Equilibrium in the Environment

Paper mill on the Potomac River near Westernport, Maryland, neutralizes acid mine drainage in the water. Upstream of the mitt, the river is acidic and lifeless below the mitt, the river teems with life. 

Reaction A is the same reaction by which ocean acidification (Box 11-1) can destroy marine life in the ocean. 

I art of the North Branch of the Potomac River runs crystal clear through the scenic Appalachian Mountains but it is lifeless—a victim of acid drainage from abandoned coal mines. As the river passes a paper mill and wastewater treatment plant near Westernport, Maryland, the pH rises from a lethal value of 4.5 to a neutral value of 7.2, at which fish and plants thrive. This fortunate accident comes about when calcium carbonate by-product from papermaking exits the paper mill and reacts with massive quantities of carbon dioxide from bacterial respiration at the sewage treatment plant The resulting soluble bicarbonate neutralizes the acidic river and restores life downstream of the plant In the absence of CO2, solid CaCOj would be trapped at the treatment plant and would never enter the river. 

CaCOjCs) + COiiaq) + HjOC/) Calcium carbonate trapped at treatment plant 

Ca +(o9) + 2HCOJ(a ) Dissolved calcium bicarbonate enters river and neutralizes add 

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The effectiveness of the polymer reflects several of its chemical and structural properties. Its cationic charge and apolar groups endow it with a very strong affinity for small molecules, particularly anions, as has been known for some time.20 For oxalacetate specifically, binding by polymer is also directly evident in the ultraviolet and infrared spectroscopic changes accompanying the shift in tautomeric equilibrium in the environment of the macromolecule. 

Ethyl acrylate is a volatile (38hPa at 20°C) liquid under normal environmental conditions. At equilibrium in the environment, ethyl acrylate will partition primarily to air (94%) with lesser amounts to water (5.6%), soil (<1%), and sediment (<0.1%). In air, ethyl acrylate will be removed by reaction with photochemically produced hydroxyl radicals (11.8 h half-life) and ozone (33 h half-life). When released to water, ethyl acrylate will volatilize to air (Henry s law constant of 25 Pa m mol -1) or be biodegraded (57% removal in 28 days). Based on its relatively low octanol-water partition coefficient (logKow of 1-18), ethyl acrylate does not pose a significant bioaccumulation hazard. 

Metal surfaces in a well-designed, well-operated cooling water system will establish an equilibrium with the environment by forming a coating of protective corrosion product. This covering effectively isolates the metal from the environment, thereby stifling additional corrosion. Any mechanical, chemical, or chemical and mechanical condition that affects the ability of the metal to form and maintain this protective coating can lead to metal deterioration. Erosion-corrosion is a classic example of a chemical and mechanical condition of this type. A typical sequence of events is ... 

Adsorption — An important physico-chemical phenomenon used in treatment of hazardous wastes or in predicting the behavior of hazardous materials in natural systems is adsorption. Adsorption is the concentration or accumulation of substances at a surface or interface between media. Hazardous materials are often removed from water or air by adsorption onto activated carbon. Adsorption of organic hazardous materials onto soils or sediments is an important factor affecting their mobility in the environment. Adsorption may be predicted by use of a number of equations most commonly relating the concentration of a chemical at the surface or interface to the concentration in air or in solution, at equilibrium. These equations may be solved graphically using laboratory data to plot "isotherms." The most common application of adsorption is for the removal of organic compounds from water by activated carbon. 

Here Bq is the steady flow availability function at the so-called dead state, where the fluid is in equilibrium with the environment, at state (po> T )- The maximum work obtainable between states X and Y may then be written as... 

Kotas [3] has drawn a distinction between the environmental state, called the dead state by Haywood [1], in which reactants and products (each at po. To) are in restricted thermal and mechanical equilibrium with the environment and the truly or completely dead state , in which they are also in chemical equilibrium, with partial pressures (/)j) the same as those of the atmosphere. Kotas defines the chemical exergy as the sum of the maximum work obtained from the reaction with components atpo. To, [—AGo], and work extraction and delivery terms. The delivery work term is Yk k kJo ln(fo/pt), where Pii is a partial pressure, and is positive. The extraction work is also Yk kRkTo n(po/Pk) but is negative. 

