Formation of chlorinated hydrocarbons from the reaction of chlorine atoms and activated carbon

The reactions of chlorine atoms and activated carbon have been studied over the temperature range of 200-400 degrees C using an isothermal flow reactor in conjunction with 337 nm laser photolysis of Cl2. These studies have shown that carbon tetrachloride is the major product, with chloroform, methylene chloride, and methyl chloride being formed in progressively decreasing yields. Trace quantities of methane, ethane, and dichloroethylenes were also observed. Mechanisms of carbon fragmentation by successive addition of chlorine atoms are proposed. The formation of small chlorinated hydrocarbons by the direct reaction of chlorine with carbon may be a key link in both the de novo and precursor pathways of formation of PCDD/F.     

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.     

Dechlorination pathways of ortho-substituted PCBs by UV irradiation in n-hexane and their correlation to the charge distribution on carbon atom

The photodegradations of 22 individual polychlorinated biphenyl (PCB) congeners (including 21 non-coplanar ortho substituted and one non-ortho substituted) by irradiation with ultraviolet lamp in n-hexane solution were studied. Photoproducts were identified by matching their retention times and mass spectra with those of authentic standards. PCB congener with less than two chlorides was photodegraded within half an hour, if more than three chlorine on ring, the photodechlorination time for PCB needs one and half hours or more, sometimes even longer than 15 h. The half-life of PCB degradation by photodechlorination was much shorter than that by anaerobic biological dechlorination. Charge distribution on carbon atom combined with the monitoring products of individual PCB congeners were used to deduce the photodegradation pathways. The higher the charge distribution for carbon to which chlorine is attached, the easier for photodechlorination to occur. A lot of chlorine atoms attached PCB, the dechlorination was found to occur prior to the carbon with higher charge distribution at the benzene ring with more chlorine atoms attached.     

Degradation of polychlorinated dibenzo-p-dioxins in aqueous solution by Fe(II)/H2O2/UV system

The photodegradation of polychlorinated dibenzo-p-dioxins (PCDDs), which include tetra- to octa-CDDs (TeCDD, PeCDD, HxCDD, HpCDD and OCDD), was carried out in the presence of Fe(II) and H2O2 mixed reagent. The degradation efficiency was strongly influenced by UV irradiation, and the initial concentrations of H2O2 and Fe(II). An initial TeCDD concentration of 10 ng l(-1) was completely degraded within 20 min under the optimum conditions. All PCDDs tested were successfully degraded by Fe(II)/H2O2/UV treatment and complete degradation of TeCDD, PeCDD and HxCDD was achieved within 120 min. PCDD photodegradation rates decreased with the number of chlorine atoms. The degradation process of TeCDD by this system seems to be initiated by an oxidative reaction (OH* radical attack) because less chlorinated DDs as intermediate products were not detected. From the Frontier electron density calculation, the first OH* radical attack positions on TeCDD were found to be four C atoms neighboring two O atoms. The decomposition of TeCDD gave 4,5-dichlorocatechol as an intermediate product. A TeCDD degradation scheme was proposed based on the identified intermediate and the values of Frontier electron density. Based on these results, Fe(II)/H2O2/UV system could be useful technology for the treatment of wastewater containing persistent pollutants such as dioxins and polychlorinated biphenyls.     

The internal barriers of rotation for the 209 polychlorinated biphenyls

The internal barrier of rotation (Erot) was calculated for all 209 polychlorinated biphenyls (PCBs) by using a semi-empirical method, viz. the Austin Model 1 (AMI) Hamiltonian. The difference in total energy between a forced planar state and an optimised twisted structure was defined as Erot. The Erot values were in the range of 8.33 to 483 kj/mol, and were significantly influenced by the number of chlorine atoms inortho position. An additional structural characteristic of the PCBs influencing Erot ofortho substituted congeners was substitution by chlorine atoms in vicinalmeta positions, which is assumed to prevent outward bending ofortho substituents. This so-called buttressing effect contributed with 4 to 31 kj/mol per added chlorine atom. In conclusion, the internal barrier of rotation, calculated for all 209 PCBs, provides an important structure dependent physico-chemical parameter for multivariate modelling of future quantitative structure-activity and structure-property relationships (QSARs/QSPRs).     

