Nanoparticles as Sorbents for Removal of Heavy Metal Ions from Aqueous Solutions - Nanotechnologies for Water Environment Applications - page 144

Various synthetic methodologies producing metallic or metal oxide nanoparticles
are available (Cushing et al., 2004). Due to the straightforwardness and scalability,
chemical synthesis methods, such as liquid-phase reduction and controlled chemical
coprecipitation, have long been favored by environmental technologies and scientists
over traditional top-down approach such as mechanical milling (Huber, 2005; Li et al.,
2006). Standard “wet-chemistry” techniques can be used to alter the surface
functionality of the synthesized nanoparticles. The following sections briefly discuss the
synthesis and applications of various iron-based nanoparticles. Those representative
examples are included in Table 6.3 for the reader's convenience.

6.2.2 Recent Research Activities

6.2.2.1 Pristine Zero-Valent Iron (Fe 0 ) Nanoparticles

Iron nanoparticles are often prepared by liquid-phase reduction through reducing
an iron salt or an iron oxide, with or without the presence of a surfactant. The surfactants,
if present, could self-assemble in the solution into micelles and prevent the
agglomeration of the formed iron nanoparticles. The most commonly used reductant in
both basic research and industry is sodium borohydride (NaBH 4 ). Core/shell nanoscale
zero-valent iron was synthesized via popular borohydride reduction of ferric or ferrous
salts and studied with respect to the mechanism of heavy metal removal (Kanel et al . ,
2005; Kanel et al . , 2006; Li and Zhang, 2006; Li and Zhang, 2007). Kanel et al . (2005,
2006) have demonstrated that the pristine zero-valent iron nanoparticles with diameters
of 1120 nm can remove As(III) and As(V) via a rapid adsorption step followed by co-
precipitation with the surface corrosion by-products. The aging of the pristine zero-
valent iron yields magnetite (Fe 3 O 4 ), ferrous hydroxide and ferric hydroxide (Fe(OH) 2 ,
Fe(OH) 3 ) which react with As(III) and convert the neutral HAsO 4 0 to oxyanions,
H 2 AsO 4 1- and HAsO 4 2- at pH = 6-9. The negatively-charged species are then adsorbed
and undergo surface complexation on the positively-charged surface of iron
nanoparticles (pH pzc ~8.0). During this process, the high redox activity present in the
iron-nanoparticle system is essential for transformation of toxic contaminants into the
adsorbable form or co-precipitation products. Nevertheless, leaching of adsorbed As(III)
and As(V) was observed due to the weak electrostatic attraction between the adsorbed
species and binding sites.

The composition of various corrosion by-products continually changes as aging
proceeds, hence contributing to the variable mixed-valence nature of the core/shell
nanoparticles. Li and Zhang (2007) have further demonstrated that the oxidized shell
layer of the iron nanoparticles contains the adsorption sites whereby the heavy metal
removal occurs. Heavy metal ions such as Ni(II), diffusing through the oxide-shell,
would not only be adsorbed, but also be sequestrated by the zero-valent iron core via