A Targeted Mitigation Strategy for Freshwater Harmful Algal Blooms through the Release of a Small-Molecule Polyphenolic Algaecide from Clay Nanotubes.

Harmful algal blooms (HABs) caused by Microcystis aeruginosa (MC) pose serious threats to freshwater ecosystems, public health, and water treatment infrastructure. Here, we present a targeted mitigation technology involving flocculation and settling of MC using a flocculant, polyaluminum chloride (PAC), and a nanotubular clay, halloysite, which serves both as a ballast to aid settling of the flocs and as a reservoir to deliver algaecides. The tubular morphology of halloysite allows loading of algaecide, thus constituting a targeted release of algaecide to MC cells in the floc. The specific algaecide chosen is a small-molecule polyphenol, propyl gallate (PG), whose release can be modulated by coating the halloysite nanotubes (HNT) with a thin layer of paraffin wax. Pulse amplitude modulation fluorometry (PAM) measurements confirm that PG disrupts the photosynthesis efficiency of MC in doses as low as 25 mg/L. Importantly, the extensive extracellular polymeric substance (EPS) produced by MC forms a hydrated and adhesive matrix that physically entraps HNTs within the floc through entanglement and interfacial interactions, thereby enhancing localized algaecide delivery while limiting exposure to nontarget organisms. Our findings demonstrate the potential of HNT-based delivery systems as scalable, environmentally benign technologies for managing freshwater HABs caused by Microcystis aeruginosa, with promising implications for long-term bloom suppression and ecological safety.