THE FATE OF TRACE SYNTHETIC ORGANICS IN
BIOSOLIDS APPLIED TO SOIL
Introduction
Man-made trace synthetic organic compounds are present in municipal biosolids because
they may be discharged in the sewage by households and industries, because they decay
slowly in the treatment process, and because they don't dissolve well in water and tend to
attach to the organic compounds that make up a large part of the biosolids. These properties
also make trace synthetic organic compounds stable when biosolids is applied to soil.
Movement of trace synthetic organic compounds in soil through leaching into groundwater is
limited to less than one percent of the amount applied. Somewhat higher losses have been
reported for the more volatile organic compounds, which escape into the air. Questions
remain regarding the movement of forms of trace synthetic organic compounds which have
undergone partial degradation with a resulting higher solubility in water.
Concentrations of trace synthetic organics in biosolids vary greatly, as they are influenced
by the sources in the collection area. Thus polychlorinated biphenyl (PCB) concentrations
range from 0.32-23.1 ppm with the high value reported from Schenectady, where PCBs were
long used in manufacture by the General Electric Co. (Furr et al., 1976). Dieldrin, a pesticide,
was reported to range from 0.03-2.2 ppm.
Since trace synthetic organic compounds have low solubility and adsorb strongly onto
other organics, they leach slowly into water as it moves through biosolids and soil. Losses of
PCBs into groundwater are less than 0.1 percent per year (District, Madison Metropolitan
Sewerage, 1988; Moza et al., 1979b). Studies with radiolabeled PCBs show that losses of
partially degraded PCBs are also less than 0.1 percent per year (Moza et al., 1979b).
The degradation rates of trace synthetic organic compounds also vary greatly, but the
compounds likely to appear in treated biosolids naturally have lower degradation rates since
they must have persisted during passage through the degradative wastewater and biosolids
treatment processes. Degradation of organic compounds can be chemical, photochemical, or
biological. Although photochemical degradation is important for some compounds, most
degradation of trace synthetic organics in soils is biological. Biological degradation takes
place in aerobic (with oxygen; for example, in well-tilled, aerated soils) or in anaerobic (for
example, waterlogged soils) environments. The biosolids digestion process is usually
anaerobic, while the biosolids/soil environment is, for the most part, aerobic. The presence or
absence of oxygen has a dominant effect on the type of microorganisms present and on how
and to what degree and rate organic compounds are degraded. Microorganism also require
nutrients (such as the nitrogen and phosphorus found in fertilizers or in the biosolids itself)
and the proper pH. The bacteria in biosolids, which is at a neutral pH, may not adapt well to
the lower pH of forest soils, e.g. (4-5.5). The effect of pH changes on the biodegradation of
trace synthetic organics in biosolids application to forest soil has not been studied. Another
NBMA Summary - Organics
factor that can prevent biodegradation is the presence of amounts of chemicals that are toxic
to the degrading organisms themselves. The amounts in municipal biosolids are not high
enough to pose this problem.
One of the roadblocks to rapid biodegradation of man-made trace synthetic organic
compounds is the presence of chlorine atoms in their structure. Chlorine atoms tend to
prevent the oxidation of organics. In general, degradation rates are slower in anaerobic
environments and degradation of complex trace synthetic organics is often incomplete.
However, the removal of chlorine from highly chlorinated compounds sometimes proceeds
faster anaerobically than it does aerobically.
Another type of structure which can interfere with biodegradation is the benzene ring
characteristic of aromatic compounds. Aromatic structures degrade slowly in aerobic
environments, but they are particularly resistant to anaerobic degradation.
Since compounds like PCBs and many pesticides contain both aromatic structures and
chlorines, their persistence in the environment is to be expected. Thus only a few percent of
applied PCBs degrade in soil (Focht and Brunner, 1985) and less chlorinated PCBs degrade
faster than highly chlorinated PCBs (District, Madison Metropolitan Sewerage, 1988).
Dieldrin (with eight chlorines) degrades slowest of the common pesticides (Sharom et al.,
1980). Dioxins, with several chlorines and an aromatic structure degrade very slowly in the
environment (Matsumura et al., 1983; Young, 1983).
