bluuebunny/crossref_metadata_2025 huggingface dataset:
Scandium, Yttrium and the Lanthanides: Applications
Authors: Cotton Simon A.
Published: December 2011

New applications are increasingly found for scandium, yttrium, and the lanthanides and their compounds. Among reagents in synthetic organic chemistry, LaNi
5
is a versatile hydrogenation catalyst. SmI
2
is an important one‐electron reducing agent, and lanthanide triflates, Ln(CF
3
OSO
3
)
3
(Ln = Sc, Y, or lanthanide), are recyclable Lewis acid catalysts, which operate in aqueous conditions. Organocerium compounds are valuable, mild alkylating agents, while using cerium chloride in combination with NaBH
4
and LiAlH
4
permits a range of reductions to be carried out. (NH
4
)
2
[Ce(NO
3
)
6
] is an established oxidant for which new applications are being found. The alloys SmCo
5
and Sm
2
Co
17
have been used to produce magnetic materials that are very powerful for their size, though partly superseded by cheaper Nd
2
Fe
14
B material, while strongly paramagnetic salts of Dy
3+
or Gd
3+
permit attainment of extremely low temperatures through adiabatic demagnetization. The mixed metal oxide YBa
2
Cu
3
O

7−
x

was the first material to superconduct above 77 K, revolutionizing understanding; new materials are being developed, most recently fluoride‐doped oxypnictides LaFeAsO. In addition to gadolinium complexes as MRI contrast agents, other lanthanide compounds find medicinal application, including cerium compounds in wound and burn treatment. In terms of quantity, the most important application of lanthanide compounds is lanthanide‐exchanged zeolites as catalysts in the petrochemical industry, notably in cracking, but cerium oxide is an important component of catalytic converters in automobile exhausts. The ability of LaNi
5
to absorb hydrogen reversibly has been vital to nickel–metal hydride (NiMH) batteries, while yttrium aluminum garnet (Y
3
Al
5
O
12
) is widely used as a host material in lasers and phosphors. Yttria‐stabilized zirconia (YSZ) is a ceramic material used as the electrolyte material in solid oxide fuel cells (SOFCs) and in sensors in car exhaust systems.
Medicinal Inorganic Chemistry: An Introduction
Authors: Thompson Katherine H.
Published: December 2011

Metal ions in clinical use today include bismuth compounds, which have been in use for over a hundred years, lithium carbonate, which has been in use for over 50 years, and cisplatin, which was introduced almost 40 years ago. “Metals in medicine” also covers a wealth of newer diagnostic and therapeutic compounds, such as gadolinium‐based contrast agents for magnetic resonance imaging (MRI), samarium‐based compounds for relief of bone pain in intractable cancer, and lanthanum carbonate for the treatment of hyperphosphatemia. In the process, mystical/magical use of metals in medicine has given way to rational design and testing of new metallopharmaceutical compounds. Advances in our understanding of bioinorganic chemistry and increasingly sensitive analytical instrumentation enable an accelerated pace of development in medicinal inorganic chemistry.
Calculation of Bonding Properties
Authors: Frenking Gernot , von Hopffgarten Moritz
Published: December 2011

The bonding situation in four classes of compounds is analyzed with modern charge and energy decomposition methods. The nature of the bonding in the diatomic species H
2
, N
2
, CO, and BF is investigated with the energy decomposition analysis (EDA). The electronic structure of the heavy‐atom homologs of acetylene HEEH (E = SiPb) is investigated with the help of an orbital analysis and EDA calculations. Metal–ligand bonding in isoelectronic metal hexacarbonyls TM(CO)
6
q
(TM
q
 = Hf
2−
, Ta
−
, W, Re
+
, Os
2+
, Ir
3+
) is the subject of a detailed EDA study. The bonding situation in the unusual 12‐fold coordinated transition‐metal compound [Mo(ZnH)
12
] is analyzed with the AIM (atoms in molecules), EDA, and MO methods. The results of the electronic structure analyses build a bridge between the heuristic bonding models, which are ubiquitously used in inorganic chemistry and accurate quantum chemical calculations.
Multiconfigurational Quantum Mechanics (QM) for Heavy Element Compounds
Authors: Roos Björn O.
Published: December 2011

One frequently used quantum chemical approach for the studies of transition metal and other heavy element compounds is the complete active space self consistent field (CASSCF) method in combination with multiconfigurational second‐order perturbation theory (CASPT2). Here, we describe this method and some of its typical applications. The basic idea is to construct a wave function that gives a qualitatively correct description of the electronic structure already at the lowest level of theory. This should be possible for all sorts of electronic states independent of spin multiplicity and excitation level. Strong mixing of configurations makes it necessary to use a multiconfigurational approach in such cases. It should also be possible to use for all atoms of the periodic system. The CASSCF wave function fulfills, in principle, this requirement because it is full configuration interaction (CI), albeit in a limited space of active orbitals. The addition of dynamic electron correlation is necessary to give quantitative results. The suggested solution is to compute this energy using second‐order perturbation theory (CASPT2) because it is relatively simple and allows applications to a wide variety of systems and many electrons. Relativistic corrections are included using a Douglas‐Kroll‐Hess Hamiltonian. The method is scrutinized and illustrated by some applications in transition metal and actinide chemistry.
Approximate Density Functionals: Which Should I Choose?
Authors: Rappoport Dmitrij , Crawford Nathan R. M. , Furche Filipp , Burke Kieron
Published: December 2011

