Heterogeneous Nucleation and Growth Kinetics of Struvite in Anaerobic Digestate: Role of Diatomaceous Earth as a Model Floc

Anaerobic digestion (AD) is widely applied for sludge stabilization and energy recovery, yet its operation is frequently affected by phosphate scaling, particularly the crystallization of struvite (MgNH ₄ PO ₄ ·6H ₂ O). To quantitatively examine the role of floc-like solid interfaces in this process, diatomite was employed as an interfacial model material in a constant-pH, constant-composition crystallization system. The results show that diatomite induced systematic and non-linear variations in struvite crystallization kinetics. The induction time increased with diatomite concentration, consistent with ion adsorption and diffusion boundary layer effects. The nucleation rate exhibited a rise-decline-plateau trend, reflecting the accumulation and subsequent saturation of heterogeneous nucleation sites. In contrast, the crystal growth rate displayed a non-linear inhibition-promotion response, shifting from slight suppression at low diatomite loadings to enhanced growth at higher concentrations, associated with shortened diffusion paths and facilitated surface integration. Characterization by SEM, XRD, FTIR, and XPS indicates that diatomite modified crystal morphology and interfacial chemical environments without altering the crystal phase or lattice structure of struvite. These findings demonstrate that suspended solid interfaces can actively modulate struvite crystallization behavior under digestion-relevant aqueous conditions and provide quantitative insight into heterogeneous crystallization processes involving particulate matter.