Thermogravimetry of soil humic compounds

Abstract

The thermogravimetry under nitrogen of a humic and a fulvic acid extracted from a podzol soil was investigated. Samples were heated on the thermobalance, withdrawn at regular temperature intervals and characterized by ultimate and functional group analyses and by infrared spectroscopy. In contrast to thermogravimetry under air, the differential thermogravimetric curves of the two humic compounds failed to show maxima indicative of definitive decomposition reactions. Furthermore, higher temperatures were required for the removal of functional groups and for complete decomposition. Chars heated to temperatures close to complete decomposition contained less carbon but more oxygen than chars similarly heated under air. Infrared spectra of the chars were in accord with ultimate and functional group analyses and showed a gradual increase in aromaticity with increase in temperature. Using the H C vs. O C graphical-statistical relationship of Van Krevelen, the main reactions governing the thermogravimetry under nitrogen of the humic acid were: 1. (a) dehydrogenation (between room temperature and 200°C); 2. (b) dehydration (between 200 and 500°C) and 3. (c) dehydrogenation (between 500 and 650°C). Similarly, for the fulvic acid the main reactions were: 1. (a) dehydration (between room temperature and 400°C) and 2. (b) a complex reaction involving both dehydration and deoxygenation (between 400 and 625°C). Atomic H C vs. O C plots for humic and related compounds showed that humic acid can be formed from lignin primarily by oxidation and partially by demethanation. Fulvic acid can result from humic acid by a combination of demethanation and oxidation. Fulvic acid can also arise from lignin via peat by demethanation and oxidation. The conversion of fulvic acid to brown coal can occur by dehydration. The genetic relationships between lignin, humic compounds and brown coal discussed in this paper are shown to be generally consistent with published experimental data.