Characterization and acid-base properties of fulvic and humic acids isolated from two horizons of an ombrotrophic peat bog

Peat humic and fulvic acids were isolated from a surface horizon (0-60 cm) and deeper horizon (>60 cm) of an ombrotrophic peat bog located in Galicia (NW Spain) and which constitutes a record of paleoenvironmental contamination. The humic and fulvic acids were characterized and compared using chemical methods and spectroscopic techniques, including ultraviolet visible (UV-vis), Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (13C NMR). Comparison of the E4/E6 ratios showed that in the deeper horizon the peat humic substances have a greater degree of aromatic condensation than in the surface horizon. The alkyl C content of the humic substances remained practically unchanged, whereas the O-alkyl-C content of humic acids decreased with depth. No variation with depth in the ionization constants of the carboxylic groups or of the phenolic groups was observed. The values of the ratios of the carboxylic/phenolic contents were similar to those reported in the available literature, with a higher content of acid groups in the humic substances isolated from the lower horizon mainly attributable to the carboxylic groups.