Radiation removals of low-concentration halomethanes in drinking water

Gamma radiation induced removals of four halomethanes, with low initial concentrations in drinking water were investigated. The results show that absorbed dose and solution pH are important factors in affecting halomethanes removals. High-absorbed dose and solution pH drive halomethanes removals. The reactions of halomethanes with e(aq)(-) play a crucial role in their removal processes. Halomethanes removal during the radiation follow a pseudo-first-order kinetics model. Gamma radiation results in a slight decrease in pH and TOC values of drinking water.

Mechanistic aspects regarding the direct aqueous environmental photochemistry of phenol and its simple halogenated derivatives. A review

We have reviewed the mechanistic aspects regarding the direct aqueous phase environmental photochemistry of phenol and its simple halogenated derivatives. These compounds are important industrial and natural products, are ubiquitous in aquatic systems, and their acute and chronic toxicity makes their environmental fate of interest. Work over the past two decades has unified the photochemistry of phenol and its simple halogenated derivatives. In general, three photochemical pathways dominate in aqueous solution depending on the nature of the substrate: (1) photoionization, (2) photochemical aryl-halogen bond homolysis, and (3) photochemical aryl-halogen bond heterolysis. Photoionization typically results in an array of biaryl radical coupling products which are only relevant for highly concentrated waste streams. Photolytic aryl-halogen bond homolysis will primarily give photoreduction products where reducing agents such as dissolved organic matter or reduced metal cations are present, and radical coupling products in highly concentrated waste streams. The 2- and 4-substituted halophenols may undergo photochemical aryl-halogen bond heterolysis upon irradiation to give an aryl cation. The aryl cation can be attacked by water to give the corresponding hydroxylated derivative, or may deprotonate to generate alpha- and gamma-ketocarbenes, respectively. Following their formation, the singlet alpha-ketocarbenes may undergo Wolff rearrangements to cyclopentadiene-ketenes that are subsequently hydrolyzed to cyclopentadiene carboxylic acids. The triplet alpha- and gamma-ketocarbenes are attacked by oxygen and hydrolyzed to give benzoquinones, directly hydrolyzed to yield hydroquinones, reduced to give phenols, or could take part in coupling reactions in highly concentrated waste streams to give dimers and hydroxybiaryl complexes. Additional studies in natural water samples are required to assess the relative importance of these direct irradiation mechanisms relative to indirect photolysis and other abiotic and biotic degradation and environmental partitioning pathways across the continuum of marine, freshwater, and wastewater biogeochemical signatures.

Combined vacuum ultraviolet laser and synchrotron pulsed field ionization study of CH2BrCl

The pulsed field ionization-photoelectron (PFI-PE) spectrum of bromochloromethane (CH2BrCl) in the region of 85,320-88,200 cm-1 has been measured using vacuum ultraviolet laser. The vibrational structure resolved in the PFI-PE spectrum was assigned based on ab initio quantum chemical calculations and Franck-Condon factor predictions. At energies 0-1400 cm-1 above the adiabatic ionization energy (IE) of CH2BrCl, the Br-C-Cl bending vibration progression (nu1+=0-8) of CH2BrCl+ is well resolved and constitutes the major structure in the PFI-PE spectrum, whereas the spectrum at energies 1400-2600 cm-1 above the IE(CH2BrCl) is found to exhibit complex vibrational features, suggesting perturbation by the low lying excited CH2BrCl+(A 2A") state. The assignment of the PFI-PE vibrational bands gives the IE(CH2BrCl)=85,612.4+/-2.0 cm-1 (10.6146+/-0.0003 eV) and the bending frequencies nu1+(a1')=209.7+/-2.0 cm-1 for CH2BrCl+(X2A'). We have also examined the dissociative photoionization process, CH2BrCl+hnu-->CH2Cl++Br+e-, in the energy range of 11.36-11.57 eV using the synchrotron based PFI-PE-photoion coincidence method, yielding the 0 K threshold or appearance energy AE(CH2Cl+)=11.509+/-0.002 eV. Combining the 0 K AE(CH2Cl+) and IE(CH2BrCl) values obtained in this study, together with the known IE(CH2Cl), we have determined the 0 K bond dissociation energies (D0) for CH2Cl+-Br (0.894+/-0.002 eV) and CH2Cl-Br (2.76+/-0.01 eV). We have also performed CCSD(T, full)/complete basis set (CBS) calculations with high-level corrections for the predictions of the IE(CH2BrCl), AE(CH2Cl+), IE(CH2Cl), D0(CH2Cl+-Br), and D0(CH2Cl-Br). The comparison between the theoretical predictions and experimental determinations indicates that the CCSD(T, full)/CBS calculations with high-level corrections are highly reliable with estimated error limits of <17 meV.

