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Environmental fate & pathways

Hydrolysis

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Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Qualifier:
no guideline required
Principles of method if other than guideline:
The kiniteics of hydrolysis of pyrophosphate and tripolyphosate in sterile lake water was investigated.
GLP compliance:
not specified
Analytical monitoring:
yes
Buffers:
Buffers were not used in this study as the purpose was to investigate the rate of hydrolysis of pyrophosphate and tripolyphosphate in natural lake water. The lake water was taken from Lake Mendota, Wisconsin, USA.
Details on test conditions:
To ensure that hydrolysis was due to chemical reaction rather than a reaction induced by microbes, the lake water was sterilised by filtering through a sterile mebrane filter into a sterile filter flask. The flask was used as the reaction vessel for monitoring hydrolysis with aliquotes of 50 ml withdrawn aseptically for analysis.

Pyrophosphate - 15 analytical timepoints over 350 hours, pH 8.3, temperature 25°C. Initial concentration 0.5 mg P/l
Tripolyphosphate - 15 analytical timepoints over 400 hours, pH 8.3, temperature 25°C. Initial concentration 0.5 mg P/l

The results for the rate of hydrolysis in sterile lake water were compared to published rate constants for the hydrolysis of pyrophosphate and tripolyphosphate in distilled water at various temperatures and hydrogen ion concentrations. Rate cionctants at temperatures other than 25°C were transformed to rate constants at 25°C by using Arrhenius's equation.

Number of replicates:
Pyrophosphate - 2
Tripolyphosphate - 1
Positive controls:
no
Negative controls:
no
Transformation products:
not measured
Details on hydrolysis and appearance of transformation product(s):
Tripolyphosphate is hydrolysed to pyrophosphate and this undergoes hydrolysis to orthophosphate.
pH:
8.3
Temp.:
25 °C
Hydrolysis rate constant:
0 min-1
Type:
not specified
Remarks on result:
other: Pyrophosphate
pH:
8.3
Temp.:
25 °C
Hydrolysis rate constant:
0 min-1
Type:
not specified
Remarks on result:
other: Tripolyphosphate
Details on results:
The logarithm of substance remaining plotted against time was linear indicating first order kinetics.

The first-order rate constant for pyrophosphate is 6.1 x 10-5 min-1.
The first-order rate constant for tripolyphosphate is 1.4 x 10-4 min-1

The rate of hydrolysis of tripolyphosphate is approximately 2.5 times greater than the rate of hydrolysis of pyrophospahte.

The rate constants obtaiend are approiximately 1000 times more rapid than those obtained with distilled water.
Conclusions:
The rate of hydrolysis of pyrophosphate and tripolyphosphate in sterile lake water is approximately 1000 times faster than in distilled water.
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Qualifier:
no guideline required
Principles of method if other than guideline:
The kinetics of hydrolysis of pyrophosphate and tripolyphophate in sterile and non-sterile lake water was investigated.
GLP compliance:
not specified
Analytical monitoring:
yes
Details on test conditions:
The rates of hydrolysis of tripolyphosphate and pyrophosphate were determined in lake Mendota water.

The water was collected in March from an intake located approximatley 600ft from shore, and 8ft below the water surface where the total depth is 20ft. The phopshates were added to 10l of lake water in 3 gallon carboys which were used as reaction vessels. The solutions were aerated and illuminated with fluorescent lamps. Gluocose (50mg/l) was added to half of the reaction vessels to encourage microbial activity. Each day for a period of 8 days. aliquotes were taken from the reaction vessles and the orthophosphate and "total phosphate" contents were determined. However, before the phosphate content of the aliquot was determined, the aliquotes were membrane-filtered so that only the non filterable phosphorus was measured.

Typical experiment 1 - hydrolysis in lake water.
Temperature 25°C, pH range 7.9-8.7. Initial phosphorus concentration 0.5 mg P/l. Duration 160 hours, analysis at 7 timepoints.

