Low-dose high-efficiency sedimentation of bloom-forming cyanobacterium Raphidiopsis raciborskii using modified clays

This study evaluated the low-dose, high-efficiency sedimentation of the bloom-forming cyanobacterium Raphidiopsis raciborskii using five clays (kaolinite, laterite, diatomite, bauxite, and bentonite) modified with polyaluminum chloride (PAC), polyamine (LSC), or polydimethyldiallyl ammonium chloride (PDLS). Removal efficacy was assessed via turbidity, chlorophyll-a, and permanganate index (COD_Mn, using potassium permanganate as the oxidant). A minimum modifier dosage of 3 mg/L was required, with organic modifiers (LSC and PDLS) performing significantly better than PAC. Bauxite showed the best performance among the clays. PAC+LSC and PAC+PDLS exhibited synergy, achieving approximately 79.48% and 74.41% chlorophyll a removal within 240 min, and maximum COD_Mn removal of 90.61% and 86.08%, respectively. Floc properties analyzed by high-speed video and image analysis revealed a strong correlation between particle size, fractal dimension, and settling velocity. Furthermore, after 4 h of pre-oxidation with 3 mg/L H₂O₂, the Fv/Fm continuously decreased from 0.49 to 0.11 over 240 h, representing only 26.19% of the untreated control group. This significantly suppressed the "rebound viability" of the settled cells. The modified clay coupled pre-oxidation process enables rapid cyanobacterial cell removal and sustained photosynthetic viability inhibition with minimal chemical input, providing a technological basis for emergency control of cyanobacterial blooms.