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Environmental fate & pathways

Biodegradation in water and sediment: simulation tests

Administrative data

Endpoint:
biodegradation in water: sewage treatment simulation testing
Type of information:
experimental study
Adequacy of study:
key study
Study period:
No data
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
This study was performed according to modified OECD Test Guidelines to permit prolonged measurements with no GLP statement. This study was reliable with restrictions because all validity criteria could not be verified. No information on oxygen depletion in the inoculum blank is available

Data source

Reference
Reference Type:
study report
Title:
Unnamed
Year:
2011
Report Date:
2011

Materials and methods

Test guideline
Qualifier:
equivalent or similar to
Guideline:
OECD Guideline 303 A (Simulation Test - Aerobic Sewage Treatment. A: Activated Sludge Units)
Deviations:
yes
Remarks:
Tests were modified to include studies on the effect of reducing agents and to allow measurements after 28 days.
Principles of method if other than guideline:
Not applicable.
GLP compliance:
no
Remarks:
Tests were performed under internal quality systems.

Test material

Reference
Name:
Unnamed
Type:
Constituent
Type:
Constituent
Test material form:
other: liquid
Details on test material:
Name of test material (as cited in study report): Trigonox 101 (2,5-dimethyl-2,5-di(tert-butylperoxy)hexane).
Specific details on test material used for the study:
Details on properties of test surrogate or analogue material (migrated information):
Not applicable.
Radiolabelling:
no

Study design

Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on source and properties of surface water:
River water was sampled from the Rhine near Heveadorp, The Netherlands. River water was aerated for 7 days before use and particles were removed by sedimentation.
Details on inoculum:
Secondary activated sludge was obtained from the wastewater treatment plant Nieuwgraaf in Duiven, The Netherlands. This plant is an activated sludge plant treating predominantly domestic wastewater. The activated sludge was preconditioned to reduce the endogenous respiration rates. To this end, 400 mg Dry Weight (DW)/L of activated sludge was aerated for one week. The sludge was diluted in the biological oxygen demand (BOD) bottles (van Ginkel and Stroo, 1992).
Initial test substance concentration
Initial conc.:
1 mg/L
Based on:
test mat.
Details on study design:
In the SCAS test, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane was exposed to activated sludge maintained by daily addition of primary settled sewage for a period of 8 weeks. The test substance caused no reduction of the biodegradation of the organic compounds present in primary settled sewage. Therefore 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is considered to be non-inhibitory to the activated sludge.

Results and discussion

% Degradation
% Degr.:
0
Parameter:
DOC removal
Sampling time:
8 wk
Transformation products:
no
Details on results:
SCAS Test: The test substance caused no reduction of the removal of organic carbon present in the primary settled sewage. Therefore 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is considered to be non-inhibitory to the activated sludge. These test conditions allow biodegradation of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane by microorganisms present in activated sludge. Before the additions of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, the effluent NPOC values obtained from the test compound unit and the control unit are comparable and constant. Additions of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane did not result in an increase of the organic carbon concentration. This was expected due to the insolubility of the test substance in water. The NPOC values of the test unit did not increase during the test period demonstrating that water soluble persistent substances were not formed.

Any other information on results incl. tables

The calculated organic carbon concentration of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane in the influent of the SCAS test was 9 mg/L. The incubation temperature of the SCAS units ranged from 18 to 22°C. The pH of the effluent of both SCAS units varied from 6.5 to 7.3. The SCAS test was started with a high concentration of aerobic microorganisms (2 g DW/L) maintained by the daily additions of primary settled sludge. The test substance caused no reduction of the removal of organic carbon present in the primary settled sewage. Therefore 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is considered to be non-inhibitory to the activated sludge. These test conditions allow biodegradation of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane by microorganisms present in activated sludge.

Before the additions of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane, the effluent NPOC values ob­tai­ned from the test compound unit and the control unit are com­para­ble and constant. Additions of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane did not result in an increase of the organic carbon concentration. This was expected due to the insolubility of the test substance in water. The NPOC values of the test unit did not in­crease during the test period demonstrating that water soluble persistent substances were not formed.

Based on these results it is concluded that it is very unlikely that 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is biodegraded by microorganisms capable of growing on this test substance under completely aerobic conditions.

2,5-Dimethyl-2,5-hexanedioland tert butanol are potential intermediates in the biodegradation process of the organic peroxide.2,5-Dimethyl-2,5-hexanedioland tert butanol are degraded >80% within 56 days in the Closed Bottle test.

Both potential intermediates were also as expected degraded in Closed Bottle tests inoculated with sludge exposed to 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. The degradation of 2,5 -dimethyl-2,5 -hexanediol with sludge exposed to the peroxide started almost immediately whereas with unacclimated sludge, lag periods of 2 to 4 weeks were noted. The absence of a lag period indicates that2,5-dimethyl-2,5-hexanediolis formed in the SCAS unit fed with 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane. This strongly indicates that2,5-dimethyl-2,5-hexanediolwas produced from 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane in the SCAS unit.

It is hypothized that 2,5 -dimethyl-2,5-di(tert-butylperoxy)hexane is converted chemically in the SCAS unit during the settle period or with the help of chemicals excreted by the activated sludge present in the unit. During the settle period anaerobic conditions (reducing conditions) are created because aeration is stopped. Conversion of highly oxidized peroxide bonds is more likely under reducing anaerobic conditions. It is also possible that the peroxide bond of 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane-2,5-di(tert-butylperoxy)hexane is reduced by microorganisms possessing reductases (enzymes) capable of inactivating toxic lipid peroxides. Microorganisms are present at very low concentrations in ready biodegradability tests. At these low concentrations, activity of these reductases may be insufficient to reduce the organic peroxides administered in the Closed Bottle test.

Applicant's summary and conclusion

Conclusions:
Based on these results it is concluded that it is very unlikely that 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane is biodegraded by microorganisms capable of growing on this test substance under completely aerobic conditions.
Executive summary:

In the SCAS test, 2,5-dimethyl-2,5-di(tert-butylperoxy)hexane was exposed to activated sludge maintained by daily addition of sewage for a period of 8 weeks. The test substance caused no reduction of the biodegradation of the organic compounds present in primary settled sewage.