Positive controls:
no
Negative controls:
no
Transformation products:
not measured
Details on hydrolysis and appearance of transformation product(s):
Tripolyphosphate is hydrolysed to pyrophosphate and this undergoes hydrolysis to orthophosphate.

pH:
7.9
Temp.:
25 °C
DT50:
ca. 60 h
Details on results:
The studies on the hydrolysis of pyrophosphate and tripolyphosphate at 0.5 mgP/l have shown that these compounds are hydrolysed to orthophophate in a period of several days. Hydrolysis proceeded more rapidly in lake water supplemented with glucose with tripolyphosphate being affected to a greater extent than pyrophosphate. The final concentration of phosphorus was lower than expected in water containing glucose due to uptake of phosphorus by microorganisms as the glucose was metabolised. Tripolyphosphate was more stable than pyrophosphate in lake water (but less stable in lake water containing glucose)

Conclusions:
Pyrophospahte hydrolysed over a period of several days in non-sterile lake water.
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
A robust and well documented study. Not conducted to a standard method or to any quality assurance but the work conducted is fully described and experimental details are given. The work is well carried out to a high scientific standard and the test conditions used are representative of actual conditions encountered in the life cycle of the substance.
Qualifier:
no guideline required
Principles of method if other than guideline:
Hydrolysis of triphosphate in a rural waste water system was investigated
GLP compliance:
no
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
Five litre aliquots of raw sewage were collected each hour from the influent pipe entering the local sewage treatment plant. Samples were immediately pre-filtered through a 150 µm nylon mesh screen, followed by filtration on site through a 0.2 µm membrane filter (Millipore) using a tangential flow filtration apparatus. A 100 ml aliquot of this filtrate was randomly selected and stored on ice for not more than 24 h prior to analysis.
Buffers:
Not applicable (field study)
Details on test conditions:
The waste water facility selected for this study was the Whittlesea Local Treatment Plant (LTP) located on the outskirts of a rural township near Melbourne, Australia.The plant uses an extended aeration activated sludge process and the transit time from household to treatment plant is 2.6 hours.
Number of replicates:
One replicate per sample point. 12 samples analysed every hour over an 11 hour period.
Transformation products:
not measured
Temp.:
15 °C
DT50:
7.42 h
Temp.:
20 °C
DT50:
2.97 h
Details on results:
Detergent phosphates were found to rapidly degrade in raw sewage. Unfiltered sewage samples containing triphosphate at the time of sampling contained no detectable levels of diphosphate or triphosphate 24 h later. The degradation of tripho­ sphate spiked into unfiltered raw sewage at 15 and 20°C exhibited pseudo-first-order reaction kinetics with rate constants of (2.59 ± 0.05) x 10-5 s-1 and (6.47±0.08) x 10-5 s- 1, respectively. Half-lives for triphosphate of 7.42 ± 0.15 and 2.97 ± 0.04 h at 15 and 20°C respectively were calculated from the rates given above.

The major factor contributing to triphosphate degradation in waste water was shown to be biological in nature, with the most likely mechanism being enzymatic hydrolysis. Sewage samples that were spiked with triphosphate and sterilised by autoclaving showed little degradation over a 10 h period, compared with rapid triphosphate degradation that occurred over the same period in unsterilised samples.

The study shows that triphosphate undergoes rapid degradation in sewage by hydrolysis first to diphosphate (pyrophosphate) then to orthophosphate. Although the study indicates that hydrolysis of diphosphate to orthophosphate will be intrinsically slower than hydrolysis of triphosphate to diphosphate there is no attempt to evaluate the difference.

Conclusions:
The half-life time for triphosphate in the sewage has been determined as 7.42 hours at 15°C and 2.97 hours at 20°C.
Endpoint:
hydrolysis
Type of information:
experimental study
Adequacy of study:
weight of evidence
Study period:
16 December 2003 and 11 March 2004
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: GLP study conducted in accordance with an internationally recognised method
Qualifier:
according to guideline
Guideline:
OECD Guideline 111 (Hydrolysis as a Function of pH)
GLP compliance:
yes (incl. QA statement)
Radiolabelling:
no
Analytical monitoring:
yes
Details on sampling:
Aliquots of the sample solutions were taken from the flasks at various times and the pH of each solution recorded.