Multi-component behavior of fixed-bed adsorption of dioxins by activated carbon fiber

The multi-component behavior of fixed-bed adsorption of dioxins (DXNs) was examined through detailed analyses of the concentration profiles of isomers in fixed-bed activated carbon fiber (ACF). Regularities in both adsorption rates and strengths were clarified. (1) The rate of transfer in the adsorption of polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCCDs/DFs) tends to increase with decreasing number of chlorine substituents. Axial dispersion also tends to increase with a decreasing number of chlorine substituents under our experimental conditions. (2) Homologues with the same number of chlorine substituents in PCDDs/DFs have similar adsorption strengths. The adsorption strength of PCDD/DF isomers is probably greater than that of co-planar polychlorinated biphenyls (co-PCB) isomers when the number of chlorine substituents is identical. (3) The adsorption strength of isomers depends on their molecular structure. In PCDDs/DFs the toxic isomers, all of which have vicinal chlorine substituents at the 2, 3, 7 and 8 positions, are relatively strong. It is clear, especially in TeCDDs, that isomers with vicinal chlorine substituents are stronger than isomers without. In co-PCBs, isomers without chlorine substituents at ortho positions are stronger than those with, and (4) A close analogy exists between the adsorption strength order for ACF and the elution order in gas chromatography (GC).     

Mechanism of the formation of polychlorinated dibenzo-p-dioxins and dibenzofurans from chlorophenols in gas phase reactions

The pyrolysis of chlorinated phenates at a temperature of about 280 degrees C results in the formation of definite chlorinated dibenzodioxin (PCDD) congeners [1-3]. It is shown that in gas phase reactions chlorophenols react in the presence of oxygen above 340 degrees C not only to PCDD but also to chlorinated dibenzofurans (PCDF). The mechanism of this reaction of chlorophenols to PCDD and PCDF was elucidated. In a first step phenoxyradicals are formed which are capable of forming PCDDs and PCDFs. This is confirmed by the oxygen dependency of the reaction. In an argon atmosphere no dimerization of chlorophenols could be observed at 420 degrees C. By the identification of intermediates and by analyzing the PCDF isomers formed from individual chlorophenols the reaction pathway is elucidated. As intermediates in the formation of PCDFs polychlorinated dihydroxybiphenyls (DOHB) were identified. These are most likely formed by the dimerization of two phenoxy radicals at the hydrogen substituted carbons in ortho-positions under simultaneous movement of the hydrogen atoms to the phenolic oxygen PCDDs are formed in the gas phase via ortho-phenoxyphenols (POP) analogous to the pyrolysis of phenates, but due to the radical mechanism in the first condensation step to POPs not only a chlorine atom is capable for substitution but also the hydrogen atoms. The formation of the DOHBs and their condensation to PCDFs and hydroxylated PCDFs as well as the ratio of PCDD to PCDF formed show a strong dependency on the reaction temperature, the substitution pattern of the chlorophenols and the oxygen concentration.     

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.     

酚类化合物对硝化颗粒污泥活性抑制的定量结构-活性相关研究

测定了酚类化合物对硝化颗粒污泥活性抑制的logIC50值,以量子化学参数为自变量,应用偏最小二乘法（PLS）,建立了酚类化合物对硝化颗粒污泥活性抑制的定量结构-活性相关（QSAR）模型。模型所提取的PLS主成分所能解释的因变量总方差的比例Qc2um为0.820,表明模型具有较好的稳定性和预测能力。模型的结果表明,影响酚类化合物对硝化颗粒污泥活性抑制的主要因素是logkow、CCR和Ehomo,酚类化合物对硝化颗粒污泥活性抑制的logIC50随着分子logkow的增大而减小,随着Ehomo和CCR的增大而增大。     

Photolysis of vinclozolin

Following photolysis of vinclozolin in methanol five products were detected and identified: 3,5-dichlorophenylisocyanate, 3,5-dichloroaniline, methyl 3,5-dichlorophenylcarbamate, 3-(3-chlorophenyl)-5-methyl-5-vinyl-oxazolidine-2,4-dione and methyl (3,5-dichlorophenyl) (2-hydroxy-2-methyl-1-oxo-buten-3-yl) carbamate. The major component identified from photolysis in benzene solution, 3-(3-chlorobiphenyl)-5-methyl-5-vinyl-oxazolidine-2,4-dione, was produced by replacement of one chlorine atom by a solvent molecule.