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EC number: 202-705-0
CAS number: 98-83-9
highly volatile from aqueous solution (Henry's Law constant calculated
using EPIWIN v3.11, HENRYWIN v3.10: 438.63 Pa x m³/mole and calculation
according to the equation H=vp/ws: 282 Pa x m³/mole at 25°C each). The
Henry's Law constant calculated according to the equation H=vapor
pressure/water solubility and using reliable experimental derived values
also indicates that alpha-methylstyrene is highly volatile from aqueous
solution (H=258.32 Pa x m³/mole at 25°C).
distribution of alpha-methylstyrene according to the fugacity model
Mackay Level I v3.00 reveals that in the equilibrium state the substance
is distributed to 98.4 % into the atmosphere (cf. chapter 5.4.3),
whereas only minor amounts of the substance will be transferred to water
(1.1 %), sediment (0 %) and soil (0 %). Furthermore,
SIMPLEBOX results indicate that the main removal pathway in sewage
treatment plants (STP) is volatilisation. It could be demonstrated by
the exposure estimation specified in the quantitative environmental
exposure assessment using the SIMPLETREAT model (cf.
Quant_Env_ERA_AMS-CSR; attached document in section 13) that the major
part of alpha-methylstyrene introduced into STP is directed to air.
The assumption, that the
atmosphere has to be regarded as the dominant environmental target
compartment of alpha-methylstyrene is underlined by monitoring data from
surface water, sediment and air in Japan (1997-2005, cited in CHRIP
database, cf. chapter 5.5.1). Whereas the substance was detected in none
of more than 100 surface water (detection limit: 0.009 - 4 µg/L) or
bottom sediments samples (detection limit: 0.0007 - 0.01 µg/g dw), it
was found in 20 out of 26 air samples (detection limit: 1.9 ppb).
Furthermore, alpha-methylstyrene was not detected in the sludge from the
wastewater treatment plant of the Emscher Genossenschaft receiving the
wastewater of INEOS Phenol (one sample collected 2010; limit of
determination: 0.5 mg/kg dw).
Once introduced in the
atmosphere, alpha-methylstyrene is rapidly degraded by OH-radicals.
The rates of the reactions
of OH radicals with styrene, alpha-methylstyrene, and beta-methylstyrene
were measured by irradiating mixtures of these aromatic olefins and NO
in an environmental chamber at 298K. Based on the experimentally
determined reaction rate constant k(OH) =5.3 +/- 0.6 x 10E-11
cm³/molecule x sec (25°C) and assuming a tropospheric OH radical
concentration of 5 x 10E5 radicals/cm³ (24 h day) a half-life of 7.27 h
can be estimated for alpha-methylstyrene. Based on the results,
alpha-methylstyrene present in air will be relative quickly
photodegraded by reaction with OH radicals. Acetophenone and
formaldehyde were observed as main breakdown products which in turn
undergo further decomposition. Similar results were obtained using the
generally accepted calculation model EPIWIN v3.11, AOPWIN v1.91
(t1/2=7.2 h). The reaction of alpha-methylstyrene with ozone is a
further elimination pathway (t1/2=2 h, calculated via EPIWIN). In
addition, due to the spectral properties of alpha-methylstyrene
(absorption of light in the environmental UV spectrum) direct photolysis
is to be expected.
In a study conducted by CITI
(Japan) according to OECD Guideline 111 (Hydrolysis as a Function of
pH), alpha-methylstyrene proved to be stable to hydrolysis at pH 4, 7,
and 9 at 25°C.
For the the ready and
inherent biodegradability of alpha-methylstyrene, reliable results from
studies conducted according to OECD guidelines (OECD 301C, 301D, 301F
and 302C) are available.
The ready biodegradation of
alpha-methylstyrene was investigated in a study conducted according to
OECD Guideline 301 D (Closed Bottle Test) employing a mixed microbial
inoculum obtained from the effluent of a wastewater treatment plant and
soil. The test substance proved to be not inhibitory at the
concentration tested. In this close-bottle test 56 % biodegradation were
obtained within 21 days, which is very close to the 60 % threshold for
ready biodegradability. Regarding this test, a number of uncertainties
have to be taken into account. During application of the volatile AMS to
the medium in the Closed Bottle test, the spiked medium was left at
least for some time in an „open system“ and so a part of the applied
amount of AMS might evaporate from the medium before transferred into
the test vessels or at least before the vessels were gas-tight sealed.
In this case, the actual loading would have been below the nominal one
and a part of the nominal concentration would have been not available
for microbial biodegradation.
The latter uncertainty may
also be related to the results of a study conducted according to OECD
Guideline 301 F, in which alpha-methylstyrene underwent 8 % and 21 %
biodegradation after 28 d at 100 and 20 mg/L, respectively, under the
specific test conditions. The part of applied test substance, which
might have evaporated from the test solution, would have been no longer
available to degradation by the inoculum, leading to an underestimation
of the degradation rate. This is supported by the higher percentage
observed with the lowest nominal concentration, suggesting that the
actual level of dissolved substance (and consequently actual ThOD) is
closest to 20 than 100 mg/L, and probably lower than 20 mg/L.
In the study conducted
according to OECD Guideline 301C (Ready Biodegradability: Modified MITI
Test (I)), alpha-methylstyrene was not degraded under the test
conditions employed (0% biodegradation after 14 d; initial
concentration: 100 mg/l). The test method was identified by OECD as
being in principal appropriate for volatile substances and an ‘improved
type of study design for volatile substances’ was applied for testing
AMS (not further details specified). The reason for the significant
difference compared to the results of all other studies available on
biodegradability can not be explained by the available references.
However, it seems questionable, whether the applied MITI-I method is the
best choice for testing ready biodegradation of substances exhibiting
both, high volatility as well as limited water solubility.
