| Water pollution by Pb(Ⅱ)is a serious environmental problem.Wastewaters from various industrial activities usually contain considerable amounts of Pb(Ⅱ)which can be a severe threat to public health.Adsorption is considered as an effective and economic technology to remove Pb(Ⅱ)from water prior to its discharge into the natural environment.Manganese oxides have been proven to be promising adsorbents to capture Pb(Ⅱ)from wastewaters.In nature,MnO2 can be found in different crystalline structures,as the basic structure unit,[MnO6]octahedron,links in different conditions.The crystalline structures are generally believed to be responsible for their chemical and physical properties.However,the effect of crystal structure on their adsorption performance remains unclear.To gain a better understanding of the effects of MnO2 structure on adsorption behaviors,the adsorption comparison of Pb(Ⅱ)on α-,β-,γ-,δ-,and λ-MnO2 were made in our study.The adsorbents were prepared and characterized by X-ray diffraction(XRD),transmission electron microscopy(TEM),Fourier transform infrared(FT-IR),X-ray photoelectron spectroscopy(XPS),X-ray fluorescence analysis(XRF),zeta potential measurements and N2 adsorption-desorption.The adsorption properties of the adsorbents toward Pb(Ⅱ)in aqueous solution were systematically examined for adsorption capacity and effect of water chemistry.Column adsorption tests were also carried out to elucidate the applicability of δ-MnO2 in Pb(Ⅱ)removal from synthetic wastewater effluent.Manganese oxides functionalized by plasma,ultrasonic and loaded on magnetic materials were synthesized and Pb(Ⅱ)adsorption on these adsorbents was investigated.The main results were as follows:1.In this study,five manganese oxides with different crystallographic phases,α-,β-,β-,δ-,and λ-MnO2 were prepared and characterized by XRD,TEM,XPS,N2 adsorption-desorption,FT-IR and zeta potential measurements.Adsorptive removal of aqueous Pb(Ⅱ)was investigated using these manganese oxides as adsorbents.Results showed that adsorption capacities of manganese oxides for Pb(Ⅱ)varied with BET surface area and crystalline structure,following the order of δ-MnO2>α-MnO2>λ-MnO2>γ-MnO2>β-MnO2.δ-MnO2 displayed the highest capacity for Pb(II),and the adsorption was scarcely influenced by the presence of coexisting cation Na+.The surface complexation model was used to describe the Pb(Ⅱ)adsorption on the MnO2 adsorbents.In column adsorption test δ-MnO2 was capable of continuously treating 25,000 bed volumes synthetic wastewater stream with an influent concentration of 20 mg L-1 Pb(Ⅱ)to the effluent concentration below 0.5 mg L-1.This work indicates thatδ-MnO2 has great potential to be used as effective adsorbent for Pb(Ⅱ)removal.2.Al2O3-pillared layered MnO2(A-MnO2)was synthesized using δ-MnO2 as precursor and Pb(Ⅱ)adsorption on A-MnO2 and δ-MnO2 was investigated.To clarify the adsorption mechanism,Al2O3 was also prepared as an additional sorbent.The adsorbents were characterized by XRF,XRD,TEM,XPS and N2 adsorption-desorption.Results showed that in comparison with pristine δ-MnO2,Al2O3 pillaring led to increased BET surface area of 166.3 m2 g’1 and enlarged basal spacing of 0.85 nm.Accordingly,A-MnO2 exhibited a higher adsorption capacity of Pb(Ⅱ)than δ-MnO2.The adsorption isotherms of Pb(Ⅱ)on 8-MnO2 and Al2O3 pillar fitted well to the Freundlich model,while the adsorption isotherm of Pb(Ⅱ)on A-MnO2 could be well described using a dual-adsorption model,attributing to Pb(Ⅱ)adsorption on both δMnO2 and Al2O3.Additionally,Pb(Ⅱ)adsorption on δ-MnO2 and A-MnO2 followed the pseudo second-order kinetics,and a lower adsorption rate was observed on p-MnO2 than δ-MnO2.The Pb(Ⅱ)adsorption capacity of A-MnO2 increased with solution pH and co-existing cation concentration,and the presence of dissolved humic acid(10.2 mg L-1)did not markedly impact Pb(Ⅱ)adsorption.A-MnO2 also displayed good adsorption capacities for aqueous Cu(Ⅱ)and Cd(Ⅱ).3.Mesoporous β-MnO2(m-MnO2)was prepared using KIT-6 as the templates and was further treated by plasma(β-MnO2)and ultrasonic(u-MnO2)techniques,and Pb(Ⅱ)adsorption on the adsorbents was investigated.For comparison,bulk β-MnO2 was prepared using hydrothermal method(h-MnO2)and pyrolysis method(t-MnO2),respectively.The adsorbents were characterized by XRD,TEM,N2 adsorption-desorption,measurement of zeta potential and XPS.The results showed that in comparison with h-MnO2 and t-MnO2,m-MnO2 displayed a higher BET surface area and ordered mesoporous pore structures.After plasma and ultrasonic treatments,the mesoporous pore structures were remained and the surface hydroxyl groups increased.Accordingly,p-MnO2 and u-MnO2 exhibited higher adsorption capacities of Pb(Ⅱ)than m-MnO2.The adsorption isotherms of Pb(Ⅱ)on the adsorbents fitted well to the Langmuir model,and Pb(Ⅱ)adsorption kinetics followed the pseudo second-order kinetics.The Pb(Ⅱ)adsorption capacities of p-MnO2 and u-MnO2 increased with solution pH and co-existing cation concentration,and the presence of dissolved humic acid(10.2 mg L-1)did not markedly impact Pb(Ⅱ)adsorption.4.The Fe3O4@SiO2 core-shell composite material was prepared using the hydrothermal method,and surface functionalization of the composite by MnO2 was conducted using the deposition precipitation method.The characterization results showed that the composite material was magnetic and could be recovered under an external magnetic field.Additionally,the adsorption capacity for Pb(Ⅱ)increased with the amount of MnO2 loading,and the adsorption followed the Langmuir adsorption isotherm.The adsorption capacity of the sorbent increased with MnO2 content,while the MnO2 content-normalized adsorption decreased with MnO2 content.This is probable because the highly dispersed MnO2 at lowered loading level.Pb(Ⅱ)adsorption on MFS@MnO2 increased with the increase of pH,and the adsorption was enhanced with the presence of co-existing ions.After four consecutive adsorption-desorption cycles,MFS@MnO2-1.54 exhibited good adsorption capacity,reflecting an excellent potential for cycling utilization. |
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