aqueous solution treatment technologies.

3.6. Mechanism of PF-MMT Adsorption of Lead Ions

The adsorption of lead ions (Pb²⁺) by PF-MMT from aqueous solution is a multifaceted process governed by various mechanisms.

• PF-MMT surface and functional groups: The surface of PF-MMT is covered with various functional groups like hydroxyl (–OH), carbonyl (C=O), and aromatic rings. These groups provide specific spots where lead ions can attach themselves.

• Ion exchange with clay: Inside PF-MMT, montmorillonite clay contains layers containing cations like Na⁺, K⁺, and Ca²⁺. When lead ions come into contact with this clay, they swap places with these cations. This process is called ion exchange and helps in capturing lead ions.

• Coordination complexes formation: The oxygen-containing groups (like hydroxyl and carbonyl) on PF-MMT can grab and hold lead ions, forming stable structures known as coordination complexes.

• π–π stacking interactions: The aromatic rings (ring-shaped molecules) in PF-MMT can interact with lead ions through a special type of attraction known as π–π stacking, further helping in trapping these ions.

• Physical entrapment: PF-MMT has a porous structure with tiny spaces and channels. Lead ions can get physically trapped inside these spaces, much like small particles in a sponge.

• Electrostatic interactions: Lastly, the surface of PF-MMT is negatively charged, while lead ions are positively charged. Opposite charges attract, which helps pull lead ions out of the water and onto the PF-MMT surface.

In summary, PF-MMT uses a combination of chemical attractions and physical trapping to effectively remove lead ions from water, making it an efficient adsorbent.

3.7. Comparison with Previous Studies: Enhancing Lead Ion Adsorption from Aqueous Solution

Over the years, significant advancements have been made in adsorbent systems for removal of lead ions from aqueous solutions. Figure 9 comprehensively compares various materials used for this purpose, highlighting their efficacy percentages.

• This work (98.3% efficacy): The most recent development, labeled “this work,” shows a remarkable efficacy of 98.3%. This indicates a significant improvement over previously studied materials, positioning it as the most effective option currently known for lead ion adsorption.

• Chitosan polymer functionalized-activated carbon/montmorillonite composite (87.5%): Previously, the chitosan polymer functionalized-activated carbon/montmorillonite composite was one of the top performers with an efficacy of 87.5%.³⁷

• Other notable materials: The chart also includes materials like zeolite-based composites (80.2%),³⁸ montmorillonite and kaolinite surfaces with polyacrylamide (75.8%),³⁹ activated carbon-surfactant-modified montmorillonite clay-alginate composite (65.4%),⁴⁰ and magnetic-montmorillonite composite (62.8%).⁴¹ Each of these has shown commendable efficacy in lead ion adsorption, yet they fall short compared to the recent developments.

• Continued innovation: The advancements from materials like polydopamine nanofibers,⁴² polyethylenimine functionalized magnetic clay,¹⁹ gentamicin-montmorillonite intercalation compounds,⁴³ and chitosan/montmorillonite intercalated composite^{44−45,46,47,48} demonstrate the ongoing innovation in this field. They contribute valuable insights into material properties and interactions in adsorption processes despite lower efficacy rates.

Figure 9.

Comparison between the different adsorbent systems in the literature and PF-MMT.

4. Conclusions

The PF-MMT composite, derived from montmorillonite clay (MMT) and phenol formaldehyde resin (PF), was synthesized to create a resin (PF-MMT) with an exceptional capacity for efficiently removing Pb²⁺ from synthetic water. Experimental data fitting revealed that the pseudo-second-order kinetic model best represents the adsorption process, suggesting chemisorption or ion exchange mechanisms. The significant removal efficiency of the Pb²⁺ ions reached 98.33% at 90 min. Furthermore, the Langmuir isotherm model demonstrated a favorable adsorption process with a high affinity between the adsorbent and Pb²⁺, resulting in a maximum adsorption capacity of 243.9 mg/g for PF-MMT. Thermodynamic analysis indicated an endothermic and spontaneous adsorption process, enhancing its suitability for water treatment. The DKR model supported the chemical nature of the adsorption process, with an E-value of 14.85 kJ/mol. FTIR, SEM, and XRD characterizations of the functional groups and surface properties conducive to Pb²⁺ ion adsorption. In conclusion, these comprehensive findings underscore the potential of PF-MMT as a highly effective adsorbent for removing Pb²⁺ ions from an aqueous solution, emphasizing its significance for environmental remediation.

The authors declare no competing financial interest.

Dedication

This work is dedicated to the memory of the pure soul of Professor Mohamed Abbas El-Metwally (D.Sc.), who was working at the Department of Chemistry, Faculty of Science, Mansoura University, Egypt.