The hidden power of microalgal extracellular polymeric substances: A natural source of persistent free radicals for aquatic environmental remediation

Persistent free radicals (PFRs) hold significant potential for environmental remediation via pollutant degradation. In microalgae-based wastewater treatment systems, extracellular polymeric substances (EPS) secreted by microalgae have emerged as a crucial, yet underexplored, source of environmental PFRs. This review introduces a novel perspective by systematically evaluating the composition of microalgal EPS, and analyzes the classification and formation mechanisms of PFRs. Advanced detection methodologies, including electron paramagnetic resonance spectroscopy, mass spectrometry-based techniques and colorimetric assays, are also critically examined. The production and stability of PFRs are governed by key environmental factors, including temperature, light, pH, dissolved oxygen, organic substances and metal oxides (e.g., Cu and Fe). EPS-derived PFRs function as efficient electron shuttles. They activate oxidants (e.g., persulfate and H₂O₂) or molecular oxygen to generate reactive oxygen species, thereby driving remediation processes. Moreover, applications for degrading organic contaminants (e.g., antibiotics and dyes) and remediating heavy metals (e.g., Cr(VI) and As(III)) are comprehensively discussed, demonstrating their broad efficacy. Despite these advances, significant knowledge gaps remain. These include the precise formation mechanisms within EPS matrices, the long-term stability and fate of PFRs in aquatic environments, and their ecological impacts. This review identifies these research needs and provides future perspectives to advance the application of PFRs in microalgae-based systems, paving the way for more efficient and sustainable wastewater treatment technologies.