Sodium Salts of (1-Hydroxyethylidene)bisphosphonic acid (2-3 Na:1)

Biodegradation in water: screening tests

Link to relevant study record(s)

Reference

Reference 1

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 The sample was allowed to settle for 30 minutes and the supernatant withdrawn for test.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.

Reference 2

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)

The registrants consider that the possible benefits to the CSA of conducting further studies of the formation of degradation products are not significant in comparison with the foreseeable difficulties to conduct and interpret the study.

Isolating and identifying degradation products presents a significant analytical challenge. There is substantial evidence across most types of phosphonates of rapid and irreversible binding to solids, particularly inorganic substrates (please refer to Section 4.2.1 of the Category CSR). It is difficult to envisage an analytical system suitable for extracting and analysing the substances which could not be affected by this. Secondly, the relevance of the data must be considered. This CSR discusses the environmental fate of HEDP and other analogous phosphonates. Whilst there is limited degradation in the environment, it is not extensive or rapid under standard conditions. Removal processes from natural waters are attributed to the typically rapid, irreversible adsorption to solid matrices. As such the chemical safety assessment for the environment focuses on the parent substance. There are no unacceptable risks (please refer to CSR Chapter 10). The substance is not classified for environmental hazard, and is not PBT or vPvB. The organophosphonate impurities are predicted to have the same properties as HEDP and not be of higher toxicity. Inorganic impurities present are not biodegradable.

Description of key information

The substance is not readily biodegradable, based on the results of a BOD5 test.

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, aerobic and anaerobic studies using radiolabelling etc (Henkel, 1979a and b; Henkel, 1981; Steber and Wierich, 1986). For HEDP (2-3Na) 0.4% and 4.8% biodegradation was observed after 79 days under aerobic conditions in a modification of the OECD TG 304A study (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 was observed after 140 days (Steber and Wierich, 1986a) and 3.8% after 28 days under anaerobic conditions (Steber and Wierich, 1986b), and 33-48% biodegradation was observed in algae-innoculated medium in a US EPA algal bioassay (Steber and Wierich, 1986c).

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(2-3Na) is 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.