Environmental Engineering Reference
In-Depth Information
0.018
Np without HS
Exposure time (h)
0.33
0.75
0.016
0.014
1.00
0.012
2.27
0.010
2.33
5.52
21.10
22.78
25.08
27.28
46.40
0.008
0.006
0.004
0.002
0.000
950
960
970
980
Wavelength (nm)
990
1000
1010
1020
Figure 29.5
Typical spectra of NpO
2
+
and a Np(V)-humic complex (NpO
2
+
HS); c(Np)
tot
= 3.8 10
−5
m, c(HS) = 250 mg/l. The spectrum
of Np(V)-humic complex is shown for the dynamics during 46,4 h of exposure. It can be seen that within the time the absorption intensity of
both NpO
2
+
and the HS complex decreased as a result of Np(V) reduction and complexation by the quinonoid-enriched humic derivatives.
Np(V)-HS complex at 981.5 and 987 nm, respectively (Fig. 29.5). The molar absorbance of the NpO
2
+
ion was determined to
be 377.7 l mol
−1
cm
−1
, which is close to the 395 l·mol
−1
cm
−1
value reported by Sachs and Bernhard [33], and Keller [34].
The Np(V) reduction by humic derivatives containing different quinonoid moieties is shown in Figure 29.6. The most effec-
tive reduction of Np(V) was observed for a hydroquinone-enriched humic substance followed by catechol and benzoquinone
(Fig. 29.6a). As in the case of Pu(V), a direct relationship was observed between the reducing performance of the derivative and
its degree of modification (Fig. 29.6b).
The reduction rate for a hydroquinone derivative with a moderate degree of modification (HQ250) was on the same level as
with the samples enriched with catechol and benzoquinone units with higher degrees of modifications (cT-500 and BQ-500).
In addition, the hydroquinone derivatives brought about the most complete reduction of Np(V).
The results show better Np(V) reduction with hydroquinone humic derivatives, which suggests that they may function better
as sequestering agents for waterborne actinides in their higher-valence states. With regard to a contaminated site remediation,
a humic-based technology would function best under conditions that were both anoxic and acidic (e.g., at pH < 5). These
conditions are met, for example, at the Field Research center of the Oak Ridge National laboratory, u.S. DOe [35], where high
concentrations of uranium (mg/l) and technetium are also found. These conditions would be considered acceptable for the effec-
tive use of modified humic derivatives for the purpose of actinide reduction, thereby reducing their mobility in the subsurface
environment.
For actinide contaminated sites having oxygenated groundwaters and pH values close to neutral, such as the Siberian
chemical Industrial complex, Russia (ScIc) [36], and another highly contaminated Russian site—the Production Association
“mayak” in Western ural [37]—the proposed humic-based technology might not provide for desired lowering of neptunium
mobility, but will be still efficient for immobilizing Pu(V).
29.2.3 interactions of np with Hydroquinone-enriched Humic materials on the surface
of colloidal goethite Particles
To assess redox-mediating activities of the hydroquinone-enriched humic materials under environmental conditions, they were
introduced to neptunium(V)-goethite (α-FeOOH) colloid systems. The latter were to mimic omnipresent mineral colloids [38].
For this investigation, goethite suspensions were spiked with
237
Np(V) to create a concentration of (6.0 ± 0.6)⋅10
−7
m. The
parental and modified humic materials (cHP and cHP-HQ100, respectively) were added separately to the pre-equilibrated
Np-goethite suspensions at concentrations of 43 mg/l and continuously stirred for a month. The experiments were run at two
different pH values: 3.5 and 7.5. It was found that in the presence of hydroquinone derivatives of HS, neptunium-HS complexes
formed on the surface of the goethite, whereas they were not observed with only the parent humic materials. The Np-HS
