11.5 Nanomaterials in Hybrid Processes
Some other hybrid processes have been described to upgrade the oxidation ability of
the EF and PEF methods (Brillas and Sirés 2018). However, the use of nanomaterials
is rather limited to date, mainly being reduced to different kinds of carbon materials
as adsorbents. Recently, Zhao et al. (2017) proposed a hybrid method involving
electrosorption/EF to treat 100 mL of an O 2 -saturated solution with 50 mg/L
dimethyl phthalate in 0.05 M Na 2 SO 4 at natural pH 6 using an undivided cell with
a Pt anode and an Fe-doped carbon aerogel with Gr sheets as cathode at I ¼ 135 mA.
It was found that about 90% of the substrate was adsorbed on the cathode surface,
whereas 98% of it was removed after 150 min of hetero-EF. The authors suggested a
degradation mechanism involving the destruction of physisorbed dimethyl phthalate
with heterogeneous
•
OH formed from the reaction in Eq. 11.8. Wang et al. (2018)
synthesized nanomaterials like La 1-x Nd x FeO 3 @activated carbon as anodes with a
high adsorption ability. In only 10 min of EF treatment of Methyl Orange at pH 2.0
using GDE as cathode at I ¼ 60 min, 99% color and 97% chemical oxygen demand
(COD) was found, attaining 17% and 36% removal of such by direct adsorption at
the same time. Direct removal of the adsorbed dye by heterogeneous
•
OH formed
from water oxidation at the anode and homogeneous
•
OH from Fenton’s reaction
(Eq. 11.2) between the anodically leached Fe
2+ and the produced H 2 O 2 was proposed as the main degradation mechanism.
11.6 Conclusions
This chapter has reviewed the main current developments in the synthesis, modification, use, and reuse of key nanomaterials that enhance the performance of EF and
PEF processes for the removal of organic pollutants from water. Since Fenton’s
reaction relies on the decomposition of H 2 O 2 upon the reaction with Fe(II) to
produce large amounts of
•
OH in the bulk solution, efforts are particularly concentrated on new Fe-based nanocatalysts. We believe that research should especially
focus on nanomaterials used as suspended catalysts since catalysts supported on the
carbonaceous cathode surface tend to be detrimental for the two-electron ORR, and
they may affect the material’s stability and diminish the exposed carbon surface area
for H 2 O 2 electrogeneration. Investigation into potential Fe-based catalysts should
pay special attention to their robustness, reducing the iron leaching to ensure a
minimum secondary contamination and a greater reusability with reproducible
performance. The use of MOFs in heterogeneous electrochemical Fenton-based
EAOPs is forecast as a relevant topic in the near future. Novel cathodes, particularly
those based on non-metal and non-ferrous nanocatalysts, may become the material
of choice to electrogenerate H 2 O 2 on site, and new electrode configurations might
contribute to a significant improvement of such in situ electrosynthesis. The preparation of all these mentioned catalysts should, hopefully, be based on green synthesis
280
I. Sirés and E. Brillas
Some other hybrid processes have been described to upgrade the oxidation ability of
the EF and PEF methods (Brillas and Sirés 2018). However, the use of nanomaterials
is rather limited to date, mainly being reduced to different kinds of carbon materials
as adsorbents. Recently, Zhao et al. (2017) proposed a hybrid method involving
electrosorption/EF to treat 100 mL of an O 2 -saturated solution with 50 mg/L
dimethyl phthalate in 0.05 M Na 2 SO 4 at natural pH 6 using an undivided cell with
a Pt anode and an Fe-doped carbon aerogel with Gr sheets as cathode at I ¼ 135 mA.
It was found that about 90% of the substrate was adsorbed on the cathode surface,
whereas 98% of it was removed after 150 min of hetero-EF. The authors suggested a
degradation mechanism involving the destruction of physisorbed dimethyl phthalate
with heterogeneous
•
OH formed from the reaction in Eq. 11.8. Wang et al. (2018)
synthesized nanomaterials like La 1-x Nd x FeO 3 @activated carbon as anodes with a
high adsorption ability. In only 10 min of EF treatment of Methyl Orange at pH 2.0
using GDE as cathode at I ¼ 60 min, 99% color and 97% chemical oxygen demand
(COD) was found, attaining 17% and 36% removal of such by direct adsorption at
the same time. Direct removal of the adsorbed dye by heterogeneous
•
OH formed
from water oxidation at the anode and homogeneous
•
OH from Fenton’s reaction
(Eq. 11.2) between the anodically leached Fe
2+ and the produced H 2 O 2 was proposed as the main degradation mechanism.
11.6 Conclusions
This chapter has reviewed the main current developments in the synthesis, modification, use, and reuse of key nanomaterials that enhance the performance of EF and
PEF processes for the removal of organic pollutants from water. Since Fenton’s
reaction relies on the decomposition of H 2 O 2 upon the reaction with Fe(II) to
produce large amounts of
•
OH in the bulk solution, efforts are particularly concentrated on new Fe-based nanocatalysts. We believe that research should especially
focus on nanomaterials used as suspended catalysts since catalysts supported on the
carbonaceous cathode surface tend to be detrimental for the two-electron ORR, and
they may affect the material’s stability and diminish the exposed carbon surface area
for H 2 O 2 electrogeneration. Investigation into potential Fe-based catalysts should
pay special attention to their robustness, reducing the iron leaching to ensure a
minimum secondary contamination and a greater reusability with reproducible
performance. The use of MOFs in heterogeneous electrochemical Fenton-based
EAOPs is forecast as a relevant topic in the near future. Novel cathodes, particularly
those based on non-metal and non-ferrous nanocatalysts, may become the material
of choice to electrogenerate H 2 O 2 on site, and new electrode configurations might
contribute to a significant improvement of such in situ electrosynthesis. The preparation of all these mentioned catalysts should, hopefully, be based on green synthesis
280
I. Sirés and E. Brillas