Topics in Current Chemistry (2020) 378:7
1 3
pre-oxidation step before a biological process, it has some drawbacks. One of the
main limitations is that the process relies on acidic operating conditions (optimal
at pH < 3) to avoid catalyst precipitation. Thus conventional homogeneous photoFenton can improve process efficiency but does not solve pH-related problems
(pH pre-acidification, pH post-neutralization, and sludge production, treatment,
and disposal). This restriction makes this option attractive only for the treatment of acidic wastewater, because application to neutral or alkaline wastewater
would increase operating costs. The improvement of photo-Fenton performance
under neutral conditions has been a subject of high interest among the scientific
community in recent years. In particular, photo-Fenton has been investigated for
the removal of CECs from urban wastewater and inactivation of bacteria, and
its operation under mild conditions (pH 5–6) resulted in good efficiency [159,
160]. Other possible approaches include the use of heterogeneous and homogeneous (photo)-Fenton-like processes. In homogeneous processes, Fe
2+
is replaced
by other metals or possibly combined with organic or inorganic ligands to form
complexes and/or to stabilize the metals over a wide pH range [161]. Different
ligands including nitriloacetic acid, ethylenediaminetetraacetic acid, oxalic acid,
tartaric acid [162], and ethylenediamine-N-N0-disuccinic acid [163], as well as
metal–organic complexes, have been investigated so far. Another option for overcoming homogeneous photo-Fenton drawbacks includes heterogeneous catalytic
systems based on macroscopic supports (such as corundum, cordierite) [164].
Heterogeneous photo-Fenton methods have been investigated for the removal
of dyes from aqueous matrices [165] through the immobilization of Fe ions on
clays, bentonite, and laponite [166], or by using iron oxides such as goethite or
hematite under non-controlled pH conditions [167]. To overcome the drawback
of powder catalyst removal after treatment, macroscopic supports appear to be an
attractive alternative. Structured catalysts can be purposefully designed to optimize fluid dynamics. The application of such a catalyst (i.e., LaFeO 3 loaded on
corundum honeycomb monolithic support) for the removal of food azo dyes from
aqueous solutions resulted in complete discoloration and mineralization (evaluated in terms of TOC removal) of two food dyes [Allura Red (RED) and tartrazine (TRZ)] (Fig. 5); notably, the process proved to be extremely efficient at
spontaneous pH values (equal to 6) [67].
Among food industrial wastewater, dairy wastewater is a category not easily managed through conventional biological process alone [168]. While it does not contain
particularly toxic substances, dairy wastewater is characterized by high organic content including fats and proteins, which can decompose. Their treatment is generally
managed through biological processes, but several problems have been noted related
to operating pH, variations in the organic loading, and high sludge volume produced
[169]. Some attempts to treat such wastewater by HPC have been carried out but
were not so successful.
HPC was combined with a flocculation process to remove COD and inactivate
bacteria from dairy wastewater [170]. Although a TiO 2 P25 Evonik photocatalyst
resulted in total inactivation of E. Coli after 5 h of treatment (120 W/m
2
of UV–Vis
light intensity), an increase in COD and TOC was also observed, which the authors
attributed to the formation of organic oxidation intermediaries.
248
Reprinted from the journal
1 3
pre-oxidation step before a biological process, it has some drawbacks. One of the
main limitations is that the process relies on acidic operating conditions (optimal
at pH < 3) to avoid catalyst precipitation. Thus conventional homogeneous photoFenton can improve process efficiency but does not solve pH-related problems
(pH pre-acidification, pH post-neutralization, and sludge production, treatment,
and disposal). This restriction makes this option attractive only for the treatment of acidic wastewater, because application to neutral or alkaline wastewater
would increase operating costs. The improvement of photo-Fenton performance
under neutral conditions has been a subject of high interest among the scientific
community in recent years. In particular, photo-Fenton has been investigated for
the removal of CECs from urban wastewater and inactivation of bacteria, and
its operation under mild conditions (pH 5–6) resulted in good efficiency [159,
160]. Other possible approaches include the use of heterogeneous and homogeneous (photo)-Fenton-like processes. In homogeneous processes, Fe
2+
is replaced
by other metals or possibly combined with organic or inorganic ligands to form
complexes and/or to stabilize the metals over a wide pH range [161]. Different
ligands including nitriloacetic acid, ethylenediaminetetraacetic acid, oxalic acid,
tartaric acid [162], and ethylenediamine-N-N0-disuccinic acid [163], as well as
metal–organic complexes, have been investigated so far. Another option for overcoming homogeneous photo-Fenton drawbacks includes heterogeneous catalytic
systems based on macroscopic supports (such as corundum, cordierite) [164].
Heterogeneous photo-Fenton methods have been investigated for the removal
of dyes from aqueous matrices [165] through the immobilization of Fe ions on
clays, bentonite, and laponite [166], or by using iron oxides such as goethite or
hematite under non-controlled pH conditions [167]. To overcome the drawback
of powder catalyst removal after treatment, macroscopic supports appear to be an
attractive alternative. Structured catalysts can be purposefully designed to optimize fluid dynamics. The application of such a catalyst (i.e., LaFeO 3 loaded on
corundum honeycomb monolithic support) for the removal of food azo dyes from
aqueous solutions resulted in complete discoloration and mineralization (evaluated in terms of TOC removal) of two food dyes [Allura Red (RED) and tartrazine (TRZ)] (Fig. 5); notably, the process proved to be extremely efficient at
spontaneous pH values (equal to 6) [67].
Among food industrial wastewater, dairy wastewater is a category not easily managed through conventional biological process alone [168]. While it does not contain
particularly toxic substances, dairy wastewater is characterized by high organic content including fats and proteins, which can decompose. Their treatment is generally
managed through biological processes, but several problems have been noted related
to operating pH, variations in the organic loading, and high sludge volume produced
[169]. Some attempts to treat such wastewater by HPC have been carried out but
were not so successful.
HPC was combined with a flocculation process to remove COD and inactivate
bacteria from dairy wastewater [170]. Although a TiO 2 P25 Evonik photocatalyst
resulted in total inactivation of E. Coli after 5 h of treatment (120 W/m
2
of UV–Vis
light intensity), an increase in COD and TOC was also observed, which the authors
attributed to the formation of organic oxidation intermediaries.
248
Reprinted from the journal