Characterization of Black Carbon by Advanced NMR.

The characteristics of the aromatic rings in two charcoal samples, in grassland humic acids and whole soils, as well as in a peat humic acid have been examined in detail employing various advanced solid-state nuclear magnetic resonance (NMR) techniques. Black carbon is distinguished from multiply substituted aromatic rings, for instance in lignin, based on the chemical shift and fraction of nonprotonated C as well as the large average H-to-C distance in the fused aromatic ring systems of black carbon. The C-H distance distribution is probed using the strongly distance dependent C-H dipolar couplings, in long-range C-H recoupled dipolar dephasing or ¹³C{¹H} HARDSHIP NMR. From the edge aromatic carbon fraction, the maximum average aromatic cluster size in each sample has been estimated while the long-range C-H dephasing provides information on the size distribution of fused ring systems. It is concluded that the average aromatic clusters contain fewer than 100 carbons in charcoal and significantly less in the humic acids.