Light scattering study of algal floc growth and structure: alum <i>vs.</i> polymeric plant-derived flocculant

The flocculation dynamics of <i>Microcystis aeruginosa</i> algal cultures using alum and aqueous <i>Moringa oleifera</i> seed extracts as flocculants were analyzed through light scattering and fractal analysis. Floc growth in continuously stirred <i>M. aeruginosa</i> suspensions, with cell densities ranging from 200 to 800 μg L^-1 chlorophyll <i>a</i> (Chl <i>a</i>), exhibited distinct patterns in fractal dimension (<i>d</i>_F) evolution relative to floc size: a smooth, monotonic increase; stochastic increase; and stabilization or leveling off. <i>d</i>_F values ranged from 1.3 to 2.6, with floc diameters (<i>D</i>_4,3 volume-weighted mean) spanning 30 to 300 μm. Alum (0.1 to 0.4 g L^-1) induced fast diffusion-limited flocculation, initially producing lower <i>d</i>_F values, which progressively increased due to structural rearrangement at a slower rate. In contrast, at sufficiently high concentrations (0.1 to 0.2 g L^-1 BSA equivalent), <i>M. oleifera</i> seed proteins facilitated stable, high <i>d</i>_F ≈ 2.0 early on, evidently through patch charge interactions. Flocs formed with alum were prone to shear-induced breakage, limiting both their size and stability, whereas <i>M. oleifera</i> extract produced larger, more stable flocs with greater resilience to shear due to robust particle network formation by the polymer. Both flocculants effectively treated 800 μg L^-1 Chl <i>a M. aeruginosa</i> suspensions, but <i>M. oleifera</i> extract demonstrated better performance in terms of floc size at similar mass concentrations. These findings highlight the potential of <i>Moringa</i> seed extract as a sustainable and effective alternative to conventional flocculants like alum, offering insights into their mechanisms and performance in flocculation processes.