Chemical and biological studies on environmental humic acids II. ¹H-nmr and IR spectra of humic acids

Abstract

Forty humic acids from various types of soils were examined by ¹H nuclear magnetic resonance (NMR) and infrared (IR) spectroscopy. The ¹H-NMR spectra were characterized by peaks which were attributed to aromatic, lactone, methoxyl, and aliphatic protons a, ß, and y to aromatic carbon rings. Andisol humic acid showed high percentages of aromatic protons and low percentages of aliphatic and methoxyl protons. Sediment and Inceptisol humic acids showed low aromatic proton and high aliphatic proton percentages. Based on the relationships between the H/C ratio and various proton percentages, it was found that with the progression of humiflcation, aliphatic side chain and methoxyl groups split off, and condensed aromatic rings were introduced into the humic molecules. The characteristic absorption of lignin appeared in the IR spectra of Entisol, Histosol, and Inceptisol humic acids. Entisol, Oxisol, and sediment humic acids showed a strong absorption arising from peptides. Andisol humic acid showed no evidence of the presence of a lignin moiety or peptide linkages.

Key Words:

• ¹H-NMR spectra
• humic acid
• humiflcation process
• IR spectra
• soil types