281
TiO 2 catalyst used, any variation in the operating pH is known to affect the isoelectric point or the surface charge of the photocatalyst used. Many reports have used
the point-of-zero charge (PZC) of TiO 2 to study the pH impact on the photocatalytic
oxidation performance [48, 61, 62, 247, 306]. The PZC is a condition where the
surface charge of TiO 2 is zero or neutral that lies in the pH range of 4.5–7.0, depending on the catalysts used. At PZC of TiO 2 , the interaction between the photocatalyst
particles and water contaminants is minimal due to the absence of any electrostatic
force. When operating pH < PZC(TiO 2 ), the surface charge for the catalyst becomes
positively charged and gradually exerts an electrostatic attraction force toward the
negatively charged compounds. Such polar attractions between TiO 2 and charged
anionic organic compounds can intensify the adsorption onto the photon-activated
TiO 2 surface for subsequent photocatalytic reactions [112, 275, 336]. This is particularly significant when the anionic organic compounds are present at a low
concentration level. At pH > PZC(TiO 2 ), the catalyst surface will be negatively
charged and will repulse the anionic compounds in water. Different pH will affect
the surface charge density of the TiO 2 catalyst, according to the following water
equilibrium equations (Eqs. 13.19 and 13.20):
At pH PZC TiOH H
TiOH
<
+
↔
+
+
:
2
(13.19)
At pH PZC TiOH OH
TiO H O
>
+
↔
+
−
−
:
2
(13.20)
The surface charge density distribution for these TiO 2 catalyst clusters is highly
dependent on the operating pH. It was reported that the distribution of TiOH is
≥80% at 3 < pH < 10; TiO
−
≥ 20% at pH > 10; and TiOH 2 20
+
≥ % at pH < 3. The
equilibrium constants for these reactions at different pH are p TiOH
K
2
2 4
+ = . and
pK TiOH = 8.0 [162]. During photocatalytic reaction, the initial operating pH usually
drops slightly from the formation of multitude intermediate by-products that may
pose different chemical functional groups and affect the water pH indifferently [295].
A similar electrostatic interaction enhancement for photo-disinfection of microorganisms was observed during the photocatalytic process [112]. During the photodisinfection, the initial photoinduced damage to the microorganisms takes place on
the lipopolysaccharide layer of the external cell wall and on the peptidoglycan layer.
This is followed by lipid membrane peroxidation and the subsequent oxidation of the
membrane protein and polysaccharides. An increased density of TiOH 2
+ (at low pH)
can form electrostatic link with the bacteria of negatively charged surfaces, resulting
in increasing rate of photo-disinfection. Herrera Melián et al. [119] observed that the
bacterial inactivation rate was enhanced at pH 5.0. It should be noted that the
enhanced bactericidal activity of TiO 2 at a low pH is due solely to the TiO 2 -mediated
photo-killing and not acidification of the cell. Heyde and Portalier [123] explained
that the negligible E. coli reaction to acid conditions was from the presence of an
acid tolerance response to the bacterium itself, which secreted acid- induced proteins
for acid-shock protection. However, Rincón and Pulgarin [277] did not find any differences in E. coli inactivation rates when the initial pH varied between 4.0 and 9.0.
To date, various types of microorganisms have been successfully inactivated using
Recent Developments in Photocatalytic Water Treatment Technology
TiO 2 catalyst used, any variation in the operating pH is known to affect the isoelectric point or the surface charge of the photocatalyst used. Many reports have used
the point-of-zero charge (PZC) of TiO 2 to study the pH impact on the photocatalytic
oxidation performance [48, 61, 62, 247, 306]. The PZC is a condition where the
surface charge of TiO 2 is zero or neutral that lies in the pH range of 4.5–7.0, depending on the catalysts used. At PZC of TiO 2 , the interaction between the photocatalyst
particles and water contaminants is minimal due to the absence of any electrostatic
force. When operating pH < PZC(TiO 2 ), the surface charge for the catalyst becomes
positively charged and gradually exerts an electrostatic attraction force toward the
negatively charged compounds. Such polar attractions between TiO 2 and charged
anionic organic compounds can intensify the adsorption onto the photon-activated
TiO 2 surface for subsequent photocatalytic reactions [112, 275, 336]. This is particularly significant when the anionic organic compounds are present at a low
concentration level. At pH > PZC(TiO 2 ), the catalyst surface will be negatively
charged and will repulse the anionic compounds in water. Different pH will affect
the surface charge density of the TiO 2 catalyst, according to the following water
equilibrium equations (Eqs. 13.19 and 13.20):
At pH PZC TiOH H
TiOH
<
+
↔
+
+
:
2
(13.19)
At pH PZC TiOH OH
TiO H O
>
+
↔
+
−
−
:
2
(13.20)
The surface charge density distribution for these TiO 2 catalyst clusters is highly
dependent on the operating pH. It was reported that the distribution of TiOH is
≥80% at 3 < pH < 10; TiO
−
≥ 20% at pH > 10; and TiOH 2 20
+
≥ % at pH < 3. The
equilibrium constants for these reactions at different pH are p TiOH
K
2
2 4
+ = . and
pK TiOH = 8.0 [162]. During photocatalytic reaction, the initial operating pH usually
drops slightly from the formation of multitude intermediate by-products that may
pose different chemical functional groups and affect the water pH indifferently [295].
A similar electrostatic interaction enhancement for photo-disinfection of microorganisms was observed during the photocatalytic process [112]. During the photodisinfection, the initial photoinduced damage to the microorganisms takes place on
the lipopolysaccharide layer of the external cell wall and on the peptidoglycan layer.
This is followed by lipid membrane peroxidation and the subsequent oxidation of the
membrane protein and polysaccharides. An increased density of TiOH 2
+ (at low pH)
can form electrostatic link with the bacteria of negatively charged surfaces, resulting
in increasing rate of photo-disinfection. Herrera Melián et al. [119] observed that the
bacterial inactivation rate was enhanced at pH 5.0. It should be noted that the
enhanced bactericidal activity of TiO 2 at a low pH is due solely to the TiO 2 -mediated
photo-killing and not acidification of the cell. Heyde and Portalier [123] explained
that the negligible E. coli reaction to acid conditions was from the presence of an
acid tolerance response to the bacterium itself, which secreted acid- induced proteins
for acid-shock protection. However, Rincón and Pulgarin [277] did not find any differences in E. coli inactivation rates when the initial pH varied between 4.0 and 9.0.
To date, various types of microorganisms have been successfully inactivated using
Recent Developments in Photocatalytic Water Treatment Technology