Multimethod characterization of lake aquatic humic matter isolated with sorbing solid and tangential membrane filtration

Abstract

According to this work, it is evident that structural alterations take place in the organic solutes defined as humic substances, in addition to the removal of certain “impurities”, during specific isolation of aquatic humic solutes at preadjusted acidity by non-ionic sorbing solids (e.g. XAD-resins). The acidic partitioning of the humic solutes at strongly acidic conditions into humic- (HA) and fulvic-acid (FA) fractions does not lead to a chemically unambiguous result, especially in the case of humic solutes with smaller molecular sizes (most significant fraction of humic solutes). The combined molecular formula (C,H,O) recalculated from distinct HA- and FA-entities, after acid precipitation of the smaller size humic solutes, was ca. 50% greater than that obtained for their corresponding unseparated HA↔FA mixtures. Also, the combined relative unsaturation (mainly due to aromatics and in particular cycloalkyl groups) recalculated from distinct HA- and FA-entities after acid precipitation of the smaller size humic solutes was ca. 10% greater than that obtained for their corresponding unseparated mixture. The combined degree of substitution of aromatic rings recalculated from distinct HA- and FA-entities after acid precipitation was, on an average, 40% greater than that estimated for their total unseparated mixture, and even ca. 90% greater in the case of smaller size humic solutes. The combination of the distinct HA- and FA-subfractions with different molecular sizes generates a molecular formula close to that obtained for the corresponding individual HA- and FA-solutes, respectively (relative differences of C, H and O: 5±2%). Also, the composition, based on the NMR spectra of different types of carbons and hydrogens, was particularly similar ( r=0.98 and 0.82, respectively) for the computationally combined distinct HA- and FA-subfractions with different molecular sizes and for the corresponding individual HA- and FA-solutes, respectively. Isolation/fractionation of aquatic humic solutes with the XAD technique appears to occur with nearly the same mechanism independent of the molecular size or of the degree of polydispersity of the solutes. Therefore the isolated humic solutes (especially the unseparated mixture of HA and FA) must play a role as certain definite entities in the dissolved organic matter (DOM) and they cannot be merely accidental products of the isolation processes, though they do not seem to be fully in accord with the original DOM. With a computational method, the most probable carbon distribution was generated for a humus sample.