| A large amount of Cr(VI)-containing wastewater exceeding the emission standard is a potential hazard to the environmental and human health duo to its extremely toxic and carcinogenic properties. Adsorption is recognized as one of the most effective methods for the removal of Cr(VI), with the advantages of simple technological equipment, easy operation, recovery of metal ions and reusability of the adsorbents. However, the traditional adsorption processes are not easy for continuous operation, and have many problems such as channeling, which brings on a low efficiency of production. Magnetic separation technology has the advantages of convenient and quick separation, easy for continuous process and high treatment efficiency, which can effectively solve the defects of the traditional adsorption processes. The foundation of magnetic separation technology is the preparation of magnetic separation materials. However, the reported magnetic adsorbents for Cr(VI) removal were mainly focus on weakly basic materials, and were only suitable for Cr(VI) removal at low pH values. Besides, most of these adsorbents had the dlisadvantages of low adsorption capacity, slow adsorption rate, the asymmetry of magnetic responsiveness and adsorption property, weak acid resistance of magnetic cores, no deep study of adsorption mechanism and real systems. To address these problems mentioned above, this thesis develops a applied basic research on magnetic adsorption of Cr(VI) from the points of exploring influencing factors of adsorption properties, developing the traditional adsorbents with high adsorption properties, investigating the adsorption mnechanisms of strongly and weakly basic anion adsorbents, developing magnetic adsorbents based on synthetic and native polymers and evaluating the adsorption properties for simulated and real Cr(VI)-containing wastewater.Firstly, micron- and millimeter-size poly(glycidyl methacrylate) microspheres (PGMA) were prepared and modified with ethylenediamine (EDA), with significant property difference in size, porosity, specific surface area and amino group capacity. According to the comparison of the adsorption rate and capacity, the properties of micron-size microspheres were remarkably higher than that of the millimeter-size microspheres. These results demonstrated that reducing the size and pore structure of adsorbents was able to decrease the intra-particle diffusion so as to increase the adsorption rate, and increasing the active group content was more benefit to improve the adsorption capacity. Besides, the primary and secondary amines have a higher adsorption capacity than tertiary amine, indicating that polyamines are effective modification reagents due to plenty of primary and secondary amines on the molecular chains. Furthermore, polyethylenimine (PEI) modified PGMA microspheres (PGMA-PEI) were prepared on the basis of PGMA-EDA microspheres, which exhibited a high adsorption property. The maximum amino group capacity and adsorption capacity were up to 6.5 mmol/g and 505.1 mg/g, respectively. The adsorption equilibrium was reached within 4 min. When the initial Cr(VI) concentrations were 150 and 50 mg/L, the removal efficiency was up to 97% after the first time adsorption, and the Cr(VI) concentration can be reduced to 0.06 mg/L and below 0.05 mg/L after the second and third time adsorption.Secondly, the adsorption mechanisms of strongly and weakly basic anion adsorbents were deeply discussed through the analysis of adsorption-desorption properties and the characterization of Zeta potential, FT-IR and XPS. The PGMA-PEI microspheres served as the model of weakly basic adsorbent, while the N-methylimidazole (MIM) modified polystyrene microspheres (Pst-MIMC1) served as the model of strongly basic adsorbent. The results illuminated that the adsorption mechanism of PGMA-PEI was electrostatic attraction and hydrogen bond interaction, and the microspheres exhibited a high pH-dependence, with the optimized pH at 2.0. Meanwhile, the adsorption mechanism of Pst-MIMC1 was electrostatic attraction-assisted ion exchange interaction, and the microspheres showed a weak pH-dependence and high adsorption capacity in the pH range of 1.0-6.0.Thirdly, to realize the target of continuous magnetic treatment of Cr(VI)-containing wastewater, the weakly (PGMA-PEI, PVA-PEI) and strongly (Pst-MIMCl) basic magnetic adsorbents were developed using the modified suspension polymeration and in-situ precipitation methods, in which the PGMA, poly(vinyl alcohol) (PVA) and Pst microspheres were used as magnetic carriers, and PEI and MIM were used as modification reagents. These microspheres exhibited superparamagnetic nature, and the saturated magnetization values were evaluated as 8.6,36.7 and 12.5 emu/g for PGMA-PEI, PVA-PEI and Pst-MIMCl magnetic microspheres, respectively. Besides, the maximum adsorption capacities were 492.6,88.4 and 104.0 mg/g, and the equilibrium time were 10, 8and 30 min. Compared with the literatures, these three kinds microspheres simultaneously had the advantages of strong magnetic responsiveness, high adsorption capacity and rapid adsorption rate, suggesting that the PGMA-PEI, PVA-PEI and pst-MIMCl have a potential use for the removal of Cr(VI) from wastewater. Especially, the PGMA-PEI had the highest adsorption capacity among the reported magnetic adsorbents. In addition, the Cr(VI)-adsorbed PGMA-PEI and PVA-PEI can be effectively desorbed by a 0.1 mol/L NaOH solution, and Cr(VI)-adsorbed Pst-MIMC1 can be successfully eluted using a 0.3 mol/L NaOH+0.3 mol/L NaCl solution. These three microspheres exhibited a good reusability.Fourthly, to improve the acid resistance of magnetic cores, the protective layer of SiO2 was coated on the surface of Fe3O4 nanoparticles. Signal coated magnetic SiO2 particles were synthesized using a modified method, and the acid resistance of magnetic cores was highly improved. No satisfactory results were achieved after silylation modification and initiated polymerization, which were attributed to the low grafting capacity of organic group. Therefore, magnetic natural polymers with core-shell-shell structure Fe3O4@SiO2@CE and Fe3O4@SiO2@CTS) were first developed using the sol-gel method and emulsion cross-linking method, in which the magnetic silica nanoparticles and natural polymers (cellulose and chitosan) were used as magnetic cores and coating layers, respectively. Furthermore, weakly (Fe3O4@SiO2@CE-EDA, Fe3O4@SiO2@CTS-PEI) and strongly (Fe3O4@SiO2@CTS-GTMAC) basic magnetic adsorbent were prepared, with EDA, PEI and glycidyl trimethyl ammonium chloride (GTMAC) as the modification reagents. The maximum adsorption capacities of these three adsorbents were estimated as 171.5,236.4 and 233.1 mg/g, respectively, and the adsorption capacity of Fe3O4@SiO2@CTS-PEI had the highest adsorption capacity among the reported magnetic chitosan materials. Besides, the equilibrium time was 10,60-120 and 40-120 min, respectively. In addition, these three adsorbents showed a good reusability.Finally, the developed magnetic polymer composites were used for the removal of Cr(VI) from electroplating wastewater, and the optimal adsorbent was selected based on the removal efficiency and metal ions selectivity. Among these magnetic adsorbents, the Pst-MIMCl exhibited the highest removal efficiency and metal ions selectivity. When the adsorbent dose was 7 g/L, the removal efficiency was up to 99.7%, and the Cr(?) concentration was reduced to 0.26 mg/L, which was below the emission standard (0.5 mg/L). In addition, only a small amount of Fe3+ ions could be adsorbed on the adsorbent. The interference of other heavy metal cations can be resolved by adjusting the stock pH value to 7.0. |
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