NCD980602163_19831201_Warren County PCB Landfill_SERB C_Environmental Transport and Transformation of PCBs-OCR

ENVIRONMENTAL TRANSPORT AND TRANSFORMATION
OF POLYCHLORINATED BIPHENYLS
by
Asa Leifer, Robert H. Brink, Gary C. Thom, and
Kenneth G. Partymiller
U.S. Environmental Protection Agency
Office of Toxic Substances
Washington, DC 20460
December, 1983
EPA 560/5-83-025
I.
I I.
III.
IV.
v.
VI.
VII.
VIII.
IX.
C8APTER 9
BIODEGRADATION OF CHLORINATED BIPHENYLS
by
Robert H. Brink
Contents
I:-lTRODUCTION AND SUMMARY.
1"10NITORING EVIDENCE ..
BIODE.GRADATION RATES.
A. General •••
Anaerobic ...... .
Aerobic Aquatic.
B.
c.
D.
E.
Biological Waste Treatment •.................•
Soil.
ANAEROBIC BIODEGRADATION,
PURE CULTURE STUDIES .....
SORPTION.
VOLATILIZATION.
OTHER FACTORS,
REFERENCES,,,,
9 -i-
P?.l.ge No.
9-1
9-3
9-4
9-4
9-5
9-5
9-8
9-11
9-12
9-13
9-15
9-16
9-17
9-19
' ' ..
I. HTTRODUCTION AND SUMMARY
A review of the available literature on the degradation of
chlorinated hiphenyls by microorganisms discloses many
conflicting findings and conclusions. There are, however, some
discernable patterns. It is quite clear that there are numerous
aerobic microorganisms in the environment that are capable of
deqrading most of the chlorinated biphenyls oresent in commercia l
PCB products and that such organisms are wi rl ely distrihute~ in
t he environment. It is also evi dent t hat rates of h i odeararla~ion
are related to both the cieqree of chlorination of the hipheny 1
structure and the positioning of those chlorines on the biphenyl
rings.
There is no evidence for biodegradation of PCBs under
anaerobic conditions and this might be quite important given the
high degree of sorption to solids and the li keli_hood that many of
those solids will reside in the environment under anaerobic
conditions.
With respect to the degree of chlorination, as a hroad
generalization biodegradation proceeds more slowly as more
chlorines are added to the biphenyl. Given the info:rmation in
section III on biodegradation rates, it is possihle to arrive at
some general conclusions about potential biorlegradation rates in
various environments, but it must be emphasized that these are
9-1
broad generalizations and that half lives in particular
environments for specific chlorinated bi~henyls may be much
larger due to certain limiting environmental variables (e.g. low
temperatur~s, low moisture, pH extremes) or the s9ecific PCB
structure.
.~robic
Surface Waters-
E'!'esh.
Oceanic
~tivated Sludge
SOil
Anaerobic
aalf Lives Resultin:; fran 3iodeqracation
Mono-& dioloro Trichloro Tetrachloro ~ntachloro
and h is;her
2-4 days S-40 days l wk-2-+ffOs. >l year
----several 11Cnths----
1-~ days
6-10 days
2-3 days 3-5 days
12-30 days
OQ·
>l year ---
>l year
..
-1r. It is not clear how lon·g the highly chlorinated PCBs would last
under activated sludge treatment but there a1;,i_:,ears to. be no
significant biodegradation during typical residence times.
These half-life approximations also must be tempered by the
kna,,1ledge that positioning of the chlorine atoms on the biphenyl
rings can be important. It has been shown, for example, that
( l) PCBs containing all of the chlorines on one -ring are degraded
faster than PCBs c.ontaining all of the chlorines distributed ovel:'
both rings, ( 2) PCBs containing chlorine on 2 or more ortho
9-2
~ositions degrade ve~J slowly, (3 ) c:~e resistance of tetrachloro
?C3s is Jreacer when there are 2 chlorines on each ring and, (41
c:he initial b iodegrada t ion step occurs on the b ipheny 1 ::-i ng wi t:i.
the fewest chlorines.