Density functional methods have a long tradition in inorganic and bioinorganic chemistry. We introduce the density functional machinery and give an overview of most popular approximate exchange‐correlation (XC) functionals. We present comparisons of density functionals for energies, structures, and reaction barriers of inorganic and bioinorganic systems, giving guidance on the title question. New development directions and current trends in density functional theory (DFT) are reviewed.
Broken Symmetry States of Iron—Sulfur Clusters
Authors: Noodleman Louis , Case David A.
Published: December 2011

After presenting a general introduction to the structures and physical properties of dinuclear and polynuclear iron sulfur clusters, we discuss the relationship between the broken symmetry model within density functional theory (DFT), and the energies and properties of related pure spin states. The methodology for calculating and the physical significance of Heisenberg coupling parameters (J) and spin‐dependent delocalization parameters (B) are developed. The interaction energy of the active site quantum cluster with the protein and solvent environment is treated with a single step procedure based on the classical Poisson–Boltzmann (PB equation). Redox potentials of various high‐potential 4Fe4S and ferredoxin proteins are calculated with DFT/PB methods. The electronic and spin‐coupling structure of oxidized high‐potential (HIPIP) proteins is examined, and it is shown that two distinct electronic structures are possible. These contrasting electronic structures may have implications for the structural stability of oxidized HIPIP. For the Fe protein of nitrogenase, the redox potential and spin state of the super‐reduced form are calculated, and it is argued that this all‐ferrous state is probably not physiologically accessible. The redox potential and electronic structure of the FeMo cofactor of the MoFe protein of nitrogenase are examined for the “resting” state. A very small spin density at the central atom is very feasible. The central atom could be XC, N, O, but C, N are most probable, despite the absence of an observable ENDOR signal for the central atom. The spin‐density for a central atom should be much higher for the 2e‐reduced FeMo cofactor when a ligand is bound, and we have tested this for bound allylic alcohol. Three different ligand‐cluster binding structures are characterized by geometry, energy, and spin distribution.
Modeling Metalloenzymes with Density Functional and Mixed Quantum Mechanical/Molecular Mechanical (QM/MM) Calculations: Progress and Challenges
Authors: Friesner Richard A.
Published: December 2011

We discuss the application of density functional theory (DFT)‐based quantum chemical and mixed quantum mechanics/molecular mechanics (QM/MM) methods to the modeling of metalloenzyme chemistry. The state of the art of current approaches in terms of accuracy and reliability is considered, primarily in the context of two example systems, methane monooxygenase (MMO) and cytochrome P450. Key issues include the intrinsic accuracy of current DFT functionals for metal‐containing systems, modeling of conformational changes in protein structure, treatment of environmental electrostatic effects, use of spectroscopic calculations to help in identifying the structures of intermediates, and the validity of standard transition state theory approximations using a small number of optimized structures, as opposed to performing full statistical mechanical sampling with methods such as free energy perturbation theory. Considerable insight into the catalytic cycles of both MMO and P450 have been obtained over the past decade using existing computational methods, in close coupling with experimental efforts. However, significant challenges remain, both for the two systems that we consider here and for the modeling of metalloenzyme chemistry in the general case. A number of the most important of these challenges are identified and prospects for addressing them via improvements of the theoretical methodology are discussed.
Electronic Structure Calculations: Dinitrogen Reduction in Nitrogenase and Synthetic Model Systems
Authors: Tuczek Felix
Published: December 2011

Theoretical studies on the binding and reduction of dinitrogen and alternative substrates at the iron‐molybdenum cofactor (FeMoco) of nitrogenase are reviewed. Before considering the enzymatic pathways, the available mechanistic scenarios for N
2
reduction in small‐molecule model systems are discussed. These systems are divided into those involving an end‐on terminal (Sections 2.1 and 2.2) and those involving a bridging coordination of N
2
(Section 2.3). For the enzyme, calculations of Noodleman
et al
., Nørskov
et al
., Blöchl
et al
., Dance, and others are reviewed and compared to each other. Conclusions are drawn with respect to the most probable scenario of N
2
reduction at the FeMoco and the open mechanistic questions in this research area.
Electronic Structure of Metal—Metal Bonds
Authors: McGrady John E.
Published: December 2011

The nature of bonding between two metal centres is extremely variable, spanning formal bond orders from 1 to 6, and this leads to a great diversity of chemical properties in metal cluster compounds. The bonds themselves have components with σ, π, δ, and ϕ symmetry, each of which has its own distinct properties. This article reviews the key features of these different bonding components.
Computational Studies: Boranes
Authors: Shameema Oottikkal , Jemmis Eluvathingal D.
Published: December 2011

