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Environmental fate & pathways

Biodegradation in soil

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Description of key information

Concluding from the reliable (without restriction) key study performed according to OECD 307 and compliant to GLP (RL 1) DMA category members are expected to degrade rapidly and ultimately in soil: A median (3 soils) half-life of 8.9 d at 20 degree C was determined for supporting substance hexadecanamine and found to be applicable for DMA category. This corresponds to a median half-life of 16.9 d at an environmental temperature of 12 degree C. It is important to note that despite the fact that cationic surfactants sorb considerably biodegradation rate is still high meaning rapid biodegradation. This experimental results do not support the assumption given in REACH Guidance R.16 Table R16.6 that the biodegradation half-life in soil is as lower as higher the sorption is.

Key value for chemical safety assessment

Half-life in soil:
16.9 d
at the temperature of:
12 °C

Additional information

The key study was performed with the supporting substance [1-14C]-hexadecanamine according to OECD 307 and compliant to GLP (RL 1). The test item is of high similarity to DMA category members (for details, see Category Document, chapter 1). Samples containing 100 g dry soil were treated with [1-14C]-hexadecanamine at the concentration of 0.45 mg a.i./kg dry soil (= 0.045 mg a.i./100 g dry soil), sufficient to determine its degradation rate. The application rate was based on an exposure modelling using realistic use rates. In order to achieve a realistic exposure regime the test item was added to the soil via sewage sludge. The amount of sludge added to the soil reflected normal sludge loading to agricultural soils according to REACH Guidance (5000 kg/ha) and corresponds to about 0.17 g dry sludge / 100 g dry soil. Although this C16 amine is strongly sorbing to soil (median Kp soil of 3875 L/kg at lowest measured concentration) the following half-lives at 20 degree C were determined

Soil 1 t1/2 = 9.0 d

Soil 2 t1/2 = 8.1 d

Soil 3 t1/2 = 8.9 d

The median Half-life of 8.9 d at 20 degree C corresponds to a median Half-life of 16.9 d at an environmental temperature of 12 degree C (see REACH Guidance R.16, equation R.16-9). This study demonstrates that 1-hexadecanamine (C16 amine) is rapidly degraded in various soils and that the assumption of low degradation rates for strongly sorbing substances could be unjustified.

 

The same holds true for the supporting study performed with the supporting substance HYEQS according to OECD 307 and compliant to GLP (RL 1). HYEQS is of high similarity to C12-14 DMA in that it has essentially the same alkyl chain distribution and – like DMA category members - two methyl substituents at the amino group. The only difference is an additional hydroxyethyl substituent at the amino group rendering it a quaternary amine being permanently positively charged. This is however a minor difference as due to protonation at environmentally relevant pH values also DMA category members are predominantly positively charged at the amino group (for details, see category document, chapter 1).The degradation rate of 14C-HYEQS in three aerobic soils was investigated during 62 days. 14C-HYEQS was applied at a rate of 0.45 mg a. i. /kg soil dw. using sewage sludge as carrier. The application rate was determined from an exposure modelling using realistic use rates. Soil sampling was done after 2, 4, 8, 15, 28 and 62 days. Further sampling up to 120 days was not necessary due to rapid biodegradation. Significant amounts of radioactive carbon dioxide and bound residues were formed. The total mean recoveries of radioactivity were in the range of 91.2 to 94.9% for the three soils. From the measurements the following DT50 were calculated:

DT50 soil 1 = 6.2 d (median of all 3 soils),

DT50 soil 2 = 6.0 d,

DT50 soil 3 = 13.6 d.

 

A further supporting study performed with the supporting substance C22 ATQ according to OECD 307 and compliant to GLP (RL 1) is available. C22 ATQ may be regarded as worst case for DMA category members due to its longer alkyl chain. A further difference is that it has - in addition to the two methyl substituents at the amino group inherent to all DMA category members - an additional methyl substituent at the amino group, rendering it a quaternary amine being permanently positively charged. This is however a minor difference as due to protonation at environmentally relevant pH values also DMA category members are predominantly positively charged at the amino group (for details, see category document, chapter 1).The degradation rate of 14C-C22 -ATQ in three aerobic soils was investigated during 124 days. The 14C-labelled substance was applied at a rate of 0.2 mg a. i. /kg soil dw. using sewage sludge as carrier. The application rate was determined from an exposure modelling using realistic use rates. Soil sampling was done after 3, 7, 14, 29, 62 and 124 days. Significant amounts of radioactive carbon dioxide and bound residues were formed. The total mean recoveries of radioactivity were in the range of 105 to 107% for the three soils. From the measurements the following DT50 for biotransformation were calculated:

DT50 soil 1: 23.2 d;

DT50 soil 2: 24.9 d;

DT50 soil 3: 41.4 d;

 

In conclusion, for DMA category members fast and ultimate biodegradation in soil is expected.Because the key study (performed with hexadecanamine) and the supporting study on HYEQS (which is of nearly identical chain length distribution like C12-14 DMA) produce nearly identical results (geometric mean DT50 values for 3 soils: hexadecanamine = 8.7; HYEQS = 8.0),the median half-life of 8.9 d at 20 degree C (corresponding to a median half-life of 16.9 d at an environmental temperature of 12 degree C) from hexadecanamine will be used for the exposure assessment of DMA category members.The longer half-lives found for supporting substance C22-ATQ is most probably due to the longer alkyl chain and therefore less relevant compared to the results of the two other studies. It is important to note that despite the fact that cationic surfactants sorb considerably biodegradation rate is still high meaning rapid biodegradation. This experimental results do not support the assumption given in REACH Guidance R.16 Table R16.6 that the biodegradation half-life in soil is as lower as higher the sorption is.