Adsorption of CO₃^2-/HCO₃^- on a quartz surface: cluster formation, pH effects, and mechanistic aspects

Soluble inorganic carbon is an important component of a soil carbon pool, and its fate in soils, sediments, and underground water environments has great effects on many physiochemical and geological processes. However, the dynamical processes, behaviors and mechanism of their adsorption by soil active components, such as quartz, are still unclear. The aim of this work is to systematically address the anchoring mechanism of CO₃^2- and HCO₃^- on a quartz surface at different pH values. Three pH values (pH 7.5, pH 9.5 and pH 11) and three carbonate salt concentrations (0.07, 0.14 and 0.28 M) are considered, and molecular dynamics methods are used. The results indicate that the pH value regulates the adsorption behavior of CO₃^2- and HCO₃^- on the quartz surface by affecting the CO₃^2-/HCO₃^- ratio and the surface charge of quartz. In general, both HCO₃^- and CO₃^2- ions were able to adsorb on the quartz surface and the adsorption capacity of CO₃^2- is higher than that of HCO₃^-. HCO₃^- ions tended to uniformly distribute in an aqueous solution and contact the quartz surface in the form of single molecules instead of clusters. In contrast, CO₃^2- ions were mainly adsorbed as clusters which became larger as the concentration increased. Na⁺ ions were essential for the adsorption of HCO₃^- and CO₃^2-, because some of the Na⁺ and CO₃^2- ions spontaneously associated together to form clusters, promoting the clusters to be adsorbed on the quartz surface through cationic bridges. The local structures and dynamics trajectory of CO₃^2- and HCO₃^- showed that the anchoring mechanism of carbonate solvates on quartz involved H-bonds and cationic bridges, which changed in relation to the concentration and pH values. However, the HCO₃^- ions mainly adsorbed on the quartz surface via H-bonds while the CO₃^2- ions tended to be adsorbed through cationic bridges. These results may help in understanding the geochemical behavior of soil inorganic carbon and further the processes of the Earth's carbon chemical cycle.