Environmental Engineering Reference
In-Depth Information
table 17.2
general reactions for the remediation of several types of contaminants in water by nanoscale iron particles
General reactions
contaminants
equations
references
c
2
cl
4
+ 4Fe
0
+ 4H
+
→ c
2
H
4
+ 4Fe
2 +
+ 4cl
−
Tetrachloroethene
17.3
[9]
rcl + Fe
0
+ H
+
→ rH + Fe
2 +
+ cl
−
p-chlorophenol
17.4
[25]
nO
3
−
+ 4Fe
0
+ 10H
+
→ nH
4
+
+ 4Fe
2 +
+ 3H
2
O
nitrate
17.5
[32]
2Fe
0
+ 2H
2
crO
4
+ 3H
2
O → 3(cr
.67
Fe
.33
)(OH)
3
+ FeOOH
chromium
17.6
[30]
2Fe
0
+ 3pb(c
2
H
3
O
2
)
2
+ 4H
2
O → 3pb
0
+ 2FeOOH + 4(Hc
2
H
3
O
2
) + 2H
+
lead
17.7
[30]
c
2
cl
4
+ 5Fe
0
+ 6H
+
→ c
2
H
6
+ 5Fe
2 +
+ 4cl
−
Tetrachloroethene
17.8
[36]
c
2
Hcl
3
+ 4Fe
0
+ 5H
2
O → c
2
H
6
+ 4Fe
2 +
+ 5OH
−
+ 3cl
−
Trichloroethene
17.9
[38]
cheng et al. [25] revealed that nanoscale Fe
0
successfully reduced chlorophenols (rcl) to phenol (rH), while some of the
chlorophenols were transformed to oxidation compounds, as given by equation 17.4.
2
Fe HO Fe
0
++→+
4
+
2
2
+
2
HO
(17.1)
2
2
Fe HO Fe H H
0
+ →++
2
2
+
2
−
(17.2)
2
2
In the past few years, nanoscale zero-valent iron (nZVI), consisting of an Fe
0
core and a predominantly magnetite shell [31],
has become one of the most common adsorbents for the rapid removal of arsenic, including both arsenite (As(III)) and arsenate
(As(V)), [26, 27, 29] as well as halogenated hydrocarbons, such as trichloroethylene (Tce) [31], nO
3
32-
, cr, and pb [30]. Kanel
et al. [27] revealed that the removal of As(III) is due to the spontaneous adsorption and coprecipitation of As(III), with the reac-
tive surface sites of nZVI as oxidation products. X-ray diffractograms demonstrated that nZVI was converted to amorphous iron
(III) oxide (Fe
2
O
3
), iron (III) hydroxide, magnetite (Fe
3
O
4
), or maghemite (γ-Fe
2
O
3
) corrosion products, mixed with lepidocroc-
ite (γ-FeOOH). Hence, As(III) adsorption was enhanced with the increase of corrosion products.
To achieve a higher adsorption rate and removal efficiency, Hwang et al. [32] investigated the optimum conditions for the
reduction of nO
3
−
using nZVI. Their results showed that the supply of excessive thermal energy to the system increased the rate
of nitrate reduction to nitrite and ammonia, where the activation energy barrier was overcome. In addition, a lower aqueous pH
favors nitrate removal, in which ferrous hydroxide (Fe(OH)
2
) and other protective layers deposited on the nZVI surface were
dissolved, thereby yielding fresh reactive sites for the reduction process. The general reduction process of nitrate is given by
equation 17.5. Suzuki et al. [34] reported that the reduction of nitrate can be further enhanced by augmenting aqueous iron ions
(Fe
2+
), where the role of Fe
2+
has been revealed to facilitate the electron transfer from the nZVI core to nitrate. Furthermore, a
higher adsorbent dose can increase the removal efficiency of arsenate, and complete arsenic removal is achievable in acidic and
neutral pH ranging from pH 3 to pH 7, as reported by Kanel et al. [29].
Although the addition of nZVI to the aquifer showed no effect on the geochemistry and indigenous microbial communities
during Tce removal, as reported by Kirschling et al. [31], materials such as Ac are recommended as support to nZVI to avoid
loss and prevent the agglomeration of nZVI nanoparticles in the treated water. The use of support materials can also enhance
the specific surface area of nZVI, thereby leading to higher adsorption capacity [30]. Zhu et al. [26] reported that Ac-supported
nZVI was easily regenerated by elution with 0.1 M sodium hydroxide (naOH) for 12 h. Great reusability was shown because
there was no observed significant reduction in As removal efficiency after eight cycles of adsorption and desorption. Meanwhile,
ponder et al. [30] revealed that supported nZVI showed great performance in the remediation of cr(VI) and pb(II) from
aqueous solutions. Fe
0
was rapidly oxidized to goethite (α-FeOOH), while cr(VI) and pb(II) were reduced to cr(III) and zero-
valent lead (pb
0
), respectively, as given by equations 17.6 and 17.7.
Fe becomes oxidized when it is in contact with a less active metal, such as Ag and palladium (pd). Hence, the said concept is
applied in water remediation to enhance the reactivity of monometallic nZVI [35]. bimetallic nZVI, such as Ag/Fe, pd/Fe, and
ni/Fe, have been shown to be effective in the transformation of chlorinated compounds [4, 36-38]. Xu et al. [4] evaluated the
effectiveness of Ag/Fe bimetallic nZVI in the transformation of hexachlorobenzenes to tetra-, tri-, and dichlorobenzenes in an
aqueous solution at room temperature. The results showed that the transformation rate was positively correlated to the metal
loading, as well as the location of chlorine on the benzene ring. lien et al. [36] and Tee et al. [38] demonstrated the utilization of
pd/Fe and ni/Fe bimetallic nZVI on the degradation of chlorinated ethenes. Trichloroethene (c
2
Hcl
3
) and c
2
cl
4
were reduced
completely, with c
2
H
4
as the primary reduction product, in accordance with equations 17.8 and 17.9, as given in Table 17.2. The
presence of metal on the nZVI surface significantly increased its reactivity due to the increase in the specific surface area for
reactivity. The degradation of Tce in an aqueous solution is shown in Figure 17.1. Moreover, Tian et al. [37] demonstrated that
