Silicon (Si) is the second most abundant element (after oxygen) in the Earth’s
crust [84, 85]. Silica (SiO 2 ), the oxide form of Si, has many applications in science
and engineering because it is chemically inert, thermodynamically stable, non-toxic,
and low-cost [86]. Silica can be crystalline and non-crystalline. In crystalline silica,
the Si and O atoms are arranged in a geometric structure. In non-crystalline (amorphous) silica, no spatial ordering of Si and O atoms is observed. Crystalline forms of
silica include quartz, cristobalite, and tridymite [87]. Of these, quartz is the most
common and is easily attainable from natural sources, such as rocks and soil,
including arable loess and clay soils. The three types of amorphous silica are
naturally occurring silica, silica formed under uncontrolled conditions, and synthetic
silica [87]. Silica fumes comprising very fine amorphous silica particles are a
by-product of the manufacture of silicon and ferrosilicon [88]. Studies have demonstrated that silica fumes can fix heavy metals due to their large specific surface area
and high amorphous silica content [89–91]. Amorphous and crystalline silicon
compounds are also commonly present in sewage sludge ash and MSW fly ash
[92]. Most silicates have good physical and chemical stability and thus have a wide
range of industrial applications [93]. Silicates can be prepared by various methods,
including sol-gel, sintering, and solid-state diffusion. It has been reported that Cd can
react with silicates via solid-state reactions, forming different silicate products
[23, 94], and thus the use of silicates to stabilize Cd can be a promising strategy.
Aluminum (Al) is the third most abundant element in the Earth’s crust and
accounts for 8 wt.% of its solid surface [95]. Alumina (Al 2 O 3 ) is the oxide form of
Al. The common forms of alumina are α, γ, η, δ, θ, κ, and χ forms. α-Al 2 O 3 is the
most stable phase and the other forms can be obtained by the thermal decomposition
of aluminum hydroxides or oxyhydroxides [96]. Aluminas are widely applied as raw
materials in the manufacturing of various functional and conventional ceramic
products. Among these aluminas, γ-Al 2 O 3 is the most common oxide form of Al
in nature. It has great application potential in many fields because of its unique
crystal structure, large surface area, highly reactive nature, low cost, and attainability
[97, 98]. γ-Al 2 O 3 has a cubic lattice in space group Fd3. Its crystal structure is often
defective, which may promote the incorporation of metals [99–102].
Iron (Fe) is also one of the most abundant elements on Earth and is common in the
raw materials of ceramic manufacturing [17, 103]. Several iron oxides exist, including hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), β-Fe 2 O 3 , εFe 2 O 3 , and wustite (FeO). Both β-Fe 2 O 3 and ε-Fe 2 O 3 are uncommon in nature
[104]. Wustite is an intermediate in the reduction process of iron ores and is not
thermodynamically stable [103–105]. Hematite and magnetite are the two most
widespread iron oxides in nature [103, 104]. Hematite has a corundum structure
and is the most thermodynamically stable of all the iron oxides. The close-packed
arrangement of Fe and O atoms in hematite means that it has no charge excess or
deficit. Magnetite is an inverse spinel and can be easily oxidized to maghemite and
finally turned into hematite [103, 104, 106]. Few studies have revealed the effects of
different iron oxides on the formation of CdFe 2 O 4 . Hence, it will be of great interest
to investigate the reaction of different iron oxides with Cd to form ceramic products.
7 Advances in Cadmium Detoxification/Stabilization by Sintering with Ceramic. . .
307
crust [84, 85]. Silica (SiO 2 ), the oxide form of Si, has many applications in science
and engineering because it is chemically inert, thermodynamically stable, non-toxic,
and low-cost [86]. Silica can be crystalline and non-crystalline. In crystalline silica,
the Si and O atoms are arranged in a geometric structure. In non-crystalline (amorphous) silica, no spatial ordering of Si and O atoms is observed. Crystalline forms of
silica include quartz, cristobalite, and tridymite [87]. Of these, quartz is the most
common and is easily attainable from natural sources, such as rocks and soil,
including arable loess and clay soils. The three types of amorphous silica are
naturally occurring silica, silica formed under uncontrolled conditions, and synthetic
silica [87]. Silica fumes comprising very fine amorphous silica particles are a
by-product of the manufacture of silicon and ferrosilicon [88]. Studies have demonstrated that silica fumes can fix heavy metals due to their large specific surface area
and high amorphous silica content [89–91]. Amorphous and crystalline silicon
compounds are also commonly present in sewage sludge ash and MSW fly ash
[92]. Most silicates have good physical and chemical stability and thus have a wide
range of industrial applications [93]. Silicates can be prepared by various methods,
including sol-gel, sintering, and solid-state diffusion. It has been reported that Cd can
react with silicates via solid-state reactions, forming different silicate products
[23, 94], and thus the use of silicates to stabilize Cd can be a promising strategy.
Aluminum (Al) is the third most abundant element in the Earth’s crust and
accounts for 8 wt.% of its solid surface [95]. Alumina (Al 2 O 3 ) is the oxide form of
Al. The common forms of alumina are α, γ, η, δ, θ, κ, and χ forms. α-Al 2 O 3 is the
most stable phase and the other forms can be obtained by the thermal decomposition
of aluminum hydroxides or oxyhydroxides [96]. Aluminas are widely applied as raw
materials in the manufacturing of various functional and conventional ceramic
products. Among these aluminas, γ-Al 2 O 3 is the most common oxide form of Al
in nature. It has great application potential in many fields because of its unique
crystal structure, large surface area, highly reactive nature, low cost, and attainability
[97, 98]. γ-Al 2 O 3 has a cubic lattice in space group Fd3. Its crystal structure is often
defective, which may promote the incorporation of metals [99–102].
Iron (Fe) is also one of the most abundant elements on Earth and is common in the
raw materials of ceramic manufacturing [17, 103]. Several iron oxides exist, including hematite (α-Fe 2 O 3 ), magnetite (Fe 3 O 4 ), maghemite (γ-Fe 2 O 3 ), β-Fe 2 O 3 , εFe 2 O 3 , and wustite (FeO). Both β-Fe 2 O 3 and ε-Fe 2 O 3 are uncommon in nature
[104]. Wustite is an intermediate in the reduction process of iron ores and is not
thermodynamically stable [103–105]. Hematite and magnetite are the two most
widespread iron oxides in nature [103, 104]. Hematite has a corundum structure
and is the most thermodynamically stable of all the iron oxides. The close-packed
arrangement of Fe and O atoms in hematite means that it has no charge excess or
deficit. Magnetite is an inverse spinel and can be easily oxidized to maghemite and
finally turned into hematite [103, 104, 106]. Few studies have revealed the effects of
different iron oxides on the formation of CdFe 2 O 4 . Hence, it will be of great interest
to investigate the reaction of different iron oxides with Cd to form ceramic products.
7 Advances in Cadmium Detoxification/Stabilization by Sintering with Ceramic. . .
307