Deciphering Competitive Interactions of Natural Organic Matter Components at Metal Oxides: Insights from Experiments and Modeling

Natural organic matter (NOM) is a heterogeneous mixture, including humic acid (HA) and fulvic acid (FA), that competitively interacts with metal (hydr)oxides. Despite its environmental importance, this competition has not yet been measured extensively, and mechanistic modeling is lacking. The present work examined the competitive adsorption to goethite and the corresponding molecular fractionation of HA and FA using UV-vis spectroscopy, acid precipitation, and size exclusion chromatography (SEC). Our findings reveal that on a mass basis, FA particles effectively remove HA particles from the surface. This efficiency can be mainly attributed to an interfacial space limitation in which FA restricts HA adsorption, as evidenced by mechanistic modeling with the Consistent Competitive Ligand and Charge Distribution (LCD_cc) approach for the heterogeneous adsorption of NOM. The adsorbed FA particles occupying part of the surface prevent HA from accessing the corresponding double-layer space, disproportionally reducing HA adsorption. This restriction leads to a high HA/FA mass exchange ratio (∼2.4 ± 0.6), consequently affecting the mobility and transport of oxyanions (arsenate and phosphate) in the environment. The difference in the partitioning of NOM is also relevant for soil carbon sequestration via the selective preservation of NOM by association with oxide minerals.