Characterization of organic matter of plants from lakes by thermal analysis in a N2 atmosphere

Organic matter (OM) is important as complexing agents or adsorbents of environmental pollutants found in surface waters. OM influences the forms, toxicity, and bioavailability of pollutants; however, conventional methods of characterizing OM are unsatisfactory because they do not capture the complete continuum of behavior of OM1. Thermal methods, which have simple pretreatment and short experimental times, have been shown to be easy, fast, relatively inexpensive yet reliable for characterizing OM2. To date, studies of applications of differential scanning calorimetry (DSC) and thermal gravimetric analysis (TGA) on OM have focused on thermal oxidation of soil organic matter (SOM) or plant biomass in aerobic atmospheres3. Thermal experiments in anaerobic atmospheres focus primarily on mechanisms and products of rapid thermal composition of biomass at stable temperature; however, few investigations of OM by use of slow thermal degradation in N2 atmospheres (STDN) have been conducted to date. Behavior of soil organic matter (SOM) during slow thermal oxidation (STO) in aerobic atmospheres has been well characterized, and stages of the STO process have been qualitatively described. SOM can be divided into three categories during thermal oxidation: labile organic matter (200–380 °C), recalcitrant organic matter (380–475 °C), and refractory organic matter (475–650 °C), including black carbon4,5,6,7. Thermal analyses focus on the two former categories, corresponding to the first two peaks of the DSC curve, which are considered to be aliphatic and carboxylic groups (300 °C) and aromatic groups (450 °C), respectively8,9,10. Furthermore, thermal analysis of extracted humus can be used to evaluate the degree of humification4,11. In recent years, TGA and DSC have been applied widely in studies of stability of SOM and thermal indexes have been shown to be related to land-use types, tillage practices, and even forest fires7,12,13,14,15,16,17,18. The relationship between thermal and biochemical indexes has also been found to be a potential, rapid method for inspection of SOM7. Moreover, thermal analysis has been demonstrated to be useful for identification of types of vegetation from which organic matters in soils and sediments have derived10. However, slow thermal degradation in the N2 atmosphere (STDN) of OM is poorly understood. Pyrolysis is a rapid, thermal, degradation technique conducted under a N2 atmosphere that has been widely applied to characterization of structures of natural macromolecules. Although pyrolysis has been thoroughly investigated, few studies of STDN have investigated the process qualitatively19. For example, when thermal degradation characteristics of pistachio shells in an N2 atmosphere at different rate of heating were studied by use of a thermo-gravimetric analyzer, two main peaks were observed before and after 350 °C in the DTG curves20. A similar study was conducted by using DSC and TGA to monitor the composting of solid wastes such as vegetable waste and sludge2,21. Nevertheless, studies of anaerobic fermentation of biomass have usually been based on thermal oxidation data22,23,24,25, so advanced understanding of STDN of OM is still needed. Ongoing thermal analyses of OM have revealed differences between DSC curves of thermal oxidation and STDN. The former is composed of two or three exothermic peaks, while the latter is composed of two or three endothermic peaks and one exothermic peak, and there are several indistinct endothermic peaks associated with exothermic processes2,21. STDN has the advantages of being rapid, convenient, and quantifiable. Parameters characterizing the pyrolysis process, such as change in mass, absorbed heat (amount and rate), and released heat (amount and rate) can be determined throughout the heating period, and the results can be combined with other biochemical indexes. Thus, it is an effective method for quantitative characterization of OM and identification of their sources. However, DSC scanning is a difficult process because of the complexity of OM. Specifically, fusions, decomposition and polymerization occur as the temperature increases in reactions that could be endothermic or exothermic. Moreover, superposition can occur during each process. Accordingly, it is important to identify each peak during analysis. The test method used for plant samples is more mature than that used for natural organic matter. Thus, in this study, 17 samples of plant materials were selected to interpret the characteristics at each stage of the STDN process based on comparison to standard materials. Specifically, this study investigated: a) whether the characteristic peak at 300 °C is a shoulder peak generated from two superposed exothermic peaks or a transition stage between an endothermic peak and an exothermic peak; b) whether the characteristic peak at 460 °C is an endothermic peak or a transition stage between two exothermic peaks; c) the relationship between peaks of OM at different stages and their components; d) the relationship between characteristic peaks of OM at different stages and other biochemical indexes.