Iron oxyhydroxide coating

The presence of iron oxyhydroxide coatings (i.e., Fe plaque, often dominated by ferrihydrite) on the surface of wetland plant roots is visual evidence that subsurface iron oxidation is occurring in otherwise anoxic wetland soils and sediments. Oxygen delivered via radial O2 loss may react with reduced iron in soil pore spaces to form oxidized iron that can be deposited on the plant roots as Fe plaque. Despite a long history of observing Fe plaque on wetland plant roots and understanding the basics of plaque formation [i.e., reaction of plant-transported O2 with Fe(II) in soils and sediments], it was largely assumed that plaque formation is predominately an abiotic (i.e., chemical) process because the kinetics of chemical oxidation can be extremely rapid (Mendelssohn et al., 1995). However, recent evidence has demonstrated that populations of lithotrophic FeOB are associated with Fe plaque and may play a role in plaque deposition. 

The materials were ultrasonically cleaned and wet sieved to maximize the removal of fines and then treated with hydrogen peroxide and sodium dithionite to remove organic compounds and any iron oxyhydroxide coating which could contribute to increased surface areas. This clean size fraction was further split by isodynamic magnetic separation into predominantly quartz-feldspar and mafic... 

Huminicki DMC, Rimstidt JD (2009) Iron oxyhydroxide coating of pyrite for add mine drainage control. Appl Geochem 24 1626-1634... 

Example 7.6. Iron oxyhydroxide coating on pyrite for AMD mitigation... 

Huininicki and Rimstidt (2009) suggested that treating pyritic mine wastes with bicarbonate solutions would produce an iron oxyhydroxide coating on the pyrite surfaces, which would inhibit the development of acid mine drainage. The overall reaction converts iron from the pyrite into an iron oxyhydroxide coating. 

Figure 7.9. (a)Thickness of an iron oxyhydroxide coating on pyrite undergoing oxidation in a solution buffered to a near-neutral pH. (b) Rate of oxygen consumption by the pyrite oxidation reaction as a coating of iron oxyhydroxide grows in thickness. 

Jackson, T.A. and Bistricki, T. (1995) Selective scavenging of copper, zinc, lead, and arsenic by iron and manganese oxyhydroxide coatings on plankton in lakes polluted with mine and smelter wastes results of energy dispersive X-ray micro- analysis. Journal of Geochemical Exploration, 52(1-2), 97-125. 

Figure 5 (bottom, opposite page). Transmission electron microscope of stalks produced by Gallionella sp. Coated by few-nanometer-diameter iron oxyhydroxide particles (see Fig. 2). The iron oxyhydroxides also form semi-spherical colloids, seen adhering to the stalks (Banfield, unpubhshed).

Figure 6. Low-magnification scanning electron microscope image of products of microbial iron oxidation at near-neutral pH. The elongate tubes produced by Leptothrix sp. and twisted stalks produced by Gallionella sp. (see Fig. 3) are coated in nano-scale iron oxyhydroxides (unpublished data reproduced with permission of Clara Chan).

Many chemicals, however, do sorb onto the grains of aquifers, or onto iron oxyhydroxide or organic coatings on the grains. As discussed in Section 1.8.3, the term sorb includes both adsorptive processes, in which a chemical sticks to the two-dimensional surface of a solid, and absorptive processes, in which... 

The three examples of the effects of pedochemical weathering on the surface structures in soil clays just described illustrate the complexity of the reactive solid materials in natural soils. To these examples can be added many others, including the formation of iron oxyhydroxide or calcium carbonate coatings on the external surfaces (as opposed to interlayer surfaces) of phyllosilicates, the development of thick envelopes of colloidal organic matter on aggregates of metal oxides and aluminosilicates, and the... 

Fig. 6. Scanning Electron photomicrographs illustrating key mineral textures in core samples. A, Plagioclase surface showing dissolution textures (DH-3/-484m) B, plagioclase surface coated by kaolinite (DH-3/-484m) C and D, biotite and iron oxyhydroxide mineral (DH-3/-484m) E, pyrite dissolution texture (DH-4/80m) ...

Hydrolysis reactions and the precipitation of iron oxyhydroxide phases also play a key role in the corrosion of metallic iron. Corrosion reactions can involve both ferrous and ferric iron hydrolysis species and the formation of surface coatings of either iron(ll) or iron(lll) (oxy)hydroxide phases. The understanding of corrosion and its effects has received a considerable amount of attention. 

Figure 9 shows the cyclic voltammetry of an FePc/XC-72 dispersion, prepared in such fashion, in the form of a thin porous Teflon-bonded coating electrode in a 1 M NaOH solution. A description of the methodology involved in the preparation of this type of electrode may be found in Ref. 15. As can be clearly seen, the voltammetry of this specimen exhibits two sharply defined peaks separated by about 330 mV. The potentials associated with these features are essentially identical to those found by other workers for the reduction and oxidation of films of iron oxyhydroxide formed on a number of host surfaces, including iron and carbon. 

Hydrous Oxides. This term is generally taken to include the oxides, hydroxides, and oxyhydroxides of aluminium, iron and manganese, which form in soil when these elements are released from primary minerals by weathering. They exist mainly as small particles in the claysized fraction of a soil (<2 pm), and also as coatings on other soil minerals or as components of larger aggregates. 

Grantham M. C., Dove P. M., and DiChristina T. J. (1997) Microbially catalyzed dissolution of iron and aluminum oxyhydroxide mineral surface coatings. Geochim. Cosmochim. Acta 61, 4467—4477. 

Nanoparticles of iron and aluminum oxides and oxyhydroxides transport both organic and inorganic contaminants in the environment. The systematics developed here may be applied to understanding such natural nanocomposites. For example, it may be possible to treat coating of amorphous uranium or chromium oxides on nanophase (Fe, Al)OOH particles as a mixture of nanophase end-members from the point of view of energetics. 

Iron film Lithobiontic coatings Nitrate crust Composed primarily of iron oxides or oxyhydroxides Organic remains form the rock coating, for example lichens, moss, fungi, cyanobacteria, algae Potassium and calcium nitrate coatings on rocks, often in caves and rock shelters in limestone areas... 

Coprecipitation of copper can occur within the larger mass of ferric oxyhydroxide. Most mineral soils contain significant amounts of active iron, up to 1%. The iron can be reduced to its soluble ferrous form when the system becomes anaerobic or it can be oxidized to the insoluble ferric form when the system is oxidized. As the ferrous ion is precipitated into more insoluble ferric form, it forms coatings on clay and silt particles often as a ferric oxyhydroxide layer, which can potentially coprecipitate trace metals. This is an important sink apparently for copper as well. The copper trapped in this material is released only under reducing conditions. 

The starting materials were soluble salts, cobalt acetate (Co(C2H302)2 H2O) and iron nitrate (Fe(N03)2 9H2O). These salts produce hydroxides (M(OH)2), oxyhydroxides (MOOH) or hydrated oxides in water, where M is Co or Fe. These solutions were reacted with lithium hydroxide. Diluted ammonium hydroxide (3M) was added to form stable colloids (Barboux, 1991). Lithium hydroxide and cobalt acetate were dissolved separately in distilled water. These two solutions were then mixed together and stirred vigorously. The hydrolysis of the mixture was promoted by slow addition of 3M ammonium hydroxide. Similarly, sols with ferric nitrate, or ferric nitrate plus cobalt acetate, were prepared. The sols used for coating were diluted to give a 2 1 ratio of moles water to moles oxide. 

---