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Environmental fate & pathways

Phototransformation in water

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Endpoint:
phototransformation in water
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
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: supporting information
Reason / purpose:
read-across: supporting information
Reason / purpose:
read-across: supporting information
% Degr.:
82
Sampling time:
17 d
Test condition:
Light conditions; pH 7 (in presence of ferric nitrate)
% Degr.:
12
Sampling time:
17 d
Test condition:
Light conditions; pH7
% Degr.:
52.7
Sampling time:
300 min
Test condition:
UV only
DT50:
ca. 100 d
Test condition:
(in absence of sensitisers)
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: Test procedure in accordance with national (draft) standard methods with acceptable restrictions.
Study type:
direct photolysis
Principles of method if other than guideline:
Method: other (measured)
GLP compliance:
no
Light source:
sunlight
Type of sensitiser:
water with additives
% Degr.:
12
Sampling time:
17 d
Test condition:
Light conditions; pH7
% Degr.:
82
Sampling time:
17 d
Test condition:
Light conditions with Ferric Nitrate; pH7
DT50:
ca. 100 d
Test condition:
Half-lives not determined in report but would be equivalent to approx 100 d in absence of sensitisers.
Transformation products:
yes
Details on results:
12% transformation (phosphonate to orthophosphate) was measured after 17 days at pH 7 (9% at pH 4 and 4% at pH10). Levels of degradation in the presence of ferric (Fe III) nitrate were higher, with 82% transformation after 17 days at pH 7 (64% at pH 4 and 19% at pH10). The effect of other metals (chromic, zinc and cupric ions) was insignificant.

Table 1 presents the results seen for Dequest 2051 under the various conditions of this study.

Table 1: Degradation results for Dequest 2051

Test condition

pH

% transformation to orthophosphate

Day 3

Day 9

Day 17

Light conditions

4

2

6

9

7

2

6

12

9

0

1

4

Dark conditions

4

 

 

1

7

 

 

0

9

 

 

1

With Ferric nitrate (light)

4

32

47

64

7

28

50

82

9

3

12

19

With Ferric nitrate (dark)

4

 

 

2

7

 

 

0

9

 

 

3

With Chromic nitrate (light)

4

 

 

15

7

 

 

14

9

 

 

14

With Chromic nitrate (dark)

4

 

 

0

7

 

 

0

9

 

 

1

With Zinc nitrate (light)

4

 

 

13

7

 

 

15

9

 

 

13

With Zinc nitrate (dark)

4

 

 

0

7

 

 

0

9

 

 

1

With Cupric nitrate (light)

4

 

 

7

7

 

 

11

9

 

 

8

With Cupric nitrate (dark)

4

 

 

0

7

 

 

0

9

 

 

1

Validity criteria fulfilled:
not applicable
Conclusions:
Direct photolysis of HMDTMP is not extensive in the absence of sensitisers. Sensitised photolysis was observed in the presence of ferric nitrate.
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Study type:
other: both direct and sensitised photolysis were studied
Principles of method if other than guideline:
Study of photodegration (5-hr duration) with UV light and catalyst Mn and H2O2 (at 1%). The degradation products phosphate (PO4), ammonium (NH4) and total organic carbon (TOC) were analyzed in order to indirectly verify the cleavage efficiency of the C-P bond and C-N bond.
Also, the influence of the concentration of the catalyst Mn was tested for HDTMP.
GLP compliance:
no
Analytical method:
other: spectrophotometry
Details on sampling:
The samples were collected after initial 10 minutes, then after 30 minutes and subsequently in an interval of 30 minutes until the end of the study (full duration 5 hours).
Light source:
other: lamp
Light spectrum: wavelength in nm:
200
Relative light intensity:
600
Details on light source:
450 W lamp with wavelength from 200 to 600 nm (Heraeus Noblelight, Germany)
Details on test conditions:
The liquid volume of 350 ml containing phosphonates were exposed to UV light in a reactor with quartz cooling jacket and a 450 W lamp with wavelength from 200 to 600 nm. Concentrations: 100 mg/L, 0.20 mmol of HDTMP were used.

The degradation products phosphate (PO4), ammonium (NH4) and total organic carbon (TOC) were analyzed in order to indirectly verify the cleavage efficiency of the C-P bond and C-N bond.