Computational studies on boranes, in recent years, are summarized with the special emphasis on the macropolyhedral boranes. Computational studies to understand the energetics, geometry, aromaticity, and reactivity of boranes and substituted boranes are reviewed. There is a recent interest in macropolyhedral boranes because of their uses in boron neutron capture therapy (BNCT) of cancer, ionic liquids, weakly coordinating ions, and materials chemistry. The
mno
rule, which is the electronic counting rule for the macropolyhedral boranes, is explained. Even though the electron counting rule explains the electronic structure of macropolyhedra, it cannot explain the relative stability of isomers since different structures are possible with the same electron count. There have been many computational studies to explain the relative stability of various structural patterns in macropolyhedral boranes. We have used the concept of orbital compatibility to explain the relative energies of different macropolyhedral structural patterns. Computations on the
closo
‐
closo
macropolyhedral boranes show that a large polyhedral borane prefers to condense with a smaller polyhedron owing orbital compatibility. Even though only a few
closo
‐
closo
macropolyhedral structures have been synthesized experimentally, calculations suggest optimism for the synthesis of many condensation products. A brief discussion about the use of the computational methods for the rearrangement of the B
20
H
16
skeleton reacting with ligands is also provided.
Molecular Mechanics in Bioinorganic Chemistry
Authors: Deeth Robert J.
Published: December 2011

Molecular mechanics (MM) in bioinorganic chemistry separates into applications involving s‐block cations and those involving the d‐block elements. The former behave essentially as hard, charged spheres and can be treated efficiently and accurately with conventional force fields with little or no additional parameterization. The d‐block elements are significantly more challenging. The apparently complex electronic structures associated with the open d shell, the challenges of modeling reaction pathways within empirical methods, the lack of suitable software and MM parameters, and the rise of hybrid quantum mechanics/molecular mechanics (QM/MM) have all contributed to MM for transition metal systems being relatively underdeveloped. Given that metal‐ligand binding in coordination complexes and metalloenzyme active sites is essentially identical, this has significant consequences for bioinorganic applications. However, a critical handicap of any QM‐based method, including QM/MM, is that it is computationally expensive to carry out the configurational sampling or molecular dynamics necessary to get an accurate structure or reaction pathway. In contrast, MM is orders of magnitude faster and therefore has the potential to make a significant impact in bioinorganic chemistry. Encouraging progress is being made, especially with respect to d electron effects where ligand field molecular mechanics has demonstrated QM‐like performance at MM‐like speeds.
Gaussian Basis Sets for Quantum Mechanical (
QM
) Calculations
Authors: Peterson Kirk A.
Published: December 2011

In addition to the selection of a particular electronic structure method, the choice of basis set to use in an ab initio calculation can also strongly influence the accuracy of the results. The possibility of decoupling the errors associated with the chosen method and basis set is central to any systematic treatment of electronic structure. With this in mind, the availability, characteristics, and use of accurate Gaussian basis sets across the entire periodic table are reviewed with special emphasis to series of basis sets that improve in quality in a systematic fashion toward the complete basis set limit.
Theoretical Aspects of Main Group Multiple Bonded Systems
Authors: Silaghi‐Dumitrescu Ioan , Petrar Petronela , Nemeş Gabriela , Bruce King R.
Published: December 2011

Computational data on homonuclear multiple bonding between elements of group 13 and heavier elements of group 14 are reviewed. For R

n

EER

n

, there is an increased tendency of bending the RER planes (
n
 = 2) or the RE bonds (
n
 = 1) toward the EE axis upon descending a group. This is related to the mixing of the valence σ–π orbitals reflecting the tendency of the heavier elements to retain nonbonding character for their
n
s
2
orbital. As a result, the bond orders are also reduced and the EE interaction weakened.
Radionuclides: Anthropogenic
Authors: Mietelski Jerzy W.
Published: December 2011

Following the definition of anthropogenic radioactivity, the main discoveries in this field made in the 1930s are briefly presented. The main facts about radioisotopes classification, radioactive decay, and artificial radionuclide production are then described. The article then provides a general overview on anthropogenic sources of radioactivity such as nuclear explosions, nuclear accidents, routine releases from nuclear reactors, releases due to nuclear weapon production programs or nuclear fuel reprocessing installations, and nuclear medicine. A description of modern techniques applied in determining radioactivity in the environment as well as a very general introduction to the concept of radioecology and some basic information on environmental remediation are presented.
Speciation: Radionuclides
Authors: Salbu Brit
Published: December 2011

