Adsorptive removal of lead, copper, and nickel using natural and activated Egyptian calcium bentonite clay

Characterization of natural and activated bentonite samples

X‑ray fluorescence (XRF)

The findings revealed an improvement in the properties of the activated bentonite (AB). As shown in Table 1, the iron content was reduced as a result of the magnetic separation process. The chemical analysis indicated that calcium montmorillonite was the primary clay mineral present in the natural bentonite sample, as evidenced by its calcium content. However, after activation, calcium levels decreased while sodium concentration increased. Additionally, the reduction in iron content in the activated sample further confirmed the effectiveness of the magnetic separation technique.

X‑ray diffraction (XRD)

XRD analysis of the natural and activated bentonite samples confirmed that montmorillonite was the primary mineral, accompanied by smaller amounts of kaolinite and quartz. The absence of the dolomite peak in the activated sample suggests its removal during the acetic acid treatment, as illustrated in Fig. 3. The structural alterations identified in the XRD patterns of activated bentonite significantly contribute to improving its adsorption capability. The expansion of interlayer spacing following alkali activation, as evidenced by the shift in diffraction peaks, creates more available sites for heavy metal ion uptake. These modifications enhance the material’s surface area and cation exchange capacity (CEC), leading to more effective adsorption of Cu^2⁺, Pb²⁺, and Ni²⁺ from aqueous solutions. Additionally, the improved crystallinity and phase composition of activated bentonite strengthen its ability to retain metal ions by supporting monolayer adsorption.

XRD analysis for natural and activated bentonite samples.

Fourier‑transform infrared spectroscopy (FTIR)

The FTIR spectra of natural and activated bentonite samples, presented in Fig. 4, illustrate the presence of characteristic functional groups. Notably, the stretching vibrations associated with Si–O and Si–O–Al bonds appear at 540 cm^-121, while the peak at 905 cm⁻¹ is attributed to Si–O–Si–O–Al stretching²¹. The peak observed at 800 cm⁻¹ corresponds to O–Si–O stretching. A broad band near 3433 cm⁻¹ is associated with the stretching vibrations of OH or H₂O molecules, possibly present on the clay surface, and the peak at 1633 cm⁻¹ is linked to H–O–H bending vibrations²². The O–H stretching vibration of structural hydroxyl groups in montmorillonite appears at 3633 cm^-1. Furthermore, both natural and activated bentonite samples display distinct peaks near 995 cm^-1 and 912 cm^-1, corresponding to Si–O and Al–O stretching vibrations, respectively²³. The modifications observed in the FTIR spectra after activation indicate the formation of additional hydroxyl (–OH) and silanol (Si–OH) functional groups, which play a vital role in the adsorption mechanism. These functional groups serve as active binding sites for metal ions, promoting both surface complexation and ion exchange. The enhanced presence of hydroxyl groups following activation strengthens the interaction between metal ions and the bentonite surface, ultimately boosting adsorption efficiency.

FTIR spectra of natural (NB) and activated (AB) bentonite samples.

Swelling index

The swelling index results indicate a significant improvement in the bentonite sample after activation. As shown in Table 2, the swelling index of activated bentonite increased to 27 ± 1.1 mL/2 g, compared to 7 ± 0.4 mL/2 g for natural bentonite.

Cation exchange capacity

The cation exchange capacity (CEC) was evaluated using BaCl₂ as the saturating cation. As shown in Table 2, activation with 5% Na₂CO₃ significantly enhanced the CEC, increasing from 45 ± 1.4 meq/100g in natural bentonite to 60 ± 2.0 meq/100g in activated bentonite.

Point of zero charge determination

Figure 5 depicts the point of zero charge (pzc) assessment for both natural and activated bentonite using the pH drift method. The pzc, defined as the pH at which the surface has no net charge (ΔpH = 0), was determined to be 7.3 for natural bentonite and 7.9 for activated bentonite. This parameter is essential for understanding how bentonite’s surface charge changes with pH. When the solution pH is below the pzc, protonation of surface hydroxyl groups occurs, creating a net positive charge on the edge sites, while the basal surfaces remain negatively charged. This leads to electrostatic repulsion, reducing metal cation adsorption. In contrast, at pH levels above the pzc, deprotonation of surface hydroxyl groups results in a net negative surface charge, which strengthens as pH increases. This negative charge enhances electrostatic attraction, facilitating metal cation adsorption.

