Conformational Analysis of Humic Acids from Amazonian Dark Earth Soils observed by AFM

Conformational analysis of humic acid (HA), extracted from an Amazon anthropogenic soil and an adjacent soil which did not have anthropogenic A horizon, was done by atomic force microscopy (AFM), through the deposition of humic acid layers on muscovite mica strips using drop-casting deposition technique. This short communication presents, for the first time, AFM images of the sub-micron level structure of humic acid of the Amazonian Dark Earths, compared to natural pedogenic Amazonian soil. The adsorbed anthropogenic HA form ring-shaped aggregates with diameters on the scale of several tens of nanometers, possibly showing evidences of a supramolecular formation. The formation of these structures was not verified for HA from pedogenic soil. In this case, it was observed particles with globular shape and a homogenous distribution of them on the mica surface. Structural characteristics of each sample were observed by nuclear resonance magnetic spectroscopy (NMR). The spectra analysis indicated that pedogenic HA are richer in aliphatic groups, as methoxylic and in polysaccharides structures, while the higher concentrations of aromatic carbon, including also phenolic carbon, were observed in the anthropogenic HA, justifying its higher hydrophobic character.