Another option to minimize the sludge production in the Fenton process is to
utilize insoluble iron catalysts instead of soluble ferrous ions (Nidheesh 2017, 2015).
Ferrous or ferric ion in solid form reduces the complexation reactions and will help
to improve the mineralization rate rather than coagulation of pollutants. Waste
materials, such as fly ash (Bansal and Verma 2017; Narayani et al. 2017); iron
minerals, including pyrite (Khataee et al. 2016), hematite (Changotra et al. 2018),
magnetite (Nidheesh et al. 2014; Xavier et al. 2016), goethite (Wang et al. 2015),
ferrite (Kurian and Nair 2015; Sharma and Singhal 2018), iron molybdophosphate
(Baiju et al. 2018), and ferrihydrite (Xu et al. 2016); and zero-valent iron (Devi et al.
2016; Malik et al. 2018), are found as effective heterogeneous Fenton catalysts.
These solid materials contain iron and can decompose hydrogen peroxide. Another
type of heterogeneous Fenton catalysts includes the iron or iron minerals loaded over
stable support. Supports, such as activated carbon (Nidheesh and Rajan 2016),
alumina (di Luca et al. 2015), biosorbents (Laiju et al. 2014), biochar (Rubeena
et al. 2018), zeolite (Sruthi et al. 2018), fiber (Su et al. 2018), alginate beads
(Hammouda et al. 2016), silica (Jinisha et al. 2018; Kumar et al. 2016; Thirumoorthy
and Satishkumar 2019), and sewage sludge, are found effective for iron and iron
oxides. The performances of these catalysts are detailed in the review articles
published recently (Mirzaei et al. 2017; Munoz et al. 2015; Nidheesh 2015;
Nidheesh et al. 2018a, b).
Pollutant removal mechanism, as well as reusability of heterogeneous Fenton
catalyst, purely depends on the iron leaching. Effective Fenton reaction rate depends
on the amount of exposed iron, not on the iron content in the solid catalyst. If the iron
leaching rate of solid catalyst is high, reusability of the material is less. The leached
iron will undergo Fenton reactions in the water medium and follows the principles of
homogeneous Fenton process. Pollutants will be removed via degradation and
coagulation. At the same time, a solid catalyst with lesser iron leaching is generally
highly reusable, and reactions will take place on the surface of the catalyst. Here, the
pollutant removal will be mainly by degradation because of the attack of hydroxyl
radicals, and adsorption of pollutants over the surface also improves the performance
of heterogeneous Fenton catalyst.
14.3 Soil as Heterogeneous Fenton Catalyst
Soil can be considered as an ideal heterogeneous catalyst with higher catalytic
activity and stability. They are rich in minerals and other oxide contents. Higher
availability, environmental friendliness, and lower cost make them an ideal heterogeneous catalyst for pollutant degradation. The higher surface area and low leaching
of iron put it into perspectives of many researchers for using it a suitable heterogeneous Fenton catalyst. Among the different soil samples, laterite, clay, and volcanic
soils have been successful for heterogeneous Fenton process.
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A. V. Karim et al.
utilize insoluble iron catalysts instead of soluble ferrous ions (Nidheesh 2017, 2015).
Ferrous or ferric ion in solid form reduces the complexation reactions and will help
to improve the mineralization rate rather than coagulation of pollutants. Waste
materials, such as fly ash (Bansal and Verma 2017; Narayani et al. 2017); iron
minerals, including pyrite (Khataee et al. 2016), hematite (Changotra et al. 2018),
magnetite (Nidheesh et al. 2014; Xavier et al. 2016), goethite (Wang et al. 2015),
ferrite (Kurian and Nair 2015; Sharma and Singhal 2018), iron molybdophosphate
(Baiju et al. 2018), and ferrihydrite (Xu et al. 2016); and zero-valent iron (Devi et al.
2016; Malik et al. 2018), are found as effective heterogeneous Fenton catalysts.
These solid materials contain iron and can decompose hydrogen peroxide. Another
type of heterogeneous Fenton catalysts includes the iron or iron minerals loaded over
stable support. Supports, such as activated carbon (Nidheesh and Rajan 2016),
alumina (di Luca et al. 2015), biosorbents (Laiju et al. 2014), biochar (Rubeena
et al. 2018), zeolite (Sruthi et al. 2018), fiber (Su et al. 2018), alginate beads
(Hammouda et al. 2016), silica (Jinisha et al. 2018; Kumar et al. 2016; Thirumoorthy
and Satishkumar 2019), and sewage sludge, are found effective for iron and iron
oxides. The performances of these catalysts are detailed in the review articles
published recently (Mirzaei et al. 2017; Munoz et al. 2015; Nidheesh 2015;
Nidheesh et al. 2018a, b).
Pollutant removal mechanism, as well as reusability of heterogeneous Fenton
catalyst, purely depends on the iron leaching. Effective Fenton reaction rate depends
on the amount of exposed iron, not on the iron content in the solid catalyst. If the iron
leaching rate of solid catalyst is high, reusability of the material is less. The leached
iron will undergo Fenton reactions in the water medium and follows the principles of
homogeneous Fenton process. Pollutants will be removed via degradation and
coagulation. At the same time, a solid catalyst with lesser iron leaching is generally
highly reusable, and reactions will take place on the surface of the catalyst. Here, the
pollutant removal will be mainly by degradation because of the attack of hydroxyl
radicals, and adsorption of pollutants over the surface also improves the performance
of heterogeneous Fenton catalyst.
14.3 Soil as Heterogeneous Fenton Catalyst
Soil can be considered as an ideal heterogeneous catalyst with higher catalytic
activity and stability. They are rich in minerals and other oxide contents. Higher
availability, environmental friendliness, and lower cost make them an ideal heterogeneous catalyst for pollutant degradation. The higher surface area and low leaching
of iron put it into perspectives of many researchers for using it a suitable heterogeneous Fenton catalyst. Among the different soil samples, laterite, clay, and volcanic
soils have been successful for heterogeneous Fenton process.
522
A. V. Karim et al.