To assess the environmental impact of radioactive contamination of different ecosystems, information on source terms, mobility, biological uptake, and associated effects is needed. However, naturally occurring and artificially produced radionuclides can be present in different physicochemical forms (ions, colloids, nanoparticles, pseudocolloids, particles) influencing the mobility and biological uptake. Following severe nuclear events associated with the nuclear weapon or fuel cycles, a major fraction of refractory radionuclides will be present as radioactive particles; radionuclides are also present as colloids or low molecular mass (LMM) ionic species in effluents from nuclear installations. Thus, the distribution of radionuclide species depends on the source and release scenarios. LMM radionuclide species are believed to be mobile and potentially bioavailable, while high molecular mass (HMM) species such as nanoparticles, colloids, polymers, and pseudocolloids are mobile in water. Radioactive particles are considered biologically inert, but can be retained in filtering organisms as well as in soils and sediments. If mobile species are present, ecosystem transfer is relatively fast, whereas the ecosystem transfer is delayed if particles are present. Owing to time‐dependent transformation processes such as interactions of mobile and reactive radionuclide species with components in soils and sediments or particle weathering and subsequent remobilization of associated radionuclide species, the original distribution of radionuclides deposited in ecosystems will change over time. Therefore, information on radionuclide species released and deposited, as well as on time‐dependent transformation processes influencing the distribution of radionuclide species, is essential for the assessment of the environmental impact and risk associated with radionuclide contamination. This article summarizes available speciation techniques that should be utilized within radioecology, including fractionation and solid‐state speciation, to characterize radionuclide species in waters, soils, and sediments.
Potassium: Radonuclides
Authors: Jun Taehong , Tsay Olga G. , Churchill David G.
Published: December 2011

There are currently 28 known nuclides of potassium (K). Three of these are naturally occurring:
39
K,
40
K, and
41
K of which potassium‐40 is the only naturally occurring radionuclide. With its long half‐life of 1.28 × 10
9
years and an abundance of 0.012%,
40
K is a discrete source of natural radiation. Thus, in potassium‐rich foods, minerals, etc., a distinct natural dose is unavoidable for humans and all living creatures on Earth. Thus, it may be quantified in soil, plants, and animals and allows for various studies. Potassium‐40 may decay to calcium‐40 (89% probability) or may undergo electron capture (EC) to give argon‐40 (11% probability). The
biological
half‐life (excretion time decay) of
40
K, however, is 30 days. As it is often found in many mineral forms, potassium‐40 is a common constituent of soil and water, but not air. There are various reports in which
40
K is tracked migrating from soil(s) into various plants. In countries such as Europe and Brazil, drinking water was also scrutinized and found to contain
abnormal
levels of
40
K. The abundance of
40
K in human diet and foods from places such as China, Pakistan, and Ukraine has also been studied.
Sulfur: Radionuclides
Authors: Tsay Olga G. , Churchill David G.
Published: December 2011

35S is a radionuclide with a half‐life of 87.5 days and it decays via β‐emission. It is available from natural and anthropogenic sources. It is cosmogenic, that is, it is produced by the collision of atmospheric argon with cosmic rays. Anthropogenic molecular sources of35S include carbonyl sulfide (CO35S) and hydrogen sulfide (H235S), which are released from nuclear reactors. The impact of the incorporation of these compounds into plants and animals is of particular interest and concern with regard to the accumulation of35S in human food. Radioisotopes of sulfur can be detected by a Geiger–Müller counter and liquid scintillation counting techniques.
Indium: Radionuclides
Authors: Wehmschulte Rudolf J.
Published: December 2011

Although the major isotope
115
indium (95.7% abundance) is a β‐emitter, it has a very long half‐life at 6 × 10
14
(600 trillion) years and hence can be considered practically nonradioactive. Other radioactive indium isotopes are short‐lived and are either part of the decay processes of other radioactive nuclei or are prepared artificially for medicinal and analytical purposes in very small amounts. The environmental impact of radioactive indium isotopes is very minor and standard safety procedures for low‐level wastes are sufficient.
Polonium: Radionuclides
Authors: Alam Lubna , Rahim Mohamed Che Abd
Published: December 2011

All polonium isotopes are radioactive with only three having appreciable half‐lives:
208
Po,
209
Po, and
210
Po. It is a highly toxic element and is a health hazard if taken into the body. Polonium is a naturally occurring radionuclide and is present in the environment and in people at very low concentrations. Polonium concentrations in the environment can vary by orders of magnitude.
210
Po concentration in soil varies according to soil strata, depth, sedimentation and biological processes, fertilization and pH. Incomplete burning of fuels of coal loading plays an important role in the concentration of
210
Po in air. Usually, the activity concentration of
210
Po in surface air is low. Polonium can be separated by irradiating stable bismuth‐209 with thermal neutrons resulting in the formation of radioactive
210
Bi, which decays into
210
Po.
Chlorine: Radionuclides
Authors: Ashworth Daniel , Shaw George
Published: December 2011

Chlorine is ubiquitous in the environment and is an essential element for higher plants and animals. The primary radioactive isotope is
36
Cl, which has a half‐life of 3.01 × 10
5
years and which is produced both naturally and as a result of anthropogenic nuclear activity. This article considers the important sources of
36
Cl in the environment and the subsequent behavior of the radionuclide. The chemistry of radio‐ and stable chlorine is discussed with particular emphasis on its ability to not only exist as the chloride ion but also as organic chlorine. Procedures for the fractionation/speciation and analytical determination of
36
Cl in environmental samples are also reported. Owing to the high solubility of the chloride ion, the potential for a large degree of
36
Cl migration within soils, and from soils into plants is described. The most significant source of
36
Cl is likely to be nuclear waste buried in underground rock repositories. Over extended time periods, if such a repository were to leak,
36
Cl has the potential to move upward with the flow of water into soils and plants. The article describes risk modeling that assesses this important pathway by which humans may become exposed to
36
Cl. Such modeling suggests that
36
Cl leaked from a buried nuclear waste repository has the potential to eventually contaminate soils, plants, well water, and surface waters. However, the radioactive dose associated with this contamination would apparently be relatively low.
Xenon: Radionuclides
Authors: Saey Paul R.J.
Published: December 2011

