312
C. Minero . E. Pelizzetti . M.R. Preston
present. If the C signal is independent on the sample composition, quantification of
Doe is possible. Other than the detection of CO 2 formed, the signal can be made independent of the sample composition using C+ signal obtained by ICP-MS. The inductively-coupled plasma ionization technique ensures an effective matrix decomposition. Using isotope dilution techniques and MS, the quantification (De Bievre and
Peiser 1997) of Doe is possible without need of absolute intensity determination, as
required by atomic emission spectroscopy, which can suffer from some matrix effects.
This principle was recently proposed for the determination of Toe content of HPLC
fractions of heavy metal complexes with humic substances (Vogi and Heumann 1998).
However, the search for efficient oxidation systems which can be operated unattended, remotely or in restricted and mobile environments, with limited energy consumption and without the use of chemical reagents and bottles of gases, led in recent
years to the development of Toe measuring systems based on the photocatalytic oxidation capabilities of organics to CO2 in the presence of the anatase form of titanium
dioxide.
It has long been recognized that titanium dioxide illuminated with band gap energy of greater than 3.2 eV «380 nm) efficiently mineralizes most organic compounds
(Pelizzetti et al. 1993). Semiconductor-assisted photocatalysis as a method for destroying pollutants has been the subject of extensive investigations in the last 20 years.
Books, reviews and exhaustive collections are available, and dedicated conferences and
workshops have been held (Schiavello 1985; Pelizzetti and Serpone 1986; Serpone and
Pelizzetti 1989; Anpo 1989; Pelizzetti and Minero 1994; Blake 1994,1995,1997; Hoffmann
et al. 1995; Bolton 1996; AI-Ekabi 1997).
The key of the process is the absorption of light from the semiconductor creating
valence band holes (strong oxidants) and conduction band electrons (mild reductants)
(see Fig. 16.4). Heterogeneous photocatalytic processes involve reactions at the surface-solution interface, where the oxidizing species can be holes (more oxidizing than
hv
, , ,
, ,
, ,
, , ,
e- ....... ---t--. . . . . .
~~--.---~: ~conduction
:
- NAthermodynamically
permissible
reduction
band
:
,
, ,
h+
Ox
Y 2 valence band
1 .... ------_ D/Dh+jf//"':
thermodynamically
permissible
oxidation
!
Illuminated
Electrolyte
Red2
semiconductor
Electrolyte
a
b
Fig. 16.4. a Schematic representation of electron-hole formation and electron transfer reaction at the
semiconductor particle; b Thermodynamic constraints for electron exchange at illuminated semiconductor-electrolyte interfaces (A = electron acceptor, D = electron donor)
C. Minero . E. Pelizzetti . M.R. Preston
present. If the C signal is independent on the sample composition, quantification of
Doe is possible. Other than the detection of CO 2 formed, the signal can be made independent of the sample composition using C+ signal obtained by ICP-MS. The inductively-coupled plasma ionization technique ensures an effective matrix decomposition. Using isotope dilution techniques and MS, the quantification (De Bievre and
Peiser 1997) of Doe is possible without need of absolute intensity determination, as
required by atomic emission spectroscopy, which can suffer from some matrix effects.
This principle was recently proposed for the determination of Toe content of HPLC
fractions of heavy metal complexes with humic substances (Vogi and Heumann 1998).
However, the search for efficient oxidation systems which can be operated unattended, remotely or in restricted and mobile environments, with limited energy consumption and without the use of chemical reagents and bottles of gases, led in recent
years to the development of Toe measuring systems based on the photocatalytic oxidation capabilities of organics to CO2 in the presence of the anatase form of titanium
dioxide.
It has long been recognized that titanium dioxide illuminated with band gap energy of greater than 3.2 eV «380 nm) efficiently mineralizes most organic compounds
(Pelizzetti et al. 1993). Semiconductor-assisted photocatalysis as a method for destroying pollutants has been the subject of extensive investigations in the last 20 years.
Books, reviews and exhaustive collections are available, and dedicated conferences and
workshops have been held (Schiavello 1985; Pelizzetti and Serpone 1986; Serpone and
Pelizzetti 1989; Anpo 1989; Pelizzetti and Minero 1994; Blake 1994,1995,1997; Hoffmann
et al. 1995; Bolton 1996; AI-Ekabi 1997).
The key of the process is the absorption of light from the semiconductor creating
valence band holes (strong oxidants) and conduction band electrons (mild reductants)
(see Fig. 16.4). Heterogeneous photocatalytic processes involve reactions at the surface-solution interface, where the oxidizing species can be holes (more oxidizing than
hv
, , ,
, ,
, ,
, , ,
e- ....... ---t--. . . . . .
~~--.---~: ~conduction
:
- NAthermodynamically
permissible
reduction
band
:
,
, ,
h+
Ox
Y 2 valence band
1 .... ------_ D/Dh+jf//"':
thermodynamically
permissible
oxidation
!
Illuminated
Electrolyte
Red2
semiconductor
Electrolyte
a
b
Fig. 16.4. a Schematic representation of electron-hole formation and electron transfer reaction at the
semiconductor particle; b Thermodynamic constraints for electron exchange at illuminated semiconductor-electrolyte interfaces (A = electron acceptor, D = electron donor)