The self-assembly of a new series of amphiphilic polystyrene-b-poly(4-vinyldipicolinic acid) PS-b-PVDPA diblock copolymers in aqueous solution is reported in order to obtain core-shell nanoparticles composed of a PS core and a PVDPA shell. Diblock copolymers were synthesized by Supplemental Activation Reducing Agent-Atom Transfer Radical Polymerization (SARA-ATRP) with a degree of polymerization (DP) of the PS block in the range 112-260 and a DP of PVDPA block of 10, 30 or 50. Anionic latex nanoparticles of PS-b-PVDPA were prepared by solvent displacement methods. All the nanoparticle suspensions had a narrow size distribution (0.014 <= PDI <= 0.144) with zeta potential in the range -27 mV to -38 mV indicating electrostatic repulsions due to carboxylate anions and therefore high colloidal stability. DLS was used to determine nanoparticle size, with SEM and TEM used to determine and confirm both size and spherical shape. All three methods found the size for these nanoparticles to be in the range 75-120 nm. Using fluorimetry and DLS methods, Critical Aggregation Concentration (CAC) for each type of nanoparticle was determined to be within the range of 33-69 mg/L. Micelles were pH-responsive with stability in aqueous conditions for pH > 3.5. Micelles were stable at pH 5.5 for up to 40 days and at temperature up to 60 degrees C.

The development of actinide decorporation agents with high complexation affinity, high tissue specificity, and low biological toxicity is of vital importance for the sustained and healthy development of nuclear energy. After accidental actinide intake, sequestration by chelation therapy to reduce acute damage is considered as the most effective method. In this work, a series of bis- and tetra-phosphonated pyridine ligands have been designed, synthesized, and characterized for uranyl (UO22+) decorporation. Owing to the absorption of the ligand and the luminescence of the uranyl ion, UV-vis spectroscopy and time-resolved laser-induced fluorescence spectroscopy (TRLFS) were used to probe in situ complexation and structure variation of the complexes formed by the ligands with uranyl. Density functional theory (DFT) calculations and X-ray absorption fine structure (XAFS) spectroscopy on uranyl-ligand complexes revealed the coordination geometry around the uranyl center at pH 3 and 7.4. High affinity constants (log K ∼17) toward the uranyl ion were determined by displacement titration. A preliminary in vitro chelation study proves that bis-phosphonated pyridine ligands can remove uranium from calmodulin (CaM) at a low dose and in the short term, which supports further uranyl decorporation applications of these ligands.

Oceans uranium reserve of 4.5 billion tons can be the answer for the next-generation sustainable nuclear energy. However, extracting the extremely diluted uranium (3.3 ppb) is a difficult task and adsorption materials with high selectivity and uptake capacity are still to be found. Here, we propose the use of poly(4-vinyldipicolinic acid) (PVDPA) as a new highly promising polymer for uranium harvesting from seawater. PVDPA showed a uranium uptake capacity of 597 mg/g in simulated seawater conditions, even at high ionic strength and in the presence of the challenging vanadium species, that tend to limit the performance of other existing materials. PVDPA is also built from a solid PVC-based substrate using an easy and oxygen tolerant strategy. The used PVDPA-modified fibers showed a uranium uptake capacity of 392 mg/g and reached the adsorption equilibrium in less than 3 h, the fastest and highest reported, to the best of our knowledge. The cheap, easy and fast preparation, combined with fast and high uranium recovery, make PVDPA highly promising, not only for uranium harvesting from seawater, but also for treating waters contaminated with uranium. (C) 2019 Elsevier Ltd. All rights reserved.

