*2.2.4 Catalytic degradations*

*Water Chemistry*

techniques.

wastewater.

*2.2.2 Advanced oxidation processes (AOPs)*

reactive hydroxyl free radicals (\*

situ free hydroxyl radicals (OH\*

The term advanced oxidation processes is actually coined to address the process of oxidation of organic pollutants primarily via in situ generations of highly

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

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

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

*2.2.3 Microbiological and enzymatic degradation techniques*

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

OH) from H2O2. These hydroxyl free radicals are

) [23]. AOPs show a tremendously high degree of

**116**

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 catalyst materials causes issues.
