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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

Photoirradiation Techniques

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 → 2OH (1)

O3 + H2O + hν → OH + OH + O2 (2)


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.

Chemical Methods

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 → 2OH (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.

Irradiation of High Energy Radiation

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 Hand eaq- in the medium, N2O purging of the reaction medium is generally carried out for generating an oxidizing medium (N2O quantitatively converts eaq- in to OH)

N2O + eaq- → 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).


  1. P. Calza, C. Massolino and E. Pelizzetti, Light induced transformations of selected organophosphorus pesticides on titanium dioxide: Pathways and by-products evaluation using LC–MS technique. J. Photochem. Photobiol., A 2008, 199, 42-49.
  2. W. Changlong and G. L. Karl, Phototransformation of selected organophosphorus pesticides: Roles of hydroxyl and carbonate radicals. Water Research 2010, 44, 3585-3594.
  3.  T. S. Ahmad and A. A. M. Fares, Photocatalytic treatment of water soluble pesticide by advanced oxidation technologies using UV light and solar energy. Solar Energy 2010, 84, 1157-1165.
  4.  A. T. Jacob, C. T. Aravindakumar, R. Flyunt, J. von Sontag and C. von Sontag, Fenton Chemistry of 1,3-Dimethyluracil. J. Am. Chem. Soc. 2001, 123, 9007-9014.
  5. S. Rabindranathan, S. Devipriya and S. Yesodharan, Photocatalytic degradation of phosphamidon on semiconductor oxides. J. Hazard. Mater. 2003, 102, 217-229.
  6.  C. L. Duarte, M. N. Mori, Y. Kodama, H. Oikawa and M. H. O. Sampa, Decontamination of pesticide packing using ionizing radiation. Radiat. Phys. Chem. 2007, 76, 1885-1889.
  7. F. Fang, L. Chen, C. Yu-Biao and W. Li-Ming, Synthesis and Photocatalysis of ZnIn2S4 Nano/Micropeony, J. Phys. Chem. C 2010, 114, 2393–2397
  8. S. Wu, H. Cao, S. Yin, X. Liu and X. Zhang, Amino Acid-Assisted Hydrothermal Synthesis and Photocatalysis of SnO2 Nanocrystals, J. Phys. Chem. C 2009, 113, 17893–17898
  9. F. Chen, H. Huang and X. P. Zhou, Characterization of Photodegradation Catalyst 9.1% VOx/MgF2, J. Phys. Chem. C 2009, 113, 21106–21113
  10. F. Thevenet, O. Guaïtella, J. M. Herrmann, A. Rousseau and C. Guillard, Photocatalytic degradation of acetylene over various titanium dioxide-based photocatalysts, Applied Catalysis B: Environmental 2005, 61, 58–68
  11. M. Karkmaz, E. Puzenat, C. Guillard and J. M. Herrmann, Photocatalytic degradation of the alimentary azo dye amaranth: Mineralization of the azo group to nitrogen, Applied Catalysis B: Environmental, 2004, 51, 183–194
  12. S. MalatoA. Aguera, Detoxification of pesticide in water using solar photocatalysis, Pesticide Decontamination and Detoxification, 2004, 863, 113-126
  13. P. Fernández, J. Blanco, C. Sichel and S. Malato, Water disinfection by solar photocatalysis using compound parabolic collectors, Catalysis Today, 2005, 101, 345–352
  14. W. Choi, J. Lee, S. Kim, S. Hwang, M. C. Lee and T. K. Lee, Nano Pt particles on TiO2 and their effects on photocatalytic reactivity, J. Ind. Eng. Chem. 9, 96-101
  15. O. Gonzalez, Oscar, M. Navarro, B. Bayarri, J. Gimenez and S. Esplugas, Pollutants removal in wastewaters by advanced photooxidation processes, Afinidad, 2007, 64, 171-176
  16. B. Balci, N. Oturan, R. Cherrier and M. A. Oturan, Degradation of atrazine in aqueous medium by electrocatalytically generated hydroxyl radicals. A kinetic and mechanistic study, Water Research, 2009, 43, 1924–1934