Plant uptake of trace synthetic organics such as PCBs, pesticides (applied to soil), and
dioxins is small. There is no reported concentration of trace synthetic organics from the soil
to plants. Less than one percent of the PCBs applied to a forest soil were recovered in needles
of spruce (Moza et al., 1979b) and less than 6 percent of PCBs were applied to a agricultural
soil were found in carrots (Iwata and Gunther, 1976; Moza et al., 1979a). Plants take up
PCBs with fewer chlorines to a greater extent than they take up highly chlorinated PCBs
(Iwata and Gunther, 1976; Moza et al., 1979a; Moza et al., 1979b). Less than 1 percent of the
dioxin applied to a soil was taken up by various agricultural crops (Isensee and Jones, 1971;
Sacchi et al., 1989). Less than 3 percent of phthalates applied to an agricultural soil in
biosolids were taken up by food plants such as lettuce and carrots (Aranda et al., 1989).
Despite many reports of the bioconcentration of trace synthetic organics in the aquatic
food chain, there are few studies of the uptake of trace synthetic organic compounds by
terrestrial mammals exposed to contaminated soils. In a single study of dioxin uptake, rats
and guinea pigs concentrated dioxin in their livers at three times the concentration in the soil
that they were fed (McConnell et al., 1984).
Although many trace synthetic organic compounds are resistant to biodegradation in the
wastewater treatment process, two factors cause the levels of certain trace synthetic
compounds that appear in municipal sewage to be very low in municipal biosolids. Volatile
compounds such as benzene, trichloroethylene, and chloroform are lost to the atmosphere in
sewers and in the aeration basins of wastewater treatment process. Regulation of pesticide
2
NBMA Summary - Organics
and herbicide use in the last 15 years has shifted use away from the resistant compounds,
such as DDT, dieldrin, and 2,4,5-T, toward compounds such as 2,4-D and the pyrethrins,
which degrade more easily in the wastewater treatment and biosolids digestion process.
Public interest in reducing use of pesticides has played a role in the reduction of such
compounds in wastewaters and runoffs. Reductions of PAH from combustion processes and
runoff may also have occurred for similar reasons.
Thus, except for cities with specific industrial discharges, the trace synthetic organic
contaminants that appear in greatest concentration in municipal biosolids are the PCBs and
the phthalates. Phthalates are used as binders in plastics. The manufacture of PCBs has been
curtailed, but they are still in the urban environment in older electrical equipment such as
transformers. Phthalate toxicity to mammals is relatively low (Battersby and Wilson, 1989;
Keith and Telliard, 1979; Sommers et al., 1984) and their degradation in soil and biosolids,
while slow, is more complete that of PCB (Nozawa and Maruyama, 1988). PCBs, however,
are persistent and are thought to be more potent carcinogens. Thus most concern over trace
synthetic organic compounds in biosolids has focused on PCBs. The following is a review of
the published studies on PCB mobility, degradation, and uptake in soils.
Polychlorinated Biphenyls
The fate of PCBs in soils has attracted more study than that of other trace synthetic
organics, but much of the work has been concerned with sites that contain higher levels of
PCBs than is common in biosolids applications. Nevertheless several studies can be used to
draw general conclusions about PCBs in soil. These studies will be discussed in detail as
follows.
Radiolabeled PCBs (di-, tri-, and tetrachlorinated) were added to biosolids applied to nine
three-year-old spruce trees (Picea abies) growing in forest soil in a box (Munich, Germany)
and the movement of these lightly chlorinated PCBs and their degradation products was
traced over a period of four years (Moza et al., 1979b). Initially 7 L of biosolids was applied
to the soil (area = 0.36 m2) and mixed to a depth of 10 cm (total soil volume = 0.252 m3).
The total mass of PCB applied was 14.6 mg, mixed into 3.5 L applied to the top of the
soil/biosolids mix (4.17 mg L-1 in the top-dressing biosolids).
Less than 0.1 percent of the total applied PCB radioactivity was recovered in the leachate
from the site in the first two years, then no radiolabel was detectable in the leachate. Needles
and stems were found to contain 0.8 percent of the total radioactivity applied. More than 90
percent of the radioactivity recovered from the trees were transformed from hydrophobic
PCBs to unidentified hydrophilic substances. This study showed that PCBs and their
transformation products are taken up by trees and leached from the soil to a negligible degree
(total losses of less than one percent).
A study of the fate of PCBs in biosolids applied to agricultural soils was conducted by the
Madison, Wisconsin, Metropolitan Sewerage District as detailed in an interim report
published in October, 1988 (District, Madison Metropolitan Sewerage, 1988). Biosolids
3
NBMA Summary - Organics
containing 25 or 75 ppm PCB was applied at rates ranging from 1.8-16.8 dry Mg ha-1
annually and disced three times to a uniform 30.5 cm depth. Samples were obtained at four
different soil depths two to three weeks after biosolids application. Biosolids PCB loadings
ranged from 20 to 240 ppb total PCBs in the soil.
