Environmental Engineering Reference
In-Depth Information
reaction with ZVI has been found to effectively reduce perchlorate under anaerobic condi-
tions. Biological reduction of perchlorate by autotrophic microorganisms attached onto
ZVI was investigated in both batch experiments and low-through columns [129,130]. The
hydrogen gas produced from the corrosion of ZVI could be used as an electron donor
by the autotrophic microorganisms to reduce perchlorate to chloride. The degradation
followed Monod kinetics with a normalized maximum utilization rate and half-velocity
constants of 9200 μg (g h)
−1
and 8900 μg L
−1
, respectively. In addition, an increase in the
biomass measured by optical density at 660 nm (OD
660
), from 0.025 to 0.08, led to a corre-
sponding 4-fold increase in perchlorate reduction rate. In the presence of nitrate, however,
the perchlorate reduction rate was reduced but not completely inhibited. In addition, the
mass of microorganisms attached on the solid ZVI/solid in the low-through column was
found to be 3 orders of magnitude greater than that in the pore liquid, indicating that the
combination of autotrophs with ZVI is a promising alternative for perchlorate reduction in
the contaminated subsurface environments.
The removal eficiency of perchlorate was strongly dependent on the dosage and surface
area of ZVI. Similar surface area-normalized perchlorate removal rates were calculated
for the batch and long-term column studies, which means that the perchlorate removal
occurs at the iron surface [125,128]. It has been found that the perchlorate is initially sorbed
to the iron surface, followed by a reduction to chloride. However, the reaction mechanism
for perchlorate reduction is not well established. Moore et al. [125] found that perchlorate
reduction was not observed on electrolytic sources of iron or on a mixed-phase magnetite
oxide, suggesting that the reactive iron phase for perchlorate reduction is neither pure ZVI
nor mixed oxide alone. They proposed that a mixed valence iron hydr(oxide) coating or a
sorbed Fe
2+
surface complex represents the most likely sites for the reaction. Perchlorate is
a strong oxidant with the standard redox potential of 1.389 V. Therefore, perchlorate ion
is loathe to accept an additional electron because it has no low-lying unilled electronic
orbitals and the transfer of an oxygen atom is thus required for a reduction reaction [131].
Huang and Sorial [128] proposed that the perchlorate ion consists of one Cl atom in the
center of a tetrahedral structure surrounded by four oxygen atoms. The outside iron atoms
at the oxide ilm have the potential to associate with oxygen atoms in perchlorate and may
eventually distort the structure that perhaps allows the loss of one oxygen atom to become
chlorate. Since chlorate can be further reduced by ZVI [125], this oxygen transfer process is
the rate-limiting step for perchlorate reduction.
4.6.4 Removal of Heavy Metals
In addition to the reduction of halogenated organic compounds and anions, both microscale
and nanoscale ZVI have been demonstrated to be effective materials for the removal of dis-
solved metal ions such as Se(VI), As(V), Cr(VI), and Pb(II) [38,132,133]. Removal of divalent
ions containing Cu(II), Ni(II), Cd(II), Zn(II), and Pb(II) has been well documented [10,134].
The removal of divalent metal ions in acid mine drainage by ZVI was also reported under
acidic conditions [10]. In addition, the reduction of Cr(VI), Se(VI), and As(V) by ZVI has
been widely examined, and it was found that these toxic metals could be reduced by ZVI
to their low oxidation states and then coprecipitated with ferric oxyhydroxide on the sur-
face of the ZVI particles [135]. Ponder et al. [38] developed supported ZVI nanoparticles
(ferrogels), which are 10-30 nm in diameter, for the remediation of metal ions in aqueous
solution. The ferrogels were rapidly separated and immobilized Cr(VI) and Pb(II) from
aqueous solution by reducing the Cr(VI) to Cr(III) and the Pb(II) to Pb(0) while oxidizing
ZVI to goethite. In addition, the rates of remediation of Cr(VI) and Pb(II) by ferrogels are
