Unusual Behavior of MgFeO During Regeneration: Desorption Versus Specific Adsorption

The reusability of spent adsorbents is the most important characteristic for their practical application. The process of MgFeO regeneration after methylene blue (MB) adsorption was studied. The effect of the nature (HCl, HNO and MgCl) and the concentration (10-10 M) of regeneration agents was established. All the regeneration agents at 10 and 10 M had high efficiency and adsorption capacity recovery reached 80-90%, whereas for 10 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 MgFeO during regeneration could be due to different mechanisms of regeneration by OH and Mg ions: (i) for acidic regeneration the main process was the non-specific adsorption of OH ions in a diffusion layer and the substitution of adsorbed MB due to electrostatic forces; (ii) in the case of Mg as a regeneration agent, there was specific adsorption due to the completion of a crystal lattice of MgFeO nanoparticles by Mg ions (according to the rules of Fayans-Pannet) with the formation of new Mg-OH adsorption sites and the super-equivalent adsorption of Mg ions (according to DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory) accompanied by a recharge of the MgFeO surface. These phenomena of MgFeO regeneration using Mg ions must be taken into account in the theory and practice of adsorption.