Speaker
Mr
Laszlo Wojnarovits
(Centre for Energy Research, MTA, Hungary)
Description
Background of the study
In the so-called advanced oxidation processes (AOP) and in water
treatment by ionizing radiation, which belongs to the class of AOP’s,
too, the main goal is to destroy, or at least to deactivate harmful
water contaminants: pharmaceuticals, pesticides, surfactants,
health-care products, etc. The chemical transformations are suggested
to be initiated by hydroxyl radicals. However, some other inorganic
radicals may also contribute to initiating the degradation.
Methodology
The changes due to irradiation were followed by taking the UV-Vis
spectra and by measuring the chemical oxygen demand (COD) before and
after irradiation. The degradation products were identified after LC
separation with MS-MS detection. To obtain information about the
degradation, kinetics pulse radiolysis with kinetic spectroscopic
detection was used. The spectra of the intermediates were calculated
from the kinetic curves (radical concentration versus time, on the
$\mu$s time scale) taken at different wavelengths. These spectra give
information about the structure of the intermediates.
Results
The kinetics and reaction mechanisms of a large variety of inorganic
radicals with organic molecules were studied. It was shown that other
inorganic radicals as $^\bullet$OH also contribute highly to the
initiation of degradation in most AOP’s. Cl$^{-}$ and HCO$_3^{-}$ in
the treated water reacting with $^\bullet$OH transform to the
Cl$_2^{\bullet-}$ and CO$_3^{\bullet-}$ oxidizing. Reactions of
$\mathrm{e}_{\mathrm{aq}}^-$ and H$^\bullet$ water radiolysis
intermediates may also contribute to the degradation. In the primary
reactions of all these radicals with organic molecules,
carbon-centred radicals are produced. The reactions of the
carbon-centred radicals with dissolved oxygen (DO) basically
determine the oxidation rate. The peroxy radicals formed in the
reactions of aliphatic carbon-centred radicals with DO may transform
to peroxides and hydroperoxides, with the intervention of these
intermediates gradual degradation takes place. Aromatic
carbon-centred radicals (cyclohexadienyl radicals) in reversible
processes react with DO, where the ring degradation is suggested to
take place from the aromatic peroxi radicals. The primary
carbon-centred radicals in uni- or bi-molecular processes may
transform to other, e.g., oxygen or nitrogen centred radicals. These
intermediates (e.g., phenoxy or anilino radicals) do not react with
DO. Therefore, the initial degradation rate is low when, during the
degradation reactions, there is a possibility for the formation of
these radicals.
Conclusion
The carbon-centred radical formed in the reaction of a one-electron
oxidizing radical with an organic molecule undergoes a second
oxidation step when it reacts with DO. This reaction may be
followed by further oxidations starting from the peroxy radical thus
formed, or from the peroxide/hydroperoxide stabilization products.
These reactions increase the degradation efficiency with a result
that the one-electron oxidants induce $2$–$4$ electron oxidations. When
the radical does not react with DO, the degradation rate is low.
| Country/Organization invited to participate | Hungary |
|---|
Primary author
Mr
Laszlo Wojnarovits
(Centre for Energy Research, MTA, Hungary)
