Characterisation and coagulation performance of a tannin-based cationic polymer: A preliminary assessment

Abstract

A commercial tannin-based cationic polymer (TBP) has been investigated to establish its basic chemical properties and its coagulation performance in preliminary trials with a model water containing a kaolin suspension. The TBP appears to be a moderate-to-high molecular weight polymer (∼600,000 g mol −1), with approximately 1000–2000 repeating units, and a reasonably narrow molecular weight distribution. The cationic nature of the polymer is believed to be a single tertiary amine group per monomer, giving a charge density of approximately 3 mequiv. g −1. The TBP is substantially affected by hydrolysis/hydration processes which lead to changes in its charge density and solubility with solution time and pH. The loss of cationic charge (by amine de-protonation) and solubility with pH in the typical pH range for water treatment (i.e. pH 6–9) makes the behaviour of TBP similar to that of conventional aluminium and iron salt coagulants. Laboratory experiments were carried out using a photo-dispersion analyzer (PDA) to evaluate the coagulation performance of TBP at different pH values, and in comparison with alum and polyDADMAC. The results showed that the optimal TBP dose increased with increasing pH, principally because of polymer precipitation, but there was a corresponding increase in coagulation performance. In comparison with alum and polyDADMAC, at optimal doses, the performance of the TBP in treating a kaolin suspension was inferior at neutral pH. However, in higher solution pH conditions the coagulation performance of TBP appears to improve substantially, albeit with a greater dose required, and at pH 9 it was arguably superior to the other two coagulants.