Molecular-level insights into the effects of Ca2+-induced aggregation and CaCO3 coprecipitation on humic acid components in groundwater.

During the groundwater recharge to reclaimed water, dissolved organic matter (DOM) might be affected by Ca2+-induced aggregation and coprecipitation with CaCO3. In this study, humic acid (HA) was taken as the representative of DOM. Batch experimental methods combined with three-dimensional fluorescence and FT-ICR MS were used to investigate the changes in DOM components during the aggregation and coprecipitation in water at the molecular level. The results showed that Ca2+ tended to bind with high aromatic and carboxyl-rich components in HA, leading to aggregation and precipitation. During this process, the contribution of number of carboxyl groups in alicyclic molecules was more significant than that of the hydrophobicity of aromatic molecules. Among alicyclic molecules, those containing CHONS exhibit a stronger tendency to aggregate and precipitate than those containing CHO, CHON, or CHOS. As the initial HA concentration decreased from 24 to 6 mg C/L, the CaCO₃-normalized partition coefficient of residual HA in the supernatant decreased after aggregation and precipitation, due to the removal of highly unsaturated oxygen-containing components prone to coprecipitation with CaCO3. In this process, the effects of pH decrease and Ca-induced further aggregation were negligible. These findings are helpful to understand the effects of typical hydrogeochemical processes on HA components during reclaimed water recharge to groundwater.