Читать книгу Encyclopedia of Renewable Energy - James Speight G., James G. Speight - Страница 100

The Earth is a unique planet insofar as there is an abundance of water that is necessary to sustaining life on the Earth, and helps tie together the atmosphere, the land (the geosphere), and the oceans and rivers (the aquasphere) into an integrated system. Precipitation, evaporation, freezing and melting, and condensation are all part of the hydrological cycle, which is a never-ending global process of water circulation from clouds to land, to the ocean, and back to the clouds. This cycling of water is intimately linked with energy exchanges among the atmosphere, ocean, and land that determine the climate of the Earth and cause much of natural climate variability. The impacts of climate change and variability on the quality of human life occur primarily through changes in the water cycle.

The water systems of the Earth, often referred to as the aquasphere or the hydrosphere, refer to water in various forms: oceans, lakes, streams, snowpack, glaciers, the polar ice caps, and water under the ground (groundwater). An important aspect of the water systems is an aquifer which is a water-bearing (water-rich) subsurface formation (a subsurface zone) that yields water to wells. An aquifer may be porous rock, unconsolidated gravel, fractured rock, or cavernous limestone. Aquifers are important reservoirs storing large amounts of water which, in theory, should be relatively free from evaporation loss or pollution. However, in practice, this is not always the case.

The aquasphere consists of a variety of non-oceanic water systems (Table A-23) that are essential to life, and all are interrelated, and the many interactions between the systems of the Earth are complex, and they are occurring constantly and simultaneously although the effects are not always obvious.

Table A-23 Sources of non-oceanic water and water systems.

Source	Description
Surface water	Water in a river, lake or fresh water wetland; the water is naturally replenished by precipitation and naturally lost through discharge to the oceans, evaporation, evapotranspiration, and groundwater recharge.
Groundwater	Fresh water located in the subsurface pore space of soil and rocks; also water that is flowing within aquifers below the water Table.
Aquifer water	Fresh water in a layer of sediment or rock that is highly permeable; usually water in a layer of sand and gravel that have high permeability.
Unconfined aquifer	An aquifer that is overlaid by permeable earth materials and which is recharged by water seeping down from above in the form of rainfall and snow melt.
Confined aquifer	An aquifer which is sandwiched between two impermeable layers of rock or sediments and are recharged only in those areas where the aquifer intersects the land surface.

The oceans play a key role in the water cycle insofar as the oceans hold 97% v/v of the total water on the Earth and 78% v/v of the global precipitation occurs over the oceans, and it is the source of 86% v/v of global evaporation. Besides affecting the amount of atmospheric water vapor and hence rainfall, evaporation from the sea surface is important in the movement of heat in the climate system. Water evaporates from the surface of the ocean, mostly in warm, cloud-free subtropical seas. This continuing event cools the surface of the ocean, and the large amount of heat absorbed the ocean partially buffers the greenhouse effect from increasing carbon dioxide and other gases. Water vapor carried by the atmosphere condenses as clouds and falls as rain, mostly in the intertropical convergence zone (ITCZ), far from where it evaporated. Condensing water vapor releases latent heat which drives much of the atmospheric circulation in the tropics. This latent heat release is an important part of the heat balance of the Earth, and it couples the energy and water cycles of the Earth.

The intertropical convergence zone, known by sailors as the doldrums or the calms because of the monotonous, windless weather, is the area where the northeast and southeast trade winds converge. The zone encircles Earth near the thermal equator, although the specific position of the zone can vary on a seasonal basis. When the zone lies near to the geographic equator. it is referred to as the near-equatorial trough. When the intertropical convergence zone is drawn into and merges with a monsoonal circulation, the zone is sometimes referred to as a monsoon, a usage that is more common in Australia and parts of Asia.

The major physical components of the global water cycle include the evaporation from the ocean and land surfaces, the transport of water vapor by the atmosphere, precipitation onto the ocean and land surfaces, the net atmospheric transport of water from land areas to ocean, and the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.

On land, the situation is more complex, and includes the deposition of rain and snow on land; water flow in runoff; infiltration of water into the soil and groundwater; storage of water in soil, lakes and streams, and groundwater; polar and glacial ice; and use of water in vegetation and human activities. Processes labeled include precipitation, condensation, evaporation, evapotranspiration (from tree into atmosphere), radiative exchange, surface runoff, ground water and stream flow, infiltration, percolation, and soil moisture. Furthermore, in the water systems (particularly in the lakes) the term eutrophication becomes important. Eutrophication is the deterioration of the esthetic and lifesupporting qualities of lakes and estuaries, caused by excessive fertilization from effluents high in phosphorus, nitrogen, and organic growth substances. Algae and aquatic plants become excessive, and when they decompose, a sequence of objectional features arises.

Water for human consumption (and, in many cases as on farms for animal consumption) from such lakes must be filtered and treated. Diversions of sewage, better utilization of manure, erosion control, improved sewage treatment, and harvesting of the surplus aquatic crops alleviate the symptoms.

