Effects of Photochemical Oxidants on Materials

The data base on health effects of photochemical oxidants and ozone was reviewed by the Subcommittee on Ozone and Other Photochemical Oxidants in a report prepared in September 1974 for the Committee on Public Works, U.S. Senate.The following discussion repeats some of the material in that report, to exemplify the need for further work, including controlled human studies. 

Laboratory studies of effects of photochemical oxidants other than ozone—e.g., PAN, peroxybenzoylnitrate, atomic oxygen, excited molecular oxygen, Oj( A ), and hydroperoxy and hydroxyl radicals—on specific materials should be conducted. 

Approximately the first third of this report is concerned with the origins and measurement of ozone and other photochemical oxidants and the relationship of atmospheric concentrations to emissions. The middle third deals with toxicologic studies and effects on humans, and the last with effects on plants, ecosystems, and materials. 

Based on the evidences from literature review in Chapter 2, the optimal scientific and engineering approaches to remove Aromatic Dyes are still unclear. The optimal pH, sensitizer, oxidant concentration for Advanced Chemical Oxidation by UV-irradiation and ozonation were needed for further exploration. Moreover, there is still an unknown for the possible use of surfactants as facilitators of soil remediation, as both extracting agents and as a medium (i.e. micelle) in which to carry out photochemical decay within micellar solution of the solubilized Chlorinated Aromatic Dyes (CADs). More specifically, there is only little information in the photochemical decay of CADs at such micellar medium. In addition, the potential quenching or catalytic effect of humic materials in photochemical decay of CADs at micellar medium is still an unknown. Hence, in this research, the tasks were focused on the following areas ... 

Zeolite catalysts incorporated or encapsulated with transition metal cations such as Mo, or Ti into the frameworks or cavities of various microporous and mesoporous molecular sieves were synthesized by a hydrothermal synthesis method. A combination of various spectroscopic techniques and analyses of the photocatalytic reaction products has revealed that these transition metal cations constitute highly dispersed tetrahedrally coordinated oxide species which enable the zeolite catalysts to act as efficient and effective photocatalysts for the various reactions such as the decomposition of NO into N2 and O2 and the reduction of CO2 with H2O into CH3OH and CH4. Investigations on the photochemical reactivities of these oxide species with reactant molecules such as NOx, hydrocarbonds, CO2 and H2O showed that the charge transfer excited triplet state of the oxides, i.e., (Mo - O ), - O ), and (Ti - O ), plays a significant role in the photocatalytic reactions. Thus, the present results have clearly demonstrated the unique and high photocatalytic reactivities of various microporous and mesoporous zeolitic materials incorporated with Mo, V, or Ti oxide species as well as the close relationship between the local structures of these transition metal oxide species and their photocatalytic reactivities. 

In conclusion, over 130 semiconductors are known to catalyze the photochemical water-splitting reaction according to eq 1 or either water oxidation or reduction in the presence of sacrificial agents. Even though the principle activitycontrolling factors in semiconductor-heterostructures have been identified, many aspects of the function of inorganic photocatalysts are still unclear. Most importantly, the molecular mechanism of water reduction and oxidation on the semiconductor surface has not yet been elucidated in sufficient detail. ° Many questions about charge transfer between semiconductor and cocatalysts, and its dependence on the structural and electronic features of the interface are still open. The effect of variable material preparations and surface impurities on the catalytic activity of semiconductors (e.g. sulfur and oxide on... 

The mechanisms of photochemical degradation depend peculiarly on the type of polymer, as well as on the specificity of environmental factors. Usually, whereas the applications of polymaic materials run in the presence of air and of atmospheric oxygen, the polymo- degradation lead to a variety of physical and chemical effects. This damaging process is practically an oxidative photodegradation (photo-oxidation). 

---