Sustainable cationization of bark tannins from native Brazilian tree species using hexamine as a formaldehyde substitute for water treatment applications

The growing concern over the environmental and health risks of conventional coagulants has driven interest in bio-based alternatives. This study evaluated the potential of tannins extracted from the bark of Stryphnodendron adstringens and Mimosa tenuiflora, chemically modified by cationization, for application as natural coagulants in water treatment. Chemical characterization included total extractives, phenolic content, flavonoids, condensed tannins, and the Stiasny index. Cationization was performed using the Mannich reaction (with formaldehyde) and an alternative method with hexamethylenetetramine (hexamine). FTIR was used to confirm structural changes, and coagulation performance was assessed in water with 150 NTU turbidity. M. tenuiflora showed higher phenolic content (135.74 mg GAE/g), while S. adstringens had a higher condensed tannin content (15.23%) and Stiasny index (93.18%), indicating greater tannin purity. Tannins from M. tenuiflora cationized via the Mannich reaction removed over 99% turbidity, outperforming ferric chloride and matching commercial tannins. Hexamine-based cationization yielded removals of 59% (M. tenuiflora) and 64% (S. adstringens). FTIR confirmed successful incorporation of amine groups and structural differences between the modification methods. These results demonstrate the potential of native tannins as effective and sustainable natural coagulants. Although hexamine-based modification showed slightly lower performance than formaldehyde, it presents a viable, less toxic alternative for future development.