Oxygen in fresh water

The general terms that are used to describe solubility for solids and liquids do not apply to gases in the same way. For example, oxygen is described as soluble in water. Oxygen from the air dissolves in the water of lakes and rivers. The solubility of oxygen in fresh water at 20°C is only 9 mg/L, or 0.0009 g/100 mL. This small amount of oxygen is enough to ensure the survival of aquatic plants and animals. A solid solute with the same solubility, however, would be described as insoluble in water. 

An approximate equation to calculate the mg/L of oxygen in fresh waters at 1 atm, as a function of temperature (in degrees Celsius) is... 

Dole, M. (1936) The relative atomic weight of oxygen in water and air II. A note on the relative atomic weight of oxygen in fresh water, salt water and atmospheric water vapor. J. Chem. Phys., 4, 778-780. 

Oxygen is soluble in water. However, the solubility is temperature dependent. In Fig. 1.5 the solubility of oxygen in fresh water is given, for sea water the curve is parallel but shifted to lower values. 

Graphite has an electron conductivity of about 200 to 700 d cm is relatively cheap, and forms gaseous anodic reaction products. The material is, however, mechanically weak and can only be loaded by low current densities for economical material consumption. Material consumption for graphite anodes initially decreases with increased loading [4, 5] and in soil amounts to about 1 to 1.5 kg A a at current densities of 20 A m (see Fig. 7-1). The consumption of graphite is less in seawater than in fresh water or brackish water because in this case the graphite carbon does not react with oxygen as in Eq. (7-1),... 

The corrosion product is predominantly carbon dioxide, but considerable amounts of free oxygen are produced at the anode surface, particularly in fresh-water applications, and can attack both the carbon and any organic binders used to reduce its porosity. For this reason carbon anodes for underground service are used in conjunction with a carbonaceous backfill. 

The performance of graphite in seawater, where chlorine is the principal gas evolved, is considerably better than in fresh water where oxygen is produced. Graphite is immune to chlorine and has a long history in the chemical industry in this and similar applications . 

Chemical Formation. A potential chemically mediated source of H202 would be the presence of reduced metals in oxygenated waters (15, 21, 32-34, 36-38). This pathway has never been demonstrated in fresh waters, although Miles and Brezonik (35) showed that 02 concentrations varied over 24 h in humic waters with iron present. No measurements of H202 were made, but most likely H202 was formed as the 02 was consumed. The net impact of these processes on H202 concentration in fresh waters is not likely to be important in waters rich in humic substances. However, this assumption has not been verified experimentally. 

Fig. 8.1 Dissolved oxygen (mg/1) in fresh water as a function of the ambient temperature.

Natural fresh waters are saturated with air and contain dissolved oxygen in the range of 8-10ppm at room temperatures. The dissolved oxygen in the water causes corrosion by the reactions. 

R.M. Baxter, J.H. Carey (1982). Reactions of singlet oxygen in humic waters. Fresh-wat. Biol, 12, 285-292. 

D.-Z. Dan, R.C. Sandford, P.J. Worsfold, Determination of chemical oxygen demand in fresh waters using flow injection with on-line UV-photocatalytic oxidation and spectrophotometric detection, Analyst 130 (2006) 227. 

The level of soluble thallium present in the sea (e.g. Pacific Ocean, Atlantic Ocean, Irish Sea, Australian Coast) is between 9 and 16 ng/L (Matthews and Riley, 1970). This is remarkably lower than in fresh waters. In natural sea water (pH 8.1), the oxygen content is sufficient to oxidize Tl(l) to Tl(lll). because formation of chloro-complexes stabilizes the trivalent state. In the Pacific Ocean, 80% of the thallium was found to occur as Tl(lll), and only 20% as the sum of Tl(l) and alkylthallium compounds (Batley and Florence, 1975). As Tl(lll) is easily adsorbed and coprecipitated, it continuously moves down to the sediments. 

The scatter diagram of TS vs. TOC illustrates the S/C ratios of our samples (Fig. 7). Although the study area is a normal marine environment Le. clastic sediments overlain by oxic waters of typical oceanic salinity), most of the S/C ratios in the sediments are considerably lower than the average ratio of 1/2.8 obtained for reduced sulfur and organic carbon in sediments beneath oxygenated seawater (Berner, 1982). By contrast, the S/C ratios in our samples are mostly lower than 1/10, typically occurring in fresh water environments. Similarly, low S/C ratios have been observed in Amazon inner shelf muds (Alter et al, 1986 Alter and Blair, 1996). The authors attribute the low ratios to the oxidation power of iron oxides and reworking of sediments. Unlike the Amazon case, our samples are mostly from the slope and submarine canyons rather than the shelf and, therefore, deserve further discussion. 

The effects of high flow velocity and erosion on the reaction kinetics are in principle described in Figure 7.40. Both the effect on the cathodic curve (increased transport of oxygen to the surface) and on the anodic curve (increased activation of the metal in the corrosion range of potential) are shown. The indicated passivation (case d) is possible only under certain conditions, like for instance on steel in fresh water without particles and at very high velocity, as frequently occurs in sections of water turbines. 

Although carbon content of a steel has no effect on the corrosion rate in fresh waters, a slight increase in rate (maximum 20%) has been observed in seawater as the carbon content is raised from 0.1 to 0.8% [41]. The cause of this increase is probably related to greater importance of the hydrogen evolution reaction in chloride solution (with complexing of Fe " by Cl ) supplementary to oxygen depolarization as the cathodic surface of cementite (FesC) increases. 

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