76
nanoparticles contained in polyvinyl alcohol microspheres with an adsorption
capacity of 87.18 mg/g in the pH range 2–5 (Santos et al. 2012). Ramirez-Muñiz
et al. (2018) used a composite formed by natural goethite and polyacrylamide
hydrogel, which has mechanical properties and chemical stability for its application
in continuous systems. However, the authors observed that the content and dispersion of goethite in the hydrogel are variable and reduce the specific surface area of
the composite, compared to goethite powder, affecting its adsorption capacity.
In the case of fluoride removal, zeolite has been used primarily as a natural
adsorbent material (Přech et al. 2019; Suárez et al. 2019). Zeolites are highly crystalline hydrated aluminosilicates with a porous structure with pore diameters
between 3 and 10 Å. The zeolites are widely used in ion-exchange processes,
because of their low-cost and large internal and external surface (Lindlar et al.
2001). The structure of the zeolites consists of a three-dimensional framework
formed by tetrahedra of SiO 4
4−
and AlO 4
5−
that give a negative charge to the structure (Doong 2012; Siddiqui et al. 2019a). This charge can be neutralized by interchangeable cations like Na
+
, Ca
2+
, K
+
, Mg
2+
, and NH
4+
. The fact that the exchangeable
cations of the zeolites are practically harmless makes them suitable for application
in the processes of removal of undesirable heavy metals from water (Motsi et al.
2009). The zeolites have been modified with various metal oxides (iron, lanthanum,
zirconium, and manganese) for fluoride removal (Sun et al. 2011; Lai et al. 2018).
Velazquez-Peña et al. (2017) modified zeolites with oxides of iron, zirconium,
and iron–zirconium to evaluate their fluoride removal efficiency from aqueous solutions. The fluoride adsorption capacities of the zeolites modified with zirconium and
iron were 2.5 and 1.6 mg/g, respectively. Zirconium–iron-modified zeolite had the
highest adsorption capacities as high as 3.5 mg/g. In addition, the authors determined by thermodynamic studies that fluoride adsorption is carried out by an exothermic process.
3.4.2 Bimetallic Oxyhydroxides
In recent years, there has been a particular interest in applying bimetallic oxides to
remove fluorides and arsenic from water. This is because the bimetallic oxides inherit
the advantages of pristine oxides, such as considerable high surface area, adsorption
capacity, and selectivity. Also, the bimetal oxides show a synergistic effect, high
specific area and high concentration of active groups, which results in an improved
adsorption capacity (Mandal et al. 2011; Yamani et al. 2012; Xu et al. 2013). For
example, to increase fluoride adsorption capacity, alumina (Liu et al. 2010) has been
modified with different types of oxides, including cerium, titanium, lanthanum, and
zirconium. Hybrid oxides, alum–lanthanum and alum–zirconium have an adsorption capacity by around 6.6% and 33%, respectively. On the other hand, alum modified with manganese and cobalt has also shown a high adsorption capacity
E. Vences-Alvarez et al.
nanoparticles contained in polyvinyl alcohol microspheres with an adsorption
capacity of 87.18 mg/g in the pH range 2–5 (Santos et al. 2012). Ramirez-Muñiz
et al. (2018) used a composite formed by natural goethite and polyacrylamide
hydrogel, which has mechanical properties and chemical stability for its application
in continuous systems. However, the authors observed that the content and dispersion of goethite in the hydrogel are variable and reduce the specific surface area of
the composite, compared to goethite powder, affecting its adsorption capacity.
In the case of fluoride removal, zeolite has been used primarily as a natural
adsorbent material (Přech et al. 2019; Suárez et al. 2019). Zeolites are highly crystalline hydrated aluminosilicates with a porous structure with pore diameters
between 3 and 10 Å. The zeolites are widely used in ion-exchange processes,
because of their low-cost and large internal and external surface (Lindlar et al.
2001). The structure of the zeolites consists of a three-dimensional framework
formed by tetrahedra of SiO 4
4−
and AlO 4
5−
that give a negative charge to the structure (Doong 2012; Siddiqui et al. 2019a). This charge can be neutralized by interchangeable cations like Na
+
, Ca
2+
, K
+
, Mg
2+
, and NH
4+
. The fact that the exchangeable
cations of the zeolites are practically harmless makes them suitable for application
in the processes of removal of undesirable heavy metals from water (Motsi et al.
2009). The zeolites have been modified with various metal oxides (iron, lanthanum,
zirconium, and manganese) for fluoride removal (Sun et al. 2011; Lai et al. 2018).
Velazquez-Peña et al. (2017) modified zeolites with oxides of iron, zirconium,
and iron–zirconium to evaluate their fluoride removal efficiency from aqueous solutions. The fluoride adsorption capacities of the zeolites modified with zirconium and
iron were 2.5 and 1.6 mg/g, respectively. Zirconium–iron-modified zeolite had the
highest adsorption capacities as high as 3.5 mg/g. In addition, the authors determined by thermodynamic studies that fluoride adsorption is carried out by an exothermic process.
3.4.2 Bimetallic Oxyhydroxides
In recent years, there has been a particular interest in applying bimetallic oxides to
remove fluorides and arsenic from water. This is because the bimetallic oxides inherit
the advantages of pristine oxides, such as considerable high surface area, adsorption
capacity, and selectivity. Also, the bimetal oxides show a synergistic effect, high
specific area and high concentration of active groups, which results in an improved
adsorption capacity (Mandal et al. 2011; Yamani et al. 2012; Xu et al. 2013). For
example, to increase fluoride adsorption capacity, alumina (Liu et al. 2010) has been
modified with different types of oxides, including cerium, titanium, lanthanum, and
zirconium. Hybrid oxides, alum–lanthanum and alum–zirconium have an adsorption capacity by around 6.6% and 33%, respectively. On the other hand, alum modified with manganese and cobalt has also shown a high adsorption capacity
E. Vences-Alvarez et al.