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Title: Redox and complexation interactions of neptunium(V) with quinonoid-enriched humic derivatives

Abstract

Actinides in their higher valence states (e.g., MO{sub 2}{sup +} and MO{sub 2}{sup 2+}, where M can be Np, Pu, etc) possess a higher potential for migration and in turn pose a substantial environmental threat. To minimize this potential for migration, reducing them to lower oxidation states (e.g., their tetravalent state) can be an attractive and efficient remedial process. These lower oxidation states are often much less soluble in natural aqueous media and are, therefore, less mobile in the environment. The research presented here focuses on assessing the performance of quinonoid-enriched humic derivatives with regards to complexing and/or reducing Np(V) present in solution. These 'designer' humics are essentially derived reducing agents that can serve as reactive components of a novel humic-based remediation technology. The derivatives are obtained by incorporating different quinonoid-moieties into leonardite humic acids. Five quinonoid-derivatives are tested in this work and all five prove more effective as reducing agents for selected actinides than the parent leonardite humic acid, and the hydroquinone derivatives are better than the catechol derivatives. The reduction kinetics and the Np(V) species formed with the different derivatives are studied via a batch mode using near-infrared (NIR)-spectroscopy. Np(V) reduction by the humic derivatives under anoxic conditionsmore » at 293 K and at pH 4.7 obeys first-order kinetics. Rate constants range from 1.70 x 10{sup -6} (parent humic acid) to 1.06 x 10{sup -5} sec{sup -1} (derivative with maximum hydroquinone content). Stability constants for Np(V)-humic complexes calculated from spectroscopic data produce corresponding Log{beta} values of 2.3 for parent humic acid and values ranging from 2.5 to 3.2 at pH 4.7 and from 3.3 to 3.7 at pH 7.4 for humic derivatives. Maximum constants are observed for hydroquinone-enriched derivatives. It is concluded that among the humic derivatives tested, the hydroquinone-enriched ones are the most useful for addressing remedial needs of actinide-contaminated aquifers.« less

Authors:
 [1];  [2];  [2];  [2];  [1];  [3]
  1. Vernadsky Inst. of Geochemistry and Analytical Chemistry, Moscow, Russia
  2. Lomonosov Moscow State University, Moscow, Russia
  3. {Dick} G [ORNL
Publication Date:
Research Org.:
Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
930924
DOE Contract Number:
DE-AC05-00OR22725
Resource Type:
Journal Article
Resource Relation:
Journal Name: Environmental Science & Technology; Journal Volume: 41; Journal Issue: 20
Country of Publication:
United States
Language:
English
Subject:
54 ENVIRONMENTAL SCIENCES; ACTINIDES; AQUIFERS; HUMIC ACIDS; KINETICS; PERFORMANCE; REDUCING AGENTS; STABILITY; VALENCE

