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

This study evaluates the efficiency of alkali-activated Egyptian calcium bentonite, obtained from the El Alamein region in northern Egypt, for the removal of copper (Cu^2⁺), lead (Pb^2⁺), and nickel (Ni^2⁺) from synthetic wastewater. The bentonite samples underwent a series of preparation steps, including crushing, ball milling, magnetic separation, acid treatment with 0.1N acetic acid, and alkali activation using 5% sodium carbonate (Na₂CO₃). Various analytical techniques, such as X-ray fluorescence (XRF), X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), cation exchange capacity (CEC) measurements, scanning electron microscopy (SEM), and free swelling analysis, were employed to characterize the materials. Absorption experiments were performed to examine the effects of pH, temperature, starting metal concentration, bentonite dose, and contact duration on heavy metal removal. The characterization results confirmed that montmorillonite was the predominant mineral in both the natural and activated bentonite samples. Adsorption studies indicated a significant improvement in heavy metal removal efficiency after activation. Under optimal conditions (pH 7, 1 g/L adsorbent dose, 120 min contact time, 20 mg/L initial metal concentration, and 20 °C), the maximum adsorption capacities of the activated bentonite were determined as 14 ± 0.03 mg/g for Cu²⁺, 13 ± 0.04 mg/g for Pb²⁺, and 12.2 ± 0.05 mg/g for Ni²⁺, exceeding those of the natural bentonite, which recorded capacities of 9.2 ± 0.04 mg/g, 9 ± 0.03 mg/g, and 8 ± 0.02 mg/g, respectively. Adsorption equilibrium data according to the Langmuir isotherm model, exhibiting high correlation values (R² = 0.9979 for Cu²⁺, 0.9972 for Pb²⁺, and 0.9973 for Ni²⁺). Moreover, kinetic modeling demonstrated that the adsorption followed a pseudo-second-order mechanism, suggesting an intense chemisorption process. The thermodynamic analysis indicated that the adsorption process was spontaneous and endothermic, demonstrating enhanced adsorption at higher temperatures.