Within 400 days of application, dichlorobiphenyl concentration fell to 10 percent of the
starting values and trichlorobiphenyl soil concentrations fell to approximately 50 percent of
starting concentrations. Tetra-, penta-, hepta- and octachlorobiphenyl concentrations were
uniform throughout the observation period (500 days).
Since the majority of the PCB congeners were in the tetra- and above class, total PCB
concentrations varied little during the 500-day study period. Of the PCBs applied, 44 percent
were trichlorobiphenyls and 50 percent of these were lost; 7 percent were dichlorobiphenyls
and they were almost completely degraded. Therefore, an average of 35 percent total PCB
loss occurred over 500 days in these aerobic soils. The total average half life of 2-, 3-, 4PCBs were 99, 280, and 2300 days, respectively.
Based on preliminary examination of PCB profiles and depth, it appeared that PCB
movement in the soil profile was not occurring. Runoff samples from infiltrometer tests
showed 0.066 mg L-1 PCB in the liquid fraction and 0.25 mg L-1 PCB in the solid fraction.
Physical-chemical characteristics of PCB
The solubility of PCB in water varies from 6 ppm for monochlorobiphenyls to 0.03 ppm
for pentachlorobiphenyls to 0.015 ppm for PCBs with 10 chlorines (Alford-Stevens, 1986;
Haque and Schmedding, 1975). This low level limits the availability of PCBs for
biodegradation.
The tendency of PCB congeners to volatilize is given by their vapor pressure (9x10-40.9x10-4 mm Hg) (Alford-Stevens, 1986) and their Henry's law constants (average 3x10-4
atm m3 mole-1) (Dunnivant et al., 1988). Henry's law constant is proportional to the number
of chlorines, not the molecular weight; but the constant is related to the chlorine substitution
pattern, increasing with greater ortho substitution.
Although PCBs sorb strongly to organics in biomass (octanol:water coefficient, Kow =
4.5-8.1) (Choi and Chen, 1976; Rapaport and Eisenreich, 1984), sediments and soils that
adsorption is apparently reversible (Bell and Tsezos, 1987; Choi and Chen, 1976; Gschwend
and Wu, 1985). Care must be taken in sample handling to account for nonfilterable
microparticles and organic macromolecules, which may account for significant PCB sorption.
PCBs in marine sediments are sorbed primarily on particles less than 8 µm diameter with the
amount linearly related to organic content and to the fulvic and humic acid fractions (Choi
and Chen, 1976). The partition coefficient is constant over a wide range of solid/solution
ratios: Kp= 1-100 L g-1 for lake water (Gschwend and Wu, 1985), 5-26 L g-1 for a Condie silt
loam (Anderson and Pankow, 1986). The best predictor of the extent of PCB sorption on
organics and biomass is the octanol:water partition coefficient (Bell and Tsezos, 1987).
4
NBMA Summary - Organics
Movement of PCB in Soil
PCB movement in soils is greatly inhibited by its tendency to sorb strongly to soil
organics as is shown in the following studies. Table 1 summarizes the factors which affected
PCB movement in these studies.
In Madison, Wisconsin, biosolids was applied to an agricultural silt loam soil at a rate of
1.8-16.8 dry Mg ha-1 and disced three times to a depth of 30.5 cm. Biosolids PCB
concentrations ranged from 25-75 ppm and total PCBs in the soil ranged from 0.020 to 0.240
ppm. Based on preliminary examination of PCB profiles with soil depth, it appeared that
PCB movement in the soil profile was not occurring. Runoff samples from infiltrometer tests
showed 0.066 mg L-1 PCB in the liquid fraction and 0.25 mg L-1 PCB in the solid fraction
(District, Madison Metropolitan Sewerage, 1988).
Removal of PCBs from wastewater in a simulated overland flow treatment system was
studied by applying 100 ppm 14C-PCB (Aroclor 1242) directly to an alluvial soil in troughs
(8 cm deep) (Pardue et al., 1988). 99.9 percent of the applied PCB was removed at an
application rate of 7.5 L m-2 d-1. About 58 percent of the total applied PCBs were recovered
from the soil with 96 percent of the recovered PCBs found in the upper 20 percent of the
slope, and 82 percent found in the top 2 cm. Highly chlorinated and less chlorinated
congeners were distributed similarly. 32 percent of the 14C radiolabeled materials recovered
from the soils were more polar compounds than PCBs and may have been products of
biodegradations. There were more polar compounds in the lower horizons, suggesting that
such compounds were more mobile.