The quantification of He and Ne diffusion behavior in crystals rich in U and Th such as zircon is key for the interpretation of (U-Th)/He-4 and (U-Th)/Ne-21 thermochronometric ages. Multiple parameters such as chemical substitution, channel obstruction and damage can modify the diffusivity compared to a pristine structure. To investigate the impact of these parameters, we have conducted a theoretical diffusion study combining a series of methods and approaches to address the problem across the necessary range of scales (atomic to crystal size). First, using quantum calculation, we determine the different He and Ne insertion sites, insertion energies and diffusion pathways at the atomic scale for an ideal pristine zircon structure (i.e. damage free). These results serve as input for a 3D random walk simulation of atomic trajectories that provides diffusion coefficients for damage-free zircon crystals. Second, as natural zircon crystals are not perfect, we model the impact of different types of damage and diffusion pathway obstruction at the atomic level on He and Ne diffusion in 3D. The calculated He and Ne diffusion coefficients for pure ZrSiO4 exhibit strongly anisotropic behavior and very high diffusivity along the taxis, and with 3D, closure temperatures of -197 degrees C and -202 degrees C respectively. The results for He are comparable to previous DFT studies but strongly different from experimental diffusion results; results for Ne are similar in this respect. Modelling the impact of different types of damage (vacancies, recoil, fission, voids or fluid inclusions) and obstruction on He and Ne diffusion reveals important implications for the (U-Th)/He and (U-Th)/Ne thermochronometers. First, obstruction alone does not significantly modify He and Ne diffusion except to reduce anisotropy. Second, trapping is the primary mechanism altering He and Ne diffusion even at low dose, and we predict the maximal trapping energies for He and Ne to be 164 and 320 kJ/mol, similar to values inferred from experimental data. We also propose that the closure temperature increases non-linearly with damage, with effective trapping energy increasing with dose until a threshold, possibly corresponding to a percolation transition, after which retentivity decreases. Based on field data sets we also anticipate a value for this threshold of around similar to 2-5 x 10(17) alpha/g, lower than previously proposed. We show Ne to be highly blocked by damage and predict similar diffusion behavior to He, but with higher retentivity. We demonstrate the importance of investigating rare gas diffusion at the atomic level for comparison with experimental data, in order to build a predictive diffusion law at different scales. (C) 2019 Elsevier Ltd. All rights reserved.

On the recommendation of the United Nations General Assembly and the specific resolve of the United Nations Educational, Scientific and Cultural Organisation (UNESCO), the year 2019 was declared as the International Year of the Periodic Table of Chemical Elements (IYPT 2019). This decision was based on a strong appreciation of international character of scientific cooperation pursued in several frontier areas of basic and applied sciences related to the Periodic Table. It should commemorate the 150th anniversary of the creation of that Table. As expected, the decision was hailed with great enthusiasm by a large number of national, regional and international chemical societies and other scientific organisations. The IYPT was officially launched on 29 January 2019 at the UNESCO headquarters in Paris, France. Since then different types of functions, e. g. conferences, symposia, popular lectures, exhibitions and social gatherings, etc. have been going on around the world, with the aim to increase the public awareness of science in general, and chemistry in particular. Furthermore, a large number of journals are publishing editorials, commentaries or special issues, and a few publishers are bringing out special pamphlets to mark the occasion. Even a few newspapers and magazines have printed relevant articles. Since radiochemistry has contributed substantially to the extension of the Periodic Table as well as to the development of various applications related to it, this special issue of Radiochimica Acta is being published as a part of international celebrations. In this Editorial we shortly describe the origin and development of the Periodic Table and give a brief overview of its present status, discussing some related areas of particular significance to radiochemistry.

Humic acid (HA) in the environment may exist in either dissolved or fixed forms. However, laboratory studies usually take only the former into account. Here we synthesized a hybrid of HA and aluminum hydroxide (Al(OH)3) to mimic fixed HA, compared the effects of fixed and dissolved HA on Eu(III)/Yb(III) adsorption on Al(OH)3, and analyzed the adsorption mechanisms using time resolved laser induced fluorescence spectroscopy (TRLFS), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS). It was found that dissolved HA affected significantly Eu(III)/Yb(III) adsorption on Al(OH)3, whereas fixed HA showed little apparent effect on the adsorption. The spectra of TRLFS, XPS and EXAFS for Eu(III)/Yb(III) adsorption on Al(OH)3 in the presence of dissolved/fixed HA could be reproduced by those for Yb(III)/Eu(III) adsorption in the absence of HA and those for Yb(III)/Eu(III) binding to dissolved HA, respectively. Spectroscopic analyses indicated that the different effects of fixed and dissolved HA on Eu(III)/Yb(III) adsorption were due to different surface speciation with the same surface species, which could be interpreted by the decrease of available sites on fixed HA as compared to those on dissolved HA. This study implied that the effects of HA on the adsorption of Eu(III)/Yb(III) as well as other trivalent lanthanides/actinides in the environment might be overestimated if the differences between dissolved and fixed HA were not considered.