Linear polarization instruments provide an instantaneous corrosion-rate data, by utilizing polarization phenomena. These instruments are commercially available as two-electrode Corrater and three electrode Pairmeter (Figure 4-472). The instruments are portable, with probes that can be utilized at several locations in the drilling fluid circulatory systems. In both Corrater and Pairmeter, the technique involves monitoring electrical potential of one of the electrodes with respect to one of the other electrodes as a small electrical current is applied. The amount of applied current necessary to change potential (no more than 10 to 20 mV) is proportional to corrosion intensity. The electronic meter converts the amount of current to read out a number that represents the corrosion rate in mpy. Before recording the data, sufficient time should be allowed for the electrodes to reach equilibrium with the environment. The corrosion-rate reading obtained by these instruments is due to corrosion of the probe element at that instant [184]. 

The situation is more complicated for nonisolated systems consisting of strongly interacting particles and when the system is no longer in equilibrium with the environment. Kauffman [kauff93] notes that the second law really states that any system will tend to the maximum disorder possible, within the constraints due to the dynamics of the system. ... 

Table I summarizes some typical distribution coefficients. Sediments become enriched in plutonium with respect to water, usually with a factor of vlO5. Also living organisms enrich plutonium from natural waters, but usually less than sediments a factor of 103 - 101 is common. This indicates that the Kd-value for sediment (and soil) is probably governed by surface sorption phenomena. From the simplest organisms (plankton and plants) to man there is clear evidence of metabolic discrimination against transfer of plutonium. In general, the higher the species is on the trophic level, the smaller is the Kd-value. One may deduce from the Table that the concentration of plutonium accumulated in man in equilibrium with the environment, will not exceed the concentration of plutonium in the ground water, independent of the mode of ingestion.

Research into the aquatic chemistry of plutonium has produced information showing how this radioelement is mobilized and transported in the environment. Field studies revealed that the sorption of plutonium onto sediments is an equilibrium process which influences the concentration in natural waters. This equilibrium process is modified by the oxidation state of the soluble plutonium and by the presence of dissolved organic carbon (DOC). Higher concentrations of fallout plutonium in natural waters are associated with higher DOC. Laboratory experiments confirm the correlation. In waters low in DOC oxidized plutonium, Pu(V), is the dominant oxidation state while reduced plutonium, Pu(III+IV), is more prevalent where high concentrations of DOC exist. Laboratory and field experiments have provided some information on the possible chemical processes which lead to changes in the oxidation state of plutonium and to its complexation by natural ligands. 

Equilibrium data on different systems are largely missing. Systematic studies must be undertaken to understand the species distribution in the systems studied or in the environment. 

For water, there is local equilibrium between the water in the disk and in the environment at the surface. 

Such a relationship describes how a chemical will partition between water and the atmosphere under equilibrium conditions and is appropriate only for dilute solutions which are typically observed in the environment. Certain hydrocarbons despite possessing relatively low vapor pressures, may tend to partition significantly toward the air. This is largely a result of their correspondingly low water solubilities which result in low values for Kw. Therefore, chemicals which have low values for Kw have a greater tendency to partition towards the air and volatilize from solution. 

If this binding does occur, then one would expect very strongly bound compounds to show an unusual affinity for the aqueous phase. This could increase the mobility of these compounds in the environment. It is likely that the bound fraction will undergo phase transfers and degradation at different rates than the free truly dissolved fraction of a dissolved pollutant. If this is the case, then an observed equilibrium between a pollutant in the free and bound states could significantly affect its environmental behavior. 

The inorganic elements in aqueous solution reactions, both acid-base complex formation, precipitation and oxidation/reduction, frequently come rapidly to equilibrium when no more reactions are possible. The implication is that in the environment and in organisms many of their properties cannot change unless circumstances change, for example the introduction of new components. 

In summary, certain equilibrium constants of complex formation, of solubility products and of redox potentials form a set of fixed values that must be looked at in the context of the compartment which contains the components and which controlled evolution in fair part, against a background of rising amounts of environmental oxidised elements. The other factors were the rates of synthesis as dictated by supply of energy and of reactants in the environment. 

Sibley TH, Morgan JJ (1975) Equilibrium speciation of trace metals in freshwater-seawater mixtures. In Hutchinson HC (ed) Proceedings of international conference on heavy metals in the environment, University of Toronto, Toronto, Ontario pp 310-338... 

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