Differences in Humic Acid Characteristics as Determined by Carbon‐13 Nuclear Magnetic Resonance, Scanning Electron Microscopy, and Infrared Analysis

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Abstract
An investigation was conducted on the usefulness of 13C nuclear magnetic resonance (NMR) spectroscopy in the characterization of humic acids (HA) isolated from selected soils. Soil samples for extraction of HA were collected from Indonesia, USA, and Argentina. The analyses were conducted by liquid 13C NMR, scanning electron microscopy (SEM), infrared (IR), and chemical analysis. The NMR spectra indicated that all HA samples had in common aliphatic, polysaccharide, aromatic, and carboxyl groups. However, the NMR signals for the chemical groups were different in intensities, producing a distinctive spectrum for each HA studied. The NMR analysis demonstrated that HA from the tropical region soils were highly aliphatic in nature, with a combined carbohydrate and polysaccharide C content of 59.5 to 67.6%. On the other hand, HA from the seven temperate region soils contained more COOH and aromatic C. The highest aromatic C concentration together with the strongest NMR signals for aromaticity were exhibited by HAs of the Mascotte (Ultic Haplaquod) and the Argentina (suborder unknown) soil. The differences in NMR spectra were accompanied by differences in SEM micrographs and IR spectra. A significant correlation was noted between NMR and especially IR spectra. Humic acid samples with strong NMR signals for aromatic C exhibited IR spectra with strong bands at 1240 cm−1, whereas HA with weak NMR signals for aliphatic C had IR spectra with weak bands at 2980 to 2900 cm−1. Therefore, IR spectroscopy can also be an important tool in the characterization of humic matter when used in conjunction with 13C NMR.
Funding Information
  • State and Hatch funds