Thermal stability of humic substances and their metal forms: an investigation using FTIR emission spectroscopy

The thermal decomposition of humic substances extracted from swamp water and their metal forms (Fe(III), Mn(II), Ca(II), Na(I)) in the temperature range of 50-600 degrees C was investigated using infrared emission spectroscopy, a technique which provides in situ information on the mechanism of thermal decomposition of the materials studied. From their IR spectrum, humic substances appear to be relatively stable up to about 300 degrees C. However all spectral information is lost by 600 degrees C. Decarboxylation is observed as the temperature is increased. Anhydrides are seen to form in the temperature range from 150 degrees C to 400 degrees C. There is also the loss of aliphatic hydrogen and the formation of aromatic hydrogen in the temperature range from 400 to 600 degrees C. Upon heating the metal forms of humic substances are less stable and decompose at a faster rate than humic substances, with a concomitant release of carbon monoxide. The thermal stability of the metal forms is in the order: Fe(III) < Mn(II) < Ca(II) < Na(I). The thermal stabilities of sodium and calcium ion adduct of humic substances tend to be related to the ionic radii with the divalent ion being less thermally stable than the monovalent ion. Stronger bonding of the transition metal adducts to humic substances gives metal forms of humic substances with lower thermal stability because of the effect of strain due to the formation of a metal chelate. (C) 1997 Elsevier Science B.V.