Deep-well environment

The relative merits of platinised titanium and niobium in a deep-well environment, in comparison with those of other anode materials, have been given by Stephens . 

This section examines the major processes that affect the fate of deep-well-injected hazardous wastes. The focus is on processes that (1) are known to occur in the deep-well environment or (2) have not been directly observed but are theoretically possible. 

Chemical reactions may result from interactions among and between the three phases of matter solid, liquid, and gas. The major interactions that occur in the deep-well environment are those between different liquids (injected waste with reservoir fluids) and those between liquids and solids (injected wastes and reservoir fluids with reservoir rock). Although gases may exist, they are usually dissolved in liquid at normal deep-well pressures. 

Two chemical properties important in predicting fate in the deep-well environment are homogeneity and reversibility. Chemical processes can be broadly classified as either homogeneous or heterogeneous and either reversible or irreversible. 

Table 20.3 lists the reversible and irreversible processes that may be significant in the deep-well environment.3 The characteristics of the specific wastes and the environmental factors present in a well strongly influence which processes will occur and whether they will be irreversible. Irreversible reactions are particularly important. Waste rendered nontoxic through irreversible reactions may be considered permanently transformed into a nonhazardous state. A systematic discussion of mathematical modeling of groundwater chemical transport by reaction type is provided by Rubin.30... 

Fate-Influencing Processes in the Deep-Well Environment... 

Transformation processes alter the chemical structure of a substance. In the deep-well environment, the transformation processes that may occur are largely determined by the conditions created by partition processes and the prevalent environmental factors. Transport processes do not need to be considered if transformation processes irreversibly change a hazardous waste to a nontoxic form. 

Table 20.4 presents the partition and transformation processes known to occur in the near-surface environment along with the special factors that should be considered when evaluating data in the context of the deep-well environment. Geochemical processes affecting hazardous wastes in deep-well environments have been studied much less than those occurring in near-surface environments (such as soils and shallow aquifers). Consequently, laboratory data and field studies for a particular substance may be available for near-surface conditions, but not for deep-well conditions. 

As Table 20.4 shows, several processes can occur in both the near-surface and deep-well environments. For example, neutralization of acidic or alkaline wastes is a straightforward process, and although temperature differences between the two environments may need to be considered, no other factors make the deep-well setting distinctly different. The same holds true for oxidation-reduction (redox) processes. 

The remaining processes, although they occur under near-surface and deep-well conditions, are less applicable to the latter. Distinct differences between the two environments, however, can lead to significant differences in how the processes affect a specific hazardous substance. Compared with the near-surface environment, the deep-well environment is characterized by higher temperatures, pressures, and salinity, and lower organic matter content and Eh (oxidation-reduction potential). 

Table 20.5 lists the partition and transformation processes applicable in the deep-well environment and indicates whether they significantly affect the toxicity or mobility of hazardous wastes. None of the partition processes results in detoxification (decomposition to harmless inorganic constituents), but all affect mobility in some way. All transformation processes except complexation can result in detoxification however, because transformation processes can create new toxic substances, the mobility of the waste can be critical in all processes except neutralization. 

Adsorption-desorption Partly Mechanisms for adsorption on similar materials will be similar. Soil adsorption data generally do not reflect the saturated conditions of the deep-well environment. Organic-matter content is a major factor affecting adsorption in the near-surface its significance in the deep-well environment is less clear. Fate studies involving artificial recharge are probably useful, but differences between fresh waters and deep brines may reduce relevance. 

Precipitation-dissolution Partly Higher temperatures, pressures, and salinity of the deep-well environment may result in significant differences between reactions in the two environments. 

Immiscible-phase separation Transformation Processes No Fluids (such as gasoline) that are immiscible in water are a significant consideration in near-surface contamination. Deep-well injection is limited to wastestreams that are soluble in water. Well blowout from gaseous carbon dioxide formation is an example of this process that is distinct to the deep-well environment. 

Biodegradation Partly Some near-surface bacteria appear capable of entering and surviving in the deep-well environment. However, in general, temperature and pressure conditions in the deep-well environment are unfavorable for microbiota that are adapted to near-surface conditions. Biological transformations are primarily anaerobic. 

