Dyes – History, Classification, toxicity and degradation
Use of algae for treating waste water and industrial effluents is termed as Phycoremediation which can also utilize and remove waste CO2 from the air through photosynthesis.
Micro algae are superior organisms for remediation because a wide range of toxic and other wastes can be treated with algae and they are non pathogenic. The risk of accidental release into the atmosphere causing health safety and environmental problems is avoided when algae are employed for remediation. Algae utilize the wastes as nutritional sources and enzymatically degrade the pollutants. The xenobiotics and heavy metals are known to be detoxified/ transformed/or volatilized by algal metabolism.
Ever since the beginning of humankind, people have been using colorants for painting and dyeing of their surroundings, their skins and their clothes. Until the middle of the 19th century, all colorants applied were from natural origin. Inorganic pigments such as soot, manganese oxide, hematite and ochre have been utilised within living memory. Palaeolithic rock paintings, such as the 30,000 year old drawings that were recently discovered in the Chauvet caves in France, provide ancient testimony of their application. Organic natural colorants have also a timeless history of application, especially as textile dyes. These dyes are all aromatic compounds, originating usually from plants (e.g. the red dye alizarin from madder and indigo from woad) but also from insects (e.g. the scarlet dye kermes from the
shield-louse Kermes vermilio), fungi and lichens. Synthetic dye manufacturing started in 1856, when the English chemist W.H. Perkin, in an attempt to synthesise quinine, obtained instead a bluish substance with excellent dyeing properties that later became known as aniline purple, Tyrian purple or mauveine. Perkin, 18 years old, patented his invention and set up a production line. This concept of research and development was soon to be followed by others and new dyes began to appear on the market, a process that was strongly stimulated by Kékulé.s discovery of the molecular structure of benzene in 1865. In the beginning of the 20th century, synthetic dyestuffs had almost completely supplanted natural dyes.
Dye classification
All aromatic compounds absorb electromagnetic energy but only those that absorb light with wavelengths in the visible range (~350-700 nm) are coloured. Dyes contain chromophores, delocalised electron systems with conjugated double bonds, and auxochromes, electron-withdrawing or electrondonating substituents that cause or intensify the colour of the chromophore by altering the overall energy of the electron system. Usual chromophores are -C=C-, -C=N-, -C=O, -N=N-, -NO2 and quinoid rings, usual auxochromes are -NH3, -COOH, -SO3H and -OH. Based on chemical structure or chromophore, 20-30 different groups of dyes can be discerned. Azo (monoazo, disazo, triazo, polyazo), anthraquinone, phthalocyanine and triarylmethane dyes are quantitatively the most important groups. Other groups are diarylmethane, indigoid, azine, oxazine, thiazine, xanthene, nitro, nitroso, methine, thiazole, indamine, indophenol, lactone, aminoketone and hydroxyketone dyes and dyes of undetermined structure (stilbene and sulphur dyes).
The vast array of commercial colorants is classified in terms of colour, structure and application method in the Colour Index (C.I.) which is edited since 1924 (and revised every three months) by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. The Colour Index (3rd Edition, issue 2) lists about 28,000 commercial dye names, representing ~10,500 different dyes, 45,000 of which are currently produced. Each different dye is given a C.I. generic name determined by its application characteristics and its colour. The Colour Index discerns 15 different application classes:
1.Acid dyes
The largest class of dyes in the Colour index is referred to as Acid dyes (~2300 different acid dyes listed, ~40% of them are in current production). Acid dyes are anionic compounds that are mainly used for dyeing nitrogen-containing fabrics like wool, polyamide, silk and modified acryl. They bind to the cationic NH4 +-ions of those fibres. Most acid dyes are azo (yellow to red, or a broader range colours in case of metal complex azo dyes), anthraquinone or triarylmethane (blue and green) compounds. The adjective .acid. refers to the pH in acid dye dyebaths rather than to the presence of acid groups (sulphonate, carboxyl) in the molecular structure of these dyes.
2 Reactive dyes
Reactive dyes are dyes with reactive groups that form covalent bonds with OH-, NH-, or SH-groups in fibres (cotton, wool, silk, nylon). The reactive group is often a heterocyclic aromatic ring substituted with chloride or fluoride, e.g. dichlorotriazine. Another common reactive group is vinyl sulphone (as in Reactive Orange 7). The use of reactive dyes has increased ever since their introduction in 1956, especially in industrialised countries. In the Colour Index, the reactive dyes form the second largest dye class with respect to the amount of active entries: about 600 of the ~1050 different reactive dyes listed are in current production. During dying with reactive dyes,
hydrolysis (i.e. inactivation) of the reactive groups is an undesired side reaction that lowers the degree of fixation. In spite of the addition of high quantities of salt and ureum (up to respectively 60 and 200 g/l) to raise the degree of fixation, it is estimated that 10 to 50% will not react with the fabric and remain .hydrolysed. in the water phase. The problem of coloured effluents is therefore mainly identified with the use of reactive dyes. Most (~80%) reactive dyes are azo or metal complex azo compounds but also anthraquinone and phthalocyanine reactive dyes are applied, especially for green and blue.
3 (Metal complex dyes)
Among acid and reactive dyes, many Metal complex dyes can be found (not listed as a separate category in the Colour Index). These are strong complexes of one metal atom (usually chromium, copper, cobalt or nickel) and one or two dye molecules, respectively 1:1 and 1:2 metal complex dyes. Metal complex dyes are usually azo compounds. About 1/6 of all azo dyes listed in the Colour Index are metal complexes 37 but also phthalocyanine metal complex dyes are applied.
4 Direct dyes
Direct dyes are relatively large molecules with high affinity for especially cellulose fibres. Van der Waals forces make them bind to the fibre. Direct dyes are mostly azo dyes with more than one azo bond or phthalocyanine, stilbene or oxazine compounds. In the Colour Index, the direct dyes form the second largest dye class with respect to the amount of different dyes: About 1600 direct dyes are listed but only ~30% of them are in current production.
5 Basic dyes
Basic dyes are cationic compounds that are used for dyeing acid-group containing fibres, usually synthetic fibres like modified polyacryl. They bind to the acid groups of the fibres. Most basic dyes are diarylmethane, triarylmethane, anthraquinone or azo compounds. Basic dyes represent ~5% of all dyes listed in the Colour Index.
6 Mordant dyes
Mordant dyes are fixed to fabric by the addition of a mordant, a chemical that combines with the dye and the fibre. Though mordant dyeing is probably one of the oldest ways of dyeing, the use of mordant dyes is gradually decreasing: only ~23% of the ~600 different mordant dyes listed in the Colour Index are in current production. They are used with wool, leather, silk, paper and modified cellulose fibres. Most mordant dyes are azo, oxazine or triarylmethane compounds. The mordants are usually dichromates or chromium complexes.
7 Disperse dyes
Disperse dyes are scarcely soluble dyes that penetrate synthetic fibres (cellulose acetate, polyester, polyamide, acryl, etc.). This diffusion requires swelling of the fibre, either due to high temperatures (>120 °C) or with the help of chemical softeners. Dying takes place in dyebaths with fine disperse solutions of these dyes. Disperse dyes form the third largest group of dyes in the Colour Index: about 1400 different compounds are listed, of which ~40% is currently produced. They are usually small azo or nitro compounds (yellow to red), anthraquinones (blue and green) or metal complex azo compounds (all colours).
8 Pigment dyes
Pigment dyes (i.e. organic pigments) represent a small but increasing fraction of the pigments, the most widely applied group of colorants. About 25% of all commercial dye names listed in the Colour Index are pigment dyes but these ~6900 product names stand for less than 800 different dyes. These insoluble, non-ionic compounds or insoluble salts retain their crystalline or particulate structure throughout their application. Pigment dyeing is achieved from a dispersed aqueous solution and therefore requires the use of dispersing agents. Pigments are usually used together with thickeners in print pastes for printing diverse fabrics. Most pigment dyes are azo compounds (yellow, orange, and red) or metal complex phthalocyanines (blue and green). Also anthraquinone and quinacridone pigment dyes are applied.
9 Vat dyes
Vat dyes are water-insoluble dyes that are particularly and widely used for dyeing cellulose fibres. The dyeing method is based on the solubility of vat dyes in their reduced (leuco) form. Reduced with sodium dithionite, the soluble leuco vat dyes impregnate the fabric. Next, oxidation is applied to bring back the dye in its insoluble form. Almost all vat dyes are anthraquinones or indigoids. Indigo itself is a very old example of a vat dye, with about 5000 years of application history. .Vat. refers to the vats that were used for the reduction of indigo plants through fermentation.
