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Environmental fate & pathways

Endpoint summary

Administrative data

Description of key information

Additional information

Ready biodegradation studies:

6PPD is not readily biodegradable. In a biodegradation test according to a modified MITI test (OECD TG 301 C), N-(1,3-dimethylbutyl)-N'-phenylbenzene-1,4-diamine (6PPD) showed 2% degradation after 4 weeks. (MITI, 1994)

In another test comparable to an US.EPA 40 CFR method, measuring biodegradation from CO2 release. After 32 days, 6PPD showed 7.2 % degradation. (Monsanto Industrial Chemicals Co., 1979).

The primary biodegradation for biological and chemical transformations of 6PPD was studied using Mississippi river water under aerobic conditions. Controls of this biodegradation study were made with sterile and with deionized water. After 22 h, when the experiment was finished, 97 % of 6PPD had disappeared from the active river water, 96 % from the sterile river water, and 88 % from the deionized water. The estimated half-lives due to primary transformation ranged from 2.9 to 6.8 hours. (

Monsanto Industrial Chemicals Co. 1981).

Soil Simulation studies

1. Aerobic soil study with analogous substance 7PPD(N-(1,4-dimethylpentyl)-N'-phenylbenzene-1,4-diamine, CAS No. 3081-01-4)

a)   Parent compound 7PPD

The results of the aerobic test show that the parent compound disappears with a half-life of <1.9 days. As shown below, the main pathway is strong adsorption or binding to soil.

b)   Metabolites

No significant metabolites have been formed.

c)   Characterisation of the non-extractables (bound residues)

Most of the radioactivity (about 80% at day 56) was found as non-extractable.

 

The outcome of recent scientific developments considering bound residues are reflected in the following guidance documents:

 

European Commission (2012), Health and Consumers Directorate, “DG SANCO working document on “evidence needed to identify POP, PBT and vBvP properties for pesticides”.:

Unextractable residues should be excluded from further assessment. They can be considered degradation loss, not bioavailable and therefore unable to exert toxicity. This approach is consistent with the SCHER opinion on aclonifen EQS (30 march 2011). Future Guidance might foresee taking into account “adsorbed unextractable residues”, which could be mobilised in the long-term and become relevant for further assessment.”

 

US Environmental Protection Agency (2014) “Guidance for Addressing Unextracted Residues in Laboratory Studies,http://www.epa.gov/pesticides/science/efed/policy_guidance/team_authors/environmental_fate_tech_team/Unextracted_Residues_in_Lab_Studies.htm:

“Extraction solvent systems should include solvents in which the parent compound and/or its transformation products are highly soluble. Systems for the extraction of neutral organic compounds should include non-polar solvents. Combinations of solvents, including a weak acid or weak base, may enhance the extraction efficiency. Some example polar solvents with dielectric constants ranging from 18 to 80 at environmental temperatures include water, formic acid, methanol, ethanol, isopropanol, acetone, acetonitrile, and dimethyl sulfoxide. Some example polar solvents with lower dielectric constants ranging from 6.0 to 9.1 include acetic acid, ethyl acetate, tetrahydrofuran, and dichloromethane. Example nonpolar solvents include hexane, benzene, toluene, 1,4-dioxane, chloroform, and diethyl ether (dielectric constants range from 1.9 to 4.8). Judgment should be used in the choice of solvents since factors other than dielectric constant may be important. Generally, unless there is a reason for a different approach, at least one solvent from each of the three groups identified above by range of dielectric constant should be used when there are a high proportion of unextracted residues (i.e., greater than 10% of the applied). Also, the solvent system pH should be adjusted to maximize recovery of compounds known to exhibit acid-base behavior.”

 

In order to cover the groups of solvents proposed by EPA, the 7PPD soil simulation used a solvent with high dielectric constant (acetone) and one solvent in the lower dielectric constant group (dichlormethane). A very unipolar solvent was considered but not used as the nature of the chemical was not thought to result in very unpolar residues. However, as the substance has amino groups and was expected to perhaps form ionic structures, an alkalic solvent system including ammonium hydroxide was additionally used.

