Source: Adapted from Verschueren (1983).
subsurface availability of oxygen. The key variable in this case is Henry's constant, KH. Low-molecular-weight hydrocarbons, such as the BTEX compounds (see le 16.18), tend to have high KH values, and as a result these types of contaminants can be removed from contaminated soils merely be providing thorough aeration or sparging [i.e., using an approach known as soil vapor extraction (SVE)]. The difficulty encountered with biore-mediating high KH compounds stems from their motivation to leave the soil by means of physical volatilization at a rate beyond that of its biochemical degradation. Although the soil is remediated, the contaminant is not degraded or concentrated, and may be emitted to the atmosphere.
As for the level of halogenation, the degree of substitution also tends to progressively reduce the biodegradability of a compound. Carbon tetrachloride offers a good example of this phenomenon Figure 16.66. Starting with methane as the carbon precursor, the successive halogen-based oxidations of this parent compound with chlorine will eventually shift the carbon oxidation state from a fully reduced (—4) to a fully oxidized (+4) level, at which point this CCl4 compound no longer offers any energy whatsoever as an electron donor during aerobic degradation. Carbon tetrachloride consequently represents a highly recalcitrant compound in terms of its resistance to aerobic degradation. Halogenated compounds with lower levels of substitution may, however, be amenable to aerobic metabolism. At the same time, many of these compounds can also be degraded through reductive anaerobic pathways in lieu of aerobic oxidations.
Theoretically, soil and groundwater bioremediation systems can be engineered and maintained to provide a considerable variety of different biochemical mechanisms, the specific mode and implementation of which may then be tailored on a site-specific basis according to the target chemical(s) and the soil properties involved (Rittman and McCarty, 2001; National Research Council, 1993). Although the engineering requirements of these systems are rapidly becoming better established, the fact remains that this technology remains a rather inexact science. Simply put, our efforts to manipulate microbial reactions remotely within soils whose characteristics usually have a high degree of spatial inconsistency are seldom easy and are often quite difficult to complete at a desired rate or degree.
To date, the majority of bioremediation operations have been developed with the intent of using aerobic degradation mechanisms, but here again the reality of soil and pore space
Figure 16.66 Oxidative methane (C — 4) halogenation to carbonte trachloride (C + 4).
Figure 16.66 Oxidative methane (C — 4) halogenation to carbonte trachloride (C + 4).
ecosystems is that they probably experience a considerable range of aerobic, anoxic, and anaerobic reactions. At least for readily oxidizable contaminants such as the lighter hydrocarbons, this interplay is probably a moot issue given the fact that the end result usually involves a successfully remediated soil. These operations are engineered to provide aeration in the hopes of promoting aerobic remediation, and any commensurate subsurface reactions involving anoxic or anaerobic metabolism are essentially masked by the dominant effectiveness of the aerobic reactions.
In this case, therefore, it is aerobic bacteria and fungi that are relied upon to catalyze the degradation of the contaminants successfully using an array of hydroxylation, dealky-lation, decarboxylation, and epoxidation reactions. The underlying thrust with these reactions is that of using the contaminants as a primary energy source, drawing electrons from the targeted contaminant and then releasing them back to oxygen as the final electron acceptor.
However, in certain instances there may well be nonaerobic reactions that offer degra-dative enzyme pathways that are better suited to a variety of specialized contaminants. For example, the nitroreductase enzyme typically associated with denitrification can reduc-tively attack the nitro groups found on nitroaromatic compounds. In this case, therefore, an in situ remediation process designed to deal with soils contaminated with these types of otherwise recalcitrant energetic munitions residuals would benefit from the introduction of nitrates rather than oxygen, such that subsurface denitrification could be promoted. Extending beyond this particular application with nitroaromatics, denitrifying Pseudomonas, Thiobacillus, and Bacillus bacteria have also been shown to be effective metabolic oxidizers of xylene isomers.
The bioremediation of halogenated compounds involves yet another set of potential remediation pathways covering both aerobic and anaerobic conversions. One such aerobic option for dehalogenating these types of contaminants can be maintained by a cometabo-lism sequence facilitated by a special group of aerobes using monooxygenase enzymes. This generic group of bacteria includes methanotrophic (methane-oxidizing) and ammonia-oxidizing forms that employ methane-monooxygenase (MMO) and ammonia-monooxygenase (AMO) enzymes, respectively. For those sites at which this cometabolic scheme was selected, the soils would have to be supplied with the suitable substrates (e.g., dissolved methane or ammonia, and oxygen) to induce and promote the growth of the involved bacteria at levels sufficiently high to enrich the targeted soils with their mono-oxygenase enzymes.