10 Anionic dyes and ingrain dyes
Azoic dyes and Ingrain dyes (naphthol dyes) are the insoluble products of a reaction between a
coupling component (usually naphthols, phenols or acetoacetylamides; listed in the Colour Index as C.I. azoic coupling components) and a diazotised aromatic amine (listed in the Colour Index as C.I. azoic diazo components). This reaction is carried out on the fibre. All naphthol dyes are azo compounds.
11 Sulphur dyes
Sulphur dyes are complex polymeric aromatics with heterocyclic S-containing rings. Though
representing about 15% of the global dye production, sulphur dyes are not so much used in Western Europe. Dyeing with sulphur dyes involves reduction and oxidation, comparable to vat dyeing. They are mainly used for dyeing cellulose fibres.
12 Solvent dyes
Solvent dyes (lysochromes) are non-ionic dyes that are used for dyeing substrates in which they can dissolve, e.g. plastics, varnish, ink, waxes and fats. They are not often used for textile-processing but their use is increasing. Most solvent dyes are diazo compounds that underwent some molecular rearrangement. Also triarylmethane, anthraquinone and phthalocyanine solvent dyes are applied.
13 Fluorescent brighteners
Fluorescent brighteners (or bluing agents) mask the yellowish tint of natural fibres by absorbing ultraviolet light and weakly emitting visible blue. They are not dyes in the usual sense because they lack intense colour. Based on chemical structure, several different classes of fluorescent brighteners are discerned: stilbene derivatives, coumarin derivatives, pyrazolines, 1,2-ethene derivatives, naphthalimides and aromatic or heterocyclic ring structures. Many fluorescent brighteners contain triazinyl units and water-solubilising groups.
14 Other dye classes
Apart from the dye classes mentioned above, the Colour Index also lists Food dyes and Natural dyes. Food dyes are not used as textile dyes and the use of natural dyes (mainly anthraquinone, indigoid, flavenol, flavone or chroman compounds that can be used as mordant, vat, direct, acid or solvent dyes) in textile-processing operations is very limited.
Dyes, environmental concern
Many dyes are visible in water at concentrations as low as 1 mg l-1. Textile-processing wastewaters, typically with a dye content in the range 10 . 200 mg l-1 248, are therefore usually highly coloured and discharge in open waters presents an aesthetic problem. As dyes are designed to be chemically and photolytically stable, they are highly persistent in natural environments. The release of dyes may therefore present an ecotoxic hazard and introduces the potential danger of bioaccumulation that may eventually affect man by transport through the food chain.
a. Bioaccumulation
The bioaccumulation tendency of dyestuffs in fish has been comprehensively investigated in research promoted by ETAD, the Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers. The bioconcentration factors (BCF.s) of 75 dyes from different application classes were determined and related to the partition coefficient n-octanol/water (KOW) of each different compound. Water-soluble dyes with low KOW, i.e. ionic dyes like acid, reactive and basic dyes, did not bioaccumulate (generally log BCF < 0.5). For these water-soluble dyes, log P (log KOW) showed a linear relationship with log BCF so it was expected that dyestuffs with higher KOW would bioaccumulate. However, water-insoluble organic pigments with extremely high partition coefficients did not bioaccumulate probably due to their extremely low water and fat solubilities and also the BCF values for disperse dyes, i.e. scarcely soluble compounds with a moderately lipophilic nature, were much lower than expected. In all cases, log BCF < 2, which indicates that none of the dyes tested
showed any substantial bioaccumulation.
b. Toxicity of dyestuffs
Dyestuff toxicity has been investigated in numerous researches. These toxicity (i.e. mortality,
genotoxicity, mutagenicity and carcinogenicity) studies diverge from tests with aquatic organisms (fish, algae, bacteria, etc.) to tests with mammals. Furthermore, research has been carried out to effects of dyestuffs and dye containing effluents on the activity of both aerobic and anaerobic bacteria in wastewater treatment systems.
The acute toxicity of dyestuffs is generally low. Algal growth (photosynthesis), tested with
respectively 56 and 46 commercial dyestuffs, was generally not inhibited at dye concentrations below 1 mg/l. The most acutely toxic dyes for algae are cationic basic dyes. Fish mortality tests showed that 2% out of 3000 commercial dyestuffs tested had LC50 values below 1 mg/l. The most acutely toxic dyes for fish are basic dyes, especially those with a triphenylmethane structure. Fish also seem to be relatively sensitive to many acid dyes 65. Mortality tests with rats showed that only 1% out of 4461 commercial dyestuffs tested had LD50 values below 250 mg/kg body weight. Therefore, the chance of human mortality due to acute dyestuff toxicity is probably very low. However, acute sensitisation reactions by humans to dyestuffs often occurs. Especially some disperse dyestuffs have been found to cause allergic reactions, i.e. eczema or contact dermititis. Chronic effects of dyestuffs, especially of azo dyes, have been studied for several decades. Researchers were traditionally mostly focused on the effects of food colorants, usually azo compounds. Furthermore, also the effects of occupational exposure to dyestuffs of human workers in
dye manufacturing and dye utilising industries have received attention. Azo dyes in purified form are seldom directly mutagenic or carcinogenic, except for some azo dyes with free amino groups. However, reduction of azo dyes, i.e. cleavage of the dye.s azo linkage(s), leads to formation of aromatic amines and several aromatic amines are known mutagens and carcinogens. In mammals, metabolic activation (= reduction) of azo dyes is mainly due to bacterial activity in the anaerobic parts of the lower gastrointestinal tract. Various other organs, especially the liver and the kidneys, can, however, also reduce azo dyes.
After azo dye reduction in the intestinal tract, the released aromatic amines are absorbed by the intestine and excreted in the urine. The acute toxic hazard of aromatic amines is carcinogenesis, especially bladder cancer. The carcinogenicity mechanism probably includes the formation of acyloxy amines through N-hydroxylation and N-acetylation of the aromatic amines followed by O-acylation. These acyloxy amines can be converted to nitremium and carbonium ions that bind to DNA and RNA, which induces mutations and tumour formation.
The mutagenic activity of aromatic amines is strongly related to molecular structure. In 1975 and in 1982, the International Agency for Research on Cancer (IARC) summarised the literature on suspected azo dyes, mainly amino-substituted azo dyes, fat-soluble azo dyes and benzidine azo dyes, but also a few sulphonated azo dyes. Most of the dyes on the IARC list were taken out of production. Generally stated, genotoxicity is associated with all aromatic amines with benzidine moieties, as well as with some aromatic amines with toluene, aniline and naphthalene moieties. The toxicity of aromatic amines depends strongly on the spatial structure of the molecule or .in other words. the location of the amino-group(s). For instance, whereas there is strong evidence that 2-naphthylamine is a carcinogen, 1-naphthylamine is much less toxic 47. The toxicity of aromatic amines depends furthermore on the nature and location of other substituents. As an example, the substitution with nitro, methyl or methoxy groups or halogen atoms may increase the toxicity, whereas substitution with carboxyl or sulphonate groups generally lowers the toxicity. As most soluble commercial azo dyestuffs contain one or more sulphonate groups, insight in the potential danger of sulphonated aromatic amines is particularly important. In an extensive review of literature data on genotoxicity and carcinogenicity of sulphonated aromatic amines, it was concluded that sulphonated aromatic amines, in contrast to some of their unsulphonated analogues, have generally no or very low genotoxic and tumorigenic potential
Dye removal techniques
Various physical, chemical and biological pre treatment, main treatment and post treatment techniques can be employed to remove colour from dye containing wastewaters. Physicochemical techniques include membrane filtration, coagulation/flocculation, precipitation, flotation, adsorption, ion exchange, ion pair extraction, ultrasonic mineralisation, electrolysis, advanced oxidation (chlorination, bleaching, ozonation, Fenton oxidation and photocatalytic oxidation) and chemical reduction. Biological techniques include bacterial and fungal biosorption and biodegradation in aerobic, anaerobic, anoxic or combined anaerobic/aerobic treatment processes.
Several factors determine the technical and economic feasibility of each single dye removal technique:
- dye type
- wastewater composition
- dose and costs of required chemicals
- operation costs (energy and material)
- environmental fate and handling costs of generated waste products
In general, each technique has its limitations. The use of one individual process may often not be sufficient to achieve complete decolourisation. Dye removal strategies consist therefore mostly of a combination of different techniques.