This paper presents the main results of a study conducted to quantify and to model the degradation state of zirconium-based fuel claddings submitted to severe-accident conditions in a nuclear reactor core: high temperatures and either pure steam or an air-steam mixture. Due to the progressive thickening of a dense and protective ZrO2 layer, the oxidation kinetics of zirconium-based claddings in steam at high temperatures typical of severe nuclear accidents are generally cubic or parabolic. However, for some temperature domains, this oxide layer may crack, becoming porous and no longer protective. In these “breakaway” conditions, the oxidation kinetics change from (sub)parabolic to linear or even accelerated. In addition, the temperature increase can lead core materials to melt and to relocate down to the vessel lower head, threatening its integrity. If it fails, and for specific conditions, air ingress may take place into the reactor. Hence, oxygen and nitrogen both react with zirconium-based claddings successively through the oxidation of zirconium (forming a ZrO2 layer), nitriding of zirconium (forming zirconium nitride particles), and the oxidation of zirconium nitride (forming ZrO2 and releasing nitrogen). These self-sustained chemical reactions enhance the deterioration of zirconium-based claddings and their ZrO2 layers, inducing an increase in their open porosity. To quantify this porosity, a series of two-step experiments was conducted. First, ZIRLO™ cladding samples were isothermally oxidized in pure steam or in a 50:50 mol% air-steam mixture at several different temperatures and durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation were all confirmed by experimental results. Second, pioneering porosimetry measurements by mercury intrusion were realized for the first time on such corroded cladding samples. In both atmospheres, it was pointed out that 1,200 and 1,250 K led to particularly porous oxide layers, especially due to strong breakaway effects. Moreover, we confirmed that the presence of air strongly enhances the oxide cracking: cladding samples were more porous when oxidized in the air-steam mixture than under pure steam. Finally, we observed that in all conditions, the open porous volume fraction of ZIRLO claddings continuously rose during their corrosion process. Hence, for each experimental condition, porosity correlations were determined through linear regressions, and porosity increase rates were deduced by derivation versus time and validated against porosimetry results of cladding samples corroded in transient (nonisothermal) conditions.

Better understanding of uranyl-protein interactions is a prerequisite to predict uranium chemical toxicity in cells. The EF-hand motif of the calmodulin site I is about thousand times more affine for uranyl than for calcium, and threonine phosphorylation increases the uranyl affinity by two orders of magnitude at pH 7. In this study, we confront X-ray absorption spectroscopy with Fourier transform infrared (FTIR) spectroscopy, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and structural models obtained by molecular dynamics simulations to analyze the uranyl coordination in the native and phosphorylated calmodulin site I. For the native site I, extended X-ray absorption fine structure (EXAFS) data evidence a short U-Oeq distance, in addition to distances compatible with mono- and bidentate coordination by carboxylate groups. Further analysis of uranyl speciation by TRLFS and thorough investigation of the fluorescence decay kinetics strongly support the presence of a hydroxide uranyl ligand. For a phosphorylated site I, the EXAFS and FTIR data support a monodentate uranyl coordination by the phosphoryl group and strong interaction with mono- and bidentate carboxylate ligands. This study confirms the important role of a phosphoryl ligand in the stability of uranyl-protein interactions. By evidencing a hydroxide uranyl ligand in calmodulin site I, this study also highlights the possible role of less studied ligands as water or hydroxide ions in the stability of protein-uranyl complexes.

The threat of a dirty bomb which could cause internal contamination has been of major concern for the past decades. Because of their high chemical toxicity and their presence in the nuclear fuel cycle, uranium and neptunium are two actinides of high interest. Calmodulin (CaM) which is a ubiquitous protein present in all eukaryotic cells and is involved in calcium-dependent signaling pathways has a known affinity for uranyl and neptunyl ions. The impact of the complexation of these actinides on the physiological response of the protein remains, however, largely unknown. An isothermal titration calorimetry (ITC) was developed to monitor in vitro the enzymatic activity of the phosphodiesterase enzyme which is known to be activated by CaM and calcium. This approach showed that addition of actinyl ions (AnO2n+), uranyl (UO22+) and neptunyl (NpO2+), resulted in a decrease of the enzymatic activity, due to the formation of CaM-actinide complexes, which inhibit the enzyme and alter its interaction with the substrate by direct interaction. Results from dynamic light scattering rationalized this result by showing that the CaM-actinyl complexes adopted a specific conformation different from that of the CaM-Ca2+ complex. The effect of actinides could be reversed using a hydroxypyridonate actinide decorporation agent (5-LIO(Me-3,2-HOPO)) in the experimental medium demonstrating its capacity to efficiently bind the actinides and restore the calcium-dependent enzyme activation.

