Chemical Synonyms, Molecular Structure and Toxicological Risk Assessment of Synthetic Textile Dyes: A Critical Review

Since the discovery of the brilliant fuchsia color, or mauve in 1856, thousands of synthetic dyes have been manufactured all over the world [1]. Actually, there are approximately 10,000 different synthetic dyes available in the market [2] with a global annual production of almost 800,000 tons [3]. Such dyes may be defined as colored matters and when applied they are to substrates may provide them with a permanent color that by any means will not be affected by sun rays, soap and water [1,4]. Specifically, every dye stuff consists of a group of atoms, called ‘chromophore’ that is responsible for the substrates’ coloration by absorbing certain wavelengths of light from the nearby ultraviolet region. It also contains an ‘auxochrome’ which helps the chromophore attach to the fiber by means of stable chemical bonds. The most important chromophores are: N=O, -NO2, -N=N-, -C=O, C=S, -C= N and (CH-CH)n and the compounds that bear them are known as chromogens [4].


Introduction
Since the discovery of the brilliant fuchsia color, or mauve in 1856, thousands of synthetic dyes have been manufactured all over the world [1]. Actually, there are approximately 10,000 different synthetic dyes available in the market [2] with a global annual production of almost 800,000 tons [3]. Such dyes may be defined as colored matters and when applied they are to substrates may provide them with a permanent color that by any means will not be affected by sun rays, soap and water [1,4]. Specifically, every dye stuff consists of a group of atoms, called 'chromophore' that is responsible for the substrates' coloration by absorbing certain wavelengths of light from the nearby ultraviolet region. It also contains an 'auxochrome' which helps the chromophore attach to the fiber by means of stable chemical bonds. The most important chromophores are: N=O, -NO 2 , -N=N-, -C=O, C=S, -C= N and (CH-CH) n and the compounds that bear them are known as chromogens [4].
These organic chemicals are usually classified as azo, anthraquinone, vat, phtalocyanine, indigo, polymethilene, carbonium and nitro dyes [5][6][7]. Azo dyes which have an azo bond (R 1 -N = N-R 2 ), where R 1 and R 2 are aromatic groups, can be substituted by sulphonated groups. R 1 and R 2 represents the biggest and most versatile group and composes about one-half of all dyes produced [8,9]. However, anthraquinone dyes constitute the second most important category of textile dyes and are often used for dyeing cellulosic fabric (such as cotton), wool and polyamide fibers. It is to be noted that a large diversity of chemical structures of anthraquinone colorants exist [10]. Concerning the nitro dyes, it was the first to be manufactured and rarely used. In terms of its atomic structure, the nitroso dyes consist on an NO 2 group in orthoposition to an electron-donating substituent such as usually NH 2 and an OH group [11].
Each year, nearly 140 000 tons of synthetic dyes are lost into the environment because of the dyeing process [11,12]. The discharge of dye-containing effluents has obvious negative effects. Those effluents are characterized by strong colors, high pH variations, high chemical oxygen demand (COD) and increased biotoxicity against bacteria [13]. Even at very low concentration (10-50 mg/L) water-soluble dyes may, intensively, affect the aquatic organisms [14][15][16] and interfere with the transmission of sun beams into streams and, therefore, reduce photosynthetic activity [17]. It is worth noting that these chemicals show high neutrality to light, temperature and microbial attacks [18]. They are also known to be persistent in the environment [15].
There has been increasing concern in recent years the occurrence, fate and toxicity of textile dyes products in the environment. This paper is a state of the art on the toxicological effects of those chemical substances.

Harmful Effects of Textile Dyes and their Metabolite
Textile dyes extensively used in several manufacturing process have been proved to be harmful to the human health as well as to the environment. Moreover, these chemicals, especially the azo dyes, could raise potential environmental concerns considering their toxic, mutagenic and carcinogenic effects [19][20][21][22].
As the discharge of azo dyes into water bodies presents human and ecological risks, a few synthetic dyes have been tested in order to evaluate their potential toxicity. The results have shown that these dyes have toxic effects on a variety of organisms such us aquatic animals [23]. Appendix1 displays about 85 toxic textile dyes.
Textile dyes and its metabolite carcinogenic effect, mutagenic effect and DNA damages [24,25]. Those compounds are by-products of cleavage of azo bond by microorganisms, and reported to be carcinogenic and mutagenic [24,26]. They were proved to be more dangerous than the parent compound [27].
In this context, the benzidine-based azo Direct Red 28 intermediate metabolites, the benzidine and 4-aminobiphenyl were reported to be the real causes of its toxicity [25]. The reduction of textile azo dyes may breed DNA binding motifs [28,29] and may cause multiple toxic effects.
Along with the aforementioned problems, some dyes have been shown to have a propensity to bio accumulate in fish [15,30]. Moreover heavy-metal ions that are originally present in textile effluents have been detected to be higher in algae and plants which are exposed to such effluents [15]. Some experiments have revealed that wastewater generated by textile industry has high amount of Total Organic Carbon (TOC), high salt content and extremes in pH [31,32]. It was reported that high pH values are registered in reactive dye bathes and low ones in acid dye baths. Lastly, it was observed that colored effluents decrease soil fertility and inhibit several plants (Appendix 1).

Conclusion
Environmental textile dyes hazards are associated to alarming human and animal's health side effects. Through this study we tried to highlight the fact that textile dyes discharged into the environment have alarming effects as both parent compound of dyes and their breakingdown products. They induce various cytotoxic, genotoxic, mutagenic and carcinogenic effects. Even at low concentration, these substances are proved to be responsible for harmful effects. Therefore, textile dyes treatment before eventual release in the environment has become a serious preoccupation. Several physical, chemical and biological processes are nowadays used in order to remove those compounds. However, their efficiency discussed as the danger associated to those compounds as well as to their degradation products which are generated through those treatment and unknown is of real concern.