Pzc determination for natural and activated bentonite.

Scanning electron microscope (SEM)

Figure 6 presents SEM images and EDX spectra of natural and activated bentonite, providing insight into the adsorption mechanism. Natural bentonite has a dense and less porous structure, while activation results in a more fragmented and highly porous form. The alkali activation process alters the material’s structure, expanding its surface area and increasing the number of active adsorption sites, thereby improving its capacity to trap heavy metal ions. The enhanced porosity of activated bentonite promotes better metal ion diffusion, ensuring greater interaction with adsorption sites²⁴. Additionally, the rougher surface texture and increased pore volume create more opportunities for ion exchange and surface complexation, enhancing its efficiency in adsorbing Cu²⁺, Pb²⁺, and Ni²⁺. The EDX spectra confirm the elemental makeup of the samples, indicating significant changes in element distribution post-activation.

SEM & EDX images of (A) activated and (B) natural bentonite.

Adsorption studies using natural and activated bentonite samples

Effect of pH

To examine the impact of pH on the adsorption of copper, lead, and nickel onto bentonite clay, a series of single-component aqueous solutions (50 mL each) containing 20 mg/L of Cu²⁺, Pb²⁺, and Ni²⁺ were prepared at varying pH levels ranging from 1 to 9. The pH adjustments were made using HCl and NaOH. A 0.05 g portion of both natural and activated bentonite (equivalent to 1 g/L) was introduced as the adsorbent, and the mixtures were agitated at 20 °C for 120 min in a water bath shaker. The remaining metal concentrations in the solutions were analyzed using a GBC atomic absorption SavantAA spectrometer after generating a reference calibration curve. The adsorption capacities of activated bentonite for each metal ion were determined as follows: Cu²⁺; 2.6 ± 0.01, 4.6 ± 0.03, 10.6 ± 0.03, 14 ± 0.03, and 19 ± 0.05 mg/g; Pb²⁺; 2.2 ± 0.03, 3.4 ± 0.02, 9.2 ± 0.04, 13 ± 0.04, and 18.4 ± 0.05 mg/g; and Ni²⁺;1.4 ± 0.008, 2.9 ± 0.02, 8.16 ± 0.03, 12.2 ± 0.05, and 15 ± 0.04 mg/g at pH levels of 1, 3, 5, 7, and 9, respectively. The results indicated that activated bentonite exhibited superior adsorption performance compared to its natural counterpart.

Overall, adsorption capacity was observed to decrease at lower pH levels. This trend can be attributed to the competitive interaction between metal cations and hydronium (H₃O⁺) ions for available adsorption sites on the bentonite surface. At highly acidic conditions, the abundance of H₃O⁺ ions overshadow metal ions, leading to surface saturation with H₃O⁺, thereby reducing metal uptake. As pH increases, H₃O⁺ ions gradually vacate the surface, making more binding sites accessible for metal ions, ultimately enhancing their adsorption onto bentonite²⁵. The sharp increase of adsorption of heavy metals at pH > 8 may be because of their hydroxide precipitation on adsorbent surface and these results agreed with results of previous study²⁶. The adsorbent capacity of the examined heavy metals was determined in the following order: Cu²⁺  > Pb²⁺  > Ni²⁺, as seen in Fig. 7.

Effect of pH on the adsorption capacity of Cu²⁺, Pb²⁺ and Ni²⁺ on (A): activated and (B): natural bentonite.