1. Membrane filtration
Nanofiltration and reverse osmosis, using membranes with a molecular weight cut-off (MWCO) below ~10,000 Dalton, can be applied as main or post treatment processes for separation of salts and larger molecules including dyes from dyebath effluents and bulk textile-processing wastewaters. Filtration with bigger membranes, i.e. ultrafiltration and microfiltration, is generally not suitable as the membrane pore size is too large to prevent dye molecules passing through but it can be successful as pre treatment for further nanofiltration or reverse osmosis. Membrane filtration is a quick method with low spatial requirement. Another advantage is that the permeate, as well as some of the concentrated compounds, including non-reactive dyes, can be reused. This reuse, however,
applies mostly only for smaller waste flows 331. The disadvantages of membrane techniques are flux decline and membrane fouling, necessitating frequent cleaning and regular replacement of the modules. Another important drawback is that the generated concentrate must be processed further, for instance by ozonation. The capital costs of membrane filtration are therefore generally rather high. Filtration techniques for the treatment of textile wastewaters are especially widely applied in South Africa.
2. Coagulation/flocculation
Coagulation/flocculation is often applied in the treatment of textile-processing wastewater, either to partly remove Chemical Oxygen Demand (COD) and colour from the raw wastewater before further treatment, to polish the final effluents of biologically or otherwise treated wastewater or even as the main treatment process. The principle of the process is the addition of a coagulant followed by a generally rapid chemical association between the coagulant and the pollutants. The thus formed coagulates or flocs subsequently precipitate or are to be removed from the water phase by flotation.
Various inorganic coagulants are used, mostly lime, magnesium, iron and aluminium salts. Inorganic compounds are, however, generally not very suitable to remove highly soluble (= sulphonated) dyes from solution unless rather large quantities are dosed. Coagulation/flocculation with inorganic chemicals generates considerable volumes of useless or even toxic sludge that must be incinerated or handled otherwise. This presents a serious drawback of the process. Recently developed organic polymers have been proven highly effective as dye coagulants, even for coagulation of reactive dyes, while the sludge production associated with polymer dosing is relatively low. Most of the polymers used for colour removal are, however, cationic and may be toxic to aquatic life at very low concentrations (less than 1 mg/l) and in biological wastewater treatment plants, some cationic polymers have been found to inhibit the nitrification process.
3. Sorption and ion exchange
Activated carbon or other materials can be used to remove dyes from wastewater, either by adsorption (anionic dyes) or by combined adsorption and ion exchange (cationic dyes). Sorption techniques yield waste sludge, i.e. dye-saturated material, that should be disposed off or regenerated. As there are nonionic, anionic and cationic dyes, most adsorbents do not remove all different dye types. Activated carbon is capable of adsorbing many different dyes with high adsorption capacity but it is expensive and the costs of regeneration are high because desorption of the dye molecules is not easily achieved. Various other (mostly low-cost) adsorbents have therefore been investigated as an alternative to activated carbon. Those adsorbents include:
- non-modified cellulose (plant) biomass, e.g. corn/maize cobs, maize stalks, wheat straw, linseed straw, rice husks, wood chips, sawdust, bark, coirpith, banana pith, bagasse pith, palm fruit bunch particles, peat moss, peat, linseed cake, sugar beet pulp, sugar industry mud, cotton waste and cellulose;- modified cellulose biomass, e.g. carbonised coirpith, carbonised coconut-tree sawdust , chemically modified sunflower stalks, polyamide-epichlorohydrin-cellulose, carbamoyl-cellulose, quaternised-cellulose, quaternised-lignocellulose; sugarcane bagasse derived anion exchange resin; - bacterial biomass, e.g. Aeromonas , actinomycetes, activated sludge; dried and powdered biogas waste slurry - fungal biomass - yeast biomass; - chitin, a material that can be found in e.g. shells, insect shields and fungal cell walls; chitosan, deacylated chitin; cross-linked chitosan fibres;- soil material, e.g. sand, silica, natural clay, bentonite clay, diatomite clay, montmorillonite clay, vermiculite clay, fullers earth, synthetic clay; - wood charcoal, bone charcoal, barbecue charcoal, magnetic charcoal; - activated bauxite, activated alumina;- other materials, e.g. pressed sludge cake (pulp mill waste), pyrolysed tire, leather hide powder, dealginated seaweed, coal dust , chrome sludge, steel plant slag, fly ash and hair . Some of these materials show high dye removal capacities, comparable or .especially in the case of disperse dyes. even higher than activated carbon. This depends strongly on the dye class. Many of the materials listed, e.g. rice husks, bark, cotton waste and hair, have a high capacity for binding (cationic) basic dyes but hardly remove dyes from other classes 209. Acid and reactive dyes are generally the most difficult to remove: some materials, e.g. bentonite clay, bind several dye types except acid dyes 209, 277 whereas Fuller.s earth, an adsorbent capable of binding dyes from many classes including acid dyes 207 fails to bind reactive dyes. Chitin and chitosan have extremely high acid and reactive dye binding capacity. Based on adsorption capacity for two basic dyes and one acid dye, it was calculated that the use of
natural clay, bagasse pith and maize cob would require only about 2-10% of the costs of activated carbon, even though the adsorption capacity of these low-cost materials was considerably lower than that of activated carbon. To evaluate the feasibility of a potential dye adsorbent, not only its costs and its dye-binding capacity should be considered, but also its adsorption kinetics, its regeneration properties and its requirements and limitations with respect to environmental conditions like pH, temperature and salt concentration. In a review of the literature on the removal of acid dyes by using dead plant and animal matter, it was concluded that cross-linked chitosan and quaternised lignocellulose were the best materials with respect to adsorption or ion exchange capacity, adsorption kinetics and costs. Most non-modified biological materials had a low adsorption capacity, adsorbents with a high adsorption capacity like chitin, chitosan and polyamide-epichlorohydrin-cellulose had the
drawback of very slow kinetics, and quaternised cellulose was too expensive. Despite the large number of publications on dye adsorption, full-scale application is limited to combinations, e.g. combined adsorption and biodegradation in activated carbon amended activated sludge systems or anaerobic bioreactors, or combined sorption and coagulation by a synthetic clay slurry
4. Electrolysis
Electrolysis is based on applying an electric current through to the wastewater to be treated by using electrodes. The anode is a sacrificial metal (usually iron) electrode that withdraws electrons from the electrode material, which results in the release of Fe(II)-ions to the bulk solution and precipitation of Fe(OH)2 at the electrode surface. Moreover, water and chloride ions are oxidised, resulting in the formation of O2, O3 and Cl2. The cathode is a hydrogen electrode that produces H2 gas from water. Organic compounds like dyes react through a combination of electrochemical oxidation, electrochemical reduction, electrocoagulation and electroflotation reactions: - at the anode sorption onto precipitated iron, direct electrochemical oxidation forming oxidised radicals and oxidation by the produced O3 and Cl2 gases; - at the cathode electrochemical reduction forming reduced radicals and
- in the bulk solution chemical reduction or coagulation by the released Fe(II) ions, followed (in case of coagulation) by flotation by bubbles of the produced H2 gas.
In several studies, electrochemical methods have been successfully applied to achieve decolourisation of dye solutions and dye containing wastewaters. However, the process is
expensive due to large energy requirements and the limited lifetime of the electrodes 330.
Furthermore, as radical reactions are involved, uncontrolled formation of unwanted breakdown products may occur. Another possible drawback is foaming.
5. Advanced oxidation processes
Advanced oxidation can be defined as oxidation by compounds with an oxidation potential (E0) higher than that of oxygen (1.23 V), i.e. hydrogen peroxide (E0 = 1.78 V), ozone (E0 = 2.07 V) and the hydroxyl radical (E0 = 2.28 V). Hydrogen peroxide alone is, however, usually not powerful enough. Advanced oxidation processes (AOPs) are therefore mostly based on the generation of highly reactive radical species (especially the hydroxyl radical HO.) that can react with a wide range of compounds, also with compounds that are otherwise difficult to degrade, e.g. dye molecules. The four AOPs that have been most widely studied are ozonation, UV/H2O2, Fenton.s reagent (Fe2+/H2O2) and UV/TiO2. In the ozonation process, hydroxyl radicals are formed when O3 decomposes in water. Though ozone itself is a strong oxidant, hydroxyl radicals are even more reactive. Decomposition of ozone requires high pH (>10). Ozone treatment of organic molecules proceeds therefore faster in alkaline solutions than at neutral or acidic pH where ozone is the main oxidant. Ozone rapidly decolourises water-soluble dyes but non-soluble dyes (vat dyes and disperse dyes) react much slower. Textile-processing wastewater furthermore usually contains many refractory constituents other than dyes (e.g. surfactants) that will react with ozone, thereby increasing the ozone
demand 102. It is advised, therefore, to pre-treat the wastewater before ozonation is applied. For example, in Leek, England, ozonation is used as the final stage (after biological treatment and filtration) for treating textile-processing wastewater at full-scale. This concept is, however, not logical as ozonation seldom leads to complete oxidation. Instead, ozone converts the organic compounds into smaller (usually biodegradable) molecules like dicaroboxylic acids and aldehydes. The reduction of COD is therefore low, while some of the ozonation products (especially the aldehydes) are highly toxic. It is better, therefore, to treat the effluent of the ozonation stage, logically by using inexpensive biological methods.
