Fractionation of aquatic natural organic matter upon sorption to goethite and kaolinite

Natural organic matter (NOM) consists of a complex mixture of organic molec ules; previous studies have suggested that preferential sorption of higher molecular weight, more hydrophobic, and more aromatic components may lead t o fractionation of the NOM pool upon passage through porous media. Our work expands upon previous studies by quantifying the change in solution-phase weight average molecular weight (M-w) upon sorption of bulk (rather than is olated) surface water NOM from the Suwannee River (SR) and the Great Dismal Swamp (GDS) to goethite and kaolinite at different sorption densities and at pH 4, 22 degrees C. High pressure size exclusion chromatography (HPSEC) was used to quantify changes in M-w upon sorption, and molar absorptivities at lambda = 280 nm were used to approximate changes in solution NOM aromat icity. Two SR water samples were used, with M-w = 2320 and 2200 Da; a singl e GDS sample was used, with M-w = 1890 Da. The SR NOM was slightly more hyd rophobic and aromatic. These differences were reflected in greater sorption of SR NOM than GDS NOM. Both surface water NOMs showed a much greater affi nity for goethite than for kaolinite. HPSEC analysis of the NOM remaining i n solution after 24 h reaction time with goethite revealed that the largest changes in solution phase M(w)s (decreases by 900-1700 Da) occurred at rel atively low equilibrium sorbate concentrations (approximately 5-20 mg C 1(- 1)); the decrease in solution M-w suggested that reactive surface sites wer e occupied disproportionately by large and intermediate size NOM moieties. At higher equilibrium NOM concentrations (> 20 mg C 1(-1)), as percent adso rption decreased, M-w in solution was similar to original samples. A smalle r decrease in solution NOM M-w (300-500 Da at 10-20 mg C 1(-1) similar to 1 00 Da at > 20 mg) also occurred upon sorption to kaolinite. Overall, our re sults showed that factors las related to NOM composition, clay mineral surf ace properties, and position along the sorption isotherm) which promote a h igher percent sorption lead to the most pronounced decreases in solution M- w. (C) 1999 Elsevier Science B.V. All rights reserved.