Environmental Engineering Reference
In-Depth Information
1.4.1.2 Reduction of Type B Remediation Costs
Type B remediation (Appendix 1.B, Fig. 1.3) can be undertaken using
n-ZVM or ZVM corrosion products (Fig. 1.2). The remediation occurs over a long timeframe (days to years), which is controlled
by the ZVM concentration, Eh, pH, and ion type. The amount of contaminant removed increases with time, and is typically in
the range less than 0.01-0.3 g contaminant g
−1
n-Fe
0
. The galvanic model allows the timeframe required to remove specific pol-
lutants (Appendix 1.B), and the total amount of pollutant removed to be reduced by controlling the Eh, pH environment, and
ZVM composition with time. Remediation rates during active galvanic management can potentially exceed 1 g pollutant g
−1
n-Fe
0
. Active subsurface Eh:pH management using the galvanic model may be able to reduce the treatment costs to less than
$2 MM/acre (i.e., $5-$150 m
3
soil/aquifer).
1.5
conclusions
Groundwater remediation (Type A and B) using ZVM is typically undertaken by ZVM infiltration, or pneumatic injection of
ZVM [17, 137] using a passive process of injection followed by monitoring over a number of years. This approach, which
assumes that the catalytic model applies, provides little, or no, effective day to day control over the rate of remediation. The
observation that bimetallic ZVM (e.g., n-Fe
0
+ one or more metals where
E
° < E
o
Fe
II
(Appendix 1.C)) shows increased reactivity
(and delayed rates of Fe
0
oxidation) when compared with n-Fe
0
[179] is consistent with the galvanic model. The close proximity
of the cathodic and anodic species coupled with diabatic oscillations results in continual oscillating reduction and oxidation of
the bi-metal species. In mono n-Fe
0
the initial oxidation (formation of Fe-(OH)
2
) (associated with galvanic oscillation between
Fe
II
and Fe
III
(Appendix 1.C)) results initially in exponential particle growth [179]. This switches to logarithmic particle growth
as the cathodic species Fe(OH)
3
, FeOOH, and Fe
x
O
y
start to form [179]. The associated by-products, which react [10] to remove
contaminants, are [e.g., 103] e
−
, H, H
+
, OH, OH
−
, O
2
H, O
2
H
−
, H
2
O
2
, O, O
−
, O
2
−
, and O
2−
. Particle growth and agglomeration is
rapid with 50 nm particles forming agglomerations of greater than 1 mm within 21 days (e.g., Fig. 1.4e, see also [10]). n-FeO
x
H
y
expulsion with (H
2
, O
2
) gas bubbles results in a rapid and effective dispersion of colloidal Fe
II
-Fe
III
galvanic cells throughout
the water column. These grow with time (Figs. 1.2 and 1.3) to form colloidal particles greater than 1 mm in diameter, which
settle on the ZVM-water interface [10]. The n-colloid clouds in the water within the diffusion environment tend to be mono-
specific, color coordinated (e.g., white = Fe(OH)
2
; yellow/orange = Fe(OH)
3
; blue-green = green rust; dark-red brown/
black = FeOOH; oxygenated blood red = Fe
2
O
3
; grey/black = Fe
3
O
4
), and indicate the galvanic charge status within the reaction
environment (Fig. 1.2). The dominant colloid species changes with Eh, pH, and charge status of the water. The accreting
growing colloidal particles, which can grow from 50 nm to greater than 5 mm, obtain buoyancy from H and O, which are pre-
sent on the active sites.
An understanding of the corrosion of n-Fe
0
in the remediation environment and the controls that allow the net reaction directions
(Fig. 1.1e-h) to be switched between recharge (formation of Fe
III
ion adducts) and discharge (formation of Fe
II
ion adducts) (Figs. 1.2
and 1.3) is an essential prerequisite to understanding how to reduce the cost and increase the efficiency of the remediation program.
The galvanic model requires active post-injection management of the groundwater Eh, pH temperature, and oxygenation levels.
It has the potential to allow 15-100 nm Fe
0
, Cu
0
, Al
0
(typically spherical/blocky) particles with a surface area of 10-80 m
2
g
−1
, and
costing $20,000-$850,000
t
−1
, to be restructured and replaced by specific galvanic components (5-80 nm) with a layered structure
[(e.g., Fe(OH)
x
, FeOOH, etc. (Fig. 1.2)) and a surface area of <100 to >30,000 m
2
g
−1
Fe
0
, costing around $300-$15,000
t
−1
]. The
net effect of this restructuring is to reduce the amount of n-Fe
0
required, the rate of remediation, the time frame for remediation,
and the overall cost of the remediation while increasing the amount of pollutant removed g
−1
n-Fe
0
.
appendix 1.a
list of abbreviations and equation symbols
1.
a
,
b, d
,
e, m
,
n
, and
p
are constants which are determined experimentally. In a simple non-catalytic example where
b
= 0,
m
= the reaction order [130]. The reaction order is calculated as m + n + p [130].
2. A' = a constant (0.509 dm
1.5
mol
−0.5
at 298K);
3.
A
f
= pre-exponential factor [E
a(sa)
& A
(sa)
= normalized for p
m
and a
s
; E
a(observed)
&
A
(observed)
=
E
a
and
A
f
calculated without
correction or normalization for
p
m
and
a
s
.
4.
a
s(
t
=
n
)
= ZVM surface area(m
2
gm) at time t. a
s(t=n)
decreases with increasing time as the ZVM surfaces become oxidized;
5. B' = a constant; a = radius of the ion;
6.
C
a
= catalyst (e.g., ZVM);
7.
C
t
=
n
= contaminant concentration at time,
t
= n (seconds) [mg l
−1
, M l
−1
];
