Preparation Of MnFe₂O₄ Nanoparticles Based On High-gravity Technology And Application For The Removal Of Lead

Due to its excellent physical-chemical properties,Mn Fe₂O₄ magnetic nanomaterials have been widely studied and applied in the fields of adsorption,catalysis,electrode materials,and biomedicine.At present,the mainstream method for preparing Mn Fe₂O₄ magnetic nanomaterials is the liquid phase-coprecipitation method,which has the disadvantages of large and uneven particle size,and the inability to continuous production,which limits the industrial application of Mn Fe₂O₄ magnetic nanomaterials.The impinging flow-rotating packed bed(IS-RPB)as a high-gravity reactor has been favored in the field of nano-powder material preparation in recent years because of its excellent intensification effect for liquid-liquid micro-mixing and mass transfer efficiency.In this paper,IS-RPB is used as the reactor to continuously prepare Mn Fe₂O₄ magnetic nanomaterials with small particle size and narrow particle size distribution.In view of the agglomeration problem caused by the high surface energy attributed to the too-small particle size of Mn Fe₂O₄ nanoparticles,and the magnetic dipole interaction between the particles,the preparation of Mn Fe₂O₄ magnetic nanomaterials was coupled with the surface modification by L-cysteine(L-Cys),L-Cys modified Mn Fe₂O₄ magnetic nanomaterial(Mn Fe₂O₄-Cys)was obtained by one-step preparation.The effects of the operating conditions including crystallization time,L-Cys concentration,high-gravity factor,and flow rate on the properties of the prepared materials were experimentally studied.The experiment determined optimum conditions for the preparation of Mn Fe₂O₄-Cys nanoparticles in the IS-RPB are:crystallization time of 12 h,L-Cys concentration of 0.6 wt.%,high-gravity factor of 105.81,and flow rate of60 L·h^-1 Under these conditions,a series of characterization analyses and comparisons of Mn Fe₂O₄-Cys nanoparticles prepared by IS-RPB and stirred tank reactor(STR)were carried out.The results show that the Mn Fe₂O₄-Cys prepared by IS-RPB has a small particle size(average particle size is 7.25 nm),uniform particle size(the monodisperse characteristic),high crystallinity,large specific surface area(169.63 m~2·g^-1),and high saturation magnetization(49.28 emu·g^-1).The strong liquid-phase micro-mixing and mass transfer efficiency of IS-RPB provide a more uniformly-mixed liquid-phase environment for the nucleation and growth of nanoparticles;Compared with STR,the Mn Fe₂O₄-Cys nanomaterials prepared by IS-RPB have more excellent crystallinity and magnetic properties.The addition of L-Cys surface modifier can stabilize the growth of Mn Fe₂O₄ nuclei,inhibiting agglomeration and reducing particle size and particle size distribution.The common Pb(?)pollutant in water was selected as the research object,and the adsorption performance of Mn Fe₂O₄-Cys was studied.The influences of a variety of factors included solution p H value,adsorption time,and initial Pb(?)concentration on adsorption capacity were investigated.Under the conditions of initial Pb(?)concentration of 300 mg·g^-1,solution p H value of 6.0,and adsorbent dose of 1 g·L^-1,Pb(?)reaches the adsorption equilibrium on Mn Fe₂O₄-Cys after 180 min,and its equilibrium adsorption capacity is 138.60mg·L^-1,which is 2.06 times that of unmodified Mn Fe₂O₄ nanoparticles(adsorption capacity of67.40 mg·g^-1).The adsorption kinetic model studies show that the adsorption mechanism of Mn Fe₂O₄-Cys and Mn Fe₂O₄ was mainly the complexation between Pb(?)and surface functional groups(-OH,-COOH,-NH₂).Under the same experimental conditions,the maximum adsorption capacities of Mn Fe₂O₄-Cys and Mn Fe₂O₄ nanoparticles for Pb(?)are137.45 and 69.50 mg·g^-1,respectively,showing excellent adsorption performance.The results of regeneration-adsorption experiments show that the adsorption capacity of Mn Fe₂O₄-Cys for Pb(?)remains above 90%after three cycles,showing good reusability.In order to overcome the problem that L-Cys is easy to be oxidized and biodegraded,the modification method of in-site support by multiwalled carbon nanotube(CNT)was adopted.The agglomeration of particles can be eased by the steric stabilization of support and the CNT-supported Mn Fe₂O₄ nano-adsorbent(Mn Fe₂O₄/CNT)with more stable physical-chemical properties was obtained.Under the optimal Mn Fe₂O₄ loading(83.30 wt.%),the obtained Mn Fe₂O₄/CNT specific surface area is 176.14 m~2·g^-1,which is higher than the surface areas of pure CNT(133.16 m~2·g^-1)and Mn Fe₂O₄(129.26 m~2·g^-1),indicating that the in-situ support of CNTs can effectively improve the dispersion of nanoparticles.The adsorption behavior of Pb(?)on Mn Fe₂O₄/CNT nanocomposites conforms to the Elovich kinetic model and the Freundlich adsorption isotherm model,indicating the existence of the surface complex adsorption mechanism and the occurrence of the multi-layer adsorption process on the heterogeneous surface.The maximum Pb(?)adsorption capacity obtained under the experimental conditions is 106.20 mg·g^-1,showing an excellent adsorption performance.In this study,IS-RPB reactor was applied to the preparation of modified Mn Fe₂O₄nanoparticles,which not only further expands the application of IS-RPB in the field of nano-powder materials'preparation,but also lay a theoretical and practical foundation for the industrial production of Mn Fe₂O₄ nanoparticles.In addition,the as-prepared Mn Fe₂O₄-Cys and Mn Fe₂O₄/CNT nanocomposite exhibit an excellent Pb(?)adsorption performance and have good application potential in the field of water resources remediation.