Aminomethylation of spruce tannins and their application as coagulants for water clarification

Abstract

This study explored the potential for using tannin extract from spruce bark (Picea abies) as a coagulant. Spruce tannins were extracted with hot water and pulverized through spray drying and freeze drying. The pulverized tannins were cationized via the Mannich reaction with formaldehyde and diethanolamine or ethanolamine, and the coagulation performances of the spruce tannin coagulants were compared with an industrially extracted quebracho tannin, which was cationized with the same method. Jar test experiments with kaolin/river water indicated that all of the tannin coagulants were able to enhance particle settling significantly, although the quebracho tannin coagulants were slightly more efficient than the spruce tannin coagulants. Since the phenolic groups play the major role in the modification, the higher coagulation efficiency was probably related to the higher amount of proanthocyanidin in the quebracho tannin, and subsequently the higher cationic charge density obtained for the quebracho tannin coagulants. Nevertheless, the spruce tannin coagulants could still be considered to be effective products because they possessed more stable turbidity and a total surface charge close to zero over a wide dosage range. The ethanolamine was a better amine source, at least for the coagulation of river water with a positive charge demand. Moreover, the study established the importance of charge density as an essential indicator for coagulant performance.