[S-GC38] Solid Earth Geochemistry, Cosmochemistry

[SGC38-06] Enhanced weathering of lizardite-rich sand in seawater: Implications for climate change mitigation in coastal and open sea environments in Oman

*Arshad Ali¹, Usman Taura¹, Sami Al Khamisi¹, Amrou Al Alawi¹, Sobhi Nasir¹, Talal Al Hosni¹, Anas Al-Rashidi¹, Zulfiqar Ahmad Rehan¹ (1.Sultan Qaboos University)

Keywords:Peridotite, Lizardite, Serpentinization, CO2 Sequestration, Ocean Acidification

Enhanced Weathering (EW) of peridotites for ex situ CO2 sequestration has recently been investigated for potential applications in coastal and open sea environments using powders of varying grain sizes [1]. The study observed the formation of brucite, serpentine, and carbonates following the EW of olivine-rich peridotite. In situ experiments have suggested that serpentinite rocks may be more reactive due to their preferential dissolution during EW [2]. The present study investigates the use of lizardite-rich sand derived from milling the F-1 rock sample, collected from the Fanja area. Ten grams of sand were added to a flask containing 500 ml of seawater, collected from Al Hail North Beach in Seeb, Muscat. The sand-seawater mixture (SSM) was maintained on a hot plate at 60 degrees Celsius for 30 days, with continuous stirring provided by a magnetic stirrer operating at 250 rpm. The flask was covered with perforated aluminum foil to minimize excessive seawater evaporation while allowing air to interact with the SSM. Over the course of 30 days, about 700 ml of seawater evaporated and was periodically refilled to maintain the 500 ml level. Simultaneously, a separate experiment was conducted under high CO2 pressure (200 psi) in a 500 ml reactor containing 8 grams of rock sand and 400 ml of seawater, with about 100 ml of headspace. The temperature and duration of the experiment were identical for both setups. Finally, the SSMs were filtered using Whatman filter paper #54, and the residues (i.e., EW2-R represents ambient pressure conditions, while EW4-R corresponds to 200 psi) were dried in an oven at 80 degrees Celsius. The anticipated formation of hydrated and carbonated minerals was qualitatively evaluated through loss on ignition (LOI) measurements of the experimental residues at 950 degrees Celsius and XRD analysis. The LOI increase for EW2-R (3.5%) is higher than that for EW4-R (2.0%) relative to the LOI of the untreated F-1 sand. This LOI increase likely indicates the formation of hydrated and carbonated minerals, as identified by XRD analysis, including aragonite, magnesite, and shelkovite in EW2-R, and aragonite, magnesite, and vermiculite in EW4-R (see image 1). Additionally, halite and natrooxalate were observed in EW2-R. The difference in LOI between the two residues is also reflected in the reduction of peak height of the two main lizardite peaks compared to the F-1 untreated sand, indicating the weathering of lizardite into secondary minerals. Furthermore, no amorphisation is observed, in contrast to the findings of [3], as the XRD pattern shows no evidence of peak broadening. On the other hand, the pH values of the respective filtrates decreased from 8.01 (seawater) to 7.21 for EW2-F and 7.13 for EW4-F. This suggests that the water pH changed similarly in both scenarios, leading to acidification. This change may be due to the reduction of bicarbonate species, which act as a buffer in seawater. However, a decrease in bicarbonate is observed only in EW2-F (-29%), whereas EW4-F shows a substantial increase (+2400%). Meanwhile, both filtrates exhibit a similar reduction in carbonates (-96%). The decrease in pH may also be linked with changes in salinity, as suggested by the presence of two halite peaks in EW2-R. However, halite is not detected in EW4-R, despite the pH decrease being similar in both filtrates. The consistent pH reduction in both filtrates cannot be attributed solely to the decrease in salinity. It is likely influenced by either a decrease in salinity in EW2-F and an increase in dissolved inorganic carbon (DIC) in EW4-F and/ or by variations in other ionic species. These findings highlight the potential of EW of lizardite for CO2 sequestration in coastal and open sea environments. However, the observed decrease in pH suggests that the process may exacerbate ocean acidification, making highly serpentinized rock sands unsuitable for ex situ applications.