11.4 Suspended Nanocatalysts
The synthesis of nanocatalysts to be suspended in solution, with the ability to form
heterogeneous
•
OH upon the reaction with electrogenerated H 2 O 2 , has widened the
applicability of Fenton-based EAOPs for wastewater treatment through the development of hetero-EF and hetero-PEF. Similarly to what has been discussed in the
case of Fe-loaded carbon nanomaterials used as cathodes, the main advantage of
suspended nanocatalysts is that they allow operating within a larger pH range, and,
therefore, neutralization of the final effluent may be avoided (Ganiyu et al. 2018;
Poza-Nogueiras et al. 2018). This allows the minimization of sludge formation and
prevents an excessive accumulation of iron ions in the treated effluent, which are
common handicaps in the homogeneous processes. Moreover, the nanocatalyst is
easy to handle, safe to store, it can be efficiently recovered and has the possibility of
being reused (Ganiyu et al. 2018). As main drawbacks, suspended nanocatalysts
may lose some of their active sites, thus reducing their catalytic ability, and they can
undergo partial solubilization. This means that, on many occasions, there exists the
conjunction of both homogeneous and heterogeneous processes during the treatment, even if this is disregarded by many authors.
A large number of nanomaterials has been synthesized and tested for hetero-EF,
including: (i) Fe-based nanoparticles, like Fe 3 O 4 (He et al. 2014; Es’haghzade et al.
2017), Pd/Fe 3 O 4 (Luo et al. 2014; Huang et al. 2017), Fe molybdophosphate (Baiju
et al. 2018), and zero-valent iron (Babuponnusami and Muthukumar 2012);
(ii) Fe-carbon nanoparticles such as Fe-C/PTFE (Zhang et al. 2015b, c) and
Fe 3 O 4 -CNTs (Shen et al. 2014); (iii) mineral-like nanocatalysts such as martite
(Khataee et al. 2017); (iv) MOFs, only used in non-electrochemical systems so far,
as described in detail in recent reviews (Dias and Petit 2015; Cheng et al. 2018);
(v) supported Fe catalysts, such as Fe–zeolite (Rostamizadeh et al. 2018), Fe 3 O 4 –
chitosan (Rezgui et al. 2018), Fe–silica (Jinisha et al. 2018), and Fe 2 O 3 –kaolin
(Özcan et al. 2017); and (vi) non-Fe-based, like Cu/C (Xu et al. 2013). Table 11.4
collects selected results obtained in these works.
Successful degradations can be observed within the pH range 2–7, which confirms the viability of nanomaterials to operate at neutral pH—the condition required
for the treatment of wastewater under real conditions.
In the case of Fe-based nanocatalysts, the proposed heterogeneous mechanism
involves the oxidation of organics with heterogeneous
•
OH produced via the reaction
in Eq. 11.8, where H 2 O 2 is generated at a suitable carbonaceous cathode (Ganiyu
et al. 2018; Poza-Nogueiras et al. 2018). The regeneration of Fe
II at the catalyst
surface can occur either by the reaction in Eq. 11.9, when the material comes into
contact with the cathode upon stirring or recirculation, or by the heterogeneous
Fenton-like reaction in Eq. 11.18.
Fe
III
þ H 2 O 2 ! Fe
II
þ HO 2
•
þ H
þ
ð11:18Þ
276
I. Sirés and E. Brillas
The synthesis of nanocatalysts to be suspended in solution, with the ability to form
heterogeneous
•
OH upon the reaction with electrogenerated H 2 O 2 , has widened the
applicability of Fenton-based EAOPs for wastewater treatment through the development of hetero-EF and hetero-PEF. Similarly to what has been discussed in the
case of Fe-loaded carbon nanomaterials used as cathodes, the main advantage of
suspended nanocatalysts is that they allow operating within a larger pH range, and,
therefore, neutralization of the final effluent may be avoided (Ganiyu et al. 2018;
Poza-Nogueiras et al. 2018). This allows the minimization of sludge formation and
prevents an excessive accumulation of iron ions in the treated effluent, which are
common handicaps in the homogeneous processes. Moreover, the nanocatalyst is
easy to handle, safe to store, it can be efficiently recovered and has the possibility of
being reused (Ganiyu et al. 2018). As main drawbacks, suspended nanocatalysts
may lose some of their active sites, thus reducing their catalytic ability, and they can
undergo partial solubilization. This means that, on many occasions, there exists the
conjunction of both homogeneous and heterogeneous processes during the treatment, even if this is disregarded by many authors.
A large number of nanomaterials has been synthesized and tested for hetero-EF,
including: (i) Fe-based nanoparticles, like Fe 3 O 4 (He et al. 2014; Es’haghzade et al.
2017), Pd/Fe 3 O 4 (Luo et al. 2014; Huang et al. 2017), Fe molybdophosphate (Baiju
et al. 2018), and zero-valent iron (Babuponnusami and Muthukumar 2012);
(ii) Fe-carbon nanoparticles such as Fe-C/PTFE (Zhang et al. 2015b, c) and
Fe 3 O 4 -CNTs (Shen et al. 2014); (iii) mineral-like nanocatalysts such as martite
(Khataee et al. 2017); (iv) MOFs, only used in non-electrochemical systems so far,
as described in detail in recent reviews (Dias and Petit 2015; Cheng et al. 2018);
(v) supported Fe catalysts, such as Fe–zeolite (Rostamizadeh et al. 2018), Fe 3 O 4 –
chitosan (Rezgui et al. 2018), Fe–silica (Jinisha et al. 2018), and Fe 2 O 3 –kaolin
(Özcan et al. 2017); and (vi) non-Fe-based, like Cu/C (Xu et al. 2013). Table 11.4
collects selected results obtained in these works.
Successful degradations can be observed within the pH range 2–7, which confirms the viability of nanomaterials to operate at neutral pH—the condition required
for the treatment of wastewater under real conditions.
In the case of Fe-based nanocatalysts, the proposed heterogeneous mechanism
involves the oxidation of organics with heterogeneous
•
OH produced via the reaction
in Eq. 11.8, where H 2 O 2 is generated at a suitable carbonaceous cathode (Ganiyu
et al. 2018; Poza-Nogueiras et al. 2018). The regeneration of Fe
II at the catalyst
surface can occur either by the reaction in Eq. 11.9, when the material comes into
contact with the cathode upon stirring or recirculation, or by the heterogeneous
Fenton-like reaction in Eq. 11.18.
Fe
III
þ H 2 O 2 ! Fe
II
þ HO 2
•
þ H
þ
ð11:18Þ
276
I. Sirés and E. Brillas