Effects of mn-doping on structure, surface properties and selenium adsorption of goethite

Abstract

【Objective】Selenium (Se) is an essential micronutrient for human and animals. Ingestion of either an inadequate or excessive amount of Se tends to cause hazard to their health. Bioavailability of Se in soil depends on its forms. Iron oxide is an important component of soil and may interact with Se through desorption/adsorption. Iron and manganese oxides in soil are often cemented together forming binary metal oxides or Mn-doped iron oxides, thus significantly affecting translocation and transformation of nutrient elements and contaminants in the soil. However, so far little has been reported in-depth in the literature about effects of Mn-doped iron oxides on speciation and bioavailability of Se in soil. 【Method】Samples of pure goethite (Goe) and Mn-doped products (G-Mn0.1, G-Mn0.2, G-Mn0.3 and G-Mn0.5) were prepared under set hydrothermal conditions and were characterized with the aid of X-ray diffraction (XRD), transmission electron microscopy (TEM), nitrogen physical adsorption, Zeta potential analysis and potentiometric titrations. Moreover, selenite (Se(IV)) and selenate (Se(VI)) adsorption characteristics of the samples were investigated through batch adsorption experiments. 【 Result 】 Mn-doping at a low rate with RMn/Fe(Mn(II)/Fe(III) molar ratio being 0.1-0.2) promoted significantly formation of goethite crystals along Axis b, in the form of flat needles big in length-to-diameter ratio instead of short spindles, whereas Mn-doping at a high rate, 0.3-0.5 in RMn/Fe, which means increased Mn(II) content, inhibited significantly formation of goethite crystals along Axis b, while promoting their growth along Axis a by a certain degree and moreover making them tenuous. Additionally, a mass of Mn-doped magnetite appeared in the samples. Of Goe, G-Mn0.2 and G-Mn0.5, the specific surface area was 36.78、53.22 and 71.33 m2·g–1; the surface fractal dimension D, 2.31, 2.53 and 2.59; the mean pore diameter, 13.73, 15.59 and 6.92 nm; the isoelectric point, 7.36, 6.58 and 5.31; and the surface zeta potentials at pH=5.0 40.5, 35.3 and 4.92 mV, respectively. In terms of surface hydroxyl content, the three types of the samples followed the order of Goe < G-Mn0.2