Increase in stability against thermal oxidation of soil humic substances as a result of self association

The stability to thermal oxidation of soil humic extracts saturated with H, Na, Ca, or Al, was followed after treatment with relatively polar organic compounds, such as methanol, formic acid, and acetic acid. While thermal characteristics of H-humates did not change upon addition of the polar molecules, thermal decomposition (oxidation) of Na-humates was shifted to much higher temperatures (750–830 °C) than control. Substantially less dramatic was the effect on Ca-humates, whereas hardly any alteration was observed when polar organic compounds were added to Al-humates. These results can be explained by considering the forces that hold humic molecules together. Humic components are strongly bound to each other by hydrogen bonding in H-humates and by electrostatic bridges in Ca- and Al-humates. These binding forces were not overcome by the simple addition of polar organic molecules, and their stability remained generally unchanged. In Na-humates, associations of humic molecules are held together only by non-specific hydrophobic interactions. Our results showed that wetting the relatively more flexible Na-humates with organic solvents slightly less polar than water caused a significant increase in thermal stability. Because most polysaccharide-C has largely disappeared at 400 °C, this thermal behaviour can be explained by the rearrangement of largely hydrophobic humic components in methanol, leading to an increase in association energy. The intensity and reversibility of thermal stabilization indicate that association occurred among relatively small molecules rather than among macromolecules. Also Na-saturated humus of bulk samples showed an increase in stability against oxidation upon addition of methanol. These findings suggest that, counter-ions and amphiphilic organic compounds may affect organic matter stability also in natural soils.