Impact of Operational Parameters on the CO2 Absorption Rate and Uptake in MgO Aqueous Carbonation—A Comparison with Ca(OH)2

The CO2 absorption rate and total uptake by MgO aqueous suspensions were investigated in batch experiments by systematically varying MgO concentrations (0.5–5 wt.%), CO2 flow rates (0.5–2 L/min), temperatures (278–363 K), NaCl salinities (0–7 wt.%), Na2SO4 and K2SO4 concentrations (0–10.5 wt.%), and gas–liquid mixing systems (pipe outlet and porous stone sparger). Results show that temperature strongly controls the carbonation process: increasing temperature above 303 K consistently reduced both the CO2 absorption rate ηt and the total CO2 uptake VCO2 due to the destabilization of metastable Mg(HCO3)2 solutions and accelerated precipitation of less soluble hydrated magnesium carbonates. Under optimal low-temperature conditions (278–283 K, 1–1.5 wt.% MgO, sparger mixing, pure system), the average capture efficiency reached ≈ 35%, with maximum peaks over 70% and total CO2 uptakes of ≈ 12–17 L. Adding NaCl at typical seawater levels (3.5–7 wt.%) slightly increased CO2 uptake at temperatures above 323 K. Sulfate ions (Na2SO4 and K2SO4) were found to enhance the absorption rate at low concentrations (<2 wt.%) but reduce it at higher levels, with no significant impact on the total CO2 uptake observed in this study. Using a CO2 sparger significantly improved gas–liquid contact, achieving average CO2 capture efficiencies ηmaxt above 70% at low temperatures, compared to <20% with simple pipe bubbling. A direct comparison with Ca(OH)2 aqueous carbonation confirmed that, despite its lower solubility and slower kinetics, MgO can outperform Ca-based systems under specific conditions. These results provide practical experimental benchmarks and process guidance for designing Mg-based aqueous carbonation systems, including applications that use brines, industrial wastewater or seawater.