In-situ technologies for groundwater treatment: the case of arsenic - In-Situ Remediation of Arsenic-Contaminated Sites - page 15

supports, or they are obtained through the treatment of particles of higher size in colloidal dis-
persions, by means of colloidal mills, ultrasound, etc. In the second group, the main method is
reduction of metal ions in solution in conditions that favor the ulterior formation of small metal
aggregates. The main disadvantage of the chemical synthesis in aqueous phase is the ample dis-
tribution of sizes of the metal particles and their relative low stability (Chatterjee, 2008; Ponder,
2003).
Numerous methods have been developed for the manufacture of metallic nanoparticles, includ-
ing chemical vapor deposition, inert gas condensation, pulsed laser ablation, spark discharge
generation, sputtering gas-aggregation, thermal decomposition, thermal reduction of oxide
compounds, hydrogenation of metallic complexes and aqueous reduction of iron salts. These
manufacturing methods can be considered as either “bottom up” or “top down” approaches. The
former involves physical or chemical methods to construct a nanomaterial from basic building
blocks, such as atoms or molecules. The latter involves physical or chemical methods to breakdown
or restructure a bulk material to the nanoscale (Crane and Scott, 2012).
A simple method of synthesis of nanoparticles is in solution, from iron salts (for exam-
ple, FeCl 3 4H 2 O), which are reduced by a reductant such as

hydrazine, sodium borohydride or hydrogen, or another especial reductive medium (Chatterjee,
2008). The borohydride reduction of ferrous salts is the most widely studied method within
academia (Wang and Zhang, 1997). Sodium borohydride has a reductive power adequate for
several metals in normal conditions. For Fe(III) compounds:

· 6H 2 O, Fe 2 (SO 4 ) 3 · 5H 2 O or FeCl 2 ·

Fe(H 2 O) 3 6 3BH 4 + + 3H 2 O → Fe(0)(s) + 3B(OH) 3 + 10 . 5H 2 (g) (1.14)

For Fe(II) compounds:

Fe(H 2 O) 2 4 + 2BH 4

+ 2H 2 O → Fe(0)(s) + 2B(OH) 3 + 7H 2 (g) (1.15)

The sodium borohydride can be added in high excess with respect of the ferric or ferrous ion in
order to achieve a rapid and uniform growth of iron crystals. The synthesis at lower concentrations
is also satisfactory (Ponder, 2003). Although it is a physically simple process, reduction of aqueous
metal salts through borohydrides is a complex reaction, sensitive to a variety of parameters, such
as pH (which affects the particle size), and the concentration of the borohydride solution and
its addition rate (which alter the composition of the reaction product). In some studies, it has
been proved that the obtained particles were generally lower that 0.2 µ m (mostly between 1 and
100 nm) (Elliott and Zhang, 2001), with a specific area around 35 m 2 g − 1 (Shen et al ., 1993) or
close to 60 m 2 g − 1 (Morgada et al ., 2009), whereas the commercial iron only attains 0.9 m 2 g − 1
(Nowack and Bucheli, 2007). Some precautions must be taken during the process of synthesis
of the nanoparticles to avoid the oxidation of iron due to the presence of oxygen, water and salts
formed in the process of reduction (García et al ., 2008).The method produces highly reactive
nZVI; however, the nanoparticles are often highly polydispersed, ranging over tens to hundreds of
nanometers in size and thus significantly prone to agglomeration (Nurmi et al ., 2005; Scott et al .,
2010; Sun et al ., 2006). Expensive reagents and production of large volumes of hydrogen gas
also preclude its industrial application (Hoch et al ., 2008). Scanning electron microscopy images
of Fe(s) particles obtained by reduction with sodium borohydride shown particle size between 20
and 120 nm, with a medium size of 77 nm for those obtained from Fe(III) salts and of 87 nm for
those obtained from Fe(II) salts (Elliott and Zhang, 2001).
The main obstacle in the preparation of nanoparticles is their tendency to aggregate and form
particles of higher size, reducing their high surface energy. To avoid this, they can be prepared
in the presence of a surfactant forming a microemulsion keeping the particles separated. The
microemulsions form reverse micelles and are of especial interest because they can be introduced
into a variety of reagents in the aqueous domains of nanometric size to produce reactions confined
in the reverse micelles, achieving materials of controlled size and shape (Egorova and Revina,
2000). In these systems, the aqueous phase is dispersed in form of microdrops, in whose nuclei