Valorization of Industrial Spruce Bark by Alkaline Extraction

Industrial bark is an abundant side-stream from the debarking of wood logs by the forest industries. Nowadays the bark is mainly treated as energy source in the mills but other applications of bark are also being sought. Softwood bark is composed of cellulose, non-cellulosic polysaccharides, lignin, high amounts of tannins and other extractives, and inorganic compounds. Alkaline extractions of spruce bark conducted under a wide range of process conditions showed that between 20-27 % of the bark could be extracted as polyphenols, mainly tannins and some lignin, with a molar mass of about 2-3 KDa (Borrega et al., 2022). For comparison, the polyphenol yield from water extractions conducted under similar temperature and time was only about 4 % on bark.

The polyphenols in the black liquor from two selected extractions, conducted at 100 ˚C with 15 % NaOH and at 160 ˚C with 24 % NaOH, were recovered by acidic precipitation and tested as surfactants. The composition of the polyphenol-rich precipitates was dependent on the extraction conditions; lower extraction temperature increased the share of carbohydrates while higher temperature increased the share of lignin. Nonetheless, despite the different composition, the polyphenol-rich precipitates were able to decrease the surface tension in aqueous solutions and showed surface activity similar to that of a commercial biosurfactant (Borrega et al., 2022). The polyphenol-rich materials could also be used to partly replace phenol in resins for wood adhesives. Moreover, the utilization of alkali extracted polyphenols as preservative component in construction wood is currently being investigated.

In addition to extraction of polyphenols in high yield, alkaline extractions of bark leave a celluloserich bark residue that may be exploited in various applications. For instance, the utilization of residual spruce bark (after alkaline extraction) in saccharification trials demonstrated that up to 94% of the polysaccharide fraction could be hydrolyzed into monosugars (Borrega et al., 2022). The bark residues may also be used in pulping or to prepare nanofibrillated cellulose films with barrier properties. These results, coupled with preliminary techno-economic analyses, demonstrate that alkaline extraction is a promising and scalable technology for valorization of industrial bark side-streams.