Magnetic ternary layered double hydroxides for efficient removal of 1-naphthalene acetic acid from waters: Adsorption behavior and mechanism

The removal of pollutants requires environmental protection technology to use the least energy consumption, realize the reuse of substances, and ensure the health of the ecosystem and human body. Layered double hydroxides (LDHs) are widely used as adsorbents due to their excellent tunable structure properties and biocompatibility. In this study, magnetic CoMgAl LDH (C-MLDH) and magnetic FeMgAl LDH (F-MLDH) were prepared by coprecipitation method with adding Co element and Fe element respectively on the basis of classical MgAl LDH for the adsorption of 1-naphthoacetic acid (NAA) in water. The morphology and characteristics of the synthesized LDHs were investigated. The pH, adsorbent concentration, kinetics, temperature, isotherm and thermodynamics of the adsorbent were studied to explore the adsorption mechanism. The results showed that the Liu isotherm model and Avrami kinetic model can fit the adsorption isotherm and kinetic data well, respectively. The both synthesized magnetic ternary LDHs had high adsorption capacity of NAA, which reached 258.5 mg/g and 288.5 mg/g for C-MLDH and F-MLDH, respectively. The adsorption of NAA by C-MLDH and F-MLDH was both spontaneous and endothermic. There were two main adsorption mechanisms, one was adsorption by forming hydrogen bonds with hydroxyl groups on the surface of the adsorbent, and the other was anion exchange. The adsorption capacity of magnetic ternary LDHs mainly depended on the shell LDHs, the addition of Co and Fe elements could promote the adsorption reaction, enhance the adsorption capacity, and the promotion effect of Fe element was better than Co.