Aromatic condensation of black carbon : its measurement and importance.

Abstract

Black carbon (BC) is an important constituent of soils and sediments due to its role in a range of biogeochemical processes. However, since BC represents a continuum of materials with no clear-cut boundaries, it is challenging to identify and quantify. This limits our understanding of its contribution to terrestrial, marine and atmospheric carbon cycles and the role it plays in influencing climate. The focus of this study was on first improving and refining a novel solid-state ¹³C nuclear magnetic resonance (NMR) spectroscopic that offers great potential for the characterisation of BC. Once optimised, this technique was applied to a variety of BC samples in order to address some key questions in BC science. The technique developed here is a simple and rapid method for gauging the degree of aromatic condensation of chars, a molecular-scale property that affects both their degradability and sorption affinity. The foundation for the technique has been described previously. Its basis is the effect that “ring currents” that are induced in aromatic structures have on the ¹³C NMR chemical shift of probe molecules when sorbed to the char. The improvement in the technique described here involves a direct addition of the probe molecule (¹³C₆-benzene) directly to the dry char. This is demonstrated to be a much more efficient method of loading the probe molecule, both in terms of the amount of expensive ¹³C-labelled compound require and also in the time required for sample preparation and NMR acquisition. Following the optimisation of the ring current method, it was applied to three sample sets of BC materials to address in detail three important aspects of BC composition: (i) the effect of temperature on BC; (ii) the effect of feedstock on BC; and (iii) the nature and variability of charcoal produced in natural fires. The parameter derived from the ring current technique, Δδ, was demonstrated to be a good measure of aromatic condensation. This was most evident for a thermosequence of twelve chestnut wood chars produced at temperatures from 200 to 1000°C, as it clearly captured the variations in char composition with increasing temperature. Through the use of the ring current method that it became clear that there are two distinct phases in charcoal formation: first an increase in aromaticity, and second a structural rearrangement creating condensed aromatic structures. The use of different feedstock resulted in critical differences in aromatic condensation between biochars produced at the same temperature. In particular, feedstocks with higher lignin contents, such as woody materials, were found to form more condensed aromatic structures with a higher degree of aromaticity compared to biochars from mineral-rich feedstocks (e.g. crop residues) and waste materials (e.g. manures, food waste and papermill waste). Lastly, the variability in chars produced in natural vegetation fires was gauged through the analysis of 53 natural chars collected from the soil surface six to thirty years after natural fires. The aromatic condensation was found to vary considerably among 4-5 char samples collected at each of the fire sites, despite efforts to sample only the most carbonized char from burnt-out tree stumps. This demonstrates that there is great degree of variability in the composition of the char produced in such fires, which is likely to be reflected in widely varying rates of char decomposition.