63
of the soluble arsenate is converted to particulate arsenic. This formed by adsorption
during rapid mixing coagulation, removing near to 95% of soluble arsenate.
In contrast, arsenite with a neutral charge remains in solution due to the lack of
interaction with positive coagulants (Gregor 2001). Thus, the ability to remove arsenic from water during clarification depends on the proportion of arsenate to arsenite
in the water source. The results are similar when iron-based coagulants are
employed, having a similar removal mechanism (Hering et al. 1996). Additional
strategies can be implemented to increase the arsenic removal during coagulation
and flocculation, including the aeration of the water source to transform all arsenite
to arsenate, pH control, and optimal coagulant dose. Further information can be
found elsewhere (Mondal et al. 2013).
A similar mechanism applies for fluoride removal from water during coagulation, being mostly aluminum salts employed for this purpose (Bhatnagar et al. 2011;
Loganathan et al. 2013). The efficiency of fluoride removal by aluminum salt
depends on water composition, pH, and other solution characteristics (Hao Oliver
and Huang 1986). The most appropriate pH for defluorination during coagulation is
between 5.5 and 6.5 (Shen et al. 2003).
3.2.2 Ion Exchange
The ion exchange is an adsorption process that employs granular materials, characterized by having exchangeable ions in the material surface. These ions can be
exchanged by ionic pollutants, lowering the contaminant concentration in the treated
water (Fig. 3.4). The ion- exchange materials are usually cross-linked polymeric resins with chemical functionalities linked to the main polymeric chain. The chemical
functional groups are characterized by having a stable positive or negative charge
when equilibrated in water. The common functional groups are strong acid such as
sulfonate, -SO 3
−
; weak acid such as carboxylate, -COO
−
; strong base such as quaternary amine, -N(R) 3
+
; and weak base such as tertiary amine, -N(R) 2 (Edzwald 2010).
For the removal of arsenate and fluoride anions, the anionic exchange materials are
appropriate. In the process, the pollutants are exchanged for the negative ions that balance the charge in the ion exchanger, generally hydroxide or chloride. Resins have
been reported that can interchange arsenic for more than 4000 equivalent bed volumes
before becoming exhausted (DeMarco et al. 2003). In particular, for arsenic removal,
there are hybrid resins that incorporate metal hydroxides in addition to the anionic
exchange functional groups. Iron is the metal often used for the hybrid ion-exchange
resins and represents some of the more successful materials for arsenic removal from
water. Cumbal and SenGupta reported the treatment of up to 15,000 bed volumes of
arsenite or arsenate polluted groundwater (Cumbal and SenGupta 2005).
On the other hand, some examples of materials that have been used as ion
exchangers to remove fluoride from water are zeolite and activated carbon. Zeolites
are often used as a cation exchanger material; hence its arsenic and fluoride removal
is negligible. However, by means of the anchorage of high-valence cations such as
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal
of the soluble arsenate is converted to particulate arsenic. This formed by adsorption
during rapid mixing coagulation, removing near to 95% of soluble arsenate.
In contrast, arsenite with a neutral charge remains in solution due to the lack of
interaction with positive coagulants (Gregor 2001). Thus, the ability to remove arsenic from water during clarification depends on the proportion of arsenate to arsenite
in the water source. The results are similar when iron-based coagulants are
employed, having a similar removal mechanism (Hering et al. 1996). Additional
strategies can be implemented to increase the arsenic removal during coagulation
and flocculation, including the aeration of the water source to transform all arsenite
to arsenate, pH control, and optimal coagulant dose. Further information can be
found elsewhere (Mondal et al. 2013).
A similar mechanism applies for fluoride removal from water during coagulation, being mostly aluminum salts employed for this purpose (Bhatnagar et al. 2011;
Loganathan et al. 2013). The efficiency of fluoride removal by aluminum salt
depends on water composition, pH, and other solution characteristics (Hao Oliver
and Huang 1986). The most appropriate pH for defluorination during coagulation is
between 5.5 and 6.5 (Shen et al. 2003).
3.2.2 Ion Exchange
The ion exchange is an adsorption process that employs granular materials, characterized by having exchangeable ions in the material surface. These ions can be
exchanged by ionic pollutants, lowering the contaminant concentration in the treated
water (Fig. 3.4). The ion- exchange materials are usually cross-linked polymeric resins with chemical functionalities linked to the main polymeric chain. The chemical
functional groups are characterized by having a stable positive or negative charge
when equilibrated in water. The common functional groups are strong acid such as
sulfonate, -SO 3
−
; weak acid such as carboxylate, -COO
−
; strong base such as quaternary amine, -N(R) 3
+
; and weak base such as tertiary amine, -N(R) 2 (Edzwald 2010).
For the removal of arsenate and fluoride anions, the anionic exchange materials are
appropriate. In the process, the pollutants are exchanged for the negative ions that balance the charge in the ion exchanger, generally hydroxide or chloride. Resins have
been reported that can interchange arsenic for more than 4000 equivalent bed volumes
before becoming exhausted (DeMarco et al. 2003). In particular, for arsenic removal,
there are hybrid resins that incorporate metal hydroxides in addition to the anionic
exchange functional groups. Iron is the metal often used for the hybrid ion-exchange
resins and represents some of the more successful materials for arsenic removal from
water. Cumbal and SenGupta reported the treatment of up to 15,000 bed volumes of
arsenite or arsenate polluted groundwater (Cumbal and SenGupta 2005).
On the other hand, some examples of materials that have been used as ion
exchangers to remove fluoride from water are zeolite and activated carbon. Zeolites
are often used as a cation exchanger material; hence its arsenic and fluoride removal
is negligible. However, by means of the anchorage of high-valence cations such as
3 Metal Oxyhydroxide Composites for Halogens and Metalloid Removal