干旱区生态环境知识资源中心

Fe-Mn cutans are important hosts for Fe and Mn in yellow brown earth. They have been playing important roles in heavy metals fixation. However, most researches on Fe-Mn cutans have focused on their chemical characteristics rather than the mineralogical features. In this study, the mineralogical characteristics of Fe-Mn cutans from Wuhan, China are thoroughly studied. Inductive coupled plasma atomic emission spectrometer ( ICP-AES) shows the content of Fe2O3 in cutan is 7.4% (WT), slightly higher than that in soil matrix (5.2%), and the content of MnO2 is 3.3% (WT), 11 times higher than that in soil matrix (0.3%). Optical microscope (OM) and polarization microscope (POM) show that the cutan with the thickness in range of 180-500 mu m closely attaches to the soil matrix. The outer part of the cutan is black while the inner part is brown red. Environment scanning electron microscope (ESEM) and energy dispersive X-ray spectrum (EDX) show that Mn mainly distributes in the outer part while Fe mainly distributes in the inner part. On the whole, the ratio of Mn and Fe decreases from the outer edge to the inner side. X-ray powder diffraction (XRD) shows that the black cutans are composed of quartz (65%), muscovite (20%), kaolinite (similar to 5%), Fe and Mn (hydr) oxides (10%) such as hollandite, goethite, birnessite, feitknechtite and vernadite, while the brown red cutans are mainly composed of quartz (65%), muscovite (22%), goethite (8%) and kaolinite (similar to 5%). Transmission electron microscope (TEM) shows that the Mn oxide particles vary from 5-100 nm with laminated, leaf and columnar shape. As compared to Mn oxides, the nanocrystals of goethite have higher crystallinity but still with the size in nanometer scale. The poor crystallinity and small particle size of Fe and Mn (hydr) oxides endow the cutans with high adsorption capacity of heavy metals.