No definitive proof of ready
biodegradability has been obtained from the experimental study results
available for AMS on this endpoint, when strictly applying the criteria
of the OECD 301 guidelines. However, from the diversity of degradation
rates it can be concluded that the substance, while being in principal
not recalcitrant to biodegradability, requires a specific experimental
design taking into account its significant volatility as well as limited
water solubility (100-116 mg/L in pure water, i.e. lower in test medium
that contains salts). The potential for biodegradability is supported by
inherent test biodegradation, and ability of isolated strains to grow on
AMS as sole source of carbon and energy, as shown hereafter.
to be inherently biodegradable in a study conducted according to OECD
Guideline 302 C (Modified MITI Test (II)) using municipal activated
sludge as inoculum. A biodegradation rate of 56% was determined after 28
d incubation by measurement of oxygen consumption. Based on the results
and the validity criteria of the guideline the test substance can be
assumed to be not inhibitory to the inoculum, providing evidence of
biodegradability up to 70 % in 28 days, as shown in one of test
duplicates. From a long lag phase (minimum 10 days) in the other
duplicate it can be concluded that biodegradation of the AMS-molecule
requires the presence of specific bacterial species that may be
relatively infrequent, or as seen in other tests, that actual substance
concentrations achieved were irregular in different replicates. In other
tests it was shown that various bacterial strains, mostly isolated from
contaminated sites, were able to grow on alpha-methylstyrene and use the
test substance as sole carbon and energy source.
On another hand, the ready
biodegradability of alpha-methylstyrene would be supported by the
structure analogue substances styrene, acetophenone and cumene
Styrene is structurally
closely related to alpha-methylstyrene, the only difference is the
presence of a methyl group in the latter. Therefore, it is expected that
the same enzyme causes the initial step of biodegradation. When the
methyl group is removed, the same metabolism pathway is expected for
both styrene and alpha-methylstyrene. As revealed in several valid
standard or near-standard tests, styrene proved to be readily
biodegradable under aerobic conditions meeting the 10-day window
Acetophenone is structurally
closely related to alpha-methylstyrene, the only difference is the
presence of an oxygen atom instead of the methylene group. This gives
acetophenone lower volatility and higher water solubility., Acetophenone
was shown to be readily biodegradable (64,7 % after 14 d in OECD 301C),
and this was confirmed by water simulation test giving a DT50 from 6 to
Cumene is structurally
closely related to alpha-methylstyrene, the only difference is a double
bond in the side chain in the latter. Cumene was shown to undergo >60 %
biodegradation after 10 d in a test according to US-APHA guideline.
In Annex IX of Regulation
(EC) No 1907/2006, it is laid down that biodegradation simulation
testing in surface water and/or sediment as
well as soil shall be proposed by the registrant if the chemical safety
assessment according to Annex I indicates the need to investigate
further the degradation of the substance and its degradation products in
appropriate media. Experimental testing need not be conducted if direct
and indirect exposure of surface water, sediment and/ or soil is
unlikely or technically not feasible. Alpha-methylstyrene is a highly
volatile substance, for which the atmosphere is considered to be the
dominant environmental target compartment, whereas hydrosphere and
terrestrial compartment are of minor importance. This is underlined by
monitoring data from waste water treatment plants (wwtp) in Germany as
well as from surface water, sediment and air in Japan, where the
substance was detected in none of more than 100 surface water, bottom
sediments and wwtp samples while it was found in 20 out of 26 air
samples (see below). Moreover, biodegradation testing in surface water,
soil according to generally accepted OECD standard guidelines is
technically not feasible. Due to its high volatility alpha-methylstyrene
clearly does not fall into the applicability domain of either of the
OECD methods. Therefore, according to Annex IX of Regulation (EC) No
1907/2006, biodegradation simulation tests in surface water, sediment
and/or soil are considered to be scientifically unjustified for
In a bioaccumulation study
conducted according to OECD Guideline 305 C, fish were exposed to
alpha-methylstyrene concentrations of 0.03 and 0.3 mg/L at 25°C for 56 d
under flow through conditions. BCF values of 12 -113 (0.03 mg/L) and 15
-140 (0.3 mg/L) were determined. A BCF= 95 was calculated via EPIWIN.
These results indicate a low potential for accumulation of
alpha-methylstyrene in fish.
The soil adsorption
coefficient of alpha-methylstyrene was investigated in a study conducted
according to OECD Guideline 121 and EU Method C.19. The soil adsorption
coefficient log Koc was determined to be 2.8 (Koc=692). The calculation
(EPIWIN v3.11, PCKOC v1.66) of the soil sorption coefficient Koc of
alpha-methylstyrene yielded a Koc=817 (log Koc=2.9). Based on the
results and taking into account the classification scheme of Blume and
Ahlsdorf (1993), alpha-methylstyrene can be regarded as a substance with
high sorption potential onto soil organic matter. However, it has to be
underlined again that environmental distribution of alpha-methylstyrene
according to the fugacity model Mackay Level I v3.00 reveals that in the
equilibrium state the substance is distributed to 98.4 % into the
atmosphere (cf. chapter 5.4.3).
Alpha-methylstyrene was not
detected in the sludge from the wastewater treatment plant of the
Emscher Genossenschaft receiving the wastewater of INEOS Phenol (one
sample collected 2010; limit of determination: 0.5 mg/kg dw).
Alpha-methylstyrene was monitored in Japan in surface water, sediment
and air in Japan during the years 1977-2005. Whereas the substance was
detected in none of more than 100 samples from surface water (detection
limit: 0.009 - 4 µg/L) or bottom sediment (detection limit: 0.0007 -
0.01 µg/g dw), it was found in 20 out of 26 air samples (detection
limit: 1.9 ppb).
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