Study on the so-called noncrystalline free iron oxides in soils

(1)赋存状态：采用选择性溶解结合差示X-射线衍射法和穆斯堡尔谱对火山灰和非火山灰土进行了研究。结果表明，灰土中存在结晶度不同的亚铁水合物，而在非灰土中则不存在或含量很少。它在灰土中的存在被认为与风化早期火山玻璃对硅和铁的充足供应有关。野外和实验室观察表明，当含有亚铁的地下水暴露在空气中时，亚铁水合物迅速形成。用这种方法形成的水合铁酸盐通常是无序的，因为它在大约0.25和0.15 nm处只有两个X射线衍射峰。(2)合成工作：从含有铁和硅的溶液中合成出结晶良好的水合铁酸盐，其比例为10：1-2，pH范围为6-8。在低于或高于这一界限的范围内，没有形成亚铁水合物，或与其他铁矿物共存，如形成的针铁矿或鳞铁石。在pH范围内形成&gt；11铁氧石，表明该矿物不能在正常pH范围的土壤中形成。用Ge代替Si，结果完全如上所述。对合成的硅和锗铁水合物的d间距的比较表明，硅(和Ge)不存在于铁水合物的结构中，而是存在于晶域的外部。有机阴离子，如醋酸盐和柠檬酸盐，干扰了铁水合物的形成，尽管这种干扰可能因阴离子的种类不同而不同。(3)由于缺乏适当的方法，测定土壤中铁水合物的CEC和AEC，P-保持率和比表面积的尝试失败了。然而，据推测，磷的保留率和比表面积可能大于或等于粘土成分(铝硅酸盐)的大小。

(1) Occurrence: Volcanic ash and non-volcanic ash soils were examined by means of selective dissolution combined with difference X-ray diffraction and Moessbauer spectroscopy. It was shown that ferrihydrite of various crystallinity were presented in ash soils, but it was absent or very small in amount in non-ash soils. Its presence in ash soils was considered to be related to ample supply of Si, as well as Fe, from volcanic glass in the early stage of weathering.Field and laboratory observations suggested that ferrihydrite forms rapidly when the underground water containing ferrous iron exposed to air. Ferrihydrite formed this way is usually poorly-ordered in the sense that it shows only two X-ray diffractions at about 0.25 and 0.15 nm.(2) Synthetic work: Well-crystallized ferrihydrite was synthesized from the solution containing Fe and Si in the ratios 10:1-2 and in the pH range 6-8. In the range lower or higher than these limits, ferrihydrite was not formed, or was coexisted with other iron minerals such as goethite or lepidocrocite formed. In the pH range > 11 feroxyhite was formed, suggesting that the mineral cannot form in the soils of usual pH range. Use of Ge, in the place of Si, results completely in the same as above. Comparison of d-spacings between synthetic Si- and Ge-ferrihydrite indicated that Si (and Ge) are not held in the structure of ferrihydrite, but held in the "outsides" of the crystal domains.Organic anions, such as acetate and citrate, interfered with the formation of ferrihydrite, even though the interference may be different among the kind of anions.(3) Attempts of measuring CEC and AEC, P-retention, and specific surface area for ferrihydrite in soils failed due to the lack of appropriate procedure. However, it was inferred that P-retention and specific surface area may be larger than or equal to, in magnitude, to clay constituents (aluminosilicates).