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Environmental fate & pathways

Biodegradation in water: screening tests

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Endpoint:
biodegradation in water: ready biodegradability
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Justification for type of information:
HYPOTHESIS FOR THE CATEGORY APPROACH
HMDTMP acid is a member of the HMDTMP category, which is comprised of the acid form and various salts of the same acid. In the context of the RAAF, read-across of environmental, ecotox, human health and physicochemical data within the HMDTMP Category is considered as Scenario 6.

SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
This category covers various sodium and potassium salts of [hexane-1,6-diylbis-[nitrilobis(methylene)]tetrakisphosphonic acid. The different salts are prepared by neutralising the acid to a specific pH and accordingly the constituents proportions and degree of ionisation are comparable between substances under similar conditions (in vivo and in the environment). All category members are based on the HMDTMP structure.
Since the salts are prepared from the acid, the impurity profile for HMDTMP acid is also typical of the salts in this Category, although acidic impurities would also be present as salts. Typically, impurities include residual inorganic acids/salts and organic by-products from manufacturing.

CATEGORY APPROACH JUSTIFICATION
The category hypothesis is that all the members are various ionised forms of the same parent acid. This category covers various sodium and potassium salts of [hexane-1,6-diylbis-[nitrilobis(methylene)]tetrakisphosphonic acid. The different salts are prepared by neutralising the acid to a specific pH and accordingly the constituents proportions and degree of ionisation are comparable between substances under similar conditions (in vivo and in the environment). All category members are based on the HMDTMP structure. Data are available for the acid form and some salts. Only acid aqueous solutions are commercially available. The properties of the members of the category are consistent across all endpoints.
Contributing effect of counter-ions:
The approach assumes in general that alkaline metal (sodium and potassium) counter-ions are not significant in respect of all the properties under consideration.
In an environmental context, the speciation will be controlled by the prevailing conditions and ions present, and will be the same regardless of the starting form. In dilute aqueous conditions of defined pH and mineral composition, a salt or complex introduced into this medium will behave no differently to the parent acid, at identical concentration of the particular speciated form present and will be fully dissociated. Hence some properties (measured or expressed in aqueous media, e.g. ecotoxicity) for a salt can be directly read-across (with suitable mass correction) to the parent acid and vice versa, and from one salt to another.
Where data are available across different salt forms of HMDTMP, this approach is supported.

DATA MATRIX
A data matrix table of all the data available on the HMDTMP Category members is available in Section 13 of the technical dossier.
Reason / purpose:
read-across source
Parameter:
% degradation (DOC removal)
Value:
0
Sampling time:
28 d
Endpoint:
biodegradation in water: ready biodegradability
Type of information:
experimental study
Adequacy of study:
key study
Study period:
1984
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: The study was conducted according to the appropriate OECD test guideline, and in compliance with GLP.
Qualifier:
according to
Guideline:
OECD Guideline 301 E (Ready biodegradability: Modified OECD Screening Test)
Deviations:
no
GLP compliance:
yes
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, non-adapted
Details on inoculum:
Activated sludge from the HRC sewage treatment plant. The nature of the influent to this treatment plant is not known.

The activated sludge was filtered through Whatman No 1 paper (first 200 ml discarded) and the filtrate was kept aerated until used. 
Duration of test (contact time):
28 d
Initial conc.:
40 other: mg C/l
Based on:
DOC
Parameter followed for biodegradation estimation:
other: dissolved organic carbon removal
Reference substance:
other: sodium benzoate
Parameter:
% degradation (DOC removal)
Value:
0
Sampling time:
28 d
Details on results:
Briquest 462-25S attained 0% degradation after 28 days and
was not considered readily biodegradable under OECD
Guideline No. 301-E. The control substance attained 85%
degradation after 14 days and 97% after 28 days, confirming
suitability of the inoculum and culture conditions.

Degradation products: no
Results with reference substance:
Reference:
Day 7: 79%
Day 14: 85%
Day 28: 97%
Validity criteria fulfilled:
yes
Interpretation of results:
under test conditions no biodegradation observed

Description of key information

No study of ready biodegradation of potassium salts of HMDTMP are available. 0% removal (based on BOD) was observed in a reliable study conducted using an analogous sodium salt of HMDTMP.

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

This result is supported by findings in a further reliable test of ready biodegradability, measured with the acid form, showing extremely low levels of biodegradation over 28 days.

Two studies of inherent biodegradation are also available for the acid form. Both show significant but highly variable levels of removal over the test period, probably due to adsorption combined with slow abiotic and biotic degradation mechanisms.

In dilute aqueous conditions of defined pH, a salt will behave no differently to the parent acid, at identical concentration of the particular speciated form present, and will be fully dissociated to yield HMDTMP phosphonate anion and Na+(or K+). Binding behaviour is not affected by acid or alkali metal salt form. Hence some properties (measured or expressed in aqueous media) for a salt can be directly read across (with suitable mass correction) to the parent acid and vice versa, or from one salt form to another (here, sodium salt to potassium salt) (see CSR section 1 for mass correction values). In the present context the effect of the alkaline metal counter-ion (sodium/potassium) will not be significant and has been extensively discussed in the public literature. In biological systems and the environment, polyvalent metal ions will be present, and the phosphonate ions show very strong affinity to them.