Citation Formats

Shcherbina, Natalia S., Perminova, Irina V., Kalmykov, Stephan N., Kovalenko, Anton N., Novikov, Alexander P., and Haire, Richard. Redox and complexation interactions of neptunium(V) with quinonoid-enriched humic derivatives. United States: N. p., 2007. Web. doi:10.1021/es070415l.
Shcherbina, Natalia S., Perminova, Irina V., Kalmykov, Stephan N., Kovalenko, Anton N., Novikov, Alexander P., & Haire, Richard. Redox and complexation interactions of neptunium(V) with quinonoid-enriched humic derivatives. United States. doi:10.1021/es070415l.
Shcherbina, Natalia S., Perminova, Irina V., Kalmykov, Stephan N., Kovalenko, Anton N., Novikov, Alexander P., and Haire, Richard. Mon . "Redox and complexation interactions of neptunium(V) with quinonoid-enriched humic derivatives". United States. doi:10.1021/es070415l.
@article{osti_930924,
title = {Redox and complexation interactions of neptunium(V) with quinonoid-enriched humic derivatives},
author = {Shcherbina, Natalia S. and Perminova, Irina V. and Kalmykov, Stephan N. and Kovalenko, Anton N. and Novikov, Alexander P. and Haire, Richard},
abstractNote = {Actinides in their higher valence states (e.g., MO{sub 2}{sup +} and MO{sub 2}{sup 2+}, where M can be Np, Pu, etc) possess a higher potential for migration and in turn pose a substantial environmental threat. To minimize this potential for migration, reducing them to lower oxidation states (e.g., their tetravalent state) can be an attractive and efficient remedial process. These lower oxidation states are often much less soluble in natural aqueous media and are, therefore, less mobile in the environment. The research presented here focuses on assessing the performance of quinonoid-enriched humic derivatives with regards to complexing and/or reducing Np(V) present in solution. These 'designer' humics are essentially derived reducing agents that can serve as reactive components of a novel humic-based remediation technology. The derivatives are obtained by incorporating different quinonoid-moieties into leonardite humic acids. Five quinonoid-derivatives are tested in this work and all five prove more effective as reducing agents for selected actinides than the parent leonardite humic acid, and the hydroquinone derivatives are better than the catechol derivatives. The reduction kinetics and the Np(V) species formed with the different derivatives are studied via a batch mode using near-infrared (NIR)-spectroscopy. Np(V) reduction by the humic derivatives under anoxic conditions at 293 K and at pH 4.7 obeys first-order kinetics. Rate constants range from 1.70 x 10{sup -6} (parent humic acid) to 1.06 x 10{sup -5} sec{sup -1} (derivative with maximum hydroquinone content). Stability constants for Np(V)-humic complexes calculated from spectroscopic data produce corresponding Log{beta} values of 2.3 for parent humic acid and values ranging from 2.5 to 3.2 at pH 4.7 and from 3.3 to 3.7 at pH 7.4 for humic derivatives. Maximum constants are observed for hydroquinone-enriched derivatives. It is concluded that among the humic derivatives tested, the hydroquinone-enriched ones are the most useful for addressing remedial needs of actinide-contaminated aquifers.},
doi = {10.1021/es070415l},
journal = {Environmental Science & Technology},
number = 20,
volume = 41,
place = {United States},
year = {Mon Jan 01 00:00:00 EST 2007},
month = {Mon Jan 01 00:00:00 EST 2007}
}
  • The redox and complexation interactions of neptunium with humic acid and chemical analogs are studied in this research by solvent extraction and spectrophotometry. In the redox study, dicarboxylic acids and substituted phenolic compounds are selected as models for humic acids. Np(VI) is found to be reduced to Np(V) by both series of compounds, but phenolic compounds reduce Np(VI) more rapidly under the same experimental conditions. Reaction products and various effects on the reaction rate were studied, including the kinetic solvent isotope, salt, photoexcitation, concentration and micelle effects. Results suggest that the reduction of Np(VI) by dicarboxylic acids proceeds via themore » formation of 1:1 complex (NpO[sub 2]L) while the formation of 1:2 complex (NpO[sub 2]L[sub 2][sup 2[minus]]) inhibits the redox reaction. The magnitude of the rate constants depends on the stability of the 1:1 complex and the presence of a methylene proton in those dicarboxylic acids. Results also suggest that the reduction of Np(VI) by phenolic compounds occurs through the interaction of Np(VI) with the hydroxy group, which results in the formation of phenoxy radicals. The rate constant