Pollution Control In Dye Industry
A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. The dye is generally applied in an aqueous solution, and may require a mordant to improve the fastness of the dye on the fiber
Both dyes and pigments appear to be colored because they absorb some wavelengths of light preferentially. In contrast with a dye, a pigment generally is insoluble, and has no affinity for the substrate.
σ → σ*
π → π*
n → σ*
n → π*
aromatic π → aromatic π*
Chromophore: it is a functional groups which are unsaturated and they cause a compound to become coloured.
Examples of chromophores are –N=N-, -C=C-, -C=N- and -C=O.
- Transitions involving σ, π, and n electrons
- Transitions involving charge-transfer electrons
- Transitions involving d and f electrons
Auxochromes are groups that does not impart color to the compound but increase the color of the compound. Functional groups such as hydroxyl (–OH), amino ( -NH2), nitro (-NO2), alkyl (-R), —OH, —OR, —NH2, —NHR, —NR2, —SH are common examples for auxochrome. The effect of the auxochrome is due to its ability to extend the conjugation of a chromophore by the sharing of non-bonding electrons.
Classification of Dyes
- Acid Dye
- Basic Dye
- Direct Dye
- Sulphur Dye
- Azo Dye
- Vat Dye
Dyestuff Industry Treatment
The presence of residual color, high levels of electrolytes, toxic substances (e.g., metals and unreacted raw materials) in dye application processes produce wastewaters that poses unacceptable environmental risks.
In the case of textile dyeing operations, the concerns raised can arise from incomplete dye bath exhaustion and the presence of dyeing auxiliaries and metal ions that are toxic to aquatic life.
Sources of effluent
- Dyeing and printing
- Textile industries
- Paper and ink manufacturing industries
Properties of effluent
- Impart colour to water bodies even if present in small quantity
- Reduces light penetration and photosynthesis
- Carcinogenic or mutagenic
- Azo dyes are more toxic as they affect microbes thereby affecting biological degradation treatment.
- Dyes increases BOD of effluent thereby affecting aquatic life
- Toxic to fish & microbial organisms
- The discharge of heavy metals into aquatic ecosystemsIncrease in alkalinity of water
- The turbidity and colour along with oil and scum create an unsighty appearance.
- The mineral materials, mostlysodium salts increase salinity of the water.
Volume of effluent
- The volume of effluent generated in dyeing is comparatively more.
- It contains dyes, mordants, acids (acetic acid), alkalis, nitrites, chromium salts, sodium chloride and soaps.
- These effluents are usually hot, highly coloured with a high pH and sulfide content
- Care must be taken while neutralising these liquors as acid may liberate hydrogen sulfide gas.
- Removal of Sulfides by treatment with chlorine or hypochlorites
- Spent vat dyes are strongly alkaline and have fairly high permanganate value
Effluent Treatment Methods
Activated carbon adsorption has been widely studied as a waste treatment method for the removal of different classes of dyes from wastewater.
Factors such as the choice of activated carbon, temperature, pH, contact time, and dosage must be taken into consideration for optimum removal of dyes from
The most commonly used method of dye removal by adsorption. It can be Effective for adsorbing cationic, mordant and acid dyes and reactive dyes
Performance depends on the type of carbon used and the characteristics of the wastewater.
Adsorptive Bubble Separation
Surface-active material, which may be ionic, molecular, colloidal, or macroparticulate in nature, is selectively adsorbed at the surface of bubbles rising through the liquid.
98—99.8% of Direct Blue1 was removed from wastewater
Fenton’s reagent (Fe2/H2O2) has been used commercially to oxidize and decolorize effluents containing a number of azo, anthraquinone, and reactive dyes.
Dyes removed in these studies include reactive azo and anthraquinone dyes.
Titanium Dioxide (TiO2)
The cationic dye Rhodamine B, Basic Violet 10 and the anthraquinone mordant dye Alizarine Red (Mordant Red 3) can be decomposed by Ti02 in the presence of visible light. Degradation occurs via the interaction of dye with hydroxyl radicals (OH.). Two of the final degradation products are phthalic acid and carbon dioxide.
- Sodium borohydride is one of the strongest water soluble reducing agent commercially available
- SO2 is produced when borohydride is used in combination with bisulfite catalysis in the pH rang e of 5 – 8 which readily reduces azo dyes
- The utility of this process involving a variety of azo dyes, giving percent colour reduction of 83-99% for acid, direct, and reactive dyes and 74-99.9% for metalized direct and phthalocyanine dyes
- This method was extended to industrial wastewater containing either a mixture of azo disperse dyes or azo reactive black dyes.
- These solutions were treated with a mixture of 400—500 mg/L Na2S2O5, 12—25 mg/L NaBH4, and 0—200 mg cationic flocculant, reducing color by 93 – 99%
Bioaugmentation and Bioremediation
These methods are currently used by the textile and dyestuff industries to decolorize waste water using enzymes.
In this regard, enzymes such as laccases, lignin peroxidases, and manganese peroxidases have proved effective in cleaving aromatic rings
Metal Complexed Dyes
One of the most serious environmental problems in the dye, textile, and leather industries is associated with the manufacture and use of metallized azo dyes that are complexed with chromium or cobalt to obtain desirable fastness properties
various chemical, physical, and biological methods, such treatments can be expensive and may result in sludges that must be disposed by incineration or land filling
In view of an emphasis on pollution prevention instead of waste treatment, the merits of substituting iron (Fe) for chromium (Cr) and cobalt (Co) in a group of commercially important acid dyes.
Fe-complexed dyes as environmentally friendly alternatives to widely used Cr and Co complexed acid dyes
The ionic components (heavy metals) are separated through the use of semi-permeable ionselective membranes.
Application of an electrical potential between the two electrodes causes a migration of cations and anions towards respective electrodes
Because of the alternate spacing of cation and anion permeable membranes, cells of concentrated and dilute salts are formed
Membrane Bioreactor in Textile Waste Water Treatment
Utilization of membrane filtration results in the retention of active micro-organisms, extra cellular enzymes generated by these micro-organisms for degradation of the organics present in the effluent
Some micro-organisms, especially nitrifiers, are slowly growing one, their loss shall reduce the efficiency of the treatment system and nutrient removal
In the MBR, these organisms are retained and a better treatment is achieved
In addition retention of active enzymes secreted by mico organisms taking part in the metabolization of organics present in the textile waste water is an important aspect of MBR technology
Maintenance of higher concentration of these enzymes shall result in rapid and better degradation of complex organic molecules present in the textile waste water.
The overall efficiency of BOD (Biological Oxygen Demand) and COD (Chemical Oxygen Demand) removal is improved
Advantages of MBR
- Y. R. Sharma, Elements of organic spectroscopy, S. Chand & Company LTD, New Delhi, India.
- Kirk – Othmer, Encyclopedia of chemical technology, 5th
Edition, 9th Volume, Wiley Interscience, John Wiley & Sons, Inc., Publication
- S.Eshwaramoorthi, K. Dhanapal and D.S.Chauhan, Environment With People’s Involvement & Co-ordination in India, Coimbatore, India, www.ecpconsulting.in
- http://www.scribd.com/doc/12949482/Dyestuff-Industry-Treatment, 2003