The concentration of the sample solution was determined by high performance liquid chromatography (HPLC, organic component) and ion chromatography (pyrophosphate component).
Buffers:
pH 1.2
Hydrochloric acid 65.0 mmol.dm-3
Potassium chloride 50.0 mmol.dm-3

pH 4
Monopotassium citrate 5.0 mmol.dm-3
Sodium hydroxide 0.9 mmol.dm-3

pH 7
Disodium hydrogen orthophosphate (anhydrous) 3.0 mmol.dm-3
Potassium dihydrogen orthophosphate 2.0 mmol.dm-3
Sodium chloride 2.0 mmol.dm-3

pH 9

Disodium tetraborate 1.0 mmol.dm-3
Sodium chloride 2.0 mmol.dm-3
Details on test conditions:
TEST SYSTEM
- Type, material and volume of test flasks, other equipment used: glass flasks
- Sterilisation method: buffers filtered through 0.2µm membrane filter
- Measures taken to avoid photolytic effects: solutions were shielded from light
- Measures to exclude oxygen: solutions were subjected to ultrasonication and degassing with nitrogen to minimise dissolved oxygen content.
- Test system: closed (stoppered test flasks)
- Indication of the test material adsorbing to the walls of the test apparatus: none reported
Number of replicates:
2
Positive controls:
no
Negative controls:
no
Preliminary study:
Organic component at pH 4, 7 and 9:
Less than 10% hydrolysis occurred over 5 days at 50°C. This is equivalent to a half-life of greater than 1 year at 25°C and therefore the organic component of MPP is considered stable to hydrolysis and no further testing is necessary.

Pyrophosphate at pH 7 and 9:
Less than 10% hydrolysis occurred over 5 days at 50°C. This is equivalent to a half-life of greater than 1 year at 25°C and therefore pyrophosphate is considered stable to hydrolysis under these conditions and no and no further testing is necessary.

Pyrophosphate at pH 4:
Greater than 10 % hydrolysis (actual value 24.2%) occurred over 5 days at 50°C and therefore pyrophosphate cannot be considered to be stable at this pH and temperature and further testing is necessary.

Test performance:
Main test:

As pyrophosphate was found to undergo hydrolysis at pH 4 at 50°C in the preliminary test, it was extended to 385 hours until 40% degradation was observed, from which a half-life of 12.34 days at 50°C was measured.

A further two tests were conducted at 60°C for 75 hours and 70°C for 72 hours from which half life times of 80 hours and 21.6 hours, resepectively, were measured. Using the rate constants obtained at the three temperatures, it was possible to extrapolate a half-life for 25°C (at pH4). The extrapolation showed that the half-life time for pyrophosphate at 25°C is 1.49 years.
Transformation products:
not measured
No.:
#1
Details on hydrolysis and appearance of transformation product(s):
The hydrolysis product was not identified in the hydrolysis test but based on mechanistic consideration must be phosphoric acid.
Key result
pH:
4
Temp.:
25 °C
DT50:
1.49 yr
Type:
(pseudo-)first order (= half-life)
Key result
pH:
1.3
Temp.:
37 °C
DT50:
24 h
Type:
(pseudo-)first order (= half-life)
Other kinetic parameters:

The kinetics of the study have been determined to be consistent with that of a pseudo-first order reaction as the graphs of log10 concentration versus time are straight lines.

A further test was conducted on pyrophosphate under physiological conditions of pH 1.3 and 37°C. The results indicate that under these conditions 14.4% hydrolysis occurred in 24 hours.

Validity criteria fulfilled:
yes
Conclusions:
The organic component of MPP is stable to hydrolysis at pH 4, 7 and 9.

The inorganic component is stable to hydrolysis at pH 7 and 9 but undergoes hydrolysis at pH 4 with a half-life of 1.49 years at 25°C. Under physiological conditions of pH 1.3 and 37°C hydrolysis occurs at a rate of 14.4 % degradation in 24 hours.

Description of key information

The organic component of MPP is stable to hydrolysis but the pyrophosphate component shows potential for hydrolysis under environmental and physiological conditions.  

