Experimental studies on CO2 sequestration via enhanced rock weathering in seawater: Insights for climate change mitigation strategies in coastal and open ocean environments

Abstract

Enhanced weathering (EW) of ultramafic rocks from the Muslim Bagh Ophiolite, Pakistan, has been studied in laboratory experiments to explore carbon sequestration as a climate change mitigation strategy for coastal and open sea environments. The research focused on a cost-effective ex situ experiment to examine the effects of EW reaction pathways arising from the interactions among rock powder, seawater and CO₂. The experimental filtrates from different milled peridotite samples exhibit a decrease in the Mg/Ca ratio as the specific surface area increases, which accelerates reaction rates. This suggests that the leached Mg from the original rock may have been consumed in the formation of brucite, serpentine and carbonates during EW. Similar reaction pathways are also responsible for the chemical alterations observed in amphibolite, albeit to varying degrees. On the other hand, the experimental residues showed an increase in loss on ignition compared to the original rock, indicating that EW has facilitated the incorporation of H₂O and CO₂ into secondary mineral structures through various reaction pathways, leading to the formation of brucite, serpentine and carbonates. Thermal gravimetric analysis of the experimental residues confirms the presence of these minerals based on their decomposition temperatures. Additionally, XRD analysis identified a range of carbonates in the residues of both peridotite and amphibolite samples, validating the occurrence of carbonation reactions. SEM images reveal textural changes in both samples, supporting the formation of secondary minerals through EW, consistent with observations from the petrographic study of untreated samples. Control experiments on CO₂ absorption in seawater showed a decrease in pH, highlighting ocean acidification from increased CO₂ emissions. However, when rock powder was added to the seawater-CO₂ mixture, the pH increased. This suggests that the EW of ultramafic rock powders can sequester CO₂ while raising seawater pH through the formation of secondary minerals. This research could serve as an analog for EW applications, considering the worldwide abundance of ultramafic rocks and the availability of coastal and open ocean environments. However, further research is required to understand the behavior of other elements and their impacts on ocean chemistry in EW processes before applying CO₂ sequestration strategies.