Glycine-mediated leaching-mineralization cycle for CO2 sequestration and CaCO3 production from coal fly ash: Dual functions of glycine as a proton donor and receptor

[Display omitted] •Dual functions of glycine as a proton donor and receptor for CO2 mineralization.•Glycine promoted metal leaching, CO2 mineralization, and regulable CaCO3 production.•Spontaneously Glycine regeneration by consuming the H+ produced in carbonation. The state-of-the-art CO2 mineralization technologies involved Ca/Mg leaching and ensued mineralization still struggle with the slow reaction kinetics and frequent pH swings assisted by exogenous chemicals consumption (i.e., low pH for Ca/Mg leaching but high pH for CO2 dissolution and solid carbonates precipitation). This study proposed a glycine-mediated leaching-mineralization cycle (LMC) process, which can simultaneously achieve promising Ca/Mg leaching efficiency, high mineral carbonation efficiency, and production of high-purity CaCO3 from coal fly ash (CFA) at mild operating conditions in an in-situ recyclable amino acid solution. The technical feasibility of the process was initially investigated in individual leaching and mineral carbonation experiments in glycine (Gly0) solutions using a typical CFA. A Ca2+ leaching efficiency of 42.17% and a CaCO3 yield of 89.10 g/kg were achieved in the Gly0 solution. Mineralogy and morphology analysis revealed that the CaCO3 obtained after the carbonation reaction was mainly present as vaterite. The mechanism exploration revealed that Gly-species acted as a proton donor and chelating agent in the leaching step which enhanced the Ca2+ leaching, a proton receptor in mineralization step which accelerated CO2 mass transfer, and a crystal regulator in carbonates precipitation. In addition, the cyclic performance of the LMC process was investigated in multicycle leaching-carbonation experiments. Results showed that the leaching capacities, CaCO3 yield, and Gly0 loss were similar in five cycles of LMC experiments, verifying that the process is stable.