Fenton oxidation is based on the generation of hydroxyl radicals from Fenton’s reagent (Fe2+/H2O2) when ferrous iron is oxidised by hydrogen peroxide:
Fe2+ + H2O2 ! Fe(OH)2+ + HO. [1.2] Also higher oxidised iron species like [Fe(OH)2(H2O)5]2+ may be formed and it may even be possible that these species are the main oxidants in Fenton oxidation processes. In addition, re-reduction of ferric iron (redox cycling) can take place, thereby enabling iron to act as a catalyst in the generation of radicals:
Fe3+ + H2O2 ! Fe2+ + HO2. + H+ [1.3]
However, reaction (1.3) proceeds much slower than reaction (1.2), unless at very high temperatures or when the reaction is catalysed by UV-light. In the latter case, both Fenton’s reagent and Fenton-like reagent (Fe3+/H2O2) can be used. Another enhanced Fenton-like process uses H2O2 in combination with iron powder. The oxidation reaction here is the conventional dark Fenton’s process but adsorption of dyes to the iron powder increases its effectiveness. Fenton or Fenton-like oxidation can decolourise a wide range of dyes. In comparison to ozonation, the process is relatively cheap and results generally in a larger
COD reduction, although post-treatment (by for instance activated sludge) may still be required. A drawback for application of Fenton or Fenton-like oxidation for the treatment of .the usually highly alkaline- textile-processing wastewaters is that the process requires low pH (2 . 5). At higher pH, large volumes of waste sludge are generated by the precipitation of ferric iron salts and the process loses effectiveness as H2O2 is catalytically decomposed to oxygen 13. Fenton or Fenton-like oxidation will furthermore be negatively affected by the presence of radical scavengers and strong chelating agents in the wastewater.
Photocatalytic oxidation processes (UV/H2O2, UV/TiO2; UV/Fenton’s reagent; UV/O3 and other) are all based on the formation of free radicals due to UV irradiation. Typically, as UV light does not penetrate sufficiently in highly coloured waste streams, application of photocatalytic processes is limited to the post-treatment stage. When UV is used in combination with hydrogen peroxide, hydroxyl radicals are formed according to
the following (simplified) reaction:
H2O2 + h? ! 2 HO. [1.4]
Drawbacks of the UV/H2O2 process are the relatively high costs and the occasional lack of
effectiveness. Faster, cheaper and more effective photocatalytic processes receive therefore
increasing attention, especially those based on catalysis by solid semiconductor materials, mostly TiO2 particles. When this material is irradiated with photons of less than 385 nm, the band gap energy is exceeded and an electron is promoted from the valence band to the conduction band. The resultant electron-hole pair has a lifetime in the space-charge region that enables its participation in chemical reactions. In general, oxygen is used to scavenge the conduction band electron to produce a superoxide anion radical (O2.), while adsorbed water molecules are oxidised to hydroxyl radicals:
With TiO2 catalysed UV treatment, a wide range of dyes can be oxidised. The dyes are generally not only decolourised but also highly mineralised.
6. Biological techniques
Biological dye removal techniques are based on microbial biotransformation of dyes. As dyes are designed to be stable and long-lasting colorants, they are usually not easily biodegraded. Nevertheless, many researches have demonstrated partial or complete biodegradation of dyes by pure and mixed cultures of bacteria, fungi and algae.
a. Bacterial biodegradation
For a general evaluation of dye biodegradability, the dyes. chemical structures, rather than their application classes, should be considered. Investigations to bacterial dye biotransformation have so far mainly been focused to the most abundant chemical class, that of the azo dyes. The electron withdrawing nature of the azo linkages obstructs the susceptibility of azo dye molecules to oxidative reactions. Therefore, azo dyes generally resist aerobic bacterial biodegradation. Only bacteria with specialised azo dye reducing enzymes were found to degrade azo dyes under fully aerobic conditions. In contrast, breakdown of azo linkages by reduction under anaerobic conditions is much less specific. This anaerobic reduction implies decolourisation as the azo dyes are converted to -usually colourless but potentially harmful (section 1.4.2)- aromatic amines. Aromatic amines are generally not further degraded under anaerobic conditions. Anaerobic treatment must therefore be considered merely as the first stage of the complete degradation of azo
dyes. The second stage involves conversion of the produced aromatic amines. For
several aromatic amines, this can be achieved by biodegradation under aerobic conditions. Bacterial biodegradation of non-azo dyes has received little attention so far:
Anthraquinone dyes. Anthraquinone dyes may possibly be aerobically degraded analogous to
anthraquinone or anthraquinone-2-sulphonate. At least it has been demonstrated that three bacterial strains could grow with the anthraquinone dye Acid Blue 277:1 as sole source of energy. Under anaerobic conditions, the transformation of anthraquinone dyes is presumably limited to reduction of quinone to hydroquinone, a reaction that reverses once the molecule is again exposed to oxygen. Some anthraquinone dyes have been observed to be removed from the water phase by formation of an .insoluble pigment. under anaerobic conditions. This is in line with the observation that electrochemical reduction of an anthraquinone dye increased its adsorptive properties.
Triphenylmethane dyes. Aerobic decolourisation of triphenylmethane dyes has been demonstrated repeatedly but it has also been stated that these dyes resist degradation in activated sludge systems. Under anaerobic conditions, the transformation of triphenylmethane dyes is presumably limited to reversible reactions like the reduction of malachite green (Basic Green 4) to leucomalachite green.
Phthalocyanine dyes.
Phtalocyanine dyes are probably not biodegradable. Reversible reduction and decolourisation occurs under anaerobic conditions.
It should be noticed that the brief overview in this section did not include dye degradation by Streptomyces and other actinomycetes, i.e. bacteria that produce extracellular oxidative enzymes like white-rot fungi. Those extracellular oxidative enzymes are relatively non-specific enzymes catalysing the oxidation of a variety of dyes.
b. Fungal biodegradation
Lignin-degrading fungi, white-rot fungi, can degrade a wide range of aromatics. This property is mainly due to the relatively non-specific activity of their lignolytic enzymes, such as lignin
peroxidase, manganese peroxidase and laccase. The reactions catalysed by these extracellular enzymes are oxidation reactions, e.g. lignin peroxidase catalyses the oxidation of non-phenolic aromatics, whereas manganese peroxidase and laccase catalyse the oxidation of phenolic compounds. The degradation of dyes by white-rot fungi was first reported in 1983 and has since then been the subject of many research papers. An exhaustive review of these papers was recently published. Virtually all dyes from all chemically distinct groups are prone to fungal oxidation but there are large differences between fungal species with respect to their catalysing power and dye selectivity. A clear
relationship between dye structure and fungal dye biodegradability has not been established so far. Fungal degradation of aromatic structures is a secondary metabolic event that starts when nutrients (C, N and S) become limiting. Therefore, while the enzymes are optimally expressed under starving conditions, supplementation of energy substrates and nutrients are necessary for propagation of the cultures. Other important factors for cultivation of white-rot fungi and expression of lignolytic activity are the availability of enzyme cofactors and the pH of the environment. Although stable operation of continuous fungal bioreactors for the treatment of synthetic dye solutions has been achieved, application of white-rot fungi for the removal of dyes from textile wastewater faces many problems. As wastewater is not the natural environment of white-rot fungi, the enzyme production may be unreliable and the biomass growth and retention in bioreactors will be a matter of concern. As treatment of large water volumes may be difficult, extraction and concentration of dyes prior to fungal treatment, may be necessary. Furthermore, the low optimum pH for lignin peroxidase (4.5 . 5) requires extensive acidification of the usually highly alkaline textile wastewater and causes inhibition of other useful microorganisms like bacteria. Moreover, other wastewater constituents, especially aromatics, may interfere with fungal dye degradation.
c. Algal biodegradation
Degradation of a number of azo dyes by algae has been reported in a few studies. The degradation pathway is thought to involve reductive cleavage of the azo linkage followed by further degradation (mineralisation) of the formed aromatic amines. Hence, algae have been demonstrated to degrade several aromatic amines, even sulphonated ones. In open wastewater treatments systems, especially in (shallow) stabilisation ponds, algae may therefore contribute to the removal azo dyes and aromatic amines from the water phase.
