**2. Dyes – Definition and classification**

Dyes are commonly defined as organic compounds of natural or synthetic origin which absorb visible light selectively in the range 400–700 nm, being capable of dying various materials (fabrics, paper, leather, wood, plastic, food, cosmetics) [1]. Dye molecules possess chromo‐ phoric groups, e.g. –N=N–, –NO2, –NO, –CH=CH–, owing to which these compounds selec‐ tively absorb electromagnetic radiation in the visible range and auxochromic one (e.g. –NH2, –OH, –OR) responsible for dyes affinity for dyed materials [1, 2]. Dye molecules can also contain other substituents which impart specific properties affecting their solubility or stability improvement.

There are two basic ways of dyes classification. Based on the chemical structure of dye molecule, there can be distinguished carbocyclic and heterocyclic dyes. Considering the occurrence of chromophore, carbocyclic dyes include, among others, azo, nitro, anthraquinone ones and heterocyclic dyes include xanthene, acridine, indigoid ones. Using the chemical classification of dyes is convenient while discussing methods of their synthesis, chemical structure, dependence between the structure and different properties. Technical classification of dyes is based on the way of dyeing taking into account their solubility and chemical properties (Table 1) [2]. It is useful in the case of different applications of dyes. Not all dyes are universal. The way of their application depends on different methods of dye bonding with the material. Thus dyes are divided into reactive, direct, acid, basic, ice, vat, mordant, sus‐ pended, sulfurous, oxidative, pigments and lakes. From a practical point of view, there are two groups of dyes. The first one are dyes soluble in water with a created coloured cation (basic dyes) or anion (acid, acid-chromic, reactive, direct, and metal complex dyes). The second group includes dyes insoluble in water (suspended and those formed on the fibre, e.g. ice, oxidative, mordant) as well as pigments. Among them, there are dyes whose salts dissolve in water (sulfur and vat ones) and those which dissolve in organic solvents, e.g. fatty dyes.


**Table 1.** Technical classification of dyes

fluents such as inorganic electrolytes and different surfactants on the amounts of dyes re‐ tained by the anion exchangers was presented. The adsorption behaviour of the polyacrylic Amberlite IRA 958 demonstrates that it can be a promising adsorbent for the textile wastewater treatment. The results obtained with raw textile wastewaters purifica‐

Wastewaters originating from the textile, cellulose, paper, chemical, tanning, food and cosmetic industries containing dyes are a hazardous source of natural environment contami‐ nation. They are troublesome in purification process due to complex structure of dye mole‐ cules. Even small amounts of dye are undesirable as they colour water making it look unaesthetic and disturb life processes in water. Most dyes do not undergo biodegradation, deteriorate light penetration into water and inhibit photosynthesis, increase chemical and biological demand for oxygen. Some dyes are toxic and sometimes even carcinogenic and mutagenic towards living organisms and they should be carefully removed [1]. Purification of wastewaters containing dyes becomes more and more important and is aimed at avoiding potential threat for the environment and legal consequences. Decolourization of these wastewaters before their reaching water outlets is a must. Conventional methods of waste‐ waters purification do not remove colour completely. Therefore it is advisable to work out a more effective methods of wastewaters purification which would reduce the amount of discharged impurities but also contribute to recovery of water and raw materials used in technological processes. The difficulties in elaboration of both simple and economical methods of dyes removal are due to frequent changes in technology of their production as well as the use of various dyes in technological processes. In the case of impurities whose creation cannot be prevented, there should be applied highly effective technologies for their rendering harmless which can be combined into multi-stage purification systems. Such possibilities are provided by adsorption processes using, among others, ion exchangers allowing not only to separate substances dissolved based on selective interactions but also to concentrate amounts

of impurities and formed closed circulation of water in the technological process.

Dyes are commonly defined as organic compounds of natural or synthetic origin which absorb visible light selectively in the range 400–700 nm, being capable of dying various materials (fabrics, paper, leather, wood, plastic, food, cosmetics) [1]. Dye molecules possess chromo‐ phoric groups, e.g. –N=N–, –NO2, –NO, –CH=CH–, owing to which these compounds selec‐ tively absorb electromagnetic radiation in the visible range and auxochromic one (e.g. –NH2, –OH, –OR) responsible for dyes affinity for dyed materials [1, 2]. Dye molecules can also

**2. Dyes – Definition and classification**

**Keywords:** anion exchangers, dyes sorption, removal, textile effluents

tion confirmed this statement.

38 Ion Exchange - Studies and Applications

**1. Introduction**

There is a special register called *Colour Index* for dyes first published in 1924. The dual classification system *Colour Index* includes: *Colour Index Generic Name (CIGN)*, e.g. Disperse Yellow 1, which is the name describing the kind of dye as regards its technical application and colour as well as chronological notation in the register and for the known and published chemical structure, so-called *Colour Index Constitution Number (CICN)* in this case C.I. 10345 [3]. Their largest use was recorded in 2008 in Asia, particularly in China and India (Figure 1) [4]. The yearly world production of dyes is 700, 000–1000, 000 tones, which corresponds to over 100, 000 commercial products whereby azo dyes constitute 70% [5-11].

Natural dyes such as indigo, woad, or madder lost their position due to dynamic development of textile industry in the XVIIIth century. In the second half of the XIXth century, there started a synthesis of intermediate products and new dyes not having equivalents in nature of various colours, shades, and high quality (resistance to chemical factors, light, friction).

**Figure 1.** World dyes demand by the regions in 2008

The first synthetic dyes were fuchsine (prepared in 1855 by J. Natanson) and mowein (prepared in 1856 by W. Perkin) [1]. Development of dyes synthesis is promoted by their huge application in many fields of industry. The large tonnage consumer of dyes is textile industry. Moreover, they are applied in electronic industry for the production of liquid crystal and electrochromium visual indicators as well as in technology of optical recording, photographic (for creating coloured pictures), reprography (ink, non-carbon paper), food dyeing as well as for dyeing biological material, paper, leather, wood and cosmetics and also as indicators (e.g. redox, pH).
