Amino acid assisted Ca2+ leaching-CO2 mineralization: Thermodynamic equilibrium analysis and experimental investigation

Abstract

An amino acid-mediated Ca2+ leaching-CO2 mineralization process was proposed to address the challenges associated with slow kinetics and extensive consumption of exogenous reagents in CO2 mineralization. While the underlying reaction mechanism between amino acid and alkaline minerals/metals remains unclear, this study explored the interactions between amino acid-species and Ca-bearing minerals/Ca2+ by investigating the evolution of active compounds from the perspectives of thermodynamic equilibrium analysis and experimental study. Thermodynamic equilibrium analysis showed that the active Ca-bearing minerals of coal fly ash (CFA) varied in the systems of ammonium salt, acid, and amino acid, e.g., the active phases in glycine were portlandite (Ca(OH)2), lime (CaO), and gehlenite (2CaO*Al2O3*SiO2). High Ca2+ concentrations of 0.26 M, 0.14 M, and 0.1 M can be achieved after 3 min of leaching assisted by amino acids of Glycine (Gly), L-Alanine (Ala), and L-Arginine (Arg), respectively, which implied the kinetics and thermodynamical feasibility. This phenomenon was demonstrated to relate to the fact that Gly and Ala had minimal pH buffering capacity in the early leaching stage. The deprotonated forms of Gly and Ala with a higher affinity with Ca2+ increased from 0.03 % and 0.04 % to 33.9 % and 12.88 %, respectively, thereby promoting Ca2+ leaching. The Ca2+ utilization efficiencies from CFA were 26.45 %, 15.0 %, and 13.3 % for Gly, Ala, and Arg, respectively. Gly achieved the best CaCO3 yield of 114.8 g/kg, due to the combined effects of Gly’s buffering capacity and strong chelation effect for Ca2+, which drove forward the dissolution of the active phase and precipitation of CaCO3.