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DIVISION S-2—SOIL CHEMISTRY

Chemical and Molecular Heterogeneity of Humic Acids Repetitively Extracted from a Peat

^a State Key Lab. of Organic Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
^b Dep. of Civil, Architectural, and Environmental Engineering, Drexel Univ., Philadelphia, PA 19104

^* Corresponding author (weilin.huang{at}drexel.edu)

Eight humic acid (HA) fractions obtained by repetitive base extraction of a peat soil were characterized in terms of chemical compositions, functionalities, and molecular sizes using an elemental analyzer, infrared spectrometry (IR), solid-state ¹³C-nuclear magnetic resonance spectrometry (¹³C-NMR), and high performance size-exclusion chromatography (HPSEC). Large variations in chemical, functional, and molecular properties were observed among the eight humic fractions. The elemental analysis data showed that, from the first (Fr1) to the eighth fraction (Fr8), the O/C atomic ratio decreased from 0.52 to 0.36 whereas the H/C atomic ratio increased from 1.1 to 1.5. The measured average apparent molecular size (M_w) increases from 7.7 to 22.1 kDa as a function of the extent of extraction. Carbon-13 NMR and FTIR spectra of the fractions indicated that the contents of oxygen-containing and aromatic functional groups decreased and the contents of aliphatic groups increased from Fr1 to Fr8. The results suggested that two subunits of HAs might exist: an aliphatic subunit having larger apparent M_w and an aromatic subunit having smaller apparent M_w. We proposed that each of the eight HA fractions is a mixture of these two subunits.

Abbreviations: ¹³C-NMR, solid-state ¹³C-nuclear magnetic resonance spectrometry • HA, humic acids • HPSEC, high performance size exclusion chromatography • IHSS, International Humic Substances Society • IR, infrared spectrometry • M_W, apparent molecular size

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