| In this study,several pure iron oxides and manganese and chromium-doped iron oxides were prepared in the hydrothermal systems.The microstructures and surface properties of the samples were analyzed by means of modern test techniques.The adsorption characteristics for Se(Ⅳ)and Se(Ⅵ)were studied using isothermal adsorption experiments,and the adsorption mechanisms were discussed too.The main results were displayed as follows:(1)In the system of preparing goethite,the low ratio of manganese-doping(R=0.1~0.2,R is the molar ratio of manganese to iron)promoted the formation of goethite.With increasing the doping ratio of manganese,the length of the products became shorter and shorter.At R=0.3~0.5,both the crystallinity and the particle size of the products decreased with increasing the R,the manganese-doped magnetite with an irregular morphology was detected in the products.In the system of preparing hematite,increasing the R from 0.1 to 0.5,the crystallinity of the hematite in the products decreased and the particle size reduced.At R=0.5,the manganese-doped magnetite with an irregular morphology and small nona-size was detected in the products.In the systems of preparing goethite and hematite,when the R of chromiumdoped ratio was 0.1~0.5,the formation of goethite and hematite were inhibited significantly,and no other crystals was detected in the products.The crystallinity and particle size of chromium-doped goethite and hematite decreased gradually with increasing the R.At R=0.5,large amounts of amorphous particles appeared in the products.(2)The nitrogen isothermal adsorption/desorption analysis showed that there were a certain amount of mesopores in goethite,hematite and manganese and chromiumdoped products.So the samples used in this study should belong to the mesoporous materials.The specific surface area of goethite(Goe)and manganese and chromiumdoped products(G-Mn0.2 and G-Cr0.2)was 46.25,83.45 and 101.33 m2·g-1 respectively,and the average pore diameters of the three samples were 23.63,14.29 and 2.43 nm respectively.The specific surface area of hematite(Hem)and manganese and chromium-doped products(H-Mn0.2 and H-Cr0.2)were 12.29,169.62 and 99.55 m2·g-1 respectively,and their average pore diameters were 6.75,0.75 and 0.97 nm respectively.The doping of manganese and chromium had a little effect on the surface fractal degree(SFD)of the products.The SFD values of Goe and manganese and chromium-doped products were between 2.43 and 2.54,and the SFD of Hem and manganese and chromium-doped products were between 2.56 and 2.73.(3)The Zeta potentials of Goe,G-Mn0.2 and G-Cr0.2 were 7.36,6.58 and 4.74 mV respectively,and those of Hem,H-Mn0.2 and H-Cr0.2 were 6.41,5.42 and 5.71 mV respectively.This showed that the doping of both manganese and chromium reduced the surface potential of iron oxides significantly.Laser particle size analysis of the samples showed that the average particle sizes of Goe,G-Mn0.2 and G-Cr0.2 were 630,915 and 765 nm respectively,and those of Hem,H-Mn0.2 and H-Cr0.2 were 1025,534 and 523 nm respectively.This indicated that the particle size of manganese and chrome-doped goethite increased at R=0.2,but the size of manganese and chrome-doped hematite reduced significantly.(4)Isothermal adsorption experiments showed that the adsorption capacities for Se(Ⅳ)onto the samples were significantly higher than those for Se(Ⅵ).As for goethite and hematite,the doping of both manganese and chromium increased the adsorption capacities for Se(Ⅳ)and Se(Ⅵ)onto the manganese and chrome-doped products,and the effect on the adsorption capacities of Se(Ⅳ)was more obvious.At the initial Se(Ⅳ)concentration of 80 mg·g-1,the adsorption capacities of Goe,G-Mn0.2 and G-Cr0.2 were 10,16 and 25 mg·g-1 respectively,and those of Hem,H-Mn0.2 and H-Cr0.2 were 6,23 and 24 mg·g-1 respectively.This showed that the adsorption capacities of Se(Ⅵ)onto chromium-doped goethite and hematite were higher than those onto the manganese-doped products.(5)At the initial pH value of 4.0,the pH value of reaction system increased significantly after absorbing for Se(Ⅳ)and Se(Ⅵ)onto the samples,and the changes of the pH values were more obvious in the adsorption systems containing Se(Ⅳ).Zeta potential analysis of the samples showed that the adsorption for Se(Ⅳ)and Se(Ⅵ)onto the different samples reduced the pH value at the Zeta zero potential of the samples.The pH values at the Zeta zero potential of Goe and Hem after the adsorption for Se(Ⅳ)were slightly lower than those after the adsorption for Se(Ⅵ).The pH values at the Zeta zero potential of manganese and chromium-doped products after the adsorption for Se(Ⅳ)were higher than those after the adsorption for Se(Ⅵ).This showed that the mechanisms of the adsorption for Se(Ⅳ)and Se(Ⅵ)onto the samples including electrostatic attraction,anion exchange,surface coordination,etc. |
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