Removal of heavy metals from aqueous solution using novel nanoengineered sorbents: Self-assembled carbamoylphosphonic acids on mesoporous silica

Self-assembled monolayers of carbamoylphosphonic acids (acetamide phosphonic acid and propionamide phosphonic acid) on mesoporous silica supports were studied as potential absorbents for heavy and transition metal ions in aqueous wastes. The adsorption capacity, selectivity, and kinetics of the materials in sequestering metal ions, including Cd²⁺, Co ²⁺, Cu²⁺, Cr³⁺, Pb²⁺, Ni ²⁺, Zn²⁺, and Mn²⁺, were measured in batch experiments with excess sodium ion. The solution pH ranged from 2.2 to 5.5. The kinetics study shows that the adsorption reached equilibrium in seconds, indicating that there is little resistance to mass transfer, intraparticle diffusion, and surface chemical reaction. The competitive adsorption study found the phosphonic acid-SAMMS to have an affinity for divalent metal ions in decreasing order of Pb²⁺>Cu²⁺>Mn²⁺> Cd²⁺>Zn²⁺>Co²⁺>Ni²⁺. The measured Cd²⁺ adsorption isotherm was of the Langmuirian type and had a saturation binding capacity of 0.32 mmol/g.