81
the pore structure of activated carbon. The As(V) adsorption capacity of modified
material was 4.9 mg/g, this being greater than the reported by activated carbon
modified with iron (2.9 mg/g) under similar conditions (Nieto-Delgado et al. 2019).
Velazquez-Jimenez et al. (2014) used oxalic acid as a capping agent to maximize
the zirconium dispersion on activated carbon. The presence of oxalic acid and zirconium on the activated carbon surface at a molar ratio of 1.05 increased the fluoride adsorption capacity to a factor of 3 on activated carbon modified only with
zirconium. Our work suggests that oxalic acid limits the growth of ZrO 2 crystals.
The above is attached to the complexation of oxalic acid with zirconium ions. The
reduction in the particles size of zirconium increases the active surface area favoring
the fluoride ion adsorption.
3.5 Conclusions
Arsenic and fluoride are considered as priority pollutants, due to the high toxicity
they present in living organisms. Therefore, they must be removed from the water
for human consumption in such a way that they comply with the recommended
concentrations established by the World Health Organization. The technologies currently implemented for this purpose have several disadvantages, such as low efficiency and high costs. Since water with high arsenic and fluoride concentrations is
mainly found in low-income communities, the development of new materials for the
removal of these pollutants is of great interest in the scientific community. Metal
oxyhydroxides have shown to have high levels of arsenic and fluoride removal from
water. Metal and bimetallic oxides have proven to be highly efficient materials with
adsorption capacities in the range of 2–200 and 2–300 mg/g for arsenic and fluoride,
respectively.
Regardless of the chemical nature of the metal oxyhydroxides, the adsorption
mechanism reported for fluoride is the ligand exchange to form an inner-sphere
complex. Interestingly, rare earth metal oxides like La 2 O 3 have a superior adsorption capacity, due to their capability to accommodate more than one fluorine ligand
per metal cation. In contrast, the arsenic adsorption mechanism depends on the
chemical nature of the metal oxyhydroxides.
There is a significant effort in the development of experimental procedures for
the synthesis of monometallic and bimetallic oxyhydroxides, with small particle
size, high reactivity, and selectivity for the target pollutants. Studies suggest that the
synthesis solution chemistry should be considered when thermal hydrolysis is
employed for the precipitation of metal oxyhydroxides: an adequate selection of the
synthesis condition ensures the growth of specific crystalline structures. On the
other hand, it is essential to consider the chemical and physical properties of the
metal oxyhydroxide support. For instance, the activated carbon functional groups
have an indispensable role during the precipitation of oxyhydroxides by thermal
hydrolysis, leading the condensation reactions of the metal hydroxides. In addition,
the addition of surface modifiers allows the growth of specific crystalline planes
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal
the pore structure of activated carbon. The As(V) adsorption capacity of modified
material was 4.9 mg/g, this being greater than the reported by activated carbon
modified with iron (2.9 mg/g) under similar conditions (Nieto-Delgado et al. 2019).
Velazquez-Jimenez et al. (2014) used oxalic acid as a capping agent to maximize
the zirconium dispersion on activated carbon. The presence of oxalic acid and zirconium on the activated carbon surface at a molar ratio of 1.05 increased the fluoride adsorption capacity to a factor of 3 on activated carbon modified only with
zirconium. Our work suggests that oxalic acid limits the growth of ZrO 2 crystals.
The above is attached to the complexation of oxalic acid with zirconium ions. The
reduction in the particles size of zirconium increases the active surface area favoring
the fluoride ion adsorption.
3.5 Conclusions
Arsenic and fluoride are considered as priority pollutants, due to the high toxicity
they present in living organisms. Therefore, they must be removed from the water
for human consumption in such a way that they comply with the recommended
concentrations established by the World Health Organization. The technologies currently implemented for this purpose have several disadvantages, such as low efficiency and high costs. Since water with high arsenic and fluoride concentrations is
mainly found in low-income communities, the development of new materials for the
removal of these pollutants is of great interest in the scientific community. Metal
oxyhydroxides have shown to have high levels of arsenic and fluoride removal from
water. Metal and bimetallic oxides have proven to be highly efficient materials with
adsorption capacities in the range of 2–200 and 2–300 mg/g for arsenic and fluoride,
respectively.
Regardless of the chemical nature of the metal oxyhydroxides, the adsorption
mechanism reported for fluoride is the ligand exchange to form an inner-sphere
complex. Interestingly, rare earth metal oxides like La 2 O 3 have a superior adsorption capacity, due to their capability to accommodate more than one fluorine ligand
per metal cation. In contrast, the arsenic adsorption mechanism depends on the
chemical nature of the metal oxyhydroxides.
There is a significant effort in the development of experimental procedures for
the synthesis of monometallic and bimetallic oxyhydroxides, with small particle
size, high reactivity, and selectivity for the target pollutants. Studies suggest that the
synthesis solution chemistry should be considered when thermal hydrolysis is
employed for the precipitation of metal oxyhydroxides: an adequate selection of the
synthesis condition ensures the growth of specific crystalline structures. On the
other hand, it is essential to consider the chemical and physical properties of the
metal oxyhydroxide support. For instance, the activated carbon functional groups
have an indispensable role during the precipitation of oxyhydroxides by thermal
hydrolysis, leading the condensation reactions of the metal hydroxides. In addition,
the addition of surface modifiers allows the growth of specific crystalline planes
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal