Ecotoxicity and biodegradation of phthalate monoesters

Article abstract of DOI:10.1016/S0045-6535(03)00668-4

Little is known about the fate and the effects of phthalic acid monoesters. Various of these monoesters ranging from n-butyl to isononyl monoester have been evaluated in respect to their biodegradation behaviour and their acute aquatic toxicity. All esters are readily biodegradable, achieving degradation rates of 90% and more. The acute toxicity values strongly depend on the carbon chain length of the alcohol moiety. The short chain specimen have LC/EC ₅₀ around and above 100 mg/l, with values levelling off to around 30 mg/l for the isononyl monoester.

Full text of DOI:10.1016/S0045-6535(03)00668-4

Chemosphere 53(2003) 921–926
www.elsevier.com/locate/chemosphere
Ecotoxicity and biodegradation of phthalate monoesters
*
Norbert Scholz
OXENO Product Safety, Oxeno Olefinchemie GmbH, Paul-Baumann-Strasse 1, P.O. Box 1320, D-45764 Marl, Germany
Received 6 November 2002; received in revised form 13June 2003; accepted 2 July 2003
Abstract
Little is known about the fate and the effects of phthalic acid monoesters. Various of these monoesters ranging from
n-butyl to isononyl monoester have been evaluated in respect to their biodegradation behaviour and their acute aquatic
toxicity. All esters are readily biodegradable, achieving degradation rates of 90% and more. The acute toxicity values
strongly depend on the carbon chain length of the alcohol moiety. The short chain specimen have LC/EC50 around and
above 100 mg/l, with values levelling off to around 30 mg/l for the isononyl monoester.
Ó 2003Elsevier Ltd. All rights reserved.
Keywords: Phthalate monoester; Biodegradation; Aquatic toxicity
1. Introduction
term (eco-) toxicological studies the effects observed are
the result of the internal concentrations of the diesters
and their corresponding monoesters, nothing is known
on the short term effects of phthalic acid monoesters.
The following paper describes the short term effects of
various of these substances. Phthalate esters are readily
mineralized to carbon dioxide and water, so one would
not expect any stable intermediates. However, the bio-
degradation behaviour of the monoesters have been
evaluated, too.
Phthalates are diesters of phthalic acid and an alco-
hol moiety. The carbon chain length of the alcohol for
commercially relevant and available phthalates on the
market can vary from one carbon atom up to 18 carbon
atoms. Depending on their lengths and their degree of
branching phthalates display different use scenarios.
These range from adhesives, paints and lacquers to
plasticiser for PVC, the latter application being the most
prominent use category for the longer chained phthalate
esters (Wickson, 1993; Wilson, 1995). The monoesters
are of no commercial value, they only exist as a tran-
sient step during synthesis, but are not isolated. On the
other hand, the (long-term) toxicological properties of
the diesters are mainly determined through the corre-
sponding monoester (Yagi et al., 1980). These can be
formed through microbial processes, biotransformation
by higher organism as well as through abiotic degrada-
tion processes. Whereas one can assume that in long
2. Material and methods
2.1. Synthesis of monoesters
Monoesters of phthalic acid are not commercially
available. They were synthesised by thoroughly mixing
1 mol of phthalic acid anhydride and 1 mol of the corre-
sponding alcohol at room temperature. The temperature
in the reaction vessel then slowly is raised to approxi-
mately 115 °C. Monoesters are formed auto catalytically,
with exothermic heat formation. Once the temperature
has reached 135 °C, the reaction mixture is cooled down
rapidly in an ice bath. This prevents the excessive for-
mation of diesters.
* Tel.: +49-2365-49-2663; fax: +49-2365-49-2670.
E-mail address: norbert.scholz@degussa.com (N. Scholz).
0045-6535/$ - see front matter Ó 2003Elsevier Ltd. All rights reserved.
doi:10.1016/S0045-6535(03)00668-4