3iodegradation possibilities are also complicatect by the
fact that much of the ?CSs released to the environment is li~ely
to become tightly bound to sewage solids and sediments that are
under anaerobic conditions where biodegradation ~ill not be
si~ni:icant and by t~e 9ossibility tiat a substantial portion of
the more hi;hly-chlorinated, less ~ater-soluble PCSs will
evaporate into the at~osphere.
II. MONITORING SV!DENCE
Among the most convincing evidence for· the 9ersistence of
the more highly chlorinated bi9henyls in the environment is
actual monitoring data on various samples. There is a large
number of re9orts, mostly on samples of biota, that might he
cited and those presented here are not intended as a
compr~hensive listing.
Tucker et al. (1975) noted that the PCBs generally found in
the environment are those containing Sor more chlorine atoms per
molecule. This is strong evidence that the less highly
chlorinated bi9henyls degrade more rapidly because t,e l ess
highly chlorinated isomers constit'..lted about n5% of all c.lf the
PCBs manufactured.
9-3
Ballschmiter et al. (1978 ) identified more than 80 ?C3s in.
marine fish and found the ratios of ten major PCB components
(pentachloro and higher) in the fish were the same as the ratios
of those cong~ners found in ~roclor 1254 and Aroclor 1260. They
speculated' t.:1at this is t.:-ue because the differences in the
degradation rates of these highly chlorinated PCSs are too small
to produce any changes in their relative occurrence during the
ti:ne that they have been in the en11ironment ( up to 40 years).
Wszolek et al. (1979) analyzed lake trout in 197U and again
in 1978, from the same lake. They found PCBs similar to Arocl~r
1254, but with a higher proportion of more chlorinated congeners
at about 13 ppm at both sampling times. Moein et al. (1976)
reported no reduction in Aroclor 1254 concentration over a 2-year
period in a soil contaminated by a spill of transformer fluid.
III. BIODEGRADATION RATES
A. General
Biodegradation studies that re~ort rates of degradation come
in many sizes and shapes. Some used commercial mixtures, some
pure congeners and some used both. PCB concentrations employed
range from 5 ug/1 up to 500 mg/1. Many analyzed only for the
disappearance of parent compound(s), some for potential
intermediates, and a iew tor mineralization to co 2 and water.
Studies have been conducted using lake water, sea water, soils
and sewage, as well as various synthetic media, with a variety of
9-4
~icrobes and culture conditions. rhis hodge-9odge of approaches
.
makes it difficult to compare results and leads to skepticism
about some of the 9rocedures and conclusions. ~evertheless, i t
is possible to arrive at some conclusions with respect to
biodegradation rates. Those conclusions, prescnt~d below, are
mostly based on studies that used mixed microbial populations
• obtained from the environment and not those that employed pure
cultures. The pure culture work is discussed in section V.
B. Anaerobic
Biodegradation of the PCBs under anaerobic conditions is
probably very slow or nonexistent. This is discussed in more
detail in section IV.
C. Aerobic Aquatic
Biodegradation of mono-, di-and trichlorinated biphenyls is
probably moderately fast in most surface waters. Clark et al.
