Facilitating non-radical peroxymonosulfate oxidation of tetracyclines driven by struvite crystallization

Eliminating residual antibiotics during struvite recovery from livestock wastewater is essential to prevent the spread of antibiotic resistance. In this work, the effects of in-situ struvite crystallization on the degradation of typical veterinary antibiotics, tetracyclines (TCs), by non-radical peroxymonosulfate (PMS) oxidation were investigated. Results revealed that the process of struvite crystallization enhanced PMS oxidation of TCs with k_obs values reaching 1.95-2.15 times those of PMS alone. ¹O₂ and electron transfer processes were identified as the dominant non-radical pathways. Electrochemical analyses indicated that electron transfer processes would be more active through crystallization than pre-formed crystals in PMS oxidation due to the higher interfacial reactivity. Mg:P molar ratio and pH affected TCs degradation through regulating struvite crystallization and altering the speciation of TCs and PMS, with Mg:P ratio of 1.0 and pH 9.2-9.8 being optimal for facilitating ¹O₂ generation and PMS oxidation. In the presence of humic acid (HA), HA-TCs complexes caused an inhibitory effect on TCs degradation, and excess Mg²⁺ mitigated this inhibition by reducing the direct association between TCs and HA. Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) and fluorescence spectroscopy analyses revealed that HA underwent selective molecular restructuring, with nitrogen-containing and highly condensed aromatic components being converted into more oxygenated and less aromatic products through oxygenation, which could further favor electron transfer pathways. This study highlights that struvite crystallization enhances non-radical oxidation and provides new insights into antibiotic removal with simultaneous phosphorus recovery in livestock wastewater.