The interaction of salicylic acid with zirconium diphosphate surface and its reactivity toward uranium (VI) was investigated. The interaction of salicylic acid with zirconium diphosphate was firstly studied using several analytical techniques including atomic force microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. The sorption of uranium (VI) onto surface modified zirconium diphosphate was evaluated by the classical batch method at room temperature. This study showed that the uranium (VI) sorption onto zirconium diphosphate is influenced by the presence of salicylic acid. A fluorescence spectroscopy study revealed the presence of a uranyl specie onto the modified solid surface. The spectroscopy results were then used to restrain the modeling of experimental sorption data, which are interpreted in terms of a constant capacitance model using the FITEQL code. The results indicated that interaction between the uranium (VI) and the surface of zirconium diphosphate modified with salicylic acid leads to the formation of a ternary surface complex.

Because of their presence in the nuclear fuel cycle, neptunium and uranium are two actinides of main interest in case of internal contamination. Complexation of U(VI) and Np(V) by the target protein calmodulin (CaM(WT)) was therefore studied herein. Both actinides have two axial oxygen atoms, which, charge aside, makes them very similar structurally wise. This work combines spectroscopy and theoretical density functional theory (DFT) calculations. Structural characterization was performed by extended X-ray absorption fine structure (EXAFS) at the L(III)-edge for each studied actinide. Models for the binding site of the protein were developed and then refined by using DFT to fit the obtained experimental EXAFS data. The effect of hydrolysis was also considered for both actinides (the uranyl experiment was performed at pH 3 and 6, while the neptunyl experiment was conducted at pH 7 and 9). The effect of the pH variation was apparent on the coordination sphere of the uranyl complexes, while the neptunyl complex characteristics remained stable under both studied conditions. The DFT calculations showed that at near physiological pH the complex formed by CaM(WT) with the neptunium ion is more stable than the one formed with uranyl.

Migration of radionuclides in aqueous system is a matter of great environmental concern due to their acute and long-term toxicity. This study seeks to address the sorption of uranyl on rutile in presence of short-chain aliphatic carboxylic acids by zeta potential analysis and in situ ATR-IR spectroscopy. Point of zero charge of rutile was significantly shifted with the addition of carboxylic acids/uranyl ions separately in solution but it was negligible when organics and uranyl ions were added in the suspension. In situ ATR-IR data for uranyl sorption was evidenced by an absorption band of uranyl as(UO2) at 915 cm(-1). (C) 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

The complexation of uranium and europium, in oxidation states +VI and +III, respectively, was investigated with pertinent bio-inorganic systems. Three aspartate-rich pentapeptides with different structural properties were selected for study to rationalize the structure-affinity relationships. Thermodynamic results, crosschecked by both isothermal titration calorimetry and time-resolved laser fluorescence spectroscopy, showed different affinity depending on the peptide for both Eu(III) and U(VI). The thermodynamic aspects were correlated to structural predictions, which were acquired by density functional theory quantum chemical calculations and from IR and extended X-ray absorption fine structure experiments. The combination of these microscopic properties revealed that carbonyl-metal interactions affected the entropy in the case of europium, while the larger uranyl cation was mostly affected by preorganization and steric effects, so that the affinity was enhanced through enthalpy. The approach described here revealed various microscopic aspects governing peptide actinide affinity. Highlighting these mechanisms should certainly contribute to the rational synthesis of higher affinity biomimetic aspartic ligands.

