Environmental Engineering Reference
In-Depth Information
anions (Fig. 1.3) results in pollutant cation/anion removal from the water, with precipitation within a n-Fe-[O
x
H
y
] structure-Adsorption
Model [101, 102].
At any specific (constant) pH (e.g., pH = 7), both cations and anions are removed [143]. The total amount of cations removed
may increase, or decrease, with changing temperature [144], and may be a function of both pollutant concentration, and the
concentration of other anions (e.g., humic acids) within the groundwater [144]. Anion and cation removal increases with
time [145], and the ratio of cations:anions removed by incorporation/substitution varies with the pollutant ion adduct:Fe
n
+
ratio
in the water [144]. Fe(II) ions (and other cations) hydrolize on the surface of FeOOH particles [146].
1.3.3.2.2.1 galvanic type b reactions: pom and hpom products Polyoxometalates (POMs) (Fig. 1.2) are self-assembly
accretionary molecules that take the form of a sandwich composed of a central core fragment Fe
II
n
Fe
III
m
O
z
.(H
2
O)
y
surrounded
by external fragments of Fe
II
n
Fe
III
m
O
z
.H
2
O
y
linked by two distinct edge sharing dimeric clusters of (Fe(OH)
2
[147, 148]. The
formation of POMs greatly increases the rate of n-ZVM water remediation by serving as an electron shuttle and ion chelating
agent [149]. A POM (Fig. 1.2) may potentially remove (Fig. 1.3) greater than 10 g Cation g
−1
n-Fe
0
. The associated by-product
Type A reactions, involving e
−
catalysis, may remove greater than 1 g pollutant g
−1
n-Fe
0
.
Heteropolyoxometallates (HPOM) are derived from metal cages of the form (MO
n
)
x
, which incorporate anion templates of
the form (AO
x
n
−
) [150]. However, their pentagonal building blocks form around a pentagonal bipyramidal core (MO
n
), which
can be hydrated [150]. A typical HPOM nucleates around a cluster of 2 Fe ions (oxidation state 2
+
or 3
+
or 4
+
). They seed a
linkage, which allows clusters of pentagonal, or another structural form, of M(1)O
n
to accrete [150]. In saline water, the
monomer may take the form [K
8+
x
na
9+
y
H
29+
z
[H
34
M(1)
119
M(2)
8
Fe
2
O
420
(H
2
O)
34+
n
]]
(8−
x
−
y
−
z
)−
; the diamer may take the form [K
16+
x
na
1
9+
y
H
57+
z
[H
34
M(1)
119
M(2)
8
Fe
2
O
420
(H
2
O)
74+
n
]]
(16−
x
−
y
−
z
)−
[150]. M(1) and M(2) are different metal cations incorporated in the HPOM
from the water. An individual HPOM molecule may have a size of less than 3 nm [150]. HPOM formation is slow and conversion
of 4% of the n-Fe
0
to HPOM may take greater than 4 weeks [150]. However, they are highly effective remediation agents [149]
with a potential absorption capacity of greater than 100 g pollutant cation g
−1
n-Fe [150]. Injection of 100 kg n-Fe
0
into ground-
water can potentially result in greater than 400 kg of pollutant cations being removed in HPOM structures over a 4-week period.
1.3.3.2.2.2 galvanic type b reactions: impact of hydrogen In poorly oxygenated water, the n-Fe
0
gradually corrodes
(Appendix 1.C, Fig. 1.2) to form a corrosion zone of Fe-hydroxides and peroxides at the n-Fe
0
-water interface [10]. The inter-
face acts as a hydrogen electrode (cathode) and the Fe
0
acts as a current electrode (anode). During remediation, Fe/Cu acts as a
cathode to an Al anode. The Cu acts as a cathode to a Fe anode [151-154]. The basic process involves charge transfer (and OH
ion formation) at the ZVM-water interface and includes electron transfer via conduction, electron insertion into active sites, and
conduction by hopping through electron-deficient lattice sites within the active material [151].
In a diabatic environment, the perpetual oscillation and change in temperature (Fig. 1.1i and j), results in a perpetual
oscillation between forward and backward reactions (Fig. 1.2). This oscillation allows the hydrides/hydroxides/peroxides/
oxides (Fig. 1.2) to be used as stores of protons (H
+
) and electrons (e
−
) [151-154], which can be accessed for Type A remedia-
tion reactions. All changes that increase the oxidation number of the ZVM ion adducts, effectively result in electron storage
(charging) occurring and vice versa [151] (Fig. 1.2). Ions (aqueous or solid) that contain an oxidation number greater than the
stoichiometric charge are overcharged [151] (Fig. 1.2).
1.3.3.2.2.3 galvanic type b reactions: discharge During discharge, electrons flow from the current electrode (Fe
0
particles [Fe
0
= Fe
n
+
+
n
e
−
] and other ZVM and ZVM adducts (Figs. 1.2 and 1.3, Appendix 1.C)), through the hydroxides, per-
oxides (Fig. 1.2)-charge transfer sites [e.g., FeOOH + H
2
O + e
−
= Fe(OH)
3
+ e
−
; FeOOH + OH
−
+ H
+
+ e
−
= Fe(OH)
3
+ e
−
] to the
hydrogen electrode (Cu
0
particles) [151]. Hydrogen generation occurs at the particles, which act as a hydrogen electrode
[2H
2
O + 2e
−
= 2H
+
+ 2OH
−
+ 2e
−
; 2H
2
O + 2e
−
= H
2
+ 2OH
−
].
1.3.3.2.2.4 galvanic type b reactions: recharge During recharge, the electron flow is reversed [151] and oxygen forms
at the cathode as a by-product of electron generation [cathode-electron generation: 4OH
−
= O
2
(g) + 2H
2
O + 4e
−
; hydroxide
reduction to peroxide in the charge transfer sites: Fe(OH)
3
= FeOOH + H
2
O + e
−
; Fe ion reduction to Fe
0
] [151].
1.3.3.2.2.5 galvanic type b reactions: gas evolution During recharge, oxygen accumulates in the charge transfer sites
[151]. During discharge, hydrogen accumulates in the charge transfer sites [151]. Both gases show very different morphologies
at the ZVM-water interface [10]. Oxygen bubbles tend to form in, and are commonly encased by, the cathodic particles (e.g.,
Cu) [10], and form rapidly after a ZVM mixture (Fe+Cu, Fe+Cu+Al (Fig. 1.4a-d)) is placed in the reactor. The initial corrosion
reactions are recharge reactions forming FeOOH. The FeOOH forms active charge sites. The formation of hydrogen gases
initially results in the adsorption of the O
2
gas bubbles, with no hydrogen gas discharge (i.e., 2H
2
+ O
2
= 2H
2
O + heat) [151].
