Valorization of tannin from spruce bark and its application as biocoagulant in water treatment

One of the most significant side streams of the forest industry is tree bark. Spruce accounts for about 30 per cent of Finnish forest resources and represents a substantial source of tree bark. Spruce bark is typically combusted to produce energy but has a relatively low heating value. Spruce bark contains valuable compounds such as condensed tannins and other phenolic components. Tannins can be easily extracted with hot water and used to produce sustainable bio-based coagulants. The thesis work aims to valorize extracts from spruce bark as the tannin source for the synthesis of tannin-based coagulants. Depending on the aminomethylation pathway, amine source and purity of the extracts, tannin coagulants with charge densities between 0.83 meq/g and 3.38 meq/g can be produced from spruce extracts. The study showed that winter-harvested spruce bark contained a higher concentration of condensed tannins, and a preliminary cold-water extraction (23 °C) before an actual hot-water extraction (90 °C) increased the purity of the tannin extracts (WCHW) sufficiently, resulting in tannin-based coagulants with superior charge properties and improved coagulative performance. Coagulative experiments with a kaolin/river water mixture showed that a mere 15 mg/l dosage of WCHW coagulant was required to achieve a turbidity reduction of 90%. Further, the efficacy of the developed spruce tannin coagulant was tested for vanadium removal from a mine effluent in different pH ranges (pH 9, 7 and 4). The results showed that acidic conditions favoured the vanadium removal capability of the tannin coagulant due to an increased electrostatic attraction between vanadates and the protonated amine groups in the coagulant matrix. This thesis provides a comprehensive study on the utilization of tannin extracts from spruce bark as a tannin source for bio-based coagulants and the high potential of the developed coagulants in different water treatment applications.