There are some extremely dramatic examples of Earth systems interacting, such as volcanic eruptions and ocean tsunamis, but there are also slow, nearly undetectable changes that alter ocean chemistry, the content of the atmosphere, and the microbial biodiversity in soil. Each part of the Earth, from inner core to the top of the atmosphere, has a role in making Earth suitable for the existence of billions of lifeforms (1 billion = 1 x 10⁹). Within the aquasphere, the phenomenon known as (i) precipitation, (ii) evaporation, (iii) freezing, (iv) melting, and (v) condensation are part of the hydrological cycle – also known as the water cycle – which is a continuous global process of water circulation from clouds to land, to the ocean, and back to the clouds.

The water cycle describes the means by which water evaporates from the surface of the Earth, rises into the atmosphere, cools, condenses to form clouds, and falls again to the surface as precipitation. This system of the cycling of water is intimately linked with energy exchanges among the atmosphere, the ocean, and land that determine the climate of the Earth and cause much of natural climate variability. For example, approximately 75% of the energy (or heat) in the global atmosphere is transferred through the evaporation of water from the surface of the Earth. On land, water evaporates from the ground, mainly from soils, plants (through transpiration), lakes, and streams. In fact, approximately 15% v/v of the water entering the atmosphere is from evaporation from the land surfaces and evapotranspiration from plants which (i) cools the surface of the Earth, (ii) cools the lower atmosphere, and (iii) provides water to the atmosphere to form clouds.

The major physical components of the global water cycle include (i) the evaporation from the ocean and land surfaces, (ii) the transport of water vapor by the atmosphere and precipitation onto the ocean and land surfaces, (iii) the net atmospheric transport of water from land areas to ocean, and (iv) the return flow of fresh water from the land back into the ocean. The additional components of oceanic water transport are few, including the mixing of fresh water through the oceanic boundary layer, transport by ocean currents, and sea ice processes.

Water pollution has become a widespread phenomenon and has been known for centuries, particularly the pollution of rivers and groundwater. By way of example, in ancient time up to the early part of the 20^th century, many cities deposited waste into the nearby river or even into the ocean. It is only very recently (because of serious concerns for the condition of the environment) that an understanding of the behavior and fate of chemicals, which are discharged to the aquatic environment as a result of these activities, is essential to the control of water pollution. In rivers, the basic physical movement of pollutant molecules is the result of advection, but superimposed upon this are the effects of dispersion and mixing with tributaries and other discharges. Some of the chemicals discharged are relatively inert, so their concentration changes only due to advection, dispersion, and mixing. However, many substances are not conservative in their behavior and undergo changes due to chemical or biochemical processes, such as oxidation.

In addition, there are many indications that the chemical materials in the aquasphere (also called, when referring to the sea, the marine aquasphere) are subject to intense chemical transformations and physical recycling processes imply that a total carbon approach is not sufficient to resolve the numerous processes occurring. The transport of anthropogenically produced or distributed compounds such as crude oil hydrocarbon derivatives and halogenated hydrocarbon derivatives, including the polychlorobiphenyl derivatives the DDT family, and the Freon derivatives and the chemistry of these chemicals in water is not fully understood.

The effects of a chemical released into the marine environment (or any part of the aquasphere) depends on several factors such as (i) the toxicity of the chemical, (ii) the quantity of the chemical, (iii) the resulting concentration of the chemical in the water column, (iv) the length of time that floral and faunal organisms are exposed to that concentration, and (v) the level of tolerance of the organisms, which varies greatly among different species and during the life cycle of the organism. Even if the concentration of the chemical is below what would be considered as the lethal concentration, a sub-lethal concentration of an chemical can still lead to a long-term impact within the aqueous marine environment. For example, chemically-induced stress can reduce the overall ability of an organism to reproduce, grow, feed, or otherwise function normally within a few generations. In addition, the characteristics of some chemicals can result in an accumulation of the chemical within an organism (bio-accumulation) and the organism may be particularly vulnerable to this problem. Furthermore, subsequent bio-magnification may also occur if the chemical (or a toxic product produced by one or more transformation reactions) can be passed on, following the food chain up to higher flora or fauna.

In terms of a chemical spill into the environment, the complex processes of transformation start developing almost as soon as the chemical contacts the land (or the water) although the progress, duration, and result of the transformations depend on the properties and composition of the chemical, parameters of the spill, and environmental conditions. The major operative processes are (i) physical transport, (ii) dissolution, (iii) emulsification, (iv) oxidation, (v) sedimentation, (vi) microbial degradation, (vii) aggregation, and (viii) self-purification.

