Quantitative elemental and structural analysis of dissolved organic carbon fractions from lakes near Sudbury, Ontario

Dissolved organic carbon (DOC) was quantitatively isolated as five separate fractions from two Sudbury area lakes (Ontario, Canada) using the tandem XAD-8/XAD-4 resin technique. Fulvic acids made up approximate to=48% Of the total, humic acids approximate to 8%, hydrophobic neutrals approximate to 8%, XAD-4 acids approximate to 11%, and XAD-4 acetonitrile extracts approximate to 13%. Hydrophobic:hydrophilic ratios mere about 65:35. Solid-stare CPMAS C-13 NMR indicated a primarily aliphatic nature in all fractions. Percent aromaticity was in the order humic acids,fulvic acids > XAD-4 acetonitrile extracts approximate to XAD-4 acids approximate to hydrophobic neutrals. Titration analysis showed that the C-13 NMR peaks of fulvic acids, humic acids, and XAD-4 acids from 160-190 ppm were primarily carboxyl in nature. The XAD-4 acids were the most acidic, suggesting that they may be important geochemical agents. C-13 NMR indicated that polysaccharides formed <20% of any fraction. The two neutral fractions (hydrophobic neutrals and XAD-4 acetonitrile extracts) had relatively high N contents, possibly due to the acetonitrile used to extract them. However, the C-13 NMR spectra did not show significant quantities of acetonitrile. The elemental and structural composition covaries in the three acid fractions, such chat if one acid is enriched or depleted, so are the others. This suggests that the humic and non-humic acids form a continuum in which all fractions are affected by the same formative processes. Compared to published data for other freshwater samples, the three acid fractions from the Sudbury area lakes have low or very low aromaticity. This may indicate that the acid and metal loadings in the Sudbury area have affected the structure of the organic acids, despite the circumneutral character of the study lakes. These are the first quantitative C-13 NMR results published for the hydrophobic neutrals and XAD-4 acetonitrile ex-tracts, so comparisons are not possible.