301
the addition of oxyanion oxidants such as ClO 2
−
, ClO 3
−
, IO 4
−
, S 2 O 8
−
, and BrO 3
−
increased photoreactivity by scavenging conduction-band electrons and reducing
the charge-carrier recombination [208]. The presence of salts is also known to
diminish the colloidal stability as screening effects become more profound. This
was followed by the double-layer compression and surface charge neutralization,
which increases the mass transfer limitations and reduces surface contacts between
the pollutants and catalysts. Other inorganic ions also affect photodegradation rates,
where the presence of SO 4
2−
in a TiO 2 -coated glass spiral reactor could double the
disappearance rate of the pesticide monocrotophos [359]. The Mn
2+
improved the
photoactivity by simultaneously increasing the electron/hole pairs and preventing
their recombination [233].
Several mechanisms for fouling effects of inorganic ions on TiO 2 photoactivity
have been proposed [36, 279]. These include UV screening, competitive adsorption
to surface-active sites, competition for photons, surface deposition of precipitates
and elemental metals, radical and hole scavenging, and direct reaction with the photocatalyst. The NO 3
−
ion was reported to UV screen the photocatalyst than inhibit
the TiO 2 surface [36]. The competition for surface-active sites involves the constant
displacement of hydroxide ions from TiO 2 surface and thus further reduces the
generation of radicals. Quantum yield is reduced as a result of the direct competition of inorganic ions for light photons (lowered number of photons entering the
reactor). A similar decrease in photonic efficiency was observed when precipitates
were formed and deposited onto the TiO 2 surface, blocking the accessibility of both
photons and organic compounds. The ultimate inorganic anions that were determined to scavenge both the hole and radicals include Cl
−
, HCO 3
−
, SO 4
2−
, and PO 4
3−
[79]. A mechanism for Cl
−
and HCO 3
−
in inhibiting photocatalysis via hydroxyl
radical and hole scavenging was proposed by Matthews and McEnvoy [213] and
Lindner et al. [187], respectively:
Cl OH Cl OH
−
−
+
→ +
(13.47)
Cl h
Cl
−
+
+ →
(13.48)
The Cl
−
accounted for its inhibitory effect on TiO 2 photocatalysis through a preferential adsorption displacement mechanism over the surface-bound OH
−
ions. This
reduces the number of OH
−
ions available on the TiO 2 surface, and the substituted
Cl
−
further increases the recombination of electron/hole pairs. Among other chlorinated molecules, Cl
−
ions are readily released in the solution. This effect could be of
benefit in a process where photocatalysis is associated with a biological depuration
system, which is generally not efficient for chlorinated compounds [121]. Other
ions such as PO 4
3−
are known to avert adsorption of amino acids over the TiO 2 catalysts, while CO 3
2−
and other ionic species react with OH
−
radicals to compete with
microorganisms and further reduce the efficiency. Thus, the presence of inorganic
ions in water subjected to TiO 2 photocatalytic treatment is an important factor in
determining its successful implementation.
Recent Developments in Photocatalytic Water Treatment Technology
the addition of oxyanion oxidants such as ClO 2
−
, ClO 3
−
, IO 4
−
, S 2 O 8
−
, and BrO 3
−
increased photoreactivity by scavenging conduction-band electrons and reducing
the charge-carrier recombination [208]. The presence of salts is also known to
diminish the colloidal stability as screening effects become more profound. This
was followed by the double-layer compression and surface charge neutralization,
which increases the mass transfer limitations and reduces surface contacts between
the pollutants and catalysts. Other inorganic ions also affect photodegradation rates,
where the presence of SO 4
2−
in a TiO 2 -coated glass spiral reactor could double the
disappearance rate of the pesticide monocrotophos [359]. The Mn
2+
improved the
photoactivity by simultaneously increasing the electron/hole pairs and preventing
their recombination [233].
Several mechanisms for fouling effects of inorganic ions on TiO 2 photoactivity
have been proposed [36, 279]. These include UV screening, competitive adsorption
to surface-active sites, competition for photons, surface deposition of precipitates
and elemental metals, radical and hole scavenging, and direct reaction with the photocatalyst. The NO 3
−
ion was reported to UV screen the photocatalyst than inhibit
the TiO 2 surface [36]. The competition for surface-active sites involves the constant
displacement of hydroxide ions from TiO 2 surface and thus further reduces the
generation of radicals. Quantum yield is reduced as a result of the direct competition of inorganic ions for light photons (lowered number of photons entering the
reactor). A similar decrease in photonic efficiency was observed when precipitates
were formed and deposited onto the TiO 2 surface, blocking the accessibility of both
photons and organic compounds. The ultimate inorganic anions that were determined to scavenge both the hole and radicals include Cl
−
, HCO 3
−
, SO 4
2−
, and PO 4
3−
[79]. A mechanism for Cl
−
and HCO 3
−
in inhibiting photocatalysis via hydroxyl
radical and hole scavenging was proposed by Matthews and McEnvoy [213] and
Lindner et al. [187], respectively:
Cl OH Cl OH
−
−
+
→ +
(13.47)
Cl h
Cl
−
+
+ →
(13.48)
The Cl
−
accounted for its inhibitory effect on TiO 2 photocatalysis through a preferential adsorption displacement mechanism over the surface-bound OH
−
ions. This
reduces the number of OH
−
ions available on the TiO 2 surface, and the substituted
Cl
−
further increases the recombination of electron/hole pairs. Among other chlorinated molecules, Cl
−
ions are readily released in the solution. This effect could be of
benefit in a process where photocatalysis is associated with a biological depuration
system, which is generally not efficient for chlorinated compounds [121]. Other
ions such as PO 4
3−
are known to avert adsorption of amino acids over the TiO 2 catalysts, while CO 3
2−
and other ionic species react with OH
−
radicals to compete with
microorganisms and further reduce the efficiency. Thus, the presence of inorganic
ions in water subjected to TiO 2 photocatalytic treatment is an important factor in
determining its successful implementation.
Recent Developments in Photocatalytic Water Treatment Technology