Relevant Publications from our Group

  1. Sreekanth, K.P. Prasanthkumar, M.M. Sunil Paul, Usha K. Aravind,, C.T. Aravindakumar, Oxidation Reactions of 1- and 2-naphthols: An Experimental and Theoretical Study, J. Phys. Chem. A., 2013 (in press)
  2. Olvera-Vargas, N. Oturan, C. T. Aravindakumar, M. M. Sunil Paul, V.r K. Sharma, M. A. Oturan, Electro-oxidation of the Dye Azure B: Kinetics, mechanism and by-products, Current Organic Chemistry, 2013 (in press)
  3. V. Divyalakshmi, S. Sreedhanya, G. Akhil, C. T. Aravindakumar and Usha K. Aravind, Sub-picomolar sensing of hydrogen peroxide with ovalbumin embedded CHI/PSS multilayer membrane, Analytical Biochemistry,2013 (in press)
  4. Venu, D.B. Naik, S.K. Sarkar, Usha K. Aravind A.Nijamudheen, C.T.Aravindakumar, ,Oxidation Reactions of Thymol: A Pulse Radiolysis and Theoretical Study, J. Phys. Chem. A., 2013, 117, 291–299
  5. M. Sunil Paul, Usha K. Aravind, G. Pramod, C.T. Aravindakumar, Oxidative Degradation of Fensulfothion by Hydroxyl Radical in Aqueous Medium, Chemosphere, 2013, 91, 295–301
  6. Prasanthkumar, K. P.; Suresh, C. H.; Aravindakumar, C. T., Theoretical study of the addition and abstraction reactions of hydroxyl radical with uracil. Radiation Physics and Chemistry 2012, 81, (3), 267-272.
  7. Disha, V. J.; Aravindakumar, C. T.; Aravind, U. K., Phosphate Recovery by High Flux Low Pressure Multilayer Membranes. Langmuir 2012, 28, (35), 12744-12752.
  8. Baburaj, M. S.; Aravindakumar, C. T.; Sreedhanya, S.; Thomas, A. P.; Aravind, U. K., Treatment of model textile effluents with PAA/CHI and PAA/PEI composite membranes. Desalination 2012, 288, 72-79.
  9. Manoj, V. M.; Aravind, U. K.; Mohan, H.; Aravindakumar, C. T., Reaction of hydroxyl radicals with S-nitrosothiols: Formation of thiyl radical (RS center dot) as the intermediate. Research on Chemical Intermediates 2011, 37, (8), 1113-1122.
  10. Aravind, U. K.; George, B.; Baburaj, M. S.; Thomas, S.; Thomas, A. P.; Aravindakumar, C. T., Treatment of industrial effluents using polyelectrolyte membranes. Desalination 2010, 252, (1-3), 27-32.
  11. Prasanthkumar, K. P.; Mohan, H.; Pramod, G.; Suresh, C. H.; Aravindakumar, C. T., Anomalous reaction of oxide radical ion with 5-azacytosines: An experimental and theoretical study. Chemical Physics Letters 2009, 467, (4-6), 381-386.
  12. Mathew, J.; Aravindakumar, C. T.; Aravind, U. K., Effect of ionic strength and protein concentration on the transport of proteins through chitosan/polystyrene sulfonate multilayer membrane. Journal of Membrane Science 2008, 325, (2), 625-632.
  13. Manoj, P.; Min, C. K.; Aravindakumar, C. T.; Joo, T., Ultrafast charge transfer dynamics in 2-aminopurine modified double helical DNA. Chemical Physics 2008, 352, (1-3), 333-338.
  14. Varghese, R.; Aravind, U. K.; Aravindakumar, C. T., Fenton-enhanced gamma-radiolysis of cyanuric acid. Journal of Hazardous Materials 2007, 142, (1-2), 555-558.
  15. Manoj, P.; Prasanthkumar, K. P.; Manoj, V. M.; Aravind, U. K.; Manojkumar, T. K.; Aravindakumar, C. T., Oxidation of substituted triazines by sulfate radical anion (SO4 center dot-) in aqueous medium: a laser flash photolysis and steady state radiolysis study. Journal of Physical Organic Chemistry 2007, 20, (2), 122-129.
  16. Manoj, P.; Mohan, H.; Mittal, J. P.; Manoj, V. M.; Aravindakumar, C. T., Charge transfer from 2-aminopurine radical cation and radical anion to nucleobases: A pulse radiolysis study. Chemical Physics 2007, 331, (2-3), 351-358.
  17. Aravind, U. K.; Mathew, J.; Aravindakumar, C. T., Transport studies of BSA, lysozyme and ovalbumin through chitosan/polystyrene sulfonate multilayer membrane. Journal of Membrane Science 2007, 299, (1-2), 146-155
  18. Varghese, R.; Mohan, H.; Manoj, P.; Manoj, V. M.; Aravind, U. K.; Vandana, K.; Aravindakumar, C. T., Reactions of hydrated electrons with triazine derivatives in aqueous medium. Journal of Agricultural and Food Chemistry 2006, 54, (21), 8171-8176
  19. Pramod, G.; Prasanthkumar, K. P.; Mohan, H.; Manoj, V. M.; Manoj, P.; Suresh, C. H.; Aravindakumar, C. T., Reaction of hydroxyl radicals with azacytosines: A pulse radiolysis and theoretical study. Journal of Physical Chemistry A 2006, 110, (40), 11517-11526
