**5. Methods of coloured wastewaters purification**

Wastewaters from paper factories containing dyes particularly aniline and sulfur ones are generated from the paper treatment process consisting in the addition of fillers and dyes to the bleached cellulose material. As a result of multi-stage purification of wastewaters from the cellulose plants, there is obtained a low value of BOD (biochemical oxygen demand) – 4 mg/L but COD (chemical oxygen demand) is maintained on the level 75 mg/L. These waste‐ waters contain a small amount of suspended matter – 5 mg/L, but they are characterized by

**Indicator Tannery Furriery plant**

/L) 1600–4000 30000

Wastewaters containing dyes are troublesome in purification processes due to a complex structure of dye molecules. Even small amounts of dyes (of a few ppm order) are undesirable; they colour water, making it look unaesthetic and disturb life processes in water. Most dyes do not undergo biodegradation, deteriorate penetration of light into water and inhibit photosynthesis processes, increase chemical and biological demand for oxygen. Some dyes exhibit toxic and even cancerogenic as well as mutagenic action towards living organisms and therefore they should be carefully removed [5, 17]. Most dyes have a harmful effect (directly or indirectly) on fish. Direct activity consist in colouring of water and changing its composition, which significantly deteriorates the living conditions of fish and plankton, but indirect activity consist in poisonous properties of many dyes. Studies of dyes toxicity for fish seem to be particularly interesting not only with respect for estimation of water purity but also for the fact that they are a valuable source of food for people. Based on the ETAD (Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers) tests made for 3000 commonly used dyes in 27 cases, the registered LC50 (Lethal Dose50) values were of the order 0.05 mg/L (compared with, e.g. for DDT [dichlorodiphenyltrichloroethane] the LC50 value was 0.006 mg/L) [22]. It proved that 98% of studied dyes reveal LC50 toxicity over 1 mg/L. High toxicity was found in C.I. Basic Violet 1 with LC50 equal to 0.05 mg/L and C.I. Basic Yellow 37

Reaction pH 7.8–9.8 3–10

Sulfates (mg SO4 2–/L) 500–2000 1000 Chromium(III) (mg/L) 30–80 3000 Chromium(VI) (mg/L) 300–500 500 Surface active substances (mg/L) 60–200 2000 COD (mg O2/L) 1000–9000 - BOD (mg O2/L) 500–4500 -

**Table 5.** Concentration of impurities in wastewaters from the tannery and furriery plants

**4. Impact of dyes on natural environment**

with LC50 equal to 0.8 mg/L [22].

intensive colour – 40 mg Pt/L [21].

44 Ion Exchange - Studies and Applications

Chlorides (mg Cl–

The dye removal technologies can be divided into three categories: biological, chemical and physical. Possible decolourization methods of textile wastes along with advantages and disadvantages are listed in Figure 3 [23]. Because of high cost and disposal problems, many of these conventional methods for treating dye wastewaters have not been widely applied on a large scale in the textile industry. At present, there is no single process capable of treatment, mainly due to the complex nature of effluents, combination of the above mentioned techniques provides effective treatment of coloured wastewaters [12, 13, 17]. According to Babu et al. [24], more than 100 references in the bibliographical review of textile wastewater treatment prove that combination techniques permit not only the reduction of suspended solids, organic substances and colour but also the recovery of process chemicals. Currently, the main methods of textile dye treatment are by physical and chemical means with research concentrating on cheap and effective sorbents such as sandy soils, hen feathers, bottom ash, rice husk ash, orange peel, sugarcane dust, etc. However, it should be stressed that they are characterized by relatively low sorption capacity towards dyes compared with activated carbons or ion exchangers and what is more they need to be dumped.

**Figure 3.** Advantages and disadvantages of the current methods of dye removal from industrial effluents

Ion exchange is a very versatile and effective tool for treatment of aqueous hazardous wastes. The role of ion exchange in dye effluents treatment is to reduce the magnitude of hazardous load by converting them into a form in which they can be reused, leaving behind less toxic substances in their places or to facilitate ultimate disposal by reducing the hydraulic flow of the stream bearing toxic substances. Another significant feature of the ion exchange process is that it has the ability to separate as well as to concentrate pollutants.

Ion exchange resins known as reactive polymers are highly ionic, covalently cross-linked, insoluble polyelectrolytes, usually supplied as beads. Ion exchange resins have been classified based on the charge of the exchangeable counter-ion (cation exchanger or anion exchanger) and the ionic strength of the bound ion (strong exchanger or weak exchanger). Thus, there are four primary types of ion exchange resins: (a) strong cation exchange resins, containing — SO3 - H+ groups or the corresponding salts, (b) weak cation exchange resins, containing — COO-H+ groups or the corresponding salts, (c) strong anion exchange resins of quaternary ammonium groups (type I resins contain —CH2N(CH3)3 + Cl– groups and type II resins contain —CH2N(CH3)2(CH2CH2OH)+ Cl– groups), (d) weak anion exchange resins of primary (-NH2), secondary (=NH) or tertiary-amine (≡N) functional groups in the chloride or hydroxide form.

The resin beads have either a dense internal structure with no discrete pores (gel resins, also called microporous) or a porous, multichannelled structure (macroporous or macroreticular resins). They are commonly prepared from styrene and various levels of the cross-linking agent – divinylbenzene, which controls the particles' porosity. Popular ion exchangers available on the market are those of acrylic, epoxy-amine and phenol-formaldehyde matrices. The common choice is between styrene-divinylbenzene or acrylic-divinylbenzene copolymer. Disregarding structural features (gel or macroporous) for the time being, the acrylic matrix is more elastic than the rigid styrene-based copolymer. However, the elastic resilience of acrylic matrix could be of concern where the columns of resin operate under a high net compression force.

The internal structure of the resin beads, i.e. whether microporous (gel-type) or macropo‐ rous, is important in the selection of an ion exchanger. Macroporous resins, with their high effective surface area, facilitate the ion exchange process. They also give access to the exchange sites for larger ions, can be used with almost any solvent, irrespective of whether it is a good one forthe uncross-linked polymer, and take up the solvent with little or no change in volume. They make more rigid beads, facilitating the ease of removal from the reaction system. In the case of the microporous resins, since they have no discrete pores, solute ions diffuse through the particle to interact with the exchange sites. Despite diffusion limitations on the reaction rates, these resins offer certain advantages: they are less fragile, requiring less care in handling, react faster in functionalization and application reactions as well as possess higher loading capacities [25].

Taking into account high capacity and selectivity of ion exchange resins for different dyes, they seem to be proper materials for dyes sorption from textile effluents. Applicability of the anion exchange resins in the removal of acid, reactive, direct dyes widely used in the textile industry, from aqueous solutions and wastewaters, was confirmed in some papers [2, 15, 20, 23, 25–28].
