Enhancement of p,p'-DDT photodegradation on soil surfaces using TiO2 induced by UV-light.

Enhancement of p,p'-DDT photodegradation on soil surfaces using TiO2 induced by UV-light was mainly investigated in this work. After being spiked with p,p'-DDT, soil samples loaded with different doses of TiO2 (0%, 0.5%, 1%, 2%, and 3% wt) were exposed to UV-light irradiation for 24 h. The results indicated that the photodegradation of p,p'-DDT followed the pseudo-first-order kinetics. TiO2 accelerated the photodegradation of p,p'-DDT significantly as indicated by the half-life reduction from 23.3 h to 10.4 h, corresponding to the TiO2 content from 0% to 3% respectively. In addition, the effects of soil pH, photon flux and humic substances on p,p'-DDT degradation were investigated. The photodegradation rate increased with the increase of the soil pH and photon flux. The humic substances (2% wt) inhibited the p,p'-DDT photodegradation by reducing the amount of light available to excite the p,p'-DDT and TiO2 or by quenching radicals capable of oxidizing p,p'-DDT. p,p'-DDE, p,p'-DDD and DDMU were main degradation intermediates and they were further degraded in the presence of TiO2.

[Chemical-toxicological characteristic property of organochlorine pesticide transformation products formed under the effect of intensive fluxes of gamma-irradiation]. / Khimiko-toksikologicheskaia kharakteristika produktov prevrashcheniia khlororganicheskikh pestitsidov, obrazuiushchikhsia pri vozdeistvii intensivnykh potokov gamma-izlucheniia.

The results of formation of stable products of organochlorinated pesticides (OCP) under gamma-irradiation (dose 10 kGy, dose rate 0.026, 0.14 kGy/min) are discussed in the article. The model solutions of alpha-,gamma-hexachlorocyclohexane (alpha-,gamma-HCH), p,p-dichlorodiphenyl-trichloroethane (p,p-DDT) and p,p-dichlorophenyl-dichloroethylene (DDE), with initial concentrations of 1.0-0.1 ppm, were exposed to gamma-radiation. Among the products of OCP radiolysis such compounds as DDE, 2,2-di(chlorophenyl)-1-chloroethylene, dichlorophenyl-dichloroethane and various isomers of HCH were found. It was shown that taking in account the whole data on the formed metabolites the toxicological characteristics of pesticide solutions cannot be improved by exposure to gamma-radiation.

Photodecomposition of DDA.

The photodecomposition of aqueous solutions of 2,2-bis (p-chlorophenyl) acetic acid (DDA) was slow in sunlight and rapid in the laboratory, producing p,p'-dichlorobenzophenone (DCB), p-chlorobenzaldehyde, p-chlorophenol, and several unidentified polar products. p,p'-Dichlorobenzilic acid, and p,p'-dichlorobenzhydrol gave rise to the same photoproducts, while bis-(p-chlorophenyl) methane (DDM) and chlorogenzilate were converted only to DCB. DCB and p-chlorogenzaldehyde proved to be resistant to photodegradation but gradually produced p-chlorobenzoic acid which, in turn, formed p-hydroxybenzoic and benzoic acids, probably the last environmentally detectable links in the long chain of DDT degradation to CO2 and water. High pressure liquid chromatography (HPLC) proved to be ideal for separating and quantitating the parent compounds and their photoproducts directly from the aqueous photolysates or from methanol solutions of the isolates and standards.

PCBs and environmental contamination.

A survey is given on reported PCB-residues and accumulation in various environmental media including water and aquatic environment, flora, animals, food, and humans. Model studies with technical PCB, as well as with pure individual components, are presented from various working groups. As examples, metabolic studies and photochemical experiments are discussed. According to our present knowledge, a major metabolic pathway in animals and plants is hydroxylation, often followed by methylation or conjugation. By UV-irradiation, however, oxygenation, dechlorination and chlorination, polymerization and isomerization may occur.

Induced photolysis of DDT.

Photolysis of mixtures of certain alkyl halides and aromatic amines produces dehalogenation of the halide. These reactions involve a photoinduced charge transfer from the amine to the halide. Photolysis of tritolylamine and carbon tetrachloride produces tritolylaminium chloride. Photolysis of 1,1,1-trichloro-2,2-bis(p-chlorophenyl)ethane (DDT) and diethylaniline at 3100 angstroms yields 1,1-dichloro-2,2-bis(p-chlorophenyl)ethylene (DDE), 1,1-dichloro-2,2-bis(p-chlorophenyl)ethane (DDD), p,p'-dichlorobenzophenone (DDCO), and hydrogen chloride. Photolysis of DDT does not occur unless an inducer which has a low ionization potential, such as diethylaniline, is present. The DDT-diethylaniline mixture is stable in the dark, and the induced photolysis is not affected by triplet quenchers.

Photooxidation of DDT and DDE.

The pesticide DDT [1,1,1-trichloro-2,2-bis(p-chlorophenyl) ethane] and its metabolite DDE [ 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene] can be photoOxidized in methanol. Photolytic generation of free radicals that may abstract hydrogen from solvent, react with oxygen, or abstract hydrogen from unreacted substrate occurs. Further decomposition of short-lived intermediates yields many compounds. Oxidation products include benzoic acids, aromatic ketones, and chlorinated phenols. The DDE also undergoes photocyclization to give dichlorofluorene derivatives.

Photochemical decompsosition of DDT by a free-radical mechanism.

Both as a pure solid and in hexane solution, DDT readily decomposed when irradiated with ultraviolet light (2537 angstroms). Principal products identified by gas-liquid and thin-layer chromatography from irradiations of the solid phase were 1,1-dichloro-2,2-bis (p-chlorophenyl) ethane, 1,1-dichloro-2,2-bis(p-chlorophenyl) ethylene, and 4,4'-dichlorobenzophenone. 1,1-Dichloro-2,2-bis(p-chlorophenyl) ethane and hydrochloric acid were identified from irradiated solutions of DDT in hexane. On the basis of products obtained, quantum yields, scavenger experiments, and other chemical tests, a nonchain, free-radical mode of degradation is proposed.