Pcb

Pesticides Heavy metals

Yes Yes Yes No Yes

Yes No

Yes Yes No No Yes

"Contaminants that tend to be volatile (with high Kh values) are listed as +. 'Contaminants that tend to be soluble in water (with low to moderate Kow values) are listed as +. cContaminant solubility can be increased by detergents (for hydrocarbons) or acidification (for heavy metals). Source: Adapted from Staps (1990).

compound's molecular weight and configuration, solubility, volatility, and degree of halogenation. For example, the list of suitable hydrocarbon contaminants extends from lighter fuels such as gasoline well into the heavier fuel range of 10 to 20 carbon atoms (Kostecki and Calabrese, 1991). However, beyond this level, the heavy carbon compounds (e.g., lubricants) are considerably less amenable to successful biodegradation. This pattern extends to the family of aromatic hydrocarbons, where smaller two- to three-ring forms may be biodegradable, whereas larger, four- and five-ring compounds are likely to be highly recalcitrant.

The biochemically resistant nature of the larger aliphatic and aromatic hydrocarbons stems not just from their size but also from their corresponding shift toward lower levels of water solubilities, as quantified by a determination of their octanol-to-water partitioning coefficient (KOW). This variable qualifies the extent to which any given compound is attracted to (and miscible within) water as compared to its corresponding solubility in a nonpolar solvent, octanol. Contaminants regarded as water-loving or hydrophilic will have negative log KOW values, by which their solubility naturally ensures a maximal opportunity for metabolic uptake and degradation. However, as the KOW increases, the prospects for successful bioremediation begin to drop.

Moderately hydrophobic compounds with log KOW values extending up to 1 to 2 will still be suitably available. The data given in le 16.18 for the BTEX (benzene, toluene, ethylbenzene, and xylene) compounds exemplify the upper end of these permissible values. Even though their log KOW figures range from 2.12 (benzene) to 3.26 (xylene), these contaminants are all amenable to full aerobic catabolism.

Nonpolar organic liquids that have low aqueous solubility, and thus form immiscible phases, are commonly referred to as nonaqueous phase liquids (NAPLs), of which there are two versions, dense NAPL (DNAPL) and light NAPL (LNAPL), depending on whether they are more or less dense than water, respectively. Most hydrocarbon solvents are LNAPLs (e.g., benzene or mixtures such as gasoline), whereas most chlorinated hydrocarbon solvents (e.g., trichloroethylene, perchloroethylene) are DNAPLs. The sorp-tive preference of these NAPL forms to coalesce on subsurface soils substantially limits their available surface area and correspondingly constrains their effective bioavailability. There are engineering strategies to counter this behavior, however, by which surfactant (e.g., soaplike agents) chemicals can be introduced to increase the relative solubility of low-solubility contaminants (e.g., NAPLs) in a fashion which then enhances their prospective availability for metabolic degradation.

The volatility of a contaminant may also affect its rate of biodegradation, particularly given the fact that many bioremediation systems require aeration to enhance the

TABLE 16.18 Representative Aromatic Hydrocarbon (BTEX) Properties

Property

Units g/cm3

mg/L

Benzene

Ethylbenzene

Toluene

Xylene

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