72
5. The dissociative adsorption of Bronsted acids such as alcohols, in which the
oxygen in the metal oxyhydroxides surface acts as proton acceptor and the
deprotonated molecule binds to the metal cation (Parkinson and Diebold 2016).
In the following section, we revise the adsorption mechanism of arsenic and
fluorine in selected metal oxyhydroxides.
3.3.1 Fluoride Adsorption on Metal Oxyhydroxides
Due to the chemical characteristics of fluoride, the most relevant adsorption mechanism implies the coordination of fluoride anions in the metal surface. Fluoride has a
negative charge and bears eight free electrons that can be easily donated to unsaturated cations in the metal oxide surface. The fluoride has a small size (0.133 nm), so
it can be easily accommodated in the coordination geometry of transition metals.
There are scientific reports that provide experimental evidence to support this
adsorption mechanism. Tang and coworkers studied the adsorption of fluoride on
granular iron hydroxide using X-ray photoelectron spectroscopy and infrared spectroscopy (Tang et al. 2009). The results indicated the complexation of fluoride
anions in the surface of the metal oxide. In addition, it was possible to detect hydrogen bonding between the surface hydroxides and fluoride.
On the other hand, Shao-Xiang Teng and coworkers reported the adsorption of
fluoride in hydrous-manganese-oxide-coated activated alumina (Teng et al. 2009).
The authors evaluated the solution pH change during the adsorption process, noticing that the pH increases with the adsorption of fluoride. The author associates this
process with an ion-exchange mechanism in which the surface hydroxides in the
material surface are exchanged by fluoride anions that become part of the innersphere complexes. Similar results were obtained by Ayoob and coworkers using
alumina granules (Ayoob et al. 2008a). Therefore, OH groups play an essential role
in the adsorption process of fluoride ions (Jin et al. 2016; Yu et al. 2018; Wang et al.
2018; Kang et al. 2018). However, the interactions between fluoride and the surface
of adsorbent do not lead to chemisorption since, in several studies, it has been shown
that fluoride adsorption is reversible (Medellin-Castillo et al. 2014).
Conventionally, aluminum and iron oxyhydroxides have been used for the fluoride adsorption; however, recent research has reported the use of rare earth metal for
the fluoride adoption, including lanthanum and cerium (Huo et al. 2011; VencesAlvarez et al. 2015, 2019; Zhu et al. 2017; Hernández-Campos et al. 2018; Chigondo
et al. 2018; Zhang et al. 2019). Rare earth-based metal oxides provide a slightly
different adsorption mechanism. Zhang and coworkers recently reported that lanthanum oxide (La 2 O 3 ) could bond fluoride anions without the exchange of hydroxy
ligands (Zhang et al. 2019). Results of X-ray photoelectron spectroscopy showed no
change of the peak attributed to M–OH after the fluoride adsorption. However, the
relative area of the peak attributed to M–OH 2 decreased considerably. Magnetic
resonance spectroscopy analysis and theoretical calculations demonstrated the configuration of the adsorbed fluoride. A study on the coordination chemistry of
E. Vences-Alvarez et al.
5. The dissociative adsorption of Bronsted acids such as alcohols, in which the
oxygen in the metal oxyhydroxides surface acts as proton acceptor and the
deprotonated molecule binds to the metal cation (Parkinson and Diebold 2016).
In the following section, we revise the adsorption mechanism of arsenic and
fluorine in selected metal oxyhydroxides.
3.3.1 Fluoride Adsorption on Metal Oxyhydroxides
Due to the chemical characteristics of fluoride, the most relevant adsorption mechanism implies the coordination of fluoride anions in the metal surface. Fluoride has a
negative charge and bears eight free electrons that can be easily donated to unsaturated cations in the metal oxide surface. The fluoride has a small size (0.133 nm), so
it can be easily accommodated in the coordination geometry of transition metals.
There are scientific reports that provide experimental evidence to support this
adsorption mechanism. Tang and coworkers studied the adsorption of fluoride on
granular iron hydroxide using X-ray photoelectron spectroscopy and infrared spectroscopy (Tang et al. 2009). The results indicated the complexation of fluoride
anions in the surface of the metal oxide. In addition, it was possible to detect hydrogen bonding between the surface hydroxides and fluoride.
On the other hand, Shao-Xiang Teng and coworkers reported the adsorption of
fluoride in hydrous-manganese-oxide-coated activated alumina (Teng et al. 2009).
The authors evaluated the solution pH change during the adsorption process, noticing that the pH increases with the adsorption of fluoride. The author associates this
process with an ion-exchange mechanism in which the surface hydroxides in the
material surface are exchanged by fluoride anions that become part of the innersphere complexes. Similar results were obtained by Ayoob and coworkers using
alumina granules (Ayoob et al. 2008a). Therefore, OH groups play an essential role
in the adsorption process of fluoride ions (Jin et al. 2016; Yu et al. 2018; Wang et al.
2018; Kang et al. 2018). However, the interactions between fluoride and the surface
of adsorbent do not lead to chemisorption since, in several studies, it has been shown
that fluoride adsorption is reversible (Medellin-Castillo et al. 2014).
Conventionally, aluminum and iron oxyhydroxides have been used for the fluoride adsorption; however, recent research has reported the use of rare earth metal for
the fluoride adoption, including lanthanum and cerium (Huo et al. 2011; VencesAlvarez et al. 2015, 2019; Zhu et al. 2017; Hernández-Campos et al. 2018; Chigondo
et al. 2018; Zhang et al. 2019). Rare earth-based metal oxides provide a slightly
different adsorption mechanism. Zhang and coworkers recently reported that lanthanum oxide (La 2 O 3 ) could bond fluoride anions without the exchange of hydroxy
ligands (Zhang et al. 2019). Results of X-ray photoelectron spectroscopy showed no
change of the peak attributed to M–OH after the fluoride adsorption. However, the
relative area of the peak attributed to M–OH 2 decreased considerably. Magnetic
resonance spectroscopy analysis and theoretical calculations demonstrated the configuration of the adsorbed fluoride. A study on the coordination chemistry of
E. Vences-Alvarez et al.