Effects of soil moisture content on CO₂ triggered soil physicochemical properties in a near-surface environment

Purpose: Carbon capture and storage (CCS) has been frequently discussed as a strategy for meeting CO₂ emission reduction and its targets. However, some critical issues have been raised with regard to the potential hazards to near-surface environments by CO₂ leakage. Hence, this study was conducted to examine the effects of soil moisture content on the potential risks associated with a high concentration of CO₂ to change physicochemical properties of a near-surface soil. Materials and methods: Soil samples for a batch experiment were collected from an abandoned gold mine, South Korea. Batch experiments were conducted in a specially designed CO₂ glove box (95–99 vol.% CO₂) at four different moisture contents (0, 15, 27.5 wt.%, and a suspension of soil to water ratio 1:10) and different reaction times up to 30 days. A control experiment was conducted at the ambient conditions open to the atmosphere otherwise the same conditions. Physicochemical properties of the soil samples (i.e., pH, mineralogy, organic matter (OM), inorganic carbon (IC), and cation exchange capacity (CEC)) as well as leachability of metal(loid)s were examined before and after the CO₂ incubation. Results and discussion: Results showed that the pH of the CO₂-incubated partially wet (15 and 27.5 wt.%) and suspension samples rapidly decreased by about one unit (P < 0.01) in 2 h and gradually recovered in 10 days. It was noteworthy that CO₂ was adsorbed to the dry soil (0 wt.%) and led to acidification as the soil pH dropped from 5.56 ± 0.02 to 4.5 ± 0.03. Soil organic matter content and cation exchange capacity decreased with increasing moisture content in the CO₂-incubated samples. A noticeable increase in the leachability of Fe, Al, Ca, Mg, Mn, K, and Zn were observed in the CO₂-incubated soil suspensions. In contrast, the leachability of metal(loid)s in the dry and partially wet samples were not remarkable, although the soil pH decreased significantly. Conclusions: Our findings highlighted the effect of CO₂ exposure on the soil acidification, even to soils in unsaturated zones, and stressed to monitor simultaneously soil physicochemical properties and metal(loid)s leaching induced by CO₂ exposure in the near-surface environment. [Figure not available: see fulltext.].