922
N.Scholz / Chemosphere 53 (2003) 921–926
The following monoesters were synthesised: mono-
solids, the longer chain monoesters form clear, slightly
viscous fluids with a specific weight >1. The octanol
water partition coefficient can be taken as a first surro-
gate for the bioaccumulation potential of a substance.
As can be seen from Table 1, the calculated log Pow
values (EPIWIN) increase with increased carbon num-
ber of the alcohol chain.
n-butyl phthalate (MNBP), mono-isobutyl phthalate
(MIBP), mono-2-ethylhexyl-phthalate (MEHP), mono-
isononyl phthalate (MINP), mono-n-hexyl/n-octyl/
n-decyl-phthalate (M6=8=10P) and mono-n-octyl/n-decyl-
phthalate (M8=10P). Both the short chained monoesters
were white and solid at room temperature, whereas the
long chained ones appeared as clear, slightly viscous
fluids. The purity ranged between 92% and 94%, the
remainder mainly being diesters, phthalic acid and al-
cohol.
Fish (Cyprinus carpio), daphnids (Daphnia magna) and
algae Scenedesmus (Desmodesmus) subspicatus served as
test species in the acute toxicity tests with various of the
monoesters. All data obtained are summarised in Table 2.
The three species seem to be equally sensitive with the
algae species being slightly more on the less sensitive side.
Carbon chain length has a marked influence on the tox-
icity. It is obvious that with increasing carbon chain
length of the alcohol moiety toxicity also increases. This
influence is such strong that the linear C6=8=10 monoester is
less toxic than the corresponding C8=10 monoester, which
lacks the hexyl moiety in its composition. The modulating
effects of the carbon chain length are more clearly pro-
nounced in fish and daphnids than in the alga species. A
difference between the toxicity values of branched and
linear monoesters is not obvious. For that reason, the
mean LC50/EC50 values had been taken for calculating
the relationship between the alcohol carbon chain length
and threshold value. Fish and daphnids were evaluated
separately.
2.2. Biodegradation
The ultimate biodegradation of a substance can be
measured through the generation of carbon dioxide or
through oxygen consumption. The test system employed
here, was the CO2 evaluation test (OECD, 1992) which
forms part of the OECD biodegradation test battery for
ready biodegradability. The low number of non-accli-
matised bacteria do not only prevent false positive re-
sults, they also allow a direct comparison with diester
results published previously. All tests were performed
following the guidelines of GLP (1999).
2.3. Ecotoxicity
All tests were performed following the corresponding
OECD testing guidelines (OECD-TG). For the acute
fish toxicity (OCED, 1982) Cyprinus carpio (common
carp) served as test specimen. Daphnia magna (water
flea) represented the invertebrate species, the test fol-
lowed the OCED (1984, guideline 202 part 1). Scene-
desmus (Desmodesmus) subspicatus was employed as alga
species, representing the producer level in the aquatic
environment. The test followed OCED (1984, guideline
201).
Fig. 1a and b visualise these relationships. The values
in brackets in Table 2 are calculated according to this
graph. Despite the low number of actual measured val-
ues the exponential relationship between carbon and
chain length and LC/EC50 values gives a fairly good
correlation.
3.1. Biodegradation
The biodegradability of any chemical finally deter-
mines its ultimate fate. Generally, it is differentiated
between primary and ultimate (mineralization) biode-
gradation. For the latter, the evolution of carbon diox-
ide or the consumption of oxygen best serves as the basis
for the calculation of the degree of degradation. The
CO2 evolution test (OECD, 1992) employs a low num-
ber of non-adapted bacteria as inocculum. The test
substance serves as the only energy source in the test
system, thus preventing bacteria from adaptation. Bio-
degradation rates of more than 60% are assumed to
demonstrate ultimate complete degradation under en-
vironmentally realistic conditions. As can be seen from
Table 3, the degradation rates reach values between 89%
and 94%.
2.4. Analytic and test concentrations
The test concentrations have been chosen such that
reliable LC50 respectively EC50 values could be calcu-
lated. Test concentrations have been verified by analy-
sing the organic carbon content of the test solutions
prior and throughout the test (Shimadzu TOC analyser).
They remained constant over the test duration.
3. Results
Little is known about the acute environmental effects
of phthalate monoesters. However, they form the first
transient metabolite during the biodegradation and
metabolism of their corresponding diesters. Table 1
summarises some physico-chemical properties of these
substances. With the low molecular ones being white
In order to classify a substance as readily biode-
gradable, the time course of the degradation process is
critical. The final minimal 60% rate must have been
reached within 10 days. The 10% value is serving as the
starting point for this period. Fig. 2 displays the time

