Abstract :
Despite the ubiquitous nature of black carbon (BC) and its importance in global element cycles and as a sorbent for organic chemicals, methodological discordance exists for its identification, characterisation and quantification. Here, oxidative differential scanning calorimetry (DSC) was adopted to characterise BC from charcoal, soot, graphite, BC-containing soils, charred plant tissues and to compare its thermal stability to that of other carbonaceous materials (lignite and bituminous coal) and possible interfering substances (melanoidin, humic acid, shale and plant tissues). Peak temperatures were the most reliable attribute to characterise the thermal stability, increasing from wood (as the most stable non-BC material investigated, maximum peak temperatures Tp < 505 °C) < charred wood (Tp 500–513 °C) < charcoals (Tp 520–525°C) < soots (Tp 620–630 °C) < graphite (Tp > 850 °C). From the comparison with NMR data, it is inferred that the oxidative stability of BC is more controlled by the degree of order (crystallinity) than by its aromaticity. Charring increased the oxidative stability of wood as indicated by a complete loss of thermally labile compounds below 400 °C and a shift of the stable peak from 484 °C up to 513 °C. However, thermal regions for charred and native wood interfered, thus preventing a definite assignment of BC in that temperature region. Bituminous coal (Tp 546 °C) was the only non-BC material interfering with BC peak temperatures in the charcoal region. Together, DSC measurements provided further evidence for a continuum of BC in terms of thermal stability. Standard addition experiments with charcoal and soot to soils revealed a strong linear relationship (R2 0.99) between the amounts of BC added and peak height at the respective temperature, indicating that DSC is a suitable screening method for quantifying thermally stable BC in soil.