When di-, tri-, and tetra-chlorobiphenyls were applied to a forest soil in biosolids (4.2
ppm) less than 0.1 percent of the PCBs and any radiolabeled products leached in 2 years, then
there were no further losses (Moza et al., 1979b).
Plant Uptake of PCB from Soil
Several studies have confirmed the minor degree of PCB uptake by plants. These factors
which affected PCB uptake by plants in these studies are summarized in Table 2. The details
of the plant studies are given in the following paragraphs.
Corn yields were unaffected by application of biosolids containing PCBs (District,
Madison Metropolitan Sewerage, 1988). Corn grown on fields receiving PCB biosolids was
sampled in order to determine the extent of PCB uptake. PCB concentrations in corn were
approximately 20-400 times lower than the lowest FDA tolerance for finished animal feed
(200 ppb).
14C-labeled
PCB congeners (tri-, tetra-, and penta-chlorobiphenyl) were applied to soils
of soybean plants growing in pots (total PCB in the soil = 2-3 ppm) (Fries and Marrow,
1981). Surface application of PCB was compared to subsurface application. After 52 d
radioactive residues were detected in the leaves of plants in which PCB was applied at the
soil surface. Most of the plant radioactivity was found in the lower leaves. There was no
5
NBMA Summary - Organics
significant root uptake or translocation of radiolabeled compounds within the plants. There
was increased plant uptake of more highly chlorinated PCB congeners, up to 11.5 percent for
pentachlorobiphenyl. 20-30 percent of the applied PCB was lost by volatilization from the
surface applications. These data suggest that plant uptake was due to sorption by foliage of
PCB vaporized from the soil surface.
The importance of vaporization as a route of PCB uptake by plants is supported by
Suzuki et al. (Suzuki et al., 1977). Soybeans were grown in uncontaminated sand with their
roots penetrating a barrier into sand containing 100 ppm of Aroclor 1242 or 1254. Contrary
to the results with surface application in which highly chlorinated PCBs were preferentially
sorbed (Fries and Marrow, 1981), less chlorinated PCBs were more absorbed than highly
chlorinated PCBs, possibly due to their greater water solubility.
When Aroclor 1254 was applied to a sandy loam (100 ppm mixed in 15.25 cm soil)
(Iwata and Gunther, 1976). Carrot roots absorbed 30-50 percent of the applied PCB
congeners with low retention times in 23 months, but only 3-4 percent of PCB congeners
with high retention times. Carrot foliage contained 1-6 percent as much PCB as soil.
Carrots and sugar beets grown in a sandy soil to which 1.3 kg ha-1 14C-labeled
trichlorobiphenyl and 1.1 kg ha-1 pentachlorobiphenyl had been mixed into the top 10 cm in
lysimeter boxes (1.3 and 1.1 ppm, respectively) (Moza et al., 1979a). In first year 67 percent
of the trichlorobiphenyl and 42 percent of the pentachlorobiphenyl was lost by volatilization.
3.1 percent of the trichlorobiphenyl and 1.4 percent of the pentachlorobiphenyl was absorbed
by the carrots. The remaining radioactivity was dispersed to a depth of 40 cm. 20 percent of
this trichlorobiphenyl was unextractable from the soil (Soxhleted with methanol for 48 hr),
possibly due to conjugation with the humic materials. 1.6 percent of the trichlorobiphenyl
was converted to soluble, oxygenated metabolites. Less than 1 percent of the
pentachlorobiphenyl was converted to soluble metabolites. In the second year the beets took
up 0.2 percent of the remaining radioactivity.
The same eight PCB mono-, di-, tri-, and tetra-chlorobiphenyl congeners were taken up
preferentially by four species of corn and beans grown in soils containing 0.145 ppm total
PCB (Shane and Bush, 1989). Fruit uptake was 100 times less than leaf uptake.
There is some evidence that some PCB congeners are degraded by plant cells. Rose root
cells in culture transformed 0.045 nmol of 2,2',4,4'-tetrachlorobiphenyl h-1 (g DW cells)-1,
but transformation was incomplete as no CO2 was formed (Fletcher et al., 1987).
