Amino-modified pillared adsorbent from water-treatment solid wastes applied to CO₂/N₂ separation

In this work, solid wastes from the flocculation step of a drinking water treatment plant were used as starting material to obtain CO₂ adsorbents upon thermal treatment and a pillaring strategy. Thermally treated solid waste and pillared materials were characterized by XRD, FTIR, TGA, N₂ adsorption–desorption isotherms at 77 K, XPS and elemental analysis. From the characterization results, it was found that the introduction of silica pillars was successful, leading to heterostructured materials with higher specific surface area and pore volume. In order to improve the CO₂ adsorption capacity, pillared materials were functionalized by grafting with (3-aminopropyl)triethoxysilane (APTES)and impregnation with polyethylenimine (PEI). Pillared materials could be loaded with up to 50 wt% PEI. PEI-impregnated pillared adsorbents were tested for CO₂ adsorption at 298 and 348 K and CO₂/N₂ adsorption at 348 K using a Magnetic Suspension Balance equipped with a mixture gas dosing (Rubotherm, Germany). Langmuir and Sips models were used to fit monocomponent and mixture experimental data, considering two different sites for CO₂ adsorption. At 1 bar, 50PEI-SiFe (loaded with 50 wt% PEI) adsorbed 2.5 and 3.6 mmol CO₂ g⁻¹ at 298 and 348 K, respectively and exhibited an extremely high CO₂/N₂ selectivity (above 150 mol CO₂/mol N₂ at 348 K). Nevertheless, PEI loaded adsorbents still exhibit a lower amine efficiency as compared to APTES-grafted counterparts, due to diffusion restrictions caused by the high viscosity of the impregnated polymer.