Pore-water Fe and

Seasonality of Pore-Water Fe and Mn Profiles Near the Sediment-Water Interface. . 382... 

Pore-water Fe and Mn from gravity cores at each site are given in Fig. 

Fig. 1. Pore-water Fe and Mn profiles from gravity cores at FOAM, NWC, and DEEP. The region near the sediment-water interface should not be compared because of different collection times of cores.

The removal of Fe " or Mn from anoxic pore waters by such reactions can be readily demonstrated by incubating sediment and following the Fe " or Mn " concentrations as the products of anaerobic metabolism buildup (e.g., Norvell, 1974). This is illustrated for Mn in Fig. 14, which shows pore-water Mn " and SO4" concentrations as a function of time during the anoxic incubation of salt-marsh sediment from LIS [The incubation experiments and SO4" analyses are those reported in detail by Martens and Berner (1974) who kindly made pore-water subsamples available for Mn determination.] Mn concentrations rapidily respond to the presence of anaerobic metabolities formed during SOJ consumption, and once SO4 is used up, come to a relatively constant level suggestive of saturation with respect to a reduced phase. 

The precipitation of (authigenic) iron sulfides resulting from the reaction between H S and Fe phases exerts an important control on the distribution of HjS in marine pore waters (Goldhaber and Kaplan 1974 Canfield 1989 Canfield et al. 

Respiratory sulphate reduction ideally takes place when all other electron acceptors are exhausted, but significant overlap may occur between the zones of microbial Fe(III) reduction and sulphate reduction due to kinetic constraints, as discussed before. Sulphate concentrations typically decrease to zero within the upper sediment layer (Fig. 1). In freshwater sediments, reduced S formed mainly by reduction of pore-water sulphate, is predominantly present as inorganic S in the form of AVS. Although pyrite is the most stable sulphide mineral, its formation in permanently submerged freshwater sediments is subject to controversy (Rickard et al., 1995). Because, contrary to marine sediments (S-dominated), there is an excess of Fe liberation over HS production in freshwater sediments (Fe-dominated), FeC03 as well as FeS may control pore-water Fe concentrations in the anoxic sediment layer. 

Dissolution of most aluminosilicate minerals also consinnes H ions and contributes base cations (Ca, Mg, Fe(II)), alkali elements (Na, K) and dissolved Si and Al to the tailings pore water (Blowes and Ptacek, 1994). Though, dissolution of almninosilicate minerals is slower than of metal hydroxides and much slower than that of carbonates. Feldspar weathering is mainly controlled by pH, silica, Na, K and Ca concentrations. One possible reactions path is ... 

Global uranium flux calculations have typically been based on the following two assumptions (a) riverine-end member concentrations of dissolved uranium are relatively constant, and (b) no significant input or removal of uranium occurs in coastal environments. Other sources of uranium to the ocean may include mantle emanations, diffusion through pore waters of deep-sea sediments, leaching of river-borne sediments by seawater," and remobilization through reduction of a Fe-Mn carrier phase. However, there is still considerable debate... 

In 2004, the pH of pore water in the reduced tailings was 4.4 at <10 cm depth, increasing to 7 by 30 cm. Dissolved constituents follow pH, with concentrations of 36,000 ppm S042", 18,000 ppm Fe, and 9,600 ppm Zn at <10 cm depth. These concentrations decrease steadily with depth. Below 30 cm, most metals drop to below detection limits. 

The tailings comprise 5-10 wt. % pyrrhotite a highly reactive sulfide mineral that releases protons and Fe3+ into adjacent pore waters on oxidation. Further, the concentration of carbonate minerals in the tailings is low providing little buffering capacity above pH 5. Therefore, the tailings continued to acidify until they reach the pH of AI(OH)3 (pH 4-4.5) and Fe(OH)3 (pH 2.5-3.5) buffering. 

