| Humic substances are the natural organic macromolecules and ubiquitous in the environment, which play an important role on the global carbon cycle; the formation of disinfection byproducts in drinking water processing; the movement, transformation and fate of the organic and inorganic compounds; the improvements of the soil fertility. All these behaviors were related to their structural composition. Therefore, it was important to fully understand their compositional characterization. However humic substances are heterogeneous in nature with complex composition. It will be necessary to divide them into relative homogeneous molecules for the better understanding of these substances.In this work, humic acid extracted from a Pahokee peat was divided into eight fractions by progressively ultrafiltration (UF1:<1K; UF2:1-3K; UF3:3-5K; UF4:5-10K; UF5:10-30K; UF6:30-50K; UF7:50-100K: UF8:>300K). The whole humic acid and the ultrafiltration fractions were characterized by various techniques: high performance size exclusion chromatography(HPSEC), elemental composition, Fourier transformed infrared spectra (FTIR), UV-Vis spectra, solid state 13C -nuclear magnetic resonance (13C-NMR) spectrometry and pyrolysis-gas chromatography-mass spectrometry (Py-GC-MS). All the studies showed that the eight ultrafiltration fractions differ from each other on chemical composition. After ultrafiltration, a narrow size distribution was found in the UF fractions by HPSEC, indicating an improved relative homogeneous. Along with the increasing molecular weight(Mw), the content of C^ H and H/C atomic ratio was increased, but content of O and 0/C atomic ratio was reduced; more oxygen-containing groups and aromatic structures but little aliphatic structures were found in the low Mw fractions by FTIR> '3C-NMRx Py-GC-MS, and reversed for high Mw fractions; Py-GC-MS also indicated that the aromatic structures were derived from the higher terrestrial plants and aliphatic structure were derived from aliphatic biopolymers both of microbes and higher plants.In addition, HA fractions obtained by progressively base extraction were also studied. The results showed that as a function of the extent of extraction, the content of C> H and H/C atomic ratio was increased, but O and O/C atomic ratio was reduced; the apparent Mw exhibited an increasing tendency; oxygen-containing groups and aromatic structures reduced, aliphatic structures increased and turned to finite molecular configurations; pyrolysis results showed that the aromatic structures were derived from the higher terrestrial plants and aliphatic structure were derived from aliphatic biopolymers, it farther demonstrated that the humic acid extracted earlier contained more gymnosperm lignin, but the humic acid extracted latter contained more grass lignin.The sorption isotherm of humic acid and the different fractions by different separationmethod to the hydrophobic organic compounds-phenanthrene showed that a linear sorption behavior was found in the low concentration but nonlinear was found in the higher concentration. Moreover, the humic composition had an important effect on its sorption to hydrophobic organic compounds: a higher sorption potential and more nonlinear sorption behavior was found in the humic with the higher contents of C and higher Mw, but reversed for the increment of polarity. Furthermore, the hydrophobic aliphatic structure was more effective than the aromatic structure for the sorption of phenanthrene. The sorption potential was enhanced with the increased content of aliphatic chains and decreased contents of aromatic structure.The result of this work indicated that the humic macromolecule was composed of many small humic molecules with different Mw and chemical composition and playing different role on the environment. The smaller humic molecules exhibiting a lower Mw, higher content of O, more lignin-derived aromatic structure, and a higher polarity, so they were more easily be extracted and will have more effect on the polar compounds and metal ions; while, lar...
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