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, 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.

Nanochitin/manganese oxide-biodegradable hybrid sorbent for heavy metal ions.

The article presents the results of the synthesis and research of a new biodegradable hybrid nanosized polymer-inorganic system possessing the double nature of heavy metal ions extraction from solutions. Firstly, the sorption of metal ions takes place by the ion-exchange properties of porous manganese oxide and, secondly, due to specific interaction and chelating with functional groups of polysaccharides in terms of nanochitin (ChNC). The synthesis is based on the colloid-chemical processes of interaction between polysaccharide and manganese oxide nanosized particles. Using the mathematical model of the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory, it has been shown that this interaction occurs due to the formation of hydrogen bonds and electrostatic interactions of oppositely charged particles. Hybrid sorbents were characterised by X-ray diffraction, Fourier transform infrared spectroscopy, Transmission electron and Atomic force microscopy. The highest value of sorption capacity for hybrid materials was obtained for Ni2+ extraction and reached 114.0 ± 1.1 mg/g. It was shown that biodegradation of hybrid sorbents increases with the concentration of manganese oxide nanoparticles and almost complete degradation of the organic part can be performed within 6 days. The obtained biodegradable sorbents were designed to address ecological problems connected with the pollution of natural waters by toxic metallic ions.

Kinetic and thermodynamic studies of the Co(II) and Ni(II) ions removal from aqueous solutions by Ca-Mg phosphates.

The aim of this work was to study the sorption kinetics and thermodynamics of Co(II) and Ni(II) from aqueous solutions by sorbents on the basis of hydrogen (PD-1) and tertiary (PD-2) Ca-Mg phosphates depending on the solution temperature and sorbents chemical composition. Kinetic studies of adsorption of Co(II) and Ni(II) ions onto samples of phosphate sorbents were performed in batch experiment at the temperatures 288, 303, 318 and 333 K. The sorbent dose was fixed at 10 g L-1, initial pH value 2.6, and contact time varied from 5 to 600 min. The kinetics of Co(II) and Ni(II) adsorption were analyzed by using pseudo-first order, pseudo-second order and intraparticle diffusion models. Thermodynamic parameters (ΔG°, ΔH° and ΔS°) for the sorption of Co(II) and Ni(II) were determined using the Gibbs-Helmholtz equation. The calculated kinetic parameters and corresponding correlation coefficients revealed that Co(II) and Ni(II) uptake process followed the pseudo-second order rate expression. Thermodynamic studies confirmed the spontaneous and endothermic nature of removal process which indicate that sorption of Co(II) and Ni(II) ions onto both phosphate sorbents is favoured at higher temperatures and has the chemisorptive mechanism. The data thus obtained would be useful for practical application of the low cost and highly effective Ca-Mg phosphate sorbents.