Coupled adsorption-catalytic reduction of permanganate on Co–Al-layered double hydroxide for dye removal

Abstract

Permanganate (MnO4−) ions were catalytically reduced to MnO2 by water molecules after adsorption on the surface of carbonate intercalated Co–Al-LDH. This reaction was very slow under experimental conditions in the absence of catalyst and was catalyzed by Co–Al-LDH and Co(II) ion had a key role in this process in the pH range of 2.8–13. But, at pH = 14, MnO4− ions reacted with Co(II) ion of adsorbent and produced Co(III). At all pHs, adsorption sites of Co–Al-LDH for MnO4− ions were Co–OH groups. These adsorption sites were located in micropores and mesopores of Co–Al-LDH and were called MI and ME adsorption sites, respectively. MnO4− ions were adsorbed on both adsorption sites in neutral aqueous solutions and only on ME sites at other pHs. Adsorption experiments were carried out at different temperatures, ionic strengths, pHs and initial adsorbate concentrations. Co–Al-LDH acted as a superadsorbent for MnO4− ions and maximum adsorption capacity was 347.9 mg g−1 (2.2 mmol g−1) in neutral aqueous solution at 318 K. The adsorption binding constants and thermodynamic parameters of adsorption of MnO4− ions on Co–Al-LDH were analyzed by ARIAN (adsorption isotherm regional analysis) model. This model determined the boundaries of adsorption of MnO4− ions on Co–Al-LDH under different conditions and showed that this process was endothermic in regions I and II of its isotherms in neutral solution and at pH = 13. This process obeyed ARIAN–Hinshelwood mechanism and water molecules in the pH range of 2.8–13 and hydroxide ions at pH = 14 participated as nucleophile reagents. Reaction in the pH range of 2.8–13 was as follows2MnO4−+ H2O →2MnO2(ad)+32O2+2OH− and at pH = 14 was as2MnO4−+2Co(II)–OH+H2O→2Co(III)–OH+2MnO2ad+32O2+2OH−This adsorbent was used for selective separation of permanganate from cationic safranin O and methylene blue dyes. However, it could not separate anionic methyl orange dye from MnO4− ions.This work further successfully demonstrated an approach to recycle the used Co–Al-LDH using a reducing agent.