processes has been reported to be crucial in the mobilization of arsenic in alluvial
aquifers of Terai Region, Nepal (Diwakar et al. 2015). However, high concentrations
of Fe, Mn, HCO 3
À ions, NH 4
+ ions and CH 4 gas and an absence of oxidized species,
such as NO 3
À and SO 4
2À ions, are the indicators of strongly reducing conditions
(Lockwood et al. 2014). Moreover, the microbial mechanisms of arsenic release
require the presence of abundant of organic matter in subsurface environment.
2.3.1.2 Alkali Desorption
Arsenic attachment in the subterranean environment onto the surface of iron oxides
is an example of adsorption, and its detachment occurs through desorption. Its
adsorption and release in the aquifer systems mainly occur through solid-phase
precipitation and solid-phase dissolution, respectively. In alluvium plains, arsenic
is present as FeAsS (arsenopyrite) along with its transformed phases, such as
FeAsO 4 (ferric arsenate) and FeAsO 3 (ferric arsenite) (Bhattacharya et al. 1997). It
has been reported that the presence of an oxidizing agent, most commonly atmospheric oxygen (as O 2 ), controls the oxidation rate of arsenic containing sulphide
minerals (Chakraborty et al. 2015; Poreda et al. 2002). Thus, increment in the
discharge rate of groundwater makes the arsenic-bearing sulphide mineral exposed
to oxygen, which causes their oxidation and further release of arsenic to aquifer
system.
2.3.1.3 Geothermal Trigger
The elevated levels of arsenic have been reported in the hot springs on Qinghai-Tibet
Plateau that caused thorough leaching of rocks under high temperature conditions
(Rodríguez-Lado et al. 2013). The generation of these conditions occurred due to
either circulation of groundwater (deep and rapid) or shallow volcanism. Incidentally, the rivers and groundwater of Chile have also been reported with severe arsenic
pollution caused due to seepage triggered by geothermal forces from the mountain,
Andes, located hundreds of kilometres from the point of groundwater abstraction
areas (Bundschuh et al. 2012).
2.4
Conventional Treatment Methods and Their Limitations
Several technologies have been developed so far to remove the arsenic from
groundwater. However, the most common removal methods which have been
widely explored include oxidation and adsorption (Luo et al. 2012; Luo et al.
2013), chemical precipitation/sedimentation and filtration (Meng et al. 2001;
Sheoran and Sheoran 2006), use of ion-exchange resins (Greenleaf et al. 2006)
and membrane technology, including reverse osmosis (Ning 2002). Recently, the
in situ arsenic removal technology using permeable reactive barriers (PRBs) has
drawn considerable attention among the research communities and is being practiced
in few countries. All these methods have been discussed briefly below:
2 Nanotechnology-Based Treatment Systems for Arsenic Sequestration in. . .
39
aquifers of Terai Region, Nepal (Diwakar et al. 2015). However, high concentrations
of Fe, Mn, HCO 3
À ions, NH 4
+ ions and CH 4 gas and an absence of oxidized species,
such as NO 3
À and SO 4
2À ions, are the indicators of strongly reducing conditions
(Lockwood et al. 2014). Moreover, the microbial mechanisms of arsenic release
require the presence of abundant of organic matter in subsurface environment.
2.3.1.2 Alkali Desorption
Arsenic attachment in the subterranean environment onto the surface of iron oxides
is an example of adsorption, and its detachment occurs through desorption. Its
adsorption and release in the aquifer systems mainly occur through solid-phase
precipitation and solid-phase dissolution, respectively. In alluvium plains, arsenic
is present as FeAsS (arsenopyrite) along with its transformed phases, such as
FeAsO 4 (ferric arsenate) and FeAsO 3 (ferric arsenite) (Bhattacharya et al. 1997). It
has been reported that the presence of an oxidizing agent, most commonly atmospheric oxygen (as O 2 ), controls the oxidation rate of arsenic containing sulphide
minerals (Chakraborty et al. 2015; Poreda et al. 2002). Thus, increment in the
discharge rate of groundwater makes the arsenic-bearing sulphide mineral exposed
to oxygen, which causes their oxidation and further release of arsenic to aquifer
system.
2.3.1.3 Geothermal Trigger
The elevated levels of arsenic have been reported in the hot springs on Qinghai-Tibet
Plateau that caused thorough leaching of rocks under high temperature conditions
(Rodríguez-Lado et al. 2013). The generation of these conditions occurred due to
either circulation of groundwater (deep and rapid) or shallow volcanism. Incidentally, the rivers and groundwater of Chile have also been reported with severe arsenic
pollution caused due to seepage triggered by geothermal forces from the mountain,
Andes, located hundreds of kilometres from the point of groundwater abstraction
areas (Bundschuh et al. 2012).
2.4
Conventional Treatment Methods and Their Limitations
Several technologies have been developed so far to remove the arsenic from
groundwater. However, the most common removal methods which have been
widely explored include oxidation and adsorption (Luo et al. 2012; Luo et al.
2013), chemical precipitation/sedimentation and filtration (Meng et al. 2001;
Sheoran and Sheoran 2006), use of ion-exchange resins (Greenleaf et al. 2006)
and membrane technology, including reverse osmosis (Ning 2002). Recently, the
in situ arsenic removal technology using permeable reactive barriers (PRBs) has
drawn considerable attention among the research communities and is being practiced
in few countries. All these methods have been discussed briefly below:
2 Nanotechnology-Based Treatment Systems for Arsenic Sequestration in. . .
39