*2.2.1 Chemical techniques*

Over the past two decades, chemical techniques are also withdrawing the attention of environmental chemists; these techniques include chemical oxidation by using hypochlorite (OCl<sup>−</sup><sup>1</sup> ), chlorine dioxide or ozone which has always shown high efficiency and reproducibility. But this technique has a major disadvantage with respect to relatively high prices and production of secondary pollutants like chlorinated hydrocarbons which are known carcinogens; similarly the difficulty in storage and transportation of reactants causes a substantial inconvenience for safe operation [20]; thus environmental scientists are looking for more, convenient, advanced, safe, efficient and sophisticated techniques which must be incorporated for removal of dyes from wastewater [12] .

### *2.2.2 Advanced oxidation processes (AOPs)*

The term advanced oxidation processes is actually coined to address the process of oxidation of organic pollutants primarily via in situ generations of highly reactive hydroxyl free radicals (\* OH) from H2O2. These hydroxyl free radicals are strong oxidizing species by virtue of powerful oxidation potential of (2.80 V) vs. SHE. Different advanced oxidation processes are ozonation; photocatalytic methods using O3/UV, TiO2/UV and H2O2/UV; and Fenton's reagent (H2O2/Fe+2). Fenton's reagent offers a novel dye degradation technique [21]. The phenomenal Fenton's reagent was named after Fenton who introduced it for the first time almost 100 years ago in 1884. Fenton's reagent is a precursor of hydroxyl free radicals; these madly reactive hydroxyl free radicals attack dye substrate molecules at the sites of multiple bonds and carry out either excessive hydroxylation of dye contaminants or causes dehydrogenation of dye molecules, hence producing stable inorganic materials directly or indirectly converting the dye pollutant into biodegradable and safe materials. Fenton's processes offer a remarkable efficiency and can be extended to much broader spectrum of dyes due to its fabulous non-selectivity [22]. Recently photo-Fenton's process, i.e. Fenton's coupled with light (UV or visible), electro-Fenton, sono-Fenton and sono-electro-Fenton are emerging water treatment techniques.

Ozonation is also reliable, safe and effective wastewater treatment tool which utilizes ozone gas as a strong oxidant usually used to disinfect water in swimming pools. In aqueous medium the mode of action of ozone is quite complicated. Molecular ozone can also oxidize the dyestuff in water by virtue of selective, direct or by indirect decomposition through a chain reaction mechanism by generating in situ free hydroxyl radicals (OH\* ) [23]. AOPs show a tremendously high degree of decolourization efficiency with associated photocatalytic degradation of dyestuff [24]. But their sky kissing prices and operational difficulties put a question mark and limit the use of these photocatalytic techniques for decolourization of wastewater.

#### *2.2.3 Microbiological and enzymatic degradation techniques*

On the contrary to AOPs microbiological methods like activated sludge process, aerobic and anaerobic decomposition of pure and mixed cultures using fungi and bacteria and enzymatic degradation techniques have so far shown excellent decolourization and degradation efficiencies of dyes in wastewater; furthermore these methods are gaining popularity by virtue of their simplicity, ease of operation and applicability [25]. Biodegradation processes may be aerobic and anaerobic, and sometimes the combinations of both aerobic and anaerobic biological treatments are used for dye removal from water [26]. The main mechanism of dye removal in biological treatment is the adsorption of dyestuff on to biomass, but at saturation point, the adsorption potential of dyes by biomass drops [27]. Unfortunately the utility of these microbiological methods is quite inadequate because most of the dyes are stable and resistant towards biodegradation due to their huge sizes, complicated and conjugated benzenoid structures with extensive electron delocalization and high degree of stability. Similarly due to high degree of specificity and sensitivity of the enzymes of the microorganisms, these techniques cannot be extended over a broader spectrum of dyes and needs an extensive study and homework of enzymes, their nature, selection and applicability. Thus biological abatement of dyes in wastewater has become difficult owing to the bio-refractory nature and stability of dyes. That is why environmental chemists were chewing their nails to explore more advanced, effective and non-selective

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*Contamination of Water Resources by Food Dyes and Its Removal Technologies*

techniques to achieve dye abatement goals. Thanks to electrochemical techniques which have taken the bull of organic pollutants by horns and have controlled the

Catalytic degradations assisted by suitable promoter and coupled techniques like photocatalytic, sonocatalytic and photoelectrocatalytic degradations of dyes have so far shown excellent efficiencies. But here again catalytic poisoning and recovery of

The term electrochemical techniques refers to application of DC current from a suitable source and carrying out an entire degradation of pollutants by electro-oxidation or reduction processes to inorganic materials. The main pollutants for electrochemical methods are not only dyes, but also other pollutants like pharmaceuticals, pesticides, herbicides, herbicides, detergents and many other harmful contaminants are the subjects of investigation. Electrochemical techniques usually incorporated for wastewater abatements are electrochemical reduction, electrochemical oxidation, electrocoagulation methods, photoelectrocatalytic and photo-assisted Fenton's oxidation techniques. Electrochemical oxidation techniques are further subdivided into two types which include direct and indirect oxidation techniques [28]. Over the past 10 years, the electrochemical methods have received a remarkable attention. This is because these methods have promising water decontamination potentials, have shown great novelty due to their versatility and potential cost-effectiveness and offer the most promising, clean, safe, efficient and green technologies for the decolourization of wastewater. Wastewater abatement using electrochemical methods has a fantastic advantage of environmental compatibility because its sole reagent, i.e. the electron, is a safe, clean and green reagent produced in situ and works most efficiently. Furthermore these methods are gaining attention of environmental chemists due to their excellent efficiencies, flexibility of automation and safe applicability over a broad spectrum of organic dyes [28]. These techniques need mild conditions for their operation, no heating of the samples are required and work under ambient conditions of both parameters, i.e. temperature and pressure. Nowadays a vast variety of electrochemical techniques are in practice such as electrochemical oxidation (EO), electrocoagulation (EC) using a variety of anodes, active chlorine indirect oxidation, etc. Recently emerging techniques which utilize twin technologies of both electrochemical cells and suitable light like UV light or sono-electrochemical degradations of dyes in wastewater are gaining much attention and appreciation [29, 30]. These photo- and sono-assisted electrochemical setups have been categorized as electrochemical advanced oxidation processes (EAOPs) [21]. The use of electrochemical methods for abatement of contaminants in wastewater was pioneered by Nilsson et al. 1973 [31] by electrochemical oxidation of phenolic-based wastes; later in the early 1980s, these studies were proceeded

*DOI: http://dx.doi.org/10.5772/intechopen.90331*

aquatic pollution to a great extent [22].

*2.2.4 Catalytic degradations*

catalyst materials causes issues.

*2.2.5 Electrochemical techniques*

in collaboration with Chettier and Watkinson [32].

Over the past two decades, much of the research regarding decolourization of wastewater by electrochemical oxidation has been focused on the use of different anodic materials, their relative efficiencies, exploration of various factors affecting process efficiency (like PH, temperature, nature and concentration of electrolytes, etc.), kinetics and mechanism of oxidation of a variety of pollutants in water. The electrochemical oxidation is of two types, the first type is direct oxidation also called anodic oxidation by using suitable anode material. Direct oxidation is carried techniques to achieve dye abatement goals. Thanks to electrochemical techniques which have taken the bull of organic pollutants by horns and have controlled the aquatic pollution to a great extent [22].