Effect of the adsorbent dose

The effect of adsorbent dosage was studied to identify the optimal amount for heavy metal removal. The findings indicate that increasing the adsorbent dose enhances the adsorption capacity, as anticipated, owing to the increased availability of adsorption sites on the adsorbent surface. Concentrations of natural and activated bentonite were tested at 0.25, 0.5, 0.75, 1, and 1.25 g/L, using 50 mL of a 20 mg/L single solution of Cu²⁺, Pb²⁺ and Ni²⁺ at pH 7 and 20 °C for 120 min, as shown in Fig. 8. The residual solution was analyzed using GBC AA. It was found that increasing the adsorbent dosage from 0.25 to 1.25 g/L significantly improved the adsorption capacities (3.56 ± 0.04, 6.86 ± 0.03, 10.18, 14 ± 0.5 and 16.84 ± 0.05 mg/g for Cu²⁺, 3.04 ± 0.02, 6.38 ± 0.03, 9.18 ± 0.04, 13 ± 0.04 and 15.62 ± 0.05 mg/g for Pb²⁺ and 2.86 ± 0.03, 5.92 ± 0.04, 8.54 ± 0.04, 12.2 ± 0.05 and 14.76 ± 0.04 mg/g for Ni²⁺), which can be attributed to the increased availability of exchangeable sites in the bentonite clay structure²⁷. The findings demonstrated that the activation of bentonite enhanced the adsorption capacity and removal efficiency.

Effect of initial adsorbent dosage on the adsorption capacity of Cu²⁺, Pb²⁺and Ni²⁺ on (A): activated and (B): natural bentonite.

Effect of contact time

The adsorption capacity of activated bentonite steadily increased with time, reaching equilibrium at 120 min under controlled conditions: 50 mL of a 20 mg/L single-metal solution, pH 7, a temperature of 20 °C, and an adsorbent dosage of 0.05 g (1 g/L), as illustrated in Fig. 9. The influence of contact time on Cu²⁺, Pb²⁺, and Ni²⁺ adsorption was examined at different time intervals. The results indicated a rapid adsorption rate within the first 20 min, aligning with previous studies that have reported swift metal ion uptake within 10–20 min.

Effect of contact time on the adsorbent capacity of Cu²⁺, Pb²⁺ and Ni²⁺ on (A): activated and (B): natural bentonite.

At the initial phase of adsorption, a large number of available surface sites facilitate metal ion binding. However, as time progresses, occupying the remaining vacant sites becomes more challenging due to repulsive interactions between the solute molecules. The quick electrostatic attraction of heavy metal ions at neutral pH is primarily due to the negatively charged surface of the bentonite, which enhances metal ion removal from the solution¹⁸. At equilibrium, after 120 min at 20 °C, the adsorption capacities were 9.2 ± 004, 11.4 ± 0.03, 12.6 ± 0.04, 13.8 ± 0.02 and 14 ± 0.03 mg/g for Cu²⁺, 8.2 ± 0.03, 10.6 ± 0.02, 11.8 ± 0.04,12.6 ± 0.05 and 13 ± 0.04 mg/g for Pb²⁺, and 7.4 ± 0.01, 10 ± 0.03, 11 ± 0.04, 11.8 ± 0.04 and 12.2 ± 0.05 mg/g for Ni²⁺, respectively, with activated bentonite which showed improvement more than natural bentonite.

Effect of initial metal ion concentration

The findings indicated that the adsorption capacity (mg/g) diminished as the initial concentration increased, as seen in Fig. 10. It was observed that adsorption capacity of heavy metals on the activated and natural bentonite samples decreased by increasing the initial metal concentration (50 ml of 10, 20, 30, 40, 50 mg/L of single component solutions) and these results agreed with previous study²⁸ While keeping the other variables constant (1 g/L bentonite, 20 °C, pH 7, and 120 min), this could be attributed to the insufficient number of active sites available for interaction with the adsorbate. The adsorption capacity of activated bentonite was 19 ± 0.04, 14 ± 0.03, 9.94 ± 004, 7.9 ± 0.02 and 6.52 ± 0.03 mg/g for Cu²⁺, 17.8 ± 0.04, 13 ± 0.04, 6.27 ± 0.03, 4.85 ± 0.05 and 3.92 ± 0.04 for Pb²⁺, and 16 ± 0.05, 12.2 ± 0.05, 5.67 ± 0.04, 4.35 ± 0.03 and 3.76 ± 0.04 mg/g for Ni²⁺.

Effect of initial metals concentration on the adsorption capacity of Cu²⁺, Pb²⁺ and Ni²⁺ on (A): activated and (B): natural bentonite.

