Nikola Getoff

Radiation-induced degradation of water pollutants—state of the art☆

Abstract The radiation-induced decomposition of biological resistant pollutants in drinking as well as in wastewater is briefly reviewed. First, some important units, definitions etc., radiation sources, as well as dose-depth curves in water as functions of the electron energy and 60Co-γ-rays are mentioned. Following is a schematical presentation of water radiolysis and of characteristics of primary free radicals. Then the degradation of some aliphatic and aromatic chlorinated compounds in the presence of air is presented. Some spectroscopic and kinetic data of transients resulting from chlorinated phenols are also quoted in order to illustrate and to explain the rather complicated degradation mechanisms. In this respect the synergistic effect of radiation and oxygen as well as that of ozone is also discussed. Finally, a scheme for technical application of high energy electron beam is presented.

Factors influencing the efficiency of radiation-induced degradation of water pollutants

Abstract The efficiency of the radiation-induced degradation of water pollutants depends on several factors, such as kind and energy of radiation, absorbed dose, dose rate, pollutant concentration as well as synergistic effects of radiation and ozone or/and catalysts (e.g. TiO2) and of the molecular structure of the pollutants. The role of the individual factors is illustrated by examples. The application of pulse radiolysis in addition to chemical analysis for elucidation of reaction mechanisms and optimization of the degradation treatment is also mentioned.

Photoelectrochemical and photocatalytic methods of hydrogen production : a short review

Abstract A critical overview on photoelectrochemical and photocatalytic methods for hydrogen production by water splitting is presented. First, the solar spectral irradiance and energy distribution as well as the direct water photolysis are briefly mentioned. This is followed by the liquid-junction transducer comprising semiconductor photoelectrodes, photogalvanic cells, homogeneous photoredox systems and hybrid water splitting methods utilizing solar energy. Presently the most promising technique seems to be the photoelectrochemical and the hybrid devices.

PATHWAYS FOR FORMATION OF HYDRATED ELECTRONS FROM EXCITED PHENOL AND RELATED COMPOUNDS

Abstract. The dependence of primary photophysical and photochemical processes, especially of electron photoejection, in phenol and related compounds in aqueous solution on excitation intensity and excitation energy is examined. Theoretical and experimental evidence is presented for the possibility of three pathways for electron ejection: (1) A monophotonic pathway via the fluorescent state, which most probably does not involve the lowest triplet state; (2) a monophotonic pathway requiring higher excitation energies, which takes place in competition with internal conversion to the fluorescent state; and (3) a consecutive biphotonic pathway in which the lowest triplet state absorbs the second photon, and which can become predominant at high intensities, e.g. under flash conditions. It is shown that this model reconciles apparently conflicting results published in the literature.

Primary yields of CH3•O and •CH2OH radicals resulting in the radiolysis of high purity methanol

Abstract The triethylsilane radical R3•Si, produced by radiolysis in an airfree methanol/silane-system, acts as a specific scavenger for the CH3•O and •CH2OH transients with rate constants, k14(R3•Si + CH3•O) = 1.1 x 108 dm3 mol-1s-1 and k 15 ( R 3 • Si + • CH 2 OH) = 0.7 x 10 8 dm 3 mol -1 s -1 , resulting in R3Si—OCH3 (triethylmethoxysilane) and R3Si—CH2OH (triethylsilylmethanol). By increasing the silane concentration (range: 10-2-6 mol dm-3 R3SiH) the formation of the otherwise major products of methanol radiolysis, formaldehyde and glycol, is successively reduced to nil. The yield of R3Si—CH2OH, studied under the same conditions, passes a maximum at about 0.8 mol dm-3 R3SiH and then also diminishes. On the other hand, the yield of R3Si—OCH3 is increased correspondingly and reaches an interpolated value of G = 3.75 ± 0.1 at 4 mol dm-3R3SiH. This indicates that the radical CH3•O (G = 3.75 ± 0.1) is the primary radiolytic transient of methanol in addition to H, e-sol etc., but not •CH2OH species. The latter one is obviously formed by the secondary reaction: CH3•O + CH3OH→ CH3OH + •CH2OH.