Radioxenon isotopes are noble gases mainly produced in nuclear fission e.g., uranium‐235. In nuclear medicine, xenon‐133 isotopes are used for measuring the physiological parameters of lung ventilation and to image the lungs. They are further used in isotonic solutions to image blood flow, particularly cerebral blood flow. Most radioactive isotopes of this element are produced by a nuclear fission reaction of uranium‐235, uranium‐238, or plutonium‐239. Xenon‐133 is the most abundant radioxenons observed in environmental samples, although contributions of
131m
Xe and
135
Xe can be determined down to a few percent of the total β‐activity. The minimum detectable concentration (MDC) for
133
Xe in routine samples is about 1 mBq m
−3
. This article will describe the production, applications, occurrence, and measurement of the various xenon isotopes.
Radon: Radionuclides
Authors: Tracy Bliss L.
Published: December 2011

Radon is a naturally occurring radioactive gas formed by the decay of uranium and thorium in the Earth's crust. At atomic number 86, it is the heaviest member of the noble or inert gas family, which also includes helium, neon, argon, krypton, and xenon. Unlike the other members of the family, radon has no stable isotopes. Of the 34 known radioactive isotopes of radon, only three are supported by the decay of primordial radionuclides, and only two occur with any significant abundance in nature. The three isotopes, along with their half‐lives, are222Rn (3.8235 days),220Rn (55.6 s), and219Rn (3.96 s). Both222Rn and220Rn decay by the emission of α‐particles and give rise to a number of short‐lived progeny.222Rn and220Rn are the only ones regarded as having any health impacts and this is due to their short‐lived progeny—218Po,214Pb,214Bi, and214Po from222Rn;216Po,212Pb,212Bi,212Po, and208Tl from220Rn. Being a gas, radon diffuses readily into the atmosphere and may build up to hazardous concentrations in confined spaces in homes and underground mines. Radon is the largest single contributor to natural background radiation exposure worldwide. Radon is now recognized as the second leading cause of lung cancer, following tobacco smoking, and is responsible for up to 20 000 deaths per year in the United States and 1900 deaths per year in Canada. Many countries have now established programs to reduce radon exposures to the population.
Protactinium: Radionuclides
Authors: Hasan Asamuddin Abu , Rahim Mohamed Che Abd
Published: December 2011

Protactinium (Pa) is one of the rarest elements that can be found or extracted from the natural source. Protactinium can be found in soil, rocks, water, plants, and animals, but in a very low concentration. There are no uses outside of basic and experimental scientific research and no commercial applications for protactinium.
231
Pa is the most important natural isotope, while
223
Pa is the only artificial isotope.
231
Pa has become important to the scientists because
231
Pa/
230
Th in the ocean can perform as an ocean circulation proxy. Because of its low concentrations, measurement of protactinium using thermal ionization mass spectrometry (TIMS) has become the finest and most promising technique.
Uranium: Radionuclides
Authors: Vandenhove Hildegarde , Hurtgen Christian , Payne Timothy E.
Published: December 2011

This article describes the occurrence, chemistry, and bioavailability of uranium (U) in terrestrial and aquatic environments, its analysis in environmental samples, and remedial measures applicable to uranium‐contaminated water and soil. Uranium is widely distributed throughout the world. There are three main isotopes present in natural uranium, which comprises234U (0.0055%),235U (0.72%), and238U (99.27%). Uranium can occur either in its reduced state (U(IV)), which is generally highly immobile, or in its more soluble and mobile (U(VI)) state. Uranium mobility and bioavailability are governed by oxidation state, complexation by organic and inorganic ligands, pH, sorption by minerals including clays and hydroxides, and interactions with organic matter. In uncontaminated surface waters uranium content is generally low (<1 ppb). Most groundwaters are low in uranium but the concentration range is large (<0.001–2600 ppb), resulting from the interaction of groundwater with naturally occurring uranium‐bearing materials or anthropogenic contamination sources. Since groundwater is a major source for drinking water, various methods have been developed to remove uranium from potable water. Similar methods are applied in wastewater treatment. Extraction and processing of uranium ore or minerals containing natural radionuclides has resulted in the generation of waste streams. The major challenge associated with these contaminated waste streams, residues, or sites is typically the larger volumes of material and the relatively low specific activities.
Neptunium: Radionuclides
Authors: Runde Wolfgang , Goff George S.
Published: December 2011

