Iron in sediments

Davison, W., G. W. Grime, J. A. W. Morgan, and K. Clarke (1991), "Distribution of Dissolved Iron in Sediment Pore Waters at Submillimetre Resolution," Nature352, 323-325. 

Availability of reactive iron in sediments also has been postulated to control S retention. Reactive iron may limit fluxes of S recycled from sediments by rendering sulfide immobile and less amenable to oxidation by bacteria or chemical agents. Availability of iron strongly influences total S content and isotopic signature of marine sediments (198). Canfield (94) ob-... 

The extent to which H2S contributes to the release of ferrous iron into pore-water solution through dissolution of reactive ferric oxides such as lepidocrocite or amorphous ferrihydrite remains unclear. According to Can-field (19), liberation of ferrous iron in sediments stems mainly from microbial dissolution of ferric oxides. The release rates of Fe2+ measured in his study range between 3 X 10"6 and 4 X 10 5 M per day, at the lower limit of the theoretical interval. 

In summary, it seems meaningful to consider both the formation of pyrite from the reaction of H2S with reactive ferric oxides and sulfate recycling as a result of this process in any discussion of the early diagenesis of sulfur and iron in sediments. 

Preliminary work (10) on the transition from oxidized surface sediment to reduced subsurface sediment in Milltown Reservoir showed that the redox transition occurs in the upper few tens of centimeters. Strong chemical gradients occur across this boundary. Ferrous iron in sediment pore water (groundwater and vadose water) is commonly below detection in the oxidizing surface zone and increases with depth. Arsenic is also low in pore water of the oxidized zone, but increases across the redox boundary, with As(III) as the dominant oxidation state in the reduced zone. Copper and zinc show the opposite trend, with relatively high concentrations in pore water of the oxidized surface sediment decreasing across the redox boundary. 

The Relationship Between Manganese and Iron in Sediment Ponds. To under-stand the behavior in and removal of iron and manganese from water, it is important to know the interactions of these two metals. A common occurrence in sediment ponds is the sudden development of a dissolved manganese problem. The cause may be ferrous iron from the incoming water due to the disturbance of a new site. The ferrous iron can react with insoluble manganese oxide (Mn02) in the sediments at the bottom of the pond according to Equations 12.5 and 12.6 ... 

Eitaor M. I. and Keigley R. B. (1991) Geochemical equilibria of iron in sediments of the Roaring river alluvial fan, Rocky Mountain National Park, Colorado. Earth Surf. Process. Landforms 16, 533—546. 

Davison, W., Grimes, G.W., Morgan, J. A. W. Clarke, K. (1991) Distribution of dissolved iron in sediment pore waters at submillimetre resolution. Nature 252, 323-5. 

Holmer, M., Kristensen, E., Banta, G., Hansen, K., Jensen, M.H. and Bussawarit, N. (1994) Biogeo-chemical cycling of sulfur and iron in sediments of a south-east Asian mangrove, Phuket Island, Thailand. Biogeochemistry, 26,145-161. 

Parkhurst (1995, example 10) also did calculations of sorbent concentrations in his surface complexation modeling for the central Oklahoma aquifer. The amount of extractable iron in sediments is from 1.6 to 4.4%. Porosity is 22%, and rock density is 2.7 gem-3. For a 2 wt.% iron, the solid concentrations should be calculated as follows. The total weight for a volume of 1000 cm3 aquifer is... 

K., 1991. Distribution of dissolved iron in sediment pore waters at submillimetre resolution. Nature, 352 323-324. 

Fig. 5.1 A) Pore water gradients of nitrate, dissolved manganese, and iron in sediments from the eastern equatorial Atlantic at 5000 m depth. The gradients indicate that Fe is oxidized hy NO, whereas Mn may he oxidized hy (no data). (Data from Froelich et al. 1979 Station lOGCl). B) Anaerobic bacterial oxidation of ferrous to ferric iron with nitrate in an enrichment culture. Filled symbols show results from a growing culture, open symbols shows a control experiment with killed cells (no concentration changes). Bacteria are clearly needed for the fast iron oxidation with nitrate. Symbols show ferric iron ( + O) and nitrate ( + ). (Data from Straub et al. 1996).

X lO and 280 X 10 mol of O2, respectively, to form these reservoirs from sulfide derived from igneous rocks. The inventory of excess ferric iron in sediments, 50 X 10 mol of Fe203, required 25 x lO mol of O2 to form from FeO derived from igneous rocks [15]. The inventory of excess ferric iron in hard rocks, 4000 x 10 mol of Fe203, required 2000 x 10 mol of O2 to form from FeO over geological time [15]. Much of this hard rock reservoir is over 2 billion years old. 

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