Different mechanisms driving the preferential adsorption of dissolved organic matter by goethite and montmorillonite

Molecular fractionation of dissolved organic matter (DOM) at the mineral-water interface governs its role in biogeochemical processes in both terrestrial and aquatic environments. However, adsorptive fractionation of DOM onto phyllosilicates lacks molecular-scale investigation. Moreover, the mechanisms driving DOM adsorptive fractionation onto typical mineral surface remains unknown. Using ultra-high resolution mass spectrometry, traditional spectroscopic method, and different background electrolytes (CaCl2, NaCl and NaCl-NaH2PO4), we investigated the molecular fractionation of DOM by typical phyllosilicate (montmorillonite) and iron oxyhydroxide (goethite) and provided direct and quantitative evidence for the specific driving mechanisms for the adsorptive fractionation. Montmorillonite selectively adsorbed the comparatively low molecular weight (MW) and high O/C constituents having aliphatic moieties (mainly protein- and carbohydrate-like species) or one benzene ring as first identified using a new aromaticity equivalent. This structural preference was more remarkable in the adsorption system with large contribution of “Ca2+ bridging” reaction (45.6–76.0%). By contrast, goethite preferentially stabilized the constituents with high MW components containing aromatics with ≥3 condensed benzene rings and high O/C, which was notable in the system where “ligand exchange” mechanism predominated (55.5–71.4%). For both montmorillonite and goethite, non-selective adsorption of DOM was detected when van der Waals dominated. These findings would advance the mechanistic understanding of biogeochemical processes of DOM in the environments.