PCBs were found at levels of 0.018 ppm in the bark of trees growing near a landfill site
contaminated with PCB and to 0.0005 ppm in trees 14 km away (Meredith and Hites, 1987).
No PCB was detected in the wood of the trees. PCB was found in the atmosphere at low
levels near the landfill and the PCB congeners in the bark were present in amounts
proportional to their affinity for lipids, suggesting that PCBs were transferred by
volatilization and sorption.
6
NBMA Summary - Organics
Soil microfungi take up and are affected by soil PCBs. Aspergillus flavus cultures
accumulated PCBs from Aroclor 1254 as the soil concentration and chlorine content
increased (Murado et al., 1976). The fungal growth rate falls as the Aroclor 1254
concentration increased from 5 to 50 ppm, mostly due to increased lag and with greater
effects for Aroclor 1232, with less chlorine content than Aroclor 1254.
PCB Uptake by Animals
There have been few studies of bioaccumulation of PCB in animals living in or near
contaminated soils. This despite the extensive literature documenting the intensive
bioaccumulation of PCB by animals in the aquatic environment (Nisbet and Sarofim, 1972,
Oliver and Niimi, 1988). Birds feeding in terrestrial environments have been noted to have
elevated PCB levels (e.g., 4-7 ppm in woodcocks feeding primarily on earthworms, Nisbet
and Sarofim, 1972, Prestt et al., 1970). More study is needed of this potential route of PCB
mobilization.
Microbial Degradation of PCBs
Although PCBs are relatively immobile in the environment their ultimate fate depends on
their rates of decomposition. Under environmental conditions PCBs are stable chemically and
resist photodecomposition (Nisbet and Sarofim, 1972). Thus the primary mechanisms for the
biodegradation of PCBs are biological. In some cases, primarily aerobic and for a limited
range of PCB congeners, PCBs serve as carbon and energy sources for cell growth. In other
cases, primarily aerobic, PCBs may be partially degraded by nonspecific enzymatic activity in
conjunction with cell growth on other carbon sources (co-metabolism). PCBs may also be
attacked by enzymatic or other biological catabolic activities associated with but not
dependent on active microbial growth (as has been proposed as the mechanism of anaerobic
dehalogenation).
Degradation in soils
Agricultural and upland forest soils are typically aerobic. General aerobic bacterial
metabolism is considered in more detail in a subsequent section. Several studies have
determined the biodegradability of PCBs in the soil environment, as summarized in Table 3.
Some studies have suggested an enhancement of PCB biodegradation in soils by the
addition of an growth substrate with an structure analogous to that of PCB, specifically
biphenyl. Biodegradation of PCB in an agricultural soil was enhanced by addition of 3,300
ppm biphenyl (Focht and Brunner, 1985). Without the addition of biphenyl, only 2 percent of
the added PCB was mineralized to carbon dioxide and 92 percent was recovered; with
biphenyl 48 percent was mineralized and 25-35 percent remained. Inoculation with a
bacterial strain known to degrade certain PCB congeners, Acinetobacter P6 (1x109 cells g-1),
did not enhance the rate or amount of PCB degradation, but did cause the onset of
degradation to occur 5 d earlier.
7
NBMA Summary - Organics
In parallel work, the rate of biodegradation of PCB in soil was not enhanced by
inoculation with PCB Acinetobacter P6 (Brunner et al., 1985). Lag times were reduced by
10-30 d, but the ultimate amount of PCB degraded was not changed. Total carbon dioxide
production from PCB was small in the unamended soils (1.3 percent) and increased by
additions of 5g kg-1 biosolids (3 percent) or 3.7 g kg-1 biphenyl (20-27 percent); but not 20 g
N kg-1 straw. Anaerobic incubations of this normally aerobic soil resulted in no PCB
transformations, or production of CO2 or CH4 from PCB.
Negligible biodegradation was observed when PCBs were applied in an aerobic simulated
overland flow soil system (Pardue et al., 1988). Less than 2 percent of the total PCBs applied
were recovered as carbon dioxide (using radiolabeled PCBs). However since 32 percent of
the radioactivity recovered from the soils was more hydrophilic than expected of PCBs the
authors suggested that biotransformation had occurred. These putative products were more
mobile than the authentic PCBs, since more were found in lower soil horizons
Aerobic bacterial degradation
General aerobic bacterial metabolism of PCBs will be discussed in four parts: degradation
in mixed microbial suspensions, degradation in pure cultures, cometabolic degradation, and
the products of bacterial degradation. The factors which affect bacterial degradation of PCBs
are summarized in Table 4.
