Effect of the chemical composition of soil humic acids on their viscosity, surface pressure, and morphology

Humic acids were extracted from two histosols, an andosol, an inceptisol, a rice field soil, and a sludge-amended cambisol and characterized by elemental analysis, acidic groups titration, E4/E6 determination, Fourier transform infrared (FTIR) and 13C nuclear magnetic resonance (13C NMR) spectroscopy, viscosity and surface pressure measurements, and scanning electron microscopy. Relevant compositional differences between humic acids depending on their formation conditions were observed. Humic acids from the histosols and the andosol were largely aromatic and rich in acidic functional groups. The inceptisol humic acid contained about the same amount of aliphatic and aromatic moieties and was particularly abundant in soil, probably because it is stabilized by strong interactions with the inorganic components. The paddy soil humic acid was mainly aliphatic and poor in acidic groups because of the partial anaerobic conditions of the rice field. The sludge-amended soil humic acid was similar to humic acids from rapidly decomposed organic matter: low content of carboxylic groups and high content of proteinaceous material. The chemical composition affected, to some extent, the physical behavior of the humic acids. The surface tension was negatively correlated with the concentration of carboxylic groups. The presence of polysaccharides and proteinaceous moieties promoted elongated conformations of humic molecules in solution and directly related to the intrinsic viscosity and consequently to the molecular dimension of humic molecules. The largest aggregates in dry state were observed in humic acids with, besides a high aromaticity, a high content of carboxylic groups.