Synthesis and characterization of geopolymers based on phosphate mining tailings and its application for carbon dioxide and nitrogen adsorption

Abstract

Phosphate sludge, a byproduct of phosphate mining, poses environmental challenges due to its large-scale generation and storage requirements, impacting land use and aesthetics. The need to investigate sustainable recycling practices for cleaner phosphate production has been emphasized, and in this work the reuse of mine tailings as raw materials to produce geopolymeric adsorbents was proposed. The synthesis and characterization of granular and spherical geopolymer particles produced by the incorporation of 19 % of phosphate mine tailings were deeply investigated to produce adsorbents suitable for CO₂/N₂ separation and capture process. The geopolymers were characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), Nuclear magnetic resonance (NMR), BET surface area, Scanning electron microscopy (SEM) and compressive strength analysis. Equilibrium isotherms for carbon dioxide and nitrogen were determined within in a pressure range of 1–25 bar and at temperatures ranging from 35 to 250 °C. The adsorption cycles were evaluated at 21 °C with a pressure variation of up to 10 bar and an adsorption capacity of 3.15 mmol CO₂∙g⁻¹ was obtained. The SIPS model was found to fit the experimental data well. The granular geopolymer has a higher CO₂ adsorptive capacity than the geopolymer beads due to the higher BET surface area (298 m² g⁻¹ and 4.2 m² g⁻¹, respectively). At low temperatures (<35^oC), CO₂ is physically adsorbed, while a different mechanism is involved in CO₂ removal at high temperatures. The presence of iron oxides in the geopolymeric matrix could result in a chemical interaction with CO₂ and the formation of iron bicarbonate, monodentate carbonate and bidentate carbonate, which could be favored by increasing temperature and pressure. The phosphate waste-based geopolymer developed in this study has an adsorption capacity ∼15 % higher than those reported in the literature using wastes. In addition, the production process is straightforward and cost-effective.