Biphenyl, monochlorinated PCBs, Aroclor 1221 in lake water samples were degraded
with biphenyl being degraded the fastest, followed by 2-monochlorobiphenyl, then 4monochlorobiphenyl, and Aroclor 1221 (Wong and Kaiser, 1975). Aroclors 1221 and 1242
stimulated bacterial growth, but Aroclor 1254 did not stimulate growth or undergo any
detectable breakdown.
In aerobic activated biosolids suspensions from municipal wastewater treatment less
chlorinated PCBs were degraded but highly chlorinated PCBs persisted (Tucker et al., 1975).
After a 5 month acclimation period mono- and di-chlorinated congeners were easily
degraded; but tri- and tetrachlorinated congeners were attacked slowly. The percent
degradation decreased from 80 to 20 percent as the percent chlorination increased from 20 to
55 percent.
Degradation of PCBs by aerobic bacterial isolates generally follows the 2,3-dioxygenasemeta-cleavage pathway, as shown by the results of the following studies.
Achromobacter strains BP and pCB were isolated on biphenyl or p-chlorobiphenyl,
respectively, as the sole source of carbon (Ahmed and Fotch, 1973). Both of these organisms
were able to oxidize biphenyl, p-, m-, and o-chlorobiphenyl, o,o'- and p,p'-dichlorobiphenyl.
There was no Cl- produced, so net destruction of PCBs did not take place. Intermediate
products included an unknown, UV-absorbing compound produced from biphenyl,
metacleavage products, and benzoic and p-chlorobenzoic acids.
8
NBMA Summary - Organics
Two bacterial species, Nocardia and Pseudomonas strains, were grown on unchlorinated
biphenyl and were able to degrade 50-100 percent of 2,4'-; 2,3-; 3,4-di-; 2,3,2'-; 2,3,4'; and
3,4,3'-tri-chlorobiphenyl; but could not degrade 4,4'-di-; 2,5,4'-; 2,4,6-tri-; or any tetra- or
hexa-chlorobiphenyl (Baxter et al., 1975). The less chlorinated Aroclor 1016 was degraded
more completely than Aroclor 1242, which has an average of four chlorines per biphenyl.
The Pseudomonas strain could degrade fewer congeners than the Nocardia strain.
Bacterial isolates, Alcaligenes Y42 and Acinetobacter P6 were found to degrade 23 and
33 PCB congeners with 1-5 chlorines, respectively (Furukawa et al., 1979). As shown in
Figure 1 degradation generally proceeded via a metacleavage pathway after attack by 2,3dioxygenase: through 2',3'-dihydro-2',3'-diol, by dehydrogenation of the 2'3'-dihydro
compounds, to 2',3' metacleavage to chlorinated 2-hydroxy-6-oxo-6-phenyl-hexa-2,4-dienoic
acids, ending in chlorobenzoic acids. As is typical of the aerobic degradation of chlorinated
aromatics ring cleavage occurs before dechlorination. Chlorinated hydroxy compounds
accumulated mostly from 2,4'-PCBs. Alcaligenes Y42 degraded 2,4,6-tetrachlorobiphenyl to
3-chlorobenzoic acid.
Figure 1
Products of aerobic degradation of PCBs by Alcaligenes Y42 and Acinetobacter P6 using the
metacleavage pathway
Cl n
Cl n
Cl n
Cl n
Cl n
H
OH
Cl
Cl
OH
H
O
OH
COOH
OH
Cl
OH
COOH
Cl
Acinetobacter strain P6 was grown on 4-chlorobiphenyl and found to be capable of
degrading tri- and di-chlorobiphenyls in Kaneclors KC 200, 300, and 400 (similar to Aroclor
1221, 1016 and 1242), producing mono-, di-, and tri-chlorinated benzoates, dihydroxy
biphenyls with 1-4 chlorines, and ring-cleavage compounds with 1-3 chlorines (Furukawa et
al., 1983). Since chlorinated benzoates are not further degraded by Acinetobacter strain P6,
they accumulate. Additional unknown chlorinated products of tetrachlorobiphenyls
accumulated during the degradation of KC 400, which was largely incomplete after 4 hr.
Kaneclor KC 500 was not significantly degraded although some dihydroxy products of some
pentachlorobiphenyls were detected.