Although the abundance of silver in the Earth s crust is comparatively low (0.07 pgg-1), it is considered an environmental contaminant and is toxic at the nanomolar level. As an environmental pollutant it is derived from mining and smelting wastes and, because of its use in the electrical and photographic industries, there are considerable discharges into the aquatic environment. Consequently, there have been studies on the geochemistry and structure of silver-sulfur compounds [31]. Silver, either bound to large molecules or adsorbed on to particles, is found in the colloidal phase in freshwater. In anoxic sediments Ag(I) can bind to amorphous FeS, but dissolved silver compounds are not uncommon. A more detailed study of silver speciation in wastewater effluent, surface and pore waters concluded that 33-35% was colloidal and ca. 15-20% was in the dissolved phases [32]. 

MOR hydrothermal component, but that the very low 5 Fe values inferred for Fe(II)aq from low 5 Fe magnetite reflect interstitial pore waters and/or bottom waters that were closely associated with DIR bacteria, and not those of the open oceans. A substantial biomass of DIR bacteria is still required to process the very large inventory of Fe that is sequestered in BIFs as Iow-5 Fe magnetite, although not so extensive as that which would be required to lower the 5 Fe values of the open oceans if the oceans were rich in Fe(II)aq. 

Croal LR, Johnson CM, Beard BL, Newman DK (2004) Iron isotope fractionation by anoxygenic Fe(II)-phototrophic bacteria. Geochim Cosmochim Acta 68 1227-1242 Curtis CD, Coleman ML, Love LG (1986) Pore water evolution during sediment burial from isotopic and mineral chemistry of calcite, dolomite and siderite concretions. Geochim Cosmochim Acta 50 2321-2334... 

Most commonly observed pore-water concentration profiles, (a) A nonreactive substance, such as chloride (b) a chemical, such as O2, which undergoes removal in the surface sediment as a result of aerobic respiration (c) a chemical that is consumed by a reaction that occurs in a subsurface layer, such as Fe2+(aq) precipitating with S2-(aq) to form FeS2(s) (d) a chemical released in surface sediments, such as silica via dissolution of siliceous hard parts (e) a chemical released into pore waters from a subsurface layer, such as Mn +(aq) by the reduction of Mn02(s) and (f) a chemical released at one depth (reactive layer 1), such as Fe2+(aq) by reduction of FeOOFI(s), and removal at another depth (reactive layer 2), such as Fe +(aq) precipitating as FeS2(s). Source From Schulz,... 

Recent research has identified some other microbial routes for denitrification that are not heterotrophic. One, called the anammox reaction, involves the oxidation of ammonium to N2 using either nitrite or nitrate as the electron donor. The second has bacteria using Mn " to reduce nitrate to N2. As noted earlier, N2 is generated by the oxidation of ammonium using Mn02 as the electron acceptor. [Denitrification may also be supported by Fe " (aq) oxidation.] These reactions are summarized in Table 12.2. The overall consequence of these reactions is that ammonium does not accumulate in the pore waters where Mn respiration and denitrification are occurring. 

Iron and manganese are initially supplied to the sediments as a component of the sinking flux of POM and particifiate oxyhydroxides. Remineralization of the POM releases iron and manganese to the pore waters. In the presence of O2, the solubilized metals are oxidized and precipitate as oxyhydroxides, thereby increasing the inorganic particifiate phase in the oxic layer. Continuing sedimentation eventually carries this particulate Mn and Fe below the oxic zone. 

In the anoxic zone, heterotrophic respiration of particulate Mn02 and Fc203 or FeOOH causes manganese and iron to be reduced to Mn (aq) and Fe (aq). As dissolved ions, these trace metals diffuse through the pore waters. The ions that diffuse upwards will reenter the oxic zone, where they react with O2 to reform the oxyhydroxides. This produces a metal-enriched layer that lies just above the redox... 

Subsurface environments under anoxic conditions may contain high levels of Fe(II) on the solid phase or dissolved within immobile pore water or groundwater. The role of Fe(II) species in reductive transformation reactions of organic and inorganic contaminants in the subsurface was reviewed by Haderlein and Pecher (1988). A major finding of current studies is that Fe(II) associated with solid phases is much more reactive than Fe(II) present in dissolved forms (e.g., Erbs et al. 1999 Hwang and Batchelor 2000). 

---