Unusual behavior of MgFe₂O₄ during regeneration: Desorption versus specific adsorption

The reusability of spent adsorbents is the most important characteristic for their practical application. The process of MgFe₂O₄ regeneration after methylene blue (MB) adsorption was studied. The effect of the nature (HCl, HNO_3, and MgCl₂) and the concentration (10^-3–10^-1 M) of regeneration agents was established. All the regeneration agents at 10^-3 and 10^-2 M had high efficiency and adsorption capacity recovery reached 80–90%, whereas for 10^-1 M concentration the adsorption efficiency was in the range of 4.5–36.2%. It was shown that the concentration of desorbed MB was much less than what had been previously adsorbed and did not correlate with regeneration efficiency. The unusual behavior of MgFe₂O₄ during regeneration could be due to different mechanisms of regeneration by OH₃ ^þ and Mg^2þ ions: (i) for acidic regeneration the main process was the non-specific adsorption of OH₃ ^þ ions in a diffusion layer and the substitution of adsorbed MB due to electrostatic forces; (ii) in the case of Mg^2þ as a regeneration agent, there was specific adsorption due to the completion of a crystal lattice of MgFe₂O₄ nanoparticles by Mg^2þ ions (according to the rules of Fayans-Pannet) with the formation of new Mg-OH adsorption sites and the super-equivalent adsorption of Mg^2þ ions (according to DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory) accompanied by a recharge of the MgFe₂O₄ surface. These phenomena of MgFe₂O₄ regeneration using Mg^2þ ions must be taken into account in the theory and practice of adsorption.