Preparation Of Amino-modified Fe₃O₄ Nanoparticles As Adsorbents For Removal Of Cr（Ⅵ） And Pb（Ⅱ） From Aqueous Solutions

Fe₃O₄ magnetic nanoparticles（MNPs）have been received considerable attention in wastewater treatment for not only their excellent reusability and quick separation from aqueous solution but also the facile functionalization process.Polyethyleneimine（PEI）is a water soluble cationic polyelectrolyte with abundant amino groups,thus it has the ability to adsorb anions through electrostatic interaction as well as neutralization and chelate heavy metal ions such as Pb（Ⅱ）.In this study,MNPs were applied as the core and modified by PEI with different molecular weight to prepare a series of amino-modified magnetic nanoparticles（PEI-MNPs）.The novel adsorbents are low-cost,easy-to-separate with high adsorption capacity,good reusability and storability.For synthesizing the PEI-MNPs,（1）MNPs were obtained by an improved co-precipitation method using NaOH as precipitant;（2）oleic acid was applied as surfactant to modify the MNPs to improve their stability and lipophilicity;（3）epoxy groups were grafted onto the surfaces of the nanoparticles;（4）PEI with different molecular weights（600,1800 and 10000 g·mol-1）was anchored via ring-opening reaction between epoxy groups and amino groups to obtain PEI-MNPs.The fabricated nanoparticles were characterized by FT-IR,XRD,VSM,SEM,TEM,Zeta potential analyzer and TGA.The results show that PEI-MNPs are superparamagnetic spheres with an average diameter of about 12 nm.With saturation magnetization of over 38.75 emu·g-1,the adsorbents can be easily separated from water using a magnet within 2 min.What’s more,the PEI-MNPs remain positively charged in alkaline solutions due to their high isoelectric（10.41）which benefits the adsorption of anion ions.The adsorption properties of PEI-MNPs towards Cr（Ⅵ）and Pb（Ⅱ）are concerned.The effects of the initial solution pH,solution ironic strength,contact time and the storage time of nanoparticles on the adsorption process are investigated.Furthermore,the adsorption capacity of PEI10000-MNPs in natural water is studied.At last,the reusability of PEI-MNPs is evaluated.It is found that in terms of adsorption efficiency,PEI10000-MNPs > PEI1800-MNPs > PEI600-MNPs > MNPs.The Cr（Ⅵ）and Pb（Ⅱ）adsorption capacity of PEI10000-MNPs achieves 95.29 and 114.85 mg g-1 at pH 2.0 and 5.0,respectively.The Cr（Ⅵ）and Pb（Ⅱ）adsorption processes achieve equilibrium in 30 and 60 min at 25°C respectively and the adsorption isotherm data fit the Langmuir adsorption model better than the Freundlich model.Based on the correlation coefficient（R2）,the pseudo-second-order model matches the experimental data better than the pseudo-first-model for both the adsorption processes.The coexisting ions in the natural water have insignificant effects on Cr（Ⅵ）and Pb（Ⅱ）adsorption.The adsorption capacity drops about 20% in 40 days.After five consecutive adsorption-desorption cycles using 0.5mol L-1NaOH as desorption solution,the adsorption efficiency remains about 80% for all the PEI-MNPs,indicating good reusability of PEI-MNPs.