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

Abstract

The flocculation dynamics of Microcystis aeruginosa algal cultures using alum and aqueous Moringa oleifera seed extracts as flocculants were analyzed through light scattering and fractal analysis. Floc growth in continuously stirred M. aeruginosa suspensions, with cell densities ranging from 200 to 800 μg L⁻¹ chlorophyll a (Chl a), exhibited distinct patterns in fractal dimension (d_F) evolution relative to floc size: a smooth, monotonic increase; stochastic increase; and stabilization or leveling off. d_F values ranged from 1.3 to 2.6, with floc diameters (D_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 d_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), M. oleifera seed proteins facilitated stable, high d_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 M. oleifera 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 a M. aeruginosa suspensions, but M. oleifera extract demonstrated better performance in terms of floc size at similar mass concentrations. These findings highlight the potential of Moringa seed extract as a sustainable and effective alternative to conventional flocculants like alum, offering insights into their mechanisms and performance in flocculation processes.