290
under the variation of its parameters. Other DOE approaches to photocatalytic reactor
optimization have also been applied over the years, such as central composite design,
Bayesian design, and Plackett–Burman designs [50, 140, 319].
Kinetics and Modeling
Kinetics and mechanistic studies on the photomineralization or photo-disinfection
rate of the water contaminants are useful for process scaleup. The appropriate utilization of kinetic models for the interpretation of experimental data enables the
design and optimization of photoreactor systems with sufficient capacity and minimal non-illuminated reactor volume. In this section, the different kinetics and rate
model for both photomineralization and photo-disinfection are discussed along with
the common misconceptions in kinetic modeling.
Photomineralization Kinetics
In most of the photocatalytic studies, kinetic or mechanistic studies over the irradiated TiO 2 surfaces usually only involve a single constituent model organic compound. The kinetics of different organic compounds ranging from dye molecules,
pesticides, herbicides, and phenolic compounds to simple alkanes, haloalkanes, aliphatic alcohols, and carboxylic acids have been investigated [19, 103, 121, 205].
The nonselective nature of the OH
•
radicals means that the disappearance rate of the
studied compound with irradiation time should not be referenced as a standard for
reactor design purpose. This is because numerous intermediates are formed en route
to complete mineralization and neglecting this aspect is a common mistake in portraying the photomineralization kinetics. In this instance, the organic concentrations
can be expressed collectively in COD or TOC to yield an in-depth understanding on
the photomineralization kinetics.
As the L–H model is surface area dependent, the reaction rate is expected to
increase with irradiation times since less organic substrate will remain after
increased irradiation times with higher surface availability. A zero rate of degradation is associated with the total decomposition achieved. Numerous assumptions for
the L–H saturation kinetics type exist and for the applicability in the rate of photomineralization, any of the four possible situations is valid: (1) reactions take place
between two adsorbed components of radicals and organics; (2) the reactions are
between the radicals in water and adsorbed organics; (3) reactions take place
between the radical on the surface and organics in water; and (4) reaction occurs
with both radical and organics in water.
Some researchers found that simpler zero- or first-order kinetics is sufficient
to model the photomineralization of organic compounds. This was, however,
only applicable for limited conditions where the solute concentration was inadequately low. In most kinetic studies, a plateau type of kinetic profile is usually
13 Wastewater
under the variation of its parameters. Other DOE approaches to photocatalytic reactor
optimization have also been applied over the years, such as central composite design,
Bayesian design, and Plackett–Burman designs [50, 140, 319].
Kinetics and Modeling
Kinetics and mechanistic studies on the photomineralization or photo-disinfection
rate of the water contaminants are useful for process scaleup. The appropriate utilization of kinetic models for the interpretation of experimental data enables the
design and optimization of photoreactor systems with sufficient capacity and minimal non-illuminated reactor volume. In this section, the different kinetics and rate
model for both photomineralization and photo-disinfection are discussed along with
the common misconceptions in kinetic modeling.
Photomineralization Kinetics
In most of the photocatalytic studies, kinetic or mechanistic studies over the irradiated TiO 2 surfaces usually only involve a single constituent model organic compound. The kinetics of different organic compounds ranging from dye molecules,
pesticides, herbicides, and phenolic compounds to simple alkanes, haloalkanes, aliphatic alcohols, and carboxylic acids have been investigated [19, 103, 121, 205].
The nonselective nature of the OH
•
radicals means that the disappearance rate of the
studied compound with irradiation time should not be referenced as a standard for
reactor design purpose. This is because numerous intermediates are formed en route
to complete mineralization and neglecting this aspect is a common mistake in portraying the photomineralization kinetics. In this instance, the organic concentrations
can be expressed collectively in COD or TOC to yield an in-depth understanding on
the photomineralization kinetics.
As the L–H model is surface area dependent, the reaction rate is expected to
increase with irradiation times since less organic substrate will remain after
increased irradiation times with higher surface availability. A zero rate of degradation is associated with the total decomposition achieved. Numerous assumptions for
the L–H saturation kinetics type exist and for the applicability in the rate of photomineralization, any of the four possible situations is valid: (1) reactions take place
between two adsorbed components of radicals and organics; (2) the reactions are
between the radicals in water and adsorbed organics; (3) reactions take place
between the radical on the surface and organics in water; and (4) reaction occurs
with both radical and organics in water.
Some researchers found that simpler zero- or first-order kinetics is sufficient
to model the photomineralization of organic compounds. This was, however,
only applicable for limited conditions where the solute concentration was inadequately low. In most kinetic studies, a plateau type of kinetic profile is usually
13 Wastewater