(1979), using bacteria isolated from soils in shake flask
cultures, found 100% primary biodegradation (loss of parent
compound) in less than 5 days for monochlorobiphenyl and 9U to
99% degradation of dichlorobiphenyl, 42 to 87% degradation of
trichlorobiphenyls and 6 to 61% degradation of tetrachlorobi~henyls
after 5 days incubation. They also examined the biodegradation
of Aroclor 1242 and presented dac.a on the J:Jercent biode<Jradation
of 38 congeners (identified by gc peaks) showing very substantial
loss of most of the mono-, di-and trichlorinated biphenyls
9-5
~ithin a 4a-hour incubation tine. Some oi the trichloro isomers
did not degrade r3pidly and these may be isomers with chlorines
in the ortho positions. Baxter et al. (1975), in shake flask
studies, found that most dichlorinated biphenyls had half lives
of less than 10 days and the trichlorobi~henyls were half gone in
20 to 40 days. ~ong and Kaiser (1975 ), using lake water in
sto99ered shake flasks found that Aroclor 1221 was degraded
completely, in about one month, to lower molecular weight
metabolites. They also demonstrated that biQnenyl deyraded
faster than 2-chlorobiphenyl which, in turn, degraded faster than
4-chlorobiphenyl. Shiaris and Sayler (1982), however, have shown
that the biodegradat..ion of the lesser chlorinated biphenyls in
natural waters may lead to the accumulation of chlorobenzoic acid
transformation products.
While t.he -evidence is that the less chlorinated bi~henyls
degrade readily in .asrobic freshwater, the same may not be true
for seawater. Carey and Harvey (1978) found very _low rates of
biodegradation of 2,5,2'-trichlorobiphenyl with only 1 to 4% loss
after· 25 days .in stoppered shake flasks. And Reichardt et al.
(1981), using closed bottles of seawater at 10°, estimated the
half-lives of biphenyl, 2-chlorobiphenyl, 3-chlorobiphenyl and
4-chlorobiphenyl. The biphenyl half-life, in their seawater, was
about 3 months, and that for the monochlorobiphenyls was about 8
months. Observati.ons of considerably slower biodegradations in
the oceans are not confined to biphenyls and may be related to
the low concentrations in seawater of certain essential elements,
especially nitrogen.
9-6
With respect to those PCBs with Sor more chlorines, it
ai;,9ears that: biodegradation is very slow in all environments
including surface waters. Oloffs et ~l. (1972) incubated Aroclor
1260 in ri~er water for u~ to 12 weeks and found no evidence of
biodegrada~ion. They did, however, observe significant losses by
evaporation. Shiaris et al. (1980 ) used re~ervoir water and
found no a9parent biodegradation of Aroclor 1254 during 2 months
incubation. They us~d sealed vessels and did ooserve that
significant amounts of the Aroclor sorbed tightly to the glass
vessel walls and to the suspended solids. Only about 20% of the
PCBs, initially added to a concentration of 0.1 mg /1, remain in
the aqueous phase. Wong and _Kaiser (1975) compared Aroclors
1221, 1242 and 1254 and found that the microorganisms in lake
water samples could use 1221 and 1242 for growth but were not
able to utilize 1254. In contrast to most reports, Sayler et al.
(1977), usi~g a pure culture of Pseudomonas sp., reported 70 to
85% biodegradation of 2,4,5,2,'4, ',5'-hexachlorobiphenyl in 10 to
15 days. This finding does ~ot seem to be consistent with the
evidence from other studies.
There is little information on the biodegradation of
tatrachlorobiphenyls in surface waters. In other media the
tetrachloro congeners dn the average, have biodegradation rates
that are intermediate to those with fewer chlorines, most of
which degrade quite readily, and those with 5 or more chlorines,
which are quite persistent. The data presented by Clark et al.
(1979) show that most of the tetrachloro congeners did not
degrade significantly in 48 hours in shake flasks. The work by
9-7
Carey and Harvey (1978) included 2,S,2' ,5'-tetrachlorobiphenyl
and they found lit~le, if any, biodegradation after 25 days. in
seawater. Given the results 6f studies u sing o ther media, it i s
9robably safe -to assume that the biodegradation rates of the
tetrachloro congeners are highly de9endent upon the positioning
of t he chlorines on the bi~henyl rings.
D. Biological Waste Treatment
Stud i es us ing acti vated s ludge o r sewige o r g anisms and
simulating biological waste treatment processes have shown that
the biodegradation rates of PCSs are de~endent upon both the
degree of chlorination and the location of the chlorines~ As
might be expected, however, the rates of biodeyradation, for the
readily biodegradable PC8s, are higher under waste treatment
conditions than in surface waters or soil.
