Nanoparticles for Treatment of Chlorinated Organic Contaminants - Nanotechnologies for Water Environment Applications - page 101

preparation. The fabrication of NZVI is based on a chemical reaction between ferric
chloride and sodiumborohydride as follows:
2FeCl 3 + 6NaBH 4 + 18H 2 O Æ 2Fe(s) + 21H 2 + 6B(OH) 3 + 6NaCl
The procedure of fabricating NZVI starts with slowly adding freshly made ferric
chloride solution to a reaction vessel containing solid NaBH 4 (Li et al., 2003) or adding
a NaBH 4 solution into a ferric chloride solution (Lien and Zhang 2001) under an inert
atmosphere of nitrogen during the entire process. To achieve rapid and uniform growth
of iron nanoparticles, the excessive mass ratio of borohydrate to ferric ions is necessary.
When the reaction is finished, the black precipitates containing iron nanoparticles can be
removed by a magnetic field. After washing the formed NZVI from the precipitates with
DI water, the iron nanoparticles are air-dried and ready for use.
Another method that received special attention is via microemulsion. In this
method, the aqueous reverse micelles are dispersed in a continuous oil phase. Because
the aqueous reaction is confined within the reverse micelles, the size and shape of the
formed nanoparticles can be controlled to produce non-aggregated nanoparticles. The
microemulsion system is constituted of cetyltrimethyl-ammonium bromide (CTAB) as
surfactant, 1-butanol as the co-surfactant and n-octane as the oil phase (Li et al., 2003).
By adding a ferric chloride solution into the microemulsion and mixing thoroughly, a
transparent yellow solution is formed. After adding solid NaBH 4 into the yellow
solution, the reaction occurs, and the black solid is dispersed in the solution under a
nitrogen atmosphere. The new-formed black NZVI can be separated by centrifugation.
The solution method usually produces amorphous iron nanoparticles, the size of
which ranges from 20 to 70 nm with a mean size of around 50 nm. These iron particles
are often lined up as a chain. In contrast, the iron nanoparticles produced by
microemulsion have a uniform spherical shape with an average particle size of 10 nm.
For stable dispersion of NZVI, a novel method was recently proposed to use
polyvinyl alcohol-co-vinyl acetate-co-itaconic acid (PV3A) as a surfactant (Sun et al.,
2007). In this method, the freshly made iron nanoparticles by the solution method were
mixed with PV3A by a mechanical stirrer at a speed of 60 rpm. The results of particle
size distribution measurements showed that iron particles in the presence of PV3A had a
median size of 7.9 nm and a mean size of 15.5 nm while iron particles without PV3A
had a median size of 59.4 nm and a mean size of 105.7 nm. Therefore, PV3A can
effectively improve iron nanoparticle stabilization. NZVI particles with PV3A treatment
exhibit improved delivery capability for environmental remediation applications.