Geoscience Reference
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pollutant. Thus, the global process must be considered as a series of basic physical and chemical
processes:
•
mass transport of dissolved pollutant from the solution to the metallic surface;
•
sorption of the pollutant onto the metal surface;
•
electron transfer from the metal surface to the pollutant;
•
desorption of the pollutant from the metal surface.
Any of these processes may be the determining step of the pollutant reduction rate, and this
depends on each specific pollutant.
Fe(0) is efficient for As(III) and As(V) removal; an advantage of ZVI is that it is affordable and
non-toxic (Cundy
et al
., 2008; Su and Puls, 2003; Wilkin
et al
., 2009; Zouboulis and Katsoyiannis,
2005). Although not totally understood, the main mechanism seems to be superficial complexation
and precipitation on surface and adsorption. The involved reaction can be depicted as follows:
when iron is oxidized, FeOOH is produced at the surface, having the capacity to adsorb metals
and metalloids such as As (Nikolaidis
et al
., 2003). As Fe(0) is very efficient as reducing agent,
it can remove both inorganic and organic As. ZVI is especially efficient for As removal at low
pH in waters with high sulfide concentrations. In spite of the fact that the reduction capacity
of elemental iron decreases significantly at neutral pH, the hydroxylated species formed on the
Fe(0) surface are effective sites for adsorption of As(III) and As(V) at neutral and alkaline pH.
Adsorption of As onto Fe(0) is widely influenced by the following anions, mentioned in decreasing
influence order: phosphate, silicate, chromate and molybdate, followed by carbonate and nitrate
and, finally, borate and sulfate.
In the transformation process, iron suffers several oxidation reactions (Powell and Puls, 1997):
Fe
2
+
+
2e
−
E
0
Fe(0)
→
=−
0
.
447 V
(1.1)
Fe
3
+
+
3e
−
E
0
Fe(0)
→
=−
0
.
037 V
(1.2)
Fe
2
+
Fe
3
+
+
e
−
E
0
→
=
0
.
771 V
(1.3)
In the absence of strong oxidants, there are two reduction half-reactions that, together with
iron oxidation, originate spontaneous oxidation in water; in aerobic conditions, the preferential
oxidant is oxygen:
4e
−
4OH
−
E
0
O
2
+
2H
2
O
+
→
=
0
.
401 V
(1.4)
2Fe
2
+
+
4OH
−
2Fe(0)
+
O
2
+
2H
2
O
→
(1.5)
4H
+
2Fe
2
+
+
2Fe(0)
+
O
2
+
→
2H
2
O
(1.6)
while, in anaerobic conditions, water acts as the oxidant:
2e
−
2OH
−
2H
2
O
+
→
H
2
(g)
+
(1.7)
Fe
2
+
+
2OH
−
Fe(0)
+
2H
2
O
→
H
2
(g)
+
(1.8)
Fe(II) oxidation follows:
4Fe
2
+
+
O
2
(g)
+
10H
2
O
→
4Fe(OH)
3
(s)
+
8H
+
(1.9)
The common ions of these solutions may affect the efficiency of Fe(0) barriers to remove As
and the competitive coprecipitation-sorption processes through the formation of the following
phases (Su and Puls, 2003):
3Fe
2
+
+
Fe
3
+
+
Cl
−
+
8H
2
O
Fe
4
(OH)
8
Cl(s)
+
8H
+
(1.10)
4Fe
2
+
+
2Fe
3
+
+
SO
2
4
+
12H
2
O
Fe
6
(OH)
12
SO
4
(s)
+
12H
+
(1.11)
4Fe
2
+
+
2Fe
3
+
+
CO
2
3
+
12H
2
O
Fe
6
(OH)
12
CO
3
(s)
+
12H
+
(1.12)
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