Environmental Engineering Reference
In-Depth Information
up to 30× higher than those for iron ilings or iron powder on an Fe molar basis. Kanel et al.
[133] used nZVI for the removal of As(V). Mössbauer spectroscopy showed that the nZVI is
a core-shell structure in which 19% were in zerovalent state with a coat of 81% iron oxides.
A total of 25% As(V) were reduced to As(III) by nZVI after 90 days. As(V) adsorption
kinetics were rapid and occurred within minutes following a pseudo-irst-order expres-
sion with a
k
obs
of 0.02-0.71 min
−1
. In addition, a bimetallic Ni/Fe system was employed for
the removal of arsenate [136]. The pseudo-irst-order rate constants (
k
obs
) for As(V) removal
by Ni/Fe was 0.091 min
−1
, which is 2.5× faster than that by nZVI alone (0.037 min
−1
).
Several parameters, including temperature, initial metal concentration, pH, and inor-
ganic anions, inluence the removal eficiency and rate of metal ions by ZNI. The increase
in reaction temperature from 25°C to 65°C increased the removal rates of As(VI), while
competitive adsorption of phosphate and sulfate was also observed [136]. At high initial
metal concentration, the removal rate shifted from pseudo-irst-order rate kinetics to zero-
order rate expression. The pH value of the solution is also an important factor inluencing
the removal capacity of Se(VI). High removal eficiency of Se(VI) by both nanosized ZVI
and bimetallic Ni/Fe were observed at pH <8.0. Kumpiene et al. [134] evaluated the effects
of ive parameters including pH, oxidation-reduction potential (ORP), liquid-to-solid
(L/S) ratio, organic matter, and microbial activity on the mobility of heavy metals in ZVI-
stabilized soils. They found that the pH value was the most important factor inluencing
the mobility of Cr, Cu, and Zn in the ZVI-stabilized soil, while the L/S ratio and microbial
activity were important factors for As stability. In addition, the studied environmental fac-
tors mostly affect the mobility of Zn, followed by Cu, As, and Cr. Also, the coexistence of
metal ions inluences the removal eficiency of another metal ion by ZVI. Zhang et al. [137]
found that the removal of Se(VI) by ZVI appeared to be partially attributed to the reduc-
tion of Se(VI) to Se(IV) by Fe(II) oxidized from ZVI, followed by rapid adsorption of Se(IV)
onto iron oxyhydroxide. The coexistence of As(V) has little inluence on Se(VI) removal
because As(V) was removed at much faster rates than Se(VI). However, addition of Mo(VI)
signiicantly decreased the removal eficiency of Se(VI) by ZVI.
Several mechanisms, including adsorption, precipitation, and biologically mediated
transformation, have been proposed to explain the removal of inorganic contaminants
[23,132,138-140]. Direct reduction of metal ions occurring at the iron surface results in the
electrochemical reduction of metal species onto the iron surface or cementation, which
can be theoretically predicted by the standard reduction potentials of the metals [41].
Immobilization of the metal ions occurs by sorption to the reactive medium or precipita-
tion from the dissolved phase. Precipitation of metals by the reduction to a less soluble
form is a combination of transformation process followed by an immobilization process.
Moreover, sorption is an abiotic reaction where the contaminant is attracted to the sur-
face by hydrophobic interaction, electrostatic attraction, and/or surface complexation [17].
Usually the basis for a sorption reaction is the corrosion of ZVI by the formation of mixed-
valent and crystalline ferric oxides such as green rust, lepidocrocite, and magnetite. The
iron oxides on the surface of ZVI are generally covered with hydroxyl groups in aqueous
solutions, and subsequently undergo the surface complexation reaction with divalent cat-
ionic metal ions [59].
≡ S-OH + Me
2+
↔ ≡ S-OHMe
+
+ H
+
(4.4)
2 ≡ S-OH + Me
2+
↔ ≡ (S-OH)
2
Me
+
+ 2 H
+
(4.5)
≡ S-OH + Me
2+
+H
2
O ↔ ≡ S-OHMeOH + 2 H
+
(4.6)
