Topics in Current Chemistry (2020) 378:7
1 3
rate of the water inside the reactors would require a catalyst to be immobilized to
avoid powdered catalyst sedimentation.
In summary, there are tangible benefits in risk reduction with tertiary treating to
a high standard, especially for the case of wastewater for agricultural reuse. The ripest area for development is the combination of cheap raceway pond reactors with
immobilized solar-active photocatalysts being employed as the need arises to alleviate temporal water scarcity bouts.
Even at equivalent outcomes, a consolidated system is always favorable to a
newer system. Thus, for HPC to be competitive and turn the tide on other AOPs,
it does not need to simply match the benefits of currently employed AOPs, but to
exceed them. A possible strong point for immobilized solar HPC is that it would not
require additional material input, and would require low energy input only for mixing, as opposed to the more consolidated ozonation or Fenton processes which have
these requirements.
Another pathway that can lead to increased acceptance of photocatalysis as
a treatment is by combining it with other treatments. There are two main ways to
combine treatments: simultaneously and in cascade/sequentially. The most common simultaneous use of HPC is its coupling with ozonation. This type of treatment was recently reviewed [134]. The premise of such a combination is that the
combined process is more efficient than either of the separate processes, by means
of additional pathways that lead to ROS formation through the interaction of the
photocatalyst and dissolved ozone as well as direct reactions with molecular ozone
[134]. Sequential applications of HPC and other water treatments are not at all well
studied. Examples exist of other AOPs that are applied in combination with or after
another treatment such as chlorination exist [135, 136], but no suitable examples for
HPC have been noted. The benefit of using HPC in a cascade with other treatments
includes the fact that the intensity of treatments can be lower than what would be
used individually; for example, if chlorination is to be used prior to a photocatalytic
process, it is possible to reduce the quantity of chlorine applied and thus reduce the
concentration of chlorinated by-products, while meeting the same targets that would
not be possible with only one of the sequential treatments. An additional benefit, due
to the different oxidation mechanisms, is that one process may be more active on a
certain type of contaminant, while the other process is more active on another type;
thus when used in cascade, the overall efficiency is higher.
A different approach for improving photocatalytic performance is to reduce the
recombination of electron–hole pairs. A particularly interesting solution, in contrast to doping with metals, is represented by the modification (or combination) of
the semiconductor with graphitic materials [137]. These materials are interesting
because they have excellent electrical conductivity and are considered promising
materials for photocatalysis [138]. In recent years, graphene-based photocatalytic
processes have also been implemented through the combination of photocatalysis with other techniques. In particular, several studies report interesting results
regarding the application of photoelectrocatalysis as an effective process used to
suppress the recombination of charges [137]. In an photoelectrocatalytic process,
an external bias potential is used. The electrons generated are accumulated on
the external cathode and the holes on the photocatalyst. The electrodes used in
244
Reprinted from the journal
1 3
rate of the water inside the reactors would require a catalyst to be immobilized to
avoid powdered catalyst sedimentation.
In summary, there are tangible benefits in risk reduction with tertiary treating to
a high standard, especially for the case of wastewater for agricultural reuse. The ripest area for development is the combination of cheap raceway pond reactors with
immobilized solar-active photocatalysts being employed as the need arises to alleviate temporal water scarcity bouts.
Even at equivalent outcomes, a consolidated system is always favorable to a
newer system. Thus, for HPC to be competitive and turn the tide on other AOPs,
it does not need to simply match the benefits of currently employed AOPs, but to
exceed them. A possible strong point for immobilized solar HPC is that it would not
require additional material input, and would require low energy input only for mixing, as opposed to the more consolidated ozonation or Fenton processes which have
these requirements.
Another pathway that can lead to increased acceptance of photocatalysis as
a treatment is by combining it with other treatments. There are two main ways to
combine treatments: simultaneously and in cascade/sequentially. The most common simultaneous use of HPC is its coupling with ozonation. This type of treatment was recently reviewed [134]. The premise of such a combination is that the
combined process is more efficient than either of the separate processes, by means
of additional pathways that lead to ROS formation through the interaction of the
photocatalyst and dissolved ozone as well as direct reactions with molecular ozone
[134]. Sequential applications of HPC and other water treatments are not at all well
studied. Examples exist of other AOPs that are applied in combination with or after
another treatment such as chlorination exist [135, 136], but no suitable examples for
HPC have been noted. The benefit of using HPC in a cascade with other treatments
includes the fact that the intensity of treatments can be lower than what would be
used individually; for example, if chlorination is to be used prior to a photocatalytic
process, it is possible to reduce the quantity of chlorine applied and thus reduce the
concentration of chlorinated by-products, while meeting the same targets that would
not be possible with only one of the sequential treatments. An additional benefit, due
to the different oxidation mechanisms, is that one process may be more active on a
certain type of contaminant, while the other process is more active on another type;
thus when used in cascade, the overall efficiency is higher.
A different approach for improving photocatalytic performance is to reduce the
recombination of electron–hole pairs. A particularly interesting solution, in contrast to doping with metals, is represented by the modification (or combination) of
the semiconductor with graphitic materials [137]. These materials are interesting
because they have excellent electrical conductivity and are considered promising
materials for photocatalysis [138]. In recent years, graphene-based photocatalytic
processes have also been implemented through the combination of photocatalysis with other techniques. In particular, several studies report interesting results
regarding the application of photoelectrocatalysis as an effective process used to
suppress the recombination of charges [137]. In an photoelectrocatalytic process,
an external bias potential is used. The electrons generated are accumulated on
the external cathode and the holes on the photocatalyst. The electrodes used in
244
Reprinted from the journal