Application of Clay–Polymer Nanocomposites for the Removal of Toxic Cyanobacteria and Other Phytoplankton from Water—A Laboratory Scale Study

The increasing global frequency of harmful cyanobacterial blooms (CyanoHABs), driven by nutrient enrichment and climate change, poses a severe threat to aquatic ecosystems and public health. This study evaluates the effectiveness of novel clay–polymer nanocomposites (CPCs) that combine the charge-neutralizing capabilities of polydiallyldimethylammonium chloride (polyDADMAC) with the high clay mineral density (kaolinite and sepiolite) for rapid removal of toxic cyanobacteria from water. Laboratory experiments were performed using Microcystis aeruginosa, Aphanizomenon ovalisporum, and Chlorella sp., with treatment doses determined by particle charge detector (PCD) measurements to identify the “nominal dose” required for full charge neutralization. Results show that clay–polymer nanocomposites achieve over 95% removal of turbidity and chlorophyll in M. aeruginosa at doses significantly lower (15–20%) than the calculated nominal dose, likely due to specific physical bridging interactions with the cyanobacteria’s external exopolysaccharide fibers. In contrast, A. ovalisporum and Chlorella sp. required doses closer to full charge neutralization for optimal removal. Among the materials tested, kaolinite-based nanocomposites (DKG24) showed slightly superior, more stable performance than sepiolite-based nanocomposites. Notably, application at or above the nominal dose was associated with increased soluble microcystin levels, suggesting that excessive polymer concentrations may compromise cell integrity and lead to toxin leakage. These findings suggest that engineered nanocomposites offer highly efficient, scalable technology for CyanoHAB management, provided that operational doses are carefully optimized to maximize biomass removal while minimizing toxin release.