In terms of physical transport, the distribution of oil spilled on the sea surface occurs under the influence of gravitational forces and is controlled by the viscosity of the (liquid) chemical as well as the surface tension of the water. In addition, during the first several days after a spill of a liquid chemical or a mixture of liquid chemicals, a part of spilled chemical may be lost through evaporation and any water-soluble constituents disappear into the water. The portion of the chemical mixture that remains is the more viscous fraction. Further changes take place under the combined impact of meteorological and hydrological factors.

Many organic chemicals are not soluble in water, although some constituents may be water-soluble to a certain degree, especially low-molecular-weight aliphatic and aromatic hydrocarbon derivatives. Polar compounds formed as a result of oxidation of some oil fractions in the marine environment also dissolve in seawater. Compared to evaporation process, the dissolution of organic chemicals in water is a slow process. However, the emulsification of a chemical (or chemicals) in the aquasphere does occur but depends predominantly on the presence of organic functional groups in the spilled material which can increase with time due to oxidation. The rate of emulsification process can be decreased by use of emulsifiers – surface-active chemicals with strong hydrophilic properties used to eliminate oil spills – which help to stabilize oil emulsions and promote dispersing oil to form microscopic (invisible) droplets that accelerates the decomposition of the chemicals in the water.

Oxidation is a complex process that can ultimately results in the destruction of the crude boil constituents. The final products of oxidation (such as hydroperoxide derivatives, phenol derivatives, carboxylic acid derivatives, ketone derivatives, and aldehyde derivatives) usually have increased water solubility. This can result in the apparent disappearance of the chemicals from the surface of the water. This is due to the incorporation of oxygen-containing functional groups into the chemicals which results in a change in density with an increase in the ability of the transformed chemicals to become miscible (or emulsify) and sink to various depths of the water system as these changes intensify. These chemical changes also result in an increases in the viscosity of the chemicals which promotes the formation of solid oil aggregates. The reactions of photo-oxidation, photolysis in particular, also initiates transformation of the more complex (polar) chemicals.

As these processes occur, some of the chemicals are adsorbed on any to suspended material and deposited on the floor of the water system (sedimentation), the rate of which is dependent upon the ocean depth – in deeper areas remote from the shore, sedimentation of oil (except for the heavy fractions) is a slow process. Simultaneously, the process of biosedimentation occurs – in this process, plankton and other organisms absorb the emulsified chemical – and the transformed chemical is sent to the bottom of the water system as sediment with the metabolites of the plankton and other organisms. However, this situation radically changes when the suspended chemical(s) oil reaches the bottom – the decomposition rate of the chemical ceases abruptly especially under the prevailing anaerobic conditions, and any chemicals that have accumulated inside the sediments can be preserved for many months and even years. These products can be swept to the edge of the water system (the river bank, the lake shore, or the beach in the case of a spill into an ocean) by turbulent condition at some later time.

Self-purification is a result of the processes previously described above in which a chemical in the environment rapidly loses the original properties and disintegrates into various products. These products may have different a chemical composition and structure to the original chemical and exist in different migrational forms, and they undergo chemical transformations that slow after reaching thermodynamic equilibrium with the environmental parameters. Eventually, the original and intermediate compounds disappear, and carbon dioxide and water form. This form of self-purification inevitably happens in water ecosystems if the amount of toxic chemicals spilled into the system does not exceed acceptable limits.

While acid-base reactions are not the only chemical reactions important in aquatic systems, they do present a valuable starting point for understanding the basic concepts of chemical equilibria in such systems. Carbon dioxide (CO₂), a gaseous inorganic chemical substance of vital importance to a variety of environmental processes, including growth and decomposition of biological systems, climate regulation, and mineral weathering, has acid-base properties that are critical to an understanding of its chemical behavior in the environment. The phenomenon of acid rain is another example of the importance of acid-base equilibria in natural aquatic systems.

Furthermore, the almost unique physical and chemical properties of water as a solvent are of fundamental concern to aquatic chemical processes. For example, in the liquid state, water has unusually high boiling point and melting point temperatures compared to its hydride analogues from the periodic table of the elements (Table 8.10) such as ammonia (NH₃), hydrogen fluoride (HF), and hydrogen sulfide (H₂S). Hydrogen bonding between water molecules means that there are strong intermolecular forces making it relatively difficult to melt or vaporize.

In addition, pure water has a maximum density at 4°C (39°F), higher in temperature than its freezing point (0°C, 32°F), and that ice is substantially less dense than liquid water, and is important (and fortunate) in several contexts. Thus, as water in a lake is cooled at its surface by loss of heat to the atmosphere, the ice structures formed will float. Furthermore, a dynamically stable water layer near 4°C (39°F), will tend to accumulate at the bottom of the lake, and the overlying, less dense water able to continue cooling down to the freezing point. This means that ice will eventually coalesce at the surface, forming an insulating layer that greatly reduces the rate of freezing of the underlying water. This situation is obviously important for plants and animals that inhabit lake waters.

In addition, the presence of salt components means that the temperature of maximum density for seawater is shifted to lower temperatures: in fact, the density of seawater continues to increase right down 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.