CHAPTER 16 • Oceanic DOC Measurements
313
aqueous hydroxyl radical) or trapped holes (less oxidizing than aqueous hydroxyl radical), and where the reducing species can be conduction band electrons or trapped electrons, both being much less reducing than aqueous hydrated electrons (Ollis and AlEkabi 1993; Pelizzetti and Schivallo 1991).
A series of events lead, in the presence of oxygen, to the conversion of the organic
species into CO2 and water, the other heteroatoms being transformed into inorganic
species (Pelizzetti and Minero 1993). Besides the solvent, other species are present at
the interface or in solution (e.g. H02·1 02" °2, H20 2) which can contribute to the complex degradation scheme leading to the final mineralization of the organic compounds.
Several degradation mechanisms have been reported for hydrocarbons, halohaliphatic,
halo aromatics, surfactants, herbicides and several other classes of organic compounds,
respectively.
The process may operate either with artificial or with solar light. This has stimulated very active research (Schiavello 1988). The impressive amount of scientific contributions and reports has led to pilot-scale work and testing of solar driven plants
for water decontamination (Minero et al. 1996) and also for marine oil spill removal
(Minero et al. 1997a).
By contrast only a few analytical applications of this process have been developed
(Low and McEvoy 1996) and only recently detectors for TOC monitoring of ultrapure
water and a benchtop TOC analyser based on this principle have been marketed. For
the last, the organic species are photocatalytically decomposed using a Ti02 slurry to
carbon dioxide, with measurement as carbonate by conductivity. The system is depicted
in Fig. 16.5. The graph of the logarithm of the conductivity against the logarithm of
the amount of organic carbon present is essentially linear for amounts of carbon greater
than 100 flM C and can be further linearized to less than 5 flM C by subtracting a constant linearizing factor (Matthews et al. 1990). Conductivity measurements are almost
incompatible with waters of high salinity. However, the photocatalytic oxidation has
been coupled also with a NDIR analyser (Abdullah and Eek 1996).
Until now the performance of the new oxidation technique has not been verified
by the scientific community, and the discussion of the benefits and limits of the new
concept can be based only on the work of developers and the knowledge of the technology for pollution abatement (i.e. organic compound oxidation).
Titanium dioxide is preferred among all other possible photocatalyst because it is
stable to photo corrosion, insoluble in a wide pH range, non-toxic and cheap. Among
the several inherent advantages of photocatalytic oxidation with respect to UV or PS
and PS/Uv, the following are probably the most important. The destruction of organic
compounds is achieved in the absence of dissolved oxidants other than oxygen (from
air), either by oxidation or reduction processes, with the possibility of transforming
into CO2, also of organics in which carbon is at the highest oxidation number (Pelizzetti
and Minero 1994,1999). The oxidation of quite recalcitrant organics, stable even to other
oxidation processes, as well as hydrophilic solutes and semivolatile compounds
(e.g. lA-dioxane (Maurino et al. 1997), ethylene glycol (Parent et al. 1996), halo carbons
(Calza et al. 1997a, 1997b» is possible at rates comparable to other organic compounds.
The reported conversions for a TOC system are on average 99% for a variety of compounds (Matthews et al.1990). Being a heterogeneous process, the catalyst surface may
adsorb organic species (Crittenden et al. 1997) favouring their degradation. The oxidation process is also very effective for ppb levels of water contaminants (Pelizzetti
313
aqueous hydroxyl radical) or trapped holes (less oxidizing than aqueous hydroxyl radical), and where the reducing species can be conduction band electrons or trapped electrons, both being much less reducing than aqueous hydrated electrons (Ollis and AlEkabi 1993; Pelizzetti and Schivallo 1991).
A series of events lead, in the presence of oxygen, to the conversion of the organic
species into CO2 and water, the other heteroatoms being transformed into inorganic
species (Pelizzetti and Minero 1993). Besides the solvent, other species are present at
the interface or in solution (e.g. H02·1 02" °2, H20 2) which can contribute to the complex degradation scheme leading to the final mineralization of the organic compounds.
Several degradation mechanisms have been reported for hydrocarbons, halohaliphatic,
halo aromatics, surfactants, herbicides and several other classes of organic compounds,
respectively.
The process may operate either with artificial or with solar light. This has stimulated very active research (Schiavello 1988). The impressive amount of scientific contributions and reports has led to pilot-scale work and testing of solar driven plants
for water decontamination (Minero et al. 1996) and also for marine oil spill removal
(Minero et al. 1997a).
By contrast only a few analytical applications of this process have been developed
(Low and McEvoy 1996) and only recently detectors for TOC monitoring of ultrapure
water and a benchtop TOC analyser based on this principle have been marketed. For
the last, the organic species are photocatalytically decomposed using a Ti02 slurry to
carbon dioxide, with measurement as carbonate by conductivity. The system is depicted
in Fig. 16.5. The graph of the logarithm of the conductivity against the logarithm of
the amount of organic carbon present is essentially linear for amounts of carbon greater
than 100 flM C and can be further linearized to less than 5 flM C by subtracting a constant linearizing factor (Matthews et al. 1990). Conductivity measurements are almost
incompatible with waters of high salinity. However, the photocatalytic oxidation has
been coupled also with a NDIR analyser (Abdullah and Eek 1996).
Until now the performance of the new oxidation technique has not been verified
by the scientific community, and the discussion of the benefits and limits of the new
concept can be based only on the work of developers and the knowledge of the technology for pollution abatement (i.e. organic compound oxidation).
Titanium dioxide is preferred among all other possible photocatalyst because it is
stable to photo corrosion, insoluble in a wide pH range, non-toxic and cheap. Among
the several inherent advantages of photocatalytic oxidation with respect to UV or PS
and PS/Uv, the following are probably the most important. The destruction of organic
compounds is achieved in the absence of dissolved oxidants other than oxygen (from
air), either by oxidation or reduction processes, with the possibility of transforming
into CO2, also of organics in which carbon is at the highest oxidation number (Pelizzetti
and Minero 1994,1999). The oxidation of quite recalcitrant organics, stable even to other
oxidation processes, as well as hydrophilic solutes and semivolatile compounds
(e.g. lA-dioxane (Maurino et al. 1997), ethylene glycol (Parent et al. 1996), halo carbons
(Calza et al. 1997a, 1997b» is possible at rates comparable to other organic compounds.
The reported conversions for a TOC system are on average 99% for a variety of compounds (Matthews et al.1990). Being a heterogeneous process, the catalyst surface may
adsorb organic species (Crittenden et al. 1997) favouring their degradation. The oxidation process is also very effective for ppb levels of water contaminants (Pelizzetti