Tucker et al. (1975) studied primary biodegradation by
activated iludge microorganisms using Aroclors 1016, 1221, 1242
and 1254 plus a non-commercial mixture, MCS-1043, containing
ahout 30% chlorine. After several weeks of acclimation, the PC8s
were tested in semi-continuous activated sludge units operated on
two 48-hour and one 72-hour cycles per week. They observed 100%
biodegradation of biphenyl, 80% for 1221, 55% for MCS-1043, 35%
for 1016, 25% for 1242 and 15% for 1254 in 48 hours. They also
claim no significant losses of 1221, 1043 or 1016 through
9-8
volatilization. Zitco (1979) noted that the data ot Tucker
et al. (1975) show a decreasing rate of biodegradation with
increasing chlorine content that has the foll owing relationshi~-
R = 1 0 6 (± 6 ) -1.7 (:: 0.2 )0
where R =%degraded in 4 8 hours and D =, chlorine
The evidence, however, indicates t h at t hose ?C8s wit h ., o r
more chlorines degrade too slowly to allow any practical
application of that relationship to them. Also, it must be noted
that individual tri-and tetrachloro congeners degrade at rates
that are highly dependent upon the location of the chlorines on
the biphenyl rings.
In contrast to the work of Tucker et al. (1975), Kaneko
et al. (1976), using semi-continuous activated sludge units,
following 3 months of acclimation, found no biodegradation of
Kanechlor-500 (KC-500), a Japanese PCB product with an average
chlorine content of 50%. In this paper the authors claim that
the PCBs sorbed rapidly to the sludge solids and that losses of
PCBs from their units were due almost entirely to losses by
evaporation and losses of material sorbed to wasted sludge
sol ids. Herbst et al. ( 19 77) also concluded that there was
little biodegradation of Aroclor 1221 in activated sludge and
that the PCBs were distributed unchanged between the water and
9-9
sludge solids. However, they used relatively short test ~eriods
of 6 hours and there is no discussion of 9rior acclimation, which
~ay be important.
Tulp et al. (1978) described the use of activated sludge
inocula in shake flask and PCBs at 50 mg /1 (well above the water
solubility ). Some of the flasks were su~9lemented with 500 mg /1
additions of other carbon sources such as glucose, 9e~tone or
h u~ic acid. T~ey reported that the microbial ~opulations did not
degrade any of the ?C8s during 14 days of incubation when there
were three or more chlorines on the bi~henyl rings. They also
noted that the additions of other carbon sources dramatically
reduced the biodegr~dation of 4,4'-dichlorobiphenyl. There are
too few details on their experimental work to permit a good
evaluation of their findings, but it is interesting to note that
their tes~ PCBs with three or more chl~ri~es were the 2,4'5-
trichloro-, 2,2' ,S,S'-tetrachloro-, 2,2' ,3,4,5,5'-hexachloro-and
decachlorobiphenyls. Other evidence (Furukawa et al. 1978b)
shows that those congeners with chlorines in any two ortho
positions are degraded poorly.
Liu (1981), using a bench-scale fermenter and sewage
inoculum, found that the half-life of Aroclor 1221 was highly
dependent on the rate of mixing in the fermenter. His data show
that the half-life was a logarithmic function of impeller speed
between O and 800 r?m. At the top speeds, the half-life of 1221
was about 2 days. Liu also claimed no more than 10~ loss of
Aroclor 1221 through volatilization, over a 10-day test period.