N.Scholz / Chemosphere 53 (2003) 921–926
923
Table 1
Physical properties of selected phthalate monoesters
Monoester
Physical property
Purity (%)
91
Water solubilitya
893White, solid
Physical appearance
>1
Density
2.84
Log pow
b
O
O
OH
O
MNBP
92.1
977
White, solid
Colourless clear fluid
4.80
>1
2.77
O
O
i-C4H9
OH
O
MiBP
93.1
111
>1
4.73
O
O
OH
O
MEHP
1
9313Colourless clear fluid
>1
O
O
n-CnH2n+1
OH
O
n=6, 8, 10
MC6/8/10
P
94
56
Colourless clear fluid
>1
5.22
O
i-C9H19
O
OH
O
MINP
9346
Colourless clear fluid
0 >1
5.3
O
O
n-CnH2n+1
OH
n=8, 10
O
MC8/10
P
a Under test conditions.
b Calculated acc. EPIWIN (1999).
course of the degradation curve for MINP as an ex-
ample. The formation of CO2 nearly starts immediately
after the onset of the experiment. All other monoesters
tested did behave in the same way. All the monoesters
can be termed as readily and ultimately biodegradable.
4. Discussion
Monoesters of phthalic acid are considered respon-
sible for the toxicological and ecotoxicological proper-
ties of their corresponding diesters. As can be deduced

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N.Scholz / Chemosphere 53 (2003) 921–926
Table 2
Phthalate monoesters: acute LC50/EC50 ecotoxicity data with fish, daphina and algae
Monoester
Threshold values (mg/l)a
Cyprinus carpio
Daphina magna
Desmodesmus subspicatus
72 h EC50 (mg/l)
134
96 h LC50 (mg/l)
48 h EC200 (mg/l)
NOEC (mg/l)
44
MNBP
MiBP
MEHP
133b
125
62b
86
163 (139–191)
141 (112–162)
73(71–100)
50 (32–61)
MC6=8=10
MINP
MC8=10P
P
P 119
P 51
40
40
51b
29 (22–31)
30 (21–42)
Concentrations refer to measured concentration data.
a Values in brackets refer to the standard deviation ( ¼ range).
b Interpolated values.
160
140
120
100
80
60
40
20
-0.1895x
y = 284.37e
0
(a)
0
2
4
6
8
10
12
14
160
140
120
100
80
60
40
20
-0.2681x
y= 441.5e
0
0
5
10
15
(b)
Carbon chain length
Fig. 1. Phthalate monoesters: (a) relationship between fish acute LC50 values and carbon chain length of the alcohol moiety (pooled
entries for n- and iso-specimen), (b) relationship between Daphnia acute EC50 values and carbon chain length of the alcohol moiety
(pooled entries for n- and iso-specimen).
from experimental degradation data of the diesters, the
monoesters were not expected to form a stable inter-
mediate in the breakdown process, neither in the envi-
ronment nor in the organisms themselves, thus posing a
hidden risk. As can be demonstrated here, the degra-
dation of the monoesters reaches values of ꢀ90%, well
above 60%, the minimum target in this test system. This
can be regarded as direct proof that phthalate mono-
esters are completely biodegradable. However, a con-
tinuous formation via continuous influx/emissions of
diesters may lead the relevant concentrations in the en-
vironment.

