Topics in Current Chemistry (2020) 378:7
1 3
attractive strategy in the context of a circular economy. The use of photocatalysis for hydrogen production is one of the most widely investigated and interesting approaches for simultaneously treating and deriving value (through renewable energy production) from wastewater [172]. For example, hydrogen recovery
through photocatalytic treatment of wastewater containing high concentrations of
sugars (particularly glucose) has been increasingly investigated [11, 173–175].
In particular, for the photocatalytic production of hydrogen, the use of suspended
and dispersed photocatalysts has unique advantages such as efficient utilization of
light energy and fast transport of pollutants between the powder surface and the
aqueous medium. An interesting solution for overcoming photocatalyst recovery
problems, while at the same time preserving the characteristics and advantages
of a slurry reactor configuration with the photocatalyst dispersed in solution, is
the use of magnetic particles as catalyst support materials [176]. For example, a
Ru-doped LaFeO 3 photocatalyst coupled with magnetic Fe 2 O 3 particles was proposed for the photocatalytic production of hydrogen (5460 μmol/L) from glucose
degradation (complete removal after 4 h treatment) under visible light irradiation [176]. Notably, the photocatalyst recovered from the photoreactor using an
external magnetic force showed high stability, and its activity did not change even
after seven cycles of use (Fig. 6).
The hydrogen production observed was competitive relative to the results of other
studies in the literature (e.g., 1580 μmol/L under UV irradiation in the presence of
ethanol as sacrificial agent and using a perovskite-based catalyst) [177]. It is not
possible to make a comparison in the case of glucose as sacrificial agent, since this
last compound has thus far been investigated only in the presence of noble metalbased catalysts [178, 179]. Moreover, the magnetic photocatalyst was also tested on
real agrifood industrial wastewater (cherry washing process), yielding significant
hydrogen production (12,344 μmol/L).
4.3.3 Tannery Wastewater
The tannery industry is one of the most productive sectors for the economies of
some countries, but also one of the most environmentally impacting processes due
to the huge consumption of water resources and chemicals and the high-polluting
wastewater [180]. Tannery wastewater is characterized by a dark brown color, high
COD and BOD 5 , and the presence of chromium(III) and phenols [181]. The application of HPC with different semiconductors has been intensively studied because
of its ability to degrade the pollutants into nontoxic molecules [182, 183]. ZnO
supported on glass spheres, through dip-coating technique without using complexing chemicals, have been effective in the discoloration and mineralization of nonbiodegradable tannery dyes under UV light irradiation, reaching discoloration and
mineralization values higher than 70% [184]. Moreover, the developed structured
photocatalyst was also effective in the treatment of real wastewater characterized
as having a high COD value (11 g/L) (Fig. 7). COD removal as high as 70% was
achieved after 180 min of UV irradiation [184].
Another reported application of ZnO-based photocatalysts (doped with rare earth
praseodymium) is in the purification of industrial wastewater from the dyeing and
250
Reprinted from the journal
1 3
attractive strategy in the context of a circular economy. The use of photocatalysis for hydrogen production is one of the most widely investigated and interesting approaches for simultaneously treating and deriving value (through renewable energy production) from wastewater [172]. For example, hydrogen recovery
through photocatalytic treatment of wastewater containing high concentrations of
sugars (particularly glucose) has been increasingly investigated [11, 173–175].
In particular, for the photocatalytic production of hydrogen, the use of suspended
and dispersed photocatalysts has unique advantages such as efficient utilization of
light energy and fast transport of pollutants between the powder surface and the
aqueous medium. An interesting solution for overcoming photocatalyst recovery
problems, while at the same time preserving the characteristics and advantages
of a slurry reactor configuration with the photocatalyst dispersed in solution, is
the use of magnetic particles as catalyst support materials [176]. For example, a
Ru-doped LaFeO 3 photocatalyst coupled with magnetic Fe 2 O 3 particles was proposed for the photocatalytic production of hydrogen (5460 μmol/L) from glucose
degradation (complete removal after 4 h treatment) under visible light irradiation [176]. Notably, the photocatalyst recovered from the photoreactor using an
external magnetic force showed high stability, and its activity did not change even
after seven cycles of use (Fig. 6).
The hydrogen production observed was competitive relative to the results of other
studies in the literature (e.g., 1580 μmol/L under UV irradiation in the presence of
ethanol as sacrificial agent and using a perovskite-based catalyst) [177]. It is not
possible to make a comparison in the case of glucose as sacrificial agent, since this
last compound has thus far been investigated only in the presence of noble metalbased catalysts [178, 179]. Moreover, the magnetic photocatalyst was also tested on
real agrifood industrial wastewater (cherry washing process), yielding significant
hydrogen production (12,344 μmol/L).
4.3.3 Tannery Wastewater
The tannery industry is one of the most productive sectors for the economies of
some countries, but also one of the most environmentally impacting processes due
to the huge consumption of water resources and chemicals and the high-polluting
wastewater [180]. Tannery wastewater is characterized by a dark brown color, high
COD and BOD 5 , and the presence of chromium(III) and phenols [181]. The application of HPC with different semiconductors has been intensively studied because
of its ability to degrade the pollutants into nontoxic molecules [182, 183]. ZnO
supported on glass spheres, through dip-coating technique without using complexing chemicals, have been effective in the discoloration and mineralization of nonbiodegradable tannery dyes under UV light irradiation, reaching discoloration and
mineralization values higher than 70% [184]. Moreover, the developed structured
photocatalyst was also effective in the treatment of real wastewater characterized
as having a high COD value (11 g/L) (Fig. 7). COD removal as high as 70% was
achieved after 180 min of UV irradiation [184].
Another reported application of ZnO-based photocatalysts (doped with rare earth
praseodymium) is in the purification of industrial wastewater from the dyeing and
250
Reprinted from the journal