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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:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Study was conducted in accordance with recognized scientific methods and procedures. The study meets national and international scientific methods and provides sufficient information to support the conclusion.
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 301 D (Ready Biodegradability: Closed Bottle Test)
Version / remarks:
BOD5 method
Principles of method if other than guideline:
The method followed the recommendations as laid down in the Her Majesty's Stationery Office publication "5 Day Biochemical Oxygen Demand (BOD5) Second Edition 1988 (with Dissolved Oxygen in Waters, Amendments 1988), Methods for the Examination of Waters and Associated Materials".  Method is very similar to OECD 301.
GLP compliance:
yes (incl. QA statement)
Remarks:
GLP for COD method: yes
Oxygen conditions:
aerobic
Inoculum or test system:
activated sludge, domestic, non-adapted
Details on inoculum:
Inoculum: Sewage works final effluent, Severn Trent Water Plc., Belper sewage treatment works, Belper, Derbyshire.  Date of collection 2 April 1992. The sample was allowed to settle for 30 minutes and the supernatant withdrawn for test.
Duration of test (contact time):
5 d
Initial conc.:
120 mg/L
Based on:
other: Test substance
Parameter followed for biodegradation estimation:
DOC removal
Details on study design:
BOD bottles were filled by siphon and firmly stoppered to exclude all air bubbles. Sufficient bottles were prepared to allow a single oxygen determination per bottle to be made at day 0 and day 5 for each test medium (duplicate bottles at each sampling time). Bottles were incubated in a water bath at 20 degrees C for 5 days. 

Chemical Oxygen Demand (COD) was determined using a semi-micro sample digestion (Hach) technique. Reaction vials containing premeasured amounts of sulfuric acid, potassium dichromate, silver catalyst plus 2 ml water sample were heated at 150 degrees C for 2 hours and the COD values read from a Hach DR/2000 Direct Reading Spectrophotometer.
Reference substance:
other: glucose/glutamic acid (1:49)
Remarks on result:
other: refer to BOD results
Parameter:
BOD5
Value:
15.1 other: mg/L
Parameter:
COD
Value:
66 other: mg O2 per litre
Parameter:
BOD5*100/COD
Results with reference substance:
BOD5 240.1 mg O2/L (at nominal test concentration 105 mg/L)
Validity criteria fulfilled:
yes
Remarks:
DOC variation in the blank fulfilled the requirements
Interpretation of results:
other: Not readily biodegradable
Conclusions:
It was determined in a reliable BOD5 study conducted in compliance with GLP that HEDP is not readily biodegradable.
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:
Please refer to Annex 3 of the CSR and IUCLID Section 13 for justification of read-across between members of the HEDP category.
Reason / purpose for cross-reference:
read-across source
Remarks on result:
other: refer to BOD results
Parameter:
BOD5
Value:
15.1 other: mg/L
Parameter:
COD
Value:
66 other: mg O2 per litre
Parameter:
BOD5*100/COD
Results with reference substance:
BOD 240.1 mg O2/L (at nominal concentration 105 mg/L)

Description of key information

The substance is not readily biodegradable, based on the results of the BOD5 test (read-across from HEDP-H).

Key value for chemical safety assessment

Biodegradation in water:
under test conditions no biodegradation observed

Additional information

A ready biodegradability study using HEDP-H was selected as the key study (Handley & Mead, 1992). This study followed the 5-day Biological Oxygen Demand (BOD5) method (similar to OECD 301). The study is considered to be a reliable study to demonstrate that the substance is not readily biodegradable.

A number of supporting studies are available for HEDP-H and its salts associated with WWTP simulation, including aerobic and anaerobic studies with some using radiolabelling (Saeger, 1978; Henkel, 1979a and b; Henkel, 1981; Steber and Wierich, 1986a-c; Schoberl and Huber, 1988; Horstmann and Grohmann, 1980). For HEDP-H, 10.17% degradation was observed for a 24-hour cycle and 6.97% degradation for a 72-hour cycle in a SCAS procedure (Saeger, 1978) and <10% degradation was observed based on CO2 production (Sturm test) (Schoberl and Huber, 1988). For HEDP (2-3Na), minimal biodegradation (<5%) was observed under aerobic conditions in water and soil environments after 10 weeks and 79 days, respectively (Henkel, 1979a and b). 1.6% biodegradation was observed after 27 days under anaerobic conditions in a 14C-digester slurry simulation test (Henkel, 1981), ca. 3.8% biodegradation in anaerobic sludge was observed after 140 days under anaerobic conditions (Steber and Wierich, 1986a) and 3.8% after 28 days under anaerobic conditions (Steber and Wierich, 1986b), 33-48% biodegradation in 28 days was observed in algae-innoculated medium in a US EPA algal bioassay (Steber and Wierich, 1986c) and 35 -75% degradation after 126 days in a SCAS procedure (Horstmann and Grohmann, 1980).

In summary, all of the available evidence supports a lack of biodegradability even where pre-adapted inoculum is used, though in some cases removal from the system by adsorption processes has occurred. These studies support the conclusion that HEDP-H and its salts are not biodegradable in water.

The acid, sodium and potassium salts in the HEDP category are freely soluble in water and, therefore, the HEDP anion is fully dissociated from its sodium or potassium cations when in solution. Under any given conditions, the degree of ionisation of the HEDP species is determined by the pH of the solution. At a specific pH, the degree of ionisation is the same regardless of whether the starting material was HEDP-H, HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK or another salt of HEDP.

 

Therefore, when a salt of HEDP is introduced into test media or the environment, the following is present (separately):

  1. HEDP is present as HEDP-H or one of its ionised forms. The degree of ionisation depends upon the pH of the system and not whether HEDP (1-2Na), HEDP (2-3Na), HEDP-4Na, HEDP-xK salts, HEDP-H or another salt was added.
  2. Disassociated sodium/potassium cations. The amount of sodium/potassium present depends on which salt was added.
  3. Divalent and trivalent cations have much higher stability constants for binding with HEDP than the sodium or potassium ions, so would preferentially replace them. These ions include calcium (Ca2+), magnesium (Mg2+) and iron (Fe3+). Therefore, the presence of these in the environment or in biological fluids or from dietary sources would result in the formation of HEDP-dication (e.g. HEDP-Ca, HEDP-Mg) and HEDP-trication (e.g. HEDP-Fe) complexes in solution, irrespective of the starting substance/test material.

In this context, for the purpose of this assessment, read-across of data within the HEDP Category is considered to be valid.