Seawater/freshwater

Luther et al. [92] have described a procedure for the direct determination of iodide in seawater. By use of a cathodic stripping square-wave voltammetry, it is possible to determine low and sub-nanomolar levels of iodide in seawater, freshwater, and brackish water. Precision is typically 5% (la). The minimum detection limit is 0.1 - 0.2 nM (12 parts per trillion) at 180 sec deposition time. Data obtained on Atlantic Ocean samples show similar trends to previously reported iodine speciation data. This method is more sensitive than previous methods by 1-2 orders of magnitude. Triton X-100 added to the sample enhances the mercury electrode s sensitivity to iodine. 

At this junction it seems appropriate to consider some further thermodynamic and kinetic arguments that bear on carbonate diagenetic problems discussed in this chapter and in Chapters 7 and 8. Because most carbonate diagenetic reactions take place in an aqueous environment (seawater, freshwater, brines) over a range of temperature and pressure, it is rapidly becoming... 

Goldenberg, L.C., Margaritz, M., and Mandel, S. (1983) Experimental investigation of irrevesible changes of hydraulic conductivity on the seawater-freshwater interface in coastal aquifers. Water Resourc. Res. 19, 77-85. 

Boron is an ubiquitous element that is present, naturally combined with oxygen, in seawater, freshwater, rocks, soils and all plants (Woods 1994, International Programme on Chemical Safety 1998, European Centre for Ecotoxicology and Toxicology of Chemicals 1996). Boron does not accumulate in any environmental compartment, neither does it bioaccumulate. Boron in the environment eventually is transported to the oceans which have a relatively high content of borates. Seawater contains about 4.5 mg Bkg h... 

Methylation of inorganic and methyltin compounds has been reported with the formation of mono-, di-, tri-, and tetramethyltin compounds. In addition, tributylmethyltin and dibutylmethyltin species have been found in harbor sediments, which suggests that some butyltin compounds may be methylated in aquatic systems. Methyltin formation in the environment is due mainly to methyl donation from methylcobalamin and methyl iodide. Photochemical reaction and transalkylation of inorganic tins produce methyltins methylation of tin increases the toxicity of their original metal form due, in part, to their higher volatility and lipophilicity. Methyltins are ubiquitous in the environment and have been measured in seawater, freshwater, rain, wastewaters, sediments, fish, invertebrates, birds, and humans. 

We examine the global water cycle, which describes how water moves from the ground to surface water to the atmosphere and back into the ground. We compare the chemical compositions of seawater, freshwater, and groundwater. 

A large wealth of literature is available for biological degradation of polymers in soil or compost, whereas little information is available on the degradation processes in aqueous environments. Since the applications of polymers in contact with aqueous environments are now gaining popularity, biodegradation under aqueous conditions becomes a matter of concern. The durability and lifetime of polymers certainly depends on the water body composition wherein they are used (seawater, freshwater, etc.) and also on the availability of methods that can detect the level of degradation in such conditions. 

The softened seawater is fed with dry or slaked lime (dolime) to a reactor. After precipitation in the reactor, a flocculating agent is added and the slurry is pumped to a thickener where the precipitate settles. The spent seawater overflows the thickener and is returned to the sea. A portion of the thickener underflow is recirculated to the reactor to seed crystal growth and improve settling and filtering characteristics of the precipitate. The remainder of the thickener underflow is pumped to a countercurrent washing system. In this system the slurry is washed with freshwater to remove the soluble salts. The washed slurry is vacuum-filtered to produce a filter cake that contains about 50% Mg(OH)2. Typical dimensions for equipment used in the seawater process may be found in the Hterature (75). 

Sea.wa.ter, Many offshore wells are drilled usiag a seawater system because of ready availabiHty. Seawater muds geaerally are formulated and maintained ia the same way that a freshwater mud is used. However, because of the preseace of dissolved salts ia seawater, more additives are aeeded to achieve the desired flow and filtration (qv) properties. 

Most surfaces in prolonged contact with seawater and freshwater are susceptible to the attachment of marine growths, eg, algae and barnacles. 

In some places and under certain conditions, freshwater can be obtained more cheaply by desalination of seawater than by transporting water. This is tme when all the costs of extremely large monetary investments in dams, reservoirs, conduits, and pumps to move the water are considered. Before the rapid escalation of fuel costs between 1973 and 1980, the cost of desalination of seawater to adequately supply southern California would have been less than that of transport to the Peripheral Canal. This would have been the case even if there were an unlimited supply of water in the mountains of northern California, a condition that does not appear to exist. It has been shown that before 1973 a seacoast town could have been suppHed with 7-12 x lO" /d of freshwater more cheaply by desalination than by damming and piping water a distance of >160 km km (7). Indeed, the 1987—1992 drought in California has compelled the city of Santa Barbara to constmct a water desalination plant, and a 76,000-m /d plant is plaimed for the western coast of Florida (8). 

Pipes, valves, fittings, and almost all other components of small equipment are now available in plastic or ceramics, which do not corrode in salt water and are less expensive than the metals now used. Synthetic detergents are now available for use with seawater, although a final rinse with freshwater may be desired. Saltwater sewage can be treated successfully. Dual water systems using freshwater and seawater are already in use on ships and in many island resort hotels. Many of these also have seawater systems for fire fighting. This trend will grow. 

In addition to freshwater, seawater is also a source for sodium, magnesium, chlorides, iodine, bromine, and magnesium metal (see Sodium coLD>ouNDS Magnesium coLD>ouNDS Iodine Bromine Magnesiumand magnesium alloys). Many other elements are certain to be economically obtained from the ocean as technology for the recovery improves. 

Zinc, like most metals, is found in all natural waters and soils as well as the atmosphere and is an important trace element in plant and animal life (see Mineral nutrients). Rocks of various kinds contain 20—200 ppm zinc and normal soils 10—30 ppm (average ca 50 ppm) in uncontaminated areas. The average zinc content of coal is 33 ppm. Seawater contains 1—27 )-lg/L (median ca 8 p.g/L), and uncontaminated freshwater usually <10 / g/L. 

Calcium chloride is found in the marine environment. The elemental composition of seawater is 400 ppm calcium, 18,900 ppm chlorine, and many organisms and aquatic species are tolerant of these concentrations. Toxicity arises either from the invasion of freshwater in otherwise saltwater environments or possible toxic doses of calcium chloride from spills, surface mnoff, or underground percolation into typically freshwater streams or aquifers. Various agencies have guidelines for calcium and chloride in potable water (41). The European Economic Community (EEC) is the only agency to have a minimum specification for calcium in softened water. 

Galvanic or impressed current anodes are used to protect these components. The anode material is determined by the electrolyte zinc and aluminum for seawater, magnesium for freshwater circuits. Platinized titanium is used for the anode material in impressed current protection. Potential-regulating systems working independently of each other should be used for the inlet and outlet feeds of heat exchangers on account of the different temperature behavior. The protection current densities depend on the material and the medium. 

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