Effect of temperature

The adsorption efficiency of activated bentonite for heavy metals improved with increasing temperature, likely due to enhanced molecular interactions and accelerated reaction kinetics at the adsorbent-adsorbate interface. Higher temperatures may also induce structural expansion in the bentonite layers, facilitating metal ion diffusion²⁹. This effect was examined by adjusting the temperature to 20, 30, 40, 50, and 60 °C while maintaining constant parameters: 50 mL solution containing 20 mg/L of heavy metal, pH 7, an adsorbent dose of 1 g/L, and a contact period of 120 min. The adsorption capacities recorded for Cu²⁺ were 14 ± 0.03, 14.4 ± 0.06, 15 ± 0.05, 15.5 ± 0.06, and 15.8 ± 0.04 mg/g; for Pb²⁺, 13 ± 0.04, 13.6 ± 0.06, 14.2 ± 0.05, 14.6 ± 0.03 and 15.2 ± 0.05 mg/g; and for Ni²⁺, 12.2 ± 0.05, 12.6 ± 0.04, 13.06 ± 0.05, 13.4 ± 0.06 and 13.7 ± 0.05 mg/g, as illustrated in Fig. 11.

Effect of initial temperature on the adsorption capacity of Cu²⁺, Pb²⁺ and Ni²⁺ on (A): activated and (B): natural bentonite.

Adsorption kinetics

To better understand the mechanism of heavy metal adsorption onto activated bentonite, the subsequent kinetic models were employed to interpret the experimental results.

Pseudo first order model

This kinetic model can be expressed as follows³⁰:

where q_e and q_t denote the quantity of metal ion adsorbed on the adsorbent (mg/g) at equilibrium and at time t, respectively, K1 signifies the rate constant for first-order adsorption (min^-1). By plotting ln (q_e–q_t) against time (t), a linear relationship is obtained, from which the first-order rate constant (K1) and equilibrium adsorption capacity (q_e) are determined from the slope and intercept, respectively. As shown in Fig. 12, the R² values for Cu²⁺, Pb²⁺, and Ni²⁺ are 0.6619, 0.6924, and 0.7278, respectively.

Pseudo first order plot for Cu²⁺, Pb²⁺, and Ni²⁺ adsorption onto activated bentonite.

Pseudo second order model

The adsorption data were analyzed using the pseudo-second-order model, represented by the following equation³⁰:

Figure 13 demonstrates that the correlation coefficient (R²) values were 0.9079 for Cu²⁺, 0.9949 for Pb²⁺, and 0.9963 for Ni²⁺. The results indicate that the adsorption of Cu²⁺, Pb²⁺, and Ni²⁺ ions onto the activated bentonite surface conforms to the pseudo-second-order model. This implies that the adsorption process is primarily controlled by chemical sorption, suggesting the involvement of valence forces, ion exchange, or covalent bond formation³¹.

Pseudo second order plot for Cu²⁺, Pb²⁺, and Ni²⁺ adsorption onto activated bentonite.

Adsorption isotherm models

The adsorption isotherm illustrates the equilibrium relationship between the concentration of the adsorbate in the liquid phase and its concentration on the adsorbent surface under specific conditions. Langmuir and Freundlich isotherm models were used to establish the relationship between the amounts of Cu²⁺, Pb²⁺, and Ni²⁺ ions adsorbed by activated bentonite clay and their equilibrium concentrations in the aqueous solution.

Langmuir isotherm model

The linear representation of the Langmuir isotherm model, which characterizes monolayer adsorption on a surface with a finite number of uniform binding sites, is articulated by the subsequent equation³²:

In this context, qmax signifies the maximum monolayer adsorption capacity (mg/g), Ce indicates the equilibrium concentration of metal ions in solution (mg/L), q_e refers to the amount of metal ions adsorbed per gram of activated bentonite at equilibrium (mg/g), and KL represents the Langmuir constant associated with the affinity of the binding sites (L/mg). The Langmuir isotherm assumes the formation of a single monolayer during the reaction, with fixed positions for the adsorbate, no interaction between the adsorbate and adsorbent, and no overlap between adsorbate molecules on the surface³³. The correlation coefficients for the Langmuir isotherm were 0.9979 for copper, 0.9972 for lead, and 0.9973 for nickel, as shown in Fig. 14. This signifies that the Langmuir isotherm effectively characterizes Cu²⁺, Pb²⁺, and Ni²⁺ adsorption onto bentonite clay, implying that the adsorption process transpires as anticipated. The calculated maximum monolayer capacities (q_max) were 14 ± 0.03 mg/g for Cu²⁺, 13 ± 0.04 mg/g for Pb²⁺, and 12.2 ± 0.05 mg/g for Ni²⁺.

