Soil iron oxides

Because the appearance of the superparamagnetic effect depends on the particle size and on the anisotropy constant, it is often displayed at room temperature by iron oxides <10 nm in size, for example, soil iron oxides. Superparamagnetic relaxation may be counteracted by lowering the temperature and thereby increasing x. Superparamagnetic particles will usually be ordered below a blocking temperature,Tb, which is ... 

Iron oxides in soils have in common that they are of extremely small crystal size and/or low crystal order. This, in combination with their low concentration (only tens g kg in most soils) explains why soil iron oxides have escaped identification for a long time in spite of their obvious existence as seen from the soil colour. In the past, therefore, Fe oxides in surface environments have been considered to be amorphous to X-rays and often called limonite , which mineralogically, is an obsolete term. Furthermore, in order to identify the clay minerals in soils properly, Fe oxides are usually removed before X-ray diffraction methods are applied (Alexander et al., 1939 Mehra Jackson, 1960). 

Coey, J.M.D. (1988) Magnetic properties of iron in soil iron oxides and clay minerals. In Stucki, J.W. Goodman, B.A. Schwert-mann, U. (eds.) Iron in soils and clay minerals. D. Reidel Publ. Co., 397-466... 

In general, minerals in sedimentary and meta-morphic rocks contain ferrous iron (Velde, 1985) which is destined to become iron oxide under conditions of weathering. Oxidation under surface conditions has a tendency to produce iron in the ferric state. Most often the process takes iron out of the silicates and puts it into an oxide phase. In the uppermost layers of mature soils, iron oxide and various silicates, usually non-iron-bearing, are produced. In silicates containing iron, the majority is in the ferric state. The extent of the transformation of iron oxidation state is a rough measure of the maturity of the soil. In the extremely weathered soils one finds only ferric iron and aluminum oxides and hydroxides. These soils are typically red. 

In soils, iron oxides usually represent the smallest particle fraction present and are often closely associated with clay minerals, their concentrations are usually low and the crystallinity of these phases may be very poor. The poor crystallinity and low concentrations of iron oxides in soils are generally responsible for the difficulty in their identification and quantification, for instance, poor ciystallinity and low concentration of iron oxides usually yield broad low-intensity XRD peaks, and these peaks often overlap with high-intensity peaks... 

J. Arocena, G. De Geyter, C. Landuydt, U. Schwertmaim, Dissolution of soil iron oxides with ammonium oxalate Comparison between bulk samples and thin sections. Pedologie... 

Minerals. Iron-bearing minerals are numerous and are present in most soils and rocks. However only a few minerals are important sources of iron and thus called ores. Table 2 shows the principle iron-bearing minerals. Hematite is the most plentiful iron mineral mined, followed by magnetite, goethite, siderite, ilmenite, and pyrite. Siderite is unimportant in the United States, but is an important source of iron in Europe. Tlmenite is normally mined for titania with iron as a by-product. Pyrite is roasted to recover sulfur in the form of sulfur dioxide, leaving iron oxide as a by-product. 

Another important environmental issue is the fate of cyanide. Hydrogen cyanide, if spilled, evaporates quite readily. That which does not evaporate is soon decomposed or rendered nonha2ardous by complexing with iron in the soil, biological oxidation, or polymeri2ation. 

The use of nanoscale materials in the dean-up of hazardous waste sites is termed nanoremediation. Remediation of soil contaminated with pentachloro phenol using NZVI was studied [198]. In a separate study, soils contaminated with polychlorinated biphenyls was treated using iron nanopartides [194], NZVI and iron oxide have been suggested to be used as a colloidal reactive barrier for in situ groundwater remediation due to its strong and spedfic interactions with Pb and As compounds [199]. 

The effects of the removal of organic matter and iron oxides on Zn adsorption on soils are also influenced by Zn concentration. At low concentrations (5-10 mg L initial concentration), both treated soils (removed organic matter and iron oxides) behaved similarly. At high Zn concentration, however, treated soils behaved differently. When the initial Zn concentration was between 5 and 10 mg kg-1, adsorption of Zn by soils without organic matter and without both organic matter and iron oxides were 2-2.5 times that of the untreated soil. With an increase in initial Zn concentration, the soil without both iron oxides and organic matter adsorbed more Zn than the soil without organic matter. This indicates that the available sites for Zn decrease with increases in the initial Zn concentration. 

Chromium has a similar electron configuration to Cu, because both have an outer electronic orbit of 4s. Since Cr3+, the most stable form, has a similar ionic radius (0.64 A0) to Mg (0.65 A0), it is possible that Cr3+ could readily substitute for Mg in silicates. Chromium has a lower electronegativity (1.6) than Cu2+ (2.0) and Ni (1.8). It is assumed that when substitution in an ionic crystal is possible, the element having a lower electronegativity will be preferred because of its ability to form a more ionic bond (McBride, 1981). Since chromium has an ionic radius similar to trivalent Fe (0.65°A), it can also substitute for Fe3+ in iron oxides. This may explain the observations (Han and Banin, 1997, 1999 Han et al., 2001a, c) that the native Cr in arid soils is mostly and strongly bound in the clay mineral structure and iron oxides compared to other heavy metals studied. On the other hand, humic acids have a high affinity with Cr (III) similar to Cu (Adriano, 1986). The chromium in most soils probably occurs as Cr (III) (Adriano, 1986). The chromium (III) in soils, especially when bound to... 

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