Oxic/anoxic boundaries

Redox Reactions of Iron and Manganese at the Oxic Anoxic Boundary in Waters and Soils... 

At the oxic-anoxic boundaries a rapid turnover of iron takes place. This oxic-anoxic boundary may occur in deeper layers of the water column of fresh and marine waters, at the sediment-water interface or within the sediments. 

Transformation of Fe(II,III) at an oxic-anoxic boundary in the water or sediment column (Modified from Davison, 1983)... 

NO 3-Reducing. Fig. 9.15 shows data on groundwater below agricultural areas. The sharp decrease of 02 and NO3 at the redox cline indicate that the kinetics of the reduction processes are fast compared to the downward water transport rate. Postma et al., 1991 suggest that pyrite, present in small amounts is the main electron donor for NO3 reduction (note the increase of SOJ immediately below the oxic anoxic boundary). Since NO3 cannot kinetically interact sufficiently fast with pyrite a more involved mechanism must mediate the electron transfer. Based on the mechanism for pyrite oxidation discussed in Chapter 9.4 one could postulate a pyrite oxidation by Fe(III) that forms surface complexes with the disulfide of the pyrite (Fig. 9.1, formula VI) subsequent to the oxidation of the pyrite, the Fe(II) formed is oxidized direct or indirect (microbial mediation) by NO3. For the role of Fe(II)/Fe(III) as a redox buffer in groundwater see Grenthe et al. (1992). 

The iron cycle shown in Fig. 10.14 illustrates some redox processes typically observed in soils, sediments and waters, especially at oxic-anoxic boundaries. The cycle includes the reductive dissolution of iron(lll) hydr)oxides by organic ligands, which may also be photocatalyzed in surface waters, and the oxidation of Fe(II) by oxygen, which is catalyzed by surfaces. The oxidation of Fe(II) to Fe(III)(hydr)-oxides is accompanied by the binding of reactive compounds (heavy metals, phosphate, or organic compounds) to the surface, and the reduction of the ferric (hydr) oxides is accompanied by the release of these substances into the water column. 

Fe(III)(hydr)oxides introduced into the lake and formed within the lake - Strong affinity (surface complex formation) for heavy metals, phosphates, silicates and oxyanions of As, Se Fe(III) oxides even if present in small proportions can exert significant removal of trace elements. - At the oxic-anoxic boundary of a lake (see Chapter 9.6) Fe(III) oxides may represent a large part of settling particles. Internal cycling of Fe by reductive dissolution and by oxidation-precipitation is coupled to the cycling of metal ions as discussed in Chapter 9. 

The freshly precipitated manganese and iron oxides which precipitate at an oxic-anoxic boundary within the lake water column or in the top layers of the sediment, form small particles with high surface area they cause an additional scavenging at... 

Von Gunten, U., and W. Schneider (1991), "Primary Products of the Oxygenation of Iron(II) at an Oxic-Anoxic Boundary Nucleation, Aggregation, and Aging", J. Colloid and Interface Science 145, 127-139. 

Gunneriusson, L. (1994) Composition and stability of Cd(II) chloro and Cd(II) hydroxo complexes at the goethite (a-FeOOH)/water interface. J. Colloid Interface Sd. 163 484-492 Gunten, U. von Schneider, W. (1991) Primary produds of oxygenation of iron(II) at an oxic/ anoxic boundary nucleation, agglomeration and ageing. J. Colloid Interface Sd. 145 127-139... 

Dissimilatory reduction by anaerobic bacteria occurs in the anoxic hy-polimnion of stratified lakes and in sediments just below the oxic-anoxic boundary. It produces H2S,... 

The observations in Lake Greifen are in line with other studies of As(III) and As(V) at oxic-anoxic boundaries (16, 17, 76). In Saanich Inlet (16) and in Lake Pavin (17), increasing As(III) concentrations were found below the 02-H2S boundary, although the reduction of As(V) was not complete. A similar situation is encountered in Lake Greifen, in which sulfide is only an intermittent species. The formation of reduced and methylated As species in the upper layers of various lakes has also been described in ref. 76. 

