Mechanistic limits of ethylenediamine-assisted CO₂ mineralization in alkaline brines

This study investigates whether enhanced CO₂ capture by ethylenediamine (Amine 4) promotes carbonate mineralization in CaCl₂- and MgCl₂-based brines. Total inorganic carbon (TIC) increased systematically with ethylenediamine concentration, reaching 1.28 g L⁻¹ at 0.3 wt% and 1.55 g L⁻¹ at 0.6 wt% amine for both CaCl₂- and MgCl₂-based formulations, confirming enhanced CO₂ uptake without a corresponding increase in carbonate precipitation. Although amine addition significantly increased CO₂ uptake, it did not improve carbonate formation because captured CO₂ was predominantly stabilized as carbamate, thereby reducing the availability of free CO₃^2– ions required for mineralization. In Mg-based systems, no MgCO₃ was detected: at 0.3 wt% amine, the precipitate consisted entirely of halite, while at 0.6 wt% amine Mg²⁺ was removed primarily as brucite, and further alkalinity led to almost exclusive salt crystallization. In Ca-based systems, limited carbonate formation occurred only within a narrow window: at 0.3 wt% amine, a small carbonate fraction (∼7%) composed of vaterite and calcite was observed, while at 0.6 wt% amine carbonate was dominated by metastable vaterite (∼71%) through kinetically driven nucleation. However, this carbonate window collapsed under increased alkalinity, yielding hydroxide-dominated solids and suppressing permanent CO₂ storage. Overall, the results demonstrate that under ethylenediamine-assisted and highly alkaline conditions, increased CO₂ capture does not translate into enhanced mineralization, revealing a strong chemical decoupling between carbamate-based capture and carbonate formation in Ca-Mg brines.