2.5 Examples of Processes for Dyes Removal from Water
Using TiO 2 Photocatalyst
In most of processes developed for photocatalytic water treatment, TiO 2 is usually
introduced as slurry in the reactor. This can lead to several problems of filtration
and/or micro-reactor clogging and light transport in the reaction medium (BouguerLambert-Beer law) (Cambié et al. 2016). Up to now, the main drawback of industrial
TiO 2 -based advanced oxidation processes is due to postoperational separation of the
catalyst (filtration, coagulation-flocculation-sedimentation) from the reaction
medium (Wang et al. 2014a). This drawback can be overcome, thanks to processes
such as sedimentation, membrane filtration, or cross-flow filtration, which allow the
physical separation of particles from the slurry (Molinari et al. 2002a, b; FernándezIbáñez et al. 2003; Doll and Frimmel 2005). An alternative is to use a packed-bed
reactor, but in this case, the light is not able to reach the center of the reactor. Another
alternative allows overcoming such limitation by coating a thin catalyst layer on the
reactor’s walls of a continuous-flow photochemical micro-reactor.
2.5.1 Photocatalytic Membrane Reactors (PMRs)
Membrane Process
Membrane process is a separation technique extensively implemented in numerous
industries such as cosmetics, chemical, electronic, food, pharmaceutical, as well as
for desalination and wastewater treatment industries. The main advantages of
membrane-based processes lie in the low energy cost, suitability for implementing
continuous processes, low-cost maintenance, the production of stable quality of
water almost independent of the wastewater composition, and the easy scale-up by
simply increasing the number of membrane modules (Mozia 2010). On the other
hand, among the main drawbacks of such technology, one can specify the membrane
lifetime and the membrane fouling with particles and colloids present in the feed.
The two main technologies used are the pressure-driven membrane processes and
those where the concentration difference is the driving force (dialysis, pervaporation,
and direct contact membrane distillation) (Mozia 2010). In the case of pressuredriven membrane processes, the smaller the pore size is, the smaller the molecules
separated are. Pore sizes can be classified as follows: microfiltration (MF) > ultrafiltration (UF) > nanofiltration (NF) > reverse osmosis (RO). MF membranes allow the
separation of particles >0.1 μm; UF membranes are efficient for particles >2 nm, and
NF membrane allows the separation of particles <2 nm. In reverse osmosis process,
almost all salts, metal ions, and small organic molecules are separated (Koros et al.
1996). As expected, decreasing the pore size leads to an increase of the resistance of
the membrane, and consequently the pressure to apply in the process increases while
decreasing the pore size.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
65
Using TiO 2 Photocatalyst
In most of processes developed for photocatalytic water treatment, TiO 2 is usually
introduced as slurry in the reactor. This can lead to several problems of filtration
and/or micro-reactor clogging and light transport in the reaction medium (BouguerLambert-Beer law) (Cambié et al. 2016). Up to now, the main drawback of industrial
TiO 2 -based advanced oxidation processes is due to postoperational separation of the
catalyst (filtration, coagulation-flocculation-sedimentation) from the reaction
medium (Wang et al. 2014a). This drawback can be overcome, thanks to processes
such as sedimentation, membrane filtration, or cross-flow filtration, which allow the
physical separation of particles from the slurry (Molinari et al. 2002a, b; FernándezIbáñez et al. 2003; Doll and Frimmel 2005). An alternative is to use a packed-bed
reactor, but in this case, the light is not able to reach the center of the reactor. Another
alternative allows overcoming such limitation by coating a thin catalyst layer on the
reactor’s walls of a continuous-flow photochemical micro-reactor.
2.5.1 Photocatalytic Membrane Reactors (PMRs)
Membrane Process
Membrane process is a separation technique extensively implemented in numerous
industries such as cosmetics, chemical, electronic, food, pharmaceutical, as well as
for desalination and wastewater treatment industries. The main advantages of
membrane-based processes lie in the low energy cost, suitability for implementing
continuous processes, low-cost maintenance, the production of stable quality of
water almost independent of the wastewater composition, and the easy scale-up by
simply increasing the number of membrane modules (Mozia 2010). On the other
hand, among the main drawbacks of such technology, one can specify the membrane
lifetime and the membrane fouling with particles and colloids present in the feed.
The two main technologies used are the pressure-driven membrane processes and
those where the concentration difference is the driving force (dialysis, pervaporation,
and direct contact membrane distillation) (Mozia 2010). In the case of pressuredriven membrane processes, the smaller the pore size is, the smaller the molecules
separated are. Pore sizes can be classified as follows: microfiltration (MF) > ultrafiltration (UF) > nanofiltration (NF) > reverse osmosis (RO). MF membranes allow the
separation of particles >0.1 μm; UF membranes are efficient for particles >2 nm, and
NF membrane allows the separation of particles <2 nm. In reverse osmosis process,
almost all salts, metal ions, and small organic molecules are separated (Koros et al.
1996). As expected, decreasing the pore size leads to an increase of the resistance of
the membrane, and consequently the pressure to apply in the process increases while
decreasing the pore size.
2 Dyes Depollution of Water Using Porous TiO 2 -Based Photocatalysts
65