Cationic-hydrophobic functionalization of starch and pullulan enables high-performance, bio-sourced, and biodegradable flocculants

We present efficient, biodegradable, and putatively non-toxic alternatives to conventional synthetic flocculants. This study demonstrates the potential of high-performance polysaccharide-based flocculants through the synergistic interplay of cationic and hydrophobic modifications. Starch and pullulan backbones were first functionalized with the cationic group betaine, followed by grafting of fatty acids (C5, C8, C12 with 0.2 and 0.6 equivalents of fatty acids) to tune hydrophobicity. Coagulation-flocculation performance was investigated in a real sludge system, and a novel image-based in situ setup was developed for fast, real-time quantitative monitoring of floc formation and stability under controlled (high) shear. Characterization of materials confirmed successful cationic and hydrophobic functionalization, with increased charge density after cationization and hydrophobicity increasing with longer fatty acid chains. Cationic modification alone improved flocculation performance, particularly for pullulan, which formed reformed flocs of ∼15 mm under shear. When combined, cationic-hydrophobic modifications substantially enhanced floc size and shear stability. Moderately hydrophobic pullulan derivatives (0.2 equivalents of fatty acids) reached initial floc diameters of ∼23 mm and retained ∼13-18 mm after shear, outperforming synthetic pDADMAC (∼5 mm) and matching or exceeding cPAM (∼13 mm). In contrast, excessive hydrophobic substitution reduced solubility and performance. At comparable initial molecular weights of 400 kDa, and substitution levels, linear pullulan derivatives consistently exhibited greater performance than branched starch analogues. Best performing flocculants were evaluated for biodegradability via OECD 301F testing and most demonstrated ready degradability, whereas cPAM and pDADMAC remained below 20% degradation. Composed entirely of bio-based building blocks, these materials represent competitive alternatives for sustainable water treatment.