Effects of Zn2+ on Limestone Weathering and Carbon Sink in the Chaotian River Basin, Guilin, China

In this paper, the generation of inorganic carbon (mainly HCO3−) in a karst system (soil-limestone system) under karst soil and non-karst soil conditions was investigated using two Zn2+ concentrations and water flow rates. The results showed that (1) the dynamic equilibrium state of the chemical weathering of limestone is altered by Zn2+, which is the primary cause of the change in HCO3− in soil-limestone systems; (2) ion exchange and adsorption are the primary characteristics of Zn2+ depletion under 1 mg/L ZnCl2 settings, whereas Zn2+ under 50 mg/L ZnCl2 conditions created two new solid phases (Zn5(OH)6(CO3)2, ZnCO3) in the soil-limestone system; (3) the dissolution rate of limestone increases with the water flow rate, which facilitates the dissolution process; (4) the notable difference in ion release between non-karst and karst soil conditions could potentially be attributed to variations in the mineral composition, specific surface area, and particle size of the two soil types; (5) the combination of SEM, XPS, FT-IR, and XRD microstructure observation methods reveals that when limestone is exposed to a high flow rate (1.23 mL/min) and a high concentration (50 mg/L) of ZnCl2, it experiences obvious dissolution and surface precipitation phenomena, as well as a significant change in HCO3− content.