302
To resolve TiO 2 photocatalyst fouling issue, preventative or regenerative strategies can be adopted, depending on the nature of the photocatalyst deactivation in the
water matrix. Fouling preventative strategies by means of water pretreatment, complexation, and photocatalyst surface modifications can be addressed, while rinsing
the TiO 2 surface with different chemical solutions constitutes the regenerative strategies. Water pretreatment with ion-exchange resins is one of a few methods that can
be employed to ensure minimal disturbances by inorganic ions. Burns et al. [36]
discussed that the operational cost of water pretreatment with ion-exchange resins
can be minimized if the fouling ions were identified and selectively removed. Other
preventative strategies such as complexation of fouling agents after “escorting” the
ions through the reactor can be utilized, provided that the strong fouling ions are
hard to remove from the feed water stream. Modification of the TiO 2 surface to
increase the hydrophobicity and adsorption capacity is an upstream preventative
way to enhance the rate of photodegradation in the presence of fouling ions.
However, this method is unstable and impractical compared to the others, where the
modifications are reported to be displaced away with time. As for the regenerative
strategies, different types of chemical rinsing were reported to potentially resolubilize surface deposits, precipitates, and reduced metals. The fouled ions usually do
not form strong surface complexes and can be easily displaced by an ion-exchange
rinse. Abdullah et al. [1] reported that TiO 2 fouling with SO 4
2−
and PO 4
3−
can be
displaced by NaOH, KOH, and NaHCO 3 , while Cl
−
can be easily regenerated with
water. More complex mixtures of inorganic ions need to be investigated to better
mimic real water matrices in photocatalytic water treatment or after investigation of
water matrices with one known inorganic ion composition at a time.
Heavy and Noble Metals
Heavy metals that might be present in trace amount in the wastewater stream are
highly toxic in some of their valence states [146]. Due to the pliable nature of biological treatment, these intoxicant metals can remain and permeate through the
treatment process. To treat such metals, TiO 2 photocatalytic process has been
reported to simultaneously convert these metals into nontoxic ionic states and further reduce them into their corresponding elemental form on the TiO 2 surface for
metal recovery. Prairie et al. [265] reported that metals of Ag(I), Cr(IV), Hg(II), and
Pt(II) were easily treated with TiO 2 of 0.1 wt% whereas Cd(II), Cu(II), and Ni(II)
could not be removed. The extent of such metal conversion and recovery process is
highly dependent on the standard reduction potential of the metals for the reduction
reactions. It was reported that for an efficient removal of the metals, a positive
potential of greater than 0.4 V or the flat band potential of TiO 2 was required [121,
146]. Since the rates of both oxidation (organics) and reduction (metals) on the TiO 2
surface are intrinsically interrelated, the presence of sufficient organics in the water
matrices was found to facilitate metal recovery. The redox process for the metal
reduction on the TiO 2 surface is given below:
13 Wastewater
To resolve TiO 2 photocatalyst fouling issue, preventative or regenerative strategies can be adopted, depending on the nature of the photocatalyst deactivation in the
water matrix. Fouling preventative strategies by means of water pretreatment, complexation, and photocatalyst surface modifications can be addressed, while rinsing
the TiO 2 surface with different chemical solutions constitutes the regenerative strategies. Water pretreatment with ion-exchange resins is one of a few methods that can
be employed to ensure minimal disturbances by inorganic ions. Burns et al. [36]
discussed that the operational cost of water pretreatment with ion-exchange resins
can be minimized if the fouling ions were identified and selectively removed. Other
preventative strategies such as complexation of fouling agents after “escorting” the
ions through the reactor can be utilized, provided that the strong fouling ions are
hard to remove from the feed water stream. Modification of the TiO 2 surface to
increase the hydrophobicity and adsorption capacity is an upstream preventative
way to enhance the rate of photodegradation in the presence of fouling ions.
However, this method is unstable and impractical compared to the others, where the
modifications are reported to be displaced away with time. As for the regenerative
strategies, different types of chemical rinsing were reported to potentially resolubilize surface deposits, precipitates, and reduced metals. The fouled ions usually do
not form strong surface complexes and can be easily displaced by an ion-exchange
rinse. Abdullah et al. [1] reported that TiO 2 fouling with SO 4
2−
and PO 4
3−
can be
displaced by NaOH, KOH, and NaHCO 3 , while Cl
−
can be easily regenerated with
water. More complex mixtures of inorganic ions need to be investigated to better
mimic real water matrices in photocatalytic water treatment or after investigation of
water matrices with one known inorganic ion composition at a time.
Heavy and Noble Metals
Heavy metals that might be present in trace amount in the wastewater stream are
highly toxic in some of their valence states [146]. Due to the pliable nature of biological treatment, these intoxicant metals can remain and permeate through the
treatment process. To treat such metals, TiO 2 photocatalytic process has been
reported to simultaneously convert these metals into nontoxic ionic states and further reduce them into their corresponding elemental form on the TiO 2 surface for
metal recovery. Prairie et al. [265] reported that metals of Ag(I), Cr(IV), Hg(II), and
Pt(II) were easily treated with TiO 2 of 0.1 wt% whereas Cd(II), Cu(II), and Ni(II)
could not be removed. The extent of such metal conversion and recovery process is
highly dependent on the standard reduction potential of the metals for the reduction
reactions. It was reported that for an efficient removal of the metals, a positive
potential of greater than 0.4 V or the flat band potential of TiO 2 was required [121,
146]. Since the rates of both oxidation (organics) and reduction (metals) on the TiO 2
surface are intrinsically interrelated, the presence of sufficient organics in the water
matrices was found to facilitate metal recovery. The redox process for the metal
reduction on the TiO 2 surface is given below:
13 Wastewater