Deciphering the interplay between wastewater compositions and oxytetracycline in recovered struvite: Unveiling mechanisms and introducing control strategies

Struvite recovery from wastewater offers a sustainable phosphorus and nitrogen source, yet it harbors the challenge of variable antibiotic residues, notably oxytetracycline (OTC), increasing the ecological risk during subsequent use. Despite the need, mechanisms behind these residues and regulatory solutions remain obscure. We characterized OTC in recovered struvite and showed that increased dissolved organic matter (DOM) enhanced OTC accumulation, while PO4 3-suppressed it. NH4+ modulated OTC levels through the saturation index (SI), with a rise in SI significantly reducing OTC content. Additionally, excess Mg2+ formed complexes with OTC and DOM (humic acid, HA), leading to increased residue levels. Complexation was stronger at higher pH, whereas electrostatic interactions dominated at lower pH. The primary binding sites for antibiotics and DOM were Mg-OH and P-OH groups in struvite. OTC's dimethylamino, amide, and phenolic diketone groups primarily bound to struvite and DOM, with the carboxyl group of DOM serving as the main binding site. Mg2+ complexation was the primary pathway for OTC transportation, whereas electrostatic attraction of PO43 dominated during growth. Controlling magnesium (Mg) dosage and adjusting pH were effective for reducing OTC in recovered products. Our findings provided insights into the intricate interactions between struvite and antibiotics, laying the groundwork for further minimizing antibiotic residues in recovered phosphorus products.