Dye Decoloration and Degradation
Catalytic Advanced Oxidation reactions developed by Hydrogen Link for the decoloration of the dye wastewater result in very effective oxidative degradation of a variety of dyes from food and textile industry effluents. Even the most stubborn dyes such as azo dyes can be eliminated from the colored wastewater by our approach.
The treatment involves catalytic decomposition of hydrogen peroxide (H2O2) with the formation of uniquely efficient hydroxyl radicals, which are formed due to a specifically designed atomic coordination of our catalyst - Oxycatalyst. It is a heterogeneous catalyst (in a solid state, not liquid) which can be reused and does not contribute to the effluent impurities. In combination with hydrogen peroxide (which is the most eco-friendly chemical because it decomposes only into water and oxygen) – the Catalytic Advanced Oxidation has unsurpassed environmental quality of a truly non-polluting and non-toxic method for the dye decoloration and degradation
The Catalytic Advanced Oxidation treatment works in the whole pH range, and can be conveniently performed at neutral pH and room temperature, without the need for specialized equipment and installations.
Colorants – dyes and pigments
are a growing pollution problem in the industrial and household wastewater as a result of their extensive use and relative stability towards degradation. The dyes, (especially textile dyes) are engineered to be resistant to all kinds of treatments without fading. They need to sustain either alkaline or acidic environment; they need to withstand washing with soaps and bleaching agents, microbiological fading, be resistant to light and ultraviolet irradiation etc. Obviously, the better stability of dyes achieved in the consumer products, the worse problem they cause in the wastewater stream when subjected to decoloration and degradation
are able to color water even in concentrations as low as 1mg/liter. Textile wastewater contains typically a much higher amount of the dye content: 10 -200 mg/liter, which gives intense coloration. While color is easily recognizable in the water stream, an additional environmental hazard comes from the fact that many dyes are either toxic or become toxic when being gradually decomposed in the ecosystem. The dyes undergo bioaccumulation in living organisms - their natural degradation is much slower than the amounts added to the environment.
Some dyes are potentially carcinogenic and mutagenic (can cause mutations in organisms), as well as genotoxic (can damage DNA). Azo dyes may be toxic after metabolic reduction of the azo bond, producing aromatic amines. Dyes preserve their ability to absorb sunlight in water and as a result they reduce the photosynthetic capability of aquatic plants and microorganisms, even when the dilution of the wastewater camouflages the presence of the dye.
The problem of bioaccumulation of dyes in the aquatic organisms is mounting because of the multiply sources of te dye contamination, such as textile industry, food technology, paper and printing, cosmetic, pharmaceutical, detergent, pesticide and leather tanning industry, which in total consume more than one million tons of dyes annually.
Dyes are aromatic compounds which have the ability to absorb light in the visible wavelengths range (400–700 nm). The color of dyes results from their specific chemical structure, which involves aryl rings which have delocalised electron systems. The dye molecule is a combination of
- a chromophore (a part of the molecule that is able to absorb light, i.e. the colour-absorbing coordination group
- a conjugated system, i.e. a structure with alternating double and single bonds
Examples of chromophores are: C=C and C=O (carbonyl), as well as azo group -N=N- or nitro group (-NO2).
Azo dyes are characterized by the presence of nitrogen double bonds (-N=N-) which connect aromatic rings in the complex organic molecule. The name comes from the double nitrogen group N=N which is derived from a French word for nitrogen – “azote”. Thel azo dye molecule can be described in general as ( aryl - N = N - R ), where R can be an aryl, heteroaryl, or -CH = C(OH) alkyl derivatives.
Amongst all types of dyes, the azo dyes are the most common, constituting 60-70% of all dyes production They are the largest class of dyestuff, with a wide variety of colours and structures, suitable for numerous applications.
The breakdown of azo dyestuff results in cleavage of the bonds in the molecule and releasing the component amines, which consist mainly of aromatic amines (with amine group connected to the aryl group). The aromatic amines are known to be toxic and carcinogenic.. Therefore, degradation of the dyestuff effluent needs to proceed with the opening of the aromatic ring, but not just the azo bond cleavage. Azo group cleavage alone might eliminate color, but not the toxicity of the aromatic amines.
Dyestuff degradation and decoloration
The necessity of preventing the dyestuff effluent from entering the ecosystem cannot be exaggerated. Both international and national regulations for industrial wastewater require substantial elimination of the dyestuff content form the effluent. However, it has been evaluated that as much as at least 12% of the dyestuff is still being released to the ecosystem. The effluent treatment systems develop several approaches, but none of them is still sufficiently effective and a combination approach seems to be so far the most efficient.
The typical dyestuff treatment include physical and chemical methods, such as coagulation/flocculation, activated carbon, adsorption and bio-treatment, ozonation, sodium hypochlorite treatment, photochemical decolourization. Amongst the disadvantages of these methods are either formation of large amounts of sludge , or generation of toxic byproducts (aromatic amines) when the degradation is incomplete, i.e. without the opening of the aromatic ring of the dye molecule. These limitations of conventional wastewater treatment methods can be overcome by the application of the advanced oxidation processes (AOP), which has the unique ability to fully mineralize the dyes, including the opening of the aryl ring.
Dye Degradation by Catalytic Advanced Oxidation
Catalyzed decomposition of hydrogen peroxide allows effective decoloration treatment of dye wastewater. Our technology allows to fully degrade all kinds of industrial dyes, such as those commonly used in the food and textile industry (including the most notorious azo dyes) even in high concentration. The technology is environmentally friendly and does not use any harsh solutions or reagents such as chlorine, or any kind of bleach, strong alkaline or acidic solutions. No pH adjustment is needed for the treatment.
The process, which is based on the most efficient hydroxyl radicals, is much more powerful than any of the conventional methods. Our technology is able to fully remove color contaminants of such dyes as: Tartrazine, Erythrosine, Allura Red (red 40), Orange II, Bengal Rose, Fast Green, Brilliant Blue, Brilliant Green, Methyl Violet, Naphthol Yellow S, Erioglaucine, Rhodamine, Congo Red and other dyes. The technology is scalable and can be also used to treat other organic impurities.
Decoloration and degradation of Orange II
ALLURA RED - RED no 40
Degradation of Allura Red - Red 40 azo dye
TARTRAZINE - YELLOW
Degradation of Tartrazine Yellow
Degradation of Methyl Violet Dye (gentian violet)
ERIOCHROME BLACK T
Decoloration and degradation of Eriochrome Black T azo dye