Using selected operational descriptors to examine the heterogeneity within a bulk humic substance

The heterogeneity within a bulk humic substance (HS) was investigated for ultrafiltration (UF) fractions of purified Aldrich humic acid (PAHA) using specific ultraviolet absorbance (SUVA), weight- and number-average molecular weights (MWw and MWn), and organic carbon-normalized pyrene binding coefficients (Koc). As expected, variations in these selected operational descriptors were found for the PAHA UF fractions, but the variations were smaller than the ranges observed for a small subset of different aquatic and terrestrial HS. In general, correlations among the operational descriptors for the different HS failed to predict the correct trends for the PAHA UF fractions. More variation in Koc was observed for the different HS versus the PAHA UF fractions despite comparable molecular weight ranges, suggesting that the extent of pyrene binding is more strongly influenced by HS source and chemical structure than merely by physical structural features. Size-exclusion chromatography (SEC) confirmed the structural stability of PAHA UF fractions, as demonstrated by a near-identical mathematical reconstruction of the original bulk chromatogram from a weighted summation of the individual UF fraction chromatograms. An ideal mixture model accurately predicted some operational descriptors of PAHA properties (i.e., SUVA and MWw), but failed to predict the PAHA reactivity for pyrene binding. Our Koc results at pH 4 and 7 suggest that the relative contribution of two mechanisms, partitioning and specific interactions, may account for the interactions between HS and pyrene.