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, HNO3, 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+ and Mg2+ ions: (i) for acidic regeneration the main process was the non-specific adsorption of OH3+ ions in a diffusion layer and the substitution of adsorbed MB due to electrostatic forces; (ii) in the case of Mg2+ as a regeneration agent, there was specific adsorption due to the completion of a crystal lattice of MgFe2O4 nanoparticles by Mg2+ ions (according to the rules of Fayans-Pannet) with the formation of new Mg-OH adsorption sites and the super-equivalent adsorption of Mg2+ ions (according to DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory) accompanied by a recharge of the MgFe2O4 surface. These phenomena of MgFe2O4 regeneration using Mg2+ ions must be taken into account in the theory and practice of adsorption.

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