Regulatory effects of extracellular polymeric substances on the nucleation and growth kinetics of struvite crystallization

This study examined how different extracellular polymeric substance (EPS) fractions affect the homogeneous nucleation and growth of struvite (MgNH₄PO₄·6H₂O) under a constant pH-constant composition crystallization condition. Soluble EPS (S-EPS), loosely bound EPS (LB-EPS), and tightly bound EPS (TB-EPS) were extracted from municipal activated sludge, and their impacts on induction time (IT), nucleation rate (NK), and growth rate (K) were tracked. The three EPS fractions behaved differently. LB-EPS generally shortened IT and increased NK, suggesting easier nucleus formation, whereas TB-EPS prolonged IT and lowered NK, implying a higher nucleation barrier and weaker effective ion activity. After nucleation, however, TB-EPS produced more regular prismatic crystals and showed the strongest apparent growth promotion; S-EPS caused only modest changes. FTIR, XRD, SEM, and XPS together indicate that EPS does not change the struvite phase, but may interact with crystal surfaces through oxygen-/nitrogen-containing functionalities, thereby affecting interfacial processes and leading to the observed kinetic differences. These results link EPS fraction properties with nucleation resistance and growth behaviour, and may help guide controllable phosphorus recovery and scale control in wastewater systems.