THERMAL-STABILITY OF HUMIC SUBSTANCES AND THEIR METAL FORMS - AN INVESTIGATION USING FTIR EMISSION-SPECTROSCOPY

The thermal decomposition of humic substances extracted from swamp wat er and their metal forms (Fe(III), Mn(II), Ca(II), Na(I)) in the tempe rature range of 50-600 degrees C was investigated using infrared emiss ion spectroscopy, a technique which provides in situ information on th e mechanism of thermal decomposition of the materials studied. From th eir 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 degree s 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, wit h a concomitant release of carbon monoxide. The thermal stability of t he 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 substan ces 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 hum ic substances with lower thermal stability because of the effect of st rain due to the formation of a metal chelate. (C) 1997 Elsevier Scienc e B.V.