Iron sediments oxide

Nelson, M. B., Davis, J. A., Benjamin, M. M. and Leckie, J. O. (1977). The Role of Iron Sulfides in Controlling Trace Heavy Metals in Anaerobic Sediments Oxidative Dissolution of Ferrous Monosulfides and the Behavior of Associated Trace Metals." Air Force Weapons Laboratory, Technical Report 425. 

In the latter case, electron uptake occurs after decarboxylation of the ketoacid via Fe3+, which is able to take up electrons in strongly oxidized regions of the black-band iron sediments. Fe-ions act catalytically on the process of thioester formation, which can then occur without the help of enzymes. Thus, it was solar UV irradiation which carried the prebiotic thioesters across the energy threshold. 

Reactions between Fe(ll) in contaminated groundwater (5.8 mg/L) and oxic sediment also affected As mobility. Ferrous iron was oxidized by manganese oxides to ferric iron which precipitated as hydrous ferric oxide, creating additional sorption sites. Evidence for this reaction included an increase in ferric oxide concentrations in reacted column sediments and manganese concentrations in leachate that were greater than in the initial eluent. 

Two types of metal-rich hydrogenous deposits are formed on the seafloor iron-manganese oxides and polymetallic sulfides. The iron-manganese oxides have been deposited as nodules, sediments, and crusts. They are enriched in various trace elements, such as manganese, iron, copper, cobalt, nickel, and zinc, making them a significant repository for some of these metals. Most of the metals in the polymetallic sulfides are of hydrothermal origin. These sulfides have been deposited as metalliferous sediments aroimd hydrothermal systems and as rocks that infill cracks within former... 

Table 18.1 Average Compositions of the Earth s Upper Continental Crust, Shale, Iron-Manganese Oxides, Phosphorite, and Various Types of Marine Sediments (All in Units of ppm. Unless Noted otherwise), along with Seawater and a Hydrothermal Vent Solution from the East Pacific Rise (both in Units of 10 g L ). 

Nevin, K.P. Lovley, D.R. (2000) Potential for nonenzymatic reduction of Fe(III) via electron shuttling in subsurface sediments. Environ. Sci. Techn. 34 2472-2479 Nickel, E.H. Mandarino, J.A. (1977) Mineral nomenclature. Am. Min. 62 188 Nightingale, E.R. Benck, R.F. (1960) Precipitation of crystalline iron(IIl) oxide from homogeneous solution. Anal. Chem. 32 566-567... 

Petersen,W. Wallmann, K. Schroer, S. Schroeder, F. (1993) Studies on the adsorption of cadmium on hydrous iron (III) oxides in oxide sediments. Anal. Chim. Acta 273 323-327... 

Pryor, M.J. Evans, U.R. (1950) The reductive dissolution of ferric oxide in acid. I. The reductive dissolution of oxide films present on iron. J. Chem. Soc., 1259-1266 Puchelt, H. (1973) Recent iron sediment formation at the Kameni Islands, Santorini (Greece). In Amstutz, G.C. Bernard, A.J. (eds.) Ores in sediments. Springer Berlin, 227-245... 

Oxidation by manganese oxides Laboratory system Marine sediments Oxidation by iron oxides Unidentified anaerobic oxidation Lake sediments... 

In sediments, oxidized iron is commonly present as the oxides magnetite, hematite, goethite, limonite, and as silicates such as ferric chamo-site. In reduced form iron is present as siderite, several iron (II) silicates, sulfides, such as pyrite, FeS2, and as undefined compounds of the general type FeS nH20, sometimes referred to as hydrotroilite. In rare cases iron occurs in native form in wood in bogs, in petrified wood mixed with limonite and organic matter, and in carbonaceous sediments. 

Manganese(III/IV) oxides also play an important role in oxidizing cations such as As(III) to As(V) (Oscarson et al., 1980,1981a,b). This is a very beneficial effect, since As(III) is an extremely toxic pollutant and is more soluble and mobile (Deuel and Swoboda, 1972) than As(V). Iron(III) oxides in soils and sediments may also play a role in oxidizing As(III), but the kinetics of this reaction is very slow (Oscarson et al., 1980). Phyllosilicates and calcite are not effective in oxidizing As(III) to As(V) (Oscarson et al., 1981b). 

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... 

Iron oxide sediments are formed in the absence of or at low concentrations of active forms of silicic acid, carbonic acid, and sulfur in the waters. According to the data of investigations of the composition of recent ferric iron sediments, they consist mainly of amorphous hydroxides of Fe " " (Strakhov, 1947, 1960 Ostroumov, 1955 Volkov and Tikhomirova, 1966 ... 

Thermodynamic analysis of the relationships between the primary iron sediments confirms the geologic data. On the /i-pH diagram (Fig. 39) it is seen that in oxidizing environments iron is deposited in the form of amorphous Fe " " hydroxides, and in reducing environments, finely divided magnetite is formed. Hydroxides of divalent iron are deposited only in a narrow field of highly reducing environments near the limit of stability of water at atmospheric pressure. 

Carbonate iron sediments are formed when iron is precipitated in the presence of dissolved carbonic acid or as a result of interaction of the primary sediments with organic matter in the course of diagenesis. The determination of the stability of primary iron carbonates with respect to iron oxides and hydroxides was made on the basis of using the functional dependence of and on pH, calculated by the method of Garrels and Christ (1968) for the system carbonate-water. According to this dependence, in conditions of FeCOj in equilibrium with water the value of partial pressure of carbon dioxide decreases in proportion to increasing pH. 

Oxide-carbonate-silicate-sulfide iron sediments can be formed under conditions of joint interaction of iron compounds with reactive forms of silicic... 

In the analysis of the deposition of iron sediments it has already been mentioned that quite likely both iron silicates and carbonates and amorphous iron hydroxide were formed, which could convert to other forms both during the formation of the sediment and in subsequent diagenesis. Reduction of hydroxide could have been controlled by external (atmospheric) or internal (organic matter, free carbon in the sediment) oxidation-reduction buffer systems. All these variants need additional consideration in the thermodynamic analysis of diagenetic processes. 

On the basis of thermodynamic constants obtained for hydroxide compounds of iron with different aging time and also of experimental data, the physicochemical character of the diagenetic transformations of iron sediments of various compositions (oxide, silicate, carbonate, sulfide) can be traced. The results obtained are represented graphically in the form of stability diagrams of iron compounds as a function of variations in the main parameters governing the physicochemical character of the environment of diagenesis—pH, Eh, activity of iron and dissolved forms of sulfur and carbon dioxide. 

Diagenesis of oxide-silicate-carbonate-sulfide sediments. If reactive sulfur, carbonic acid, and silicic acid are present at the same time in original iron sediments of any composition, the mineral associations formed are determined by the concentrations of these active forms. Figure 59a gives a diagram of the relationships between the crystalline iron compounds in the system Fe-Si02-C02-S-H,0 for a — 10 and = 10 g- ion/1. 

On the basis of the isotopic data obtained, the hypothesis of chemogenic deposition of cherty iron sediments of complex composition in sea basins seems to be most applicable. Only by such a process was it possible for compounds to form which already differed sharply in oxygen isotopic composition—oxides (goethite or hematite, magnetite) and carbonates (siderite, sideroplesite)—in the original iron sediment. 

Schippers A. and Jorgensen B. B. (2002) Biogeochemistry of pyrite and iron sulfide oxidation in marine sediments. Geochim. Cosmochim. Acta 66(1), 85-92. 

---