From empiricism to rational design: Decoding structure-activity relationships in natural organic flocculants

Flocculation plays a critical role in water treatment, but conventional inorganic and synthetic flocculants can pose environmental and health risks. Natural organic flocculants, derived from renewable biomass such as cellulose, starch, lignin, and chitosan, provide eco-friendly alternatives. Their application, however, is limited by inconsistent performance and lack of understanding of structure-activity relationships (SARs). This review systematically analyses SARs of natural organic flocculants, focusing on how molecular features (chain configuration, chain length, molecular weight, charge density, and hydrophilic-hydrophobic balance) affect flocculation mechanisms and efficiency. Life cycle assessment highlights their environmental benefits over conventional flocculants. Key research gaps are identified, such as insufficient exploration of synergistic effects among parameters and discrepancies between model systems and real wastewater. We propose future research directions to unravel structure-mechanism links and enable rational design of high-performance, sustainable flocculants.