75
3.4 Synthesis of Metal Oxyhydroxide-Based Materials
for Adsorption
As reviewed in Sect. 3.3, metal oxyhydroxides have ideal chemical properties for
the arsenic and fluoride adsorption from water. Currently, several research groups
that are working on the development of new adsorbents based on metal oxyhydroxides that can be ultimately applied in the elimination of arsenic and fluoride purification of drinking water. The efforts are oriented in two main areas: the increment
of the adsorption capacity of the metal oxides and the conformation of granular
materials. For the increment of adsorption capacity, authors have developed new
synthesis methodologies in which the particle size and specific crystalline structures
(crystalline habit) can be controlled. Also, the development of binary and ternary
metal oxide is a strategy already tested to increase the reactive surface of the metal
oxyhydroxide. Regarding the formation of granular materials, the agglomeration of
the metal oxyhydroxides with binders and the anchorage of the metal oxides to
granular materials are the most relevant strategies. In the following sections, we
review the recent advances in both areas.
3.4.1 Natural Metal Oxyhydroxides
There are natural materials with a high potential to be used as adsorbent due to its
physical properties. Giménez et al. (2007) evaluated the arsenic adsorption capacity
in natural hematite, magnetite, and goethite. Within the physicochemical characteristics of natural oxides, the surface area was determined. The natural oxides have a
relatively low surface area, between 0.381 and 2.009 m
2
/g, compared to synthetic
oxides: 11–140 m
2
/g for goethite, 2–14 m
2
/g for hematite, and 60 m
2
/g for magnetite
(Grossl and Sparks 1995; Singh et al. 1996; Matis et al. 1997; Manning et al. 1998;
Dixit and Hering 2003; Yang et al. 2010). The points of zero charge for magnetite,
goethite, and hematite are very similar (6.5, 6.8, and 6.7, respectively), so the oxides
tend to have a positive surface charge at acidic pH and a negative charge at alkaline
pH. The adsorption capacity of natural oxides is usually low ̴ 1.31 mg/g (RamirezMuñiz et al. 2018).
Another natural granular material studied for the removal of arsenic is volcanic
rock. The soils and volcanic rocks are composed mainly of silicon, iron, and aluminum oxides (Alemayehu and Lennartz 2009). Therefore, arsenic removal by soils
and volcanic rocks such as pumice and slag can be considered as low-cost adsorbents (Yazdani et al. 2016). However, both slag and pumice have quite low adsorption capacity of As(III and V) (Kwon et al. 2010; Turan et al. 2014).
Hybrid adsorbents composed of iron–polymer oxide have been developed
(Rahim and Mas Haris 2015). Within these materials, we find iron oxyhydroxide
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal
3.4 Synthesis of Metal Oxyhydroxide-Based Materials
for Adsorption
As reviewed in Sect. 3.3, metal oxyhydroxides have ideal chemical properties for
the arsenic and fluoride adsorption from water. Currently, several research groups
that are working on the development of new adsorbents based on metal oxyhydroxides that can be ultimately applied in the elimination of arsenic and fluoride purification of drinking water. The efforts are oriented in two main areas: the increment
of the adsorption capacity of the metal oxides and the conformation of granular
materials. For the increment of adsorption capacity, authors have developed new
synthesis methodologies in which the particle size and specific crystalline structures
(crystalline habit) can be controlled. Also, the development of binary and ternary
metal oxide is a strategy already tested to increase the reactive surface of the metal
oxyhydroxide. Regarding the formation of granular materials, the agglomeration of
the metal oxyhydroxides with binders and the anchorage of the metal oxides to
granular materials are the most relevant strategies. In the following sections, we
review the recent advances in both areas.
3.4.1 Natural Metal Oxyhydroxides
There are natural materials with a high potential to be used as adsorbent due to its
physical properties. Giménez et al. (2007) evaluated the arsenic adsorption capacity
in natural hematite, magnetite, and goethite. Within the physicochemical characteristics of natural oxides, the surface area was determined. The natural oxides have a
relatively low surface area, between 0.381 and 2.009 m
2
/g, compared to synthetic
oxides: 11–140 m
2
/g for goethite, 2–14 m
2
/g for hematite, and 60 m
2
/g for magnetite
(Grossl and Sparks 1995; Singh et al. 1996; Matis et al. 1997; Manning et al. 1998;
Dixit and Hering 2003; Yang et al. 2010). The points of zero charge for magnetite,
goethite, and hematite are very similar (6.5, 6.8, and 6.7, respectively), so the oxides
tend to have a positive surface charge at acidic pH and a negative charge at alkaline
pH. The adsorption capacity of natural oxides is usually low ̴ 1.31 mg/g (RamirezMuñiz et al. 2018).
Another natural granular material studied for the removal of arsenic is volcanic
rock. The soils and volcanic rocks are composed mainly of silicon, iron, and aluminum oxides (Alemayehu and Lennartz 2009). Therefore, arsenic removal by soils
and volcanic rocks such as pumice and slag can be considered as low-cost adsorbents (Yazdani et al. 2016). However, both slag and pumice have quite low adsorption capacity of As(III and V) (Kwon et al. 2010; Turan et al. 2014).
Hybrid adsorbents composed of iron–polymer oxide have been developed
(Rahim and Mas Haris 2015). Within these materials, we find iron oxyhydroxide
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal