However, there are also several problems with the homogeneous iron activation
of PS, i.e., the selection of an appropriate dose and the type of the iron catalyst since,
when it is applied in excess, scavenging of sulfate radicals may become problematic
(Zhao et al. 2014).
Moreover, Fe(II) can be quickly oxidized to Fe(III); although there are several
researchers claiming that the Fe(III)/PS process can be efficient, the activation
mechanism is still being questioned and investigated. For example, an interesting
theory proposed by Ike et al. (2017) explains that a possible reaction considering the
E
0 of PDS/Fe may involve Fe(III) oxidation to higher valence iron, i.e., ferrates.
However, the main problem is the low solubility of Fe(III) in the absence of suitable
complexation. Literature data indicates that, under standard conditions, the concentration of Fe(III) in solution can be less than 1.8 Â 10
À4 mmol/L. This is due to the
strong tendency of Fe(III) to hydrolyse and form hydroxide complexes (that have a
low solubility in H 2 O).
During the last eight years these problems have been thoroughly investigated and
several solutions have been found, e.g.,:
• chelation of iron for effective control of the amount of Fe(II) available to react
with PS (Rastogi et al. 2009)
• regeneration of Fe(II) by electrolysis. This technique may be very effective and
may be competitive where sustainably generated electricity is abundant and cheap
(Wacławek et al. 2016; Yuan et al. 2014)
• heterogeneous catalysis of persulfates that (in case of the zero-valent iron) provides slow release of Fe(II) (Oh et al. 2016; Xiao et al. 2018)
Especially the last point has aroused significant interest lately since the heterogeneous catalyst can often be easily separated from the reactant solution and have
other interesting properties (Oh et al. 2016; Xiao et al. 2018). Moreover, carbonaceous catalysis of PS (Chen and Carroll 2016; Duan et al. 2015; Lee et al. 2015) was
found very popular recently, however, this topic still remains controversial (Lee
et al. 2017). Several oxidants can also be heterogeneously activated by subsurface
minerals (Ahmad et al. 2010), which makes their application for, e.g., groundwater
remediation more facile.
As mentioned before, iron is one of those elements that can be incorporated to the
heterogeneous catalysis of PS. Adopting the use of solid iron particles where the
release of iron species being responsible for the activation of PDS occurred smoothly
without the risk of sulfate radical quenching was reported by Naim and Ghauch
(2016). On the other hand, Ayoub and Ghauch (2014) demonstrated that the
activation of PDS can be sustained better in solution especially while using bimetallic and trimetallic iron-based particles, which make the process more efficient in
long-term application. Such great efficiency of heterogeneous activation by solid
iron has led to investigating this material also in smaller dimensions, i.e., nanoscale
zero-valent iron (nZVI). nZVI is not a new material in the environmental chemistry
field and has already been applied to many contaminated sites in situ, where it can
9 Radical Reactions and Their Application for Water Treatment
211
of PS, i.e., the selection of an appropriate dose and the type of the iron catalyst since,
when it is applied in excess, scavenging of sulfate radicals may become problematic
(Zhao et al. 2014).
Moreover, Fe(II) can be quickly oxidized to Fe(III); although there are several
researchers claiming that the Fe(III)/PS process can be efficient, the activation
mechanism is still being questioned and investigated. For example, an interesting
theory proposed by Ike et al. (2017) explains that a possible reaction considering the
E
0 of PDS/Fe may involve Fe(III) oxidation to higher valence iron, i.e., ferrates.
However, the main problem is the low solubility of Fe(III) in the absence of suitable
complexation. Literature data indicates that, under standard conditions, the concentration of Fe(III) in solution can be less than 1.8 Â 10
À4 mmol/L. This is due to the
strong tendency of Fe(III) to hydrolyse and form hydroxide complexes (that have a
low solubility in H 2 O).
During the last eight years these problems have been thoroughly investigated and
several solutions have been found, e.g.,:
• chelation of iron for effective control of the amount of Fe(II) available to react
with PS (Rastogi et al. 2009)
• regeneration of Fe(II) by electrolysis. This technique may be very effective and
may be competitive where sustainably generated electricity is abundant and cheap
(Wacławek et al. 2016; Yuan et al. 2014)
• heterogeneous catalysis of persulfates that (in case of the zero-valent iron) provides slow release of Fe(II) (Oh et al. 2016; Xiao et al. 2018)
Especially the last point has aroused significant interest lately since the heterogeneous catalyst can often be easily separated from the reactant solution and have
other interesting properties (Oh et al. 2016; Xiao et al. 2018). Moreover, carbonaceous catalysis of PS (Chen and Carroll 2016; Duan et al. 2015; Lee et al. 2015) was
found very popular recently, however, this topic still remains controversial (Lee
et al. 2017). Several oxidants can also be heterogeneously activated by subsurface
minerals (Ahmad et al. 2010), which makes their application for, e.g., groundwater
remediation more facile.
As mentioned before, iron is one of those elements that can be incorporated to the
heterogeneous catalysis of PS. Adopting the use of solid iron particles where the
release of iron species being responsible for the activation of PDS occurred smoothly
without the risk of sulfate radical quenching was reported by Naim and Ghauch
(2016). On the other hand, Ayoub and Ghauch (2014) demonstrated that the
activation of PDS can be sustained better in solution especially while using bimetallic and trimetallic iron-based particles, which make the process more efficient in
long-term application. Such great efficiency of heterogeneous activation by solid
iron has led to investigating this material also in smaller dimensions, i.e., nanoscale
zero-valent iron (nZVI). nZVI is not a new material in the environmental chemistry
field and has already been applied to many contaminated sites in situ, where it can
9 Radical Reactions and Their Application for Water Treatment
211