Size-based selectivity of magnetic-layered double hydroxide (MLDH) for efficient removal of oxytetracycline in the presence of natural organic matter

Abstract

While the adsorption process plays an essential role in antibiotics removal from water, one major problem for applying absorbents in actual wastewater is competitive adsorption that is commonly caused by natural organic matter (NOM). Therefore, we aimed to investigate size-based selectivity of magnetic layered double hydroxides (MLDH) as the absorbent in the mixture of oxytetracycline (OTC) and NOM, as MLDH are generally characterized by high specific surface area, limited intercalation, and ion-exchange properties. MLDH were synthesized by the one-step solvothermal method with four different Fe:Mg ratios and then applied in a series of adsorption experiments. In addition, the role of limited interlayer space of MLDH in the adsorption of OTC and NOM was investigated based on molar mass distribution of those organic components. As results, the adsorption behavior of OTC/NOM on MLDH in the binary systems was found to involve chemisorption and multi-layer coverage. Although the competitive adsorption reduced OTC adsorption, Freundlich coefficient of MLDH (KF) for OTC was higher than KF for NOM, indicating that MLDH selectively adsorbed OTC molecules in the presence of NOM. Moreover, the selectivity (α) value of all MLDH, except MLDH with a Fe:Mg ratio of 1:4, indicating substantial selective adsorption. Among the four MLDH, MLDH11 exhibited the highest α, because its smallest interlayer space offers size-based selectivity, which was also supported by the molar mass distribution. The type of NOM did not significantly affect the OTC adsorption efficiency on MLDH. The results confirmed the benefits of limited interlayer space of MLDH, which offers selectivity for adsorption of OMP in water and shows potential applicability for removing other organic micropollutants.