  20. Pramod, G.; Mohan, H.; Manoj, P.; Manojkumar, T. K.; Manoj, V. M.; Mittal, J. P.; Aravindakumar, C. T., Redox chemistry of 8-azaadenine: a pulse radiolysis study. Journal of Physical Organic Chemistry 2006, 19, (7), 415-424.
  21. Manoj, V. M.; Mohan, H.; Aravind, U. K.; Aravindakumar, C. T., One-electron reduction of S-nitrosothiols in aqueous medium. Free Radical Biology and Medicine 2006, 41, (8), 1240-1246
  22. Manoj, V. M.; Aravindakumar, C. T., Oxidative and reductive decomposition of S-nitrosothiols. Nitric Oxide-Biology and Chemistry 2004, 11, (1), 53-53.
  23. Larsen, O. F. A.; van Stokkum, I. H. M.; de Weerd, F. L.; Vengris, M.; Aravindakumar, C. T.; van Grondelle, R.; Geacintov, N. E.; van Amerongen, H., Ultrafast transient-absorption and steady-state fluorescence measurements on 2-aminopurine substituted dinucleotides and 2-aminopurine substituted DNA duplexes. Physical Chemistry Chemical Physics 2004, 6, (1), 154-160.
  24. Larsen, O. F. A.; Somsen, O. J. G.; van Stokkum, I. H. M.; de Weerd, F. L.; Vengris, M.; Aravindakumar, C. T.; van Grondelle, R.; Geacintov, N. E.; van Amerongen, H., Ultrafast spectroscopy on 2-aminopurine DNA oligonucleotides provides new insights into mechanism of fluorescence quenching. Biophysical Journal 2004, 86, (1), 312A-312A.
  25. Manoj, V. M.; Aravindakumar, C. T., Reaction of hydroxyl radicals with S-nitrosothiols: determination of rate constants and end product analysis. Organic & Biomolecular Chemistry 2003, 1, (7), 1171-1175.
  26. Aravindakumar, C. T.; Schuchmann, M. N.; Rao, B. S. M.; von Sonntag, J.; von Sonntag, C., The reactions of cytidine and 2 '-deoxycytidine with SO4 center dot- revisited. Pulse radiolysis and product studies. Organic & Biomolecular Chemistry 2003, 1, (2), 401-408.
  27. Manoj, P.; Varghese, R.; Manoj, V. M.; Aravindakumar, C. T., Reaction of sulphate radical anion (SO4 center dot-) with cyanuric acid: A potential reaction for its degradation? Chemistry Letters 2002, (1), 74-75.
  28. Luke, T. L.; Jacob, T. A.; Mohan, H.; Destaillats, H.; Manoj, V. M.; Manoj, P.; Mittal, J. P.; Hoffmann, M. R.; Aravindakumar, C. T., Properties of the OH adducts of hydroxy-, methyl-, methoxy-, and amino-substituted pyrimidines: Their dehydration reactions and end-product analysis. Journal of Physical Chemistry A 2002, 106, (11), 2497-2504.
  29. Aravindakumar, C. T.; De Ley, M.; Ceulemans, J., Kinetics of the anaerobic reaction of nitric oxide with cysteine, glutathione and cysteine-containing proteins: implications for in vivo S-nitrosation. Journal of the Chemical Society-Perkin Transactions 2 2002, (3), 663-669.
  30. Theruvathu, J. A.; Flyunt, R.; Aravindakumar, C. T.; von Sonntag, C., Rate constants of ozone reactions with DNA, its constituents and related compounds. Journal of the Chemical Society-Perkin Transactions 2 2001, (3), 269-274
  31. Theruvathu, J. A.; Aravindakumar, C. T.; Flyunt, R.; von Sonntag, J.; von Sonntag, C., Fenton chemistry of 1,3-dimethyluracil. Journal of the American Chemical Society 2001, 123, (37), 9007-9014.
  32. Joseph, J. M.; Luke, T. L.; Aravind, U. K.; Aravindakumar, C. T., Photochemical production of hydroxyl radical from aqueous iron(III)-hydroxy complex: Determination of its reaction rate constants with some substituted benzenes using deoxyribose-thiobarbituric acid assay. Water Environment Research 2001, 73, (2), 243-247.
  33. Manoj, V. M.; Aravindakumar, C. T., Hydroxyl radical induced decomposition of S-nitrosoglutathione. Chemical Communications 2000, (23), 2361-2362.
  34. Joseph, J. M.; Jacob, T. A.; Manoj, V. M.; Aravindakumar, C. T.; Mohan, H.; Mittal, J. P., Oxidative degradation of triazine derivatives in aqueous medium: A radiation and photochemical study. Journal of Agricultural and Food Chemistry 2000, 48, (8), 3704-3709.
  35. Aravindakumar, C. T.; Ceulemans, J.; De Ley, M., Steric effect and effect of metal coordination on the reactivity of nitric oxide with cysteine-containing proteins under anaerobic conditions. Biophysical Chemistry 2000, 85, (1), 1-6.
  36. Aravindakumar, C. T.; Ceulemans, J.; De Ley, M., Nitric oxide induces Zn2+ release from metallothionein by destroying zinc-sulphur clusters without concomitant formation of S-nitrosothiol. Biochemical Journal 1999, 344, 253-258.