Following the element uranium, neptunium is the fourth element in the actinide series. Although primordial neptunium isotopes formed during the creation of the solar system billions of years ago have long since decayed away, neptunium continues to be formed by the nuclear reactions in naturally occurring uranium deposits. Currently, trace amounts of neptunium isotopes can be isolated from uranium ore minerals. Most of present‐day inventory of neptunium originates from human activities, such as nuclear weapons testing, nuclear accidents, or irradiation of uranium fuel for producing nuclear energy. All neptunium isotopes are highly radioactive and pose extreme health hazards to humans when ingested or inhaled. Neptunium exhibits five oxidation states (III–VII) in solution and solid state, but only the IV and V oxidation states are relevant under most environmental conditions. In the +IV state, neptunium is insoluble and forms stable complexes with ligands abundant in natural environments (such as hydroxide, carbonate, or humic substances), and can adsorb onto mobile colloidal matter. In contrast, neptunium(V) forms a soluble dioxo‐cation in solution (NpO$_{2}^{+}$), which exhibits an increased solubility, weaker complex formation, and higher stability in the aqueous phases of aquatic systems. Thus, neptunium(V) is mostly characterized as the dominant solution species, while Np(IV) is found to be adsorbed on sediments, minerals, and colloidal particles. There is no large‐scale use of neptunium in industrial applications.
Plutonium: Radionuclides
Authors: Neu Mary P. , Goff George S. , Runde Wolfgang
Published: December 2011

Plutonium is the most debated actinide element because of its dual use in devastating nuclear weapons and beneficial generation of nuclear energy. Plutonium metal exists in several allotropic phases and plutonium in solution exhibits a rich and complex chemistry. The ability to exist in up to four oxidation states makes plutonium unique among all elements. Pu(III) and Pu(VI) form only under reducing or oxidizing conditions, respectively; whereas Pu(IV) and Pu(V) have broad stability ranges within the geochemical bounds of environmental systems. When Pu(IV) is above trace concentration and particulate matter is present, then solids or suspended colloids are the predominant chemical forms. In contrast, Pu(V), as PuO$_{2}^{+}$, has relatively higher solubility as an aquo species or a monomeric complex of natural ligands (hydroxide, chloride, sulfate, etc.). Stronger complexing, polydentate ligands, such as carbonate and organics, tend to increase plutonium solubility by forming very stable Pu(IV) complexes. Minerals, high molecular weight organic matter, and bacteria can either increase or decrease the solubility of plutonium, depending upon their chemical properties, mechanisms of interaction, and the geochemical conditions of the surrounding environment. Plutonium generally has very low solubility, low mobility, and few health consequences from environmental forms, and therefore poses little risk to living systems under nearly all conditions. This article provides a brief overview of the chemistry and behavior of plutonium in aqueous solutions and in the environment.
Americium and Curium: Radionuclides
Authors: Runde Wolfgang
Published: December 2011

Following the element plutonium, americium and curium are the sixth and seventh elements in the actinide series. The primordial americium and curium isotopes that formed during the creation of the solar system billions of years ago have decayed away. Today, residual amounts of americium and curium isotopes present in nature are anthropogenic, originating from human activities such as nuclear weapons testing or nuclear energy generation. All americium and curium isotopes are highly radioactive and pose extreme health hazards to humans, especially when ingested or inhaled. Complexation reactions with environmentally relevant ligands, separation chemistry, the mechanistic studies of the processes that Am(III) and Cm(III) undergo at the solid–liquid interface in nature, and innovative process applications have been the focus of many studies during recent decades.

In general, the trivalent oxidation state of americium and curium is the most stable under environmental conditions. As hard cations, the Am3+and Cm3+ions form strong complexes with hard donor ligands and adsorb readily on surfaces. In the absence of carbonate, hydrolysis leads to sparingly soluble Am(III) and Cm(III) hydroxide complexes. Americium and curium are commonly used in home smoke detectors and for oil‐well logging (241Am), and as radioisotope thermoelectric generators (244Cm).
PacificOcean: Radionuclides
Authors: Aoyama Michio
Published: December 2011

The main sources of anthropogenic radioactivity to the Pacific Ocean are fallout from atmospheric nuclear weapon testing. The releases from the nuclear fuel reprocessing plant and from the Chernobyl accident in 1986 are minimal in this region. Consequently, the radionuclide levels in the Pacific Ocean are low, about a few becquerel per cubic meter for137Cs in the surface water.

137Cs,239, 240Pu (other transuranics), and90Sr are discussed in this article for the following reasons: (i)137Cs and90Sr, conservative in the open sea, are the most abundant anthropogenic radionuclides and the main contributors to human exposure and (ii)239, 240Pu are very long‐lived, nonconservative, and the most abundant among transuranics. The distributions and temporal change in these radionuclides in seawater and sediment are described. The current radionuclide levels in the Pacific Ocean are low and, therefore, of no concern from the radiological point of view.
IndianOcean: Radionuclides
Authors: Povinec Pavel P.
Published: December 2011

The distribution of global fallout radionuclides in Indian Ocean waters about four decades after their main injection from atmospheric nuclear weapons tests on the ocean surface is discussed. Recent data obtained in the framework of the WOMARS (Worldwide Marine Radioactivity Studies) and SHOTS (Southern Hemisphere Ocean Tracer Studies) projects are evaluated and compared with previous investigations carried out in the northern and southern Indian Ocean. The results indicate transport of surface waters labeled with radionuclides from the Mediterranean Sea, as well as from the Northwestern Pacific through the Indonesian Seas to the Indian Ocean. After passing the equator the waters circulate in the South Indian Ocean subtropical gyre, which accumulates radionuclide tracers and other contaminants on timescales of decades. This accumulation of contaminants in the Southern Indian Ocean is of environmental importance for the protection of the marine environment from land‐based contamination sources.
Arctic Ocean: Radionuclides
Authors: Zaborska Agata , Carroll JoLynn
Published: December 2011

