Selective adsorption of Pb (II) and Cu (II) on mercapto-functionalized aerogels: Experiments, DFT studies and LCA analysis

Mercapto-functionalized aerogels (MA-X) were fabricated using γ-mercaptopropyltrimethoxysilane (MPTMS) as a modification reagent to eliminate Pb (II) and Cu (II) ions from wastewater. Mercapto-functionalized aerogel (MA2) with the MPTMS/TEOS molar ratio of 0.5 exhibited the maximum adsorption amounts of 163.99 mg/g for Pb (II) and 172.41 mg/g for Cu (II) in single ion system, respectively. In binary ion system, selective adsorption experiments revealed that the equilibrium adsorption capacity for Cu (II) (161.29 mg/g) was significantly greater than Pb (II) (90.42 mg/g), and the selectivity factor α showed greater selectivity for Cu (II), demonstrating that Cu (II) was more readily adsorbed on MA2. The results showed that adsorption was consistent with pseudo- second order model and Langmuir model. Thermodynamic results demonstrated that adsorption phenomenon was an exothermic reaction that occurred spontaneously. XPS analysis and density functional theory (DFT) simulations showed that the main mechanism for the adsorption of Pb (II) and Cu (II) on MA2 was through coordination chelation of the –SH groups with Pb (II) and Cu (II). DFT calculations showed a lower adsorption energy (Eads) of Cu (II) ( 2.72 eV) with respect to Pb (II) ( 0.80 eV), indicating that Cu (II) was more stably adsorbed on MA2 and more difficult to exchange by Pb (II). In order to determine the environmental impact of the MA2 preparation process, a life cycle assessment (LCA) was conducted and contribution of each material to MA2 production was analyzed. Finally, a strategy that is environmentally friendly and effective has been pro-posed in order to facilitate MA-X adsorbents production and to improve their application for the treatment of heavy metal polluted wastewater.