soil/compost mixture can thus be optimized in terms of aeration, nutrients and
temperature, to achieve the most efficient degradation.
As part of its operations between 1955 and 1977, a Finnish sawmill had been
impregnating timber with a preservative to inhibit microbial degradation.
This product, called Ky-5, contained a mixture of chlorophenols, namely,
2,4,6-trichlorophenol (7–15%), 2,3,4,6-tetrachlorophenol (~80%) and
pentachlorophenol (6–10%). Ky-5 also contained traces of polychlorinated
phenoxyphenols and dibenzo-p-dioxins as impurities. Over the years this product
had contaminated the soils around the sawmill. A cost-effective bioremediation
strategy was needed that could be used at this site but also throughout Finland
where 800 other sites of this type existed.
The bioremediation strategy used compost and composting materials mixed
with the excavated soil in heaps known as biopiles. Two-parts contaminated soil
were mixed with one-part inoculant that contained straw-compost, bark chips,
lime (to adjust the pH) and nutrients (supplied by a commercial fertilizer). The
biopiles contained 7500 kg of material (volume of 13 m^3 ) and were built on a layer
of bark chips to provide insulation. The entire biopile was covered with a plastic
sheet and moisture content was adjusted by watering.
Chlorophenol degradation proceeds via dechlorination (removal of Cl-groups)
with hydroxylation (addition of OH-groups) at the dechlorinated sites. The
microbes are effectively manipulating the molecule to make it susceptible to
degradation by cleavage of the benzene ring.eqn. 4.21
In the case of pentachlorophenol (eqn. 4.21) the Cl-group opposite the OH-
group on the benzene ring is replaced by an OH-group first. The next dechlori-
nation/hydroxylation reaction yields a molecule with a total of three OH-groups,
two of which are adjacent to each other. At this stage ring cleavage occurs between
these adjacent OH-groups (see benzene degradation in Fig. 4.33) and the resul-
tant straight-chain hydrocarbon degrades to derive smaller chlorinated and
unchlorinated products.
The extent of degradation in the biopiles was proportional to the
starting chlorophenol concentration. Overall, however, chlorophenol loss was
between 80 and 90%. Where highly contaminated soil was treated (original
chlorophenol concentration 850 mg kg-^1 ) the rate of loss was between 2 and 5 mg
(kgdry wt)-^1 d-^1. This is a fast rate of chlorophenol loss and the remediation process
was complete within 3 months.OH
ClCl
ClClClOH
ClCl
OHClClOH
OHCl
OHClCl Cleavage of
benzene ring
(see Fig. 4.33)pentachlorophenol134 Chapter Four