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

Abstract

Conformat ional analysis of humic acid (HA), extracted fro m an A mazon 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 commun ication presents, for the first time, AFM images of the sub-micron level structure of humic acid o f the A mazonian 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 supramo lecular formation. The formation of these structures was not verified for HA fro m pedogenic soil. In this case, it was observed particles with g lobular 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 metho xylic and in polysaccharides structures, while the h igher concentrations of aro matic carbon, including also pheno- lic carbon, were observed in the anthropogenic HA, justifying its higher hydrophobic character.