Vivekananda Institute of Algal Technology:
Research work carried out by VIAT both in the laboratory conditions and in the field conditions revealed the potentials of a few selected micro algae for degrading dyes in textile industry effluent. VIAT research group has successfully established the degrading capabilities of micro algae at SUNTEX Processing Mills Pvt Ltd near Gummidippoondy in pilot scale trials. Scaling-up of the technology is in progress to not only degrade dyes but also correct the pH and effectively prevent sludge formation by simplifying the effluent treatment process using algal remediation process (Phycoremediation).
All aromatic compounds absorb electromagnetic energy but only those that absorb light with wavelengths in the visible range (~350-700 nm) are coloured. Dyes contain chromophores, delocalised electron systems with conjugated double bonds, and auxochromes, electron-withdrawing or electrondonating substituents that cause or intensify the colour of the chromophore by altering the overall energy of the electron system. Usual chromophores are -C=C-, -C=N-, -C=O, -N=N-, -NO2 and quinoid rings, usual auxochromes are -NH3, -COOH, -SO3H and -OH. Based on chemical structure or chromophore, 20-30 different groups of dyes can be discerned. Azo (monoazo, disazo, triazo, polyazo), anthraquinone, phthalocyanine and triarylmethane dyes are quantitatively the most important groups. Other groups are diarylmethane, indigoid, azine, oxazine, thiazine, xanthene, nitro, nitroso, methine, thiazole, indamine, indophenol, lactone, aminoketone and hydroxyketone dyes and dyes of undetermined structure (stilbene and sulphur dyes).
The vast array of commercial colorants is classified in terms of colour, structure and application method in the Colour Index (C.I.) which is edited since 1924 (and revised every three months) by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. The Colour Index (3rd Edition, issue 2) lists about 28,000 commercial dye names, representing ~10,500 different dyes, 45,000 of which are currently produced. Each different dye is given a C.I. generic name determined by its application characteristics and its colour. The Colour Index discerns 15 different application classes:
1.Acid dyes
The largest class of dyes in the Colour index is referred to as Acid dyes (~2300 different acid dyes listed, ~40% of them are in current production). Acid dyes are anionic compounds that are mainly used for dyeing nitrogen-containing fabrics like wool, polyamide, silk and modified acryl. They bind to the cationic NH4 +-ions of those fibres. Most acid dyes are azo (yellow to red, or a broader range colours in case of metal complex azo dyes), anthraquinone or triarylmethane (blue and green) compounds. The adjective .acid. refers to the pH in acid dye dyebaths rather than to the presence of acid groups (sulphonate, carboxyl) in the molecular structure of these dyes.
2 Reactive dyes
Reactive dyes are dyes with reactive groups that form covalent bonds with OH-, NH-, or SH-groups in fibres (cotton, wool, silk, nylon). The reactive group is often a heterocyclic aromatic ring substituted with chloride or fluoride, e.g. dichlorotriazine. Another common reactive group is vinyl sulphone (as in Reactive Orange 7). The use of reactive dyes has increased ever since their introduction in 1956, especially in industrialised countries. In the Colour Index, the reactive dyes form the second largest dye class with respect to the amount of active entries: about 600 of the ~1050 different reactive dyes listed are in current production. During dying with reactive dyes,
hydrolysis (i.e. inactivation) of the reactive groups is an undesired side reaction that lowers the degree of fixation. In spite of the addition of high quantities of salt and ureum (up to respectively 60 and 200 g/l) to raise the degree of fixation, it is estimated that 10 to 50% will not react with the fabric and remain .hydrolysed. in the water phase. The problem of coloured effluents is therefore mainly identified with the use of reactive dyes. Most (~80%) reactive dyes are azo or metal complex azo compounds but also anthraquinone and phthalocyanine reactive dyes are applied, especially for green and blue.
3 (Metal complex dyes)
Among acid and reactive dyes, many Metal complex dyes can be found (not listed as a separate category in the Colour Index). These are strong complexes of one metal atom (usually chromium, copper, cobalt or nickel) and one or two dye molecules, respectively 1:1 and 1:2 metal complex dyes. Metal complex dyes are usually azo compounds. About 1/6 of all azo dyes listed in the Colour Index are metal complexes 37 but also phthalocyanine metal complex dyes are applied.
4 Direct dyes
Direct dyes are relatively large molecules with high affinity for especially cellulose fibres. Van der Waals forces make them bind to the fibre. Direct dyes are mostly azo dyes with more than one azo bond or phthalocyanine, stilbene or oxazine compounds. In the Colour Index, the direct dyes form the second largest dye class with respect to the amount of different dyes: About 1600 direct dyes are listed but only ~30% of them are in current production.
5 Basic dyes
Basic dyes are cationic compounds that are used for dyeing acid-group containing fibres, usually synthetic fibres like modified polyacryl. They bind to the acid groups of the fibres. Most basic dyes are diarylmethane, triarylmethane, anthraquinone or azo compounds. Basic dyes represent ~5% of all dyes listed in the Colour Index.
6 Mordant dyes
Mordant dyes are fixed to fabric by the addition of a mordant, a chemical that combines with the dye and the fibre. Though mordant dyeing is probably one of the oldest ways of dyeing, the use of mordant dyes is gradually decreasing: only ~23% of the ~600 different mordant dyes listed in the Colour Index are in current production. They are used with wool, leather, silk, paper and modified cellulose fibres. Most mordant dyes are azo, oxazine or triarylmethane compounds. The mordants are usually dichromates or chromium complexes.
7 Disperse dyes
Disperse dyes are scarcely soluble dyes that penetrate synthetic fibres (cellulose acetate, polyester, polyamide, acryl, etc.). This diffusion requires swelling of the fibre, either due to high temperatures (>120 °C) or with the help of chemical softeners. Dying takes place in dyebaths with fine disperse solutions of these dyes. Disperse dyes form the third largest group of dyes in the Colour Index: about 1400 different compounds are listed, of which ~40% is currently produced. They are usually small azo or nitro compounds (yellow to red), anthraquinones (blue and green) or metal complex azo compounds (all colours).
8 Pigment dyes
Pigment dyes (i.e. organic pigments) represent a small but increasing fraction of the pigments, the most widely applied group of colorants. About 25% of all commercial dye names listed in the Colour Index are pigment dyes but these ~6900 product names stand for less than 800 different dyes. These insoluble, non-ionic compounds or insoluble salts retain their crystalline or particulate structure throughout their application. Pigment dyeing is achieved from a dispersed aqueous solution and therefore requires the use of dispersing agents. Pigments are usually used together with thickeners in print pastes for printing diverse fabrics. Most pigment dyes are azo compounds (yellow, orange, and red) or metal complex phthalocyanines (blue and green). Also anthraquinone and quinacridone pigment dyes are applied.
9 Vat dyes
Vat dyes are water-insoluble dyes that are particularly and widely used for dyeing cellulose fibres. The dyeing method is based on the solubility of vat dyes in their reduced (leuco) form. Reduced with sodium dithionite, the soluble leuco vat dyes impregnate the fabric. Next, oxidation is applied to bring back the dye in its insoluble form. Almost all vat dyes are anthraquinones or indigoids. Indigo itself is a very old example of a vat dye, with about 5000 years of application history. .Vat. refers to the vats that were used for the reduction of indigo plants through fermentation.
10 Anionic dyes and ingrain dyes
Azoic dyes and Ingrain dyes (naphthol dyes) are the insoluble products of a reaction between a
coupling component (usually naphthols, phenols or acetoacetylamides; listed in the Colour Index as C.I. azoic coupling components) and a diazotised aromatic amine (listed in the Colour Index as C.I. azoic diazo components). This reaction is carried out on the fibre. All naphthol dyes are azo compounds.
11 Sulphur dyes
Sulphur dyes are complex polymeric aromatics with heterocyclic S-containing rings. Though
representing about 15% of the global dye production, sulphur dyes are not so much used in Western Europe. Dyeing with sulphur dyes involves reduction and oxidation, comparable to vat dyeing. They are mainly used for dyeing cellulose fibres.