9-10
E. Soil
As in water. and sewage sludge, the biodegradation of PCBs in
most soils ap9ears to be rapid for the less chlorinated ones and
increasingly difficult with increasing chlorination. In their
review on the fate of PCBs, Pal et al. (1980) categorized
decomposition rates in soils in three groups. Grou9 l is for
chlorinated bi~henyls with 2 or fewer chlorines ~er molecule and
Baxter et al. (1 975 ) have shown that these de~rade ra9idly with
half-lives of about 8 days. The second group contains the tri-
and tetrachloro PCBs which have half-lives of 12 to 30 days. The
third group, those with 5 or more chlorines, have half-l~ves in
excess of one year. As with the biodegradation of any chemical
in soils, biodegradation rates will vary greatly and depend upon
the nature and viability of the microbial populations, the
presence of other degradable organic matter, the moisture and
oxygen content of the soils, pH, temperature and other
environmental variables.
Griffin et al. (1978) also described the fate of PCBs in
soils but the section on biodegradation is not easy to follow and
presents data indicating a high percentage of biodeyradation of
tetrachloro PCB congeners (up to 99%) in only 20 hours. This
seems unlikely. Fries and Marrow (1982), on the other hand,
reported that only about 20% of labelled biphenyl and
monochlorobiphenyls could be accounted for as 14co 2 after 98 days
in silt loam soil.
9-11
!V. ANAEROBIC BIODEGRADATION
There is no evidence in the literat~re that the ?CBs ara
degraded by microorganisms under anaerobic conditions. Carey and
narvey (1978), :<aneko et al. (1976 ) and Pal et al. (1980) discuss
anaerobic studies with PCBs, and there is no indication of
anaer~bic ~iodegradation. This seems somewhat sur?rising since
dehalogenation is a commonly observed :-eaction for o'ther organics
under anaerobic conditions, for exam9le ~ich DDT and he9tachlor.
On the other hand, when DDT is transformed anaerobically to DOE
or DOD, the dehalogenation removes only one chlorine from the
ethane grou9 and the products are more stable than the original
DDT. It may be that, even if there is some reductive
dehalogena tion with PC'Bs-, the trans format ion ;;,reduct would be
very stable and that the investigations conducted to date have
not looked for those kinds of transformations. At any. rate, the
resistance to biodegradation under anaerobic conditions is
;irobably quite significant. rt is likely that much of the PCBs
released to the environment are rapidly bound to particulate
matter and stored under anaerobic conditions in sewage sludges
and sediments. McIntyre et al. ( l98lb) found that about 83~ of
~roclor 1260 found in digested sludge was retained on that sludge
after chemical conditioning and dewatering ste9s and, as
discussed in section VI, there is considerable ~vidence that P~3s
can sorb ra9idly to the sludge solids in sewage and waste
I treatment plants. Overall, it apl:Jears from the evidence that an
im9ortant fraction of the PCBs released to the envi:onment will
9-12
r
become tightly bound to particulate matter in sewage sludges, in
sediments and in flooded soils, where anaerobic conditions will.
prevent or greatly slow degradation by microorganisms.
V. PURE CULTURE STUDIES
~uch of the literature on the biodegradation of PCBs
describes studies made using 9ure cultures of microorganisms.
Those studies are of considerable value in elucidating the
potential .pathways of biodegradation and the relative rates of
biodegradation of various isomers. They do not 9rovide much of
value in assessing the environmental biodegradation rates of
specific congeners.
Lunt and Evans (1970), using pure cultures of gram-negative
bacteria, described the transformaton of biphenyl into
2,3-dihydroxybiphenyl. Ahmed and Focht (1973) subsequently
demonstrated the biodegradation of 3-chloro-, 4-chloro-,
2,2' dichloro-and 4,4'-dichlorobiphenyl by Achromobacter sp. and
proposed a hypothetical pathway going from the PCB to a
dihydroxychlorobiphenyl followed by ring opening and degradation
to chlorinated benzoic acids. Other pure culture studies
(Furukawa and Matsumura 1976, Furukawa et al. 1978a, Yagi and
Sudo 1980, Ballschmiter 1977, Ohrnori et al. 1973, and Wallnofer
et al. 1973) tend to confirm this general pathway and have
sup9lied additional details. The potential pathways of aerobic
biodegradation are not very relevant to this review and will not
be described in any detail. It does appear, however, that the
9-13
dihydroxylation requires oxygen and occurs on the less
chlorinated ring when there is uneven distribution of the
chlorines. It may be that the a~pearance of chlorines in two or
more ortho positions sterically hinders th~ dihydroxylation
step. It also seems that the dihydroxylation may be accomplished
by an electrophilic form of oxygen and that the electron-
withdrawing nature of the chlorines su9resses that initial
biodegradation ste9.
