Physico-chemical and adsorption study of hydrothermally treated zeolite A and FAU-type zeolite X prepared from LD (Linz–Donawitz) slag of the steel industry

Sodium-rich zeolite A and zeolite X (FAU-type) samples were synthesised from LD-slag via fusion-assisted hydrothermal treatment. The physicochemical and thermal stability of the prepared samples were examined with the help of various characterisation techniques, namely, Fourier Transform Infrared (FTIR) spectroscopy, X-ray diffraction (XRD) and thermogravimetric analysis (TGA) analysis at three different pH conditions and treatment time. Moreover, the sustainability of the crystalline phase and the corresponding zeolite network was evaluated from XRD, FTIR, TGA and Field-Emission Scanning Electron Microscopy (FESEM) analysis. Zeolite A depicts a unique cubical structure and is thermally more stable as compared to zeolite type-X. Also, zeolite A showed the highest dye removal efficiency of 98.13%, as compared to 94.47% for zeolite X, along with equilibrium sorption capacities of 25.30 and 23.57 mg g−1, respectively. In addition, the study proposes that both the synthesised adsorbents are effective and economically sustainable for cationic methylene blue adsorption. Furthermore, methylene blue adsorption was regulated by a multistage diffusion process that agreed with a pseudo-second-order kinetic model (R2 = 0.999 and 0.996 for A- and X-type zeolites). The Langmuir isotherm model best suited the equilibrium data, with monolayer adsorption capacities of 20 and 25.40 mg g−1, respectively.