Schiff base functionalized silica aerogels for enhanced removal of Pb (II) and Cu (II): Performances, DFT calculations and LCA analysis

Schiff base silica aerogels (SCA-X) were synthesized using amino-containing organosilanes and salicylic aldehyde as functional monomers with ethyl orthosilicate hydrolysis condensation as carrier. The influence of SCA-X on the adsorption of Pb (II) and Cu (II) under different adsorption conditions was evaluated, including the effect of solution pH, isotherm, kinetics, thermodynamics and adsorption mechanism. The batch adsorption experiments showed that SCA2 had the optimum adsorption capacity for Pb (II) (357.1 mg/g) and Cu (II) (243.9 mg/g), leading to adsorption equilibrium within 120 min and 360 min, respectively. After six adsorption–desorption cycles, SCA2 still possessed satisfactory adsorption for Pb (II) and Cu (II), demonstrating the reusability of the SCA2 adsorbent material. Kinetic studies indicated that the adsorption process could be described by a pseudo- second-order kinetic model, adsorption isotherms were in accordance with the Langmuir model, indicative of monomolecular layer adsorption. Thermodynamics evaluation revealed the nature of the adsorption process was an endothermic spontaneous process. XPS analysis combined with DFT calculations confirmed that the inter- action mechanism between SCA2 and Pb (II) occurred through the coordination between the nitrogen atom donor in the Schiff base and the oxygen atom donor in the benzene ring, while the interaction between SCA2 and Cu (II) occurred mainly through the coordination between the nitrogen atom in the Schiff base and Cu (II). Life Cycle Assessment (LCA) was introduced to analyze the environmental impact of the SCA2 fabrication process and eco-friendly approaches were provided, which eventually provided theoretical evidence for the application of as-prepared material in the handling of heavy metal effluents.