285
Contaminants and Their Loading
Previous investigations have reported the dependency of the TiO 2 photocatalytic
reaction rate on the concentration of water contaminants [61, 62, 247, 306]. Under
similar operating conditions, a variation in the initial concentration of water contaminants will result in different irradiation times necessary to achieve complete
mineralization or disinfection. Owing to the photonic nature of the photocatalysis
reaction, excessively high concentration of organic substrates is known to simultaneously saturate the TiO 2 surface and reduce the photonic efficiency leading to photocatalyst deactivation [281].
Not all organic substrates will have such a profound effect on the irradiation
time, and this also depends on the corresponding chemical nature of the targeted
compounds for TiO 2 photocatalysis reaction. For instance, 4-chlorophenol will
undergo a degradation pathway with constant evolution of intermediate(s) product
(i.e., hydroquinone and benzoquinone) while oxalic acid will undergo direct
transformation to carbon dioxide and water [19]. In the case of 4-chlorophenol, such
evolution of intermediate(s) will further prolong the irradiation time necessary for
total mineralization owing to the direct competition over unselective TiO 2 surfaces.
In the development of mathematical model that represents the kinetics of mineralization while relating to the TiO 2 loading required, commonly used water quality
parameters such as chemical oxygen demand (COD), total organic carbon (TOC),
or dissolved organic carbon (DOC) could be more appropriate to account for such
competitiveness of intermediate(s) with its predecessor compounds. Also organic
substrates with electron-withdrawing nature such as benzoic acid and nitrobenzene
were found to strongly adhere and to be more susceptible to direct oxidation than
those with electron-donating groups [27]. Most of the TiO 2 studies conducted to
date utilize a range of model organic substrates with different substituent groups but
these rarely convey any useful information and merely test the photo-efficiency of a
new photocatalyst or an integrated reactor column. Some field kinetics of the photomineralization of real wastewater has also been reported [206, 209, 268, 316]. It
was observed that owing to the persistency of the dissolved organic in the real
wastewater, a slow photomineralization kinetics is attained with prolonged irradiation times to achieve complete mineralization. Slow kinetics turnover of the photocatalytic water treatment as a stand-alone process means that a higher initial cost on
the reactor volume and site area is required. Recently, this heterogeneous photocatalytic technology has been coupled with biological treatment to increase its industrial
feasibility [256]. Such coupling allows the retention time in biological treatment
stages to be reduced, where the nonbiodegradable compounds of the wastewater can
be turned into biodegradable compounds with the aid of photocatalytic treatment.
Similarly, the photo-disinfection efficiency of various microorganisms has been
assessed for the possible application of photocatalytic technology to replace the
chemical disinfectant methods. In general, the mechanism involved in the microbial
disinfection includes the destruction of the microbial protein structures and inhibition of their enzymatic activities [207]. Compared to the persistency during organic
Recent Developments in Photocatalytic Water Treatment Technology
Contaminants and Their Loading
Previous investigations have reported the dependency of the TiO 2 photocatalytic
reaction rate on the concentration of water contaminants [61, 62, 247, 306]. Under
similar operating conditions, a variation in the initial concentration of water contaminants will result in different irradiation times necessary to achieve complete
mineralization or disinfection. Owing to the photonic nature of the photocatalysis
reaction, excessively high concentration of organic substrates is known to simultaneously saturate the TiO 2 surface and reduce the photonic efficiency leading to photocatalyst deactivation [281].
Not all organic substrates will have such a profound effect on the irradiation
time, and this also depends on the corresponding chemical nature of the targeted
compounds for TiO 2 photocatalysis reaction. For instance, 4-chlorophenol will
undergo a degradation pathway with constant evolution of intermediate(s) product
(i.e., hydroquinone and benzoquinone) while oxalic acid will undergo direct
transformation to carbon dioxide and water [19]. In the case of 4-chlorophenol, such
evolution of intermediate(s) will further prolong the irradiation time necessary for
total mineralization owing to the direct competition over unselective TiO 2 surfaces.
In the development of mathematical model that represents the kinetics of mineralization while relating to the TiO 2 loading required, commonly used water quality
parameters such as chemical oxygen demand (COD), total organic carbon (TOC),
or dissolved organic carbon (DOC) could be more appropriate to account for such
competitiveness of intermediate(s) with its predecessor compounds. Also organic
substrates with electron-withdrawing nature such as benzoic acid and nitrobenzene
were found to strongly adhere and to be more susceptible to direct oxidation than
those with electron-donating groups [27]. Most of the TiO 2 studies conducted to
date utilize a range of model organic substrates with different substituent groups but
these rarely convey any useful information and merely test the photo-efficiency of a
new photocatalyst or an integrated reactor column. Some field kinetics of the photomineralization of real wastewater has also been reported [206, 209, 268, 316]. It
was observed that owing to the persistency of the dissolved organic in the real
wastewater, a slow photomineralization kinetics is attained with prolonged irradiation times to achieve complete mineralization. Slow kinetics turnover of the photocatalytic water treatment as a stand-alone process means that a higher initial cost on
the reactor volume and site area is required. Recently, this heterogeneous photocatalytic technology has been coupled with biological treatment to increase its industrial
feasibility [256]. Such coupling allows the retention time in biological treatment
stages to be reduced, where the nonbiodegradable compounds of the wastewater can
be turned into biodegradable compounds with the aid of photocatalytic treatment.
Similarly, the photo-disinfection efficiency of various microorganisms has been
assessed for the possible application of photocatalytic technology to replace the
chemical disinfectant methods. In general, the mechanism involved in the microbial
disinfection includes the destruction of the microbial protein structures and inhibition of their enzymatic activities [207]. Compared to the persistency during organic
Recent Developments in Photocatalytic Water Treatment Technology