Experimental and theoretical approach to probe the aquatic speciation of transuranic (neptunyl) ion in presence of two omnipresent organic moieties

Highlights

• Neptunium forms stable complexes with pyrazine carboxylates.

• The coordination mode dictates the complexation strength.

• The spectral features are decided by configuration of species.

• The species are aqueous soluble over a wide Eh-pH region.

Abstract

Pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas. The present studies aimed at aquatic speciation of the neptunyl ion (NpO₂⁺) with two pyrazine compounds namely pyrazine monocarboxylic acid (PMC) and pyrazine dicarboxylic acid (PDC). Absorption spectrophotometry was used to probe the stability, speciation and spectral properties for the complexation process. NpO₂⁺ forms a more stable complex with PMC than PDC for 1:1 (ML), while for 1:2 (ML₂) the opposite trend is observed. The extent of shift in λ_max, which is also an indicator for the strength of complexation, reflected similar trends for the complexation process. Isothermal titration calorimetry was employed to determine the enthalpies of complex formation, which is found to be endothermic. The complexation process is entropy driven. Linear free energy correlations were established to retrieve the coordination modes of the complexes. The variation in peak potentials (the cyclic voltammograms) with change in pH and metal to ligand ratio were explored to understand redox speciation, electron transfer kinetics and Eh-pH characteristics for the interaction of NpO₂⁺ with pyrazine carboxylate ligands. Density functional theory calculations were employed to optimize the geometries and to calculate the bond distances and partial charges on key atoms of the optimized geometries. The theoretical calculations helped to reveal the contributions from two different configurations of the same geometry towards the optical absorption. The bond distances and partial charges estimated theoretically helped to understand the aqueous interactions at the molecular level.

Introduction

The reprocessing and disposal of nuclear spent fuel and its waste are the major challenges in nuclear industry for public acceptance (Editorial, 2019). Uranium and plutonium are recovered from the spent fuel in major portions by PUREX process, while the discharges containing unrecoverable uranium, plutonium, minor actinides (Np, Am and Cm) along with the fission products contribute to the high level waste (NEA-OECD, 2012). The integrity and safe disposal of high level waste in vitrified form is the prime concern with respect to environmental and human health hazards (Ojovan et al., 2019a). Inhalation and ingestion are the primary pathways for neptunium with respect to human exposure (Peterson et al., 2007). The nuclear weapon test fall outs and accidental releases from reprocessing plants and nuclear waste storage sites are major routes of neptunium exposure to human beings (Denecke et al., 2018). The radiotoxicity of neptunium is majorly due to its alpha emitting nature while a portion is also contributed by gamma radiation of its other isotopes (²³⁶Np, ²³⁹Np) and the daughter product (²³³Pa) (Ojovan et al., 2019b). The personnel dealing with spent nuclear fuel processing at reprocessing facilities and the method developers at research laboratories are also prone to neptunium exposure at working premises.

The integrity of waste storage sites for extremely long durations is debatable regarding the accidental leakages of radioactivity to the geosphere. Neptunium exists as stable NpO₂⁺ in most of the waste storage environments, and is the major contributor to the transportation and migration of the radioactivity from the source to far fields in long term scenarios (Runde and Goff, 2010). Aquifers are the major routes for the migration and transportation of these radionuclides from the source in near or far fields of the disposal facilities. Dissolved organic matter and its constituents facilitate the mobility of these radionuclides by complexation phenomena. The complexation processes alter the aquatic speciation and solubility of the radionuclides. In cases, where the sorption of complexed species occurs, this will contribute to retarding the radionuclide (Silva and Nitsche, 1995). The organic reagents used in reprocessing strategies, and decontamination processes are co-disposed with the radionuclides in the nuclear waste storage sites. The common organic chelates present in natural and marine aquifers may also complex actinide ions and facilitate the movement of the radioactivity (IAEA, 2004; Choppin, 2006; Maher et al., 2013).

Nitrogen compounds such as ethylenediamminetetraacetic acid (EDTA), nitrilotriacetic acid (NTA), picolinic acid (PA) are used as decontaminating agents (Kinnunen, 2008); while the pyridine and pyrazine based organic chelators are well known for their application in lanthanide-actinide separation and purification methodologies (Dam et al., 2007). The pyrazines are diaza-benzenes in which both the para-positions of the benzene ring are occupied by nitrogen atoms. The pyrazines are omnipresent in nature and have their occurrence in plants, microbes, food supplies, marine arenas (Müller and Rappert, 2010). Pyrazine is an important pharmacophore to access the biological functionality of a moiety. Pyrazine derivatives like phenazine and tetramethylpyrazine have their applications as antitumor, antibiotic agents and as superoxide ions scavengers respectively (Zhang et al., 2003; Dickschat et al., 2005; Lewis et al., 2007; Kao et al., 2013). It is metabolized primarily in the liver and excreted largely in the urine (Kumar, 2017).

Mitchell and et al. (1945) studied the chemical composition of an adult human body and reported that the total water content is 67.85% of the total weight of the human being. The pyrazines are the base moieties of most of the flavoring agents used in food and the amount of pyrazines in food supplies spans over 0.01–725 μg/g depending on the kind and origin of food supply (Maga et al., 1973; Fan et al., 2007). The microbial action on pyrazine derivatives results in formation of pyrazine mono and dicarboxylic acids and is majorly present in effluents from the food, pesticide, and pharmaceutical industries (Dickschat et al., 2005). Thus, in case of neptunium exposure and its absorption into the human body, there is some probability for the interaction of neptunium with pyrazine carboxylic acids. Hence, aqueous phase studies on neptunium interaction with pyrazine carboxylic acids at acidic conditions would be highly helpful to understand the stability of neptunium complexes and their speciation in presence of pyrazine carboxylic acids. The studies on aqueous speciation of neptunium in the presence of pyrazine carboxylates are not available in the literature. Thermodynamic and detailed speciation studies could give crucial insights for development of formulations for detoxication and excretion of neptunium. Also, aqueous speciation studies on the interaction of neptunium ions with pyrazine carboxylic acid provide insight into the chemistry behind the uptake of neptunium by plants and microorganism in aquatic environment in general and the marine flora and fauna in particular. The probability for occurrence of these species is high in aquatic environments and pyrazine carboxylic acid forms strong water soluble species with Np(V) over a wide range of pH. It is a chelating agent and exists abundantly in aquatic environments. Thus, the complexation leads to the transportation and migration of neptunium from the place of origin.

The poor nucleophiliicity, non-coordinating nature, and inertness anionic species to redox processes make the perchlorate anion an ideal electrolyte to carry out metal ion interaction and speciation studies (Brown and Gu, 2006). Thus, the present studies aimed at determining the stability, speciation, energetics of interaction for neptunium-pyrazinoic acid system in aqueous perchlorate medium. Spectrophotometric titrations are employed to determine the stability constants for neptunium complexes by pyrazine carboxylic acids, while the speciation plots are generated with the help of determined stability constants for the same system in acidic pH range. Isothermal titration calorimetric methodology was employed to determine the enthalpy of species formed. The redox properties of neptunyl in the presence of pyrazine carboxylates is determined to reveal the electron-transfer kinetics, Eh-pH diagrams. Rigorous theoretical calculations at density functional theory (DFT) level of theory are carried out to obtain optimized geometries, the bond distances and partial charges for the predicted species of neptunyl with pyrazine carboxylic acids to have gain further insight into the interaction process at molecular level.