- Email: [email protected]
Ecotoxicity and biodegradation of phthalate monoesters
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 13 June 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 2003 Elsevier Ltd. All rights reserved. Keywords: Phthalate monoester; Biodegradation; Aquatic toxicity
1. Introduction Phthalates are diesters of phthalic acid and an alcohol 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 transient step during synthesis, but are not isolated. On the other hand, the (long-term) toxicological properties of the diesters are mainly determined through the corresponding monoester (Yagi et al., 1980). These can be formed through microbial processes, biotransformation by higher organism as well as through abiotic degradation processes. Whereas one can assume that in long
*
Tel.: +49-2365-49-2663; fax: +49-2365-49-2670. E-mail address: [email protected] (N. Scholz).
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 biodegradation behaviour of the monoesters have been evaluated, too.
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 corresponding alcohol at room temperature. The temperature in the reaction vessel then slowly is raised to approximately 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 formation of diesters.
0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00668-4
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N. Scholz / Chemosphere 53 (2003) 921–926
The following monoesters were synthesised: monon-butyl phthalate (MNBP), mono-isobutyl phthalate (MIBP), mono-2-ethylhexyl-phthalate (MEHP), monoisononyl phthalate (MINP), mono-n-hexyl/n-octyl/ n-decyl-phthalate (M6=8=10 P) and mono-n-octyl/n-decylphthalate (M8=10 P). 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 alcohol. 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-acclimatised bacteria do not only prevent false positive results, 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 followed the OCED (1984, guideline 202 part 1). Scenedesmus (Desmodesmus) subspicatus was employed as alga species, representing the producer level in the aquatic environment. The test followed OCED (1984, guideline 201). 2.4. Analytic and test concentrations The test concentrations have been chosen such that reliable LC50 respectively EC50 values could be calculated. Test concentrations have been verified by analysing 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
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 surrogate for the bioaccumulation potential of a substance. As can be seen from Table 1, the calculated log Pow values (EPIWIN) increase with increased carbon number of the alcohol chain. 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 toxicity. 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 pronounced 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. 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 values 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 determines its ultimate fate. Generally, it is differentiated between primary and ultimate (mineralization) biodegradation. For the latter, the evolution of carbon dioxide 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 number of non-adapted bacteria as inocculum. The test substance serves as the only energy source in the test system, thus preventing bacteria from adaptation. Biodegradation rates of more than 60% are assumed to demonstrate ultimate complete degradation under environmentally realistic conditions. As can be seen from Table 3, the degradation rates reach values between 89% and 94%. In order to classify a substance as readily biodegradable, 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
O
Purity (%)
Water solubilitya
Physical appearance
Density
Log pow b
91
893
White, solid
>1
2.84
92.1
977
White, solid
>1
2.77
93.1
111
Colourless clear fluid
>1
4.73
93
131
Colourless clear fluid
>1
4.80
94
56
Colourless clear fluid
>1
5.22
93
46
Colourless clear fluid
>1
5.30
O OH O
MNBP O i-C4H9 O OH O
MiBP O O OH O
MEHP O n-CnH2n+1 O OH O
n=6, 8, 10
MC 6/8/10 P O i-C9H19 O OH O
MINP O n-CnH2n+1 O OH O
n=8, 10
MC8/10 P a b
Under test conditions. Calculated acc. EPIWIN (1999).
course of the degradation curve for MINP as an example. 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 responsible for the toxicological and ecotoxicological properties 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 Threshold values (mg/l)a
Monoester
MNBP MiBP MEHP MC6=8=10 P MINP MC8=10 P
Cyprinus carpio 96 h LC50 (mg/l)
Daphina magna 48 h EC200 (mg/l)
Desmodesmus subspicatus 72 h EC50 (mg/l)
NOEC (mg/l)
133b 125 62b 86 40 51b
163 (139–191) 141 (112–162) 73 (71–100) 50 (32–61) 29 (22–31) 30 (21–42)
134
44
P 119 P 51
40
Concentrations refer to measured concentration data. a Values in brackets refer to the standard deviation ( ¼ range). b Interpolated values.
