| Cutans are formed on the surface of peds and the walls of voids in soils. They are common fabric features in soil and represent foci of chemical and biological reaction., and they are one of the most active microzones of interaction among peds, soil solution, plants root and soil air. The geochemical characteristics of some elements, compositions of clay minerals and types of Fe oxides in cutans and matrix soils in central China were investigated. The character of Al-substitution of goethite, adsorptions of heavy metal ions and oxidations of Cr(III) on samples were studied. The main results were shown as follows:1. The pH values and contents of clays, bases, Fe2O3, P2O5, Feph, Feo, Fed, Feo/Fed, MnO2, Co, Pb, Li, Cu, Zn, Ni, Cd in cutans were significantly higher than those in corresponding matrix soils. MnO2, Co, Pb were remarkably enriched in cutans, and the contents of them were 4.9, 9.7 and 7.0 times of those in matrix soils, respectively.2. Correlation matrix analysis of soil properties and elements contents between matrix soils and cutans showed: 1) In both cutans and matrix soils, there were remarkably positive correlations between base elements and pH values. This suggsted that pH values increased as the amounts of base elements increased. 2) Mn oxides enriched Co and Ni in both cutans and matrix soils, but Mn oxides could enrich more other heavy metals in cutans than in matrix soils. 3) In matrix soils, Co and Ni were accreted and existed in Mn oxides and Feph. However, they were not accreted in cutans. The forms of Ni existed in matrix soils were not changed in cutans, but the forms of Co which existed in clay and Fed as well as in Mn oxides in cutans had been changed. 4) Cu, Zn and Cd grew together and consisted in phyllosilicate in matrix soils, whereas they existed in Mn oxides in cutans. 5) There wasn't correlation between Co and Pb in matrix soils, but there was remarkably positive correlation between them in cutans. The correlation coefficient was 0.95**. This showed that Co and Pb grew together and integrated with Mn oxides, clay and Fed in cutans.3. Factors analysis of soil properties and element contents of matrix soils and cutans indicated: 1) In matrix soils, common factors F1, F2, F3 and F4 represented the functionof pedology, Feo/Fed, enrichment of clays and Fed, substitution and adsorption of Cd by phyllosilicate seperately. 2) In cutans, common factors Fl represented the function of pedology, F2 and F3 represented enrichment of some heavy metals by clays and Mn oxides, and F4 the relationship between Feo/Fed and Zn.4. The compositions of clay minerals in cutans and matrix soils of yellow cinnamon soils and yellow brown earths were hydromica, 1.4nm minerals and kaolinite. The contents of them in order were hydromica>1.4nm minerals>kaolinite. In matrix soils, the 1.4nm minerals were 1.4nm intergrade minerals and vermiculites. While in cutans, the 1.4nm minerals were totally vermiculites and no 1.4nm intergrade minerals. This illustrated that the conditions of chelation with organic matter, high pH values and rebasification resulted in transforming 1.4nm intergrade minerals into vermiculites in cutans.5 The types of Fe oxides in cutans and matrix soils were only goethite and no hematite The contents of Al-substitution of goethite in cutans were 9.4mole%Al to 17.9mole%Al. but 12.0mole%Al to 19.9mole%Al of goethite in matrix soils. The amounts of Al-substitution of goethite in cutans were lower than those in matrix soils. These were probably due to restrain available Al of activity by rebasification and the action of organic chelation in cutans6. The max amounts of heavy metals adsorbed by cutans which were deprived of Mn oxides decreased over 50% comparing those adsorbed by cutans in red soil, yellow brown earth or yellow cinnamon soil. These showed that Mn oxides were the main sorbents in adsorbing heavy metal ions.7 The max amounts of Cr(VI) oxidized by cutans which were deprived of Mn oxides decreased 88.6% to 100% comparing those oxidized... |
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