Environmental Engineering Reference
In-Depth Information
7.5.1 Conclusions from flask studies ................................................... 285
7.5.2 Conclusions from LTUs................................................................ 286
7.5.3 Conclusions from trough study.................................................. 286
7.5.4 Utility to remediation of highly contaminated soil................. 287
7.6 Recommendations for transitional research .......................................... 288
7.6.1 PAH availability in soil and regulatory cleanup levels.......... 288
7.6.2 Phytoremediation of PAHs.......................................................... 288
7.7 Technology transfer ................................................................................... 289
References............................................................................................................. 290
7.1 Land-farming background
7.1.1 Polycyclic aromatic hydrocarbons
7.1.1.1 Chemical structure and source of contamination
Polycyclic aromatic hydrocarbons (PAHs) are multiringed, organic com-
pounds, characteristically nonpolar, neutral, and hydrophobic. PAHs have
two or more fused benzene rings in a linear, stepped, or cluster arrangement.
Although there are more than 100 known PAHs, Table 7.1 provides the
chemical structure, abbreviated name, and molecular weight for the 15 PAHs
that were analyzed in this study.
PAHs occur naturally as components of incompletely burned fossil fuels,
and they are also manufactured. Several of these manufactured homologues
are used in medicines, dyes, and pesticides, but most are found in coal tar,
roofing tar, and creosote, a commonly used wood preservative. PAHs are
major chemical constituents of a wide variety of contaminants found at
Department of Defense (DOD) installations. They are found in burning pits
and as spills of creosote, fungicides, heavy oils, Bunker C fuels, and other
petroleum-based products. The higher-molecular-weight (HMW) homo-
logues are particularly recalcitrant and toxic. Some lower-molecular-weight
PAHs are volatile, readily evaporating into the air. Others will undergo
photolysis. Because they are hydrophobic and neutral in charge, PAHs are
strongly adsorbed onto soil particles, especially clays. Park et al. (1990)
studied the degradation of 14 PAHs in two soils. They found air phase
transfer (volatilization) an important means of contaminant reduction only
for naphthalene and 1-methylnaphthalene (the two-ring compounds). Abi-
otic mechanisms accounted for up to 20% of the total reduction but involved
only two-and three-ring compounds. Biotic mechanisms were responsible
for the removal of PAHs over three rings. The persistence of PAHs in the
environment, coupled with their hydrophobicity, gives them a high potential
for bioaccumulation.
7.1.1.2 Toxicity and benzo(a)pyrene toxic equivalent factors
The 15 compounds examined in this study (Table 7.2) are grouped together
because (1) more information is available on them and (2) they are suspected
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