160
LC 50 (mg/l)
140 120 100 80 60 40 20
-0.1895x
y = 284.37e
0
(a)
0
2
4
6
8
10
12
14
160
EC 50 (mg/l)
140 120 100 80 60 40 20
-0.2681x
y= 441.5e
0 0
(b)
5 10 Carbon chain length
15
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 intermediate in the breakdown process, neither in the environment nor in the organisms themselves, thus posing a hidden risk. As can be demonstrated here, the degradation 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 monoesters are completely biodegradable. However, a continuous formation via continuous influx/emissions of diesters may lead the relevant concentrations in the environment.
N. Scholz / Chemosphere 53 (2003) 921–926 Table 3 Phthalate monoesters: results of biodegradation experiments obtained with the CO2 evolution test (OECD, 1992) Monoester
MNBP MIBP MINP MC8=10 P
Biodegradation (OECD, 1992) %
Time delay (days)
Remarks
93 93 89 94
2 2 2 2
Readily Readily Readily Readily
biodegradable biodegradable biodegradable biodegradable
Formation of carbon dioxide nearly starts immediately 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 behaviour 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 corresponding 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 formation of stable intermediates formed under environmental conditions thus seems rather untenable. The acute aquatic toxicity of the monoesters is considerably less pronounced than their parent compounds. A direct comparison can be made based on the compilation of ecotoxicity data from Staples et al. (1997). For the diester of n-butyl phthalate, acute toxicity data for
925
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). 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 (Suzuki et al., 2001). Based on these, one might not assume an immediate and unacceptable risk posed through phthalate esters and/or their degradation products released 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 concentrations 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 3 and 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 bioaccumulation rates.
Acknowledgements Michael Woelk-F€ ahrmann was helpful in synthesising the monoester samples.
Degradation in (%)
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’’.
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N. Scholz / Chemosphere 53 (2003) 921–926
References ECETOC, 1993. Aquatic toxicity data evaluation. Technical Report no. 56, 66 pp. EPIWIN, 1999. EPA version for windows. Available from Syracuse Research Corporation, Syracuse, NY. GLP, 1987. 87/18/EEC, Council directive on the harmonisation of the regulations and administrative provisions relating to the application of the principles of good laboratory praxis and the verification of their application for tests on chemical substances, most recent version. OECD 201, 1984. Alga, growth inhibition test. OECD 202, 1984. Daphnia sp. Acute immobilisation test and reproduction test, Part 1. OECD 203, 1982. Fish, acute toxicity test. OECD 301 b, 1992. CO2 evolution test. Scholz, N., Diefenbach, R., Rademacher, I., Linnemann, D., 1997. Biodegradation of DEHP, DBP, and DINP: purely water soluble and widely used phthalates plastizisers. Bull. Environ. Contam. Toxicol. 58, 527–534.
Staples, C.A., Adams, W.J., Parkerton, T.F., Gorsuch, J.W., Biddinger, G.R., Reinhard, K.H., 1997. Aquatic toxicity of 18 phthalate esters. Environ. Toxicol. Chem. 16, 875–891. Suzuki, T., Yagushi, K., Suzuki, S., Suga, T., 2001. Monitoring of phthalic acid monoesters in river water by solid-phase extraction and GC–MS determination. Environ. Sci. Technol. 35, 3757–3763. Wickson, E.J., 1993. Handbook of PVC Formulating. John Wiley & Sons, New York. Wilson, A.S., 1995. Plastizisers, Principles and Practice. The University Press, Cambridge. Yagi, Y., Nakamura, Y., Tomita, L., Tsuchikawa, K., Shimoi, N., 1980. Teratogenic potential of di- and mono-2-ethylhexylphthalate in mice. J. Environ. Pathol. Toxicol. 4, 533– 544. 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 Toxicology and Chemistry), Norbert Scholz chairs the Env. WG of the European Plastiziser and Intermediate Producers (ECPI).