sequence is explained by the electronic effect of various substitution groups. A correlation between the rate constant and the basicity of the hydroxy group is discussed. Np(VI) is reduced rapidly by humic acid to Np(V), which exists as free Np(V) and Np(V)-humate complex. In the subsequent study of complexation between Np(V) and humic acid, Np(V) is found to form a single type of complex with humic acid in the pH range of 4.5-7.5, which is probably the 1:1 Np(V)-carboxylate species. There is a slight trend that humic acids with lower molecular weights form weaker complexes with Np(V). Different approaches for correction of the apparent binding constants are discussed. The binding strength of Np(V)-humate is compared with those of other metal-humate or Np(V)-dicarboxylate systems.« less
  • The solid-liquid equilibrium of Np(V) has been studied in NaCl at 25{degrees}C and 0.01 atm CO{sub 2} The equilibrium solids were characterized using X-ray powder diffraction, and the Np(V) solution species were characterized using NIR absorption spectroscopy. The solid phases NaNpO{sub 2}CO{sub 3} {center_dot} nH{sub 2}O at [CO{sub 3}{sup 2{minus}}] < 10{sup {minus}3} M and Na{sub 3}NpO{sub 2}CO{sub 3}{sub 2}{center_dot}nH{sub 2}O at [CO{sub 3}{sup 2{minus}}]>10{sup {minus}3} M were found as solubility-limiting solid equilibrium phases. Pure Np(V) carbonato complexes were formed in solution; hydroxo, bicarbonato, or mixed hydroxocarbonato Np(V) complexes were determined not to be significant. The comparison of Np(V) solubilitymore » data in NaCl and NaClO{sub 4} solutions indicated a stabilization of Np(V) in solution due to the interaction with chloride ions. Absorption spectra of NpO{sub 2}{sup +} and NpO{sub 2}CO{sub 3}{sup {minus}} in 5 M NaCl were shifted slightly towards higher wavelengths, also suggesting an interaction with chloride ions. The Pitzer approach was applied to parameterize experimental data and to predict Np(V) solubility in brine solutions. The activity of water changes with electrolyte concentration; thus incorporation of the reported molecules of hydration water in NaNpO{sub 2}CO{sub 3} {center_dot}nH{sub 2}O (n = 3.5) to determine the chemical potential of this solid improved the agreement between predicted solubility and experimental data. NaNpO{sub 2}CO{sub 3}{center_dot}nH{sub 2}O and NaAmO{sub 2}CP{sub 3}{center_dot}nH{sub 2}O showed the same solubility in 3 and 5 M NaCl. 31 refs., 6 figs., 3 tabs.« less
  • Over the past several decades, the production and testing of nuclear weapons in the U.S. have created significant amounts of high-level nuclear wastes (HLW) that are currently stored in underground tanks across the U.S. DOE (Department of Energy) sites. Eventually, the HLW will be made into the waste form and disposed of in geological repositories for HLW. Among the radioactive materials, neptunium is of great concern in the post-closure chemical environment in the repository because of the long half-life of 237Np (2.14•106 years) and the high mobility of Np(V), the most stable oxidation state of neptunium. It is estimated thatmore » 237Np, together with 129I and 99Tc, will be the major contributors to the potential total annual dose from the repository beyond 10000 years [1]. Due to the high radiation energy released from the HLW, the postclosure repository is expected to remain at elevated temperatures for thousands of years [1]. If the waste package is breached and becomes in contact with groundwater, neptunium, as well as other radioactive materials will be in aqueous solutions at elevated temperatures. Interactions of radioactive materials with the chemical components in groundwater play an important role in determining their migration in the repository. To predict the migration behavior of neptunium, it is necessary to have sufficient and reliable thermodynamic data on its complexation with the ligands that are present in the groundwater of the repository (e.g., OH–, F–, SO42– ,PO43– and CO32) at elevated temperatures. However, such data are scarce and scattered for 25°C, and nearly nonexistent for elevated temperatures [2]. To provide reliable thermodynamic data, we have conducted investigations of the complexation of actinides, including thorium, uranium, neptunium and plutonium, at elevated temperatures. Thermodynamic parameters, including formation constants, enthalpy and heat capacity of complexation are experimentally determined. This paper summarizes the results of the complexation of Np(V) with fluoride in aqueous solutions at 10–70°C studied by spectrophotometry and microcalorimetry.« less