Survey of azo-colorants in Denmark

Executive Summary

Background

The Danish Environmental Protection Agency (Danish EPA) has in 1996 published a position paper on their standpoint regarding the status and perspectives of chemicals (Miljøstyrelsen, 1996c). With reference to the position paper and in the light of the general international legislative development, a list of chemicals of concern, including azo colorants, has been proposed by the Danish EPA.

Objective

The objective of the survey has been to summarise present knowledge concerning toxicological and environmental properties of the azo colorants. Furthermore, the objective has been to establish an overview of consumption and use of azo colorants in Denmark, aiming at establishment of a preliminary mass balance.

Based on the overview of consumption and use, the survey also aims at, on a provisional and qualitative level, identifying and assessing the human and environmental risks.

Scope of the survey

The survey has been limited/confined to include the trades manufacturing azo colorants, i.e. the dye industry, and the primary users of colorants, the plastics processing industry, leather and leather products, textiles, pulp and paper, printing, paints and lacquers.

Azo colorants consumed and applied in the drug, cosmetic and food industries are omitted, because they are subject to legislation.

The overview of consumption and use does not include either intermediates or metabolites. However, the survey encompasses their toxicological and environmental properties.

Content

The survey covers:

	Technical aspects of azo colorants.
	Consumption and use in Denmark and mass balances for dyes and pigments, respectively.
	Physico-chemical properties, toxicity, environmental fate and ecotoxicity of azo dyes.
	Physico-chemical properties, toxicity, environmental fate and ecotoxicity of azo pigments.
	Conclusions and recommendations.

Technical aspects

Azo colorants are the most numerous and widely manufactured group of synthetic colorants encompassing both azo dyes and azo pigments. The chemical organic synthesis of azo colorants is relatively simple and cheap.

Azo colorants have a chromophore group, the azo linkage. Although all the azo colorants share this group, they exhibit a great variety of physical, chemical and technological properties. Azo dyes may be further divided into ionic and non-ionic dyes.

The azo linkage of azo dyes easily undergoes enzymatic, thermal or photochemical breakdown, whereas the linkage of azo pigments is stable, except with regards to thermal breakdown. Cleavage of azo dyes results in free component aromatic amines.

The main difference between azo dyes and azo pigments, is that azo dyes are soluble in water and/or in substrate, whereas pigments are only sparingly soluble.

Impurities may be found in almost all commercial available formulations of azo colorants. They may be introduced during the manufacturing process and/or as a result of thermal or photochemical decomposition of the native colorants.

The industrial production and use of pigments, including azo pigments, are expanding world-wide. Today, most likely 50% of organic colorants applied within industrial processes are organic pigments.

Mass balance

Danish azo pigments are mainly used in the processing industries in: paints, lacquers, printing and printing inks and in plastics. Azo dyes are predominantly used in the colouring of textiles and to some extent in plastics and leather.

Production of pigments takes place in Denmark (approximately 18,000 tonnes/year), whereas all dyes are imported. Mixing of dye formulations is, however, carried out in Danish dye houses.

The total input is 2,400 tonnes of dyes and 22,600 tonnes of pigments annually.

Imported goods account for an important share of the mass flow of azo colorants in Denmark: 3/4 of the azo dyes and 1/5 of the azo pigments are imported in manufactured products, especially in textiles and in printing inks.

The exports of azo colorants are 1,400 tonnes and 17,400 tonnes for dyes and pigments, respectively.

The survey has revealed that the major importers and producers of azo colorants do not import and/or sell azo colorants, restricted abroad, in Denmark. However, registrations in the Product Register indicate that some of these colorants are in use. In addition, the restricted compounds may be present in textiles and leather products from Asia, Eastern Europe and South America. The imports from Asia alone account for 430 tonnes of azo dyes, primarily in textiles and 40 tonnes of azo pigments in leather products. Thus, at least 20% of the azo dyes associated with imported goods stem from regions where there may be a potential use of the restricted dyes.

About 70 tonnes of dyes and more than 10 tonnes of pigments may be released to waste water during processing of textiles and to a minor extent leather. Presumably most of this does not reach the municipal sewage treatment plants, as most of the industries concerned are submitted to restrictions with respect to their emissions.

Washing of textiles in the use-phase, on the other hand, may cause a release of about 70 tonnes of azo dyes and 10 tonnes of pigments which are emitted directly to the municipal sewage treatment plant.

Emissions to the atmosphere during production, processing and incineration are insignificant, approximately 0.

Most of the azo colorants are disposed by incineration, however, approximately 1,000 tonnes are landfilled and 50 tonnes of the azo pigments from the paper recycling are associated with sludge, applied on soil.

Physico-chemical properties

The azo colorants share some common physico-chemical properties like absorption maxima in the range of visible and UV-light and low vapour pressures. The non-ionic dyes and pigments are sparingly soluble in water and have, in general, high octanol-water partition coefficients (log K_ow 3 to 8). In contrast hereto are the ionic dyes, which are characterised by being very soluble in water and having low partition coefficients (-3 to 2.5).