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 13 June 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 2003 Elsevier Ltd. All rights reserved. Keywords: Phthalate monoester; Biodegradation; Aquatic toxicity
1. Introduction Phthalates are diesters of phthalic acid and an alcohol 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 transient step during synthesis, but are not isolated. On the other hand, the (long-term) toxicological properties of the diesters are mainly determined through the corresponding monoester (Yagi et al., 1980). These can be formed through microbial processes, biotransformation by higher organism as well as through abiotic degradation processes. Whereas one can assume that in long
*
Tel.: +49-2365-49-2663; fax: +49-2365-49-2670. E-mail address: [email protected] (N. Scholz).
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 biodegradation behaviour of the monoesters have been evaluated, too.
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 corresponding alcohol at room temperature. The temperature in the reaction vessel then slowly is raised to approximately 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 formation of diesters.
0045-6535/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0045-6535(03)00668-4
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N. Scholz / Chemosphere 53 (2003) 921–926
The following monoesters were synthesised: monon-butyl phthalate (MNBP), mono-isobutyl phthalate (MIBP), mono-2-ethylhexyl-phthalate (MEHP), monoisononyl phthalate (MINP), mono-n-hexyl/n-octyl/ n-decyl-phthalate (M6=8=10 P) and mono-n-octyl/n-decylphthalate (M8=10 P). 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 alcohol. 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-acclimatised bacteria do not only prevent false positive results, 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 followed the OCED (1984, guideline 202 part 1). Scenedesmus (Desmodesmus) subspicatus was employed as alga species, representing the producer level in the aquatic environment. The test followed OCED (1984, guideline 201). 2.4. Analytic and test concentrations The test concentrations have been chosen such that reliable LC50 respectively EC50 values could be calculated. Test concentrations have been verified by analysing 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
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 surrogate for the bioaccumulation potential of a substance. As can be seen from Table 1, the calculated log Pow values (EPIWIN) increase with increased carbon number of the alcohol chain. 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 toxicity. 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 pronounced 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. 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 values 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 determines its ultimate fate. Generally, it is differentiated between primary and ultimate (mineralization) biodegradation. For the latter, the evolution of carbon dioxide 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 number of non-adapted bacteria as inocculum. The test substance serves as the only energy source in the test system, thus preventing bacteria from adaptation. Biodegradation rates of more than 60% are assumed to demonstrate ultimate complete degradation under environmentally realistic conditions. As can be seen from Table 3, the degradation rates reach values between 89% and 94%. In order to classify a substance as readily biodegradable, 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
O
Purity (%)
Water solubilitya
Physical appearance
Density
Log pow b
91
893
White, solid
>1
2.84
92.1
977
White, solid
>1
2.77
93.1
111
Colourless clear fluid
>1
4.73
93
131
Colourless clear fluid
>1
4.80
94
56
Colourless clear fluid
>1
5.22
93
46
Colourless clear fluid
>1
5.30
O OH O
MNBP O i-C4H9 O OH O
MiBP O O OH O
MEHP O n-CnH2n+1 O OH O
n=6, 8, 10
MC 6/8/10 P O i-C9H19 O OH O
MINP O n-CnH2n+1 O OH O
n=8, 10
MC8/10 P a b
Under test conditions. Calculated acc. EPIWIN (1999).
course of the degradation curve for MINP as an example. 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 responsible for the toxicological and ecotoxicological properties of their corresponding diesters. As can be deduced
924
N. Scholz / Chemosphere 53 (2003) 921–926
Table 2 Phthalate monoesters: acute LC50 /EC50 ecotoxicity data with fish, daphina and algae Threshold values (mg/l)a
Monoester
MNBP MiBP MEHP MC6=8=10 P MINP MC8=10 P
Cyprinus carpio 96 h LC50 (mg/l)
Daphina magna 48 h EC200 (mg/l)
Desmodesmus subspicatus 72 h EC50 (mg/l)
NOEC (mg/l)
133b 125 62b 86 40 51b
163 (139–191) 141 (112–162) 73 (71–100) 50 (32–61) 29 (22–31) 30 (21–42)
134
44
P 119 P 51
40
Concentrations refer to measured concentration data. a Values in brackets refer to the standard deviation ( ¼ range). b Interpolated values.