Decay processes of singlet excited phenol in solution

Abstract The yields of fluorescence at 295 and 77 K and of phosphorescence at 77 K of phenol and anisole were determined in solvents of different polarities (hydrocarbons, ether, ethanol). Also intersystem crossing yields are reported for phenol and anisole in cyclohexane solution at 295 K. The data show that a non-radiative process, different from intersystem crossing, competes with the decay of S 1 phenol in liquid hydrocarbons. The nature of this deactivation pathway is discussed.

Radiation chemistry and the environment

Abstract The rather strong and many-sided pollution of the environment (atmosphere, water resources, soil) as a consequence of human activity is summarized. The solution of the arised problems by application of radiation chemistry methods and the utilization of modern environmentally “clean” and economical technologies, founded on electron beam processing, are mentioned. Some basic environmental problems and their solution are briefly discussed. (i) Removal of CO2 from flue gases and its radiation induced utilization. (ii) Principals for degradation of aqueous pollutants by electron beam processing in the presence of ozone (synergistic effect). The radiation chemistry as a modern and manifold discipline with very broad applications can also essentially contribute in the conservation of the environment.

Radiolysis and pulse radiolysis of chlorinated phenols in aqueous solutions

Abstract The radiation induced decomposition of 2-, 3- and 4-chlorophenols was studied under steady-state conditions. The rate constants for the reactions of H, OH and e - ag with 2-Cl-phenol were determined. Absorptions spectra of the H- and OH- adducts are presented.

Radiation induced decomposition of chlorinated phenols in water

Abstract Experiments with 4-Cl-phenol as a model compound for pesticides were performed under steady-state conditions using deoxygenated solutions as well as such saturated with air, oxygen or oxygen mixed with ozone. The yield of Cl - ions serviced as an indicator for the degradation process. As main products of the first step of decomposition were identified: polyhydroxybenzenes, aldehydes and acids. The yield of aldehydes was studied as a function of the absorbed dose and substrate concentration. In the presence of ozone a chain-reaction of the oxidative pollutant degradation takes place. Transient absorption spectra and kinetics obtained by preliminary pulse radiolysis studies of 4-Cl-phenol in the presence of oxygen as well as probable reaction mechanisms are also presented.

Resolved multisite OH-attack on aqueous aniline studied by pulse radiolysis

Abstract The individual formation and decay kinetics as well as the absorption characteristics of the simultaneously formed primary species by OH attack on aniline in aqueous (pH 8–9.6), saturated with N2O, have been determined by pulse radiolysis combined with a computer optimization procedure. It was established that 36% OH radicals react directly with the -NH2 moiety (k = (5.0 ± 0.6) × 109dm3mol-1s-1 resulting in anilino radical, C6H5NH (ϵ300 = 2500 ± 100, ϵ355 = 950 ± 50, ϵ420 = 420 ± 40 dm3 mol-1cm-1)), which decays by a second order reaction with 2k = (1.8 × 0.2) × 109dm3mol-1s-1. The main primary process (54%) is the formation of the ortho-directed OH-adduct (k = (7.5 ± 0.8) × 109dm3mol-1s-1, ϵ300 = 1800±100, ϵ355 =3300±170dm3mol-1cm-1), which subsequently decays by water splitting resulting again in C6H5NH. The remaining 10% OH attack most probably the para position of the aniline ring (k = (1.5±0.2) × 109dm3mol-1s-1) forming the corresponding OH-adduct (ϵ300 = 1700±100, ϵ355 = 3600±200 dm3mol-1cm-1), which disappears with 2k = (1.1±0.3) × 109dm3mol-1cm-1; reactions with meta- and ipso-positions, however, cannot be excluded. H-atoms react with the neutral form of aniline with k =(1.9 ± 0.1) × 109dm3mol-1cm-1, forming H-adducts. Further the rate constant of e-aq with aniline was determined to (3.0 ± 0.1) × 107dm3mol-1cm-1. Qualitative analysis of final products were also performed.