Variation in the degree of aromatic condensation of chars

Char is ubiquitous in the environment and contributes significantly to the inert soil organic carbon pool. Its roles as a carbon sink and a strong sorbent of organic contaminants have come under increasing attention. However, quantifying these roles is difficult because properties such as degradability and sorption affinity vary greatly amongst chars, especially those produced at different temperatures. Here we describe a simple method for gauging the degree of aromatic condensation of chars, a molecular-scale property that affects both their degradability and sorption affinity. The method involves adding 13C6 benzene directly to char. The 13C nuclear magnetic resonance (NMR) chemical shift of the sorbed benzene is affected by diamagnetic ring currents in the fused aromatic structures in the char. These ring currents increase in magnitude with increasing extent of aromatic condensation. Seventeen heat-treated materials were analysed in this way. Our results confirm that aromatic condensation increases with increasing heat treatment temperature (HTT) and that activated chars contain the most highly condensed aromatic structures, but also show the importance of starting material and heat treatment time on aromatic condensation. We also show for four of the materials that the ring current effect on chemical shift was similar for other 13C-labelled molecules, including MeOH, CH3CN and toluene.