Designing an eco-efficient sonocatalytic process: iron-modified bamboo biochar for cyanobacteria elimination and...

Cyanobacterial blooms have emerged as a persistent threat to global aquatic ecosystem stability and drinking water safety. Conventional algae removal technologies (e.g., coagulation, photocatalysis) generally face two long-standing bottlenecks: the trade-off between removal efficiency and secondary pollution from cell lysis-triggered microcystin release, and the lack of valorization pathways for hazardous algal residues. In this study, a low-cost iron/potassium hydroxide co-modified bamboo biochar (BC-900-2, prepared via one-step pyrolysis at 900 °C with a Fe/KOH molar ratio of 2:1) was developed as a sonosensitizer to construct a synergistic sonocatalytic algae removal system. Under optimal conditions (400 mg/L dosage, 40 kHz ultrasound, 10 min), the integrated sonocatalytic process achieved over 90% inactivation of Microcystis aeruginosa (initial concentration: 2 × 10⁶ cells/mL). Critically, the residual dissolved microcystin-LR concentration was reduced from an initial 0.00105 mg L^-1 to < 0.0002 mg L^-1, thereby falling below the Chinese drinking water standard (0.001 mg L^-1, GB 5749-2022). However, further validation at higher cyanobacterial densities and in natural water matrices is required. Mechanistic investigations revealed that the physical-chemical coupling yielded a synergy index of 1.82, originating from enhanced cavitation and sustained Fe²⁺/Fe³⁺ cycling, with hydroxyl radical (OH) identified as the dominant reactive oxygen species responsible for algal cell inactivation and microcystin degradation. Beyond pollutant removal, molecular characterization by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) indicated that the treated products were enriched in protein/amino sugar- and lignin-like components with moderate oxidation states, consistent with molecular signatures often associated with bioavailable organic matter. Pot experiments demonstrated that the treated algal products significantly increased soybean sprout root length and total length by 34% and 31%, respectively, supporting the feasibility of converting hazardous algal waste into a potential plant growth-promoting resource. This work proposes an integrated treatment-and-valorization paradigm for cyanobacterial bloom management that simultaneously addresses secondary pollution control and waste disposal. It should be noted that the current study was conducted at the laboratory scale; pilot-scale validation in real eutrophic water bodies is required to evaluate its practical applicability.