Study On The Formation Of Iron Cutan Coated On The Surface Of Quartz Sand And The Influential Factors

Soil cutans are important components of the soil interface. As one of the soil cutans, iron-manganese cutans are widely distributed in Alfisol and Ultisols in subtropical zone in China, and they have active surface chemical properties. Compared with manganese, iron content of soil is high and the deposition amount of iron is great in iron-manganese cutans, which has important influence on the formaton of iron-manganese cutans. In this study, iron cutans coated on the surface of quartz sand were synthetized to investigate the influence of initial pH(5,6,7), iron mole ratio R(Fe(II)/Fe(III)), temperature(30℃,50℃,70℃,90℃), quartz sand amount(5g,10g,20g,30g,40g,50g) and the aging time(2d,4d, l1d,18d,25d,32d,40d) of the synthesis systems on the formation of iron oxides in the cutans, further provided the formation mechanism of iron cutans. The results were as follows:1) At initial pH7, the mineral composition of iron cutan was ferrihydrite with weak crystallization when R was0and it was hematite when R was0.01in the synthesis systems. The mineral composition was a mixture of goethite and magnetite when R was0.5and only Fe(II) existed at initial pH7in the synthesis systems. Goethite was the only iron oxide in the cutans when R values were0.02,0.04,0.06and0.1at all initial pH. With the increase of R, the peak intensity of goethite in X-Ray Diffraction (XRD) patterns increased successively and the crystalline shape of iron oxide in the cutans analysed by Scanning Electronic Microscope (SEM) became more and more big, and the crystal was the biggest when R was0.1at initial pH7compared with other treatments.2) When R remained constant in the goethite cutans synthetized system, with the increase of the initial pH from5to7, the peak intensity of goethite in XRD patterns increased successively and the crystal of iron oxides in the cutan became bigger and bigger, and it reached the maximum at initial pH7compared with other initial pHs. The morphology of goethite in iron cutans were short rod and acicular at initial pH5with about50nm length and20nm width, while they were acicular at initial pH6with about150nm length and20nm width, and they were lath-shaped at initial pH7with the length range of200-400nm and width range of5-100nm. 3) When the initial pH was7, R was0.1, quartz sand amount was lOg and the aging time was25d, the peak intensity of goethite in XRD patterns increased successively and the crystalline shape of iron oxide in the cutans analysed by SEM became more and more big with the increase of temperature from30℃to90℃.4) When the initial pH was7, R was0.1, quartz sand amount was lOg and the temperature was60℃, the peak intensity of goethite in XRD patterns increased successively during the first18days, then decreased and tended towards stability. The crystalline shape of goethite in the cutans analysed by SEM increased first then decreased, and reached the maximum on18st day.5) The presence of quartz sand in the synthetized system could delay the crystallization of goethite. The difference of quartz amount had no significant influence on the types of iron oxides in cutans when the initial pH was7, R was0.1and the temperature was60℃.6) The contents of iron in cutans of different conditions ranged from2.0to10.0mg Fe/g iron cutans. The iron content reached the maximum when the temperature was70℃in the processing of temperature, while it reached the maximum of8.5mg Fe/g cutans after about18days.7) The iron cutans coated on the surface of quartz sand were great when the initial pH was7, R was0.1and the temperature was60℃, at this moment, the PZC of the iron cutan was5.45, which almost doubled compared with quartz sand of2.88.