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.
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