| The structure, mineralogy, elemental geochemistry of Fe-Mn nodules, and heavy metals adsorbed and Cr(III) oxidized by them were studied. Formation and elemental accumulation mechanisms of Fe-Mn nodules, relationship between composition of nodules and soil environment, interaction of ion adsorption and redox on Mn oxide minerals were discussed.1 The method-pH3.0 of 0.1 moI/L NH2OH HC1 solution (sample:solution = 1:100) treatment with 2h-proposed by the paper could dissolve 80.6%-86.3% of total Mn, 1.9%-3.8% of total Fe and < 0.36% of total Al in Fe-Mn nodules. The method could effectively separate Mn, Fe and Al oxide minerals from Fe-Mn nodules in soil and estimate their content.2 Comparison with soils, Mn02, Ba, Cd, Co, Cu, Ni, Pb, Zn and Sr were strongly accumulated, Fe2O3 were obviously accumulated, CaO, P2O5, Na2O were slightly accumulated, and SiO2, A12O3, K2O, MgO, TiO2 were reduced in Fe-Mn nodules. There was remarkable positive correlation between the contents of Ba, Cd, Co, Cu, Ni, Pb, Zn and MnO2 in Fe-Mn nodules, whereas there was remarkable negative correlation between the contents of Cd, Cu, Ni, Pb, Zn and Fe2O3 in soils. The chemical elements could be divided into three groups according to factor analysis: (1) Si, K, Mg, Ca, Na, Ti and Sr which were elemental composition of phyllosilicate, (2) Mn, Ba, Cd, Co, Cu, Li, Ni, Pb and Zn which were related to Mn oxide minerals, (3) Fe, Cr and P which were related to Fe oxide minerals. The results of XPS showed that valences of Mn in Fe-Mn nodules were +3 and +4.3 From south to north, the band structure of Fe-Mn nodules was from clear to obscure, the extent of banded Mn and Fe decreased, whereas the extent of banded Ca increased in backscattered electron image (BSE) of SEM. The line distribution of MnO2 and Fe2O3 contents in Fe-Mn nodule profile was wave-like curve, and the content of Fe203 increased when the content of MnO2 declined or the content of Fe203 decreased when the content of Mn02 increased. With the latitude increasing, the wave-like curve of Fe2O3 and MnO2 contents changed frequently, and the thickness of band was from thick to thin. The coefficient variation (C.V.%) of MnO2 content in Fe-Mn nodules profile of red soil and yellow-brown soil were higher than that of Shajiang black soil and brown soil, but C.V.% of Fe2O3 content of them was reverse. Ca, Ba were accumulated in Mn oxide minerals regardless of soil types. However, Co, Ni, Pb were accumulated in Mn oxide minerals of acid-slight acid soils, and precipitated in Fe-Mn nodules of alkaline-slight alkaline soils. The alkaline micro-region model of Mn-Fe nodule in soil was proposed. Two periods for growth of Fe-Mn nodules in soils were flooding-water and lacking-water. The band structures of Fe-Mn nodules wouldprobably form when the landscape and soil environment experienced greatly change which were affected by global climate change, neotectonics and so on.4 From red soil to Shajiang black soil and brown soil, the main phyllosilicate composition of soils changed from 1:1 kaolinite to 2:1 minerals, from 1.4 nm intergrade mineral to vermiculite, montmorillonite from none to present, and gibbsite was only in red soil. The phyllosilicate composition of their Fe-Mn nodules was similar to the corresponding soil. However, Fe-Mn nodules contained no gibbsite, no 1.4 nm intergrade mineral. The content of 2:1 minerals in nodules was higher than that in soils, whereas the content of kaolinite in them was reverse. With the increasing of diameter of nodules, the content of kaolinite decreased, whereas the content of hydromica increased, and the differences of phyllosilicate composition between nodules and soils were increasing. The ratios of Feo/Fed and Fed/Fet of soils were lower than that of Fe-Mn nodules. From south to north, the composition of Fe oxide minerals changed from goethite-hematite mixture type to goethite (G) type, and the contents of goethite were increased as the contents of hematite (H) were decreased in soils. The changes of types and content...
|
PDF Full Text Request