Azocolorants in Textiles and Toys

4 Environmental Hazard Assessment of Azo-colorants

4.1.1 Introduction
4.1.2 Description of sulfonated azo dyes and pigments
4.1.3 Degradation of sulfonated azo dyes and pigments
4.1.4 Ecotoxicology of sulfonated azo dyes and pigments
4.1.5 Conclusions

Based on the human health assessment pointing out the sulfonated azo dyes and pigments (azo-colorants) as the least toxic azo dye components for dyeing textile, fabrics, etc., this environmental hazard assessment will focus on the sulfonated type. lt is presumed that the main emission to the aquatic en­vironment during the use of dyed textiles originates from the washing process with subsequent aqueous, effluent to the drain. The hazard assessment will be based on the intrinsic properties of the parent chemicals and to some extent the degradation products of the azo-colorants.

In general, after completion of the dyeing process, loss of the dyestuff to the aquatic environment is considered as negligible as the dyestuff is effectively irreversibly bound to the substrate. However, excess dye in fabrics etc. is washed out during initial washes and afterwards conveyed to domestic wastewater.

Azo dyestuffs and pigments are generally resistant to biodegradation under aerobic conditions. The fate of azo-colorants in wastewater treatment plants has been investigated and the general observation is that these are adsorbed to the sludge in biological treatment plants (Clarke & Anliker, 1980). The adsorption of time organic colorants is favoured in the case of azo-groups, whereas the adsorption may be reduced by the presence of sulfo-groups. This is observed in the case of acid dyes but not in time case of reactive and direct dyes (Clarke & Anliker, 1980). In general, all pigments are likely to adsorb onto organic particles. In many wastewater treatment plants the sludge is treated only by anaerobic digestion and afterwards the sludge may be used as fertilizer on agriculture land. Due to the adsorption ability, it is most likely that the dyes, at least initially, will end up in anaerobic environments after discharge to the environment. Degradation under anaerobic conditions are therefore an important factor in assessing the fate of the azo-colorants. The azo dyestuffs are normally primarily degradable under anaerobic conditions, at least slowly, whereas the pigments may be degraded slowly if at all. The azo dyes are reduced to form aromatic amines that are resistant to further biodegradation in the absence of oxygen, and thus accumulation in the environment is likely to occur. Focus on degradation products is therefore very important from a risk assessment point of view. Low water solubility of the azo compounds complicates the ecotoxicology testing, and ecotoxicity data are very sparse. The bioavailability of the colorants for aquatic organisms is also very low because of the low solubility and this results in low toxicity properties. A survey of the fish toxicity of 3000 commercial products by ETAD, members showed that LC50 values were higher than 1 mg/l for 98% of the products (Clarke & Anliker, 1980).

Sulfonated azo pigments are typically beta-oxy-naphthoic acids or betanaphthols. The pigments are water insoluble. The sulfonated azo dyes are water soluble to some extent. These dyes are used for dyeing textile and leather, among others. The acid dyes are anionic dyes with typically 1-3 sulfonic groups. They are applied to wool, silk and polyamide, whereas direct dyes and mordant dyes often composed of polyazo groups are applied for cotton, viscose, etc. (Zollinger, 1991). The sulfonated azo dyes may be composed of naphthalene sulfonic acids, naphthols, naphthoic acids, benzidines etc. Benzidine based azo dyes are in focus because of the carcinogenicity of benzidine. Sulfonated azo dyes based on benzidine are e.g.(Brown & Hamburger, 1987):

Direct Red 28
Direct Blue 1, 14, 15
Acid Red 114

Degradation studies of azo pigments are very sparse while more studies have been conducted for the water soluble azo dyes. Results from degradation studies of sulfonated azo dyes are listed in Table 4.1.

Aerobic degradation

Azo dyestuffs and pigments are not readily aerobic degradable. The biological degradation of organic dyes (87 different acid, basic, direct, and reactive dyes) was studied using different aerobic degradation tests (Pagga & Brown, 1986). The general conclusion was that the dyestuffs were not readily biodegradable but some were inherently degradable. None of the sulfonated azo dyes Acid Orange 3, Acid Yellow 36, Direct Blue 14, 15, and Direct Red 7 were biodegradable except Direct Blue 14 that was partly degradable (Pagga & Brown, 1986). Bacterial strains with the ability of reducing the azo-bond under aerobic conditions have been isolated from a wastewater treatment plant receiving azo dye-containing effluents (Coughlin et al., 1996). This indicates that rnicroorganisms may develop the ability of degrading azo components after an adaptation period. The degradability potential of the isolated bacterial strains was limited to dyes with only one sulfonate group, indicating that highly charged compounds have difficulties in crossing the cell membrane for degradation (Coughlin et al., 1996).