Langmuir plot of Cu²⁺, Pb²⁺and Ni²⁺ onto activated bentonite.

The Langmuir isotherm’s essential properties can be represented by R_L, a dimensionless constant referred to as the separation factor or equilibrium parameter. The value of R_L is determined using the following equation³⁴:

Co indicates the initial concentration of metal ions (mg/L), whereas K_L signifies the Langmuir adsorption equilibrium constant (L/mg). The R_L parameter serves as a more reliable indicator of the adsorption process. It specifies the arrangement of the isotherm as follows: unfavorable (R_L > 1), linear (R_L = 1), favorable (0 L L = 0)³⁵. The computed R_L values for Cu²⁺, Pb²⁺, and Ni²⁺ in this investigation were determined to be within the interval of 0 L L varied from 0.0097 to 0.0466 for Cu²⁺; while it ranged from 0.014 to 0.066 for Pb²⁺, and it spanned from 0.0306 to 0.1361 for Ni²⁺, as seen in Fig. 15.

The Separation factor R_L versus initial concentration for Cu²⁺, Pb²⁺, and Ni²⁺ adsorption onto activated bentonite.

Freundlich isotherm model

The following equation expresses the linear form of the Freundlich model³⁶:

As presented in Fig. 16, the correlation coefficients (R²) for Cu²⁺, Pb²⁺, and Ni²⁺ were 0.9661, 0.9910, and 0.9954, respectively. These values suggest that the Freundlich isotherm model provides a less accurate fit for the adsorption data. Therefore, the Langmuir isotherm provided a better fit for the adsorption of Cu²⁺, Pb²⁺, and Ni²⁺.

Freundlich plot of Cu²⁺, Pb²⁺and Ni²⁺ onto activated bentonite.

Thermodynamics studies

The thermodynamic parameters ΔH° and ΔS° were determined from the slope and intercept of the Van’t Hoff plot of ln K_L versus 1/T (Fig. 17). The negative values of Gibbs free energy change (ΔG°) for Cu²⁺, Pb²⁺, and Ni²⁺ adsorption confirm that the process is spontaneous and thermodynamically favorable. These values also indicate the presence of electrostatic interactions between the metal ions and the activated bentonite surface. Furthermore, the magnitude of ΔG° reflects the strength of metal ion binding, where more negative values correspond to stronger interactions. This adsorption behavior is influenced by the electrical, chemical, and structural properties of activated bentonite. The enthalpy change (ΔH°) provides insights into the energy dynamics of the adsorption process³⁷. The positive ΔH° values for Cu²⁺, Pb²⁺, and Ni²⁺ as illustrated in Table 3 confirm that the process is endothermic, meaning that adsorption efficiency improves with increasing temperature. Similar studies have been previously observed³⁸. In general, ΔH° values below 20 kJ/mol suggest a physical adsorption mechanism, while values exceeding this range indicate additional processes such as ion exchange, which involves weak chemical interactions. The entropy change (ΔS°) provides further insight into adsorption behavior, particularly in terms of molecular arrangement. A negative ΔS° suggests that metal ions assume a more ordered configuration on the bentonite surface, leading to reduced system randomness. Conversely, a positive ΔS° implies structural modifications in both the adsorbent and adsorbate during the adsorption process. Overall, the thermodynamic analysis confirms that heavy metal adsorption onto activated bentonite is spontaneous, endothermic, and influenced by electrostatic interactions and ion exchange.

Thermodynamic behaviors of adsorption of Cu²⁺, Pb²⁺ and Ni²⁺ onto activated bentonite.