Response surface methodology for heavy metals removal by tioglycolic-modified Zn–Fe layer double hydroxide as a magnetic recyclable adsorbent

A new nanoadsorbent was synthesized through intercalation of thioglycolic acid (TGA) in the magnetic layered double hydroxide nanocomposite (Fe3O4/Zn–Fe LDH/TGA) and used for the removal of heavy metal ions such as Hg2+ and Pb2+ from aqueous solutions. The morphology and structure of the synthesized nanoadsorbent were investigated by Fourier transform infrared spectroscopy, X-ray diffraction, field-emission scanning electron microscopy and transmission electron microscopy. Also, the quantitative measurements of metal ions were determined using atomic fluorescence spectroscopy. The simultaneous impact of important parameters on the Hg2+ removal including pH, adsorbent dosage and removal time were investigated using response surface methodology. Based on the obtained results, pH 5.5 was achieved, and also 9 mg as adsorbent dosage and 30 min for removal time were selected as optimum conditions in Hg2+ removal experiments. The adsorption kinetic results fitted well with the pseudo-second-order model, and the R2 value = 0.991 while the estimated qe was matched well with the experimental data. Thermodynamic results showed that the adsorption process of Hg2+ was endothermic, feasible and spontaneous. Also, the efficiency of the magnetic nanocomposite was investigated for adsorption of the metal ions. The selectivity experiments showed the following order: Hg2+ ⋙ Pb2+ > Fe2+ > Cu2+ > Zn2+ > Cd2+. Finally, the obtained results verified the excellent ability of the magnetic nanoadsorbent for Hg2+ removal from the aqueous media for at least three times recycle with 90% recovery.