12 Solvent dyes
Solvent dyes (lysochromes) are non-ionic dyes that are used for dyeing substrates in which they can dissolve, e.g. plastics, varnish, ink, waxes and fats. They are not often used for textile-processing but their use is increasing. Most solvent dyes are diazo compounds that underwent some molecular rearrangement. Also triarylmethane, anthraquinone and phthalocyanine solvent dyes are applied.
13 Fluorescent brighteners
Fluorescent brighteners (or bluing agents) mask the yellowish tint of natural fibres by absorbing ultraviolet light and weakly emitting visible blue. They are not dyes in the usual sense because they lack intense colour. Based on chemical structure, several different classes of fluorescent brighteners are discerned: stilbene derivatives, coumarin derivatives, pyrazolines, 1,2-ethene derivatives, naphthalimides and aromatic or heterocyclic ring structures. Many fluorescent brighteners contain triazinyl units and water-solubilising groups.
14 Other dye classes
Apart from the dye classes mentioned above, the Colour Index also lists Food dyes and Natural dyes. Food dyes are not used as textile dyes and the use of natural dyes (mainly anthraquinone, indigoid, flavenol, flavone or chroman compounds that can be used as mordant, vat, direct, acid or solvent dyes) in textile-processing operations is very limited.
Dyes, environmental concern
Many dyes are visible in water at concentrations as low as 1 mg l-1. Textile-processing wastewaters, typically with a dye content in the range 10 . 200 mg l-1 248, are therefore usually highly coloured and discharge in open waters presents an aesthetic problem. As dyes are designed to be chemically and photolytically stable, they are highly persistent in natural environments. The release of dyes may therefore present an ecotoxic hazard and introduces the potential danger of bioaccumulation that may eventually affect man by transport through the food chain.
a. Bioaccumulation
The bioaccumulation tendency of dyestuffs in fish has been comprehensively investigated in research promoted by ETAD, the Ecological and Toxicological Association of Dyes and Organic Pigments Manufacturers. The bioconcentration factors (BCF.s) of 75 dyes from different application classes were determined and related to the partition coefficient n-octanol/water (KOW) of each different compound. Water-soluble dyes with low KOW, i.e. ionic dyes like acid, reactive and basic dyes, did not bioaccumulate (generally log BCF < 0.5). For these water-soluble dyes, log P (log KOW) showed a linear relationship with log BCF so it was expected that dyestuffs with higher KOW would bioaccumulate. However, water-insoluble organic pigments with extremely high partition coefficients did not bioaccumulate probably due to their extremely low water and fat solubilities and also the BCF values for disperse dyes, i.e. scarcely soluble compounds with a moderately lipophilic nature, were much lower than expected. In all cases, log BCF < 2, which indicates that none of the dyes tested
showed any substantial bioaccumulation.
b. Toxicity of dyestuffs
Dyestuff toxicity has been investigated in numerous researches. These toxicity (i.e. mortality,
genotoxicity, mutagenicity and carcinogenicity) studies diverge from tests with aquatic organisms (fish, algae, bacteria, etc.) to tests with mammals. Furthermore, research has been carried out to effects of dyestuffs and dye containing effluents on the activity of both aerobic and anaerobic bacteria in wastewater treatment systems.
The acute toxicity of dyestuffs is generally low. Algal growth (photosynthesis), tested with
respectively 56 and 46 commercial dyestuffs, was generally not inhibited at dye concentrations below 1 mg/l. The most acutely toxic dyes for algae are cationic basic dyes. Fish mortality tests showed that 2% out of 3000 commercial dyestuffs tested had LC50 values below 1 mg/l. The most acutely toxic dyes for fish are basic dyes, especially those with a triphenylmethane structure. Fish also seem to be relatively sensitive to many acid dyes 65. Mortality tests with rats showed that only 1% out of 4461 commercial dyestuffs tested had LD50 values below 250 mg/kg body weight. Therefore, the chance of human mortality due to acute dyestuff toxicity is probably very low. However, acute sensitisation reactions by humans to dyestuffs often occurs. Especially some disperse dyestuffs have been found to cause allergic reactions, i.e. eczema or contact dermititis. Chronic effects of dyestuffs, especially of azo dyes, have been studied for several decades. Researchers were traditionally mostly focused on the effects of food colorants, usually azo compounds. Furthermore, also the effects of occupational exposure to dyestuffs of human workers in
dye manufacturing and dye utilising industries have received attention. Azo dyes in purified form are seldom directly mutagenic or carcinogenic, except for some azo dyes with free amino groups. However, reduction of azo dyes, i.e. cleavage of the dye.s azo linkage(s), leads to formation of aromatic amines and several aromatic amines are known mutagens and carcinogens. In mammals, metabolic activation (= reduction) of azo dyes is mainly due to bacterial activity in the anaerobic parts of the lower gastrointestinal tract. Various other organs, especially the liver and the kidneys, can, however, also reduce azo dyes.
After azo dye reduction in the intestinal tract, the released aromatic amines are absorbed by the intestine and excreted in the urine. The acute toxic hazard of aromatic amines is carcinogenesis, especially bladder cancer. The carcinogenicity mechanism probably includes the formation of acyloxy amines through N-hydroxylation and N-acetylation of the aromatic amines followed by O-acylation. These acyloxy amines can be converted to nitremium and carbonium ions that bind to DNA and RNA, which induces mutations and tumour formation.
The mutagenic activity of aromatic amines is strongly related to molecular structure. In 1975 and in 1982, the International Agency for Research on Cancer (IARC) summarised the literature on suspected azo dyes, mainly amino-substituted azo dyes, fat-soluble azo dyes and benzidine azo dyes, but also a few sulphonated azo dyes. Most of the dyes on the IARC list were taken out of production. Generally stated, genotoxicity is associated with all aromatic amines with benzidine moieties, as well as with some aromatic amines with toluene, aniline and naphthalene moieties. The toxicity of aromatic amines depends strongly on the spatial structure of the molecule or .in other words. the location of the amino-group(s). For instance, whereas there is strong evidence that 2-naphthylamine is a carcinogen, 1-naphthylamine is much less toxic 47. The toxicity of aromatic amines depends furthermore on the nature and location of other substituents. As an example, the substitution with nitro, methyl or methoxy groups or halogen atoms may increase the toxicity, whereas substitution with carboxyl or sulphonate groups generally lowers the toxicity. As most soluble commercial azo dyestuffs contain one or more sulphonate groups, insight in the potential danger of sulphonated aromatic amines is particularly important. In an extensive review of literature data on genotoxicity and carcinogenicity of sulphonated aromatic amines, it was concluded that sulphonated aromatic amines, in contrast to some of their unsulphonated analogues, have generally no or very low genotoxic and tumorigenic potential
Dye removal techniques
Various physical, chemical and biological pre treatment, main treatment and post treatment techniques can be employed to remove colour from dye containing wastewaters. Physicochemical techniques include membrane filtration, coagulation/flocculation, precipitation, flotation, adsorption, ion exchange, ion pair extraction, ultrasonic mineralisation, electrolysis, advanced oxidation (chlorination, bleaching, ozonation, Fenton oxidation and photocatalytic oxidation) and chemical reduction. Biological techniques include bacterial and fungal biosorption and biodegradation in aerobic, anaerobic, anoxic or combined anaerobic/aerobic treatment processes.
Several factors determine the technical and economic feasibility of each single dye removal technique:
- dye type
- wastewater composition
- dose and costs of required chemicals
- operation costs (energy and material)
- environmental fate and handling costs of generated waste products
In general, each technique has its limitations. The use of one individual process may often not be sufficient to achieve complete decolourisation. Dye removal strategies consist therefore mostly of a combination of different techniques.
1. Membrane filtration
Nanofiltration and reverse osmosis, using membranes with a molecular weight cut-off (MWCO) below ~10,000 Dalton, can be applied as main or post treatment processes for separation of salts and larger molecules including dyes from dyebath effluents and bulk textile-processing wastewaters. Filtration with bigger membranes, i.e. ultrafiltration and microfiltration, is generally not suitable as the membrane pore size is too large to prevent dye molecules passing through but it can be successful as pre treatment for further nanofiltration or reverse osmosis. Membrane filtration is a quick method with low spatial requirement. Another advantage is that the permeate, as well as some of the concentrated compounds, including non-reactive dyes, can be reused. This reuse, however,
applies mostly only for smaller waste flows 331. The disadvantages of membrane techniques are flux decline and membrane fouling, necessitating frequent cleaning and regular replacement of the modules. Another important drawback is that the generated concentrate must be processed further, for instance by ozonation. The capital costs of membrane filtration are therefore generally rather high. Filtration techniques for the treatment of textile wastewaters are especially widely applied in South Africa.
