Boreal forest side streams for natural colorants and growing media

Bark is the largest by-product of the sawmill industry and could potentially serve as a sustainable raw material for added value chemicals and materials. Currently, most bark residue is directly utilized in energy production, but cascading use would increase its value and lengthen the lifetime of the carbon bound to it. This study addressed the use of freeze- and spray-dried bark extracts of Norway spruce (Picea abies (L.) H. Karst), Scotch pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth) as bio-based colorants in fabric printing, and utilization of the extraction residues as peat substitute in growing media for plants. To evaluate the suitability in fabric printing, hot water extraction, drying, screen printing, colorimetric measurements and standardized accelerated lightfastness tests were carried out. Total phenolic compounds of extracts were analyzed. The function and phytotoxicity of the barks and extraction residues as growing media constituents were studied with germination test in a short-term laboratory experiment. Spruce and pine produced an equally uniform and strong color in screen-printing, and good color fastness was observed for both. The best yield of phenolic compounds was obtained for spruce bark, whereas for birch bark, the yield was modest. Pine bark was found to be more phytotoxic than spruce and birch, but the growth-inhibiting compounds were removed with extraction, whereas the extraction of phytotoxic compounds of spruce was insufficient. The obtained results encourage further investigation of cascade use of bark as a bio-based colorant as well as growing media.