Ultrasound (US), Ultraviolet light (UV) and combination (US + UV) assisted semiconductor catalysed degradation of organic pollutants in water: Oscillation in the concentration of hydrogen peroxide formed in situ

Abstract

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).

MnO2 AND MnO2/TiO2 MEDIATED, PERSULPHATE ENHANCED PHOTOCATALYSIS FOR THE REMOVAL OF INDIGO CARMINE DYE POLLUTANT FROM WATER

“…The concentration of in situ formed H2O2 increases initially, reaches an optimum and decreases after that. This is consistent with the earlier reports 57 on the oscillation in the concentration of in situ formed H2O2 in semiconductor-mediated photocatalytic systems which are attributed to concurrent formation and decomposition [reactions (53)- (60)]. …”

“…OHads. 57 This will lead to decrease in the oxidation rate and the quantum efficiency in the case of stable organic molecules. When there are more H2O2 in the system, it can also trap the h + as in reaction (51) and quench .…”

Comparison of catalytic activities for photocatalytic and sonocatalytic degradation of organic dye in the presence of ternary Fe3O4/ZnO/CuO magnetic heteregenous nanocatalyst

“…The sonocatalytic efficiency increases with increasing nanocatalyst dosage. It is known that sonocatalytic degradation of dyes in the presence of nanocatalyst can be explained by mechanism of hot spot followed by sonoluminescence [15][16]. The observed trend in sonocatalytic efficiency can be attributed to the increase of the formation of bubles with the quantity of nanocatalyst.…”

Photocatalytic, sonocatalytic, and photosonocatalytic of Fe3O4/CuO/ZnO nanocomposites with addition of 2 different types of carbon

“…7), the photosonocatalysis is about ~2 to 3 times higher than sonocatalytic and photocatalytic activity. This result could be due to the synergetic effect of UV light and ultrasound irradiation in photosonocatalytic process [20]. In Fig.…”

“…The different results may indicate that photocatalysis and sonocatalysis have different mechanisms. Based on the theories, ultrasonic irradiation could create a hotspot which produces hydroxyl radicals that could degrade organic pollutant [20] while in photocatalysis the light directly activates the catalyst. For the FCZ/graphene we obtained the same trend as FCZ/NGP in photo-,sono-, and photosonocatalysis which indicated no mechanism change by using graphene or NGP.…”