Organic pollutants photooxidation

For describing the kinetics of indirect photolysis of organic pollutants involving well-defined photooxidants (e.g., HO , 02, CO] ), we adapt the approach suggested by Hoigne et al. (1989) and Mill (1989). The rate of formation, rf ox(A), of a given photooxidant (Ox) by radiation of wavelength A may be described by ... 

Why is the hydroxyl radical (HO ) a more important photooxidant in the atmosphere as compared to surface waters What kind of reactions do organic pollutants undergo with HO How structure specific are these reactions ... 

WTiy is 02 a more selective photooxidant than HO With what type of organic pollutants does 02 primarily react ... 

Safarzaden-Amiri, A., Bolton, J.R., and Cater, S.R., Ferrioxalate-mediated photooxidation of organic pollutants in contaminated water, Water Res., 31(4), 787-798, 1997. 

Kochany J, Bolton JR (1991) Mechanism of Photodegradation of Aqueous Organic Pollutants. 1. EPR Spin-Trapping Technique for the Determination of "OH Radical Rate Constants in the Photooxidation of Chloro-phenols following the Photolysis of H2O2, J. Phys. Chem. 95 5116-5120. 

Photooxidation of Organic Pollutants by Transition Metal Complexes in Hydrosphere and Soils 316... 

J. H. Wang and M. B. Ray, Application of ultraviolet photooxidation to remove organic pollutants in the gas phase, Separation and Purification Technol. 19, 11-20 (2000). 

Most of the reactor designs tested for the photooxidation of organic pollutants by solar radiation are Ti02 slurry reactors. The implementation of solar photocatalytic reactors has occurred concurrently with advances in thedesign of solar thermal collectors, given the important characteristics shared by these units. There are, however, specific constraints for the design of solar photocatalytic reactors. 

Abstract A brief overview on the main photoprocesses applied to the treatment of water and wastewater is presented. The photodegradation methods that have been applied to the oxidation of organic pollutants are described. A review on advanced oxidation processes (AOP s) and photooxidation mechanisms in homogeneous and heterogeneous solution is presented and some practical applications discussed. Combinations of biological and chemical treatments are considered to be a good approach to improve the removal efihciencies and reduce costs. 

The catalytic membranes were successfully applied in the aerobic photooxidation of phenol, one of the main organic pollutants in wastewater, providing stable and recyclable photoeatalytic systems. The catalytic tests were carried out in a photoeatalytic membrane reactor operating with flow-through at different transmembrane pressures (TMP). 

Shortly after the discovery of photochemical air pollution, Stephens and his co-workers (Stephens et al., 1956 Stephens, 1987) applied long-path infrared spectroscopy to identifying and measuring products in the photooxidation of organic-NO, mixtures. In the photooxidations of 3-methylheptane and, to a larger extent, 2,3-butanedione, a set of infrared bands that could not be assigned to known products was observed. These were assigned to a previously unobserved species, which was initially called compound X. It was ultimately shown to be peroxyacetyl nitrate (PAN) ... 

Table 15.5 lists concentrations of the major photooxidants in surface waters, diurnally averaged over 24 hours. Note that, even if kox(i) values are measured or estimated accurately (within a factor two or three), oxidant concentrations in the environment vary widely, and averaged values have a variance of five- to tenfold for any given location. In extreme locations, such as pristine marine waters, or heavily polluted surface waters, oxidant concentrations may be 100 times smaller or larger than the values Table 15.5 lists. Table 15.6 lists rate constants (kox) for various photooxidants in their reaction with major classes of organic compounds. To estimate the rate of an indirect photoreaction for chemical C (Equation (18)), either a measured or estimated value of kox is required, specific for each oxidant and for each class of organic compounds. Methods for estimating kox from molecular structure with structure-activity relationships (SARs) have been developed for many photooxidants and are discussed below. 

Krauss and Wilcke examined the TiC -photocatalyzed oxidation of 12 PCB congeners [and 20 polycyclic aromatic hydrocarbons (PAHs)] on various soil samples (four mineral topsoil horizons, six organic horizons, and four particle-site fractions in three different soils) [107]. When the Ti02/soil mixture was irradiated in the absence of H2O, no photooxidation of the chlorobiphenyls occurred. When slurried with water, however, chlorobiphenyl concentrations decreased by 40-50% after 48 hours of irradiation, while the PAH concentrations were unchanged. By way of contrast, PAHs and PCBs doped onto quartz sand diminished by 95-100% after 8 hours of photolysis. The pollutants are clearly more accessible to hydroxyl radicals on sand than on soil. It is also clear that the photooxidation occurred in the soil and not in solution. Thus, OH is generated on one surface (Ti02), diffuses in the water to the other surface (soil), where the oxidation occurs. 

Most of the photo-initiated processes in the atmosphere are radical reactions. However, the hydroxyl radical ( OH) is of special significance for the chemistry of the atmosphere (Ehhalt, 1999). This reactive species is mainly responsible for the photooxidation of trace organic chemicals in the troposphere and hence for the oxidative cleansing of the atmosphere (Fabian, 1989). It is nature s atmospheric detergent (Comes, 1994, Ravishankara, et al. 1998). Furthermore, several years ago it was well established that the interaction of UV/VIS radiation and environmental pollution seems to be responsible for the dramatic forest decline that has been observed, for example in the higher areas of the Black Forest or the Ore Mountains in Germany (Schenck, 1985). 

In a sunlit natural surface water receiving incident light with intensity I (A), dissolved organic material (DOM) acts as a sensitizer for producing the singlet oxygen (for details, see Section 2.2). In this case, the rate of photooxidation of a pollutant P with this photooxydant can be described by the kinetic expression ... 

In the S02->H2S04 transformation organic materials play an important part. Thus, if organic substances are added to air containing S02, H20, NOz and 03 the S02 oxidation rate is observed to accelerate. According to the laboratory experiments of Cox and Penkett (1972) the photooxidation rate of S02 is as large as 1-10 % per hr if the air contains appropriate olefins and ozone. Olefins can be detected, however, only in air polluted by exhaust gases. However, Cadle (1972) has... 

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