The quantification of the different parameters influencing He diffusion in apatite is an important issue for the interpretation of (U-Th)/He thermochronometric ages. Key issues include understanding the role of chemical composition and the mechanism modifying diffusivity by radiation damage, both requiring a realistic description at the atomic level. In this contribution, we restrict ourselves on the influence of the chemical composition especially on the effect of Cl-atoms on the He diffusion in the damage-free apatite crystal. For this purpose, a multi-scale theoretical diffusion study has been conducted using periodic Density Functional Theory calculations for two different apatite compositions (pure fluorine apatite and apatite with one chlorine and 3 fluorine atoms per cell called Cl0.25-apatite) representative of damage-free crystals. Different He insertion sites and diffusion pathways are first investigated. The Density Functional Theory approach coupled to the Nudged Elastic Band method is used to determine the energy barriers between the insertion sites. A statistical method, based on Transition State Theory, is used to compute the jump rate between sites and the different results are used as output for a 3D random walk simulation, which determines the diffusion trajectories and the diffusion coefficients. The calculated diffusion coefficients for pure F-apatite exhibit a slightly anisotropic behavior with an activation energy Ea=95.5kJ/mol and a frequency factor D0=1.9×10−3cm2/s along the c axis; Ea=106.1kJ/mol and D0=4.1×10−3cm2/s in the plane orthogonal to c. Closure temperatures for a 60μm grain radius and 10°C/Ma cooling rate range from 33 to 36°C and depend on crystal geometry for a given grain size. Surprisingly, even though He diffusion is strongly blocked across the Cl atoms in Cl0.25-apatite, where Ea is significantly higher (166.7kJ/mol), He atoms can still diffuse along the c axis through workaround pathways. Closure temperatures are dependent on the Cl content in the crystal lattice and can be ∼12°C higher for Cl0.25-apatite than for F-apatite. These results show that various Cl contents lead to a more He retentive diffusivity in addition to their impact on damage-annealing rate. The results of this study are in good agreement with experimental results and demonstrate that a proper Density Functional Theory treatment allows to characterize He diffusion in damage-free apatite. This opens new avenues to a reliable method of quantifying rare gas diffusion in mineral structures.

In case of a nuclear event, contamination (broad or limited) of the population or of specific workers might occur. In such a senario, the fate of actinide contaminants may be of first concern, in particular with regard to human target organs like the skeleton. To improve our understanding of the toxicological processes that might take place, a mechanistic approach is necessary. For instance, ∼50% of Pu(IV) is known from biokinetic data to accumulate in bone, but the underlining mechanisms are almost unknown. In this context, and to obtain a better description of the toxicological mechanisms associated with actinides(IV), we have undertaken the investigation, on a molecular scale, of the interaction of thorium(IV) with osteopontin (OPN) a hyperphosphorylated protein involved in bone turnover. Thorium is taken here as a simple model for actinide(IV) chemistry. In addition, we have selected a phosphorylated hexapeptide (His-pSer-Asp-Glu-pSer-Asp-Glu-Val) that is representative of the peptidic sequence involved in the bone interaction. For both the protein and the biomimetic peptide, we have determined the local environment of Th(IV) within the bioactinidic complex, combining isothermal titration calorimetry, attenuated total reflectance Fourier transform infrared spectroscopy, theoretical calculations with density functional theory, and extended X-ray absorption fine structure spectroscopy at the Th LIII edge. The results demonstrate a predominance of interaction of metal with the phosphate groups and confirmed the previous physiological studies that have highlighted a high affinity of Th(IV) for the bone matrix. Data are further compared with those of the uranyl case, representing the actinyl(V) and actinyl(VI) species. Last, our approach shows the importance of developing simplified systems [Th(IV)-peptide] that can serve as models for more biologically relevant systems.

