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.
The application of magnetic metal ferrites to treat environmental pollution has attracted a great deal of interest (Reddy & Yun 2016). In particular, ferrites are widely used in the preparation of adsorbents and catalysts for removing heavy metal ions and organic pollutants from wastewater (Kefeni et al. 2017; Basheer 2018; Singh et al. 2018; Park et al. 2019) due to high sorption capacity and kinetic parameters, the possibility of rapid separation from the purified solution, and multiple applications. It is clear that the spent sorbent must be regenerated. Inorganic acids (Tu et al. 2013, 2017; Ghobadi et al. 2018; Khan et al. 2019) and bases (Tu et al. 2014; Zhou et al. 2014; Kaur et al. 2015; Wu et al. 2018), organic solvents (Hou et al. 2010; Konicki et al. 2013; Sayğili 2015; Lu et al. 2016) and chelating agents (Kraus et al. 2009; Zhao et al. 2015) were most often used for the regeneration of ferrite-based sorbents. At the same time, the success of the regeneration process was confirmed by the recovery of sorption characteristics. However, the regeneration mechanism often fails to conform to the description of a regeneration mechanism.
In their previous works (Ivanets et al. 2018; Ivanets et al. 2019), the authors of this report showed that MgFe2O4 nanoparticles can be used for heavy metal ions and methylene blue (MB) adsorption as well as from single- and multi-component solutions. Firstly, MgCl2 solution was proposed for the regeneration of ferrite-based sorbents. It was shown that, at very low concentrations (MgCl2 10−3 M) of the regenerating agent, the sorption capacity of MgFe2O4 nanoparticles was almost completely recovered. This short communication reports on the in-depth study of the regeneration mechanism of MgFe2O4 nanoparticles by HCl, HNO3 and MgCl2 solutions after MB adsorption.
MATERIALS AND METHODS
MgFe2O4 adsorbent was obtained by a self-combustible glycine-nitrate synthesis method with the addition of an inert additive, NaCl, as described earlier (Ivanets et al. 2018). The characteristic of the MgFe2O4 nanoparticles was studied using X-ray diffraction analysis, Fourier-transform infrared spectroscopy, differential thermogravimetric analysis, scanning electron and transmittance electron microscopy and low-temperature (196 °C) nitrogen adsorption–desorption analysis as is described in detail in Ivanets et al. (2018, 2019).
During the adsorption experiment, the concentration of the MB dye in model solutions was determined using a Lambda 45 scanning spectrophotometer (Perkin Elmer Instruments, USA). The pH of model solutions was measured using a pH meter 340i (±0.02) (Mettler Toledo, USA). In each experiment, the sorbent was kept in aliquots of the MB solution with constant stirring at a speed of 300 rpm.
Magnesium ferrite adsorbent regeneration after sorption of MB dye was performed using HCl, HNO3 and MgCl2 solutions with concentrations of 10−1, 10−2 and 10−3 M. An amount of magnesium ferrite (0.025 g) after sorption of the MB dye was placed in 50.0 mL of the corresponding regeneration solutions and kept for 120 min at room temperature and constant stirring at 300 rpm. A regeneration test for MgFe2O4 adsorbent was repeated for at least two cycles of adsorption–desorption.
RESULTS AND DISCUSSION
Isotherm and kinetic studies of MB adsorption
The isotherm of MB adsorption on MgFe2O4 adsorbent is type L according to the Giles classification (Limousin et al. 2007). The ratio between the concentration of the compound remaining in the solution and adsorbed on the solid decreases when the solute concentration increases (Figure 1(a)). This suggests a progressive saturation of the solid.
The Redlich–Peterson model was well fitted to the MB adsorption isotherm (R2 > 0.99), a hybrid isotherm featuring both Langmuir and Freundlich isotherms, which incorporates three parameters into an empirical equation. The model has a linear dependence on concentration in the numerator and an exponential function in the denominator to represent adsorption equilibrium over a wide concentration range. This equation is widely used as a compromise between Langmuir and Freundlich systems. There are some discrepancies in the literature about the definitions of the Redlich–Peterson parameters. In some cases, they are said to be constants without any physical meaning. When the value of g is equal to 1, the above equation is reduced to the Langmuir isotherm, while it is reduced to a Freundlich isotherm when the value of the parameter aRPCeg is much bigger than 1. The ratio of KRP/aRP indicates the adsorption capacity (Foo & Hameed 2010).