Key value for chemical safety assessment

Additional information

Hydrolysis of organic component of MPP

In a GLP study conducted in accordance with OECD Test Guideline 111, the organic component of MPP was found to have a hydrolytic half-life of greater than 1 year at pH 4, 7 and 9 at 25°C and is therefore regarded as stable to hydrolysis under environmental conditions.

Hydrolysis of Pyrophosphate

In a GLP study conducted in accordance with OECD Test Guideline 111, the Pyrophosphate component of MPP was found, in a preliminary test at 50°C carried out over 5 days, to be stable to hydrolysis at pH 7 and 9 but was shown to be unstable at pH 4 with a half-life of 12 day. Under physiological conditions of pH 1.3 and 37°C hydrolysis occurred at a rate of 14.4 % degradation in 24 hours. In the main test at pH 4 at 25°C pyrophosphate was found to be stable with a half-life of 1.49 years, contrary to the results of the preliminary test. The test report does not offer an explanation for this behaviour and from the evidence available it is concluded that pyrophosphate undergoes rapid hydrolysis under physiological conditions (pH 1.3, 37°C) and appears to stable to hydrolysis under environmental conditions at 25°C and pH 7 and 9 at 25°C, but there is some evidence from the preliminary test that pyrophosphate can undergo hydrolysis under some conditions.

Further investigations on hydrolysis of phosphate:

While the above test was conducted under laboratory, sterile conditions, there is evidence that pyrophosphate in fact undergoes rapid hydrolysis in the environment.

Surface water

Clesceri and Lee (1965 -II) showed that tripolyphosphate and pyrophosphate hydrolysed 1000 times quicker in sterile lake water than in distilled water. The increase in the rate of hydrolysis was attributed to dissolved substances such as calcium. Note: in sterile medium, triphosphate hydrolysed slightly quicker than pyrophosphate.

Clesceri and Lee (1965 -I) determined the rate of hydrolysis of tripolyphophate and pyrophosphate in non-sterile lake water at 25°C at an initial concentration of 0.5 mg P/l. The results showed that  tripolyphosphate and pyrophosphate were hydrolysed to orthophosphate in a period of several days. The rate of hydrolysis increased with the addition of glucose, providing evidence that microbial activity is the primary mechanism of hydrolysis. In non-sterile medium pyrophosphate hydrolysed slightly quicker than triphosphate. A rate constant and half-life time for triphosphate was determined to be :

k (25°C): – 0.2369 d-1

Half-life time: 70.2 hrs

Sewage treatment plant

Halliwell et al. (2001) studied the hydrolysis of Sodium Triphosphate in the raw sewage entering the waste water treatment plant of a small rural township treating exclusively domestic sewage. Triphosphate was spiked to give an initial concentration of 2 mg P/l and the degradation of triphosphate in the sewage was found to exhibit pseudo-first-order reaction kinetics, from which the following rate constants were obtained:

k (15°C) = 2.59 x 10-5s-1= 0.093 h-1
k (20°C) = 6.47 x 10-5s-1= 0.233 h-1

which corresponds to half-lives of 7.42 h and 2.97 h at 15 °C and 20 °C respectively.

The major factor contributing to triphosphate degradation in waste water was shown to be biological in nature, with the most likely mechanism being enzymatic hydrolysis.

The study shows that triphosphate undergoes rapid degradation in sewage by hydrolysis first to diphosphate (pyrophosphate) then to orthophosphate. Although the study indicates that hydrolysis of diphosphate to orthophosphate will be intrinsically slower than hydrolysis of triphosphate to diphosphate there is no attempt to evaluate the difference. Taking into consideration that the hydrolysis data obtained using non-sterile lake water above suggests the opposite, and that this is based on experimental data, it can be concluded that the rapid degradation of triphosphate in sewage water is indicative of similar rapid hydrolysis of pyrophosphate in sewage treatment plant waste water.

Hydrolysis of MPP 

It is clear that under both environment and physiological conditions the pyrophosphate component of MPP will undergo rapid degradation to phosphate whereas the organic component will be stable under such conditions.