Environmental Engineering Reference
In-Depth Information
Both sorption and precipitation processes are reversible, and therefore may require
removal of the reactive materials and accumulated products, which is dependent on the
stability of the immobilized compounds and the geochemistry of the groundwater. Surface
precipitation and adsorption have been demonstrated to be predominant mechanisms for
the removal of As(V) and As(III) by both microscale and nanoscale ZVI. The spontane-
ous oxidation of ZVI in aqueous solution leads to the formation of Fe(II) and ferric oxides,
resulting in the uptake of As(V) on iron oxyhydroxides by a ligand-exchange mechanism
that replaces the surface-bonded hydroxide ion with an irreversible and stable arsenate or
inner-sphere bidentate complex [136].
4.7 Stability and Mobility of Nanoscale Iron Particles
In general, the mobility of particles in saturated aquifers is directly related to the number
of particle collisions with the porous media and Brownian diffusion [7,141]. It is also com-
monly assumed that the nanoscale iron particles allow the particles to overcome the limi-
tations of gravitational force and adhere to Brownian motion for particle movement and
dispersion. However, the mobility of nZVI particles in saturated porous media is usually
limited and the practical transport distances of nZVI are only a few centimeters or less for
bare and unsupported nanoparticles, presumably attributed to the iltration from solution
by attachment to aquifer materials and aggregation and sedimentation of nZVI to plug
pores [142,143]. Phenrat et al. [143] depicted that the nZVI particles at 20 nm aggregated
to micron-size colloids only within 10 min, subsequently assembling into fractal, chain-
like clusters due to the magnetic forces between nZVI particles. At an initial nanoparticle
concentration of 60 mg L
−1
, cluster sizes increase from 20 nm to 20-70 μm within 30 min
and rapidly precipitate from solution. This result also implies that the use of a stabilizer
or support to modify the nZVI surface for inhibiting particle aggregation and improving
mobility is needed.
Several studies have demonstrated the effectiveness of employing organic polymer or
support to homogeneously disperse nZVI nanoparticles to enhance their stability and
mobility, as shown in Table 4.4. Various stabilizers, including thiols, carboxylic acids,
surfactants, and polymers, have been used to prevent agglomeration of nanoscale iron
oxide and ZVM particles [30,144]. Sun et al. [145] found that the addition of polyvinyl
alcohol-co-vinyl acetate-co-itaconic acid (PV3A) could stabilize the synthesized nZVI for
>6 months. However, not all stabilizers can be applied to the nZVI because thiols and
carboxylic acid can be reduced by nZVI [146]. Schrick et al. [142] used anionic support
materials such as hydrophilic carbon (Fe/C) and polyacrylic acid (Fe/PAA) as stabilizers to
inhibit the aggregation and reduce the sticking coeficient of nZVI. Column tests showed
that nanoparticle diffusion is the dominant iltration mechanism, and the anionic surface
charges of Fe/PAA and Fe/C facilitated transport through sand- and clay-rich Chagrin
soil. Saleh et al. [147] employed block copolymer consisting of a hydrophobic inner shell
and a hydrophilic outer shell to modify a commercial nZVI. More recently, Xu et al. [29]
synthesized the bimetallic Ni/Fe and Pd/Fe nanoparticles in the PAA/polyether sulfone
(PES) composite membrane for the reductive dechlorination of TCE. Cross-linked PAA/
PES composite membranes containing metal ions as particle precursors were prepared by
heat treatment with ethylene glycol as the cross-linker. The average particle sizes of the
synthesized Ni/Fe nanoparticles were 5 ± 0.8 nm (TEM image,
n
= 150), which is a much
