FRACTIONATION OF AQUATIC NATURAL ORGANIC MATTER UPON SORPTION TO GOETHITE AND KAOLINITE

Abstract

Natural organic matter (NOM) consists of a complex mixture of organic molecules; previous studies have suggested that preferential sorption of higher molecular weight, more hydrophobic, and more aromatic components may lead to 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 (Mw) upon sorption of bulk (rather than isolated) 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°C. High pressure size exclusion chromatography (HPSEC) was used to quantify changes in Mw upon sorption, and molar absorptivities at λ=280 nm were used to approximate changes in solution NOM aromaticity. Two SR water samples were used, with Mw=2320 and 2200 Da; a single GDS sample was used, with Mw=1890 Da. The SR NOM was slightly more hydrophobic and aromatic. These differences were reflected in greater sorption of SR NOM than GDS NOM. Both surface water NOMs showed a much greater affinity for goethite than for kaolinite. HPSEC analysis of the NOM remaining in solution after 24 h reaction time with goethite revealed that the largest changes in solution phase Mws (decreases by 900-1700 Da) occurred at relatively low equilibrium sorbate concentrations (approximately 5-20 mg C l-1); the decrease in solution Mw suggested that reactive surface sites were occupied disproportionately by large and intermediate size NOM moieties. At higher equilibrium NOM concentrations (>20 mg C l-1), as percent adsorption decreased, Mw in solution was similar to original samples. A smaller decrease in solution NOM Mw (300-500 Da at 10-20 mg C l-1 ~100 Da at >20 mg) also occurred upon sorption to kaolinite. Overall, our results showed that factors (as related to NOM composition, clay mineral surface properties, and position along the sorption isotherm) which promote a higher percent sorption lead to the most pronounced decreases in solution Mw.