Quaternized chitosan-coordinated metal-azolate framework: Core-bridge-shell composite for electrostatic flocculation...

Harmful algal blooms dominated by Microcystis aeruginosa (M. aeruginosa) threaten aquatic ecosystems and public health. Traditional treatment methods had limitations of temporary effects or slow response. To address these issues, this study designed a multifunctional CTA-CS/M composite with flocculation and long-term algal inhibition, which constructed "metal-azolate frameworks (M) core - chitosan (CS) bridge - 3-chloro-2-hydroxypropyl trimethylammonium chloride (CTA) shell" hierarchical structure constructed via CS/M coordination and covalent grafting of CTA onto CS. CTA-CS/M was systematically characterized by zeta potential, XRD, FTIR, XPS, SEM and other techniques. Flocculation experiments showed that CTA-CS/M exhibited superior aggregation of M. aeruginosa, at a dosage of ≥ 10 mg L^-1, complete sedimentation was achieved within 30 min, outperforming the commercial flocculant polyaluminium chloride (PAC) and similar flocculants reported. The composite maintained clear supernatants for 60 days, demonstrating long-term inhibition of algal regeneration. At a concentration of ≥ 10 mg L^-1, it showed strong adaptability to pH (7.0-10.0) and algal concentrations (0.8-5.5 ×10⁸ cells L^-1), and could be combined with low-cost cotton towel filtration for efficient floc separation. Mechanistic studies revealed that the adsorption process followed pseudo-second-order kinetics and the Langmuir isotherm, with -N(CH₃)₃⁺ groups on CTA-CS/M as the key functional groups, driving single-layer chemical adsorption through electrostatic attraction with negatively charged groups on M. aeruginosa. This work upgrades flocculation into a comprehensive long-term strategy for cyanobacterial bloom control.