High-efficiency preparation of formaldehyde-free tannin-based flocculants and their algae removal mechanism.

The development of green and high-efficiency flocculants for controlling harmful algal blooms (HABs) is currently a critical research focus in the field of water treatment. Plant tannins have garnered significant attention as a sustainable biomass resource; however, their application in high-salinity environments, such as seawater, remains relatively underexplored. In this study, four plant tannins were systematically investigated to select black wattle tannin (BWT) as the optimal precursor due to its superior comprehensive performance. A formaldehyde-free alkaline amination modification route was established, with synthesis parameters optimized via response surface methodology (RSM). The results indicated that the modified tannin prepared under optimal conditions (25 g/L BWT, ammonia-to-tannin molar ratio of 5.656, aerobic conditions, 20°C, and stirring at 100 rpm for 4 h) achieved a stable removal efficiency of over 85% for Heterosigma akashiwo. Furthermore, this modification effectively overcame the application bottlenecks of high residual color and turbidity associated with natural tannin treatment. Mechanism analysis revealed that the performance enhancement was attributed to the synergistic effect of oxidation, polymerization, and amination reactions, resulting in the formation of a high-efficiency flocculant with enhanced bridging capabilities.