Seawater boosts oxytetracycline (OTC) residues in struvite via hydroxyapatite (HAP) and dissolved organic matter (DOM) co-precipitation

Seawater, as an alternative magnesium (Mg) source, has the potential to improve the overall economic and environmental footprint of struvite production compared to the use of pure Mg salts. However, the presence of other ions in seawater may affect the migration of tetracyclines (TCs), commonly found in wastewater, potentially reducing the quality of the recovered product, and posing environmental risks. But these effect has not been illustrated before. This study investigated the impact of seawater on oxytetracycline (OTC) migration during struvite recovery from swine wastewater. Under varying pH levels and Mg/P molar ratios, the OTC content in recovered struvite was ranged from 19 to 103 μg/g, which was 6 to 43 μg/g when using MgCl₂ as Mg source. The key factor influencing OTC content was identified as Ca²⁺ in seawater. The co-precipitation of hydroxyapatite (HAP) and the incorporation of dissolved organic matter (DOM) were the primary reasons for increased OTC content. Interactions between Ca²⁺ on HAP surfaces and the carbonyl oxygen of OTC, along with ternary complex formations involving Ca²⁺, DOM, and OTC, facilitated OTC migration through adsorption onto struvite. Notably, increasing the Mg/P molar ratio reduced OTC enrichment due to the competition between Ca²⁺ and Mg²⁺. Density functional theory (DFT) calculations supported these interfacial interactions. These findings enhanced the understanding of antibiotic migration during phosphorus (P) recovery using seawater as alternative Mg source and suggested solutions for reducing P recovery contamination in the future.