Chemically Surface-Engineered Mesoporous Silica for the Toxic Metal Ions Uptake: Insights from Experiment and Density Functional Calculations

Robust chemical modification of mesoporous silica was conducted using 3-mercaptopropyltriethoxysilane via a chemical surface-engineered postgrafting of mesoporous silica KIT-6 with acidic propylsulfonate groups to obtain mesoporous KIT-6-SO₃H. A fabricated meso-KIT-6-SO₃H-modified quartz crystal microbalance sensor with KIT-6-SO₃H layers coating on a QCM electrode is utilized to detect Pb²⁺, Cd²⁺, and Cs⁺ ions with high sensing affinity. The functionalized KIT-SO₃H exhibits adsorption capacities (Q_e) of 123.5 mg g^–1, 117.5 mg g^–1, and 90.6 mg g^–1 for Pb²⁺, Cd²⁺, and Cs⁺, respectively, as determined by UV–vis measurements. These values coincide well with those obtained from the QCM sensor and ICP-OES measurements. The remarkable ability to adsorb metal ions is achieved by the synergistic cooperation of the large pore volume, high surface area, and abundant acidic −SO₃H groups within the mesoporous structure of KIT-6-SO₃H. A comprehensive study was carried out to investigate the influence of pH on the adsorption uptake of metal ions. Kinetic and isotherm studies demonstrate that the adsorptive removal of metal ions by KIT-6-SO₃H follows a second-order kinetic model and is well described by the Langmuir isotherm, reflecting monolayer adsorption behavior. Density functional calculations reveal that the adsorption of these metals is highly exothermic from a thermodynamic perspective, which aligns with and supports the experimental findings. All metals were exothermically adsorbed with binding energies of −1.790 eV for Pb²⁺, −0.181 eV for Cd²⁺, and −3.113 eV for Cs⁺, confirming the exergonic adsorption of the investigated metals on KIT-6-SO₃H.