In Situ Remediation Technologies - Dealing with Contaminated Sites: From Theory Towards Practical Application - page 959

Chemical Structure

Here emphasis is on mobile compounds like chlorinated ethenes and TPH reme-
diation. In the anaerobic conversion of PCE to ethene nearly always cis-DCE is
found as intermediate (Kennedy et al. 2006 ). Therefore, if trans-DCE is present
at a site biodegradation is inhibited (Agteren et al. 1998 ). For TPH remediation
the effect of chemical structure on biodegradation is largely responsible for the
incomplete degradation of TPH, which consists of over 500 different components.
Biodegradation rates of the various compounds decrease in the following sequence:
alkanes > branched alkanes > aromatics (low molecular weight) > cyclic alkanes
(Geerdink et al. 1996 ).
For Hydrophobic Organic Compounds (HOC) the chemical structure plays an
important role in their persistence to biodegradation (Agteren et al. 1998 ).
When chemical structure leads to persistence, regulators and site operators will
most often use physical techniques to control a specific site.

Toxicity

Several organic contaminants may be toxic for micro-organisms. The effects of
many organic contaminants seem to be related to the octanol-water partitioning
coefficient ( K ow ). The value of this coefficient gives the ratio of a contaminant
between the organic fraction (octanol) and water. Many organic contaminants have
very high values of this partitioning coefficient, therefore often the log K ow is given
for organic contaminants. In general, it is found that organic solvents with log
K ow > 4.0 do not inhibit microbial degradation. Organic solvents with log K ow <2.0
show a negative impact on micro-organisms (Laane et al. 1987 ). In specific cases
contaminants with K ow < 2.0 can be degraded, whereas contaminants with a high
log K ow sometimes may lead to effects. In Table 21.2 some log K ow values are given
for non chlorinated organic contaminants.
For chlorinated ethenes the K ow decreases during conversion from PCE to TCE,
cis-DCE and VC, therefore the toxicity in fact increases. However this is only a
temporary effect as the final product is the harmless ethane (Agteren et al. 1998 ).

Table 21.2 Log K ow values
are given for non chlorinated
organic contaminants

Contaminant Log K ow

Dioxane − 1.1

Acetone 0.23

Pyridine 0.71

Benzene 2.0

Toluene 2 . 4

Xylene 3.1

Diethyl phthalate 3.3

diphenyl ether 4.3

Decane 5.6

Tetradecane 7.6

Dioctyl phthalaat 8.8

Source: Laane et al. ( 1987 )