300
either slurry or fixed-bed configuration to treat real waters with different inorganic
ions, photocatalyst deactivation was usually observed. This resulted from a strong
inhibition from the inorganic ions on the surface of the TiO 2 semiconductor used.
Thus, the presence of these inorganic ions together with their permissible levels on
the photocatalytic performance of TiO 2 in water treatment has to be determined to
ensure minimal disturbances on the efficient operation of the TiO 2 -based treatment
process. With such data cost-effective fouling prevention with inorganic ions and
photocatalyst regeneration strategies can be customized.
A number of studies have been conducted on the effects of different inorganic
anions or cations on both TiO 2 photomineralization and photo-disinfection reactions
[46, 114, 116, 175, 251, 273, 275, 282, 320, 329]. It must be emphasized, however,
that most of these studies have concentrated on how different inorganic ions affect
the rates of photocatalytic reactions with a model surrogate organic compound. The
surrogate model organic compound can be biased toward its photocatalytic performance owing to its underlying chemical properties and the main constituent groups
that form the compound. For instance, the model compounds with either electronwithdrawing or -donating groups will contribute to a different degree of interaction
to degradation pathways [121]. Due to the zwitterionic nature of the TiO 2 particles
used, it is also possible that the operating pH might have a profound effect on the
selective inhibition of inorganic ions on the surface of the TiO 2 particles [114].
However, few discussions have been centered on considering how the chemical
nature of the model organic used and the operating pH in the photocatalytic reactor
impact the inhibition of different inorganic ions in photocatalytic water treatment.
To date, the effects of both inorganic cations (i.e., Na
+
, K
+
, Ca
2+
, Cu
2+
, Mn
2+
,
Mg
2+
, Ni
2+
, Fe
2+
, Zn
2+
, Al
3+
) and inorganic anions (i.e., Cl
−
, NO
3−
, HCO
3−
, ClO
4−
,
SO 4
2−
, HPO 4
2−
, PO 4
3−
) on the photocatalytic water treatment have been investigated
[46, 114, 116, 175, 251, 273, 275, 282, 320, 328]. A general consensus from these
studies concludes that Cu
2+
, Fe
2+
, Al
3+
, Cl
−
, and PO 4
3−
at certain levels may decrease
photomineralization reaction rates while Ca
2+
, Mg
2+
, and Zn
2+
may have negligible
effects. This is because Ca
2+
, Mg
2+
, and Zn
2+
are at their maximum oxidation states
resulting in their inability to inhibit the photocatalysis reaction. The presence of Fe
2+
can catalyze both the Fenton and photo-Fenton reactions. However Choi et al. [58]
observed that Fe
2+
fouled the photocatalyst surface by introducing a rusty orange
color change via the formation of Fe(OH) 3 , while PO 4
3−
in the nominal pH range
remains strongly adsorbed onto the TiO 2 surface and further inhibits its photoactivity [1, 153]. Some research groups observed that NO 3
−
, SO 4
2−
, ClO 4
−
, and HCO 3
−
inhibit the surface activity of the photocatalysts, while others suggest no such
impact. Both NO 3
−
and SO 4
2−
have detrimental effect on the photo-disinfection rate
[116]. The Cu
2+
can enhance the photocatalytic activity at its concentration up to
0.1 mM, while further increases in its concentration reduce the reaction rate [248].
Nitrogen-containing molecules are mineralized into NH 4
+
and mostly NO 3
−
ammonium ions are relatively stable and their proportion depends mainly on the initial
oxidation degree of nitrogen on the irradiation time. Pollutants containing sulfur
atoms are mineralized into sulfate ions. Cl
−
has no inhibition on the photocatalytic
degradation of trichloroethylene at a concentration up to 3.0 mM. On the contrary,
13 Wastewater
either slurry or fixed-bed configuration to treat real waters with different inorganic
ions, photocatalyst deactivation was usually observed. This resulted from a strong
inhibition from the inorganic ions on the surface of the TiO 2 semiconductor used.
Thus, the presence of these inorganic ions together with their permissible levels on
the photocatalytic performance of TiO 2 in water treatment has to be determined to
ensure minimal disturbances on the efficient operation of the TiO 2 -based treatment
process. With such data cost-effective fouling prevention with inorganic ions and
photocatalyst regeneration strategies can be customized.
A number of studies have been conducted on the effects of different inorganic
anions or cations on both TiO 2 photomineralization and photo-disinfection reactions
[46, 114, 116, 175, 251, 273, 275, 282, 320, 329]. It must be emphasized, however,
that most of these studies have concentrated on how different inorganic ions affect
the rates of photocatalytic reactions with a model surrogate organic compound. The
surrogate model organic compound can be biased toward its photocatalytic performance owing to its underlying chemical properties and the main constituent groups
that form the compound. For instance, the model compounds with either electronwithdrawing or -donating groups will contribute to a different degree of interaction
to degradation pathways [121]. Due to the zwitterionic nature of the TiO 2 particles
used, it is also possible that the operating pH might have a profound effect on the
selective inhibition of inorganic ions on the surface of the TiO 2 particles [114].
However, few discussions have been centered on considering how the chemical
nature of the model organic used and the operating pH in the photocatalytic reactor
impact the inhibition of different inorganic ions in photocatalytic water treatment.
To date, the effects of both inorganic cations (i.e., Na
+
, K
+
, Ca
2+
, Cu
2+
, Mn
2+
,
Mg
2+
, Ni
2+
, Fe
2+
, Zn
2+
, Al
3+
) and inorganic anions (i.e., Cl
−
, NO
3−
, HCO
3−
, ClO
4−
,
SO 4
2−
, HPO 4
2−
, PO 4
3−
) on the photocatalytic water treatment have been investigated
[46, 114, 116, 175, 251, 273, 275, 282, 320, 328]. A general consensus from these
studies concludes that Cu
2+
, Fe
2+
, Al
3+
, Cl
−
, and PO 4
3−
at certain levels may decrease
photomineralization reaction rates while Ca
2+
, Mg
2+
, and Zn
2+
may have negligible
effects. This is because Ca
2+
, Mg
2+
, and Zn
2+
are at their maximum oxidation states
resulting in their inability to inhibit the photocatalysis reaction. The presence of Fe
2+
can catalyze both the Fenton and photo-Fenton reactions. However Choi et al. [58]
observed that Fe
2+
fouled the photocatalyst surface by introducing a rusty orange
color change via the formation of Fe(OH) 3 , while PO 4
3−
in the nominal pH range
remains strongly adsorbed onto the TiO 2 surface and further inhibits its photoactivity [1, 153]. Some research groups observed that NO 3
−
, SO 4
2−
, ClO 4
−
, and HCO 3
−
inhibit the surface activity of the photocatalysts, while others suggest no such
impact. Both NO 3
−
and SO 4
2−
have detrimental effect on the photo-disinfection rate
[116]. The Cu
2+
can enhance the photocatalytic activity at its concentration up to
0.1 mM, while further increases in its concentration reduce the reaction rate [248].
Nitrogen-containing molecules are mineralized into NH 4
+
and mostly NO 3
−
ammonium ions are relatively stable and their proportion depends mainly on the initial
oxidation degree of nitrogen on the irradiation time. Pollutants containing sulfur
atoms are mineralized into sulfate ions. Cl
−
has no inhibition on the photocatalytic
degradation of trichloroethylene at a concentration up to 3.0 mM. On the contrary,
13 Wastewater