Environmental Engineering Reference
In-Depth Information
constant (1/h) and t is time (h). It has been recognized that the degradation rate is
directly proportional to the amount of available iron surface (Johson et al., 1996) and
Eq. 7.4 can be expanded as follows:
dC
=
k
a
C
(Eq. 7.5)
SA
m
s
dt
where k SA is defined as the surface area normalized rate constant (L/m 2 h), ρ m is the
metal loading (g/L), and a s is the specific surface area of metals (m 2 /g). The degradation
of carbon tetrachloride follows the behavior of first-order kinetics (R 2 > 0.98) in the
presence of bimetallic Fe/Al particles. The observed rate constant for bimetallic Fe/Al
particles was calculated to be approximately 0.07 1/h, corresponding to a surface area
normalized rate constant ( k SA ) of 5.9 × 10 -4 L/m 2 h. It has been reported that the k SA
value of micro-sized iron, iron nanoparticles and bimetallic Pd/Fe nanoparticles for the
carbon tetrachloride degradation is 1
10 -3 L/m 2 h, respectively
(Lien and Zhang, 2001). It is clear that bimetallic Fe/Al particles increased the reactivity
toward carbon tetrachloride by a factor of 6 as compared to the micro-sized ZVI.
Although the reaction rate of bimetallic Fe/Al particles is about 15 times less than that of
bimetallic Pd/Fe nanoparticles, bimetallic Fe/Al particles still possess reactivity
comparable with iron nanoparticles.
7.2.1.3 Reaction Mechanisms
0 -4 , 5.3
10 -4 , and 9
×
×
×
Dissolved iron and aluminum ions were measured during the course of carbon
tetrachloride degradation. Concentrations of dissolved aluminum ion initially increased
to 2.1 mg/L at 7 h, and then it was removed from solution gradually with time to reach
0.04 mg/L at 70 h. However, dissolved iron concentration quickly increased to about 8
mg/L at 1 h while slightly decreasing to 5 mg/L at the end of the experiment. Unlike ZVI
increasing pH significantly in aqueous solutions (e.g., a typical increase in pH of 23
standard units), bimetallic Fe/Al particles resulted in only a minor change in the solution
pH during the reaction. It was found that the solution pH increased from 6.7
0.5 at the end of the experiment. At the neutral pH, an insoluble form of polymeric
aluminum hydroxide (Al(OH) 3 ) and dissolved ferrous iron dominate in reducing regions
(Stumm and Morgan, 1996; Deltombe and Pourbaix, 1958). Consequently, a low
concentration of dissolved aluminum and a relatively high concentration of dissolved
iron can be found in the bimetallic Fe/Al aqueous system.
Based on the XRD study, product analysis of the carbon tetrachloride
degradation, and the measurement of dissolved metals, a conceptual model accounting
for the roles of aluminum and iron is proposed in Figure 7.4. Carbon tetrachloride was
first adsorbed at iron surface where a surface-mediated dechlorination took place
(Matheson and Tratnyek, 1994). Iron oxidation was involved in the carbon tetrachloride
degradation and water dissolution. The dechlorination and water dissolution are dynamic
±
0.5 to 7.3
±
 
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