Sorbent effect on the sorption of Cr(VI) on a Mg₆AlFe-layered double hydroxide and its calcined product in aqueous solutions

In this study, the sorption of hexavalent chromium, Cr(VI), on a Mg₆AlFe-layered double hydroxide (LDH) and its calcined product (layered double oxide, LDO) in aqueous solutions was investigated using a batch technique at various sorbent dosages (C _s). A significant sorbent effect (C _s-effect) was observed in the sorption systems; the sorption isotherms declined as C _s increased. The Langmuir and Freundlich isotherms can describe the sorption equilibriums at each given C _s well but cannot describe the C _s-effect observed. These C _s-effect data can be described by the surface component activity (SCA) model, namely, the Langmuir-SCA and Freundlich-SCA isotherms. The C _s-effect in the LDO system is stronger than that in the LDH system, which can be attributed to the higher specific surface area of the LDO than that of the LDH. Furthermore, the characteristic saturation sorption capacity of the LDO for Cr(VI), which was obtained from the SCA model, is higher than that of the LDH, indicating that LDOs are more effective sorbents than LDHs are for heavy metal removal. In addition, the influences of pH and electrolyte (NaNO₃) concentration (C _NaNO3) on the C _s-effect were examined; no significant effects were observed upon changing either the solution pH (5–9) or C _NaNO3 (0.001–0.100 mol/L). This work improves our understanding of the C _s-effect phenomenon and confirms the applicability of the SCA model in describing sorption equilibriums with the C _s-effect.