Adsorption of tetracycline from an aqueous solution on a CaMgAl-layer double hydroxide/red mud composite: Kinetic, isotherm, and thermodynamic studies

This study focused on CaMgAl layered double hydroxide (LDH) based nanocomposite synthesized by a co-precipitation method combined with red mud (RM). The prepared red mud-supported CaMgAl-LDH was characterized by powder X-ray diffraction patterns (XRD), Fourier transform infrared spectra (FTIR), Field-emission scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), and Brunauer-Emmett-Teller analysis (BET), then subsequently used to remove tetracycline (TEC) from an aqueous solution. The TEC maximum adsorption capacity of nanocomposite was 108.753 mg/g. The maximum removal efficiency of 96.91 % was reached under the optimum conditions: pH 6, agitation speed 150 rpm, dosage 0.4 g/100 ml, and initial concentration 70 mg/l with a contact time of 90 min. The experimental adsorption data were fitted well by the Langmuir isotherm model and pseudo second-order model well describe the kinetics of TEC adsorption onto CaMgAl-LDH/RM sites. The adsorption mechanisms were controlled by external mass transfer as well as intra-particle diffusion. The thermodynamic parameters (ΔG°, ΔS°, and ΔH°) indicated that the adsorption of TEC onto red mud-CaMgAl was spontaneous and exothermic in nature. Only a 27 % reduction in the prepared adsorbent′s removal efficiency was noted after six sequential regeneration cycles. This work demonstrates that red mud-supported CaMgAl-LDH offers a potentially efficient adsorbent for the removal of antibiotics from aqueous solution.