The study of the influence of contact time on MB adsorption shows that the adsorption was rapid and, during the first 10 mins, the kinetic curve reached equilibrium (Figure 1(b)). The pseudo-second order model well described the kinetic of MB adsorption (R2 1.00). This gives an indication of the chemisorption mechanism (Tan & Hameed 2017). The adsorption capacities from isotherm and kinetic studies were in good agreement and equaled 60.2 ± 0.1 mg g−1.
Regeneration of MgFe2O4 nanoparticles after MB adsorption
The efficiency of MgFe2O4 nanoparticles regeneration is highly dependent on the nature and concentration of regeneration agents. As shown in Figure 2 and Table 1, the highest regeneration efficiency (≈80–90%) for all solutions was achieved at 10−2 and 10−3 M. For the most concentrated regeneration solutions, 10−1 M, only 25.9–36.2 and 4.5% of adsorption capacity was recovered for inorganic acids (HCl and HNO3) and MgCl2, respectively. Additionally, the MB concentration after MgFe2O4 adsorbent regeneration did not exceed 9.4–11.4 and 0.5 mg L−1 for an acid solution at 10−2–10−1 and 10−3 M, respectively. This corresponds to the desorption of approximately 10 and 0.5% of previously adsorbed MB. In the case of MgCl2 regeneration solutions for all range concentrations, the desorbed MB concentration was <2 mg L−1.
|Regeneration solution .||Concentration, M .||qreg/qinitial, % .||Cdes(MB), mg L−1 .||αdes(MB), % .|
|Regeneration solution .||Concentration, M .||qreg/qinitial, % .||Cdes(MB), mg L−1 .||αdes(MB), % .|
qreg: sorption capacity after sorbent regeneration; qinitial: sorption capacity for the first adsorption cycle; Cdes: MB concentration in regeneration solution; αdes: ratio between adsorbed MB and released MB in solution during adsorbent regeneration.
To understand these phenomena, we tried to interpret the obtained results in the modern terms of the double electric layer structure. Thus, the unusual behavior of MgFe2O4 during regeneration could be due to different mechanisms of regeneration by OH3+ and Mg2+ ions. 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 (Lyklema 1995). In the case of Mg2+ ions 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 super-equivalent adsorption of Mg2+ ions (according to DLVO (Derjaguin, Landau, Verwey, and Overbeek) theory) accompanied by a recharge of the MgFe2O4 surface (Park & Seo 2011).
Competitive MB adsorption in the presence of the Mg2+ and 3d-metal ions
To support the proposed mechanism of MgFe2O4 adsorbent regeneration, the experiment for competitive MB adsorption in the presence of the Mg2+ and transition metal ions (Mn2+, Co2+, Ni2+, Cu2+) ions was carried out (Figure 3).
The results obtained demonstrated that Mg2+ had a great negative effect on MB adsorption. For all ranges of concentration (10−3–10−1 M) of MgCl2, the adsorption capacity was three times less than for adsorption from single-component MB solution. On the other hand, for a multi-component model solution containing Mn2+, Co2+, Ni2+, and Cu2+ cations at 10−2 M concentration for each ion, the adsorption capacity of MgFe2O4 towards MB was close to the results obtained for the single-component solution. This indicates specific adsorption of Mg2+ ions.
These phenomena of MgFe2O4 regeneration using Mg2+ ions must be taken into account in the theory and practice of adsorption.
The regeneration efficiency of MgFe2O4 by using HCl, HNO3 and MgCl2 solutions as regeneration agents is investigated for the first time in this study. It was established that the regeneration efficiency of MgFe2O4 adsorbent did not correlate with the concentration of desorbed MB. The possible regeneration mechanisms in the modern terms of a double electric layer structure were suggested. Specific and super-equivalent adsorptions of Mg2+ ions on MgFe2O4 nanoparticles were proposed for the interpretation of the discovered phenomena.