This article presents an overview of the major sources, transport pathways, and fate of anthropogenic radionuclides in the Arctic marine environment. The dominant anthropogenic radionuclides present in Arctic seawater, sediments, and marine biota are cesium‐137 (
137
Cs), plutonium‐239 (
239
Pu), plutonium‐240 (
240
Pu), technetium‐99 (
99
Tc), and strontium‐90 (
90
Sr). Major sources contributing to the inventory of radionuclides in the Arctic Ocean and surrounding shelf seas include atmospheric fallout, European nuclear reprocessing facilities, the Chernobyl accident, and discharges from major Arctic rivers. Anthropogenic radionuclides from these sources are supplied to and redistributed within the Arctic Ocean via atmospheric, sea ice, currents, sediment, and river transport pathways. With the exception of a few locations, e.g., the underwater nuclear test site at Chernaya Bay, sediment, seawater, and biota activity concentrations in the Arctic are today very low, posing negligible biological and human‐health risks. However, the rate of warming of the Arctic is three times faster than other parts of the globe, raising the prospect of significant reorganization of system properties and processes with consequent changes to both the sources and fate of radionuclides in the Arctic in the decades to come.
OakRidgeReservation: Radionuclides
Authors: Jardine Philip M. , Brooks Scott C. , Watson David B.
Published: December 2011

The Department of Energy (DOE) facilities on the Oak Ridge Reservation (ORR) have a rich history of radionuclide separation technologies and research involving the conversion of uranium to plutonium for thermonuclear weapon parts during WWII and the Cold War era. During this period, tens of millions of curies of radioactive waste was typically disposed in unlined surface impoundments and shallow subsurface trenches. The disposal methods have resulted in a daunting challenge of remediating huge below‐ground inventories of legacy radioactive waste, since the scale of the problem is at the watershed level. The focus of this article is to provide an overview of past and current activities on the ORR that involve the use, disposal, and remediation of radionuclide isotopes. The aim of the article is to provide an improved understanding of why radioactive material was generated, how it was used and disposed of, and how it continues to interact with the environment.
BlackSea: Radionuclides
Authors: Egorov Victor , Gulin Sergey , Polikarpov Gennady , Osvath Iolanda
Published: December 2011

Anthropogenic radionuclide pollution of the Black Sea originated primarily from the atmospheric nuclear weapon tests carried out during the late 1950s and early 1960s before the signing of the Test Ban Treaty in 1963, and after the Chernobyl nuclear power plant (NPP) accident in April 1986. Chernobyl atmospheric fallout deposited 1.7–2.4 PBq of137Cs into the Black Sea surface, which temporarily increased the137Cs inventory of the 0–50 m surface layer by a factor of 6–10 in comparison with its pre‐Chernobyl value. The contribution of Chernobyl‐origin90Sr (0.1–0.3 PBq) from atmospheric fallout was lower in comparison with that of137Cs. A subsequent90Sr input from the Danube and the Dnieper Rivers (about 0.16 PBq) was an important contribution to the budget of this radionuclide in the Black Sea, while the riverine137Cs input (0.02–0.03 PBq) was insignificant. The decrease of the137Cs inventory in the surface layer after Chernobyl accident has been mainly controlled by vertical mixing, loss through the Bosporus Strait, and radioactive decay. The loss through the Bosporus accounted for 2–2.5% of the137Cs inventory. In the case of90Sr, these processes have been compensated by river inputs from the Dnieper and Danube up to 1994–1995 and partially after 2000. The vertical mixing of137Cs and90Sr was mainly effective within the 0–200 m layer. Sediment inventories of137Cs in the Danube and Dnieper delta regions exceeded by one order of magnitude the values in the slope zone and two orders of magnitude those in the deep basin. Because of the self‐cleaning processes, the current levels of anthropogenic radionuclides are relatively low in all the components of the Black Sea environment, and therefore do not pose any radiological problem. Doses to marine biota from137Cs and90Sr are several orders of magnitude lower than those from natural210Po.
Fuel Cells: Enzymes and Microbes for Energy Production
Authors: Barrière Frédéric
Published: December 2011

