Characterization of Charcoal Using Advanced Solid-State NMR Spectroscopy.

The characteristics of charcoal samples and of aromatic rings in humic acids were investigated in detail by various advanced solid-state nuclear magnetic resonance (NMR) techniques. These techniques included quantitative direct polarization/magic angle spinning (DP/MAS), C-H recoupled dipolar dephasing, and two-dimensional ¹H-¹³C heteronuclear correlation NMR. The samples were two biochar samples from prairie grass and wood, and several humic acids from soil. Compared with cross polarization/magic angle spinning, DP/MAS gave quantitative aromaticities and other structural information on charcoal samples. In humic substances, charcoal is distinguished from other aromatic rings, such as those of lignin, based on the chemical shift, fraction of nonprotonated C, and the large average H-to-C distance in the fused aromatic ring systems of charcoal, which is probed using the strongly distance-dependent C-H dipolar couplings in long-range C-H recoupled dipolar dephasing NMR. The aromatic cluster size in these samples has been estimated from the fraction of aromatic carbons along the edge of a cluster (bonded to H, O, or alkyl C) combined with analysis of the long-range C-H dephasing data.  The average aromatic cluster in charcoal consists of fewer than 40 carbons and significantly less in the humic acids.