The physico-chemical properties of the metabolites vary within the same range as the colorants, except with respect to their absorption maxima, which are generally below the range of visible and UV-light.

Human toxicity

Azo colorants exhibit an extremely wide variety of toxicological properties. Certain azo colorants, all azo dyes, belong to the first organic compounds associated with human cancer, although many of the azo dyes are not carcinogenic.

The azo linkage of azo dyes, but not of azo pigments, may undergo metabolic cleavage resulting in free component aromatic amines. 22 of these amines are recognised as potential human carcinogens and/or several of them have shown carcinogenic potential in experimental animals. The toxicity (carcinogenicity) of azo dyes is therefore mainly based on the toxicity of the component amines.

Aromatic amines are one of the first classes of organic compounds in which the structural and molecular bases for carcinogenicity are well understood.

The apparent generality of the metabolic cleavage of azo linkage has raised concern about the potential hazards associated with exposure to azo colorants, inclusive azo pigments.

Extensive toxicological investigation on experimental animals have been carried out in the past decades. The investigations have mainly been related to carcinogenicity and the mechanism behind, whereas to the remaining toxicological end-points only very limited attention has been given.

Based on the experiences with azo dyes, the probable carcinogenicity of azo pigments has been of main concern. Although epidemiological studies have not revealed any risks, several carcinogenicity studies have been carried out on experimental animals. Azo pigments are, due to their very low solubility in water, in practice, not available for metabolic activity. Consequently, metabolic cleavage to the component aromatic amines has not been found for the pigments.

Although the metabolic cleavage of azo dyes is the main source of aromatic amines, aromatic amines may also be present as impurities in both azo dyes and azo pigments.

Despite a very broad field of application and exposure, sensitising properties of some groups of azo colorants have been identified in relatively few reports. The allergenic potential of azo colorants seems to be very low.

Due to a strong relationship between exposure to azo dyes and/or aromatic amines and evidence for human cancer and/or cancer in experimental animals, the aromatic amines account for the greatest hazard to health. Consequently, exposure to azo dyes based on aromatic amines, which are known or suspected human carcinogens, encompasses the greatest risk for health.

Azo pigments do not show carcinogenic potential neither in humans nor in experimental animals. However, the presence of aromatic amines as impurities in azo pigments may, depending on the actual exposure, constitute a risk for human health.

Environmental fate and ecotoxicity

Adsorption seems to be the major route of removal of azo colorants in the environment. This applies for the metabolites, as well.

Abiotic degradation (photolysis and hydrolysis) does not play a dominant role in the environmental fate of azo colorants or their metabolites.

In contrast, biotic degradation of the azo dyes may take place in an anaerobic environment. Biodegradation of azo dyes, in general, varies from hours to several months or more indicating that they are at least inherent biodegradable. The pigments, however, do not seem to be biodegradable, neither ready nor inherent. The metabolites are primarily biodegraded under aerobic conditions. Some of the metabolites are ready biodegradable and some are inherent biodegradable.

In general, it is indicated that the ionic dyes do not have any significant bioaccumulation potential. However, when looking at the log BCFs (bioconcentration factor) of the dyes encompassed in the survey, it is indicated that some may bioaccumulate in fish. The non-ionic dyes and pigments, on the other hand, have a potential risk for bioaccumulation. But for the pigments, experimentally assessed BCFs indicate that the immediate concern for bioaccumulation is very low.

The metabolites, generally, have a bioaccumulation potential.

Generally, it is indicated that the ecotoxicity of azo pigments to aquatic organisms, compared to the azo dyes, is lower.

Some of the ionic dyes, i.e. acid and basic, are acute toxic to aquatic organisms. Reactive dyes are not considered to be toxic to aquatic organisms.

Furthermore, it is indicated that the non-ionic dyes are toxic or potentially toxic. Solvent dyes may even be acute toxic to aquatic organisms. The mordant dyes may, according to the present findings, not be of immediate concern.

Short term studies imply that azo pigments, in general, do not give rise to immediate concern about aquatic toxicity, but e.g. Pigment Yellow 83 is potentially toxic.

In general, it is indicated that the effects of the metabolites to aquatic organisms, except for algae, are at levels where potential toxicity is re-cognised. This applies for all metabolites with moieties of: anilines, benzidines and toluidines. Anilines and benzidines are both acute toxic and toxic depending on the specific species. The findings of the toluidines indicate potential toxicity for various aquatic organisms.

The estimated PEC (Predicted Environmental Concentration) and PNEC (Predicted No Effect Concentration) and the subsequent ratios indicate that there is a need of additional information on the potential environ-mental risks for sewage treatment plant and for the aquatic compartment, except for sediment, in association with processing and use of dyes and with production of pigments, whereas sludge applied on soil does not present immediate concern.