We investigated how molecular-weight–resolved dissolved organic matter (DOM) in anaerobic digester supernatant regulates struvite (MgNH 4 PO 4 ·6H 2 O), homogeneous nucleation and crystal growth. DOM was separated by sequential ultrafiltration into six size fractions, and induction time (IT), nucleation rate (NK), and apparent growth rate (K) were quantified using constant-composition crystallization at constant pH. A pronounced nonlinear “inhibition–promotion–re-inhibition” dependence on DOM size was observed. High-molecular-weight DOM (>100 kDa) imposed steric and transport constraints, causing mass-transfer limitation that prolonged IT and suppressed NK and K. The 30–10 kDa fraction maximized crystallization performance, giving the shortest IT and the highest NK and K, consistent with improved mass transport and surface integration. Further decreasing DOM (<10 kDa) re-inhibited crystallization through Mg 2+ complexation, competitive adsorption at active sites, and a strengthened interfacial hydration layer, increasing ion dehydration and surface integration barriers. FTIR, XRD, SEM, and XPS corroborated size-dependent changes in functional-group interactions and facet exposure, shifting rate-controlling steps between nucleation and growth. These results guide pretreatment and process control to enhance phosphorus recovery and mitigate struvite scaling in organic-rich digestate. • DOM MW fractions drive nonlinear struvite nucleation and growth in supernatant. • Constant-composition kinetics show inhibition–promotion–re-inhibition behavior. • 30–10 kDa yields minimum IT (~1000 s) and maximum NK and K. • FTIR/XRD/SEM/XPS reveal interfacial control of pathway switching.
Zhao et al. (Sun,) studied this question.