Environmental Engineering Reference
In-Depth Information
53. ΔG
o
{A[ZVM]}
= free energy change on forming the encounter pair;
54. ΔG
*
= free energy of activation from the encounter pair;
55. ΔG = ΔH−T ΔS;
56. ΔH = heat of reaction;
57. ΔS = entropy;
appendix 1.b ions (oxides, Hydrides, peroxides, and Hydroxides) removed by
precipitation due to tHe alteration of eh and pH in Groundwater by zvm
Data Sources: [10, 103, 104, 167-175]
In the simplest case, n-ZVM addition leaves pH effectively unaltered (e.g., Fig. 1.1c).
Eh prior to addition of n-ZVM = Eh [103, 104, 131] = ΔE
o
+ (0.0591/n) log([B]
b
/[A]
a
)
t
= 0
. For an example contaminant
removal reaction,
Cd
2+
+ H
2
= Cd(s) + 2H
+
(the half reactions are Cd
2+
+ 2e
−
= Cd
0
and H
2
= 2H
+
+ 2e
−
; see Appendix 1.C); K = Q = [H
+
]
2
/([Cd
2+
]
P
H2
) = B
b
/A
a
[131]. After
n-ZVM addition, at time
t
=
m
, the Eh changes (Fig. 1.1b-d) result in a new equilibria, where the new log([B]
b
/[A]
a
)
t
=
m
= (Eh−ΔE
o
)/
(0.0591/
n
); ΔE
o
is corrected to the actual groundwater temperature. In this example, if the groundwater at t = 0 contains a 0.001 M
Cd
2+
l
−1
and an Eh of 0.13 V (Fig. 1.1c), then Eh = 0.13 = ΔE
o
(−0.4 V—Appendix 1.B) + 0.0591/2 log Q; that is, log Q = 18; if −log
(H
+
) = pH [103, 131], then for pH = 6.5, at t = 0,
P
H2
= 10
−22
. Changing the Eh to −0.2 V (Fig. 1.1c) after 1 month, while maintaining
a pH of 6.5, changes log Q to 6.7. The Cd
2+
concentration in the water at time, t = 1 month, is therefore a function of
P
H2
in the
groundwater resulting from the presence of n-Fe
0
(Fig. 1.4e). Increasing
P
H2
to 10
−10
could achieve the observed Eh (−0.2 V)
while leaving the Cd
2+
concentration unchanged. Increasing
P
H2
to 10
−8
reduces the Cd
2+
concentration in water to 0.00001 M Cd
2+
l
−1
from 0.001 M Cd
2+
l
−1
; that is, the effectiveness of the n-Fe
0
treatment program for any specific Eh and pH, where the product
is a zero valent metal (Appendix 1.B), is maximized by increasing the H
2
partial pressure. The alternative remediation strategy of
using O
2
injection to oxidize cations (e.g., Cd
2+
+ 0.5O
2
+ H
2
O = Cd(OH)
2
, where 0.5O
2
+ H
2
O + 2e
−
= 2OH
−
; Cd
2+
+ 2OH
−
= Cd(OH
2
),
and H
2
= 2H
+
+ 2e
−
) effectively changes Q to Q = [H
+
]
2
/([Cd
2+
]
P
H2
P
O2
), and ΔE
o
to 0.4V [177]. This alternative strategy uses the
n-Fe
0
to control the groundwater pH (i.e., H
+
and
P
H2
) and the
P
O2
associated with O
2
injection to control the rate and degree of
remediation [139-141]. For example, if at t = 0, Eh = 0.13V, pH = 6.5, and the water contains 0.001 M Cd
2+
l
−1
and
P
H2
= 10
−22
,
P
O2
= 0, then instigation of an oxygen injection scheme following n-Fe
0
injection into the groundwater, while maintaining a
constant Eh and pH, will result in both
P
H2
and
P
O2
increasing [e.g., [139-141]]. Once
P
H2
and
P
O2
have exceeded a critical level
(e.g., 10
−11
), any subsequent increases in partial pressure will be compensated for by either decreases in Eh, or the removal of Cd
2+
as Cd(OH)
2
. Increasing
P
H2
and
P
O2
to 10
−9
, will reduce the molar concentration of Cd
2+
to 0.0000001 M Cd
2+
l
−1
(i.e., 0.146 g
Cd(OH)
2
l
−1
H
2
O will have been precipitated into the ZVM bed). This simple example has been used to demonstrate how a tradi-
tional ZVM remediation program [e.g., [17]] can be modified using the galvanic model [138, 2, 139-141] to both accelerate and
control the rate of remediation. Once the bulk of the cations have been converted to oxides/hydroxides/peroxides, the diabatic
galvanic model (Figs. 1.2 and 1.3) controls the rate of remediation.
Contaminant Ion/Ion Adduct
Potentially precipitated by ZVM as
Ac
3+
, AcOH
2+
, Ac(OH)
2
+
Ac(OH)
3
, AcOOH
Ag
n
+
, AgO
+
, AgO
−
, AgOH, AgOH
2
−
, AgCl
2
−
Ag, AgCl, AgOH, Ag
2
O, Ag
2
O
2
, Ag
2
O
3
Al
n
+
, HAlO
2
, AlO
2
−
, AlOH
2+
, AlOH3, Al(OH)
2
+
, Al(OH)
4
−
Al(OH)
3
, AlOOH, Al
2
O
3
Am
n
+
, AmOH
2+
, AmO
2
+
, Am(OH)
2
+
Am(OH)
3
, Am(OH)
4
, AmO
2
AsH
3
, HAsO
2
, AsO
+
, H
3
AsO
4
, H
2
AsO
4
−
, HAsO
4
2−
, AsO
2
−
, AsO
4
3−
As, AsO
3
Au
n
+
, H
2
AuO
3
, H
2
AuO
3
−
, HAuO
2
2−
Au, Au(OH)
3
, AuOOH, AuO
2
Ba
2+
, BaOH
+
Ba(OH)
2
, BaO
2
Be
2+
, Be
2
O
2
−
Be(OH)
2
, BeO, Be
2
O(OH)
2
Bi
3+
, BiOH
2+
, BiO
+
, BiO
2
−
, BiO
3
−
Bi, Bi(OH)
3
, BiOOH, Bi
2
O
3
, Bi
2
O
5
, Bi
4
O
7
, Bi
2
O
4
Ca
2+
, CaOH
+
Ca(OH)
2
, CaO
2
, CaCO
3
, CaSO
4
Cd
2+
, CdOH
+
, HCdO
2
−
Cd, Cd(OH)
2
Ce
3+
, CeO
+
, Ce(OH)
3+
, Ce(OH)
2
2+
Ce(OH)
3
, CeOOH, Ce
2
(CO
3
)
3
, CeO
2
Cm
3+
, CmOH
2+
, Cm(OH)
2
+
Cm(OH)
3
, CmOOH