Pure culture studies have also hel~ed in demonstratin~ not
only that increasing levels of chlorine lead, in general, to
decreasing rates of biodegradation, but also that the
biodegradability is influenced by the 9ositioning of the
chlorines on the biphenyl rings. Furukawa et al. (1978b) studied
31 PCB congeners and demonstrated that '(l) the resistance of
tetrachloro PCBs is greater when there are cwo chlorines on each
ring, (2) PCBs c~ntaining chlorine on 2 or more ortho ~ositions
(of either ring or both) are very resistant, (3) PCBs containing
all Qf the chlorines on one ring are degraded faster than those
with the same number of chlorines distributed over both rings,
and (4) hydroxylation and ring fission occur preferentialli on
the biphenyl ring with the fewest chlorines. Furukawa et al.
(1979), using Alcaligenes and Acinetobacter sp., have also shown
that the positioning of chlorines has an effect on the metabolic
pathways and kinds of degradation ~roducts formed. Liu (1982),
using a Pseudomonas species in a closed fermenter, also observed
9-14
· that the number of chlorines and the 9osition of the chlorines on
the rinys are important factors in the. relative biodegradatian
rates of ?Ci::!s.
VI. SORP'l'ION
The 9receding sections on biodegradatian in various
environments are complicated by the fact that a large ~roportion
a f the PCBs re leased ta the environment ·,; i 11 sorb tightly to the
surfaces of sewage solids, sus9ended matter in surface ~aters and
various sediments and may not be available eor degradation by
microorganisms in sewage treatment ;ilants or in natural .waters.
Whila the subject of adsor?tion of PCBs is covered in detail
elsewhere in this reviewr it is important to keep this ~henomenon
in mind when considering biadegradation potential and to consider
some of the findings of those who were ~rimarily investigating
b iodegrada t ion.
furukawa et al. (1978a), Bourquin and Cassidy (1973),
Gresshoff et al. (1977), Kaneko et al. (1976) and McIntyre et al.
(1981b) all noted, in connection with microbial studies, the
highly SOr?tive nature of the PCBs. Gresshoff et al. (1977)
Sl:)eculated that much of the PCBs in the envi:::-onment ,,;ould tend to
adsorb to rocks or sand or soil surfaces and to resistant
organisms. They go on to suggest that those sorbed ?CBs might ~e
remobilized by other organic pollutants, such as ()il spills.
Colwell and Sayler (1977) noted that ?CBs in the environment will
be ~artitianed into sus~ended sediments, ails and surface
9-15
f il.ms. Studies at sewage treatment plants have shown that PCBs
are ~rin~i9ally remove9 form wastewater during sludge settling
steps. McIntyre et al. (1981b) demonstrated that PCBs will be
closely associated with the settled solids in sewage treatment
and that those PCBs will be retained on the solirls during
chemical conditioning and dewatering steps. fifty 9ercent or
~ore of the ?CBs in raw sewage may be associated with the solids
removed in ;::irimary sedi:nentation. ( :<1cintyt'e et al. 198 la,
Garcia-Gutierrez et al. 1982). Shiaris et al. ( 1980 1 , in a study
on extraction techniques for residual ?C3s, found that when J.l
mg/1 ;::ireparations of Aroclor 1254 were incubated for 4 to 8
weeks, most of the PCB became tightly bound to 11essel walls and
9articulates in the water. Marinucci and Bartha, in a study on
the accumulation of Aroclor 1242 in 9ercolators containing a
shredded :narshgrass (Soartina sp.) demonstrated that ?CB
accumulation in the litter was significantly enhanced by the
~resence of litter -decaying microbes and concluded that a
significant fraction of the PCB in the litter was contained in
the microbiota.