Also, the influence of the concentration of the catalyst Mn was tested for HDTMP. 40 ml of phosphonate (100 mg/L) were treated with Mn at concentrations, 0.5%, 1%, 2%, 3%, 4%. The solutions were left continuously stirred for 24 hours and NH4 was measured after 1 h, 2 h, 4 h, 8 h and 24 h.
Duration:
5 h
% Degr.:
10.7
Sampling time:
10 min
Test condition:
UV only
% Degr.:
52.7
Sampling time:
300 min
Test condition:
UV only
% Degr.:
58.8
Sampling time:
10 min
Test condition:
UV with Mn
% Degr.:
63.5
Sampling time:
300 min
Test condition:
UV with Mn
% Degr.:
55.8
Sampling time:
10 min
Test condition:
UV with H2O2
% Degr.:
92.5
Sampling time:
300 min
Test condition:
UV with H2O2
% Degr.:
38.2
Sampling time:
10 min
Test condition:
UV with Mn and H2O2
% Degr.:
79.6
Sampling time:
300 min
Test condition:
UV with Mn and H2O2
DT50:
293.3 min
Test condition:
UV only
DT50:
14.9 min
Test condition:
UV in presence of Mn
DT50:
6.7 min
Test condition:
UV in presence of H2O2
DT50:
18.3 min
Test condition:
UV in presence of Mn and H2O2
Validity criteria fulfilled:
not applicable
Endpoint:
phototransformation in water
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: The study was well documented and meets generally accepted scientific principles, but was not conducted in compliance with GLP.
Study type:
direct photolysis
Principles of method if other than guideline:
Concentrations of 1 mg/l (3.2 uM) irradiated by a middle pressure mercury lamp emitting between 190 and 600 nm. pH 3, 5-6 and 10. Effect of presence of iron at 3.6uM investigated, matching the encountered concentrations in the environment (between 0.2 and 2.5uM Fe in ocean water and an average of 12uM Fe in river). The degradation of phosphonates measured by the release of orthophosphates (PO4-P) and aminomethylphosphonic
acid (AMPA).
GLP compliance:
no
Analytical method:
high-performance liquid chromatography
other: UV-VIS
Light source:
other: mercury lamp
Light spectrum: wavelength in nm:
190 - 600
Details on light source:
- Emission wavelength spectrum: 190nm - 600 nm

- Filters used and their purpose: quartz glass

- Relative light intensity based on intensity of sunlight: similar intensity to natural light in the visible range.
Details on test conditions:
TEST SYSTEM

- Type, material and volume of test apparatus/vessels: reactor is a round-bottomed 2L flask containing the mercury lamp. Flask is wrapped in aluminium foil to assure light-insulation.

TEST MEDIUM

- Kind and purity of water: distilled water

- Preparation of test medium: iron added to a concentration of 3.6µM

-pH range: 3-10
Duration:
1.5 h
Initial conc. measured:
1 mg/L
DT50:
15 min
Test condition:
pH3
DT50:
6 min
Test condition:
pH3 in presence of Fe
DT50:
18 min
Test condition:
pH5-6
DT50:
6 min
Test condition:
pH5-6 in presence of Fe
DT50:
50 min
Test condition:
pH10
DT50:
35 min
Test condition:
pH10 in presence of Fe
Validity criteria fulfilled:
not applicable

Description of key information

Photodegradation in water (HMDTMP-H, CAS 23605-74-5): 12 -82% transformation (phosphonate to orthophosphate) after 17 days under a range of conditions.

Key value for chemical safety assessment

Additional information

Three studies are available focussing on stability in water due to photodegradation mechanisms. Whilst this is not a conventional pathway for study it brings useful evidence for environmental fate in the real environment.

Photodegradation of HMDTMP-H (CAS 23605-74-5) in water was examined (Saeger, Monsanto (undated, believed to be 1979), reliability 2). 12% transformation (phosphonate to orthophosphate) was measured after 17 days at pH 7 (9% at pH 4 and 4% at pH 10). Levels of degradation in the presence of ferric nitrate were higher, with 28% transformation by day 3 in the presence of ferric nitrate at pH 7 (by day 17: 82% at pH 7, 64% at pH 4 and 19% at pH 10). The effect of other metals (chromic, zinc and cupric ions) was insignificant.

In a separate test (Lesueur et al, 2010), half-lives less than 1 hour were measured in water at pH 3, pH 5-6 and at pH 10, irradiated

by a middle pressure mercury lamp emitting between 190 and 600 nm. Half-lives were found to be shorter in the presence of iron ions at environmentally relevant concentrations. Degradation in the presence of ferric (Fe III) ions reflects the ability of that ion to absorb light, and because it can be strongly complexed by HMDTMP, that energy can be transferred to the complexing anion, resulting in degradation. It is possible that ferrous (Fe II) ions would be formed in this process, and, due to the presence of oxygen, ferric ions would be regenerated. The degradation product identified in this study is orthophosphate. No specific reaction pathway is proposed by the study authors.

In a third study (Brandenburg University of Technology, 2010), a half-life of approximately 5 hours was measured in water irradiated with a 450 W lamp with wavelength from 200 to 600 nm. In the presence of Mn ions and/ or peroxide, the half-lives were much shorter (6.7 to 18.3 minutes). The degradation product identified in this study is orthophosphate. No specific reaction pathway is proposed by the study authors.

Discussion of trends in the HMDTMP category

Photodegradation in the presence of common metal ions has been observed. Based on evidence from a number of studies members of this group are considered to be partially degradable over short time periods, and with evidence of mineralisation, particularly in the light, over longer periods.