Enhanced Adsorption of Cd(II) and Pb(II) Using Amine-Modified MCM-41 Mesoporous Silica

This research investigates the synthesis and characterization of molecular sieve MCM-41 functionalized with 3-aminopropyltriethoxysilane to assess its efficacy in adsorbing heavy metal ions such as Cd(II) and Pb(II) from aqueous environments. A series of amine-modified MCM-41 samples, incorporating 3, 5, 8, and 15 wt% amine groups, were fabricated through a co-condensation approach. This synthesis process utilized cetyltrimethylammonium bromide (CTAB) as the template agent, 3-aminopropyltriethoxysilane (APTES) for functionalization, and tetraethyl orthosilicate (TEOS) as the silica precursor.
Comprehensive characterization of the adsorbents was conducted using analytical techniques, including X-ray diffraction (XRD), nitrogen adsorption-desorption isotherms (BET), thermogravimetric analysis (TG-DTG), Fourier-transform infrared spectroscopy (FT-IR), and transmission electron microscopy (TEM). Batch adsorption tests were performed to examine the influence of various factors such as initial metal ion concentration, solution pH, adsorbent dosage, and temperature. Findings revealed that all synthesized NH2-MCM-41 variants exhibited superior adsorption capacities for Cd(II) and Pb(II) relative to unmodified MCM-41, irrespective of amine concentration. The enhanced adsorption was attributed to the presence of amine groups near the pore openings and on the external surfaces of the mesoporous silica.
For the optimized adsorbent (5 mL-NH2-MCM-41), the maximum adsorption efficiencies were observed under specific pH conditions—89.1% removal of Cd(II) at pH 5–6 and 95.3% removal of Pb(II) at pH 3.0–3.5. Adsorption isotherm analyses confirmed that the experimental data closely aligned with the Langmuir model, indicating monolayer adsorption behavior. Kinetic modeling was carried out using pseudo-first-order and pseudo-second-order approaches, with adsorption rates for Cd(II) and Pb(II) demonstrating a strong fit with the pseudo-second-order model.