Environmental Engineering Reference
In-Depth Information
A non-catalytic redox remediation reaction can be expressed as
a
[A] +
m
H
+
+
n
e
−
=
b
[B] +
c
[H
2
O]. This generic equation
allows the relationship between Eh, pH,
E
, Δ
G
°, and ([B]
b
/[A]
a
) associated with each remediation reaction to be summarized
[103, 131] as
1. when A is an aqueous ion (oxide, hydroxide, peroxide) and B is a precipitate as Eh = Δ
G
°/(
nF
) + (0.0591/
n
) log([B]
b
/
[A]
a
) + ([−0.0591
m
/
n
[pH]]); where Δ
G
°/(
nF
) = Δ
E
° = −
RT
ln[K]: pH = −log H
+
; (H
2
O)
c
= 1
2. when [A] and [B] are dissolved substances (M L
−1
), and
m
> 0 and
n
= 0, then, log([B]
b
/[A]
a
) = Δ
E
° +
m
[pH]
3. when [A] and [B] are dissolved substances (M L
−1
), and
m
= 0 and
n
> 0, then, Eh = Δ
E
° + (0.0591/
n
) log([B]
b
/[A]
a
)
4. when [A] and [B] are dissolved substances (M L
−1
), and
m
= >0 and
n
> 0, then, Eh = Δ
E
° + (0.0591/
n
) log([B]
b
/
[A]
a
) + ([−0.0591
m
/
n
[pH]]
5. when [A] and [B] are solid substances, and
m
> 0 and
n
> 0, then, Eh = Δ
E
° + ([−0.0591
m
/
n
[pH]])
6. when [B] is a solid substance and [A] is a dissolved substance (M l
−1
), and
m
> 0 and
n
= 0, then, log([A]) = Δ
E
° +
m
[pH]
7. when [B] is a solid substance and [A] is a dissolved substance (M l
−1
), and
m
> 0 and
n
> 0, then, Eh = Δ
G
°/(nF) + (0.0591/
n
)
log([A]) + ([−0.0591
m
/
n
[pH]]).
The partial pressure of the gaseous reactants/ions (e.g., H, O, CO, CO
2
, C
x
H
y
, etc.) alters
k
observed
, as
k
observed
=
k
(P
p
)
xm
and
K
p
=
K
(RP)
cp
, [21, 131].
The interactions between Eh, pH, partial pressure of (
p
H
2
), and partial pressure of [O
2
] (
p
O
2
) are defined by the relationships
[103]: (i) Hydrogen: Eh = 0.00-0.0591 pH-0.0295 log (
p
H
2
) [2H
+
+ 2e
−
= H
2
(g, aq)], (ii) Oxygen: Eh = 1.228-0591 pH + 0.0147
log (
p
O
2
) [2H
2
O = O
2
(g, aq) + 4H
+
+ 4e
−
].
These relationships imply [10, 103, 104] that if ZVM is able to alter the Eh and pH of water, that the resultant remediation
(e.g., Appendix 1.B) is both non-catalytic, and a natural consequence of an Eh, pH modification of pore water chemistry. This
model assumes that the primary role of ZVM during the remediation process is to alter the water Eh and pH [10].
1.3.3
Galvanic model
The presence of ZVM creates two primary products in water [103, 104]. They are e
−
and H
+
. Secondary products include H, H
2
,
O, O
2
, O
2
−
, O
2−
, OH, OH
−
, O
2
H, O
2
H
−
, H
2
O
2
[103, 104, 132]. The ZVM gradually degrades to produce ZVM ions [Fe
n
+
, Al
3+
,
Cu
n
+
] and associated ion adducts [103, 104, 132].
1.3.3.1 Diabatic Environment
Remediation by ZVM injection into soil, or groundwater (<25 m depth), takes place in a
diabatic environment where the temperature,
T
, is a function of atmospheric temperature [133, 134]. T varies during the day
and seasonally over the year [133, 134]. Daily variations in
T
decrease with increasing depth [133]; daily variations of
T
are
within the range <1 to >15°C; annual variations are within the range <1 to >50°C. Changing
T
will change the partial pressures
of H
2
(and O
2
) and one or more of pH, Eh, K, Q,
k
observed
[103, 104, 131, 132]. Where the remediation reaction is reversible, and
E
a
> 0, decreases in temperature may result in
k
d
<
k
−
d
and reversal of the remediation reaction (and vice versa). When Eh and pH
are largely unaffected by changes in
T
, and ion removal is by precipitation (Appendix 1.B) then, a change in
T
of 1°C changes
log([B]
b
/[A]
a
) by (
R
ln[K])/(0.0591/
n
) [103]. Consideration of temperature variation is therefore a major variable when predict-
ing the effectiveness of a groundwater remediation program.
1.3.3.1.1 Redox Trajectory
Placement of n-ZVM in a diabatic groundwater environment results in a gradual change in Eh, pH
over time [10, 17, 135] as the oxidation state of the Fe
0
increases (Fig. 1.1b-d). The redox trajectory is a function of Fe
0
particle
size [10, 17, 135] (Fig. 1.1b and c), Fe
0
:water ratio [10, 17] (Fig. 1.1b and c) and ZVM composition [10, 135] (Fig. 1.1d) [10,
135]. Daily variations in temperature [134] force an oscillation in both Eh and pH [10, 96, 135] (Fig. 1.1e and f), while maintaining
a relatively constant hydrogen partial pressure (
p
H
2
) (Fig. 1.1g).
p
H
2
can be independent of ZVM composition (Fig. 1.1g).
The general redox oscillation (Fig. 1.1e and f) is accompanied by a cyclic oscillation in EC [10, 135] (Fig. 1.1h), which
reflects adjusting Fe(OH)
x
, FeOOH, Fe
x
O
y
composition [10, 96, 135]. Each oscillation cycle commences with a large swing in
EC, which dampens with time (Fig. 1.1h). These EC oscillations (Fig. 1.1h) reflect oscillations in Eh, pH, K, log(B]
b
/[A]
a
) and
are directly linked to cyclic changes in temperature (Fig. 1.1i and j).
1.3.3.2 Remediation Types
Fe
0
remediation reactions fall into two basic groups: (i) irreversible, ZVM, or e
−
, catalyzed
reactions, or reaction sequences (Type A) (e.g., nitrate, PCE removal [10]), and (ii) reversible redox, or ZVM (oxide, hydroxide,