Although 4-chlorobenzoate is dead-end product of other aerobic bacterial PCB
degradations, a Pseudomonas strain has recently been isolated that grew on 4-chlorobiphenyl
and used 4-chlorobenzoate as a substrate (Barton and Crawford, 1988). In these cultures 4'-
9
NBMA Summary - Organics
chloroacetophenone accumulated along with 2-hydroxy-2-[4'-chlorophenyl]-ethane and 2oxo-2-[4'-chlorophenyl]-ethanol.
An aerobic oxidation pathway for PCB degradation that did not involve metacleavage
was found in Alcaligenes eutrophus strain H850 (Bedard et al., 1987a), which attacked 2-,
2,4-, 2,5-, 2,4,5-chlorobiphenyl; but not 4-chlorobiphenyl as does the 2,3-dioxygenase of
Corynebacter MB1 (Bedard et al., 1986). Strain H850 oxidized 2,5-dichlorobiphenyl to 2,3dichlorobenzoate (not to 2,5-dichlorobenzoate as does MB1) and 2,3,2',5-tetrachlorobiphenyl
to 2',3'-dichloroacetophenone. 2,4,5,2',4',5'-Hexachlorobiphenyl, which is highly recalcitrant
since it has no adjacent unsubstituted carbons, was also oxidized to 2,4,5trichloroacetophenone. Acinetobacter P6 also attacked congeners without an open 2,3position (Kohler et al., 1988).
PCB degrading isolates were grown on biphenyl and classified by the type of PCB
degraded (Bedard et al., 1986). Several defined congener mixes were tested, representing
PCB structure classes: single ring substitution; double para position; no free 3,4; no free 2,3
position; no adjacent unchlorinated; 2 or more ortho chlorines, steric hindrance, and PCBs
that are not attacked by Acinetobacter P6. Most of the 25 strains screened used 2,3dioxygenase in their attack on PCBs. LB400 and H850 were the most competent, but MB1
was the best degrader of 2,4,5,2',3'-pentachlorobiphenyl and 2,4,3',4'-tetrachlorobiphenyl.
There was little or no degradation of 2,4,5,2',4',5'-hexachlorobiphenyl by any strain.
Strain 850 of Alcaligenes eutrophus was isolated from PCB-contaminated dredge spoils
from the Hudson River by aerobic growth on biphenyl (Bedard et al., 1987b). Strain 850
grew on 2-dichlorobiphenyl and degraded tetra-, penta-, and hexa-chlorobiphenyls (including
ortho congeners) and 85 percent of Aroclor 1242 (38 out of 41 components) and 35 percent
(15 out of 44) Aroclor 1254 in 2 days.
Aerobic cometabolism of PCB
The studies cited above have delineated the pathways of PCB degradation by resting,
non-growing cells. Most PCBs are not used as a growth substrate during their degradation. A
second substrate, such as biphenyl, is required to support growth. Such no-growth
decomposition is called cometabolism. The presence of the growth substrate would be
expected to affect PCB degradation. Nocardia and Pseudomonas strains were able to degrade
mono-, di- and trichlorobiphenyls faster and 2,5,4'-trichlorobiphenyl was degraded only in the
presence of biphenyl (Baxter et al., 1975). Kohler et al. (1988) found that PCB degradation
by Acinetobacter and Arthrobacter species actively growing on biphenyl was more extensive
and faster than that of resting cells (without biphenyl). Growing Acinetobacter P6
suspensions attacked 25 out of 40 Aroclor 1254 congeners, but only 19 congeners were
attacked by resting cells. Growth of Acinetobacter P6 on biphenyl was inhibited by 10 ppm
Aroclor 1254.
Some of the PCB degradation in soils is due to bacteria working in concert. One species
may partially oxidize PCBs to an intermediate which is further oxidized to carbon dioxide by
10
NBMA Summary - Organics
another species. An example of this cooperation of bacterial cocultures is the combined
activity of Acinetobacter strains P6 and 4CB1 (Adriaens et al, 1989). Acinetobacter 4C1 was
isolated by growth on 4-chlorobenzoate, which accumulates from the degradation of 4,4'dichlorobiphenyl. In the presence of both strains 4,4'-dichlorobiphenyl is mineralized to
carbon dioxide. In similar experiments, cocultures of Acinetobacter P6 or Arthrobacter M5
(both capable of degrading PCBs to chlorobenzoates) and a chlorobenzoate-degrading
Pseudomonad were able to completely degrade mono- and 3,5-di-chlorobiphenyl to carbon
dioxide (Furukawa and Chakrabarty, 1982).
