Characterization of humic like substances obtained by chemical oxidation of eucalyptus charcoal

Charcoal of Eucalyptus grandis and Eucalyptus saligna was produced with different final charring temperatures. Ground charcoal samples (120 mesh) were treated with HNO3 25% (w/w) during 4 h under reflux conditions. Humic (HA) and fulvic acids (FA) were extracted from the reaction products utilizing the standard method recommended by the International Humic Substances Society (IHSS). The HA and FA yields were obtained by quantifying carbon in each fraction. The highest HA yields were obtained at 450 °C final charring temperature (430 mg C-HA g−1charcoal), and the highest FA yields at 300–350 °C (310–304 mg C-FA g−1 charcoal). Elemental analysis, thermogravimetry, FTIR and 13C NMR were used for HA characterization. Significant chemical modifications were observed after acid oxidation, such as increments in nitrogen and oxygen contents, and formation of carboxylic groups. Based on the characteristics of the obtained products, we conclude that charcoal synthetic HAs and natural pyrogenic derived HAs have similar characteristics.