Precipitation is a phase-partitioning process whereby solids separate from a solution.34 Dissolution involves movement from the solid or gaseous phase to the aqueous phase. Solids dissolve into ions, whereas gases retain their original chemical structure when dissolved. The solubility of a compound (its tendency to dissolve in water or other solutions) is the main property affecting the precipitation-dissolution process.
The concentration of a compound in water is controlled by its equilibrium solubility or solubility constant (the maximum amount of a compound that will dissolve in a solution at a specified temperature and pressure). Equilibrium solubility will change with environmental parameters such as temperature, pressure, and pH; for example, the solubility of most organic compounds triples when temperature rises from 0°C to 30°C. Each type of waste has a specific equilibrium solubility at a given temperature and pressure. The solubility of toxic organic compounds is generally much lower than that of inorganic salts. This characteristic is particularly true of nonpolar compounds because of their hydrophobic character.
Precipitation usually occurs when the concentration of a compound in solution exceeds the equilibrium solubility, although slow reaction kinetics may result in "supersaturated" solutions. For organic wastes in the deep-well environment, precipitation is not generally a significant partitioning process; in certain circumstances, however, it may need to be considered. For example, pentach-lorophenol precipitates out of solution when the solution has a pH of <5,35,36 and polychlorophenols form insoluble precipitates in water high in Mg2+ and Ca2+ ions.37 Also, organic anions react with such elements as Ca2+, Fe2+, and Al3+ to form slowly soluble to nearly insoluble compounds.
Precipitation may be significant for heavy metals and other inorganic constituents in injected wastes. For example, sulfide ions have a strong affinity for metal ions, precipitating as metal sulfides. The dissolved constituents in injected wastes and reservoir fluids would not be in equilibrium with the in situ brines because of the fluids' different temperature, pH, and Eh. When the fluids are mixed, precipitation reactions can lead to injection-well plugging.
Coprecipitation is a partitioning process whereby toxic heavy metals precipitate from the aqueous phase even if the equilibrium solubility has not been exceeded. This process occurs when heavy metals are incorporated into the structure of silicon, aluminum, and iron oxides when these latter compounds precipitate out of solution. Iron hydroxide collects more toxic heavy metals (chromium, nickel, arsenic, selenium, cadmium, and thorium) during precipitation than aluminum hydroxide.38 Coprecipitation is considered to effectively remove trace amounts of lead and chromium from solution in injected wastes at New Johnsonville, Tennessee.39 Coprecipitation with carbonate minerals may be an important mechanism for dealing with cobalt, lead, zinc, and cadmium.
Dissolution of carbonates (acidic wastes), sand (alkaline wastes), and clays (both acidic and alkaline wastes) can neutralize deep-well-injected wastes.39 Because precipitation-dissolution reactions are highly dependent on environmental factors such as pH and Eh, changes in one or more factors as a result of changes in injected-waste characteristics, or varying percentages of injected waste and reservoir fluids concentrations, may result in re-solution or reprecipitation of earlier reaction products. This sensitivity to environmental factors increases the complexity of predicting precipitation-dissolution reactions, because different equilibrium solubilities of a compound may exist in different parts of the injection zone depending on the proportions of waste and reservoir fluid. Similarly, a sequence of precipitation and dissolution reactions may take place at a given location of the injection zone as the concentration of injected wastes increases.
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