The genes for unusual degradative pathways in bacteria may often be carried on plasmids
that can be transmitted from one species or strain to another. A self-transmissible plasmid
was found to mediate complete mineralization of 4-chlorobiphenyl in two PCB-degrading
strains: Acinetobacter P6 and Arthrobacter M5 (Furukawa and Chakrabarty, 1982; Hooper et
al., 1989). Plasmids of unknown function have been found in other PCB-degrading strains
(Chatterjee and Chakrabarty, 1976; Bedard et al., 1987b). Sequence similarities have been
found in the genes encoding PCB degradation by Pseudomonas LB400 and A. eutrophus
H850, which have both 3,4- and 2,3-dioxygenases (Yates and Mondello, 1989). It was
suggested that there are two classes of plasmids coding for PCB degradation.
Anaerobic degradation
Reductive dehalogenation under anaerobic conditions has been found for organics
containing three or more chlorines, starting with observations of the conversion of the
pesticides DDT and lindane to less chlorinated forms in anaerobic sediments (Fries, 1972).
The degradation of PCBs under anaerobic conditions has recently been discovered for
sediments that are heavily contaminated with PCBs. Sediments that contain more than 50
ppm PCB were found to be depleted in highly chlorinated congeners and enriched in monoand dichlorinated biphenyls (Brown et al., 1987; Brown et al., 1984). Earlier findings of no
anaerobic PCB transformations in environments with lower levels of PCBs (Fries, 1972;
Kaneko et al., 1976) and the lack of activity in pristine sediments exposed to PCBs for the
first time (Quensen et al., 1988) suggest that anaerobic PCB degradation is a rare
phenomenon requiring extended acclimation.
Anaerobic microorganisms that were eluted from PCB-dehalogenating Hudson River
sediments reduced the content of tetrachlorobiphenyls by 90 percent in four months (Quensen
et al., 1988). Reductive dehalogenation was observed at 700 ppm PCB, but not at 14 ppm.
Para and meta congeners decreased while ortho (2,6 substitutions) congeners increased. It
was suggested that reductive dechlorination serves as a sink for electrons produced during
anaerobic fermentation. Although the observed rates are slow the authors proposed that
sequential anaerobic/aerobic biodegradation of PCBs would yield more thorough reduction of
all congeners and reduce the toxicity of the highly chlorinated PCBs. A problem with this
concept is that anaerobic degradation is halted at concentrations less than 50 ppm, a level
which is environmentally unacceptable.
11
NBMA Summary - Organics
Other workers have found that PCB degradation in anaerobic sediments is enriched by
addition of biphenyl and that dehalogenation of highly chlorinated PCBs proceeded without
accumulation of lower chlorinated PCBs (Rhee et al., 1989). There was no dehalogenation
under an atmosphere of H2/CO2. All biodegraded congeners lacked adjacent chlorines on
both rings and the best degraded congeners had no adjacent chlorines on either ring.
Degradation by ligninase
The remaining class of PCB biodegradation is the nonspecific attack on aromatics catalyzed
by the ligninase enzyme produced by Phanerochaete chrysosporium, the white-rot fungus.
Tetra- and hexachlorobiphenyls were degraded to carbon dioxide by Ph. chrysosporium
(Bumpus et al., 1985; Eaton, 1985). Ph. chrysosporium ligninase was also capable of
mineralizing dioxin, pentachlorophenol, p-cresol, naphthalene, phenanthrene,
benzo[a]pyrene, chlorobenzoic, chlorophenol, chloroguaiacol, chloroaniline, chlorovanillin,
DDT, Lindane, Chlordane, Mirex, and Methoxychlor (Bumpus et al., 1985; Bumpus and
Aust, 1987; Aust and Bumpus, 1987). Application of this promising method is hindered by
the stringent requirements for the expression of ligninase by Ph. chrysosporium: low nitrogen
and high glucose.
Reference: Henry, C., and R. Harrison. 1998. Environmental Effects of Biosolids
Management. Trace Metals: Potential for Movement and Toxicity from Biosolids
Application, Effects on Wildlife and Domestic Animals from Biosolids Application, Air
Emissions and Ash Resulting from Incineration of Biosolids, Nitrogen Cycle and Nitrate
Leaching from Biosolids Application, Microbial Activity, Survival and Transport in Soils
Amended with Biosolids, The Fate of Trace Synthetic Organics in Biosolids Applied to
Soil, Runoff Water Quality from Biosolids Application, Effects of Organic Residuals on
Poplars. Northwest Biosolids Management Association.
12