N.Scholz / Chemosphere 53 (2003) 921–926
925
Table 3
Phthalate monoesters: results of biodegradation experiments
obtained with the CO2 evolution test (OECD, 1992)
fish range from 0.4 to around 4 mg/l, thus being 30–300
times more toxic than the corresponding monoesters.
Nearly the same ratio holds true for the invertebrates.
The same relationship might be assumed valid also for
the longer chain esters. However, as the water solubility
is decreasing more rapidly than the increase in toxicity,
this relationship obviously does not take effect in reality
(Staples et al., 1997).
Monoester Biodegradation (OECD, 1992)
%
Time delay
(days)
Remarks
MNBP
MIBP
MINP
MC8=10
932
932
89
Readily biodegradable
Readily biodegradable
In order to evaluate the possible risks posed through
monoesters in the environment, a comparison between
effect data and environmental concentration is necessary.
There are only very few monitoring data available (Su-
zuki et al., 2001). Based on these, one might not assume
an immediate and unacceptable risk posed through
phthalate esters and/or their degradation products re-
leased into the environment. Possible hidden effects
caused through longer term exposure may partly be ruled
out with the results from long term tests. In these at least
the internal concentrations would have reached their
maximum levels based on external exposure concentra-
tions of the diester, their uptake and subsequent turnover
metabolic rates. Based on an extensive analysis of acute
and chronic aquatic toxicity data (ECETOC, 1993), an
overall acute to chronic ratio of 10 (range between 3and
28) roughly could predict long term NOEC-values. Low
toxicity values would not be anticipated. It might be
helpful to perform some targeted testing here. The same
might hold true for the evaluation of the actual bioac-
cumulation rates.
2
2
Readily biodegradable
Readily biodegradable
P
94
Formation of carbon dioxide nearly starts immedi-
ately after the onset of test, contrasting sharply to the
behaviour of the diester. In the latter a clear lag phase
could be observed, increasing with increasing carbon
chain length (Scholz et al., 1997). This different behav-
iour underpins the interpretation that two mechanisms
control the degradation behaviour of phthalate esters.
The first one is achieving bioavailability of the diesters,
which definitely is slower the longer the alcohol chain is.
This might explain the prolonged lag phase. The second
step then is hydrolysis of the diester into the corre-
sponding monoester. Once the latter had been formed,
degradation gets off immediately. This behaviour further
underlines the assumption, that an accumulation of
monoesters under environmentally realistic conditions is
highly unlikely. Assuming possible risks due to the for-
mation of stable intermediates formed under environ-
mental conditions thus seems rather untenable.
The acute aquatic toxicity of the monoesters is con-
siderably less pronounced than their parent compounds.
A direct comparison can be made based on the compi-
lation of ecotoxicity data from Staples et al. (1997). For
the diester of n-butyl phthalate, acute toxicity data for
Acknowledgements
€
Michael Woelk-Fahrmann was helpful in synthesis-
ing the monoester samples.
100
90
80
70
60
50
40
30
20
10
0
0
5
10
15
20
25
30
35
Test duration in days
Fig. 2. Phthalate monoesters: degradation rate of MINP in relation to test duration of the experiment. The rectangular box displays
the ‘‘10 day window’’.

926
N.Scholz / Chemosphere 53 (2003) 921–926
Staples, C.A., Adams, W.J., Parkerton, T.F., Gorsuch, J.W.,
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Norbert Scholz is with the Oxeno Olefinchemie GmbH since
1998. He is responsible for product safety issues. Beside various
functions at SETAC level (Society of Environmental Toxicol-
ogy and Chemistry), Norbert Scholz chairs the Env. WG of the
European Plastiziser and Intermediate Producers (ECPI).
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