Impact of emerging contaminants on phosphorus recovery in batch and fluidized-bed reactors: Kinetic and mechanistic...

Struvite crystallization is a sustainable strategy for phosphorus (P) recovery, yet the presence of emerging contaminants (ECs) poses significant obstacles by disrupting struvite crystallization kinetics and compromising product purity and thus its potential agronomic safety. This study systematically investigates the impacts of tetracycline (TC), polyethylene terephthalate (PET), Cu^2⁺, and humic acid (HA) on struvite formation using both batch reactors (BRs) and fluidized-bed reactors (FBRs). Batch experiments revealed that HA and Cu+TC act as potent inhibitors through a combination of surface-mediated interactions and Mg^2⁺ complexation, reducing the apparent crystallization rate of struvite. Conversely, FBR results demonstrated that specific hydraulic conditions can mitigate the kinetic inhibition of Cu+TC, yielding larger pellets (∼ 1342 vs ∼ 1138 μm in diameter) with apparent Cu^2⁺ incorporation into the pellet matrix. Trials with real swine wastewater verified that excessive dissolved organic matter and Ca^2⁺ favor the formation of amorphous phases rather than crystalline struvite (84.9% vs 2.7% in the amorphous fraction). Density functional theory calculations indicated that the Mg^2⁺ complexation is primarily governed by non-covalent ion-dipole (Mg-O) interactions between HA/TC with Mg²⁺. Besides, ternary complexes bridged by metal-O were identified as the most stable species (HA-Cu/Mg-TC) within these matrices, which may enhance the mobility or persistence of such ECs in aquatic environments. Consequently, robust P recovery requires integrated strategies-combining pre-treatment to remove strong inhibitors with specific FBR conditions (hydraulics and extended retention time) that leverage crystal growth and conglomeration. This work provides the kinetic and mechanistic insights necessary for developing such integrated solutions, advancing our understanding of EC-mediated crystallization of struvite and the viability of P recovery from increasingly complex waste streams.