Environmental Engineering Reference
In-Depth Information
(b)
(a)
100
100
CHP
80
80
HQ100
CHP
HQ100
HQ250
60
60
HQ250
HQ500
40
40
HQ500
20
20
0
0
0
1
2
Exposure time (h)
3
4
5
0
5
Exposure time (h)
20
25
30
Figure 29.4 Reduction of Pu(V) by hydroquinone-enriched humic derivatives. conditions: Ar atmosphere, C tot (Pu) = 2.3·10 −9 m,
C tot (HS) = 10 mg·l −1 , I = 0 m. (a) pH 7.5; (b) pH 4.5. Reproduced with permission from Ref. [30]. © elsevier B.V.
500 mg of the monomer per 1 g of the parent HS using phenolformaldehyde polycondensation for HQ and cT, and oxidative
polymerization for BQ. It should be noted that contrary to HQ and cT monomers that were joined to aromatic rings of HS via
cH2 bridges, BQ produced biphenyl-type structures with c-c bonds between humic and hydroquinone rings. The redox
capacities of these derivatives measured as described by matthiessen [29] were substantially higher (up to 4 mmol/g) as
compared to the parent humic material (0.6 mmol/g). The largest increase was observed within the hydroquinone-enriched
derivatives as opposed to catechol or benzoquinone products. Of particular importance is that the redox capacity was proportional
to the incorporated amount of quinonoid centers, which enables the manufacturing of quinonoid-enriched derivatives with
predicted redox capacities.
29.2.2
reduction of actinides in the Higher-valence state by the Quinonoid-enriched Humic materials
The performance of quinonoid-enriched humic derivatives was tested with regard to the reduction of Pu(V) and Np(V).
corresponding data were reported by Shcherbina et al. [30, 31]. Since qiononoid monomers are sensitive to the presence of
oxygen, it was important to study the redox properties of the quinonoid-enriched humic derivatives both under oxic and anoxic
conditions and at different pH values. The actinide concentrations were set at 10 −9 m for Pu(V) and 10 −5 m for Np(V). Such a
substantial difference in actinide concentrations was predicated on different sensitivities of analytical techniques used for
monitoring Pu(V) and Np(V) speciation in the solution: liquid-liquid extraction followed by liquid scintillation counting in the
case of Pu(V) [23, 32], and spectrophotometry in the ultraviolet-visible-near-infrared (uV/vis/NIR) range for Np(V) [33].
consequently, the concentrations of HS used in the experiments with Pu(V) were set to a lower value (10 mg/l) compared to
Np(V) (250 and 500 mg/l).
It was found that, in general, all humic derivatives studied (including the parental HS) were more effective at reducing Pu(V)
than Np(V). As shown in Figure  29.4a, under anoxic conditions, the complete reduction of Pu(V) to Pu (IV) was observed
within 4-5 h. liquid extraction did not reveal Pu(III) in the solution once equilibrium was achieved given its stabilty in such
solutions. In the presence of oxygen, Pu(V) reduction was still detected, although with much slower kinetics: complete Pu(V)
reduction occurred within 145 h (not shown).
For the quinonoid-enriched derivatives, a significant relationship was observed between the degree of modification and the
reduction performance. This can be clearly seen for the case of plutonium from Figure 29.4b. The derivatives could be placed
in the following ascending order according to their reducing efficiency: cHP < HQ100 ≤ HQ250 < HQ500.
As the reduction of An(V) is pH-dependent, the kinetics of Pu(V) reduction under varying pH conditions was also studied.
Faster reduction rates occurred in acidic (4.5) versus neutral pH (7.5).
For Np(V), the reduction was much slower, reaching 30% (by HQ500) within 120 h [30]. The reduction of Np(V) was mon-
itored as described by Sachs and Bernhard, and Keller [33, 34], by measuring the absorbance of the aqueous NpO 2 + ion and the
 
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