Study On The Preparation And Adsorption-regeneration Of Magnetic Nanocomposites For The Removal Of Methylene Blue Dyes From Wastewater

A novel mesoporous magnetic nanocomposites（MNCs） adsorbent was synthesized for the removal of methylene blue（MB） dyes from wastewater. The inner core Mn0.6Zn0.4Fe2O4 magnetic nanoparticles（Fe-MNPs） were coated with an outer shell of silica based on a facile method using tetraethylorthosilicate（TEOS） as a precursor material. The optimum conditions of preparation were gotten. Silica coating has been confirmed by FT-IR. All the peaks in XRD of Fe-MNPs and silica-coated Mn-Zn ferrite MNPs（Si-Fe-MNCs） confirm the formation of spinel structure systems of Mn-Zn ferrite（Mn0.6Zn0.4Fe2O4）. The XRD indicates that all the MNPs of Mn0.6Zn0.4Fe2O4 are nucleated in the amorphous matrix of silica. The observed BET surface area and pore volume of MNCs all increased, which makes it useful for adsorption of MB dyes. It was confirmed that the mesoporous Si-Fe-MNCs adsorbent was synthesized with large surface area, suitable poresize, and uniform pore size distribution, but the material possessed irregular pore structures by HRTEM. It is observed that the agglomeration as compared to Fe-MNPs is reduced due to the silica coating by SEM and TEM. The synthesized Si-Fe-MNCs were nanosized microspheres with a special “core–shell” structure. The cores of Fe-MNPs are coated with a continuous shell of silica with a thickness of approximately 2.7 nm. The selected area electron diffraction（SAED） pattern reveals diffused ring pattern, typical for amorphous silica coating on the Mn-Zn ferrite nanoparticles. The various values of dhkl estimated from the SAED pattern are in good agreement with those obtained from XRD studies. The Si-Fe-MNCs exhibited a good dispersibility and magnetic separability in water. The nanosize and a fast separation from water make Si-Fe-MNCs adsorbent a potential claimant for environmental applicationsThe specific surface area, morphology and magnetic properties of “core–shell” structured adsorbents were systematically characterized by BET surface area measurement, transmission electron microscopy（TEM） and vibrating sample magnetometer（VSM）, respectively. The synthesized magnetic adsorbent exhibited good performance for the removal of methylene blue（MB） from water, with the maximum adsorption capacity ranging from 40.31 mg·g-1 to 184.1 mg·g-1 at 318 K. The adsorption equilibrium was reached at 120 min. The adsorption kinetics of MB onto Si-Fe-MNCs could be simulated by the pseudo-second-order model, and the adsorption isotherms could be expressed by Langmuir model. Thermodynamic calculations indicated that the adsorption of MB onto Si-Fe-MNCs was a spontaneous endothermic physical process on the surface. Infrared spectra revealed that hydrogen bonding was the major interaction forces between the surface functional groups of Si-Fe-MNCs and MB. The regeneration of Si-Fe-MNCs after MB adsorption was conducted using H₂O₂, and the adsorption capacity of Si-Fe-MNCs after five cycles was still maintained higher equilibrium adsorption capacity.The regeneration of Si-Fe-MNCs after MB adsorption was conducted using H₂O₂, formed as heterogeneous Fenton-like reaction by the “core – shell” structured adsorbents. The influencing factors on the regeneration of Si-Fe-MNCs including the regeneration time, H₂O₂ concentration, adsorption dosage of MB and temperature were investigated. It was found that the optimum conditions were H₂O₂ concentration of 1.305 mol·L-1 and temperature of 313 K. The regeneration rate reached to 98%, on the condition of more adsorption dosage of MB on Si-Fe-MNCs. In the process of regeneration, Si-Fe-MNCs was employed as an effectiveand stable heterogeneous catalyst for the Fenton-like reaction without adjusting pH, catalytic decomposition of H₂O₂ into hydroxyl radical（?OH）. By the good degradation capacity of MB, after the reaction, the aqueous solution was colourless and TOC of 80 mg·L-1. The degradation mechanism of methylene blue was explored that the Fenton-like oxidation processesis a microscopic heterogeneous interfacial reaction. The Si-Fe-MNCs and regenerated Si-Fe-MNCs of crystal structure, specific surface area, micromorphology and magnetic property were characterized by XRD, BET, SEM, TEM and VSM. The regenerated Si-Fe-MNCs can be used repeatedly for many times with high equilibrium adsorption capacity and magnetic separation capacity.