Environmental Engineering Reference
In-Depth Information
peroxide) substitution reactions (Type B) [10, 101-104]. Type A reactions are described by the Catalyst model, Redox model,
and Galvanic model, while Type B reactions are described by the Redox model, Galvanic model, and Adsorption model.
1.3.3.2.1 Galvanic Type A Reactions
Type A reactions require e
−
, or H
+
, or O
2
(e.g., electron shuttle and Fenton Reactions
[10, 135]) to produce a product. They are favored by changes to the redox (Eh:pH) environment as [103]
b
log
[]
[]
B
A
=−°+
−
[.
0 0591
0 0591
m
/[ ]]
n
n
pH
(1.9)
Eh
∆
E
a
.
/
Type A reactions are theoretically reversible, but in practice many are effectively irreversible (e.g., nitrate removal [10, 24-33,
129], TCE removal [10, 17, 54-57, 136]). For example [57], PCE (C
2
Cl
4
) degrades to C
2
Cl
2
and TCE (C
2
CH
3
H). TCE
degrades to DCE (C
2
Cl
2
H
2
) and C
2
ClH. DCE degrades to VC (C
2
ClH
3
), C
2
H
2
, and C
2
H
4
. VC degrades to C
2
H
4
. C
2
Cl
2
degrades
to C
2
ClH, which then degrades to C
2
H
2
, which is then hydrogenated to C
2
H
4
. C
2
H
2
and C
2
H
4
are hydrogenated to C
2
H
6
and C
x
H
y
[94, 96].
E
a
for nitrate removal is in the range 21-46 kJ mol
−1
[6, 30].
E
a
for PCE/TCE/chlorinated hydrocarbon removal is in the
range 9.8-80 kJ mol
−1
[11, 136]. Since Δ
E
o
= −
RT
ln[K] and ln[K]= Δ
E
°/
RT
[103, 104], it follows that increasing temperature,
while maintaining a constant Eh and pH (when Δ
E
° > 0), will decrease the equilibrium ratio ([B]
b
/[A]
a
). It will also increase the
reaction rate (
k
observed
) (Eq. 1.4).
From Equation 1.3, it follows that the principal controls on a Type A remediation program are ZVM particle size, particle
type, mass ratio of pollutant:injected ZVM, and the injected ZVM:water/gas slurry concentration (g l) [137]. A relatively small
reduction in particle size (from >1000 to 50-300 nm) can allow a major reduction in the amount of ZVM required to remove
greater than 99% of the TCE in the groundwater (Fig. 1.1b and c).
From Equation 1.9, it follows that remediation is enhanced by increasing the availability of e
−
by increasing the O
2
saturation
of the pore water [138, 2, 139-141], while maintaining a constant, or decreasing, pH, and/or decreasing the aquifer pH by injec-
tion of CO
2
[94, 96, 2, 139-141] or addition of acidic components, for example, FeCl
y
, while maintaining a constant or
decreasing Eh [[10], [21], [95], [103], [142]]. It also follows (from Eqs. 1.3-1.5) that increasing the groundwater temperature
by water injection, steam injection, or gas injection may reduce the time and amount of n-ZVM required to achieve a specific
level of remediation from, for example, 100 days, to between <1 day and >50 days.
1.3.3.2.1.1 galvanic type a reactions: impact of oxygenation In oxygenated water, n-Fe
0
behaves as an iron-oxygen
redox cell [138], where the overall reaction is Fe
0
+ 0.5O
2
+ H
2
O = Fe(OH)
2
[Cathode {+} reaction : 0.5O
2
+ H
2
O + 2e
−
= 2OH
−
;
Anode [−] reaction: Fe
0
+ 2OH
−
= Fe(OH)
2
+ 2e
−
; pH = <10.53]. Fe
0
= Fe
2+
+ 2e
−
; Fe
2+
+ 2OH
−
= Fe(OH)
2
when the Fe
2+
concentration is greater than (log (Fe
2+
) = 13.29-2pH [103]. At a pH > 10.53, FeOOH
−
+ H
+
= Fe(OH)
2
when the FeOOH
−
concentration is greater than (log (FeOOH
−
) = −18.30-pH [103]. The relative stability of the Fe
2+
and FeOOH
−
ions is provided
by the molar relationship log[FeOOH
−
/Fe
2+
] = −31.58 + 3pH [103]. The addition of oxygen into the iron-air cell modifies the
standard redox cell used to produce Fe(OH)
2
from: (i) Fe + 2H
2
O = Fe(OH)
2
+ 2H
+
+2e
−
(Eh for phase boundary is [103]:
Eh = −0.047-0.0591 pH) to; (ii) Fe
0
+ 0.5O
2
+ H
2
O = Fe(OH)
2
(Eh for phase boundary is [103]: Eh = −1.29-0.0591 pH). The net
effect is an increase in the availability of e
−
, and an increase in the associated remediation rates. At any given time, the
concentration of e
−
in the water is [103]: e
−
[M l
−1
] = 10
((Eh (water) + 1.125)/0.0295)-pH (water))
. Magnetised n-Fe
0
will preferentially attract O
2
(e.g., Fe
0
+ O
2
+ 2H
+
= Fe
2+
+ H
2
O
2
; Fe
2+
+ H
2
O
2
= Fe
3+
+ 2HO + e
−
) [2]. Chlorinated organics are removed from oxygenated water
by an electron shuttle mechanism using Fe
0
(for Al
0
. A simple shuttle mechanism, where e
−
acts as a catalyst [130], is provided
as H
z
C
x
Cl
y
+ e
−
+ H = [H
z
+1
C
x
Cl
y
−1
] + Cl + e
−
. The electron shuttle model predicts that increasing the availability of e
−
by oxygen-
ation, or another mechanism, will increase the remediation rate. Experiments have established that oxygenation increases the
rate of remediation reaction (for As removal) by greater than 4 fold (over a 60-min period) but does not necessarily reduce Eh
[139-141], through the reversible equilibrium reactions Fe
0
+ 2H
2
O = Fe
2+
+ H
2
+ 2OH
−
; Fe
2+
+ H + e
−
= FeH
+
; FeH
+
+ O = FeOH
+
;
Fe
0
+ 2H
2
O + O
2
= 2Fe
2+
+ 4OH
−
; 2Fe
2+
+ nOH
−
= Fe(OH)
n
, etc. (Fig. 1.2). Effective anion removal (e.g., As) is enhanced in an
acidic environment [101-104, 2, 139-141]. This can be achieved by acidifying the water by CO
2
injection [139-141] or acid
injection [2, 139-141], prior to n-Fe
0
injection, and oxidation [139-141]. e
−
generation through a strategy of cyclic n-Fe
0
oxidation and reduction appears to be effective over greater than 4000 redox cycles [138].
1.3.3.2.2 Galvanic Type B Reactions
Type B remediation reactions occur when (i) the interaction of T, Eh, pH changes
resulting from the presence of ZVM, results in a change in K, which allows pollutant ions to be precipitated as oxides, peroxides,
hydroxides, sulfides, carbonates, etc (e.g., Appendix 1.A), and (ii) when the Fe
0
corrodes to one or more of n-FeH
n
+
, n-Fe(OH)
x
,
n-FeOOH, n-Fe-[O
x
H
y
]
(
n
+/−)
) (Fig. 1.2). Subsequent Fe ion substitution/adsorption (or Fe ion adduct formation) of cations and
