Encyclopedia of Renewable Energy. James G. Speight

to the freezing point. The high concentration of electrolytes in seawater assist in breaking up the open, hydrogen-bonded ice-like structure of water near its freezing point. Because the salt components tend to be excluded from the ice formed by freezing seawater, sea ice is relatively fresh and still floats on water. Much of the salt it contains is not truly part of the ice structure but contained in brines that are physically entrained by small pockets and fissures in the ice.

      The dielectric constant of water (78.2 at 25°C, 77°F) is high compared to most liquids. Of the common liquids, few have comparable values at this temperature, e.g., hydrogen cyanide (HCN, 106.8), hydrogen fluoride (HF, 83.6), and sulfuric acid (H₂SO₄, 101). By contrast, most non-polar liquids have dielectric constants on the order of 2. The high dielectric constant helps liquid water to solvate ions, making it a good solvent for ionic substances, and arises because of the polar nature of the water molecule and the tetrahedrally-coordinated structure in the liquid phase.

      In many electrolyte solutions of interest, the presence of ions can alter the nature of the water structure. Ions tend to orient water molecules that are near to them. For example, cations attract the negative oxygen end of the water dipole toward them. This reorientation tends to disrupt the ice-like structure further away. This can be seen by comparing the entropy change on transferring ions from the gas phase to water with a similar species that does not form ions.

      As an example or chemical transformation that can occur in a water system, the chemistry of methyl iodide (which is thermodynamically unstable in seawater) is known and its chemical fate is kinetically controlled. The equations showing the fate of methyl iodide are as follows:

      In this equation, X = C1^-, Br^-, I^-

      Chloride ion was theoretically predicted to be the most kinetically reactive species, with water second, and other anions of lesser importance. This suggested that methyl iodide in seawater would react predominantly via a nucleophilic substitution reaction with chloride ion to yield methyl chloride. Methyl iodide and the methyl chloride produced by would also react with water, although more slowly, to yield methanol and halide ions. According to these experiments, substantial amounts of methyl chloride should be formed in seawater. Methyl chloride has a long half-life for decomposition by known reactions in seawater. Hence, its presence could be a useful label for some surface-derived water masses. Methyl chloride is in fact found in the atmosphere, where compared to methyl iodide, it is less stable to photo-degradation reactions.

Steroids are a class of biogenic compounds which may serve as an indicator of certain processes transforming matter in seawater and sediments. The steroid hydrocarbon structure (Figure A-2) forms a relatively stable nucleus which may incorporate functional groups such as alcohols (sterol derivatives and stanol derivatives), ketone derivatives (stanone derivatives) and olefin linkages (sterene derivatives) either in the four ring system or on the side chain originating at C-17.

Figure A-2 The hydrocarbon framework of the steroid system (ring lettering and atom numbering are shown).

      These compounds are produced by a wide variety of marine and terrestrial organisms and often have specific species sources. Diagenetic alteration of steroids by geochemical and biochemical processes can lead to the accumulation of transformed products in seawater and sediments.

      Within the group of chlorinated compounds, chlorinated ethylene derivatives are the most often detected groundwater pollutants. Tetrachloroethylene (PCE) is the only chlorinated ethylene derivative that resists aerobic biodegradation. Trichloroethylene (TCE), all three isomers of dichloroethylene (CCl₂=_CH2 and the cis/trans isomers of CHCl=CHCl), and vinyl chloride (CH₂=CHC1) are mineralized in aerobic co-metabolic processes by methanotropic or phenol-oxidizing bacteria. Oxygenase derivatives with broad substrate spectra are responsible for the co-metabolic oxidation. Vinyl chloride is furthermore utilized by certain bacteria as carbon and electron source for growth. All chlorinated ethylene derivatives are reductively dechlorinated under anaerobic conditions with possibly ethylene or ethane as harmless end-products.

      Tetrachloroethylene (CCl₂=CCl₂) is dechlorinated to trichloroethylene (CCl₂=CHCl) in a co-metabolic process by methanogens, sulfate reducers, homoacetogen derivatives, and others. Furthermore, tetrachloroethylene and trichloroethylene serve in several bacteria as terminal electron acceptors in a respiration process. The majority of these isolates dechlorinate tetrachloroethylene and trichloroethylene to cis-l,2-dichloroethene, although they have been isolated from systems where complete dechlorination to ethene occurred.

      If chemicals have become subsurface contaminants that threaten important drinking water resources. A strategy to remediate such polluted subsurface environments is with the help of the degradative capacity of bacteria.

See also: Alicyclic Hydrocarbons, Alkaloids.

      Aquatic Organisms

      Aquatic organisms can be classified into each varying in the biological characteristics, habitat, and adaptations, but linked within a complex network of ecological roles and relationships.

      Microorganisms (algae, bacteria, and fungi) are living catalysts that enable a vast number of chemical processes to occur in water and soil. The living organisms (biota) in an aquatic ecosystem may be classified as either autotrophic or heterotrophic. Autotrophic organisms utilize solar or chemical energy to fix elements from simple, nonliving inorganic material into complex life molecules that compose living organisms. Algae are typical autotrophic aquatic organisms. Generally, carbon dioxide (CO₂), nitrate (NO₃), and phosphate derivatives (PO₄3-) are sources of carbon, nitrogen, and phosphorus, respectively, for autotrophic organisms. Organisms that utilize solar energy to synthesize organic matter from inorganic materials are called producers.

      Macrophytes are individual aquatic plants that can be seen by the unaided eye and can be categorized based on where and how they grow. Rooted macrophytes are rooted in the riverbed or lake substrate, and are thus restricted to areas where flow is low enough to permit fine sediments to accumulate. Rooted macrophytes may have leaves entirely submerged (under the water), floating on the surface, or emergent above the surface. In turbid water, little light penetrates and photosynthesis is restricted; hence, only plants with floating or emergent leaves can thrive. Rooted macrophytes may extract nutrients from the substrate as well as absorbing them from the water as algae do. Floating aquatic macrophytes are rootless plants that persist only in backwater areas where the flow slackens—otherwise, they are carried downstream. Because their photosynthetic surfaces are above the water surface, these plants can grow in deep, turbid water and places where rooting sites are sparse.

      Macrophyte abundance can fluctuate seasonally as a result of scouring of the bottom sediments and washout of plants during heavy rains. For this reason, the number of macrophytes in river channels generally peaks during periods of low flow. Aquatic macrophytes are important in many aquatic systems, especially wetlands, slower moving water in streams and rivers, and in shallower areas of lakes. Aquatic macrophytes add three-dimensional complexity to aquatic habitat, and can provide habitat, refuge, and spawning areas for animals such as aquatic insects and fish, as well as a surface for periphyton growth. As they are primary producers, aquatic macrophytes produce organic matter which can be eaten by some fish; however, most of this plant material is unpalatable to herbivores while it is alive.

      Large populations of aquatic macrophytes can have negative effects on aquatic