Some authors have suggested more complex reaction mechanisms for hetero-EF
when H 2 O 2 is not cathodically produced. Figure 11.6a schematizes the steps proposed for the removal of humic acids (HAs) and Cr(VI) in a BDD/Pt cell using
Pd/Fe 3 O 4 as catalyst (Huang et al. 2017). These include: (i) H 2 and O 2 gas generation
at the cathode and anode, respectively, (ii) diffusion of O 2 to the Pd/Fe 3 O 4
nanoparticles to be reduced to the oxidant O 2
•- , (iii) H 2 O 2 formation from the
reaction between H 2 and O 2 at the catalyst surface, (iv) generation of heterogeneous
•
OH from heterogeneous Fenton’s reaction (Eq. 11.8) between Fe
II at the Pd/Fe 3 O 4
surface and produced H 2 O 2 , with subsequent Fe
II regeneration via the reaction in
Eq. 11.9, and (v)
•
OH formation at the BDD surface from a reaction similar to the
one in Eq. 11.3. HAs are then oxidized by O 2
•- and
•
OH. According to step 6 shown
in Fig. 11.6a, Cr(VI) is reduced to Cr(III) at the cathode, and by atomic H at the
nanocatalyst surface, whereas as shown in step 7, Cr(III) can be removed upon the
deposition of chromite (FeCr 2 O 4 ). Similarly, for the treatment of phenol solutions
with Cu/C nanoparticles (Xu et al. 2013), the H 2 O 2 production from the heterogeneous reaction of H 2 and O 2 gases at the catalyst surface has been suggested. Hence,
the oxidant
• OH is formed homogeneously from Fenton’s reaction (Eq. 11.2) thanks
to the addition of Fe
2+ ion to the solution.
Several authors have also compared the homogeneous EF and hetero-EF treatments to show the excellent performance of the latter method. Figure 11.6b illustrates the normalized TOC abatement from a 0.25 mM enoxacin solution at pH 3.0
by homogeneous EF with 0.3 mM Fe
2+ and hetero-EF with 0.1 g of Fe 2 O 3 –kaolin
Table 11.4 (continued)
Nanocatalyst Substrate
Experimental remarks Best performance Ref.
catalyst, pH ¼ 3,
Pt/carbon felt cell,
I ¼ 100 mA
80% TOC decay
(360 min)
Fe–silica
Rhodamine B
750 mL of 10 mg/L
drug in 20 mg/ L
Na 2 SO 4 , 15 mg/L catalyst, pH ¼ 2, graphite/graphite cell,
E cell ¼ 8 V
98% color and
35% TOC
removals
(180 min)
Jinisha et al.
(2018)
Fe 2 O 3 –
kaolin
Enoxacin
175 mL of 0.25 mM
drug in 0.05 M
Na 2 SO 4 , 0.3 g/L catalyst, pH ¼ 5.1,
Pt/carbon felt cell,
I ¼ 100 mA
100% drug
removal
(15 min), 99%
TOC decay
(360 min)
Özcan et al.
(2017)
Cu/C
Phenol
200 mL of 2 mg/L
substrate in 0.01 M
Na 2 SO 4 , 1 g/L catalyst, 10 mg/L Fe
2+ ,
pH ¼ 3, Pt/Pt cell,
I ¼ 50 mA
80% (pH ¼ 3)
and 50% (pH ¼ 5
and 7) substrate
removals
(180 min)
Xu et al. (2013)
a E cell —potential difference between anode and cathode
278
I. Sirés and E. Brillas
when H 2 O 2 is not cathodically produced. Figure 11.6a schematizes the steps proposed for the removal of humic acids (HAs) and Cr(VI) in a BDD/Pt cell using
Pd/Fe 3 O 4 as catalyst (Huang et al. 2017). These include: (i) H 2 and O 2 gas generation
at the cathode and anode, respectively, (ii) diffusion of O 2 to the Pd/Fe 3 O 4
nanoparticles to be reduced to the oxidant O 2
•- , (iii) H 2 O 2 formation from the
reaction between H 2 and O 2 at the catalyst surface, (iv) generation of heterogeneous
•
OH from heterogeneous Fenton’s reaction (Eq. 11.8) between Fe
II at the Pd/Fe 3 O 4
surface and produced H 2 O 2 , with subsequent Fe
II regeneration via the reaction in
Eq. 11.9, and (v)
•
OH formation at the BDD surface from a reaction similar to the
one in Eq. 11.3. HAs are then oxidized by O 2
•- and
•
OH. According to step 6 shown
in Fig. 11.6a, Cr(VI) is reduced to Cr(III) at the cathode, and by atomic H at the
nanocatalyst surface, whereas as shown in step 7, Cr(III) can be removed upon the
deposition of chromite (FeCr 2 O 4 ). Similarly, for the treatment of phenol solutions
with Cu/C nanoparticles (Xu et al. 2013), the H 2 O 2 production from the heterogeneous reaction of H 2 and O 2 gases at the catalyst surface has been suggested. Hence,
the oxidant
• OH is formed homogeneously from Fenton’s reaction (Eq. 11.2) thanks
to the addition of Fe
2+ ion to the solution.
Several authors have also compared the homogeneous EF and hetero-EF treatments to show the excellent performance of the latter method. Figure 11.6b illustrates the normalized TOC abatement from a 0.25 mM enoxacin solution at pH 3.0
by homogeneous EF with 0.3 mM Fe
2+ and hetero-EF with 0.1 g of Fe 2 O 3 –kaolin
Table 11.4 (continued)
Nanocatalyst Substrate
Experimental remarks Best performance Ref.
catalyst, pH ¼ 3,
Pt/carbon felt cell,
I ¼ 100 mA
80% TOC decay
(360 min)
Fe–silica
Rhodamine B
750 mL of 10 mg/L
drug in 20 mg/ L
Na 2 SO 4 , 15 mg/L catalyst, pH ¼ 2, graphite/graphite cell,
E cell ¼ 8 V
98% color and
35% TOC
removals
(180 min)
Jinisha et al.
(2018)
Fe 2 O 3 –
kaolin
Enoxacin
175 mL of 0.25 mM
drug in 0.05 M
Na 2 SO 4 , 0.3 g/L catalyst, pH ¼ 5.1,
Pt/carbon felt cell,
I ¼ 100 mA
100% drug
removal
(15 min), 99%
TOC decay
(360 min)
Özcan et al.
(2017)
Cu/C
Phenol
200 mL of 2 mg/L
substrate in 0.01 M
Na 2 SO 4 , 1 g/L catalyst, 10 mg/L Fe
2+ ,
pH ¼ 3, Pt/Pt cell,
I ¼ 50 mA
80% (pH ¼ 3)
and 50% (pH ¼ 5
and 7) substrate
removals
(180 min)
Xu et al. (2013)
a E cell —potential difference between anode and cathode
278
I. Sirés and E. Brillas