The tetracyclines removal by MgAl layered double oxide in the presence of phosphate or nitrate: Behaviors and mechanism exploration

Pollution of tetracyclines (TCs) in swine wastewater has been a critical concern worldwide. Notably, multiple anions (e.g. PO₄ ³⁻, NO₃ ⁻) coexist in the actual environments, which could significantly influence the TCs removal. In the current study, MgAl layered double oxide (MgAl-LDO) was adopted for investigating the TC removal performance with/without PO₄ ³⁻ or NO₃ ⁻. In all systems, the adsorption performance exhibited two different approaches between low and high TC concentrations. In the single system, pseudo-second-order and the Freundlich model fitted well to the equilibrium adsorption data when TC concentration was below 125 mg·L⁻¹, while the pseudo-first-order and the linear model could describe the removal process at high TC concentration (>125 mg·L⁻¹). The maximum adsorption capacity was 83.56 mg·g⁻¹. In the co-existing system, the adsorption capacity was slightly enhanced when TC concentration below 150 mg·L⁻¹ however was inhibited at high concentration (>150 mg·L⁻¹). Combined with the characterization analyses, the interaction mechanism at low concentration was primarily surface adsorption on reconstructed LDH from LDO in the TC-alone system. It is worth mention that both PO₄ ³⁻ and NO₃ ⁻ facilitated the formation of LDH via rehydration of LDO which enhanced surface adsorption in the co-existing system. At high TC concentration, the formation of tetracycline-metal complexes played a dominant role in TC removal in the single system, whereas diminished complexation in the binary system led to the decreased TC removal. This study provides a theoretical and practical guidance for MgAl-LDO on the efficient remediation of actual tetracyclines wastewater.