2. Coagulation/flocculation
Coagulation/flocculation is often applied in the treatment of textile-processing wastewater, either to partly remove Chemical Oxygen Demand (COD) and colour from the raw wastewater before further treatment, to polish the final effluents of biologically or otherwise treated wastewater or even as the main treatment process. The principle of the process is the addition of a coagulant followed by a generally rapid chemical association between the coagulant and the pollutants. The thus formed coagulates or flocs subsequently precipitate or are to be removed from the water phase by flotation.
Various inorganic coagulants are used, mostly lime, magnesium, iron and aluminium salts. Inorganic compounds are, however, generally not very suitable to remove highly soluble (= sulphonated) dyes from solution unless rather large quantities are dosed. Coagulation/flocculation with inorganic chemicals generates considerable volumes of useless or even toxic sludge that must be incinerated or handled otherwise. This presents a serious drawback of the process. Recently developed organic polymers have been proven highly effective as dye coagulants, even for coagulation of reactive dyes, while the sludge production associated with polymer dosing is relatively low. Most of the polymers used for colour removal are, however, cationic and may be toxic to aquatic life at very low concentrations (less than 1 mg/l) and in biological wastewater treatment plants, some cationic polymers have been found to inhibit the nitrification process.
3. Sorption and ion exchange
Activated carbon or other materials can be used to remove dyes from wastewater, either by adsorption (anionic dyes) or by combined adsorption and ion exchange (cationic dyes). Sorption techniques yield waste sludge, i.e. dye-saturated material, that should be disposed off or regenerated. As there are nonionic, anionic and cationic dyes, most adsorbents do not remove all different dye types. Activated carbon is capable of adsorbing many different dyes with high adsorption capacity but it is expensive and the costs of regeneration are high because desorption of the dye molecules is not easily achieved. Various other (mostly low-cost) adsorbents have therefore been investigated as an alternative to activated carbon. Those adsorbents include:
- non-modified cellulose (plant) biomass, e.g. corn/maize cobs, maize stalks, wheat straw, linseed straw, rice husks, wood chips, sawdust, bark, coirpith, banana pith, bagasse pith, palm fruit bunch particles, peat moss, peat, linseed cake, sugar beet pulp, sugar industry mud, cotton waste and cellulose;- modified cellulose biomass, e.g. carbonised coirpith, carbonised coconut-tree sawdust , chemically modified sunflower stalks, polyamide-epichlorohydrin-cellulose, carbamoyl-cellulose, quaternised-cellulose, quaternised-lignocellulose; sugarcane bagasse derived anion exchange resin; - bacterial biomass, e.g. Aeromonas , actinomycetes, activated sludge; dried and powdered biogas waste slurry - fungal biomass - yeast biomass; - chitin, a material that can be found in e.g. shells, insect shields and fungal cell walls; chitosan, deacylated chitin; cross-linked chitosan fibres;- soil material, e.g. sand, silica, natural clay, bentonite clay, diatomite clay, montmorillonite clay, vermiculite clay, fullers earth, synthetic clay; - wood charcoal, bone charcoal, barbecue charcoal, magnetic charcoal; - activated bauxite, activated alumina;- other materials, e.g. pressed sludge cake (pulp mill waste), pyrolysed tire, leather hide powder, dealginated seaweed, coal dust , chrome sludge, steel plant slag, fly ash and hair . Some of these materials show high dye removal capacities, comparable or .especially in the case of disperse dyes. even higher than activated carbon. This depends strongly on the dye class. Many of the materials listed, e.g. rice husks, bark, cotton waste and hair, have a high capacity for binding (cationic) basic dyes but hardly remove dyes from other classes 209. Acid and reactive dyes are generally the most difficult to remove: some materials, e.g. bentonite clay, bind several dye types except acid dyes 209, 277 whereas Fuller.s earth, an adsorbent capable of binding dyes from many classes including acid dyes 207 fails to bind reactive dyes. Chitin and chitosan have extremely high acid and reactive dye binding capacity. Based on adsorption capacity for two basic dyes and one acid dye, it was calculated that the use of
natural clay, bagasse pith and maize cob would require only about 2-10% of the costs of activated carbon, even though the adsorption capacity of these low-cost materials was considerably lower than that of activated carbon. To evaluate the feasibility of a potential dye adsorbent, not only its costs and its dye-binding capacity should be considered, but also its adsorption kinetics, its regeneration properties and its requirements and limitations with respect to environmental conditions like pH, temperature and salt concentration. In a review of the literature on the removal of acid dyes by using dead plant and animal matter, it was concluded that cross-linked chitosan and quaternised lignocellulose were the best materials with respect to adsorption or ion exchange capacity, adsorption kinetics and costs. Most non-modified biological materials had a low adsorption capacity, adsorbents with a high adsorption capacity like chitin, chitosan and polyamide-epichlorohydrin-cellulose had the
drawback of very slow kinetics, and quaternised cellulose was too expensive. Despite the large number of publications on dye adsorption, full-scale application is limited to combinations, e.g. combined adsorption and biodegradation in activated carbon amended activated sludge systems or anaerobic bioreactors, or combined sorption and coagulation by a synthetic clay slurry
4. Electrolysis
Electrolysis is based on applying an electric current through to the wastewater to be treated by using electrodes. The anode is a sacrificial metal (usually iron) electrode that withdraws electrons from the electrode material, which results in the release of Fe(II)-ions to the bulk solution and precipitation of Fe(OH)2 at the electrode surface. Moreover, water and chloride ions are oxidised, resulting in the formation of O2, O3 and Cl2. The cathode is a hydrogen electrode that produces H2 gas from water. Organic compounds like dyes react through a combination of electrochemical oxidation, electrochemical reduction, electrocoagulation and electroflotation reactions: - at the anode sorption onto precipitated iron, direct electrochemical oxidation forming oxidised radicals and oxidation by the produced O3 and Cl2 gases; - at the cathode electrochemical reduction forming reduced radicals and
- in the bulk solution chemical reduction or coagulation by the released Fe(II) ions, followed (in case of coagulation) by flotation by bubbles of the produced H2 gas.
In several studies, electrochemical methods have been successfully applied to achieve decolourisation of dye solutions and dye containing wastewaters. However, the process is
expensive due to large energy requirements and the limited lifetime of the electrodes 330.
Furthermore, as radical reactions are involved, uncontrolled formation of unwanted breakdown products may occur. Another possible drawback is foaming.
5. Advanced oxidation processes
Advanced oxidation can be defined as oxidation by compounds with an oxidation potential (E0) higher than that of oxygen (1.23 V), i.e. hydrogen peroxide (E0 = 1.78 V), ozone (E0 = 2.07 V) and the hydroxyl radical (E0 = 2.28 V). Hydrogen peroxide alone is, however, usually not powerful enough. Advanced oxidation processes (AOPs) are therefore mostly based on the generation of highly reactive radical species (especially the hydroxyl radical HO.) that can react with a wide range of compounds, also with compounds that are otherwise difficult to degrade, e.g. dye molecules. The four AOPs that have been most widely studied are ozonation, UV/H2O2, Fenton.s reagent (Fe2+/H2O2) and UV/TiO2. In the ozonation process, hydroxyl radicals are formed when O3 decomposes in water. Though ozone itself is a strong oxidant, hydroxyl radicals are even more reactive. Decomposition of ozone requires high pH (>10). Ozone treatment of organic molecules proceeds therefore faster in alkaline solutions than at neutral or acidic pH where ozone is the main oxidant. Ozone rapidly decolourises water-soluble dyes but non-soluble dyes (vat dyes and disperse dyes) react much slower. Textile-processing wastewater furthermore usually contains many refractory constituents other than dyes (e.g. surfactants) that will react with ozone, thereby increasing the ozone
demand 102. It is advised, therefore, to pre-treat the wastewater before ozonation is applied. For example, in Leek, England, ozonation is used as the final stage (after biological treatment and filtration) for treating textile-processing wastewater at full-scale. This concept is, however, not logical as ozonation seldom leads to complete oxidation. Instead, ozone converts the organic compounds into smaller (usually biodegradable) molecules like dicaroboxylic acids and aldehydes. The reduction of COD is therefore low, while some of the ozonation products (especially the aldehydes) are highly toxic. It is better, therefore, to treat the effluent of the ozonation stage, logically by using inexpensive biological methods.