Figure 16.67 schematically depicts one such cometabolic conversion of TCE, a widely observed soil and groundwater contaminant used historically in many industries as a solvent and degreasing agent. This particular reaction is known to be mediated by a number of bacteria, including Alcaligenes and Pseudomonas (Harker and Kim, 1990). These types of monooxygenase reactions convert the ethylene backbone (— C=C—) of TCE into an epoxide structure (i.e., TCE-epoxide) by oxidatively introducing a single mole of oxygen. This product is chemically unstable and subsequently breaks down into smaller fragments, which are then amenable to direct microbial breakdown.
The actual circumstance of these cometabolic reactions is that they occur due to inherent inefficiencies of the involved enzymes. In this case, these catalysts inadvertently attack and oxidize secondary compounds (e.g., TCE) other than the energy-releasing primary substrate (methane or ammonia) for which the enzyme had originally been produced. The difficulty with this process is that the level of inherent inefficiency for these cometabolic reactions is so low that far more primary substrate has to be introduced
to the soil than the mass of secondary contaminant. In turn, the level of growth by the induced bacteria can be so high that it can clog the soil's pore space and restrict the migration of the added substrates away from the point of injection.
Dehalogenating pathways are also used with bioremediation systems that involve anaerobic reactions, including those of hydrolytic dehalogenation and reductive dehalogena-tion. The latter process of reductive dehalogenation has drawn the most attention, as it has proven to be widely applicable to the uncoupling of attached halogen species. One such process is depicted schematically in Figure 16.68, where the backbone carbon unit is
Carbon Oxidation States
reductively converted during the course of its dehalogenation. The end products created by this latter sort of anaerobic transformation may not have been suitably transformed to qualify the bioremediation sequence to this point as an outright success. With a highly halogenated compound such as pentachlorophenol (PCP), several such reactions may be necessary. In the particular case of TCE, a second anaerobic repetition of the reductive dehalogenation reaction can lead to the formation of vinyl chloride (H2C=CHCl). Given the fact that the latter product has been linked to carcinogenic impacts, therefore, this reactive sequence is not considered to be a suitable bioremediation mechanism. However, in many instances, a coordinated linkage of cyclic, sequential anaerobic-aerobic conversions could well achieve overall conversions of the targeted contaminants to a degree of transformation that is considerably higher than either option used independently.
The structural character and environmental conditions of a soil can play a sizable role by promoting or restricting the success of bioremediation. Certainly, one of the most important factors is that of a soil's hydraulic conductivity (K) (le 16.19), measured in velocity units (e.g., cm/s). This property ranges from highly permeable gravel and sand whose open void space is readily conducive to water transfer (i.e., with a K value of 102 cm/s or higher) to extremely tight, heavy clay soils with permeabilities (at or below 10=5 cm/s) which are so low that they restrict the necessary hydraulic throughput of moisture, nutrients, and substrates required for metabolic activity. In fact, the majority of successful in situ bioremediation systems have tended to fall with a hydraulic conductivity range of 10=3 to 10=1 cm/s (Staps, 1990).
Soil bioremediation depends on providing the necessary factors for microbial activity: moisture, nutrients including substrate and electron acceptors, and the presence of species of microbes capable of conducting the desired reaction. Moisture should be at a level of at least 40% of saturation (i.e., 40% of the soil pore space is filled with water and the remainder by air). Higher moisture levels would be acceptable for anaerobic systems, and may even be helpful, but in the case of aerobic bioremediation, a blockage of oxygen movement in soils with high pore water percentages would be decidedly harmful to the process. The oxidation of a readily biodegradable aliphatic hydrocarbon would, for example, require a supply of oxygen many times larger (in mass) than that of the oxidized contaminant, to the point where the soil would have had to be readily open to oxygenating air movement. The presence, or absence, of this soil moisture in remediation systems may also be affected in those aerobic systems that employ an aeration header or well, where the continuous addition of atmospheric air could induce an evaporative water loss that
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