Iron Hydr Oxides

Natural iron oxides show isomorphic substitution like phyllosilicates however, the charge differences are usually compensated internally (e.g., by a change in the 

Most Important Iron Oxide and Hydroxide Minerals 

Mineral Formula System Color (gcm-5) Soil Environment Where It Is Found 

Hematite a-Fe203 Trigonal Reddish gray, black, blackish red 5.3 Aerobic soils of tropics, subtropic, arid, semiarid regions, especially warm areas 

Maghemite y-Fe Oj Isometric Dark brown, brick red 4.9 Highly weathered sods, trebles and subtropics, from magnetite 

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S. Hiradate and K. Inoue, Interaction of mugineic acid with iron(hydr)oxides Sul-fate and phosphate influences. Soil Sci. Soc. Am. J. 62 159 (1998). 

Table 16.2 Characteristics of common iron(hydr)oxide minerals. Reprinted with permission from Pecher K, Haderline SB, Schwarzenbach RP (2002) Reduction of polyhalogenated methanes by surface-bound Fe(II) in aqueous suspensions of iron oxides. Environ Sd Technol 36 1734-1741. Copyright 2002 American Chemical Society...

A.I. Bordia, R.K. Korshin, G.V. Christensen T.H. (2000 a) Aging of iron (hydr)oxides by heat treatment and effects on heavy metal binding. Environ. Sci. Techn. 34 3991-4000 Sorensen, S. Thorling, L. (1991) Stimulation by lepidocrocite (y-FeOOH) of Fe(II)-depen-... 

Venema P. Charging and ion adsorption behaviour of different iron (hydr) oxides PhD. Thesis, Wageningen Agricultural University Wageningen, The Netherlands. 

In laboratory model systems using reduced DOM constituents (Dunnivant et al., 1992), Fe(II) adsorbed to iron (hydr)oxides (Klausen et al., 1995), or zero-valent iron metal (Agrawal and Tratnyek, 1996) as reductants, the nitroso- and particularly the hydroxylamino compounds have been observed as reaction intermediates, but were ultimately also converted to the corresponding amino-compounds. 

It has been established that the pH needs to be above about 4 for reaction 9.14 to proceed. Thus, in the case of Fe(III), where nucleation already starts at pH around 2, the silica surface is not able to compete, and Fe(OH)3 particles are generated in solution. These particles do not subsequently react with the silica surface upon further increasing the pH, resulting eventually in a separate, rather than supported, iron (hydr)oxide phase. However, iron(III) can be deposited on Si02 by injecting a weakly acidic Fe(III) solution into a silica suspension, whose pH is kept above 5. 

Nelson, Y.M. et al., Effect of oxide formation mechanisms on lead adsorption by biogenic manganese (hydr)oxides, iron (hydr)oxides, and their mix lures, Environ. Sci. Technol., 36, 421, 2002. 

Other examples of redox-sensitive elements include heavy elements such as uranium, plutonium, and neptunium, all of which can exist in multiple oxidation states in natural waters. Redox conditions in natural waters are also indirectly important for solute species associated with redox-sensitive elements. For example, dissolution of iron (hydr)oxides under reducing conditions may lead to the solubilization and hence mobilization of associated solid phase species, e.g. arsenate, phosphate (see Sections 3.3.2.1, 3.3.3.2, and 3.3.4.1). 

Many such studies of sedimentary phosphorus profiles, also incorporating pore water measurement of soluble reactive phosphate, have demonstrated that redox-controlled dissolution of iron (hydr)oxides under reducing conditions at depth releases orthophosphate to solution. This then diffuses upwards (and downwards) from the pore water maximum to be re-adsorbed or co-precipitated with oxidized Fe in near-surface oxic sections. The downwards decrease in solid phase organic phosphorus indicates increasing release of phosphorus from deposited organic matter with depth, some of which will become associated with hydrous iron and other metal oxides, added to the pool of mobile phosphorus in pore water or contribute to soluble unreactive phosphorus . The characteristic reactions involving inorganic phosphorus in the sediments of Toolik Lake, Alaska, are shown in... 

With redox control largely responsible for phosphorus mobility in sediments, what might the consequences of oxygen depletion in the hypolimnion be If conditions in the surface sediments are not sufficiently oxidizing to precipitate iron (hydr)oxides and thereby adsorb the phosphate i.e. the redox boundary for iron may be in the overlying... 

The solubility of magnetite was studied in [117] and that of iron(iii) hydroxide in [118], Figure 2 of [119] shows the solubilities of different iron (hydr)oxides amorphous goethite hematite. The lowest, pH-independent solubility is at pH 6-10, and ranges from 10M (amorphous) to 10 M (hematite). 

Nowadays, measures against silica contamination are often undertaken, and this has become a standard procedure, too obvious to be specified in scientific papers. Plastic labware was used in the synthesis of iron (hydr)oxides in [462]. Contact with sources of silica was avoided in a cleaning procedure of alumina flat plates in [463]. Attempts to avoid silica are not always successful. An abnormally low lEP was found in a study of the force between two single-crystal sapphire platelets [464]. The authors explained the positive surface potential at pHs as low as 6.7 by the sorption of silica or other species from the solution. 

Venema, P. et al.. Intrinsic proton affinity of reactive surface groups of metal (hydr) oxides Application to iron (hydr)oxides, J. Colloid Interf. Sci., 198, 282, 1998. 

Manasse, A. and Mellini, M., Iron(hydr)oxide nanocrystals in raw and burnt sienna pigments, Eur J. Mineral., 18, 845, 2006. 

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