Figure 8.6 Size distributions (particles below lpm) based on particle number for different natural water systems Gulf of Mexico (Harris, 1977), foraminifera and diatoms from near-surface South-lndian Ocean (Lai and Lerman, 1975), coastal surface waters of North Pacific Ocean (off Tokyo Bay) (Koike et al., 1990), Grimsel test site groundwater (Switzerland) (Degueldre, 1990), Markham Clinton groundwater (UK) (Longworth et al., 1990), amorphous iron oxy(hydroxo)phos-phate at the oxic/anoxic boundary of Lake Bret (Switzerland) (Buffle et al., 1989), Rhine River (The Netherlands) (van de Meentef al., 1983), Rhine River (Basle, Switzerland) (Newman etal., 1994), St Lawrence River (Canada) (Comba and Kaiser, 1990). Distributions recalculated from the original data as explained in Filella and Buffle (1993) (reproduced from Filella and Buffle, 1993, by permission of the copyright holders, Elsevier Science Publishers BV, Amsterdam).

Microbial consumption of DMS in the hypolimnion is more difficult to estimate since our laboratory experiments showing degradation have not been rigorous enough to yield degradation rates applicable to natural conditions. However, if we assume that DMS is transported into the hypolimnion by eddy diffusion, with an arbitrary eddy diffusion coefficient of 1 m2/d, the observed concentration gradient across the oxic/anoxic boundary would support a sink for 30 pmol/m2/d DMS produced in the hypolimnion. We therefore hypothesize that anaerobic microbial consumption in the hypolimnion of Salt Pond may be the major sink for DMS produced in the metalimnion. At this point we cannot estimate the potential for DMS oxidation in the epilimnion. 

Figure 2 Oxic-anoxic boundary in the water or sediment columf ...

At the oxic-anoxic boundary in the deeper portions of a stagnant water body or a sediment, a redox cycling of sulfur compounds of different oxidation states may occur (Figure 8.20) because reduced S may become reoxidized. 

At the oxic-anoxic boundary of a lake, Fe(IlI) oxides may represent a large part of settling particles. Internal cycling of Fe by reductive dissolution and by oxidation-precipitation is coupled to the cycling of metal ions. 

Cowie, G. L and J. I. Hedges (1991) Organic carbon and nitrogen geochemistry of Black Sea surface sediments from stations spanning the oxic anoxic boundary. In Black Sea Oceanography (ed. E. Izdar and J.W. Murray), pp. 343-9. Boston, MA Kluwer. 

The light-induced processes on the dissolution of hydrous Fe(III) oxides are discussed in this volume by Sulzberger (Chapter 14). Here, we present our views on the reactions of reductive dissolution of Fe(III) (hydr)oxides that occur at the oxic-anoxic boundary in oceans and lakes. This is schematically presented in Figure 19. As shown by Sulzberger et al. (1989) the important processes are ... 

Figure 19. Transformations of Fe(II, III) at an oxic anoxic boundary in the water or sediment column (modified from Davidson, 1985). Peaks in the concentration of solid Fe(III) (hydr)oxides and of dissolved Fe II) are observed at locations of maximum Fe(III) and Fe(II) production, respectively. The combination of ligands and Fe(ll) produced in underlying anoxic regions are most efficient in dissolving Fe(III) (hydr)oxides. Redox reactions of iron—oxidation accompanied by precipitation, reduction accompanied by dissolution—constitute an important cycle at the oxic-anoxic boundary which is often coupled with transformations (adsorption and desorption) or reactive elements such as heavy metals, metalloids, and phosphates.

The tendency for a redox reaction to occur is reflected by the tendency for electrons to flow and produce an electrical current, which can be measured as a voltage, the redox potential (Eh). At the oxic/anoxic boundary in sediments Eh = 0, and conditions become more oxidizing (i.e. organic matter is more vigorously attacked by oxygen) as Eh becomes... 

Wakeham S.G. (1989) Reduction of sterols to stanols in particulate matter at oxic-anoxic boundaries in sea water. Nature 342, 787-90. 

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