Pollution Control In Dye Industry

Introduction

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

Auxochrome

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.

Bathochromic Effect

Hypsochromic shift

Hyperchromic Effect

Hypochromic Effect

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
industries
Textile industries
Paper and ink manufacturing industries
Cosmetics
Pharmaceuticals
Food

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

Physical Methods

Adsorption

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

wastewater

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

Chemical methods

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.

Borohydride Reduction

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

Electrodialysis

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

References

www.wikipedia.org
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