The use of PLS algorithms and quantum chemical parameters derived from PM3 hamiltonian in QSPR studies on direct photolysis quantum yields of substituted aromatic halides

The Partial Least-Squares (PLS) method was used to further study Quantitative Structure-Property Relationship (QSPR) for photohydrolysis quantum yields of selected aromatic halides. The cluster of substituted bromobenzenes and iodobenzenes was further grouped into two clusters, substituted bromobenzenes and substituted iodobenzenes, for which QSPRs were obtained, respectively. The QSPRs obtained by PLS are more significant than those obtained in previous studies. The studies showed that only when the compounds belonging to different groups are studies separately, inherent relationships between molecular properties and their structure characteristics can be revealed. It can be generally concluded that the photohydrolysis quantum yields of the substituted aromatic halides are dependent on the overall characters of the molecules, the characters of the carbon–halogen bonds to be broken in the photolysis, and the character of the halogen atoms to be replaced in the photolysis. Electronegativity has great relevance to the photohydrolysis processes of the compounds. Substituted aromatic halides with large average molecular polarizability and molecular weight values tend to have low photolysis quantum yields.     

Determination of quantum yields for the photolysis of Fe(III)-hydroxo complexes in aqueous solution using a novel kinetic method

The determination of quantum yields for the photolysis of Fe(III)-hydroxo complexes is important for the quantitative investigation of hydroxyl radical (*OH) production, not only in a natural water body, but also in the photo-Fenton process. A novel kinetic method, using a *OH probe compound, was established for the determination of the quantum yields in this study. The method was based on measuring the pseudo-first-order rate constant of the photodecomposition of dimethylsulfoxide (DMSO) in which DMSO and its primary products scavenged the *OH at an identical rate. The preliminary experiments for the photodecomposition kinetics supported the suitability of DMSOs as a probe compound for determining quantum yields. The individual quantum yields for the photolysis of the monomeric Fe(III) complexes, in the wavelength range 240-380 nm, were determined by the photodecomposition kinetics of the hydroxyl radical (*OH) probe compound (DMSO). The determined values of the individual quantum yields were 0.046+/-0.00052 for Fe3+ (H2O)6 (hexaaquo ion) and 0.69+/-0.025 for Fe(OH)2+ (H2O)5 (hydroxypentaaquo ion) at 254 nm, and showed decreasing values with increasing wavelength, in the ranges of 240-380 nm. The quantum yields between 240 and 280 nm were newly reported in this study, and the values obtained between 280 and 380 nm were in good agreement with the literature values.     

Photodegradation of polychlorinated dibenzo-p-dioxins and dibenzofurans in aqueous solutions and in organic solvents

Aqueous solutions of selected polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs) were prepared using a generator column and exposed to UV (300 nm) light in the laboratory and to sunlight in an outdoor environment. In the laboratory, additional exposures were also carried out using 60% acetonitrile/water solutions. At 300 nm di- and tetra PCDDs had higher first-order photodegradation rate constants in 60% acetonitrile/water than in pure water. The solvent effect was reversed for PCDFs. These results may be a reflection of the higher polarity of PCDFs compared to PCDDs. In both the indoor and outdoor exposures photodegradation rates decreased with increasing concentrations of chlorination. However, OCDF exposed to 300 nm light in 60% acetonitrile/water and to sunlight in pure water photodegraded more rapidly than tetra CDF. Photolysis rates in sunlight were considerably slower (t(1/2) of 6.4-23 h) than photolysis rates at 300 nm in the laboratory (t(1/2) of 4.3-680 min), reflecting the lower intensity of sunlight in the 300 nm region of the UV/Vis spectrum. The extent of dechlorination of the PCDDs/PCDFs was less than 20% and reductive dechlorination does not appear to be a major process in the photodegradation of PCDDs/PCDFs in aqueous solutions.     

Phototransformation of polychlorinated dibenzo-p-dioxins from photolysis of pentachlorophenol on soils surface

The phototransformation of polychlorinated dibenzo-p-dioxins (PCDDs) by photolysis of pentachlorophenol (PCP) on soil surface under irradiation of UV light in the laboratory has been investigated. Octachlorodibenzo-p-dioxin (OCDD) and heptachlorodibenzo-p-dioxin (H7CDD) were detected from the products of the photoreactions. The effects of soil types with different basic soil physicochemical properties were varied from silt loam, silt clay and clay soil on the formation of PCDDs by irradiation of PCP on soils surface. Fulvic acids can prevent phototransformation of PCDDs by photolysis of PCP on soil surface.     

Sensitized photodecomposition of high disperse pesticide chemicals exposed to sunlight and irradiation from halogen or mercury lamp

The kinetics of sensitized photolysis for four pesticides, deltamethrin (Decis), acrinathrin (Rufast), s-fenvalerate (Sumialpha), and propiconazol (Tilt) in thin films and aerosol particles was investigated under sunlight or the irradiation from halogen or mercury lamp. The peculiarities of principle of the photochemistry of pesticides in high disperse substance state were discussed. The quantum yields of sensitized and direct photolysis were measured, and the phenomenological kinetic mechanism of photolysis was proposed and examined. Four relations between six rate constants of reaction steps of the kinetic mechanism were determined. The use of sensitizers to accelerate the decomposition rates of pesticide pollutants under sunlight could minimize the contamination of agricultural commodities and the adjacent environment. The Shirvanol 2 sensitizer (a by-product of the full treatment of Caspian oils) can regulate decomposition of many pesticides in a wide range of photolysis rates, so that practically any desired lifetime could be really available for either pesticide residues on plant foliage or pesticide aerosol particles in the air.