CHAPTER 16 • Oceanic DOC Measurements
315
Specific areas in which it is possible to envisage improvements in the efficiency of
the photocatalytic method for Toe measurements are the improvement of the catalyst performance (Anpo 1989; Hoffmann et al.1995; Ollis and Al-Ekabi 1993) and catalyst immobilisation on solid support. Suspended catalysts require a gas-liquid separator for CO 2 measure, adding to the cost and complexity of the overall process, with
the possibility of incomplete equilibrium distribution (Matthews et al. 1990). Immobilised materials (Zeltner et al. 1993) would facilitate the engineering of the system, although these materials will probably be more costly and subject to fouling. The suspension is in general more suited for the analysis of dirty water. Titanium dioxide has
been attached to a variety of supporting materials (glass (Matthews et al.1990), Teflon ®
(Low and Matthews 1990), silica, quartz). The performance of the supported catalyst
is generally less than in suspension, by a factor of two or more. The decrease has been
assigned to a diminution in the number of active sites and to the mass transfer limitation (Bideau et al. 1995). Attention has to be paid also to the extent of mineralization
when porous supports are adopted (Matthews 1991). The pumping speed has thus a
significant effect on the rate of oxidation (Matthews et al. 1990).
Oxygen is essential for the complete mineralization of organic compounds, so it is
generally crucial that oxygen depletion not occur during the photo oxidation process.
However, when the DOC is highly oxidated (e.g. heavily halogenated hydrocarbons,
carbonyl compounds, polyhydroxo- or polycarboxyl compounds), the reductive pathways may initially predominate and, since oxygen is a competitor for the conduction
band electrons, a detrimental effect of oxygen concentration on the degradation rate
is observed (Pelizzetti and Minero 1999).
Other electron scavengers, such as peroxydisulfate, generally, but not always, show
a beneficial effect on the photo oxidation processes. For example, whereas peroxydisulfate shows an astonishing increase of the rate of formation of cyanuric acid as the
end product of atrazine degradation (Pelizzetti et al. 1991), a detrimental effect is reported for ethylene glycol (Parent et al. 1996). The need of PS (1.5 mM) for photocatalytic Toe measurement of urea was examined, suggesting a little beneficial effect
(Abdullah and Eek 1996). Hydrogen peroxide shows an even more complex behavior
dependent on several operational parameters (Pichat et al. 1995). The use of these additives may be envisaged when DOC amount is high to reach rapidly the total conversion to CO2,
The fouling and poisoning of the catalyst, as well as the catalyst regeneration procedure, are under current research, and severely depend on the real composition of
DOC. It was reported (Parent et al. 1996) that humic acids reduce the activity of titanium dioxide toward the target substances, but it was recently shown that the mineralization process still occurs efficiently and chloride evolution ensures a complete
dehalogenation of the chlorinated compounds (Sega et al. 1999). The photocatalytic
DOC oxidation seems effective on macromolecular components isolated by ultrafiltration of sea water (Abdullah and Eek 1996).
Simple inorganic ions, such as chloride, bromide, phosphate and sulfate have been
found to reduce the photocatalytic performances (Abdullah et al.1990). Recent studies
have shown the formation of halogenated (Cl and Br) organics if photocatalytic degradation occurs in the presence of the correspondent halides (Minero et al. 1997b). This
is of major importance for marine DOC measurements. However, Matthews et al. (1990)
reported that the inhibition is not dramatic when working with sea water at acidic pH.
315
Specific areas in which it is possible to envisage improvements in the efficiency of
the photocatalytic method for Toe measurements are the improvement of the catalyst performance (Anpo 1989; Hoffmann et al.1995; Ollis and Al-Ekabi 1993) and catalyst immobilisation on solid support. Suspended catalysts require a gas-liquid separator for CO 2 measure, adding to the cost and complexity of the overall process, with
the possibility of incomplete equilibrium distribution (Matthews et al. 1990). Immobilised materials (Zeltner et al. 1993) would facilitate the engineering of the system, although these materials will probably be more costly and subject to fouling. The suspension is in general more suited for the analysis of dirty water. Titanium dioxide has
been attached to a variety of supporting materials (glass (Matthews et al.1990), Teflon ®
(Low and Matthews 1990), silica, quartz). The performance of the supported catalyst
is generally less than in suspension, by a factor of two or more. The decrease has been
assigned to a diminution in the number of active sites and to the mass transfer limitation (Bideau et al. 1995). Attention has to be paid also to the extent of mineralization
when porous supports are adopted (Matthews 1991). The pumping speed has thus a
significant effect on the rate of oxidation (Matthews et al. 1990).
Oxygen is essential for the complete mineralization of organic compounds, so it is
generally crucial that oxygen depletion not occur during the photo oxidation process.
However, when the DOC is highly oxidated (e.g. heavily halogenated hydrocarbons,
carbonyl compounds, polyhydroxo- or polycarboxyl compounds), the reductive pathways may initially predominate and, since oxygen is a competitor for the conduction
band electrons, a detrimental effect of oxygen concentration on the degradation rate
is observed (Pelizzetti and Minero 1999).
Other electron scavengers, such as peroxydisulfate, generally, but not always, show
a beneficial effect on the photo oxidation processes. For example, whereas peroxydisulfate shows an astonishing increase of the rate of formation of cyanuric acid as the
end product of atrazine degradation (Pelizzetti et al. 1991), a detrimental effect is reported for ethylene glycol (Parent et al. 1996). The need of PS (1.5 mM) for photocatalytic Toe measurement of urea was examined, suggesting a little beneficial effect
(Abdullah and Eek 1996). Hydrogen peroxide shows an even more complex behavior
dependent on several operational parameters (Pichat et al. 1995). The use of these additives may be envisaged when DOC amount is high to reach rapidly the total conversion to CO2,
The fouling and poisoning of the catalyst, as well as the catalyst regeneration procedure, are under current research, and severely depend on the real composition of
DOC. It was reported (Parent et al. 1996) that humic acids reduce the activity of titanium dioxide toward the target substances, but it was recently shown that the mineralization process still occurs efficiently and chloride evolution ensures a complete
dehalogenation of the chlorinated compounds (Sega et al. 1999). The photocatalytic
DOC oxidation seems effective on macromolecular components isolated by ultrafiltration of sea water (Abdullah and Eek 1996).
Simple inorganic ions, such as chloride, bromide, phosphate and sulfate have been
found to reduce the photocatalytic performances (Abdullah et al.1990). Recent studies
have shown the formation of halogenated (Cl and Br) organics if photocatalytic degradation occurs in the presence of the correspondent halides (Minero et al. 1997b). This
is of major importance for marine DOC measurements. However, Matthews et al. (1990)
reported that the inhibition is not dramatic when working with sea water at acidic pH.