The use of redox enzymes and bacteria as catalysts at the electrodes of fuel cells is reviewed with an emphasis on some inorganic aspects of the research. In biofuel cells, redox enzymes catalyze the oxidation of organic substrates like glucose at the anode or the reduction of electron acceptors like dioxygen at the cathode. Many of the redox enzymes have metal‐active sites that are of interest to the bioinorganic chemist. Moreover, efficient electrical linking between a redox enzyme and an electrode requires the mediation of a small electroactive molecule, capable of diffusing and shuttling electrons between the core of the enzyme and the electrode surface. These redox mediators may be coordination or organometallic complexes. In microbial fuel cells, the biocatalysts are living microorganisms, most often bacteria. At the anode, the bacteria metabolize organic substrates like sugars or acetate. In the absence of dioxygen or other natural electron acceptors, some species of bacteria may transfer electrons to the anode of a fuel cell. Different mechanisms for bacterial anodic electron transfer are proposed and discussed in this article. One common feature of these processes is the direct or indirect linking of the electrode surface to the outer‐membrane‐bound electron‐transfer chain of the respiratory system of bacteria. This system contains a chain of redox enzymes containing cytochrome‐type active sites that may be directly electrochemically addressed in some cases. At the cathode of microbial fuel cell, some bacteria may also accept electrons and catalyze the reduction of electron acceptors like nitrate, sulfate, and dioxygen. Given the importance of dioxygen reduction in most fuel cells, a perspective is given on the way inorganic chemistry may contribute to the design of efficient electrode catalysts through molecular, material, enzymatic, or biological approaches. Finally, the prospect of self‐sustained biological fuel cell is discussed with integration of plants with their anolyte as an in situ fuel provider in the form of root exudates, or enrichment of dioxygen in their catholyte with photosynthetic activity of algae. Although the emphasis is on the inorganic themes of biological fuel cell research, the important multidisciplinary approaches of the topic are put forward. This article also contains discussions on the relative performances and applications of both types of biological fuel cells.
Hydrogen Economy
Authors: Wells Stephen A. , Sartbaeva Asel , Kuznetsov Vladimir L. , Edwards Peter P.
Published: December 2011

Hydrogen technologies and fuel cells evoke, for many people, vision of a future in which electrical energy and heat can be generated not only cleanly but also noiselessly and efficiently—with only water as by‐product at the point of use. These technologies—particularly for transport applications—are viewed as a bridge to a sustainable and secure energy future. But despite its clear promise, hydrogen is today still largely only a potential energy carrier of the future; this chemical element is usually a key component of long‐term energy policies. But, as recently noted by Eames and McDowall: “…it is expectations about, and visions of, the future of hydrogen that are currently driving investment and research.”

This article explores some of these expectations; in particular, we examine important scientific and technological issues concerning the production, storage, and utilization of hydrogen, as well as outlining the interrelated—and equally challenging—socioeconomic issues for a future hydrogen energy economy. We hope that this approach allows the reader to assess the complexities, challenges—and, of course, the considerable potential—of hydrogen and fuel cells in long‐term transport strategies. We illustrate the very considerable central role of chemistry in building innovative solutions to key areas of a future hydrogen energy economy.
Fuel Cells: Proton Exchange Membranes
Authors: Devanathan Ram
Published: December 2011

Proton exchange membrane, also known as polymer electrolyte membrane, fuel cells (PEMFCs) offer the promise of efficient conversion of chemical energy of fuel, such as hydrogen or methanol, into electricity with minimal pollution. Their widespread use to power zero‐emission automobiles as part of a hydrogen economy can contribute to enhanced energy security and reduction in greenhouse gas emissions. However, the commercial viability of PEMFC technology is hindered by high cost associated with the membrane‐electrode assembly (MEA) and poor membrane durability under prolonged operation at elevated temperature. Membranes for automotive fuel cell applications need to perform well over a period comparable to the life of an automotive engine and under heavy load cycling including start‐stop cycling under subfreezing conditions. The combination of elevated temperature, changes in humidity levels, physical stresses, and harsh chemical environment contribute to membrane degradation. Perfluorinated sulfonic acid (PFSA)‐based membranes, such as Nafion
®
, have been the mainstay of PEMFC technology. Their limitations, in terms of cost and poor conductivity at low hydration, have led to continuing research into membranes that have good proton conductivity at elevated temperatures above 120 °C and under low‐humidity conditions. Such membranes have the potential to avoid catalyst poisoning, simplify fuel cell design, and reduce the cost of fuel cells. Hydrocarbon‐based membranes are being developed as alternatives to PFSA membranes, but concerns about chemical and mechanical stability and durability remain. Novel anhydrous membranes based on polymer gels infused with protic ionic liquids have also been recently proposed, but considerable fundamental research is needed to understand proton transport in novel membranes and evaluate durability under fuel cell operating conditions. In order to advance this promising technology, it is essential to rationally design the next generation of PEMs on the basis of an understanding of chemistry, membrane morphology, and proton transport obtained from experiment, theory, and computer simulation.
Oxygen Production from Water: Molecular Catalysts
Authors: Llobet Antoni , Romain Sophie
Published: December 2011

This article presents an overview of the established basis needed for the oxidation of water to dioxygen, and thus the demands a catalyst needs to fulfill to be able to perform this reaction. It further describes the structure, redox properties, and performance of selected molecular catalysts that have been reported to be able to perform this difficult reaction and that can be considered as references in the field. The scarce reaction mechanism studies that have been described are also summarized and discussed. The article mainly deals with Ru complexes, because they constitute most of the available literature at the moment, and are classified as a function of their nuclearity. Other transition metal complexes of relevance based on Ir, Co, and Mn are also described. Finally, a few conclusions are drawn from the global description of all of the catalysts presented here, and the challenges and opportunities in the field are also discussed.