CO₂ sequestration by gas hydrates in marine environment: investigation of seawater and sediment chemistry

Objective Due to rising atmospheric CO₂ levels, significant scientific efforts have focused on reducing its concentration. One novel approach involves the use of Gas Hydrates (GHs) as a long-term reservoir for CO₂ sequestration, a promising strategy for mitigating climate change by storing CO₂ in marine environments. GHs are crystalline structures formed under lowtemperature and high-pressure conditions, consisting of water molecules arranged in cages that encapsulate various guest molecules, such as CO₂ [1]. This study focuses on the characterization of naturally occurring compounds present in real seawater and natural sediments, aiming to analyze and reveal their effects on CO₂-GHs formation. This characterization is crucial, as most existing studies concentrate on the addition of synthetic chemicals, which may compromise the properties of GHs for CO2 storage and potentially lead to further environmental emissions. Methods Natural sand and seawater sampled on Adriatic Sea were characterized by FE-SEM microscopy and BET measurements. Seawater and pore water samples have been analysed by HPLC-MS and UHPLC-QTOF. The analysis was conducted in both positive and negative ionization modes, with data acquired in SCAN and auto MS/MS modes In addition, Raman analysis on seawater with and without sand has been performed. Results Sewater and pore water samples obtained from SPE extraction were analysed using UHPLC-QTOF. More than 50 different compounds were identified with a total score higher than 75% when compared to NIST library. The analysis revealed certain natural compounds in seawater that could act as kinetic promoters for gas hydrate formation, potentially enhancing CO₂ absorption. Raman analysis was used to analyse the influence of natural sand on the structure and formation of CO₂-GHs [2]. The Raman spectra showed that the presence of sand affects the hydrogen-bond interactions among water molecules, enhancing the symmetric vibration components at lower temperatures, suggesting sand contributes to more ordered structure. Conclusions The study focus on the importance of characterizing seawater and pore water to understand the behavior of gas hydrates (GHs) and the potential for CO₂ storage beneath marine sediments. The proposed method evaluates various organic compounds in real seawater using HPLC-MS and UHPLC-QTOF. Additionally, Raman spectroscopy assesses the structural influence of natural sand sediments, confirming that sand stabilizes the ice-like structure of hydrates through hydrogen bonding interactions between water molecules and silanol groups (Si–OH) ACKNOWLEDGMENTS This study is part of the PRIN 2022 PNRR project entitled “Reliable long-term CO₂ storage as clathrate hydrates in seawater and marine sediments”, funded by the European Union-Next Generation EU.