treatment (Gogate and Pandit 2004a). At the end of the process, it is common to
alkalinize the waters, with simultaneous addition of a flocculant to eliminate the
remaining iron (Pignatello 1992).
The application of the Fenton process for destroying toxic organics began in 1960
(Huang et al. 1993).The method is effective in treating waters containing pharmaceuticals, dyes, insecticides, water from petroleum refineries and fuel terminals,
nitroaromatics and explosives such as trinitrotoluene (TNT), chlorinated aliphatic
and aromatic compounds (chlorobenzene, pentachlorophenol, phenols, chlorinated
phenols, octachloro-p-dioxin, polychlorinated biphenyls (PCBs)), formaldehyde,
etc. Only some compounds cannot be attacked by this reagent: acetone, acetic
acid, oxalic acid, or paraffins. The technology has been successfully applied to the
COD reduction of municipal water and groundwater, in treating leachates, and as a
pretreatment of non-biodegradable compounds. It has been effectively applied as a
good oxidant of herbicides, hexadecane, or dieldrin in the area of soil treatment (e.g.,
Safarzadeh-Amiri et al. 1996b; Bigda 1995; Lin and Lo 1997; Watts et al. 2002;
Quan et al. 2003).
Generally, a complete mineralization cannot be attained in Fenton processes
because resistant intermediates such as carboxylic acids, which react very slowly
with HO
•
, are formed, and the unproductive Eq. 7.24 predominates. Occasionally,
products that are more toxic than the initial ones are formed; this obliges to careful
monitoring of the process until the complete depletion of these noxious byproducts is
achieved (Sedlak and Andren 1991).
As said, the use of Fe(II)/Fe(III) in the solution for Fenton processes suffers from
considerable disadvantages, such as the formation of a high amount of iron sludge
that has to be removed, the requirement of large amounts of Fe
2+ (ca. 50–80 ppm),
and a high H 2 O 2 /Fe
2+ molar ratio. In addition, the acidification of the effluents before
the reaction and the neutralization of the treated solutions before disposal should be
part of the process. Due to these problems, in the mid-1990s, heterogeneous Fenton
catalysts, i.e., solid iron-containing compounds or solid materials rich in iron, started
to be developed (Pignatello et al. 2006; Mukherjee et al. 2016; Tang and Chen 1996;
Fajerwerg and Debellefontaine 1996; Pera-Titus et al. 2004; Navalon et al. 2010; Li
et al. 2015; Dhakshinamoorthy et al. 2012; Zhao et al. 2016). Several ironcontaining materials such as clays, activated carbon, silicas, zeolites, hydrotalcitelike compounds, fly ashes, iron-oxide minerals, iron oxide nanocatalysts, metalexchange resins and layered materials, Nafion films or Nafion resins containing iron,
iron-coated pumice particles, and iron immobilized aluminates have been tested;
additionally, iron-containing polyethylene copolymers, alginate gel beads, structured silica fabrics, or brick grain were also essayed (Domènech et al. 2004; PeraTitus et al. 2004; Dhakshinamoorthy et al. 2012 and references therein). Zero-valent
state metals have also been proposed, especially zero-valent iron (ZVI), which will
be treated in Sect. 7.2.11. Pure and metal-modified iron oxides such as magnetite,
goethite, hematite, etc., can be used as heterogeneous Fenton catalysts because they
are relatively less expensive, allow magnetic separation, and present both high
stability and improved adsorption capacity (Teel et al. 2001; Pouran et al. 2014).
130
M. I. Litter
alkalinize the waters, with simultaneous addition of a flocculant to eliminate the
remaining iron (Pignatello 1992).
The application of the Fenton process for destroying toxic organics began in 1960
(Huang et al. 1993).The method is effective in treating waters containing pharmaceuticals, dyes, insecticides, water from petroleum refineries and fuel terminals,
nitroaromatics and explosives such as trinitrotoluene (TNT), chlorinated aliphatic
and aromatic compounds (chlorobenzene, pentachlorophenol, phenols, chlorinated
phenols, octachloro-p-dioxin, polychlorinated biphenyls (PCBs)), formaldehyde,
etc. Only some compounds cannot be attacked by this reagent: acetone, acetic
acid, oxalic acid, or paraffins. The technology has been successfully applied to the
COD reduction of municipal water and groundwater, in treating leachates, and as a
pretreatment of non-biodegradable compounds. It has been effectively applied as a
good oxidant of herbicides, hexadecane, or dieldrin in the area of soil treatment (e.g.,
Safarzadeh-Amiri et al. 1996b; Bigda 1995; Lin and Lo 1997; Watts et al. 2002;
Quan et al. 2003).
Generally, a complete mineralization cannot be attained in Fenton processes
because resistant intermediates such as carboxylic acids, which react very slowly
with HO
•
, are formed, and the unproductive Eq. 7.24 predominates. Occasionally,
products that are more toxic than the initial ones are formed; this obliges to careful
monitoring of the process until the complete depletion of these noxious byproducts is
achieved (Sedlak and Andren 1991).
As said, the use of Fe(II)/Fe(III) in the solution for Fenton processes suffers from
considerable disadvantages, such as the formation of a high amount of iron sludge
that has to be removed, the requirement of large amounts of Fe
2+ (ca. 50–80 ppm),
and a high H 2 O 2 /Fe
2+ molar ratio. In addition, the acidification of the effluents before
the reaction and the neutralization of the treated solutions before disposal should be
part of the process. Due to these problems, in the mid-1990s, heterogeneous Fenton
catalysts, i.e., solid iron-containing compounds or solid materials rich in iron, started
to be developed (Pignatello et al. 2006; Mukherjee et al. 2016; Tang and Chen 1996;
Fajerwerg and Debellefontaine 1996; Pera-Titus et al. 2004; Navalon et al. 2010; Li
et al. 2015; Dhakshinamoorthy et al. 2012; Zhao et al. 2016). Several ironcontaining materials such as clays, activated carbon, silicas, zeolites, hydrotalcitelike compounds, fly ashes, iron-oxide minerals, iron oxide nanocatalysts, metalexchange resins and layered materials, Nafion films or Nafion resins containing iron,
iron-coated pumice particles, and iron immobilized aluminates have been tested;
additionally, iron-containing polyethylene copolymers, alginate gel beads, structured silica fabrics, or brick grain were also essayed (Domènech et al. 2004; PeraTitus et al. 2004; Dhakshinamoorthy et al. 2012 and references therein). Zero-valent
state metals have also been proposed, especially zero-valent iron (ZVI), which will
be treated in Sect. 7.2.11. Pure and metal-modified iron oxides such as magnetite,
goethite, hematite, etc., can be used as heterogeneous Fenton catalysts because they
are relatively less expensive, allow magnetic separation, and present both high
stability and improved adsorption capacity (Teel et al. 2001; Pouran et al. 2014).
130
M. I. Litter