Effect of different magnesium sources on tetracycline migration during struvite recovery

Struvite crystallization is one of the commonly used methods for phosphorus (P) recovery from livestock and poultry wastewater. The recovered struvite has been proved to be an effective fertilizer, but the antibiotics residues of in the recovered struvite products may bring potential ecological risks in subsequent agricultural applications. Although using different magnesium (Mg) sources for the optimization of struvite recovered have been widely investigated, their influence on antibiotic transportation and its accumulation in the final product has been rarely reported. This study systematically discussed the effects of different Mg sources, Mg dosage, wastewater salinity and tetracycline (TC) concentration on the struvite production and the migration behavior of TC in the process of phosphorus recovery. The results showed that when MgCl₂, MgO, small size Mg alloy, large size Mg alloy were used as magnesium sources, the recovery of PO₄^3-- P reached 81.43-98.81 %, 30.11-92.90 %, 7.28-20.07 %, 77.85-85.38 %, respectively. The TC content in the crystal was significantly positively correlated with the initial TC concentration. At initial TC concentrations of 200 μg/L, when MgCl₂, MgO, small size Mg alloy, large size Mg alloy were used as magnesium sources, TC content in struvite were 4.04-4.49 μg/g, 2.73-5.105 μg/g, 4.025-4.705 μg/g, 2.455-3.04 μg/g, respectively. At initial TC concentrations of 20 μg/L, they were 0.085-0.125 μg/g, 0.095-0.155 μg/g, 0.09-0.115 μg/g, 0.045-0.085 μg/g, respectively. When MgCl₂ or Mg alloy was used as the Mg source, the content of TC in the crystal increased with the increase of wastewater salinity. Conversely, when MgO was used as the Mg source, TC content decreased with the increase of wastewater salinity. This phenomenon may be attributed to Mg supply and electrostatic effects. In the MgCl₂ and Mg alloy systems, rapid or continuous Mg²⁺ supply and electrical double-layer compression at higher salinity reduced electrostatic repulsion, promoting TC incorporation. In contrast, the slow dissolution of MgO was further inhibited at higher salinity, lowering Mg²⁺ availability and supersaturation, which limited TC incorporation into struvite. Notably, utilizing the large-size Mg alloy as the Mg source, TC content in recovered struvite is the lowest. Moreover, pH adjustment using acid or alkali solutions is unnecessary in the process of P recovery using Mg alloy, contributing to reduced production costs. These findings highlight Mg alloy as a promising Mg source for struvite recovery, offering dual benefits of providing alkaline conditions and minimizing antibiotic residues, making it a recommended choice for future applications.