Montmorillonite clay decorated with Al-Mg LDH as a strong and retrievable nanocomposite for decontamination of wastewater containing chromium(VI) and arsenic(III) toxic ions

The aim of this study is removing chromium(VI) and arsenic(III) from water and wastewater using montmorillonite clay/Al-Mg layered double hydroxide (LDH) nanocomposite. TEM, FT-IR, Mapping/FESEM, EDX, BET, and XRD analyses were utilized for studying the characterization of the aforementioned sorbent. According to the results, the best pH values for uptake of Cr(VI) and As(III) were obtained at pH 2 and 7, respectively, which in these conditions, the highest sorption yield of Cr(VI) and As(III) were 98.36% and 95.76%, respectively. Also, the equilibrium and kinetic study of the adsorption process illustrated that the Langmuir isotherm model and pseudo-second-order kinetic model are better able to express the absorption process. According to the Langmuir model, the highest sorption capacity of montmorillonite clay/Al-Mg LDH in adsorbing As(III) and Cr(VI) ions was 34.1 and 37.6 mg g⁻¹, respectively. Furthermore, the sorption capacity of As(III) and Cr(VI) ions obtained using the PFO model was lower than the PSO model. Moreover, the uptake of As(III) and Cr(VI) ions utilizing montmorillonite clay/Al-Mg LDH is physical, favorable, thermosetting (ΔH° < 0), and owing to the weak Van der Waals forces (based on the FTIR analysis). The recyclability study indicated that montmorillonite clay/Al-Mg LDH can be reutilized in 5 steps with considerable reduction in uptake yield. Additionally, clay/Al-Mg composite could remove co-existing ions (e.g., Ni(II), Pb(II), Hg(II), Cr(III), Cu(II), Co(II), Cr(VI), and As(III)) with excellent uptake performance (> 80% for each ion separately). Eventually, montmorillonite clay/Al-Mg LDH was able to eliminate BOD₅, COD, As(III), and Pb(II) from a real effluent with considerable efficiencies.