| Industrial wastewater contains high levels of heavy metals and can reduce the water quality when being directly discharged into natural water, bringing series of environmental issues. Heavy metals in aqueous solution mainly exist in ion state, which can be easily absorbed by aquatic organisms through food intake, thus producing ecological-toxic effects, and even threatening human health. Therefore, removal of heavy metals is a significant task in wastewater treatment.In the treatment of heavy metal, adsorption is widely employed because of its simple operation, low cost and short processing cycle. So it is very important to select and synthesize suitable adsorption materials for heavy metal removal. Among those materials, hydrate iron oxide is frequently used to remove heavy metals from wastewater due to its simple synthesis method, various sources and the better adsorption performance, but it is difficult to separate from solution. However, magnetic iron oxides, such as Fe3O4 and Fe2O3, have better adsorption behavior of heavy metals and can be also easily recycled under external magnetic field. It is, therefore, very significant to synthesize magnetic iron oxides owning high adsorption capacity. Based on the fact that different crystal forms of magnetic iron oxides have different physical and chemical properties, and then affect the adsorption efficiency of heavy metal, this paper proposed a new synthesis method of crystalline magnetic iron oxides, which was conducted under different temperatures. And the synthesized iron oxides were applied to the removal of heavy metals in wastewater, the main results were as follows:Firstly, three different crystal types of magnetic iron oxides were prepared via sol-gel method. Hydrate iron oxides from sol-gel method was dried, then placed in three calcining temperatures(200, 350 and 500 ℃) for 2 h, and finally ground into fine particles. Many technologies including scanning electron microscope(SEM), transmission electron microscopy(TEM), X-ray diffraction(XRD) and Fourier infrared transform spectrometer(FT-IR) were employed to characterize the surface morphology and structure. Results showed that in the process of calcination, sodium dodecyl sulfate(SDS) was removed and nanoparticles surface turned rough. According to XRD spectrum, as the increase of calcining temperature, amorphous iron oxide(or non-crystalline) Fe3O4 transformed into(α+γ)-Fe2O3, and finally into α-Fe2O3. At the same time, specific surface area and pore volume of as-obtained iron oxides decreased with the increase of calcination temperature. Additionally, the saturation magnetization of Fe3O4,(α+γ)-Fe2O3 and α-Fe2O3 were 69.80, 60.10 and 49.80 emu/g with isoelectric point of 4.0, 6.5 and 7.0 respectively.Secondly, those three kinds of magnetic iron oxides were applied to remove Pb(Ⅱ) and Cd(Ⅱ) from simulated wastewater. Simulated wastewater used in the experiment was prepared by deionized water, coupled with a certain concentration of heavy metals. The performance of magnetic iron oxides as adsorbents for Pb(Ⅱ) and Cd(Ⅱ) adsorption were investigated in isothermal adsorption, adsorption kinetics and thermodynamics, etc. Experimental results showed that amorphous Fe3O4 had higher adsorption capacity towards Pb(Ⅱ) and Cd(Ⅱ) than(α+γ)-Fe2O3 and α-Fe2O3: for Pb(Ⅱ) 22.83, 19.52, and 15.51 mg/g, for Cd 7.40, 3.50, and 2.79 mg/g, respectively. Freundlich isothermal model was fitted well with the kinetics results and the fitting degree R2 > 0.99. Intraparticle diffusion model was also used and divided the adsorption process into three stages, i.e., rapid adsorption, slow adsorption and adsorption equilibrium phase. The adsorption mechanism was detected by X-ray spectrometer(XPS), which showed that ion exchange, hydrolysis and electrostatic attraction contributed to the removal of Pb(Ⅱ) and Cd(Ⅱ) by magnetic iron oxides.Thirdly, adsorption-desorption and regeneration of materials was conducted by addition of 0.5 M Na OH solution to magnetic iron oxides after adsorption equilibrium. Desorption time was 2 h, ionic strength was adjusted to 0.01 M, and adsorption temperature was 25 ± 2℃. Results showed that after five cycles, removal efficiency of Pb(Ⅱ) and Cd(Ⅱ) by all as-obtained iron oxides still remained above 79%. |
PDF Full Text Request