Oxygen surface waters

Loss rates of both CCI4 and Fll in anoxic waters are probably due to biological rather than chemical removal (Lee et al., 1999). It also seems likely that some of the chlorofluorocarbons are removed in fully oxygenated surface waters. Observations show that there is a deficit of CCI4 in the Antarctic surface and bottom waters (Meredith et al., 1996). Finally, fluorinated compounds such as CFC-113 are degraded in warm surface waters of the temperate North Atlantic, the tropical western Pacific, the Eastern Mediterranean, and even the Weddell Sea (Roether et al., 2001). CFC-113 depletions were —3% yr, with possibly accelerated rates in the mixed layer or near the surface. 

Coal mining exposes suffides (primarily pyrite) in coal and associated rocks to oxygen and moisture. These oxidize the sulfides and form sulfuric acid. The resulting acidic waters (referred to as acid mine drainage (AMD)) adversely impact the biota in watersheds downstream from active and abandoned mines. Oxidation of the sulfides also releases chalcophyllic trace elements into the water. Many of these elements precipitate in oxygenated surface waters and are concentrated in stream sediments (Goldhaber et at, 2001). 

Metal release from tidal Elbe river sediments by a process of oxidative remobilization has been described by Kersten (1989) (Figure 8.2). Short (30-cm) sediment cores were taken from a site, where diurnal inundation of the fine-grained fluvial deposits take place. In the upper part of the sediment column, total particulate cadmium content was 10 mg kg whereas in the deeper anoxic zone the total particulate concentration of Cd was 20 mg kg h Sequential extractions indicate that in the anoxic zone 60 -80% of the Cd was associated with the sulfidic/organic fraction. In the upper (oxic and transition) zone, the association of Cd in the carbonatic and exchangeable fractions simultaneously increase up to 40% of total Cd. This distribution suggests that the release of metals from particulate phases into the pore water and further transfer into biota is controlled by the frequent downward flux of oxygenated surface water. From the observed concentrations, it would be expected that long-term transfer of up to 50% of the Cd from the sediment subsurface would take place either into the anoxic zone located further below the sediment-water interface or released into the open water. 

Figure A3.10.1 (a) A schematic illustration of the corrosion process for an oxygen-rich water droplet on an iron surface, (b) The process can be viewed as a short-circuited electrochemical cell [4],...

In the presence of oxygen and water the oxides of most metals are more thermodynamically stable than the elemental form of the metal. Therefore, with the exception of gold, the only metal which is thermodynamically stable in the presence of oxygen, there is always a thermodynamic driving force for corrosion of metals. Most metals, however, exhibit some tendency to passivate, ie, to form a protective oxide film on the surface which retards further corrosion. 

Oxygen corrosion only occurs on metal surfaces exposed to oxygenated waters. Many commonly used industrial alloys react with dissolved oxygen in water, forming a variety of oxides and hydroxides. However, alloys most seriously affected are cast irons, galvanized steel, and non-stainless steels. Attack occurs in locations where tuberculation also occurs (see Chap. 3). Often, oxygen corrosion is a precursor to tubercle development. 

In addition to its presence as the free element in the atmosphere and dissolved in surface waters, oxygen occurs in combined form both as water, and a constituent of most rocks, minerals, and soils. The estimated abundance of oxygen in the crustal rocks of the earth is 455 000 ppm (i.e. 45.5% by weight) see silicates, p. 347 aluminosilicates, p. 347 carbonates, p. 109 phosphates, p. 475, etc. 

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