The original birch and spruce wood samples, as well as the biochar samples were analyzed by FTIR spectroscopy (Figs. 13.2, 13.3, respectively). Infrared band assignments for wood and biochar samples are given in Table 13.1 [30-32]. The spruce and birch biochar FTIR spectra were similar to those reported by Sharma et al. [32] of lignin-derived biochar, suggesting that the biochar was lignin derived.

The band at * 1,390 cm-1 was attributable to H-bonded hydroxyl groups (O-H stretch) in both wood and biochar samples. The OH peak intensity decreased after SCM treatment of both birch and spruce. The aliphatic bands (2,850-2,950 cm-1) increased after SCM treatment due to dehydration reactions. The C=O ester band at 1,730-1,735 cm-1 was eliminated after SCM treatment as a result of hemi — cellulose deacetylation. The presence of a new C=O signal at *1,698 cm-1 suggests the presence of a conjugated ketone (Hibbert’s ketones) resulting from lignin demethylation of aromatic methoxy groups or b-O-4 cleavage after SCM treatment [33]. The new peak in the biochar samples at *1,640 cm-1 was assigned to a C=C bond. The increase in peak intensities of bands at 1,600 and 1,510 cm-1 (aromatic skeletal vibrations) suggests that cleavage of the aliphatic side chains and condensation reaction occurred in lignin during biochar formation [31, 34]. Other bands in the biochar spectra correspond to a lone aryl C-H wag (*860 cm-1). The band at 1,373 cm-1 is probably due to both OH in-plane bending and CH bending [32].

To assess the level of lignin cross-linking during biochar formation lignin CIs were determined. For spruce and birch wood the CI was 0.511 and 0.433, respectively. The lower CI value for birch wood is due to the less condensed structure of hardwood lignin relative to the spruce softwood lignin [31]. The CI values for the SCM biochar from spruce and birch were respectively, 0.681 and 0.666 suggesting lignin cross-linking had occurred to a significant extent. For

comparison softwood kraft lignin (Indulin AT, Mead Westvaco) has a CI of 0.624 which is known to be highly condensed [29, 35].