Sorption of natural organic matter fractions to goethite (α-FeOOH): effect of chemical composition as revealed by liquid-state 13C NMR and wet-chemical analysis

Abstract

In order to quantify the potential influence of the chemical composition of natural organic matter (NOM) on the extent of its sorption to mineral surfaces, I compared the interaction with synthetic goethite (α-FeOOH) of 18 different NOM (hydrophilic and hydrophobic fractions, fulvic and humic acids) samples from soil and terrestrial surface waters. The chemical composition of NOM was characterised by elemental analysis, titration of acidity, wet chemical analyses of lignin and carbohydrate constituents, and liquid-state 13C NMR spectroscopy. The proportions of resonances due to carboxyl C ranged from 7 to 21% and signals for total aromatic C varied from 0 to 38%. Signals due to O,N-alkyl C comprised 5–65% of the spectral area. The sorption of all tested NOM was adequately described by the Langmuir model. In general, hydrophobic fractions and fulvic and humic acids rich in carboxyl and aromatic C, probably derived from lignin, sorbed much more strongly to goethite than the hydrophilic fractions which were characterised by high abundance of O,N-alkyl C, probably due to carbohydrates, and low abundance of aromatic C. A sample of fulvic acids rich in alkyl C sorbed more weakly than samples rich in aromatic C. Consequently, the strong sorption of the hydrophobic fractions and fulvic acids seemed to be caused by the presence of aromatic structures. Within the groups of samples, however, the major determinant of the extent of the sorption proved to be acidic groups, as revealed by the close relationship between maximum sorption and total acidity and the content of carboxyl C. The maximum sorption of hydrophobic, fulvic and humic acids was inversely related to the ratio of aromatic to carboxyl C, which suggests that the number (and probably the position) of acidic groups attached to aromatics controls the interactions of hydrophobic, fulvic and humic acids with goethite.