Modeling Sorptive Fractionation of Organic Matter at the Mineral-Water Interface

Sorptive fractionation of organic matter (OM) at mineral-water interfaces alters the composition of OM and therefore affects the fate, transport, and bioavailability of nutrients and contaminants in terrestrial and aquatic environments. It is essential to identify the fractions of OM that are selectively sorbed on minerals and to elucidate the mechanisms of sorptive fractionation. In this study, the partition between mineral and water for a large set of organic molecules covering the most relevant OM functional groups and building blocks were predicted using the mechanistic Polyparameter Linear Free Energy Relationships (pp-LFERs). The results were used to infer the sorptive fractionation tendency of OM moieties. It is found that for a OM moiety belonging to the chemical classes of alkanes, alkenes, or benzenes, its sorption to bentonite is the strongest, followed by kaolinite, talc or hematite, and Al2O3 or quartz, relative to water. For a moiety with either double bond, or -OH, or -COON, or R-COO-R', or R-O-R', or R-COH, or R-CO-R', or -NH2, its partition to water is enhanced as compared to the respective alkane moiety. Opposite is found for thiols and sulfides. Meanwhile, a correlation is developed, that is, as the double bond equivalence decreases and the number of C atoms increases, the degree of sorption increases. Overall, the key factors controlling the sorptive fractionation are polarity, H-bonding, van de Waals dispersion, and the energy for cavity formation. This work provides insights into the OM sorption at the mineral and water interface at the molecular level.