Oxygen-driven shift from Pb(II) lattice incorporation to reversible surface complexation: Exploiting hydrothermal sludge DOM for high-purity vivianite recovery.

Recovering phosphorus from sewage sludge hydrolysate via vivianite crystallization is a promising route for resource recovery, yet Pb(II) enrichment can undermine product purity and stability. Here we reveal a ligand-directed mechanism by which sludge dissolved organic matter (DOM) regulates Pb(II) partitioning between mineral lattices and interfacial complexes. Increasing the hydrothermal temperature to 220 °C promoted the transformation of DOM toward oxygen-rich aromatic components, including highly oxidized carboxyl-rich alicyclic molecules and polyphenolic species, which preferentially bind Pb(II) through O-donor functional groups. FT-ICR MS combined with Pb L3-edge synchrotron XAFS indicates that Pb coordination shifts from Pb-O-P environments associated with Pb-phosphate precipitation/lattice incorporation to predominantly inner-sphere Pb-O-C bonding at the vivianite-solution interface. Density functional theory and molecular dynamics simulations further support that these O-ligand complexes provide both energetic stabilization and steric hindrance, thereby suppressing Pb(II) penetration into the vivianite lattice. Leveraging this interfacial confinement, we demonstrate a "decoupled recovery" approach in which surface-associated Pb is selectively removed by mild acidification (pH 3.5) while maintaining vivianite lattice integrity. This work links DOM molecular evolution to metal-mineral interfacial chemistry and suggests a feasible strategy to improve vivianite-based phosphorus recovery from Pb-impacted sludge streams.