Adsorption of Cr(VI) on calcined MgAl-layered double hydroxides and recycling of spent adsorbents for the removal of organic dyes

Adsorption is considered an effective strategy for removing Cr(VI) from wastewater, but disposal of spent adsorbents is still a thorny problem. This work aims to develop an efficient adsorbent for Cr(VI) removal and recycling spent adsorbent as catalyst for the removal of organic dyes. In this study, MgAl-mixed oxides adsorbents (MgAlO) were synthesized via hydrothermal and calcination steps to effectively adsorb Cr(VI) from wastewater. The influence of initial pH and temperature on the adsorption performance of MgAlO was investigated. The maximum adsorption capacity for Cr(VI) was 95.2 mg/g at a MgAlO dosage of 1.0 g/L and a pH value of 5.5. The combination of XRD, FT-IR, and UV-vis DRS analyses revealed that CrO₄²⁻ anions were intercalated into the interlayer spaces of layered double hydroxides, and high temperatures can accelerate the reconstruction of MgAlO. The adsorption of Cr(VI) by MgAlO followed the pseudo-second-order kinetic model, which included intra-particle diffusion, film diffusion, and chemical reaction. Furthermore, the resulting spent adsorbent (Cr-MgAlO) after adsorption was reused as a catalyst for methyl orange (MO) removal after adsorption (75.6% removal rate) and showed no significant decrease in removal rate after 5 cycles. The results of this study provide insights into the reuse of spent Cr(VI) adsorbent for environmental catalytic applications, which are of great importance for the disposal of waste adsorbent.