Characterization and diagenesis of strong-acid carboxyl groups in humic substances

• Published in: Applied geochemistry Vol. 18; no. 3; pp. 471 - 482
• Main Authors: LEENHEER, J. A, WERSHAW, R. L, BROWN, G. K, REDDY, M. M
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier 01.03.2003
• Subjects: Earth sciences; Earth, ocean, space; Exact sciences and technology; Freshwater; Soils; Surficial geology; United States; Georgia; USA, Suwannee R; USA, Florida, Suwannee R; diagenesis; fractionation; models; processes; density; stabilization; humic substances; North America; nuclear magnetic resonance; fulvic acids; molecular structure; acidity; C-13; carboxylic acids; pH; protons; NMR spectra; oxygen; hydrogen bond; infrared spectroscopy
• ISSN: 0883-2927; 1872-9134
• DOI: 10.1016/s0883-2927(02)00100-2

A small fraction of carboxylic acid functional groups in humic substances are exceptionally acidic with pK sub(a) values as low as 0.5. A review of acid-group theory eliminated most models and explanations for these exceptionally acidic carboxyl groups. These acidic carboxyl groups in Suwannee River fulvic acid were enriched by a 2-stage fractionation process and the fractions were characterized by elemental, molecular-weight, and titrimetric analyses, and by infrared and super(13)C- and H-nuclear magnetic resonance spectrometry. An average structural model of the most acidic fraction derived from the characterization data indicated a high density of carboxyl groups clustered on oxygen-heterocycle alicyclic rings. Intramolecular H-bonding between adjacent carboxyl groups in these ring structures enhanced stabilization of the carboxylate anion which results in low pK sub(a1) values. The standard, tetrahydrofuran tetracarboxylic acid, was shown to have similar acidity characteristics to the highly acidic fulvic acid fraction. The end products of 3 known diagenetic pathways for the formation of humic substances were shown to result in carboxyl groups clustered on oxygen-heterocycle alicyclic rings.