Complexation Partly Humic substances are very significant factors in near-surface complexation processes, probably less so in die deep-well environment. Data on complexation in saline waters are probably most relevant. 

Hydrolysis Partly Basic processes will be the same. Higher salinity of deep-well environment may affect rate constants. 

Oxidation-reduction Partly The deep-well environment tends to be more reducing than the near-reduction surface environment, but equally reducing conditions occur in the near-surface. Some adjustments may be required for pressure/temperature effects. 

Partition processes determine how a substance is distributed among the liquid, solid, and gas phases and determine the chemical form or species of a substance. Partitioning usually does not affect the toxic properties of the substance. Partitioning can, however, affect the mobility of the waste, its compatibility with the injection zone, or other factors that influence fate in the deep-well environment. The major partition processes are as follows ... 

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. 

Two other processes that may transform hazardous wastes are photolysis and volatilization, but they are not considered here because they do not occur in the deep-well environment. 

The solubility of most metals is much higher when they exist as organometallic complexes.4445 Naturally occurring chemicals that can partially complex with metal compounds and increase the solubility of the metal include aliphatic acids, aromatic acids, alcohols, aldehydes, ketones, amines, aromatic hydrocarbons, esters, ethers, and phenols. Several complexation processes, including chelation and hydration, can occur in the deep-well environment. 

Hydrolysis occurs when a compound reacts chemically with water (i.e., new chemical species are formed by the reaction), and can be a significant transformation process for certain hazardous wastes in the deep-well environment (see Table 20.7). Hydrolysis reactions fall into two major categories replacement and addition. The rates at which these reactions occur are also significant in a fate assessment because some take so long to occur that they will not take place during the analytical time frame (10,000 years). 

Polar organic compounds such as amino acids normally do not polymerize in water because of dipole-dipole interactions. However, polymerization of amino acids to peptides may occur on clay surfaces. For example, Degens and Metheja51 found kaolinite to serve as a catalyst for the polymerization of amino acids to peptides. In natural systems, Cu2+ is not very likely to exist in significant concentrations. However, Fe3+ may be present in the deep-well environment in sufficient amounts to enhance the adsorption of phenol, benzene, and related aromatics. Wastes from resinmanufacturing facilities, food-processing plants, pharmaceutical plants, and other types of chemical plants occasionally contain resin-like materials that may polymerize to form solids at deep-well-injection pressures and temperatures. 

Smith and Raptis53 have suggested using the deep-well environment as a wet-oxidation reactor for liquid organic wastes. This process, however, does not involve deep-well injection of wastes but rather uses temperatures and pressures in the subsurface to increase the oxidation rate of organic wastes, which are then returned to the surface. 

The previous chapter examined the geochemical processes that can occur in the deep-well environment. The type and outcome of reactions that will actually occur when a waste is injected, however, depend on its chemical characteristics and on injection-zone conditions. This chapter examines six major environmental factors that must be taken into consideration. 

The pH of a system greatly influences what chemical processes will occur in the deep-well environment. Directly or indirectly, pH also affects most of the other environmental factors. Table 20.12 summarizes the significance and some major effects of changes in pH on chemical processes and environmental factors in the deep-well environment. 

Reducing conditions predominate in the deep-well environment for several reasons ... 

Higher temperatures in the deep-well environment are associated with decreases in Eh. 

Neutral to slightly alkaline water in the deep-well environment favors lower Eh values. 

Temperature and pressure are the primary influences on the rate of chemical reactions. Both temperature and pressure increase with depth below the Earth s surface. Consequently, temperatures and pressures in the deep-well environment are significantly higher than those in the near-surface environment. 

This section relates the chemical characteristics of inorganic and organic hazardous wastes to the important fate-influencing geochemical processes occurring in the deep-well environment. 

Acid-base equilibrium is very important to inorganic chemical reactions. Adsorption-desorption and precipitation-dissolution reactions are also of major importance in assessing the geochemical fate of deep-well-injected inorganics. Interactions between and among metals in solution and solids in the deep-well environment can be grouped into four types1 2 3 4 ... 

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