Advanced oxidation processes (AOPs) is a common name refers to a number of chemical treatment procedures designed for the removal of organic pollutants from aqueous medium. AOPs (generate highly reactive hydroxyl radical in-situ, second most powerful oxidizing agent reported), are found to be cost effective and less expensive. AOPs are more environmentally accepted techniques because in many case it leads to complete mineralization of the organic pollutants. The presence of organic micro pollutants in water has been one of the most discussed environmental topics in the last few decades. Compounds such as conventional pesticides used in agricultural and house hold industries, coloring compounds (aromatic azo dyes - textile industries) and pharmaceutically active compounds (so-called emerging pollutants – from pharmaceutical industries) are considered as organic micro pollutants. Other than the parent compounds, few of their degradation products are also reported to be contributing the toxicity. The detection of these compounds from aquatic environment and other environmental matrices is a difficult task due to their low concentrations but they are reported to be still potentially harmful. Even though they are very low in concentration, high sensitive analytical techniques such as GC-MS, LC-MS/MS and LC-Q-TOF offers a convenient way to detect these compounds along with other organic and inorganic matrices.

Due to the presence of organic pollutants in aquatic environment (many of them are reported to be stable in aqueous medium) and due to their environmental impacts, the removal of these compounds is very important for drinking and other water use/reuse applications. Conventional water purification techniques such as membrane filtration, adsorption on activated charcoal and reverse osmosis are not effective for the complete removal of these compounds and many of them are very expensive. The efficiency of advanced oxidation processes (AOPs) against many organic pollutants is reported in literature but the mechanism of oxidative transformation leading to the degradation of these compounds is still under investigation. The adverse effects of these compounds and their degradation products in aquatic and non-aquatic organisms are being extensively studied and still a long way to go for completely understanding their impact.

Fenton and photo-Fenton reaction, irradiation with high energy radiation (gamma rays and high energy electrons), photoirradiation (ultraviolet) in the presence of H2O2, ozone, ferric perchlorate and metal oxides (titanium dioxide, zinc oxide) etc. are some of the widely experimented AOP techniques. Different methods used for the generation of hydroxyl radicals (in-situ) are

H2O2/UV and O3/UV photolysis. Photolysis in presence of H2O2 and ozone are two well-studied methods for the production of ^●OH. In this method, hydroxyl radicals (^●OH) are formed by the dissociation of H2O2 and ozone respectively (1,2).

H₂O₂ + hν → 2^●OH (1)
O₃ + H₂O + hν → ^●OH + ^●OH + O₂ (2)

Photocatalysis. Photocatalysis using metal oxides is a well-known technique to produce highly oxidizing conditions in aqueous medium. UV (sometimes visible also) light can be used to create electron-hole pairs in semiconductor such as TiO₂ and ZnO. The electrons are converted into superoxide radical anion (O₂ ^●–) by reacting with molecular oxygen I n the medium and holes react with surface hydroxyl groups to form hydroxyl radicals ( ^●OH). The radical species as well as the hole may react with the organic molecules which would be eventually oxidized to CO2, H2O. Degradation of various organic water pollutants using photocatalysis with TiO2 has been an area of intense research in the last many decades. Nano-TiO2 photocatalysis is a better methodology for the degradation of organic water pollutants and several recent reports demonstrated that the degradation in the nano dimension is much more effective than normal TiO2. Photocatalysis using metal doped or carbon nanotube/graphene-modified TiO2 is also an emerging topic. The relevant equations are shown in III.