Photoinduced One-Electron Reduction of Alkyl Halides by Dirhodium(II,II) Tetraformamidinates and a Related Complex with Visible Light

Various substituted dirhodium tetraformamidinate complexes, Rh(2)(R-form)(4) (R = p-CF(3), p-Cl, p-OCH(3), m-OCH(3); form = N,N'-diphenylformamidinate), and the new complex Rh(2)(tpgu)(4) (tpgu = 1,2,3-triphenylguanidinate) have been investigated as potential agents for the photoremediation of saturated halogenated aliphatic compounds, RX (R = alkyl group). The synthesis and characterization of the complexes is reported, and the crystal structure of Rh(2)(tpgu)(4) is presented. The lowest energy transition of the complexes is observed at approximately 870 nm and the complexes react with alkyl chlorides and alkyl bromides under low energy irradiation (lambda(irr) > or = 795 nm), but not when kept in the dark. The metal-containing product of the photochemical reaction with RX (X = Cl, Br) is the corresponding mixed-valent Rh(2)(II,III)X (X = Cl, Br) complex, and the crystal structure of Rh(2)(p-OCH(3)-form)(4)Cl generated photochemically from the reaction of the corresponding Rh(2)(II,II) complex in CHCl(3) is presented. In addition, the product resulting from the dimerization of the alkyl fragment, R(2), is also formed during the reaction of each dirhodium complex with RX. A comparison of the dependence of the relative reaction rates on the reduction potentials of the alkyl halides and their C-X bond dissociation energies are consistent with an outer-sphere mechanism. In addition, the relative reaction rates of the metal complexes with CCl(4) decrease with the oxidation potential of the dirhodium compounds. The mechanism of the observed reactivity is discussed and compared to related systems.

Time-resolved kinetic studies on quenching of HCF(A1A'') by alkane and alcohol molecules

HCF(X1A') radicals were produced by laser photolysis of CHFBr2 at 213 nm and were electronically excited from the ground state to A1A''(030) at 492.7 nm with a dye laser pumped by a Nd:YAG laser. With the analysis of the lifetime of the time-resolved total fluorescence signals collected in the reaction cell where the total pressure was fixed to be 14.0 Torr, the quenching data of HCF(A1A'') by alkane and alcohol molecules at room temperature were derived from variation of pseudo-first-order rate constant with different quencher pressures. It is found that the quenching rate constants are close to the collision rate constants (10(-10) cm3 molecule(-1) s(-1)), indicating the long-range attractive forces between the collision partners play an important role in the entrance channel of quenching process. Several kinetic models were applied to analyze the mechanism of the quenching process. The complex formation cross sections are calculated with the collision complex model. Correlations of the quenching rate constant for the removal of the HCF(A1A'') state with ionization potential of the quenching partners show that the insertion reactive mechanism is probably the dominant reaction channel, which is analogous to the behaviors of other three-atom carbenes in corresponding electronic states.

Enhanced clastogenicity of contaminated groundwater following UV irradiation detected by the Tradescantia micronucleus assay

The Tradescantia micronucleus (Trad-MCN) assay was used to determine clastogenic effects of contaminated groundwater collected near a hazardous waste landfill. Water samples were taken from a purification plant (activated charcoal filtration, UV irradiation) which was built to avoid groundwater contamination by this landfill. Five series of experiments were conducted during approximately 4 months. In addition, water samples were irradiated under laboratory conditions with increasing doses of UV light. Several field water samples gave positive, dose-dependent effects before filtration and irradiation. Maximal values (6.1 +/- 4.7 micronuclei (MCN)/100 tetrads) were six-fold above controls. UV irradiation of activated charcoal-filtered water resulted in an enhancement of MCN frequencies. Exposure of groundwater to UV irradiation in the laboratory led to a dose-dependent increase of micronuclei. At the highest dose (1500 J/m2) the MCN frequency was more than six times higher than in the unirradiated sample (5.4 +/- 1.0 vs. 0.8 +/- 0.4 MCN/100 tetrads). The clastogenicity of UV-irradiated samples decreased with a half-life of approximately 1 day. Irradiation of tap water did not increase the MCN frequency. Our results indicate that irradiation of water with UV light for disinfection purposes might lead to a transiently increased genotoxicity of chemically polluted water samples.

Electron spin resonance spin trapping studies on the photolytic generation of halocarbon radicals

The technique of free radical spin trapping has been used to study the photolytic behaviour of anaerobic solutions of aliphatic halocarbons such as CCl4, CBrCl3 and halothane in either toluene or water at room temperature. Use of the spin traps N-tert-butyl-alpha-phenylnitrone (PBN) and 2-methyl-2-nitrosopropane provides evidence for photolytic cleavage of carbon-halogen bonds yielding radicals such as .CCl3 (from CCl4 and CBrCl3) and .CHClCF3 (from halothane) which are readily trapped; this method of generating biologically important radicals may be of use in studying model reactions of these species. Under aerobic conditions evidence is obtained, by use of the spin trap PBN, for the production of the corresponding halocarbon peroxyl radicals (such as CCl3O2.) by addition of oxygen to the initially produced halocarbon radical. Though the direct detection of the peroxyl radical adducts to the spin trap proved impossible under these experimental conditions, the observed oxidation products of the spin trap (acyl nitroxides) are shown to be indicative of the presence of such species.