A mixture of: O,O-di(1-methylethyl)trithio-bis-thioformate; O,O-di(1-methylethyl)tetrathio-bis-thioformate; O,O-di(1-methylethyl)pentathio-bis-thioformate

PBT assessment

PBT assessment: overall result

PBT status:

the substance is not PBT / vPvB

Justification:

Persistence Assessment

P criteria:

– the half-life in marine water is higher than 60 days, or

– the half-life in fresh- or estuarine water is higher than 40 days, or

– the half-life in marine sediment is higher than 180 days, or

– the half-life in fresh- or estuarine water sediment is higher than 120 days, or

– the half-life in soil is higher than 120 days

OECD 309 Testing has shown that AS100 and its hydrolysis degradant DIXD do not show any persistence characteristics over a 28 day period. The hydrolysis of AS100, using naturally-sourced river/lake water, had a half-life of approximately 48 hours indicating that AS100 is readily biodegrable. At its peak DIXD was also shown to undergo ready biodegradation and formed CO2 and mineralised species and again the half life of the biodegradation was approximately 48 hours. Therefore it can be concluded that AS100 and its degradant - DIXD - are not persistent and readily biodegrable. This also leads to the conclusion that AS100 cannot be assigned with the characteristics of either a PBT or a vPvB substance.

A Hydrolysis Study (Abiotic Degradation, Hydrolysis as a Function of pH) conducted on AS100 showed the substance hydrolyses in water, with the following half-lives determined:

16.7 days at pH4,

15 days at pH7

6.88 hours at pH9 (very similar to the OECD 309 test which used naturally-sourced local river/lake water of a pH of 7.8 and half-life was approximately 48 hours).

The rate of hydrolysis increases with an increase in pH.

Based on these hydrolysis results, the test substance (AS100) is not considered to meet the criteria for P or vP.

The main hydrolysis products were confirmed to be di-isopropyl xanthogen disulphide (DIXD) and sulphur. These results corroborate what was observed in the Aerobic Mineralisation test.

Bioaccumulation Assessment

B criteria: BCF > 2000.

vB criteria: BCF > 5000

 

For the PBT and vPvB assessment a screening criterion has been established, which is log Kow greater than 4.5.

The substance (AS100/DIXT) initially screens as meeting the B criteria, based on the partition coefficient result of log Pow >5.9.

However, at very high log Kow (>6), a decreasing relationship between the two parameters (log BCF increasing linearly with log Kow ) is observed. 

In order to further assess the bioaccumulation potential of AS100, estimates of BCF were made for the substance using a computer program.The BCFBAF v3.01 model of US EPI suite v4.10 was used to predict the BCF in Fathead Minnow of AS100.

BCFBAF v 3.01 estimates the bioconcentration factor (BCF) of an organic compound in the Fathead Minnow using the compounds Log Octanol- water partition coefficient (Kow).The octanol water partition coefficient used in the predictions for the three main constituents of AS100 was Log Kow = 5.9.

The predicted BCF value for the test material is 165.7 (for all three constituents) which suggest that the material is unlikely to bioaccumulate in the fish.

It is therefore considered, that although the log Pow is >5.9, based on the predicted BCF values, the criteria for B and vB may not be met.

Toxicity Assessment

 

T criteria:

– the long-term no-observed effect concentration (NOEC) for marine or freshwater organisms is

less than 0.01 mg/l, or

– the substance is classified as carcinogenic (category 1 or 2), mutagenic (category 1 or 2) or toxic

for reproduction (category 1, 2, or 3), or

– there is other evidence of chronic toxicity, as identified by the classifications: T, R48, or Xn, R48 according to Directive 67/548/EEC.

 The water solubility of AS100 has previously given variable results – in-house it was 0.3mg/L, in 1988 it was measured as 0.752mg/L and more recently it was measured as 1.3mg/L (most likely a super-saturated system so an exaggerated result). Using the average value of 0.752mg/L – it can be concluded that there is no aquatic test to which we can achieve a dissolved dose above the cut-off value of 1mg/L and achieve a negative result in the EC50 values (i.e. not environmentally toxic). It must be inferred that as a consequence of use of sterile test environments, which we know do not reflect the true aquatic environment, and the limit of water solubility of AS100 will always result in an acute environmental toxicity. Therefore, the classification is a result of inadequacies of the simulation testing and not a true measurement of the innate properties of AS100. Previous testing to determine EC50 values used the methodologies available for “poorly soluble/difficult substances” – such as use of detergents and solvents – but their use has resulted in inadvertent reaction with said substances leading to adverse reaction products which have produced adverse effect in the EC50 tests and its determination.

Two long-term studies in freshwater organisms have been conducted, a 14 day toxicity study in fish and a Daphnia reproduction study.

In the 14 day fish study the threshold levels of lethal and other observed effects for Gobiocypris rarus exposed to the test substance for 14 days were higher than the concentration of the saturated solution (the measured concentration was less than the limit of detection (LOD=1.12 ng/ml)) under tested conditions.

In the Daphnia reproduction study there was no observed inhibition on production of Daphnia magna during the test. In the definitive test, the measured concentration of test substance was less than Limit of detection (LOD).

In the Daphnia reproduction study there was no observed inhibition on production of Daphnia Magna during the test. In the definitive test, the measured concentration of the test substance was less than the Limit of Detection (LOD). Therefore, the two long-term studies do not give a definitive NOEC or full test substance concentration data on which to compre against teh criteria for T, as measured concentrations were below the Limit of Detection. However, both tests used saturated solutions of the test item and no toxic effects were observed during the studies. It is therefore concluded that the T criteria may not be met.

An algal inhibition study gave a NOEC of 0.10 mg/l (or 0.00084 mg/l based geometric mean measured test concentrations), which was the highest test concentration possible to test.Although this was a short-term algal test, it can be viewed as providing both acute and chronic endpoints, based on the short life cycle of the algae.The NOEC of 1.0 mg/l (based on nominal test concentration) and lack of toxicity observed in the study, indicates that the T criteria would not be met.

 

The substance is not currently classified as carcinogenic, mutagenic (category 1 or 2) or toxic for reproduction (category 1, 2 and 3)or classified as STOT RE (category 1 or 2) (nor R48).

The substance is therefore not considered to meet the Toxicity (T) criteria. However, as a precaution, the GHS classification still holds the H400 for acute environmental toxic effects.

 

Summary and overall Conclusions on PBT or vPvB Properties

 

Based on the information available, it is considered that the test substance (AS100/DIXT) does not meet the criteria to be classed as a PBT or vPvB substance, purely on the basis of persistence.