TiO₂ + hν → e^- + h⁺ (3a)
O₂+ e^- → O₂ ^●- (3b)
Ti(IV)-OH + h⁺ → Ti(IV)-OH ^● (3c)
Ti(IV)-H₂O + h⁺ → Ti(IV)-OH + H⁺ (3d)
Because of the advantages associated with TiO₂ photocatalysis, it is one of the very advanced and frequently updated areas in the field of pollutant degradation. Other than TiO₂, ZnIn₂S₄ nano/micro particles (Fang et al. in 2010), SnO₂ nanocrystals (S. Wu et al. in 2009), 9.1% VOx/MgF₂ (F. Chen et al. in 2009) are also reported as photo catalysts. Among the different research groups working in the area, C. Guillard and co-workers (Université Claude Bernard Lyon-1, France) reports the degradation of different types of pollutants ranging from acetylene to big molecules such as amaranth (dyes). The research group of Dr. S. Malato (Centro de Investigaciones Energéticas, Medioambientales y Tecnológicas (CIEMAT), Plataforma Solar de Almería, Almería, Spain) are interested in developing solar photocatalytic techniques for the degradation of different class of pollutants. Few of the recent result from Malato’s group discuss the solar photocatalytic degradation of some real systems. However Prof. W. Choi (POSTECH, S. Korea) and co-workers are try to improve the catalytic activity of TiO2 by doping metals such as Pt in to the photocatalyst.

In 1894, H. J. H. Fenton discovers the capability of producing hydroxyl radical by certain metals by an electron transfer reaction with H2O2. The mechanism possibly involves an oxoiron(IV) intermediate. The hydroxyl radical is used to oxidize organic pollutants in water. The reaction is named after H. J. H. Fenton (Fenton’s reaction) and the mechanism of hydroxyl radical generation in this reaction is given below.

Fe²⁺ + H₂O₂ → Fe³⁺ + ^●OH + OH^- (4)
Other than the main reaction, a number of side reactions are also reported in the case of Fenton’s reaction and few of them decrease the efficiency of Fenton’s reaction by reducing the concentration of ^●OH radical.

H₂O₂ +^●OH → H₂O + HO₂• k₂ = 2.7 × 10⁷ M^-1s^-1 (5a)
Fe²⁺+^●OH → Fe³⁺++ OH^- k₂ = 3.2 × 108 M^-1s^-1 (5b)
In addition with these reactions, hydroxyl radical is formed from the following routes in the case of photo-Fenton’s reaction.
Fe³⁺ + H₂O + hν → ^●OH + Fe²⁺ + H⁺ (6a)
H₂O₂ + hν → 2^●OH (6b)

Electro-Fenton’s reaction is one of the areas under intense research in the field of water treatment. A research group from University of Paris, led by Prof. M. A. Oturan, publishes many reports related with different Fenton’s techniques for water purification. One of the recent reports from Prof. Oturan’s group discusses the degradation of atrazine by electro-Fenton reaction. In this technique, H2O2 and Fe2+are produced in the medium and are reacted together to form ^●OH. Prof. S. Esplugas and co-workers (University of Barcelona, Spain) reports the degradation of, 4-dichlorophenol (DCP) and sulfamethoxazol (SMX) in aqueous solution by different AOPs including H2O2/UV photolysis, Fenton’s and photo-Fenton’s reactions.

Along with other radical and molecular radiolysis products, hydroxyl radical was also produced from water radiolysis. The equation describing the radiolysis of water is given below.

H₂O /\/\/\-> ^●OH (2.7), H^● (0.06), eaq^- (2.6)
H⁺, H₂, H₂O₂ (16)

The values in brackets are known as radiation chemical yield or G-values (defined as number of species produced per 100 eV of absorbed radiation. G-value depends on the type of the radiation used for irradiation. Due to the formation of reducing radicals such as H^● and e_aq- in the medium, N₂O purging of the reaction medium is generally carried out for generating an oxidizing medium (N₂O quantitatively converts e_aq- in to ^●OH)

N₂O + e_aq- → ^●OH + OH^- + N₂ (17)

Along with many other groups, our group also contributed in the degradation studies of organic pollutants by using high energy radiation. In our research, we are mainly deals with investigation of mechanism responsible for the oxidative transformation of pollutants by detecting the transient species by time resolved pulse radiolysis techniques as well as detecting stable transformation products by mass spectrometric techniques. In 2000, we report the degradation of triazine derivatives in aqueous medium by γ-irradiation (Joseph et al. in 2000) and in 2007, we report the degradation of cyanuric acid by a Fenton-enhanced γ-radiolysis technique (Rani et al).