This paper presents the main results and conclusions from a study conducted to quantify the degradation state of Zr-based fuel claddings submitted to severe accident conditions in a nuclear reactor core: high temperatures and either pure steam or air-steam mixture.At high temperatures typical of nuclear severe accidents, the oxidation kinetics of Zr-based claddings in steam is generally cubic or parabolic due to the progressive thickening of a protective ZrO2 layer. However, at temperatures below 1300 K, this oxide layer may crack, becoming porous and non-protective anymore. In such conditions, the oxidation kinetics of Zr-based claddings change from (sub-)parabolic to linear or even accelerated. Additionally, the temperature increase can lead core materials to melt and to relocate down to the vessel lower head, threatening its integrity. If it fails, and for specific conditions, air ingress into the reactor may take place: oxygen and nitrogen both react with Zr-based claddings, causing not only oxidation of Zr but also formation and re-oxidation of ZrN. These self-sustained chemical reactions enhance the deterioration of ZrO2 layers and claddings, inducing a rise of their porosity.In order to quantify this porosity, a series of two-step experiments was conducted. First, Zr alloy cladding samples were oxidized in various conditions: at several temperatures, in steam or air-steam mix, and for different durations. The main thermal effects on reaction kinetics and the high impact of air on the cladding degradation are all confirmed by the experimental results. Then, porosity measurements by Hg intrusion were realized for the first time on such oxidized cladding samples. In all atmospheres, it is pointed out that 1200 and 1250 K lead to particularly porous oxide layers, especially due to strong 'breakaway' effects. Moreover, it is confirmed that the presence of air strongly enhances the oxide cracking: cladding samples are more porous when oxidized in the air-steam mixture than under pure steam. Finally, it is shown that in most of conditions, the porous volume fraction of Zr-based claddings seems to continuously rise during their oxidation process.

Periodic Density Functional Theory (DFT) calculations on apatite lattice have been performed to investigate the chemical composition effect on He diffusion and its impact on the (U-Th)/He thermochronometer. Two preferential diffusion directions in both structures have been identified, one along the fluorine atoms and the other one in the plane orthogonal to the later direction. A NEB has been used to determine the activation energies, which range from 95,500 to 106,100 kJ/mol for the F-apatite and from 79,118 to 166,920 kJ/mol for the Cl0.25-apatite. According to the energy barriers a small anisotropy is noticed in the case of the pure F-apatite and a more pronounced anisotropy in Cl0.25-apatite. Consequently He diffuses preferentially in the plane in case of Cl0.25-apatite while a 3 dimension (3D) diffusion process is observed in the pure F-apatite at low temperature. In a second part, Kinetic Monte Carlo calculations have been performed to simulate the He 3D diffusion in the two-apatite lattices composition. From these calculations the Arrhenius law gives us access to the diffusion coefficient for infinite crystal such as:

The speciation of uranium(VI) present in aqueous solutions of 0.73–7.08mol.L−1 phosphoric acid or extracted from these solutions in various organic phases was investigated by time-resolved laser-induced fluorescence spectroscopy (TRLFS). The organic phases consisted of bis-(2-ethyl-hexyl) phosphoric acid (D2EHPA), bis(1,3-dibutyloxypropan-2-yl) phosphoric acid (BiDiBOPP) and synergistic mixtures of tri-n-octylphosphine oxide (TOPO) or di-n-hexyl octyl methoxy phosphine oxide (di-n-HMOPO) with D2EHPA or BiDiBOPP in Isane IP 185 (an aliphatic diluent). The present paper indicates that uranium(VI) exists as a unique species in aqueous solutions of 0.78–7.08mol.L−1 H3PO4 likely as UO2(H3PO4)n(H2PO4)m2-m where n+m=3, and that a predominant uranium(VI) species (more than 90%) is extracted by the D2EHPA/TOPO synergistic mixture. This species, whose formula is likely UO2(HL)2L2TOPO with HL=monomeric D2EHPA and L=deprotonated form of D2EHPA, is characterized by the fact that the TOPO molecule is located in the first solvation shell of uranium(VI) as also confirmed by DFT calculations. Furthermore, TRLF spectra of uranium(VI) in D2EHPA/di-n-HMOPO and BiDiBOPP/di-n-HMOPO are quite different from those obtained with the D2EHPA/TOPO mixture. Indeed, the vibronic peaks of uranium(VI) TRLF spectra have partially (D2EHPA/di-n-HMOPO) or completely (BiDiBOPP/di-n-HMOPO) disappeared, which could be due to the presence of oxygen atoms in the hydrophobic chains of BiDiBOPP and di-n-HMOPO. As in the case of the D2EHPA/TOPO mixture, the fluorescence lifetime of uranium(VI) in the BiDiBOPP/di-n-HMOPO mixture indicates that uranium(VI) is extracted by this mixture from 5.3mol.L−1 H3PO4 as a unique or at least a predominant species.