7-acetamido-4-hydroxy-3-[[4-[[2-(sulphooxy)ethyl]sulphonyl]phenyl]azo]naphthalene-2-sulphonic acid, sodium salt

Endpoint summary

Description of key information

Stability

Hydrolysis

Based on results of the hydrolysis study, the half life at 25°C is estimated to be > 1 year, 6 days, and < 1 day at pH 4, 7, and 9, respectively.

 

Biodegradation

Biodegradation in water: screening test

The ready biodegradability of the test substance was tested in the modified OECD 301E screening test and the inherent biodegradability in a modified Zahn-Wellens test. No biodegradation was observed in either test system.

Transport and distribution

Adsorption/desorption

A study on adsorption and desorption does not need to be conducted because the test substance has a low octanol water partition coefficient (log Kow <-2.61) and the adsorption potential of this substance is related to this parameter.

Additional information

It should be noted that the test substance is not considered as posing a hazard to the aquatic environment.

The test substance, Reactive Orange 72/78, is a solid under all environmental conditions and is highly soluble in water. It has a low volatility (based on the high melting / boiling point and high molecular weight) and a low affinity for soil / sediment (based on the partition coefficient value of log Kow ≤ -2.61). As such, any environmental release will result in virtually all of the substance compartmentalising into water compartments, with little release directly to atmosphere or compartmentalising to soil/sediment compartments.

Any potential exposure to the environment is predicted to result in rapid redistribution to water; due to its low volatility, high water solubility and partitioning values indicates that the majority of the substance would eventually partition to water rather than to soil and sediment should it be released to the environment. 

A Level III fugacity model was conducted in the US EPA EPISUITE which assumes steady-state but not equilibrium conditions.The Level III model in EPI Suite predicts partitioning between air, soil, sediment and water using a combination of default parameters and various input parameters.This model has been used to calculate the theoretical distribution of the highest % component substance between four environmental compartments (air, water, soil, sediment) at steady state in a unit world.

Table 1. Partitioning model Reactive Orange 72/78

Compartment	Distribution [%]	Half-life [h]
Air	3.8E-7	15.1
Water	7	4.32E3
Soil	92.8	8.64E3
Sediment	0.186	3.89E4

It is proposed that although the majority of the substance distributes to the soil compartment within the model, the high solubility in water indicates that the substance is more likely to distribute to water – e. g. soil pore water.

This assumption is confirmed by the model for the STP Overall Chemical Mass Balance in the US EPA EPI SUITE, which shows that 98% of the influent of the dye is in the water phase:

Table 2. STP Overall Chemical Mass Balance

	g/h	mol/h	percent
Influent	1.00E+001	1.6E-002	100.00
Primary sludge	2.50E-002	4.0E-005	0.25
Waste sludge	1.50E-001	2.4E-004	1.50
Primary volatilization	5.45E-023	8.8E-026	0.00
Settling volatilization	1.49E-022	2.4E-025	0.00
Aeration off gas	3.66E-022	5.9E-025	0.00
Primary biodegradation	1.76E-003	2.8E-006	0.02
Settling biodegradation	5.27E-004	8.5E-007	0.01
Aeration biodegradation	6.93E-003	1.1E-005	0.07
Final water effluent	9.82E+000	1.6E-002	98.15
Total removal	1.85E-001	3.0E-004	1.85
Total biodegradation	9.22E-003	1.5E-005	0.09

Reactive Orange 72/78 displays a low ready biodegradability in that it achieved < 10% biodegradation in a modified OECD screening test and was not inherently biodegradable, achieving 4% biodegradation in a Zahn-Wellens test. Microbial decolourization of azo dyes in an anaerobic environment occurred as a result of reduction of azo bonds, leading to decolourized metabolites. No ready biodegradation of these metabolites was expected in the anaerobic system. However, these metabolites could be further diminished by means of either biodegradation or autoxidation under subsequent aerobic treatment. The mixed bacterial cultures decolorized three structurally dissimilar azo dyes, suggesting that anaerobic decolourization was not a specific process (Supaka 2003). It was clear that the majority of the colour removal occurred in the anaerobic stage.

The substance is expected to hydrolyse under normal environmental conditions. Experimental studies on hydrolytic effects of a structural analogue demonstrated that the substance does undergo rapid hydrolysis at environmentally relevant pHs, (t_½= 6 days at pH7 and < 1 day at pH9), indicating the potential for significant removal by hydrolysis. At use conditions during dyeing (pH > 10 at ≥ 60°C) the substance is hydrolytically unstable. At 50°C and pH9, more than 99% of the substance was degraded at the first measure point of 1.5 hours. As such, degradation is anticipated via this route. Studies on direct phototransformation in water are not available but it was found that the sulfonated azo dyes can be destroyed by UV photooxidation process (Saliha 2004). The kinetics of the degradation depends on the azo, benzene and naphthalene groups of the dyes. It was found that the first step of the degradation is related to cleavage of azo bond of the molecule and naphthalene ring which leads to further degradation until complete mineralization. It is concluded, therefore, that abiotic processes would contribute significantly to the depletion of the substance within the environment.

Reactive Orange 72/78 has an estimated log Kow of ≤ -2.61. This value indicates that possible bioaccumulation in the food chain is not anticipated. Given the fact that the substance is subject to hydrolysis at biologically relevant pHs, it is anticipated that bioaccumulation of the substance itself would not occur, as hydrolytic effects in association with metabolic effects would result in removal of the substance. Based on its high water solubility, low partition coefficient and fairly rapid hydrolysis rate at environmentally relevant pHs, it can be concluded that it is unlikely that Reactive Orange 72/78 could potentially be persistent within the environment.

Adsorption to soil is deemed to be low, based on the very low partition coefficient value and high water solubility. Such a low potential indicates that the substance is unlikely to bind tightly to soils and sediments and instead partition almost exclusively to water. As such, significant exposure related effects to sediment and soil dwelling organisms are considered to be negligible.

Based on its high water solubility and low partition coefficient it can be concluded that it is unlikely that Reactive Orange 72/78 could potentially be persistent within the environment in its registered form. Abiotic effects within the environment will result in eventual removal from the environment and hence significant contact with the organisms in the food chain can considered to be minimised.

Finally, Reactive Orange 72/78 demonstrates low acute toxicity in mammalian studies therefore in the event of exposure to environmental organisms. Effects due to secondary poisoning can be excluded.