 

The following extraction procedures had been used in the study in order to cover the requirements of EPA (2014 (Supercritical fluid extraction: tested in pretest and found not useful):

 

·        Ambient extraction with acetone / dichloromethane (3x)

·        Soxhlet-extraction with acetone / dichloromethane

·        Ammonium hydroxide / acetone / dichloromethane

ECHA (2014) REACH Guidance document R11 “Endpoint specific guidance”, November 2014

With regard to persistence it is insufficient to consider removal alone where this may simply represent the transfer of a substance from one environmental compartment to another (e.g. from the water phase to sediment). Degradation may be biotic (e.g. hydrolysis) or abiotic and result in complete mineralization, or simply in the transformation of the parent substance (primary degradation). Where only primary degradation is observed, it is necessary to identify the degradation products and to assess whether they possess PBT/vPvB properties.”

 

If binding to the matrix is understood as primary degradation, the bound residue should be assessed for PBT.

 

ECHA (2012) REACH Guidance document R7b “PBT/vPvB assessment”, November 2012:

Knowledge of bound residues and incorporation into biomass also needs to be considered and should be seen as a potential removal pathway. The OECD 308 (2002) guideline advises as follows: Bound residues represent compounds in soil, plant or animal that persists in the matrix in the form of the parent substance or its metabolites after extractions. The extraction method must not substantially change the compounds themselves or the structure of the matrix….. In general, the formation of bound residues reduces the bioaccessibility and the bioavailability significantly. Extraction of the sample, often with a suitable organic solvent is generally repeated 3 or 4 times until no further yield is achieved. Typically a range of solvents are used of increasing polarity…..Finally the use of strong acids, bases or refluxing could alter both the compounds of interest and the matrices…

When a substance is not fully mineralized but degraded to more persistent degradation products, the PBT/vPvB properties of these should be evaluated before a final judgement of whether the substance fulfils the P criteria.”

 

The last paragraph is understood that also bound residues (which are indeed more persistent than the parent substance) should be taken into consideration. In order to characterise the bound residues the following investigations were performed:

 

Binding of non-extractables to different soil components

·       7.8 - 16.4 % AR was bound to fulvic acids,

·        9.8 - 30.5 % AR to the humic acids and

·        23.3 - 57.3% AR to insoluble humins

Harsh acidic reflux extraction, following ambient and Soxhlet-extraction had an altering effect. 9.7 to 14.9 % radioactivity could be extracted. No parent substance was found.

 

Taking the different non-destroying extraction methods and the characterization of the bound residue into account, it can be concluded that all efforts have been undertaken in order to bring the residues into solution. Further characterization of the bound residues indicate that they are strongly (chemically) bound and only a small portion can be released even with exhaustive, altering extraction methods.

 

2. Anaerobic soil study with analogous substance 7PPD(N-(1,4-dimethylpentyl)-N'-phenylbenzene-1,4-diamine, CAS No. 3081-01-4)

 

Within the aerobic phase (0 to 4h), extractables (mainly parent) diminish with a DT50 of 1.5 days. In the anaerobic phase parent decreases slowly from 23.2% AR on day 1 to 13 -17 % until day 120. One metabolite ("7QDI", the oxidised form of 7PPD) is formed only during the anaerobic phase with a maximum of 38% (day 3) and reduction to 7.5% (day 120) with a DT50 of 66.9 days.

Considering the exposure pathway of the rubber additives mainly via TRWP and TWP, the main process in soil for degradation loss is aerobic. Those particles are expected to remain on soil surfaces for a rather long period. During this period, strong adsorption or binding to soil occurs. Transport to anaerobic parts of soil is unlikely.

According to European Commission (2012), Health and Consumers Directorate, “DG SANCO working document on “evidence needed to identify POP, PBT and vBvP properties for pesticides”.,“anaerobic data should be used, but only as additional information. As regards the initial establishment of a list of CFS, anaerobic data should not be considered.”