160
LC 50 (mg/l)
140 120 100 80 60 40 20
-0.1895x
y = 284.37e
0
(a)
0
2
4
6
8
10
12
14
160
EC 50 (mg/l)
140 120 100 80 60 40 20
-0.2681x
y= 441.5e
0 0
(b)
5 10 Carbon chain length
15
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 intermediate in the breakdown process, neither in the environment nor in the organisms themselves, thus posing a hidden risk. As can be demonstrated here, the degradation 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 monoesters are completely biodegradable. However, a continuous formation via continuous influx/emissions of diesters may lead the relevant concentrations in the environment.
N. Scholz / Chemosphere 53 (2003) 921–926 Table 3 Phthalate monoesters: results of biodegradation experiments obtained with the CO2 evolution test (OECD, 1992) Monoester
MNBP MIBP MINP MC8=10 P
Biodegradation (OECD, 1992) %
Time delay (days)
Remarks
93 93 89 94
2 2 2 2
Readily Readily Readily Readily
biodegradable biodegradable biodegradable biodegradable
Formation of carbon dioxide nearly starts immediately 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 behaviour 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 corresponding 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 formation of stable intermediates formed under environmental conditions thus seems rather untenable. The acute aquatic toxicity of the monoesters is considerably less pronounced than their parent compounds. A direct comparison can be made based on the compilation of ecotoxicity data from Staples et al. (1997). For the diester of n-butyl phthalate, acute toxicity data for
925
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). 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 (Suzuki et al., 2001). Based on these, one might not assume an immediate and unacceptable risk posed through phthalate esters and/or their degradation products released 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 concentrations 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 3 and 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 bioaccumulation rates.
Acknowledgements Michael Woelk-F€ ahrmann was helpful in synthesising the monoester samples.
Degradation in (%)
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
References ECETOC, 1993. Aquatic toxicity data evaluation. Technical Report no. 56, 66 pp. EPIWIN, 1999. EPA version for windows. Available from Syracuse Research Corporation, Syracuse, NY. GLP, 1987. 87/18/EEC, Council directive on the harmonisation of the regulations and administrative provisions relating to the application of the principles of good laboratory praxis and the verification of their application for tests on chemical substances, most recent version. OECD 201, 1984. Alga, growth inhibition test. OECD 202, 1984. Daphnia sp. Acute immobilisation test and reproduction test, Part 1. OECD 203, 1982. Fish, acute toxicity test. OECD 301 b, 1992. CO2 evolution test. Scholz, N., Diefenbach, R., Rademacher, I., Linnemann, D., 1997. Biodegradation of DEHP, DBP, and DINP: purely water soluble and widely used phthalates plastizisers. Bull. Environ. Contam. Toxicol. 58, 527–534.
Staples, C.A., Adams, W.J., Parkerton, T.F., Gorsuch, J.W., Biddinger, G.R., Reinhard, K.H., 1997. Aquatic toxicity of 18 phthalate esters. Environ. Toxicol. Chem. 16, 875–891. Suzuki, T., Yagushi, K., Suzuki, S., Suga, T., 2001. Monitoring of phthalic acid monoesters in river water by solid-phase extraction and GC–MS determination. Environ. Sci. Technol. 35, 3757–3763. Wickson, E.J., 1993. Handbook of PVC Formulating. John Wiley & Sons, New York. Wilson, A.S., 1995. Plastizisers, Principles and Practice. The University Press, Cambridge. Yagi, Y., Nakamura, Y., Tomita, L., Tsuchikawa, K., Shimoi, N., 1980. Teratogenic potential of di- and mono-2-ethylhexylphthalate in mice. J. Environ. Pathol. Toxicol. 4, 533– 544. 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 Toxicology and Chemistry), Norbert Scholz chairs the Env. WG of the European Plastiziser and Intermediate Producers (ECPI).