Nanoparticles for Treatment of Arsenic - Nanotechnologies for Water Environment Applications - page 117

Environmental nanotechnologies for treatment of arsenic have been attracting
increasing attention in the last few years. Although the area of nanoscience is relatively
new, nanotechnology will play an essential role in the development of novel arsenic
treatment processes. This chapter is intended to bring together the various experimental
aspects of nanoparticles of interest to environmental scientists and engineers and to
show how the subject works. The chapter begins with a discussion of environmental
chemistry of arsenic followed by sections dealing with synthesis and characterization of
nanoparticles, and treatment of arsenic.

5.2 Environmental Chemistry of Arsenic

The chemistry of arsenic in aquatic environments is complex because of its
multiple oxidation states and its association with a variety of minerals through
adsorption and precipitation. Arsenic exists in inorganic and organic forms and in
different oxidation states depending on the redox environment. Inorganic arsenate
[As(V)] and arsenite [As(III)], monomethylarsonic acid (MMA) and dimethylarsinic
acid (DMA) are four commonly found arsenic species in the environment (Cullen and
Reimer, 1989). Inorganic arsenic is the predominant species; however, the presence of
MMA and DMA in natural waters has also been reported (Anderson and Bruland, 1991).
The distribution of arsenic species as a function of pH is shown in Figure 5.1 with the
inserted molecular structure for fully protonated species. As(V) is the stable oxidation
state under oxic conditions or in oxygenated waters. In a neutral pH range, As(V) exists
in oxyanionic forms of H 2 AsO 4 - and HAsO 4 2- . In a moderately reducing environment
As(III) becomes stable. It is present predominantly as H 3 AsO 3 when pH is less than 9.
As(III) is often present in anoxic systems such as groundwater, sediment porewater, and
geothermal water. In aquatic environments with neutral pH, arsenite has higher mobility
than arsenate because of its lower sorptive affinity to sediment and soil particles. As(III)
is considered more toxic than As(V).

The redox transformation of arsenic has been well documented (Meng et al.,
2002). Arsenic can be methylated by bacteria and fungi (McBride and Wolfe, 1971;
Woolson, 1977), and concentrations of DMA and MMA have been reported to increase
in the summer due to the increase in microbial activities (Hasegawa, 1997).

A variety of technologies have been used for the removal of arsenic from water,
such as coagulation with ferric and aluminum salts, filtration with anion ion exchange
resins and activated alumina, and reverses osmosis ( Hering et al., 1996; Chen et al., 1999;
Amy et al., 2000; Meng et al., 2000, 2001). In recent years, granular ferric hydroxide
(GFH) and granular ferric oxide (GFO) with high adsorption capacity have been
developed and marketed (Driehaus et al., 1998; Hatch, 2002; Seven Trent Services,
2002). The coagulation processes require flocculation, sedimentation, and filtration