Formation of oxalic acid in alkaline peroxide treatment of different wood components

Oxalic acid (OA), formed as an oxidation product in alkaline peroxide bleaching (P) of mechanical pulps, can form a sparingly soluble salt with calcium and cause severe scale deposit problems. The focus of this work was the question how much OA is formed from the different components of spruce (Picea abies (L.) Karst.), i.e., from cellulose, lignin, hemicelluloses, extractives, and bark, during the P-stage of bleaching. Isolated wood components and monosaccharides typical for spruce hemicelluloses and pectins were treated with chemicals of P-stage for 90 min at 73 degrees C, and the released OA was analyzed by ion chromatography. Most OA was formed from galacturonic and glucuronic acids, xylans, and lignin. Taking into account the content of these substances in spruce wood, lignin can be regarded as the major source of oxalate. Untreated spruce bark was found to contain substantial amounts of oxalate, and an additional amount of oxalate was formed in the P-stage. It is evident that effective debarking is essential, not only for pulp brightness, but also for the control of calcium oxalate formation.