Fenton oxidation is based on the generation of hydroxyl radicals from Fenton’s reagent (Fe2+/H2O2) when ferrous iron is oxidised by hydrogen peroxide:
Fe2+ + H2O2 ! Fe(OH)2+ + HO. [1.2] Also higher oxidised iron species like [Fe(OH)2(H2O)5]2+ may be formed and it may even be possible that these species are the main oxidants in Fenton oxidation processes. In addition, re-reduction of ferric iron (redox cycling) can take place, thereby enabling iron to act as a catalyst in the generation of radicals:
Fe3+ + H2O2 ! Fe2+ + HO2. + H+ [1.3]
However, reaction (1.3) proceeds much slower than reaction (1.2), unless at very high temperatures or when the reaction is catalysed by UV-light. In the latter case, both Fenton’s reagent and Fenton-like reagent (Fe3+/H2O2) can be used. Another enhanced Fenton-like process uses H2O2 in combination with iron powder. The oxidation reaction here is the conventional dark Fenton’s process but adsorption of dyes to the iron powder increases its effectiveness. Fenton or Fenton-like oxidation can decolourise a wide range of dyes. In comparison to ozonation, the process is relatively cheap and results generally in a larger
COD reduction, although post-treatment (by for instance activated sludge) may still be required. A drawback for application of Fenton or Fenton-like oxidation for the treatment of .the usually highly alkaline- textile-processing wastewaters is that the process requires low pH (2 . 5). At higher pH, large volumes of waste sludge are generated by the precipitation of ferric iron salts and the process loses effectiveness as H2O2 is catalytically decomposed to oxygen 13. Fenton or Fenton-like oxidation will furthermore be negatively affected by the presence of radical scavengers and strong chelating agents in the wastewater.
Photocatalytic oxidation processes (UV/H2O2, UV/TiO2; UV/Fenton’s reagent; UV/O3 and other) are all based on the formation of free radicals due to UV irradiation. Typically, as UV light does not penetrate sufficiently in highly coloured waste streams, application of photocatalytic processes is limited to the post-treatment stage. When UV is used in combination with hydrogen peroxide, hydroxyl radicals are formed according to
the following (simplified) reaction:
H2O2 + h? ! 2 HO. [1.4]
Drawbacks of the UV/H2O2 process are the relatively high costs and the occasional lack of
effectiveness. Faster, cheaper and more effective photocatalytic processes receive therefore
increasing attention, especially those based on catalysis by solid semiconductor materials, mostly TiO2 particles. When this material is irradiated with photons of less than 385 nm, the band gap energy is exceeded and an electron is promoted from the valence band to the conduction band. The resultant electron-hole pair has a lifetime in the space-charge region that enables its participation in chemical reactions. In general, oxygen is used to scavenge the conduction band electron to produce a superoxide anion radical (O2.), while adsorbed water molecules are oxidised to hydroxyl radicals:
With TiO2 catalysed UV treatment, a wide range of dyes can be oxidised. The dyes are generally not only decolourised but also highly mineralised.
6. Biological techniques
Biological dye removal techniques are based on microbial biotransformation of dyes. As dyes are designed to be stable and long-lasting colorants, they are usually not easily biodegraded. Nevertheless, many researches have demonstrated partial or complete biodegradation of dyes by pure and mixed cultures of bacteria, fungi and algae.
a. Bacterial biodegradation
For a general evaluation of dye biodegradability, the dyes. chemical structures, rather than their application classes, should be considered. Investigations to bacterial dye biotransformation have so far mainly been focused to the most abundant chemical class, that of the azo dyes. The electron withdrawing nature of the azo linkages obstructs the susceptibility of azo dye molecules to oxidative reactions. Therefore, azo dyes generally resist aerobic bacterial biodegradation. Only bacteria with specialised azo dye reducing enzymes were found to degrade azo dyes under fully aerobic conditions. In contrast, breakdown of azo linkages by reduction under anaerobic conditions is much less specific. This anaerobic reduction implies decolourisation as the azo dyes are converted to -usually colourless but potentially harmful (section 1.4.2)- aromatic amines. Aromatic amines are generally not further degraded under anaerobic conditions. Anaerobic treatment must therefore be considered merely as the first stage of the complete degradation of azo
dyes. The second stage involves conversion of the produced aromatic amines. For
several aromatic amines, this can be achieved by biodegradation under aerobic conditions. Bacterial biodegradation of non-azo dyes has received little attention so far:
Anthraquinone dyes. Anthraquinone dyes may possibly be aerobically degraded analogous to
anthraquinone or anthraquinone-2-sulphonate. At least it has been demonstrated that three bacterial strains could grow with the anthraquinone dye Acid Blue 277:1 as sole source of energy. Under anaerobic conditions, the transformation of anthraquinone dyes is presumably limited to reduction of quinone to hydroquinone, a reaction that reverses once the molecule is again exposed to oxygen. Some anthraquinone dyes have been observed to be removed from the water phase by formation of an .insoluble pigment. under anaerobic conditions. This is in line with the observation that electrochemical reduction of an anthraquinone dye increased its adsorptive properties.
Triphenylmethane dyes. Aerobic decolourisation of triphenylmethane dyes has been demonstrated repeatedly but it has also been stated that these dyes resist degradation in activated sludge systems. Under anaerobic conditions, the transformation of triphenylmethane dyes is presumably limited to reversible reactions like the reduction of malachite green (Basic Green 4) to leucomalachite green.
Phthalocyanine dyes.
Phtalocyanine dyes are probably not biodegradable. Reversible reduction and decolourisation occurs under anaerobic conditions.
It should be noticed that the brief overview in this section did not include dye degradation by Streptomyces and other actinomycetes, i.e. bacteria that produce extracellular oxidative enzymes like white-rot fungi. Those extracellular oxidative enzymes are relatively non-specific enzymes catalysing the oxidation of a variety of dyes.
b. Fungal biodegradation
Lignin-degrading fungi, white-rot fungi, can degrade a wide range of aromatics. This property is mainly due to the relatively non-specific activity of their lignolytic enzymes, such as lignin
peroxidase, manganese peroxidase and laccase. The reactions catalysed by these extracellular enzymes are oxidation reactions, e.g. lignin peroxidase catalyses the oxidation of non-phenolic aromatics, whereas manganese peroxidase and laccase catalyse the oxidation of phenolic compounds. The degradation of dyes by white-rot fungi was first reported in 1983 and has since then been the subject of many research papers. An exhaustive review of these papers was recently published. Virtually all dyes from all chemically distinct groups are prone to fungal oxidation but there are large differences between fungal species with respect to their catalysing power and dye selectivity. A clear
relationship between dye structure and fungal dye biodegradability has not been established so far. Fungal degradation of aromatic structures is a secondary metabolic event that starts when nutrients (C, N and S) become limiting. Therefore, while the enzymes are optimally expressed under starving conditions, supplementation of energy substrates and nutrients are necessary for propagation of the cultures. Other important factors for cultivation of white-rot fungi and expression of lignolytic activity are the availability of enzyme cofactors and the pH of the environment. Although stable operation of continuous fungal bioreactors for the treatment of synthetic dye solutions has been achieved, application of white-rot fungi for the removal of dyes from textile wastewater faces many problems. As wastewater is not the natural environment of white-rot fungi, the enzyme production may be unreliable and the biomass growth and retention in bioreactors will be a matter of concern. As treatment of large water volumes may be difficult, extraction and concentration of dyes prior to fungal treatment, may be necessary. Furthermore, the low optimum pH for lignin peroxidase (4.5 . 5) requires extensive acidification of the usually highly alkaline textile wastewater and causes inhibition of other useful microorganisms like bacteria. Moreover, other wastewater constituents, especially aromatics, may interfere with fungal dye degradation.
c. Algal biodegradation
Degradation of a number of azo dyes by algae has been reported in a few studies. The degradation pathway is thought to involve reductive cleavage of the azo linkage followed by further degradation (mineralisation) of the formed aromatic amines. Hence, algae have been demonstrated to degrade several aromatic amines, even sulphonated ones. In open wastewater treatments systems, especially in (shallow) stabilisation ponds, algae may therefore contribute to the removal azo dyes and aromatic amines from the water phase.
Vivekananda Institute of Algal Technology:
Research work carried out by VIAT both in the laboratory conditions and in the field conditions revealed the potentials of a few selected micro algae for degrading dyes in textile industry effluent. VIAT research group has successfully established the degrading capabilities of micro algae at SUNTEX Processing Mills Pvt Ltd near Gummidippoondy in pilot scale trials. Scaling-up of the technology is in progress to not only degrade dyes but also correct the pH and effectively prevent sludge formation by simplifying the effluent treatment process using algal remediation process (Phycoremediation).