Environmental Engineering Reference
In-Depth Information
1
Groundwater water remediation by static
diffusion usinG nano-zero valent metals (fe
0
,
cu
0
, al
0
), n-feH
n
+
, n-fe(oH)
x
, n-feooH, n-fe-[o
x
H
y
]
(
n
+/−)
David D.J. Antia
DCA Consultants Ltd., Scotland, UK
1.1
introduction
Zero valent metals (ZVM) and ZVM combinations, including magnetic ZVM [1-4] and reduced ZVM [5-7], are highly efficient
water treatment agents [8-12]. They will remove ions, chemicals, compounds, and biota from water. The principal ZVM used
in pilot and commercial water treatment are Fe
0
and Fe
0
+ Cu
0
[13-17]. Prices for ZVM powders are a function of commodity
prices, particle size, particle shape (e.g., dendritic, spherical, platy, irregular, etc), manufacturing method, source, and quantity
ordered. Current (June 2014) prices (k$ FOB
t
−1
) for powders are (i) 13,000-400,000 nm (Fe = 0.6-1.6; Cu = 1-7; Al = 5-20), (ii)
<3,000 to >8,000 nm (Fe = 5-15; Cu = 9-18; Al = 9-850), (iii) 10-1000 nm (Fe = 30-1100; Cu = 9-1000; Al = 9-900). n-ZVM
powders are either used to treat water in a reactor [10], or are injected into an aquifer [10, 17], or are placed in a permeable
reactive barrier (PRB) within the aquifer [10, 18].
Use of n-ZVM in a fixed (packed) bed reactor (where all the feed water flows through the n-ZVM) is impractical, as n-Fe
0
rapidly corrodes and expands to form hydrated, low-density hydroxides (Fe(OH)
2
, Fe(OH)
3
) and peroxides (FeOOH). This
results [10] in a decrease in porosity, decrease in permeability, increase in the proportion of dead end pores, and a decrease in
pore throat size. Associated gas bubble formation (O
2
, H
2
[10]) results in permeability reduction [18, 19] due to gas occlusion
switching water flow from viscous flow to Knudsen diffusion [20-22]. The net effect is a major reduction in permeability (and
water flow rate) over a short time period (Fig. 1.1a) in the reactor, or aquifer [10]. Fluid flow (Q
f
, m
3
m
−2
s
−1
) =
k
p
D
f
[20-22]. A
list of abbreviations is provided in Appendix 1.A. The dominant fluid flow mechanism switches over a period of 2-6 weeks from
viscous flow to Knudsen diffusion due to the generation and presence of trapped nano/micron-sized gas bubbles [10, 20-22].
Changing the reactor type to a diabatic diffusion reactor (where a body of water overlies a static body of ZVM, and all water
enters and leaves the reactor through the water body), mimics the situation that occurs in an aquifer, during remediation,
following n-ZVM injection. A policy of groundwater abstraction, treatment, and reinjection allows the remediation to be under-
taken in a short time period in a controlled environment without creating long-term damage to the aquifer [10]. In an uncon-
fined, diabatic, diffusion environment (e.g., shallow contaminated aquifer or soil), the principal controls on Eh, pH, EC
(electrical conductivity), and remediation are [10]: (i) flowing water space velocity (SV = Q
fr
/W
zvm
); (ii) the stored water to ZVM
ratio, [SWZ = S
w
/W
zvm
]; (iii) the water composition; (iv) atmospheric/groundwater temperature fluctuations; (v) atmosphere
composition variation (principally humidity); (vi) atmospheric pressure fluctuation; (vii) vertical infiltration recharge (associ-
ated with storm events) into the aquifer/soil; (viii) water losses from the aquifer/soil, due to evaporation, leaching, and the
interaction of the n-ZVM (and n-ZVM products) with minerals and biota; and (ix) porosity occlusion resulting from the
movement of displaced air as the water levels rise and fall (during and following infiltration recharge) [20-23].