VII. VOLATILIZATION LOSSE~
There are many questions that come to mind when reviewing
the literature on PCB biodegradation. An important one co~ce~ns
the possibility that losses due to volatilization may have been
reported as losses due to biodegradation. Baxter et al. (1975 )
and Tucker et al. (1975) asset"t that their studies contained
checks on vol3.tilization losses and that there 'N'ece no
9-16
significant losses to the air. Liu (1981) demonstrated no more
than 10% loss of Aroclor 1221 due to volatilization during 10
days of stirring in a bench-top fet"mentor. It should :Je noted,
howevec, that theic claims for n·o significant volatilization
losses are limited to the less-chlocinated, more Nater-soluble
PCBs. On the other hand, Kaneko et al. (19 7 6) and Oloffs (1972 )
cepocted very high levels of e•,aporative losses of :<anechlor-300
and Arocloc 1260 i.n their studies. :1any of the o i odegradation
studies i.n the literature (e.g. furukawa et al. 1978b, Sayle::-
et al. 1977, Tul;, et al. 1978 ) ar-e not desc:-ioed i.n suffic i ent
detail to allow the reader to detec:nine whether oc not the
investigators accounted for potential evapocative losses.
VIII. OTHER FACTORS
Th~re are several other factors in the literature on PCB
biodegradation that caise questions about the validity of certain
studies or present the reviewer -,;,ith conflicting conclusions.
Most of them do not alter significantly the general conclusions
~resented above, but they should be kept in mind by those who
might attempt to obtain better evaluations of biodegradation
possibilities oc more reliable cate predictions.
Among the moce interesting factocs is the indication that
PCBs can affect the metabolic Qrocesses of microocganisms.
Kaneko et al. (1976) and Wong and Kaisec (1975) ce9orted the
stimulation of microbial cespiration by PC9s at concentrations as
low as l ug/ l. Kaneko et al. ( 19 7 i5 l suggested that the PCBs
9-17
might act as uncou9let's of oxidative 9hosphor:ylation. These
findings cast some doubt on the work of others who used
:-es9 irome tric techniques to study PCB b iodegi:ada t ion (e.g. .;hmed
and E'ocht 1973 and Sayle!:' et al. 1977).
Othet' unresolved factot's include the influence of other
degi:adable organic matter. Clack et al. (1979 ) an<i Y'agi and .Sudo
( 1980 ) :ound t:1at ?CBs degraded better in the 9resence of othe::-
substi:aces (acetate, meat extract or ~e9tone ). Tul9 et al.
( l '3 78 ) , on the other hand, found that the 9resence of othe::-
:arjon sources (glucose, 9e9tone, glycerol, yeast extract or
humic acid) led to dramatically reduced biodegradation. Some
C'esearchers have obseeved fastet' biodegradation by 9ure cultures
than by mixed cultures (Tulp et al. _1978 and Sayler et al. 1977)
while others have noted the opposite (Clark et .al. 1979). Liu
(1981) ::-eported the isolation of a Pseudomonas sp. that degraded
Aroclor 1221 ten times faster than sewage organisms, and he
proposed the use of that steain to seed biological treatment
plants.
One area that needs to be investigated mote fully is the
::-ale that acclimation may play in enhancing the rate and extent
of biodegradation of those PCBs that are relatively
biode~radable. It would also be very intet'esting to investigate
anaerobic ~recesses more fully to find out if t'eductive
dechlorinations do occur and what would hap9en to PCBs in
environments exposed to alternating aerobic and anaet'obic
conditions.
I 9-18