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Application of Advanced Oxidation Processes (AOP) such as sono, photo and sonophoto catalysis in the purification of polluted water under ambient conditions involve the formation and participation of Reactive Oxygen Species (ROS) like ·OH, HO2·, O2(-), H2O2 etc. Among these, H2O2 is the most stable and is also a precursor for the reactive free radicals. Current investigations on the ZnO mediated sono, photo and sonophoto catalytic degradation of phenol pollutant in water reveal that H2O2 formed in situ cannot be quantitatively correlated with the degradation of the pollutant. The concentration of H2O2 formed does not increase corresponding to phenol degradation and reaches a plateau or varies in a wave-like fashion (oscillation) with well defined crests and troughs, indicating concurrent formation and decomposition. The concentration at which decomposition overtakes formation or formation overtakes decomposition is sensitive to the reaction conditions. Direct photolysis of H2O2 in the absence of catalyst or the presence of pre-equilibrated (with the adsorption of H2O2) catalyst in the absence of light does not lead to the oscillation. The phenomenon is more pronounced in sonocatalysis, the intensity of oscillation being in the order sonocatalysis>photocatalysis⩾sonophotocatalysis while the degradation of phenol follows the order sonophotocatalysis>photocatalysis>sonocatalysis>sonolysis>photolysis. In the case of sonocatalysis, the oscillation continues for some more time after discontinuing the US irradiation indicating that the reactive free radicals as well as the trapped electrons and holes which interact with H2O2 have longer life time (memory effect).

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SummaryRh-and Ru-loaded Ti02-particles were produced from cluster precursors (Rh6 (CO)16, Ru3 (CO),,) and their activity in mediating H20-decomposition through band-gap excitation was investigated. Activity increases in the order Ru < RuO2 < Rh * Rh203. Bifunctional Rh/Ru02-loaded Ti02 exhibits optimal performance with overall light-to-chemical-energy conversion efficiency of 0.13%. Lack of Orappearance in the gas phase during photolysis observed with closed systems is due to photo-uptake of O2 by the Ti02-particles. In alkaline solution the capacity for 02-uptake is surprisingly high and the nature of the stored O2 is tentatively identified as a p-peroxo-bridged Ti-species.Introduction. -Supported metal catalysts have previously been shown to be active in microheterogeneous systems that achieve H20-cleavage by visible and UV light [l-lo]. Noble metals deposited onto semiconductors such as Ti02 [l-31171, SrTi03 [4-61 and CdS [8] [9] appear to be particularly promising. To achieve optimum activity in H20-decomposition, high dispersion of the noble metal onto the support is generally required. This increases the H20-reduction and -oxidation rates per unit mass of catalyst employed. Moreover, there is recent evidence [ 111 that the recombination of H2 with O2 might be affected by the size of the noblemetal particle. Thus, it has been shown that the reduction of O2 on Pt-cathodes tested for fuel cell application is almost totally inhibited once the Pt-crystallite size decreases below 30 A. Reduction of O2 to H 2 0 is an undesirable back reaction in devices that afford simultaneous generation of H2 and O2 by light. In close relation to this effect stands the observation of Turkevich et al. [I21 that the activity of Pt-colloids in the catalytic decomposition of H202 drops from an extremely high value (close to that of catalase) to practically zero when the particle size was decreased below 30 A.A particularly favorable situation for H20-decomposition would arise if these ultrafine Pt-particles, though inactive in 02-reduction, would still remain powerful catalysts for the generation of H2 from H20. That this concept is viable was shown by us in a recent investigation [13] where a systematic test of the effect of Pt-particle size on the rate of photochemical H2-generation in Pt/Ti02-dispersions was carried

MnO2 is identified as a highly efficient sonocatalyst and sonophotocatalyst for the complete removal of even very small concentration of Indigo carmine (IC) dye pollutant from water. The effect of various reaction parameters, viz. dosage of the catalyst, concentration of pollutant, volume of reaction system, pH, dissolved gases, presence of anions/salts and oxidants etc. on the rate of degradation is evaluated and optimum parameters are identified. The degradation follows variable kinetics depending on the concentration of the substrate. The rate of degradation is facilitated by acidic pH. Classic oxidants H2O2 and S2O82− behave differently, with the former inhibiting and the latter enhancing the degradation. The effect of anions/salts on the degradation is complex and ranges from ‘inhibition’ (PO43−, CO32−, HCO3−) and ‘no effect’ (SO42−, Cl−) to ‘enhancement’ (NO3−, CH3COO−). The high affinity of MnO2 for O2 and its extremely efficient adsorption of H2O2 and the substrate play key roles in the efficiency of the process. Participation of lattice oxygen from MnO2 in the reaction, whenever the dissolved or adsorbed oxygen is deficient, is an important highlight of the process. Major transient intermediates formed during the process are identified by LC–MS. Combination of sonocatalysis with UV photolysis (sonophotocatalysis) enhances the efficiency of degradation and mineralization of IC.

Heterogeneous photocatalysis using UV/VIS light or natural solar radiation as the energy source is one of the most efficient advanced oxidation processes (AOP) for the removal of chemical and bacterial pollutants from water. One of the least investigated oxides in this context, i.e. MnO2 and its combination MnO2/TiO2 are examined as potential photocatalysts for the removal of Indigo carmine (IC) dye pollutant from the water. The catalysts are characterized by XRD, FTIR, SEM, TEM, adsorption and surface area measurements. While MnO2 is very efficient for the decolorization of the dye, it is not effective enough for mineralization. MnO2 /TiO2 as the photocatalyst at the optimized ratio of 9:1 combines the advantages of both the oxides, i.e., rapid decolorization and efficient mineralization. Persulphate (S2O8 2-) enhances the degradation while H2O2 inhibits the same. The degradation is dependent on pH with higher degradation under extremely acidic conditions. The influence of dissolved salts/anions in the water on the degradation varies from 'moderate inhibition' to 'no effect' or even 'enhancement' depending on the chemistry of the anion and reaction conditions. Effect of various parameters such as reaction time, substrate concentration, catalyst dosage, the presence of O2, recycling of the catalyst, etc. on the efficiency of degradation is investigated. The results are critically analyzed, and a tentative mechanism is proposed.

Contamination of water by chemical and bacterial pollutants as well as ‘white pollution’ caused by carelessly discarded waste plastics are major environmental problems. In the current study, the possibility of using semiconductor photocatalysis for the removal of last traces of organic water pollutants of different types is investigated. Semiconductors ZnO and TiO