Anaerobic degradation

The reductive cleavage of the azo-bond may lead to the formation of two aromatic amines. In the case of sulfonated azo dyes, at least one of the metabolites is sulfonated whereas the other part theoretically is similar to the degradation products from non-sulfonated azo dyes. The anaerobic degradation of azo dyes has been studied with subsequent aerobic treatment in order to study the degradation of the metabolites. The first study in a series of studies on the degradation of azo dyestuffs showed that the sulfonated azo dyes Acid Orange 3, Acid Red 114, Direct Blue 15, Di­rect Red 7, Direct Yellow 12, and Mordant Black 9 were primarily degradable under anaerobic conditions (Brown & Laboureur, 1983). Degradation of sulfonated azo dyes (Acid Orange 7, 8,10, Acid Red 14,18) in a two stage anaerobic/aerobic treatment has been investigated (Seshadri et al., 1994; FitzGerald & Bishopo 1995). The dyes were readily reduced in time anaerobic stage, whereas the degradation of the metabolites was not clarifired. The results showed that the metabolites may be absorbed to the biomass (FitzGerald & Bishop, 1995).

Degradation of metabolites

Another study showed that primary anaerobic degradation of the sulfonated azo dyestuffs (Acid Orange 7, Acid Yellow 25, 36, Acid dye, Acid Red 114, Acid Black 24, Direct Red 7, Direct Blue 14, 15, Direct Yellow 12, 50, Mordant Black 9, 11) took place with formation of metabolites that were not further degraded in the absence of oxygen (Brown & Hamburger, 1987). The metabolites were subsequently treated with activated sludge under aerobic conditions. Amino benzene sulfonic acid, amino ethmoxy benzene, diamino dimethoxy biphenyl, and amino methylbenzene phenyl sulfonamid were degraded (>75%), whereas dichloroanilin, aminonaphthalene sulfonic acid, and amino methyl (sulfophenyl) pyrazole were only partly or not degraded (<25%). Tests for inherent biodegradability showed that amino benzene sulfonic acid, amino naphthalene sulfonic acid, and amino ethoxybenzene were inherently degraded, whereas amino napthalene disulfonic acid, amino methylbenzenesulfonic acid and dichloroanilin were not inherently biodegradable (See Table 4.1) (Brown & Hamburger, 1987). Degradation of the metabolites was also investigated under anaerobic conditions and only diamino dimethmoxy biphenyl was readily degraded (Brown & Hamburger, 1987).

According to MITI (1992), no components with structural similarities to the typical metabolites, aromatic amines (sulfonated or not) are readily biodegradable under aerobic conditions.

Abiotic degradation

Photolysis is considered as insignificant for the degradation of the dyestuffs in the aquatic environment (Clarke & Anliker, 1980).

In general, organic pigments exhibit a low acute aquatic toxicity probably because of the low water solubility of the pigments. However, in presence of fairly high concentrations of pigments, a lethal effect to fish may occur arising from clogging of the gills by the aqueous pigment dispersion (Anliker & Clarke, 1980). The ecotoxicity of the sulfonated azo dyes and pigments is in general very low. The survey of fish toxicity for 3000 azo dyestuff products collected by ETAD summarises the LC0 values (no effect concentration) for a range of dyestuffs. Acid dyes exhibit the highest toxicity level (LC0 values < 100 mg/I observed for 48% of 371 dyes) whereas only 9% of the direct dyes (of 224 dyes) and 38% of the mordant dyes (of 21 dyes) have LC0 values lower than 100 mg/l. Data for some of the sulfonated azo dyes are listed in Table 4.1. Only Mordant Black 11 shows a high fish toxicity in addition to inhibition effects on algal growth and the activated sludge process (Burg & Charest, 1980).

lt is beyond the scope of this preliminary assessment of the environmental effects of azo dyes to consider the toxicity of the metabolites from the anaerobic degradation. In general, the hydrophilic sulfonated aromatic amines have a low toxicity (Anliker, 1989). However, some of the metabolites not sulfonated might have a high toxicity in the aquatic environment e.g. the chlorinated aromatics.

This preliminary hazard assessment of sulfonated azo dyes and pigments is based on the most available literature, so the results here may not necessarily cover the whole spectrum of sulfonated azo colorants. The azo dyes and pigments are likely to adsorb onto organic particles as sludge and sediments and consequently they are likely to end up in anaerobic environments where they normally are primarily degraded. In aerobic wastewater treatment, some dyes may be inherently biodegraded after an adaptation period of the biomass. The anaerobic degradation leads to the formation of metabolites that are resistant to biological degradation in the absence of oxygen. In general, the sulfonated azo dyes and pigments exhibit a low toxicity. Mordant Black 11 is the only dyestuff observed here with a fish toxicity lower than 100 mg/l. The biodegradability and the toxicity of the metabolites are highly dependent on the structural formula, which is very diverse. Some of the metabolites (e.g. chlorinated aromatic amines) may exhibit a high toxicity. In order to clarify the fate and the effects of the metabolites, which may be important for the risk assessment of the azo dyes in the aquatic environment, further studies have to be done.

Table 4.1
Biodegradability and ecoloxicity of some sulfonated azo dyes and pigments and some potential metabolites.

¹Pagga & Brown, 1986 ²Brown & Laboureur, 1983 ³Seshadri et al., 1994,
⁴FitzGerald & Bishop, 1995, ⁵Brown & Hamburger, 1987, ⁶AQUIRE, 1993, ⁷Burg & Charest, 1980,⁸ IUCLID, 1996